TCP Header Flags 

For further information see [RFC3168]

(last updated 2003-07-09)

The Transmission Control Protocol (TCP) included a 6-bit Reserved
field defined in RFC 793, reserved for future use, in bytes
13 and 14 of the TCP header, as illustrated below.  The other six
Control bits are defined separately by RFC 793.

     0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |               |                       | U | A | P | R | S | F |
   | Header Length |        Reserved       | R | C | S | S | Y | I |
   |               |                       | G | K | H | T | N | N |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   RFC 3168 defines two of the six bits from the Reserved field to be
   used for ECN, as follows:

     0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |               |               | C | E | U | A | P | R | S | F |
   | Header Length |    Reserved   | W | C | R | C | S | S | Y | I |
   |               |               | R | E | G | K | H | T | N | N |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

TCP Header Flags

Bit      Name                                        Reference
---      ----                                        ---------
  7      NS (Nonce Sum)                              [RFC3540]
  8      CWR (Congestion Window Reduced)             [RFC3168]
  9      ECE (ECN-Echo)                              [RFC3168]


References
----------

[RFC3168]  K. Ramakrishnan, S. Floyd, and D. Black, "The Addition of 
           Explicit Congestion Notification (ECN) to IP", RFC 3168,
           September 2001.

[RFC3540]  N. Spring, D. Wetherall, and D. Ely, "Robust ECN Signaling 
           with Nonces", RFC 3540, June 2003.

(created 2001-08-15)

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