Network Working Group L. Eggert
Internet-Draft S. Schuetz
Expires: March 11, 2005 S. Schmid
NEC
September 10, 2004
TCP Extensions for Immediate Retransmissions
draft-eggert-tcpm-tcp-retransmit-now-01
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document describes a modification to TCP's standard
retransmission scheme that improves performance across intermittently
connected paths. In addition to the regular retransmission attempts
scheduled at exponentially increasing intervals, this extension
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causes additional, speculative retransmission attempts upon receiving
external connectivity indicators. One example of such a connectivity
indicator is "first hop router reachable." This document does not
define the specifics of such connectivity indicators, although it
describes some examples. Instead, it defines how a conforming TCP
implementation operates when it receives a connectivity indicator.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Examples of Connectivity Indicators . . . . . . . . . . . . . 5
5. TCP Immediate Retransmission Extension . . . . . . . . . . . . 6
5.1 Variant Based on Fast Retransmit . . . . . . . . . . . . . 8
5.2 Variant Based on Retransmission Option . . . . . . . . . . 9
6. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1 Normative References . . . . . . . . . . . . . . . . . . . . 12
11.2 Informative References . . . . . . . . . . . . . . . . . . . 12
Editorial Comments . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
A. Document Revision History . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . 16
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1. Introduction
Depending on the specific path between two nodes in the Internet,
disruptions in connectivity may be frequent. Host mobility and other
factors can further increase the likelihood of connectivity
disruptions. When hosts communicate with the Transmission Control
Protocol (TCP) [1], their connections may abort during periods of
disconnection.
The main reason for connection aborts during periods of disconnection
is TCP's "user timeout." It defines the maximum amount of time that
transmitted segments may remain unacknowledged. If a disconnection
lasts longer than the user timeout, the TCP connection will abort.
Many TCP implementations default to user timeout values of a few
minutes [7]. The proposed TCP Abort Timeout Option [8] allows
conforming TCP implementations to use longer user timeout values and
consequently tolerate long disconnections without disruption.
Although the TCP Abort Timeout Option enables TCP connections to
survive extended periods of disconnections, experiments have shown
that TCP connections perform significantly worse when operating along
paths with frequent disconnections [9][10]. This decrease in
performance is caused by TCP's retransmission behavior after
connectivity is restored.
This document describes a modification of TCP's retransmission scheme
to improve performance over a path with frequent disconnections. The
basic idea is to trigger a speculative retransmission attempt when a
TCP implementation receives an indication that connectivity to a
previously disconnected peer node may have been restored.
[Comment.1]
Section 3 discusses TCP performance over intermittently connected
paths in more detail, comparing it to similar proposals [11][12][13],
and Section 4 describes the proposed "immediate retransmission"
extension to TCP. Section 7 investigates security aspects of the
proposed modification and Section 8 summarizes and concludes this
document.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2].
3. Background
When a disconnection occurs along the path between a host and its
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peer while the host is transmitting data, it stops receiving
acknowledgments. After the retransmission timeout (RTO) expires, the
host attempts to retransmit the first unacknowledged segment. TCP
implementations that follow the recommended RTO management proposed
in RFC 2988 [3] double the RTO after each retransmission attempt
until it exceeds 60 seconds. This scheme causes a host to attempt to
retransmit across established connections roughly once a minute.
(More frequently during the first minute or two of the disconnection,
while the RTO is still being backed off.)
When the disconnection ends, standard TCP implementations still wait
until the RTO expires before attempting retransmission. Figure 1
illustrates this behavior. Depending on when connectivity becomes
available again, this can waste up to a minute of connection time for
TCPs that implement the recommended RTO management described in RFC
2988 [3]. For TCP implementations that do not implement RFC 2988,
even more connection time may be lost. For example, Linux uses 120
seconds as the maximum RTO.
sequence
number X = successfully transmitted segment
^ O = lost segment
| : : : X
| : : :X
| XO O O O O : X
| X: : :
| X : :<------------>:
| X : : wasted :
| X : : connection :
|X : : time :
+-----:---------------------:--------------:-------->
: : : time
connectivity connectivity TCP
gone back retransmit
Figure 1: Standard TCP behavior in the presence of a disconnection
This retransmission behavior is not efficient, especially in
scenarios where connected periods are short and disconnections
frequent [14]. Experiments show that TCP performance across a path
with frequent disruptions is significantly worse compared to a
similar path without disruptions [9][10].
In the ideal case, TCP would attempt a retransmission as soon as
connectivity to its peer was re-established. Figure 2 illustrates
the ideal behavior.
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sequence
number X = successfully transmitted segment
^ O = lost segment
| : : X :
| : :X :
| XO O O O O X :
| X: : :
| X : :<------------>:
| X : : efficiency :
| X : : improvement :
|X : : :
+-----:---------------------:--------------:-------->
: : : time
connectivity connectivity next
gone back = immediate scheduled
TCP retransmit retransmit
Figure 2: Ideal TCP behavior in the presence of a disconnection
The ideal behavior is difficult to achieve for arbitrary connectivity
disruptions. One obviously problematic approach would use
higher-frequency retransmission attempts to enable earlier detection
of whether connectivity was restored. This can generate significant
amounts of extra traffic. Other proposals attempt to trigger faster
retransmissions by retransmitting buffered or newly-crafted segments
from inside the network [11][12][13]. Section 6 compares these
approaches to the "immediate retransmission" extension.
4. Examples of Connectivity Indicators
This section describes examples of connectivity indicators, which the
retransmission mechanism described in the next section acts upon.
This document does not define the specifics of such connectivity
indicators but merely discusses them to illustrate the operation of
the "immediate retransmission" extension.
Connectivity indicators signal TCP when connectivity to a previously
disconnected peer may have been restored. They depend on the
specifics of a node and its environment, for example network-layer
mechanisms such as DHCP [15], MobileIP [16] or HIP [17]. The IETF's
Detection of Network Attachment (DNA) working group currently
investigates the specifics of providing such connectivity indicators
[18].
One example of a connectivity indicator is "next hop reachable." This
indicator could occur if a combination of the following conditions is
true, depending on host specifics:
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o Network-layer connectivity along the path to the destination is
restored, e.g., the outbound interface has an IP address and a
next-hop router is known, maybe due to DHCP [15] or IPv6 router
advertisements [19].
o Link-layer connectivity of the link to the next-hop router along
the path to the destination is restored (e.g., link-layer "link
up").
o Other local conditions that affect reachability of the destination
are satisfied (e.g., IKE exchanges [20], MobileIP binding updates
[16] or HIP readdressing [21] have completed).
The "next hop reachable" connectivity indicator only depends on
locally determinable information (e.g., state of directly-connected
links, etc.) and does not require network cooperation. It can signal
TCP to restart active connections across intermittently connected
links where disruptions occur on the first or last hop. This simple
indicator has the potential to improve TCP performance in many cases,
because connection disruptions at the first or last hop are arguably
the most common cause of disconnections in today's Internet.
A second, more general example of a connectivity indicator would be
"end-to-end connectivity restored." If hosts have the ability to
detect or be notified of connectivity changes inside the network
(i.e., not only at the first or last hop), a more general
retransmission mechanism could act on those pieces of information.
This can improve TCP performance across intermittently connected
paths where disruptions occur at arbitrary links along the path, even
inside the network. However, providing this more general
connectivity indicator is problematic due to its dependence on remote
information and its related issues, such as trust.
Connectivity indicator are generally asymmetric, i.e., they may occur
on one peer host but not the other. As discussed above, a local
event at one host may trigger the "immediate retransmission"
mechanism, while the other host is unable to detect this event across
the network. Symmetric connectivity indicators are a special case
and always occur concurrently at both communicating hosts. Examples
for such symmetric connectivity indicators are handshake events such
as IKE exchanges or HIP readdressing. Symmetric indicators are an
important special case, because the retransmission procedure required
in response to a symmetric indicator is simpler than that for an
asymmetric one. The next section will describe this in detail.
5. TCP Immediate Retransmission Extension
This section describes the main contribution of this document, i.e.,
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TCP extensions for immediate retransmission in response to
connectivity indicators. The basic idea behind the "immediate
retransmission" extension is to allow TCP to restart stalled
connections as soon as it receives an indicator that connectivity to
previously disconnected peers may have been restored.
This document does not specify how TCP determines which connections
are affected by a specific connectivity indicator, i.e., for which
connections it should initiate retransmission attempts. This is a
property of individual connectivity indicators. For example, the
"next hop reachable" indicator described in the previous section
affects connections to all destinations routed through that hop.
It is important to note that this retransmission extension does not
modify TCP's basic congestion control, fairness properties or
slow-start algorithms. The only difference in TCP behavior is the
timing of retransmission events and, in some cases, a minor, fixed
increase in the number of initially retransmitted segments. The
"immediate retransmission" extensions increases performance through
better utilization of connected periods, not through sending traffic
at a faster rate or modifying TCP's congestion control mechanisms.
Hosts that implement the "immediate retransmission" TCP extension
MUST implement the following retransmission mechanism whenever a
connectivity indicator is received:
When receiving a symmetric or asymmetric connectivity indicator,
conforming TCP implementations MUST immediately initiate the standard
retransmission procedure for connections affected by the connectivity
indicator - just as if the RTO for those connections had expired.
If the connectivity indicator is symmetric, i.e., all peers receive
it concurrently; this simple change is sufficient to kick-start the
relevant TCP connections.
If the connectivity indicator is asymmetric, this simple extension is
not always sufficient, because only one peer has received the
indicator. In case the host receiving the connectivity indicator has
no (or too little) unacknowledged data awaiting retransmission, it
will not emit enough segments to cause its peer node, which may have
unacknowledged data as well, to attempt retransmission. Transmission
would thus only resume in one direction, which is ineffective for
two-way communication.
To avoid this issue, conforming TCP implementation MUST perform a
different retransmission procedure in response to an asymmetric
connectivity indicator. The following sections describe two
alternative TCP modifications that aim to improve retransmission
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behavior after receiving an asymmetric connectivity indicator.
Section 5.1 describes the first variant. As described in an earlier
revision of this document, this variant generates duplicate ACKs to
activate the peer's fast retransmit algorithm. Section 5.2 describes
the second variant, based on an explicit, new TCP "immediate
retransmission" option.
5.1 Variant Based on Fast Retransmit
This variant of improving TCP retransmission scheme based on
connectivity indicators uses duplicate ACKs. Conforming TCPs MUST
send at least four segments that all acknowledge the last segment
received from a peer for all connections affected by the connectivity
indicator. These triple-duplicate ACKs will activate the peers' fast
retransmit algorithms and cause them to immediately restart
communication in the reverse direction, i.e., before their next
scheduled retransmission.
In this variant, if a TCP connection affected by a connectivity
indicator has four or more unacknowledged data segments in the
retransmission queue, it SHOULD piggyback the triple-duplicate ACK to
the regular retransmissions of those data segments. In this case,
the "immediate retransmission" TCP extension does not require
additional messages, compared to standard TCP.
For connections where the retransmission queue contains only three or
less unacknowledged data segments, TCP implementations supporting the
"immediate retransmission" TCP extension MUST send additional pure
ACKs until a complete triple-duplicate ACK has been sent. In the
worst case, when the retransmission queue is empty, this scheme
requires four additional ACKs, compared to standard TCP.
After the peer's fast retransmit algorithm sends the assumed missing
segment, TCP performs either fast recovery or a slow-start [4],
depending on the length of the disconnection. If the connectivity
indicator occurs before the RTO, i.e., for very short disconnections,
TCP has not yet lost its ACK clock and can thus perform fast
recovery. After longer disconnections, TCP falls back to slow-start
to restart the ACK clock, just as it does at the beginning of a
connection.
The result of this modification is twofold. First, TCP connections
receiving the connectivity indicator attempt retransmission of their
unacknowledged segments before the next scheduled RTO. This
increases utilization of connected periods. Second, TCP connections
receiving the connectivity indicator use an existing TCP mechanism
(triple-duplicate ACK) to signal their peer. Although the peer may
not have received a connectivity indicator itself (e.g., the
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indicator was asymmetric), this causes it to attempt faster
retransmission as well.
As mentioned above, the "immediate retransmission" scheme can
generate up to four additional segments, compared to standard TCP.
All additional segments are pure ACKs and hence small, resulting in a
minor total overhead. Furthermore, measurements have shown that
increasing TCP's initial window is not problematic [22]; this may
indicate that a minor increase in traffic at retransmission time may
be tolerable as well.
5.2 Variant Based on Retransmission Option
Unlike the mechanism described in the previous section, the second
variant described in this section does not overload an existing TCP
mechanism - i.e., fast retransmit - to improve retransmission after a
disconnection. Generating duplicate ACKs in the manner described in
Section 5.1 was criticized by some working group participants as an
abuse of a well-defined TCP mechanism for an unrelated purpose.
The variant described in this section uses a newly defined TCP
"Immediate Retransmission" Option to explicitly signal the remote
peer to activate its fast retransmit algorithm instead of generating
duplicate ACKS. It was suggested by Kacheong Poon [23].
In this variant, conforming TCP implementations MUST send a single
segment to each peer affected by a connectivity indicator. This
segment will contain the TCP Immediate Retransmission Option and may
either be a retransmission or a pure ACK if the connection has no
data awaiting retransmission. Upon reception of such an option,
conforming TCPs MUST immediately initiate their fast retransmit
algorithm.
The TCP Immediate Retransmission Option could be a single-byte
option. Use of this option MUST be negotiated during the SYN
handshake in the usual way. [Comment.2]
One major drawback of this variant compared to the one described in
Section 5.1 is that it requires both communicating TCPs to implement
this modification. Triggering a peer's fast retransmit with
duplicate ACKs only requires the triggering local peer to support
this extension - the triggered remote peer may run an unmodified TCP
stack. Additionally, firewalls may block segments carrying unknown
TCP options. Finally, TCP option space is becoming limited.
6. Related Work
Several other approaches try to improve TCP performance in the
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presence of connectivity disruptions [11][12][13]. They attempt to
improve TCP startup after a disconnection by retransmitting buffered
or newly-crafted segments from inside the network.
These proposals can be problematic, because TCP is built on the
assumption that segments older than the maximum segment lifetime
(MSL) of 2 minutes [1] will never be received. When a disconnection
lasts longer than the MSL, these proposals will either become
ineffective or risk leaking buffered old segments onto new
connections, violating TCP's semantics.
The "immediate retransmission" modification also improves performance
over a path with frequent disconnections. The basic idea is to
schedule an additional, speculative retransmission attempt when a TCP
implementation receives an indication that connectivity to a peer
node has been restored. Unlike the other proposals, the "immediate
retransmission" scheme uses regular retransmissions, i.e.,
retransmits data that is buffered at the end systems. Because that
data has not entered the network yet, it is not subject to the
problematic MSL rule. Consequently, the "immediate retransmission"
scheme remains effective even for disconnections longer than the MSL,
without the risk of compromising connection integrity.
Other transport-layer approaches such as the Explicit Link Failure
Notification [24] or TCP-F [25] use specific messages generated by
intermediate routers to inform TCP senders about disrupted paths.
The former extends the TCP state machine with a new "stand by" state
during which the standard retransmission timers are disabled. In
this state, TCP periodically probes the network to detect
connectivity reestablishment. Depending on the frequency of the
probes and the network environment, this can cause significant
amounts of extra traffic. TCP-F completely suspends ongoing
connections until receiving "route reestablishment notifications"
that indicate peer reachability. Both proposals are primarily
designed for ad hoc networks and rely on changes to intermediate
routers, whereas the "immediate retransmission" extension only
requires end system support.
ATCP [26] uses a similar approach as the Explicit Link Failure
Notification, but discovers link failures through ICMP Destination
Unreachable messages. Caceres and Iftode [27] propose and evaluate a
solution similar to the TCP "immediate" retransmission extension that
improves performance during MobileIP handoffs. Unlike the solution
proposed in this paper, the handoff mechanism is targeted at
disconnections of a few seconds.
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7. Security Considerations
To protect against abuse of the TCP "immediate retransmission"
extension, e.g., denial-of-service attacks by flooding TCP with
connectivity indicators, a control mechanism that "rate-limits" these
indicators may be effective. This document does not currently
discuss the security aspects of connectivity indicators and the
"immediate retransmission" extension to TCP.
8. Conclusion
This document described the "immediate retransmission" extension to
TCP's standard retransmission scheme. The new extension improves
performance across intermittently connected paths through additional,
speculative retransmission attempts upon receiving external
connectivity indicators. One example of such a connectivity
indicator is "first hop router reachable." This document did not
define the specifics of such connectivity indicators, although it
described some examples to illustrate the operation of the "immediate
retransmission" extension, which is its main contribution.
9. IANA Considerations
This section is to be interpreted according to [5].
This document does not define any new namespaces. It uses an 8-bit
TCP option number maintained by IANA at
http://www.iana.org/assignments/tcp-parameters.
10. Acknowledgments
The following people have helped to improve this document through
thoughtful suggestions and feedback: Marcus Brunner, Kacheong Poon,
Juergen Quittek and Joe Touch.
Part of this work is a product of the Ambient Networks project,
partially supported by the European Commission under its Sixth
Framework Programme. It is provided "as is" and without any express
or implied warranties, including, without limitation, the implied
warranties of fitness for a particular purpose. The views and
conclusions contained herein are those of the authors and should not
be interpreted as necessarily representing the official policies or
endorsements, either expressed or implied, of the Ambient Networks
project or the European Commission.
11. References
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11.1 Normative References
[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[4] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion Control",
RFC 2581, April 1999.
[5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[6] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
11.2 Informative References
[7] Stevens, W., "TCP/IP Illustrated, Volume 1: The Protocols",
Addison-Wesley , 1994.
[8] Eggert, L., "TCP Abort Timeout Option",
draft-eggert-tcpm-tcp-abort-timeout-option-01 (work in
progress), July 2004.
[9] Schuetz, S., "Network Support for Intermittently Connected
Mobile Nodes", M.S. Thesis, University of Mannheim, Germany,
June 2004.
[10] Schuetz, S., Eggert, L., Schmid, S. and M. Brunner, "Protocol
Enhancements for Intermittently Connected Hosts", under
submission (work in progress), July 2004.
[11] Scott, J. and G. Mapp, "Link layer-based TCP optimisation for
disconnecting networks", ACM Computer Communication Review,
Vol. 33, No. 5, October 2003.
[12] Dawkins, S., "End-to-end, Implicit 'Link-Up' Notification",
draft-dawkins-trigtran-linkup-01 (work in progress), October
2003.
[13] Karn, P., "Advice for Internet Subnetwork Designers",
draft-ietf-pilc-link-design-15 (work in progress), December
2003.
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[14] Ott, J. and D. Kutscher, "Drive-Thru Internet: IEEE 802.11b for
Automobile Users", Proc. INFOCOM 2004, March 2004.
[15] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[16] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[17] Moskowitz, R., "Host Identity Protocol Architecture",
draft-moskowitz-hip-arch-06 (work in progress), June 2004.
[18] Choi, J., "Detecting Network Attachment in IPv6 Goals",
draft-ietf-dna-goals-00 (work in progress), June 2004.
[19] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[20] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[21] Nikander, P., "End-Host Mobility and Multi-Homing with Host
Identity Protocol", draft-nikander-hip-mm-02 (work in
progress), July 2004.
[22] Allman, M., Hayes, C. and S. Ostermann, "An Evaluation of TCP
with Larger Initial Windows.", ACM Computer Communication
Review, Vol. 28, No. 3, July 1998.
[23] Poon, K., "Personal Communication", August 2004.
[24] Holland, G. and N. Vaidya, "Analysis of TCP Performance over
Mobile Ad Hoc Networks", Proc. 5th Annual ACM/IEEE
International Conference on Mobile Computing and Networking,
1999.
[25] Chandran, K., Raghunathan, S., Venkatesan, S. and R. Prakash,
"A Feedback Based Scheme For Improving TCP Performance In
Ad-Hoc Wireless Networks", IEEE Personal Communication Systems
(PCS) Magazine: Special Issue on Ad Hoc Networks, Vol. 8, No.
1, February 2001.
[26] Liu, J. and S. Singh, "ATCP: TCP for Mobile Ad Hoc Networks",
IEEE Journal on Selected Areas in Communication, Vol. 19, No.
7, July 2001.
[27] Caceres, R. and L. Iftode, "Improving the Performance of
Reliable Transport Protocols in Mobile Computing Environments",
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IEEE Journal on Selected Areas in Communication, Vol. 13, No.
5, 1995.
Editorial Comments
[] LE: The authors have seen the idea of triggering
retransmits based on connectivity events of
directly-connected links attributed to Phil Karn, but
were unable to locate a specific reference. Pointers are
highly appreciated.
[] LE: If this variant is seen as superior, the details of
the negotiation must be described here.
Authors' Addresses
Lars Eggert
NEC Network Laboratories
Kurfuerstenanlage 36
Heidelberg 69115
Germany
Phone: +49 6221 90511 43
Fax: +49 6221 90511 55
EMail: lars.eggert@netlab.nec.de
URI: http://www.netlab.nec.de/
Simon Schuetz
NEC Network Laboratories
Kurfuerstenanlage 36
Heidelberg 69115
Germany
Phone: +49 6221 90511 10
Fax: +49 6221 90511 55
EMail: simon.schuetz@netlab.nec.de
URI: http://www.netlab.nec.de/
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Stefan Schmid
NEC Network Laboratories
Kurfuerstenanlage 36
Heidelberg 69115
Germany
Phone: +49 6221 90511 54
Fax: +49 6221 90511 55
EMail: stefan.schmid@netlab.nec.de
URI: http://www.netlab.nec.de/
Appendix A. Document Revision History
+-----------+-------------------------------------------------------+
| Revision | Comments |
+-----------+-------------------------------------------------------+
| 00 | Initial version. |
| 01 | Updated terminology according to [10]. Added |
| | "retransmission option" variant as Section 5.2. |
+-----------+-------------------------------------------------------+
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Eggert, et al. Expires March 11, 2005 [Page 16]