Re: [tram] I-D Action: draft-ietf-tram-stun-pmtud-20.txt

Marc Petit-Huguenin <marc@petit-huguenin.org> Sun, 08 August 2021 20:40 UTC

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From: Marc Petit-Huguenin <marc@petit-huguenin.org>
To: Magnus Westerlund <magnus.westerlund@ericsson.com>, "tram@ietf.org" <tram@ietf.org>, "draft-ietf-tram-stun-pmtud@ietf.org" <draft-ietf-tram-stun-pmtud@ietf.org>
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Subject: Re: [tram] I-D Action: draft-ietf-tram-stun-pmtud-20.txt
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Hi Magnus,

Sorry for the delay -- unexpected issue restricted my time in front of a computer until few days ago.

I think that your most important point is that you (and others before) want this draft to follow RFC 8899.  I am starting to feel bullied into accepting that RFC 8899 is the only way to do DPLPMTUD which, as explained multiple times, is it not.

These explanations have been conveniently ignored, so I decided to prove what I think are the weaknesses of RFC 8899 by doing a formal analysis of that protocol.  If I can prove that then I will publish my findings and continue using a better algorithm than RFC 8899.  If I can't then I'll do the modifications requested.

Now a formal analysis is not a small task so it will take some time to do that especially as my plate is already quite full until the end of the year.  But I should be able to spare half a day each week for that, so expect periodic updates here.

Thanks.

On 7/18/21 7:16 AM, Marc Petit-Huguenin wrote:
> Hi Magnus,
> 
> Thank you very much for that review.
> 
> I'll work on a response and an update to the draft in the next two weeks, probably to be uploaded after the IETF meeting.
> 
> On 7/14/21 8:08 AM, Magnus Westerlund wrote:
>> Hi,
>>
>> I have reviewed -20 and have the following feedback.
>>
>> Some of the issues have been resolved. However, my individual conclusion is
>> that this document would be shorter, have fewer issues if only the simple
>> probing was retained and defined as a RFC 8899 PL solution for UDP based
>> application protocols that wants DPLPMTUD and not use protocol internal
>> methods. This could shorten section 4 significantly. I think the alternative
>> is to simply declare the document dead.
>>
>> However, I would take a look at the examples of RFC 8899 PL definitions that
>> exists in RFC 8899, RFC9000 (QUIC) and for UDP Options
>> (https://datatracker.ietf.org/doc/draft-fairhurst-tsvwg-udp-options-dplpmtud
>> /) and rewrite Section 4 based on that and only keep the simple method. Then
>> rewrite intro to adjust it as RFC 8899 based, and also clarify the STUN
>> server on the port for the whole session aspect and the demultiplexing need.
>> Then section 7 also can be deleted. It would be good to have a clearer rate
>> limiting specification for the probe packets in section 4, as the STUN
>> retransmission timer gives exponential back-off, and the ICE is not really
>> applicable here. The probe sending implementation will have a RTT estimate
>> when some response has been received. Based on that one can limit the probes
>> to be sent only every n*RTT, possible with a MAX(n*RTT, Minimal_Interval).
>>
>> The main reason I write this is that I think RFC 8899 have resolved some
>> corner cases that could cause issues in a naïve implementation that think
>> that just getting the probe through means that one should immediately update
>> the MTU. And as RFC 8899 is improved this usage could also get that
>> improvement without a need for update.
>>
>> In addition as can be seen there are some unclarities that I think makes
>> implementation challenging from the current spec.
>>
>>
>> Section 1:
>>
>>     The Packetization Layer Path MTU Discovery (PMTUD) specification
>>     [RFC4821] describes a method to discover the Path MTU, but does not
>>     describe a practical protocol to do so with UDP.  Many application
>>     layer protocols based on the transport layer protocol UDP do not
>>     implement the Path MTU discovery mechanism described in [RFC4821].
>>
>> Wouldn't it be better do rewrite this in relation to RFC 8899? Which doesn't
>> have a PL definition for UDP, and the only "competing" proposal is based on
>> UDP Options which have no real deployment yet and requires OS level changes.
>>
>>
>> Section 1:
>>
>>     These application layer protocols can make use of the probing
>>     mechanisms described in this document instead of designing their own
>>     adhoc extension.  These probing mechanisms are implemented with
>>     Session Traversal Utilities for NAT (STUN), but their usage is not
>>     limited to STUN-based protocols.
>>
>> Yes, UDP based protocols that previously haven't been using STUN can use
>> this mechanism, but they need to be compatible and accept the multiplexing
>> solution that is implied. I think that could be clarified. They also needs
>> the STUN Server to be deployed in the peer endpoint which could be made
>> clearer. It is a given for any ICE supporting application, but I find no
>> reference to this. I would also note that ICE (RFC 8445) once it conclude
>> allows the peers to stop responding to STUN request, thus this method needs
>> to be clear that the STUN Server needs to maintained during the whole
>> session lifetime to enable DPLPMTUD.
>>
>>
>> Section 1:
>>
>>     Complementary techniques can be used to discover additional network
>>     characteristics, such as the network path (using the STUN Traceroute
>>     mechanism described in [I-D.martinsen-tram-stuntrace]) and bandwidth
>>     availability (using the mechanism described in
>>     [I-D.martinsen-tram-turnbandwidthprobe]).
>>
>> Is this text really relevant as written. Neither of these individual
>> proposal have been updated since 2015.
>>
>>
>> Section 2:
>>
>> When a
>>     probe succeeds with a larger size than the current PMTU, the PMTU is
>>     increased.
>>
>> There is a point to verification. If one looks at the search algorithm
>> behavior it updates its probe sizes, but it doesn't conclude it searching
>> nor update the upper layer's MTU until the search has concluded. There are
>> good reasons for doing it this way and thus I would suggest reformulation.
>>
>> Section 4.1:
>>
>>    The Simple Probing Mechanism uses only STUN Requests/Responses, which
>>     are subject to the congestion control mechanism in [RFC8489] section
>>     6.2.1.  The default Rc and Rm values may be defined differently for a
>>
>>     combination of the Simple Probing Mechanism and the protocol running
>>     on the same port.
>>
>> I would not call it a congestion control mechanism, rather a retransmission
>> timer mechanism. Thus a rate limiting mechanism makes more sense.
>>
>>
>> Section 4.1.1:
>>
>>     The client adds a PADDING attribute with a length that, when added to
>>     the IP and UDP headers and the other STUN components, is equal to the
>>     Selected Probe Size, as defined in [RFC4821] Section 7.3.
>>
>> Why referencing RFC 4821, when RFC 8899 has simpler and clearer interface
>> suitable for Datagram and where the simple probe is an excellent match to
>> just define as PL for probing.
>>
>>
>> Section 4.1.3:
>>
>>     A client receiving a Probe Response MUST process it as specified in
>>     section 6.3.3 of [RFC8489] and MUST ignore the PADDING attribute.  If
>>     a response is received this is interpreted as a Probe Success, as
>>     defined in [RFC4821] Section 7.6.1.
>>
>> More reliance on RFC 4821 rather than RFC 8899.
>>
>> RFC 8899 is not perfect but it handles a number of corner cases that can
>> occur and should produce more stable, and fewer updates to the MTU than what
>> RFC 4821 does.
>>
>> Section 4.2:
>>
>>        The Simple Probing Mechanism uses STUN indications, which are not
>>
>>          subject to the congestion control mechanism in [RFC8489] section
>>
>>          6.2.1.  As it will have to be intricately related to the protocol
>>
>>          that runs on the same port, each implementation of the Complete
>>
>>        Probing Mechanism in association MUST define the congestion
>> control
>>          that will be applied to the STUN Indications.  The default Rc and
>> Rm
>>          values for the STUN Requests/Responses may be defined differently
>> for
>>          a combination of the Simple Probing Mechanism and the protocol
>>
>>          running on the same port.
>>
>> Once more a full blown congestion control is not really needed here. The
>> point is that the PMTUD probe traffic will be a small fraction of the
>> application traffic, alternatively such a low rate application that it is
>> extremely unlikely that the probe will overload any network. I would note
>> that RFC 8899 do specify some normative statement about this in Section 3,
>> Bullet 7.
>>
>> In addition this does not really give an implementor a clear answer to what
>> they should implement. Some basic rate limiting would be more simple to
>> implement.
>>
>> My high level comment is that I don't see what the benefits of the complete
>> method compared to running simple probes as the PL probes in the algorithm
>> of RFC 8899. The only potentially benefit is that one sometime will get an
>> indication of a burst loss across a probe when a prior or following
>> application protocol packet as well as the probe is lost, indicating
>> potentially having loss for other reasons. I think RFC 4899 probing a
>> multiple times gives significant high probability that congestion or random
>> loss of probes will rarely affect the DPLPMTUD results.
>>
>> Section 4.2.3:
>>
>>     The server creates a Report Response and adds an IDENTIFIERS
>>     attribute that contains the chronologically ordered list of all
>>     identifiers received so far.  The server MUST add the FINGERPRINT
>>     attribute.  The server then sends the response to the client.
>>
>> This doesn't discuss what a server should do if the IDENTIFIERS attribute
>> does not fit in the packet.
>>
>>
>> Section 5.1:
>>
>> Based on that the peer need to keep a STUN server following this spec
>> running on the ports being used. Isn't the need for explicit signaling more
>> clear. Or is the inclusion of any STUN PMTUD attribute a sufficient
>> indication that the peer will not remove its STUN Server when ICE concludes?
>>
>>
>> Cheers
>>
>> Magnus Westerlund
>>
> 
> 


-- 
Marc Petit-Huguenin
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