Re: [v6ops] IPv6 fragmentation experience

[Jen Linkova <[e-mail]>]

On Fri, Mar 23, 2018 at 3:58 AM, Tom Herbert <[e-mail]> wrote:
> That's one of the problems with intermediate nodes that attempt to
> infer or participate in transport layer semantics.

I agree it's most unfortunate that almost every intermediate network
device is interested in transport layer semantics
of transit packets. However I'm not sure it can be changed.
Obviously the situations when such behavior is causing the packets
being dropped need to be fixed, and I totally agree
that it's not end points but intermediate devices fault.
On the other hand it's unrealistic to expect that  any modern network
of decent size would be able to operate w/o taking transport layer
into account
(well, there was an awesome idea of deprecating L4 ports and using
IPv6 addresses instead but...;)))

So from operational perspective we might need to think about how to
limit the damage. As an example, network operators might want to look
into
how ECMP and L2 balancing work in their networks and maybe talk to
vendors about adding special treatment for EHs - as well as
reconsidering filtering policies.

The expectations need to be set and documented. I mean, it's
reasonable to expect that an ISP might use L4 for balancing and/or
applying QoS policies (which in MPLS TE networks, for example, might
mean 'your RTT will vary'). Filtering fragments to/from your DNS
servers? Not such a brilliant idea..

>Similar behavior
> can happen if a device is using flow label for load balancing and the
> flow label changes mid flow, or if there is a routing change an
> packets of a flow hit a different load balancer that implements a
> different hash. In these cases it's not IP protocol that is broken,
> but technically it's incorrect and convenient assumptions being made
> within the network that fail under certain circumstances.

Exactly why it's a good idea to document most common scenarios and
discuss who we can ...well, maybe not to solve them completely but
make hurt less.

-- 
SY, Jen Linkova aka Furry