Network Working Group M. Duke Internet-Draft Boeing Phantom Works Expires: July 21, 2005 R. Braden USC Information Sciences Institute W. Eddy NASA GRC/Verizon FNS E. Blanton Purdue University January 20, 2005 A Roadmap for TCP Specification Documents draft-ietf-tcpm-tcp-roadmap-01 Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. 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 become aware will be disclosed, in accordance with RFC 3668. 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 July 21, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document contains a "roadmap" to the Requests for Comments (RFC) documents relating to the Internet's Transmission Control Protocol Duke, et al. Expires July 21, 2005 [Page 1] Internet-Draft TCP Roadmap January 2005 (TCP). This roadmap provides a brief summary of the documents defining TCP and various TCP extensions that have accumulated in the RFC series. This serves as a guide and quick reference for both TCP implementers and other parties who desire information contained in the TCP-related RFCs. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Basic Functionality . . . . . . . . . . . . . . . . . . . . . 5 3. Standard Enhancements . . . . . . . . . . . . . . . . . . . . 7 3.1 Congestion Control and Loss Recovery Extensions . . . . . 7 3.2 SACK-based Loss Recovery and Congestion Control . . . . . 9 3.3 Dealing with Forged Segments . . . . . . . . . . . . . . . 9 4. Experimental Extensions . . . . . . . . . . . . . . . . . . . 11 5. Historic Extensions . . . . . . . . . . . . . . . . . . . . . 13 6. Support Documents . . . . . . . . . . . . . . . . . . . . . . 15 6.1 Foundational Works . . . . . . . . . . . . . . . . . . . . 15 6.2 Difficult Network Environments . . . . . . . . . . . . . . 16 6.3 Implementation Advice . . . . . . . . . . . . . . . . . . 18 6.4 Management Information Bases . . . . . . . . . . . . . . . 19 6.5 Tools and Tutorials . . . . . . . . . . . . . . . . . . . 20 6.6 Case Studies . . . . . . . . . . . . . . . . . . . . . . . 21 7. Security Considerations . . . . . . . . . . . . . . . . . . . 23 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25 9.1 Basic Functionality . . . . . . . . . . . . . . . . . . . 25 9.2 Standard Enhancements . . . . . . . . . . . . . . . . . . 25 9.3 Experimental Extensions . . . . . . . . . . . . . . . . . 26 9.4 Historic Extensions . . . . . . . . . . . . . . . . . . . 27 9.5 Support Documents . . . . . . . . . . . . . . . . . . . . 27 9.6 Informative References Outside the RFC Series . . . . . . 30 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30 Intellectual Property and Copyright Statements . . . . . . . . 32 Duke, et al. Expires July 21, 2005 [Page 2] Internet-Draft TCP Roadmap January 2005 1. Introduction A correct and efficient implementation of the Transmission Control Protocol (TCP) [RFC0793] is a critical part of the software of most Internet hosts. As TCP has evolved over the years, many distinct documents have become part of the accepted standard for TCP. At the same time, a large number of more experimental modifications to TCP have also been published in the RFC series, along with informational notes, case studies, and other advice. As an introduction to newcomers and an attempt to organize the plethora of information for old hands, this document contains a "roadmap" to the TCP-related RFCs. It provides a brief summary of the RFC documents that define TCP. This should provide guidance to implementers on the relevance and significance of the standards track extensions, informational notes, and best current practices that relate to TCP. This roadmap includes a brief description of the contents of each TCP-related RFC. In some cases, we simply supply the abstract or a key summary sentence from the text as a terse description. In addition, a letter code after each RFC number indicates its category in the RFC series: S - Standards Track (Proposed Standard, Draft Standard, or Standard) E - Experimental B - Best Current Practice I - Informational Note that the category of an RFC does not necessarily reflect its current relevance. For instance, RFC 2581 is nearly universally deployed although it is only a Proposed Standard. Similarly, some Informational RFCs contain significant technical proposals for changing TCP. This roadmap is divided into four main sections. Section 2 lists the RFCs that describe absolutely required TCP behaviors for proper functioning and interoperability. Further RFCs that describe strongly encouraged, but not essential, behaviors are listed in Section 3. Experimental extensions which are not yet standard practices, but potentially could be in the future, are described in Section 4. The reader probably notices that these three sections are broadly equivalent to MUST/SHOULD/MAY specifications, and while the authors support this intuition, this document is merely descriptive; it does not represent a binding standards track position. An individual Duke, et al. Expires July 21, 2005 [Page 3] Internet-Draft TCP Roadmap January 2005 implementor still needs to examine the standards documents themselves to evaluate specific requirement levels. A small number of older experimental extensions which have not caught on are noted in Section 5. Many other supporting documents that are relevant to the development, implementation, and deployment of TCP are described in Section 6. Within each section, RFCs are listed in chronological order. Duke, et al. Expires July 21, 2005 [Page 4] Internet-Draft TCP Roadmap January 2005 2. Basic Functionality A small number of documents compose the core specification of TCP. These define the required basic functionalities of TCP's header parsing, state machine, congestion control, and retransmission timeout computation. These base specifications must be correctly followed for interoperability. RFC 793 S: "Transmission Control Protocol", STD 7 (September 1981) This is the fundamental TCP specification document [RFC0793]. Written by Jon Postel as part of the Internet protocol suite's core, it describes the TCP packet format, the TCP state machine and event processing, and TCP's semantics for data transmission, reliability, flow control, multiplexing, and acknowledgement. Section 3.6 of RFC 793, describing TCP's handling of the IP precedence and security compartment, is mostly irrelevant today. RFC 2873 changed the IP precedence handling, and the security compartment portion of the API is no longer implemented or used. In addition, RFC 793 did not describe any congestion control mechanism. Otherwise, however, the majority of this document still acurately describes modern TCPs. RFC 793 is the last of a series of developmental TCP specifications, starting from IENs and continuing in the RFC series. RFC 1122 S: "Requirements for Internet Hosts - Communication Layers" (October 1989) This document [RFC1122] updates and clarifies RFC 793, fixing some specification bugs and oversights. It also explains some features such as keep-alives and Karn's and Jacobson's RTO estimation algorithms [Karn][VJ88]. ICMP interactions are mentioned and some tips are given for efficient implementation. RFC 1122 is an Applicability Statement, listing the various features that MUST, SHOULD, MAY, SHOULD NOT, and MUST NOT be present in standards-conforming TCP implementations. RFC 2147 S: "TCP and UDP over IPv6 Jumbograms" (May 1997) IPv6's support for longer datagrams than were allowed in IPv4, necessitated some changes to the way that TCP's MSS and Urgent fields (both 16 bits) are treated. RFC 2460 S: "Internet Protocol, Version 6 (IPv6) Specification (December 1998) This document [RFC2460] makes a slight update to the way the Duke, et al. Expires July 21, 2005 [Page 5] Internet-Draft TCP Roadmap January 2005 pseudo-header for checksum computation is derived, defining the process for IPv6 in addition to the previous practice for IPv4. RFC 2581 S: "TCP Congestion Control" (April 1999) Although RFC 793 did not contain any congestion control mechanisms, today congestion control is a required component of TCP implementations. This document [RFC2581] defines the current versions of Van Jacobson's congestion avoidance and control mechanisms for TCP, based on his 1988 SIGCOMM paper [VJ88]. RFC 2001 was a conceptual precursor that was obsoleted by RFC 2581. A number of behaviors that together comprise what the community refers to as "Reno TCP", are described in RFC 2581. The name "Reno" comes from the Net/2 release of the 4.3 BSD operating system. This is generally regarded as the least common denominator among TCP flavors currently found running on Internet hosts. Reno TCP includes the congestion control features of slow start, congestion avoidance, fast retransmit, and fast recovery. RFC 2873 S: "TCP Processing of the IPv4 Precendence Field" (June 2000) This document [RFC2873] removes from the TCP specification all processing of the precedence bits of the TOS byte of the IP header. This resolves a conflict over the use of these bits between RFC 793 and Differentiated Services. RFC 2988 S: "Computing TCP's Retransmission Timer" (November 2000) Abstract: "This document defines the standard algorithm that Transmission Control Protocol (TCP) senders are required to use to compute and manage their retransmission timer. It expands on the discussion in section 4.2.3.1 of RFC 1122 and upgrades the requirement of supporting the algorithm from a SHOULD to a MUST." [RFC2988] Duke, et al. Expires July 21, 2005 [Page 6] Internet-Draft TCP Roadmap January 2005 3. Standard Enhancements This section describes recommended TCP modifications that improve performance and security. RFCs 1323 and 3168 represent fundamental changes to the protocol. RFC 1323, based on RFCs 1072 and 1185, allows better utilization of high bandwidth-delay product paths by providing some needed mechanisms for high-rate transfers. RFC 3168 describes a change to the Internet's architecture, where routers signal end-hosts of growing congestion levels, and can do so before packet losses are forced. Section 3.1 lists improvements in the congestion control and loss recovery mechanisms specified in RFC 2581. Section 3.2 describes further refinements that make use of selective acknowledgements. Section 3.3 deals with the problem of preventing forged segments. RFC 1323 S: "TCP Extensions for High Performance" (May 1992) This document [RFC1323] defines TCP extensions for window scaling, timestamps, and protection against wrapped sequence numbers, for efficient and safe operation over paths with large bandwidth-delay products. These extensions are commonly found in currently-used systems; however, they may require manual tuning and configuration. Some "corner cases" in this specification are still under discussion. RFC 3168 S: "The Addition of Explicit Congestion Notification (ECN) to IP" (September 2001) This document [RFC3168] defines a means of detecting congestion without resorting to packet loss. Although congestion notification takes place at the IP level, ECN requires support at the transport level (e.g., in TCP) to echo the bits and adapt the sending rate. This document updates RFC 793 to define two previously-unused flag bits in the TCP header for ECN support. RFC 3540 provides a supplementary (experimental) means for making ECN use more secure, and RFC 2884 provides some sample results from using ECN. 3.1 Congestion Control and Loss Recovery Extensions Two of the most important aspects of TCP are its congestion control and loss recovery features. Since TCP traditionally (in the absence of ECN) uses losses to infer congestion, there is a rather intimate coupling between congestion control and loss recovery mechanisms. There are several extensions to both features, and more often than not, a particular extension applies to both. In this sub-section, we group enhancements to either congestion control, loss recovery, or Duke, et al. Expires July 21, 2005 [Page 7] Internet-Draft TCP Roadmap January 2005 both, which can be performed unilaterally - without negotiating support between endpoints. In the next sub-section, we group the extensions which specify or rely on the SACK option, whose use must be negotiated bilaterally. TCP implementations should include the enhancements from both sub-sections so that they can perform well without regard to the feature sets of other hosts they connect to. For example, if SACK use is not successfully negotiated, a TCP should use the NewReno behavior as a fall-back. RFC 3042 S: "Enhancing TCP's Loss Recovery Using Limited Transmit" (January 2001) Abstract: "This document proposes [Limited Transmit,] a new Transmission Control Protocol (TCP) mechanism that can be used to more effectively recover lost segments when a connection's congestion window is small, or when a large number of segments are lost in a single transmission window." [RFC3042] RFC 3390 S: "Increasing TCP'S Initial Window" (October 2002) This document [RFC3390] updates RFC 2581 to permit an initial TCP window larger that one packet during in the slow-start phase. RFC 3782 S: "The NewReno Modification to TCP's Fast Recovery Algorithm" (April 2004) This document [RFC3782] specifies a slight modification to the standard Reno fast recovery algorithm, whereby a TCP sender can use partial acknowledgements to make inferences determining the next segment to send in situations where SACK would be helpful, but isn't available. Work in progress: The Eifel Response Algorithm for TCP (Internet Draft name: draft-ietf-tsvwg-tcp-eifel-response) At the time of this writing, work on this document (from authors Reiner Ludwig and Andrei Gurtov) had stabilized within the Transport Area Working Group, and the document was planned to become a Proposed Standard, pending IESG review, but was not yet a part of the RFC series. This document describes the response portion of the Eifel algorithm, which can be used in conjunction with one of several methods of detecting when loss recovery has been spuriously entered, such as the Eifel detection algorithm in RFC 3522, the algorithm in RFC 3708, or F-RTO. Duke, et al. Expires July 21, 2005 [Page 8] Internet-Draft TCP Roadmap January 2005 Abstract: "Based on an appropriate detection algorithm, the Eifel response algorithm provides a way for a TCP sender to respond to a detected spurious timeout. It adapts the retransmission timer to avoid further spurious timeouts, and can avoid - depending on the detection algorithm - the often unnecessary go-back-N retransmits that would otherwise be sent. In addition, the Eifel response algorithm restores the congestion control state in such a way that packet bursts are avoided." 3.2 SACK-based Loss Recovery and Congestion Control The base TCP specification in RFC 793 provided only a simple cumulative acknowledgment mechanism. However, a selective acknowledgment (SACK) mechanism provides significant performance improvement in the presence of packet losses, more than outweighing the modest increase in complexity. A TCP should be expected to implement SACK, however SACK is a negotiated option and is only used if support is advertised by both sides of a connection. RFC 2018 S: "TCP Selective Acknowledgement Options" (October 1996) This document [RFC2018] defines the basic selective acknowledgement (SACK) mechanism for TCP. RFC 2883 S: "An Extension to the Selective Acknowledgement (SACK) Option for TCP" (July 2000) This document [RFC2883] extends RFC 2018 to cover the case of acknowledging duplicate packets. RFC 3517 S: "A Conservative Selective Acknowledgement (SACK)-based Loss Recovery Algorithm for TCP" (April 2003) This document [RFC3517] describes a relatively sophisticated algorithm that a TCP sender can use for loss recovery when SACK reports more than one segment lost from a single flight of data. While support for the exchange of SACK information is widely implemented, not all implementations use an algorithm as sophisticated as that described in RFC 3517. 3.3 Dealing with Forged Segments By default, TCP lacks any cryptographic structures to differentiate legitimate segments and those spoofed from malicious hosts. Spoofing valid segments requires correctly guessing a number of fields. The documents in this sub-section describe ways to make that guessing Duke, et al. Expires July 21, 2005 [Page 9] Internet-Draft TCP Roadmap January 2005 harder, or prevent it from being able to negatively impact a connection. RFC 1948 I: "Defending Against Sequence Number Attacks" (May 1996) This document [RFC1948] describes the TCP vulnerability based upon guessing sequence numbers and as well as defenses against this exploit. Some variation is implemented in most currently-used operating systems. RFC 2385 S: "Protection of BGP Sessions via the TCP MD5 Signature Option" (August 1998) From document: "This document describes currrent existing practice for securing BGP against certain simple attacks. It is understood to have security weaknesses against concerted attacks. This memo describes a TCP extension to enhance security for BGP. It defines a new TCP option for carrying an MD5 [RFC1321] digest in a TCP segment. This digest acts like a signature for that segment, incorporating information known only to the connection end points. Since BGP uses TCP as its transport, using this option in the way described in this paper significantly reduces the danger from certain security attacks on BGP." [RFC2385] TCP MD5 options are currently only used in very limited contexts, primarily for defending BGP exchanges between routers. Some deployment notes for those using TCP MD5 are found in the later RFC 3562, "Key Management Considerations for the TCP MD5 Signature Option" [RFC3562]. Work in progress: Transmission Control Protocol Security Considerations (Internet Draft name: draft-ietf-tcpm-tcpsecure) At the time of this writing, the TCP Maintenance and Minor Extensions Working Group is producing a document (edited by Mitesh Dalal) which describes a challenge-response mechanism for securing TCP against spoofed control segments. This document is expected to become an RFC in the near future. Duke, et al. Expires July 21, 2005 [Page 10] Internet-Draft TCP Roadmap January 2005 4. Experimental Extensions The RFCs in this section are still experimental, but may become proposed standards in the future. At least part of the reason that they are still experimental is to gain more wide-scale experience with them before making a standards track decision. RFC 2140 I: "TCP Control Block Interdependence" (April 1997) This document [RFC2140] suggests how TCP connections between the same endpoints might share information, such as their congestion control state. To some degree, this is done in practice by a few operating systems; for example, Linux has a destination cache. A related proposal, the Congestion Manager, is specified in RFC 3124 [RFC3124]. The idea behind the Congestion Manager, moving congestion control outside of individual TCP connections, represents a modification to the core of TCP. Although a Proposed Standard, some pieces of the Congestion Manager support architecture have not been specified yet, and it has not achieved use or implementation beyond experimental stacks. RFC 2861 E: "TCP Congestion Window Validation" (June 2000) This document [RFC2861] suggests reducing the congestion window over time when no packets are flowing. RFC 3465 E: "TCP Congestion Control with Appropriate Byte Counting (ABC)" (February 2003) This document [RFC3465] suggests that congestion control use the number of bytes acknowledged rather than the number of acknowledgements received. This has been implemented in Linux. The ABC mechanism behaves differently than the standard means when there is not a one-to-one relationship between data segments and acknowledgements. ABC still operates within the accepted guidelines, but is more robust to delayed ACKs and ACK-division [Savage]. RFC 3522 E: "The Eifel Detection Algorithm for TCP" (April 2003) This document [RFC3522] suggests using timestamps to detect spurious timeouts. RFC 3540 E: "Robust Explicit Congestion Notification (ECN) signaling with Nonces" (June 2003) This document [RFC3540] suggests a modified ECN to address Duke, et al. Expires July 21, 2005 [Page 11] Internet-Draft TCP Roadmap January 2005 security concerns, and updates RFC 3168. RFC 3649 E: "HighSpeed TCP for Large Congestion Windows" (December 2003) This document [RFC3649] suggests a modification to TCP's steady-state behavior to efficiently use very large windows. RFC 3708 E: "Using TCP Duplicate Selective Acknowledgement (DSACKs) and Stream Control Transmission Protocol (SCTP) Duplicate Transmission Sequence Numbers (TSNs) to Detect Spurious Retransmissions" (February 2004) Abstract: "TCP and Stream Control Transmission Protocol (SCTP) provide notification of duplicate segment receipt through Duplicate Selective Acknowledgement (DSACKs) and Duplicate Transmission Sequence Number (TSN) notification, respectively. This document presents conservative methods of using this information to identify unnecessary retransmissions for various applications." [RFC3708] RFC 3742 E: "Limited Slow-Start for TCP with Large Congestion Windows" (March 2004) This document [RFC3742] describes a more conservative slow-start behavior to prevent massive packet losses when a connection uses a very large window. Work in progress: Forward RTO-Recovery (F-RTO): An Algorithm for Detecting Spurious Retransmission Timeouts with TCP and SCTP (Internet Draft name: draft-ietf-tcpm-frto) The F-RTO detection algorithm provides another option for inferring spurious retransmission timeouts. At the time of this writing, the TCP Maintenance and Minor Extensions Working Group had completed a document describing F-RTO (by Pasi Sarolahti and Markku Kojo), and planned to make this an Experimental part of the RFC series, pending IESG review. Duke, et al. Expires July 21, 2005 [Page 12] Internet-Draft TCP Roadmap January 2005 5. Historic Extensions The RFCs listed here define extensions that have thus far failed to arouse substantial interest, or were found to be defective. RFC 1106 "TCP Big Window and NAK Options" (June 1989) This RFC [RFC1106] defined an alternative to the Window Scale option for using large windows, and described the "negative acknowledgement" or NAK option. There is a comparison of NAK and SACK methods, and early discussion of TCP over satellite issues. The options described in this document have not been adopted by the larger community, although NAKs are used in the SCPS-TP adaptation of TCP, developed by the Consultive Committee for Space Data Systems (CCSDS). RFC 1110 "A Problem with the TCP Big Window Option" (August 1989) Abstract: "The TCP Big Window option discussed in RFC 1106 will not work properly in an Internet environment which has both a high bandwidth * delay product and the possibility of disordering and duplicating packets. In such networks, the window size must not be increased without a similar increase in the sequence number space. Therefore, a different approach to big windows should be taken in the Internet." [RFC1110] RFC 1146 E "TCP Alternate Checksum Options" (March 1990) This document [RFC1146] defined more robust TCP checksums than the 16-bit ones-complement in use today. A typographical error in RFC 1145 is fixed in RFC 1146, otherwise the documents are the same. RFC 1263 "TCP Extensions Considered Harmful" (October 1991) This interesting document [RFC1263] argues against "backwards compatible" TCP extensions. Specifically mentioned are several TCP enhancements that have been successful, including timestamps, window scaling, PAWS, and SACK. RFC 1263 presents an alternative approach called "protocol evolution", whereby several evolutionary versions of TCP would exist on hosts. These distinct TCP versions would represent upgrades to each other and could be header-incompatible. Interoperability would be provided by having a virtualization layer select the right TCP version for a particular connection. This idea did not catch on with the community, while the type of extensions RFC 1263 specifically targeted as harmful did become popular. Duke, et al. Expires July 21, 2005 [Page 13] Internet-Draft TCP Roadmap January 2005 RFC 1379 I "Extending TCP for Transactions -- Concepts" (November 1992) See RFC 1644. RFC 1644 E "T/TCP -- TCP Extensions for Transactions Functional Specification" (July 1994) The inventors of TCP believed that cached connection state could have been used to eliminate TCP's 3-way handshake, to support two-packet request/response exchanges. RFCs 1379 [RFC1379] and 1644 [RFC1644] show that this is far from simple. Furthermore, T/TCP floundered on the ease of denial-of-service attacks that can result. RFC 1693 E "An Extension to TCP: Partial Order Service" (November 1994) This document [RFC1693] defines a TCP extension for applications that do not care about the order in which application-layer objects are received. Examples are multimedia and database applications. In practice, these applications either accept the possible performance loss because of TCP's strict ordering, or they use more specialized transport protocols. Duke, et al. Expires July 21, 2005 [Page 14] Internet-Draft TCP Roadmap January 2005 6. Support Documents This section contains several classes of documents that do not necessarily define current protocol behaviors, but are nevertheless of interest to TCP implementors. Section 6.1 describes several foundational RFCs that give modern readers a better understanding of the principles underlying TCP's behaviors and development over the years. The documents listed in Section 6.2 provide advice on using TCP in various types of network situations that pose challenges above those of typical wired links. Some implementation notes can be found in Section 6.3. The TCP Management Information Bases are described in Section 6.4. RFCs that describe tools for testing and debugging TCP implementations or contain high-level tutorials on the protocol are listed Section 6.5, while Section 6.6 lists a number of case studies that have explored TCP performance. 6.1 Foundational Works The documents listed in this section contain information that is largely duplicated by the standards documents previously discussed. However, some of them contain a greater depth of problem statement explanation or other context. Particularly, RFCs 813-817 (known as the "Dave Clark Five"), describe some early problems and solutions (RFC 815 only describes the reassembly of IP fragments, and is not included here). RFC 813: "Window and Acknowledgement Strategy in TCP" (July 1982) This document [RFC0813] contains an early discussion of Silly Window Syndrome and its avoidance, and motivates and describes the use of delayed acknowledgements. RFC 814: "Name, Addresses, Ports, and Routes" (July 1982) Suggestions and guidance for the design of tables and algorithms to keep track of various identifiers within a TCP/IP implementation are provided by this document [RFC0814]. RFC 816: "Fault Isolation and Recovery" (July 1982) In this document [RFC0816], TCP's response to indications of network error conditions such as timeouts or received ICMP messages. RFC 817: "Modularity and Efficiency in Protocol Implementation" (July 1982) This document [RFC0817] contains implementation suggestions that Duke, et al. Expires July 21, 2005 [Page 15] Internet-Draft TCP Roadmap January 2005 are general and not TCP-specific. However they have been used to develop TCP implementations and describe some performance implications of the interactions between various layers in the Internet stack. RFC 872: "TCP-ON-A-LAN" (September 1982) Conclusion: "The sometimes-expressed fear that using TCP on a local net is a bad idea is unfounded." [RFC0872] RFC 896: "Congestion Control in IP/TCP Internetworks" (January 1984) This document [RFC0896] contains some early experiences with congestion collapse and some initial thoughts on how to avoid it using congestion control in TCP. RFC 964: "Some Problems with the Specification of the Military Standard Transmission Control Protocol" (November 1985) This document [RFC0964] was prepared by the US Military to define TCP in greater detail than RFC 793. A few serious specification bugs are detailed in RFC 964, reminding us of the difficulty in specification writing (even when working from existing documents!). RFC 1072: "TCP Extensions for Long-Delay Paths" (October 1988) This document [RFC1072] contains early explanations of the mechanisms that were later described by RFCs 1323 and 2018, which obsolete it. RFC 1185: "TCP Extension for High-Speed Paths" (October 1990) This document [RFC1185] builds on RFC 1072 to describe more advanced strategies for dealing with sequence number wrapping and detecting duplicates from earlier connections. This document was obsoleted by RFC 1323. RFC 2914 B: "Congestion Control Principles" (September 2000) This document [RFC2914] motivates the use of end-to-end congestion control for preventing congestion collapse and providing fairness to TCP. 6.2 Difficult Network Environments As the internetworking field has explored wireless, satellite, Duke, et al. Expires July 21, 2005 [Page 16] Internet-Draft TCP Roadmap January 2005 cellular telephone, and other kinds of link-layer technologies, a large body of work has built up on enhancing TCP performance for such links. The RFCs listed in this section describe some of these more challenging network environments and how TCP interacts with them. RFC 2488 B: "Enhancing TCP Over Satellite Channels using Standard Mechanisms" (January 1999) From abstract: "While TCP works over satellite channels there are several IETF standardized mechanisms that enable TCP to more effectively utilize the available capacity of the network path. This document outlines some of these TCP mitigations. At this time, all mitigations discussed in this document are IETF standards track mechanisms (or are compliant with IETF standards)." [RFC2488] RFC 2757 I: "Long Thin Networks" (January 2000) Several methods of improving TCP performance over long thin networks, such as geosynchronous satellite links, are discussed in this document [RFC2757]. A particular set of TCP options is developed that should work well in such environments, and be safe to use in the global Internet. RFC 2760 I: "Ongoing TCP Research Related to Satellites" (February 2000) This document [RFC2760] discusses the advantages and disadvantages of several different experimental means of improving TCP performance over long-delay or error-prone paths. These include: T/TCP, larger initial windows, byte counting, delayed acknowledgements, slow start thresholds, NewReno and SACK-based loss recovery, FACK [FACK], ECN, various corruption-detection mechanisms, congestion avoidance changes for fairness, use of multiple parallel flows, pacing, header compression, state sharing, and ACK congestion control, filtering, and reconstruction. While RFC 2488 looks at standard extensions, this document focuses on more experimental means of performance enhancement. RFC 3135 I: "Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations" (June 2001) From abstract: "This document is a survey of Performance Enhancing Proxies (PEPs) often employed to improve degraded TCP performance caused by characteristics of specific link environments, for example, in satellite, wireless WAN, and wireless LAN environments. Different types of Performance Enhancing Proxies Duke, et al. Expires July 21, 2005 [Page 17] Internet-Draft TCP Roadmap January 2005 are described as well as the mechanisms used to improve performance." [RFC3135] RFC 3449 B: "TCP Performance Implications of Network Path Asymmetry" (December 2002) From abstract: "This document describes TCP performance problems that arise because of asymmetric effects. These problems arise in several access networks, including bandwidth-asymmetric networks and packet radio subnetworks, for different underlying reasons. However, the end result on TCP performance is the same in both cases: performance often degrades significantly because of imperfection and variability in the ACK feedback from the receiver to the sender. The document details several mitigations to these effects, which have either been proposed or evaluated in the literature, or are currently deployed in networks." [RFC3449] RFC 3481 B: "TCP over Second (2.5G) and Third (3G) Generation Wireless Networks" (February 2003) From abstract: "This document describes a profile for optimizing TCP to adapt so that it handles paths including second (2.5G) and third (3G) generation wireless networks." [RFC3481] RFC 3819 B: "Advice for Internet Subnetwork Designers" (July 2004) This document [RFC3819] describes how TCP performance can be negatively impacted by some particular lower-layer behaviors, and provides guidance in designing lower-layer networks and protocols to be amicable to TCP. 6.3 Implementation Advice RFC 879: "The TCP Maximum Segment Size and Related Topics" (November 1983) Abstract: 'This memo discusses the TCP Maximum Segment Size Option and related topics. The purposes is to clarify some aspects of TCP and its interaction with IP. This memo is a clarification to the TCP specification, and contains information that may be considered as "advice to implementers".' [RFC0879] RFC 2525 I: "Known TCP Implementation Problems" (March 1999) From abstract: "This memo catalogs a number of known TCP implementation problems. The goal in doing so is to improve Duke, et al. Expires July 21, 2005 [Page 18] Internet-Draft TCP Roadmap January 2005 conditions in the existing Internet by enhancing the quality of current TCP/IP implementations." [RFC2525] RFC 2923 I: "TCP Problems with Path MTU Discovery" (September 2000) From abstract: "This memo catalogs several known Transmission Control Protocol (TCP) implementation problems dealing with Path Maximum Transmission Unit Discovery (PMTUD), including the long-standing black hole problem, stretch acknowlegements (ACKs) due to confusion between Maximum Segment Size (MSS) and segment size, and MSS advertisement based on PMTU." [RFC2923] RFC 3360 B: "Inappropriate TCP Resets Considered Harmful" (August 2002) This document [RFC3360] is a plea that firewall vendors not send gratuitous TCP RST (Reset) packets when unassigned TCP header bits are used. This practice prevents desirable extension and evolution of the protocol and hence is inimical to the future of the Internet. RFC 3493 I: "Basic Socket Interface Extensions for IPv6" (February 2003) This document [RFC3493] describes the de facto standard sockets API for programming with TCP. This API is implemented nearly ubiquitously in modern operating systems and programming languages. 6.4 Management Information Bases The first MIB module defined for use with SNMP (in RFC 1066 and its update, RFC 1156) was a single monolithic MIB module, called MIB-I. This evolved over time to be MIB-II (RFC 1213). It then became apparent that having a single monolithic MIB module was not scalable, given the number and breadth of MIB data definitions that needed to be included. Thus, additional MIB modules were defined, and those parts of MIB-II which needed to evolve were split off. Eventually, the remaining parts of MIB-II were also split off, with the TCP-specific part being documented in RFC 2012. RFC 2012 is the primary document for MIB-II. MIB-I, defined in RFC 1156, has been obsoleted by the MIB-II specification in RFC 1213 (updated by 2012). Work is in progress, at the time of this writing, on a document that incorporates IPv6 and updates and obsoletes RFC 2012 (currently in the form of draft-ietf-ipv6-rfc2012-update, edited by Rajiv Raghunarayan, under submission to the IESG as a Proposed Duke, et al. Expires July 21, 2005 [Page 19] Internet-Draft TCP Roadmap January 2005 Standard). RFC 1066: "Management Information Base for Network Management of TCP/IP-based Internets" (August 1988) This document [RFC1066] was the description of the TCP MIB. It was obsoleted by RFC 1156. RFC 1156 S: "Management Information Base for Network Management of TCP/IP-based Internets" (May 1990) This document [RFC1156] describes the required MIB fields for TCP implementations, with minor corrections and no technical changes from RFC 1066, which it obsoletes. This is the standards track document for MIB-I. RFC 1213 S: "Management Information Base for Network Management of TCP/IP-based Internets: MIB-II" (March 1991) This document [RFC1213] describes the second version of the MIB in a monolithic form. RFC 2012 updates this document by splitting out the TCP-specific portions. RFC 2012 S: "SNMPv2 Management Information Base for the Transmission Control Protocol using SMIv2" (November 1996) This document [RFC2012] defines the TCP MIB, updating RFC 1213. RFC 2452 S: "IP Version 6 Management Information Base for the Transmission Control Protocol" (December 1998) This document [RFC2452] augments RFC 2012 by adding an IPv6-specific connection table. The rest of 2012 holds for any IP version. Although it is a standards track document, RFC 2452 is considered a historic mistake by the MIB community, as it is based on the idea of parallel IPv4 and IPv6 structures. Although IPv6 requires new structures, the community has decided to define a single generic structure for both IPv4 and IPv6. This will aid in definition, implementation, and transition between IPv4 and IPv6. 6.5 Tools and Tutorials Duke, et al. Expires July 21, 2005 [Page 20] Internet-Draft TCP Roadmap January 2005 RFC 1180 I: "TCP/IP Tutorial" (January 1991) This document [RFC1180] is an extremely brief overview of the TCP/IP protocol suite as a whole. It gives some explanation as to how and where TCP fits in. RFC 1470 I: "FYI on a Network Management Tool Catalog: Tools for Monitoring and Debugging TCP/IP Internets and Interconnected Devices" (June 1993) A few of the tools that this document [RFC1470] describes are still maintained and in use today, for example ttcp and tcpdump. However, many of the tools described do not relate specifically to TCP and are no longer used or easily available. RFC 2398 I: "Some Testing Tools for TCP Implementors" (August 1998) This document [RFC2398] describes a number of TCP packet generation and analysis tools. While some of these tools are no longer readily available or widely used, for the most part they are still relevant and useable. 6.6 Case Studies RFC 1337 I: "TIME-WAIT Assassination Hazards in TCP" (May 1992) This document [RFC1337] points out a problem with acting on received reset segments while in the TIME-WAIT state. The main recemmendation is that hosts in TIME-WAIT ignore resets. RFC 2415 I: "Simulation Studies of Increased Initial TCP Window Size" (September 1998) This document [RFC2415] presents results of some simulations using TCP initial windows greater than 1 segment. The analysis indicates that user-perceived performance can be improved by increasing the initial window to 3 segments. RFC 2416 I: "When TCP Starts Up With Four Packets Into Only Three Buffers" (September 1998) This document [RFC2416] uses simulation results to clear up some concerns about using an initial window of 4 segments when the network path has less provisioning. Duke, et al. Expires July 21, 2005 [Page 21] Internet-Draft TCP Roadmap January 2005 RFC 2884 I: "Performance Evaluation of Explicit Congestion Notification (ECN) in IP Networks" (July 2000) This document [RFC2884] describes experimental results that show some improvements to the performance of both short and long-lived connections due to ECN. Duke, et al. Expires July 21, 2005 [Page 22] Internet-Draft TCP Roadmap January 2005 7. Security Considerations This document introduces no new security considerations. Each RFC listed in this document attempts to address the security considerations of the specification it contains. Duke, et al. Expires July 21, 2005 [Page 23] Internet-Draft TCP Roadmap January 2005 8. Acknowledgments This document grew out of a discussion on the end2end-interest mailing list, the public list of the End-to-End Research Group of the IRTF. We thank Joe Touch, Reiner Ludwig, and Pekka Savola for their contributions, in particular. The chairs of the TCPM working group, Mark Allman and Ted Faber, have been instrumental in the development of this document. Keith McCloghrie provided some useful notes and clarification on the various MIB-related RFCs. Duke, et al. Expires July 21, 2005 [Page 24] Internet-Draft TCP Roadmap January 2005 9. References 9.1 Basic Functionality [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. [RFC2147] Borman, D., "TCP and UDP over IPv6 Jumbograms", RFC 2147, May 1997. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion Control", RFC 2581, April 1999. [RFC2873] Xiao, X., Hannan, A., Paxson, V. and E. Crabbe, "TCP Processing of the IPv4 Precedence Field", RFC 2873, June 2000. [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission Timer", RFC 2988, November 2000. 9.2 Standard Enhancements [RFC1323] Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks", RFC 1948, May 1996. [RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, October 1996. [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998. [RFC2883] Floyd, S., Mahdavi, J., Mathis, M. and M. Podolsky, "An Extension to the Selective Acknowledgement (SACK) Option for TCP", RFC 2883, July 2000. [RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's Loss Recovery Using Limited Transmit", RFC 3042, January 2001. Duke, et al. Expires July 21, 2005 [Page 25] Internet-Draft TCP Roadmap January 2005 [RFC3168] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. [RFC3390] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's Initial Window", RFC 3390, October 2002. [RFC3517] Blanton, E., Allman, M., Fall, K. and L. Wang, "A Conservative Selective Acknowledgment (SACK)-based Loss Recovery Algorithm for TCP", RFC 3517, April 2003. [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5 Signature Option", RFC 3562, July 2003. [RFC3782] Floyd, S., Henderson, T. and A. Gurtov, "The NewReno Modification to TCP's Fast Recovery Algorithm", RFC 3782, April 2004. 9.3 Experimental Extensions [RFC2140] Touch, J., "TCP Control Block Interdependence", RFC 2140, April 1997. [RFC2861] Handley, M., Padhye, J. and S. Floyd, "TCP Congestion Window Validation", RFC 2861, June 2000. [RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager", RFC 3124, June 2001. [RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte Counting (ABC)", RFC 3465, February 2003. [RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for TCP", RFC 3522, April 2003. [RFC3540] Spring, N., Wetherall, D. and D. Ely, "Robust Explicit Congestion Notification (ECN) Signaling with Nonces", RFC 3540, June 2003. [RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows", RFC 3649, December 2003. [RFC3708] Blanton, E. and M. Allman, "Using TCP Duplicate Selective Acknowledgement (DSACKs) and Stream Control Transmission Protocol (SCTP) Duplicate Transmission Sequence Numbers (TSNs) to Detect Spurious Retransmissions", RFC 3708, February 2004. Duke, et al. Expires July 21, 2005 [Page 26] Internet-Draft TCP Roadmap January 2005 [RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large Congestion Windows", RFC 3742, March 2004. 9.4 Historic Extensions [RFC1106] Fox, R., "TCP big window and NAK options", RFC 1106, June 1989. [RFC1110] McKenzie, A., "Problem with the TCP big window option", RFC 1110, August 1989. [RFC1146] Zweig, J. and C. Partridge, "TCP alternate checksum options", RFC 1146, March 1990. [RFC1263] O'Malley, S. and L. Peterson, "TCP Extensions Considered Harmful", RFC 1263, October 1991. [RFC1379] Braden, B., "Extending TCP for Transactions -- Concepts", RFC 1379, November 1992. [RFC1644] Braden, B., "T/TCP -- TCP Extensions for Transactions Functional Specification", RFC 1644, July 1994. [RFC1693] Connolly, T., Amer, P. and P. Conrad, "An Extension to TCP : Partial Order Service", RFC 1693, November 1994. 9.5 Support Documents [RFC0813] Clark, D., "Window and Acknowledgement Strategy in TCP", RFC 813, July 1982. [RFC0814] Clark, D., "Name, addresses, ports, and routes", RFC 814, July 1982. [RFC0816] Clark, D., "Fault isolation and recovery", RFC 816, July 1982. [RFC0817] Clark, D., "Modularity and efficiency in protocol implementation", RFC 817, July 1982. [RFC0872] Padlipsky, M., "TCP-on-a-LAN", RFC 872, September 1982. [RFC0879] Postel, J., "TCP maximum segment size and related topics", RFC 879, November 1983. [RFC0896] Nagle, J., "Congestion control in IP/TCP internetworks", RFC 896, January 1984. Duke, et al. Expires July 21, 2005 [Page 27] Internet-Draft TCP Roadmap January 2005 [RFC0964] Sidhu, D. and T. Blumer, "Some problems with the specification of the Military Standard Transmission Control Protocol", RFC 964, November 1985. [RFC1066] McCloghrie, K. and M. Rose, "Management Information Base for network management of TCP/IP-based internets", RFC 1066, August 1988. [RFC1072] Jacobson, V. and R. Braden, "TCP extensions for long-delay paths", RFC 1072, October 1988. [RFC1156] McCloghrie, K. and M. Rose, "Management Information Base for network management of TCP/IP-based internets", RFC 1156, May 1990. [RFC1180] Socolofsky, T. and C. Kale, "TCP/IP tutorial", RFC 1180, January 1991. [RFC1185] Jacobson, V., Braden, B. and L. Zhang, "TCP Extension for High-Speed Paths", RFC 1185, October 1990. [RFC1213] McCloghrie, K. and M. Rose, "Management Information Base for Network Management of TCP/IP-based internets:MIB-II", STD 17, RFC 1213, March 1991. [RFC1337] Braden, B., "TIME-WAIT Assassination Hazards in TCP", RFC 1337, May 1992. [RFC1470] Enger, R. and J. Reynolds, "FYI on a Network Management Tool Catalog: Tools for Monitoring and Debugging TCP/IP Internets and Interconnected Devices", RFC 1470, June 1993. [RFC2012] McCloghrie, K., "SNMPv2 Management Information Base for the Transmission Control Protocol using SMIv2", RFC 2012, November 1996. [RFC2398] Parker, S. and C. Schmechel, "Some Testing Tools for TCP Implementors", RFC 2398, August 1998. [RFC2415] Poduri, K., "Simulation Studies of Increased Initial TCP Window Size", RFC 2415, September 1998. [RFC2416] Shepard, T. and C. Partridge, "When TCP Starts Up With Four Packets Into Only Three Buffers", RFC 2416, September 1998. [RFC2452] Daniele, M., "IP Version 6 Management Information Base for Duke, et al. Expires July 21, 2005 [Page 28] Internet-Draft TCP Roadmap January 2005 the Transmission Control Protocol", RFC 2452, December 1998. [RFC2488] Allman, M., Glover, D. and L. Sanchez, "Enhancing TCP Over Satellite Channels using Standard Mechanisms", BCP 28, RFC 2488, January 1999. [RFC2525] Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner, J., Heavens, I., Lahey, K., Semke, J. and B. Volz, "Known TCP Implementation Problems", RFC 2525, March 1999. [RFC2757] Montenegro, G., Dawkins, S., Kojo, M., Magret, V. and N. Vaidya, "Long Thin Networks", RFC 2757, January 2000. [RFC2760] Allman, M., Dawkins, S., Glover, D., Griner, J., Tran, D., Henderson, T., Heidemann, J., Touch, J., Kruse, H., Ostermann, S., Scott, K. and J. Semke, "Ongoing TCP Research Related to Satellites", RFC 2760, February 2000. [RFC2884] Hadi Salim, J. and U. Ahmed, "Performance Evaluation of Explicit Congestion Notification (ECN) in IP Networks", RFC 2884, July 2000. [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914, September 2000. [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, September 2000. [RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G. and Z. Shelby, "Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations", RFC 3135, June 2001. [RFC3360] Floyd, S., "Inappropriate TCP Resets Considered Harmful", BCP 60, RFC 3360, August 2002. [RFC3449] Balakrishnan, H., Padmanabhan, V., Fairhurst, G. and M. Sooriyabandara, "TCP Performance Implications of Network Path Asymmetry", BCP 69, RFC 3449, December 2002. [RFC3481] Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A. and F. Khafizov, "TCP over Second (2.5G) and Third (3G) Generation Wireless Networks", BCP 71, RFC 3481, February 2003. [RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J. and W. Stevens, "Basic Socket Interface Extensions for IPv6", RFC 3493, February 2003. Duke, et al. Expires July 21, 2005 [Page 29] Internet-Draft TCP Roadmap January 2005 [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J. and L. Wood, "Advice for Internet Subnetwork Designers", BCP 89, RFC 3819, July 2004. 9.6 Informative References Outside the RFC Series [FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgement: Refining TCP Congestion Control", ACM SIGCOMM, August 1996. [Karn] Karn, P. and C. Partridge, "Round Trip Time Estimation", ACM SIGCOMM, August 1987. [Savage] Savage, S., Cardwell, N., Wetherall, D. and T. Anderson, "TCP Congestion Control with a Misbehaving Receiver", ACM Computer Communication Review 29 (5), October 1999. [VJ88] Jacobson, V., "Congestion Avoidance and Control", ACM SIGCOMM, August 1988. Authors' Addresses Martin Duke Boeing Phantom Works PO Box 3707, MC 3W-51 Seattle, WA 98124-2207 Phone: 253-657-8203 EMail: mduke26@comcast.net Robert Braden USC Information Sciences Institute Marina del Rey, CA 90292-6695 Phone: 310-448-9173 EMail: braden@isi.edu Wesley M. Eddy NASA GRC/Verizon FNS EMail: weddy@grc.nasa.gov Duke, et al. Expires July 21, 2005 [Page 30] Internet-Draft TCP Roadmap January 2005 Ethan Blanton Purdue University EMail: eblanton@cs.purdue.edu Duke, et al. Expires July 21, 2005 [Page 31] Internet-Draft TCP Roadmap January 2005 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. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Duke, et al. Expires July 21, 2005 [Page 32]