10. Implementation Examples

This document mandates only the sender and receiver behaviours that are necessary for interoperability. In addition, certain algorithms, such as rate control or buffer management when targeted at specific environments, may enhance the retransmission efficiency.

This section gives an overview of different implementation options allowed within this specification.

The first example describes a minimal receiver implementation. With this implementation, it is possible to retransmit lost RTP packets, detect efficiently the loss of retransmissions, and perform multiple retransmissions, if needed. Most of the necessary processing is done at the server.

The second example shows how retransmissions may be used in (small) multicast groups in conjunction with layered encoding. It illustrates that retransmissions and layered encoding may be complementary techniques.

10.1. A Minimal Receiver Implementation Example

This section gives an example of an implementation supporting multiple retransmissions. The sender transmits the original data in RTP packets using the MPEG-4 video RTP payload format. It is assumed that NACK feedback messages are used, as per [1]. An SDP description example with SSRC-multiplexing is given below:

v=0
o=mascha 2980675221 2980675778 IN IP4 host.example.net
c=IN IP4 192.0.2.0
m=video 49170 RTP/AVPF 96 97
a=rtpmap:96 MP4V-ES/90000
a=rtcp-fb:96 nack
a=rtpmap:97 rtx/90000
a=fmtp:97 apt=96;rtx-time=3000

The format-specific parameter "rtx-time" indicates that the server will buffer the sent packets in a retransmission buffer for 3.0 seconds, after which the packets are deleted from the retransmission buffer and will never be sent again.

In this implementation example, the required RTP receiver processing to handle retransmission is kept to a minimum. The receiver detects packet loss from the gaps observed in the received sequence numbers. It signals lost packets to the sender through NACKs as defined in the AVPF profile [1]. The receiver should take into account the signalled sender retransmission buffer length in order to dimension its own reception buffer. It should also derive from the buffer length the maximum number of times the retransmission of a packet can be requested.

The sender should retransmit the packets selectively; i.e., it should choose whether to retransmit a requested packet depending on the packet importance, the observed Quality of Service (QoS), and congestion state of the network connection to the receiver. Obviously, the sender processing increases with the number of receivers as state information and processing load must be allocated to each receiver.

10.2. Retransmission of Layered Encoded Media in Multicast

This section shows how to combine retransmissions with layered encoding in multicast sessions. Note that the retransmission framework is offered only for small multicast applications. Refer to RFC 2887 [10] for a discussion of the problems of NACK implosion, severe congestion caused by feedback traffic, in large-group reliable multicast applications.

Packets of different importance are sent in different RTP sessions. The retransmission streams corresponding to the different layers can themselves be seen as different retransmission layers. The relative importance of the different retransmission streams should reflect the relative importance of the different original streams.

In multicast, SSRC-multiplexing of the original and retransmission streams is not allowed as per Section 5.3 of this document. For this reason, the retransmission stream(s) MUST be sent in different RTP session(s) using session-multiplexing.

An SDP description example of multicast retransmissions for layered encoded media is given below:

m=video 8000 RTP/AVPF 98
c=IN IP4 224.2.1.0/127/3
a=rtpmap:98 MP4V-ES/90000
a=rtcp-fb:98 nack
m=video 8000 RTP/AVPF 99
c=IN IP4 224.2.1.3/127/3
a=rtpmap:99 rtx/90000
a=fmtp:99 apt=98;rtx-time=3000

The server and the receiver may implement the retransmission methods illustrated in the previous examples. In addition, they may choose to request and retransmit a lost packet depending on the layer it belongs to.