Cisco Systems 78-11741-02 manual IPC-431, RTP Header Compression

Configuring IP Multicast Routing

Configuring RTP Header Compression

Figure 70 RTP Header Compression

Before RTP header compression:

20 bytes	8 bytes 12 bytes
IP	UDP	RTP	Payload
Header			20 to 160 bytes

After RTP header compression:

3 to 5 bytes

Payload

IP/UDP/RTP header 20 to 160 bytes

S5925

The RTP header compression feature compresses the IP/UDP/RTP header in an RTP data packet from

40bytes to approximately 2 to 5 bytes, as shown in Figure 70. It is a hop-by-hop compression scheme similar to RFC 1144 for TCP/IP header compression. Using RTP header compression can benefit both telephony voice and MBONE applications running over slow links.

RTP header compression is supported on serial lines using Frame Relay, High-Level Data Link Control (HDLC), or PPP encapsulation. It is also supported over ISDN interfaces.

Enabling compression on both ends of a low-bandwidth serial link can greatly reduce the network overhead if substantial amounts of RTP traffic are on that slow link. This compression is beneficial especially when the RTP payload size is small (for example, compressed audio payloads of 20 to 50 bytes). Although the MBONE-style RTP traffic has higher payload sizes, compact encodings such as code excited linear prediction (CELP) compression can also help considerably.

Before you can enable RTP header compression, you must have configured a serial line that uses either Frame Relay, HDLC, or PPP encapsulation, or an ISDN interface. To configure RTP header compression, perform the tasks described in the following sections. Either one of the first two tasks is required.

•Enabling RTP Header Compression on a Serial Interface

•Enabling RTP Header Compression with Frame Relay Encapsulation

•Changing the Number of Header Compression Connections

You can compress the IP/UDP/RTP headers of RTP traffic to reduce the size of your packets, making audio or video communication more efficient. You must enable compression on both ends of a serial connection.

RTP header compression occurs in either the fast-switched or CEF-switched path, depending on whether certain prerequisites are met. Otherwise, it occurs in the process-switched path. For more information about where RTP header compression occurs, see the section “Enabling Express RTP Header Compression” later in this document.

Cisco IOS IP Configuration Guide

IPC-431