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

"Scheffenegger, Richard" <rs@netapp.com> Tue, 27 May 2014 11:56 UTC

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From: "Scheffenegger, Richard" <rs@netapp.com>
To: "Scharf, Michael (Michael)" <michael.scharf@alcatel-lucent.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 11:56:02 +0000
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Subject: Re: [tcpm] timestamp options (was Re: New Version Notification for draft-touch-tcpm-tcp-edo-01.txt)
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Hi Michael,


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. 

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 I should have a unpublished draft sitting around here about that aspect.

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
> 
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