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RFCs & Standards Voice over IP

SIP Extensions – Path

In vanilla RFC3261 SIP, a UA can only send a REGISTER request to a SIP Registrar.

It can’t go via any intermediary proxies.

That’s obviously a bit of a problem, as we build out our network we might have a series of load balancers that send traffic to a pool of Registrars, but according to RFC3261 this can’t be done, the SIP REGISTER request would need to go direct to one of these Registrars.

To get around this the SIP Path extensions, officially called “Session Initiation Protocol (SIP) Extension Header Field for Registering Non-Adjacent Contacts” (catchy title) was defined under RFC 3327.

An additional header is introduced called “Path:” for each proxy between the UA and the Registrar,

As the SIP REGISTER request passes through each proxy, each proxy appends the Path header with the value of it’s own SIP URI.

Let’s take a look at an example call flow from [email protected] who sends his REGISTER to atlanta.com, which is proxied by atlanta.com to registrar1.atlanta.com:

Bob to atlanta.com:

[email protected] > atlanta.com
      REGISTER sip:atlanta.com SIP/2.0
      Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
      To: Bob <sip:[email protected]>
      From: Bob <sip:[email protected]>;tag=456248
      Call-ID: [email protected]
      CSeq: 1826 REGISTER
      Contact: <Bob <sip:[email protected]>>
      Supported: path

The REGISTER request is received by atlanta.com, which forwards it to registrar1.atlanta.com after adding it’s own URI as a Path header.

atlanta.com > registrar1.atlanta.com
      REGISTER sip:atlanta.com SIP/2.0
      Via: SIP/2.0/UDP atlanta.com;branch=z9hG4bK34ghi7ab04
      Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
      To: Bob <sip:[email protected]>
      From: Bob <sip:[email protected]>;tag=456248
      Call-ID: [email protected]
      CSeq: 1826 REGISTER
      Contact: <Bob <sip:[email protected]>>
      Supported: path
      Path: <sip:atlanta.com;lr>

The response is sent back in the same way, relying on the via headers and Path headers.

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