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Re: [Sip] Fwd: I-D Action:draft-sparks-sip-invfix-03.txt

On Mar 21, 2009, at 7:59 AM, Jonathan Rosenberg wrote:


Thanks, Robert. Comments below:


It modifies state transitions in the INVITE server state
  machine to absorb retransmissions of the INVITE request after
encountering a unrecoverable transport error when sending a response.
this little tidbit is not mentioned in section 3, whereas the other parts of this section (section 4) are mentioned. I think a paragraph describing this error is appropriate. 

Edit-itis. I will add such a paragraph.




Implementations strictly conformant to [RFC3261] will process
  retransmitted initial INVITE requests as new requests.  Proxies may
  forward them to different locations than the original.  Proxies may
  also be used as anonymizing forwarders of bulk traffic.
should mention the consequences of not addressing this transport- error issue in my comment above. 

ok





7.1.  UAS Impacts
  To allow a UAS to recognize retransmissions of an INVITE as
retransmissions instead of new requests, a new state, "Accepted", is 
  added to the INVITE server transaction state machine.
this change is not JUST for a UAS - its also for proxies implementing the server transaction machine, right? Is there a reason this is worded for UAS?
Hmm. I'm going to have to go read this through fairly carefully again. The change is to the client and server transaction state machines. It may be that the prose started to erroneously use UAC and UAS as synonyms for those state machines. Let me look through it and propose correction. 





Timer L reflects the amount of time the TU will wait to
  receive an ACK for the 2xx it is emitting before considering the
  transaction failed.


but an ACK-200 is not part of this transaction...
Right, the TU is tying those two transactions together. This is the period of time that the mentioned TU can be handing down 200s for retransmission. Given your question, it looks like I should change this motivation sentence to say that instead?
also, I think its worth at least noting that this change does not 100% solve the problem. If the UAC and any server transaction between it, and the UAS, use a different value of T1, there can still be a problem. In particular if the value of T1 at the UAC is larger than the value at any of the downstream server transactions, the server transaction may be destroyed while the UAC remains in the calling state.
I agree (and think there are other places where we should more strongly point out the pitfalls of changing T1 only in one part of a system). 
Could you propose text for this instance though?





UAC Impacts
  In order to correctly distinguish retransmissions of 2xx responses
from stray 2xx responses, the INVITE client state machine is modified 
  to not transition immediately to "Terminated" on receipt of a 2xx
  response.
as above, this is not specific to UAC and covers proxy/b2b client transactions too, no? 


Proxy Considerations
A direct consequence of the change to the UAC state machine is that a 
  transaction-stateful proxy will not foward any stray INVITE
  responses.
It is not a consequence of the new state machines - but it is a change in behavior we are making.
I understand your objection, and think this may be another aspect of unintentional conflation of UAS/UAC with server and client state machines. When I propose a correction as mentioned above, I'll capture this.
Meta-comment: A drawback of this change is that proxies and b2bua will now hold transaction state for 64*t1 seconds after the transaction is done, even for calls that set up immediately. In some implementations this may be a non-trivial increase in memory usage, and there may be other impacts (for example if an implementation had a non-optimized transaction matching mechanism which only worked well with a small number of transaction machines e.g., a linear search). I think this implication needs to be called out more strongly. You mention it as a security vulnerability, but its not just a security issue.
Where in the document would you think such a highlight would have the best impact? 


-Jonathan R.

Robert Sparks wrote:
This got a fairly big touch due to better handling of transport error conditions. Please review this carefully. 
RjS
Begin forwarded message:

*From: *Internet-Drafts at ietf.org <mailto:Internet-Drafts at ietf.org>
*Date: *March 2, 2009 8:45:01 AM CST
*To: *i-d-announce at ietf.org <mailto:i-d-announce at ietf.org>
*Subject: **I-D Action:draft-sparks-sip-invfix-03.txt *
*Reply-To: *internet-drafts at ietf.org <mailto:internet-drafts at ietf.org >
A New Internet-Draft is available from the on-line Internet-Drafts directories.
Title : Correct transaction handling for 200 responses to Session Initiation Protocol INVITE requests 
Author(s)       : R. Sparks
Filename        : draft-sparks-sip-invfix-03.txt
Pages           : 20
Date            : 2009-03-02

This document normatively updates RFC 3261, the Session Initiation
Protocol (SIP), to address an error in the specified handling of
success (200 class) responses to INVITE requests. Elements following RFC 3261 exactly will misidentify retransmissions of the request as a 
new, unassociated, request.  The correction involves modifying the
INVITE transaction state machines.  The correction also changes the
way responses that cannot be matched to an existing transaction are
handled to address a security risk.

A URL for this Internet-Draft is:
http://www.ietf.org/internet-drafts/draft-sparks-sip-invfix-03.txt

Internet-Drafts are also available by anonymous FTP at:
ftp://ftp.ietf.org/internet-drafts/

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Jonathan D. Rosenberg, Ph.D.                   111 Wood Avenue South
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