Re: [Int-area] IPv6 fragmentation for IPv4

Tom Herbert <tom@herbertland.com> Tue, 23 May 2017 20:25 UTC

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From: Tom Herbert <tom@herbertland.com>
Date: Tue, 23 May 2017 13:24:59 -0700
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To: "Templin, Fred L" <Fred.L.Templin@boeing.com>
Cc: Joe Touch <touch@isi.edu>, "int-area@ietf.org" <int-area@ietf.org>
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Subject: Re: [Int-area] IPv6 fragmentation for IPv4
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On Tue, May 23, 2017 at 1:13 PM, Templin, Fred L
<Fred.L.Templin@boeing.com> wrote:
> Joe,
>
>> -----Original Message-----
>> From: Int-area [mailto:int-area-bounces@ietf.org] On Behalf Of Templin, Fred L
>> Sent: Tuesday, May 23, 2017 11:49 AM
>> To: Joe Touch <touch@isi.edu>du>; int-area@ietf.org
>> Subject: Re: [Int-area] IPv6 fragmentation for IPv4
>>
>> Hi Joe,
>>
>> > -----Original Message-----
>> > From: Joe Touch [mailto:touch@isi.edu]
>> > Sent: Tuesday, May 23, 2017 11:01 AM
>> > To: Templin, Fred L <Fred.L.Templin@boeing.com>om>; int-area@ietf.org
>> > Subject: Re: IPv6 fragmentation for IPv4
>> >
>> > Hi, Fred (et al.),
>> >
>> > On 5/23/2017 9:17 AM, Templin, Fred L wrote:
>> > > Joe, I wanted to run an idea by you. We all know that IPv4 fragmentation has
>> > > problems because of the 16-bit ID field. So, why not insert an IPv6 Fragment
>> > > Header between the IPv4 header and the upper layer protocol data, then
>> > > use IPv6-style fragmentation instead of IPv4 fragmentation?
>> >
>> > IPv4 fragmentation has several impediments:
>> >     - small ID field
>> >     - lack of a reassembly checksum
>> >     - lack of a fixed-location flow ID
>> >
>> > Using IPv6-Frag as the next header solves only the first of these. The
>> > last is significant - putting a new header would defeat IPv4 flow ECMP
>> > even for the first fragment.
>>
>> ECMP gateways could be updated to look at the ULP headers
>> following the IPv6 Frag header in the first fragment.
>>
>> > IPv6 includes a flow field that serves this
>> > purpose.
>>
>> How does it work for plain-old IPv4 fragmentation? I would think
>> that ECMP gateways would look at the IP ID and try to associate
>> the fragments so they all get equal ECMP treatment, i.e., the
>> same as for vanilla IPv4.
>
> Here's another think - since the IPv6 Frag Header already has a
> 32-bit IP ID that we are using for fragmentation, and since we
> are asking the IPv4 header to set DF=1, the 16-bit IP ID field in
> the IPv4 header is available for use as a flow field - right?
>
Fred,

I think this would over kill. Assuming fragmentation remains the rare
case, getting the ECMP hash over the addresses should be sufficient.
ECMP is a performance optimization, once you're fragmenting that's
already giving up a lot.

Tom

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