Re: [tsvwg] Last Call: <draft-ietf-tsvwg-datagram-plpmtud-15.txt> (Packetization Layer Path MTU Discovery for Datagram Transports) to Proposed Standard

Marc Petit-Huguenin <petithug@acm.org> Tue, 10 March 2020 22:00 UTC

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From: Marc Petit-Huguenin <petithug@acm.org>
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Subject: Re: [tsvwg] Last Call: <draft-ietf-tsvwg-datagram-plpmtud-15.txt> (Packetization Layer Path MTU Discovery for Datagram Transports) to Proposed Standard
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Please find below my Last Call review of draft-ietf-tsvwg-datagram-plpmtud-15.  Note that this review does not cover sections 6.2, 6.3 and 9.  Also I believe that an RFC should be implementable without reading the informative parts, so I skipped the abstract and section 1.

Let's start with the most general comments:

It seems that the goal of this standard track document is to prescribe one single method (from now on: "method") to find the effective PMTU, something that RFC 4821 did not do.  By doing so, this draft effectively restricts the number of ways that RFC 4821 can be implemented.  A non-exhaustive list of things that the method would prevent could be:

- Doing parallel probing, i.e. sending a few probes of different sizes at the same time.  Instead the method uses a lockstep mechanism so a new size can be tried only when an acknowledgement is received or the PROBE_TIMER expired MAX_PROBES times.
- Using the possibility in RFC 4821 section 6.1 to take in account the packets surrounding a Probe (including probes of different size sent at the same time) to differentiate between congestion and a probe lost because of its size.

As a software developer specialized in communication protocols, I do not particularly like the idea that my options to implement a protocol are constrained, especially when the constraints are that I can only do things sequentially.  I think that a better option would be to simply constrain RFC 4821 by defining some limits (like the number of retransmission, and the rate probes should be sent) and let developers do their job.  That said that draft certainly has value for a beginner or unsupervised developer, in which case that whole state machine would be useful in an Informative draft, as the simplest and safest way to do PLPMTUD.

Now going more in detail about the draft:

- I would suggest to say something about RFC 6864, which would rate-limits the probes sent between a pair of IPv4 addresses for a particular protocol (in that case UDP).

- MAX_PMTU is defined as the minimum of the local link MTU and the destination link MTU.  From the top of my mind I could not find a protocol that actually carries that value back to the local side, but I suppose that can be easily done.  It would be useful to say something about that, that the size of the packet used to retrieve that value (also the size of the packet used for connectivity check) should be lower than MIN_MTU, and also what happen when that value becomes available when the state machine is in another state than DISABLED.

- About MAX_PMTU, this name and others are defined after their first use.  Maybe adding all these to section 2 would make it easier to find definitions (and may even result in discovering some unnecessary aliasing).

- It could be useful to state that a probe should carry a unique identifier, and that it needs to be reflected in the acknowledgement, so to be able to process out-of-order and delayed packets.  In that case an additional variable in section 5.1.3 would contain the last probe identifier used.

- From a developer point of view, the information needed to implement PLPMTUD seems to be spread in different sections, making it difficult to get a complete picture of what is going on.  In fact I had to convert the text into a Petri Net -- a non-trivial and time-consuming task -- to be able to understand how bits from various sections fit together. 

So I would suggest to merge sections 4.6.2, 5.1.1, 5.1.2, 5.1.3, 5.2 and 5.3 into one single state machine, listing (a) the set of states, (b) the state context (aka variables, adding PLPMTU to it), (c) the list of transitions conditions (effectively merging timers and packet types received -- destination MTU size, connectivity acknowledgment, probe acknowledgement, and PTB) and finally (d) the exhaustive list of transitions between states, including for each the list of actions on the context and/or the packets sent.  I would either forgo completely the state machine diagram, or use Cosmogol (draft-bortzmeyer-language-state-machines) to include a formal state machine that can be converted into an SVG picture.

Having such exhaustive list of transitions between states would 1) put all the information needed in one single place and 2) add more clarity to the whole state machine.  E.g. it is not clear if a Probe should also be sent when entering the Base and Search state, or just when PROBE_TIMER expires (delaying the first probe by PROBE_TIMER).  There is other ambiguities like this that could be resolved by a systematic listing of the transitions actions.  And the formalization would permit to check the model for completeness and a few other properties, which cannot be a bad thing in itself.

Some minor comments:

- Section 3, Bullet point 8:  Why not "MUST NOT"?
- Section 4.4: "The MPS is smaller than the PLPMTU because of the presence of Pl headers and any IP options or extensions added to the PL packet."  Obviously also because of the presence of the IP header itself, as shown in the diagram.
- Figure 2: "UDPO" is never defined.
- Section 5.1.1: "When an acknowledged PL is used..."  I do not understand what an "acknowledged PL" is.
- Section 5.1.1: "An implementation..." Should be replaced by a more general statement saying that implementers can do whatever they want, as long as the external behavior of the implementation behaves exactly as the external behavior of how that state machine would behave.
- Section 5.1.4: "sends an acknowledged probe packet"  I do not know what that is.
- Section 5.2: "Not all changes are shown to simplify the diagram."  See above.
- Section 5.2: "uses an unacknowledged PL": I do not know what that is.

Some nits:

- Section 3, first bullet point: s/For datagram PLs,]/For datagram PLs,]/
- Section 4.3: s/MUST NOT rely soley/MUST NOT rely solely/
- Section 4.3: s/up-to-data/up-to-date/
- Section 4.6.1: s/speed at the which/speed at which/
- Section 4.6.2: s/(e. g.  PLPMTU/(e.g. PLPMTU/
- Section 4.6.2: s/to trigger enabling a resilience/to enable a resilience/
- Section 5.2: s/This state is left, once/This state is left once/
- Section 6.1.3: s/A probe packet that could/A probe packet could/
- Section 6.1.6: s/the application to check each/the application checks that/

On 2/25/20 6:14 AM, The IESG wrote:
> 
> The IESG has received a request from the Transport Area Working Group WG
> (tsvwg) to consider the following document: - 'Packetization Layer Path MTU
> Discovery for Datagram Transports'
>   <draft-ietf-tsvwg-datagram-plpmtud-15.txt> as Proposed Standard
> 
> The IESG plans to make a decision in the next few weeks, and solicits final
> comments on this action. Please send substantive comments to the
> last-call@ietf.org mailing lists by 2020-03-10. Exceptionally, comments may
> be sent to iesg@ietf.org instead. In either case, please retain the beginning
> of the Subject line to allow automated sorting.
> 
> Abstract
> 
> 
>    This document describes a robust method for Path MTU Discovery
>    (PMTUD) for datagram Packetization Layers (PLs).  It describes an
>    extension to RFC 1191 and RFC 8201, which specifies ICMP-based Path
>    MTU Discovery for IPv4 and IPv6.  The method allows a PL, or a
>    datagram application that uses a PL, to discover whether a network
>    path can support the current size of datagram.  This can be used to
>    detect and reduce the message size when a sender encounters a packet
>    black hole (where packets are discarded).  The method can probe a
>    network path with progressively larger packets to discover whether
>    the maximum packet size can be increased.  This allows a sender to
>    determine an appropriate packet size, providing functionality for
>    datagram transports that is equivalent to the Packetization Layer
>    PMTUD specification for TCP, specified in RFC 4821.
> 
>    The document updates RFC 4821 to specify the method for datagram PLs,
>    and updates RFC 8085 as the method to use in place of RFC 4821 with
>    UDP datagrams.  Section 7.3 of RFC4960 recommends an endpoint apply
>    the techniques in RFC 4821 on a per-destination-address basis.  RFC
>    4960, RFC 6951 and RFC 8261 are updated to recommend that SCTP, SCTP
>    encapsulated in UDP and SCTP encapsulated in DTLS use the method
>    specified in this document instead of the method in RFC 4821.
> 
>    The document also provides implementation notes for incorporating
>    Datagram PMTUD into IETF datagram transports or applications that use
>    datagram transports.
> 
>    When published, this specification updates RFC 4960, RFC 4821, RFC
>    8085 and RFC 8261.
> 
> 
> 
> 
> The file can be obtained via
> https://datatracker.ietf.org/doc/draft-ietf-tsvwg-datagram-plpmtud/
> 
> IESG discussion can be tracked via
> https://datatracker.ietf.org/doc/draft-ietf-tsvwg-datagram-plpmtud/ballot/
> 
> 
> No IPR declarations have been submitted directly on this I-D.
> 
> 



-- 
Marc Petit-Huguenin
Email: marc@petit-huguenin.org
Blog: https://marc.petit-huguenin.org
Profile: https://www.linkedin.com/in/petithug