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16 • Architecture
[ 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
Call-ID: 3c26701d7cb9-pady07b5783t@203-145-183-113
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
Content-Length: 0
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
Call-ID: 3c26701d7cb9-pady07b5783t@203-145-183-113
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
Content-Length: 0
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
2.