TCP Maintenance and Minor L. Eggert Extensions (tcpm) NEC Internet-Draft F. Gont Expires:
November 24, 2005January 16, 2006 UTN/FRH May 23,July 15, 2005 TCP User Timeout Option draft-ietf-tcpm-tcp-uto-00draft-ietf-tcpm-tcp-uto-01 Status of this Memo By submitting this Internet-Draft, each author represents that any 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 aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on November 24, 2005.January 16, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract The TCP user timeout controls how long transmitted data may remain unacknowledged before a connection is aborted. TCP implementations typically useforcefully 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-connectiontheir local user timeouts. LengtheningThis exchange provides an in-protocol mechanism to coordinate raising or lowering the system-wide defaulttwo user timeouttimeouts of a connection. Increase the user timeouts allows established TCP connections to survive extended periods of disconnection. On the other hand, shortening the defaultDecreasing user timeouttimeouts allows busy servers to explicitly notify their clients that they will maintain the connection state informationonly accrossacross short periods of disconnection. 1. Introduction The Transmission Control Protocol (TCP) specification [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 abortforcefully 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], and the TCP connection will be abortedclose when the user timeout occurs. The TCP specification  does not constrainIn the permitted values forabsence of an application-specified user timeouts. However,timeout, the TCP specification [RFC0793] defines a default user timeout of 5 minutes. The Host Requirements RFC  mandates a timeout[RFC1122] refines this definition by introducing two thresholds, R1 and R2 (R2 > R1), on the number of at least three minutesretransmissions 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 timeoutconnection once R2 is reached. [RFC1122] also refines the recommended values of a few minutes .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 . System-wide[SOLARIS-MANUAL]. Although applications may set their local user timeouts are a useful basic policy. However, the abilitytimeout, there is no in-protocol mechanism to selectively choose individualsignal changes in the local user timeout valuesto remote peers. This causes local changes to be ineffective, because, for different connectionsexample, 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 ,[RFC3344], HIP [I-D.ietf-hip-arch] or transport-layer mobility mechanisms ,[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 .[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-connectiontheir 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'speer's default user timeout. A second use of the TCP User Timeout Option is advertisement of shorter-than-defaultshorter-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 approachThe same benefits can be obtained through an application-layer mechanism, i.e., coordinating changes to tolerate longer periods of disconnectionthe 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 hostbusy 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 maintainconnections, reducing the amount of state associated with selected connectionsthey 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  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 .[RFC2119]. 3. Operation Sending a TCP User Timeout Option suggests tothat the remote peer to useSHOULD 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'sthe 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 . [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 incomingan exchange of TCP User Timeout Options and MAY use a different user timeout for the connection. Itbetween peers is important to note that thenot a binding negotiation. Transmission of a TCP User Timeout Option does not change the semantics ofis 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 otherin the next possible segment to its peer whenever it starts using a new user timeout durations. A TCP implementationfor the connection. This allows the peer to adapt its local user timeout for the connection accordingly. When a host that does not supportsupports the TCP User Timeout Option SHOULD silently ignorereceives one, it , 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: Formatdecides 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 ofOptions 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 Option, it first decides whether to be assigned by IANA upon publication of this documentchange its local user timeout for the connection (see Section 6.) Length (8 bits) Length of the3.3) and then decides whether to send a TCP optionUser Timeout Option to its peer in octets ;response. If it has never sent a TCP User Timeout Option to its value MUST be 4. Granularity (1 bit) Granularity bit, indicatingpeer during the granularitylifetime 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 theOption 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 Handshaketo its peer. [anchor5] A host that supports the TCP User Timeout Option MUSTSHOULD include an appropriate TCP User Timeout Optionone in its initialeach packet that carries a SYN segment to indicateflag, but need not. [MEDINA] has shown that it supportsunknown options are correctly handled by the option andvast majority of modern TCP stacks. It is thus not necessary to suggest an initial user timeout forrequire negotiation use of the TCP User Timeout Option for a connection. [Comment.2]A hostTCP implementation that supportsdoes not support the TCP User Timeout Option and receives a SYN segment that includes oneMUST respond with an appropriatesilently 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 includewith 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 contentsspecial purpose, see Section 3.4. Section 3.3 discusses potentially problematic effects of anyother receiveduser timeout durations. 3.1 Reliability Considerations The TCP User Timeout Option. 3.3 Operation During the Synchronized States Unless bothOption 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 ofspecification in this section does not define a connection containedreliability handshake for TCP User Timeout Options, both hosts participating in the connection MUST NOT sendOption exchanges. When a segment that carries a TCP User Timeout Options in any other segment. Additionally, they both MUST ignoreOption is lost, the contents of any received TCP User Timeout Option. If, however, bothoption may never reach the SYN and SYN-ACK contained TCP User Timeout Options, hostsintended peer. Implementations MAY chooseimplement local mechanisms to include additionalimprove delivery reliability, such as retransmitting the TCP User Timeout Options in segments sent duringOption 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 ofoption to a connection during its lifetime could be usefulbyte in a number of scenarios, for example: o TCP may adaptthe user timeout based on observed network characteristics. [Comment.3] o TCP may use short timeouts when connections startstream 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 acrossretransmitting the connection has succeeded . Generally,option whenever a host decidesthat byte or its ACK are retransmitted. It is important to changenote that although these mechanisms can improve transmission reliability for the local user timeout of a connection, it SHOULD include aTCP 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 alsoOption, they do not guarantee delivery (a three-way handshake would be required for this). Consequently, implementations MUST NOT assume that the exchange ofa TCP User Timeout Options during the SYN handshake is reliable. ThisOption is notreliably 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 case forTCP User Timeout Option exchanges duringFigure 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 TCPtime 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 localuser timeout accordingly. This draft does not currently describe mechanisms to ensure the reliabilitysuggestion 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 oftimeout (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 seconds1 second to over 9 hours at a granularity of seconds and from zero minutes1 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 .[TCP-ILLU]. Although the TCP User Timeout Option allows suggestion of short timeouts, applications advertising them shouldSHOULD 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'suser timeout of a connection by the local host. L_LIMIT Current lower limit imposed on the connection'suser 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 transientlonger 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 abortsprevents 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 [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 forTCP 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 .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 .[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 UTOTCP User Timeout Option. In the future, such firewalls may learn to parse the UTO optionTCP 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 keepaliveskeep-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 UTOTCP 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 ,[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 .[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 .[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 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. 8.2 Informative References  "TCP/IP Illustrated, Volume 1: The Protocols", Addison-Wesley , 1994.  Sun Microsystems, "Solaris Tunable Parameters Reference Manual", Part No. 806-7009-10, 2002.  Perkins, C., "IP Mobility Support for IPv4", RFC 3344, August 2002.  Moskowitz, R., "Host Identity Protocol Architecture", draft-ietf-hip-arch-02 (work in progress), January 2005.  Eddy, W., "Mobility Support For TCP", draft-eddy-tcp-mobility-00 (work in progress), April 2004.  Schuetz, S., "Network Support for Intermittently Connected Mobile Nodes", M.S. Thesis, University of Mannheim, Germany, June 2004.  Schuetz, S., Eggert, L., Schmid, S.,Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2434] Narten, T. and M. Brunner, "Protocol EnhancementsH. Alvestrand, "Guidelines for Intermittently Connected Hosts", under submission (workWriting an IANA Considerations Section in progress), November 2004. 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. INFOCOMInfocom , March 2004.  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [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-03the Internet Protocol", draft-ietf-ipsec-rfc2401bis-06 (work in progress), MayApril 2005. [MEDINA] Medina, A., Allman, M., and S. Floyd, "Measuring Interactions Between Transport Protocols and Middleboxes", To appear:Proc. 4th ACM SIGCOMM/USENIX Internet MeasurementConference , October 2004.  Kent, S. and K. Seo, "Security Architecture for theon 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 eitherMeasurement , 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 meansC. 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. ThisA future version takes the minimum. My rationale is that the party announcing the lower value probably had a reason for it andof this document may hence not be preparedextend per- connection user timeouts to handlethe SYN-SENT and SYN-RECEIVED states in a longer valueway that it originally indicated. [Comment.5] FG: IMO, in practice the TCP User Timeout option does not makeconforms to the situation worse:required minimum timeouts. [anchor5] LE: Should it really always send UTO when it changes the same type of attacklocal 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: firstname.lastname@example.org 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: email@example.com 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 The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in 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 made any independent effort to identify any such rights. 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