IP Telephony Cookbook by Saverio Niccolini, Jorg Ott, et al - HTML preview

RTP Streams

Figure 2.19 INVITE message flow

2.2.2.6.4 Record routing

All requests sent within a dialogue are, by default, sent directly from one user agent to the other.

Only requests outside a dialogue traverse SIP proxies.This approach makes a SIP network more scalable because only a small number of SIP messages hit the proxies.

There are certain situations in which a SIP Proxy needs to stay on the path of all further messages. For instance, proxies controlling a NAT box, or proxies doing accounting need to stay on the path of BYE requests.

The mechanism by which a proxy can inform user agents that it wishes to stay on the path of all further messages is called record routing. Such a proxy would insert a Record-Route header field into SIP messages which contain address of the proxy. Messages sent within a dialogue will then traverse all SIP proxies that put a Record-Route header field into the message.

The recipient of the request receives a set of Record-Route header fields in the message. It must mirror all the Record-Route header fields into responses because the originator of the request also needs to know the set of proxies.

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[IP Telephony Cookbook] / Technological Background

Without record routing

With record routing

UA1

SIP Proxy

UA2

UA1

SIP Proxy

UA2

BYE

BYE

200 OK

BYE

200 OK