Internet Engineering Task Force                              Mark Allman
INTERNET DRAFT                                              NASA GRC/BBN
File: draft-allman-tcp-abc-00.txt                             July, 2000
                                                  Expires: January, 2001

         TCP Congestion Control with Appropriate Byte Counting

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.

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    This document proposed a small modification to the way TCP increases
    its congestion window.  Rather than the traditional method of
    increasing the congestion window by a constant amount for each
    arriving acknowledgment, we suggest basing the increase on the
    number of previously unacknowledged bytes each ACK covers.  This
    change improves the performance of TCP, as well as closes a security
    hole TCP receivers can use to induce the sender into increasing the
    sending rate too rapidly.

1   Introduction

    This document proposes a modified algorithm for increasing TCP's
    congestion window (cwnd) that improves performance and security.
    Rather than increasing a TCP's congestion window based on the number
    of acknowledgments (ACKs) that arrive at the data sender the
    congestion window is increased based on the number of bytes
    acknowledged by the arriving ACKs.  The algorithm improves
    performance by mitigating the impact of delayed ACKs on the growth
    of cwnd.  At the same time, the algorithm provides more appropriate
    cwnd growth in response to ACKs that cover only small amounts of
    data (less than a full segment size).  More appropriate cwnd growth

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    can improve both performance and can prevent inappropriate cwnd
    growth in response to a misbehaving receiver.

    Much of the language in this document is taken from [RFC2581].

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    document are to be interpreted as described in [RFC2119].

    This document is organized as follows.  Section 2 outlines the
    modified algorithm for increasing TCP's congestion window.  Section
    3 discusses the advantages of using the modified algorithm.  Section
    4 discusses the disadvantages of the approach outlined in this
    document.  Section 5 outlines some of the fairness issues that must
    be considered for the modified algorithm.  Section 6 discusses
    security considerations.

2   A Modified Algorithm for Increasing the Congestion Window

    As originally outlined in [Jac88] and specified in [RFC2581], TCP
    uses two algorithms for increasing the congestion window (cwnd).
    During steady-state, TCP uses the Congestion Avoidance algorithm to
    linearly increase the value of cwnd.  At the beginning of a
    transfer, after a retransmission timeout or after a long idle period
    (in some implementations), TCP uses the Slow Start algorithm to
    increase cwnd exponentially.  According to RFC 2581 slow start bases
    the cwnd increase on the number of incoming acknowledgments.  During
    congestion avoidance RFC 2581 allows more latitude in increasing
    cwnd, but traditionally implementations have based the increase on
    the number of arriving ACKs.  In the following two subsections, we
    detail modifications to these algorithms to increase cwnd based on
    the number of bytes being acknowledged by each arriving ACK, rather
    than by the number of ACKs that arrive.  We call these changes
    ``Appropriate Byte Counting'' (ABC) [All99].

2.1 Congestion Avoidance

    RFC 2581 specifies that cwnd should be increased by 1 segment per
    round-trip time (RTT) during the congestion avoidance phase of a
    transfer.  Traditionally, TCPs have approximated this increase by
    increasing cwnd by 1/cwnd for each arriving ACK.  This algorithm
    opens cwnd by roughly 1 segment per RTT if the receiver ACKs each
    incoming segment and no ACK loss occurs.  However, if the receiver
    implements delayed ACKs [Bra89] the receiver returns roughly half as
    many ACKs which causes the sender to open cwnd more conservatively
    (by approximately 1 segment every second RTT).  The approach that we
    suggest is to store the number of bytes that have been ACKed in a
    bytes_acked variable in the TCP control block.  When bytes_acked
    becomes greater than or equal to the value of the congestion window,
    bytes_acked is reduced by the value of cwnd.  Next, cwnd is
    incremented by a full-sized segment.  The algorithm suggested above
    is specifically allowed by RFC 2581 during congestion avoidance
    because it opens the window by at most 1 segment per RTT.

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2.2 Slow Start

    RFC 2581 states that the sender increments the congestion window by
    at most 1*SMSS bytes for each arriving acknowledgment during slow
    start.  We propose that a TCP sender SHOULD increase cwnd by the
    number of previously unacknowledged bytes ACKed by each incoming
    acknowledgment provided the increase is not more than L bytes.
    Choosing the limit on the increase, L, is discussed in the next
    subsection.  When the number of previously unacknowledged bytes
    ACKed is less than 1*SMSS bytes or L is less than 1*SMSS bytes this
    proposal is no more aggressive (and possibly less aggressive) than
    allowed by RFC 2581.  However, increasing cwnd by more than 1*SMSS
    bytes in response to a single ACK is more aggressive than allowed by
    RFC 2581.  We believe the more aggressive version of the slow start
    algorithm still falls under the ``conservation of packets''
    principle outlined in [Jac88] and is safe for experimentation in
    shared networks provided an appropriate limit is applied (see next

2.3 Choosing the Limit

    The limit, L, chosen for the cwnd increase during slow start
    controls the aggressiveness of the algorithm.  Choosing L=1*SMSS
    bytes provides behavior that is no more aggressive than allowed by
    RFC 2581.  However, ABC with L=1*SMSS bytes is more conservative in
    a number of key ways (as discussed in the next section) and
    therefore, we believe that even though with L=1*SMSS bytes TCP
    stacks will see little performance benefit, ABC SHOULD be used.

    A very large L could potentially lead to large line-rate bursts of
    traffic in the face of a large amount of ACK loss or in the case
    when the receiver sends ``stretch ACKs'' (ACKs for more than the two
    full-sized segments allowed by the delayed ACK algorithm) [Pax97].

    This documents suggest that TCP implementations SHOULD use L=2*SMSS
    bytes to balance between being conservative (L=1*SMSS bytes) and
    potentially being very aggressive.  In addition, L=2*SMSS bytes
    exactly balances the negative impact of the delayed ACK algorithm
    (as discussed in more detail in section 3.2).  Note that when
    L=2*SMSS bytes cwnd growth is roughly the same as the case when the
    standard algorithms are used in conjunction with a receiver that
    transmits an ACK for each incoming segment.

    The exception to the above suggestion is during a slow start phase
    that follows a retransmission timeout (RTO).  In this situation, a
    TCP MUST use L=1*SMSS as specified in RFC 2581 since ACKs for large
    amount of previously unacknowledged data are common during this
    phase of a transfer.  These ACKs do not necessarily indicate how
    much data has left the network in the last RTT and therefore ABC
    cannot accurately determine how much to increase cwnd.  As an
    example, say segment N is dropped by the network and segments N+1
    and N+2 arrive successfully at the receiver.  The sender will
    receive only two duplicate ACKs and therefore must rely on the
    retransmission timer (RTO) to detect the loss.  When the RTO expires

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    segment N is retransmitted.  The ACK sent in response to the
    retransmission will be for segment N+2.  However, this ACK does not
    indicate that three segments have left the network in the last RTT,
    but rather only a single segment left the network.  Therefore, the
    appropriate cwnd increment is at most 1*SMSS bytes.

3   Advantages

    This section outlines several advantages of using the ABC algorithm
    to increase cwnd, rather than the standard ACK counting algorithm
    given in [RFC2581].

3.1 More Appropriate Congestion Window Increase

    The ABC algorithm outlined in section 2 increases TCP's cwnd in
    proportion to the amount of data actually sent into the network.
    ACK counting, on the other hand, increments cwnd by a constant upon
    the arrival of each ACK.  For instance, consider a telnet connection
    in which ACKs generally cover only a few bytes of data, but cwnd is
    increased by 1*SMSS bytes for each ACK received.  When a large
    amount of data needs to be transmitted (e.g., displaying a large
    file) the data is sent in one large burst because the cwnd grows by
    1*SMSS bytes per ACK rather than based on the actual amount of
    capacity used.  Such a line-rate burst of data can potentially cause
    a large amount of segment loss.

    Congestion Window Validation (CWV) [RFC2861] helps the above problem
    as well.  CWV limits the amount of unused cwnd a TCP connection can
    accumulate.  ABC can be used in conjunction with CWV to obtain an
    accurate measure of the network path.

3.2 Mitigate the Impact of Delayed ACKs and Lost ACKs

    Delayed ACKs [RFC1122,RFC2581] allow a TCP receiver to refrain from
    sending an ACK for each incoming segment.  However, a receiver
    SHOULD send an ACK for every second full-sized segment that arrives.
    Furthermore, a receiver MUST NOT withhold an ACK for more than 500
    ms.  By reducing the number of ACKs sent to the data originator the
    receiver is slowing the growth of the congestion window under an ACK
    counting system.  Using ABC with L=2*SMSS bytes can roughly negate
    the negative impact imposed by delayed ACKs by allowing cwnd to be
    increased for ACKs that were withheld by the receiver.  This allows
    the congestion window to grow in a manner similar to the case when
    the receiver ACKs each incoming segment, but without adding extra
    traffic to the network.  Simulation studies have shown increased
    throughput when a TCP sender uses ABC when compared to the standard
    ACK counting algorithm [All99], especially for short transfers that
    never leave the initial slow start period.

    Note that delayed ACKs should not be an issue during slow
    start-based loss recovery, as RFC 2581 recommends that receivers not
    delay ACKs that cover out-of-order segments.  Therefore, as
    discussed above, ABC with L > 1*SMSS is inappropriate for such slow
    start based loss recovery and MUST NOT be used.

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3.3 Prevents Attacks from Misbehaving Receivers

    [SCWA99] outlines several methods for a receiver to induce a TCP
    sender into violating congestion control and transmitting data at a
    potentially inappropriate rate.  One of the outlined attacks is
    ``ACK Splitting''.  This scheme involves the receiver sending
    multiple ACKs for each incoming data segment, each ACKing only a
    small portion of the original TCP data segment.  Since TCP senders
    have traditionally used ACK counting to increase cwnd, ACK splitting
    causes inappropriately rapid cwnd growth and, in turn, a potentially
    inappropriate sending rate.  A TCP sender that uses ABC can prevent
    this attack from being used to undermine standard congestion control
    because the cwnd increase is based on the number of bytes ACKed,
    rather than the number of ACKs received.

    To prevent misbehaving receivers from inducing inappropriate sender
    behavior we suggest TCP implementation use ABC, even if L=1*SMSS
    bytes (i.e., not allowing ABC to provide more aggressive cwnd growth
    than allowed by RFC 2581).

4   Disadvantages

    The main disadvantages of using ABC with L=2*SMSS bytes are an
    increase in the burstiness of TCP and a small increase in the
    overall loss rate.  [All98] discusses the two ways that ABC
    increases the burstiness of the TCP sender.  First, the ``micro
    burstiness'' of the connection is increased.  In other words, the
    number of segments sent in response to each incoming ACK is
    increased by at most 1 segment when using ABC with L=2*SMSS bytes in
    conjunction with a receiver that is sending delayed ACKs.  During
    slow start this translates into an increase from sending 2
    back-to-back segments to sending 3 back-to-back packets in response
    to an ACK for a single packet.  Or, an increase of 3 packets to 4
    packets when receiving a delayed ACK for two outstanding packets.
    Note that ACK loss can cause larger bursts.  However, ABC only
    increases the burst size by at most 1*SMSS bytes per ACK received
    when compared to the standard behavior.  This slight increase in the
    burstiness should only cause problems for devices that have very
    small buffers.  In addition, ABC increases the ``macro burstiness''
    of the TCP sender in response to delayed ACKs.  Rather than
    increasing cwnd by roughly 1.5 times per RTT, ABC roughly doubles
    the congestion window every RTT.  However, doubling cwnd every RTT
    fits within the spirit of slow start, as originally outlined

    With the increased burstiness comes a modest increase in the loss
    rate for a TCP connection employing ABC (see the next section for a
    short discussion on the fairness of ABC to non-ABC flows).  The
    additional loss can be directly attributable to the increased
    aggressiveness of ABC.  During slow start cwnd is increased more
    rapidly and therefore when loss occurs cwnd is larger and more drops
    are likely.  Similarly, a congestion avoidance cycle takes roughly
    half as long when using ABC and delayed ACKs when compared to an ACK

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    counting implementation.  In other words, a TCP sender reaches the
    capacity of the network path, drops a packet and reduces the
    congestion window by half roughly twice as often when using ABC.
    However, as discussed above, in spite of the additional loss an ABC
    TCP sender generally obtains better overall performance than a
    non-ABC TCP.

5   Fairness Considerations

    [All99] presents several simulations conducted to measure the impact
    of ABC on competing traffic (both ABC and non-ABC).  The experiments
    show that while ABC increases the drop rate for the connection using
    ABC, competing traffic is not greatly effected.  The experiments
    show that standard TCP and ABC both obtain roughly the same
    throughput regardless of the variant of the competing traffic.  The
    simulations also reaffirm that ABC outperforms non-ABC TCP in an
    environment with varying types of TCP connections.

6   Security Considerations

    As discussed in section 3.3 ABC protects a TCP from a misbehaving
    receiver that induces the sender into transmitting at an
    inappropriate rate with an ``ACK splitting'' attack.  This, in turn,
    protects the network from an overly aggressive sender.

7   Conclusions

    We RECOMMEND that all TCP stacks be modified to use ABC with
    L=1*SMSS bytes.  Furthermore, simulations of ABC with L=2*SMSS bytes
    show a promising performance improvement that we encourage
    researchers to experiment with in the Internet.


    This draft has benefited from discussions with and encouragement
    from Sally Floyd.


    [All98] Mark Allman. TCP Byte Counting Refinements. ACM Computer
        Communication Review, 29(3), July 1999.

    [All99] Mark Allman. TCP Byte Counting Refinements. ACM Computer
        Communication Review, 29(3), July 1999.

    [Jac88] Van Jacobson.  Congestion Avoidance and Control.  ACM
        SIGCOMM 1988.

    [Pax97] Vern Paxson.  Automated Packet Trace Analysis of TCP
        Implementations.  ACM SIGCOMM, September 1997.

    [RFC1122] B. Braden, ed., Requirements for Internet Hosts --
        Communication Layers, RFC 1122, Oct. 1989.

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    [RFC2119] S. Bradner, Key words for use in RFCs to Indicate
        Requirement Levels, BCP 14, RFC 2119, March 1997.

    [RFC2581] Mark Allman, Vern Paxson, W. Richard Stevens. TCP
        Congestion Control, April 1999. RFC 2581.

    [RFC2861] Mark Handley, Jitendra Padhye, Sally Floyd.  TCP
        Congestion Window Validation, June 2000.  RFC 2861.

    [SCWA99] Stefan Savage, Neal Cardwell, David Wetherall, Tom
        Anderson.  TCP Congestion Control with a Misbehaving Receiver.
        ACM Computer Communication Review, 29(5), October 1999.

Author's Addresses:

    Mark Allman
    NASA Glenn Research Center/BBN Technologies
    Lewis Field
    21000 Brookpark Rd.  MS 54-2
    Cleveland, OH  44135
    Phone: 216-433-6586
    Fax: 216-433-8705

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