Subject: RTO back off operation

From: Tatsuya Yanagibashi <[email-masked]>
Date: Tue, 23 Jun 2009 12:38:32 +0900

Dear SCTP experts,

I do apologize if we used to have same discussion in the past. I'm
currently supposed to make sure how "back off operation for RTO"
works. According to RFC, it says like following;

6.3.3 Handle T3-rtx Expiration

   Whenever the retransmission timer T3-rtx expires for a destination
   address, do the following:

   E2) For the destination address for which the timer expires, set RTO
       <- RTO * 2 ("back off the timer").  The maximum value discussed
       in rule C7 above (RTO.max) may be used to provide an upper bound
       to this doubling operation.

I'm just wondering if this RTO above is exactly calculated RTO based
on formula specified in RFC or can be rounded-up to RTO.MIN in case
calculated RTO is less than RTO.MIN. For example, we assume calculated
RTO is approx 50ms, and RTO.MIN is 200ms.

My understanding is, back off mechanism only deals with original
calculated RTO than rounded-up RTO(by RTO.MIN). Then SCTP always
evaluates RTO.MIN(and RTO.MAX) and picks proper value as selected RTO
value.

Calc RTO ; 50ms->100ms->200ms
RTO.MIN ; 200ms
Selected RTO value ; 200ms->200ms->200ms...

But if back-off mechanism is designed to deal with selected RTO value
than calculated RTO, then it should be like;

Calc RTO ; 50ms->400ms->800ms
RTO.MIN ; 200ms
Selected RTO value ; 200ms->400ms->800ms...

*assuming RTO.MAX is even bigger...

I'm interested in how much SCTP layer with given SCTP parameter sets
can be tolerant to link break. Appreciate your feedback.

Thanks and best regards,
Tatsuya

---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Mon, 22 Jun 2009 22:40:33 -0600

Tatsuya,

See rule C6 on RFC 4960 page 84.

--brian

-- 
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