Re: [Int-area] Discussion about Section 6.1 in draft-ietf-intarea-frag-fragile

Joe Touch <touch@strayalpha.com> Fri, 06 September 2019 14:14 UTC

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From: Joe Touch <touch@strayalpha.com>
In-Reply-To: <163CD364-2975-467A-8925-F114FFD9C422@employees.org>
Date: Fri, 6 Sep 2019 07:14:29 -0700
Cc: Bob Hinden <bob.hinden@gmail.com>, "int-area@ietf.org" <int-area@ietf.org>, Ron Bonica <rbonica=40juniper.net@dmarc.ietf.org>, IESG <iesg@ietf.org>, Joel Halpern <joel.halpern@ericsson.com>, "draft-ietf-intarea-frag-fragile@ietf.org" <draft-ietf-intarea-frag-fragile@ietf.org>, Suresh Krishnan <suresh@kaloom.com>, "intarea-chairs@ietf.org" <intarea-chairs@ietf.org>
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To: Ole Troan <otroan@employees.org>
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Subject: Re: [Int-area] Discussion about Section 6.1 in draft-ietf-intarea-frag-fragile
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Comments below. 

> On Sep 5, 2019, at 11:33 PM, Ole Troan <otroan@employees.org> wrote:
> 
> Bob, et al,
> 
> I have two issues with this text.
> 
> 1) It introduces something new and undescribed in paragraph 2.
>   "unless they also include mechanisms to detect that IP fragmentation isn't working
>  reliably."
>   That seems like hand-waving to me. Suggest deleting.

Fragmentation success or failure is directly testable. Any feedback mechanism will work and specific ones are mentioned elsewhere (PLPMTUD).

This differs from ICMP black-holing in path MTU detection.

> 
> 2) Paragraph 4:
>   "The risks of IP fragmentation can also be mitigated
>   through the use of encapsulation, e.g., by transmitting IP fragments
>   as payloads."
> 
>   This seems like proposing new unspecified solutions with it's own set
>   of considerations.

Specific widely-deployed methods are mentioned elsewhere in the doc, including GRE and UDP.

>   IP fragmentation is a general solution to all hosts,
>   encapsulation is certainly not in every host,

Actually, it is - unless you’re claiming hosts don’t support UDP.

> and has different
>   properties with regards to NAT traversal etc.

If by “different” you mean “much more likely to succeed”, agreed.

> vAlso if encapsulation
>   was the answer, other segmentation / reassembly that were tunnel
>   specific could be developed.

It is and is widely used - IPsec tunnels over UDP, e.g.

>  Regardless this also amounts of hand-waving
>   and doesn't seem to offer any advice that can be heeded now.
>   And of course encapsulation can also exacerbate the problem
>   by increasing packet size.

Yes, it makes the fragments smaller, which may be additional effort/performance impact, but it completely hides its impact on successful forwarding.

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