Postcards from the Bleeding Edge
Monday, July 05, 2004

  SIP is a standard, IAX is better

The complexities of implementing SIP (and H323 before it) for voice transit have given many a sysadm grey hairs.

Personally I thought that voice and videoconferencing would "just work" about 5 years ago, but nobody out there doing protocol design really thought about NAT all that hard, so slowly, slowly, SIP capable routers have become more common, and methods for STUN Nat tunneling have begun to work better, but we are still years out from being able to make a phone call from our PC simply and easily.

There IS a better way than SIP - with an open, simplified, faster protocol called "IAX".

    From: Mark Spencer
    To:
    Subject: Re: [Asterisk-Users] Re: iax or sip
    Reply-To:

    Okay, setting aside conspiracy theories, trolling, flaming, etc, let me
    summarize some differences between SIP and IAX, and it might help you
    make a decision about what is best for you.

    1) IAX is more efficient on the wire than RTP for *any* number of calls,
    *any* codec. The benefit is anywhere from 2.4k for a single call to
    approximately trippling the number of calls per megabit for G.729 when
    measured to the MAC level when running trunk mode.

    2) IAX is information-element encoded rather than ASCII encoded. This
    makes implementations substantially simpler and more robust to buffer
    overrun attacks since absolutely no text parsing or interpretation is
    required. The IAXy runs its entire IP stack, IAX stack, TDM interface,
    echo canceller, and callerid generation in 4k of heap and stack and 64k of
    flash. Clearly this demonstrates the implementation efficiency of its
    design. The size of IAX signalling packets is phenomenally smaller than
    those of SIP, but that is generally not a concern except with large
    numbers of clients frequently registering. Generally speaking, IAX2 is
    more efficient in its encoding, decoding and verifying information, and it
    would be extremely difficult for an author of an IAX implementation to
    somehow be incompatible with another implementation since so little is
    left to interpretation.

    3) IAX has a very clear layer2 and layer3 separation, meaning that both
    signalling and audio have defined states, are robustly transmitted in a
    consistant fashion, and that when one end of the call abruptly disappears,
    the call WILL terminate in a timely fashion, even if no more signalling
    and/or audio is received. SIP does not have such a mechanism, and its
    reliability from a signalling perspective is obviously very poor and
    clumsy requiring additional standards beyond the core RF3261.

    4) IAX's unified signalling and audio paths permit it to transparently
    navigate NAT's and provide a firewal administrator only a *single* port to
    have to open to permit its use. It requires an IAX client to know
    absolutely nothing about the network that it is on to operate. More
    clearly stated, there is *never* a situation that can be created with a
    firewall in which IAX can complete a call and not be able to pass audio
    (except of course if there was insufficient bandwidth).

    5) IAX's authenticated transfer system allows you to transfer audio and
    call control off a server-in-the-middle in a robust fashion such that if
    the two endpoints cannot see one another for any reason, the call
    continues through the central server.

    6) IAX clearly separates Caller*ID from the authentication mechanism of
    the user. SIP does not have a clear method to do this unless
    Remote-Party-ID is used.

    7) SIP is an IETF standard. While there is some fledgling documentation
    courtesy Frank Miller, IAX is not a published standard at this time.

    8) IAX allows an endpoint to check the validity of a phone number to know
    whether the number is complete, may be complete, or is complete but could
    be longer. There is no way to completely support this in SIP.

    9) IAX always sends DTMF out of band so there is never any confusion about
    what method is used.

    10) IAX support transmission of language and context, which are useful in
    an Asterisk environment. That's pretty much all that comes to mind at the
    moment.

    Mark


I LIKE asterisk and IAX
 
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