SIP Illustrated 5: SIP Session Routing

In the last post we have seen a basic SIP (VoLTE) session. This time we should analyze in more detail, what headers are used by network elements for their routing decisions and how they discover what port and IP to use. In practice that’s what people are not always sure about. They know the flow, order of signalling messages, but when something goes wrong, they are just guessing what could be the reason.

SIP Message Routing

Let’s recap what we have learnt so far. We use loose routing. So if a SIP message contains a Route header, we will use the top most one for the routing. If there is no Route header the routing is done based on the Request-URI, which contains the address of the final recipient. Don’t forget, network elements are able to add and modify the headers. The way how we handle the messages and modify their content within the IMS is described in 3GPP standards.

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An Éminence Grise – the DNS

In marriage we need to find a way how to make common decisions. In my case we decided that my wife will make those routine, unimportant decisions as what to cook for a dinner, how will spend a weekend or holidays or what to do with the money we have. My responsibility is to take the key, long-term decisions which will have the major influence on our well-being. So I decide what soccer team is our favorite for the next season, brand of a BBQ grill and what is our relationship towards EU.

In the IMS network there are many key network functions which take the decisions about routing, charging, applied services etc. But should I choose one which would allow me to manage the whole network, I’d go for DNS. Without DNS there would be no IMS. And don’t get me wrong. This is not only about the usage of IP protocol and symbolic address translation. DNS is responsible also for other vital mechanisms as routing, load-sharing, resiliency, service discovery and zero-configuration, provisioning, mobile number portability, timezone support, etc.

DNS is defined in many documents the most important for us are IETF RFC 1034, RFC 1035, RFC 3646 and mainly in RFC 3263 and RFC 7984 and GSMA IR.67.

Let’s take a look on routing in the IMS network.

Routing in IMS

Routing in IMS


We can start with the registration. Well, the DNS is used also during the ePC attach procedure. But we don’t discuses particular access networks here. (The Protocol Configuration Options – PCO response will contain the IPv4/IPv6 address of the P-CSCF anyway).  In the IMS the P-CSCF will take the Request-URI header of the SIP REGISTER message and based on the DNS response will identify the I-CSCF.

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