Re: [multipathtcp] MPTCP implementation feedback for RFC6824bis

V Anil Kumar <anil@csir4pi.in> Tue, 10 December 2019 20:04 UTC

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From: V Anil Kumar <anil@csir4pi.in>
To: Alan Ford <alan.ford@gmail.com>, Christoph Paasch <cpaasch@apple.com>
Cc: MultiPath TCP - IETF WG <multipathtcp@ietf.org>, mptcp Upstreaming <mptcp@lists.01.org>, Mirja Kuehlewind <ietf@kuehlewind.net>
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Date: Wed, 11 Dec 2019 01:34:12 +0530
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Subject: Re: [multipathtcp] MPTCP implementation feedback for RFC6824bis
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Hi Alan,

Please see inline.

On 12/06/19 09:28 PM, Alan Ford  <alan.ford@gmail.com> wrote: 
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> Hi all,
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> Following on from the discussion of implementation feedback with Christoph, I propose the following edits to RFC6824bis - which is currently in AUTH48 - as clarifications.
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> ADs, please can you confirm you consider these edits sufficiently editorial to fit into AUTH48.
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> WG participants, please speak up if you have any concerns.
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> Edit 1, clarifying reliability of MP_CAPABLE
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> Change the sentence reading:
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>  The SYN with MP_CAPABLE occupies the first octet of data sequence space, although this does not need to be acknowledged at the connection level until the first data is sent (see Section 3.3).
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> To:
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>  The SYN with MP_CAPABLE occupies the first octet of data sequence space, and this MUST be acknowledged at the connection level at or before the time the first data is sent or received (see Section 3.3).
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> Change the sentence reading:
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>  If B has data to send first, then the reliable delivery of the ACK + MP_CAPABLE can be inferred by the receipt of this data with an MPTCP Data Sequence Signal (DSS) option (Section 3.3). 
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> To:
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>  If B has data to send first, then the reliable delivery of the ACK + MP_CAPABLE is ensured by the receipt of this data with an MPTCP Data Sequence Signal (DSS) option (Section 3.3) containing a DATA_ACK for the MP_CAPABLE (which is the first octet of the data sequence space).
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> In my personal opinion either one of these edits would be sufficient for making the point, however clearly this has caused some confusion amongst the implementor community so making both these changes should make it absolutely clear as to the expected behaviour here.
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> Edit 2, mapping constraint
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> Change the sentence reading:
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>  A Data Sequence Mapping does not need to be included in every MPTCP packet, as long as the subflow sequence space in that packet is covered by a mapping known at the receiver.
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> To:
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>  A Data Sequence Mapping MUST appear on a TCP segment which is covered by the mapping. It does not need to be included in every MPTCP packet, as long as the subflow sequence space in that packet is covered by a mapping known at the receiver.
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As far as I understand, the proposed change introduces a “MUST” to insist that the map in a segment must cover at least some data in the segment. But the document does not talk anything about the rational behind it. I guess it is purely an 
ease of implementation?

I think the design/format of the Data Sequence Mapping permits the map to stand independent of the data being carried in a segment. So, as long as an implementation is willing to deal with the complexity of storing and processing late and early mappings (with respect to the data arrival), it could be permitted provided that the received map is for an in-window data.

Anil 

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> Best regards,
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> Alan
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