Let's say that Alice would like to make SIP call to Bob. Alice initiates the call using her mobile device over a cellular network. On the other hand, Bob sits in a local network of an organisation. The traffic should go from Alice, then to Bob's organisation SIP server/gateway, which in turns routes the call to Bob's end device. It's a requirement that the traffic on the leg between Alice's mobile device and Bob's organisation SIP server/gateway is encrypted with TLS/SRTP. Therefore the SIP server/gateway needs to decrypt any incoming SRTP traffic to RTP traffic, before routing it to Bob's end device. Obviously, it also needs to encrypt any outgoing RTP traffic to SRTP for the other direction.

What is the exact name or term of this SIP server/gateway?


  • This part of your question is probably off-topic: "Is there any open-source solution that provides this functionality?" I doubt you are looking for a yes/no answer to this question, but product and/or resource recommendations are explicitly off-topic here. You could try Software Recommendations for that question.
    – Ron Maupin
    Apr 22, 2016 at 22:50
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 8, 2017 at 3:37

2 Answers 2


SIP (Session Initiation Protocol) is used to create the session. Basically it prepares the ground for Alice and Bob to communicate directly with each other via whavever protocol they choose (in this case it would be RTP or SRTP). When this session is up, application data go directly from Alice to Bob and they take care of any encryption/decryption.

What you might have meant is that when SIP does it's initiation. Alice might be communicating with her closest SIP proxy via SIP and that proxy might be communicating with Bob's proxy via SIPS (SIP with TLS encryption). In that case the name you were looking for would be SIP proxy.

Nice explanation with a picture can be found here.


The SRTP session is between the endpoints. The gateway is unseen/transparent.

In SIP lingo, there is a Session Border Controller, SBC. A security device that injects itself between endpoints is often called a proxy.

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