Network Working Group                                          L. Eggert
Internet-Draft                                                S. Schuetz
Expires: December 30, 2005                                     S. Schmid
                                                           June 28, 2005

              TCP Extensions for Immediate Retransmissions

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document classifies connectivity disruptions along an end-to-end
   path into four types and describes how standard TCP mechanisms can
   lead to inefficient or over-aggressive sending behavior when
   connectivity resumes.  The proposed techniques for TCP mobility

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   detection and response (LMDR) can improve behavior for some types of
   disruptions.  This document describes another, complementary and
   orthogonal modification to TCP's retransmission scheme that improves
   performance for disruption types that TCP LMDR does not address.
   This extension is based on connectivity indicators, i.e., generic
   network events that may indicate that end-to-end connectivity has
   resumed.  This document focuses on TCP modifications that use
   connectivity indicators to increase performance, but does not define
   the specifics of such connectivity indicators itself.

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Conventions  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.   Classification of Connectivity Disruptions . . . . . . . . .   4
     3.1  Short Connectivity Disruptions . . . . . . . . . . . . . .   5
     3.2  Long Connectivity Disruptions  . . . . . . . . . . . . . .   5
   4.   Examples of Connectivity Indicators  . . . . . . . . . . . .   8
   5.   TCP Immediate Retransmission Extension . . . . . . . . . . .   9
     5.1  Variant Based on Fast Retransmit . . . . . . . . . . . . .  10
     5.2  Variant Based on Retransmission Option . . . . . . . . . .  11
     5.3  Discussion . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.   Related Work . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.   Security Considerations  . . . . . . . . . . . . . . . . . .  14
   8.   Conclusion . . . . . . . . . . . . . . . . . . . . . . . . .  14
   9.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  14
   10.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  14
   11.  References . . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.1   Normative References . . . . . . . . . . . . . . . . . .  15
     11.2   Informative References . . . . . . . . . . . . . . . . .  15
        Editorial Comments . . . . . . . . . . . . . . . . . . . . .  17
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  17
   A.   Document Revision History  . . . . . . . . . . . . . . . . .  18
        Intellectual Property and Copyright Statements . . . . . . .  20

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1.  Introduction

   Depending on the specific path between two nodes in the Internet,
   disruptions in connectivity may be frequent.  Host mobility and other
   factors can further increase the likelihood of connectivity
   disruptions.  When hosts communicate with the Transmission Control
   Protocol (TCP) [RFC0793], their connections may either abort during
   periods of disrupted connectivity or exhibit significant performance
   reduction compared to permanently connected paths [SCHUETZ-
   THESIS][SCHUETZ-CCR].  This decrease in performance is mainly caused
   by TCP's retransmission behavior after connectivity returns.

   While connection aborts due to TCP's "user timeout" [TCP-ILLUSTRATED]
   can be prevented with the TCP User Timeout Option [I-D.ietf-tcpm-tcp-
   uto], additional mechanisms are required to improve TCP performance
   when operating along paths with frequent connectivity disruptions.

   This document describes a modification of TCP's retransmission scheme
   to improve performance in such scenarios.  The basic idea is to
   trigger a speculative retransmission attempt when a TCP
   implementation receives an indication that connectivity to a
   previously unreachable peer node may have returned. [anchor2] These
   extensions increase TCP performance through more intelligent
   scheduling of retransmission attempts by using periods of
   connectivity more efficiently.  They do not cause significant amounts
   of additional traffic (at most three empty ACKs in some situations,
   see Section 5.1) and do not change TCP's congestion control

   Note that this draft treats connectivity indicators as relatively
   abstract events and focuses on how TCP should react when receiving
   such triggers.  Specific connectivity indicators will be described in
   separate drafts.  Simple connectivity indicators (some of which have
   already been experimentally implemented [SCHUETZ-THESIS][SCHUETZ-
   CCR]) include link- and network-layer events occurring at the first
   and last hop of a path, i.e., on those links that are observable
   directly from the end hosts.  Examples of such simple connectivity
   indicators are DHCP lease events [RFC2131], IPv6 router
   advertisements [RFC2460], or MobileIP [RFC3775] binding updates and
   HIP [I-D.ietf-hip-arch] readdressing events.  It is also conceivable
   to use events inside the network, which are not directly observable
   by the end hosts, as connectivity triggers.  However, it is currently
   unclear how to signal these events to the end hosts.

   Section 3 discusses TCP performance over intermittently connected
   paths in more detail, classifies different types of disruptions and
   describes their implications on TCP.  Section 4 presents examples for
   connectivity indicators and Section 5 describes the proposed

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   "immediate retransmission" extension to TCP.  Section 6 discusses
   related work, Section 7 investigates security aspects of the proposed
   modification and Section 8 summarizes and concludes this document.

2.  Conventions

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

3.  Classification of Connectivity Disruptions

   Connectivity disruptions occur in many different situations.  They
   can be due to wireless interference, movement out of a wireless
   coverage area, switching between access networks, or simply due to
   unplugging an Ethernet cable.  Depending on the situation in which
   they occur, the implications of connectivity disruptions are
   different and must be handled appropriately.  This section attempts
   to classify different types of connectivity disruptions and discusses
   their implications and effects on TCP.

   Two main properties affect how TCP reacts to a connectivity
   disruption: the duration of a connectivity disruption and whether the
   path characteristics have significantly changed after it ends.  This
   draft distinguishes between "short" and "long" disconnections and
   "changed" and "unchanged" path characteristics.

   Connectivity disruptions are "short" for a given TCP connection, if
   connectivity returns before the RTO first fires.  In this case,
   standard TCP recovers lost data segments through Fast Retransmit and
   lost ACKs through successfully delivered later ACKs.  Section 3.1
   briefly describes this case.

   Connectivity disruptions are "long" for a given TCP connection, if
   the RTO fires at least once before connectivity returns.  In this
   case, TCP can be inefficient in its retransmission scheme, as
   described in Section 3.2.

   Whether or not path characteristics change when connectivity returns
   is a second important factor for TCP's retransmission scheme.
   Standard TCP implicitly assumes that path characteristics remain
   unchanged for short disruptions by performing Fast Retransmit based
   on path parameters collected before the disruption.  For long
   disruptions, standard TCP is more conservative and performs slow-
   start, re-probing the path characteristics from scratch.

   These implicit assumptions can cause standard TCP to misbehave or
   perform inefficiently in some scenarios.  Figure 1 illustrates the

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   standard TCP behavior.

          Short    | Fast Retransmit      | Fast Retransmit      |
          Duration | using collected path | using collected path |
          < RTO    | characteristics      | characteristics      |
          Long     |                      |                      |
          Duration | Slow-start           | Slow-start           |
          >= RTO   |                      |                      |
                       Unchanged Path         Changed Path
                       Characteristics        Characteristics

                      Figure 1: Standard TCP behavior

3.1  Short Connectivity Disruptions

   For short disruptions, standard TCP performs Fast Retransmit based on
   the previously collected path characteristics.  When path
   characteristics remain unchanged after connectivity resumes, this is
   an effective and efficient mechanism.

   On the other hand, when path characteristics do change after a short
   disruption, TCP may be too aggressive in its sending behavior.  This
   can occur, for example, after MobileIP handovers.  A TCP extension to
   inform a peer about such events and trigger the probing behavior is
   currently being proposed [I-D.swami-tcp-lmdr].  The LMDR extension
   involves forced invalidation of collected path characteristics and
   re-probing the new path from scratch.

3.2  Long Connectivity Disruptions

   For long disruptions, standard TCP performs slow-start after
   connectivity returns.  This is a conservative strategy that avoids
   overloading the new path.  However, TCP's general exponential back-
   off retransmission strategy can time these slow-starts such that
   performance decreases.

   This section describes this issue in detail.  Later sections of this
   document propose a different method of scheduling retransmission
   attempts after connectivity returns, which results in a more
   efficient retransmission behavior.

   When a long connectivity disruption occurs along the path between a
   host and its peer while the host is transmitting data, it stops
   receiving acknowledgments.  After the retransmission timeout (RTO)

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   expires, the host attempts to retransmit the first unacknowledged
   segment.  TCP implementations that follow the recommended RTO
   management proposed in [RFC2988] double the RTO after each
   retransmission attempt until it exceeds 60 seconds.  This scheme
   causes a host to attempt to retransmit across established connections
   roughly once a minute.  (More frequently during the first minute or
   two of the connectivity disruption, while the RTO is still being
   backed off.)

   When the long connectivity disruption ends, standard TCP
   implementations still wait until the RTO expires before attempting
   retransmission.  Figure 2 illustrates this behavior.  Depending on
   when connectivity becomes available again, this can waste up to a
   minute of connection time for TCPs that implement the recommended RTO
   management described in [RFC2988].  For TCP implementations that do
   not implement [RFC2988], even more connection time may be lost.  For
   example, Linux uses 120 seconds as the maximum RTO.

          number      X = Successfully transmitted segment
           ^          O = Lost segment
           |     :                     :              : X
           |     :                     :              :X
           |     OO O  O    O        O :              X
           |    X:                     :              :
           |   X :                     :<------------>:
           |  X  :                     :    Wasted    :
           | X   :                     :  connection  :
           |X    :                     :     time     :
                 :                     :              :       Time
            Connectivity          Connectivity       TCP
               gone                  back         retransmit

       Figure 2: Standard TCP behavior in the presence of disrupted

   This retransmission behavior is not efficient, especially in
   scenarios where connected periods are short and connectivity
   disruptions are frequent [DRIVE-THRU].  Experiments show that TCP
   performance across a path with frequent disruptions is significantly
   worse, compared to a similar path without disruptions [SCHUETZ-

   In the ideal case, TCP would attempt a retransmission as soon as
   connectivity to its peer was re-established.  Figure 3 illustrates
   the ideal behavior.

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          number      X = Successfully transmitted segment
           ^          O = Lost segment
           |     :                     : X            :
           |     :                     :X             :
           |     OO O  O    O        O X              :
           |    X:                     :              :
           |   X :                     :<------------>:
           |  X  :                     :  Efficiency  :
           | X   :                     :  improvement :
           |X    :                     :              :
                 :                     :              :       Time
            Connectivity          Connectivity      Next
               gone             back = immediate  scheduled
                                 TCP retransmit   retransmit

         Figure 3: Ideal TCP behavior in the presence of disrupted

   The ideal behavior is difficult to achieve for arbitrary connectivity
   disruptions.  One obviously problematic approach would use higher-
   frequency retransmission attempts to enable earlier detection of
   whether connectivity has returned.  This can generate significant
   amounts of extra traffic.  Other proposals attempt to trigger faster
   retransmissions by retransmitting buffered or newly-crafted segments
   from inside the network [SCOTT][I-D.dawkins-trigtran-
   linkup][RFC3819].  Section 6 compares these approaches to the
   "immediate retransmission" extension.

   Note that scenarios exist where path characteristics remain unchanged
   after long connectivity disruptions.  In this case, even an
   intelligently scheduled slow-start is inefficient, because TCP could
   safely resume transmitting at the old rate instead of slow-starting.
   Although originally developed to avoid line-rate bursts, techniques
   for the well-known "slow-start after idle" case [I-D.ietf-tcpimpl-
   restart] may be useful to further improve performance after a
   disruption ends.  This document does not currently describe this
   additional optimization.

   Also note that on asymmetric paths, TCP's standard retransmission
   behavior can be over-aggressive in some scenarios.  If the forward
   and reverse paths have significantly different RTTs, a long
   disruption for the host that sends along the lower-delay path may
   still be a short disruption for its peer host, because of its
   correspondingly longer RTO.  When the sender resumes and slow-starts,
   the peer performs fast retransmit, which can lead to line-rate
   bursts.  An LMDR-like mechanism that resets collected path

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   characteristics and forces a slow-start re-probe could be effective
   in these cases [I-D.swami-tcp-lmdr].

   The following sections of this document focus on mechanisms to
   improve TCP's efficiency when resuming after long disruptions.  They
   use connectivity indicators to schedule retransmission attempts more
   effectively than TCP's standard exponential back-off scheme.  To
   improve behavior after short disruptions, the mechanisms in
   [I-D.swami-tcp-lmdr] are effective.  Eventually, the TCP-option-based
   mechanism described there may be extended to incorporate the
   retransmission option described in Section 5.2 below.

4.  Examples of Connectivity Indicators

   This section describes examples of connectivity indicators, which the
   retransmission mechanism described in the next section acts upon.
   This document does not define the specifics of such connectivity
   indicators but merely discusses them to illustrate the operation of
   the "immediate retransmission" extension.

   Connectivity indicators signal TCP when connectivity to a previously
   unreachable peer may have returned.  They depend on the specifics of
   a node and its environment, for example network-layer mechanisms such
   as DHCP [RFC2131], MobileIP [RFC3775] or HIP [I-D.ietf-hip-arch].
   The IETF's Detection of Network Attachment (DNA) working group
   currently investigates the specifics of providing such connectivity
   indicators [I-D.ietf-dna-goals].

   One example of a connectivity indicator is "next hop reachable."
   This indicator could occur if a combination of the following
   conditions is true, depending on host specifics:

   o  Network-layer connectivity along the path to the destination
      returned, e.g., the outbound interface has an IP address and a
      next-hop router is known, maybe due to DHCP [RFC2131] or IPv6
      router advertisements [RFC2460].

   o  Link-layer connectivity of the link to the next-hop router along
      the path to the destination returned (e.g., link-layer "link up").

   o  Other local conditions that affect reachability of the destination
      are satisfied (e.g., IKE exchanges [RFC2409], MobileIP binding
      updates [RFC3775] or HIP readdressing [I-D.nikander-hip-mm] have

   The "next hop reachable" connectivity indicator only depends on
   locally determinable information (e.g., state of directly-connected
   links, etc.) and does not require network cooperation.  It can signal

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   TCP to restart active connections across intermittently connected
   links where disruptions occur on the first or last hop.  This simple
   indicator has the potential to improve TCP performance in many cases,
   because connectivity disruptions at the first or last hop are
   arguably the most common cause of connectivity disruptions in today's

   A second, more general example of a connectivity indicator would be
   "end-to-end connectivity returned."  If hosts have the ability to
   detect or be notified of connectivity changes inside the network
   (i.e., not only at the first or last hop), a more general
   retransmission mechanism could act on those pieces of information.
   This can improve TCP performance across intermittently connected
   paths where disruptions occur at arbitrary links along the path, even
   inside the network.  However, providing this more general
   connectivity indicator is problematic due to its dependence on remote
   information and its related issues, such as trust.

   Connectivity indicators are generally asymmetric, i.e., they may
   occur on one peer host but not the other.  As discussed above, a
   local event at one host may trigger the "immediate retransmission"
   mechanism, while the other host is unable to detect this event across
   the network.  Symmetric connectivity indicators are a special case
   and always occur concurrently at both communicating hosts.  Examples
   for such symmetric connectivity indicators are handshake events such
   as IKE exchanges or HIP readdressing.  Symmetric indicators are an
   important special case, because the retransmission procedure required
   in response to a symmetric indicator is simpler than that for an
   asymmetric one.  The next section will describe this in detail.

5.  TCP Immediate Retransmission Extension

   This section describes the main contribution of this document, i.e.,
   TCP extensions for immediate retransmission in response to
   connectivity indicators.  The basic idea behind the "immediate
   retransmission" extension is to allow TCP to resume stalled
   connections as soon as it receives an indicator that connectivity to
   previously unreachable peers may have returned.

   This document does not specify how TCP determines which connections a
   specific connectivity indicator affects, i.e., for which connections
   it should initiate retransmission attempts.  This is a property of
   individual connectivity indicators.  For example, the "next hop
   reachable" indicator described in the previous section affects
   connections to all destinations routed through that hop.

   It is important to note that this retransmission extension does not
   modify TCP's basic congestion control, fairness properties or slow-

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   start algorithms.  The only difference in TCP behavior is the timing
   of retransmission events and, in some cases, a minor, fixed increase
   in the number of initially retransmitted segments.  The "immediate
   retransmission" extension increases performance through better
   utilization of connected periods, not through sending traffic at a
   faster rate or modifying TCP's congestion control mechanisms.

   Hosts that implement the "immediate retransmission" TCP extension
   MUST implement the following retransmission mechanism whenever they
   receive a connectivity indicator:

   When receiving a symmetric or asymmetric connectivity indicator,
   conforming TCP implementations MUST immediately initiate the standard
   retransmission procedure for connections that the connectivity
   indicator affects - just as if the RTO for those connections had

   If the connectivity indicator is symmetric, i.e., all peers receive
   it concurrently; this simple change is sufficient to kick-start the
   relevant TCP connections.

   If the connectivity indicator is asymmetric, this simple extension is
   not always sufficient, because only one peer has received the
   indicator.  In case the host receiving the connectivity indicator has
   no (or too little) unacknowledged data awaiting retransmission, it
   will not emit enough segments to cause its peer node, which may have
   unacknowledged data as well, to attempt retransmission.  Transmission
   would thus only resume in one direction, which is ineffective for
   two-way communication.

   To avoid this issue, conforming TCP implementation MUST perform a
   different retransmission procedure in response to an asymmetric
   connectivity indicator.  The following sections describe two
   alternative TCP modifications that aim to improve retransmission
   behavior after receiving an asymmetric connectivity indicator.
   Section 5.1 describes the first variant.  As described in an earlier
   revision of this document, this variant generates duplicate ACKs to
   activate the peer's fast retransmit algorithm.  Section 5.2 describes
   the second variant, based on an explicit, new TCP "immediate
   retransmission" option.

5.1  Variant Based on Fast Retransmit

   This variant of improving TCP retransmission scheme based on
   connectivity indicators uses duplicate ACKs.  Conforming TCPs MUST
   send at least four segments that all acknowledge the last segment
   received from a peer for all connections that the connectivity
   indicator affects.  These triple-duplicate ACKs will activate the

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   peers' fast retransmit algorithms and cause them to immediately
   restart communication in the reverse direction, i.e., before their
   next scheduled retransmission.

   In this variant, if a TCP connection that a connectivity indicator
   affects has four or more unacknowledged data segments in the
   retransmission queue, it SHOULD piggyback the triple-duplicate ACK to
   the regular retransmissions of those data segments.  In this case,
   the "immediate retransmission" TCP extension does not require
   additional messages, compared to standard TCP.

   For connections where the retransmission queue contains only three or
   less unacknowledged data segments, TCP implementations supporting the
   "immediate retransmission" TCP extension MUST send additional pure
   ACKs until they have sent a complete triple-duplicate ACK.  In the
   worst case, when the retransmission queue is empty, this scheme
   requires four additional ACKs, compared to standard TCP.

   After the peer's fast retransmit algorithm sends the assumed missing
   segment, TCP performs either fast recovery or a slow-start [RFC2581],
   depending on the length of the connectivity disruption.  If the
   connectivity indicator occurs before the RTO, i.e., for very short
   disruptions, TCP has not yet lost its ACK clock and can thus perform
   fast recovery.  After longer disruptions, TCP falls back to slow-
   start to restart the ACK clock, just as it does at the beginning of a

   The result of this modification is twofold.  First, TCP connections
   receiving the connectivity indicator attempt retransmission of their
   unacknowledged segments before the next scheduled RTO.  This
   increases utilization of connected periods.  Second, TCP connections
   receiving the connectivity indicator use an existing TCP mechanism
   (triple-duplicate ACK) to signal their peer.  Although the peer may
   not have received a connectivity indicator itself (e.g., the
   indicator was asymmetric), this causes it to attempt faster
   retransmission as well.

   As mentioned above, the "immediate retransmission" scheme can
   generate up to four additional segments, compared to standard TCP.
   All additional segments are pure ACKs and hence small, resulting in a
   minor total overhead.  Furthermore, measurements have shown that
   increasing TCP's initial window is not problematic [ALLMAN]; this may
   indicate that a minor increase in traffic at retransmission time may
   be tolerable as well.

5.2  Variant Based on Retransmission Option

   Unlike the mechanism described in the previous section, the second

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   variant described in this section does not overload an existing TCP
   mechanism - i.e., fast retransmit - to improve retransmission after a
   connectivity disruption.  Generating duplicate ACKs in the manner
   described in Section 5.1 was criticized by some working group
   participants as "an abuse of a well-defined TCP mechanism for an
   unrelated purpose."  The variant described in this section uses a new
   TCP "Immediate Retransmission" Option to explicitly signal to the
   remote peer that should attempt a retransmit.  It was originally
   suggested by Kacheong Poon [POON] and the specifics are currently
   under investigation; this section describes the current state of this

   When receiving a connectivity indicator, conforming TCP
   implementations MUST send a single segment to each affected peer.
   This segment MUST contain the TCP Immediate Retransmission Option and
   may either be a queued data retransmission or a pure ACK, if the
   connection has no data awaiting retransmission.

   Upon reception of the TCP Immediate Retransmission Option, conforming
   TCPs MUST immediately attempt a retransmit.  This could either be a
   fast retransmit for short disruptions or a slow-start for long

   Note that the same effect can be achieved by overloading the TCP LMDR
   option  [I-D.swami-tcp-lmdr], i.e., sending it in response to a
   connectivity indicator, and triggering a retransmission attempt upon
   reception.  Overloading LMDR in this way also has the benefit of
   being able to force a reset of accumulated path state followed by a
   slow-start.  However, a flag in the option would be needed to trigger
   a retransmit without this reset, to cover all types of disruptions
   discussed in Section 3.  Given the similar motivation and relatively
   orthogonal, complementary mechanisms proposed here and in [I-D.swami-
   tcp-lmdr], this may be attractive solution.

5.3  Discussion

   One major drawback of using a retransmission option compared to the
   one based on  fast retransmit is that it requires both communicating
   TCPs to implement this modification.  Triggering a peer's fast
   retransmit with duplicate ACKs only requires the triggering local
   peer to support this extension - the triggered remote peer may run an
   unmodified TCP stack.  Additionally, firewalls may block segments
   carrying unknown TCP options.  Finally, TCP option space is becoming

   One major advantage of using an option, however, is that it avoids
   overloading an established TCP mechanism to produce side effects.
   Using an option also allows a more careful definition of the

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   semantics of the mechanism, e.g., control of whether and when it
   should reset accumulated path state or what transmission scheme to
   schedule retransmits with.

6.  Related Work

   Several other approaches try to improve TCP performance in the
   presence of connectivity disruptions [SCOTT][I-D.dawkins-trigtran-
   linkup][RFC3819].  They attempt to improve TCP startup after a
   connectivity disruption by retransmitting buffered or newly-crafted
   segments from inside the network.

   These proposals can be problematic, because TCP is built on the
   assumption that segments older than the maximum segment lifetime
   (MSL) of 2 minutes [RFC0793] will never be received.  When a
   connectivity disruption lasts longer than the MSL, either these
   proposals will become ineffective or they risk leaking buffered old
   segments onto new connections, violating TCP's semantics.

   The "immediate retransmission" modification also improves performance
   over a path with frequent connectivity disruptions.  The basic idea
   is to schedule an additional, speculative retransmission attempt when
   a TCP implementation receives an indication that connectivity to a
   peer node has returned.  Unlike the other proposals, the "immediate
   retransmission" scheme uses regular retransmissions, i.e.,
   retransmits data that is buffered at the end systems.  Because that
   data has not entered the network yet, it is not subject to the
   problematic MSL rule.  Consequently, the "immediate retransmission"
   scheme remains effective even for connectivity disruption longer than
   the MSL, without the risk of compromising connection integrity.

   Other transport-layer approaches such as the Explicit Link Failure
   Notification [HOLLAND] or TCP-F [CHANDRAN] use specific messages
   generated by intermediate routers to inform TCP senders about
   disrupted paths.  The former extends the TCP state machine with a new
   "stand by" state during which the standard retransmission timers are
   disabled.  In this state, TCP periodically probes the network to
   detect connectivity reestablishment.  Depending on the frequency of
   the probes and the network environment, this can cause significant
   amounts of extra traffic.  TCP-F completely suspends ongoing
   connections until receiving "route reestablishment notifications"
   that indicate peer reachability.  Both proposals are primarily
   designed for ad hoc networks and rely on changes to intermediate
   routers, whereas the "immediate retransmission" extension only
   requires end system support.

   ATCP [ATCP] uses a similar approach as the Explicit Link Failure
   Notification, but discovers link failures through ICMP Destination

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   Unreachable messages.  Caceres and Iftode [CACERES] propose and
   evaluate a solution similar to the TCP "immediate" retransmission
   extension that improves performance during MobileIP hand-offs.
   Unlike the solution proposed in this paper, the hand-off mechanism
   targets connectivity disruptions of a few seconds.

7.  Security Considerations

   To protect against abuse of the TCP "immediate retransmission"
   extension, e.g., denial-of-service attacks by flooding TCP with
   connectivity indicators, a control mechanism that "rate-limits" these
   indicators may be effective.  This document does not currently
   discuss the security aspects of connectivity indicators and the
   "immediate retransmission" extension to TCP.

8.  Conclusion

   This document described the "immediate retransmission" extension to
   TCP's standard retransmission scheme.  The new extension improves
   performance across intermittently connected paths through additional,
   speculative retransmission attempts upon receiving external
   connectivity indicators.  One example of such a connectivity
   indicator is "first hop router reachable."  This document did not
   define the specifics of such connectivity indicators, although it
   described some examples to illustrate the operation of the "immediate
   retransmission" extension, which is its main contribution.

9.  IANA Considerations

   This section is to be interpreted according to [RFC2434].

   This document does not define any new namespaces.  It uses an 8-bit
   TCP option number maintained by IANA at

10.  Acknowledgments

   The following people have helped to improve this document through
   thoughtful suggestions and feedback: Marcus Brunner, Wesley Eddy,
   Kacheong Poon, Juergen Quittek and Joe Touch.

   The authors are partly funded by Ambient Networks, a research project
   supported by the European Commission under its Sixth Framework
   Program.  The views and conclusions contained herein are those of the
   authors and should not be interpreted as necessarily representing the
   official policies or endorsements, either expressed or implied, of
   the Ambient Networks project or the European Commission.

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11.  References

11.1  Normative References

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

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

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC2581]  Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
              Control", RFC 2581, April 1999.

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

11.2  Informative References

   [ALLMAN]   Allman, M., Hayes, C., and S. Ostermann, "An Evaluation of
              TCP with Larger Initial Windows.", ACM Computer
              Communication Review, Vol. 28, No. 3, July 1998.

   [ATCP]     Liu, J. and S. Singh, "ATCP: TCP for Mobile Ad Hoc
              Networks", IEEE Journal on Selected Areas in
              Communication, Vol. 19, No. 7, July 2001.

   [CACERES]  Caceres, R. and L. Iftode, "Improving the Performance of
              Reliable Transport Protocols in Mobile Computing
              Environments", IEEE Journal on Selected Areas in
              Communication, Vol. 13, No. 5, 1995.

              Chandran, K., Raghunathan, S., Venkatesan, S., and R.
              Prakash, "A Feedback Based Scheme For Improving TCP
              Performance In Ad-Hoc Wireless Networks", IEEE Personal
              Communication Systems (PCS) Magazine: Special Issue on Ad
              Hoc Networks, Vol. 8, No. 1, February 2001.

              Ott, J. and D. Kutscher, "Drive-Thru Internet: IEEE
              802.11b for Automobile Users", Proc. Infocom 2004,
              March 2004.

   [HOLLAND]  Holland, G. and N. Vaidya, "Analysis of TCP Performance

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              over Mobile Ad Hoc Networks", Proc. 5th Annual ACM/IEEE
              International Conference on Mobile Computing and
              Networking, Seattle, WA, USA, 1999.

              Dawkins, S., "End-to-end, Implicit 'Link-Up'
              Notification", draft-dawkins-trigtran-linkup-01 (work in
              progress), October 2003.

              Choi, J., "Goals of Detecting Network Attachment in IPv6",
              draft-ietf-dna-goals-04 (work in progress), December 2004.

              Moskowitz, R., "Host Identity Protocol Architecture",
              draft-ietf-hip-arch-02 (work in progress), January 2005.

              Hughes, A., Touch, J., and J. Heidemann, "Issues in TCP
              Slow-Start Restart After Idle",
              draft-ietf-tcpimpl-restart-00 (work in progress),
              March 1998.

              Eggert, L. and F. Gont, "TCP User Timeout Option",
              draft-ietf-tcpm-tcp-uto-00 (work in progress), May 2005.

              Nikander, P., "End-Host Mobility and Multi-Homing with
              Host Identity Protocol", draft-nikander-hip-mm-02 (work in
              progress), July 2004.

              Swami, Y. and K. Le, "Lightweight Mobility Detection and
              Response (LMDR) Algorithm for TCP",
              draft-swami-tcp-lmdr-05 (work in progress), February 2005.

   [POON]     Poon, K., "Personal Communication", August 2004.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

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

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   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 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.

              Schuetz, S., Eggert, L., Schmid, S., and M. Brunner,
              "Protocol Enhancements for Intermittently Connected
              Hosts", To appear: ACM Computer Communication Review, Vol.
              35, No. 3, July 2005.

              Schuetz, S., "Network Support for Intermittently Connected
              Mobile Nodes", Diploma Thesis, University of Mannheim,
              Germany, June 2004.

   [SCOTT]    Scott, J. and G. Mapp, "Link layer-based TCP optimisation
              for disconnecting networks", ACM Computer Communication
              Review, Vol. 33, No. 5, October 2003.

              Stevens, W., "TCP/IP Illustrated, Volume 1: The
              Protocols", Addison-Wesley , 1994.

Editorial Comments

   [anchor2]  LE: The authors have seen the idea of triggering
              retransmits based on connectivity events of directly-
              connected links attributed to Phil Karn, but were unable
              to locate a specific reference. Pointers to a citable
              reference are highly appreciated!

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Authors' Addresses

   Lars Eggert
   NEC Network Laboratories
   Kurfuerstenanlage 36
   Heidelberg  69115

   Phone: +49 6221 90511 43
   Fax:   +49 6221 90511 55

   Simon Schuetz
   NEC Network Laboratories
   Kurfuerstenanlage 36
   Heidelberg  69115

   Phone: +49 6221 90511 65
   Fax:   +49 6221 90511 55

   Stefan Schmid
   NEC Network Laboratories
   Kurfuerstenanlage 36
   Heidelberg  69115

   Phone: +49 6221 90511 54
   Fax:   +49 6221 90511 55

Appendix A.  Document Revision History

   | Revision  | Comments                                              |
   | 00        | Initial version.                                      |
   | 01        | Updated terminology according to [SCHUETZ-CCR]. Added |
   |           | "retransmission option" variant as Section 5.2.       |

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   | 02        | Added classification of connectivity disruptions and  |
   |           | describe the design space in relation to              |
   |           | [I-D.swami-tcp-lmdr] and [I-D.ietf-tcpimpl-restart].  |
   |           | Updated references.                                   |

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