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Obsoleted by: 6247 HISTORIC

Network Working Group                                             R. Fox
Request for Comments:  1106                                       Tandem
                                                               June 1989


                     TCP Big Window and Nak Options

Status of this Memo

   This memo discusses two extensions to the TCP protocol to provide a
   more efficient operation over a network with a high bandwidth*delay
   product.  The extensions described in this document have been
   implemented and shown to work using resources at NASA.  This memo
   describes an Experimental Protocol, these extensions are not proposed
   as an Internet standard, but as a starting point for further
   research.  Distribution of this memo is unlimited.

Abstract

   Two extensions to the TCP protocol are described in this RFC in order
   to provide a more efficient operation over a network with a high
   bandwidth*delay product.  The main issue that still needs to be
   solved is congestion versus noise.  This issue is touched on in this
   memo, but further research is still needed on the applicability of
   the extensions in the Internet as a whole infrastructure and not just
   high bandwidth*delay product networks.  Even with this outstanding
   issue, this document does describe the use of these options in the
   isolated satellite network environment to help facilitate more
   efficient use of this special medium to help off load bulk data
   transfers from links needed for interactive use.

1.  Introduction

   Recent work on TCP has shown great performance gains over a variety
   of network paths [1].  However, these changes still do not work well
   over network paths that have a large round trip delay (satellite with
   a 600 ms round trip delay) or a very large bandwidth
   (transcontinental DS3 line).  These two networks exhibit a higher
   bandwidth*delay product, over 10**6 bits, than the 10**5 bits that
   TCP is currently limited to.  This high bandwidth*delay product
   refers to the amount of data that may be unacknowledged so that all
   of the networks bandwidth is being utilized by TCP.  This may also be
   referred to as "filling the pipe" [2] so that the sender of data can
   always put data onto the network and the receiver will always have
   something to read, and neither end of the connection will be forced
   to wait for the other end.

   After the last batch of algorithm improvements to TCP, performance



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   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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   successfully been received and tells the sender the next byte of data
   of the stream that is expected.  If a packet is lost and succeeding
   packets arrive the current protocol has no way of telling the sender
   that it missed one packet but received following packets.  TCP
   currently resends all of the data over again, after a timeout or the
   sender suspects a lost packet due to a duplicate ack algorithm [1],
   until the receiver receives the lost packet and can then ack the lost
   packet as well as succeeding packets received.  On a normal low
   bandwidth*delay network this effect is minimal if the timeout period
   is set short enough.  However, on a long delay network such as a T1
   satellite channel this is catastrophic because by the time the lost
   packet can be sent and the ack returned the TCP window would have
   been exhausted and both the sender and receiver would be temporarily
   stalled waiting for the packet and ack to fully travel the data pipe.
   This causes the pipe to become empty and requires the sender to
   refill the pipe after the ack is received.  This will cause a minimum
   of 3*X bandwidth loss, where X is the one way delay of the medium and
   may be much higher depending on the size of the timeout period and
   bandwidth*delay product.  Its 1X for the packet to be resent, 1X for
   the ack to be received and 1X for the next packet being sent to reach
   the destination.  This calculation assumes that the window size is
   much smaller than the pipe size (window = 1/2 data pipe or 1X), which
   is the typical case with the current TCP window limitation over long
   delay networks such as a T1 satellite link.

   An attempt to reduce this wasted bandwidth from 3*X was introduced in
   [1] by having the sender resend a packet after it notices that a
   number of consecutively received acks completely acknowledges already
   acknowledged data.  On a typical network this will reduce the lost
   bandwidth to almost nil, since the packet will be resent before the
   TCP window is exhausted and with the data pipe being much smaller
   than the TCP window, the data pipe will not become empty and no
   bandwidth will be lost.  On a high delay network the reduction of
   lost bandwidth is minimal such that lost bandwidth is still
   significant.  On a very noisy satellite, for instance, the lost
   bandwidth is very high (see appendix for some performance figures)
   and performance is very poor.

   There are two methods of informing the sender of lost data.
   Selective acknowledgements and NAKS.  Selective acknowledgements have
   been the object of research in a number of experimental protocols
   including VMTP [3], NETBLT [4], and SatFTP [5].  The idea behind
   selective acks is that the receiver tells the sender which pieces it
   received so that the sender can resend the data not acked but already
   sent once.  NAKs on the other hand, tell the sender that a particular
   packet of data needs to be resent.

   There are a couple of disadvantages of selective acks.  Namely, in



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   some of the protocols mentioned above, the receiver waits a certain
   time before sending the selective ack so that acks may be bundled up.
   This delay can cause some wasted bandwidth and requires more complex
   state information than the simple nak.  Even if the receiver doesn't
   bundle up the selective acks but sends them as it notices that
   packets have been lost, more complex state information is needed to
   determine which packets have been acked and which packets need to be
   resent.  With naks, only the immediate data needed to move the left
   edge of the window is naked, thus almost completely eliminating all
   state information.

   The selective ack has one advantage over naks.  If the link is very
   noisy and packets are being lost close together, then the sender will
   find out about all of the missing data at once and can send all of
   the missing data out immediately in an attempt to move the left
   window edge in the acknowledge number of the TCP header, thus keeping
   the data pipe flowing.  Whereas with naks, the sender will be
   notified of lost packets one at a time and this will cause the sender
   to process extra packets compared to selective acks.  However,
   empirical studies has shown that most lost packets occur far enough
   apart that the advantage of selective acks over naks is rarely seen.
   Also, if naks are sent out as soon as a packet has been determined
   lost, then the advantage of selective acks becomes no more than
   possibly a more aesthetic algorithm for handling lost data, but
   offers no gains over naks as described in this paper.  It is this
   reason that the simplicity of naks was chosen over selective acks for
   the current implementation.

2.1  Implementation details

   When the receiver of data notices a gap between the expected sequence
   number and the actual sequence number of the packet received, the
   receiver can assume that the data between the two sequence numbers is
   either going to arrive late or is lost forever.  Since the receiver
   can not distinguish between the two events a nak should be sent in
   the TCP option field.  Naking a packet still destined to arrive has
   the effect of causing the sender to resend the packet, wasting one
   packets worth of bandwidth.  Since this event is fairly rare, the
   lost bandwidth is insignificant as compared to that of not sending a
   nak when the packet is not going to arrive.  The option will take the
   form as follows:

      +========+=========+=========================+================+
      +option= + length= + sequence number of      + number of      +
      +   A    +    7    +  first byte being naked + segments naked +
      +========+=========+=========================+================+

   This option contains the first sequence number not received and a



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   count of how many segments of bytes needed to be resent, where
   segments is the size of the current TCP MSS being used for the
   connection.  Since a nak is an advisory piece of information, the
   sending of a nak is unreliable and no means for retransmitting a nak
   is provided at this time.

   When the sender of data receives the option it may either choose to
   do nothing or it will resend the missing data immediately and then
   continue sending data where it left off before receiving the nak.
   The receiver will keep track of the last nak sent so that it will not
   repeat the same nak.  If it were to repeat the same nak the protocol
   could get into the mode where on every reception of data the receiver
   would nak the first missing data frame.  Since the data pipe may be
   very large by the time the first nak is read and responded to by the
   sender, many naks would have been sent by the receiver.  Since the
   sender does not know that the naks are repetitious it will resend the
   data each time, thus wasting the network bandwidth with useless
   retransmissions of the same piece of data.  Having an unreliable nak
   may result in a nak being damaged and not being received by the
   sender, and in this case, we will let the tcp recover by its normal
   means.  Empirical data has shown that the likelihood of the nak being
   lost is quite small and thus, this advisory nak option works quite
   well.

3.  Big Window Option

   Currently TCP has a 16 bit window limitation built into the protocol.
   This limits the amount of outstanding unacknowledged data to 64
   Kbytes.  We have already seen that some networks have a pipe larger
   than 64 Kbytes.  A T1 satellite channel and a cross country DS3
   network with a 30ms delay have data pipes much larger than 64 Kbytes.
   Thus, even on a perfectly conditioned link with no bandwidth wasted
   due to errors, the data pipe will not be filled and bandwidth will be
   wasted.  What is needed is the ability to send more unacknowledged
   data.  This is achieved by having bigger windows, bigger than the
   current limitation of 16 bits.  This option to expands the window
   size to 30 bits or over 1 gigabytes by literally expanding the window
   size mechanism currently used by TCP.  The added option contains the
   upper 15 bits of the window while the lower 16 bits will continue to
   go where they normally go [6] in the TCP header.

   A TCP session will use the big window options only if both sides
   agree to use them, otherwise the option is not used and the normal 16
   bit windows will be used.  Once the 2 sides agree to use the big
   windows then every packet thereafter will be expected to contain the
   window option with the current upper 15 bits of the window.  The
   negotiation to decide whether or not to use the bigger windows takes
   place during the SYN and SYN ACK segments of the TCP connection



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   startup process.  The originator of the connection will include in
   the SYN segment the following option:

                    1 byte    1 byte      4 bytes
              +=========+==========+===============+
              +option=B + length=6 + 30 bit window +
              +=========+==========+===============+


   If the other end of the connection wants to use big windows it will
   include the same option back in the SYN ACK segment that it must
   send.  At this point, both sides have agreed to use big windows and
   the specified windows will be used.  It should be noted that the SYN
   and SYN ACK segments will use the small windows, and once the big
   window option has been negotiated then the bigger windows will be
   used.

   Once both sides have agreed to use 32 bit windows the protocol will
   function just as it did before with no difference in operation, even
   in the event of lost packets.  This claim holds true since the
   rcv_wnd and snd_wnd variables of tcp contain the 16 bit windows until
   the big window option is negotiated and then they are replaced with
   the appropriate 32 bit values.  Thus, the use of big windows becomes
   part of the state information kept by TCP.

   Other methods of expanding the windows have been presented, including
   a window multiple [2] or streaming [5], but this solution is more
   elegant in the sense that it is a true extension of the window that
   one day may easily become part of the protocol and not just be an
   option to the protocol.

3.1  How does it work

   Once a connection has decided to use big windows every succeeding
   packet must contain the following option:

        +=========+==========+==========================+
        +option=C + length=4 + upper 15 bits of rcv_wnd +
        +=========+==========+==========================+

   With all segments sent, the sender supplies the size of its receive
   window.  If the connection is only using 16 bits then this option is
   not supplied, otherwise the lower 16 bits of the receive window go
   into the tcp header where it currently resides [6] and the upper 15
   bits of the window is put into the data portion of the option C.
   When the receiver processes the packet it must first reform the
   window and then process the packet as it would in the absence of the
   option.



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3.2  Impact of changes

   In implementing the first version of the big window option there was
   very little change required to the source.  State information must be
   added to the protocol to determine if the big window option is to be
   used and all 16 bit variables that dealt with window information must
   now become 32 bit quantities.  A future document will describe in
   more detail the changes required to the 4.3 bsd tcp source code.
   Test results of the window change only are presented in the appendix.
   When expanding 16 bit quantities to 32 bit quantities in the TCP
   control block in the source (4.3 bsd source) may cause the structure
   to become larger than the mbuf used to hold the structure.  Care must
   be taken to insure this doesn't occur with your system or
   undetermined events may take place.

4.  Effects of Big Windows and Naks when used together

   With big windows alone, transfer times over a satellite were quite
   impressive with the absence of any introduced errors.  However, when
   an error simulator was used to create random errors during transfers,
   performance went down extremely fast.  When the nak option was added
   to the big window option performance in the face of errors went up
   some but not to the level that was expected.  This section will
   discuss some issues that were overcome to produce the results given
   in the appendix.

4.1  Window Size and Nak benefits

   With out errors, the window size required to keep the data pipe full
   is equal to the round trip delay * throughput desired, or the data
   pipe bandwidth (called Z from now on).  This and other calculations
   assume that processing time of the hosts is negligible.  In the event
   of an error (without NAKs), the window size needs to become larger
   than Z in order to keep the data pipe full while the sender is
   waiting for the ack of the resent packet.  If the window size is
   equaled to Z and we assume that the retransmission timer is equaled
   to Z, then when a packet is lost, the retransmission timer will go
   off as the last piece of data in the window is sent.  In this case,
   the lost piece of data can be resent with no delay.  The data pipe
   will empty out because it will take 1/2Z worth of data to get the ack
   back to the sender, an additional 1/2Z worth of data to get the data
   pipe refilled with new data.  This causes the required window to be
   2Z, 1Z to keep the data pipe full during normal operations and 1Z to
   keep the data pipe full while waiting for a lost packet to be resent
   and acked.

   If the same scenario in the last paragraph is used with the addition
   of NAKs, the required window size still needs to be 2Z to avoid



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   wasting any bandwidth in the event of a dropped packet.  This appears
   to mean that the nak option does not provide any benefits at all.
   Testing showed that the retransmission timer was larger than the data
   pipe and in the event of errors became much bigger than the data
   pipe, because of the retransmission backoff.  Thus, the nak option
   bounds the required window to 2Z such that in the event of an error
   there is no lost bandwidth, even with the retransmission timer
   fluctuations.  The results in the appendix shows that by using naks,
   bandwidth waste associated with the retransmission timer facility is
   eliminated.

4.2  Congestions vs Noise

   An issue that must be looked at when implementing both the NAKs and
   big window scheme together is in the area of congestion versus lost
   packets due to the medium, or noise.  In the recent algorithm
   enhancements [1], slow start was introduced so that whenever a data
   transfer is being started on a connection or right after a dropped
   packet, the effective send window would be set to a very small size
   (typically would equal the MSS being used).  This is done so that a
   new connection would not cause congestion by immediately overloading
   the network, and so that an existing connection would back off the
   network if a packet was dropped due to congestion and allow the
   network to clear up.  If a connection using big windows loses a
   packet due to the medium (a packet corrupted by an error) the last
   thing that should be done is to close the send window so that the
   connection can only send 1 packet and must use the slow start
   algorithm to slowly work itself back up to sending full windows worth
   of data.  This algorithm would quickly limit the usefulness of the
   big window and nak options over lossy links.

   On the other hand, if a packet was dropped due to congestion and the
   sender assumes the packet was dropped because of noise the sender
   will continue sending large amounts of data.  This action will cause
   the congestion to continue, more packets will be dropped, and that
   part of the network will collapse.  In this instance, the sender
   would want to back off from sending at the current window limit.
   Using the current slow start mechanism over a satellite builds up the
   window too slowly [1].  Possibly a better solution would be for the
   window to be opened 2*Rlog2(W) instead of R*log2(W) [1] (open window
   by 2 packets instead of 1 for each acked packet).  This will reduce
   the wasted bandwidth by opening the window much quicker while giving
   the network a chance to clear up.  More experimentation is necessary
   to find the optimal rate of opening the window, especially when large
   windows are being used.

   The current recommendation for TCP is to use the slow start mechanism
   in the event of any lost packet.  If an application knows that it



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   will be using a satellite with a high error rate, it doesn't make
   sense to force it to use the slow start mechanism for every dropped
   packet.  Instead, the application should be able to choose what
   action should happen in the event of a lost packet.  In the BSD
   environment, a setsockopt call should be provided so that the
   application may inform TCP to handle lost packets in a special way
   for this particular connection.  If the known error rate of a link is
   known to be small, then by using slow start with modified rate from
   above, will cause the amount of bandwidth loss to be very small in
   respect to the amount of bandwidth actually utilized.  In this case,
   the setsockopt call should not be used.  What is really needed is a
   way for a host to determine if a packet or packets are being dropped
   due to congestion or noise.  Then, the host can choose to do the
   right thing.  This will require a mechanism like source quench to be
   used.  For this to happen more experimentation is necessary to
   determine a solid definition on the use of this mechanism.  Now it is
   believed by some that using source quench to avoid congestion only
   adds to the problem, not help suppress it.

   The TCP used to gather the results in the appendix for the big window
   with nak experiment, assumed that lost packets were the result of
   noise and not congestion.  This assumption was used to show how to
   make the current TCP work in such an environment.  The actual
   satellite used in the experiment (when the satellite simulator was
   not used) only experienced an error rate around 10e-10.  With this
   error rate it is suggested that in practice when big windows are used
   over the link, TCP should use the slow start mechanism for all lost
   packets with the 2*Rlog2(W) rate discussed above.  Under most
   situations when long delay networks are being used (transcontinental
   DS3 networks using fiber with very low error rates, or satellite
   links with low error rates) big windows and naks should be used with
   the assumption that lost packets are the result of congestion until a
   better algorithm is devised [7].

   Another problem noticed, while testing the affects of slow start over
   a satellite link, was at times, the retransmission timer was set so
   restrictive, that milliseconds before a naked packet's ack is
   received the retransmission timer would go off due to a timed packet
   within the send window.  The timer was set at the round trip delay of
   the network allowing no time for packet processing.  If this timer
   went off due to congestion then backing off is the right thing to do,
   otherwise to avoid the scenario discovered by experimentation, the
   transmit timer should be set a little longer so that the
   retransmission timer does not go off too early.  Care must be taken
   to make sure the right thing is done in the implementation in
   question so that a packet isn't retransmitted too soon, and blamed on
   congestion when in fact, the ack is on its way.




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RFC 1106             TCP Big Window and Nak Options            June 1989


4.3  Duplicate Acks

   Another problem found with the 4.3bsd implementation is in the area
   of duplicate acks.  When the sender of data receives a certain number
   of acks (3 in the current Berkeley release) that acknowledge
   previously acked data before, it then assumes that a packet has been
   lost and will resend the one packet assumed lost, and close its send
   window as if the network is congested and the slow start algorithm
   mention above will be used to open the send window.  This facility is
   no longer needed since the sender can use the reception of a nak as
   its indicator that a particular packet was dropped.  If the nak
   packet is lost then the retransmit timer will go off and the packet
   will be retransmitted by normal means.  If a senders algorithm
   continues to count duplicate acks the sender will find itself
   possibly receiving many duplicate acks after it has already resent
   the packet due to a nak being received because of the large size of
   the data pipe.  By receiving all of these duplicate acks the sender
   may find itself doing nothing but resending the same packet of data
   unnecessarily while keeping the send window closed for absolutely no
   reason.  By removing this feature of the implementation a user can
   expect to find a satellite connection working much better in the face
   of errors and other connections should not see any performance loss,
   but a slight improvement in performance if anything at all.

5.  Conclusion

   This paper has described two new options that if used will make TCP a
   more efficient protocol in the face of errors and a more efficient
   protocol over networks that have a high bandwidth*delay product
   without decreasing performance over more common networks.  If a
   system that implements the options talks with one that does not, the
   two systems should still be able to communicate with no problems.
   This assumes that the system doesn't use the option numbers defined
   in this paper in some other way or doesn't panic when faced with an
   option that the machine does not implement.  Currently at NASA, there
   are many machines that do not implement either option and communicate
   just fine with the systems that do implement them.

   The drive for implementing big windows has been the direct result of
   trying to make TCP more efficient over large delay networks [2,3,4,5]
   such as a T1 satellite.  However, another practical use of large
   windows is becoming more apparent as the local area networks being
   developed are becoming faster and supporting much larger MTU's.
   Hyperchannel, for instances, has been stated to be able to support 1
   Mega bit MTU's in their new line of products.  With the current
   implementation of TCP, efficient use of hyperchannel is not utilized
   as it should because the physical mediums MTU is larger than the
   maximum window of the protocol being used.  By increasing the TCP



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   window size, better utilization of networks like hyperchannel will be
   gained instantly because the sender can send 64 Kbyte packets (IP
   limitation) but not have to operate in a stop and wait fashion.
   Future work is being started to increase the IP maximum datagram size
   so that even better utilization of fast local area networks will be
   seen by having the TCP/IP protocols being able to send large packets
   over mediums with very large MTUs.  This will hopefully, eliminate
   the network protocol as the bottleneck in data transfers while
   workstations and workstation file system technology advances even
   more so, than it already has.

   An area of concern when using the big window mechanism is the use of
   machine resources.  When running over a satellite and a packet is
   dropped such that 2Z (where Z is the round trip delay) worth of data
   is unacknowledged, both ends of the connection need to be able to
   buffer the data using machine mbufs (or whatever mechanism the
   machine uses), usually a valuable and scarce commodity.  If the
   window size is not chosen properly, some machines will crash when the
   memory is all used up, or it will keep other parts of the system from
   running.  Thus, setting the window to some fairly large arbitrary
   number is not a good idea, especially on a general purpose machine
   where many users log on at any time.  What is currently being
   engineered at NASA is the ability for certain programs to use the
   setsockopt feature or 4.3bsd asking to use big windows such that the
   average user may not have access to the large windows, thus limiting
   the use of big windows to applications that absolutely need them and
   to protect a valuable system resource.

6.  References

  [1]  Jacobson, V., "Congestion Avoidance and Control", SIGCOMM 88,
       Stanford, Ca., August 1988.

  [2]  Jacobson, V., and R. Braden, "TCP Extensions for Long-Delay
       Paths", LBL, USC/Information Sciences Institute, RFC 1072,
       October 1988.

  [3]  Cheriton, D., "VMTP: Versatile Message Transaction Protocol", RFC
       1045, Stanford University, February 1988.

  [4]  Clark, D., M. Lambert, and L. Zhang, "NETBLT: A Bulk Data
       Transfer Protocol", RFC 998, MIT, March 1987.

  [5]  Fox, R., "Draft of Proposed Solution for High Delay Circuit File
       Transfer", GE/NAS Internal Document, March 1988.

  [6]  Postel, J., "Transmission Control Protocol -  DARPA Internet
       Program Protocol Specification",  RFC 793, DARPA, September 1981.



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RFC 1106             TCP Big Window and Nak Options            June 1989


  [7]  Leiner, B., "Critical Issues in High Bandwidth Networking", RFC
       1077, DARPA, November 1989.

7.  Appendix

   Both options have been implemented and tested.  Contained in this
   section is some performance gathered to support the use of these two
   options.  The satellite channel used was a 1.544 Mbit link with a
   580ms round trip delay.  All values are given as units of bytes.


   TCP with Big Windows, No Naks:


               |---------------transfer rates----------------------|
   Window Size |  no error  |  10e-7 error rate | 10e-6 error rate |
   -----------------------------------------------------------------
     64K       |   94K      |      53K          |      14K         |
   -----------------------------------------------------------------
     72K       |   106K     |      51K          |      15K         |
   -----------------------------------------------------------------
     80K       |   115K     |      42K          |      14K         |
   -----------------------------------------------------------------
     92K       |   115K     |      43K          |      14K         |
    -----------------------------------------------------------------
     100K      |   135K     |      66K          |      15K         |
   -----------------------------------------------------------------
     112K      |   126K     |      53K          |      17K         |
   -----------------------------------------------------------------
     124K      |   154K     |      45K          |      14K         |
   -----------------------------------------------------------------
     136K      |   160K     |      66K          |      15K         |
   -----------------------------------------------------------------
     156K      |   167K     |      45K          |      14K         |
   -----------------------------------------------------------------
                                Figure 1.















Fox                                                            [Page 12]

RFC 1106             TCP Big Window and Nak Options            June 1989


   TCP with Big Windows, and Naks:


               |---------------transfer rates----------------------|
   Window Size |  no error  |  10e-7 error rate | 10e-6 error rate |
   -----------------------------------------------------------------
     64K       |   95K      |      83K          |      43K         |
   -----------------------------------------------------------------
     72K       |   104K     |      87K          |      49K         |
   -----------------------------------------------------------------
     80K       |   117K     |      96K          |      62K         |
   -----------------------------------------------------------------
     92K       |   124K     |      119K         |      39K         |
   -----------------------------------------------------------------
     100K      |   140K     |      124K         |      35K         |
   -----------------------------------------------------------------
     112K      |   151K     |      126K         |      53K         |
   -----------------------------------------------------------------
     124K      |   160K     |      140K         |      36K         |
   -----------------------------------------------------------------
     136K      |   167K     |      148K         |      38K         |
   -----------------------------------------------------------------
     156K      |   167K     |      160K         |      38K         |
   -----------------------------------------------------------------
                                Figure 2.

   With a 10e-6 error rate, many naks as well as data packets were
   dropped, causing the wild swing in transfer times.  Also, please note
   that the machines used are SGI Iris 2500 Turbos with the 3.6 OS with
   the new TCP enhancements.  The performance associated with the Irises
   are slower than a Sun 3/260, but due to some source code restrictions
   the Iris was used.  Initial results on the Sun showed slightly higher
   performance and less variance.

Author's Address

   Richard Fox
   950 Linden #208
   Sunnyvale, Cal, 94086

   EMail: rfox@tandem.com










Fox                                                            [Page 13]


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