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What is the purpose of TCP sequence number checking?



Article ID: KB5599 KB Last Updated: 09 Jun 2010Version: 3.0
What is the purpose of TCP sequence number checking?


TCP sequence checking monitors and validates the serialization of bytes. Sequence checking involves continuous learning and remembering the (trusted client) TCP byte sequence counter, and validating the receipt (untrusted server) acknowledgements. It verifies the untrusted acknowledgement number is in the range of the trusted sequence number and window. (The window may use an appropriately selected fixed value; say 32 or 64K, rather than strictly monitoring the window. The implementation may also verify trusted acknowledgement number against the untrusted sequence number, but this may not be necessary).  Checking sequence numbers offers some protection against a man-in-the-middle reset attack.

Here is the problem or goal:

  • What is the purpose of TCP sequence number checking?

Applicable Products:

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