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What are Branch IDs & Tags in SIP?

Branch IDs permit proxies to correspond with responses to forked requests. Without them, a proxy wouldn't be able to distinguish which branch of a response corresponds to.

Tags are used by the UAC to differentiate multiple final responses from dissimilar UAS. UAS has no reliable way of deciding if the request has been forked or not. To be safe, it needs to add a tag. Proxies only insert tags into the final responses they produce themselves; they never insert tags into requests or responses they send.

In view of the fact that a request can be forked many times on its way to UAS, a single "tag" added to the request by one of the proxies is not adequate for the next forking proxy along the chain to match responses on its own branches; every proxy that forked the request would need to add its own special IDs to the branches it created.

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