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forwarding any incoming multicast packet to all switch ports. With high levels of audio stream traffic
this will result in network flooding and can result in a total network lock-up.
Managed switches provide multicast traffic management through IGMP
registered multicast packets are forwarded to their designation ports only. Consequently, all AES67
nodes are required to support IGMPv2 which is used to tell the network which streams are to be
received. The IGMP (join) requests need to be updated periodically in order to maintain the multicast
forwarding. This is ensured by enabling the IGMP querier function in one of the participating
switches, which the sends out periodic IGMP queries triggering the nodes to renew their IGMP
requests. Once a stream connection is terminated, an IGMP leave request will be sent out to signal
termination of a particular multicast flow to that node.
One of the benefits of managed multicast traffic is its scalability: any multicast flow is only sent once
by a particular sender into the network. If more than one receiver requests the same flow, the
network switches will clone packets as required. With IGMP the network inherently optimizes the
traffic, so that a particular multicast flow will be present on any involved link just once.
1.3
Quality of Service
Quality of Service (QoS) is another fundamental principle which needs to be supported by the
network – again, only available with managed switches. Proper QoS configuration ensures that the
most critical packets – PTP and audio stream traffic – receive prioritized forwarding on their way
through the network. AES67 mandates for support of Differentiated Services (DiffServ)
scheme where different types of traffic can be categorized into service classes. DiffServ works with
64 different priority tags – DSCP
can apply different tags to different traffic classes; switches can then examine the individual priority
tags and forward packets on a preferred basis. Put simply, with DiffServ a network works in a very
similar way to the boarding procedure at airports - priority passengers (first and business class) board
the plane first (and at any time), while economy class passengers have to wait in line as long as priority
passengers are still queuing up.
However, while recommendations exist in the standards on how to assign DSCP values to certain
types of traffic, a network administrator is free to configure these values according to the individual
application requirements. In larger network environments with a variety of shared traffic classes, QoS
configuration requires special attention.
AES67 requires the use of 3 traffic classes and recommends certain DSCP values:
•
PTP traffic should be tagged with DSCP EF (46), translating into expedited forwarding,
receiving the highest forwarding priority (first class passengers)
•
RTP traffic (audio packets) should be tagged with AF41 (34), translating into advanced
forwarding with the second-highest forwarding priority (business class passengers)
4
Internet Group Management Protocol
https://en.wikipedia.org/wiki/Internet_Group_Management_Protocol
5
https://en.wikipedia.org/wiki/Differentiated_services
6
Differentiated Services Code Point – a number in the range of 0..63
6
values – which can be applied to individual IP packets. End nodes
Page 3 of 28
AES67 Practical Guide
4
support. With IGMP,
5
, a QoS
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