Subject: [Sip] draft-ietf-sip-fork-loop-fix-05

From: "Kolomaznik Jan" <[email-masked]>
Date: Mon, 14 May 2007 09:08:31 +0200

I have two notes to the draft-ietf-sip-fork-loop-fix-05. 

1st note:

This draft states the number of stimulated request is equal 2^70 when
the involved endpoint and proxies follow RFC 3261 recommendations. It is
mentioned in sections 2 and 3.

My opinion is, that the number of stimulated requests is another and it
is 2^71 for the given example and 2^(Max-Forwards + 1) in a little
generally way (but still valid only for the given example).

Let follow next calculation example, where an endpoint sends request
with Max-Forwards reduced to 4.

                  |                      Max-Forwards: 4
                 a@P1
               /      \
             /          \                Max-Forwards: 3
           /              \
         /                  \
       a@P2                 b@P2
        / \                 / \
      /     \             /     \        Max-Forwards: 2
     /       \           /       \
   a@P1      b@P1      a@P1      b@P1
   / \       / \       / \       / \     Max-Forwards: 1
a@P2 b@P2 a@P2 b@P2 a@P2 b@P2 a@P2 b@P2
 /\   /\   /\   /\   /\   /\   /\   /\   Max-Forwards: 0

There are two messages with Max-Forwards header set to 3, four messages
with Max-Forwards header set to 2, eight messages with Max-Forwards
header set to 1 and sixteen messages with Max-Forwards header set to 0.
Altogether there are 32 stimulated messages. Request with Max-Forwards
set to 4 is not possible to include because it is stimulating message.

So, the calculation mentioned by sections 2 and 3 is valid only for the
last forking round but it is not number of all produced requests.

2nd note:

Each proxy must handle number of UAS transactions and number of UAC
transaction. Sum of UAS and UAC transactions per proxy is close to
2^(Max-Forwards + 1). When UAS transaction of the stimulating request
will not be counted, the number of transactions handled by each proxy is
the same. 

Please note that these calculations are valid only for the given
example. In another attack configuration it will be different.

Jan

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