For most Voice / Telco engineers IPsec is a VPN technology, maybe something used when backhauling over an untrusted link, etc, but voice over IP traffic is typically secured with TLS and SRTP.

IMS / Voice over LTE handles things a bit differently, it encapsulates the SIP & RTP traffic between the UE and the P-CSCF in IPsec Encapsulating Security Payload (ESP) payloads.

In this post we’ll take a look at how it works and what it looks like.

It’s worth noting that Kamailio recently added support for IPsec encapsulation on a P-CSCF, in the IMS IPSec-Register module. I’ll cover usage of this at a later date.

The Message Exchange

The exchange starts off looking like any other SIP Registration session, in this case using TCP for transport. The UE sends a REGISTER to the Proxy-CSCF which eventually forwards the request through to a Serving-CSCF.

This is where we diverge from the standard SIP REGISTER message exchange. The Serving-CSCF generates a 401 Unauthorized response, containing an authentication challenge in the WWW-Authenticate header, and also a Ciphering Key & Integrity Key (ck= and ik=) also in the WWW-Authenticate header.

The Serving-CSCF sends the Proxy-CSCF the 401 response it created. The Proxy-CSCF assigns a SPI for the IPsec ESP to use, a server port and client port and indicates the used encryption algorithm (ealg) and algorithm to use (In this case HMAC-SHA-1-96.) and adds a new header to the 401 Unauthorized called Security–Server header to share this information with the UE.

The Proxy-CSCF also strips the Ciphering Key (ck=) and Integrity Key (ik=) headers from the SIP authentication challenge (WWW-Auth) and uses them as the ciphering and integrity keys for the IPsec connection.

Finally after setting up the IPsec server side of things, it forwards the 401 Unauthorized response onto the UE.

Upon receipt of the 401 response, the UE looks at the authentication challenge.

Keep in mind that the 3GPP specs dictate that IMS / VoLTE authentication requires mutual network authentication meaning the UE authenticates the network as well as the network authenticating the UE. I’ve written a bit about mutual network authentication in this post for anyone not familiar with it.

If the network is considered authenticated by the UE it generates a response to the Authentication Challenge, but it doesn’t deliver it over TCP. Using the information generated in the authentication challenge the UE encapsulates everything from the network layer (IPv4) up and sends it to the P-CSCF in an IPsec ESP.

Communication between the UE and the P-CSCF is now encapsulated in IPsec.

If you’re leaning about VoLTE & IMS networks, or building your own, I’d suggest checking out my other posts on the topic.