Re: [tcpm] Éric Vyncke's Discuss on draft-ietf-tcpm-converters-17: (with DISCUSS and COMMENT)

[[e-mail]]

Hi Eric, 

Thank you for the review. 

Please see inline. 

Cheers,
Med

> -----Message d'origine-----
> De : Éric Vyncke via Datatracker [mailto:[e-mail]]
> Envoyé : lundi 2 mars 2020 15:59
> À : The IESG
> Cc : [e-mail]; [e-mail];
> [e-mail]; Michael Scharf; [e-mail]
> Objet : Éric Vyncke's Discuss on draft-ietf-tcpm-converters-17: (with
> DISCUSS and COMMENT)
> 
> Éric Vyncke has entered the following ballot position for
> draft-ietf-tcpm-converters-17: Discuss
> 
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> Please refer to https://www.ietf.org/iesg/statement/discuss-
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> for more information about IESG DISCUSS and COMMENT positions.
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> 
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-tcpm-converters/
> 
> 
> 
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
> 
> Thank you for the work put into this document. It is indeed useful to
> be able
> to deploy easily new TCP features.
> 
> Nevertheless, please find below two DISCUSSes and some non-blocking
> COMMENTs
> (and I would appreciate a response from the authors) and NITS.
> 
> I hope that this helps to improve the document,
> 
> Regards,
> 
> -éric
> 
> == DISCUSS ==
> 
> -- Section 1.2 --
> A trivial one: while the title and the abstract of this document
> appear as
> quite generic, the document focus is reduced later in section 1.2 to
> MPTCP:
>   "this
>    document specifies how the Convert Protocol applies for Multipath
>    TCP.  It is out of scope of this document to provide a
> comprehensive
>    list of all potential conversion services. "
> While I do not mind having a focus on MPTCP only, I would strongly
> suggest to
> reflect this focus in the title and in the abstract (the current
> filename is
> correct).
> 

[Med] As explained by Mirja, MPTCP is only meant to exemplify the approach. I added this note at the end of the abstract:

"As a sample applicability use case, this document specifies how the Convert Protocol applies for Multipath TCP." 

> -- Section 6.2.8 --
> I appreciate that this is an experimental document, but, having only 2
> occurrences of ICMP in the whole document and no real "how to process"
> TLV
> "Destination Unreachable"... and the payload of this TLV having only
> the code
> without the offending packet will probably make Path MTU discovery
> (and other
> mechanisms) impossible.
> 

[Med] Some comments here:
* The connection is split into two legs. As such, path MTU considerations are managed by the endpoints of each of these legs: (client and converter) and (converter and server). 
* We don't require specific treatment to ICMP messages, except for when there is a reachability issue to the server we decide to echo ICMP reachability errors to the client. 
* We don't need to echo the payload (*) of the ICMP error in the Convert Error Code because this is bound to the TCP connection over which the error was received. This is done in-band if you will.  

=(*)=
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Internet Header + 64 bits of Original Data Datagram      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Or 

      |                    As much of invoking packet                 |
      +                as possible without the ICMPv6 packet          +
      |                exceeding the minimum IPv6 MTU [IPv6]          |
==

* The connection is aborted upon receipt of this error. 

Please let us know if you had in mind other processing that we need to call out. Thank you. 


> While I am not a transport expert, I believe that this aspect needs to
> be
> described in this document.
> 
> 
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
> 
> Here are some COMMENTs
> -- Section 1.2 --
> Is the benefit of a transport proxy only for the clients as written in
> the 1st
> paragraph? It was probably the case for the original MPTCP proxy but
> what about
> other TCP extensions?
> 

[Med] Good point. Added "in particular" to the sentence you are referring to. 

Even with MPTCP we do have scenarios that are 'network-specific" (e.g., data-centers). 


> -- Section 4.1 --
> While the difference "(Internet-facing interface, customer-facing
> interface)"
> will probably represent the vast majority of use cases, I wonder
> whether there
> will always be a single Internet-facing ? Could probably be 0 or 2 in
> some use
> cases... Suggest to remove this part of the text.
> 

[Med] Indeed, more than one interface may be available. See for example:

   A first safeguard against the misuse of Transport Converter resources
   by illegitimate users (e.g., users with access networks that are not
   managed by the same provider that operates the Transport Converter)
   is the Transport Converter to reject Convert connections received on
   its Internet-facing interfaces.
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 

What about using Internal realm and External realm? 

> -- Section 6.1 --
> Please state the usual wording about "unassigned" field: it must be
> ignored by
> the receiver.
> 

[Med] Added. Thanks.

> Adding some explanations on why version 0 is reserved but cannot be
> used would
> be welcome.

[Med] In early versions of the spec, we don't use a dedicated port number for the protocol but relies only the IP address of the converter. Having a bit set in the version number + the length field allows to avoid interpreting a data as Convert TLVs. We even considered the use of a Magic Number to unambiguously identify Convert messages. 

Since we updated design to have a dedicated service port, we don't have those issues. Version 0 would work as well but existing implementations already use version 1.


> 
> -- Section 6.2.5 --
> Can the "remote peer IP address" be a link-local address ?

[Med] It is unlikely but I prefer to not declare it as illegal. As a general rule, the converter will return a "Network Failure" error when it can't forward a packet.  

(You may check RFC8335 which allows to probe a remote endpoint...using a link-local address!)

> 
> == NITS ==
> 
> -- Section 1.1 (and possibly others) --
> Please use a consistent means of introducing acronyms, cfr URLLC and
> ATSSS ;-)

[Med] Will double check. Than you.

> 
> -- Section 4.2 ---
> The use of "regular TCP packets" makes me wonder whether the authors
> think that
> MPTCP uses "irregular TCP packets"... The use of 'regular' seems a
> little weird

[Med] I hear you. Will update the text. 

> here but I am not a native English speaker.
> 
> -- Section 4.3 --
> The caption below figure 7 ends with "(TCP)" that is not the acronym
> of
> "Transport Session Entry". Is it expected ?

[Med] Deleted, thanks.

We used to have another example with MPTCP specifics hence that mention.