Re: Last Call: <draft-ietf-6man-rfc1981bis-04.txt> (Path MTU Discovery for IP version 6) to Internet Standard

Fernando Gont <fgont@si6networks.com> Thu, 02 February 2017 11:08 UTC

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Subject: Re: Last Call: <draft-ietf-6man-rfc1981bis-04.txt> (Path MTU Discovery for IP version 6) to Internet Standard
To: "Eggert, Lars" <lars@netapp.com>
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From: Fernando Gont <fgont@si6networks.com>
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Date: Thu, 2 Feb 2017 07:58:17 -0300
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On 02/02/2017 07:21 AM, Eggert, Lars wrote:
> On 2017-2-2, at 10:54, Fernando Gont <fgont@si6networks.com> wrote:
>> On 02/02/2017 06:37 AM, Eggert, Lars wrote:
>>> Also, even if ICMP delivery is assured, there are additional 
>>> complications for UDP, which has been seeing a lot of interest
>>> both as a tunneling encapsulation and for applications (e.g.,
>>> QUIC). Many platforms do not provide UDP-sending applications any
>>> information about arriving ICMP messages that were triggered by
>>> their transmissions. So even if the path delivers ICMP, the OS
>>> makes ICMP-based PMTUD for UDP often impossible. Another reason
>>> to recommend 4821?
>> 
>> Agreed... although in this case this would be more of an app-layer 
>> implementation than one at the transport layer?
> 
> There are two dimensions here, one is in kernel vs. in userspace, the
> other which "layer" something is at. It used to be that "transport
> layer" (or "network layer" always implied "in kernel", but those days
> are past.

I was refering to "layer" as in a reference model. i.e., PMTU assumes
that you can repacketize your data. For TCP, that can be done at the
transport layer, since it's byte-stream oriented. OTOH, UDP is
essentially record-based... so any "re-packetization" must be done at
the upper (app) layer.

Thanks,
-- 
Fernando Gont
SI6 Networks
e-mail: fgont@si6networks.com
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492