[multipathtcp] A question related to the 4th ACK in the MP_JOIN handshake

"Shuai Wang" <13211134@bjtu.edu.cn> Thu, 13 April 2017 08:43 UTC

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Date: Thu, 13 Apr 2017 16:45:00 +0800 (GMT+08:00)
From: "Shuai Wang" <13211134@bjtu.edu.cn>
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Subject: [multipathtcp] A question related to the 4th ACK in the MP_JOIN handshake
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Hi,

    As it is said in RFC 6824 that the 4th ACK packet (for responding ACK/MP_JOIN packet) is necessary in MP_JOIN handshake before sending data. And the reason is that "The initiator's authentication information is sent in its first ACK(the third packet of the handshake). This data needs to be sent reliably, since it is the only time this HMAC is sent; therefore, receipt of this packet MUST trigger a regulat TCP ACK in response, and the packet MUST be retransmitte if this ACK is not received."

    But in my opinion, the receiver can check whether this HMAC included in the third ACK is correct, so the receiver can know whether the sender is the original sender or not. For the above considerations, I think the 4th ACK packet is unnecessary.

    If my opinion is wrong, please help me to correct. Thank you very much!

Best wishes,

Shuai