Vikram,
Using invariants like call-ID is what has been suggested.
This has problems when servers fail or recover in the
middle of a transaction. DNS SRV usage in stateless proxies
is an open issue due to these reasons. You can have a look
at issue #220 (slide 39) in this presentation on bis-05
open issues:
http://www.jdrosen.net/sip/sip_bis_open_dec01.ppt
-Binu
Vikram Varma <[EMAIL PROTECTED]> on 12/12/2001 01:26:13 AM
To: K Binu/HSSBLR
cc: "'[EMAIL PROTECTED] '"
<[EMAIL PROTECTED]>
Subject: RE: [Sip-implementors] retransmissions in a stateless proxy
Binu,
I am not sure what you mean by the second paragraph.
When you do a DNS query, you get a list of IP addresses corresponding to
that domain. As you have mentioned, there may be a weighting factor.
Are you saying that the stateless proxy should have an algorithm based on
nonvariables within the dialog, such as CallID, to choose which IP from the
list the message should be sent to? i.e.
if algorithm(CallID) = 5, choose the 5th one on the list?
This is probably a good method, however, it will not work when the DNS
records are changed. (I dont know how often that will happen, but it
probably will).
Thanks
Vikram
-----Original Message-----
From: [EMAIL PROTECTED]
To: Ansar Kassim Pillai
Cc: [EMAIL PROTECTED]
Sent: 12/3/2001 8:42 PM
Subject: Re: [Sip-implementors] retransmissions in a stateless proxy
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
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