Re: Treatment of ICMP for PMTU

[Magnus Westerlund <[e-mail]>]

Hi,


Den 2018-06-29 kl. 02:30, skrev Lubashev, Igor:
>>>   • What to do, when an IPv4 ICMP (w/o on-path proof) request a PMTU reduction (after handshake)?
>> Isn't this sufficiently covered in 4821?
> Not really.  Or, at least I could not find anything more useful there then: "If an ICMP PTB message is received matching the probe packet, then search_high and eff_pmtu MAY be set from the MTU value indicated in the message."  So 4821 seems silent on what to do if ICMP is received for a non-probe packet, and it seems silent on how one could decide whether to set search_high and eff_pmtu from the MTU in ICMP or not (saying "MAY" does not really help here).  These questions are what I am trying to address for QUIC.
>
> 4821 uses MAY in the sentence above, since it does not really give any better indication.  This PR draws a distinction between ICMP w/ and w/o on-path proof and in some specific cases, it recommends a SHOULD.
>
>
If you look at the updated PLPMTUD:

https://datatracker.ietf.org/doc/draft-ietf-tsvwg-datagram-plpmtud/

3.3. Reducing the PLPMTU: Confirming Path Characteristics

    If the DPLPMTUD method detects that a packet with the PLPMTU size is
    no supported by the network path, then the DLPMTUD method needs to
    validate the PLPMTU. This can happen when a validated PTB message is
    received, or another event that indicates the network path no longer
    sustains this packet size, such as a loss report from the PL

    All implementations of DPLPMTUD are REQUIRED to provide support that
    reduces the PLPMTU when the actual PMTU supported by a network path
    is less than the PLPMTU.


Section 4.9 says:

    PROBE_DONE: The PROBE_DONE state indicates a successful end to a
       probing phase.  DPLPMTUD remains in this state until either the
       PMTU_RAISE_TIMER expires or a received PTB message is verified.


However, I have to say that the draft is a bit unclear on what to do at 
this point.

I guess the options are two.

Probe starting at PTB_SIZE or verify Base_MTU and then probe up. From my 
perspective, the important is that one uses this a signal for checking 
the PMTU.

I will start a thread on TSVWG WG mailing list regarding the lack of 
specific text in their draft for this case.

Cheers

Magnus Westerlund

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