[tram] Adam Roach's Discuss on draft-ietf-tram-stun-pmtud-10: (with DISCUSS and COMMENT)

Adam Roach <adam@nostrum.com> Sun, 30 September 2018 21:19 UTC

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From: Adam Roach <adam@nostrum.com>
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Subject: [tram] Adam Roach's Discuss on draft-ietf-tram-stun-pmtud-10: (with DISCUSS and COMMENT)
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Adam Roach has entered the following ballot position for
draft-ietf-tram-stun-pmtud-10: Discuss

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Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
for more information about IESG DISCUSS and COMMENT positions.


The document, along with other ballot positions, can be found here:
https://datatracker.ietf.org/doc/draft-ietf-tram-stun-pmtud/



----------------------------------------------------------------------
DISCUSS:
----------------------------------------------------------------------

This seems like an interesting technique that warrants collection of operational
experience.

>From a process perspective, I think we have a bit of an issue, unless I've
overlooked something relevant. This is proposed as a Standards-Track document,
but it relies on the use of the PADDING attribute defined in RFC 5780. RFC
5780 is Experimental, so this is a formal downref. And RFC 5780 does not
appear in the downref registry [1], nor did the IETF last call [2] include a
request that the IETF community consider allowing such a refernce.

>From a practical perspective, the mechanism described in this document seems
like the kind of thing that it would be useful to gather operational experience
with prior to putting it on the standards track. I have some operational
concerns (described below) that I think could be either proven out or dispelled
by experimental deployment of the technology.

My recommendation is to recategorize this mechanism as experimental, adding some
text about the desire to gather operational experience.

For avoidance of doubt: My DISCUSS is only on the process issue, and I'll
happily clear regardless of how this issue is rationalized (e.g., either by
running IETF last call again, by reclassifying this mechanism as experimental,
or perhaps some novel solution that I may not have thought of). Everything
else is merely a recommendation.

____
[1] https://datatracker.ietf.org/doc/downref/
[2] https://www.ietf.org/mail-archive/web/tram/current/msg02609.html


----------------------------------------------------------------------
COMMENT:
----------------------------------------------------------------------

In the general case, STUN servers aren't aware of the signaling protocol that is
in use. For example, when a TURN server is use with RTP and RTCP with a session
set up via SIP, there is no requirement that the TURN server itself have any
inherent knowledge of SIP or RTP or RTCP. From that perspective, the following
text in section 4.2 is a bit confusing and/or problematic:

   Some application layer protocols may already have a way of
   identifying each individual UDP packet, in which case these
   identifiers SHOULD be used in the IDENTIFIERS attribute of the Report
   Response.

It seems odd that I would have to teach my TURN server about the protocols I'm
using it with just so that it can identify the packets.

This behavior, combined with the requirement that all behavior be symmetrical
("As a result of the fact that all endpoints implementing this specification
are both clients and servers") leads me to believe that perhaps the use cases
that drove this mechanism are tightly scoped to direct peer-to-peer uses of ICE,
while the other common uses of STUN (e.g., public TURN servers used for
symmetric NAT traversal) were given no consideration. If that was intentional,
then I think the abstract and introduction need to clearly describe the
scenarios the mechanism was defined for; and, more importantly, clarify that it
does not work for the general case, including STUN servers used for NAT
traversal.

I suspect that, once this mechanism begins to be deployed, the foregoing
limitations will cause operational difficulties, which may in turn suggest
changes to the mechanism that is currently defined, hence my suggestion above
to recharacterize the mechanism as experimental.

---------------------------------------------------------------------------

§4:

>  The Probing mechanism is used to discover the Path MTU in one
>  direction only, from the client to the server.

Nit: "...only: from..."

>  Two Probing mechanisms are described, a Simple Probing mechanism and

Nit: "...described: a Simple..."

>  a more complete mechanism that can converge quicker and find an

Nit: "...converge more quickly..."

>  appropriate PMTU in the presence of congestion.  Additionally, the

Nit: Please expand "PMTU" on first use.

---------------------------------------------------------------------------

§4.2.5:

>  algorithm used for the FINGERPRINT attribute (i.e., the CRC-32 of the
>  payload XOR'ed with the 32-bit value 0x5354554e [ITU.V42.2002]).

The location of the citation in here implies that the XOR'ing described is part
of V.42. Given that 0x53545554E is ASCII for "STUN," I'm pretty sure that's not
part of the underlying CRC. Would suggest reworking as:

   algorithm used for the FINGERPRINT attribute (i.e., the CRC-32
   calculated per the algorithm defined in [ITU.V42.2002], such has
   subsequently been XOR'ed with 32-bit value 0x5354554e).