draft-ietf-tcpm-tcp-lcd-01.txt   draft-ietf-tcpm-tcp-lcd-02.txt 
TCP Maintenance and Minor A. Zimmermann TCP Maintenance and Minor A. Zimmermann
Extensions (TCPM) WG A. Hannemann Extensions (TCPM) WG A. Hannemann
Internet-Draft RWTH Aachen University Internet-Draft RWTH Aachen University
Intended status: Experimental March 30, 2010 Intended status: Experimental July 29, 2010
Expires: October 1, 2010 Expires: January 30, 2011
Making TCP more Robust to Long Connectivity Disruptions (TCP-LCD) Making TCP more Robust to Long Connectivity Disruptions (TCP-LCD)
draft-ietf-tcpm-tcp-lcd-01 draft-ietf-tcpm-tcp-lcd-02
Abstract Abstract
Disruptions in end-to-end path connectivity, which last longer than Disruptions in end-to-end path connectivity, which last longer than
one retransmission timeout, cause suboptimal TCP performance. The one retransmission timeout, cause suboptimal TCP performance. The
reason for this performance degradation is that TCP interprets reason for this performance degradation is that TCP interprets
segment loss induced by long connectivity disruptions as a sign of segment loss induced by long connectivity disruptions as a sign of
congestion, resulting in repeated retransmission timer backoffs. congestion, resulting in repeated retransmission timer backoffs.
This, in turn, leads to a delayed detection of the re-establishment This, in turn, leads to a delayed detection of the re-establishment
of the connection since TCP waits for the next retransmission timeout of the connection since TCP waits for the next retransmission timeout
before it attempts a retransmission. before it attempts a retransmission.
This document proposes an algorithm to make TCP more robust to long This document proposes an algorithm to make TCP more robust to long
connectivity disruptions (TCP-LCD). It describes how standard ICMP connectivity disruptions (TCP-LCD). It describes how standard ICMP
messages can be exploited during timeout-based loss recovery to messages can be exploited during timeout-based loss recovery to
disambiguate true congestion loss from non-congestion loss caused by disambiguate true congestion loss from non-congestion loss caused by
connectivity disruptions. Moreover, a revert strategy of the connectivity disruptions. Moreover, a reversion strategy of the
retransmission timer is specified that enables a more prompt retransmission timer is specified that enables a more prompt
detection of whether or not the connectivity to a previously detection of whether or not the connectivity to a previously
disconnected peer node has been restored. TCP-LCD is a TCP sender- disconnected peer node has been restored. TCP-LCD is a TCP sender-
only modification that effectively improves TCP performance in case only modification that effectively improves TCP performance in case
of connectivity disruptions. of connectivity disruptions.
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Connectivity Disruption Indication . . . . . . . . . . . . . . 6 3. Connectivity Disruption Indication . . . . . . . . . . . . . . 6
4. Connectivity Disruption Reaction . . . . . . . . . . . . . . . 8 4. Connectivity Disruption Reaction . . . . . . . . . . . . . . . 8
4.1. Basic Idea . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Basic Idea . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Algorithm Details . . . . . . . . . . . . . . . . . . . . 8 4.2. Algorithm Details . . . . . . . . . . . . . . . . . . . . 8
5. Discussion of TCP-LCD . . . . . . . . . . . . . . . . . . . . 11 5. Discussion of TCP-LCD . . . . . . . . . . . . . . . . . . . . 11
5.1. Retransmission Ambiguity . . . . . . . . . . . . . . . . . 12 5.1. Retransmission Ambiguity . . . . . . . . . . . . . . . . . 12
5.2. Wrapped Sequence Numbers . . . . . . . . . . . . . . . . . 13 5.2. Wrapped Sequence Numbers . . . . . . . . . . . . . . . . . 12
5.3. Packet Duplication . . . . . . . . . . . . . . . . . . . . 14 5.3. Packet Duplication . . . . . . . . . . . . . . . . . . . . 14
5.4. Probing Frequency . . . . . . . . . . . . . . . . . . . . 14 5.4. Probing Frequency . . . . . . . . . . . . . . . . . . . . 14
5.5. Reaction during Connection Establishment . . . . . . . . . 14 5.5. Reaction during Connection Establishment . . . . . . . . . 14
5.6. Reaction in Steady-State . . . . . . . . . . . . . . . . . 15 5.6. Reaction in Steady-State . . . . . . . . . . . . . . . . . 15
6. Dissolving Ambiguity Issues (the Safe Variant) . . . . . . . . 15 6. Dissolving Ambiguity Issues using the TCP Timestamps Option . 15
7. Interoperability Issues . . . . . . . . . . . . . . . . . . . 17 7. Interoperability Issues . . . . . . . . . . . . . . . . . . . 17
7.1. Detection of TCP Connection Failures . . . . . . . . . . . 17 7.1. Detection of TCP Connection Failures . . . . . . . . . . . 17
7.2. Explicit Congestion Notification . . . . . . . . . . . . . 17 7.2. Explicit Congestion Notification . . . . . . . . . . . . . 17
7.3. ICMP for IP version 6 . . . . . . . . . . . . . . . . . . 18 7.3. ICMP for IP version 6 . . . . . . . . . . . . . . . . . . 18
7.4. TCP-LCD and IP Tunnels . . . . . . . . . . . . . . . . . . 18 7.4. TCP-LCD and IP Tunnels . . . . . . . . . . . . . . . . . . 18
8. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 19 8. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20 10. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. Normative References . . . . . . . . . . . . . . . . . . . 21 12.1. Normative References . . . . . . . . . . . . . . . . . . . 21
12.2. Informative References . . . . . . . . . . . . . . . . . . 21 12.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Changes from previous versions of the draft . . . . . 24 Appendix A. Changes from previous versions of the draft . . . . . 23
A.1. Changes from draft-ietf-tcpm-tcp-lcd-00 . . . . . . . . . 24 A.1. Changes from draft-ietf-tcpm-tcp-lcd-01 . . . . . . . . . 24
A.2. Changes from draft-zimmermann-tcp-lcd-02 . . . . . . . . . 24 A.2. Changes from draft-ietf-tcpm-tcp-lcd-00 . . . . . . . . . 24
A.3. Changes from draft-zimmermann-tcp-lcd-01 . . . . . . . . . 25 A.3. Changes from draft-zimmermann-tcp-lcd-02 . . . . . . . . . 24
A.4. Changes from draft-zimmermann-tcp-lcd-00 . . . . . . . . . 25 A.4. Changes from draft-zimmermann-tcp-lcd-01 . . . . . . . . . 25
A.5. Changes from draft-zimmermann-tcp-lcd-00 . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
1. Terminology 1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
The reader should be familiar with the algorithm and terminology from The reader should be familiar with the algorithm and terminology from
[RFC2988], which defines the standard algorithm Transmission Control [RFC2988], which defines the standard algorithm Transmission Control
Protocol (TCP) senders are required to use to compute and manage Protocol (TCP) senders are required to use to compute and manage
their retransmission timer. In this document the terms their retransmission timer. In this document, the terms
"retransmission timer" and "retransmission timeout" are used as "retransmission timer" and "retransmission timeout" are used as
defined in [RFC2988]. The retransmission timer ensures data delivery defined in [RFC2988]. The retransmission timer ensures data delivery
in the absence of any feedback from the receiver. The duration of in the absence of any feedback from the receiver. The duration of
this timer is referred to as retransmission timeout (RTO). this timer is referred to as retransmission timeout (RTO).
As defined in [RFC0793], the term "acceptable acknowledgment (ACK)" As defined in [RFC0793], the term "acceptable acknowledgment (ACK)"
refers to a TCP segment that acknowledges previously unacknowledged refers to a TCP segment that acknowledges previously unacknowledged
data. The TCP sender state variable "SND.UNA" and the current data. The TCP sender state variable "SND.UNA" and the current
segment variable "SEG.SEQ" are used as defined in [RFC0793]. SND.UNA segment variable "SEG.SEQ" are used as defined in [RFC0793]. SND.UNA
holds the segment sequence number of earliest segment that has not holds the segment sequence number of earliest segment that has not
been acknowledged by the TCP receiver (the oldest outstanding been acknowledged by the TCP receiver (the oldest outstanding
segment). SEG.SEQ is the segment sequence number of a given segment. segment). SEG.SEQ is the segment sequence number of a given segment.
For the purposes of this specification we define the term "timeout- For the purposes of this specification, we define the term "timeout-
based loss recovery" that refers to the state, which a TCP sender based loss recovery" that refers to the state that a TCP sender
enters upon the first timeout of the oldest outstanding segment enters upon the first timeout of the oldest outstanding segment
(SND.UNA) and leaves upon the arrival of the *first* acceptable ACK. (SND.UNA) and leaves upon the arrival of the *first* acceptable ACK.
It is important to note that other documents use a different It is important to note that other documents use a different
interpretation of the term "timeout-based loss recovery". For interpretation of the term "timeout-based loss recovery". For
example the NewReno modification to TCP's Fast Recovery algorithm example, the NewReno modification to TCP's Fast Recovery algorithm
[RFC3782] extents the period a TCP sender remains in timeout-based [RFC3782] extents the period a TCP sender remains in timeout-based
loss recovery compared to the one defined in this document. This is loss recovery compared to the one defined in this document. This is
because [RFC3782] attempts to avoid unnecessary multiple Fast because [RFC3782] attempts to avoid unnecessary multiple Fast
Retransmits that can occur after an RTO. Retransmits that can occur after an RTO.
2. Introduction 2. Introduction
Connectivity disruptions can occur in many different situations. The Connectivity disruptions can occur in many different situations. The
frequency of connectivity disruptions depends on the property of the frequency of connectivity disruptions depends on the properties of
end-to-end path between the communicating hosts. While connectivity the end-to-end path between the communicating hosts. While
disruptions can occur in traditional wired networks too, e.g., caused connectivity disruptions can occur in traditional wired networks,
by an unplugged network cable, the likelihood of occurrence is e.g., caused by an unplugged network cable, the likelihood of their
significantly higher in wireless (multi-hop) networks. Especially, occurrence is significantly higher in wireless (multi-hop) networks.
end-host mobility, network topology changes, and wireless Especially, end-host mobility, network topology changes, and wireless
interferences are crucial factors. In the case of the Transmission interferences are crucial factors. In the case of the Transmission
Control Protocol (TCP) [RFC0793], the performance of the connection Control Protocol (TCP) [RFC0793], the performance of the connection
can experience a significant reduction compared to a permanently can experience a significant reduction compared to a permanently
connected path [SESB05]. This is because TCP, which was originally connected path [SESB05]. This is because TCP, which was originally
designed to operate in fixed and wired networks, generally assumes designed to operate in fixed and wired networks, generally assumes
that the end-to-end path connectivity is relatively stable over the that the end-to-end path connectivity is relatively stable over the
connection's lifetime. connection's lifetime.
Depending on their duration connectivity disruptions can be Depending on their duration, connectivity disruptions can be
classified into two groups [I-D.schuetz-tcpm-tcp-rlci]: "short" and classified into two groups [I-D.schuetz-tcpm-tcp-rlci]: "short" and
"long". A connectivity disruption is "short" if connectivity returns "long". A connectivity disruption is "short" if connectivity returns
before the retransmission timer fires for the first time. In this before the retransmission timer fires for the first time. In this
case, TCP recovers lost data segments through Fast Retransmit and case, TCP recovers lost data segments through Fast Retransmit and
lost acknowledgments (ACK) through successfully delivered later ACKs. lost acknowledgments (ACK) through successfully delivered later ACKs.
Connectivity disruptions are declared as "long" for a given TCP Connectivity disruptions are declared as "long" for a given TCP
connection if the retransmission timer fires at least once before connection if the retransmission timer fires at least once before
connectivity is resumed. Whether or not path characteristics, like connectivity is resumed. Whether or not path characteristics, like
the round trip time (RTT) or the available bandwidth, have changed the round trip time (RTT) or the available bandwidth, have changed
when connectivity resumes after a disruption is another important when connectivity resumes after a disruption is another important
aspect for TCP's retransmission scheme [I-D.schuetz-tcpm-tcp-rlci]. aspect for TCP's retransmission scheme [I-D.schuetz-tcpm-tcp-rlci].
This document improves TCP's behavior in case of "long connectivity This document improves TCP's behavior in case of "long connectivity
disruptions". In particular, it focuses on the period "prior" to the disruptions". In particular, it focuses on the period prior to the
re-establishment of the connectivity to a previously disconnected re-establishment of the connectivity to a previously disconnected
peer node. The document does not describe any modifications of TCP's peer node. The document does not describe any modifications to TCP's
behavior and its congestion control mechanisms [RFC5681] "after" behavior and its congestion control mechanisms [RFC5681] after
connectivity has been restored. connectivity has been restored.
When a long connectivity disruption occurs on a TCP connection the When a long connectivity disruption occurs on a TCP connection, the
TCP sender eventually does not receive any more acknowledgments. TCP sender eventually does not receive any more acknowledgments.
After the retransmission timer expires, the TCP sender enters the After the retransmission timer expires, the TCP sender enters the
timeout-based loss recovery and declares the oldest outstanding timeout-based loss recovery and declares the oldest outstanding
segment (SND.UNA) as lost. Since TCP tightly couples reliability and segment (SND.UNA) as lost. Since TCP tightly couples reliability and
congestion control, the retransmission of SND.UNA is triggered congestion control, the retransmission of SND.UNA is triggered
together with the reduction of the transmission rate. This is based together with the reduction of the transmission rate. This is based
on the assumption that segment loss is an indication of congestion on the assumption that segment loss is an indication of congestion
[RFC5681]. As long as the connectivity disruption persists, TCP will [RFC5681]. As long as the connectivity disruption persists, TCP will
repeat this procedure until the oldest outstanding segment has repeat this procedure until the oldest outstanding segment has
successfully been acknowledged, or until the connection has timed successfully been acknowledged, or until the connection has timed
out. TCP implementations that follow the recommended retransmission out. TCP implementations that follow the recommended retransmission
timeout (RTO) management of RFC 2988 [RFC2988] double the RTO after timeout (RTO) management of RFC 2988 [RFC2988] double the RTO after
each retransmission attempt. However, the RTO's growth may be each retransmission attempt. However, the RTO growth may be bounded
bounded by an upper limit, the maximum RTO, which is at least 60s, by an upper limit, the maximum RTO, which is at least 60s, but may be
but may be longer: Linux, for example, uses 120s. If connectivity is longer: Linux, for example, uses 120s. If connectivity is restored
restored between two retransmission attempts, TCP still has to wait between two retransmission attempts, TCP still has to wait until the
until the retransmission timer expires before resuming transmission, retransmission timer expires before resuming transmission, since it
since it simply does not have any means to know if the connectivity simply does not have any means to know if the connectivity has been
has been re-established. Therefore, depending on when connectivity re-established. Therefore, depending on when connectivity becomes
becomes available again, this can waste up to a maximum RTO of available again, this can waste up to a maximum RTO of possible
possible transmission time. transmission time.
This retransmission behavior is not efficient, especially in This retransmission behavior is not efficient, especially in
scenarios with long connectivity disruptions. In the ideal case, TCP scenarios with long connectivity disruptions. In the ideal case, TCP
would attempt a retransmission as soon as connectivity to its peer would attempt a retransmission as soon as connectivity to its peer
has been re-established. In this document, we specify a TCP sender- has been re-established. In this document, we specify a TCP sender-
only modification to provide robustness to long connectivity only modification to provide robustness to long connectivity
disruptions (TCP-LCD). The memo describes how the standard Internet disruptions (TCP-LCD). The memo describes how the standard Internet
Control Message Protocol (ICMP) can be exploited during timeout-based Control Message Protocol (ICMP) can be exploited during timeout-based
loss recovery to identify non-congestion loss caused by long loss recovery to identify non-congestion loss caused by long
connectivity disruptions. TCP-LCD's revert strategy of the connectivity disruptions. TCP-LCD's reversion strategy of the
retransmission timer enables higher-frequency retransmissions and retransmission timer enables higher-frequency retransmissions and
thereby a prompt detection when connectivity to a previously thereby a prompt detection when connectivity to a previously
disconnected peer node has been restored. If no congestion is disconnected peer node has been restored. If no congestion is
present, TCP-LCD approaches the ideal behavior. present, TCP-LCD approaches the ideal behavior.
3. Connectivity Disruption Indication 3. Connectivity Disruption Indication
If the queue of an intermediate router experiencing a link outage can If the queue of an intermediate router that is experiencing a link
buffer all incoming packets, a connectivity disruption will only outage can buffer all incoming packets, a connectivity disruption
cause a variation in delay, which is handled well by TCP will only cause a variation in delay, which is handled well by TCP
implementations using either Eifel [RFC3522], [RFC4015] or Forward implementations using either Eifel [RFC3522], [RFC4015] or Forward
RTO-Recovery (F-RTO) [RFC5682]. However, if the link outage lasts RTO-Recovery (F-RTO) [RFC5682]. However, if the link outage lasts
for too long, the router experiencing the link outage is forced to for too long, the router experiencing the link outage is forced to
drop packets, and finally to discard the according route. Means to drop packets, and finally to discard the according route. Means to
detect such link outages include reacting on failed address detect such link outages include reacting on failed address
resolution protocol (ARP) [RFC0826] queries, unsuccessful link resolution protocol (ARP) [RFC0826] queries, unsuccessful link
sensing, and the like. However, this is solely in the responsibility sensing, and the like. However, this is solely in the responsibility
of the respective router. of the respective router.
Note: The focus of this memo is on introducing a method how ICMP Note: The focus of this memo is on introducing a method how ICMP
messages may be exploited to improve TCP's performance; how messages may be exploited to improve TCP's performance; how
different physical and link layer mechanisms below the network different physical and link layer mechanisms below the network
layer may trigger ICMP destination unreachable messages are out of layer may trigger ICMP destination unreachable messages are out of
scope of this memo. scope of this memo.
Provided that no other route to the specific destination exists the Provided that no other route to the specific destination exists, the
router will notify the corresponding sending host about the dropped router will notify the corresponding sending host about the dropped
packets via ICMP destination unreachable messages of code 0 (net packets via ICMP destination unreachable messages of code 0 (net
unreachable) or code 1 (host unreachable) [RFC1812]. Therefore, the unreachable) or code 1 (host unreachable) [RFC1812]. Therefore, the
sending host can use the ICMP destination unreachable messages of sending host can use the ICMP destination unreachable messages of
these codes as an indication for a connectivity disruption, since the these codes as an indication for a connectivity disruption, since the
reception of these messages provide evidence that packets were reception of these messages provide evidence that packets were
dropped due to a link outage. dropped due to a link outage.
Note that there are also other ICMP destination unreachable messages Note that there are also other ICMP destination unreachable messages
with different codes. Some of them are candidates for connectivity with different codes. Some of them are candidates for connectivity
disruption indications, too, but need further investigation. For disruption indications, too, but need further investigation. For
example, ICMP destination unreachable messages with code 5 (source example, ICMP destination unreachable messages with code 5 (source
route failed), code 11 (net unreachable for TOS), or code 12 (host route failed), code 11 (net unreachable for TOS), or code 12 (host
unreachable for TOS) [RFC1812]. On the other hand, codes that flag unreachable for TOS) [RFC1812]. On the other hand, codes that flag
hard errors are of no use for the proposed scheme, since TCP should hard errors are of no use for this scheme, since TCP should abort the
abort the connection when those are received [RFC1122]. In the connection when those are received [RFC1122]. In the following, the
following, the term "ICMP unreachable message" is used as synonym for term "ICMP unreachable message" is used as synonym for ICMP
ICMP destination unreachable messages of code 0 or code 1. destination unreachable messages of code 0 or code 1.
The accurate interpretation of ICMP unreachable messages as a The accurate interpretation of ICMP unreachable messages as a
connectivity disruption indication is complicated by the following connectivity disruption indication is complicated by the following
two peculiarities of ICMP messages. Firstly, they do not necessarily two peculiarities of ICMP messages. First, they do not necessarily
operate on the same timescale as the packets, i.e., TCP segments that 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 elicited them. When a router drops a packet due to a missing route,
it will not necessarily send an ICMP unreachable message immediately, it will not necessarily send an ICMP unreachable message immediately,
but will rather queue it for later delivery. Secondly, ICMP messages but will rather queue it for later delivery. Second, ICMP messages
are subject to rate limiting, e.g., when a router drops a whole are subject to rate limiting, e.g., when a router drops a whole
window of data due to a link outage, it will hardly send as many ICMP window of data due to a link outage, it is unlikely to send as many
unreachable messages as it dropped TCP segments. Depending on the ICMP unreachable messages as dropped TCP segments. Depending on the
load of the router it may even send no ICMP unreachable messages at load of the router, it may not even send any ICMP unreachable
all. Both peculiarities originate from [RFC1812]. messages at all. Both peculiarities originate from [RFC1812].
Fortunately, according to [RFC0792], ICMP unreachable messages have Fortunately, according to [RFC0792], ICMP unreachable messages have
to contain in their body the entire Internet Protocol (IP) header to contain in their body the entire Internet Protocol (IP) header
[RFC0791] of the datagram eliciting the ICMP unreachable message, [RFC0791] of the datagram eliciting the ICMP unreachable message,
plus the first 64 bits of the payload of that datagram. This allows plus the first 64 bits of the payload of that datagram. This allows
the sending host to match the ICMP error message to the transport the sending host to match the ICMP error message to the transport
that elicited it. RFC 1812 [RFC1812] augments the requirements and connection that elicited it. RFC 1812 [RFC1812] augments these
states that ICMP messages should contain as much of the original requirements and states that ICMP messages should contain as much of
datagram as possible without the length of the ICMP datagram the original datagram as possible without the length of the ICMP
exceeding 576 bytes. Therefore, in case of TCP, at least the source datagram exceeding 576 bytes. Therefore, in case of TCP, at least
port number, the destination port number, and the 32-bit TCP sequence the source port number, the destination port number, and the 32-bit
number are included. This allows the originating TCP to demultiplex TCP sequence number are included. This allows the originating TCP to
the received ICMP message and to identify the faulty connection. demultiplex the received ICMP message and to identify the affected
Moreover, it can identify which segment of the respective connection connection. Moreover, it can identify which segment of the
triggered the ICMP unreachable message, unless there are several respective connection triggered the ICMP unreachable message, unless
segments in-flight with the same sequence number (see Section 5.1). there are several segments in-flight with the same sequence number
(see Section 5.1).
A connectivity disruption indication in form of an ICMP unreachable A connectivity disruption indication in form of an ICMP unreachable
message associated with a presumably lost TCP segment provides strong message associated with a presumably lost TCP segment provides strong
evidence that the segment was not dropped due to congestion, but was evidence that the segment was not dropped due to congestion, but was
successfully delivered to the temporary end-point of the employed successfully delivered as far as the reporting router. It therefore
path, i.e., the reporting router. It therefore did not witness any did not witness any congestion at least on that part of the path that
congestion at least on that part of the path that was traversed by was traversed by both the TCP segment eliciting the ICMP unreachable
both the TCP segment eliciting the ICMP unreachable message as well message as well as the ICMP unreachable message itself.
as the ICMP unreachable message itself.
4. Connectivity Disruption Reaction 4. Connectivity Disruption Reaction
Section 4.1 introduces the basic idea of TCP-LCD. The complete Section 4.1 introduces the basic idea of TCP-LCD. The complete
algorithm is specified in Section 4.2. algorithm is specified in Section 4.2.
4.1. Basic Idea 4.1. Basic Idea
The goal of the algorithm is to promptly detect when connectivity to The goal of the algorithm is to promptly detect when connectivity to
a previously disconnected peer node has been restored after a long a previously disconnected peer node has been restored after a long
connectivity disruption, while retaining appropriate behavior in case connectivity disruption, while retaining appropriate behavior in case
of congestion. TCP-LCD exploits standard ICMP unreachable messages of congestion. TCP-LCD exploits standard ICMP unreachable messages
during timeout-based loss recovery. This increases TCP's during timeout-based loss recovery. This increases TCP's
retransmission frequency by undoing one retransmission timer backoff retransmission frequency by undoing one retransmission timer backoff
whenever an ICMP unreachable message reports on the sequence number whenever an ICMP unreachable message is received that contains a
of a presumably lost retransmission. segment with a sequence number of a presumably lost retransmission.
This approach has the advantage of appropriately reducing the probing This approach has the advantage of appropriately reducing the probing
rate in case of congestion. If either the retransmission itself, or rate in case of congestion. If either the retransmission itself or
the corresponding ICMP message, is dropped the previously performed the corresponding ICMP message is dropped the previously performed
retransmission timer backoff is not undone, which effectively halves retransmission timer backoff is not undone, which effectively halves
the probing rate. the probing rate.
4.2. Algorithm Details 4.2. Algorithm Details
A TCP sender using RFC 2988 [RFC2988] to compute TCP's retransmission A TCP sender that uses RFC 2988 [RFC2988] to compute TCP's
timer MAY employ the following scheme to avoid over-conservative retransmission timer MAY employ the following scheme to avoid over-
retransmission timer backoffs in case of long connectivity conservative retransmission timer backoffs in case of long
disruptions. If a TCP sender does implement the following steps, the connectivity disruptions. If a TCP sender does implement the
algorithm MUST be initiated upon the first timeout of the oldest following steps, the algorithm MUST be initiated upon the first
outstanding segment (SND.UNA) and MUST be stopped upon the arrival of timeout of the oldest outstanding segment (SND.UNA) and MUST be
the first acceptable ACK. The algorithm MUST NOT be re-initiated stopped upon the arrival of the first acceptable ACK. The algorithm
upon subsequent timeouts for the same segment. The scheme SHOULD NOT MUST NOT be re-initiated upon subsequent timeouts for the same
be used in SYN-SENT or SYN-RECEIVED states [RFC0793] (i.e., during segment. The scheme SHOULD NOT be used in SYN-SENT or SYN-RECEIVED
connection establishment). states [RFC0793] (see Section 5.5).
A TCP sender that does not employ RFC 2988 [RFC2988] to compute TCP's A TCP sender that does not employ RFC 2988 [RFC2988] to compute TCP's
retransmission timer SHOULD NOT use TCP-LCD. We envision that the retransmission timer MUST NOT use TCP-LCD. We envision that the
scheme could be easily adapted to algorithms others than RFC 2988. scheme could be easily adapted to algorithms others than RFC 2988.
However, we leave this as future work. However, we leave this as future work.
In rule (2.5) RFC 2988 [RFC2988] provides the option to place a In rule (2.5), RFC 2988 [RFC2988] provides the option to place a
maximum value on the RTO. When a TCP implements this rule to provide maximum value on the RTO. When a TCP implements this rule to provide
an upper bound for the RTO, it SHOULD also be used in the following 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 algorithm. In particular, if the RTO is bounded by an upper limit
(maximum RTO), the "MAX_RTO" variable used in this scheme SHOULD be (maximum RTO), the "MAX_RTO" variable used in this scheme MUST be
initialized with this upper limit. Otherwise, if the RTO is initialized with this upper limit. Otherwise, if the RTO is
unbounded, the "MAX_RTO" variable SHOULD be set to infinity. unbounded, the "MAX_RTO" variable MUST be set to infinity.
The scheme specified in this document uses the "BACKOFF_CNT" The scheme specified in this document uses the "BACKOFF_CNT"
variable, whose initial value is zero. The variable is used to count variable, whose initial value is zero. The variable is used to count
the number of performed retransmission timer backoffs during one the number of performed retransmission timer backoffs during one
timeout-based loss recovery. Moreover, the "RTO_BASE" variable is timeout-based loss recovery. Moreover, the "RTO_BASE" variable is
used to recover the previous RTO if the retransmission timer backoff used to recover the previous RTO if the retransmission timer backoff
was unnecessary. The variable is initialized with the RTO upon was unnecessary. The variable is initialized with the RTO upon
initiation of timeout-based loss recovery. initiation of timeout-based loss recovery.
(1) Before TCP updates the variable "RTO" when it initiates timeout- (1) Before TCP updates the variable "RTO" when it initiates timeout-
skipping to change at page 10, line 11 skipping to change at page 10, line 11
else else
proceed to step (3). proceed to step (3).
(5) Extract the TCP segment header included in the ICMP unreachable (5) Extract the TCP segment header included in the ICMP unreachable
message "ICMP_DU": message "ICMP_DU":
SEG := Extract(ICMP_DU). SEG := Extract(ICMP_DU).
(6) If "SEG.SEQ == SND.UNA", i.e., if the TCP segment "SEG" (6) If "SEG.SEQ == SND.UNA", i.e., if the TCP segment "SEG"
eliciting the ICMP unreachable message "ICMP_DU" carries the eliciting the ICMP unreachable message "ICMP_DU" contains the
sequence number of a retransmission, then sequence number of a retransmission, then
proceed to step (7); proceed to step (7);
else else
proceed to step (3). proceed to step (3).
(7) Undo the last retransmission timer backoff: (7) Undo the last retransmission timer backoff:
skipping to change at page 10, line 38 skipping to change at page 10, line 38
proceed to step (R); proceed to step (R);
else else
proceed to step (3). proceed to step (3).
(A) This is a placeholder for standard TCP's behavior in case an (A) This is a placeholder for standard TCP's behavior in case an
acceptable ACK has arrived. No further processing. acceptable ACK has arrived. No further processing.
When a TCP in steady-state detects a segment loss using the When a TCP in steady-state detects a segment loss using the
retransmission timer it enters the timeout-based loss recovery and retransmission timer, it enters the timeout-based loss recovery and
initiates the algorithm (step 1). It adjusts the slow start initiates the algorithm (step 1). It adjusts the slow start
threshold (ssthresh), sets the congestion window (CWND) to one threshold (ssthresh), sets the congestion window (CWND) to one
segment, backs off the retransmission timer, and retransmits the segment, backs off the retransmission timer, and retransmits the
first unacknowledged segment (step R) [RFC5681], [RFC2988]. To first unacknowledged segment (step R) [RFC5681], [RFC2988]. To
account for the expiration of the retransmission timer the TCP sender account for the expiration of the retransmission timer, the TCP
increments the "BACKOFF_CNT" variable by one (step 2). sender increments the "BACKOFF_CNT" variable by one (step 2).
In case the retransmission timer expires again (step 3a) a TCP will In case the retransmission timer expires again (step 3a), a TCP will
repeat the retransmission of the first unacknowledged segment and repeat the retransmission of the first unacknowledged segment and
back off the retransmission timer once more (step R) [RFC2988] as back off the retransmission timer once more (step R) [RFC2988], as
well as increment the "BACKOFF_CNT" variable by one (step 2). Note 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 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 maximum value on the RTO that may result in not performing the
retransmission timer backoff. However, step (2) MUST always and retransmission timer backoff. However, step (2) MUST always and
unconditionally be applied, no matter whether or not the unconditionally be applied, no matter whether or not the
retransmission timer is actually backed off. In other words, each retransmission timer is actually backed off. In other words, each
time the retransmission timer expires, the "BACKOFF_CNT" variable time the retransmission timer expires, the "BACKOFF_CNT" variable
MUST be incremented by one. MUST be incremented by one.
If the first received packet after the retransmission(s) is an If the first received packet after the retransmission(s) is an
acceptable ACK (step 3b), a TCP will proceed as normal, i.e., slow acceptable ACK (step 3b), a TCP will proceed as normal, i.e., slow
start the connection and terminate the algorithm (step A). Later start the connection and terminate the algorithm (step A). Later
ICMP unreachable messages from the just terminated timeout-based loss ICMP unreachable messages from the just terminated timeout-based loss
recovery are ignored since the ACK clock is already restarting due to recovery are ignored, since the ACK clock is already restarting due
the successful retransmission. to the successful retransmission.
On the other hand, if the first received packet after the On the other hand, if the first received packet after the
retransmission(s) is an ICMP unreachable message (step 3c), and if retransmission(s) is an ICMP unreachable message (step 3c), and if
step (4) permits it, a TCP SHOULD undo one backoff for each ICMP step (4) permits it, TCP SHOULD undo one backoff for each ICMP
unreachable message reporting an error on a retransmission. To unreachable message reporting an error on a retransmission. To
decide if an ICMP unreachable message reports on a retransmission, decide if an ICMP unreachable message was elicited by a
the sequence number therein is exploited (step 5, step 6). The undo retransmission, the sequence number it contains is inspected (step 5,
is performed by re-calculating the RTO with the decremented step 6). The undo is performed by re-calculating the RTO with the
"BACKOFF_CNT" variable (step 7). This calculation explicitly matches decremented "BACKOFF_CNT" variable (step 7). This calculation
the (bounded) exponential backoff specified in rule (5.5) of explicitly matches the (bounded) exponential backoff specified in
[RFC2988]. rule (5.5) of [RFC2988].
Upon receipt of an ICMP unreachable message that legitimately undoes Upon receipt of an ICMP unreachable message that legitimately undoes
one backoff there is the possibility that the shortened one backoff, there is the possibility that the shortened
retransmission timer has already expired (step 8). Then, a TCP retransmission timer has already expired (step 8). Then, TCP SHOULD
SHOULD retransmit immediately, i.e., an ICMP message clocked retransmit immediately. In case the shortened retransmission timer
retransmission. In case the shortened retransmission timer has not has not yet expired, TCP MUST wait accordingly.
yet expired, TCP MUST wait accordingly.
5. Discussion of TCP-LCD 5. Discussion of TCP-LCD
TCP-LCD takes caution to only react to connectivity disruption TCP-LCD takes caution to only react to connectivity disruption
indications in form of ICMP unreachable messages during timeout-based indications in the form of ICMP unreachable messages during timeout-
loss recovery. Therefore, TCP's behavior is not altered when either based loss recovery. Therefore, TCP's behavior is not altered when
no ICMP unreachable messages are received, or the retransmission either no ICMP unreachable messages are received, or the
timer of the TCP sender did not expire since the last received retransmission timer of the TCP sender did not expire since the last
acceptable ACK. Thus, by defintion the algorithm triggers only in received acceptable ACK. Thus, by defintion, the algorithm triggers
case of long connectivity disruptions. only in the case of long connectivity disruptions.
Only such ICMP unreachable messages that report on the sequence Only such ICMP unreachable messages that contain a TCP segment with a
number of a retransmission, i.e., report on SND.UNA, are evaluated by the sequence number of a retransmission, i.e., contain SND.UNA, are
TCP-LCD. All other ICMP unreachable messages are ignored. The evaluated by TCP-LCD. All other ICMP unreachable messages are
arrival of those ICMP unreachable messages provides strong evidence ignored. The arrival of those ICMP unreachable messages provides
that the retransmissions were not dropped due to congestion but were strong evidence that the retransmissions were not dropped due to
successfully delivered to the temporary end-point of the employed congestion, but were successfully delivered to the reporting router.
path, i.e., the reporting router. In other words, there is no In other words, there is no evidence for any congestion at least on
evidence for any congestion at least on that very part of the path that very part of the path that was traversed by both the TCP segment
that was traveled by both, the TCP segment eliciting the ICMP eliciting the ICMP unreachable message as well as the ICMP
unreachable message as well as the ICMP unreachable message itself. unreachable message itself.
However, there are some situations where TCP-LCD makes a false However, there are some situations where TCP-LCD makes a false
decision and incorrectly undoes a retransmission timer backoff. This decision and incorrectly undoes a retransmission timer backoff. This
can happen, albeit the received ICMP unreachable message reports on can happen, even when the received ICMP unreachable message contains
the segment number of a retransmission (SND.UNA) because the TCP the segment number of a retransmission (SND.UNA), because the TCP
segment that elicited the ICMP unreachable message may either not be segment that elicited the ICMP unreachable message may either not be
a retransmission (Section 5.1), or does not belong to the current a retransmission (Section 5.1), or does not belong to the current
timeout-based loss recovery (Section 5.2). Finally, packet timeout-based loss recovery (Section 5.2). Finally, packet
duplication (Section 5.3) can also spuriously trigger the algorithm. duplication (Section 5.3) can also spuriously trigger the algorithm.
Section 5.4 discusses possible probing frequencies, while Section 5.6 Section 5.4 discusses possible probing frequencies, while Section 5.6
describes the motivation for not reacting on ICMP unreachable describes the motivation for not reacting to ICMP unreachable
messages while TCP is in steady-state. messages while TCP is in steady-state.
5.1. Retransmission Ambiguity 5.1. Retransmission Ambiguity
Historically, the retransmission ambiguity problem [Zh86], [KP87] is Historically, the retransmission ambiguity problem [Zh86], [KP87] is
the TCP sender's inability to distinguish whether the first the TCP sender's inability to distinguish whether the first
acceptable ACK after a retransmission refers to the original acceptable ACK after a retransmission refers to the original
transmission or to the retransmission. This problem occurs after transmission or to the retransmission. This problem occurs after
both a Fast Retransmit and a timeout-based retransmit. However, both a Fast Retransmit and a timeout-based retransmit. However,
modern TCP implementations can eliminate the retransmission ambiguity modern TCP implementations can eliminate the retransmission ambiguity
with either the help of Eifel [RFC3522], [RFC4015] or Forward RTO- with either the help of Eifel [RFC3522], [RFC4015] or Forward RTO-
Recovery (F-RTO) [RFC5682]. Recovery (F-RTO) [RFC5682].
The revert strategy of the given algorithm suffers from a form of The reversion strategy of the given algorithm suffers from a form of
retransmission ambiguity, too. In contrast to the above case, TCP retransmission ambiguity, too. In contrast to the above case, TCP
suffers from ambiguity regarding ICMP unreachable messages received suffers from ambiguity regarding ICMP unreachable messages received
during timeout-based loss recovery. With the TCP segment number during timeout-based loss recovery. With the TCP segment number
included in the ICMP unreachable message, a TCP sender is not able to included in the ICMP unreachable message, a TCP sender is not able to
determine if the ICMP unreachable message refers to the original determine if the ICMP unreachable message refers to the original
transmission or to any of the timeout-based retransmissions. That transmission or to any of the timeout-based retransmissions. That
is, there is an ambiguity which TCP segment an ICMP unreachable is, there is an ambiguity with regards to which TCP segment an ICMP
message reports on. unreachable message reports on.
However, for the algorithm this ambiguity is not considered to be a However, this ambiguity is not considered to be a problem for the
problem. The assumption that a received ICMP message provides algorithm. The assumption that a received ICMP message provides
evidence that a non-congestion loss caused by the connectivity evidence that a non-congestion loss caused by the connectivity
disruption was wrongly considered a congestion loss still holds, disruption was wrongly considered a congestion loss still holds,
regardless to which TCP segment, transmission or retransmission, the regardless to which TCP segment, transmission or retransmission, the
message refers. message refers.
5.2. Wrapped Sequence Numbers 5.2. Wrapped Sequence Numbers
Besides the ambiguity whether a received ICMP unreachable message Besides the ambiguity whether a received ICMP unreachable message
refers to the original transmission or to any of the retransmissions, refers to the original transmission or to any of the retransmissions,
there is another source of ambiguity about the TCP sequence numbers there is another source of ambiguity related to the TCP sequence
contained in ICMP unreachable messages. For high bandwidth paths numbers contained in ICMP unreachable messages. For high bandwidth
like modern gigabit links the sequence space may wrap rather quickly, paths, the sequence space may wrap quickly. This migth cause that
thereby allowing the possibility that delayed ICMP unreachable delayed ICMP unreachable messages may coincidentally fit as valid
messages - a router dropping packets due to a link outage is not input in the proposed scheme. As a result, the scheme may
obliged to send ICMP unreachable messages in a timely manner incorrectly undo retransmission timer backoffs. Chances for this to
[RFC1812] - may coincidentally fit as valid input in the proposed happen are minuscule, since a particular ICMP message would need to
scheme. As a result, the scheme may incorrectly undo retransmission contain the exact sequence number of the current oldest outstanding
timer backoffs. Chances for this to happen are minuscule, since a segment (SND.UNA), while at the same time TCP is in timeout-based
particular ICMP message would need to contain the exact sequence loss recovery. However, two "worst case" scenarios for the algorithm
number of the current oldest outstanding segment (SND.UNA), while at are possible:
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 For instance, consider a steady state TCP connection, which will be
disrupted at an intermediate router R due to a link outage. Upon the disrupted at an intermediate router R due to a link outage. Upon the
expiration of the RTO, the TCP sender enters the timeout-based loss expiration of the RTO, the TCP sender enters the timeout-based loss
recovery and starts to retransmit the earliest segment that has not recovery and starts to retransmit the earliest segment that has not
been acknowledged (SND.UNA). For some reason, router R delays all been acknowledged (SND.UNA). For some reason, router R delays all
corresponding ICMP unreachable messages so that the TCP sender corresponding ICMP unreachable messages so that the TCP sender backs
backoffs the retransmission timer normally without any undoing. At the retransmission timer off normally without any undoing. At the
the end of the connectivity disruption, the TCP sender eventually end of the connectivity disruption, the TCP sender eventually detects
detects the re-establishment, leaves the scheme and finally the the re-establishment, leaves the scheme and finally the timeout-based
timeout-based loss recovery, too. A sequence number wrap-around loss recovery, too. A sequence number wrap-around later, the
later, the connectivity between the two peers is disrupted again, but connectivity between the two peers is disrupted again, but this time
this time due to congestion and exactly at the time at which the due to congestion and exactly at the time at which the current
current SND.UNA matches the SND.UNA from the previous cycle. If SND.UNA matches the SND.UNA from the previous cycle. If router R
router R emits the delayed ICMP unreachable messages now, the TCP emits the delayed ICMP unreachable messages now, the TCP sender would
sender would incorrectly undo retransmission timer backoffs. As the incorrectly undo retransmission timer backoffs. As the TCP sequence
TCP sequence number contains 32 bits, the probability of this number contains 32 bits, the probability of this scenario is at most
scenario is at most 1/2^32. Given sufficiently many retransmissions 1/2^32. Given sufficiently many retransmissions in the first
in the first timeout-based loss recovery, the corresponding ICMP timeout-based loss recovery, the corresponding ICMP unreachable
unreachable messages could reduce the RTO in the second recovery at messages could reduce the RTO in the second recovery at most to
most to "RTO_BASE". However, once the ICMP unreachable messages are "RTO_BASE". However, once the ICMP unreachable messages are
depleted, the standard exponential backoff will be performed. Thus, depleted, the standard exponential backoff will be performed. Thus,
the congestion response will only be delayed by some false the congestion response will only be delayed by some false
retransmissions. retransmissions.
Similar to the above, consider the case where a steady state TCP Similar to the above, consider the case where a steady state TCP
connection with n segments in-flight will be disrupted at some point connection with n segments in flight will be disrupted at some point
due to a link outage by an intermediate router R. For each segment due to a link outage at an intermediate router R. For each segment in
in-flight, router R may generate an ICMP unreachable message. flight, router R may generate an ICMP unreachable message. However,
However, due to some reason it delays them. Once the link outage is due to some reason it delays them. Once the link outage is over and
over and the connection has been re-established, the TCP sender the connection has been re-established, the TCP sender leaves the
leaves the scheme and slow-starts the connection. Following a scheme and slow-starts the connection. Following a sequence number
sequence number wrap-around, a retransmission timeout occurs, just at wrap-around, a retransmission timeout occurs, just at the moment the
the moment the TCP sender's current window of data reaches the TCP sender's current window of data reaches the previous range of the
previous range of the sequence number space again. In case router R sequence number space again. In case router R emits the delayed ICMP
emits the delayed ICMP unreachable messages now, one spurious undoing unreachable messages now, spurious undoing of the retransmission
of the retransmission timer backoff is possible, if the TCP segment timer backoff is possible once, if the TCP segment number contained
number contained in ICMP unreachable messages matches the current in ICMP unreachable messages matches the current SND.UNA, and the
SND.UNA, and the timeout was a result of congestion. In the case of timeout was a result of congestion. In the case of another
another connectivity disruption, the additional undoing of the connectivity disruption, the additional undoing of the retransmission
retransmission timer backoff has no impact. The probability of this timer backoff has no impact. The probability of this scenario is at
scenario is at most n/2^32. most n/2^32.
5.3. Packet Duplication 5.3. Packet Duplication
In case an intermediate router duplicates packets, a TCP sender may In case an intermediate router duplicates packets, a TCP sender may
receive more ICMP unreachable messages during timeout-based loss receive more ICMP unreachable messages during timeout-based loss
recovery than it actually has sent timeout-based retransmissions. recovery than sent timeout-based retransmissions. However, since
However, since TCP-LCD keeps track of the number of performed TCP-LCD keeps track of the number of performed retransmission timer
retransmission timer backoffs in the "BACKOFF_CNT" variable, it will backoffs in the "BACKOFF_CNT" variable, it will not undo more
not undo more retransmission timer backoffs than were actually retransmission timer backoffs than were actually performed.
performed. Nevertheless, if packet duplication and congestion Nevertheless, if packet duplication and congestion coincide on the
coincide on the path between the two communicating hosts, duplicated path between the two communicating hosts, duplicated ICMP messages
ICMP messages could hide the congestion loss of some retransmissions could hide the congestion loss of some retransmissions or ICMP
or ICMP messages, and the algorithm may incorrectly undo messages, and the algorithm may incorrectly undo retransmission timer
retransmission timer backoffs. Considering the overall impact of a backoffs. Considering the overall impact of a router that duplicates
router that duplicates packets, the additional load induced by some packets, the additional load induced by some spurious timeout-based
spurious timeout-based retransmits can probably be neglected. retransmits can probably be neglected.
5.4. Probing Frequency 5.4. Probing Frequency
One could argue that if an ICMP unreachable message arrives for a One could argue that if an ICMP unreachable message arrives for a
timeout-based retransmission, the RTO shall be reset or recalculated, timeout-based retransmission, the RTO shall be reset or recalculated,
similar to what is done when an ACK arrives during timeout-based loss similar to what is done when an ACK arrives during timeout-based loss
recovery (see Karn's algorithm [KP87], [RFC2988]), and a new recovery (see Karn's algorithm [KP87], [RFC2988]), and a new
retransmission should be sent immediately. Generally, this would retransmission should be sent immediately. Generally, this would
allow for a much higher probing frequency based on the round trip allow for a much higher probing frequency based on the round trip
time up to the router where connectivity has been disrupted. time up to the router where connectivity has been disrupted.
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message is likely to be substantially shorter than the overall RTT to message is likely to be substantially shorter than the overall RTT to
the destination, the ICMP unreachable message may very well reach the the destination, the ICMP unreachable message may very well reach the
originating TCP while it is transmitting the current window of data. originating TCP while it is transmitting the current window of data.
In case the remaining window is large, it might seem appropriate to In case the remaining window is large, it might seem appropriate to
refrain from transmitting the remaining window as there is timely refrain from transmitting the remaining window as there is timely
evidence that it will only trigger further ICMP unreachable messages evidence that it will only trigger further ICMP unreachable messages
at the very router. Although this promises improvement from a at the very router. Although this promises improvement from a
wastage perspective, it may be counterproductive from a security wastage perspective, it may be counterproductive from a security
perspective. An attacker could forge such ICMP messages, thereby perspective. An attacker could forge such ICMP messages, thereby
forcing the originating TCP to stop sending data, very similar to the forcing the originating TCP to stop sending data, very similar to the
blind throughput-reduction attack mentioned in blind throughput-reduction attack mentioned in [RFC5927].
[I-D.ietf-tcpm-icmp-attacks].
An additional consideration is the following: in the presence of An additional consideration is the following: in the presence of
multi-path routing even the receipt of a legitimate ICMP unreachable multi-path routing, even the receipt of a legitimate ICMP unreachable
message cannot be exploited accurately because there is the option message cannot be exploited accurately, because there is the
that only one of the multiple paths to the destination is suffering possibility that only one of the multiple paths to the destination is
from a connectivity disruption, which causes ICMP unreachable suffering from a connectivity disruption, which causes ICMP
messages to be sent. Then, however, there is the possibility that unreachable messages to be sent. Then, however, there is the
the path along which the connectivity disruption occurred contributed possibility that the path along which the connectivity disruption
considerably to the overall bandwidth, such that a congestion occurred contributed considerably to the overall bandwidth, such that
response is very well reasonable. However, this is not necessarily a congestion response is very well reasonable. However, this is not
the case. Therefore, a TCP has no means except for its inherent necessarily the case. Therefore, a TCP has no means except for its
congestion control to decide on this matter. All in all, it seems inherent congestion control to decide on this matter. All in all, it
that for a connection in steady-state, i.e., not in timeout-based seems that for a connection in steady-state, i.e., not in timeout-
loss recovery, reacting on ICMP unreachable messages in regard to based loss recovery, reacting on ICMP unreachable messages in regard
congestion control is not appropriate. For the case of timeout-based to congestion control is not appropriate. For the case of timeout-
retransmissions, however, there is a reasonable congestion response, based retransmissions, however, there is a reasonable congestion
which is skipping further retransmission timer backoffs because there response, which is skipping further retransmission timer backoffs
is no congestion indication - as described above. because there is no congestion indication - as described above.
6. Dissolving Ambiguity Issues (the Safe Variant) 6. Dissolving Ambiguity Issues using the TCP Timestamps Option
Given that the TCP Timestamps option [RFC1323] is enabled for a If the TCP Timestamps option [RFC1323] is enabled for a connection, a
connection, a TCP sender MAY use the following algorithm to dissolve TCP sender SHOULD use the following algorithm to dissolve the
the ambiguity issues mentioned in Sections 5.1, 5.2, and 5.3. In ambiguity issues mentioned in Sections 5.1, 5.2, and 5.3. In
particular, both the retransmission ambiguity and the packet particular, both the retransmission ambiguity and the packet
duplication problems are prevented by the following TCP-LCD variant. duplication problems are prevented by the following TCP-LCD variant.
On the other hand, the false positives caused by wrapped sequence On the other hand, the false positives caused by wrapped sequence
numbers cannot be completely avoided, but the likelihood is further numbers cannot be completely avoided, but the likelihood is further
reduced by a factor of 1/2^32 since the Timestamp Value field (TSval) reduced by a factor of 1/2^32 since the Timestamp Value field (TSval)
of the TCP Timestamps Option contains 32 bits. of the TCP Timestamps Option contains 32 bits.
Hence, implementers may choose to implement the TCP-LCD with the Hence, implementers may choose to implement the TCP-LCD with the
following modifications. following modifications.
Step (1) is replaced by step (1'): Step (1) is replaced by step (1'):
(1') Before TCP updates the variable "RTO" when it initiates (1') Before TCP updates the variable "RTO" when it initiates
timeout-based loss recovery, set the variables "BACKOFF_CNT" timeout-based loss recovery, set the variables "BACKOFF_CNT"
and "RTO_BASE" and the data structure "RETRANS_TS" as follows: and "RTO_BASE" and the data structure "RETRANS_TS" as follows:
BACKOFF_CNT := 0; BACKOFF_CNT := 0;
RTO_BASE := RTO. RTO_BASE := RTO;
RETRANS_TS := []; RETRANS_TS := [].
Proceed to step (R). Proceed to step (R).
Step (2) is extended by step (2b): Step (2) is extended by step (2b):
(2b) Store the value of the Timestamp Value field (TSval) of the TCP (2b) Store the value of the Timestamp Value field (TSval) of the TCP
Timestamps option included in the retransmission "RET" sent in Timestamps option included in the retransmission "RET" sent in
step (R) into the "RETRANS_TS" data structure: step (R) into the "RETRANS_TS" data structure:
RETRANS_TS.add(RET.TSval) RETRANS_TS.add(RET.TSval)
Step (6) is replaced by step (6'): Step (6) is replaced by step (6'):
(6') If "SEG.SEQ == SND.UNA && RETRANS_TS.exists(SEQ.TSval)", i.e., (6') If "SEG.SEQ == SND.UNA && RETRANS_TS.exists(SEQ.TSval)", i.e.,
if the TCP segment "SEG" eliciting the ICMP unreachable message if the TCP segment "SEG" eliciting the ICMP unreachable message
"ICMP_DU" carries the sequence number of a retransmission, and "ICMP_DU" contains the sequence number of a retransmission, and
the value in its Timestamp Value field (TSval) is valid, then the value in its Timestamp Value field (TSval) is valid, then
proceed to step (7'); proceed to step (7');
else else
proceed to step (3). proceed to step (3).
Step (7) is replaced by step (7'): Step (7) is replaced by step (7'):
(7') Undo the last retransmission timer backoff: (7') Undo the last retransmission timer backoff:
RETRANS_TS.remove(SEQ.TSval); RETRANS_TS.remove(SEQ.TSval);
BACKOFF_CNT := BACKOFF_CNT - 1; BACKOFF_CNT := BACKOFF_CNT - 1;
RTO := min(RTO_BASE * 2^(BACKOFF_CNT), MAX_RTO). RTO := min(RTO_BASE * 2^(BACKOFF_CNT), MAX_RTO).
The downside of the safe variant is twofold. Firstly, the The downside of the this variant is twofold. First, the
modifications come at a cost: the TCP sender is required to store the modifications come at a cost: the TCP sender is required to store the
timestamps of all retransmissions sent during one timeout-based loss timestamps of all retransmissions sent during one timeout-based loss
recovery. Second, the safe variant can only undo a retransmission recovery. Second, this variant can only undo a retransmission timer
timer backoff if the intermediate router experiencing the link outage backoff if the intermediate router experiencing the link outage
implements [RFC1812] and chooses to include as many more than the implements [RFC1812] and chooses to include as many more than the
first 64 bits of the payload of the triggering datagram, as are first 64 bits of the payload of the triggering datagram, as are
needed to include the TCP Timestamps option in the ICMP unreachable needed to include the TCP Timestamps option in the ICMP unreachable
message. message.
7. Interoperability Issues 7. Interoperability Issues
This section discusses interoperability issues related to introducing This section discusses interoperability issues related to introducing
TCP-LCD. TCP-LCD.
7.1. Detection of TCP Connection Failures 7.1. Detection of TCP Connection Failures
TCP-LCD may have side-effects on TCP implementations that attempt to TCP-LCD may have side-effects on TCP implementations that attempt to
detect TCP connection failures by counting timeout-based detect TCP connection failures by counting timeout-based
retransmissions. RFC 1122 [RFC1122] states in Section 4.2.3.5 that a retransmissions. [RFC1122] states in Section 4.2.3.5 that a TCP host
TCP host must handle excessive retransmissions of data segments with must handle excessive retransmissions of data segments with two
two thresholds R1 and R2 measuring the number of retransmissions that thresholds R1 and R2 that measure the number of retransmissions that
have occurred for the same segment. Both thresholds might either be have occurred for the same segment. Both thresholds might either be
measured in time units or as a count of retransmissions. measured in time units or as a count of retransmissions.
Due to TCP-LCD's revert strategy of the retransmission timer, the Due to TCP-LCD's reversion strategy of the retransmission timer, the
assumption that a certain number of retransmissions corresponds to a assumption that a certain number of retransmissions corresponds to a
specific time interval no longer holds, as additional retransmissions specific time interval no longer holds, as additional retransmissions
may be performed during timeout-based-loss recovery to detect the end may be performed during timeout-based-loss recovery to detect the end
of the connectivity disruption. Therefore, a TCP employing TCP-LCD of the connectivity disruption. Therefore, a TCP employing TCP-LCD
either SHOULD measure the thresholds R1 and R2 in time units or, in either MUST measure the thresholds R1 and R2 in time units or, in
case R1 and R2 are counters of retransmissions, SHOULD convert them case R1 and R2 are counters of retransmissions, MUST convert them
into time intervals, which correspond to the time an unmodified TCP into time intervals, which correspond to the time an unmodified TCP
would need to reach the specified number of retransmissions. would need to reach the specified number of retransmissions.
7.2. Explicit Congestion Notification 7.2. Explicit Congestion Notification
By the use of Explicit Congestion Notification (ECN) [RFC3168] ECN- With Explicit Congestion Notification (ECN) [RFC3168], ECN-capable
capable routers are no longer limited to dropping packets as routers are no longer limited to dropping packets to indicate
congestion indication. Instead, they can set the Congestion congestion. Instead, they can set the Congestion Experienced (CE)
Experienced (CE) codepoint in the IP header to indicate congestion. codepoint in the IP header to indicate congestion. With TCP-LCD, it
may happen that during a connectivity disruption, a received ICMP
With TCP-LCD it may happen that during a connectivity disruption a unreachable message has been elicited by a timeout-based
received ICMP unreachable message has been elicited by a timeout- retransmission that was marked with the CE codepoint before reaching
based retransmission that was marked with the CE codepoint before the router experiencing the link outage. In such a case, a TCP
reaching the router experiencing the link outage. In such a case, we sender MUST, corresponding to [RFC3168] (Section 6.1.2), additionally
suggest that the TCP sender SHOULD additionally reset the reset the retransmission timer in case the algorithm undoes a
retransmission timer in case the algorithm undoes a retransmission retransmission timer backoff.
timer backoff.
7.3. ICMP for IP version 6 7.3. ICMP for IP version 6
RFC 4443 [RFC4443] specifies the Internet Control Message Protocol RFC 4443 [RFC4443] specifies the Internet Control Message Protocol
(ICMPv6) to be used with the Internet Protocol version 6 (IPv6) (ICMPv6) to be used with the Internet Protocol version 6 (IPv6)
[RFC2460]. From TCP-LCD's point of view, it is important to notice [RFC2460]. From TCP-LCD's point of view, it is important to notice
that for IPv6, the payload of an ICMPv6 error messages has to include that for IPv6, the payload of an ICMPv6 error messages has to include
as many bytes as possible from the IPv6 datagram that elicited the as many bytes as possible from the IPv6 datagram that elicited the
ICMPv6 error message, without making the error message exceed the ICMPv6 error message, without making the error message exceed the
minimum IPv6 MTU (1280 bytes) [RFC4443]. Thus, more information is minimum IPv6 MTU (1280 bytes) [RFC4443]. Thus, more information is
available for TCP-LCD as in the case of IPv4. available for TCP-LCD than in the case of IPv4.
The counterpart of the ICMPv4 destination unreachable message of code The counterpart of the ICMPv4 destination unreachable message of code
0 (net unreachable) and of code 1 (host unreachable) is the ICMPv6 0 (net unreachable) and of code 1 (host unreachable) is the ICMPv6
destination unreachable message of code 0 (no route to destination) destination unreachable message of code 0 (no route to destination)
[RFC4443]. As with IPv4, a router should generate an ICMPv6 [RFC4443]. As with IPv4, a router should generate an ICMPv6
destination unreachable message of code 0 in response to a packet destination unreachable message of code 0 in response to a packet
that cannot be delivered to its destination address because it lacks that cannot be delivered to its destination address because it lacks
a matching entry in its routing table. As a result, TCP-LCD can a matching entry in its routing table. As a result, TCP-LCD can
employ this ICMPv6 error messages as connectivity disruption employ this ICMPv6 error messages as connectivity disruption
indication, too. indication, too.
7.4. TCP-LCD and IP Tunnels 7.4. TCP-LCD and IP Tunnels
It is worth noting that IP tunnels, including IPsec [RFC4301], IP in It is worth noting that IP tunnels, including IPsec [RFC4301], IP in
IP [RFC2003], Generic Routing Encapsulation (GRE) [RFC2784], and IP [RFC2003], Generic Routing Encapsulation (GRE) [RFC2784], and
others are compatible with TCP-LCD, as long as the received ICMP others are compatible with TCP-LCD, as long as the received ICMP
unreachable messages can be demultiplexed and extracted appropriately unreachable messages can be demultiplexed and extracted appropriately
by the TCP sender during timeout-based loss recovery. by the TCP sender during timeout-based loss recovery.
If, for example, end-to-end tunnels like IPSec in transport mode If, for example, end-to-end tunnels like IPsec in transport mode
[RFC4301] are employed, a TCP sender may receive ICMP unreachable [RFC4301] are employed, a TCP sender may receive ICMP unreachable
messages where additional steps, e.g., decrypting in step (5) of the messages where additional steps, e.g., decrypting in step (5) of the
algorithm, are needed to extract the TCP header from these ICMP algorithm, are needed to extract the TCP header from these ICMP
messages. Provided that the received ICMP unreachable message messages. Provided that the received ICMP unreachable message
contains enough information, i.e., SEQ.SEG is extractable, these contains enough information, i.e., SEQ.SEG is extractable, this
information MAY still be used as a valid input for the proposed information can still be used as a valid input for the proposed
algorithm. algorithm.
Likewise, if IP encapsulation like [RFC2003] is used in some part of Likewise, if IP encapsulation like [RFC2003] is used in some part of
the path between the communicating hosts, the tunnel ingress node may the path between the communicating hosts, the tunnel ingress node may
receive the ICMP unreachable messages from an intermediate router receive the ICMP unreachable messages from an intermediate router
experiencing the link outage. Nevertheless, the tunnel ingress node experiencing the link outage. Nevertheless, the tunnel ingress node
may replay the ICMP unreachable messages in order to inform the TCP may replay the ICMP unreachable messages in order to inform the TCP
sender. If enough information is preserved to extract SEQ.SEG, the sender. If enough information is preserved to extract SEQ.SEG, the
replayed ICMP unreachable messages MAY still be used in TCP-LCD. replayed ICMP unreachable messages can still be used in TCP-LCD.
8. Related Work 8. Related Work
Several methods that address TCP's problems in the presence of Several methods that address TCP's problems in the presence of
connectivity disruptions have been proposed in literature. Some of connectivity disruptions have been proposed in literature. Some of
them try to improve TCP's performance by modifying lower layers. For them try to improve TCP's performance by modifying lower layers. For
example [SM03] introduces a "smart link layer", which buffers one example, [SM03] introduces a "smart link layer", which buffers one
segment for each active connection and replays these segments upon segment for each active connection and replays these segments upon
connectivity re-establishment. This approach has a serious drawback: connectivity re-establishment. This approach has a serious drawback:
previously stateless intermediate routers have to be modified in previously stateless intermediate routers have to be modified in
order to inspect TCP headers, to track the end-to-end connection, and order to inspect TCP headers, to track the end-to-end connection, and
to provide additional buffer space. This leads to an additional need to provide additional buffer space. This leads to an additional need
of memory and processing power. of memory and processing power.
On the other hand, stateless link layer schemes, as proposed in On the other hand, stateless link layer schemes, as proposed in
[RFC3819], which unconditionally buffer some small number of packets [RFC3819], which unconditionally buffer some small number of packets
may have another problem: if a packet is buffered longer than the may have another problem: if a packet is buffered longer than the
maximum segment lifetime (MSL) of 2 min [RFC0793], i.e., the maximum segment lifetime (MSL) of 2 min [RFC0793], i.e., the
disconnection lasts longer than MSL, TCP's assumption that such disconnection lasts longer than MSL, TCP's assumption that such
segments will never be received will no longer be true, violating segments will never be received will no longer be true, violating
TCP's semantics [I-D.eggert-tcpm-tcp-retransmit-now]. TCP's semantics [I-D.eggert-tcpm-tcp-retransmit-now].
Other approaches, like TCP-F [CRVP01] or the Explicit Link Failure Other approaches, like TCP-F [CRVP01] or the Explicit Link Failure
Notification (ELFN) [HV02] inform a TCP sender about a disrupted path Notification (ELFN) [HV02] inform a TCP sender about a disrupted path
by special messages generated and sent from intermediate routers. In by special messages generated and sent from intermediate routers. In
case of a link failure the TCP sender stops sending segments and the case of a link failure, the TCP sender stops sending segments and
freezes its retransmission timers. TCP-F stays in this state and freezes its retransmission timers. TCP-F stays in this state and
remains silent until either a "route establishment notification" is remains silent until either a "route establishment notification" is
received or an internal timer expires. In contrast, ELFN received or an internal timer expires. In contrast, ELFN
periodically probes the network to detect connectivity re- periodically probes the network to detect connectivity re-
establishment. Both proposals rely on changes to intermediate establishment. Both proposals rely on changes to intermediate
routers, whereas the scheme proposed in this document is a sender- routers, whereas the scheme proposed in this document is a sender-
only modification. Moreover, ELFN does not consider congestion and only modification. Moreover, ELFN does not consider congestion and
may impose serious additional load on the network, depending on the may impose serious additional load on the network, depending on the
probe interval. probe interval.
The authors of ATCP [LS01] propose enhancements to identify different The authors of ATCP [LS01] propose enhancements to identify different
types of packet loss by introducing a layer between TCP and IP. They types of packet loss by introducing a layer between TCP and IP. They
utilize ICMP destination unreachable messages to set TCP's receiver utilize ICMP destination unreachable messages to set TCP's receiver
advertised window to zero, thus forcing the TCP sender to perform advertised window to zero, thus forcing the TCP sender to perform
zero window probing with a exponential backoff. ICMP destination zero window probing with an exponential backoff. ICMP destination
unreachable messages that arrive during this probing period are unreachable messages that arrive during this probing period are
ignored. This approach is nearly orthogonal to this document, which ignored. This approach is nearly orthogonal to this document, which
exploits ICMP messages to undo a retransmission timer backoff when exploits ICMP messages to undo a retransmission timer backoff when
TCP is already probing. In principle, both mechanisms could be TCP is already probing. In principle, both mechanisms could be
combined. However, due to security considerations it does not seem combined. However, due to security considerations, it does not seem
appropriate to adopt ATCP's reaction as discussed in Section 5.6. appropriate to adopt ATCP's reaction, as discussed in Section 5.6.
Schuetz et al. describe, in [I-D.schuetz-tcpm-tcp-rlci], a set of TCP Schuetz et al. [I-D.schuetz-tcpm-tcp-rlci] describe a set of TCP
extensions that improve TCP's behavior when transmitting over paths extensions that improve TCP's behavior when transmitting over paths
whose characteristics can change rapidly. Their proposed extensions whose characteristics can change rapidly. Their proposed extensions
modify the local behavior of TCP and introduce a new TCP option to modify the local behavior of TCP and introduce a new TCP option to
signal locally received connectivity-change indications (CCIs) to signal locally received connectivity-change indications (CCIs) to
remote peers. Upon receipt of a CCI, they re-probe the path remote peers. Upon receipt of a CCI, they re-probe the path
characteristics either by performing a speculative retransmission or characteristics either by performing a speculative retransmission or
by sending a single segment of new data, depending on whether the by sending a single segment of new data, depending on whether the
connection is currently stalled in exponential backoff or connection is currently stalled in exponential backoff or
transmitting in steady-state, respectively. The authors focus on transmitting in steady-state, respectively. The authors focus on
specifying TCP response mechanisms, nevertheless underlying layers specifying TCP response mechanisms, nevertheless underlying layers
skipping to change at page 20, line 34 skipping to change at page 20, line 27
9. IANA Considerations 9. IANA Considerations
This memo includes no request to IANA. This memo includes no request to IANA.
10. Security Considerations 10. Security Considerations
The algorithm proposed in this document is considered to be secure. The algorithm proposed in this document is considered to be secure.
For example, an attacker who already guessed the correct four-tuple For example, an attacker who already guessed the correct four-tuple
(i.e., Source IP Address, Source TCP port, Destination IP Address, (i.e., Source IP Address, Source TCP port, Destination IP Address,
and Destination TCP port), can still not make a TCP modified with and Destination TCP port), can still not make a TCP modified with
TCP-LCD to flood the network just by sending forged ICMP unreachable TCP-LCD flood the network just by sending forged ICMP unreachable
messages in an attempt to maliciously shorten the retransmission messages in an attempt to maliciously shorten the retransmission
timer. The attacker additionally would need to guess the correct timer. The attacker additionally would need to guess the correct
segment sequence number of the current timeout-based retransmission, segment sequence number of the current timeout-based retransmission,
with a probability of at most 1/2^32. Even in the case of man-in- with a probability of at most 1/2^32. Even in the case of man-in-
the-middle attacks, i.e., attacks performed in scenarios in which the the-middle attacks, i.e., attacks performed in scenarios in which the
attacker can sniff the retransmissions, the impact on network load is attacker can sniff the retransmissions, the impact on network load is
considered to be low, since the retransmission frequency is limited considered to be low, since the retransmission frequency is limited
by the RTO that was computed before TCP had entered the timeout-based by the RTO that was computed before TCP had entered the timeout-based
loss recovery. Hence, the highest probing frequency is expected to loss recovery. Hence, the highest probing frequency is expected to
be even lower than once per minimum RTO, i.e. 1s as specified by be even lower than once per minimum RTO, i.e. 1s as specified by
[RFC2988]. [RFC2988].
11. Acknowledgments 11. Acknowledgments
We would like to thank Kai Jakobs, Ilpo Jarvinen, Pasi Sarolahti, We would like to thank Lars Eggert, Mark Handley, Kai Jakobs, Ilpo
Timothy Shepard, Joe Touch and Carsten Wolff for feedback on earlier Jarvinen, Pasi Sarolahti, Tim Shepard, Joe Touch and Carsten Wolff
versions of this document. We also thank Michael Faber, Daniel for feedback on earlier versions of this document. We also thank
Schaffrath, and Damian Lukowski for implementing and testing the Michael Faber, Daniel Schaffrath, and Damian Lukowski for
algorithm in Linux. Special thanks go to Ilpo Jarvinen for giving implementing and testing the algorithm in Linux. Special thanks go
valuable feedback regarding the Linux implementation. to Ilpo Jarvinen for giving valuable feedback regarding the Linux
implementation.
This work has been supported by the German National Science This work has been supported by the German National Science
Foundation (DFG) within the research excellence cluster Ultra High- Foundation (DFG) within the research excellence cluster Ultra High-
Speed Mobile Information and Communication (UMIC), RWTH Aachen Speed Mobile Information and Communication (UMIC), RWTH Aachen
University. University.
12. References 12. References
12.1. Normative References 12.1. Normative References
skipping to change at page 22, line 10 skipping to change at page 21, line 48
[HV02] Holland, G. and N. Vaidya, "Analysis of TCP performance [HV02] Holland, G. and N. Vaidya, "Analysis of TCP performance
over mobile ad hoc networks", Wireless Networks vol. 8, over mobile ad hoc networks", Wireless Networks vol. 8,
no. 2-3, pp. 275-288, March 2002. no. 2-3, pp. 275-288, March 2002.
[I-D.eggert-tcpm-tcp-retransmit-now] [I-D.eggert-tcpm-tcp-retransmit-now]
Eggert, L., "TCP Extensions for Immediate Eggert, L., "TCP Extensions for Immediate
Retransmissions", draft-eggert-tcpm-tcp-retransmit-now-02 Retransmissions", draft-eggert-tcpm-tcp-retransmit-now-02
(work in progress), June 2005. (work in progress), June 2005.
[I-D.ietf-tcpm-icmp-attacks]
Gont, F., "ICMP attacks against TCP",
draft-ietf-tcpm-icmp-attacks-12 (work in progress),
March 2010.
[I-D.schuetz-tcpm-tcp-rlci] [I-D.schuetz-tcpm-tcp-rlci]
Schuetz, S., Koutsianas, N., Eggert, L., Eddy, W., Swami, Schuetz, S., Koutsianas, N., Eggert, L., Eddy, W., Swami,
Y., and K. Le, "TCP Response to Lower-Layer Connectivity- Y., and K. Le, "TCP Response to Lower-Layer Connectivity-
Change Indications", draft-schuetz-tcpm-tcp-rlci-03 (work Change Indications", draft-schuetz-tcpm-tcp-rlci-03 (work
in progress), February 2008. in progress), February 2008.
[KP87] Karn, P. and C. Partridge, "Improving Round-Trip Time [KP87] Karn, P. and C. Partridge, "Improving Round-Trip Time
Estimates in Reliable Transport Protocols", Proceedings of Estimates in Reliable Transport Protocols", Proceedings of
the Conference on Applications, Technologies, the Conference on Applications, Technologies,
Architectures, and Protocols for Computer Communication Architectures, and Protocols for Computer Communication
skipping to change at page 23, line 40 skipping to change at page 23, line 26
Version 6 (IPv6) Specification", RFC 4443, March 2006. Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC5461] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461, [RFC5461] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
February 2009. February 2009.
[RFC5682] Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata, [RFC5682] Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
"Forward RTO-Recovery (F-RTO): An Algorithm for Detecting "Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
Spurious Retransmission Timeouts with TCP", RFC 5682, Spurious Retransmission Timeouts with TCP", RFC 5682,
September 2009. September 2009.
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.
[SESB05] Schuetz, S., Eggert, L., Schmid, S., and M. Brunner, [SESB05] Schuetz, S., Eggert, L., Schmid, S., and M. Brunner,
"Protocol enhancements for intermittently connected "Protocol enhancements for intermittently connected
hosts", SIGCOMM Computer Communication Review vol. 35, no. hosts", SIGCOMM Computer Communication Review vol. 35, no.
3, pp. 5-18, December 2005. 3, pp. 5-18, December 2005.
[SM03] Scott, J. and G. Mapp, "Link layer-based TCP optimisation [SM03] Scott, J. and G. Mapp, "Link layer-based TCP optimisation
for disconnecting networks", SIGCOMM Computer for disconnecting networks", SIGCOMM Computer
Communication Review vol. 33, no. 5, pp. 31-42, Communication Review vol. 33, no. 5, pp. 31-42,
October 2003. October 2003.
[Zh86] Zhang, L., "Why TCP Timers Don't Work Well", Proceedings [Zh86] Zhang, L., "Why TCP Timers Don't Work Well", Proceedings
of the Conference on Applications, Technologies, of the Conference on Applications, Technologies,
Architectures, and Protocols for Computer Communication Architectures, and Protocols for Computer Communication
(SIGCOMM'86) pp. 397-405, August 1986. (SIGCOMM'86) pp. 397-405, August 1986.
Appendix A. Changes from previous versions of the draft Appendix A. Changes from previous versions of the draft
A.1. Changes from draft-ietf-tcpm-tcp-lcd-00 This appendix should be removed by the RFC Editor before publishing
this document as an RFC.
A.1. Changes from draft-ietf-tcpm-tcp-lcd-01
o Incorporated feedback submitted by Lars Eggert
A.2. Changes from draft-ietf-tcpm-tcp-lcd-00
o Editorial changes. o Editorial changes.
o Clarified TCP-LCD's behaviour during connection establishment o Clarified TCP-LCD's behaviour during connection establishment
(Thanks to Mark Handley). (Thanks to Mark Handley).
A.2. Changes from draft-zimmermann-tcp-lcd-02 A.3. Changes from draft-zimmermann-tcp-lcd-02
o Incorporated feedback submitted by Ilpo Jarvinen. o Incorporated feedback submitted by Ilpo Jarvinen.
<http://www.ietf.org/mail-archive/web/tcpm/current/msg04841.html> <http://www.ietf.org/mail-archive/web/tcpm/current/msg04841.html>
o Incorporated feedback submitted by Pasi Sarolahti. o Incorporated feedback submitted by Pasi Sarolahti.
<http://www.ietf.org/mail-archive/web/tcpm/current/msg04870.html> <http://www.ietf.org/mail-archive/web/tcpm/current/msg04870.html>
o Incorporated feedback submitted by Joe Touch. o Incorporated feedback submitted by Joe Touch.
<http://www.ietf.org/mail-archive/web/tcpm/current/msg04895.html> <http://www.ietf.org/mail-archive/web/tcpm/current/msg04895.html>
<http://www.ietf.org/mail-archive/web/tcpm/current/msg04900.html> <http://www.ietf.org/mail-archive/web/tcpm/current/msg04900.html>
skipping to change at page 25, line 8 skipping to change at page 25, line 5
subsection anymore. Moreover, the section was renamed to subsection anymore. Moreover, the section was renamed to
"Dissolving Ambiguity Issues" and has now real content. "Dissolving Ambiguity Issues" and has now real content.
o An interoperability issues section (Section 7) was added. In o An interoperability issues section (Section 7) was added. In
particular comments to ECN, ICMPv6, and to the two thresholds R1 particular comments to ECN, ICMPv6, and to the two thresholds R1
and R2 of [RFC1122] (Section 4.2.3.5) were added. and R2 of [RFC1122] (Section 4.2.3.5) were added.
o Miscellaneous editorial changes. In particular, the algorithm has o Miscellaneous editorial changes. In particular, the algorithm has
a name now: TCP-LCD. a name now: TCP-LCD.
A.3. Changes from draft-zimmermann-tcp-lcd-01 A.4. Changes from draft-zimmermann-tcp-lcd-01
o The algorithm in Section 4.2 was slightly changed. Instead of o The algorithm in Section 4.2 was slightly changed. Instead of
reverting the last retransmission timer backoff by halving the reverting the last retransmission timer backoff by halving the
RTO, the RTO is recalculated with help of the "BACKOFF_CNT" RTO, the RTO is recalculated with help of the "BACKOFF_CNT"
variable. This fixes an issue that occurred when the variable. This fixes an issue that occurred when the
retransmission timer was backed off but bounded by a maximum retransmission timer was backed off but bounded by a maximum
value. The algorithm in the previous version of the draft, would value. The algorithm in the previous version of the draft, would
have "reverted" to half of that maximum value, instead of using have "reverted" to half of that maximum value, instead of using
the value, before the RTO was doubled (and then bounded). the value, before the RTO was doubled (and then bounded).
o Miscellaneous editorial changes. o Miscellaneous editorial changes.
A.4. Changes from draft-zimmermann-tcp-lcd-00 A.5. Changes from draft-zimmermann-tcp-lcd-00
o Miscellaneous editorial changes in Section 1, 2 and 3. o Miscellaneous editorial changes in Section 1, 2 and 3.
o The document was restructured in Section 1, 2 and 3 for easier o The document was restructured in Section 1, 2 and 3 for easier
reading. The motivation for the algorithm is changed according reading. The motivation for the algorithm is changed according
TCP's problem to disambiguate congestion from non-congestion loss. TCP's problem to disambiguate congestion from non-congestion loss.
o Added Section 4.1. o Added Section 4.1.
o The algorithm in Section 4.2 was restructured and simplified: o The algorithm in Section 4.2 was restructured and simplified:
 End of changes. 87 change blocks. 
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