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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 [Page 1]  RFC 1110 Comments on TCP Big Window Option August 1989

 is contained in a 32-bit field and therefore "wraps around" after the
 transmission of 2**32 bytes of data.  Two additional mechanisms are
 used to insure the effective uniqueness of sequence numbers; these
 are the TCP transmission window and bounds on packet lifetime within
 the Internet, including the IP Time-to-Live (TTL).  The transmission
 window specifies the maximum number of bytes which may be sent by the
 source in one source-destination roundtrip time.  Since the TCP
 transmission window is specified by 16 bits, which is 1/65536 of the
 sequence number space, a sequence number will not be reused (used to
 number another byte) for 65,536 roundtrip times.  So long as the
 combination of gateway action on the IP TTL and holding times within
 the individual networks which interconnect the gateways do not allow
 a packet's lifetime to exceed 65,536 roundtrip times, each sequence
 number is effectively unique.  It was believed by the TCP designers
 that the networks and gateways forming the internet would meet this
 constraint, and such has been the case.

 The proposed TCP Big Window option, as described in RFC 1106, expands
 the size of the window specification to 30 bits, while leaving the
 sequence number space unchanged.  Thus, a sequence number can be
 reused after 4 roundtrip times.  Further, the Nak option allows a
 packet to be retransmitted (i.e., potentially duplicated) by the
 source after only one roundtrip time.  Thus, if a packet becomes
 "lost" in the Internet for only about 5 roundtrip times it may be
 delivered when its sequence number again lies within the window,
 albeit a later cycle of the window.  In this case, TCP will not
 necessarily recreate at the destination an exact copy of the data
 stream generated at the source; it may replace some data with earlier
 data.

 Of course, the problem described above results from the storage of
 the "lost" packet within the net, and its subsequent out-of-order
 delivery.  RFC 1106 seems to describe use of the proposed options in
 an isolated satellite network.  We may hypothesize that this network
 is memoryless, and thus cannot deliver packets out of order; it
 either delivers a packet in order or loses it.  If this is the case,
 then there is no problem with the proposed options.  The Internet,
 however, can deliver packets out of order, and this will likely
 continue to be true even if gigabit links become part of the
 Internet.  Therefore, the approach described in RFC 1106 cannot be
 adopted for general Internet use.

McKenzie [Page 2]  RFC 1110 Comments on TCP Big Window Option August 1989

Author's Address

 Alex McKenzie
 Bolt Beranek and Newman Inc.
 10 Moulton Street
 Cambridge, MA 02238

 Phone: (617) 873-2962

 EMail: MCKENZIE@BBN.COM

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