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Re: [tcpm] timestamp options (was Re: New Version Notification for draft-touch-tcpm-tcp-edo-01.txt)

"Scharf, Michael (Michael)" <michael.scharf@alcatel-lucent.com> Tue, 27 May 2014 12:12 UTC

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From: "Scharf, Michael (Michael)" <michael.scharf@alcatel-lucent.com>
To: "Scheffenegger, Richard" <rs@netapp.com>, "Eggert, Lars" <lars@netapp.com>, Joe Touch <touch@isi.edu>
Thread-Topic: [tcpm] timestamp options (was Re: New Version Notification for draft-touch-tcpm-tcp-edo-01.txt)
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Date: Tue, 27 May 2014 12:12:17 +0000
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Cc: "tcpm@ietf.org" <tcpm@ietf.org>
Subject: Re: [tcpm] timestamp options (was Re: New Version Notification for draft-touch-tcpm-tcp-edo-01.txt)
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> Well, if it's just for Timestamps, just use an "invalid" length of 2 as
> a "Late TS permitted".
> 
> Maintaining the same option value and this would be rather straight
> forward.

Interesting proposal. But actually we are not really short of option numbers. If we get e.g. 8 bytes in a significant number of SYNs (which is our real bottleneck), I think we could also spend a new option codepoint.

> Personally, I'd like to combine this with a change in semantics, if
> SACK is negotiated on the same session as well, to allow more efficient
> loss recovery (lost retransmission detection during a loss recovery
> episode). I should have a unpublished draft sitting around here about
> that aspect.

A change in semantics would also be a reason for a new codepoint, imho.

Michael


> The TS option handling seems to be rather generous by middleboxes, in
> general.
>
> Richard Scheffenegger
> 
> > -----Original Message-----
> > From: tcpm [mailto:tcpm-bounces@ietf.org] On Behalf Of Scharf,
> Michael
> > (Michael)
> > Sent: Dienstag, 27. Mai 2014 13:49
> > To: Scheffenegger, Richard; Eggert, Lars; Joe Touch
> > Cc: tcpm@ietf.org
> > Subject: Re: [tcpm] timestamp options (was Re: New Version
> Notification
> > for draft-touch-tcpm-tcp-edo-01.txt)
> >
> > > For late TS, we would still need at least 2 Bytes during
> > > <SYN>/<SYN,ACK> to allow that capability.
> > >
> > > However, as the useful values of Window Scale only cover 0..14 [1],
> > > one approach to save SYN option space would be to negotiate up to 4
> > > binary
> > > (on/off) plus WS in a new single 3-byte option.
> >
> > I might be wrong, but I've thought so far that "replacing" the window
> > scale option would be pretty difficult. We already had reports of
> > middleboxes that parse this option because they try to understand
> rwnd (or
> > they do not parse the option even if they should, or they have
> entirely
> > broken logic to mess up TCP...). I think one example are firewalls
> that
> > try to detect whether segments are in-window. Another example could
> be
> > PEPs that try to tune rwnd (if they still exist in the wild).
> >
> > My understanding so far was that a solution only for timestamps would
> be
> > the most low-hanging fruit.
> >
> > Michael
> >
> > _______________________________________________
> > tcpm mailing list
> > tcpm@ietf.org
> > https://www.ietf.org/mailman/listinfo/tcpm

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