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We know the TCP three-way handshake, I want to know is there any limit/role for syn initial number and seq number?

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I hear about the number set random can protect server, who can help talk about this?

  • What's your aim? – Zac67 Aug 31 '19 at 20:03
  • I mean whether the syn and seq number must set to 1? in fact whether they are usual random for security? – fanhualuojin154873 Sep 1 '19 at 3:00
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syn control flag should be set to 1 and seq should be generated randomly in the first step of connection establishment as described below.

RFC 793 indicates about sequence number that:

Sequence Number: 32 bits

The sequence number of the first data octet in this segment (except when SYN is present). If SYN is present the sequence number is the initial sequence number (ISN) and the first data octet is ISN+1.

Also, RFC 6528 indicates that:

We can prevent sequence number guessing attacks by giving each connection -- that is, each four-tuple of (localip, localport, remoteip, remoteport) -- a separate sequence number space. Within each space, the ISN is incremented according to [RFC0793]; however, there is no obvious relationship between the numbering in different spaces.

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