Ansar,
A domain that has a stateful proxy server farm is not
likely to implement a round robin DNS mechanism where
a lookup returns a different IP for each query.
The right way to implement DNS based load balancing
is to use the load balancing mechanism in DNS SRV records.
In such setups, when the stateless proxy performs a query
for the next hop server, it might get multiple IP addresses
for the server with equal priority. The records will contain
a weight field that indicates the preference to be given to
the record when a random selection is made.
In this case, the proxy must make a random selection from
the alternatives such that the probability of an address
getting selected depends on its weight. Here, the stateless
proxy should not use a truly random number to decide on the
selection. Instead, it should choose a number generated
from message elements like the call-ID and sequence number,
which are invariant in retransmissions or transactions.
This will ensure that for messages in the same transaction
the same proxy is selected from the SRV results.
For more information on SRV records and the mechanism used
for selecting SRV entries, refer RFC 2782.
-Binu
"Ansar Kassim Pillai" <[EMAIL PROTECTED]> on 12/03/2001 03:55:58 PM
To: [EMAIL PROTECTED]
cc: (bcc: K Binu/HSSBLR)
Subject: [Sip-implementors] retransmissions in a stateless proxy
Hi,
Page number:65 of the RFC2543bis-05 says that...
2464 A stateless proxy MUST choose one and only one destination from the
destination set. This choice
2465 MUST only rely on fields in the message and time-invariant properties
of the server. In particular, a
2466 retransmitted request MUST be forwarded to the same destination each
time it is processed. Further-
2467 more, CANCEL and non-Routed ACK requests MUST generate the same choice
as their associated 2467 INVITE.
Relying on the message along with the time-invariant properties of the
server, we can make sure that all the retransmissions are forwarded to the
same URI. But how do we make sure that the re-transmissions are always
forwarded to the same physical location (IP Address), especially when a DNS
query returns different IP each time with his round robbin algorithm?
We need to assure that all retransmissions follow the same physical path as
there could be some one in the path maintaining the state of the call.
Can anyone please explain how do we implement this in a stateless proxy
where we cannot remember the IP address for retransmissions.
Regards,
Ansar
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