[MPTCP] Re: question on MP_CAPABLE v1 handshake

[MPTCP] Re: question on MP_CAPABLE v1 handshake

[Christoph Paasch]

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Hello,

> On Nov 5, 2019, at 4:18 AM, Paolo Abeni <pabeni(a)redhat.com> wrote:
> 
> hi,
> 
> while testing the v1 MP_CAPABLE handshake, I stumbled upon the
> following scenario:
> 
> syn + MP_CAPABLE ->
> 
> 	<- syn + ack + MP_CAPABLE(sndr key)
> 
> ack + MP_CAPABLE(sndr key, rcvr key)
> 
> 	<- ack + DSS(map,data_ack)
> 
> At this point, is the client still supposed to send MP_CAPABLE(sndr
> key, rcvr key, data_len)? or should it send a plain DSS? Since it
> received a DSS from the peer, the client knows that the MP_CAPABLE
> options has been successfully received, but reading 
> 
> https://datatracker.ietf.org/doc/draft-ietf-mptcp-rfc6824bis/?include_text=1
> 
> from section 3.1.:
> 
> """
>   If A does not
>   immediately have data to send, it MUST include the MP_CAPABLE on the
>   third ACK, but without the additional data parameters.  When A does
>   have data to send, it must repeat the sending of the MP_CAPABLE
>   option from the third ACK, with additional data parameters.
> """
> 
> it looks like the client should always use MP_CAPABLE+data for the
> first data packet, is that right?

no, not always. Once the client receives a valid DATA_ACK from the server it knows that the server did receive the keys. This is somehow mentioned at:

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 a
MPTCP Data Sequence Signal (DSS) option (Section 3.3 <https://tools.ietf.org/html/draft-ietf-mptcp-rfc6824bis-18#section-3.3>).

The draft  says that the delivery can be inferred by the receipt of this data with a DSS-option, but it should actually read "by the receipt of a DATA_ACK". I actually mentioned that to Alan (https://mailarchive.ietf.org/arch/msg/multipathtcp/Mo-bvEcMEodPph8rTpylFMQ85IE <https://mailarchive.ietf.org/arch/msg/multipathtcp/Mo-bvEcMEodPph8rTpylFMQ85IE>) and suggested text to him in private, but that text never made it into the draft.

Do you want to send an e-mail to the IETF mailing-list to correct this?

The text I suggest would be:

"If B has data to send first, then the reliable delivery of the ACK
can be inferred by the receipt of this data with an MPTCP
DATA_ACK inside the DSS option (Section 3.3)."

Christoph


> 
> Thanks,
> 
> Paolo
> 


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