Tag Archives: Via

SIP Route, Contact, From Headers – Which to use?

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SIP Proxies are simple in theory but start to get a bit more complex when implemented.

When a proxy has a response to send back to an endpoint, it can have multiple headers with routing information for how to get that response back to the endpoint that requested it.

So how to know which header to use on a new request?

Routing SIP Requests

Record-Route

If Route header is present (Like Record-Route) the proxy should use the contents of the Record-Route header to route the traffic back.

The Record-Route header is generally not the endpoint itself but another proxy, but that’s not an issue as the next proxy will know how to get to the endpoint, or use this same logic to know how to get it to the next proxy.

Contact

If no Route headers are present, the contact header is used.

The contact provides an address at which a endpoint can be contacted directly, this is used when no Record-Route header present.

From

If there is no Contact or Route headers the proxy should use the From address.

A note about Via

Via headers are only used in getting responses back to a client, and each hop removes it’s own IP on the response before forwarding it onto the next proxy.

This means the client doesn’t know all the Via headers that were on this SIP request, because by the time it gets back to the client they’ve all been removed one by one as it passed through each proxy.

A client can’t send a SIP request using Via’s as it hasn’t been through the proxies for their details to be added, so Via is only used in responding to a request, for example responding with a 404 to an INVITE, but cannot be used on a request itself (For example an INVITE).

SIP Via Header

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The SIP Via header is added by a proxy when it forwards a SIP message onto another destination,

When a response is sent the reverse is done, each SIP proxy removes their details from the Via header and forwards to the next Via header along.

SIP Via headers in action
SIP Via headers in action

As we can see in the example above, each proxy adds it’s own address as a Via header, before it uses it’s internal logic to work out where to forward it to, and then forward on the INVITE.

Now because all our routing information is stored in Via headers when we need to route a Response back, each proxy doesn’t need to consult it’s internal logic to work out where to route to, but can instead just strip it’s own address out of the Via header, and then forward it to the next Via header IP Address down in the list.

SIP Via headers in action

Via headers are also used to detect looping, a proxy can check when it receives a SIP message if it’s own IP address is already in a Via header, if it is, there’s a loop there.