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Re: [aqm] Who supports tsvwg adoption of adding ECN to L2 or tunnel protocols?

"Fred Baker (fred)" <fred@cisco.com> Fri, 08 November 2013 19:49 UTC

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From: "Fred Baker (fred)" <fred@cisco.com>
To: Andrew McGregor <andrewmcgr@gmail.com>
Thread-Topic: [aqm] Who supports tsvwg adoption of adding ECN to L2 or tunnel protocols?
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Date: Fri, 08 Nov 2013 19:49:05 +0000
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Cc: AQM IETF list <aqm@ietf.org>, "<draft-briscoe-tsvwg-ecn-encap-guidelines@tools.ietf.org>" <draft-briscoe-tsvwg-ecn-encap-guidelines@tools.ietf.org>, Bob Briscoe <bob.briscoe@bt.com>, tsvwg IETF list <tsvwg@ietf.org>
Subject: Re: [aqm] Who supports tsvwg adoption of adding ECN to L2 or tunnel protocols?
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At 23:04 04/11/2013, Andrew Mcgregor wrote:
> This seems like valuable work.  One question is, can we put in scope notification that losses are NOT due to congestion?

Speaking for myself, I'm not sure how I would do that.

A loss (or mark) due to congestion is pretty simple. The switch knows what it did. 

To know that a packet was lost for reasons other than congestion, I need to somehow know what packets I should expect, and infer that something that I expected didn't happen. In TCP, we know about data segments because they are enumerated - I know what the next octet sequence number to expect, and it doesn't arrive. Control segments (SYN, ACK, FIN, RST, and so on) are not enumerated in that sense - if my peer sends ten identical acks and nine arrive, I as the receiver have no way to know that. At the link layer, most link protocols in use today (PPP, Ethernet, and so on) do not enumerate packets in flight - they are simply there. I *might* be able to see a burst of noise on the line, but only if it looks like it might be the start of a packet and then doesn't end with the right checksum. Even if I can see it, I have no way to know whether the noise garbled one packet or many.

If you want to do some research and come up with a solution, be my guest. But in a standard discussed in 2013... let's not.

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