Re: Non-Last Small IPv6 Fragments

[Fernando Gont <[e-mail]>]

Hi, Erik,

On 14/1/19 02:19, Erik Kline wrote:
> 
>     > Ole Troan wrote on 13/01/2019 20:03:
>     >> Let’s fix path MTU discovery. There’s no fix for fragmentation.
>     >
>     > pmtu discovery is hard because it needs a way for an intermediate
>     node to be able to signal a transmitting node to dynamically drop
>     the MTU if network conditions change.  The only way to reliably work
>     around this is to transmit at 1280 and claim implementation /
>     operational breakage if this cannot get through.  This, however,
>     robs the host devices of the ability to use higher MTUs.
> 
>     “Fix” as in something different than rfc8201.
> 
> 
> How about a version of the fragment header that:
> 
>     (a) *always* has the L4 header (or a configurable number of bytes)
> in every non-first fragment
> 
> This could be documented on a per-L4+ basis how much needs to be
> included, i.e. we could have one doc for UDP and (say) a separate doc
> for information QUIC would need on a per-fragment basis. 

If you are requiring that the first frag always contains the L4 ehader,
that's already achieved by RFC7112.

If you really mean that the FH contains the L4 header, then:

* There's the issue of EHs in the fragmentable part

* It may be hard to enforce in the presence of tunnels, unless the
tunnel reassembles and refragments where necesary


Still, that doesn't solve the biggest issues with fragmention:

* a stateful operation for anotherwise stateless protocol

* i'd argue that for the IPv6 case, one of the base reasons for which
fragments are dropped is because they employ EHs



> This increases the overhead in a given fragment, but also helps to
> ensure that (eventually) intermediate systems can examine this field and
> preemptively make a drop/no-drop decision.

Then we're back on draft-gont-v6ops-ipv6-ehs-packet-drops



>     (b) state clearly that every non-last packet must contain
> $CONSENSUS_MINIMUM_BYTES of actual fragment payload

In which cases do you expect this to make a substantial difference?

Thanks,
-- 
Fernando Gont
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