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

Note: This RFC has been updated by RFC 3265, RFC 3853, RFC 4320, RFC 4916, RFC 5393, RFC 5621, RFC 5626, RFC 5630, RFC 5922, RFC 5954, RFC 6026, RFC 6141, RFC 6665, RFC 6878, RFC 7462, RFC 7463, RFC 8217, RFC 8591, RFC 8760, RFC 8898, RFC 8996

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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