Subject: Re:

From: <[email-masked]>
Date: Sun, 16 Mar 2003 22:55:52 -0800 (PST)

Hi there,
I am a student reading RFC2960. I can not understand the narrative below
in 6.1 RTO Calculation.

When a new RTT measurement R' is made, set

       RTTVAR <- (1 - RTO.Beta) * RTTVAR + RTO.Beta * |SRTT - R'| SRTT
       <- (1 - RTO.Alpha) * SRTT + RTO.Alpha * R'

My question is what RTO.Beta and RTO.Alpha are? They never appear in any
place in the file.

I will be grateful if I can answer from u.

Zhou Yu

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From: Pawel Hadam <[email-masked]>
Date: Mon, 24 Mar 2003 22:00:41 +0100

Hi

yeuc> in 6.1 RTO Calculation.
yeuc> When a new RTT measurement R' is made, set
yeuc>        RTTVAR <- (1 - RTO.Beta) * RTTVAR + RTO.Beta * |SRTT - R'| SRTT
yeuc>        <- (1 - RTO.Alpha) * SRTT + RTO.Alpha * R'

yeuc> My question is what RTO.Beta and RTO.Alpha are? They never appear in any
yeuc> place in the file.
yeuc> I will be grateful if I can answer from u.

14. Suggested SCTP Protocol Parameter Values
[...]
   RTO.Alpha                - 1/8
   RTO.Beta                 - 1/4

Regards
Pawel

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From: Sourabh Ladha <[email-masked]>
Date: Sat, 29 Mar 2003 19:43:44 -0500 (EST)

RFC 2960 says that the fragmentation of user messages is done based on the
current PMTU..Is this the case when say at an instance in an SCTP association:

(application write) > PMTU > (rcvr's a_rwnd)

Or should fragmentation at an instance be done based on min(PMTU, a_rwnd)?

Thanks,
Sourabh

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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Sun, 30 Mar 2003 03:22:56 -0700

Sourabh,

RFC 2960 is a little lacking in this area: see
draft-ietf-tsvwg-sctpimpguide-08.txt Section 2.15.

Per RFC1122 Section 4.2.3.3 a_rwnd should be held at zero (0) until it
exceeds Eff.snd.MSS or half the receive buffer, whichever is less.  A
usable Eff.snd.MSS for SCTP is 1 PMTU, so unless the receive buffer is
extremely small (RCV.BUFF < 2 * PMTU) there should seldom be a case
where PMTU > (rcvr's a_rwnd).  Nevertheless, for receivers with small
receive buffers (or paths with large MTUs) it is possible that
(RCV.BUFF < 2 * PMTU).

So, I suppose fragmentation should not be done on min(PMTU, a_rwnd)
either, but on min(PMTU, RCV.BUFF/2) considering that Silly Window
Syndrome avoidance should also be performed on the a_rwnd.  That is, if
the initial rwnd (in INIT/INI-ACK) is greater than 2 * PMTU (which
is often the case), the sender might consider ignoring a_rwnd less than
PMTU.  Several implementations already do this.

--brian

-- 
Brian F. G. Bidulock    ¦ The reasonable man adapts himself to the ¦
[email-masked]    ¦ world; the unreasonable one persists in  ¦
http://www.openss7.org/ ¦ trying  to adapt the  world  to himself. ¦
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