Subject: [Softwires] #21: Fragmentation must not be handled according to RFC 2473

From: "Lee, Yiu" <[email-masked]>
Date: Sat, 16 Feb 2013 06:35:00 +0000

I was tasked in IETF-85 to find out why mss rewrite was removed before
publishing RFC6333. Here is my finding:

Back in 2009-4-7, people in ML reported rewriting MSS would break TCP-AO if
TCP option flag set to 0. In IETF-75, Dave Thaler suggested to delete mss
rewrite from the draft. The argument was MTU is a generic issue for all
tunneling techniques, a separate draft should be written to address it
rather than including mss rewrite in a particular draft. Alain asked the WG
the question "Whether to remove fragmentation/mss discussion from the
document or to keep it". Dave Ward suggested to remove it.

Discussion could be found in IETF-75 minute:
http://ietf.10.n7.nabble.com/Softwires-WG-meeting-Notes-td112138.html

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From: Maoke <[email-masked]>
Date: Mon, 18 Feb 2013 10:07:48 +0900

dear Yiu,

i support your view on not including mss rewriting as a standard/mandatory
way to solve the tunneling MTU issue. however, i have 2 observations:

1. mss rewriting could be an optional workaround for some operators in some
circumstances. therefore it is worth implementing, IMHO.
2. regarding the issue of #21, fragmenting in IPv4 stack before entering
the MAP module is not equal to the issue of "mss rewriting". MTU discovery
can still work and help this functionality. on the other hand, as the
fragment happens before the encapsulation, RFC2473 is not involved yet. it
is equivalent to having a fragmenting box before the CE. as RFC2473 was
written when the stateless mapping was not happening, it is reasonable that
we make a new building block for the new demand of supporting anycast BR
even when the datagram is to be fragmented. (is this as the same as Remi's
proposal?)

thanks and regards,
maoke

2013/2/16 Lee, Yiu <[email-masked]>

> Back in 2009-4-7, people in ML reported rewriting MSS would break TCP-AO
> if TCP option flag set to 0. In IETF-75, Dave Thaler suggested to delete
> mss rewrite from the draft. The argument was MTU is a generic issue for all
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