Code change:
Adding Olivier's proposed text about the Fast-Close (cfr., URL) and integrated a shorter version of URL Hello, Sorry for being late with this, but the beginning of the fall semester is always too busy for me. During the summer, there was a discussion on the FASTCLOSE option that was triggered by Francois. The current text of RFC6824bis uses the FASTCLOSE option inside ACK packets, but deployment experience has shown that this results in additional complexity since the ACK must be retransmitted to ensure its reliable delivery. Here is a proposed update for section 3.5 that allows a sender to either send FASTCLOSE inside ACK or RST while a receiver must be ready to process both. Changes are marked with Fast Close Regular TCP has the means of sending a reset (RST) signal to abruptly close a connection. With MPTCP, a reguler RST only has the scope of the subflow and will only close the concerned subflow but not affect the remaining subflows. MPTCP's connection will stay alive at the data level, in order to permit break-before-make handover between subflows. It is therefore necessary to provide an MPTCP-level "reset" to allow the abrupt closure of the whole MPTCP connection, and this is the MPFASTCLOSE option. MPFASTCLOSE is used to indicate to the peer that the connection will be abruptly closed and no data will be accepted anymore. The reasons for triggering an MPFASTCLOSE are implementation specific. Regular TCP does not allow sending a RST while the connection is in a synchronized state [RFC0793]. Nevertheless, implementations allow the sending of a RST in this state, if, for example, the operating system is running out of resources. In these cases, MPTCP should send the MPFASTCLOSE. This option is illustrated in Figure 14. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +---------------+---------------+-------+-----------------------+ +---------------+---------------+-------+-----------------------+ +---------------------------------------------------------------+ Figure 14: Fast Close (MPFASTCLOSE) Option If Host A wants to force the closure of an MPTCP connection, it has two different options : Option A (ACK) : Host A sends an ACK containing the MPFASTCLOSE option on one subflow, containing the key of Host B as declared in the initial connection handshake. On all the other subflows, Host A sends a regular TCP RST to close these subflows, and tears them down. Host A now enters FASTCLOSEWAIT state. Option R (RST) : Host A sends a RST containing the MPFASTCLOSE option on all subflows, containing the key of Host B as declared in the initial connection handshake. Host A can tear the subflows and the connection down immediately. If a host receives a packet with a valid MPFASTCLOSE option, it shall process it as follows : o Upon receipt of an ACK with MPFASTCLOSE, containing the valid key, Host B answers on the same subflow with a TCP RST and tears down all subflows. Host B can now close the whole MPTCP connection (it transitions directly to CLOSED state). o As soon as Host A has received the TCP RST on the remaining subflow, it can close this subflow and tear down the whole connection (transition from FASTCLOSEWAIT to CLOSED states). If Host A receives an MPFASTCLOSE instead of a TCP RST, both hosts attempted fast closure simultaneously. Host A should reply with a TCP RST and tear down the connection. o If Host A does not receive a TCP RST in reply to its MPFASTCLOSE after one retransmission timeout (RTO) (the RTO of the subflow where the MPTCPRST has been sent), it SHOULD retransmit the MPFASTCLOSE. The number of retransmissions SHOULD be limited to avoid this connection from being retained for a long time, but this limit is implementation specific. A RECOMMENDED number is 3. If no TCP RST is received in response, Host A SHOULD send a TCP RST with the MPFASTCLOSE option itself when it releases state in order to clear any remaining state at middleboxes. o Upon receipt of a RST with MPFASTCLOSE, containing the valid key, Host B tears down all subflows. Host B can now close the whole MPTCP connection (it transitions directly to CLOSED state). The burden of the MP_FASTCLOSE is on the host that decides to close the connection (typically the server, but not necessarily). The text above allows it to opt for either ACK or RST without adding complexity to the other side. Another point that might be discussed in RFC6824bis is how a host should react when there are all the subflows associates to a Multipath TCP connection are in the CLOSED state, but the connection is still alive. At this point, it is impossible to send or receive data over the connection. It should attempt to reestablish one subflow to keep the connection alive. Olivier

Related discussion:
text: If Host A does not receive a TCP RST in reply to its MP_FASTCLOSE after one retransmission timeout (RTO) (the RTO of the subflow <del> where the MPTCP_RST has been sent), it SHOULD retransmit the </del> <ins> where the MP_FASTCLOSE has been sent), it SHOULD retransmit the </ins> MP_FASTCLOSE.  The number of retransmissions SHOULD be limited to avoid this connection from being retained for a long time, but this limit is implementation specific.  A RECOMMENDED number is 3. If no TCP RST is received in response, Host A SHOULD send a TCP <del> RST itself when it releases state in order to clear any remaining state at middleboxes. </del> <ins> RST with the MP_FASTCLOSE option itself when it releases state in order to clear any remaining state at middleboxes. Upon receipt of a RST with MP_FASTCLOSE, containing the valid key, Host B tears down all subflows.  Host B can now close the whole MPTCP connection (it transitions directly to CLOSED state). </ins> 3.6.