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Versions: 00 01 02 03 RFC 6069

TCP Maintenance and Minor                                  A. Zimmermann
Extensions (TCPM) WG                                        A. Hannemann
Internet-Draft                                    RWTH Aachen University
Intended status: Experimental                             March 30, 2010
Expires: October 1, 2010


   Making TCP more Robust to Long Connectivity Disruptions (TCP-LCD)
                       draft-ietf-tcpm-tcp-lcd-01

Abstract

   Disruptions in end-to-end path connectivity, which last longer than
   one retransmission timeout, cause suboptimal TCP performance.  The
   reason for this performance degradation is that TCP interprets
   segment loss induced by long connectivity disruptions as a sign of
   congestion, resulting in repeated retransmission timer backoffs.
   This, in turn, leads to a delayed detection of the re-establishment
   of the connection since TCP waits for the next retransmission timeout
   before it attempts a retransmission.

   This document proposes an algorithm to make TCP more robust to long
   connectivity disruptions (TCP-LCD).  It describes how standard ICMP
   messages can be exploited during timeout-based loss recovery to
   disambiguate true congestion loss from non-congestion loss caused by
   connectivity disruptions.  Moreover, a revert strategy of the
   retransmission timer is specified that enables a more prompt
   detection of whether or not the connectivity to a previously
   disconnected peer node has been restored.  TCP-LCD is a TCP sender-
   only modification that effectively improves TCP performance in case
   of connectivity disruptions.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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."

   The list of current Internet-Drafts can be accessed at



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   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on October 1, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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 BSD License.






























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Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Connectivity Disruption Indication . . . . . . . . . . . . . .  6
   4.  Connectivity Disruption Reaction . . . . . . . . . . . . . . .  8
     4.1.  Basic Idea . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  Algorithm Details  . . . . . . . . . . . . . . . . . . . .  8
   5.  Discussion of TCP-LCD  . . . . . . . . . . . . . . . . . . . . 11
     5.1.  Retransmission Ambiguity . . . . . . . . . . . . . . . . . 12
     5.2.  Wrapped Sequence Numbers . . . . . . . . . . . . . . . . . 13
     5.3.  Packet Duplication . . . . . . . . . . . . . . . . . . . . 14
     5.4.  Probing Frequency  . . . . . . . . . . . . . . . . . . . . 14
     5.5.  Reaction during Connection Establishment . . . . . . . . . 14
     5.6.  Reaction in Steady-State . . . . . . . . . . . . . . . . . 15
   6.  Dissolving Ambiguity Issues (the Safe Variant) . . . . . . . . 15
   7.  Interoperability Issues  . . . . . . . . . . . . . . . . . . . 17
     7.1.  Detection of TCP Connection Failures . . . . . . . . . . . 17
     7.2.  Explicit Congestion Notification . . . . . . . . . . . . . 17
     7.3.  ICMP for IP version 6  . . . . . . . . . . . . . . . . . . 18
     7.4.  TCP-LCD and IP Tunnels . . . . . . . . . . . . . . . . . . 18
   8.  Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 19
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 21
     12.2. Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Changes from previous versions of the draft . . . . . 24
     A.1.  Changes from draft-ietf-tcpm-tcp-lcd-00  . . . . . . . . . 24
     A.2.  Changes from draft-zimmermann-tcp-lcd-02 . . . . . . . . . 24
     A.3.  Changes from draft-zimmermann-tcp-lcd-01 . . . . . . . . . 25
     A.4.  Changes from draft-zimmermann-tcp-lcd-00 . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25

















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1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The reader should be familiar with the algorithm and terminology from
   [RFC2988], which defines the standard algorithm Transmission Control
   Protocol (TCP) senders are required to use to compute and manage
   their retransmission timer.  In this document the terms
   "retransmission timer" and "retransmission timeout" are used as
   defined in [RFC2988].  The retransmission timer ensures data delivery
   in the absence of any feedback from the receiver.  The duration of
   this timer is referred to as retransmission timeout (RTO).

   As defined in [RFC0793], the term "acceptable acknowledgment (ACK)"
   refers to a TCP segment that acknowledges previously unacknowledged
   data.  The TCP sender state variable "SND.UNA" and the current
   segment variable "SEG.SEQ" are used as defined in [RFC0793].  SND.UNA
   holds the segment sequence number of earliest segment that has not
   been acknowledged by the TCP receiver (the oldest outstanding
   segment).  SEG.SEQ is the segment sequence number of a given segment.

   For the purposes of this specification we define the term "timeout-
   based loss recovery" that refers to the state, which a TCP sender
   enters upon the first timeout of the oldest outstanding segment
   (SND.UNA) and leaves upon the arrival of the *first* acceptable ACK.
   It is important to note that other documents use a different
   interpretation of the term "timeout-based loss recovery".  For
   example the NewReno modification to TCP's Fast Recovery algorithm
   [RFC3782] extents the period a TCP sender remains in timeout-based
   loss recovery compared to the one defined in this document.  This is
   because [RFC3782] attempts to avoid unnecessary multiple Fast
   Retransmits that can occur after an RTO.


2.  Introduction

   Connectivity disruptions can occur in many different situations.  The
   frequency of connectivity disruptions depends on the property of the
   end-to-end path between the communicating hosts.  While connectivity
   disruptions can occur in traditional wired networks too, e.g., caused
   by an unplugged network cable, the likelihood of occurrence is
   significantly higher in wireless (multi-hop) networks.  Especially,
   end-host mobility, network topology changes, and wireless
   interferences are crucial factors.  In the case of the Transmission
   Control Protocol (TCP) [RFC0793], the performance of the connection
   can experience a significant reduction compared to a permanently



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   connected path [SESB05].  This is because TCP, which was originally
   designed to operate in fixed and wired networks, generally assumes
   that the end-to-end path connectivity is relatively stable over the
   connection's lifetime.

   Depending on their duration connectivity disruptions can be
   classified into two groups [I-D.schuetz-tcpm-tcp-rlci]: "short" and
   "long".  A connectivity disruption is "short" if connectivity returns
   before the retransmission timer fires for the first time.  In this
   case, TCP recovers lost data segments through Fast Retransmit and
   lost acknowledgments (ACK) through successfully delivered later ACKs.
   Connectivity disruptions are declared as "long" for a given TCP
   connection if the retransmission timer fires at least once before
   connectivity is resumed.  Whether or not path characteristics, like
   the round trip time (RTT) or the available bandwidth, have changed
   when connectivity resumes after a disruption is another important
   aspect for TCP's retransmission scheme [I-D.schuetz-tcpm-tcp-rlci].

   This document improves TCP's behavior in case of "long connectivity
   disruptions".  In particular, it focuses on the period "prior" to the
   re-establishment of the connectivity to a previously disconnected
   peer node.  The document does not describe any modifications of TCP's
   behavior and its congestion control mechanisms [RFC5681] "after"
   connectivity has been restored.

   When a long connectivity disruption occurs on a TCP connection the
   TCP sender eventually does not receive any more acknowledgments.
   After the retransmission timer expires, the TCP sender enters the
   timeout-based loss recovery and declares the oldest outstanding
   segment (SND.UNA) as lost.  Since TCP tightly couples reliability and
   congestion control, the retransmission of SND.UNA is triggered
   together with the reduction of the transmission rate.  This is based
   on the assumption that segment loss is an indication of congestion
   [RFC5681].  As long as the connectivity disruption persists, TCP will
   repeat this procedure until the oldest outstanding segment has
   successfully been acknowledged, or until the connection has timed
   out.  TCP implementations that follow the recommended retransmission
   timeout (RTO) management of RFC 2988 [RFC2988] double the RTO after
   each retransmission attempt.  However, the RTO's growth may be
   bounded by an upper limit, the maximum RTO, which is at least 60s,
   but may be longer: Linux, for example, uses 120s.  If connectivity is
   restored between two retransmission attempts, TCP still has to wait
   until the retransmission timer expires before resuming transmission,
   since it simply does not have any means to know if the connectivity
   has been re-established.  Therefore, depending on when connectivity
   becomes available again, this can waste up to a maximum RTO of
   possible transmission time.




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   This retransmission behavior is not efficient, especially in
   scenarios with long connectivity disruptions.  In the ideal case, TCP
   would attempt a retransmission as soon as connectivity to its peer
   has been re-established.  In this document, we specify a TCP sender-
   only modification to provide robustness to long connectivity
   disruptions (TCP-LCD).  The memo describes how the standard Internet
   Control Message Protocol (ICMP) can be exploited during timeout-based
   loss recovery to identify non-congestion loss caused by long
   connectivity disruptions.  TCP-LCD's revert strategy of the
   retransmission timer enables higher-frequency retransmissions and
   thereby a prompt detection when connectivity to a previously
   disconnected peer node has been restored.  If no congestion is
   present, TCP-LCD approaches the ideal behavior.


3.  Connectivity Disruption Indication

   If the queue of an intermediate router experiencing a link outage can
   buffer all incoming packets, a connectivity disruption will only
   cause a variation in delay, which is handled well by TCP
   implementations using either Eifel [RFC3522], [RFC4015] or Forward
   RTO-Recovery (F-RTO) [RFC5682].  However, if the link outage lasts
   for too long, the router experiencing the link outage is forced to
   drop packets, and finally to discard the according route.  Means to
   detect such link outages include reacting on failed address
   resolution protocol (ARP) [RFC0826] queries, unsuccessful link
   sensing, and the like.  However, this is solely in the responsibility
   of the respective router.

      Note: The focus of this memo is on introducing a method how ICMP
      messages may be exploited to improve TCP's performance; how
      different physical and link layer mechanisms below the network
      layer may trigger ICMP destination unreachable messages are out of
      scope of this memo.

   Provided that no other route to the specific destination exists the
   router will notify the corresponding sending host about the dropped
   packets via ICMP destination unreachable messages of code 0 (net
   unreachable) or code 1 (host unreachable) [RFC1812].  Therefore, the
   sending host can use the ICMP destination unreachable messages of
   these codes as an indication for a connectivity disruption, since the
   reception of these messages provide evidence that packets were
   dropped due to a link outage.

   Note that there are also other ICMP destination unreachable messages
   with different codes.  Some of them are candidates for connectivity
   disruption indications, too, but need further investigation.  For
   example, ICMP destination unreachable messages with code 5 (source



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   route failed), code 11 (net unreachable for TOS), or code 12 (host
   unreachable for TOS) [RFC1812].  On the other hand, codes that flag
   hard errors are of no use for the proposed scheme, since TCP should
   abort the connection when those are received [RFC1122].  In the
   following, the term "ICMP unreachable message" is used as synonym for
   ICMP destination unreachable messages of code 0 or code 1.

   The accurate interpretation of ICMP unreachable messages as a
   connectivity disruption indication is complicated by the following
   two peculiarities of ICMP messages.  Firstly, they do not necessarily
   operate on the same timescale as the packets, i.e., TCP segments that
   elicited them.  When a router drops a packet due to a missing route
   it will not necessarily send an ICMP unreachable message immediately,
   but will rather queue it for later delivery.  Secondly, ICMP messages
   are subject to rate limiting, e.g., when a router drops a whole
   window of data due to a link outage, it will hardly send as many ICMP
   unreachable messages as it dropped TCP segments.  Depending on the
   load of the router it may even send no ICMP unreachable messages at
   all.  Both peculiarities originate from [RFC1812].

   Fortunately, according to [RFC0792], ICMP unreachable messages have
   to contain in their body the entire Internet Protocol (IP) header
   [RFC0791] of the datagram eliciting the ICMP unreachable message,
   plus the first 64 bits of the payload of that datagram.  This allows
   the sending host to match the ICMP error message to the transport
   that elicited it.  RFC 1812 [RFC1812] augments the requirements and
   states that ICMP messages should contain as much of the original
   datagram as possible without the length of the ICMP datagram
   exceeding 576 bytes.  Therefore, in case of TCP, at least the source
   port number, the destination port number, and the 32-bit TCP sequence
   number are included.  This allows the originating TCP to demultiplex
   the received ICMP message and to identify the faulty connection.
   Moreover, it can identify which segment of the respective connection
   triggered the ICMP unreachable message, unless there are several
   segments in-flight with the same sequence number (see Section 5.1).

   A connectivity disruption indication in form of an ICMP unreachable
   message associated with a presumably lost TCP segment provides strong
   evidence that the segment was not dropped due to congestion, but was
   successfully delivered to the temporary end-point of the employed
   path, i.e., the reporting router.  It therefore did not witness any
   congestion at least on that part of the path that was traversed by
   both the TCP segment eliciting the ICMP unreachable message as well
   as the ICMP unreachable message itself.







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4.  Connectivity Disruption Reaction

   Section 4.1 introduces the basic idea of TCP-LCD.  The complete
   algorithm is specified in Section 4.2.

4.1.  Basic Idea

   The goal of the algorithm is to promptly detect when connectivity to
   a previously disconnected peer node has been restored after a long
   connectivity disruption, while retaining appropriate behavior in case
   of congestion.  TCP-LCD exploits standard ICMP unreachable messages
   during timeout-based loss recovery.  This increases TCP's
   retransmission frequency by undoing one retransmission timer backoff
   whenever an ICMP unreachable message reports on the sequence number
   of a presumably lost retransmission.

   This approach has the advantage of appropriately reducing the probing
   rate in case of congestion.  If either the retransmission itself, or
   the corresponding ICMP message, is dropped the previously performed
   retransmission timer backoff is not undone, which effectively halves
   the probing rate.

4.2.  Algorithm Details

   A TCP sender using RFC 2988 [RFC2988] to compute TCP's retransmission
   timer MAY employ the following scheme to avoid over-conservative
   retransmission timer backoffs in case of long connectivity
   disruptions.  If a TCP sender does implement the following steps, the
   algorithm MUST be initiated upon the first timeout of the oldest
   outstanding segment (SND.UNA) and MUST be stopped upon the arrival of
   the first acceptable ACK.  The algorithm MUST NOT be re-initiated
   upon subsequent timeouts for the same segment.  The scheme SHOULD NOT
   be used in SYN-SENT or SYN-RECEIVED states [RFC0793] (i.e., during
   connection establishment).

   A TCP sender that does not employ RFC 2988 [RFC2988] to compute TCP's
   retransmission timer SHOULD NOT use TCP-LCD.  We envision that the
   scheme could be easily adapted to algorithms others than RFC 2988.
   However, we leave this as future work.

   In rule (2.5) RFC 2988 [RFC2988] provides the option to place a
   maximum value on the RTO.  When a TCP implements this rule to provide
   an upper bound for the RTO, it SHOULD also be used in the following
   algorithm.  In particular, if the RTO is bounded by an upper limit
   (maximum RTO), the "MAX_RTO" variable used in this scheme SHOULD be
   initialized with this upper limit.  Otherwise, if the RTO is
   unbounded, the "MAX_RTO" variable SHOULD be set to infinity.




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   The scheme specified in this document uses the "BACKOFF_CNT"
   variable, whose initial value is zero.  The variable is used to count
   the number of performed retransmission timer backoffs during one
   timeout-based loss recovery.  Moreover, the "RTO_BASE" variable is
   used to recover the previous RTO if the retransmission timer backoff
   was unnecessary.  The variable is initialized with the RTO upon
   initiation of timeout-based loss recovery.

   (1)  Before TCP updates the variable "RTO" when it initiates timeout-
        based loss recovery, set the variables "BACKOFF_CNT" and
        "RTO_BASE" as follows:

           BACKOFF_CNT := 0;
           RTO_BASE := RTO.

        Proceed to step (R).

   (R)  This is a placeholder for standard TCP's behavior in case the
        retransmission timer has expired.  In particular, if RFC 2988
        [RFC2988] is used, steps (5.4) - (5.6) of that algorithm go
        here.  Proceed to step (2).

   (2)  To account for the expiration of the retransmission timer in the
        previous step (R), increment the "BACKOFF_CNT" variable by one:

           BACKOFF_CNT := BACKOFF_CNT + 1.

   (3)  Wait either

           for the expiration of the retransmission timer.  When the
           retransmission timer expires, proceed to step (R);

           or for the arrival of an acceptable ACK.  When an acceptable
           ACK arrives, proceed to step (A);

           or for the arrival of an ICMP unreachable message.  When the
           ICMP unreachable message "ICMP_DU" arrives, proceed to step
           (4).

   (4)  If "BACKOFF_CNT > 0", i.e., if at least one retransmission timer
        backoff can be undone, then

           proceed to step (5);

        else

           proceed to step (3).




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   (5)  Extract the TCP segment header included in the ICMP unreachable
        message "ICMP_DU":

           SEG := Extract(ICMP_DU).

   (6)  If "SEG.SEQ == SND.UNA", i.e., if the TCP segment "SEG"
        eliciting the ICMP unreachable message "ICMP_DU" carries the
        sequence number of a retransmission, then

           proceed to step (7);

        else

           proceed to step (3).

   (7)  Undo the last retransmission timer backoff:

           BACKOFF_CNT := BACKOFF_CNT - 1;
           RTO := min(RTO_BASE * 2^(BACKOFF_CNT), MAX_RTO).

   (8)  If the retransmission timer expires due to the undoing in the
        previous step (7), then

           proceed to step (R);

        else

           proceed to step (3).

   (A)  This is a placeholder for standard TCP's behavior in case an
        acceptable ACK has arrived.  No further processing.

   When a TCP in steady-state detects a segment loss using the
   retransmission timer it enters the timeout-based loss recovery and
   initiates the algorithm (step 1).  It adjusts the slow start
   threshold (ssthresh), sets the congestion window (CWND) to one
   segment, backs off the retransmission timer, and retransmits the
   first unacknowledged segment (step R) [RFC5681], [RFC2988].  To
   account for the expiration of the retransmission timer the TCP sender
   increments the "BACKOFF_CNT" variable by one (step 2).

   In case the retransmission timer expires again (step 3a) a TCP will
   repeat the retransmission of the first unacknowledged segment and
   back off the retransmission timer once more (step R) [RFC2988] as
   well as increment the "BACKOFF_CNT" variable by one (step 2).  Note
   that a TCP may implement RFC 2988's [RFC2988] option to place a
   maximum value on the RTO that may result in not performing the
   retransmission timer backoff.  However, step (2) MUST always and



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   unconditionally be applied, no matter whether or not the
   retransmission timer is actually backed off.  In other words, each
   time the retransmission timer expires, the "BACKOFF_CNT" variable
   MUST be incremented by one.

   If the first received packet after the retransmission(s) is an
   acceptable ACK (step 3b), a TCP will proceed as normal, i.e., slow
   start the connection and terminate the algorithm (step A).  Later
   ICMP unreachable messages from the just terminated timeout-based loss
   recovery are ignored since the ACK clock is already restarting due to
   the successful retransmission.

   On the other hand, if the first received packet after the
   retransmission(s) is an ICMP unreachable message (step 3c), and if
   step (4) permits it, a TCP SHOULD undo one backoff for each ICMP
   unreachable message reporting an error on a retransmission.  To
   decide if an ICMP unreachable message reports on a retransmission,
   the sequence number therein is exploited (step 5, step 6).  The undo
   is performed by re-calculating the RTO with the decremented
   "BACKOFF_CNT" variable (step 7).  This calculation explicitly matches
   the (bounded) exponential backoff specified in rule (5.5) of
   [RFC2988].

   Upon receipt of an ICMP unreachable message that legitimately undoes
   one backoff there is the possibility that the shortened
   retransmission timer has already expired (step 8).  Then, a TCP
   SHOULD retransmit immediately, i.e., an ICMP message clocked
   retransmission.  In case the shortened retransmission timer has not
   yet expired, TCP MUST wait accordingly.


5.  Discussion of TCP-LCD

   TCP-LCD takes caution to only react to connectivity disruption
   indications in form of ICMP unreachable messages during timeout-based
   loss recovery.  Therefore, TCP's behavior is not altered when either
   no ICMP unreachable messages are received, or the retransmission
   timer of the TCP sender did not expire since the last received
   acceptable ACK.  Thus, by defintion the algorithm triggers only in
   case of long connectivity disruptions.

   Only such ICMP unreachable messages that report on the sequence
   number of a retransmission, i.e., report on SND.UNA, are evaluated by
   TCP-LCD.  All other ICMP unreachable messages are ignored.  The
   arrival of those ICMP unreachable messages provides strong evidence
   that the retransmissions were not dropped due to congestion but were
   successfully delivered to the temporary end-point of the employed
   path, i.e., the reporting router.  In other words, there is no



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   evidence for any congestion at least on that very part of the path
   that was traveled by both, the TCP segment eliciting the ICMP
   unreachable message as well as the ICMP unreachable message itself.

   However, there are some situations where TCP-LCD makes a false
   decision and incorrectly undoes a retransmission timer backoff.  This
   can happen, albeit the received ICMP unreachable message reports on
   the segment number of a retransmission (SND.UNA) because the TCP
   segment that elicited the ICMP unreachable message may either not be
   a retransmission (Section 5.1), or does not belong to the current
   timeout-based loss recovery (Section 5.2).  Finally, packet
   duplication (Section 5.3) can also spuriously trigger the algorithm.

   Section 5.4 discusses possible probing frequencies, while Section 5.6
   describes the motivation for not reacting on ICMP unreachable
   messages while TCP is in steady-state.

5.1.  Retransmission Ambiguity

   Historically, the retransmission ambiguity problem [Zh86], [KP87] is
   the TCP sender's inability to distinguish whether the first
   acceptable ACK after a retransmission refers to the original
   transmission or to the retransmission.  This problem occurs after
   both a Fast Retransmit and a timeout-based retransmit.  However,
   modern TCP implementations can eliminate the retransmission ambiguity
   with either the help of Eifel [RFC3522], [RFC4015] or Forward RTO-
   Recovery (F-RTO) [RFC5682].

   The revert strategy of the given algorithm suffers from a form of
   retransmission ambiguity, too.  In contrast to the above case, TCP
   suffers from ambiguity regarding ICMP unreachable messages received
   during timeout-based loss recovery.  With the TCP segment number
   included in the ICMP unreachable message, a TCP sender is not able to
   determine if the ICMP unreachable message refers to the original
   transmission or to any of the timeout-based retransmissions.  That
   is, there is an ambiguity which TCP segment an ICMP unreachable
   message reports on.

   However, for the algorithm this ambiguity is not considered to be a
   problem.  The assumption that a received ICMP message provides
   evidence that a non-congestion loss caused by the connectivity
   disruption was wrongly considered a congestion loss still holds,
   regardless to which TCP segment, transmission or retransmission, the
   message refers.







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5.2.  Wrapped Sequence Numbers

   Besides the ambiguity whether a received ICMP unreachable message
   refers to the original transmission or to any of the retransmissions,
   there is another source of ambiguity about the TCP sequence numbers
   contained in ICMP unreachable messages.  For high bandwidth paths
   like modern gigabit links the sequence space may wrap rather quickly,
   thereby allowing the possibility that delayed ICMP unreachable
   messages - a router dropping packets due to a link outage is not
   obliged to send ICMP unreachable messages in a timely manner
   [RFC1812] - may coincidentally fit as valid input in the proposed
   scheme.  As a result, the scheme may incorrectly undo retransmission
   timer backoffs.  Chances for this to happen are minuscule, since a
   particular ICMP message would need to contain the exact sequence
   number of the current oldest outstanding segment (SND.UNA), while at
   the same time TCP is in timeout-based loss recovery.  However, two
   "worst case" scenarios for the algorithm are possible:

   For instance, consider a steady state TCP connection, which will be
   disrupted at an intermediate router R due to a link outage.  Upon the
   expiration of the RTO, the TCP sender enters the timeout-based loss
   recovery and starts to retransmit the earliest segment that has not
   been acknowledged (SND.UNA).  For some reason, router R delays all
   corresponding ICMP unreachable messages so that the TCP sender
   backoffs the retransmission timer normally without any undoing.  At
   the end of the connectivity disruption, the TCP sender eventually
   detects the re-establishment, leaves the scheme and finally the
   timeout-based loss recovery, too.  A sequence number wrap-around
   later, the connectivity between the two peers is disrupted again, but
   this time due to congestion and exactly at the time at which the
   current SND.UNA matches the SND.UNA from the previous cycle.  If
   router R emits the delayed ICMP unreachable messages now, the TCP
   sender would incorrectly undo retransmission timer backoffs.  As the
   TCP sequence number contains 32 bits, the probability of this
   scenario is at most 1/2^32.  Given sufficiently many retransmissions
   in the first timeout-based loss recovery, the corresponding ICMP
   unreachable messages could reduce the RTO in the second recovery at
   most to "RTO_BASE".  However, once the ICMP unreachable messages are
   depleted, the standard exponential backoff will be performed.  Thus,
   the congestion response will only be delayed by some false
   retransmissions.

   Similar to the above, consider the case where a steady state TCP
   connection with n segments in-flight will be disrupted at some point
   due to a link outage by an intermediate router R. For each segment
   in-flight, router R may generate an ICMP unreachable message.
   However, due to some reason it delays them.  Once the link outage is
   over and the connection has been re-established, the TCP sender



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   leaves the scheme and slow-starts the connection.  Following a
   sequence number wrap-around, a retransmission timeout occurs, just at
   the moment the TCP sender's current window of data reaches the
   previous range of the sequence number space again.  In case router R
   emits the delayed ICMP unreachable messages now, one spurious undoing
   of the retransmission timer backoff is possible, if the TCP segment
   number contained in ICMP unreachable messages matches the current
   SND.UNA, and the timeout was a result of congestion.  In the case of
   another connectivity disruption, the additional undoing of the
   retransmission timer backoff has no impact.  The probability of this
   scenario is at most n/2^32.

5.3.  Packet Duplication

   In case an intermediate router duplicates packets, a TCP sender may
   receive more ICMP unreachable messages during timeout-based loss
   recovery than it actually has sent timeout-based retransmissions.
   However, since TCP-LCD keeps track of the number of performed
   retransmission timer backoffs in the "BACKOFF_CNT" variable, it will
   not undo more retransmission timer backoffs than were actually
   performed.  Nevertheless, if packet duplication and congestion
   coincide on the path between the two communicating hosts, duplicated
   ICMP messages could hide the congestion loss of some retransmissions
   or ICMP messages, and the algorithm may incorrectly undo
   retransmission timer backoffs.  Considering the overall impact of a
   router that duplicates packets, the additional load induced by some
   spurious timeout-based retransmits can probably be neglected.

5.4.  Probing Frequency

   One could argue that if an ICMP unreachable message arrives for a
   timeout-based retransmission, the RTO shall be reset or recalculated,
   similar to what is done when an ACK arrives during timeout-based loss
   recovery (see Karn's algorithm [KP87], [RFC2988]), and a new
   retransmission should be sent immediately.  Generally, this would
   allow for a much higher probing frequency based on the round trip
   time up to the router where connectivity has been disrupted.
   However, we believe the current scheme provides a good trade-off
   between conservative behavior and fast detection of connectivity re-
   establishment.

5.5.  Reaction during Connection Establishment

   It is possible that a TCP sender enters timeout-based loss recovery
   while the connection is in SYN-SENT or SYN-RECEIVED states [RFC0793].
   The algorithm described in this document could also be used for
   faster connection establishment in networks with connectivity
   disruptions.  However, because existing TCP implementations [RFC5461]



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   already interpret ICMP unreachable messages during connection
   establishment and abort the corresponding connection, we refrain from
   suggesting this.

5.6.  Reaction in Steady-State

   Another exploitation of ICMP unreachable messages in the context of
   TCP congestion control might seem appropriate in case the ICMP
   unreachable message is received while TCP is in steady-state, and the
   message refers to a segment from within the current window of data.
   As the RTT up to the router that generated the ICMP unreachable
   message is likely to be substantially shorter than the overall RTT to
   the destination, the ICMP unreachable message may very well reach the
   originating TCP while it is transmitting the current window of data.
   In case the remaining window is large, it might seem appropriate to
   refrain from transmitting the remaining window as there is timely
   evidence that it will only trigger further ICMP unreachable messages
   at the very router.  Although this promises improvement from a
   wastage perspective, it may be counterproductive from a security
   perspective.  An attacker could forge such ICMP messages, thereby
   forcing the originating TCP to stop sending data, very similar to the
   blind throughput-reduction attack mentioned in
   [I-D.ietf-tcpm-icmp-attacks].

   An additional consideration is the following: in the presence of
   multi-path routing even the receipt of a legitimate ICMP unreachable
   message cannot be exploited accurately because there is the option
   that only one of the multiple paths to the destination is suffering
   from a connectivity disruption, which causes ICMP unreachable
   messages to be sent.  Then, however, there is the possibility that
   the path along which the connectivity disruption occurred contributed
   considerably to the overall bandwidth, such that a congestion
   response is very well reasonable.  However, this is not necessarily
   the case.  Therefore, a TCP has no means except for its inherent
   congestion control to decide on this matter.  All in all, it seems
   that for a connection in steady-state, i.e., not in timeout-based
   loss recovery, reacting on ICMP unreachable messages in regard to
   congestion control is not appropriate.  For the case of timeout-based
   retransmissions, however, there is a reasonable congestion response,
   which is skipping further retransmission timer backoffs because there
   is no congestion indication - as described above.


6.  Dissolving Ambiguity Issues (the Safe Variant)

   Given that the TCP Timestamps option [RFC1323] is enabled for a
   connection, a TCP sender MAY use the following algorithm to dissolve
   the ambiguity issues mentioned in Sections 5.1, 5.2, and 5.3.  In



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   particular, both the retransmission ambiguity and the packet
   duplication problems are prevented by the following TCP-LCD variant.
   On the other hand, the false positives caused by wrapped sequence
   numbers cannot be completely avoided, but the likelihood is further
   reduced by a factor of 1/2^32 since the Timestamp Value field (TSval)
   of the TCP Timestamps Option contains 32 bits.

   Hence, implementers may choose to implement the TCP-LCD with the
   following modifications.

   Step (1) is replaced by step (1'):

   (1')  Before TCP updates the variable "RTO" when it initiates
         timeout-based loss recovery, set the variables "BACKOFF_CNT"
         and "RTO_BASE" and the data structure "RETRANS_TS" as follows:

            BACKOFF_CNT := 0;
            RTO_BASE := RTO.
            RETRANS_TS := [];

         Proceed to step (R).

   Step (2) is extended by step (2b):

   (2b)  Store the value of the Timestamp Value field (TSval) of the TCP
         Timestamps option included in the retransmission "RET" sent in
         step (R) into the "RETRANS_TS" data structure:

            RETRANS_TS.add(RET.TSval)

   Step (6) is replaced by step (6'):

   (6')  If "SEG.SEQ == SND.UNA && RETRANS_TS.exists(SEQ.TSval)", i.e.,
         if the TCP segment "SEG" eliciting the ICMP unreachable message
         "ICMP_DU" carries the sequence number of a retransmission, and
         the value in its Timestamp Value field (TSval) is valid, then

               proceed to step (7');

         else

               proceed to step (3).

   Step (7) is replaced by step (7'):







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   (7')  Undo the last retransmission timer backoff:

               RETRANS_TS.remove(SEQ.TSval);
               BACKOFF_CNT := BACKOFF_CNT - 1;
               RTO := min(RTO_BASE * 2^(BACKOFF_CNT), MAX_RTO).

   The downside of the safe variant is twofold.  Firstly, the
   modifications come at a cost: the TCP sender is required to store the
   timestamps of all retransmissions sent during one timeout-based loss
   recovery.  Second, the safe variant can only undo a retransmission
   timer backoff if the intermediate router experiencing the link outage
   implements [RFC1812] and chooses to include as many more than the
   first 64 bits of the payload of the triggering datagram, as are
   needed to include the TCP Timestamps option in the ICMP unreachable
   message.


7.  Interoperability Issues

   This section discusses interoperability issues related to introducing
   TCP-LCD.

7.1.  Detection of TCP Connection Failures

   TCP-LCD may have side-effects on TCP implementations that attempt to
   detect TCP connection failures by counting timeout-based
   retransmissions.  RFC 1122 [RFC1122] states in Section 4.2.3.5 that a
   TCP host must handle excessive retransmissions of data segments with
   two thresholds R1 and R2 measuring the number of retransmissions that
   have occurred for the same segment.  Both thresholds might either be
   measured in time units or as a count of retransmissions.

   Due to TCP-LCD's revert strategy of the retransmission timer, the
   assumption that a certain number of retransmissions corresponds to a
   specific time interval no longer holds, as additional retransmissions
   may be performed during timeout-based-loss recovery to detect the end
   of the connectivity disruption.  Therefore, a TCP employing TCP-LCD
   either SHOULD measure the thresholds R1 and R2 in time units or, in
   case R1 and R2 are counters of retransmissions, SHOULD convert them
   into time intervals, which correspond to the time an unmodified TCP
   would need to reach the specified number of retransmissions.

7.2.  Explicit Congestion Notification

   By the use of Explicit Congestion Notification (ECN) [RFC3168] ECN-
   capable routers are no longer limited to dropping packets as
   congestion indication.  Instead, they can set the Congestion
   Experienced (CE) codepoint in the IP header to indicate congestion.



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   With TCP-LCD it may happen that during a connectivity disruption a
   received ICMP unreachable message has been elicited by a timeout-
   based retransmission that was marked with the CE codepoint before
   reaching the router experiencing the link outage.  In such a case, we
   suggest that the TCP sender SHOULD additionally reset the
   retransmission timer in case the algorithm undoes a retransmission
   timer backoff.

7.3.  ICMP for IP version 6

   RFC 4443 [RFC4443] specifies the Internet Control Message Protocol
   (ICMPv6) to be used with the Internet Protocol version 6 (IPv6)
   [RFC2460].  From TCP-LCD's point of view, it is important to notice
   that for IPv6, the payload of an ICMPv6 error messages has to include
   as many bytes as possible from the IPv6 datagram that elicited the
   ICMPv6 error message, without making the error message exceed the
   minimum IPv6 MTU (1280 bytes) [RFC4443].  Thus, more information is
   available for TCP-LCD as in the case of IPv4.

   The counterpart of the ICMPv4 destination unreachable message of code
   0 (net unreachable) and of code 1 (host unreachable) is the ICMPv6
   destination unreachable message of code 0 (no route to destination)
   [RFC4443].  As with IPv4, a router should generate an ICMPv6
   destination unreachable message of code 0 in response to a packet
   that cannot be delivered to its destination address because it lacks
   a matching entry in its routing table.  As a result, TCP-LCD can
   employ this ICMPv6 error messages as connectivity disruption
   indication, too.

7.4.  TCP-LCD and IP Tunnels

   It is worth noting that IP tunnels, including IPsec [RFC4301], IP in
   IP [RFC2003], Generic Routing Encapsulation (GRE) [RFC2784], and
   others are compatible with TCP-LCD, as long as the received ICMP
   unreachable messages can be demultiplexed and extracted appropriately
   by the TCP sender during timeout-based loss recovery.

   If, for example, end-to-end tunnels like IPSec in transport mode
   [RFC4301] are employed, a TCP sender may receive ICMP unreachable
   messages where additional steps, e.g., decrypting in step (5) of the
   algorithm, are needed to extract the TCP header from these ICMP
   messages.  Provided that the received ICMP unreachable message
   contains enough information, i.e., SEQ.SEG is extractable, these
   information MAY still be used as a valid input for the proposed
   algorithm.

   Likewise, if IP encapsulation like [RFC2003] is used in some part of
   the path between the communicating hosts, the tunnel ingress node may



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   receive the ICMP unreachable messages from an intermediate router
   experiencing the link outage.  Nevertheless, the tunnel ingress node
   may replay the ICMP unreachable messages in order to inform the TCP
   sender.  If enough information is preserved to extract SEQ.SEG, the
   replayed ICMP unreachable messages MAY still be used in TCP-LCD.


8.  Related Work

   Several methods that address TCP's problems in the presence of
   connectivity disruptions have been proposed in literature.  Some of
   them try to improve TCP's performance by modifying lower layers.  For
   example [SM03] introduces a "smart link layer", which buffers one
   segment for each active connection and replays these segments upon
   connectivity re-establishment.  This approach has a serious drawback:
   previously stateless intermediate routers have to be modified in
   order to inspect TCP headers, to track the end-to-end connection, and
   to provide additional buffer space.  This leads to an additional need
   of memory and processing power.

   On the other hand, stateless link layer schemes, as proposed in
   [RFC3819], which unconditionally buffer some small number of packets
   may have another problem: if a packet is buffered longer than the
   maximum segment lifetime (MSL) of 2 min [RFC0793], i.e., the
   disconnection lasts longer than MSL, TCP's assumption that such
   segments will never be received will no longer be true, violating
   TCP's semantics [I-D.eggert-tcpm-tcp-retransmit-now].

   Other approaches, like TCP-F [CRVP01] or the Explicit Link Failure
   Notification (ELFN) [HV02] inform a TCP sender about a disrupted path
   by special messages generated and sent from intermediate routers.  In
   case of a link failure the TCP sender stops sending segments and
   freezes its retransmission timers.  TCP-F stays in this state and
   remains silent until either a "route establishment notification" is
   received or an internal timer expires.  In contrast, ELFN
   periodically probes the network to detect connectivity re-
   establishment.  Both proposals rely on changes to intermediate
   routers, whereas the scheme proposed in this document is a sender-
   only modification.  Moreover, ELFN does not consider congestion and
   may impose serious additional load on the network, depending on the
   probe interval.

   The authors of ATCP [LS01] propose enhancements to identify different
   types of packet loss by introducing a layer between TCP and IP.  They
   utilize ICMP destination unreachable messages to set TCP's receiver
   advertised window to zero, thus forcing the TCP sender to perform
   zero window probing with a exponential backoff.  ICMP destination
   unreachable messages that arrive during this probing period are



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   ignored.  This approach is nearly orthogonal to this document, which
   exploits ICMP messages to undo a retransmission timer backoff when
   TCP is already probing.  In principle, both mechanisms could be
   combined.  However, due to security considerations it does not seem
   appropriate to adopt ATCP's reaction as discussed in Section 5.6.

   Schuetz et al. describe, in [I-D.schuetz-tcpm-tcp-rlci], a set of TCP
   extensions that improve TCP's behavior when transmitting over paths
   whose characteristics can change rapidly.  Their proposed extensions
   modify the local behavior of TCP and introduce a new TCP option to
   signal locally received connectivity-change indications (CCIs) to
   remote peers.  Upon receipt of a CCI, they re-probe the path
   characteristics either by performing a speculative retransmission or
   by sending a single segment of new data, depending on whether the
   connection is currently stalled in exponential backoff or
   transmitting in steady-state, respectively.  The authors focus on
   specifying TCP response mechanisms, nevertheless underlying layers
   would have to be modified to explicitly send CCIs to make these
   immediate responses possible.


9.  IANA Considerations

   This memo includes no request to IANA.


10.  Security Considerations

   The algorithm proposed in this document is considered to be secure.
   For example, an attacker who already guessed the correct four-tuple
   (i.e., Source IP Address, Source TCP port, Destination IP Address,
   and Destination TCP port), can still not make a TCP modified with
   TCP-LCD to flood the network just by sending forged ICMP unreachable
   messages in an attempt to maliciously shorten the retransmission
   timer.  The attacker additionally would need to guess the correct
   segment sequence number of the current timeout-based retransmission,
   with a probability of at most 1/2^32.  Even in the case of man-in-
   the-middle attacks, i.e., attacks performed in scenarios in which the
   attacker can sniff the retransmissions, the impact on network load is
   considered to be low, since the retransmission frequency is limited
   by the RTO that was computed before TCP had entered the timeout-based
   loss recovery.  Hence, the highest probing frequency is expected to
   be even lower than once per minimum RTO, i.e. 1s as specified by
   [RFC2988].







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11.  Acknowledgments

   We would like to thank Kai Jakobs, Ilpo Jarvinen, Pasi Sarolahti,
   Timothy Shepard, Joe Touch and Carsten Wolff for feedback on earlier
   versions of this document.  We also thank Michael Faber, Daniel
   Schaffrath, and Damian Lukowski for implementing and testing the
   algorithm in Linux.  Special thanks go to Ilpo Jarvinen for giving
   valuable feedback regarding the Linux implementation.

   This work has been supported by the German National Science
   Foundation (DFG) within the research excellence cluster Ultra High-
   Speed Mobile Information and Communication (UMIC), RWTH Aachen
   University.


12.  References

12.1.  Normative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, May 1992.

   [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers",
              RFC 1812, June 1995.

   [RFC2988]  Paxson, V. and M. Allman, "Computing TCP's Retransmission
              Timer", RFC 2988, November 2000.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009.

12.2.  Informative References

   [CRVP01]   Chandran, K., Raghunathan, S., Venkatesan, S., and R.
              Prakash, "A feedback-based scheme for improving TCP
              performance in ad hoc wireless networks", IEEE Personal
              Communications vol. 8, no. 1, pp. 34-39, February 2001.

   [HV02]     Holland, G. and N. Vaidya, "Analysis of TCP performance
              over mobile ad hoc networks", Wireless Networks vol. 8,
              no. 2-3, pp. 275-288, March 2002.




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   [I-D.eggert-tcpm-tcp-retransmit-now]
              Eggert, L., "TCP Extensions for Immediate
              Retransmissions", draft-eggert-tcpm-tcp-retransmit-now-02
              (work in progress), June 2005.

   [I-D.ietf-tcpm-icmp-attacks]
              Gont, F., "ICMP attacks against TCP",
              draft-ietf-tcpm-icmp-attacks-12 (work in progress),
              March 2010.

   [I-D.schuetz-tcpm-tcp-rlci]
              Schuetz, S., Koutsianas, N., Eggert, L., Eddy, W., Swami,
              Y., and K. Le, "TCP Response to Lower-Layer Connectivity-
              Change Indications", draft-schuetz-tcpm-tcp-rlci-03 (work
              in progress), February 2008.

   [KP87]     Karn, P. and C. Partridge, "Improving Round-Trip Time
              Estimates in Reliable Transport Protocols", Proceedings of
              the Conference on Applications, Technologies,
              Architectures, and Protocols for Computer Communication
              (SIGCOMM'87) pp. 2-7, August 1987.

   [LS01]     Liu, J. and S. Singh, "ATCP: TCP for mobile ad hoc
              networks", IEEE Journal on Selected Areas in
              Communications vol. 19, no. 7, pp. 1300-1315, 2001 July.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              converting network protocol addresses to 48.bit Ethernet
              address for transmission on Ethernet hardware", STD 37,
              RFC 826, November 1982.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,



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              March 2000.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, September 2001.

   [RFC3522]  Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm
              for TCP", RFC 3522, April 2003.

   [RFC3782]  Floyd, S., Henderson, T., and A. Gurtov, "The NewReno
              Modification to TCP's Fast Recovery Algorithm", RFC 3782,
              April 2004.

   [RFC3819]  Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, July 2004.

   [RFC4015]  Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm
              for TCP", RFC 4015, February 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC5461]  Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
              February 2009.

   [RFC5682]  Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
              "Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
              Spurious Retransmission Timeouts with TCP", RFC 5682,
              September 2009.

   [SESB05]   Schuetz, S., Eggert, L., Schmid, S., and M. Brunner,
              "Protocol enhancements for intermittently connected
              hosts", SIGCOMM Computer Communication Review vol. 35, no.
              3, pp. 5-18, December 2005.

   [SM03]     Scott, J. and G. Mapp, "Link layer-based TCP optimisation
              for disconnecting networks", SIGCOMM Computer
              Communication Review vol. 33, no. 5, pp. 31-42,
              October 2003.

   [Zh86]     Zhang, L., "Why TCP Timers Don't Work Well", Proceedings
              of the Conference on Applications, Technologies,



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              Architectures, and Protocols for Computer Communication
              (SIGCOMM'86) pp. 397-405, August 1986.


Appendix A.  Changes from previous versions of the draft

A.1.  Changes from draft-ietf-tcpm-tcp-lcd-00

   o  Editorial changes.

   o  Clarified TCP-LCD's behaviour during connection establishment
      (Thanks to Mark Handley).

A.2.  Changes from draft-zimmermann-tcp-lcd-02

   o  Incorporated feedback submitted by Ilpo Jarvinen.
      <http://www.ietf.org/mail-archive/web/tcpm/current/msg04841.html>

   o  Incorporated feedback submitted by Pasi Sarolahti.
      <http://www.ietf.org/mail-archive/web/tcpm/current/msg04870.html>

   o  Incorporated feedback submitted by Joe Touch.
      <http://www.ietf.org/mail-archive/web/tcpm/current/msg04895.html>
      <http://www.ietf.org/mail-archive/web/tcpm/current/msg04900.html>

   o  Extended and reorganized the discussion (Section 5):

      *  Every discussion item got its own title, so that we have a
         better overview.

      *  Extended Retransmission Ambiguity section.  Added also some
         references to the historical retransmission ambiguity problem.

      *  Heavily extended discussion about wrapped sequence numbers (see
         Joe's comments).

      *  Described the influence of packet duplication on the algorithm
         (Thanks to Ilpo).

      *  The section "Protecting Against Misbehaving Routers" is not a
         subsection anymore.  Moreover, the section was renamed to
         "Dissolving Ambiguity Issues" and has now real content.

   o  An interoperability issues section (Section 7) was added.  In
      particular comments to ECN, ICMPv6, and to the two thresholds R1
      and R2 of [RFC1122] (Section 4.2.3.5) were added.





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   o  Miscellaneous editorial changes.  In particular, the algorithm has
      a name now: TCP-LCD.

A.3.  Changes from draft-zimmermann-tcp-lcd-01

   o  The algorithm in Section 4.2 was slightly changed.  Instead of
      reverting the last retransmission timer backoff by halving the
      RTO, the RTO is recalculated with help of the "BACKOFF_CNT"
      variable.  This fixes an issue that occurred when the
      retransmission timer was backed off but bounded by a maximum
      value.  The algorithm in the previous version of the draft, would
      have "reverted" to half of that maximum value, instead of using
      the value, before the RTO was doubled (and then bounded).

   o  Miscellaneous editorial changes.

A.4.  Changes from draft-zimmermann-tcp-lcd-00

   o  Miscellaneous editorial changes in Section 1, 2 and 3.

   o  The document was restructured in Section 1, 2 and 3 for easier
      reading.  The motivation for the algorithm is changed according
      TCP's problem to disambiguate congestion from non-congestion loss.

   o  Added Section 4.1.

   o  The algorithm in Section 4.2 was restructured and simplified:

      *  The special case of the first received ICMP destination
         unreachable message after an RTO was removed.

      *  The "BACKOFF_CNT" variable was introduced so it is no longer
         possible to perform more reverts than backoffs.

   o  The discussion in Section 5 was improved and expanded according to
      the algorithm changes.















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Authors' Addresses

   Alexander Zimmermann
   RWTH Aachen University
   Ahornstrasse 55
   Aachen,   52074
   Germany

   Phone: +49 241 80 21422
   Email: zimmermann@cs.rwth-aachen.de


   Arnd Hannemann
   RWTH Aachen University
   Ahornstrasse 55
   Aachen,   52074
   Germany

   Phone: +49 241 80 21423
   Email: hannemann@nets.rwth-aachen.de































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