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