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Versions: 00

 TCP Maintenance Working Group                                   K. Yang
Internet-Draft                                               N. Cardwell
Intended status: Standards Track                                Y. Cheng
Expires: May 6, 2021                                          E. Dumazet
                                                             Google, Inc
                                                        November 2, 2020


                 TCP ETS: Extensible Timestamp Options
                         draft-yang-tcpm-ets-00

Abstract

   This document presents ETS: an Extensible TimeStamps option for TCP.
   It allows hosts to use microseconds as the unit for timestamps to
   improve the precision of timestamps, and advertise the maximum ACK
   delay for its own delayed ACK mechanism.  Furthermore, it extends the
   information provided in the [RFC7323] TCP Timestamps Option by
   including the receiver delay in the TSecr echoing, so that the
   receiver of the ACK is able to more accurately estimate the portion
   of the RTT that resulted from time traveling through the network.
   The ETS option format is extensible, so that future extensions can
   add further information without the overhead of extra TCP option kind
   and length fields.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 6, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.  In this document, these words will appear
   with that interpretation only when in UPPER CASE.  Lower case uses of
   these words are not to be interpreted as carrying [RFC2119]
   significance.

2.  Introduction

   Accurate round-trip time (RTT) estimation is necessary for TCP to
   adapt to diverse and dynamic traffic conditions.

   The TCP timestamp option specified in [RFC7323] is designed largely
   for RTT samples intended for computing TCP's retransmission (RTO)
   timer [RFC6298].

   Some congestion control algorithms may wish to use a form of RTT
   measurement as one of several congestion signals, since elevated RTT
   measurements can reflect increases in network queueing delays.  For
   example, the Swift congestion control algorithm [KDJWWM20],
   successfully deployed in data-center environments, requires precise
   and accurate measurements of both network and host delays.  However,
   the existing TCP RTT sampling mechanisms that measure the delay
   between data transmission and ACK receipt [RFC6298] do not separate
   network and host delays, and cannot measure the RTT of retransmitted
   data.  Even the TCP timestamp option specified in [RFC7323] is not
   well-suited to use as a congestion signal, for a number of reasons.

   With the TCP Timestamps Option [RFC7323], data senders can measure an
   RTT sample by computing the difference between the data sender's
   current timestamp clock value and the received TSecr value.  However,
   there are some drawbacks in this [RFC7323] measurement method:





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   1.  The TCP endpoint can compute an [RFC7323] RTT measurement only if
       the ACK advances the left edge of the send window, i.e. SND.UNA
       is increased.  (Without this rule, in one-way data flows the RTT
       measured by the receiver would be inflated by gaps in the data
       sending process; see [RFC7323] Section 4.1).

   2.  Even if an ACK advances the left edge of the send window, RTT
       measurements can be greatly inflated by delayed ACKs: the host
       receiving a data segment can delay an ACK segment in hopes of
       piggybacking an ACK on the next outgoing data segment.  This
       delay can be as large as 500 milliseconds (as in [RFC1122]
       Section 4.2.3.2).

   3.  There is delay included in the RTT measurement that is irrelevant
       to network queuing, e.g. host-side transmit and receive delays,
       including delays for waking CPUs from power management
       "C-states".

   4.  [RFC7323] specifies a "timestamp clock frequency in the range 1
       ms to 1 sec per tick" ([RFC732] Section 5.4).  But today's
       datacenter networks are capable of delivering network packets
       within tens of microseconds [BMPR17].  In such networks the lower
       bound of 1 ms limits the usefulness of [RFC7323] timestamps.
       First, [RFC7323] is incapable of accurate RTT measurements in
       such networks.  Second, [RFC7323] timestamps are not suitable for
       reverting spurious loss recovery and congestion control responses
       [RFC4015] due to packet reordering in such networks.

   In many of the cases above, an RTT sample computed using [RFC7323]
   can be inflated for reasons other than network queuing.  It is
   difficult for the ACK receiver to infer how long the non-network
   delay was, which makes it hard to use an [RFC7323] RTT measurement as
   a clean signal for congestion control.

   Delayed ACKs, as mentioned above, are particularly problematic.  TCP
   receivers typically implement a delayed ACK algorithm.  To avoid
   spurious timeouts due to these delayed ACKs, TCP senders can adapt to
   this delayed ACK behavior by guessing the maximum delayed ACK value
   of the remote receiver.  Historically, many implementations tended to
   delay ACKs by up to roughly 200ms [WS95], so some implementations
   have correspondingly used a minimum RTO of 200ms.  However, this
   imposes a latency penalty that is very large compared to RTTs in some
   of today's datacenter networks.  If receivers had the ability to
   signal their maximum delayed ACK timer delay, this could allow faster
   timer-based loss recovery, while still avoiding spurious timeouts.

   This document presents ETS: an Extensible TimeStamps option for TCP.
   ETS extends the information provided in the [RFC7323] TCP Timestamps



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   Option, adding several features.  First, ETS allows connections to
   use microseconds as the unit for timestamps, to improve the precision
   of timestamps.  Second, ETS allows endpoints to advertise the maximum
   ACK delay for their own delayed ACK mechanism.  Third, ETS allows
   connections to include information about the delay between data
   receipt and ACK generation, so that the receiver of the ACK is able
   to more accurately estimate the portion of the RTT that resulted from
   time that data and ACK segments spent traveling through the network.
   Fourth, the ETS option format is extensible, so that future
   extensions can add further information without the overhead of extra
   TCP option kind and length fields.

2.1.  High-level Design

   The ETS protocol has two phases: an exchange of ETS options (ETSopt)
   in the negotiation handshake in <SYN> and <SYN,ACK> segments, and
   then ETS options included in all following segments.

   In the negotiation handshake, the two senders exchange their
   MaxACKDel: a hint characterizing the maximum amount of a delay that
   the sender expects to schedule before acknowledging an incoming data
   segment.

   All segments include EcrDel, the delay in the TSecr echoing process
   that was inserted by the data receiver, helping the receiver of an
   ACK to estimate the portion of the RTT delay caused by the network.

   An example of a handshake exchange is illustrated below:

      TCP A (Client)                                      TCP B (Server)
      ______________                                      ______________
      CLOSED                                                      LISTEN

      #1 SYN-SENT     --- <SYN,TSval=X,TSecr=0,
                           EcrDel=0,MaxACKDel=M1> ----------->  SYN-RCVD

                          <SYN,ACK,TSval=Y,TSecr=X, ----------  SYN-RCVD
      #2 ESTABLISHED  <--  EcrDel=E1,MaxACKDel=M2>

      #3 ESTABLISHED  -- <ACK,TSval=Z,TSecr=Y,EcrDel=E2> --> ESTABLISHED

   Active connect: An actively connecting host that wishes to negotiate
   ETSopt MUST include the ETSopt in the <SYN>.  For backward
   compatibility, the endpoint performing the active connect MAY also
   include a [RFC7323] TSopt in the <SYN> segment, so that if the
   passive side or middleboxes do not support and respond to the ETSopt,
   the active and passive sides can proceed with the [RFC7323] TSopt
   negotiation for the connection.



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   Passive connect: For a passively connecting host that is willing to
   proceed with ETSopt negotiation, if the <SYN> includes an ETSopt, the
   host MUST include a TCP ETS option in the initial <SYN,ACK> segment.
   A retransmission of the <SYN,ACK> segment may omit the ETSopt, to
   increase robustness in the presence of middleboxes that block
   segments containing ETSopt.

   Processing of <SYN,ACK> for active connect: If the ETSopt is absent
   from the <SYN,ACK> segment received by the actively connecting
   endpoint, suggesting that the passive endpoint does not support
   ETSopt, or some middlebox has stripped the option from the <SYN,ACK>
   segment, then the actively connecting endpoint MUST disable ETSopt
   for this connection.  In such cases the actively connecting endpoint
   MAY fall back to using [RFC7323] timestamps if both the <SYN> and
   <SYN,ACK> segments include valid [RFC7323] timestamps.

3.  Detailed Protocol

3.1.  Definitions

   The reader is expected to be familiar with the TCP Timestamps Option
   (TSopt), including TSval, TSecr, and TS.Recent [RFC7323].

   Variables introduced by this document are described below:

   MaxACKDel: a hint characterizing the maximum amount of a delay that
   the sender expects to schedule before acknowledging an incoming data
   segment.

   TSval: the value of the sender's timestamp clock when this segment is
   scheduled for transmission, in microseconds.

   TSecr: the echo of TS.recent

   TS.Recent: the recently received TSval sent by the remote TCP
   endpoint in the TSval field of an ETSopt, updated using the TS.Recent
   rules specified in [RFC7323].

   EcrDel: the field quantifying the delay between data receipt and ACK
   generation, so that the receiver of the ACK is able to more
   accurately estimate the NetworkRTT.

   NetworkRTT: the time from when the data segment leaves the sender
   until when it arrives at the receiver, plus the time from when the
   corresponding ACK leaves the (data) receiver until the ACK arrives at
   the data sender (here sender and receiver refer to the TCP layer
   only).




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3.2.  TCP ETS option header format

   The header format for TCP ETS options (ETSopt) is as follows:

               01234567 89012345 67890123 45678901
              +--------+--------+--------+--------+
              |  Kind  | Length |       ExID      |
              +--------+--------+--------+--------+
              |               TSval               |
              +--------+--------+--------+--------+
              |               TSecr               |
              +--------+--------+--------+--------+
              |Un|   EcrDel   |R|    MaxACKDel    |
              +-----------------+-----------------+
              |                 |
             /                   \
        .---'                     `---------------------.
       /                                                 \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Unit  |            EcrDel               |Reserved|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        2 bits |            13 bits              | 1 bit

      Kind: 1 byte, has value 254,
            TCP experimental option codepoint [RFC6994]
      Length: 1 byte option length, value is 16 if SYN bit is set,
              otherwise 14 if SYN bit is not set
              (value MAY be higher in later ETSOpt protocol versions).
      ExID: 2 byte [RFC6994] experiment ID; MUST be 0x4554.
      TSval and TSecr: 32 bits each, have the same definition as
                    [RFC7323] except that both are in microseconds.
      EcrDelUnit: 2 bits, has value:
                  0: indicates EcrDel is in microsecond units
                  1: indicates EcrDel is in millisecond units
                  2: indicates EcrDel is invalid
                  3: reserved
      EcrDel: 13 bits, the value of EcrDel.
      Reserved: 1 bit, in this protocol version; sender MUST set to 0;
                receiver MUST ignore field and tolerate 0 or 1
      MaxACKDel: 16 bits, max expected ACK delay in microseconds,
                 MUST only be present when SYN bit is set.

   The semantics of the option fields are as follows:

   TSval: The TSval field contains the value of the sender host's
   timestamp clock, in microseconds, at the time the sender schedules
   transmission of the segment.




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   TSecr: The TSecr field contains the current value of TS.Recent, the
   recently received TSval sent by the remote TCP that is recorded using
   the TS.recent update algorithm described in [RFC7323], section 4.3.
   TSecr is only valid when the ACK bit is set.  When the ACK bit is not
   set, senders MUST set this field to 0, and receivers MUST ignore the
   value in this field (and MUST tolerate any value in this field).

   MaxACKDel: When the SYN bit is set, the sender advertises MaxACKDel
   as a hint characterizing the maximum amount of a delay that the
   sender expects to apply before ACKing an incoming data segment.  The
   unit of MaxACKDel is microseconds.  Field MaxACKDel MUST be set to
   0xFFFE if the value is equal to or greater than 0xFFFE.  If the
   MaxACKDel value is invalid, or the sender does not want to advertise
   this value, field MaxACKDel MUST be set to the max value allowed by
   the field, i.e. 0xFFFF.  When a host performing an active TCP
   connection attempt is willing to enable ETS, in a <SYN> segment it
   MUST include a TCP ETS option (ETSopt) with the MaxACKDel field.  If
   the SYN bit is not set, the field MaxACKDel MUST NOT be present in
   the option.

   EcrDel: Field EcrDel contains the delay inserted by the receiver in
   this TSecr echoing process.  Field EcrDel is only valid when the ACK
   bit is set, otherwise MUST be set to 0 by the sender and ignored by
   the receiver.  When the ACK bit is set, the sender computes the
   EcrDel using the following algorithm:

     (1) When a <SYN> or non-empty data segment SEG is received:
             If SYN is set, or SEG.TSval is after TS.Latest:
                 TS.Latest = SEG.TSval
                 TS.LatestClock = ArrivalTime of SEG
     (2) When an ETSopt is sent:
             TSecr = TS.Recent (as in [RFC7323])
             LatestACKDel = ACKSendTime - TS.LatestClock
             TSecrAge = TS.Latest - TSecr
             EcrDel = LatestACKDel + TSecrAge

   In (1), a non-empty data segment is defined as a segment containing a
   non-empty data payload.  The timestamps are in a modular 32-bit
   space, and a timestamp s is "after" timestamp t if and only if 0 < (s
   - t) < 2^31.

   In (2), the semantics of the two temporary variables LatestACKDel and
   TSecrAge are as follows:

   LatestACKDel, the time from the arrival of the segment with the
   latest TSval until the time when an ACK segment is sent.

   TSecrAge, the age of TSecr compared to TS.Latest.



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   We discuss how the ACK receiver can estimate the latest NetworkRTT
   using EcrDel in the next section.

3.3.  Estimating the Network RTT

   NetworkRTT is the time from when the data segment leaves the sender
   until when it arrives at the receiver, plus the time from when the
   corresponding ACK leaves the (data) receiver until the ACK arrives at
   the data sender (here sender and receiver refer to the TCP layer
   only).

   With EcrDel in ETSopt when a data sender (TCP A) receives an ACK
   segment with ETSopt from the remote endpoint (TCP B), NetworkRTT can
   be eatimated by:

     NetworkRTT = ACKArrivalTime - SEG.TSecr - SEG.EcrDel

   where ACKArrivalTime is the TCP A's timestamp clock when the ACK
   segment arrives, and SEG.TSecr and SEG.EcrDel are the values from the
   incoming ETSopt.

   This gives us the latest network RTT measurement by:

   NetworkRTT = ACKArrivalTime - SEG.TSecr - SEG.EcrDel
              = ACKArrivalTime - SEG.TSecr - B.TSecrAge - B.LatestACKDel
              = ACKArrivalTime - (SEG.TSecr + B.TSecrAge)
                - B.LatestACKDel
              = ACKArrivalTime - (SEG.TSecr + B.TS.Latest - SEG.TSecr)
                - B.LatestACKDel
              = ACKArrivalTime - B.TS.Latest - B.LatestACKDel

   In the calculation above, B.TSecrAge, B.TS.Latest and B.LatestACKDel
   are TSecrAge, TS.Latest and LatestACKDel on TCP B respectively at the
   time this ACK segment was sent.

   At the time TCP B sends an ETSopt, TS.Latest can be different from
   TS.Recent, e.g. if there is a sequence hole in B's receiving window.
   If this is the case, the NetworkRTT measures the network RTT of the
   latest segment received by TCP B.

   The following example shows how NetworkRTT is computed when TS.Latest
   is different from TS.Recent:









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      TCP A                                                        TCP B
      ______________                                      ______________
                      -- <TSval=1> -->                    arrives at t=2
                                                   TS.Latest=TS.Recent=1
                                                        TS.LatestClock=2
                      -- <TSval=2> --> lost
                      -- <TSval=3> -->                   arrives at t=10
                                                TS.Recent=1, TS.Latest=3
                                                       TS.LatestClock=10
     arrive at t=11   <-- <ACK, TSecr=1, EcrDel=2> --   Send ACK at t=10

   In this example, EcrDel is LatestACKDel + TSecrAge = (10 - 10) + (3 -
   1) = 2, and the NetworkRTT from the last <ACK> segment is estimated
   as 11 - 1 - 2 = 8, which is the network RTT of the segment sent with
   TSval = 3.

   In order to make use of this NetworkRTT estimate as a clean signal
   that more precisely reflects network queuing, it is RECOMMENDED that
   timestamps ACKArrivalTime and TS.LatestClock use the time at which
   the segment arrives at the host, e.g. the time the Network Interface
   Controller (NIC) receives the segment, if the NIC supports hardware
   receive timestamping.  Within this context, NetworkRTT is then
   defined as the time from when the data segment leaves the sender's
   TCP until when it is received at the receiver's NIC, plus the time
   from when the corresponding ACK leaves the (data) receiver's TCP
   until the ACK is received at the data sender's NIC.

4.  Interaction with other TCP Mechanisms

4.1.  Interaction with PAWS

   Protection Against Wrapped Sequences (PAWS), introduced by [RFC7323]
   Section 5, is a mechanism to reject old duplicate segments that might
   corrupt an open TCP connection.  In the PAWS mechanism, a segment can
   be discarded as an old duplicate if it is received with a timestamp
   SEG.TSval that is "before" some timestamps recently received on this
   connection.

   As in [RFC7323], ETS receivers need to exercise care to avoid
   spurious PAWS discards due to wrapping 32-bit timestamp values during
   periods in which the connection is idle.  When microsecond units are
   used, as in ETS, the 32-bit timestamp could trigger wrapping issues
   and spurious PAWS discards after 2^31 ticks of idleness, which is
   around 2147 seconds (or around 35.7 minutes).  To prevent a false
   positive PAWS rejection of a valid segment, an ETS receiver MUST skip
   the PAWS check for the first arriving segment after the timestamp
   used by PAWS, e.g.  TS.Recent, has not been updated for 2147 seconds
   or more.



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4.2.  Eifel Considerations

   The Eifel Detection Algorithm [RFC3522] detects a spurious recovery
   by comparing a received TSecr to RetransmitTS, the value of the TSval
   in the retransmit sent when loss recovery is initiated.  ETS allows
   Eifel to work as-is because the fields TSval and TSecr in the ETSopt
   have the same semantics as in TSopt [RFC7323].  Further in sub-
   millisecond environment, ETS microsecond precision is more effective
   detecting spurious retransmission compared to TSopt's more coarse
   unit.

4.3.  RTO Calculation Considerations

   The RTT measurement used in the calculation of RTO (retransmission
   timeout) [RFC6298] stays the same as described in [RFC7323].  It is
   NOT RECOMMENDED to use only NetworkRTT measurements for RTO
   calculation because RTO needs to include the host side delays to
   avoid spurious RTO events due to host delays.

   With MaxACKDel information exchanged in the ETSopt of <SYN> and
   <SYN,ACK>, senders MAY impose a minimum RTO that is greater than or
   equal to the MaxACKDel.

4.4.  SACK Considerations

   The ETSopt has a length of 14 bytes on non-<SYN> segments.  This
   leaves a remaining space of 26 bytes for other TCP options.  A SACK
   option [RFC2018] with at most 3 SACK blocks is able to coexist with
   ETSopt in a single TCP segment as with TSopt.

4.5.  Interaction with Middleboxes

   [HNRGHT11] shows that middleboxes could drop an unrecognized TCP
   option or even drop the whole segment.

   In order to fall back on [RFC7323] TSopt, the sender MAY include a
   [RFC7323] TSopt in the <SYN> and <SYN,ACK> segments, so that the
   [RFC7323] TSopt can be adopted when the ETSopt is stripped by a
   middlebox.

   Once an expected ETSopt is missing from an incoming segment, the
   sender MUST NOT include an ETSopt for all future segments of this TCP
   connection.  An implementation could negatively cache such incidents
   to avoid using ETS on these hosts or routes on future connections.







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5.  IANA Considerations

   This document specifies a new TCP option that uses the shared
   experimental options format [RFC6994], with ExID in network-standard
   byte order.

   The authors plan to request the allocation of ExID value 0x4554 for
   the TCP option specified in this document.

6.  Security Considerations

   A malicious receiver can manipulate the sender's network RTT
   estimated by forging an EcrDel value.  However this does not
   introduce a new vulnerability relative to the Timestamp option
   [RFC7323], because a malicious receiver could already forge the TSecr
   field to manipulate the RTT measured by the other side.

7.  References

7.1.  Normative References

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

   [RFC6994]  Touch, J., "Shared Use of Experimental TCP Options",
              August 2013.

7.2.  Informative References

   [BMPR17]   Barroso, L., Marty, M., Patterson, D., and P. Ranganathan,
              "Attack of the Killer Microseconds", Communications of the
              ACM , April 2017.

   [HNRGHT11]
              Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A.,
              Handley, M., and H. Tokuda, "Is it still possible to
              extend TCP?", InProceedings of the 2011 ACM SIGCOMM
              conference on Internet measurement conference 2011 Nov 2
              (pp.181-194) , 2011.












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   [KDJWWM20]
              Kumar, G., Dukkipati, N., Jang, K., Wassel, HM., Wu, X.,
              Montazeri, B., Wang, Y., Springborn, K., Alfeld, C., Ryan,
              M., and D. Wetherall, "Swift: Delay is Simple and
              Effective for Congestion Control in the Datacenter",
              InProceedings of the Annual conference of the ACM Special
              Interest Group on Data Communication on the applications,
              technologies, architectures, and protocols for computer
              communication 2020 Jul 30 (pp514-528) , 2020.

   [RFC1122]  Braden, R., "Requirements for Internet hosts-communication
              layers", October 1989.

   [RFC3522]  Ludwig, R. and M. Meyer, "The Eifel detection algorithm
              for TCP", April 2003.

   [RFC4015]  Ludwig, R. and A. Gurtov, "The Eifel response algorithm
              for TCP", February 2005.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", June 2011.

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, "TCP Extensions for High Performance",
              September 2014.

   [WS95]     Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2:
              The Implementation", 1995.

Authors' Addresses

   Kevin (Yudong) Yang
   Google, Inc

   Email:  yyd@google.com


   Neal Cardwell
   Google, Inc

   Email:  ncardwell@google.com


   Yuchung Cheng
   Google, Inc

   Email:  ycheng@google.com




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   Eric Dumazet
   Google, Inc

   Email:  edumazet@google.com















































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