RE: Adrian Farrel's No Objection on draft-ietf-6man-ext-transmit-04: (with COMMENT)

> -----Original Message-----
> From: ipv6-bounces@ietf.org [mailto:ipv6-bounces@ietf.org] On Behalf Of
> Brian E Carpenter
> Sent: Monday, October 07, 2013 5:18 PM
> To: C. M. Heard
> Cc: 6man-chairs@tools.ietf.org; Adrian Farrel; draft-ietf-6man-ext-
> transmit@tools.ietf.org; ipv6@ietf.org; The IESG
> Subject: Re: Adrian Farrel's No Objection on draft-ietf-6man-ext-
> transmit-04: (with COMMENT)
> 
> On 08/10/2013 10:28, C. M. Heard wrote:
> ...
> 
> > Maybe I'm making too much of this.  Certainly a reasonable action
> > for a middlebox that's told to pass packets with extension header
> > types 253 and 254 is to stop parsing when it encounters those next
> > header types and forward the packet in question.  Maybe that's
> > obvious to everyone -- but it seems to me that the spec, as written,
> > is actually asking for something different.
> 
> There's no header length field in an IPv6 header. That means that
> since header types 253 and 254 might be used either for experimental
> extension headers or for experimental payload types, there's no
> generic way for a middlebox to tell the difference. I think that's
> an issue that the current draft cannot resolve. I suspect we probably
> need an extra sentence to state the problem, but the authors will
> wait for direction on that.

When Wireshark encounters a header type 253 or 254, it assumes it is
an unknown extension header of length 8 bytes, then skips ahead and
attempts to parse anything that follows as additional headers.

Wireshark can be thought of as a middlebox of sorts (albeit one
that does not alter the traffic flow), and it wouldn't be hard to
imagine real middleboxes that do the same thing when they encounter
253/254.

Thanks - Fred
fred.l.templin@boeing.com

>     Brian
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