RFC 3261, "SIP: Session Initiation Protocol", June 2002

Errata ID: 2769
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT
Reported By: Iñaki Baz Castillo
Date Reported: 2011-04-08
Held for Document Update by: Robert Sparks

Section 17.1.2.2 says:

   MUST be passed to the transport layer for transmission.  If an
   unreliable transport is in use, the client transaction MUST set timer
   E to fire in T1 seconds.  If timer E fires while still in this state,
   the timer is reset, but this time with a value of MIN(2*T1, T2).
   When the timer fires again, it is reset to a MIN(4*T1, T2).  This
   process continues so that retransmissions occur with an exponentially
   increasing interval that caps at T2.

It should say:

   MUST be passed to the transport layer for transmission.  If an
   unreliable transport is in use, the client transaction MUST set timer
   E to fire in T1 seconds.  If timer E fires while still in this state,
   the timer is reset, but this time with a value of MIN(2*T1, T2).
   When the timer fires again, it is reset to a MIN(4*T1, T2). Multiplier
   on T1 doubles with each reset so that retransmissions occur with an
   exponentially increasing interval that caps at T2.

Notes:

The original text doesn't clarify that the multiplier of T1 is doubled with each timer reset, so it could be understood that the maximum value the timer takes is MIN(4*T1, T2) which is 2s (so the timer would never reach 4s and the resulting intervals would be 500ms, 1s, 2s, 2s, 2s, 2s, etc).

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