TCP User Timeout Option
draft-ietf-tcpm-tcp-uto-04

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Copyright © The Internet Society (2006).

Abstract

This document specifies a new TCP option - the TCP User Timeout Option - that allows a TCP to advertise its current user timeout for a connection. Thus, the remote TCP may modify its local user timeout based on knowledge of the peer's user timeout. The TCP user timeout controls how long transmitted data may remain unacknowledged before a connection is forcefully closed. It is a local, per-connection parameter. Increasing the user timeouts allows established TCP connections to survive extended periods of disconnection. Decreasing the user timeouts allows busy servers to explicitly notify their clients that they will maintain the connection state only across short periods of disconnection.

Table of Contents

1.  Introduction
2.  Conventions
3.  Operation
    3.1.  Changing the Local User Timeout
    3.2.  Reliability Considerations
    3.3.  Option Format
    3.4.  Special Option Values
4.  Interoperability Issues
    4.1.  Middleboxes
    4.2.  TCP Keep-Alives
5.  Security Considerations
6.  IANA Considerations
7.  Acknowledgments
8.  References
    8.1.  Normative References
    8.2.  Informative References
Appendix A.  Alternative solutions
Appendix B.  Document Revision History
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

The Transmission Control Protocol (TCP) specification [RFC0793] (Postel, J., “Transmission Control Protocol,” September 1981.)defines a local, per-connection "user timeout" parameter that specifies the maximum amount of time that transmitted data may remain unacknowledged before TCP will 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 [TCP‑ILLU] (Stevens, W., “TCP/IP Illustrated, Volume 1: The Protocols,” 1994.), and the TCP connection will close when the user timeout occurs. In the absence of an application-specified user timeout, the TCP specification [RFC0793] (Postel, J., “Transmission Control Protocol,” September 1981.) defines a default user timeout of 5 minutes.

The Host Requirements RFC [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.) refines this definition by introducing two thresholds, R1 and R2 (R2 > R1), on the number of retransmissions of a single segment. It suggests that TCP should notify applications when R1 is reached for a segment, and close the connection once R2 is reached. [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.) also defines the recommended values 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 [SOLARIS‑MANUAL] (Sun Microsystems, “Solaris Tunable Parameters Reference Manual,” 2002.).

Although some TCP implementations allow applications to set their local user timeout, there is no in-protocol mechanism to signal changes in the local user timeout to remote peers. This causes local changes to be ineffective, because the peer will still close the connection after its user timeout expires, even when the host has raised its local user timeout. The ability to suggest the remote peer a user timeout to be used for the connection can improve TCP's 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 [RFC3344] (Perkins, C., “IP Mobility Support for IPv4,” August 2002.), HIP [RFC4423] (Moskowitz, R. and P. Nikander, “Host Identity Protocol (HIP) Architecture,” May 2006.) or transport-layer mobility mechanisms [I‑D.eddy‑tcp‑mobility] (Eddy, W., “Mobility Support For TCP,” April 2004.), 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. 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 non-geostationary 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.

This document specifies a new TCP option - the User Timeout Option (UTO) - that allows a TCP to advertise its current local user timeout parameter. Thus, based on the information advertised by the remote TCP peer, a TCP may modify its own user timeout accordingly. This allows, for example, mobile hosts to maintain TCP connections across disconnected periods that are longer than their peer's default user timeout. A second use of the TCP User Timeout Option is advertisement of 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.

Use of the TCP User Timeout Option could be triggered either by an API call or by a system-wide toggle. The API could be, for example, a Socket option that would need to be explicitly set by the corresponding application. This option would default to "off". A system-wide toggle would allow a system administrator to enable the use of the TCP User Timeout Option on a system-wide basis, and set the option a desired value. This system-wide toggle would allow the use of the option by application programs that have not been explicitly coded to do so. If such a system-wide toggle were provided, it would default to "off".

In all cases, use of the TCP User Timeout Option would depend on an active decision, either by the application programmer (by means of an API call), or by a system administrator (by means of a system-wide toggle).

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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Operation

Sending a TCP User Timeout Option informs the remote peer of the current local user timeout for the connection, and suggests the TCP peer to adapt its user timeout accordingly. The user timeout value included in a TCP User Timeout Option specifies the requested user timeout during the synchronized states of a connection (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, or LAST-ACK). Connections in other states MUST the default timeout values defined in [RFC0793] (Postel, J., “Transmission Control Protocol,” September 1981.) [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.).

Note that an exchange of TCP User Timeout Options between peers is not a binding negotiation. Transmission of a TCP User Timeout Option is an advisory suggestion that the peer consider adapting its local user timeout. Hosts remain free to adopt a different user timeout, or to forcefully close or abort connections at any time for any reason, whether or not they use 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. The presence of this option is not a negotiation of the capability, but simply an advisory message specifying the currently preferred user timeout value. This allows TCP to adopt a user timeout with knowledge of that used by the peer TCP from the very beginning of the data transfer phase. Additionally, a TCP that supports the User Timeout Option and has sent a SYN segment as a result of an active OPEN SHOULD include an UTO in the first packet that does not have the SYN flag set. This helps to minimize the amount of state information a TCP must keep for connections in non-synchronized states, and is particularly useful when mechanisms such as "SYN cookies" [I‑D.ietf‑tcpm‑syn‑flood] (Eddy, W., “TCP SYN Flooding Attacks and Common Mitigations,” July 2006.) are implemented, allowing a newly-established TCP connection to benefit from the information advertised by the UTO option, even if the UTO contained in the initial SYN segment was not recorded.

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

When a host that supports the TCP User Timeout Option receives one, it will use the received value to compute the local user timeout for the connection. Generally, hosts should honor requests for changes to the user timeout (see Section 3.1 (Changing the Local User Timeout)), unless security concerns, resource constraints or external policies indicate otherwise (see Section 5 (Security Considerations)). If so, hosts may use a different user timeout for the connection.

A TCP implementation that does not support the TCP User Timeout Option MUST silently ignore it [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.), thus ensuring interoperability.

Hosts MUST impose upper and lower limits on the user timeouts they use. Section 3.1 (Changing the Local User Timeout) discusses user timeout limits, and describes a RECOMMENDED scheme to apply them. A TCP User Timeout Option with a value of zero (i.e., "now") is nonsensical and is used for a special purpose, see Section 3.4 (Special Option Values). Section 3.1 (Changing the Local User Timeout) discusses potentially problematic effects of other user timeout durations.

3.1.  Changing the Local 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. [anchor3] (Without this, UTO would modify TCP semantics, because application-requested UTOs could be overridden by peer requests.) Consequently, unless the application on the local host has requested a specific user timeout for the connection, e.g., through the OPEN or SEND calls, 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 User Timeout Option specifies the user timeout in terms of time units, rather than in terms of number of retransmissions or round-trip times (RTTs), as in most cases the periods of disconnection have to do with operation and mobility patterns, rather than with the current network conditions. 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 (Special Option Values).)

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 closes. Many TCP implementations default to user timeout values of a few minutes [TCP‑ILLU] (Stevens, W., “TCP/IP Illustrated, Volume 1: The Protocols,” 1994.). Although the TCP User Timeout Option allows suggestion of short timeouts, applications advertising them 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 (Security Considerations) discusses the security implications of long timeout values.

To protect against these effects, implementations MUST 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))

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 user timeout of a connection by the local host.

L_LIMIT

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

LOCAL_UTO

Current local user timeout of this specific connection.

REMOTE_UTO

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

This means that, provided they are within the upper and lower limits, 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 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.

It must be noted that the two endpoints of the connection will not necessarily adopt the same user timeout.

Enforcing a lower limit (L_LIMIT) 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 (Security Considerations)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 [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.) 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 (L_LIMIT) SHOULD be set to at least 100 seconds when following the RECOMMENDED scheme described in this section. Adopting a user timeout smaller than the current retransmission timeout (RTO) for the connection would likely cause the connection to be aborted unnecessarily. Therefore, the lower limit (L_LIMIT) MUST be larger than the current retransmission timeout (RTO) for the connection.

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 should 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) shows the format of the TCP User Timeout Option. It contains these fields:

Kind (8 bits)

A TCP option number [RFC0793] (Postel, J., “Transmission Control Protocol,” September 1981.) to be assigned by IANA upon publication of this document (see Section 6 (IANA Considerations)).

Length (8 bits)

Length of the TCP option in octets [RFC0793] (Postel, J., “Transmission Control Protocol,” September 1981.); 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 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.

Thus, the sender SHOULD adapt its local user timeout according to the peer's UTO, and the receiver SHOULD continue using its local user timeout. In order to achieve this, 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 3.1 (Changing the Local User Timeout), with a numeric value of zero seconds for REMOTE_UTO. The sender SHOULD perform the same calculation as described in Section 3.1 (Changing the Local User Timeout), with a numeric value of zero seconds for 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 send it and MUST ignore it upon reception.

4.  Interoperability Issues

This section discusses interoperability issues related to introducing the TCP User Timeout Option.

4.1.  Middleboxes

A TCP implementation that does not support the TCP User Timeout Option MUST silently ignore it [RFC1122] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.), thus ensuring interoperability. In a study of the effects of middleboxes on transport protocols, Medina et al. have shown that unknown TCP options are correctly handled by the vast majority of modern TCP stacks [MEDINA] (Medina, A., Allman, M., and S. Floyd, “Measuring Interactions Between Transport Protocols and Middleboxes,” October 2004.). In this study, 3% of connections failed when an unknown TCP option appeared in the middle of a connection. Because these failures violate existing requirements to ignore unknown options, they do not warrant taking special measures to handle these cases. In particular, we do not define a separate mechanism to negotiate support of the TCP User Timeout Option on the three-way handshake.

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 TCP User Timeout Option. In the future, such firewalls may learn to parse the 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 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 TCP User Timeout Option, then the keep-alive timer MUST be set to a value larger than that of the adopted USER 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. However, TCP implementations do not become more vulnerable to simple SYN flooding by implementing the TCP User Timeout Option, because user timeouts exchanged 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.

Several approaches can help mitigate this issue. First, implementations can require prior peer authentication, e.g., using IPsec [RFC4301] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.), before accepting long user timeouts for the peer's connections. Similarly, a host can start to accept long user timeouts for an established connection only after in-band authentication has occurred, for example, after a TLS handshake across the connection has succeeded [RFC2246] (Dierks, T. and C. Allen, “The TLS Protocol Version 1.0,” January 1999.). 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 3.1 (Changing the Local User Timeout), 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 [RFC2434] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” October 1998.).

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 Allman, Caitlin Bestler, David Borman, Bob Braden, Marcus Brunner, Wesley Eddy, Gorry Fairhurst, Abolade Gbadegesin, Ted Faber, Guillermo Gont, Tom Henderson, Joseph Ishac, Jeremy Harris, Phil Karn, Michael Kerrisk, Dan Krejsa, Jamshid Mahdavi, Kostas Pentikousis, Juergen Quittek, Joe Touch, Stefan Schmid, Simon Schuetz, Tim Shepard 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

8.2. Informative References

Editorial Comments

Appendix A.  Alternative solutions

The same benefits could be obtained through an application-layer mechanism, i.e., exchanging user timeout information via application messages and having the application adjust the user timeouts through the TCP API on both sides of a connection. This approach would not require a new TCP option, but would require changing all application implementations that desire to tolerate extended periods of disconnection, and in most cases would also require 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 would be to simply increase the system-wide user timeout on both peers. This approach has the benefit of not requiring a new TCP option or application changes. However, it can also significantly increase the amount of connection state a busy server must maintain, because a longer global timeout value would 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, reducing the amount of state they must maintain across disconnected periods.

Appendix B.  Document Revision History

To be removed upon publication

Authors' Addresses

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