TCP retransmissions and routers - TCP-IP

This is a discussion on TCP retransmissions and routers - TCP-IP ; Hi, I am experiencing a situation where a router does not forward retransmitted FIN packets: UNIT A router PC Unit A ( client ) connects to PC ( server ) sends some data, received some data, then the PC closes ...

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Thread: TCP retransmissions and routers

  1. TCP retransmissions and routers

    Hi,

    I am experiencing a situation where a router does not forward
    retransmitted FIN
    packets:

    UNIT A <->router<->PC

    Unit A ( client ) connects to PC ( server ) sends some data, received
    some data,
    then the PC closes the socket.
    Unit A sends an ACK to the PC , but the ack number does not acks the
    FIN sent by
    PC. Then PC retransmitts the FIN, that is correct, but the router does
    not forward
    the retransmitted FIN to unit A.

    Does routers interpret the TCP connection and try to optimized it ?
    If yes, this router probably is thinking the connection is closed, but
    it
    is not, does this makes sense ?

    Thanks,


  2. Re: TCP retransmissions and routers

    joseluismarchetti@yahoo.com.br wrote:
    > Does routers interpret the TCP connection and try to optimized it ?


    "Routers" do not do that. A "firewall" might, and a "NAT" might.
    Those last two features can be bundled into a device that is
    called/marketed a "router" though.

    > If yes, this router probably is thinking the connection is closed, but
    > it is not, does this makes sense ?


    It sounds plausible, although it would be a bug since unless the
    "router" saw the ACK of the FIN, it could not ass-u-me that the remote
    actually saw the FIN. Further, the "router" cannot ass-u-me that the
    sender of the FIN actually saw the ACK of the FIN after it
    passed-through the "router."

    rick jones
    --
    Wisdom Teeth are impacted, people are affected by the effects of events.
    these opinions are mine, all mine; HP might not want them anyway...
    feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

  3. Re: TCP retransmissions and routers

    In article <1148687543.147752.239060@i39g2000cwa.googlegroups. com>,
    joseluismarchetti@yahoo.com.br wrote:

    > Hi,
    >
    > I am experiencing a situation where a router does not forward
    > retransmitted FIN
    > packets:
    >
    > UNIT A <->router<->PC
    >
    > Unit A ( client ) connects to PC ( server ) sends some data, received
    > some data,
    > then the PC closes the socket.
    > Unit A sends an ACK to the PC , but the ack number does not acks the
    > FIN sent by
    > PC. Then PC retransmitts the FIN, that is correct, but the router does
    > not forward
    > the retransmitted FIN to unit A.
    >
    > Does routers interpret the TCP connection and try to optimized it ?
    > If yes, this router probably is thinking the connection is closed, but
    > it
    > is not, does this makes sense ?


    Does the router perform NAT or firewall functions? If so, it needs to
    keep track of open connections. NAT has a table of port translations
    for each connection, and firewalls only allow traffic on open
    connections (unless specific inbound rules allow otherwise).

    It *should* wait until FINs have been sent and ACKed in both directions
    before clearing out the connection entry, but I wouldn't be surprised if
    there are some implementations that get this wrong. How long was it
    until the retransmitted FIN -- maybe the router has a timeout for idle
    connections in this state.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

  4. Re: TCP retransmissions and routers

    Barry,

    Thanks for your response.

    "How long was it until the retransmitted FIN -- maybe the router has a
    timeout for idle
    connections in this state. "

    First retransmision = 3 seconds, second = 6 seconds

    After your and Rick answer, I think the router ( WIFI LinkSys router)
    has a bug, do you think it is a good idea to contact cisco about ?


  5. Re: TCP retransmissions and routers

    Rick,

    Thanks, I think the WIFI Linksys router has a bug.


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