RFC 1106 (rfc1106) - Page 2 of 13

TCP big window and NAK options

RFC 1106             TCP Big Window and Nak Options            June 1989


   over high bandwidth*delay networks is still very poor.  It appears
   that no algorithm changes alone will make any significant
   improvements over high bandwidth*delay networks, but will require an
   extension to the protocol itself.  This RFC discusses two possible
   options to TCP for this purpose.

   The two options implemented and discussed in this RFC are:

   1.  NAKs

      This extension allows the receiver of data to inform the sender
      that a packet of data was not received and needs to be resent.
      This option proves to be useful over any network path (both high
      and low bandwidth*delay type networks) that experiences periodic
      errors such as lost packets, noisy links, or dropped packets due
      to congestion.  The information conveyed by this option is
      advisory and if ignored, does not have any effect on TCP what so
      ever.

   2.  Big Windows

      This option will give a method of expanding the current 16 bit (64
      Kbytes) TCP window to 32 bits of which 30 bits (over 1 gigabytes)
      are allowed for the receive window.  (The maximum window size
      allowed in TCP due to the requirement of TCP to detect old data
      versus new data.  For a good explanation please see [2].)  No
      changes are required to the standard TCP header [6]. The 16 bit
      field in the TCP header that is used to convey the receive window
      will remain unchanged.  The 32 bit receive window is achieved
      through the use of an option that contains the upper half of the
      window.  It is this option that is necessary to fill large data
      pipes such as a satellite link.

   This RFC is broken up into the following sections: section 2 will
   discuss the operation of the NAK option in greater detail, section 3
   will discuss the big window option in greater detail.  Section 4 will
   discuss other effects of the big windows and nak feature when used
   together.  Included in this section will be a brief discussion on the
   effects of congestion versus noise to TCP and possible options for
   satellite networks.  Section 5 will be a conclusion with some hints
   as to what future development may be done at NASA, and then an
   appendix containing some test results is included.

2.  NAK Option

   Any packet loss in a high bandwidth*delay network will have a
   catastrophic effect on throughput because of the simple
   acknowledgement of TCP.  TCP always acks the stream of data that has



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