Re: [v6ops] I-D Action: draft-jaeggli-v6ops-pmtud-ecmp-problem-00.txt

[Brian E Carpenter <[e-mail]>]

Hi,

>    A problem common to the approach of distribution through hashing is
>    its impact on path MTU discovery.  An ICMPv6 type 2 PTB message
>    generated on the path between a client and an ECMP load balanced
>    server will have the anycast address as the destination and will be
>    statelessly load balanced to one of the anycast servers. 

This may seem picky, but I think the reader's brain will run
more smoothly if it's explicit that it's a PTB triggered by
a packet *from* the server and therefore directed *to* the server.
"on the path" doesn't quite contain that information.

>             Because of this, the results of
>    the ICMPv6 ECMP hash do not match that of the corresponding TCP or
>    UDP ECMP hash.

Again picky, but if there are (say) 2 paths, there's a 50% chance
that it gets the right path, etc. So "might not match" is probably
better.

General comment: I'm not sure what is specific to ECMP
about this problem. We identified it as a general (but out of
scope) problem in RFC 7098. We did include this comment there:

 o  Note that correct handling of ICMPv6 for Path MTU Discovery
    requires the layer 3/4 balancer to keep state for the client
    source address, independently of either the port numbers or the
    flow label.

because, indeed, the PMTU is a function of the address pair only.

    Brian