SIP - Connection reuse vs Persistent connection

It goes without saying that SIP solutions are impacted by NAT. So much that some scenarios required integration to RFC 3261, e.g. with RFC 3581, which defined the 'rport' attribute to be added in the Via header (integrating the 'received' attribute): with that information, responses could be routed to the source port of the related request, and not on the advertised port in the original Via.

That was called Symmetric Response, and applied to connection-less transports (UDP), while, as mentioned in RFC 6314, it's not necessary when using reliable transports (TCP in most cases): SIP responses can be sent back on the same connection on which the request arrived.

Also from RFC 3261, chapter 18, Transport layer, client behaviour:

"For reliable transports, the response is normally sent on the connection on which the request was received."

But the client needs to be prepared to receive the response on a new connection:

"[...] the transport layer MUST also be prepared to receive an incoming connection on the source IP address from which the request was sent and port number in the "sent-by" field."

That obviously would require the ability to create such connection from the server to the client.

Anyway when a reliable connection between two SIP entities is up, after a transaction is already concluded, there are two interesting opportunities:

- Use that same connection for more requests from the client

- Use that same connection for more requests from the server

where for "client" I refer to the entity that created the connection and sent the initial request, and "server" is the entity that accepted the connection and delivered the response(s).

The first case is mentioned in the same chapter on Transport layer:

"If a request is destined to an IP address, port, and transport to which an existing connection is open, it is RECOMMENDED that this connection be used to send the request, but another connection MAY be opened and used."

This is what's referred to as "Persistent connection", as mentioned in RFC 5923:

"The SIP protocol includes the notion of a persistent connection

   [...], which is a mechanisms to insure that

   responses to a request reuse the existing connection that is

   typically still available, as well as reusing the existing

   connections for other requests sent by the originator of the

   connection."

The second case (using the same connection from future requests from the server) is instead the subject of RFC 5923, and it is defined as "Connection reuse".

Once the connection is up, it seems a good opportunistic approach to reuse it, but an important limitation is mandated:

"Unlike TCP, TLS connections can be reused to send requests in the backwards direction since each end can be authenticated when the connection is initially set up."

In other words, only TLS connections formed by exchanging certificates can be reused, because the identities have been mutually verified.

The way the client can tell the server that connection reuse is desired is with a new parameter to be added in the Via header: 'alias'.

In general, RFC 5923 at chapter 5 clarifies:

"The act of reusing a connection needs

   the desired property that requests get delivered in the backwards

   direction only if they would have been delivered to the same

   destination had connection reuse not been employed."

Last important bit, not to be left implicit: persistent connections don't imply connection reuse, as RFC 5923 clarifies:

"[...] Persistent connections do not

      imply connection reuse."

So this post is basically sharing my own notes on this topic, which maybe somebody else (including me) can find useful in the future.