RFC 1110 (rfc1110) - Page 1 of 3

Problem with the TCP big window option

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Network Working Group                                        A. McKenzie
Request for Comments: 1110                                       BBN STC
                                                             August 1989


                A Problem with the TCP Big Window Option

Status of this Memo

   This memo comments on the TCP Big Window option described in RFC
   1106.  Distribution of this memo is unlimited.

Abstract

   The TCP Big Window option discussed in RFC 1106 will not work
   properly in an Internet environment which has both a high bandwidth *
   delay product and the possibility of disordering and duplicating
   packets.  In such networks, the window size must not be increased
   without a similar increase in the sequence number space.  Therefore,
   a different approach to big windows should be taken in the Internet.

Discussion

   TCP was designed to work in a packet store-and-forward environment
   characterized by the possibility of packet loss, packet disordering,
   and packet duplication.  Packet loss can occur, for example, by a
   congested network element discarding a packet.  Packet disordering
   can occur, for example, by packets of a TCP connection being
   arbitrarily transmitted partially over a low bandwidth terrestrial
   path and partially over a high bandwidth satellite path.  Packet
   duplication can occur, for example, when two directly-connected
   network elements use a reliable link protocol and the link goes down
   after the receiver correctly receives a packet but before the
   transmitter receives an acknowledgement for the packet; the
   transmitter and receiver now each take responsibility for attempting
   to deliver the same packet to its ultimate destination.

   TCP has the task of recreating at the destination an exact copy of
   the data stream generated at the source, in the same order and with
   no gaps or duplicates.  The mechanism used to accomplish this task is
   to assign a "unique" sequence number to each byte of data at its
   source, and to sort the bytes at the destination according to the
   sequence number.  The sorting operation corrects any disordering.  An
   acknowledgement, timeout, and retransmission scheme corrects for data
   loss.  The uniqueness of the sequence number corrects for data
   duplication.

   As a practical matter, however, the sequence number is not unique; it



McKenzie
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