[multipathtcp] rfc6824bis - RST after MP_FASTCLOSE retransmission

François Finfe <francois.finfe@tessares.net> Tue, 23 May 2017 08:55 UTC

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From: =?UTF-8?Q?Fran=c3=a7ois_Finfe?= <francois.finfe@tessares.net>
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Date: Tue, 23 May 2017 10:55:32 +0200
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Subject: [multipathtcp] rfc6824bis - RST after MP_FASTCLOSE retransmission
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I encountered an issue with MP_FASTCLOSE and a stateful firewall.

Let's explain it with the following scenario. See figure 1.
Firewalls M and N are stateful firewall which don't analyse MPTCP
options and ignore them.

- An MPTCP connection has been established between host A and host B.
- Host A sends a ACK with the MP_FASTCLOSE option.
- Firewall M forwards the packet to host firewall N.
- Firewall N forwards the packet to host B.
- Host B receives the MP_FASTCLOSE and replies with a TCP RST.
- Firewall N forwards the TCP RST packet to firewall M.
   Due to the TCP RST, the stateful firewall removes the connection
- The TCP RST is lost due to a lossy link, network congestion, etc.

- As host A didn't receive the expected TCP RST packet, a timeout fires
   a MP_FASTCLOSE retransmission.
- Firewall M forwards the packet to firewall N.
- For firewall N, this connection no more exists. It sees the
   MP_FASTCLOSE as an TCP ACK packet without any related connection.
   Firewall N drops the packet.
- MP_FASTCLOSE are retransmitted until the limit of MP_FASTCLOSE
   retransmission is reached.
- If nothing is done, firewall M will retain the connection state for
   some time until a connection tracking timeout occurs.
   In a production environment, with a lot of simultaneous connection,
   this kind of entries (erroneous connection state for an already closed
   connection) can accumulate in the firewall.
   Due to ressources limitation, this might lead to performance issue
   where new connections might be rejected.

To mitigate this issue, here is a proposal for rfc6824bis:
- When the limit of MP_FASTCLOSE retransmission is reached, a TCP RST
   could be sent by host A.
- In this scenario, firewall M forwards the TCP RST packet and removes
   the connection state.

This TCP RST packet could contain the MP_FASTCLOSE option.

   Host A                                                          Host B
    |                 Firewall M             Firewall N                |
    |                      |                      |                    |
    |                      |             TCP RST  | TCP RST            |
    |                      |           x----------|<-------------------|
    |                      |                      |                    |
    |                      |                      |                    |
    |                      |                      |                    |
    |  ACK(MP_FASTCLOSE)   |  ACK(MP_FASTCLOSE)   |                    |
    |--------------------->|--------------------->x                    |
    |                      |                      |                    |
    |                      |                      |                    |
    |  ACK(MP_FASTCLOSE)   |  ACK(MP_FASTCLOSE)   |                    |
    |--------------------->|--------------------->x                    |
    |                      |                      |                    |
    :                      :                      :                    :
    :                                                                  :
    :  Multiple MP_FASTCLOSE retransmissions until limit is reached    :
    :                                                                  :
    :                      :                      :                    :
    :                      :                      :                    :
    :                      :                      :                    :
    |  Last retransmission |                      |                    |
    |                      |                      |                    |
    |  ACK(MP_FASTCLOSE)   |  ACK(MP_FASTCLOSE)   |                    |
    |--------------------->|--------------------->x                    |
    |                      |                      |                    |
    | (timeout)            |                      |                    |
    |  RST(MP_FASTCLOSE)   |  RST(MP_FASTCLOSE)   |                    |
    |--------------------->|--------------------->x                    |
    |                      |                      |                    |

    Figure 1

Best regards,

François Finfe


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