





Internet Engineering Task Force (IETF)                     A. Zimmermann
Request for Comments: 6069                                  A. Hannemann
Category: Experimental                            RWTH Aachen University
ISSN: 2070-1721                                            December 2010


   Making TCP More Robust to Long Connectivity Disruptions (TCP-LCD)

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 reversion 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 the
   case of connectivity disruptions.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6069.




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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 Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. Connectivity Disruption Indication ..............................5
   4. Connectivity Disruption Reaction ................................7
      4.1. Basic Idea .................................................7
      4.2. Algorithm Details ..........................................8
   5. Discussion of TCP-LCD ..........................................11
      5.1. Retransmission Ambiguity ..................................12
      5.2. Wrapped Sequence Numbers ..................................12
      5.3. Packet Duplication ........................................13
      5.4. Probing Frequency .........................................14
      5.5. Reaction during Connection Establishment ..................14
      5.6. Reaction in Steady-State ..................................14
   6. Dissolving Ambiguity Issues Using the TCP Timestamps Option ....15
   7. Interoperability Issues ........................................17
      7.1. Detection of TCP Connection Failures ......................17
      7.2. Explicit Congestion Notification (ECN) ....................17
      7.3. TCP-LCD and IP Tunnels ....................................17
   8. Related Work ...................................................18
   9. Security Considerations ........................................19
   10. Acknowledgments ...............................................20
   11. References ....................................................20
      11.1. Normative References .....................................20
      11.2. Informative References ...................................21










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1.  Introduction

   Connectivity disruptions can occur in many different situations.  The
   frequency of connectivity disruptions depends on the properties of
   the end-to-end path between the communicating hosts.  While
   connectivity disruptions can occur in traditional wired networks,
   e.g., disruption caused by an unplugged network cable, the likelihood
   of their 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 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 [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 (ACKs) 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 [TCP-RLCI].

   The algorithm specified in this document improves TCP's behavior in
   the 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 to 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



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   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 growth may be bounded
   by an upper limit, the maximum RTO, which is at least 60 s, but may
   be longer: Linux, for example, uses 120 s.  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.

   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 reversion 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.

   Experimental results of a Linux implementation of TCP-LCD have been
   presented in [ZimHan09].  The implementation has been incorporated
   into mainline Linux, and is already used within the Internet.  Thus
   far, no negative experiences have been reported that could be
   attributed to the algorithm.  However, we consider TCP-LCD as
   experimental until more real-life results have been obtained.
   Nevertheless, we encourage implementation of TCP-LCD under other
   operating systems to provide for broader testing and experimentation
   opportunities.

2.  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 that 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



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   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 the 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", which refers to the state that 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] extends the period that 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.

3.  Connectivity Disruption Indication

   If the queue of an intermediate router that is 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 may remove the corresponding next hop from
   its routing table.  Means to detect such link outages include
   reacting to failed address resolution protocol (ARP) [RFC0826]
   queries, sensing unsuccessful links, and the like.  However, this is
   solely the responsibility of the respective router.

      Note: The focus of this memo is on introducing a method of 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, an
   Internet Protocol version 4 (IPv4) [RFC0791] router will notify the
   corresponding sending host about the dropped packets via ICMP
   destination unreachable messages of code 0 (net unreachable) or



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   code 1 (host unreachable) [RFC1812].  Therefore, the sending host can
   use the ICMP destination unreachable messages of these codes as an
   indication of a connectivity disruption, since the reception of these
   messages provides evidence that packets were dropped due to a link
   outage.

   For Internet Protocol version 6 (IPv6) [RFC2460], the counterpart of
   the ICMP 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.

   Note that there are also other ICMP and ICMPv6 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 route failed), code 11 (net unreachable
   for TOS (Type of Service)), or code 12 (host unreachable for TOS)
   [RFC1812]).  On the other hand, codes that flag hard errors are of no
   use for this scheme, since TCP should abort the connection when those
   are received [RFC1122].

   For the sake of simplicity, we will use, unless explicitly qualified
   with ICMPv4 or ICMPv6, the term "ICMP unreachable message" as a
   synonym for ICMP destination unreachable messages of code 0 or code 1
   and ICMPv6 destination unreachable messages of code 0.  This implies
   that all keywords from [RFC2119] that deal with the handling of
   received ICMP messages apply in the same way to ICMPv6 messages.

   The accurate interpretation of ICMP unreachable messages as a
   connectivity disruption indication is complicated by the following
   two peculiarities of ICMP messages.  First, 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.  Second, ICMP messages
   are subject to rate-limiting, e.g., when a router drops a whole
   window of data due to a link outage, it is unlikely to send as many
   ICMP unreachable messages as dropped TCP segments.  Depending on the
   load of the router, it may not even send any ICMP unreachable
   messages at all.  Both peculiarities originate from [RFC1812] for
   ICMPv4 and [RFC4443] for ICMPv6.






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   Fortunately, according to [RFC0792], ICMPv4 unreachable messages have
   to contain, in their body, the entire IPv4 header [RFC0791] of the
   datagram eliciting the ICMPv4 unreachable message, plus the first
   64 bits of the payload of that datagram.  This allows the sending
   host to match the ICMPv4 error message to the transport connection
   that elicited it.  RFC 1812 [RFC1812] augments these requirements and
   states that ICMPv4 messages should contain as much of the original
   datagram as possible without the length of the ICMPv4 datagram
   exceeding 576 bytes.  Therefore, in the 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 ICMPv4 message and to identify the affected
   connection.  Moreover, it can identify which segment of the
   respective connection triggered the ICMPv4 unreachable message,
   unless there are several segments in flight with the same sequence
   number (see Section 5.1).

   For IPv6 [RFC2460], the payload of an ICMPv6 error message 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, enough
   information is available to identify both the affected connection and
   the corresponding segment that triggered the ICMPv6 error message.

   A connectivity disruption indication in the 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 as far as 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 and the ICMP unreachable message itself.

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 is received that contains a
   segment with a sequence number of a presumably lost retransmission.




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   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 that uses 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] (see Section 5.5).

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

   RFC 2988 [RFC2988] provides in rule (2.5) 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 MUST 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 MUST be
   initialized with this upper limit.  Otherwise, if the RTO is
   unbounded, the "MAX_RTO" variable MUST be set to infinity.

   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).



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   (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) to (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

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

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

        c)  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).

   (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" contains the
        sequence number of a retransmission, then

           proceed to step (7);

        else

           proceed to step (3).






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   (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
   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, TCP SHOULD undo one backoff for each ICMP



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   unreachable message reporting an error on a retransmission.  To
   decide if an ICMP unreachable message was elicited by a
   retransmission, the sequence number it contains is inspected
   (step (5), step (6)).  The undo is performed by recalculating 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, TCP
   SHOULD retransmit immediately.  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 the 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 definition, the algorithm triggers
   only in the case of long connectivity disruptions.

   Only such ICMP unreachable messages that contain a TCP segment with
   the sequence number of a retransmission, i.e., that contain 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 reporting router.
   In other words, there is no evidence for any congestion at least on
   that very part of the path that was traversed by both the TCP segment
   eliciting the ICMP unreachable message and 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, even when the received ICMP unreachable message contains
   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 to ICMP unreachable
   messages while TCP is in steady-state.




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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 reversion 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 with regard to which TCP segment an ICMP
   unreachable message reports on.

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

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 related to the TCP sequence
   numbers contained in ICMP unreachable messages.  For high-bandwidth
   paths, the sequence space may wrap quickly.  This might cause delayed
   ICMP unreachable messages to coincidentally fit as valid input in the
   proposed scheme.  As a result, the scheme may incorrectly undo
   retransmission timer backoffs.  The chances of this happening are
   minuscule, since a particular ICMP unreachable 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 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



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   been acknowledged (SND.UNA).  For some reason, the router delays all
   corresponding ICMP unreachable messages so that the TCP sender backs
   the retransmission timer off normally without any undoing.  At the
   end of the connectivity disruption, the TCP sender eventually detects
   the re-establishment, and it 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 the
   router 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 at an intermediate router.  For each segment in
   flight, the router may generate an ICMP unreachable message.
   However, for some reason, it delays them.  Once the link outage is
   over and the connection has been re-established, the TCP sender
   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 the
   router emits the delayed ICMP unreachable messages now, spurious
   undoing of the retransmission timer backoff is possible once, if the
   TCP segment number contained in the 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 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 unreachable



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   messages could hide the congestion loss of some retransmissions or
   ICMP unreachable 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 might 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
   result in a much higher probing frequency based on the round-trip
   time 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.  TCP-LCD focuses on long-connectivity disruptions,
   i.e., on disruptions that last for several RTOs.  Thus, a much higher
   probing frequency (less than once per RTO) would not significantly
   increase the available transmission time compared to the duration of
   the connectivity disruption.

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]
   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, while TCP is in
   steady-state.  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 that very router.  Although this promises
   improvement from a wastage perspective, it may be counterproductive
   from a security perspective.  An attacker could forge such ICMP



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   messages, thereby forcing the originating TCP to stop sending data,
   very similar to the blind throughput-reduction attack mentioned in
   [RFC5927].

   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
   possibility 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 to 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 Using the TCP Timestamps Option

   If the TCP Timestamps option [RFC1323] is enabled for a connection, a
   TCP sender SHOULD use the following algorithm to dissolve the
   ambiguity issues mentioned in Sections 5.1, 5.2, and 5.3.  In
   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 employ 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 := [].



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         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" contains 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'):

   (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 this variant is twofold.  First, 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, this variant can only undo a retransmission timer backoff if
   the intermediate router experiencing the link outage implements
   [RFC1812] and chooses to include, in addition to the first 64 bits of
   the payload of the triggering datagram, as many bits as are needed to
   include the TCP Timestamps option in the ICMP unreachable message.












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7.  Interoperability Issues

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

7.1.  Detection of TCP Connection Failures

   TCP-LCD may produce side effects for TCP implementations that attempt
   to detect TCP connection failures by counting timeout-based
   retransmissions.  [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, that measure the number of retransmissions
   that have occurred for the same segment.  Both thresholds might be
   measured either in time units or as a count of retransmissions.

   Due to TCP-LCD's reversion 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 MUST measure the thresholds R1 and R2 in time units or, in
   case R1 and R2 are counters of retransmissions, MUST convert them
   into time intervals that correspond to the time an unmodified TCP
   would need to reach the specified number of retransmissions.

7.2.  Explicit Congestion Notification (ECN)

   With Explicit Congestion Notification (ECN) [RFC3168], ECN-capable
   routers are no longer limited to dropping packets to indicate
   congestion.  Instead, they can set the Congestion Experienced (CE)
   codepoint in the IP header to indicate congestion.  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, a TCP
   sender MUST, corresponding to Section 6.1.2 of [RFC3168],
   additionally reset the retransmission timer in case the algorithm
   undoes a retransmission timer backoff.

7.3.  TCP-LCD and IP Tunnels

   It is worth noting that IP tunnels, including IPsec [RFC4301], IP
   encapsulation within 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.




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   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., also performing decryption 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., SEG.SEQ is extractable,
   this information can 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
   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 SEG.SEQ, the
   replayed ICMP unreachable messages can 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
   for 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 the MSL, TCP's assumption that such
   segments will never be received will no longer be true, violating
   TCP's semantics [TCP-REXMIT-NOW].

   Other approaches, like the TCP feedback-based scheme (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 the 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



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   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 "ad hoc TCP" (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 an
   exponential backoff.  ICMP destination unreachable messages that
   arrive during this probing period are 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. [TCP-RLCI] describe 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.  Security Considerations

   Generally, an attacker has only two attack alternatives: to generate
   ICMP unreachable messages to try to make a TCP modified with TCP-LCD
   flood the network, or to suppress legitimate ICMP unreachable
   messages to try to slow down the transmission rate of a TCP sender.

   In order to generate ICMP unreachable messages that fit as an input
   for TCP-LCD, an attacker would need to guess the correct four-tuple
   (i.e., Source IP Address, Source TCP port, Destination IP Address,
   and Destination TCP port) and the exact segment sequence number of
   the current timeout-based retransmission.  Yet, the correct sequence
   number is generally hard to guess, given the probability of 1/2^32.
   Even if an attacker has information about that sequence number (i.e.,
   the attacker can eavesdrop on the retransmissions) the impact on the
   network load from the attacker may be considered low, since the
   retransmission frequency is limited by the RTO that was computed
   before TCP had entered the timeout-based loss recovery.  Hence, the



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   highest probing frequency is expected to be even lower than once per
   minimum RTO, i.e., 1 s as specified by [RFC2988].  It is important to
   note that an attacker who can correctly guess the four-tuple and the
   segment sequence number can easily launch more serious attacks (i.e.,
   hijack the connection), whether or not TCP-LCD is used.

   There may be means by which an attacker can cause the suppression of
   legitimate ICMP unreachable messages (e.g., by flooding the router
   experiencing the link outage to trigger ICMP rate-limiting).
   However, even if the attacker could suppress every legitimate ICMP
   unreachable message, the security impact of such an attack is
   negligible, since the TCP sender using TCP-LCD will behave like a
   regular TCP would.  Note that this kind of attack is
   indistinguishable from a router experiencing a link outage that is
   not sending ICMP unreachable messages at all (e.g., because of local
   policy).

   In summary, the algorithm proposed in this document is considered to
   be secure.

10.  Acknowledgments

   We would like to thank Lars Eggert, Adrian Farrel, Mark Handley, Kai
   Jakobs, Ilpo Jarvinen, Enrico Marocco, Catherine Meadows, Juergen
   Quittek, Pasi Sarolahti, Tim 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.

11.  References

11.1.  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.



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

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

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

   [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.

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

11.2.  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.

   [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, July 2001.

   [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.




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

   [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,
               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.

   [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.

   [RFC5927]   Gont, F., "ICMP Attacks against TCP", RFC 5927,
               July 2010.

   [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.




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   [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.

   [TCP-REXMIT-NOW]
               Eggert, L., Schuetz, S., and S. Schmid, "TCP Extensions
               for Immediate Retransmissions", Work in Progress,
               June 2005.

   [TCP-RLCI]
               Schuetz, S., Koutsianas, N., Eggert, L., Eddy, W., Swami,
               Y., and K. Le, "TCP Response to Lower-Layer Connectivity-
               Change Indications", Work in Progress, February 2008.

   [Zh86]      Zhang, L., "Why TCP Timers Don't Work Well", Proceedings
               of the Conference on Applications, Technologies,
               Architectures, and Protocols for Computer Communication
               (SIGCOMM'86) pp. 397-405, August 1986.

   [ZimHan09]
               Zimmermann, A., "Make TCP more Robust to Long
               Connectivity Disruptions", Proceedings of the 75th IETF
               Meeting slides, July 2009,
               <http://www.ietf.org/proceedings/75/slides/tcpm-0.pdf>.

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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