[tcpm] RFC8684: TFO with MPTCP V1 example shows incorrect data map

Krishna Khanal <erkrishna.khanal@gmail.com> Thu, 13 May 2021 18:37 UTC

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From: Krishna Khanal <erkrishna.khanal@gmail.com>
Date: Thu, 13 May 2021 11:37:19 -0700
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Subject: [tcpm] RFC8684: TFO with MPTCP V1 example shows incorrect data map
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The example for TCP Fastopen with MPTCPv1 in RFC8684, Appendix-B.3 shows
the listener sending an incorrect data-ack in its first data packet to the
initiator sent immediately after its SYN-ACK.

 initiator                                                    listener
       |                                                           |
       |    S  0(20) <MP_CAPABLE>, <TFO cookie>                    |
       | --------------------------------------------------------> |
       |                                                           |
       |    S. 0(0) ack 21 <MP_CAPABLE>                            |
       | <-------------------------------------------------------- |
       |                                                           |
       |    .  1(100) ack 21 <DSS ack=1 seq=1 ssn=1 dlen=100>      |
       | <-------------------------------------------------------- |


At this point, the listener has not received the initiator's key, so
it cannot generate the data-ack in it's response packets.


The correct behaviour would be to set flag 'A' to 0 and exclude
sending data-ack until the initiator's key is received.


initiator                                                    listener
       |                                                           |
       |    S  0(20) <MP_CAPABLE>, <TFO cookie>                    |
       | --------------------------------------------------------> |
       |                                                           |
       |    S. 0(0) ack 21 <MP_CAPABLE>                            |
       | <-------------------------------------------------------- |
       |                                                           |
       |    .  1(100) ack 21 <DSS seq=1 ssn=1 dlen=100, M=1, A=0>  |
       | <-------------------------------------------------------- |


Thanks,

Krishna