Re: [multipathtcp] RFC6824bis edits based on implementation feedback

Yoshifumi Nishida <nsd.ietf@gmail.com> Thu, 13 February 2020 09:34 UTC

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References: <C36D742F-6D76-48FA-B6D8-44DE484A9E2C@gmail.com> <882106347.533187.1578939921488@csir4pi.in> <CAAK044RLsJCFWfeP4XAzMmMhUqH1hCnDs94-3Zkrj3QkJeVx7g@mail.gmail.com> <146023054.25180934.1580661581660@csir4pi.in> <CAAK044QBO4Bjby=fkEiuQ-f-zuZG1Q=8H0p1kJoUaxyjzod3Cg@mail.gmail.com> <620489655.29573797.1580915365031@csir4pi.in> <DA9790A3-134D-4D44-9532-65EE359B0B69@gmail.com> <1411057985.31281938.1581012439446@csir4pi.in> <CAAK044QoFh_eXE+MwggOn5X0uUs3xuzTcw2kdJaKuUnEP745dA@mail.gmail.com> <F3D07C9D-12B2-45A7-B9D0-23E862DDF4BA@apple.com>
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From: Yoshifumi Nishida <nsd.ietf@gmail.com>
Date: Thu, 13 Feb 2020 01:34:29 -0800
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To: Christoph Paasch <cpaasch@apple.com>
Cc: V Anil Kumar <anil@csir4pi.in>, MultiPath TCP - IETF WG <multipathtcp@ietf.org>, mptcp Upstreaming <mptcp@lists.01.org>
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Subject: Re: [multipathtcp] RFC6824bis edits based on implementation feedback
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Hi Christoph,

Thanks for the clarification. It seems that there are several ways to deal
with this situation.
I think these are not ideal, but at the same time, it won't be fatal. I
think more thorough solutions can be discussed in future versions.
Given that there're several ways to address it, I guess the draft doesn't
have to specify the behavior for this.
--
Yoshi

On Tue, Feb 11, 2020 at 2:30 PM Christoph Paasch <cpaasch@apple.com> wrote:

> +MPTCP-upstreaming
>
> On Feb 11, 2020, at 1:05 AM, Yoshifumi Nishida <nsd.ietf@gmail.com> wrote:
>
> Hi folks,
> I guess we might want to know the behavior of existing implementations
> such as linux.
>
> When an mptcp stack tries to send a data packet and find that there's no
> enough option space for data mapping, what it will do?
>
>
> iOS, multipath-tcp.org and Linux 5.6 implementation all favor the
> DSS-option over any other option (except TCP Timestamps). And TCP
> Timestamps and DSS comfortably fit in the TCP option-space.
>
> If SACK-blocks should be added, we only add SACK-blocks up to the
> remaining space. SACK is anyways opportunistic as once there are more than
> 3 blocks they don't fit either.
>
>
> Christoph
>
> Split the packet to create more option space?
> --
> Yoshi
>
> On Thu, Feb 6, 2020 at 10:10 AM V Anil Kumar <anil@csir4pi.in> wrote:
>
>> Hi Alan,
>>
>> Thank you for your reply. I have two points to clarify. Please see them
>> in line.
>>
>> ------------------------------
>>
>> *From: *"Alan Ford" <alan.ford@gmail.com>
>> *To: *"V Anil Kumar" <anil@csir4pi.in>jk
>> *Cc: *"Yoshifumi Nishida" <nsd.ietf@gmail.com>om>, "multipathtcp" <
>> multipathtcp@ietf.org>
>> *Sent: *Thursday, February 6, 2020 2:49:29 AM
>> *Subject: *Re: [multipathtcp] RFC6824bis edits based on implementation
>> feedback
>>
>> Hi Anil,
>>
>> This would not be forbidden if the mapping was carried on a pure ACK with
>> no data.
>>
>> As far as I understand, delivery of DSM through pure ACK is not in the
>> scope of the current draft. If we intend to do this, we may need an
>> approach similar to the one proposed for delivery of ADD_ADDR option in
>> pure ACK with reliability feature.  So, packing DSM on pure ACK does not
>> seem to be an option at this stage.
>>
>>
>> I do see the point here: if a packet of 1000 bytes contains 500 bytes of
>> one mapping and 500 bytes of another mapping, then only one DSM would
>> appear on one packet, leaving the mapping for the second 500 bytes to be
>> carried somewhere else - the only option being a pure ACK. But this kind of
>> scenario would be extremely rare
>>
>> Yes, I do agree that the scenario you mentioned above would be extremely
>> rare. In fact, I wonder whether such a situation (i.e., need to cover the
>> bytes in a single packet with two different maps) would ever arise.
>> Probably there might be some corner cases, which I don't get it rightly
>> now.
>>
>> More importantly, the scenario that Yoshi and me are referring to is
>> totally different: the one I had given as part of my comments in response
>> to the proposed change 2. In this scenario, which you could see in the
>> trailing mail, data segment-1 is transmitted without including its map,
>> something which is permitted in the  mptcp framework (6824 bis). I make
>> this inference from the below text in 6824 bis:
>>
>> "...even if a mapping does not exist from the subflow space to the data-
>> level space, the data SHOULD still be ACKed at the subflow (if it is
>> in-window). This data cannot, however, be acknowledged at the data level
>> (Section 3.3.2) because its data sequence numbers are unknown.
>> Implementations MAY hold onto such unmapped data for a short while in the
>> expectation that a mapping will arrive shortly."
>>
>> One option the data sender has at this stage is to include the map for
>> data in segment-1, in segment-3 (please see the two-subflow example I gave
>> in response to the change 2 you had proposed). But the proposed change does
>> not permit this. What would happen to  subflow-1 in this case ?
>>
>> With regards,
>>
>> Anil
>>
>> and I would imagine any implementation would just split into two 500 byte
>> segments each with the PSH flag set. I don’t think we need to spell this
>> out in the spec however.
>>
>> Regards,
>> Alan
>>
>> On 5 Feb 2020, at 15:09, V Anil Kumar <anil@csir4pi.in> wrote:
>> Hi Yoshi,
>>
>> Please see in line.
>>
>> ------------------------------
>> *From: *"Yoshifumi Nishida" <nsd.ietf@gmail.com>
>> *To: *"V Anil Kumar" <anil@csir4pi.in>
>> *Cc: *"alan ford" <alan.ford@gmail.com>om>, "multipathtcp" <
>> multipathtcp@ietf.org>
>> *Sent: *Tuesday, February 4, 2020 1:08:45 PM
>> *Subject: *Re: [multipathtcp] RFC6824bis edits based on implementation
>> feedback
>>
>> Hi Anil,
>>
>> Thanks for pointing it out. I overlooked this one.
>> This looks an interesting point.
>>
>> It seems to me that whether RST is happen or not depends on the size of
>> receive window according to the text.
>> If the receive window size is big enough to accommodate segment 1 and
>> segment 3, the text "Implementations MAY hold onto such unmapped data
>> for a short while in the expectation that a mapping will arrive shortly.
>> " can be applied to the segment 1. As a result, the segment 1 won't be
>> discarded.
>>
>> *Yes.  So, segment 1 may be kept in the data receiver's buffer in
>> expectation that its mapping will arrive shortly. And in the example that
>> we are referring to, the data sender will not be able to include the map
>> for the data in segment 1 in segment 3 or any higher segment.*
>>
>> *Regards,*
>>
>> *Anil*
>>
>> However, this might be contradict with the new texts Alan proposed? Or,
>> am I missing something?
>>
>> Thanks,
>> --
>> Yoshi
>>
>>
>>
>> On Sun, Feb 2, 2020 at 8:42 AM V Anil Kumar <anil@csir4pi.in> wrote:
>>
>>> Hi Yoshi,
>>>
>>> Thanks for this point. In fact, I had initially not thought of a
>>> scenario, where the map is being delivered through a retransmitted data
>>> packet while its first transmission did not include the map. Now I am just
>>> seeing the document (RFC 6824-bis) in this context.
>>>
>>> My understanding is that in scenarios like what I described in my
>>> previous mail, RST is likely to happen whether we explicitly state so or
>>> not. Please see the paragraph containing the below text in RFC 6824-bis.
>>>
>>> "If a mapping for that subflow-level sequence space does not arrive
>>> within a receive window of data, that subflow SHOULD be treated as broken,
>>> closed with a RST, and any unmapped data silently discarded."
>>>
>>> if we assume that the map is included while retransmitting the data
>>> (even though the first transmission did not contain the map for some
>>> reasons),  we could argue that RST could be avoided provided that the
>>> retransmission is triggered within a receive window of data. But the
>>> question here would be how and when will the retransmission take place. In
>>> this case, the subflow may not initiate the retransmission of data by its
>>> own (i..e., no retransmission due to three duplicate ACKs or RTO expiry at
>>> subflow level) as there is no segment loss at subflow level sequence space.
>>> So there could be a high possibility of RST happening even before the map
>>> delivery through retransmission.
>>>
>>> With regards,
>>>
>>> Anil
>>>
>>> ------------------------------
>>> *From: *"Yoshifumi Nishida" <nsd.ietf@gmail.com>
>>> *To: *"V Anil Kumar" <anil@csir4pi.in>
>>> *Cc: *"alan ford" <alan.ford@gmail.com>om>, "multipathtcp" <
>>> multipathtcp@ietf.org>
>>> *Sent: *Saturday, February 1, 2020 3:39:51 AM
>>> *Subject: *Re: [multipathtcp] RFC6824bis edits based on implementation
>>> feedback
>>>
>>> Hi Anil,
>>>
>>> I have a question about your proposed text.
>>> I am actually wondering if we really want to terminate connection here.
>>>
>>> The packets without proper mappings will be treated as invalid and will
>>> be discarded.
>>> If an implementation failed to attach proper mapping for some reasons
>>> (e.g. option space), it might be able to attach the proper one when it
>>> retransmits the packets. This also looks ok to me.
>>>
>>> I don't have strong preference for this. But, do we have a reason to
>>> terminate connection?
>>>
>>> Thanks,
>>> --
>>> Yoshi
>>>
>>> On Mon, Jan 13, 2020 at 10:28 AM V Anil Kumar <anil@csir4pi.in> wrote:
>>>
>>>> Hi,
>>>>
>>>> I have some points related to the  modifications (Change 2) being
>>>> proposed on data sequence map. Please see them inline. Though I am
>>>> putting forward the below points, if the consensus is in favour of the
>>>> proposed change for reducing implementation complexity, I am also OK with
>>>> that as well.
>>>>
>>>> ------------------------------
>>>> *From: *"alan ford" <alan.ford@gmail.com>
>>>> *To: *multipathtcp@ietf.org
>>>> *Sent: *Thursday, January 2, 2020 4:21:32 AM
>>>> *Subject: *[multipathtcp] RFC6824bis edits based on implementation
>>>> feedback
>>>>
>>>> Hi all,
>>>> We’d love to get this to a state of completion as soon as possible, and
>>>> to this end I am starting a new thread on this topic. In discussion with
>>>> the chairs, it *is *possible to make the desired changes in AUTH48 as
>>>> long as there is WG consensus. The discussion so far has been fairly
>>>> limited in terms of participation.
>>>>
>>>> I would ask the chairs please if it was possible to specify a time
>>>> bound for this discussion and a default conclusion.
>>>>
>>>> Regarding the changes, in summary, there are two areas where changes
>>>> have been requested by the implementation community. As we are the IETF we
>>>> obviously have strong focus on “running code” and so ease of implementing
>>>> standards-compliant code is strongly desirable. However, we do not wish to
>>>> reduce functionality agreed by the IETF community if it is considered a
>>>> required feature by the community.
>>>>
>>>>
>>>> *Change 1*
>>>>
>>>> Change the sentence reading:
>>>>
>>>>
>>>> *   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). *
>>>> To:
>>>>
>>>>
>>>> *   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).*
>>>> What this means:
>>>>
>>>> The current text is concerned only with ensuring a path is MPTCP
>>>> capable, and so only cares that DSS option occurs on a data packet.
>>>> However, the MP_CAPABLE option is defined to occupy the first octet of data
>>>> sequence space and thus, if analogous to TCP, must be acknowledged. >From
>>>> an implementation point of view it would make sense not to have this
>>>> hanging around forever and instead define it is acknowledged at the
>>>> connection level as soon as received. This change ensures the first data
>>>> packet also DATA_ACKs this MP_CAPABLE octet.
>>>>
>>>>
>>>> *Change 2*
>>>>
>>>> Change the sentence reading:
>>>>
>>>> *   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.*
>>>>
>>>> To:
>>>>
>>>>
>>>> *   The mapping provided by a Data Sequence Mapping MUST apply to some
>>>> or all of the subflow sequence space in the TCP segment which carries the
>>>> option. 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.*
>>>> What this means:
>>>>
>>>> The current text does not place any restrictions on where a mapping
>>>> could appear. In theory a sender could define a thousand different mappings
>>>> up front, send them all, and expect a receiver to store this and reassemble
>>>> data according to these mappings as it arrives. Indeed, this was never
>>>> explicitly disallowed since it “might have been useful”. The implementation
>>>> community, however, has expressed concerns over the difficulty of
>>>> implementing this open-endedly. How many mappings is it reasonable to
>>>> store? Is there a DoS risk here? Instead, it has been requested that thee
>>>> specification restricts the placement of the DSS option to being within the
>>>> subflow sequence space to which it applies.
>>>>
>>>> Below are my comments on this. I had shared some of these points in a
>>>> previous thread that you had initiated in the same context.
>>>>
>>>> Transmitting large number of non-contiguous data sequence maps could be
>>>> a misbehaviour (map-flooding), though it is not clear whether this can go
>>>> to the extent of causing a potential DoS to the data receiver. So some sort
>>>> of restriction on this could be useful.  One approach could be to insist
>>>> that the data sender should ensure that the map being transmitted is for
>>>> in-window data, as per the receiver advertised window. A receiver should
>>>> anyhow be willing to store the maps for in-window data to deal with packet
>>>> loss. For example, when a window of data segments (say 1 to 64) is
>>>> transmitted, each carrying its corresponding map, and segment-1 is lost,
>>>> the maps for the remaining 63 need to be stored till the lost segment is
>>>> retransmitted. Of course, in this case the maps will be stored at the
>>>> receiver side along with their corresponding data. But the need to store
>>>> multiple maps for in-window data would still be there.
>>>>
>>>> The problem with the proposed change (restriction) is that a data
>>>> sender may find it difficult, in case a need arise to slightly delay the
>>>> map delivery by few segments, i.e., sending some data first without map,
>>>> and then send the corresponding map in a later segment, as shown below:
>>>>
>>>> subflow-1:      segment-1                   segment-3
>>>>   segment-4                       segment-7
>>>>                       bytes:1-100                 bytes:201-300
>>>>      bytes:301-400                 bytes:601-700
>>>>                       no map                        map for 1-100
>>>>        map for 201-400             map for 601-700
>>>>
>>>>
>>>>
>>>> subflow-2:       segment-2                  segment-5
>>>>   segment-6                       segment-8
>>>>                        bytes: 101-200           bytes:401-500
>>>>         bytes: 501-600                bytes:701-800
>>>>                        map for 101-200       map for 401-600
>>>>  no map                            map for 701-800
>>>>
>>>>
>>>> In the above case, segment-1 goes without map and its map is included
>>>> later in segment-3, the next data segment in the same subflow. Further,  in
>>>> the above scheduling pattern, the map in segment-3 cannot cover the  data
>>>> in segment-1 and segment-3, as some  data in between (segment-2) is
>>>> transmitted through another subflow.  With the proposed change, the map in
>>>> segment-3 will become invalid and this will eventually break subflow-1,
>>>> though this could be a corner case..
>>>>
>>>> The question at this stage is why would segment-1 be transmitted
>>>> without its map. In the case of bidirectional data transfer, there could be
>>>> a need to pack both timestamp and SACK  options in a data segment, i.e.,
>>>> piggybacking of  SACK with data. If we consider that timestamp takes 12
>>>> bytes and SACK, even with single block,  takes another 10 bytes, the
>>>> remaining 18 bytes option space is not adequate to carry data sequence
>>>> signal with map, especially when DSN is 64 bit long. So the delivery of
>>>> either of the two (SACK or map) would be delayed.
>>>>
>>>> As far as I understand, RFC 2018 (TCP Selective Acknowledgement
>>>> Options) implies that SACK should not be delayed. It states "If sent at
>>>> all, SACK options SHOULD be included in all ACKs which do not ACK the
>>>> highest sequence number in the data receiver's queue". It also says "If
>>>> data receiver generates SACK options under any circumstance, it SHOULD
>>>> generate them under all permitted circumstances".   So, as part of meeting
>>>> the RFC 2018 requirements, if the combination of SACK and timestamp is
>>>> given preference over DSS, data segments could be transmitted without their
>>>> map.
>>>>
>>>> Another case of delaying map could arise if the data sender prefers to
>>>> send ADD_ADDR option, instead of map, in a data segment. It is nice that
>>>> ADD_ADDR option can be delivered reliably in a pure ACK, but I think this
>>>> is not the case with DSS at present.
>>>>
>>>> If we adopt the proposed change, I think it might also be helpful to
>>>> spell out how the receiver is supposed to behave, if it gets maps not
>>>> meeting the MUST condition in the proposed change.  For example termination
>>>> of the subflow with MP_TCPRST option (section 3.6 in RFC 6824-bis) with
>>>> appropriate reason code and T flag value to intimate the data sender the
>>>> cause for subflow termination.
>>>>
>>>> With regards,
>>>>
>>>> Anil
>>>>
>>>> Please can members of the WG express whether they are happy with these
>>>> changes, or concerned.
>>>>
>>>> Best regards,
>>>> Alan
>>>>
>>>>
>>>> _______________________________________________
>>>> multipathtcp mailing list
>>>> multipathtcp@ietf.org
>>>> https://www.ietf.org/mailman/listinfo/multipathtcp
>>>>
>>>>
>>>>
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>>>>
>>>
>>>
>>>
>>
>>
>>
>>
>>
>>
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