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

Ole Troan <otroan@employees.org> Fri, 06 September 2019 14:50 UTC

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From: Ole Troan <otroan@employees.org>
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Date: Fri, 6 Sep 2019 16:50:21 +0200
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: Joe Touch <touch@strayalpha.com>
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Subject: Re: [Int-area] Discussion about Section 6.1 in draft-ietf-intarea-frag-fragile
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Joe,

> 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.

Can you please point me to where in the PLPMTUD document testing for IP fragmentation is described?
I thought PLPMTUD found the largest unfragmented size of a datagram.

>> 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.

Sorry, I couldn't find those either.
Inner fragmentation, firstly is only applicable to IPv4. And only applicable to tunnels.
And both those specs go to great length in avoiding fragmentation.

>>  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.

Sorry, I don't understand what you mean.
Are you saying that a new UDP applications should support the following stack:

IPv6 + UDP + IPv6 + FH + UDP + Applcation data

So to be able to hide IP fragments from the network?
While still having to do the full PLPMTUD to function correctly?

>> and has different
>>  properties with regards to NAT traversal etc.
> 
> If by “different” you mean “much more likely to succeed”, agreed.

I need to see numbers of that. But regardless I don't see the relevance to this document.

>> 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.

That's a encapsulated solution to start with.

>> 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.

You may be making a point. I'm afraid I don't get it.

Cheers,
Ole