Subject: Re: [Tsvwg] Input for draft-paxson-tcp-rto-00.txt

From: Scott Bradner <[email-masked]>
Date: Thu, 25 Nov 1999 10:00:29 -0500 (EST)

Reiner,
	this document is trying to ensure that someone who wants to
implement TCP *as it is currently defined* can do so from RFCs - this
part of the TCP spec had never been documented in RFCs (though it had been
elsewhere and in lots of code)

	It is proposed standard because it defines what the current
IETF standard is for this function

	Proposals for changes to the way TCP works for the future 
can be considered on their own merits in the future.

Scott

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From: Reiner Ludwig <[email-masked]>
Date: Fri, 26 Nov 1999 06:13:31 +0100

Scott, thanks for your reply. That makes it clearer to me.

In that case I have two questions. (I assume 4.4BSD as the de facto 
standard for TCP implementations.)

- (2.3) Why is RTTVAR computed before SRTT? Although, I don't have a 
problem with that, the 4.4BSD code computes SRTT first. I assume there is 
good reason but I think it should be written down.

- (2.4) Although I read the TSVWG minutes, I still wonder why there should 
be a *fixed* RTO minimum of one second? One second is way conservative on a 
path with RTTs << 100 ms assuming the sender can benefit from a timer 
granularity of less than 500 ms (e.g., 10ms as in some OSs or even 1ms).
I would have expected a minimum of 2 * G (timer granularity) which is also 
what 4.4BSD does because of the heartbeat timer running out of phase. On 
the other hand, not every OS might implement a heartbeat timer. Beyond 
that, I like what FreeBSD does: MIN-RTO = R' + (2 * G), i.e., having the 
last RTT measurement be part of the lower limit for the RTO.

///Reiner