wdiff draft-ietf-tcpm-tcp-uto-00.txt draft-ietf-tcpm-tcp-uto-01.txt

TCP Maintenance and Minor L. Eggert
Extensions (tcpm) NEC
Internet-Draft F. Gont
Expires: November 24, 2005 January 16, 2006 UTN/FRH
May 23,
July 15, 2005

TCP User Timeout Option
draft-ietf-tcpm-tcp-uto-00
draft-ietf-tcpm-tcp-uto-01

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Abstract

The TCP user timeout controls how long transmitted data may remain
unacknowledged before a connection is aborted. TCP implementations
typically use forcefully closed. It is a single, system-wide user timeout value.
local, per-connection parameter. The advisory TCP User Timeout
Option allows conforming TCP implementations to exchange
requests for individual, per-connection their local
user timeouts. Lengthening This exchange provides an in-protocol mechanism to
coordinate raising or lowering the system-wide default two user timeout timeouts of a connection.
Increase the user timeouts allows established TCP connections to
survive extended periods of disconnection. On the
other hand, shortening the default Decreasing user timeout timeouts
allows busy servers to explicitly notify their clients that they will
maintain the connection state information only accross across short periods of
disconnection.

1. Introduction

The Transmission Control Protocol (TCP) specification [1] [RFC0793]
defines a local, per-connection "user timeout" parameter that
specifies the maximum amount of time that transmitted data may remain
unacknowledged before TCP will abort forcefully close the corresponding
connection. Applications can set and change this parameter with OPEN
and SEND calls. If a network disconnection lasts longer than the
user timeout, no acknowledgments will be received for any
transmission attempt, including keep-alives [5], [TCP-ILLU], and the TCP
connection will be aborted close when the user timeout occurs.

The TCP specification [1] does not constrain In the permitted values for absence
of an application-specified user timeouts. However, timeout, the TCP specification
[RFC0793] defines a default user timeout of 5 minutes.

The Host Requirements RFC [2] mandates a
timeout [RFC1122] refines this definition by
introducing two thresholds, R1 and R2 (R2 > R1), on the number of at least three minutes
retransmissions of a single segment. It suggests that TCP notify
applications when R1 is reached for a segment, and close the SYN-SENT case. Many TCP
implementations default to user timeout
connection once R2 is reached. [RFC1122] also refines the
recommended values of a few minutes [5]. for R1 (three retransmissions) and R2 (100
seconds), noting that R2 for SYN segments should be at least 3
minutes. Instead of a single user timeout, some TCP implementations
offer finer-grained policies. For example, Solaris supports
different timeouts depending on whether a TCP connection is in the
SYN-SENT, SYN-RECEIVED, or ESTABLISHED state [6].

System-wide [SOLARIS-MANUAL].

Although applications may set their local user timeouts are a useful basic policy. However, the
ability timeout, there is no
in-protocol mechanism to selectively choose individual signal changes in the local user timeout values to
remote peers. This causes local changes to be ineffective, because,
for
different connections example, the peer will still close the connection after its user
timeout expires, even when a host has raised its local user timeout.
The ability to modify the two user timeouts associated with a
connection in a coordinated manner can improve TCP operation in
scenarios that are currently not well supported. One example of such
scenarios are mobile hosts that change network attachment points
based on current location. Such hosts, maybe using MobileIP [7],
[RFC3344], HIP [8] [I-D.ietf-hip-arch] or transport-layer mobility
mechanisms [9], [I-D.eddy-tcp-mobility], are only intermittently connected
to the Internet. In between connected periods, mobile hosts may
experience periods of disconnection during which no network service
is available [10][11][12]. [SCHUETZ-THESIS][SCHUETZ-CCR][DRIVE-THRU]. Other
factors that can cause transient periods of disconnection are high
levels of congestion as well as link or routing failures inside the
network.

In scenarios similar to the ones described above, a host may not know
exactly when or for how long it will be disconnected from the
network, but it might expect such events due to past mobility
patterns and thus benefit from using longer user timeouts. In other
scenarios, the length and time of a network disconnection may even be
predictable. For example, an orbiting node on a satellite might
experience disconnections due to line-of-sight blocking by other
planetary bodies. The disconnection periods of such a node may be
easily computable from orbital mechanics.

In the examples above, as well as in other cases, established TCP
connections between two peers may be aborted if a disconnection
exceeds the system-wide default user timeout.

This document specifies a new TCP option - the User Timeout Option
(UTO) - that allows conforming hosts to exchange per-connection their local, per-
connection user timeout requests. information. This allows, for example,
mobile hosts to maintain TCP connections across disconnected periods
that are longer than their system's peer's default user timeout. A second use
of the TCP User Timeout Option is advertisement of shorter-than-default shorter-than-
default user timeouts. This can allow busy servers to explicitly
notify their clients that they will maintain the state associated
with established connections only across short periods of
disconnection.

A different approach

The same benefits can be obtained through an application-layer
mechanism, i.e., coordinating changes to tolerate longer periods of disconnection the user timeout values of a
connection through application messages. This approach does not
require a new TCP option, but requires application changes.

A different approach to tolerate longer periods of disconnection is
simply increasing the system-wide user timeout on both peers. This
approach has the benefit of not requiring a new TCP option. However,
it can also significantly increase the amount of connection state
information a host busy server must maintain, because a longer global
timeout value will apply to all its connections. The proposed TCP
User Timeout Option, on the other hand, allows hosts to selectively
manage the user timeouts of individual connections. They must then only
maintain connections, reducing the
amount of state associated with selected connections they must maintain across disconnected periods.

A second benefit of the TCP User Timeout Option is that it allows
hosts to both request specific user timeouts for new connections and
to request changes to the effective user timeouts of established
connections. The latter allows connections to start with short
timeouts and only request longer timeouts when disconnection is
imminent, and only for connections considered important. The ability
to request changes to user timeouts of established connections is
also useful to raise the user timeout after in-band authentication
has occurred. For example, peers could request longer user timeouts
for the TCP connections underlying two-way authenticated TLS
connections [13] after their authentication handshakes have
succeeded.

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 [3]. [RFC2119].

3. Operation

Sending a TCP User Timeout Option suggests to that the remote peer to use
SHOULD start using the indicated user timeout value for the
corresponding connection.
Section 3.4 discusses the effects of different timeout values. The user timeout value included in a TCP
User Timeout Option specifies the requested user timeout during a connection's the
synchronized states of a connection (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, FIN-
WAIT-2, CLOSE-WAIT, CLOSING, or LAST-ACK.) LAST-ACK). Connections in other
states MUST use standard timeout values [1][2]. [Comment.1]

When a host [RFC0793][RFC1122]. [anchor4]

Note that supports the TCP User Timeout Option receives one,
it decides whether to change the connection's local user timeout
based on the received value. Generally, hosts SHOULD honor requests
for changes to the user timeout, unless security concerns or external
policies indicate otherwise (see Section 5.) If so, hosts MAY ignore
incoming an exchange of TCP User Timeout Options and MAY use a different user
timeout for the connection.

It between peers is important to note that the
not a binding negotiation. Transmission of a TCP User Timeout Option does not
change the semantics of
is an advisory suggestion that the TCP protocol. peer consider adapting its local
user timeout. Hosts remain free to forcefully close or abort
connections at any time for any reason, whether or not they use
custom user timeouts or have suggested to the peer to use them.

Hosts SHOULD impose upper and lower limits on the user timeouts they
use. Section 3.4 discusses user timeout limits.

A host that supports the TCP User Timeout Option with a value of zero (i.e., "now") is nonsensical and MUST NOT
be sent. If received, SHOULD include it MUST be ignored. Section 3.4 discusses
potentially problematic effects of other in
the next possible segment to its peer whenever it starts using a new
user timeout durations.

A TCP implementation for the connection. This allows the peer to adapt its
local user timeout for the connection accordingly.

When a host that does not support supports the TCP User Timeout Option SHOULD silently ignore receives one,
it [2], thus ensuring interoperability.

3.1 Option Format

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind = X | Length = 4 |G| User Timeout |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

(One tick mark represents one bit.)

Figure 1: Format decides whether to change its local user timeout of the connection
based on the received value. Generally, hosts should honor requests
for changes to the user timeout (see Section 3.3), unless security
concerns, resource constraints or external policies indicate
otherwise (see Section 5). If so, hosts may ignore incoming TCP User
Timeout Option

Figure 1 shows the format of Options and use a different user timeout for the connection.

When a host receives a TCP User Timeout Option. It
contains these fields:

Kind (8 bits)
A TCP option number [1] Option, it first decides
whether to be assigned by IANA upon publication of
this document change its local user timeout for the connection (see
Section 6.)
Length (8 bits)
Length of the 3.3) and then decides whether to send a TCP option User Timeout
Option to its peer in octets [1]; response. If it has never sent a TCP User
Timeout Option to its value MUST be 4.

Granularity (1 bit)
Granularity bit, indicating peer during the granularity lifetime of the "User Timeout"
field. When set (G = 1), the time interval in the "User Timeout"
field MUST be interpreted as minutes. Otherwise (G = 0), the time
interval in the "User Timeout" field MUST be interpreted as
seconds. connection or
if it has changed its local user timeout, it SHOULD send TCP User
Timeout (15 bits)
Specifies the Option with its current local user timeout suggestion for this connection. It
MUST be interpreted as a 15-bit unsigned integer. The granularity
of the timeout (minutes or seconds) depends on the "G" field.

3.2 Operation During the SYN Handshake to its peer.
[anchor5]

A host that supports the TCP User Timeout Option MUST SHOULD include an
appropriate TCP User Timeout Option one
in its initial each packet that carries a SYN segment to
indicate flag, but need not. [MEDINA] has
shown that it supports unknown options are correctly handled by the option and vast majority
of modern TCP stacks. It is thus not necessary to suggest an initial user
timeout for require
negotiation use of the TCP User Timeout Option for a connection. [Comment.2]

A host TCP implementation that supports does not support the TCP User Timeout
Option and receives a SYN
segment that includes one MUST respond with an appropriate silently ignore it [RFC1122], thus ensuring
interoperability.

Hosts SHOULD impose upper and lower limits on the user timeouts they
use. Section 3.3 discusses user timeout limits. A TCP User Timeout
Option in its SYN-ACK segment. If an incoming SYN segment
does not include with a TCP User Timeout Option, value of zero (i.e., "now") is nonsensical and is used
for a host MUST NOT include
one in the SYN-ACK segment nor in any other segment, and it MUST
ignore the contents special purpose, see Section 3.4. Section 3.3 discusses
potentially problematic effects of any other received user timeout durations.

3.1 Reliability Considerations

The TCP User Timeout Option.

3.3 Operation During the Synchronized States

Unless both Option is an advisory TCP option that does not
change processing for subsequent segments. Unlike other TCP options,
it need not be exchanged reliably. Consequently, the SYN and SYN-ACK of specification
in this section does not define a connection contained reliability handshake for TCP User
Timeout Options, both hosts participating in the connection MUST NOT
send Option exchanges. When a segment that carries a TCP User
Timeout Options in any other segment. Additionally,
they both MUST ignore Option is lost, the contents of any received TCP User Timeout
Option.

If, however, both option may never reach the SYN and SYN-ACK contained TCP User Timeout
Options, hosts intended peer.

Implementations MAY choose implement local mechanisms to include additional improve delivery
reliability, such as retransmitting the TCP User Timeout
Options in segments sent during Option when
they retransmit the synchronized states (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, segment that originally carried it or LAST-ACK).

Dynamically adapting "attaching"
the user timeout of option to a connection during its
lifetime could be useful byte in a number of scenarios, for example:

o TCP may adapt the user timeout based on observed network
characteristics. [Comment.3]
o TCP may use short timeouts when connections start stream and only suggest
longer timeouts when disconnection was imminent.

o TCP may use short user timeouts when connections start and only
raise them once in-band authentication has occurred, for example,
once a TLS handshake across retransmitting the connection has succeeded [13].

Generally, option
whenever a host decides that byte or its ACK are retransmitted.

It is important to change note that although these mechanisms can improve
transmission reliability for the local user timeout
of a connection, it SHOULD include a TCP User Timeout Option
indicating the new user timeout in its next segment to the peer.
This allows the peer to adapt its local user timeout for the
connection accordingly.

TCP's SYN handshake has specific retransmission rules to guarantee
reliability. These mechanisms also Option, they do not
guarantee delivery (a three-way handshake would be required for
this). Consequently, implementations MUST NOT assume that the exchange of a TCP User
Timeout Options during the SYN handshake is reliable. This Option is not reliably transmitted.

3.2 Option Format

(One tick mark represents one bit.)

Figure 1: Format of the case for TCP User Timeout Option exchanges during

Figure 1 shows the format of the TCP User Timeout Option. It
contains these fields:

Kind (8 bits)
A TCP option number [RFC0793] to be assigned by IANA upon
publication of this document (see Section 6).

Length (8 bits)
Length of the TCP option in octets [RFC0793]; its value MUST be 4.

Granularity (1 bit)
Granularity bit, indicating the granularity of the "User Timeout"
field. When set (G = 1), the time interval in the "User Timeout"
field MUST be interpreted as minutes. Otherwise (G = 0), the
synchronized states. When a segment carrying a TCP time
interval in the "User Timeout" field MUST be interpreted as
seconds.

User Timeout
Option is lost, (15 bits)
Specifies the peer will not update its local user timeout
accordingly. This draft does not currently describe mechanisms to
ensure the reliability suggestion for this connection. It
MUST be interpreted as a 15-bit unsigned integer. The granularity
of the option exchange in the synchronized
states, other than noting that periodic inclusion of timeout (minutes or seconds) depends on the option may
be an appropriate interim mechanism for implementations concerned
with reliability.

3.4 "G" field.

3.3 Duration of the User Timeout

The TCP User Timeout Option allows hosts to exchange user timeout
values from zero seconds 1 second to over 9 hours at a granularity of seconds and
from zero minutes 1 minute to over 22 days at a granularity of minutes. (An
option value of zero is reserved for a special purpose, see
Section 3.4.)

Very short user timeout values can affect TCP transmissions over
high-delay paths. If the user timeout occurs before an
acknowledgment for an outstanding segment arrives, possibly due to
packet loss, the connection aborts. closes. Many TCP implementations default
to user timeout values of a few minutes [5]. [TCP-ILLU]. Although the TCP
User Timeout Option allows suggestion of short timeouts, applications
advertising them should SHOULD consider these effects.

Long user timeout values allow hosts to tolerate extended periods of
disconnection. However, they also require hosts to maintain the TCP
state information associated with connections for long periods of
time. Section 5 discusses the security implications of long timeout
values.

To protect against these effects, implementations SHOULD impose
limits on the user timeout values they accept and use. The remainder
of this section describes a RECOMMENDED scheme to limit user timeouts
based on upper and lower limits. Under the RECOMMENDED scheme, each
TCP SHOULD compute the user timeout (USER_TIMEOUT) for a connection
according to this formula:

USER_TIMEOUT = min(U_LIMIT, max(LOCAL_UTO, REMOTE_UTO, L_LIMIT))
[Comment.4]

Each field is to be interpreted as follows:

USER_TIMEOUT
Resulting user timeout value to be adopted by the local TCP for a
connection.

U_LIMIT
Current upper limit imposed on the connection's user timeout of a connection by
the local host.

L_LIMIT
Current lower limit imposed on the connection's user timeout of a connection by
the local host.

LOCAL_UTO
Current local user timeout of the specific connection.

REMOTE_UTO
Last "user timeout" value suggested by the remote peer by means of
the TCP User Timeout Option.

This means that the maximum of the two announced values will be
adopted for the user timeout of the connection. The rationale is
that choosing the maximum of the two values will let the connection
survive transient longer periods of disconnection. If the TCP that announced
the lower of the two user timeout values did so in order to reduce
the amount of TCP state information that must be kept on the host, it
can, nevertheless, close or abort the connection whenever it wants.

Enforcing a lower limit (L_LIMIT) protects against connection aborts prevents connections from closing
due to transient network conditions, including temporary congestion,
mobility hand-offs and routing instabilities.

An upper limit (U_LIMIT) can reduce the effect of resource exhaustion
attacks. Section 5 discusses the details of these attacks.

Note that these limits MAY be specified as system-wide constants or
at other granularities, such as on per-host, per-user or even per-
connection basis. Furthermore, these limits need not be static. For
example, they MAY be a function of system resource utilization or
attack status and could be dynamically adapted.

The Host Requirements RFC [2] [RFC1122] does not impose any limits on the
length of the user timeout. However, a time interval of at least 100
seconds is RECOMMENDED. Consequently, the lower limit (LLIMIT)
SHOULD be set to at least 100 seconds when following the RECOMMENDED
scheme described in this section. RECOMMENDED. Consequently, the lower limit (L_LIMIT)
SHOULD be set to at least 100 seconds when following the RECOMMENDED
scheme described in this section.

3.4 Special Option Values

Whenever it is legal to do so according to the specification in the
previous sections, TCP implementations MAY send a zero-second TCP
User Timeout Option, i.e, with a "User Timeout" field of zero and a
"Granularity" of zero. This signals their peers that they support
the option, but do not suggest a specific user timeout value at that
time. Essentially, a zero-second TCP User Timeout Option acts as a
"don't care" value.

The receiver of a zero-second TCP User Timeout Option SHOULD perform
the RECOMMENDED strategy for calculating a new local USER_TIMEOUT
described in Section 3.3 with a numeric value of zero seconds for
REMOTE_UTO. The sender SHOULD perform the calculation as described
in Section 3.3. Essentially, the sender SHOULD adapt the peer's UTO
and the receiver SHOULD continue using its local UTO.

A zero-minute TCP User Timeout Option, i.e., with a "User Timeout"
field of zero and a "Granularity" bit of one, is reserved for future
use. TCP implementations MUST NOT sent it and MUST ignore it upon
reception.

4. Interoperability Issues

This section discusses interoperability issues related to introducing
the UTO option.

One meta-issue of introducing new TCP options is that header space
available for TCP options is currently limited to 40 bytes. All
negotiable options are exchanged during the SYN/SYN-ACK handshake,
where option space is becoming limited. Current proposals to extend
the available option space may mitigate this issue [14]. User Timeout Option.

4.1 Middleboxes

The large number of middleboxes (firewalls, proxies, protocol
scrubbers, etc.) currently present in the Internet pose some
difficulty for deploying new TCP options. Some firewalls may block
segments that carry unknown options, preventing connection
establishment when the SYN or SYN-ACK contains the UTO option. a TCP User Timeout
Option. Some recent results, however, indicate that for new TCP
options, this may not be a significant threat, with only 0.2% of web
requests failing when carrying an unknown option [15]. [MEDINA].

Stateful firewalls usually reset connections after a period of
inactivity. If such a firewall exists along the path between two
peers, it may close or abort connections regardless of the use of the UTO
TCP User Timeout Option. In the future, such firewalls may learn to
parse the UTO
option TCP User Timeout Option and modify their behavior or the
option accordingly.

4.2 TCP Keep-Alives

Some TCP implementations, such as the one in BSD systems, use a
different abort policy for TCP keepalives keep-alives than for user data. Thus,
the TCP keep-alive mechanism might abort a connection that would
otherwise have survived the transient period of disconnection.
Therefore, if a TCP peer enables TCP keep-alives for a connection
that is using the UTO TCP User Timeout Option, then the keep-alive timer
MUST be set to a value larger than that of the adopted USER TIMEOUT (specified by
Equation 1). TIMEOUT.

5. Security Considerations

Lengthening user timeouts has obvious security implications.
Flooding attacks cause denial of service by forcing servers to commit
resources for maintaining the state of throw-away connections. TCP
implementations do not become more vulnerable to simple SYN flooding
by implementing the TCP User Timeout Option, because user timeouts
negotiated during the handshake only affect the synchronized states
(ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK),
which simple SYN floods never reach.

However, when an attacker completes the three-way handshakes of its
throw-away connections it can amplify the effects of resource
exhaustion attacks, because the attacked server must maintain the
connection state associated with the throw-away connections for
longer durations. Because connection state is kept longer, lower-
frequency attack traffic, which may be more difficult to detect, can
already cause resource exhaustion. [Comment.5]

Several approaches can help mitigate this issue. First,
implementations can require prior peer authentication, e.g., using
IPsec [16], [I-D.ietf-ipsec-rfc2401bis], before accepting long user
timeouts for the peer's connections. Similarly, a host can only
start to accept long user timeouts for an established connection
after in-band authentication has occurred, for example, after a TLS
handshake across the connection has succeeded [13]. [RFC2246]. Although
these are arguably the most complete solutions, they depend on
external mechanisms to establish a trust relationship.

A second alternative that does not depend on external mechanisms
would introduce a per-peer limit on the number of connections that
may use increased user timeouts. Several variants of this approach
are possible, such as fixed limits or shortening accepted user
timeouts with a rising number of connections. Although this
alternative does not eliminate resource exhaustion attacks from a
single peer, it can limit their effects. Reducing the number of
high-UTO connections a server supports in the face of an attack turns
that attack into a denial-of-service attack against the service of
high-UTO connections.

Per-peer limits cannot protect against distributed denial of service
attacks, where multiple clients coordinate a resource exhaustion
attack that uses long user timeouts. To protect against such
attacks, TCP implementations could reduce the duration of accepted
user timeouts with increasing resource utilization.

TCP implementations under attack may be forced to shed load by
resetting established connections. Some load-shedding heuristics,
such as resetting connections with long idle times first, can
negatively affect service for intermittently connected, trusted peers
that have suggested long user timeouts. On the other hand, resetting
connections to untrusted peers that use long user timeouts may be
effective. In general, using the peers' level of trust as a
parameter during the load-shedding decision process may be useful.
Note that if TCP needs to close or abort connections with a long TCP
User Timeout Option to shed load, these connections are still no
worse off than without the option.

Finally, upper and lower limits on user timeouts, discussed in
Section 3.4, 3.3, can be an effective tool to limit the impact of these
sorts of attacks.

6. IANA Considerations

This section is to be interpreted according to [4]. [RFC2434].

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.

7. Acknowledgments

The following people have improved this document through thoughtful
suggestions: Mark Allmann, David Borman, Marcus Brunner, Wesley Eddy,
Ted Faber, Guillermo Gont, Tom Henderson, Joseph Ishac, Jeremy
Harris, Phil Karn, Michael Kerrisk, Dan Krejsa, Kostas Pentikousis,
Juergen Quittek, Joe Touch, Stefan Schmid, Simon Schuetz and Martin
Stiemerling.

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

8. References

8.1 Normative References

[1]

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

[2]

[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.

[3]

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

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

8.2 Informative References

[5] "TCP/IP Illustrated, Volume 1: The Protocols", Addison-Wesley ,
1994.

[6] Sun Microsystems, "Solaris Tunable Parameters Reference
Manual", Part No. 806-7009-10, 2002.

[7] Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
August 2002.

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

[9] Eddy, W., "Mobility Support For TCP",
draft-eddy-tcp-mobility-00 (work in progress), April 2004.

[10] Schuetz, S., "Network Support for Intermittently Connected
Mobile Nodes", M.S. Thesis, University of Mannheim, Germany,
June 2004.

[11] Schuetz, S., Eggert, L., Schmid, S., Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2434] Narten, T. and M. Brunner, "Protocol
Enhancements H. Alvestrand, "Guidelines for Intermittently Connected Hosts", under
submission (work Writing an
IANA Considerations Section in progress), November 2004.

[12] RFCs", BCP 26, RFC 2434,
October 1998.

8.2 Informative References

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

[13] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.

[14]

[I-D.eddy-tcp-mobility]
Eddy, W., "Extending the Space Available "Mobility Support For TCP",
draft-eddy-tcp-mobility-00 (work in progress), April 2004.

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

[I-D.ietf-ipsec-rfc2401bis]
Kent, S. and K. Seo, "Security Architecture for TCP Options",
draft-eddy-tcp-loo-03 the
Internet Protocol", draft-ietf-ipsec-rfc2401bis-06 (work
in progress), May April 2005.

[15]

[MEDINA] Medina, A., Allman, M., and S. Floyd, "Measuring
Interactions Between Transport Protocols and Middleboxes", To appear:
Proc. 4th ACM SIGCOMM/USENIX Internet Measurement Conference ,
October 2004.

[16] Kent, S. and K. Seo, "Security Architecture for the on Internet
Protocol", draft-ietf-ipsec-rfc2401bis-06 (work in progress),
April 2005.

Editorial Comments

[Comment.1] LE: A future version of this document may extend per-
connection user timeouts to the SYN-SENT and SYN-
RECEIVED states in a way that conforms to the required
minimum timeouts.

[Comment.2] LE: My original proposal was to allow hosts to choose
whether or not to include the option. It's open for
discussion whether this flexibility is worth the
additional complexity. This is the corresponding text:
"A host that supports the TCP User Timeout Option MAY
omit the TCP User Timeout Option from the initial SYN if
it will not permit custom user timeouts for the specific
connection. It SHOULD omit the TCP User Timeout Option
from the initial SYN if there is evidence that the peer
does not support the TCP User Timeout Option, for
example, if a prior connection attempt including a TCP
User Timeout Option has failed. If a host does not
include a TCP User Timeout Option in its initial SYN, it
MUST NOT include it in any other segment either
Measurement , October 2004.

[RFC2246] Dierks, T. and MUST
ignore the contents of any received TCP User Timeout
Option."

[Comment.3] FG: My original proposal suggested that TCP might adapt
the user timeout when signalled of congestion by means C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.

[RFC3344] Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
August 2002.

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

[SCHUETZ-THESIS]
Schuetz, S., "Network Support for Intermittently Connected
Mobile Nodes", Diploma Thesis, University of ECN.

[Comment.4] Mannheim,
Germany, June 2004.

[SOLARIS-MANUAL]
Sun Microsystems, "Solaris Tunable Parameters Reference
Manual", Part No. 806-7009-10, 2002.

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

Editorial Comments

[anchor4] LE: This formula takes the maximum of the two announced
values. I'd use USER_TIMEOUT = max(L_LIMIT,
min(LOCAL_UTO, REMOTE_UTO, U_LIMIT)), instead. This A future version takes the minimum. My rationale is that the
party announcing the lower value probably had a reason
for it and of this document may hence not be prepared extend per-
connection user timeouts to handle the SYN-SENT and SYN-RECEIVED
states in a longer
value way that it originally indicated.

[Comment.5] FG: IMO, in practice the TCP User Timeout option does
not make conforms to the situation worse: required minimum
timeouts.

[anchor5] LE: Should it really always send UTO when it changes the same type of attack
local timeout? I can be performed even if the default "USER TIMEOUT" is
used, since TCP requires no message exchange in order to
keep a connection open. imagine some ping-pong effect when
two hosts user different UTO adoption strategies. But
maybe that's OK?

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/
Fernando Gont
Universidad Tecnologica Nacional
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina

Phone: +54 11 4650 8472
Email: fernando@gont.com.ar
URI: http://www.gont.com.ar/

Appendix A. Document Revision History

To be removed upon publication

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