2.
[ S N O M 4 S N A T F I L T E R ]
0637ced821ef40a3;ua=c9b140ab598290e5bb491e9c3aaca440 Via: SIP/2.0/UDP 203.145.183.113:12975;branch=z9hG4bK- abx3au3mxb01;rport=17401
From: <sip:denny@snomag.de>;tag=k9p6fmeg7h
To: <sip:denny@snomag.de>;tag=epuy85kzm5
CSeq: 14 REGISTER
Contact: <sip:217.115.141.99:5082;ua=c9b140ab598290e5bb491e9c3aaca44 0>;expires=3600;gruu=”sip:denny@snomag.de;gruu=hobiv52b”
Date: Wed, 26 May 2004 16:03:33 GMT
SIP/2.0 200 Ok
Via: SIP/2.0/UDP 203.145.183.113:12975;branch=z9hG4bK- abx3au3mxb01;rport=17401
From: “denny” <sip:denny@snomag.de>;tag=k9p6fmeg7h
To: “denny” <sip:denny@snomag.de>;tag=epuy85kzm5
CSeq: 14 REGISTER
Contact: <sip:denny@203.145.183.113:12975;line=lhynyb3y>;expires=360 0;gruu=”sip:denny@snomag.de;gruu=hobiv52b”
Date: Wed, 26 May 2004 16:03:33 GMT
The SBC will make the registration short enough to keep the con- nection alive. The UA will reregister shortly after. However, because the registration binding time in the registrar is longer, the SBC will not forward the request to the registrar and answer it locally.
2.3.3 RTP Relay
When initiating a call, user agents usually include a Session De- scription Protocol (SDP) attachment that describes where they expect media. If the user agent operates on a public Internet address, there is no need to interfere in this process. In this case the SBC will just forward the request.
Operators should encourage customers to use equipment that operates on a public Internet address or properly allocates a globally routable Internet address. Because media relay is an expensive opera- tion, it reduces the overall load on the network and at the same time in- creases the quality of the service.
However, when a user agent is behind NAT, it might not be able to receive media directly. In some cases this is because the user agent is
16 • Architecture
