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List:       sip-implementors
Subject:    Re: [Sip-implementors] Transport Selection dependent on message size and NAPTR priorities (pitting r
From:       Brett Tate <brett () broadsoft ! com>
Date:       2014-04-28 9:40:04
Message-ID: f405eae34041244c13201df146f8b053 () mail ! gmail ! com
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> If I had an 1400 byte request, with a next-hop FQDN
> that resolved to NAPTR with prio "1:UDP, 2:TCP",
> what transmission attempts would you make, in what order?
> (and lets say that the srv-lookup returns the same
> two hosts, as1.examle.com, and as2.example.com)
>
> The logic you described indiicated you would first
> try UDP, then TCP?

No; I said only 1 transport type is selected by the RFC 3263's NAPTR/SRV
process.

If that process results is preferring UDP, that is the selected transport.
Then the message size can cause overriding that result to try TCP to that
ip-address/port (associated with UDP); if the TCP attempt fails as mentioned
within 18.1.1, UDP should be tried to that same ip-address/port.

This is the reason for the following RFC 3261 section 18.2.1 snippet:

"For any port and interface
 that a server listens on for UDP, it MUST listen on that same port
 and interface for TCP.  This is because a message may need to be sent
 using TCP, rather than UDP, if it is too large."

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