驗證Cisco IOS XR和BGP上的路徑MTU探索
目錄
簡介
本檔案介紹Cisco IOS® XR裝置上的傳輸控制通訊協定(TCP)路徑最大傳輸單元(MTU)探索(PMTUD)。
背景資訊
PMTUD機制會嘗試判斷不需要在兩個主機之間的路徑中的任何位置進行分段的最大網際網路通訊協定(IP)封包大小。建立的值是指定的路徑MTU，並等於每一躍點上MTU值的最小值。如果您在傳輸資訊時考慮路徑MTU，便會讓您充分利用網路容量，並避免分段和傳輸效率。使用邊界閘道通訊協定(BGP)作為使用者端通訊協定（其會逐步顯示PMTUD行為），跨各種情形引入PMTUD機制和執行。
TCP PMTUD和TCP MSS
TCP利用PMTUD結果來影響本機最大區段大小(MSS)，這表示它會動態調整到發現的路徑MTU。因此，在繼續使用PMTUD之前，您可以快速檢視TCP最大區段大小(MSS)，並瞭解其意義和用途。
根據RFC879中的MSS原始定義：MSS選項的定義如下：在沒有IP報頭選項的IP資料包中傳輸沒有TCP報頭選項的TCP資料段中，此TCP選項的傳送方可以接收的最大資料八位元數。
闡明一些方面並向實施者提供建議， RFC6691 突出顯示應如何計算MSS值：
當您計算要放入TCP MSS選項的值時，MTU值應只減小固定IP和TCP標頭的大小，而不應減小以考慮任何可能的IP或TCP選項；反之，傳送者必須減少TCP資料長度，以反映其傳送的封包中包含的任何IP或TCP選項。
有關更詳細的MSS定義，請參閱IOS XR 6.7.x版Cisco ASR 9000系列路由器路由配置指南:
MSS是電腦或通訊裝置可以在單一未分段TCP區段中接收的最大資料量。所有TCP會話都受單個資料包中可傳輸位元組數的限制；此限製為MSS。在將資料包向下傳遞到IP層之前，TCP會將資料包在傳輸隊列中分割成塊。
TCP MSS值取決於介面的MTU，這是在一個情況下通訊協定可以傳輸的最大資料長度。最大TCP資料包長度由源裝置上出站介面的MTU和目的裝置在TCP設定過程中通告的MSS共同決定。MSS越接近MTU，BGP訊息的傳輸就越有效。每個資料流方向可以使用不同的MSS值。
對於給定TCP作業階段上的MSS，TCP應該考慮的值是什麼？如何計算？
根據RFC879，您有以下預設值：主機不能傳送大於576個八位位元組的資料包，除非它們知道目的主機準備接受更大的資料包。TCP最大資料段大小是IP最大資料包大小減去40。
預設IP最大資料包大小為576。
預設TCP最大資料段大小為536。
此值會考慮IP MTU值576位元組。但是如果忽略實際IP MTU值，則TCP MSS計算可以總結如下：
- Active Peer — 計算並傳送帶SYN封包的初始MSS。
MSS = IPMTU - sizeof(minimum TCPHDR) - sizeof(minimum IPHDR)
Where,
sizeof(minimum TCPHDR) = 20 bytes.
sizeof(minimum IPHDR) = 20 bytes.
- 被動對等體 — 計算初始MSS，與從主動對等體接收的MSS進行比較，並以這些MSS值中的較低者傳送SYN、ACK。
MIN[IPMTU - sizeof(minimum TCPHDR) - sizeof(minimum IPHDR) , Received MSS value]
Where,
sizeof(minimum TCPHDR) = 20 bytes.
sizeof(minimum IPHDR) = 20 bytes.
Received MSS value = MSS value received with Active Peer TCP SYN.
沒有關於MSS選項值的交涉。每個節點確定自己的值，並在TCP會話建立時宣佈相同的值。顯然，如果可從PMTUD推匯出MSS計算時考慮的IP MTU值，則對於指定的路徑MTU，MSS值可調整為最有效的值。Cisco IOS XR行為具有一些有關MSS計算和PMTUD角色的具體資訊，概述如下。
Cisco IOS XR上預設停用PMTUD:
- 根據以下內容，本機初始MSS計算會考慮IP MTU:
- 如果直接連線的對等體 — 請考慮輸出介面IP MTU。
- 如果是非直接連線的對等體 — 請考慮使用1280位元組的IP MTU。
- MSS值受已設定的TCP選項影響。
在Cisco IOS XR上啟用PMTUD時：
- 根據以下內容，本機初始MSS計算會考慮IP MTU:
- 無論直接連線/非直接連線對等點如何 — 請考慮輸出介面IP MTU。
- MSS值受已設定的TCP選項影響。
關於PMTUD機制和實施的其他詳細資訊需要考慮，本檔案將通過下表中總結的實際例子加以介紹。此表還顯示所考慮的每個場景的主動和被動TCP對等體IP MTU以及所選MSS值。
案例 — TCP PMTUD已停用
使用預設MTU值
圖2.1.使用預設MTU值
在圖2.1所示的eBGP對等體管理TCP連線的情況下，這意味著，它充當活動角色，並在目標埠179上啟動與R5的TCP會話。對等體是直接連線的，並且兩者都在各自的介面上使用預設IP MTU值。根據本文檔開頭部分共用的資訊，此方案中的MSS計算可以總結如下：
- 兩個節點都使用預設IP MTU 1500位元組
- 預設情況下禁用TCP路徑MTU發現
- TCP對等點直接連線
- R6管理BGP連線
- R6傳送MSS為1460位元組的SYN
- 1500（介面IP MTU） — 20(minTCP_H)- 20(minIP_H)
- R5傳送SYN、ACK，MSS為1460位元組
- 傳送[已接收MSS;本地初始MSS]
- 接收的MSS 1460位元組；本地初始MSS 1460位元組
- 兩個對等點上均使用最低MSS值
TCP會話詳細資訊，如R6 - ACTIVE上所示：
! – As seen on R6 – ACTIVE
RP/0/0/CPU0:R6#show interfaces gigabitEthernet 0/0/0/0
Fri Jan 8 09:35:48.553 UTC
GigabitEthernet0/0/0/0 is up, line protocol is up
Interface state transitions: 1
Hardware is GigabitEthernet, address is fa16.3e85.3dc2 (bia fa16.3e85.3dc2)
Internet address is 10.5.6.6/30
MTU 1514 bytes, BW 1000000 Kbit (Max: 1000000 Kbit)
<snip>
RP/0/0/CPU0:R6#show tcp brief
Fri Jan 8 09:36:22.491 UTC
PCB VRF-ID Recv-Q Send-Q Local Address Foreign Address State
<snip>
0x121649fc 0x60000000 0 0 10.5.6.6:24454 10.5.6.5:179 ESTAB
<snip>
RP/0/0/CPU0:R6#show tcp detail pcb 0x121649fc
Fri Jan 8 09:37:00.888 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 09:28:28 2021
PCB 0x121649fc, SO 0x121561b8, TCPCB 0x12156f64, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 78
Local host: 10.5.6.6, Local port: 24454 (Local App PID: 1011918)
Foreign host: 10.5.6.5, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 13 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 10 2 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3757770712 snduna: 3757770960 sndnxt: 3757770960
sndmax: 3757770960 sndwnd: 32574 sndcwnd: 4380
irs: 1072103647 rcvnxt: 1072103895 rcvwnd: 32593 rcvadv: 1072136488
SRTT: 155 ms, RTTO: 540 ms, RTV: 385 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 50 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R6
TCP會話詳細資訊，如R5上所示 — PASSIVE:
! – As seen on R5 – PASSIVE
RP/0/0/CPU0:R5#show interfaces gigabitEthernet 0/0/0/0
Fri Jan 8 09:33:04.564 UTC
GigabitEthernet0/0/0/0 is up, line protocol is up
Interface state transitions: 1
Hardware is GigabitEthernet, address is fa16.3ead.518f (bia fa16.3ead.518f)
Internet address is 10.5.6.5/30
MTU 1514 bytes, BW 1000000 Kbit (Max: 1000000 Kbit)
<snip>
RP/0/0/CPU0:R5#show tcp brief
Fri Jan 8 09:33:53.221 UTC
PCB VRF-ID Recv-Q Send-Q Local Address Foreign Address State
<snip>
0x12155884 0x60000000 0 0 10.5.6.5:179 10.5.6.6:24454 ESTAB
<snip>
RP/0/0/CPU0:R5#show tcp detail pcb 0x12155884
Fri Jan 8 09:34:47.317 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 09:28:29 2021
PCB 0x12155884, SO 0x1215568c, TCPCB 0x12155a54, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 78
Local host: 10.5.6.5, Local port: 179 (Local App PID: 1044686)
Foreign host: 10.5.6.6, Foreign port: 24454
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 9 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 9 7 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 1072103647 snduna: 1072103857 sndnxt: 1072103857
sndmax: 1072103857 sndwnd: 32631 sndcwnd: 4380
irs: 3757770712 rcvnxt: 3757770922 rcvwnd: 32612 rcvadv: 3757803534
SRTT: 47 ms, RTTO: 300 ms, RTV: 170 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 219 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R5#
使用非預設MTU值 — 活動TCP對等體
映像2.2 — 活動對等體使用非預設MTU值
此場景與上一場景基本相同，唯一的區別是活動TCP對等體R6現在使用非預設IP MTU值。請注意被動TCP對等體R5是如何對MSS值進行初始計算和決策的。此場景中的TCP MSS計算可以總結如下：
- R6使用非預設IP MTU 4460位元組
- 預設情況下禁用TCP路徑MTU發現
- TCP對等點直接連線
- R6管理BGP連線
- R6傳送MSS為4420位元組的SYN
- 4460（介面IP MTU） — 20(minTCP_H)- 20(minIP_H)
- R5傳送SYN，ACK，MSS為1460位元組
- 傳送[Received MSS;本地初始MSS]
- 已接收4420位元組；本地初始MSS 1460位元組
- 兩個對等點上均使用最低MSS值
源自R6的TCP SYN:
! – TCP SYN sourced from R6
140 1598.150521 10.5.6.6 10.5.6.5 TCP 62 35502 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=4420 WS=1
Frame 140: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:85:3d:c2 (fa:16:3e:85:3d:c2), Dst: fa:16:3e:ad:51:8f (fa:16:3e:ad:51:8f)
Internet Protocol Version 4, Src: 10.5.6.6, Dst: 10.5.6.5
Transmission Control Protocol, Src Port: 35502, Dst Port: 179, Seq: 0, Len: 0
Source Port: 35502
Destination Port: 179
[Stream index: 6]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 28 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x219d [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 4420 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 4420
Window scale: 0 (multiply by 1)
End of Option List (EOL)
TCP SYN、源自R5的ACK:
! – TCP SYN, ACK sourced from R5
141 1598.154866 10.5.6.5 10.5.6.6 TCP 62 179 → 35502 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1460 WS=1
Frame 141: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:ad:51:8f (fa:16:3e:ad:51:8f), Dst: fa:16:3e:85:3d:c2 (fa:16:3e:85:3d:c2)
Internet Protocol Version 4, Src: 10.5.6.5, Dst: 10.5.6.6
Transmission Control Protocol, Src Port: 179, Dst Port: 35502, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 35502
[Stream index: 6]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 28 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xe2b4 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
TCP會話詳細資訊，如R6 - ACTIVE上所示：
! - as seen on R6 – Active
RP/0/0/CPU0:R6#show interfaces gigabitEthernet 0/0/0/0
Fri Jan 8 09:46:54.138 UTC
GigabitEthernet0/0/0/0 is up, line protocol is up
Interface state transitions: 1
Hardware is GigabitEthernet, address is fa16.3e85.3dc2 (bia fa16.3e85.3dc2)
Internet address is 10.5.6.6/30
MTU 4474 bytes, BW 1000000 Kbit (Max: 1000000 Kbit)
<snip>
RP/0/0/CPU0:R6#show tcp detail pcb 0x1215761c
Fri Jan 8 09:56:25.819 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 09:51:46 2021
PCB 0x1215761c, SO 0x12156f64, TCPCB 0x1216419c, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 886
Local host: 10.5.6.6, Local port: 35502 (Local App PID: 1011918)
Foreign host: 10.5.6.5, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 9 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 6 5 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 764231407 snduna: 764231579 sndnxt: 764231579
sndmax: 764231579 sndwnd: 32650 sndcwnd: 4380
irs: 2712512697 rcvnxt: 2712512869 rcvwnd: 32669 rcvadv: 2712545538
SRTT: 31 ms, RTTO: 300 ms, RTV: 130 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 50 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 4420, max MSS 4420
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R6#
TCP會話詳細資訊，如R5上所示 — PASSIVE:
! - as seen on R5 – Passive
RP/0/0/CPU0:R5#show tcp detail pcb 0x12155a98
Fri Jan 8 09:55:18.193 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 09:51:47 2021
PCB 0x12155a98, SO 0x12153ea0, TCPCB 0x12154e18, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 886
Local host: 10.5.6.5, Local port: 179 (Local App PID: 1044686)
Foreign host: 10.5.6.6, Foreign port: 35502
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 6 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 6 1 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 2712512697 snduna: 2712512850 sndnxt: 2712512850
sndmax: 2712512850 sndwnd: 32688 sndcwnd: 4380
irs: 764231407 rcvnxt: 764231560 rcvwnd: 32669 rcvadv: 764264229
SRTT: 107 ms, RTTO: 538 ms, RTV: 431 ms, KRTT: 0 ms
minRTT: 29 ms, maxRTT: 219 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 4420, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R5#
使用非預設MTU值 — 被動TCP對等體
映像2.3 — 被動對等體使用非預設MTU值。
仍然使用相同的eBGP方案，但現在使用非預設IP MTU配置被動TCP對等體R5，使用預設IP MTU值配置主動TCP對等體R6。與先前的方案一樣，請注意被動對等體R5如何選擇MSS值。此方案中的TCP MSS計算可總結如下：
- R5使用非預設IP MTU 4460位元組
- 預設情況下禁用TCP路徑MTU發現
- TCP對等點直接連線
- R6管理BGP連線
- R6傳送MSS為1460位元組的SYN
- 1500（介面IP MTU） — 20(minTCP_H)- 20(minIP_H)
- R5傳送SYN，ACK，MSS為1460位元組
- 傳送[Received MSS;本地初始MSS]
- 接收的MSS 1460位元組；本機初始MSS 4420位元組
- 兩個對等點上均使用最低MSS值
源自R6的TCP SYN:
! – TCP SYN sourced from R6
237 2696.666481 10.5.6.6 10.5.6.5 TCP 62 47007 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1460 WS=1
Frame 237: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:85:3d:c2 (fa:16:3e:85:3d:c2), Dst: fa:16:3e:ad:51:8f (fa:16:3e:ad:51:8f)
Internet Protocol Version 4, Src: 10.5.6.6, Dst: 10.5.6.5
Transmission Control Protocol, Src Port: 47007, Dst Port: 179, Seq: 0, Len: 0
Source Port: 47007
Destination Port: 179
[Stream index: 10]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 28 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x2025 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
TCP SYN、源自R5的ACK:
! – TCP SYN, ACK sourced from R5
238 2696.702792 10.5.6.5 10.5.6.6 TCP 62 179 → 47007 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1460 WS=1
Frame 238: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:ad:51:8f (fa:16:3e:ad:51:8f), Dst: fa:16:3e:85:3d:c2 (fa:16:3e:85:3d:c2)
Internet Protocol Version 4, Src: 10.5.6.5, Dst: 10.5.6.6
Transmission Control Protocol, Src Port: 179, Dst Port: 47007, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 47007
[Stream index: 10]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 28 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x7078 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
TCP會話詳細資訊，如R6 - ACTIVE上所示：
! - as seen on R6 - Active
RP/0/0/CPU0:R6#show tcp detail pcb 0x1215761c
Fri Jan 8 10:15:20.351 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 10:10:04 2021
PCB 0x1215761c, SO 0x12162aac, TCPCB 0x12156f64, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 103
Local host: 10.5.6.6, Local port: 47007 (Local App PID: 1011918)
Foreign host: 10.5.6.5, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 10 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 7 5 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3949093168 snduna: 3949093359 sndnxt: 3949093359
sndmax: 3949093359 sndwnd: 32631 sndcwnd: 4380
irs: 54439005 rcvnxt: 54439196 rcvwnd: 32650 rcvadv: 54471846
SRTT: 75 ms, RTTO: 459 ms, RTV: 384 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 50 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R6#
TCP會話詳細資訊，如R5上所示 — PASSIVE:
! - as seen on R5 – Passive
RP/0/0/CPU0:R5#show interfaces gigabitEthernet 0/0/0/0
Fri Jan 8 10:10:39.110 UTC
GigabitEthernet0/0/0/0 is up, line protocol is up
Interface state transitions: 1
Hardware is GigabitEthernet, address is fa16.3ead.518f (bia fa16.3ead.518f)
Internet address is 10.5.6.5/30
MTU 4474 bytes, BW 1000000 Kbit (Max: 1000000 Kbit)
<snip>
RP/0/0/CPU0:R5#show tcp detail pcb 0x121550fc
Fri Jan 8 10:14:20.105 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 10:10:05 2021
PCB 0x121550fc, SO 0x12154e18, TCPCB 0x12154304, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 103
Local host: 10.5.6.5, Local port: 179 (Local App PID: 1044686)
Foreign host: 10.5.6.6, Foreign port: 47007
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 7 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 7 2 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 54439005 snduna: 54439177 sndnxt: 54439177
sndmax: 54439177 sndwnd: 32669 sndcwnd: 4380
irs: 3949093168 rcvnxt: 3949093340 rcvwnd: 32650 rcvadv: 3949125990
SRTT: 117 ms, RTTO: 570 ms, RTV: 453 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 4420, max MSS 4420
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R5#
使用TCP選項 — XR活動
如本檔案前面所述，TCP選項(例如TCP MD5、TCP selective-ack或TCP timestamps)的使用會影響MSS計算，因為這些選項會導致在MSS計算中考慮其他位元組。
本節以及下一個目的是說明存在TCP選項時對等點進行的MSS計算。例如，使用TCP MD5身份驗證選項。請參閱映像2.4中的參考案例，如下圖所示。
影象2.4 — 使用TCP選項(MD5)- XR活動。
在此方案中，兩個對等體都使用預設IP MTU值、直接連線，並且對等體R6扮演TCP主動角色。已共用TCP MD5身份驗證帳戶的配置和使用，從而產生額外開銷。在此特定案例中，TCP MSS計算可以總結如下：
- 兩個節點都使用預設IP MTU 1500位元組
- 預設情況下禁用TCP路徑MTU發現
- TCP對等點直接連線
- 兩個節點上都啟用了TCP MD5身份驗證
- R6管理BGP連線
- R6傳送MSS為1436位元組的SYN
- 1500（介面IP MTU） — 20(minTCP_H)- 20(minIP_H)- 24位元組（IOS XR TCP選項額外負荷）
- R5傳送SYN，ACK，MSS為1436位元組
- 傳送[Received MSS;本地初始MSS]
- 收到1436位元組；本地初始MSS 1460位元組
- 兩個對等點上均使用最低MSS值
從摘要中可看出，Cisco IOS XR的行為方式並不嚴格符合RFC 879和RFC 6691，後者指出TCP選項不應計入MSS計算。
tcp標頭長度的額外因素的Cisco IOS XR帳戶進一步記錄在Cisco錯誤ID CSCvf20166中：
"(....)XR啟動BGP連線時，BGP首先建立套接字，然後設定套接字選項，包括MD5。這會使tcp選項標頭長度= 24。因此，初始MSS變為1500 - 40 - 24 = 1436。這被傳送到對等項和對等項使用min(1436, 1460)= 1436.(..)
源自R6的TCP SYN:
! – TCP SYN sourced from R6
430 5775.839420 10.5.6.6 10.5.6.5 TCP 82 24785 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1436 WS=1
Frame 430: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface 0
Ethernet II, Src: fa:16:3e:85:3d:c2 (fa:16:3e:85:3d:c2), Dst: fa:16:3e:ad:51:8f (fa:16:3e:ad:51:8f)
Internet Protocol Version 4, Src: 10.5.6.6, Dst: 10.5.6.5
Transmission Control Protocol, Src Port: 24785, Dst Port: 179, Seq: 0, Len: 0
Source Port: 24785
Destination Port: 179
[Stream index: 14]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 48 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xd62b [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (28 bytes), Maximum segment size, Window scale, No-Operation (NOP), TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1436 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1436
Window scale: 0 (multiply by 1)
No-Operation (NOP)
TCP MD5 signature
End of Option List (EOL)
TCP SYN、源自R5的ACK:
! – TCP SYN, ACK sourced from R5
431 5775.845744 10.5.6.5 10.5.6.6 TCP 82 179 → 24785 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1436 WS=1
Frame 431: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface 0
Ethernet II, Src: fa:16:3e:ad:51:8f (fa:16:3e:ad:51:8f), Dst: fa:16:3e:85:3d:c2 (fa:16:3e:85:3d:c2)
Internet Protocol Version 4, Src: 10.5.6.5, Dst: 10.5.6.6
Transmission Control Protocol, Src Port: 179, Dst Port: 24785, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 24785
[Stream index: 14]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 48 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xe83d [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (28 bytes), Maximum segment size, Window scale, No-Operation (NOP), TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1436 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1436
Window scale: 0 (multiply by 1)
No-Operation (NOP)
TCP MD5 signature
End of Option List (EOL)
TCP會話詳細資訊，如R6 - ACTIVE上所示：
! - as seen on R6 – Active
RP/0/0/CPU0:R6#show tcp detail pcb 0x1215761c
Fri Jan 8 11:14:13.599 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 11:01:21 2021
PCB 0x1215761c, SO 0x1216419c, TCPCB 0x121649fc, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 409
Local host: 10.5.6.6, Local port: 24785 (Local App PID: 1011918)
Foreign host: 10.5.6.5, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 17 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 14 13 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 1379482495 snduna: 1379482819 sndnxt: 1379482819
sndmax: 1379482819 sndwnd: 32498 sndcwnd: 4308
irs: 3750694052 rcvnxt: 3750694376 rcvwnd: 32517 rcvadv: 3750726893
SRTT: 55 ms, RTTO: 300 ms, RTV: 176 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 259 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 50 secs
State flags: none
Feature flags: MD5, Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1436, peer MSS 1436, min MSS 1436, max MSS 1436
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R6#
TCP會話詳細資訊，如R5上所示 — PASSIVE:
! - as seen on R5 – Passive
RP/0/0/CPU0:R5#show tcp detail pcb 0x12155d04
Fri Jan 8 11:12:51.984 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 11:01:22 2021
PCB 0x12155d04, SO 0x12154e18, TCPCB 0x12154304, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 409
Local host: 10.5.6.5, Local port: 179 (Local App PID: 1044686)
Foreign host: 10.5.6.6, Foreign port: 24785
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 14 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 14 3 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3750694052 snduna: 3750694357 sndnxt: 3750694357
sndmax: 3750694357 sndwnd: 32536 sndcwnd: 4308
irs: 1379482495 rcvnxt: 1379482800 rcvwnd: 32517 rcvadv: 1379515317
SRTT: 181 ms, RTTO: 443 ms, RTV: 262 ms, KRTT: 0 ms
minRTT: 29 ms, maxRTT: 219 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: MD5, Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1436, peer MSS 1436, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R5#
其他TCP選項也存在類似行為，在配置時這些選項會增加額外開銷並影響Cisco IOS XR中的MSS計算。請考慮配置TCP時間戳和TCP選擇性確認選項時記錄MSS計算的相同方案和這些示例。
TCP會話詳細資訊，如R6上所示 — ACTIVE — 已配置TCP選項時間戳和選擇性確認選項：
! - as seen on R6 – Active
! -- tcp timestamp configured
! -- 12 bytes of additional overhead
RP/0/0/CPU0:R6#show tcp detail pcb 0x1539c844
<snip>
Feature flags: Timestamp, Win Scale, Nagle
Request flags: Timestamp, Win Scale
Datagrams (in bytes): MSS 1448, peer MSS 1448, min MSS 1448, max MSS 1448
<snip>
! - as seen on R6 – Active
! -- tcp selective-ack configured
! -- 36 bytes of additional overhead
RP/0/0/CPU0:R6#show tcp detail pcb 0x1539df38
<snip>
Feature flags: Sack, Win Scale, Nagle
Request flags: Sack, Win Scale
Datagrams (in bytes): MSS 1424, peer MSS 1424, min MSS 1424, max MSS 1424
<snip>
! - as seen on R6 – Active
! -- tcp selective-ack and tcp timestamp configured
! -- 40 bytes of additional overhead
RP/0/0/CPU0:R6#show tcp detail pcb 0x1539e130
<snip>
State flags: none
Feature flags: Sack, Timestamp, Win Scale, Nagle
Request flags: Sack, Timestamp, Win Scale
Datagrams (in bytes): MSS 1420, peer MSS 1420, min MSS 1420, max MSS 1420
<snip>
! - as seen on R6 – Active
! -- MD5 and tcp selective-ack configured
! -- 36 bytes of additional overhead
RP/0/0/CPU0:R6#show tcp detail pcb 0x1539b3cc
<snip>
Feature flags: Sack, MD5, Win Scale, Nagle
Request flags: Sack, Win Scale
Datagrams (in bytes): MSS 1424, peer MSS 1424, min MSS 1424, max MSS 1424
<snip>
! - as seen on R6 – Active
! -- MD5 and tcp timestamp configured
! -- 36 bytes of additional overhead
RP/0/0/CPU0:R6#show tcp detail pcb 0x15397b4c
<snip>
Feature flags: MD5, Timestamp, Win Scale, Nagle
Request flags: Timestamp, Win Scale
Datagrams (in bytes): MSS 1424, peer MSS 1424, min MSS 1424, max MSS 1424
<snip>
! - as seen on R6 – Active
! -- MD5, tcp timestamp, and tcp selective-ack configured
! -- 40 bytes of additional overhead
RP/0/0/CPU0:R6#show tcp detail pcb 0x1539a4cc
<snip>
State flags: none
Feature flags: MD5, Timestamp, Win Scale, Nagle
Request flags: Timestamp, Win Scale
Datagrams (in bytes): MSS 1420, peer MSS 1420, min MSS 1420, max MSS 1420
<snip>
使用TCP選項 — XR被動
從先前的場景中，您可能已經注意到Cisco IOS XR節點在初始MSS計算方面處於被動角色時的不同行為。節點不考慮tcp選項標頭長度。此案例旨在突出顯示此截然不同的行為，思科錯誤ID也對此進行了描述：
"(...) — 當對等體發起連線時，它會將初始MSS作為1460傳送。XR TCP建立socket、pcb等，然後按指定順序採取以下兩個操作：
— 首先，它在減去tcp選項標頭長度後計算初始MSS。這是「0」，因為MD5選項尚未從偵聽套接字繼承到此套接字。
— 然後，它繼承「MD5」和其他選項，這使得「選項標頭位元組長度」為24。
因此，在此案例中，XR TCP將初始MSS傳送為1460，因此供兩者使用。(...)」
在此案例中，雖然活動的TCP對等體R8是Cisco IOS節點，但此事實不會帶來任何差異，也不會帶來任何場景要強調的細節。但是，有趣的是，請注意，與上一節場景所顯示的Cisco IOS XR不同，此處活動TCP對等體R8在初始MSS計算時不考慮TCP選項。
影象2.5 — 使用TCP選項(MD5)- XR被動。
兩個對等點使用預設的IP MTU值，且均已直接連線。Cisco IOS對等體R8扮演主動角色。此案例中的TCP MSS計算可總結如下：
- 兩個節點都使用預設IP MTU 1500位元組
- Cisco IOS XR R7預設禁用TCP路徑MTU發現
- Cisco IOS R8上預設啟用TCP路徑MTU發現
- TCP對等點直接連線
- 兩個節點上都啟用了TCP MD5身份驗證
- IOS R8管理BGP連線
- IOS R8傳送MSS為1460位元組的SYN
- 1500（介面IP MTU） — 20(minTCP_H)- 20(minIP_H)
- IOS XR R7傳送SYN、ACK，MSS為1460位元組
- 傳送[Received MSS;本地初始MSS]
- 接收的MSS 1460位元組；本地初始MSS 1460位元組
- 兩個對等點上均使用最低MSS值
源自R8的TCP SYN - Cisco IOS:
! – TCP SYN sourced from R8
96 5.907127 10.7.8.2 10.7.8.1 TCP 78 52975 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1460
Frame 96: 78 bytes on wire (624 bits), 78 bytes captured (624 bits) on interface 0
Ethernet II, Src: fa:16:3e:58:21:ba (fa:16:3e:58:21:ba), Dst: fa:16:3e:68:d9:e5 (fa:16:3e:68:d9:e5)
Internet Protocol Version 4, Src: 10.7.8.2, Dst: 10.7.8.1
Transmission Control Protocol, Src Port: 52975, Dst Port: 179, Seq: 0, Len: 0
Source Port: 52975
Destination Port: 179
[Stream index: 3]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 44 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xb612 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (24 bytes), Maximum segment size, TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
TCP MD5 signature
End of Option List (EOL)
TCP SYN、源自R7的ACK - Cisco IOS XR:
! – TCP SYN,ACK sourced from R7
97 0.003446 10.7.8.1 10.7.8.2 TCP 78 179 → 52975 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1460
Frame 97: 78 bytes on wire (624 bits), 78 bytes captured (624 bits) on interface 0
Ethernet II, Src: fa:16:3e:68:d9:e5 (fa:16:3e:68:d9:e5), Dst: fa:16:3e:58:21:ba (fa:16:3e:58:21:ba)
Internet Protocol Version 4, Src: 10.7.8.1, Dst: 10.7.8.2
Transmission Control Protocol, Src Port: 179, Dst Port: 52975, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 52975
[Stream index: 3]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 44 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xfb47 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (24 bytes), Maximum segment size, TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
TCP MD5 signature
End of Option List (EOL)
TCP會話詳細資訊，如R8上所示 — Cisco IOS - ACTIVE:
! – as seen from R8 - Cisco IOS
R8#show ip bgp neighbors
BGP neighbor is 10.7.8.1, remote AS 65007, external link
BGP version 4, remote router ID 192.168.0.7
BGP state = Established, up for 00:06:12
Last read 00:00:16, last write 00:00:16, hold time is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised and received
Address family IPv4 Unicast: advertised and received
Enhanced Refresh Capability: advertised
Multisession Capability:
Stateful switchover support enabled: NO for session 1
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 1 1
Keepalives: 7 7
Route Refresh: 0 0
Total: 9 9
Do log neighbor state changes (via global configuration)
Default minimum time between advertisement runs is 30 seconds
For address family: IPv4 Unicast
Session: 10.7.8.1
BGP table version 1, neighbor version 1/0
Output queue size : 0
Index 6, Advertise bit 0
6 update-group member
Slow-peer detection is disabled
Slow-peer split-update-group dynamic is disabled
Sent Rcvd
Prefix activity: ---- ----
Prefixes Current: 0 0
Prefixes Total: 0 0
Implicit Withdraw: 0 0
Explicit Withdraw: 0 0
Used as bestpath: n/a 0
Used as multipath: n/a 0
Used as secondary: n/a 0
Outbound Inbound
Local Policy Denied Prefixes: -------- -------
Total: 0 0
Number of NLRIs in the update sent: max 0, min 0
Last detected as dynamic slow peer: never
Dynamic slow peer recovered: never
Refresh Epoch: 1
Last Sent Refresh Start-of-rib: never
Last Sent Refresh End-of-rib: never
Last Received Refresh Start-of-rib: never
Last Received Refresh End-of-rib: never
Sent Rcvd
Refresh activity: ---- ----
Refresh Start-of-RIB 0 0
Refresh End-of-RIB 0 0
Address tracking is enabled, the RIB does have a route to 10.7.8.1
Connections established 6; dropped 5
Last reset 00:06:18, due to BGP Notification received of session 1, Administrative Reset
External BGP neighbor configured for connected checks (single-hop no-disable-connected-check)
Interface associated: GigabitEthernet0/1 (peering address in same link)
Transport(tcp) path-mtu-discovery is enabled
Graceful-Restart is disabled
SSO is disabled
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Connection is ECN Disabled, Mininum incoming TTL 0, Outgoing TTL 1
Local host: 10.7.8.2, Local port: 52975
Foreign host: 10.7.8.1, Foreign port: 179
Connection tableid (VRF): 0
Maximum output segment queue size: 50
Enqueued packets for retransmit: 0, input: 0 mis-ordered: 0 (0 bytes)
Event Timers (current time is 0x15DD97):
Timer Starts Wakeups Next
Retrans 10 0 0x0
TimeWait 0 0 0x0
AckHold 9 5 0x0
SendWnd 0 0 0x0
KeepAlive 0 0 0x0
GiveUp 0 0 0x0
PmtuAger 1 0 0x195465
DeadWait 0 0 0x0
Linger 0 0 0x0
ProcessQ 0 0 0x0
iss: 1154289541 snduna: 1154289755 sndnxt: 1154289755
irs: 2149897425 rcvnxt: 2149897635
sndwnd: 32612 scale: 0 maxrcvwnd: 16384
rcvwnd: 16175 scale: 0 delrcvwnd: 209
SRTT: 737 ms, RTTO: 2506 ms, RTV: 1769 ms, KRTT: 0 ms
minRTT: 7 ms, maxRTT: 1000 ms, ACK hold: 200 ms
uptime: 372981 ms, Sent idletime: 16648 ms, Receive idletime: 16431 ms
Status Flags: active open
Option Flags: nagle, path mtu capable, md5
IP Precedence value : 6
Datagrams (max data segment is 1460 bytes):
Rcvd: 18 (out of order: 0), with data: 8, total data bytes: 209
Sent: 16 (retransmit: 0, fastretransmit: 0, partialack: 0, Second Congestion: 0), with data: 9, total data bytes: 213
Packets received in fast path: 0, fast processed: 0, slow path: 0
fast lock acquisition failures: 0, slow path: 0
TCP Semaphore 0x0FBFA8A4 FREE
R8#
TCP會話詳細資訊，如R7上所示 — Cisco IOS XR - PASSIVE:
! – as seen from R7 – Cisco IOS XR
RP/0/0/CPU0:R7#show tcp detail pcb 0x12152e48
Wed Jan 13 13:03:43.363 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Wed Jan 13 12:58:16 2021
PCB 0x12152e48, SO 0x1213c130, TCPCB 0x12156060, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 947
Local host: 10.7.8.1, Local port: 179 (Local App PID: 983244)
Foreign host: 10.7.8.2, Foreign port: 52975
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 8 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 8 7 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 2149897425 snduna: 2149897616 sndnxt: 2149897616
sndmax: 2149897616 sndwnd: 16194 sndcwnd: 4380
irs: 1154289541 rcvnxt: 1154289736 rcvwnd: 32631 rcvadv: 1154322367
SRTT: 125 ms, RTTO: 552 ms, RTV: 427 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: MD5, Nagle
Request flags: none
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R7#
未直接連線的TCP對等體
當對等點沒有直接連線時，完成TCP MSS初始計算的方式將更改，如本文檔的介紹部分中所述。使用設定為預設IP MTU值的所有對等點的iBGP作業階段的案例，來逐步執行MSS計算。
映像2.6 — 未直接連線的TCP對等體 — iBGP。
需要注意的重要一點是，當禁用TCP路徑MTU發現且對等體未直接連線時，根據設計，Cisco IOS XR使用固定IP MTU值1280位元組。
在上圖中，R4扮演活動角色並管理TCP連線，R4在目的地埠179上開啟與R1的TCP會話。兩個節點在其介面上均使用預設IP MTU值。此方案中的MSS計算可概述如下：
- 所有節點均使用預設IP MTU 1500位元組
- 預設情況下禁用TCP路徑MTU發現
- TCP對等點未直接連線
- R4管理BGP連線
- R4傳送MSS為1240位元組的SYN
- 當對等點未直接連線且TCP路徑MTU探索已停用時，不會考慮介面MTU
- 根據Cisco IOS XR設計，1280位元組被認為是TCP_DEFAULT_MTU
- 1280(TCP_DEFAULT_MTU)- 20(minTCP_H)- 20(minIP_H)
- R1傳送SYN，ACK，MSS為1240位元組
- 傳送[已接收MSS;本地初始MSS]
- 接收的MSS 1240位元組；本地初始MSS 1240位元組
- 兩個對等點上均使用最低MSS值
來源為R4的TCP SYN:
! – TCP SYN sourced from R4
194 434.274181 192.168.0.4 192.168.0.1 TCP 62 37740 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1240 WS=1
Frame 194: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6), Dst: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54)
Internet Protocol Version 4, Src: 192.168.0.4, Dst: 192.168.0.1
Transmission Control Protocol, Src Port: 37740, Dst Port: 179, Seq: 0, Len: 0
Source Port: 37740
Destination Port: 179
[Stream index: 7]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 28 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x8643 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1240 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1240
Window scale: 0 (multiply by 1)
End of Option List (EOL)
源自R1的TCP SYN、ACK:
! – TCP SYN,ACK sourced from R1
195 434.277985 192.168.0.1 192.168.0.4 TCP 62 179 → 37740 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1240 WS=1
Frame 195: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54), Dst: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
Transmission Control Protocol, Src Port: 179, Dst Port: 37740, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 37740
[Stream index: 7]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 28 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xd8f7 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1240 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1240
Window scale: 0 (multiply by 1)
End of Option List (EOL)
R4上看到的TCP會話詳細資訊 — ACTIVE:
! – as seen on R4 - Active
RP/0/0/CPU0:R4#show tcp detail pcb 0x12154d3c
Fri Jan 8 12:32:41.096 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 12:17:46 2021
PCB 0x12154d3c, SO 0x12154460, TCPCB 0x1215486c, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 1577
Local host: 192.168.0.4, Local port: 37740 (Local App PID: 1052958)
Foreign host: 192.168.0.1, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 19 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 16 15 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 2075436506 snduna: 2075436868 sndnxt: 2075436868
sndmax: 2075436868 sndwnd: 32460 sndcwnd: 3720
irs: 4238127261 rcvnxt: 4238127623 rcvwnd: 32479 rcvadv: 4238160102
SRTT: 65 ms, RTTO: 300 ms, RTV: 40 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 30 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R4#
R1上看到的TCP會話詳細資訊 — PASSIVE:
! – as seen on R1 – Passive
RP/0/0/CPU0:R1#show tcp detail pcb 0x12155390
Fri Jan 8 12:23:52.041 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 12:17:43 2021
PCB 0x12155390, SO 0x121573e4, TCPCB 0x12156948, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 1577
Local host: 192.168.0.1, Local port: 179 (Local App PID: 983326)
Foreign host: 192.168.0.4, Foreign port: 37740
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 9 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 9 1 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 4238127261 snduna: 4238127471 sndnxt: 4238127471
sndmax: 4238127471 sndwnd: 32631 sndcwnd: 3720
irs: 2075436506 rcvnxt: 2075436716 rcvwnd: 32612 rcvadv: 2075469328
SRTT: 144 ms, RTTO: 578 ms, RTV: 434 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R1#
未直接連線的TCP對等體 — 使用TCP選項(MD5)
對於非直接連線對等體場景和使用TCP MD5身份驗證時，與前面所述的測試案例或場景沒有根本的差異。如之前使用TCP MD5驗證時所示，Cisco IOS XR會考慮其他額外負荷，且初始MSS值反映相同。請參閱前面的使用TCP選項 — XR主動和使用TCP選項 — XR被動部分以瞭解其他有關TCP選項對TCP MSS計算影響的詳細資訊。
映像2.7 — 未直接連線的TCP對等體 — iBGP + TCP MD5。
此案例中的TCP MSS計算可總結如下：
- 所有節點均使用預設IP MTU 1500位元組
- 預設情況下禁用TCP路徑MTU發現
- TCP對等點未直接連線
- R4管理BGP連線
- 目標R1未直接連線
- R4傳送MSS為1216位元組的SYN
- 當對等點未直接連線且TCP路徑MTU探索已停用時，不會考慮介面MTU
- 根據設計，1280位元組被視為TCP_DEFAULT_MTU
- 1280(TCP_DEFAULT_MTU)- 20(minTCP_H)- 20(minIP_H)- 24位元組（IOS XR TCP選項額外負荷）
- R1傳送SYN，ACK，MSS為1216位元組
- 傳送[已接收MSS;本地初始MSS]
- 接收的MSS 1216位元組；本地初始MSS 1240位元組
- 兩個對等點上均使用最低MSS值
來源為R4的TCP SYN:
! – TCP SYN sourced from R4
3425 3.691042 192.168.0.4 192.168.0.1 TCP 82 42135 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1216 WS=1
Frame 3425: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface 0
Ethernet II, Src: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6), Dst: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54)
Internet Protocol Version 4, Src: 192.168.0.4, Dst: 192.168.0.1
Transmission Control Protocol, Src Port: 42135, Dst Port: 179, Seq: 0, Len: 0
Source Port: 42135
Destination Port: 179
[Stream index: 10]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 48 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xc503 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (28 bytes), Maximum segment size, Window scale, No-Operation (NOP), TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1216 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1216
Window scale: 0 (multiply by 1)
No-Operation (NOP)
TCP MD5 signature
End of Option List (EOL)
源自R1的TCP SYN、ACK:
! – TCP SYN,ACK sourced from R1
3426 0.004186 192.168.0.1 192.168.0.4 TCP 82 179 → 42135 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1216 WS=1
Frame 3426: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface 0
Ethernet II, Src: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54), Dst: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
Transmission Control Protocol, Src Port: 179, Dst Port: 42135, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 42135
[Stream index: 10]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 48 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xbb05 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (28 bytes), Maximum segment size, Window scale, No-Operation (NOP), TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1216 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1216
Window scale: 0 (multiply by 1)
No-Operation (NOP)
TCP MD5 signature
End of Option List (EOL)
R4上看到的TCP會話詳細資訊 — ACTIVE:
! – as seen from R4 – Active
RP/0/0/CPU0:R4#show tcp detail pcb 0x12154490
Tue Jan 12 14:37:32.097 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Tue Jan 12 14:27:42 2021
PCB 0x12154490, SO 0x12155014, TCPCB 0x12155a84, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 1876
Local host: 192.168.0.4, Local port: 42135 (Local App PID: 1052958)
Foreign host: 192.168.0.1, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 14 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 11 9 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3124761989 snduna: 3124763317 sndnxt: 3124763317
sndmax: 3124763317 sndwnd: 32711 sndcwnd: 3648
irs: 1090344992 rcvnxt: 1090346320 rcvwnd: 32730 rcvadv: 1090379050
SRTT: 28 ms, RTTO: 300 ms, RTV: 57 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 30 secs
State flags: none
Feature flags: MD5, Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1216, peer MSS 1216, min MSS 1216, max MSS 1216
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R4#
R1上看到的TCP會話詳細資訊 — PASSIVE:
! – as seen from R1 – Passive
RP/0/0/CPU0:R1#show tcp detail pcb 0x12168df4
Tue Jan 12 14:36:38.860 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Tue Jan 12 14:27:32 2021
PCB 0x12168df4, SO 0x12156bf8, TCPCB 0x12157a44, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 1876
Local host: 192.168.0.1, Local port: 179 (Local App PID: 983326)
Foreign host: 192.168.0.4, Foreign port: 42135
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 12 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 12 1 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 1090344992 snduna: 1090346320 sndnxt: 1090346320
sndmax: 1090346320 sndwnd: 32730 sndcwnd: 3648
irs: 3124761989 rcvnxt: 3124763317 rcvwnd: 32711 rcvadv: 3124796028
SRTT: 150 ms, RTTO: 558 ms, RTV: 408 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: MD5, Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1216, peer MSS 1216, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R1#
未直接連線的TCP對等點 — 路徑段具有較低的IP MTU
在下一個場景中，目標是觀察並總結在預設條件下如果存在具有較低IP MTU的中間路徑區段會發生的情況，這表示已停用TCP PMTUD。請參閱此映像。
映像2.8 - R2/R3路徑段的IP MTU較低。
最初的情況是認為BGP資訊是最小的，也就是說，BGP對等點之間需要交換的任何內容，都可以使用適合在512位元組的最小路徑MTU之下的IP封包完成。在此假設下，MSS計算會如未直接連線的TCP對等體一節所述。R1和R4都選擇1240位元組的MSS值。
R4上看到的TCP會話詳細資訊 — ACTIVE:
! – as seen from R4 – Active
RP/0/0/CPU0:R4#show tcp detail pcb 0x15390fe8
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Wed May 12 12:09:48 2021
PCB 0x15390fe8, SO 0x15391a7c, TCPCB 0x15391368, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 835
Local host: 192.168.0.4, Local port: 39046 (Local App PID: 1196319)
Foreign host: 192.168.0.1, Foreign port: 179
(Local App PID/instance/SPL_APP_ID: 1196319/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 1267 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 1280 1235 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 1991226354 snduna: 1991250450 sndnxt: 1991250450
sndmax: 1991250450 sndwnd: 32578 sndcwnd: 2480
irs: 4276699304 rcvnxt: 4276746737 rcvwnd: 31568 rcvadv: 4276778305
SRTT: 213 ms, RTTO: 300 ms, RTV: 54 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 269 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 10, connect retry interval: 30 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
<snip>
R1上看到的TCP會話詳細資訊 — PASSIVE:
! – as seen from R1 – Passive
RP/0/0/CPU0:R1#show tcp detail pcb 0x15393770
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Wed May 12 12:09:46 2021
PCB 0x15393770, SO 0x15392224, TCPCB 0x153928cc, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 835
Local host: 192.168.0.1, Local port: 179 (Local App PID: 1192224)
Foreign host: 192.168.0.4, Foreign port: 39046
(Local App PID/instance/SPL_APP_ID: 1192224/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 1280 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 1264 1213 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 4276699304 snduna: 4276746718 sndnxt: 4276746718
sndmax: 4276746718 sndwnd: 31587 sndcwnd: 3720
irs: 1991226354 rcvnxt: 1991250431 rcvwnd: 32597 rcvadv: 1991283028
SRTT: 202 ms, RTTO: 355 ms, RTV: 153 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 309 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
<snip>
現在建立BGP作業階段時，請考慮觸發大小高於最小路徑MTU 512位元組的BGP更新訊息。從輸出中可看出，Cisco IOS XR不會使用BGP更新消息設定df位元，這表示傳輸BGP資訊時，會犧牲中間節點上的封包分段。
源自R1的BGP更新 — 被動：
! – as seen from R1 – Passive – BGP UPDATE
! – Note Total Length of 1097 bytes higher than the IP MTU value of 512 bytes at R2-R3 path segment
23 3.450878 192.168.0.1 192.168.0.4 BGP 1111 UPDATE Message
Frame 23: 1111 bytes on wire (8888 bits), 1111 bytes captured (8888 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1097
Identification: 0x5841 (22593)
Flags: 0x00
0... .... = Reserved bit: Not set
.0.. .... = Don't fragment: Not set
..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0x54a4 [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 39046, Seq: 20, Ack: 20, Len: 1057
Border Gateway Protocol - UPDATE Message
Marker: ffffffffffffffffffffffffffffffff
Length: 1057
Type: UPDATE Message (2)
Withdrawn Routes Length: 0
Total Path Attribute Length: 1034
Path attributes
Path Attribute - MP_REACH_NLRI
Path Attribute - ORIGIN: INCOMPLETE
Path Attribute - AS_PATH: empty
Path Attribute - MULTI_EXIT_DISC: 0
Path Attribute - LOCAL_PREF: 100
源自R1的BGP更新消息的分段發生在節點R2,R2介面GE0/0/0/1上的流量捕獲可以觀察到這一點。
節點R2上的IP分段：
! – as seen from R2 – GE0/0/0/1
! – Node R2 fragments original packet in three distinct packets
4 1.334852 192.168.0.1 192.168.0.4 BGP 522 UPDATE Message
5 0.000289 192.168.0.1 192.168.0.4 IPv4 522 Fragmented IP protocol (proto=TCP 6, off=488, ID=7b41)
6 0.000122 192.168.0.1 192.168.0.4 IPv4 135 Fragmented IP protocol (proto=TCP 6, off=976, ID=7b41)
! – Captured frame details
Frame 4: 522 bytes on wire (4176 bits), 522 bytes captured (4176 bits) on interface 0
Ethernet II, Src: fa:16:3e:61:25:f0 (fa:16:3e:61:25:f0), Dst: fa:16:3e:23:ab:27 (fa:16:3e:23:ab:27)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 508
Identification: 0x7b41 (31553)
Flags: 0x01 (More Fragments)
0... .... = Reserved bit: Not set
.0.. .... = Don't fragment: Not set
..1. .... = More fragments: Set
Fragment offset: 0
Time to live: 254
Protocol: TCP (6)
Header checksum: 0x14f1 [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 39046, Seq: 4276759681, Ack: 1991250830
Border Gateway Protocol - UPDATE Message
<snip>
Frame 5: 522 bytes on wire (4176 bits), 522 bytes captured (4176 bits) on interface 0
Ethernet II, Src: fa:16:3e:61:25:f0 (fa:16:3e:61:25:f0), Dst: fa:16:3e:23:ab:27 (fa:16:3e:23:ab:27)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 508
Identification: 0x7b41 (31553)
Flags: 0x01 (More Fragments)
0... .... = Reserved bit: Not set
.0.. .... = Don't fragment: Not set
..1. .... = More fragments: Set
Fragment offset: 488
Time to live: 254
Protocol: TCP (6)
Header checksum: 0x14b4 [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Data (488 bytes)
<snip>
Frame 6: 135 bytes on wire (1080 bits), 135 bytes captured (1080 bits) on interface 0
Ethernet II, Src: fa:16:3e:61:25:f0 (fa:16:3e:61:25:f0), Dst: fa:16:3e:23:ab:27 (fa:16:3e:23:ab:27)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 121
Identification: 0x7b41 (31553)
Flags: 0x00
0... .... = Reserved bit: Not set
.0.. .... = Don't fragment: Not set
..0. .... = More fragments: Not set
Fragment offset: 976
Time to live: 254
Protocol: TCP (6)
Header checksum: 0x35fa [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Data (101 bytes)
<snip>
案例 — 啟用TCP PMTUD
啟用PMTUD
啟用PMTUD後，無論對等點是直接連線還是非直接連線，MSS初始計算一律會考慮輸出介面IP MTU。
此案例可深入瞭解啟用PMTUD時的預期行為。此處，Cisco IOS XR節點R4扮演主動角色，管理TCP連線，並在目的地埠179上開啟與Cisco IOS XR節點R1的TCP會話。兩個節點在其介面上使用預設IP MTU值。
映像3.1 — 已啟用TCP PMTUD。
此方案中的MSS計算可概述如下：
- 所有節點均使用預設IP MTU 1500位元組
- 已啟用TCP路徑MTU探索
- TCP對等點未直接連線
- R4管理BGP連線
- R4傳送MSS為1460位元組的SYN
- 1500（介面IP MTU） — 20(minTCP_H)- 20(minIP_H)
- R1傳送SYN，ACK，MSS為1460位元組
- 傳送[已接收MSS;本地初始MSS]
- 接收的MSS 1460位元組；本地初始MSS 1460位元組
- 兩個對等點上均使用最低MSS值
為了突出啟用PMTUD所產生的行為變更，接下來的輸出說明事件的順序：
- 在預設停用PMTUD的情況下已建立TCP作業階段的初始狀態；
- 在TCP對等路由器R4和R1上設定和啟用PMTUD;
- TCP作業階段會重新啟動，MSS計算會發生，並受到TCP PMTUD的影響。
如R4上所示 — ACTIVE - TCP PMTUD已禁用（預設）：
! – as seen on R4 – Active
! – TCP path mtu discovery disabled (default)
! – TCP session initial state
RP/0/0/CPU0:R4#show tcp detail pcb 0x121536c8
Fri Jan 8 16:06:30.237 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 16:05:15 2021
PCB 0x121536c8, SO 0x12155370, TCPCB 0x12154f64, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 376
Local host: 192.168.0.4, Local port: 20155 (Local App PID: 1052958)
Foreign host: 192.168.0.1, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 6 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 3 2 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 357400981 snduna: 357401257 sndnxt: 357401257
sndmax: 357401257 sndwnd: 32546 sndcwnd: 3720
irs: 524019443 rcvnxt: 524019719 rcvwnd: 32565 rcvadv: 524052284
SRTT: 72 ms, RTTO: 416 ms, RTV: 344 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 30 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R4#
如R1上所示 — 被動 — TCP PMTUD已禁用（預設）：
! – as seen on R1 - Passive
! – TCP path mtu discovery disabled (default)
! – TCP session initial state
RP/0/0/CPU0:R1#show tcp detail pcb 0x12157020
Fri Jan 8 16:05:52.868 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 16:05:12 2021
PCB 0x12157020, SO 0x121565ac, TCPCB 0x121560ec, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 376
Local host: 192.168.0.1, Local port: 179 (Local App PID: 983326)
Foreign host: 192.168.0.4, Foreign port: 20155
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 3 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 3 1 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 524019443 snduna: 524019700 sndnxt: 524019700
sndmax: 524019700 sndwnd: 32584 sndcwnd: 3720
irs: 357400981 rcvnxt: 357401238 rcvwnd: 32565 rcvadv: 357433803
SRTT: 46 ms, RTTO: 300 ms, RTV: 249 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R1#
如R4上所示 — ACTIVE - TCP PMTUD已啟用：
! – 'debug tcp pmtud' output on R4
! - tcp path mtu discovery enabled and uses default Path MTU aging timer (10 min / 600000 msec)
RP/0/0/CPU0:Jan 8 16:09:28.285 : tcp[399]: [t21] Try to enable path MTU discovery(neww age timer: 10 min)
RP/0/0/CPU0:Jan 8 16:09:28.285 : tcp[399]: [t21] Path mtu is ON (age-timer: 10)
! – as seen on R4 – Active
! – TCP PMTUD is enabled
RP/0/0/CPU0:R4#show tcp detail pcb 0x121536c8
Fri Jan 8 16:11:00.138 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 16:05:15 2021
PCB 0x121536c8, SO 0x12155370, TCPCB 0x12154f64, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 376
Local host: 192.168.0.4, Local port: 20155 (Local App PID: 1052958)
Foreign host: 192.168.0.1, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 10 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 7 4 0
KeepAlive 1 0 0
PmtuAger 1 0 508096
GiveUp 0 0 0
Throttle 0 0 0
iss: 357400981 snduna: 357401333 sndnxt: 357401333
sndmax: 357401333 sndwnd: 32470 sndcwnd: 3720
irs: 524019443 rcvnxt: 524019795 rcvwnd: 32489 rcvadv: 524052284
SRTT: 116 ms, RTTO: 578 ms, RTV: 462 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 30 secs
State flags: PMTU ager
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R4#
如R1上所示 — 被動 — 已啟用TCP PMTUD:
! – 'debug tcp pmtud' output on R1
! - tcp path mtu discovery is enabled and uses default Path MTU aging timer (10 min / 600000 msec)
RP/0/0/CPU0:Jan 8 16:09:25.214 : tcp[399]: [t21] Try to enable path MTU discovery(neww age timer: 10 min)
RP/0/0/CPU0:Jan 8 16:09:25.214 : tcp[399]: [t21] Path mtu is ON (age-timer: 10)
! – as seen on R1 - Passive
! – TCP PMTUD is enabled
RP/0/0/CPU0:R1#show tcp detail pcb 0x12157020
Fri Jan 8 16:10:03.101 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 16:05:12 2021
PCB 0x12157020, SO 0x121565ac, TCPCB 0x121560ec, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 376
Local host: 192.168.0.1, Local port: 179 (Local App PID: 983326)
Foreign host: 192.168.0.4, Foreign port: 20155
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 7 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 7 4 0
KeepAlive 1 0 0
PmtuAger 1 0 562042
GiveUp 0 0 0
Throttle 0 0 0
iss: 524019443 snduna: 524019776 sndnxt: 524019776
sndmax: 524019776 sndwnd: 32508 sndcwnd: 3720
irs: 357400981 rcvnxt: 357401314 rcvwnd: 32489 rcvadv: 357433803
SRTT: 95 ms, RTTO: 528 ms, RTV: 433 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: PMTU ager
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R1#
請注意PMTU老化器計時器行為：
! – Note PmtuAger timer initial value is 10min
! – but after initial interval expires then it expires every 2min
! – As seen from 'debug tcp pmtud' output
! – TCP PMTUD is enabled
RP/0/0/CPU0:Jan 8 16:09:25.214 : tcp[399]: [t21] Try to enable path MTU discovery(neww age timer: 10 min)
RP/0/0/CPU0:Jan 8 16:09:25.214 : tcp[399]: [t21] Path mtu is ON (age-timer: 10)
RP/0/0/CPU0:Jan 8 16:19:25.233 : tcp[399]: [t21] PCB 0x12157020: Trying next higher MTU: 1240
RP/0/0/CPU0:Jan 8 16:21:25.245 : tcp[399]: [t21] PCB 0x12157020: Trying next higher MTU: 1240
RP/0/0/CPU0:Jan 8 16:23:25.256 : tcp[399]: [t21] PCB 0x12157020: Trying next higher MTU: 1240
如R4上所示 — ACTIVE - BGP Session restart - TCP SYN:
! – Once BGP session is cleared
! – TCP SYN sourced from R4 – Active
! – MSS calculation takes place and is influenced by TCP PMTUD
2734 4.810311 192.168.0.4 192.168.0.1 TCP 62 32077 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1460 WS=1
Frame 2734: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6), Dst: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54)
Internet Protocol Version 4, Src: 192.168.0.4, Dst: 192.168.0.1
Transmission Control Protocol, Src Port: 32077, Dst Port: 179, Seq: 0, Len: 0
Source Port: 32077
Destination Port: 179
[Stream index: 25]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 28 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x6398 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
如R1上所示 — 被動 — BGP會話重新啟動 — TCP SYN，ACK。
! – Once BGP session is cleared
! – TCP SYN,ACK sourced from R1 - Passive
! – MSS calculation takes place and is influenced by TCP PMTUD
2735 0.003879 192.168.0.1 192.168.0.4 TCP 62 179 → 32077 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1460 WS=1
Frame 2735: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54), Dst: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
Transmission Control Protocol, Src Port: 179, Dst Port: 32077, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 32077
[Stream index: 25]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 28 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xbf77 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
啟用TCP PMTUD並清除BGP作業階段後，在R4上看到的TCP作業階段詳細資訊 — ACTIVE:
! – BGP session re-established
! – as seen on R4 – Active
RP/0/0/CPU0:R4#show tcp detail pcb 0x121567f4
Fri Jan 8 16:45:13.928 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 16:41:49 2021
PCB 0x121567f4, SO 0x12154460, TCPCB 0x12156190, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 10
Local host: 192.168.0.4, Local port: 32077 (Local App PID: 1052958)
Foreign host: 192.168.0.1, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 8 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 5 3 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 1254100669 snduna: 1254100983 sndnxt: 1254100983
sndmax: 1254100983 sndwnd: 32508 sndcwnd: 4380
irs: 839938559 rcvnxt: 839938873 rcvwnd: 32527 rcvadv: 839971400
SRTT: 79 ms, RTTO: 485 ms, RTV: 406 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 30 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R4#
啟用TCP PMTUD並清除BGP作業階段後，在R1上看到的TCP作業階段詳細資訊 — 被動。
! – BGP session re-established
! – as seen on R1 – Passive
RP/0/0/CPU0:R1#show tcp detail pcb 0x121558cc
Fri Jan 8 16:44:59.448 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 16:41:46 2021
PCB 0x121558cc, SO 0x121556d4, TCPCB 0x121575bc, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 10
Local host: 192.168.0.1, Local port: 179 (Local App PID: 983326)
Foreign host: 192.168.0.4, Foreign port: 32077
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 6 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 6 3 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 839938559 snduna: 839938873 sndnxt: 839938873
sndmax: 839938873 sndwnd: 32527 sndcwnd: 4380
irs: 1254100669 rcvnxt: 1254100983 rcvwnd: 32508 rcvadv: 1254133491
SRTT: 76 ms, RTTO: 454 ms, RTV: 378 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 219 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R1#
PMTUD — 路徑區段的IP MTU更低
上一個案例有助於瞭解在啟用PMTUD的情況下建立初始TCP作業階段時發生的情況。此案例建立在上面，有助瞭解TCP PMTUD的運作方式及其對已建立TCP作業階段的影響。
映像3.2 — 已啟用PMTUD，但路徑區段的IP MTU較低。
將上一個映像視為參考，假設BGP會話已建立，並且R1會傳送大小大於512位元組的IP資料包所攜帶的BGP更新消息。啟用PMTUD後，現在已設定DF位元（不分段）。因此，節點R2丟棄IP資料包並傳送ICMP（網際網路控制消息協定）消息(無法到達目的地 — 型別3;需要分段 — 代碼4)傳回R1。在節點R1收到ICMP訊息後，會觸發PMTUD，並嘗試建立路徑最小的IP MTU。它使用一組定義良好的平台級別中的下一個較低值，即視為新的TCP會話MSS值。接著，TCP使用新的MSS值重新傳輸原始BGP更新，此程式會根據需要重複多次，直到ICMP訊息(無法到達目的地 — 型別3;不再接收需要分段 — 代碼4)。這表示直到使用的MSS值使得每個傳送的封包都位於最低路徑區段IP MTU之下。隨著時間的流逝，由PmtuAger計時器控制的PMTUD會反向經過平台級別，並將MSS提升回其最大值。在任何指定時間，如果ICMP消息(無法到達目的地 — 型別3 ;需要分段 — 再次收到代碼4)，然後PMTUD會如前所述運作。
接下來的輸出會瀏覽剛才介紹的PMTUD行為，並從已建立TCP作業階段的案例開始。這裡，Cisco IOS XR節點R4扮演主動角色，因此管理TCP連線，並在目的地埠179上開啟與R1的TCP會話。兩個節點在其介面上使用預設IP MTU值。此方案中的初始MSS計算可概述如下：
- R2和R3節點之間的中間網段使用非預設IP MTU 512位元組。
- R1和R4在其介面上使用預設MTU值。
- 已啟用TCP路徑MTU發現。
- TCP對等點沒有直接連線。
- R4管理BGP連線。
- R4傳送MSS為1460位元組的SYN。
- 1500（介面IP MTU） — 20(minTCP_H)- 20(minIP_H)。
- R1傳送SYN和MSS為1460位元組的ACK。
- 傳送[Received MSS ;本地初始MSS]。
- 接收的MSS 1460位元組；本地初始MSS 1460位元組。
- 兩個對等點上使用最小MSS值。
來源為R4的TCP SYN:
! – Initial TCP session establishment
! – TCP SYN sourced from R4
392 6.752774 192.168.0.4 192.168.0.1 TCP 62 32449 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1460 WS=1
Frame 392: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80), Dst: fa:16:3e:42:18:05 (fa:16:3e:42:18:05)
Internet Protocol Version 4, Src: 192.168.0.4, Dst: 192.168.0.1
Transmission Control Protocol, Src Port: 32449, Dst Port: 179, Seq: 0, Len: 0
Source Port: 32449
Destination Port: 179
[Stream index: 10]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 28 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x6858 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
源自R1的TCP SYN、ACK:
! – Initial TCP session establishment
! – TCP SYN,ACK sourced from R1
393 0.003628 192.168.0.1 192.168.0.4 TCP 62 179 → 32449 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1460 WS=1
Frame 393: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
Transmission Control Protocol, Src Port: 179, Dst Port: 32449, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 32449
[Stream index: 10]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 28 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x509e [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
建立BGP作業階段後，節點R1會傳送BGP更新訊息並接收ICMP訊息(無法到達目的地 — 型別3 ;需要分段 — 代碼4)返回源自R2節點。
之所以會出現這種情況，是因為傳送BGP更新消息的IP封包已設定DF位元，且R2/R3區段上使用的512位元組的IP MTU低於1116位元組的IP封包大小。如前所述，接收ICMP訊息會觸發PMTUD。
在R1 ICMP處，收到型別3/代碼4消息：
! – as seen from R1 – Passive
! – After session is established R1 sends BGP Update message with IP length of 1116 Bytes
! – note IP Header Flags shows DF bit set
528 5.893055 192.168.0.1 192.168.0.4 BGP 1130 UPDATE Message, KEEPALIVE Message
Frame 528: 1130 bytes on wire (9040 bits), 1130 bytes captured (9040 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1116
Identification: 0x8c37 (35895)
Flags: 0x02 (Don't Fragment)
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0xe09a [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 32449, Seq: 318, Ack: 251, Len: 1076
Border Gateway Protocol - UPDATE Message
Border Gateway Protocol - KEEPALIVE Message
<snip>
! – as seen from R1 – Passive
! – IP MTU on R2/R3 is lower than IP packet length and DF bit is set
! – R1 receives ICMP error message from R2
! – note R2 ICMP error message carries Next-Hop MTU
! – “The size in octets of the largest datagram that could be forwarded, along the path of
! the original datagram, without being fragmented at this router. The size includes the
! IP header and IP data, and does not include any lower-level headers.”
529 0.002423 10.2.3.1 192.168.0.1 ICMP 110 Destination unreachable (Fragmentation needed)
Frame 529: 110 bytes on wire (880 bits), 110 bytes captured (880 bits) on interface 0
Ethernet II, Src: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80), Dst: fa:16:3e:42:18:05 (fa:16:3e:42:18:05)
Internet Protocol Version 4, Src: 10.2.3.1, Dst: 192.168.0.1
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Total Length: 96
Identification: 0x0001 (1)
Flags: 0x00
Fragment offset: 0
Time to live: 255
Protocol: ICMP (1)
Header checksum: 0xac97 [validation disabled]
[Header checksum status: Unverified]
Source: 10.2.3.1
Destination: 192.168.0.1
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Internet Control Message Protocol
Type: 3 (Destination unreachable)
Code: 4 (Fragmentation needed)
Checksum: 0x2d52 [correct]
[Checksum Status: Good]
Length: 17
[Length of original datagram: 68]
Unused: 0011
MTU of next hop: 512
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1116
Identification: 0x8c37 (35895)
Flags: 0x02 (Don't Fragment)
Fragment offset: 0
Time to live: 254
Protocol: TCP (6)
Header checksum: 0xe19a [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 32449, Seq: 2847698730, Ack: 2130367817
Border Gateway Protocol - UPDATE Message
[Packet size limited during capture: IPv4 truncated]
在節點R1（由ICMP訊息觸發），TCP PMTUD會嘗試使用定義良好的平台(IP MTU)層中的下一個較低值來建立端對端的最低IP MTU。這些平台層級記錄在RFC1191 — 路徑MTU探索中。
MTU plateaus from RFC 1191
- values include both TCP and IP headers
65535
32000
17914
8166
4352
2002
1492
1006
508
296
68
但由於ICMP(目的地無法連線 — 型別3;需要分段 — 代碼4)節點R1收到的消息傳遞下一躍點的MTU，然後如圖所示，節點R1使用此值（在我們的示例中為512位元組），並調整TCP會話MSS值。請注意，原始TCP區段的長度為1076位元組，因此重新傳輸原始TCP區段需要三個封包。
如R1上所示 — 被動 — PMTUD操作：
! – As seen from R1 – Passive
! – Hint is provided by ICMP unreachable message MTU of next-hop field: 512 bytes
! – R1 then considers this value and retransmits BGP Update split in three distinct packets
! – Sum of TCP length = 472 + 472 + 132 = 1076 bytes
530 0.007497 192.168.0.1 192.168.0.4 TCP 526 [TCP Out-Of-Order] 179 → 32449 [ACK] Seq=318 Ack=251 Win=32593 Len=472
532 0.015374 192.168.0.1 192.168.0.4 TCP 526 [TCP Retransmission] 179 → 32449 [ACK] Seq=790 Ack=251 Win=32593 Len=472
533 0.004129 192.168.0.1 192.168.0.4 TCP 186 [TCP Retransmission] 179 → 32449 [PSH, ACK] Seq=1262 Ack=251 Win=32593 Len=132
如前所述，在一段時間內，傳送所有封包後，PMTUD會沿PmtuAger計時器所界定的相反方向走過平台層級，並嘗試根據適當的情境將MSS提升為其最大值。
如R1上所示 — PMTUD跨定義的平台：
! – As seen from R1 – Passive – 'debug tcp pmtud' and 'debug icmp' active
! – TCP PMTUD is triggered once ICMP unreachable received
RP/0/0/CPU0:May 12 09:09:22.763 UTC: ipv4_io[266]: IPv4 ICMP: Received ICMP too big from 192.168.0.1 about 192.168.0.4, MTU=512
RP/0/0/CPU0:May 12 09:09:22.763 UTC: ipv4_io[266]: ipv4_icmp_unreachable_rcvd ICMP unreach recvd: sending pak(0xb0c07d8f) to transport: 6, tid: 5
RP/0/0/CPU0:May 12 09:09:22.763 UTC: ipv4_io[266]: ip_icmp_lib_ipv4_receive: sending pak(0xb0c07d8f) to transport: 1, tid: 5
RP/0/0/CPU0:May 12 09:09:22.763 UTC: tcp[399]: [t4] PCB 0x15393770: Process ICMP Dest-unreach (next hop mtu: 512)
! – attempt new MSS 472 = MTU of next-hop(512) - TCP_H(20) - IP_H(20)
RP/0/0/CPU0:May 12 09:09:22.763 UTC: tcp[399]: [t4] PCB 0x15393770: Process ICMP Dest-unreach (next hop mtu: 512)
RP/0/0/CPU0:May 12 09:09:22.763 UTC: tcp[399]: [t4] PCB 0x15393770: Try to use new MSS: 472
RP/0/0/CPU0:May 12 09:09:22.763 UTC: tcp[399]: [t4] PCB 0x15393770, New path MTU decided to use: 472 configured tp_user_mss 0
! – over time PMTUD attempts to raise MSS as per egress interface configured MTU
RP/0/0/CPU0:May 12 09:19:22.782 UTC: tcp[399]: [t23] PCB 0x15393770: Trying next higher MTU: 966
RP/0/0/CPU0:May 12 09:21:22.793 UTC: tcp[399]: [t23] PCB 0x15393770: Trying next higher MTU: 1452
RP/0/0/CPU0:May 12 09:23:22.805 UTC: tcp[399]: [t23] PCB 0x15393770: Trying next higher MTU: 1460
在這些輸出上可觀察到最終狀態。特別要注意節點R1顯示的最小MSS值和最大MSS值，它突出顯示並顯示PMTUD已觸發。
R4上看到的TCP會話詳細資訊 — ACTIVE:
! – Final stage as seen from R4 – Active
RP/0/0/CPU0:R4#show tcp detail pcb 0x153913b8
Wed May 12 10:09:43.246 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Wed May 12 09:02:07 2021
PCB 0x153913b8, SO 0x153917f0, TCPCB 0x1538fb58, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 382
Local host: 192.168.0.4, Local port: 32449 (Local App PID: 1196319)
Foreign host: 192.168.0.1, Foreign port: 179
(Local App PID/instance/SPL_APP_ID: 1196319/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 72 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 71 69 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 2130367566 snduna: 2130368957 sndnxt: 2130368957
sndmax: 2130368957 sndwnd: 31453 sndcwnd: 2920
irs: 2847698412 rcvnxt: 2847700946 rcvwnd: 31799 rcvadv: 2847732745
SRTT: 220 ms, RTTO: 300 ms, RTV: 12 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 10, connect retry interval: 30 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
Num of peers with authentication info: 0
RP/0/0/CPU0:R4#
R1上看到的TCP會話詳細資訊 — PASSIVE:
! – Final stage as seen from R1 – Passive
RP/0/0/CPU0:R1#show tcp detail pcb 0x15393770
Wed May 12 10:12:41.432 UTC
==============================================================
Connection state is ESTAB, I/O status: 240, socket status: 0
Established at Wed May 12 09:02:05 2021
PCB 0x15393770, SO 0x15394ea0, TCPCB 0x15391c0c, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 382
Local host: 192.168.0.1, Local port: 179 (Local App PID: 1192224)
Foreign host: 192.168.0.4, Foreign port: 32449
(Local App PID/instance/SPL_APP_ID: 1192224/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 75 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 73 71 0
KeepAlive 1 0 0
PmtuAger 28 27 41595
GiveUp 0 0 0
Throttle 0 0 0
iss: 2847698412 snduna: 2847701003 sndnxt: 2847701003
sndmax: 2847701003 sndwnd: 31742 sndcwnd: 4380
irs: 2130367566 rcvnxt: 2130369014 rcvwnd: 31396 rcvadv: 2130400410
SRTT: 224 ms, RTTO: 300 ms, RTV: 23 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 259 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: PMTU ager
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 472, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x20 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
Num of peers with authentication info: 0
RP/0/0/CPU0:R1#
最後，如果在任何指定時間存在ICMP(目的地無法連線 — 型別3 ;需要分段 — 代碼4)訊息再次收到，然後PMTUD再次按之前所述運作。
如R1所示 — PMTUD已再次觸發：
! – As seen from R1 – Passive
! – TCP PMTUD is again triggered upon new ICMP unreachable received
! – Behavior can be triggered via clearing redistributed, network and aggregate routes originated
RP/0/0/CPU0:R1#clear bgp ipv4 all self-originated
Wed May 12 10:19:06.836 UTC
RP/0/0/CPU0:R1#
! – New BGP update message is sourced from R1 after clear bgp command
1707 1.712657 192.168.0.1 192.168.0.4 BGP 1121 UPDATE Message
Frame 1707: 1121 bytes on wire (8968 bits), 1121 bytes captured (8968 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1107
Identification: 0x1a38 (6712)
Flags: 0x02 (Don't Fragment)
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0x52a3 [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 32449, Seq: 2705, Ack: 1562, Len: 1067
Border Gateway Protocol - UPDATE Message
! – ICMP Destination Unreachable / Fragmentation needed is received and triggers PMTUD
1708 0.001614 10.2.3.1 192.168.0.1 ICMP 110 Destination unreachable (Fragmentation needed)
Frame 1708: 110 bytes on wire (880 bits), 110 bytes captured (880 bits) on interface 0
Ethernet II, Src: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80), Dst: fa:16:3e:42:18:05 (fa:16:3e:42:18:05)
Internet Protocol Version 4, Src: 10.2.3.1, Dst: 192.168.0.1
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Total Length: 96
Identification: 0x0002 (2)
Flags: 0x00
Fragment offset: 0
Time to live: 255
Protocol: ICMP (1)
Header checksum: 0xac96 [validation disabled]
[Header checksum status: Unverified]
Source: 10.2.3.1
Destination: 192.168.0.1
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Internet Control Message Protocol
Type: 3 (Destination unreachable)
Code: 4 (Fragmentation needed)
Checksum: 0x3b73 [correct]
[Checksum Status: Good]
Length: 17
[Length of original datagram: 68]
Unused: 0011
MTU of next hop: 512
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1107
Identification: 0x1a38 (6712)
Flags: 0x02 (Don't Fragment)
Fragment offset: 0
Time to live: 254
Protocol: TCP (6)
Header checksum: 0x53a3 [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 32449, Seq: 2847701117, Ack: 2130369128
Border Gateway Protocol - UPDATE Message
! – Note new/updated MSS value and PmtuAger
! - MSS 472 ; Aligned with “MTU of next hop” value contained in ICMP message
RP/0/0/CPU0:R1#show tcp detail pcb 0x15393770
Wed May 12 10:19:31.494 UTC
==============================================================
Connection state is ESTAB, I/O status: 240, socket status: 0
Established at Wed May 12 09:02:05 2021
PCB 0x15393770, SO 0x15394ea0, TCPCB 0x15391c0c, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 382
Local host: 192.168.0.1, Local port: 179 (Local App PID: 1192224)
Foreign host: 192.168.0.4, Foreign port: 32449
(Local App PID/instance/SPL_APP_ID: 1192224/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 83 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 80 77 0
KeepAlive 1 0 0
PmtuAger 32 30 575401
GiveUp 0 0 0
Throttle 0 0 0
iss: 2847698412 snduna: 2847702184 sndnxt: 2847702184
sndmax: 2847702184 sndwnd: 32173 sndcwnd: 944
irs: 2130367566 rcvnxt: 2130369147 rcvwnd: 32730 rcvadv: 2130401877
SRTT: 221 ms, RTTO: 300 ms, RTV: 16 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 259 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: PMTU ager
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 472, peer MSS 1460, min MSS 472, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x20 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
Num of peers with authentication info: 0
RP/0/0/CPU0:R1#
在受Cisco錯誤ID CSCvf10395影響的Cisco IOS XR版本上，會略過ICMP錯誤訊息中包含的下一個躍點，且節點會嘗試使用之前提到且由RFC1191 - Path MTU discovery所記錄的一組定義良好的平台(IP MTU)層級中的下一個較低值來建立端到端最低IP MTU。這些嘗試一直進行到成功傳輸，這表示一直到ICMP(目的地無法連線 — 型別3 ;需要分段 — 代碼4)不再接收消息。
從受Cisco錯誤ID CSCvf10395影響的Cisco IOS XR版本節點中可看出：
! – As seen from IOX XR node with a release impacted by Cisco bug ID CSCvf10395
! – Node ignores “MTU of next hop” and tries next lower plateau
! – This is observed till ICMP error messages are no longer received
! – Practical consequence is extra retransmissions occurrence
RP/0/0/CPU0:Feb 23 17:05:32.929 : tcp[399]: [t4] PCB 0x12152adc: Process ICMP Dest-unreach (next hop mtu: 33554432)
RP/0/0/CPU0:Feb 23 17:05:32.929 : tcp[399]: [t4] PCB 0x12152adc: Invalid next hop mtu (33554432), ignore it
RP/0/0/CPU0:Feb 23 17:05:34.649 : tcp[399]: [t27] PCB 0x12152adc: Trying next lower MTU: 1452 <<<<<<<< HERE: Plateau 1492
RP/0/0/CPU0:Feb 23 17:05:35.519 : tcp[399]: [t4] PCB 0x12152adc: Process ICMP Dest-unreach (next hop mtu: 33554432)
RP/0/0/CPU0:Feb 23 17:05:35.519 : tcp[399]: [t4] PCB 0x12152adc: Invalid next hop mtu (33554432), ignore it
RP/0/0/CPU0:Feb 23 17:05:37.239 : tcp[399]: [t27] PCB 0x12152adc: Trying next lower MTU: 966 <<<<<<<< HERE: Plateau 1006
RP/0/0/CPU0:Feb 23 17:05:38.109 : tcp[399]: [t4] PCB 0x12152adc: Process ICMP Dest-unreach (next hop mtu: 33554432)
RP/0/0/CPU0:Feb 23 17:05:38.109 : tcp[399]: [t4] PCB 0x12152adc: Invalid next hop mtu (33554432), ignore it
RP/0/0/CPU0:Feb 23 17:05:39.829 : tcp[399]: [t27] PCB 0x12152adc: Trying next lower MTU: 468 <<<<<<<< HERE: Plateau 508
作為下一步，請考慮所有介面上使用標籤分發協定(LDP)的相同方案。此處的目標是瞭解在支援MPLS的環境中，從以前的場景中可觀察到哪些差異。
映像3.3 — 啟用PMTUD，且路徑段具有較低的IP MTU - MPLS案例。
首先，考慮在PMTUD觸發之前建立的BGP作業階段的初始階段，如此處所示。
在R4上看到的TCP(BGP)初始狀態 — 活動 — 啟用MPLS的方案：
! – as seen on R4 – Active
! – TCP path MTU discovery enabled
! – MPLS LDP enabled
! – TCP session initial state
RP/0/0/CPU0:R4#show tcp detail pcb 0x153bdaf0
Mon May 17 08:32:16.673 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Mon May 17 08:31:57 2021
PCB 0x153bdaf0, SO 0x153acc80, TCPCB 0x153acea8, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 757
Local host: 192.168.0.4, Local port: 57400 (Local App PID: 1196319)
Foreign host: 192.168.0.1, Foreign port: 179
(Local App PID/instance/SPL_APP_ID: 1196319/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 5 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 2 1 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 1386459919 snduna: 1386460037 sndnxt: 1386460037
sndmax: 1386460037 sndwnd: 32726 sndcwnd: 4380
irs: 3874414679 rcvnxt: 3874414864 rcvwnd: 32678 rcvadv: 3874447542
SRTT: 48 ms, RTTO: 300 ms, RTV: 228 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 10, connect retry interval: 30 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 1 Label Stack: 0x5dc2
Num of peers with authentication info: 0
RP/0/0/CPU0:R4#
R1上看到的TCP(BGP)初始狀態 — 被動 — 啟用MPLS的情況：
! – as seen on R1 – Passive
! – TCP path MTU discovery enabled
! – MPLS LDP enabled
! – TCP session initial state
RP/0/0/CPU0:R1#show tcp detail pcb 0x153acc8c
Mon May 17 08:32:56.618 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Mon May 17 08:31:55 2021
PCB 0x153acc8c, SO 0x153adad4, TCPCB 0x153adcfc, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 757
Local host: 192.168.0.1, Local port: 179 (Local App PID: 1192224)
Foreign host: 192.168.0.4, Foreign port: 57400
(Local App PID/instance/SPL_APP_ID: 1192224/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 3 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 3 1 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3874414679 snduna: 3874414864 sndnxt: 3874414864
sndmax: 3874414864 sndwnd: 32678 sndcwnd: 4380
irs: 1386459919 rcvnxt: 1386460037 rcvwnd: 32726 rcvadv: 1386492763
SRTT: 45 ms, RTTO: 300 ms, RTV: 239 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x20 PD ctx: size: 0 data:
Num Labels: 1 Label Stack: 0x5dc3
Num of peers with authentication info: 0
RP/0/0/CPU0:R1#
在此啟用MPLS的場景中，觀察到已建立TCP(LDP)作業階段的詳細資訊。請注意，之前描述的所有有關TCP(BGP)會話的MSS計算的內容，同樣適用於TCP(LDP)會話。例如，節點R3和R2 TCP(LDP)會話MSS計算可以總結如下：
- R2和R3都使用512位元組的非預設IP MTU。
- 已啟用路徑MTU發現。
- TCP對等點沒有直接連線（TCP會話在環回介面之間建立）。
- R3管理LDP連線。
- R3傳送MSS為472位元組的SYN。
- 512（介面IP MTU） — 20(minTCP_H)- 20(minIP_H)。
- R2傳送SYN，ACK的MSS為472位元組。
- 傳送[已接收MSS;本地初始MSS]。
- 已接收472位元組；本地初始MSS 472位元組。
- 兩個對等點上使用最小MSS值。
TCP(LDP)會話詳細資訊，如在R3上所示 — 活動 — 啟用MPLS的方案：
! – as seen on R3 – Active
! – TCP path MTU discovery enabled
! – MPLS LDP enabled
! – TCP session initial state
RP/0/0/CPU0:R3#show tcp detail pcb 0x15393fbc
Mon May 17 08:33:30.627 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Mon May 17 08:30:04 2021
PCB 0x15393fbc, SO 0x15393d94, TCPCB 0x153941b4, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 970
Local host: 192.168.0.3, Local port: 57146 (Local App PID: 1151216)
Foreign host: 192.168.0.2, Foreign port: 646
(Local App PID/instance/SPL_APP_ID: 1151216/0/0)
Current send queue size in bytes: 0 (max 16384)
Current receive queue size in bytes: 0 (max 16384) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 60)
Timer Starts Wakeups Next(msec)
Retrans 8 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 6 4 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 2917752466 snduna: 2917752838 sndnxt: 2917752838
sndmax: 2917752838 sndwnd: 16013 sndcwnd: 944
irs: 228184383 rcvnxt: 228184763 rcvwnd: 16005 rcvadv: 228200768
SRTT: 103 ms, RTTO: 580 ms, RTV: 477 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 279 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 1, connect retry interval: 3 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 472, peer MSS 472, min MSS 472, max MSS 472
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_SEL, SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/16384
Socket send buffer : Low/High watermark 2048/16384, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 1 Label Stack: 0x5dc2
Num of peers with authentication info: 0
RP/0/0/CPU0:R3#
R2上看到的TCP(LDP)會話詳細資訊 — 被動 — 啟用MPLS的方案：
! – as seen on R2 – Passive
! – TCP path MTU discovery enabled
! – MPLS LDP enabled
! – TCP session initial state
RP/0/0/CPU0:R2#show tcp detail pcb 0x153a1f44
Mon May 17 08:34:28.843 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Mon May 17 08:30:31 2021
PCB 0x153a1f44, SO 0x153a1d1c, TCPCB 0x153a213c, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 970
Local host: 192.168.0.2, Local port: 646 (Local App PID: 1151216)
Foreign host: 192.168.0.3, Foreign port: 57146
(Local App PID/instance/SPL_APP_ID: 1151216/0/0)
Current send queue size in bytes: 0 (max 16384)
Current receive queue size in bytes: 0 (max 16384) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 60)
Timer Starts Wakeups Next(msec)
Retrans 7 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 7 5 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 228184383 snduna: 228184763 sndnxt: 228184763
sndmax: 228184763 sndwnd: 16005 sndcwnd: 944
irs: 2917752466 rcvnxt: 2917752856 rcvwnd: 15995 rcvadv: 2917768851
SRTT: 95 ms, RTTO: 561 ms, RTV: 466 ms, KRTT: 0 ms
minRTT: 0 ms, maxRTT: 219 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 472, peer MSS 472, min MSS 472, max MSS 472
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_SEL, SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/16384
Socket send buffer : Low/High watermark 2048/16384, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x60 PD ctx: size: 0 data:
Num Labels: 1 Label Stack: 0x5dc1
Num of peers with authentication info: 0
RP/0/0/CPU0:R2#
建立BGP作業階段後，R1會傳送BGP更新訊息並接收ICMP訊息(無法到達目的地 — 型別3;需要分段 — 代碼4)，返回源自R2的節點R2，該節點在節點R1處觸發TCP PMTUD。出現這種情況是因為傳送BGP更新消息的IP資料包設定了DF位元，並且R2/R3網段上使用的512位元組的IP MTU低於IP資料包大小1116位元組。與之前一樣，收到此ICMP訊息時會觸發PMTUD。與先前的非MPLS方案相比，啟用MPLS方案的差異在於節點R2 ICMP消息中包含的下一跳值(無法到達目的地 — 型別3;需要分段 — 代碼4)。 在此啟用MPLS的情況下，下一躍點的MTU值佔用4位元組的額外MPLS額外負荷，這意味著它佔用R2處的輸出MPLS標籤堆疊，如以下輸出所示。
在R1上看到的TCP路徑MTU發現 — 被動 — 啟用MPLS的情況：
! – as seen from R1 – Passive
! – R1 sends BGP Update message with IP length of 1116 Bytes
! – Note MPLS Header as packet is to be label-switched (single label ; IGP label)
! – note IP Header Flags shows DF bit set
455 0.044859 192.168.0.1 192.168.0.4 BGP 1134 UPDATE Message, KEEPALIVE Message
Frame 455: 1134 bytes on wire (9072 bits), 1134 bytes captured (9072 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
MultiProtocol Label Switching Header, Label: 24002, Exp: 6, S: 1, TTL: 255
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1116
Identification: 0xc6dd (50909)
Flags: 0x02 (Don't Fragment)
0... .... = Reserved bit: Not set
.1.. .... = Don't fragment: Set
..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0xa5f4 [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 57400, Seq: 242, Ack: 175, Len: 1076
Border Gateway Protocol - UPDATE Message
Border Gateway Protocol - KEEPALIVE Message
<snip>
! – as seen from R1 – Passive
! – IP MTU on R2/R3 of 512 bytes is lower than IP packet length and DF bit is set
! – R1 receives ICMP error message from R2
! – note R2 ICMP error message carries Next-Hop MTU
! – “The size in octets of the largest datagram that could be forwarded, along the path of
! the original datagram, without being fragmented at this router. The size includes the
! IP header and IP data, and does not include any lower-level headers.”
! – In present MPLS-enabled scenario Next-Hop MTU value is 508 bytes
! – In previous non-MPLS scenario Next-Hop MTU value was 512 bytes
456 0.014117 10.2.3.1 192.168.0.1 ICMP 182 Destination unreachable (Fragmentation needed)
Frame 456: 182 bytes on wire (1456 bits), 182 bytes captured (1456 bits) on interface 0
Ethernet II, Src: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80), Dst: fa:16:3e:42:18:05 (fa:16:3e:42:18:05)
Internet Protocol Version 4, Src: 10.2.3.1, Dst: 192.168.0.1
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Total Length: 168
Identification: 0x001f (31)
Flags: 0x00
0... .... = Reserved bit: Not set
.0.. .... = Don't fragment: Not set
..0. .... = More fragments: Not se
Fragment offset: 0
Time to live: 251
Protocol: ICMP (1)
Header checksum: 0xb031 [validation disabled]
[Header checksum status: Unverified]
Source: 10.2.3.1
Destination: 192.168.0.1
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Internet Control Message Protocol
Type: 3 (Destination unreachable)
Code: 4 (Fragmentation needed)
Checksum: 0x5199 [correct]
[Checksum Status: Good]
Length: 17
[Length of original datagram: 68]
Unused: 0011
MTU of next hop: 508
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
Transmission Control Protocol, Src Port: 179, Dst Port: 57400, Seq: 3874414921, Ack: 1386460094
Border Gateway Protocol - UPDATE Message
! – As seen from R1 – Passive
! – Hint is provided by ICMP unreachable message MTU of next-hop field: 508 bytes
! – R1 then considers this value and retransmits BGP Update split in three distinct packets
! – Sum of TCP length = 468 + 468 + 140 = 1076 bytes
457 0.006689 192.168.0.1 192.168.0.4 TCP 526 [TCP Retransmission] 179 → 57400 [ACK] Seq=242 Ack=175 Win=32669 Len=468
460 0.004001 192.168.0.1 192.168.0.4 TCP 526 [TCP Retransmission] 179 → 57400 [ACK] Seq=710 Ack=175 Win=32669 Len=468
461 0.001788 192.168.0.1 192.168.0.4 TCP 198 [TCP Retransmission] 179 → 57400 [PSH, ACK] Seq=1178 Ack=175 Win=32669 Len=140
463 0.056695 192.168.0.4 192.168.0.1 TCP 54 57400 → 179 [ACK] Seq=175 Ack=1318 Win=31545 Len=0
! – As seen from R1 – Passive – 'debug tcp pmtud' and 'debug icmp' active
! – TCP PMTUD is triggered once ICMP unreachable received
RP/0/0/CPU0:May 17 08:29:56.131 UTC: tcp[399]: [t1] Try to enable path MTU discovery(neww age timer: 10 min)
RP/0/0/CPU0:May 17 08:29:56.131 UTC: tcp[399]: [t1] Path mtu is ON (age-timer: 10)
RP/0/0/CPU0:May 17 08:35:51.726 UTC: ipv4_io[266]: ip_icmp_lib_ipv4_receive: Receiving pak(0xb0c07d8f) tid: 5
RP/0/0/CPU0:May 17 08:35:51.726 UTC: ipv4_io[266]: Entering ipv4_mtu_update_cb
RP/0/0/CPU0:May 17 08:35:51.726 UTC: ipv4_io[266]: IPv4 ICMP: Received ICMP too big from 192.168.0.1 about 192.168.0.4, MTU=508
RP/0/0/CPU0:May 17 08:35:51.726 UTC: ipv4_io[266]: ipv4_icmp_unreachable_rcvd ICMP unreach recvd: sending pak(0xb0c07d8f) to transport: 6, tid: 5
RP/0/0/CPU0:May 17 08:35:51.726 UTC: ipv4_io[266]: ip_icmp_lib_ipv4_receive: sending pak(0xb0c07d8f) to transport: 1, tid: 5
RP/0/0/CPU0:May 17 08:35:51.726 UTC: tcp[399]: [t4] PCB 0x153acc8c: Process ICMP Dest-unreach (next hop mtu: 508)
! – attempt new MSS 468 = MTU of next-hop(508) - TCP_H(20) - IP_H(20)
RP/0/0/CPU0:May 17 08:35:51.726 UTC: tcp[399]: [t4] PCB 0x153acc8c: Try to use new MSS: 468
RP/0/0/CPU0:May 17 08:35:51.726 UTC: tcp[399]: [t4] PCB 0x153acc8c, New path MTU decided to use: 468 configured tp_user_mss 0
! – over time PMTUD attempts to raise MSS as per egress interface configured MTU
RP/0/0/CPU0:May 17 08:45:51.745 UTC: tcp[399]: [t29] PCB 0x153acc8c: Trying next higher MTU: 966
RP/0/0/CPU0:May 17 08:47:51.757 UTC: tcp[399]: [t29] PCB 0x153acc8c: Trying next higher MTU: 1452
RP/0/0/CPU0:May 17 08:49:51.769 UTC: tcp[399]: [t29] PCB 0x153acc8c: Trying next higher MTU: 1460
如R1所示 — 被動 — TCP PMTUD觸發 — 啟用MPLS的情況：
! – as seen on R1 – Passive
! – R1 session details after TCP PMTUD trigger
RP/0/0/CPU0:R1#show tcp detail pcb 0x153acc8c
Mon May 17 08:43:07.077 UTC
==============================================================
Connection state is ESTAB, I/O status: 240, socket status: 0
Established at Mon May 17 08:31:55 2021
PCB 0x153acc8c, SO 0x153adad4, TCPCB 0x153adcfc, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 757
Local host: 192.168.0.1, Local port: 179 (Local App PID: 1192224)
Foreign host: 192.168.0.4, Foreign port: 57400
(Local App PID/instance/SPL_APP_ID: 1192224/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 15 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 14 9 0
KeepAlive 1 0 0
PmtuAger 1 0 164599
GiveUp 0 0 0
Throttle 0 0 0
iss: 3874414679 snduna: 3874416130 sndnxt: 3874416130
sndmax: 3874416130 sndwnd: 31412 sndcwnd: 936
irs: 1386459919 rcvnxt: 1386460246 rcvwnd: 32517 rcvadv: 1386492763
SRTT: 180 ms, RTTO: 509 ms, RTV: 329 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: PMTU ager
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 468, peer MSS 1460, min MSS 468, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x20 PD ctx: size: 0 data:
Num Labels: 1 Label Stack: 0x5dc3
Num of peers with authentication info: 0
RP/0/0/CPU0:R1#
請注意，在啟用MPLS的情況下，節點R2 ICMP消息中包含的下一跳MTU的值將計入輸出MPLS標籤堆疊。要進一步強化這一方面，請考慮下一個示例。如果在R2過濾的IP資料包與L3VPN服務相關聯，則表示乙太網幀現在帶有兩個標籤（IGP標籤和VPN標籤）。 接下來躍點的MTU會反映所需的標籤堆疊大小。請參閱這些輸出。
如R1上所示 — PASSIVE - L3 VPN服務資料包：
! – as seen from R1 – Passive
! – L3 VPN service packet is sourced by node R1 and destined to node R4
! – Note presence of MPLS label stack – both IGP and VPN label are present
! – Note IP Total Length of 610 bytes higher than the IP MTU on R2/R3 segment
! – note IP Header Flags shows DF bit set
2024 0.302370 10.1.14.1 10.1.14.14 TELNET 632 Telnet Data ...
Frame 2024: 632 bytes on wire (5056 bits), 632 bytes captured (5056 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
MultiProtocol Label Switching Header, Label: 24002, Exp: 0, S: 0, TTL: 255
0000 0101 1101 1100 0010 .... .... .... = MPLS Label: 24002
.... .... .... .... .... 000. .... .... = MPLS Experimental Bits: 0
.... .... .... .... .... ...0 .... .... = MPLS Bottom Of Label Stack: 0
.... .... .... .... .... .... 1111 1111 = MPLS TTL: 255
MultiProtocol Label Switching Header, Label: 24005, Exp: 0, S: 1, TTL: 255
0000 0101 1101 1100 0101 .... .... .... = MPLS Label: 24005
.... .... .... .... .... 000. .... .... = MPLS Experimental Bits: 0
.... .... .... .... .... ...1 .... .... = MPLS Bottom Of Label Stack: 1
.... .... .... .... .... .... 1111 1111 = MPLS TTL: 255
Internet Protocol Version 4, Src: 10.1.14.1, Dst: 10.1.14.14
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Total Length: 610
Identification: 0x7c9f (31903)
Flags: 0x02 (Don't Fragment)
0... .... = Reserved bit: Not set
.1.. .... = Don't fragment: Set
..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0xcce5 [validation disabled]
[Header checksum status: Unverified]
Source: 10.1.14.1
Destination: 10.1.14.14
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 22008, Dst Port: 23, Seq: 34755, Ack: 93250, Len: 570
如在R1上所示 — PASSIVE - L3 VPN服務 — ICMP型別3/代碼4:
! – as seen from R1 – Passive
! – IP MTU on R2/R3 of 512 bytes is lower than IP packet length and DF bit is set
! – R1 receives ICMP error message from R2
! – note R2 ICMP error message carries Next-Hop MTU
! – “The size in octets of the largest datagram that could be forwarded, along the path of
! the original datagram, without being fragmented at this router. The size includes the
! IP header and IP data, and does not include any lower-level headers.”
! – In present L3VPN MPLS-enabled scenario (dual-label) Next-Hop MTU value is 504 bytes
! – In previous MPLS scenario (single-label) Next-Hop MTU value was 508 bytes
2030 0.020299 10.2.3.1 10.1.14.1 ICMP 190 Destination unreachable (Fragmentation needed)
Frame 2030: 190 bytes on wire (1520 bits), 190 bytes captured (1520 bits) on interface 0
Ethernet II, Src: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80), Dst: fa:16:3e:42:18:05 (fa:16:3e:42:18:05)
MultiProtocol Label Switching Header, Label: 24005, Exp: 0, S: 1, TTL: 251
0000 0101 1101 1100 0101 .... .... .... = MPLS Label: 24005
.... .... .... .... .... 000. .... .... = MPLS Experimental Bits: 0
.... .... .... .... .... ...1 .... .... = MPLS Bottom Of Label Stack: 1
.... .... .... .... .... .... 1111 1011 = MPLS TTL: 251
Internet Protocol Version 4, Src: 10.2.3.1, Dst: 10.1.14.1
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Total Length: 172
Identification: 0x002b (43)
Flags: 0x00
0... .... = Reserved bit: Not set
.0.. .... = Don't fragment: Not set
..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 253
Protocol: ICMP (1)
Header checksum: 0x9821 [validation disabled]
[Header checksum status: Unverified]
Source: 10.2.3.1
Destination: 10.1.14.1
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Internet Control Message Protocol
Type: 3 (Destination unreachable)
Code: 4 (Fragmentation needed)
Checksum: 0xbbac [correct]
[Checksum Status: Good]
Length: 17
[Length of original datagram: 68]
Unused: 0011
MTU of next hop: 504
Internet Protocol Version 4, Src: 10.1.14.1, Dst: 10.1.14.14
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Total Length: 610
Identification: 0x7c9f (31903)
Flags: 0x02 (Don't Fragment)
0... .... = Reserved bit: Not set
.1.. .... = Don't fragment: Set
..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0xcce5 [validation disabled]
[Header checksum status: Unverified]
Source: 10.1.14.1
Destination: 10.1.14.14
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 22008, Dst Port: 23, Seq: 586828435, Ack: 754580617
PMTUD - TCP選項(MD5)
映像3.4 — 啟用PMTUD和TCP MD5身份驗證。
在啟用TCP MD5驗證的情況下，不會從先前案例中說明的內容區分PMTUD行為。與先前在使用中的TCP MD5驗證共用一樣，Cisco IOS XR會考慮其他額外負荷，而作用中TCP對等體的初始MSS值反映相同。請參閱前面的使用TCP選項 — XR主動和使用TCP選項 — XR被動小節，以瞭解有關使用TCP選項的影響的其他詳細資訊。此案例中的TCP MSS計算可總結如下：
- 所有節點均使用預設IP MTU 1500位元組。
- 已啟用TCP路徑MTU發現。
- TCP對等點沒有直接連線。
- 在R1和R4上啟用了TCP MD5身份驗證。
- R4管理BGP連線。
- R4傳送MSS為1436位元組的SYN。
- 1500（介面IP MTU） — 20(minTCP_H)- 20(minIP_H)- 24位元組（IOS XR TCP選項額外負荷）。
- R1傳送SYN，ACK，MSS為1436位元組。
- 傳送[Received MSS ;本地初始MSS]。
- 收到1436位元組；本地初始MSS 1460位元組。
- 兩個對等點上使用最低的MSS值。
來源為R4的TCP SYN:
! – TCP SYN sourced from R4
2408 5.695076 192.168.0.4 192.168.0.1 TCP 82 59050 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1436 WS=1
Frame 2408: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface 0
Ethernet II, Src: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6), Dst: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54)
Internet Protocol Version 4, Src: 192.168.0.4, Dst: 192.168.0.1
Transmission Control Protocol, Src Port: 59050, Dst Port: 179, Seq: 0, Len: 0
Source Port: 59050
Destination Port: 179
[Stream index: 8]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 48 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x20d7 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (28 bytes), Maximum segment size, Window scale, No-Operation (NOP), TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1436 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1436
Window scale: 0 (multiply by 1)
No-Operation (NOP)
TCP MD5 signature
End of Option List (EOL)
源自R1的TCP SYN、ACK:
! – TCP SYN,ACK sourced from R1
2409 0.004352 192.168.0.1 192.168.0.4 TCP 82 179 → 59050 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1436 WS=1
Frame 2409: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface 0
Ethernet II, Src: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54), Dst: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
Transmission Control Protocol, Src Port: 179, Dst Port: 59050, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 59050
[Stream index: 8]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 48 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xcbf8 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (28 bytes), Maximum segment size, Window scale, No-Operation (NOP), TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1436 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1436
Window scale: 0 (multiply by 1)
No-Operation (NOP)
TCP MD5 signature
End of Option List (EOL)
R4上看到的TCP會話詳細資訊 — ACTIVE:
! – as seen from R4 - Active
RP/0/0/CPU0:R4#show tcp detail pcb 0x121542c0
Tue Jan 12 13:27:23.526 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Tue Jan 12 13:25:41 2021
PCB 0x121542c0, SO 0x1213c0e4, TCPCB 0x12156010, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 359
Local host: 192.168.0.4, Local port: 59050 (Local App PID: 1052958)
Foreign host: 192.168.0.1, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 6 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 3 2 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3299472269 snduna: 3299473445 sndnxt: 3299473445
sndmax: 3299473445 sndwnd: 31646 sndcwnd: 4308
irs: 3225544359 rcvnxt: 3225545535 rcvwnd: 31665 rcvadv: 3225577200
SRTT: 89 ms, RTTO: 530 ms, RTV: 441 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 30 secs
State flags: none
Feature flags: MD5, Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1436, peer MSS 1436, min MSS 1436, max MSS 1436
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R4#
R1上看到的TCP會話詳細資訊 — PASSIVE:
! – as seen from R1 – Passive
RP/0/0/CPU0:R1#show tcp detail pcb 0x121560ec
Tue Jan 12 13:25:59.310 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Tue Jan 12 13:25:31 2021
PCB 0x121560ec, SO 0x121556d4, TCPCB 0x121575bc, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 359
Local host: 192.168.0.1, Local port: 179 (Local App PID: 983326)
Foreign host: 192.168.0.4, Foreign port: 59050
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 3 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 3 2 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3225544359 snduna: 3225545516 sndnxt: 3225545516
sndmax: 3225545516 sndwnd: 31684 sndcwnd: 4308
irs: 3299472269 rcvnxt: 3299473426 rcvwnd: 31665 rcvadv: 3299505091
SRTT: 37 ms, RTTO: 300 ms, RTV: 244 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: MD5, Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1436, peer MSS 1436, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R1#
PMTUD — 黑孔偵測
如前文PMTUD — 路徑區段的IP MTU更低一節所述，當啟用時，TCP PMTUD會透過接收ICMP(目的地無法連線 — 型別3;需要分段 — 代碼4)消息。在某些情況下，由於某些原因未收到這些消息，因此不會觸發PMTUD。在這種情況下，不會得知TCP對等路由器之間路徑的最小IP MTU。如果IP封包已設定DF位元，且它們的大小高於最低IP MTU路徑區段，則此類情況會引入潛在的黑洞。這些資料包將被靜默丟棄。
本節旨在重點介紹Cisco IOS XR如何檢測此類潛在黑洞場景並對其執行操作。為此，在R2介面GE0/0/0/0上禁用IPv4不可達功能，如下一映像和CLI輸出所示。
映像3.5 — 在R1/R4和R2 IPv4上啟用PMTUD且無法連線。
R2上禁用的IPv4無法連線：
!- R2 - IP unreachables is disabled
RP/0/0/CPU0:R2#show run interface gigabitEthernet 0/0/0/0
Thu May 13 12:09:45.483 UTC
interface GigabitEthernet0/0/0/0
ipv4 address 10.1.2.2 255.255.255.252
ipv4 unreachables disable
!
RP/0/0/CPU0:R2#show ipv4 interface gigabitEthernet 0/0/0/0
Thu May 13 12:10:04.112 UTC
GigabitEthernet0/0/0/0 is Up, ipv4 protocol is Up
Vrf is default (vrfid 0x60000000)
Internet address is 10.1.2.2/30
MTU is 1514 (1500 is available to IP)
Helper address is not set
Multicast reserved groups joined: 224.0.0.2 224.0.0.1 224.0.0.5
224.0.0.6
Directed broadcast forwarding is disabled
Outgoing access list is not set
Inbound common access list is not set, access list is not set
Proxy ARP is disabled
ICMP redirects are never sent
ICMP unreachables are never sent
ICMP mask replies are never sent
Table Id is 0xe0000000
Cisco IOS XR處理此黑洞情況的方式是重新傳輸相同封包兩次，如果仍不成功，即未收到預期的TCP ACK，則重試但使用下一個較低的、定義良好的平台值，如RFC1191 — 路徑MTU探索中所述(請參閱PMTUD — 路徑片段具有較低的IP MTU一節，瞭解平台清單)。 總而言之，Cisco IOS XR假設在到達目的地的路徑內因資料包大小而丟棄資料包，並嘗試通過資料包重新傳輸來解決這種情況。通過節點R1介面處捕獲的資料包以及debug tcp pmtud的輸出中的下一個示例，可以觀察到此行為。
R1上的IOS-XR黑洞檢測：
! – at R1
! – Original BGP Update message is sent
! – Note IP Total Length of 1116 bytes and TCP Segment Length of 1076 bytes
! – R2 filters such packet and send and ICMP error message towards R1 which triggers PMTUD
! – But because IPv4 unreachables are disabled at R2 GE0/0/0/0 ICMP message is not sent
! – Hence BGP message is silently filtered at R2
562 7.638774 192.168.0.1 192.168.0.4 BGP 1130 UPDATE Message, KEEPALIVE Message
Frame 562: 1130 bytes on wire (9040 bits), 1130 bytes captured (9040 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1116
Identification: 0x4a37 (18999)
Flags: 0x02 (Don't Fragment)
0... .... = Reserved bit: Not set
.1.. .... = Don't fragment: Set
..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0x229b [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 57082, Seq: 318, Ack: 251, Len: 1076
Border Gateway Protocol - UPDATE Message
Border Gateway Protocol - KEEPALIVE Message
<snip>
! – at R1
! – No TCP ACK is received
! – Packet retransmission is attempted (2 attempts)
! – Note initial MSS value is of 1460 bytes
563 0.560058 192.168.0.1 192.168.0.4 TCP 1130 [TCP Retransmission] 179 → 57082 [PSH, ACK] Seq=318 Ack=251 Win=32593 Len=1076
564 1.101367 192.168.0.1 192.168.0.4 TCP 1130 [TCP Retransmission] 179 → 57082 [PSH, ACK] Seq=318 Ack=251 Win=32593 Len=1076
! – at R1
! – Still no TCP ACK received; previous retransmissions failed
! – Next lower plateau value is attempted – 1492 bytes
! – Packet retransmission is attempted (2 attempts)
RP/0/0/CPU0:May 13 10:20:44.251 UTC: tcp[399]: [t1] PCB 0x15392224: Trying next lower MTU: 1452
567 1.850294 192.168.0.1 192.168.0.4 TCP 1130 [TCP Retransmission] 179 → 57082 [PSH, ACK] Seq=318 Ack=251 Win=32593 Len=1076
568 1.111361 192.168.0.1 192.168.0.4 TCP 1130 [TCP Retransmission] 179 → 57082 [PSH, ACK] Seq=318 Ack=251 Win=32593 Len=1076
! – at R1
! – Still no TCP ACK received; previous retransmissions failed
! – Next lower plateau value is attempted – 1006 bytes
! – Packet retransmission is attempted (2 attempts)
RP/0/0/CPU0:May 13 10:20:47.560 UTC: tcp[399]: [t1] PCB 0x15392224: Trying next lower MTU: 966
569 2.198327 192.168.0.1 192.168.0.4 TCP 1020 [TCP Retransmission] 179 → 57082 [ACK] Seq=318 Ack=251 Win=32593 Len=966
570 1.109602 192.168.0.1 192.168.0.4 TCP 1020 [TCP Retransmission] 179 → 57082 [ACK] Seq=318 Ack=251 Win=32593 Len=966
! – at R1
! – Still no TCP ACK received; previous retransmissions failed
! – Next lower plateau value is attempted – 508 bytes
! – Original information (TCP Length of 1076 bytes) is split in three distinct packets
! – TCP Segment Lengths 468 + 468 + 140 = 1076
! – TCP ACK is received from peer R4
RP/0/0/CPU0:May 13 10:20:50.870 UTC: tcp[399]: [t1] PCB 0x15392224: Trying next lower MTU: 468
571 2.205552 192.168.0.1 192.168.0.4 TCP 522 [TCP Retransmission] 179 → 57082 [ACK] Seq=318 Ack=251 Win=32593 Len=468
573 0.004254 192.168.0.1 192.168.0.4 TCP 522 [TCP Retransmission] 179 → 57082 [ACK] Seq=786 Ack=251 Win=32593 Len=468
574 0.002724 192.168.0.1 192.168.0.4 TCP 194 [TCP Retransmission] 179 → 57082 [PSH, ACK] Seq=1254 Ack=251 Win=32593 Len=140
! – Peer R4 TCP ACK is received
575 0.223172 192.168.0.4 192.168.0.1 TCP 54 57082 → 179 [ACK] Seq=251 Ack=1394 Win=31469 Len=0
修訂記錄
| 修訂 | 發佈日期 | 意見 |
|---|---|---|
1.0 |
16-Aug-2021 |
初始版本 |
意見