qos_hrdcomp_xe

Configuring RTP Header Compression

Header compression is a mechanism that compresses the header in a packet before the packet is transmitted. RTP header compression reduces network overhead and speeds up the transmission of Real-Time Transport Protocol (RTP) packets.

Finding Feature Information

For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for Configuring RTP Header Compression" section.

Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE Software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

•Prerequisites for Configuring RTP Header Compression

•Information About Configuring RTP Header Compression

•How to Configure RTP Header Compression

•Configuration Examples for RTP Header Compression

•Additional References

•Feature Information for Configuring RTP Header Compression

•Glossary

Prerequisites for Configuring RTP Header Compression

•Before configuring RTP header compression, read the information in the "Header Compression" module.

•You must configure RTP header compression on both ends of the network.

Information About Configuring RTP Header Compression

Before configuring RTP header compression, you should understand the following concepts:

•Configurable RTP Header-Compression Settings

•RTP Header-Compression Keywords

Configurable RTP Header-Compression Settings

With RTP header compression, you can configure the maximum size of the compressed header, the maximum time between transmitting full-header packets, and the maximum number of compressed packets between full headers. These settings are configured using the following three commands:

•ip header-compression max-header

•ip header-compression max-time

•ip header-compression max-period

The ip header-compression max-header command allows you to define the maximum size of the header of a packet to be compressed. Any packet with an header that exceeds the maximum size is sent uncompressed.

The ip header-compression max-time command allows you to specify the maximum time between transmitting full-header packets, and the ip header-compression max-period command allows you to specify the maximum number of compressed packets between full headers. With the ip header-compression max-time and ip header-compression max-period commands, the full-header packet is transmitted at the specified time period or when the maximum number of packets is reached, respectively. The counters for both the time period and the number of packets sent are reset after the full-header packet is sent.

For more information about these commands, see the Cisco IOS Quality of Service Solutions Command Reference.

RTP Header-Compression Keywords

When you configure RTP header compression, you can specify the circumstances under which the RTP packets are compressed and the format that is used when the packets are compressed. These circumstances and formats are defined by the following keywords:

•passive

•iphc-format

•ietf-format

•cisco

These keywords (described below) are available with many of the quality of service (QoS) commands used to configure RTP header compression, such as the ip rtp header-compression command. For more information about the ip rtp header-compression command, these keywords, and the other QoS commands, see the Cisco IOS Quality of Service Solutions Command Reference.

The passive Keyword

By default, the ip rtp header-compression command compresses outgoing RTP traffic. If you specify the passive keyword, outgoing RTP traffic is compressed only if incoming RTP traffic on the same interface is compressed. If you do not specify the passive keyword, all outgoing RTP traffic is compressed.

The passive keyword is ignored on PPP interfaces.

The iphc-format Keyword

The iphc-format keyword indicates that the IP Header Compression (IPHC) format of header compression will be used. For PPP and HDLC interfaces, when the iphc-format keyword is specified, TCP header compression is also enabled. Since both RTP and TCP header compression are enabled, both UDP and TCP packets are compressed.

The iphc-format keyword includes checking whether the destination port number is even and is in the ranges of 16,385 to 32,767 (for Cisco audio) or 49,152 to 65,535 (for Cisco video). Valid RTP packets that meet the criteria (that is, the port number is even and is within the specified range) are compressed using the compressed RTP packet format. Otherwise, packets are compressed using the less-efficient compressed non-TCP packet format.

The iphc-format keyword is not available for interfaces that use Frame Relay encapsulation.

Note The header compression format (in this case, IPHC) must be the same at both ends of the network. That is, if you specify the iphc-format keyword on the local router, you must also specify the iphc-format keyword on the remote router.

The ietf-format Keyword

The ietf-format keyword indicates that the Internet Engineering Task Force (IETF) format of header compression will be used. For HDLC interfaces, the ietf-format keyword compresses both TCP and UDP packets. UPD and TCP packets are compressed separately. For PPP interfaces, when the ietf-format keyword is specified, TCP header compression is also enabled. Since both RTP header compression and TCP header compression are enabled, both UDP packets and TCP packets are compressed.

With the ietf-format keyword, any even destination port number higher than 1024 can be used. Valid RTP packets that meet the criteria (that is, the port number is even and is higher than 1024) are compressed using the compressed RTP packet format. Otherwise, packets are compressed using the less-efficient compressed non-TCP packet format.

The ietf-format keyword is not available for interfaces that use Frame Relay encapsulation.

Note The header compression format (in this case, IETF) must be the same at both ends of the network. That is, if you specify the ietf-format keyword on the local router, you must also specify the ietf-format keyword on the remote router.

The cisco Keyword

The cisco keyword indicates that the Cisco-propriety or "original" format of header compression will be used.

RTP header-compression using the cisco format supports even-numbered UDP destination ports in the Cisco audio range of 16384 to 32767 or in the video range of 49152 to 65535.

The cisco keyword is only available on interfaces that use Frame Relay or HDLC encapsulation.

How to Configure RTP Header Compression

•Enabling RTP Header Compression on an Interface (required)

•Specifying the Header-Compression Settings (optional)

•Changing the Number of Header-Compression Connections (optional)

•Displaying Header-Compression Statistics (optional)

Enabling RTP Header Compression on an Interface

To enable RTP header compression on an interface, perform the following steps.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type number [name-tag]

4. encapsulation encapsulation-type

5. ip address ip-address mask [secondary]

6. ip rtp header-compression [passive | iphc-format | ietf-format | cisco] [periodic-refresh]

7. end

DETAILED STEPS

Specifying the Header-Compression Settings

With RTP header compression, you can configure the maximum size of the compressed header, the time period for an automatic resend of full-header packets, and the number of packets transmitted before a new full-header packet is sent.

To specify these header-compression settings, perform the following steps.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type number [name-tag]

4. ip header-compression max-header max-header-size

or

ip header-compression max-time length-of-time

or

ip header-compression max-period number-of-packets

5. end

DETAILED STEPS

Changing the Number of Header-Compression Connections

For PPP and HDLC interfaces, the default is 16 compression connections.

To change the default number of header-compression connections, perform the following steps.

Implications of Changing the Number of Header-Compression Connections

Each header-compression connection sets up a compression cache entry, so you are in effect specifying the maximum number of cache entries and the size of the cache. Too few cache entries for the specified interface can lead to degraded performance, and too many cache entries can lead to wasted memory. Choose the number of header-compression connections according to the network requirements.

Restrictions

Header-Compression Connections on HDLC Interfaces

For HDLC interfaces, the number of header-compression connections on both sides of the network must match. That is, the number configured for use on the local router must match the number configured for use on the remote router.

Header-Compression Connections on PPP Interfaces

For PPP interfaces, if the header-compression connection numbers on both sides of the network do not match, the number used is "autonegotiated." That is, any mismatch in the number of header-compression connections between the local router and the remote router will be automatically negotiated to the lower of the two numbers. For example, if the local router is configured to use 128 header-compression connections, and the remote router is configured to use 64 header-compression connections, the negotiated number will be 64.

Note This autonegotiation function applies to PPP interfaces only. For HDLC interfaces, no autonegotiation occurs.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type number [name-tag]

4. ip rtp compression-connections number

5. end

DETAILED STEPS

Displaying Header-Compression Statistics

You can display header-compression statistics, such as the number of packets sent, received, and compressed, by using the show ip rtp header-compression command.

To display header-compression statistics, perform the following steps.

SUMMARY STEPS

1. enable

2. show ip rtp header-compression [interface-type interface-number]

3. end

DETAILED STEPS

Configuration Examples for RTP Header Compression

•Example: Enabling RTP Header Compression on an Interface

•Example: Specifying the Header-Compression Settings

•Example: Changing the Number of Header-Compression Connections

•Example: Displaying Header-Compression Statistics

Example: Enabling RTP Header Compression on an Interface

In the following example, RTP header compression is enabled on serial interface 0.

Router> enable

Router# configure terminal

Router(config)# interface serial0

Router(config-if)# encapsulation ppp

Router(config-if)# ip address 209.165.200.225 255.255.255.224

Router(config-if)# ip rtp header-compression

Router(config-if)# end

Example: Specifying the Header-Compression Settings

In the following example, the maximum size of the compressed IP header (100 bytes) has been specified by using the ip header-compression max-header command.

Router> enable

Router# configure terminal

Router(config)# interface serial0

Router(config-if)# ip header-compression max-header 100

Router(config-if)# end

Example: Changing the Number of Header-Compression Connections

In the following example, the number of header-compression connections has been changed to 150 by using the ip rtp compression-connections command.

Router> enable

Router# configure terminal

Router(config)# interface serial0

Router(config-if)# ip rtp compression-connections 150

Router(config-if)# end

Example: Displaying Header-Compression Statistics

You can use the show ip rtp header-compression command to display header-compression statistics such as the number of packets received, sent, and compressed. The following is sample output from the show ip rtp header-compression command.

Router# show ip rtp header-compression serial0

RTP/UDP/IP header compression statistics:

  Interface Serial0 (compression on, IETF)

    Rcvd:    1473 total, 1452 compressed, 0 errors, 0 status msgs

             0 dropped, 0 buffer copies, 0 buffer failures

    Sent:    1234 total, 1216 compressed, 0 status msgs, 379 not predicted

             41995 bytes saved, 24755 bytes sent

             2.69 efficiency improvement factor

    Connect: 16 rx slots, 16 tx slots, 

             6 misses, 0 collisions, 0 negative cache hits, 13 free contexts

             99% hit ratio, five minute miss rate 0 misses/sec, 0 max

Additional References

The following sections provide references related to configuring RTP header compression.

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for Configuring RTP Header Compression

Table 1 lists the features in this module and provides links to specific configuration information.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE Software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 lists only the Cisco IOS XE Software release that introduced support for a given feature in a given Cisco IOS XE Software release train. Unless noted otherwise, subsequent releases of that Cisco IOS XE Software release train also support that feature.

Glossary

compression—The running of a data set through an algorithm that reduces the space required to store the data set or the bandwidth required to transmit the data set.

context—The state that the compressor uses to compress a header and that the decompressor uses to decompress a header. The context is the uncompressed version of the last header sent and includes other information used to compress and decompress the packet.

context-state packet—A special packet sent from the decompressor to the compressor to communicate a list of (TCP or NON_TCP/RTP) context identifiers (CIDs) for which synchronization has been lost. This packet is sent only over a single link, so it requires no IP header.

DLCI—data-link connection identifier. A value that specifies a permanent virtual circuit (PVC) or switched virtual circuit (SVC) in a Frame Relay network. In the basic Frame Relay specification, DLCIs are locally significant (connected devices might use different values to specify the same connection). In the Local Management Interface (LMI) extended specification, DLCIs are globally significant (DLCIs specify individual end devices).

encapsulation—A method of wrapping data in a particular protocol header. For example, Ethernet data is wrapped in a specific Ethernet header before network transit. Also, when dissimilar networks are bridged, the entire frame from one network is simply placed in the header used by the data link layer protocol of the other network.

full header (header refresh)—An uncompressed header that updates or refreshes the context for a packet stream. It carries a CID that will be used to identify the context. Full headers for non-TCP packet streams also carry the generation of the context that they update or refresh.

HDLC—High-Level Data Link Control. A bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). Derived from Synchronous Data Link Control (SDLC), HDLC specifies a data encapsulation method on synchronous serial links using frame characters and checksums.

header—A chain of subheaders.

IETF—Internet Engineering Task Force. A task force that consists of over 80 working groups responsible for developing Internet standards.

IPHC—IP Header Compression. A protocol capable of compressing both TCP and UDP headers.

ISDN—Integrated Services Digital Network. A communication protocol offered by telephone companies that permits telephone networks to carry data, voice, and other source traffic.

lossy serial links—Links in a network that are prone to lose packets.

packet stream—The sequence of packets whose headers are similar and share context. For example, headers in an RTP packet stream have the same source and final destination address and the same port numbers in the RTP header.

PPP—Point-to-Point Protocol. A protocol that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits.

regular header—A normal, uncompressed header. A regular header does not carry a context identifier (CID) or generation association.

RTP—Real-Time Transport Protocol. A protocol that is designed to provide end-to-end network transport functions for applications that transmit real-time data, such as audio, video, or simulation data, over unicast or multicast network services. RTP provides such services as payload type identification, sequence numbering, timestamping, and delivery monitoring to real-time applications.

subheader—An IPv6 base header, an IPv6 extension header, an IPv4 header, a UDP header, an RTP header, or a TCP header.

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Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.

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