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I thought I knew it #OSI

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  • I thought I knew it #OSI

    Hey all,

    I was just doing a bit of reading on SIP (Session Initiation Protocol) and am a little confused.

    according to wiki:
    "used for controlling multimedia communication sessions such as voice and video calls over Internet Protocol (IP) networks"

    and

    "The protocol defines the messages that are sent between peers which govern establishment, termination and other essential elements of a call"

    and it's a layer 7 protocol??????????????????

    can anybody explain this to me?? Why is it not a session layer protocol, layer 5??????

  • #2
    Re: I thought I knew it #OSI

    Originally posted by niall View Post
    Hey all,

    I was just doing a bit of reading on SIP (Session Initiation Protocol) and am a little confused.

    according to wiki:
    "used for controlling multimedia communication sessions such as voice and video calls over Internet Protocol (IP) networks"

    and

    "The protocol defines the messages that are sent between peers which govern establishment, termination and other essential elements of a call"

    and it's a layer 7 protocol??????????????????

    can anybody explain this to me?? Why is it not a session layer protocol, layer 5??????
    Because SIP is a signalling protocol that works at the application layer. The session initiation runs on top of other protocols further down the OSI model. When looking at a SIP payload its very similar to HTTP or SMTP in terms of the response codes used. Those protocols also are Layer 7. SIP is used between application API's in order to set up or tear down a SIP conversation, negotiating how the two end points will communicate.

    EDIT: Voip-Info has a better explanation extracted directly from the RFC:

    http://www.voip-info.org/wiki/view/SIP

    This document describes Session Initiation Protocol (SIP), an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences.

    SIP invitations used to create sessions carry session descriptions that allow participants to agree on a set of compatible media types. SIP makes use of elements called proxy servers to help route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users. SIP also provides a registration function that allows users to upload their current locations for use by proxy servers. SIP runs on top of several different transport protocols.

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    • #3
      Re: I thought I knew it #OSI

      Originally posted by niall View Post
      "The protocol defines the messages that are sent between peers which govern establishment, termination and other essential elements of a call"

      and it's a layer 7 protocol??????????????????

      can anybody explain this to me?? Why is it not a session layer protocol, layer 5??????
      Any protocol defining communications between client applications is by definition a layer 7 protocol. SIP deals with establishing and terminating "calls", which is an application-specific term.

      The "session" referred to in the name "Session Initiation Protocol" is in fact a VoIP session, not a session as described by the OSI model.

      Now, SIP actually handles OSI sessions as well, but that's simply because SIP normally runs on top of UDP, which is a connectionless transport protocol. Any application protocol using UDP will need to include mechanisms for session management, or do without sessions altogether.

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      • #4
        Re: I thought I knew it #OSI

        Please keep in mind the 7 layer protocol is really only "theoretical" like the answers required in Microsoft cert exams. Real world answers are usually a bit different. The Cisco stack is probably the better one to look at as the industry standard. Well that's how I look at it. YMMV.
        1 1 was a racehorse.
        2 2 was 1 2.
        1 1 1 1 race 1 day,
        2 2 1 1 2

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