[asterisk-dev] Via/maddr: (was Re: [Code Review] Properly route responses according to the Via headers in the request)

Simon Perreault simon.perreault at viagenie.ca
Tue Jan 4 07:32:50 CST 2011

Le 04/01/2011 3:34 AM, Klaus Darilion a écrit :
> why should a response be sent to a multicast
> address at all?

Some insight from RFC 3261, section 18.1.1:

    A client that sends a request to a multicast address MUST add the
    "maddr" parameter to its Via header field value containing the
    destination multicast address, and for IPv4, SHOULD add the "ttl"
    parameter with a value of 1.  Usage of IPv6 multicast is not defined
    in this specification, and will be a subject of future
    standardization when the need arises.

    These rules result in a purposeful limitation of multicast in SIP.
    Its primary function is to provide a "single-hop-discovery-like"
    service, delivering a request to a group of homogeneous servers,
    where it is only required to process the response from any one of
    them.  This functionality is most useful for registrations.  In fact,
    based on the transaction processing rules in Section 17.1.3, the
    client transaction will accept the first response, and view any
    others as retransmissions because they all contain the same Via
    branch identifier.


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