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Wednesday, March 16, 2016

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What is Via Header Field in SIP?

Leke Oyetoke - Wednesday, March 16, 2016


What is Via Header Field in SIP?


Via header field is used to record and store the SIP route taken and acquired by a request which assists to route a response back to the originator.

1. A UA producing a request, stores its own address in a Via header field.

2. A proxy sending the request adds a Via header field having its own address to the top of the list of Via header fields.

3. A proxy or UA yielding a response to a request copies all the Via header fields from the request in order into the response, then forwards the response to the address specified in the top Via header field.

4. A proxy receiving a response checks the top Via header field and matches its own address.

5. If it does not correspond to each other, the response has been discarded.

6. The top Via header field is then removed, and the response forwarded to the address specified in the next Via header field.

7. Via header fields have protocol name, version number, and transport (SIP/2.0/UDP, SIP/2.0/TCP, etc.) and may hold port numbers and parameters such as received, rport, branch, maddr, and ttl.

8. A received tag is included to a Via header field if a UA or proxy receives the request from a dissimilar address than that specified in the top Via header field.

9. A branch parameter is added to Via header fields by UAs and proxies, which is calculated as a hash function of the Request-URI, and the To, From, Call-ID, and CSeq number.


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