Audio Video Bridging (AVB) is set of standards that enable time-synchronized low latency streaming services on Ethernet networks, including wireline Ethernet networks shared with other data traffic and wireless LANs. AVB implements the set of standards developed by the IEEE Audio Video Bridging Task Group. AVB functions by reserving a fraction of the Ethernet bandwidth that is available for AVB traffic. AVB consists of the following specifications that are defined under the standard IEEE 802.1BA: Audio Video Bridging (AVB) Systems:

• IEEE 802.1AS—gPTP: Generalized Precision Time Protocol (gPTP).

• IEEE 802.1Qat: Multiple Stream Reservation Protocol (MSRP) that defines an end-to-end bandwidth reservation protocol within a bridged LAN.

• IEEE 802.1Qav: Forwarding and Queuing for Time-Sensitive Streams (FQTSS), which is AV traffic-scheduling capability for a mainstream Ethernet and other network switches.

Generalized Precision Time Protocol (gPTP) is a protocol used to enable synchronization of AVB end points so the listeners correctly use the timestamps carried in the video. gPTP is also required while switching the video streams from one source to another, on a video frame boundary. IEEE 802.1AS: Timing and Synchronization for Time-Sensitive Applications defines gPTP. Though IEEE 802.1AS is based on IEEE1588v2, there are some key differences between them. The following are the characteristics of gPTP:

• It mandates the use of only Layer-2 messages.

• It requires that all devices, including the vendor devices, in the AVB network have to be gPTP compliant.

• It requires the availability of boundary and ordinary clocks for end stations.

• It uses Peer Delay mechanism, 2-step clocks, and single gPTP domain.

• It uses Best Master Clock selection algorithm (BMCA).

Best Master Clock selection Algorithm:

• It is used to select master/slave state of the port, using PTP Announce messages.

• It enables that the best clock is selected as the master.

• Depends on the quality of the clock, particularly its stability, and configuration, such as PTP priority.

Peer Delay Mechanism:

• It is used to measure Link Delay. Link Delay is the delay in propagation of a packet or stream of packets from egress of an interface of a switch at stage N-1 to the ingress of the connected interface of the switch at stage N.

• It is used between two directly connected AVB switches/devices, which are two ends of the link, using the following peer delay PTP messages:

  • pDelay request

  • pDelay Response

  • pDelay Response Follow-up

Residence delay is the time a packet spends within a switch from the time it enters a switch through an ingress interface to the time it exits the switch through an egress interface. It is not measured but it is configured on the switch by enabling the ptp switchlatency-estimated value command. The default value is 5000 ns.

Link delay and residence delay can be used to measure end-to-end latency in an AVB network.

Link Delay for the link connected to egress of the previous node and ingress of the current node) + Residence delay (switch latency)

MSRP Signaling protocol is used to reserve resources for AVB streams. It helps in maintaining audio-video quality for AVB. It is defined by IEEE 802.1Qat. The following are the characteristics of MSRP:

• Manages bandwidth using call admission control (CAC) functionality to prevent congestion.

• MSRP is built on Multiple Registration Protocol (MRP). MSRP is attribute-based similar to Multiple Registration Protocol (MRP) and Multiple VLAN Registration Protocol (MVRP).

• Talker denotes the stream source and listener denotes the stream destination.

• The talker attributes describes the streams that are being offered.

• The listener attributes are used to subscribe to the streams that are being offered.

• The listener associates to the talker by using an identity that is known as the Stream ID, which comprises the MAC address of the talker and a talker-specific 16-bit unique ID and the MAC address of the listener.

• MSRP adds and removes per interface PTP or QoS configuration.

• MSRP determines the core ports, establishes AVB domains by exchanging domain attribute.

• MSRP propagates the talker advertisements, transform to TalkerFailed advertisements on their egress ports with insufficient bandwidth and back when bandwidth reservation is possible. See the Figure -3, "Talker Attribute Flow".

• MSRP manages listener reservations. See the Figure-4, "Talker Attribute Flow". Perform the following to enable reservations:

  • Adjust port shaper rate for traffic class

  • Add mroute, which is Layer 2 Multicast route, for traffic

• MSRP merges multiple listener reservations and propagates the results to the talker.

• MSRP uses standard specified defaults

• Rank preemption is not supported.

• The following are the MRP timers:

  • 200 milliseconds Join Timer (assumes P-to-P)

  • 1000 milliseconds Leave Timer (standard 600-1000; uses max for max scale)

  • 10000 milliseconds Leave All Timer

• Streams are multi-cast only as per the standard.

• MSRP PDU are NOT VLAN tagged

Bandwidth Management

gPTP values used by MSRP

MSRP uses the following Link Delay and Residence Delay values from gPTP.

MSRP uses Link Delay and Residence Delay values to calculate the end-to-end latency. See the figure below :

The latency between egress of the previous node and egress of the current node = Link Delay for the link connected to egress of the previous node and ingress of the current node) + Residence delay (switch latency) 

Forwarding and Queuing for Time-Sensitive Streams (FQTSS) specification defines the forwarding and queuing rules and guidelines for AVB traffic. It is defined by IEEE 802.1Qav. The following are the characteristics of FQTSS:

• Cisco NX-OS AVB implementation follows FQTSS standard.

• FQTSS supports the tables and procedures for mapping priorities to traffic classes.

• gPTP requires the availability of boundary and ordinary clocks for end stations.

Classes of Traffic

QoS is applied in AVB features to:

• Protect the control and best effort traffic, where the reserved AVB stream traffic traversing an AVB bridge has forwarding precedence over non-reserved traffic, from the SR class traffic and vice versa.

• Protect the SR class traffic from itself.

The following QoS functions are used to achieve the QoS goals:

• Supports new network QoS template for AVB

• Defines ingress and egress queues with specific properties

• Supports ingress COS re-marking and protects AVB traffic

• Boundary or non-AVB port configuration by applying ingress QoS policy

• Core port configuration by removing ingress QoS policy

• Supporting dynamic configuration of Egress Shaper Rate, for bandwidth reservation, on per class and per port basis for core ports

Network QoS Template:

The network QOS policy, which is default-nq-8021qav-4q8q-policy) defined for AVB:

The below table lists the ingress queuing policy:

The below table lists the egress queuing policy:

Note	Priority is of the order 1 > 2 > 3

Shape: $ indicates the variable.