These posts focus on the use of Diameter and SIP in an IMS / VoLTE context, however these practices can be equally applied to other networks.

The Multimedia-Authentication-Request/Answer commands are used to Authenticate subscribers / UAs using a variety of mechanisms such as straight MD5 and AKAv1-MD5.

Basics:

When a SIP Server (S-CSCF) receives a SIP INVITE, SIP REGISTER or any other SIP request, it needs a way to Authenticate the Subscriber / UA who sent the request.

We’ve already looked at the Diameter User-Authorization-Request/Answer commands used to Authorize a user for access, but the Multimedia-Authentication-Request / Multimedia-Authentication-Answer it used to authenticate the user.

The SIP Server (S-CSCF) sends a Multimedia-Authentication-Request to the Diameter server, containing the Username of the user attempting to authenticate and their Public Identity.

The Diameter server generates “Authentication Vectors” – these are Precomputed cryptographic challenges to challenge the user, and the correct (“expected”) responses to the challenges. The Diameter puts these Authentication Vectors in the 3GPP-SIP-Auth-Data (612) AVP, and sends them back to the SIP server in the Multimedia-Authentication-Answer command.

The SIP server sends the Subscriber / UA a SIP 401 Unauthorized response to the initial request, containing a WWW-Authenticate header containing the challenges.

The Subscriber / UA sends back the initial request with the WWW-Authenticate header populated to include a response to the challenges. If the response to the challenge matches the correct (“expected”) response, then the user is authenticated.

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I always find it much easier to understand what’s going on through a packet capture, so here’s a packet capture showing the two Diameter commands,

Note: There is a variant of this process allows for stateless proxies to handle this by not storing the expected authentication values sent by the Diameter server on the SIP Proxy, but instead sending the received authentication values sent by the Subscriber/UA to the Diameter server to compare against the expected / correct values.

The Cryptography

The Cryptography for IMS Authentication relies on AKAv1-MD5 which I’ve written about before,

Essentially it’s mutual network authentication, meaning the network authenticates the subscriber, but the subscriber also authenticates the network.

Other Diameter Cx (IMS) Calls

User-Authorization-Request / User-Authorization-Answer
Server-Assignment-Request / Server-Assignment-Answer
Location-Info-Request / Location-Info-Answer
Multimedia-Auth-Request / Multimedia-Auth-Answer
Registration-Termination-Request / Registration-Termination-Answer
Push-Profile-Request / Push-Profile-Answer

References:

3GPP Specification #: 29.229

RFC 4740 – Diameter Session Initiation Protocol (SIP) Application