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sip

In SIP, why the Contact header field MUST be present in the Invite request

Usually, the Contact header field in the Invite request is useless. For example, the UAC and the UAS are in different LANs. The Contact field may be:

INVITE sip:bob@sipprovider SIP/2.0
Contact: Alice<alice@192.168.1.10>
.....

There is no use of the Contact field while we can still build a dialog. Then, why the Contact header field is mandatory？

Solution

  • The contact field contains the address at which the callee can reach the caller for future requests. For example, it's necessary so that the callee can send a BYE or a re-INVITE to the caller.