Joe,
Sorry I missed this posting at the time (my mail filters moved both
cross-postings into my int-area box which I check only rarely).
On 27/04/2019 18:13, Joe Touch wrote:
Cross-posting to let both communities know:
- it would be useful for these documents to address how fragmentation
and reassembly affects these signals
(esp. if reassembling fragments with different ECN values)
[BB] This is addressed by the re-framing section
<https://tools.ietf.org/html/draft-ietf-tsvwg-ecn-encap-guidelines-12#section-4.6>
in ecn-encap-guidelines, altho it doesn't give examples of what might
have caused frame boundary misalignment, so fragmentation is not
specifically mentioned. I think I will add an explicit mention of
fragmentation (if only so a search finds that section).
Actually I've realized that this highlights an inconsistency between the
advice on ECN and fragment reassembly in RFC3168 and in
ecn-encap-guidelines.:
* RFC3168 requires that the ECN marking of a reassembled packet is the
logical OR of the ECN marks on the fragments,
* whereas ecn-encap-guidelines recommends marking the same number of
outgoing as incoming octets when reassembling L2 frames or tunnelled
packets with different boundaries - using a simple counter to track
the balance.
In fact, it was the review of RFC3168 by me and Jon Crowcroft back in
2001 that originally raised the question of how to handle reassembly of
ECN-marked fragments
<https://tools.ietf.org/html/draft-ietf-tsvwg-ecn-ip-00#section-11>.
I'll quote a passage from the review, which I think justifies the
recommendation in ecn-encap-guidelines to count marked bytes, rather
than use the logical OR of RFC3168:
To use the logical OR of the marking of all fragments might be a pragmatic
solution, particularly for congestion control protocols like TCP where one
loss per round trip is treated identically to many. However, it is becoming
more common to see large numbers of packets per round trip time as data
rates increase while packet sizes and the speed of light haven't increased
for many years. Therefore it is to be expected that newer congestion
control protocols might take more accurate account of the number of packets
marked in a round trip. Hence, the inaccuracy of a logical OR during
re-assembly at the IP layer is best avoided.
I'm not too worried about the inaccuracy of using a logical OR, but I
think it best to recommend more accurate and no less costly counting.
The only justification for the logical OR was that TCP only reacted to
one ECN mark per RTT. But that is changing now, and the behaviour of one
transport should not be embedded in lower layers anyway.
- it would be useful for these documents to consider
draft-ietf-intarea-tunnels (which relates to the above) and its
discussion on many of the protocols cited
I can't find anything in draft-ietf-intarea-tunnels that ought to be
cited from ecn-encap-guidelines or rfc6040-update-shim. Did you have
something specific in mind?
I do want to raise a question about the following sentence, which
precedes the mention of ECN:
There are exceptions to this rule that are explicitly intended to
relay signals from inside the tunnel to the network outside the
tunnel, typically relevant only when the tunnel network N and the
outer network M use the same network.
Was that last word meant to say "network protocol"?
Then, if that is what you meant, I would disagree. Many different
network protocols include concepts similar to Diffserv and/or ECN (e.g.
IEEE802.1p, MPLS and TRILL support both, etc), and there's rarely a
reason /not/ to propagate the concept between different network
protocols when they encapsulate each other, even tho it's not always
straightforward to do so.
Bob
Bob
Joe
On Apr 26, 2019, at 1:50 PM, Black, David <[email protected]
<mailto:[email protected]>> wrote:
This may be of interest to INT folks who are interested in tunnels
and encapsulations.
Comments by the WGLC deadline are encouraged, but comments after the
deadline are ok, as they’d have to be dealt with anyway at IETF Last
Call.
Thanks, --David
*From:*tsvwg <[email protected]
<mailto:[email protected]>>*On Behalf Of*Black, David
*Sent:*Wednesday, April 17, 2019 2:51 PM
*To:*[email protected] <mailto:[email protected]>
*Subject:*[tsvwg] 2nd WGLC on ecn-encap-guidelines and
rfc6040-update-shim drafts, closes 6 May 2019
[EXTERNAL EMAIL]
This email announces a 2nd TSVWG Working Group Last Call (WGLC) on
two drafts:
[1] Guidelines for Adding Congestion Notification to Protocols that
Encapsulate IP
draft-ietf-tsvwg-ecn-encap-guidelines-12
https://datatracker.ietf.org/doc/draft-ietf-tsvwg-ecn-encap-guidelines/
This draft is intended to become a Best Current Practice RFC
[2] Propagating Explicit Congestion Notification Across IP Tunnel Headers
Separated by a Shim
draft-ietf-tsvwg-rfc6040update-shim-08
https://datatracker.ietf.org/doc/draft-ietf-tsvwg-rfc6040update-shim/
This draft is intended to become a Proposed Standard RFC.
This WGLC will run through the end of the day on Monday, May 6, 2019.
Comments should be sent to [email protected]
<mailto:[email protected]>list, although purely
editorial comments may be sent directly to the author. Please cc: the
WG chairs [email protected] <mailto:[email protected]> if
you would like the chairs to
track such editorial comments as part of the WGLC process.
No IPR disclosures have been submitted directly on either draft
Thanks,
David, Gorry and Wes
(TSVWG Co-Chairs)
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Bob Briscoe http://bobbriscoe.net/
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