Re: [tcpm] increasing max window size of TCP

Hi Yoshi,

multiple times
s/archive/achieve/

One comment: A receiver that sees WS > 14 is expected to clamp WS at 14; (see the paragraph in RFC7323 right after the one that you quoted).

So, if you signal to a regular RFC7323 receiver a WS of 15, if will behave as if WS 14 was sent, limiting its maximum sending windowsize, right? No harm done except that session getting less bandwidth.

A receiver also implementing your proposal could, however, make use of the nearly twice as large windowsize. Monitoring of the evolution of the receive window signaled vs. data transmitted could allow a sender to determine passively, if the receiver actually supports the larger windowscale option. But I’m not sure if this is really necessary… (the signaled window would shrink half as fast for unprocessed data in the receive buffer, compared to WS14; however, the step change in the window would only happen when there are 16 kB (or 32kB-1) bytes of data unprocessed.

But the sender must stop sending when the receive window is signaled to be zero (except for retransmissions) either way.

Also, I guess you want to clamp every other received value than 15 down to 14 still… Not that you start scaling by (2^255 – 1).

Since you are effectively doing a special case handling with WS==15, you may want to describe the necessary operations to an implementer alike section 2.3 of RFC7323, that is, instead of only a simple left shift by 15 bits(multiply by 2^15), you need to multiply by (2^15-1) if I understand your proposal correctly. (Or course, a multiplication by 2^15-1 is a left shift by 15, followed by a subtraction of the original value; perhaps gcc even optimizes this multiplication nowadays ☺ )

BTW, I think the formula you give in the last sentence of sec  1 is off; the signaled window spans 2^16, but the maximum value of the window signaled is 2^16-1; times (2^15-1) gives a maximum window of 2147385345

I don’t think an additional option is necessary for this change, and provided I understood your intention and calculations correctly (as described above), I would support this going forward as experimental.

Best regards,
  Richard

From: tcpm [mailto:tcpm-bounces@ietf.org] On Behalf Of Yoshifumi Nishida
Sent: Mittwoch, 11. März 2015 07:35
To: tcpm@ietf.org
Cc: panda@wide.ad.jp
Subject: [tcpm] increasing max window size of TCP

Hi,

We have submitted a short draft which proposes to increase max window size of TCP.
If you're interested, please take a look at the following info.
Some may say this is a minor thing, but we still believe we need to consider this for future extensions of TCP.

We will appreciate your feedback!
--
Yoshi & Hiro

-------------------------------------------------------------------------
From: <internet-drafts@ietf.org<mailto:internet-drafts@ietf.org>>
Date: Mon, Mar 9, 2015 at 1:09 PM
Subject: I-D Action: draft-nishida-tcpm-maxwin-00.txt
To: i-d-announce@ietf.org<mailto:i-d-announce@ietf.org>

A New Internet-Draft is available from the on-line Internet-Drafts directories.

        Title           : Increasing Maximum Window Size of TCP
        Authors         : Yoshifumi Nishida
                          Hirochika Asai
        Filename        : draft-nishida-tcpm-maxwin-00.txt
        Pages           : 6
        Date            : 2015-03-09

Abstract:
   This document proposes to increase the current max window size
   allowed in TCP.  It describes the current logic that limits the max
   window size and provides a rationale to relax the limitation as well
   as the negotiation mechanism to enable this feature safely.

The IETF datatracker status page for this draft is:
https://datatracker.ietf.org/doc/draft-nishida-tcpm-maxwin/

There's also a htmlized version available at:
http://tools.ietf.org/html/draft-nishida-tcpm-maxwin-00

Please note that it may take a couple of minutes from the time of submission
until the htmlized version and diff are available at tools.ietf.org<http://tools.ietf.org>.

Internet-Drafts are also available by anonymous FTP at:
ftp://ftp.ietf.org/internet-drafts/

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