acknowledgments received. This has been implemented in Linux.
The ABC mechanism behaves differently from the standard method
when there is not a one-to-one relationship between data segments
and acknowledgments. ABC still operates within the accepted
guidelines, but is more robust to delayed ACKs and ACK-division
[SCWA99][RFC3449].
RFC 3522 E: "The Eifel Detection Algorithm for TCP" (April 2003)
The Eifel detection algorithm [RFC3522] allows a TCP sender to
detect a posteriori whether it has entered loss recovery
unnecessarily.
RFC 3540 E: "Robust Explicit Congestion Notification (ECN) signaling
with Nonces" (June 2003)
This document [RFC3540] suggests a modified ECN to address
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 use very large windows efficiently.
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.
RFC 4015 S: "The Eifel Response Algorithm for TCP" (February 2005)
This document [RFC4015] 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 in RFC 4138.
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."
RFC 4015 is itself a Proposed Standard. The consensus of the TCPM
working group was to place it in this section of the roadmap
document due to three factors.
1. RFC 4015 operates on the output of a detection algorithm, for
which there is currently no available mechanism on the
standards track.
2. The working group was not aware of any wide deployment and use
of RFC 4015.
3. The consensus of the working group, after a discussion of the
known Intellectual Property Rights claims on the techniques
described in RFC 4015, identified this section of the roadmap
as an appropriate location.
RFC 4138 E: "Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
Spurious Retransmission Timeouts with TCP and the Stream Control
Transmission Protocol" (August 2005)
The F-RTO detection algorithm [RFC4138] provides another option
for inferring spurious retransmission timeouts. Unlike some
similar detection methods, F-RTO does not rely on the use of any
TCP options.
5. Historic Extensions
The RFCs listed here define extensions that have thus far failed to
arouse substantial interest from implementers, or that were found to
be defective for general use.
RFC 1106 "TCP Big Window and NAK Options" (June 1989): found
defective
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.
RFC 1110 explains some problems with the approaches described in
RFC 1106. 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 for satellite and spacecraft use,
developed by the Consultative Committee for Space Data Systems
(CCSDS).
RFC 1110 "A Problem with the TCP Big Window Option" (August 1989):
deprecates RFC 1106
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): lack of
interest
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) - lack of
interest
This 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, although the type
of extensions RFC 1263 specifically targeted as harmful did become
popular.
RFC 1379 I "Extending TCP for Transactions -- Concepts" (November
1992): found defective
See RFC 1644.
RFC 1644 E "T/TCP -- TCP Extensions for Transactions Functional
Specification" (July 1994): found defective
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. One idea pioneered by T/TCP lives on in RFC 2140, in the
sharing of state across connections.
RFC 1693 E "An Extension to TCP: Partial Order Service" (November
1994): lack of interest
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 use
more specialized transport protocols.
6. Support Documents
This section contains several classes of documents that do not
necessarily define current protocol behaviors, but that are
nevertheless of interest to TCP implementers. 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 that contain
high-level tutorials on the protocol are listed Section 6.5, and
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 in this TCP roadmap).
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 acknowledgments.
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 is discussed.
RFC 817: "Modularity and Efficiency in Protocol Implementation" (July
1982)
This document [RFC0817] contains implementation suggestions that
are general and not TCP specific. However, they have been used to
develop TCP implementations and to 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] points out several specification bugs in
the US Military’s MIL-STD-1778 document, which was intended as a
successor to RFC 793. This serves to remind us of the difficulty
in specification writing (even when we work 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,
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. The implications of such
environments have been further discussed in RFC 3150 and RFC 3155,
and these documents should be preferred where there is overlap
between them and RFC 2757.
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
acknowledgments, slow start thresholds, NewReno and SACK-based
loss recovery, FACK [MM96], 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. Although 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
are described as well as the mechanisms used to improve
performance." [RFC3135]
RFC 3150 B: "End-to-end Performance Implications of Slow Links" (July
2001)
From abstract: "This document makes performance-related
recommendations for users of network paths that traverse "very low
bit-rate" links....This recommendation may be useful in any
network where hosts can saturate available bandwidth, but the
design space for this recommendation explicitly includes
connections that traverse 56 Kb/second modem links or 4.8 Kb/
second wireless access links - both of which are widely deployed."
[RFC3150]
RFC 3155 B: "End-to-end Performance Implications of Links with
Errors" (August 2001)
From abstract: "This document discusses the specific TCP
mechanisms that are problematic in environments with high
uncorrected error rates, and discusses what can be done to
mitigate the problems without introducing intermediate devices
into the connection." [RFC3155]
RFC 3366 "Advice to link designers on link Automatic Repeat reQuest
(ARQ)" (August 2002)
From abstract: "This document provides advice to the designers of
digital communication equipment and link-layer protocols employing
link-layer Automatic Repeat reQuest (ARQ) techniques. This
document presumes that the designers wish to support Internet
protocols, but may be unfamiliar with the architecture of the
Internet and with the implications of their design choices for the
performance and efficiency of Internet traffic carried over their
links." [RFC3366]
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 affected 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 1071: "Computing the Internet Checksum" (September 1988)
This document [RFC1071] lists a number of implementation
techniques for efficiently computing the Internet checksum (used
by TCP).
RFC 1624 I: "Computation of the Internet Checksum via Incremental
Update" (May 1994)
Incrementally updating the Internet checksum is useful to routers
in updating IP checksums. Some middleboxes that alter TCP headers
may also be able to update the TCP checksum incrementally. This
document [RFC1624] expands upon the explanation of the incremental
update procedure in RFC 1071.
RFC 1936 I: "Implementing the Internet Checksum in Hardware" (April
1996)
This document [RFC1936] describes the motivation for implementing
the Internet checksum in hardware, rather than in software, and
provides an implementation example.
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
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 thus is potentially harmful 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 Simple Network Management
Protocol (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 that needed to
evolve were split off. Eventually, the remaining parts of MIB-II
were also split off, the TCP-specific part being documented in RFC
2012.
RFC 2012 was obsoleted by RFC 4022, which is the primary TCP MIB
document today. MIB-I, defined in RFC 1156, has been obsoleted by
the MIB-II specification in RFC 1213. For current TCP implementers,
RFC 4022 should be supported.
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] defined the TCP MIB, in an update to RFC
1213. It is now obsoleted by RFC 4022.
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. RFC 2012 is now obsoleted by RFC 4022.
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.
RFC 4022 S: "Management Information Base for the Transmission Control
Protocol (TCP)" (March 2005)
This document [RFC4022] obsoletes RFC 2012 and RFC 2452 and
specifies the current standard for the TCP MIB that should be
deployed.
6.5. Tools and Tutorials
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