draft-ietf-tcpm-tcp-uto-01.txt   draft-ietf-tcpm-tcp-uto-02.txt 
TCP Maintenance and Minor L. Eggert TCP Maintenance and Minor L. Eggert
Extensions (tcpm) NEC Extensions (tcpm) NEC
Internet-Draft F. Gont Internet-Draft F. Gont
Expires: January 16, 2006 UTN/FRH Expires: April 27, 2006 UTN/FRH
July 15, 2005 October 24, 2005
TCP User Timeout Option TCP User Timeout Option
draft-ietf-tcpm-tcp-uto-01 draft-ietf-tcpm-tcp-uto-02
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
skipping to change at page 1, line 35 skipping to change at page 1, line 35
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on January 16, 2006. This Internet-Draft will expire on April 27, 2006.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2005).
Abstract Abstract
The TCP user timeout controls how long transmitted data may remain The TCP user timeout controls how long transmitted data may remain
unacknowledged before a connection is forcefully closed. It is a unacknowledged before a connection is forcefully closed. It is a
local, per-connection parameter. The advisory TCP User Timeout local, per-connection parameter. The advisory TCP User Timeout
Option allows conforming TCP implementations to exchange their local Option allows conforming TCP implementations to exchange their local
user timeouts. This exchange provides an in-protocol mechanism to user timeouts. This is an in-protocol mechanism to allow a host to
coordinate raising or lowering the two user timeouts of a connection. modify its local user timeout for a connection based on knowledge of
Increase the user timeouts allows established TCP connections to the peer's user timeout. Increasing the user timeouts allows
survive extended periods of disconnection. Decreasing user timeouts established TCP connections to survive extended periods of
allows busy servers to explicitly notify their clients that they will disconnection. Decreasing user timeouts allows busy servers to
maintain the connection state only across short periods of explicitly notify their clients that they will maintain the
disconnection. connection state only across short periods of disconnection.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Changing the Local User Timeout . . . . . . . . . . . . . 6
3.2. Reliability Considerations . . . . . . . . . . . . . . . . 8
3.3. Option Format . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Special Option Values . . . . . . . . . . . . . . . . . . 9
4. Additional Considerations . . . . . . . . . . . . . . . . . . 9
5. Interoperability Issues . . . . . . . . . . . . . . . . . . . 10
5.1. Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 10
5.2. TCP Keep-Alives . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . . 13
Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A. Document Revision History . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
1. Introduction 1. Introduction
The Transmission Control Protocol (TCP) specification [RFC0793] The Transmission Control Protocol (TCP) specification [RFC0793]
defines a local, per-connection "user timeout" parameter that defines a local, per-connection "user timeout" parameter that
specifies the maximum amount of time that transmitted data may remain specifies the maximum amount of time that transmitted data may remain
unacknowledged before TCP will forcefully close the corresponding unacknowledged before TCP will forcefully close the corresponding
connection. Applications can set and change this parameter with OPEN connection. Applications can set and change this parameter with OPEN
and SEND calls. If a network disconnection lasts longer than the and SEND calls. If a network disconnection lasts longer than the
user timeout, no acknowledgments will be received for any user timeout, no acknowledgments will be received for any
transmission attempt, including keep-alives [TCP-ILLU], and the TCP transmission attempt, including keep-alives [TCP-ILLU], and the TCP
connection will close when the user timeout occurs. In the absence connection will close when the user timeout occurs. In the absence
of an application-specified user timeout, the TCP specification of an application-specified user timeout, the TCP specification
[RFC0793] defines a default user timeout of 5 minutes. [RFC0793] defines a default user timeout of 5 minutes.
The Host Requirements RFC [RFC1122] refines this definition by The Host Requirements RFC [RFC1122] refines this definition by
introducing two thresholds, R1 and R2 (R2 > R1), on the number of introducing two thresholds, R1 and R2 (R2 > R1), on the number of
retransmissions of a single segment. It suggests that TCP notify retransmissions of a single segment. It suggests that TCP should
applications when R1 is reached for a segment, and close the notify applications when R1 is reached for a segment, and close the
connection once R2 is reached. [RFC1122] also refines the connection once R2 is reached. [RFC1122] also defines the
recommended values for R1 (three retransmissions) and R2 (100 recommended values for R1 (three retransmissions) and R2 (100
seconds), noting that R2 for SYN segments should be at least 3 seconds), noting that R2 for SYN segments should be at least 3
minutes. Instead of a single user timeout, some TCP implementations minutes. Instead of a single user timeout, some TCP implementations
offer finer-grained policies. For example, Solaris supports offer finer-grained policies. For example, Solaris supports
different timeouts depending on whether a TCP connection is in the different timeouts depending on whether a TCP connection is in the
SYN-SENT, SYN-RECEIVED, or ESTABLISHED state [SOLARIS-MANUAL]. SYN-SENT, SYN-RECEIVED, or ESTABLISHED state [SOLARIS-MANUAL].
Although applications may set their local user timeout, there is no Although some TCP implementations allow applications to set their
in-protocol mechanism to signal changes in the local user timeout to local user timeout, e.g., through the SO_SNDTIMEO socket option,
remote peers. This causes local changes to be ineffective, because, there is no in-protocol mechanism to signal changes in the local user
for example, the peer will still close the connection after its user timeout to remote peers. This causes local changes to be
timeout expires, even when a host has raised its local user timeout. ineffective, because the peer will still close the connection after
The ability to modify the two user timeouts associated with a its user timeout expires, even when a host has raised its local user
connection in a coordinated manner can improve TCP operation in timeout. The ability to modify the two user timeouts associated with
scenarios that are currently not well supported. One example of such a connection can improve TCP operation in scenarios that are
scenarios are mobile hosts that change network attachment points currently not well supported. One example of such scenarios are
based on current location. Such hosts, maybe using MobileIP mobile hosts that change network attachment points based on current
[RFC3344], HIP [I-D.ietf-hip-arch] or transport-layer mobility location. Such hosts, maybe using MobileIP [RFC3344], HIP [I-D.ietf-
mechanisms [I-D.eddy-tcp-mobility], are only intermittently connected hip-arch] or transport-layer mobility mechanisms [I-D.eddy-tcp-
to the Internet. In between connected periods, mobile hosts may mobility], are only intermittently connected to the Internet. In
experience periods of disconnection during which no network service between connected periods, mobile hosts may experience periods of
is available [SCHUETZ-THESIS][SCHUETZ-CCR][DRIVE-THRU]. Other disconnection during which no network service is available [SCHUETZ-
factors that can cause transient periods of disconnection are high THESIS][SCHUETZ-CCR][DRIVE-THRU]. Other factors that can cause
levels of congestion as well as link or routing failures inside the transient periods of disconnection are high levels of congestion as
network. well as link or routing failures inside the network.
In scenarios similar to the ones described above, a host may not know 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 exactly when or for how long it will be disconnected from the
network, but it might expect such events due to past mobility network, but it might expect such events due to past mobility
patterns and thus benefit from using longer user timeouts. In other patterns and thus benefit from using longer user timeouts. In other
scenarios, the length and time of a network disconnection may even be scenarios, the length and time of a network disconnection may even be
predictable. For example, an orbiting node on a satellite might predictable. For example, an orbiting node on a satellite might
experience disconnections due to line-of-sight blocking by other experience disconnections due to line-of-sight blocking by other
planetary bodies. The disconnection periods of such a node may be planetary bodies. The disconnection periods of such a node may be
easily computable from orbital mechanics. easily computable from orbital mechanics.
skipping to change at page 3, line 28 skipping to change at page 4, line 23
connection user timeout information. This allows, for example, connection user timeout information. This allows, for example,
mobile hosts to maintain TCP connections across disconnected periods mobile hosts to maintain TCP connections across disconnected periods
that are longer than their peer's default user timeout. A second use that are longer than their peer's default user timeout. A second use
of the TCP User Timeout Option is advertisement of shorter-than- of the TCP User Timeout Option is advertisement of shorter-than-
default user timeouts. This can allow busy servers to explicitly default user timeouts. This can allow busy servers to explicitly
notify their clients that they will maintain the state associated notify their clients that they will maintain the state associated
with established connections only across short periods of with established connections only across short periods of
disconnection. disconnection.
The same benefits can be obtained through an application-layer The same benefits can be obtained through an application-layer
mechanism, i.e., coordinating changes to the user timeout values of a mechanism, i.e., exchanging user timeout information via application
connection through application messages. This approach does not messages and having the application adjust the user timeouts through
require a new TCP option, but requires application changes. the TCP API on both sides of a connection. This approach does not
require a new TCP option, but requires changing all application
implementations that desire to tolerate extended periods of
disconnection, and in most cases also requires a modification to the
corresponding application layer protocol. With the proposed TCP
option, application changes may not be necessary at all, or may be
restricted to sender- or receiver-side only, and there is no need to
modify the corresponding application protocol.
A different approach to tolerate longer periods of disconnection is A different approach to tolerate longer periods of disconnection is
simply increasing the system-wide user timeout on both peers. This to simply increase the system-wide user timeout on both peers. This
approach has the benefit of not requiring a new TCP option. However, approach has the benefit of not requiring a new TCP option or
it can also significantly increase the amount of connection state application changes. However, it can also significantly increase the
information a busy server must maintain, because a longer global amount of connection state a busy server must maintain, because a
timeout value will apply to all its connections. The proposed TCP longer global timeout value will apply to all its connections. The
User Timeout Option, on the other hand, allows hosts to selectively proposed TCP User Timeout Option, on the other hand, allows hosts to
manage the user timeouts of individual connections, reducing the selectively manage the user timeouts of individual connections,
amount of state they must maintain across disconnected periods. reducing the amount of state they must maintain across disconnected
periods.
2. Conventions 2. Conventions
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].
3. Operation 3. Operation
Sending a TCP User Timeout Option suggests that the remote peer Sending a TCP User Timeout Option informs the remote peer of the
SHOULD start using the indicated user timeout value for the current local user timeout and suggests that the remote peer SHOULD
corresponding connection. The user timeout value included in a TCP start using the indicated user timeout value for the corresponding
User Timeout Option specifies the requested user timeout during the connection. The user timeout value included in a TCP User Timeout
synchronized states of a connection (ESTABLISHED, FIN-WAIT-1, FIN- Option specifies the requested user timeout during the synchronized
WAIT-2, CLOSE-WAIT, CLOSING, or LAST-ACK). Connections in other states of a connection (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-
states MUST use standard timeout values [RFC0793][RFC1122]. [anchor4] WAIT, CLOSING, or LAST-ACK). Connections in other states MUST the
default timeout values defined in [RFC0793][RFC1122].[anchor3]
Note that an exchange of TCP User Timeout Options between peers is Note that an exchange of TCP User Timeout Options between peers is
not a binding negotiation. Transmission of a TCP User Timeout Option not a binding negotiation. Transmission of a TCP User Timeout Option
is an advisory suggestion that the peer consider adapting its local is an advisory suggestion that the peer consider adapting its local
user timeout. Hosts remain free to forcefully close or abort user timeout. Hosts remain free to forcefully close or abort
connections at any time for any reason, whether or not they use connections at any time for any reason, whether or not they use
custom user timeouts or have suggested to the peer to use them. custom user timeouts or have suggested the peer to use them.
A host that supports the TCP User Timeout Option SHOULD include one
in each packet that carries a SYN flag, but need not. [MEDINA] has
shown that unknown options are correctly handled by the vast majority
of modern TCP stacks. It is thus not necessary to require
negotiation of the use of the TCP User Timeout Option during the
three-way handshake of a connection.
A host that supports the TCP User Timeout Option SHOULD include it in A host that supports the TCP User Timeout Option SHOULD include it in
the next possible segment to its peer whenever it starts using a new the next possible segment to its peer whenever it starts using a new
user timeout for the connection. This allows the peer to adapt its user timeout for the connection. This allows the peer to adapt its
local user timeout for the connection accordingly. local user timeout for the connection accordingly.
When a host that supports the TCP User Timeout Option receives one, When a host that supports the TCP User Timeout Option receives one,
it decides whether to change its local user timeout of the connection it decides whether to change its local user timeout of the connection
based on the received value. Generally, hosts should honor requests based on the received value. Generally, hosts should honor requests
for changes to the user timeout (see Section 3.3), unless security for changes to the user timeout (see Section 3.1), unless security
concerns, resource constraints or external policies indicate concerns, resource constraints or external policies indicate
otherwise (see Section 5). If so, hosts may ignore incoming TCP User otherwise (see Section 6). If so, hosts may ignore incoming TCP User
Timeout Options and use a different user timeout for the connection. Timeout Options and use a different user timeout for the connection.
When a host receives a TCP User Timeout Option, it first decides When a host receives a TCP User Timeout Option, it first decides
whether to change its local user timeout for the connection (see whether to change its local user timeout for the connection -
Section 3.3) and then decides whether to send a TCP User Timeout Section 3.1 discusses the specifics of this choice - and then decides
Option to its peer in response. If it has never sent a TCP User whether to send a TCP User Timeout Option to its peer in response.
Timeout Option to its peer during the lifetime of the connection or If a host has never sent a TCP User Timeout Option to its peer during
if it has changed its local user timeout, it SHOULD send TCP User the lifetime of the connection, or if it has changed its local user
Timeout Option with its current local user timeout to its peer. timeout, it SHOULD send TCP User Timeout Option with its current
[anchor5] local user timeout to its peer. [anchor4]
A host that supports the TCP User Timeout Option SHOULD include one
in each packet that carries a SYN flag, but need not. [MEDINA] has
shown that unknown options are correctly handled by the vast majority
of modern TCP stacks. It is thus not necessary to require
negotiation use of the TCP User Timeout Option for a connection.
A TCP implementation that does not support the TCP User Timeout A TCP implementation that does not support the TCP User Timeout
Option MUST silently ignore it [RFC1122], thus ensuring Option MUST silently ignore it [RFC1122], thus ensuring
interoperability. interoperability.
Hosts SHOULD impose upper and lower limits on the user timeouts they Hosts SHOULD impose upper and lower limits on the user timeouts they
use. Section 3.3 discusses user timeout limits. A TCP User Timeout use. Section 3.1 discusses user timeout limits, and describes a
Option with a value of zero (i.e., "now") is nonsensical and is used recommended scheme to apply them. A TCP User Timeout Option with a
for a special purpose, see Section 3.4. Section 3.3 discusses value of zero (i.e., "now") is nonsensical and is used for a special
potentially problematic effects of other user timeout durations. purpose, see Section 3.4. Section 3.1 discusses potentially
problematic effects of other user timeout durations.
3.1 Reliability Considerations
The TCP User Timeout 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 specification
in this section does not define a reliability handshake for TCP User
Timeout Option exchanges. When a segment that carries a TCP User
Timeout Option is lost, the option may never reach the intended peer.
Implementations MAY implement local mechanisms to improve delivery
reliability, such as retransmitting the TCP User Timeout Option when
they retransmit the segment that originally carried it or "attaching"
the option to a byte in the stream and retransmitting the option
whenever that byte or its ACK are retransmitted.
It is important to note that although these mechanisms can improve
transmission reliability for the TCP User Timeout Option, they do not
guarantee delivery (a three-way handshake would be required for
this). Consequently, implementations MUST NOT assume that a TCP User
Timeout Option is reliably transmitted.
3.2 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 of the TCP User Timeout Option
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 time
interval in the "User Timeout" field MUST be interpreted as
seconds.
User Timeout (15 bits) 3.1. Changing the Local User Timeout
Specifies the 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.
3.3 Duration of the User Timeout When a host receives a TCP User Timeout Option, it MUST decide
whether to change the local user timeout of the corresponding
connection. Application-requested user timeout values always take
precedence over timeout values received from the peer in a TCP User
Timeout Option. [anchor5] Consequently, unless the application on the
local host has requested a specific user timeout for the connection,
e.g., through a socket API call, hosts SHOULD adjust their local user
timeout in response to receiving a TCP User Timeout Option, as
described in the remainder of this section. If the local application
has requested a specific local user timeout, TCP implementations MUST
NOT change it in response to receiving a TCP User Timeout Option. In
this case, they SHOULD, however, notify the application about the
user timeout value received from the peer.
The TCP User Timeout Option allows hosts to exchange user timeout The User Timeout Option specifies the user timeout in terms of time
values from 1 second to over 9 hours at a granularity of seconds and units, rather than in terms of number of retransmissions or round-
from 1 minute to over 22 days at a granularity of minutes. (An trip times (RTTs), as in most cases the periods of disconnection have
option value of zero is reserved for a special purpose, see to do with operation and mobility patterns, rather than with the
Section 3.4.) current network conditions [anchor6][anchor7]. Thus, the TCP User
Timeout Option allows hosts to exchange user timeout values from 1
second to over 9 hours at a granularity of seconds, and from 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 Very short user timeout values can affect TCP transmissions over
high-delay paths. If the user timeout occurs before an high-delay paths. If the user timeout occurs before an
acknowledgment for an outstanding segment arrives, possibly due to acknowledgment for an outstanding segment arrives, possibly due to
packet loss, the connection closes. Many TCP implementations default packet loss, the connection closes. Many TCP implementations default
to user timeout values of a few minutes [TCP-ILLU]. Although the TCP to user timeout values of a few minutes [TCP-ILLU]. Although the TCP
User Timeout Option allows suggestion of short timeouts, applications User Timeout Option allows suggestion of short timeouts, applications
advertising them SHOULD consider these effects. advertising them SHOULD consider these effects.
Long user timeout values allow hosts to tolerate extended periods of Long user timeout values allow hosts to tolerate extended periods of
disconnection. However, they also require hosts to maintain the TCP disconnection. However, they also require hosts to maintain the TCP
state information associated with connections for long periods of state information associated with connections for long periods of
time. Section 5 discusses the security implications of long timeout time. Section 6 discusses the security implications of long timeout
values. values.
To protect against these effects, implementations SHOULD impose To protect against these effects, implementations SHOULD impose
limits on the user timeout values they accept and use. The remainder limits on the user timeout values they accept and use. The remainder
of this section describes a RECOMMENDED scheme to limit user timeouts of this section describes a RECOMMENDED scheme to limit user timeouts
based on upper and lower limits. Under the RECOMMENDED scheme, each based on upper and lower limits. Under the RECOMMENDED scheme, each
TCP SHOULD compute the user timeout (USER_TIMEOUT) for a connection TCP SHOULD compute the user timeout (USER_TIMEOUT) for a connection
according to this formula: according to this formula:
USER_TIMEOUT = min(U_LIMIT, max(LOCAL_UTO, REMOTE_UTO, L_LIMIT)) USER_TIMEOUT = min(U_LIMIT, max(LOCAL_UTO, REMOTE_UTO, L_LIMIT))
skipping to change at page 7, line 18 skipping to change at page 7, line 30
U_LIMIT U_LIMIT
Current upper limit imposed on the user timeout of a connection by Current upper limit imposed on the user timeout of a connection by
the local host. the local host.
L_LIMIT L_LIMIT
Current lower limit imposed on the user timeout of a connection by Current lower limit imposed on the user timeout of a connection by
the local host. the local host.
LOCAL_UTO LOCAL_UTO
Current local user timeout of the specific connection. Current local user timeout of this specific connection.
REMOTE_UTO REMOTE_UTO
Last "user timeout" value suggested by the remote peer by means of Last "user timeout" value suggested by the remote peer by means of
the TCP User Timeout Option. the TCP User Timeout Option.
This means that the maximum of the two announced values will be This means that the maximum of the two announced values will be
adopted for the user timeout of the connection. The rationale is adopted for the user timeout of the connection. The rationale is
that choosing the maximum of the two values will let the connection that choosing the maximum of the two values will let the connection
survive longer periods of disconnection. If the TCP that announced survive longer periods of disconnection. If the TCP that announced
the lower of the two user timeout values did so in order to reduce 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 the amount of TCP state information that must be kept on the host, it
can, nevertheless, close or abort the connection whenever it wants. can, nevertheless, close or abort the connection whenever it wants.
Enforcing a lower limit (L_LIMIT) prevents connections from closing Enforcing a lower limit (L_LIMIT) prevents connections from closing
due to transient network conditions, including temporary congestion, due to transient network conditions, including temporary congestion,
mobility hand-offs and routing instabilities. mobility hand-offs and routing instabilities.
An upper limit (U_LIMIT) can reduce the effect of resource exhaustion An upper limit (U_LIMIT) can reduce the effect of resource exhaustion
attacks. Section 5 discusses the details of these attacks. attacks. Section 6 discusses the details of these attacks.
Note that these limits MAY be specified as system-wide constants or 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- at other granularities, such as on per-host, per-user or even per-
connection basis. Furthermore, these limits need not be static. For connection basis. Furthermore, these limits need not be static. For
example, they MAY be a function of system resource utilization or example, they MAY be a function of system resource utilization or
attack status and could be dynamically adapted. attack status and could be dynamically adapted.
The Host Requirements RFC [RFC1122] does not impose any limits on the The Host Requirements RFC [RFC1122] does not impose any limits on the
length of the user timeout. However, a time interval of at least 100 length of the user timeout. However, a time interval of at least 100
seconds is RECOMMENDED. Consequently, the lower limit (L_LIMIT) seconds is RECOMMENDED. Consequently, the lower limit (L_LIMIT)
SHOULD be set to at least 100 seconds when following the RECOMMENDED SHOULD be set to at least 100 seconds when following the RECOMMENDED
scheme described in this section. scheme described in this section.
3.4 Special Option Values 3.2. Reliability Considerations
The TCP User Timeout Option is an advisory TCP option that does not
change processing of subsequent segments. Unlike other TCP options,
it need not be exchanged reliably. Consequently, the specification
in this section does not define a reliability handshake for TCP User
Timeout Option exchanges. When a segment that carries a TCP User
Timeout Option is lost, the option may never reach the intended peer.
Implementations MAY implement local mechanisms to improve delivery
reliability, such as retransmitting the TCP User Timeout Option when
they retransmit the segment that originally carried it, or
"attaching" the option to a byte in the stream and retransmitting the
option whenever that byte or its ACK are retransmitted.
It is important to note that although these mechanisms can improve
transmission reliability for the TCP User Timeout Option, they do not
guarantee delivery (a three-way handshake would be required for
this). Consequently, implementations MUST NOT assume that a TCP User
Timeout Option is reliably transmitted.
3.3. 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 of the TCP User Timeout Option
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 7).
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 time
interval in the "User Timeout" field MUST be interpreted as
seconds.
User Timeout (15 bits)
Specifies the 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.
3.4. Special Option Values
Whenever it is legal to do so according to the specification in the Whenever it is legal to do so according to the specification in the
previous sections, TCP implementations MAY send a zero-second TCP previous sections, TCP implementations MAY send a zero-second TCP
User Timeout Option, i.e, with a "User Timeout" field of zero and a User Timeout Option, i.e, with a "User Timeout" field of zero and a
"Granularity" of zero. This signals their peers that they support "Granularity" of zero. This signals their peers that they support
the option, but do not suggest a specific user timeout value at that 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 time. Essentially, a zero-second TCP User Timeout Option acts as a
"don't care" value. "don't care" value.
The receiver of a zero-second TCP User Timeout Option SHOULD perform Thus, the sender SHOULD adapt its local user timeout according to the
the RECOMMENDED strategy for calculating a new local USER_TIMEOUT peer's UTO, and the receiver SHOULD continue using its local user
described in Section 3.3 with a numeric value of zero seconds for timeout. In order to achieve this, the receiver of a zero-second TCP
REMOTE_UTO. The sender SHOULD perform the calculation as described User Timeout Option SHOULD perform the RECOMMENDED strategy for
in Section 3.3. Essentially, the sender SHOULD adapt the peer's UTO calculating a new local USER_TIMEOUT described in Section 3.1, with a
and the receiver SHOULD continue using its local UTO. numeric value of zero seconds for REMOTE_UTO. The sender SHOULD
perform the same calculation as described in Section 3.1, with a
numeric value of zero seconds for LOCAL_UTO.
A zero-minute TCP User Timeout Option, i.e., with a "User Timeout" 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 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 use. TCP implementations MUST NOT send it and MUST ignore it upon
reception. reception.
4. Interoperability Issues 4. Additional Considerations
Section 1 described that although [RFC0793] defines the API mechanism
to change the user timeout as an optional parameter for TCP's OPEN
and SEND calls, many implementations provide a different API.
Several popular TCP implementations offer the SO_SNDTIMEO socket
option, either in addition or instead of the RFC-defined OPEN and
SEND user timeout parameter.
Many implementations that offer the SO_SNDTIMEO socket option also
implement a corresponding SO_RCVTIMEO socket option. Whereas the
user timout (SO_SNDTIMEO), specifies how long data may remain
unacknowledged by the peer, i.e., how long a SEND call may take, the
SO_RCVTIMEO specifies how long a RECV call may take.
Even when two TCPs implement the TCP User Timeout Option and decide
to lengthen their local UTOs for a connection, RECV operations during
a disconnection can trigger the SO_RCVTIMEO timeout. Note that
[RFC0793] does not specify this receive timeout or how TCP reacts
when it occurs. If implementations close a connection when its
SO_RCVTIMEO times out, they SHOULD modify this parameter similarly to
how they modify SO_SNDTIMEO upon reception of a TCP User Timeout
option. [anchor10]
5. Interoperability Issues
This section discusses interoperability issues related to introducing This section discusses interoperability issues related to introducing
the TCP User Timeout Option. the TCP User Timeout Option.
4.1 Middleboxes 5.1. Middleboxes
The large number of middleboxes (firewalls, proxies, protocol The large number of middleboxes (firewalls, proxies, protocol
scrubbers, etc.) currently present in the Internet pose some scrubbers, etc.) currently present in the Internet pose some
difficulty for deploying new TCP options. Some firewalls may block difficulty for deploying new TCP options. Some firewalls may block
segments that carry unknown options, preventing connection segments that carry unknown options, preventing connection
establishment when the SYN or SYN-ACK contains a TCP User Timeout establishment when the SYN or SYN-ACK contains a TCP User Timeout
Option. Some recent results, however, indicate that for new TCP Option. Some recent results, however, indicate that for new TCP
options, this may not be a significant threat, with only 0.2% of web options, this may not be a significant threat, with only 0.2% of web
requests failing when carrying an unknown option [MEDINA]. requests failing when carrying an unknown option [MEDINA].
Stateful firewalls usually reset connections after a period of Stateful firewalls usually reset connections after a period of
inactivity. If such a firewall exists along the path between two inactivity. If such a firewall exists along the path between two
peers, it may close or abort connections regardless of the use of the peers, it may close or abort connections regardless of the use of the
TCP User Timeout Option. In the future, such firewalls may learn to TCP User Timeout Option. In the future, such firewalls may learn to
parse the TCP User Timeout Option and modify their behavior or the parse the TCP User Timeout Option and modify their behavior or the
option accordingly. option accordingly.
4.2 TCP Keep-Alives 5.2. TCP Keep-Alives
Some TCP implementations, such as the one in BSD systems, use a Some TCP implementations, such as the one in BSD systems, use a
different abort policy for TCP keep-alives than for user data. Thus, different abort policy for TCP keep-alives than for user data. Thus,
the TCP keep-alive mechanism might abort a connection that would the TCP keep-alive mechanism might abort a connection that would
otherwise have survived the transient period of disconnection. otherwise have survived the transient period of disconnection.
Therefore, if a TCP peer enables TCP keep-alives for a connection Therefore, if a TCP peer enables TCP keep-alives for a connection
that is using the TCP User Timeout Option, then the keep-alive timer that is using the TCP User Timeout Option, then the keep-alive timer
MUST be set to a value larger than that of the adopted USER TIMEOUT. MUST be set to a value larger than that of the adopted USER TIMEOUT.
5. Security Considerations 6. Security Considerations
Lengthening user timeouts has obvious security implications. Lengthening user timeouts has obvious security implications.
Flooding attacks cause denial of service by forcing servers to commit Flooding attacks cause denial of service by forcing servers to commit
resources for maintaining the state of throw-away connections. TCP resources for maintaining the state of throw-away connections.
implementations do not become more vulnerable to simple SYN flooding However, TCP implementations do not become more vulnerable to simple
by implementing the TCP User Timeout Option, because user timeouts SYN flooding by implementing the TCP User Timeout Option, because
negotiated during the handshake only affect the synchronized states user timeouts exchanged during the handshake only affect the
(ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK), synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT,
which simple SYN floods never reach. CLOSING, LAST-ACK), which simple SYN floods never reach.
However, when an attacker completes the three-way handshakes of its However, when an attacker completes the three-way handshakes of its
throw-away connections it can amplify the effects of resource throw-away connections it can amplify the effects of resource
exhaustion attacks, because the attacked server must maintain the exhaustion attacks, because the attacked server must maintain the
connection state associated with the throw-away connections for connection state associated with the throw-away connections for
longer durations. Because connection state is kept longer, lower- longer durations. Because connection state is kept longer, lower-
frequency attack traffic, which may be more difficult to detect, can frequency attack traffic, which may be more difficult to detect, can
already cause resource exhaustion. already cause resource exhaustion.
Several approaches can help mitigate this issue. First, Several approaches can help mitigate this issue. First,
implementations can require prior peer authentication, e.g., using implementations can require prior peer authentication, e.g., using
IPsec [I-D.ietf-ipsec-rfc2401bis], before accepting long user IPsec [I-D.ietf-ipsec-rfc2401bis], before accepting long user
timeouts for the peer's connections. Similarly, a host can only timeouts for the peer's connections. Similarly, a host can start to
start to accept long user timeouts for an established connection accept long user timeouts for an established connection only after
after in-band authentication has occurred, for example, after a TLS in-band authentication has occurred, for example, after a TLS
handshake across the connection has succeeded [RFC2246]. Although handshake across the connection has succeeded [RFC2246]. Although
these are arguably the most complete solutions, they depend on these are arguably the most complete solutions, they depend on
external mechanisms to establish a trust relationship. external mechanisms to establish a trust relationship.
A second alternative that does not depend on external mechanisms A second alternative that does not depend on external mechanisms
would introduce a per-peer limit on the number of connections that would introduce a per-peer limit on the number of connections that
may use increased user timeouts. Several variants of this approach may use increased user timeouts. Several variants of this approach
are possible, such as fixed limits or shortening accepted user are possible, such as fixed limits or shortening accepted user
timeouts with a rising number of connections. Although this timeouts with a rising number of connections. Although this
alternative does not eliminate resource exhaustion attacks from a alternative does not eliminate resource exhaustion attacks from a
skipping to change at page 10, line 22 skipping to change at page 12, line 22
negatively affect service for intermittently connected, trusted peers negatively affect service for intermittently connected, trusted peers
that have suggested long user timeouts. On the other hand, resetting that have suggested long user timeouts. On the other hand, resetting
connections to untrusted peers that use long user timeouts may be connections to untrusted peers that use long user timeouts may be
effective. In general, using the peers' level of trust as a effective. In general, using the peers' level of trust as a
parameter during the load-shedding decision process may be useful. parameter during the load-shedding decision process may be useful.
Note that if TCP needs to close or abort connections with a long TCP 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 User Timeout Option to shed load, these connections are still no
worse off than without the option. worse off than without the option.
Finally, upper and lower limits on user timeouts, discussed in Finally, upper and lower limits on user timeouts, discussed in
Section 3.3, can be an effective tool to limit the impact of these Section 3.1, can be an effective tool to limit the impact of these
sorts of attacks. sorts of attacks.
6. IANA Considerations 7. IANA Considerations
This section is to be interpreted according to [RFC2434]. This section is to be interpreted according to [RFC2434].
This document does not define any new namespaces. It uses an 8-bit This document does not define any new namespaces. It uses an 8-bit
TCP option number maintained by IANA at TCP option number maintained by IANA at
http://www.iana.org/assignments/tcp-parameters. http://www.iana.org/assignments/tcp-parameters.
7. Acknowledgments 8. Acknowledgments
The following people have improved this document through thoughtful The following people have improved this document through thoughtful
suggestions: Mark Allmann, David Borman, Marcus Brunner, Wesley Eddy, suggestions: Mark Allmann, David Borman, Bob Braden, Marcus Brunner,
Ted Faber, Guillermo Gont, Tom Henderson, Joseph Ishac, Jeremy Wesley Eddy, Abolade Gbadegesin, Ted Faber, Guillermo Gont, Tom
Harris, Phil Karn, Michael Kerrisk, Dan Krejsa, Kostas Pentikousis, Henderson, Joseph Ishac, Jeremy Harris, Phil Karn, Michael Kerrisk,
Juergen Quittek, Joe Touch, Stefan Schmid, Simon Schuetz and Martin Dan Krejsa, Kostas Pentikousis, Juergen Quittek, Joe Touch, Stefan
Stiemerling. Schmid, Simon Schuetz, Tim Shepard and Martin Stiemerling.
Lars Eggert is partly funded by Ambient Networks, a research project Lars Eggert is partly funded by Ambient Networks, a research project
supported by the European Commission under its Sixth Framework supported by the European Commission under its Sixth Framework
Program. The views and conclusions contained herein are those of the Program. The views and conclusions contained herein are those of the
authors and should not be interpreted as necessarily representing the authors and should not be interpreted as necessarily representing the
official policies or endorsements, either expressed or implied, of official policies or endorsements, either expressed or implied, of
the Ambient Networks project or the European Commission. the Ambient Networks project or the European Commission.
8. References 9. References
8.1 Normative References 9.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981. RFC 793, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts - [RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989. Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434, IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998. October 1998.
8.2 Informative References 9.2. Informative References
[DRIVE-THRU] [DRIVE-THRU]
Ott, J. and D. Kutscher, "Drive-Thru Internet: IEEE Ott, J. and D. Kutscher, "Drive-Thru Internet: IEEE
802.11b for Automobile Users", Proc. Infocom , March 2004. 802.11b for Automobile Users", Proc. Infocom , March 2004.
[I-D.eddy-tcp-mobility] [I-D.eddy-tcp-mobility]
Eddy, W., "Mobility Support For TCP", Eddy, W., "Mobility Support For TCP",
draft-eddy-tcp-mobility-00 (work in progress), April 2004. draft-eddy-tcp-mobility-00 (work in progress), April 2004.
[I-D.ietf-hip-arch] [I-D.ietf-hip-arch]
Moskowitz, R., "Host Identity Protocol Architecture", Moskowitz, R. and P. Nikander, "Host Identity Protocol
draft-ietf-hip-arch-02 (work in progress), January 2005. Architecture", draft-ietf-hip-arch-03 (work in progress),
August 2005.
[I-D.ietf-ipsec-rfc2401bis] [I-D.ietf-ipsec-rfc2401bis]
Kent, S. and K. Seo, "Security Architecture for the Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", draft-ietf-ipsec-rfc2401bis-06 (work Internet Protocol", draft-ietf-ipsec-rfc2401bis-06 (work
in progress), April 2005. in progress), April 2005.
[MEDINA] Medina, A., Allman, M., and S. Floyd, "Measuring [MEDINA] Medina, A., Allman, M., and S. Floyd, "Measuring
Interactions Between Transport Protocols and Middleboxes", Interactions Between Transport Protocols and Middleboxes",
Proc. 4th ACM SIGCOMM/USENIX Conference on Internet Proc. 4th ACM SIGCOMM/USENIX Conference on Internet
Measurement , October 2004. Measurement , October 2004.
skipping to change at page 12, line 22 skipping to change at page 14, line 26
[SOLARIS-MANUAL] [SOLARIS-MANUAL]
Sun Microsystems, "Solaris Tunable Parameters Reference Sun Microsystems, "Solaris Tunable Parameters Reference
Manual", Part No. 806-7009-10, 2002. Manual", Part No. 806-7009-10, 2002.
[TCP-ILLU] [TCP-ILLU]
Stevens, W., "TCP/IP Illustrated, Volume 1: The Stevens, W., "TCP/IP Illustrated, Volume 1: The
Protocols", Addison-Wesley , 1994. Protocols", Addison-Wesley , 1994.
Editorial Comments Editorial Comments
[anchor4] LE: A future version of this document may extend per- [anchor3] A future version of this document may extend per-
connection user timeouts to the SYN-SENT and SYN-RECEIVED connection user timeouts to the SYN-SENT and SYN-RECEIVED
states in a way that conforms to the required minimum states in a way that conforms to the required minimum
timeouts. timeouts.
[anchor5] LE: Should it really always send UTO when it changes the [anchor4] Should it really always send UTO when it changes the
local timeout? I can imagine some ping-pong effect when local timeout? I can imagine some ping-pong effect when
two hosts user different UTO adoption strategies. But two hosts user different UTO adoption strategies. But
maybe that's OK? maybe that's OK? Additionally, when -01 was presented in
Paris, Joe Touch has suggested that an "UTO-ACK" should
be sent when a UTO is received. I have not seen consensus
for this on the mailing list, hence -02 does not include
this suggestion.
[anchor5] Without this, UTO would modify TCP semantics, because
application-requested UTOs could be overridden by peer
requests.
[anchor6] When -01 was presented in Paris, Bob Braden suggested to
specify the UTO in terms of multiples of the RTT. Others
disagreed, hence -02 does not include this suggestion.
One reason this may be problematic is that the RTT may
change for one direction of the connection, sort of
defeating the process of exchanging UTOs.
[anchor7] Let's suppose a host is intermittently connected to a
network, and the disconnected periods last for, say,
about 15 minutes each. Let's suppose the attachment
points range from low-bandwidth/high-delay/congested
networks to high-bandwidth/low-delay networks. If the UTO
specified the user timeout in terms of number of
retransmissions or round-trip times, an UTO that is
appropriate for the high-bandwith/low-delay networks
would be too small for the low-bandwidth/high-delay/
congested networks. Also, even if the host kept connected
to the same network, if the network conditions changed,
UTO opions would need to be re-sent (as n*RTO and n*RTT
would change), unnecesarily.
[anchor10] Can we even say this much about an API that's not in the
TCP spec? Or should the SO_RCVTIMEO discussion be
removed?
Appendix A. Document Revision History
To be removed upon publication
+----------+--------------------------------------------------------+
| Revision | Comments |
+----------+--------------------------------------------------------+
| 02 | Corrected terminology by replacing terms like |
| | "negotiate", "coordinate", etc. that were left from |
| | pre-WG-document times when the UTO was a more |
| | formalized exchange instead of the advisory one it is |
| | now. Application-requested UTOs take precedence over |
| | ones received from the peer (pointed out by Ted |
| | Faber). Added a brief mention of SO_SNDTIMEO and a |
| | slightly longer discussion of SO_RCVTIMEO. |
| 01 | Clarified and corrected the description of the |
| | existing user timeout in RFC793 and RFC1122. Removed |
| | distinction between operating during the 3WHS and the |
| | established states and introduced zero-second "don't |
| | care" UTOs in response to mailing list feedback. |
| | Updated references and addressed many other comments |
| | from the mailing list. |
| 00 | Resubmission of |
| | draft-eggert-gont-tcpm-tcp-uto-option-01.txt to the |
| | secretariat after WG adoption. Thus, permit |
| | derivative works. Updated Lars Eggert's funding |
| | attribution. Updated several references. No |
| | technical changes. |
+----------+--------------------------------------------------------+
Authors' Addresses Authors' Addresses
Lars Eggert Lars Eggert
NEC Network Laboratories NEC Network Laboratories
Kurfuerstenanlage 36 Kurfuerstenanlage 36
Heidelberg 69115 Heidelberg 69115
Germany Germany
Phone: +49 6221 90511 43 Phone: +49 6221 90511 43
skipping to change at page 13, line 14 skipping to change at page 17, line 5
Fernando Gont Fernando Gont
Universidad Tecnologica Nacional Universidad Tecnologica Nacional
Evaristo Carriego 2644 Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706 Haedo, Provincia de Buenos Aires 1706
Argentina Argentina
Phone: +54 11 4650 8472 Phone: +54 11 4650 8472
Email: fernando@gont.com.ar Email: fernando@gont.com.ar
URI: http://www.gont.com.ar/ URI: http://www.gont.com.ar/
Appendix A. Document Revision History
To be removed upon publication
+-----------+-------------------------------------------------------+
| Revision | Comments |
+-----------+-------------------------------------------------------+
| 00 | Resubmission of |
| | draft-eggert-gont-tcpm-tcp-uto-option-01.txt to the |
| | secretariat after WG adoption. Thus, permit |
| | derivative works. Updated Lars Eggert's funding |
| | attribution. Updated several references. No technical |
| | changes. |
| 01 | Clarified and corrected the description of the |
| | existing user timeout in RFC793 and RFC1122. Removed |
| | distinction between operating during the 3WHS and the |
| | established states and introduced zero-second "don't |
| | care" UTOs in response to mailing list feedback. |
| | Updated references and addressed many other comments |
| | from the mailing list. |
+-----------+-------------------------------------------------------+
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79. found in BCP 78 and BCP 79.
 End of changes. 41 change blocks. 
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