> Can there be any case when CANCEL reached to UA2
> before INVITE in case od UDP? because the 100 trying
> can be sent by proxies as well.
Yes (although no longer as common); as mentioned within RFC 3261 section
28.1, RFC 2543 did not have the same mandate as RFC 3261.
"o A UA or proxy cannot send CANCEL for a transaction until it gets a
provisional response for the request. This was allowed in RFC
2543 but leads to potential race conditions."
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