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Intro to Connection Establishment in TCP

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• Establishing a TCP Connection
  • Establishing a TCP connection from A to B means:

    Make sure that: A's send seq no = B's ACK no Graphically:

    Result:

    A can transmit reliably to B because: B can use the ACK no to verify that each data item has been received in the order that was transmitted

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  • Establishing a TCP connection between A and B means:

    Make sure that: A's send seq no = B's ACK no and A's ACK no = B's send seq no Graphically:

    Result:

    A can transmit reliably to B B can transmit reliably to A

  • Fact:

    Before a sender and a receiver can exchange data with each other through TCP: they must first establish a (bi-directional) TCP connection.

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• Comment
  • Fact:

    It is possible to: Establish a uni-directional TCP connection However: TCP implementations in existing Operating Sysems will always establish a bi-directional TCP connection (i.e., 2 TCP connections) Hackers use this feature as Denial Service Attacks on networks....

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  • The SYN flood attack:

    SYN flood: click here The attacker establishes a TCP connection When the attcked machine tries to establish the reverse TCP connection: The attacker does not complete the connection

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• Why can't TCP sender/receiver both use ZERO (0) as initial number ???
  • Question:

    Why can't we always start a TCP connection with a fixed send/ACK number ???

    Answer:

    There can be situations where bogus data will be received !!!

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  • Consider the following scenario:

    P1 and P2 will always use the same starting sequence/ACK numbers: (I used ZERO (0), but it can be any fixed set of numbers P1 sends the first message, but the message is delayed (congestion): P1 times out and retransmits the first message: In the mean time, the old first transmission is still delayed (due congestion) Suppose: The retransmitted packet arrives first After the message delivery, the TCP connection is terminated (closed): (The old first transmission is still delayed) Now: another program P3 makes TCP connection to a program P4: Coincidentally: P3 and P4 use the same source and destination ports used by P1 and P2 !!! Suppose at this moment, the delayed first transmission from P1 to P2 arrives: Result: The bogus message will be accepted for delivery ny P4 !!! This is wrong and TCP will fail to guarantee reliability...

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  • Important note:

    This error is detectable if: Each TCP connection uses a random starting send sequence number !!!!

    Remember that: a TCP connection is uni-directional

    So:

    Each direction of the (bi-directional) TCP connection must pick a random send sequence number !!!

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• Minimizing the probability of communication error in TCP
  • Reducing the likelihood of accepting a bogus packet:

    Each new TCP connection must pick a random starting send sequence number The TCP connection establishment protocol must make sure that: A's send seq no = B's ACK no and A's ACK no = B's send seq no Graphically: