Re: [v6ops] new draft: draft-taylor-v6ops-fragdrop

Joe Touch <touch@isi.edu> Thu, 01 November 2012 18:35 UTC

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Date: Thu, 01 Nov 2012 11:34:34 -0700
From: Joe Touch <touch@isi.edu>
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Cc: Fernando Gont <fgont@si6networks.com>, v6ops@ietf.org
Subject: Re: [v6ops] new draft: draft-taylor-v6ops-fragdrop
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On 11/1/2012 2:35 AM, Brian E Carpenter wrote:
> On 31/10/2012 18:58, Joe Touch wrote:
>>
>>
>> On 10/31/2012 11:34 AM, Fernando Gont wrote:
>>> On 10/31/2012 04:11 PM, Joe Touch wrote:
>>>> Yes, but the whole point of the IPv6 option architecture was to avoid
>>>> the issues seen with IPv4 options.
>>>
>>> The only thing in that IPv6 would avoid is requiring routers to parse
>>> *all* options, just to find the ones that need to be processed by
>>> routers.
>>
>> Yes.
>
> No. The only extension header that *needs* to be parsed by intermediate
> routers is the hop-by-hop options header, and that is the first one (if
> present).

The first one could be:

	A. a known HBH option
		indicating there are HBH options
	B. a known E2E option
		indicating there are no HBH options
	C. an unknown option or a pad option
		indicating NOTHING

In the case of C, the router needs to keep looking at subsequent options 
until one of three things happens:

	1. a known HBH option is seen
		indicating there are HBH options
	2. a known E2E option is seen
		indicating there are no HBH options
	3. there are no more options
		indicating there are no HBH options

As a result, it's entirely possible that a router could need to parse 
the entire option chain before it can determine whether there are any 
HBH options.

> (You can legitimately argue that the hbh header and the routing header
> are effectively useless, but that doesn't break fundamental connectivity.)
>
> IPv6 routers should have nothing to do with fragmentation.

+1

> The problem is due to middleboxes that break the IPv6 spec by inspecting
> any part of the packet beyond the hop-by-hop header and discarding what
> they don't understand.

+1

Joe