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rfc:rfc6069

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.

Zimmermann & Hannemann Experimental [Page 1] RFC 6069 Making TCP More Robust to LCDs December 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 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

Zimmermann & Hannemann Experimental [Page 2] RFC 6069 Making TCP More Robust to LCDs December 2010

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

Zimmermann & Hannemann Experimental [Page 3] RFC 6069 Making TCP More Robust to LCDs December 2010

 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

Zimmermann & Hannemann Experimental [Page 4] RFC 6069 Making TCP More Robust to LCDs December 2010

 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

Zimmermann & Hannemann Experimental [Page 5] RFC 6069 Making TCP More Robust to LCDs December 2010

 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.

Zimmermann & Hannemann Experimental [Page 6] RFC 6069 Making TCP More Robust to LCDs December 2010

 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.

Zimmermann & Hannemann Experimental [Page 7] RFC 6069 Making TCP More Robust to LCDs December 2010

 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).

Zimmermann & Hannemann Experimental [Page 8] RFC 6069 Making TCP More Robust to LCDs December 2010

 (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).

Zimmermann & Hannemann Experimental [Page 9] RFC 6069 Making TCP More Robust to LCDs December 2010

 (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

Zimmermann & Hannemann Experimental [Page 10] RFC 6069 Making TCP More Robust to LCDs December 2010

 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

Zimmermann & Hannemann Experimental [Page 13] RFC 6069 Making TCP More Robust to LCDs December 2010

 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

Zimmermann & Hannemann Experimental [Page 14] RFC 6069 Making TCP More Robust to LCDs December 2010

 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 := [].

Zimmermann & Hannemann Experimental [Page 15] RFC 6069 Making TCP More Robust to LCDs December 2010

       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.

Zimmermann & Hannemann Experimental [Page 16] RFC 6069 Making TCP More Robust to LCDs December 2010

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.

Zimmermann & Hannemann Experimental [Page 17] RFC 6069 Making TCP More Robust to LCDs December 2010

 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

Zimmermann & Hannemann Experimental [Page 18] RFC 6069 Making TCP More Robust to LCDs December 2010

 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

Zimmermann & Hannemann Experimental [Page 19] RFC 6069 Making TCP More Robust to LCDs December 2010

 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.

Zimmermann & Hannemann Experimental [Page 20] RFC 6069 Making TCP More Robust to LCDs December 2010

 [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.

Zimmermann & Hannemann Experimental [Page 21] RFC 6069 Making TCP More Robust to LCDs December 2010

 [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.

Zimmermann & Hannemann Experimental [Page 22] RFC 6069 Making TCP More Robust to LCDs December 2010

 [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

Zimmermann & Hannemann Experimental [Page 23]

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