RFC 1123
Requirements for Internet Hosts - Application and Support
Pages: 98
Internet Standard: 3
→ Errata
STD 3 is also: 1122
Updates: 0822 0952
Updated by: 1349 2181 5321 5966 7766 9210
Internet Standard: 3
→ Errata
STD 3 is also: 1122
Updates: 0822 0952
Updated by: 1349 2181 5321 5966 7766 9210
Part 1 of 4 – Pages 1 to 15
Network Working Group Internet Engineering Task Force Request for Comments: 1123 R. Braden, Editor October 1989 Requirements for Internet Hosts -- Application and Support Status of This Memo This RFC is an official specification for the Internet community. It incorporates by reference, amends, corrects, and supplements the primary protocol standards documents relating to hosts. Distribution of this document is unlimited. Summary This RFC is one of a pair that defines and discusses the requirements for Internet host software. This RFC covers the application and support protocols; its companion RFC-1122 covers the communication protocol layers: link layer, IP layer, and transport layer. Table of Contents 1. INTRODUCTION ............................................... 5 1.1 The Internet Architecture .............................. 6 1.2 General Considerations ................................. 6 1.2.1 Continuing Internet Evolution ..................... 6 1.2.2 Robustness Principle .............................. 7 1.2.3 Error Logging ..................................... 8 1.2.4 Configuration ..................................... 8 1.3 Reading this Document .................................. 10 1.3.1 Organization ...................................... 10 1.3.2 Requirements ...................................... 10 1.3.3 Terminology ....................................... 11 1.4 Acknowledgments ........................................ 12 2. GENERAL ISSUES ............................................. 13 2.1 Host Names and Numbers ................................. 13 2.2 Using Domain Name Service .............................. 13 2.3 Applications on Multihomed hosts ....................... 14 2.4 Type-of-Service ........................................ 14 2.5 GENERAL APPLICATION REQUIREMENTS SUMMARY ............... 15
3. REMOTE LOGIN -- TELNET PROTOCOL ............................ 16 3.1 INTRODUCTION ........................................... 16 3.2 PROTOCOL WALK-THROUGH .................................. 16 3.2.1 Option Negotiation ................................ 16 3.2.2 Telnet Go-Ahead Function .......................... 16 3.2.3 Control Functions ................................. 17 3.2.4 Telnet "Synch" Signal ............................. 18 3.2.5 NVT Printer and Keyboard .......................... 19 3.2.6 Telnet Command Structure .......................... 20 3.2.7 Telnet Binary Option .............................. 20 3.2.8 Telnet Terminal-Type Option ....................... 20 3.3 SPECIFIC ISSUES ........................................ 21 3.3.1 Telnet End-of-Line Convention ..................... 21 3.3.2 Data Entry Terminals .............................. 23 3.3.3 Option Requirements ............................... 24 3.3.4 Option Initiation ................................. 24 3.3.5 Telnet Linemode Option ............................ 25 3.4 TELNET/USER INTERFACE .................................. 25 3.4.1 Character Set Transparency ........................ 25 3.4.2 Telnet Commands ................................... 26 3.4.3 TCP Connection Errors ............................. 26 3.4.4 Non-Default Telnet Contact Port ................... 26 3.4.5 Flushing Output ................................... 26 3.5. TELNET REQUIREMENTS SUMMARY ........................... 27 4. FILE TRANSFER .............................................. 29 4.1 FILE TRANSFER PROTOCOL -- FTP .......................... 29 4.1.1 INTRODUCTION ...................................... 29 4.1.2. PROTOCOL WALK-THROUGH ............................ 29 4.1.2.1 LOCAL Type ................................... 29 4.1.2.2 Telnet Format Control ........................ 30 4.1.2.3 Page Structure ............................... 30 4.1.2.4 Data Structure Transformations ............... 30 4.1.2.5 Data Connection Management ................... 31 4.1.2.6 PASV Command ................................. 31 4.1.2.7 LIST and NLST Commands ....................... 31 4.1.2.8 SITE Command ................................. 32 4.1.2.9 STOU Command ................................. 32 4.1.2.10 Telnet End-of-line Code ..................... 32 4.1.2.11 FTP Replies ................................. 33 4.1.2.12 Connections ................................. 34 4.1.2.13 Minimum Implementation; RFC-959 Section ..... 34 4.1.3 SPECIFIC ISSUES ................................... 35 4.1.3.1 Non-standard Command Verbs ................... 35 4.1.3.2 Idle Timeout ................................. 36 4.1.3.3 Concurrency of Data and Control .............. 36 4.1.3.4 FTP Restart Mechanism ........................ 36 4.1.4 FTP/USER INTERFACE ................................ 39
4.1.4.1 Pathname Specification ....................... 39
4.1.4.2 "QUOTE" Command .............................. 40
4.1.4.3 Displaying Replies to User ................... 40
4.1.4.4 Maintaining Synchronization .................. 40
4.1.5 FTP REQUIREMENTS SUMMARY ......................... 41
4.2 TRIVIAL FILE TRANSFER PROTOCOL -- TFTP ................. 44
4.2.1 INTRODUCTION ...................................... 44
4.2.2 PROTOCOL WALK-THROUGH ............................. 44
4.2.2.1 Transfer Modes ............................... 44
4.2.2.2 UDP Header ................................... 44
4.2.3 SPECIFIC ISSUES ................................... 44
4.2.3.1 Sorcerer's Apprentice Syndrome ............... 44
4.2.3.2 Timeout Algorithms ........................... 46
4.2.3.3 Extensions ................................... 46
4.2.3.4 Access Control ............................... 46
4.2.3.5 Broadcast Request ............................ 46
4.2.4 TFTP REQUIREMENTS SUMMARY ......................... 47
5. ELECTRONIC MAIL -- SMTP and RFC-822 ........................ 48
5.1 INTRODUCTION ........................................... 48
5.2 PROTOCOL WALK-THROUGH .................................. 48
5.2.1 The SMTP Model .................................... 48
5.2.2 Canonicalization .................................. 49
5.2.3 VRFY and EXPN Commands ............................ 50
5.2.4 SEND, SOML, and SAML Commands ..................... 50
5.2.5 HELO Command ...................................... 50
5.2.6 Mail Relay ........................................ 51
5.2.7 RCPT Command ...................................... 52
5.2.8 DATA Command ...................................... 53
5.2.9 Command Syntax .................................... 54
5.2.10 SMTP Replies ..................................... 54
5.2.11 Transparency ..................................... 55
5.2.12 WKS Use in MX Processing ......................... 55
5.2.13 RFC-822 Message Specification .................... 55
5.2.14 RFC-822 Date and Time Specification .............. 55
5.2.15 RFC-822 Syntax Change ............................ 56
5.2.16 RFC-822 Local-part .............................. 56
5.2.17 Domain Literals .................................. 57
5.2.18 Common Address Formatting Errors ................. 58
5.2.19 Explicit Source Routes ........................... 58
5.3 SPECIFIC ISSUES ........................................ 59
5.3.1 SMTP Queueing Strategies .......................... 59
5.3.1.1 Sending Strategy .............................. 59
5.3.1.2 Receiving strategy ........................... 61
5.3.2 Timeouts in SMTP .................................. 61
5.3.3 Reliable Mail Receipt ............................. 63
5.3.4 Reliable Mail Transmission ........................ 63
5.3.5 Domain Name Support ............................... 65
5.3.6 Mailing Lists and Aliases ......................... 65
5.3.7 Mail Gatewaying ................................... 66
5.3.8 Maximum Message Size .............................. 68
5.4 SMTP REQUIREMENTS SUMMARY .............................. 69
6. SUPPORT SERVICES ............................................ 72
6.1 DOMAIN NAME TRANSLATION ................................. 72
6.1.1 INTRODUCTION ....................................... 72
6.1.2 PROTOCOL WALK-THROUGH ............................. 72
6.1.2.1 Resource Records with Zero TTL ............... 73
6.1.2.2 QCLASS Values ................................ 73
6.1.2.3 Unused Fields ................................ 73
6.1.2.4 Compression .................................. 73
6.1.2.5 Misusing Configuration Info .................. 73
6.1.3 SPECIFIC ISSUES ................................... 74
6.1.3.1 Resolver Implementation ...................... 74
6.1.3.2 Transport Protocols .......................... 75
6.1.3.3 Efficient Resource Usage ..................... 77
6.1.3.4 Multihomed Hosts ............................. 78
6.1.3.5 Extensibility ................................ 79
6.1.3.6 Status of RR Types ........................... 79
6.1.3.7 Robustness ................................... 80
6.1.3.8 Local Host Table ............................. 80
6.1.4 DNS USER INTERFACE ................................ 81
6.1.4.1 DNS Administration ........................... 81
6.1.4.2 DNS User Interface ........................... 81
6.1.4.3 Interface Abbreviation Facilities ............. 82
6.1.5 DOMAIN NAME SYSTEM REQUIREMENTS SUMMARY ........... 84
6.2 HOST INITIALIZATION .................................... 87
6.2.1 INTRODUCTION ...................................... 87
6.2.2 REQUIREMENTS ...................................... 87
6.2.2.1 Dynamic Configuration ........................ 87
6.2.2.2 Loading Phase ................................ 89
6.3 REMOTE MANAGEMENT ...................................... 90
6.3.1 INTRODUCTION ...................................... 90
6.3.2 PROTOCOL WALK-THROUGH ............................. 90
6.3.3 MANAGEMENT REQUIREMENTS SUMMARY ................... 92
7. REFERENCES ................................................. 93
1. INTRODUCTION This document is one of a pair that defines and discusses the requirements for host system implementations of the Internet protocol suite. This RFC covers the applications layer and support protocols. Its companion RFC, "Requirements for Internet Hosts -- Communications Layers" [INTRO:1] covers the lower layer protocols: transport layer, IP layer, and link layer. These documents are intended to provide guidance for vendors, implementors, and users of Internet communication software. They represent the consensus of a large body of technical experience and wisdom, contributed by members of the Internet research and vendor communities. This RFC enumerates standard protocols that a host connected to the Internet must use, and it incorporates by reference the RFCs and other documents describing the current specifications for these protocols. It corrects errors in the referenced documents and adds additional discussion and guidance for an implementor. For each protocol, this document also contains an explicit set of requirements, recommendations, and options. The reader must understand that the list of requirements in this document is incomplete by itself; the complete set of requirements for an Internet host is primarily defined in the standard protocol specification documents, with the corrections, amendments, and supplements contained in this RFC. A good-faith implementation of the protocols that was produced after careful reading of the RFC's and with some interaction with the Internet technical community, and that followed good communications software engineering practices, should differ from the requirements of this document in only minor ways. Thus, in many cases, the "requirements" in this RFC are already stated or implied in the standard protocol documents, so that their inclusion here is, in a sense, redundant. However, they were included because some past implementation has made the wrong choice, causing problems of interoperability, performance, and/or robustness. This document includes discussion and explanation of many of the requirements and recommendations. A simple list of requirements would be dangerous, because: o Some required features are more important than others, and some features are optional. o There may be valid reasons why particular vendor products that
are designed for restricted contexts might choose to use
different specifications.
However, the specifications of this document must be followed to meet
the general goal of arbitrary host interoperation across the
diversity and complexity of the Internet system. Although most
current implementations fail to meet these requirements in various
ways, some minor and some major, this specification is the ideal
towards which we need to move.
These requirements are based on the current level of Internet
architecture. This document will be updated as required to provide
additional clarifications or to include additional information in
those areas in which specifications are still evolving.
This introductory section begins with general advice to host software
vendors, and then gives some guidance on reading the rest of the
document. Section 2 contains general requirements that may be
applicable to all application and support protocols. Sections 3, 4,
and 5 contain the requirements on protocols for the three major
applications: Telnet, file transfer, and electronic mail,
respectively. Section 6 covers the support applications: the domain
name system, system initialization, and management. Finally, all
references will be found in Section 7.
1.1 The Internet Architecture
For a brief introduction to the Internet architecture from a host
viewpoint, see Section 1.1 of [INTRO:1]. That section also
contains recommended references for general background on the
Internet architecture.
1.2 General Considerations
There are two important lessons that vendors of Internet host
software have learned and which a new vendor should consider
seriously.
1.2.1 Continuing Internet Evolution
The enormous growth of the Internet has revealed problems of
management and scaling in a large datagram-based packet
communication system. These problems are being addressed, and
as a result there will be continuing evolution of the
specifications described in this document. These changes will
be carefully planned and controlled, since there is extensive
participation in this planning by the vendors and by the
organizations responsible for operations of the networks.
Development, evolution, and revision are characteristic of
computer network protocols today, and this situation will
persist for some years. A vendor who develops computer
communication software for the Internet protocol suite (or any
other protocol suite!) and then fails to maintain and update
that software for changing specifications is going to leave a
trail of unhappy customers. The Internet is a large
communication network, and the users are in constant contact
through it. Experience has shown that knowledge of
deficiencies in vendor software propagates quickly through the
Internet technical community.
1.2.2 Robustness Principle
At every layer of the protocols, there is a general rule whose
application can lead to enormous benefits in robustness and
interoperability:
"Be liberal in what you accept, and
conservative in what you send"
Software should be written to deal with every conceivable
error, no matter how unlikely; sooner or later a packet will
come in with that particular combination of errors and
attributes, and unless the software is prepared, chaos can
ensue. In general, it is best to assume that the network is
filled with malevolent entities that will send in packets
designed to have the worst possible effect. This assumption
will lead to suitable protective design, although the most
serious problems in the Internet have been caused by
unenvisaged mechanisms triggered by low-probability events;
mere human malice would never have taken so devious a course!
Adaptability to change must be designed into all levels of
Internet host software. As a simple example, consider a
protocol specification that contains an enumeration of values
for a particular header field -- e.g., a type field, a port
number, or an error code; this enumeration must be assumed to
be incomplete. Thus, if a protocol specification defines four
possible error codes, the software must not break when a fifth
code shows up. An undefined code might be logged (see below),
but it must not cause a failure.
The second part of the principle is almost as important:
software on other hosts may contain deficiencies that make it
unwise to exploit legal but obscure protocol features. It is
unwise to stray far from the obvious and simple, lest untoward
effects result elsewhere. A corollary of this is "watch out
for misbehaving hosts"; host software should be prepared, not
just to survive other misbehaving hosts, but also to cooperate
to limit the amount of disruption such hosts can cause to the
shared communication facility.
1.2.3 Error Logging
The Internet includes a great variety of host and gateway
systems, each implementing many protocols and protocol layers,
and some of these contain bugs and mis-features in their
Internet protocol software. As a result of complexity,
diversity, and distribution of function, the diagnosis of user
problems is often very difficult.
Problem diagnosis will be aided if host implementations include
a carefully designed facility for logging erroneous or
"strange" protocol events. It is important to include as much
diagnostic information as possible when an error is logged. In
particular, it is often useful to record the header(s) of a
packet that caused an error. However, care must be taken to
ensure that error logging does not consume prohibitive amounts
of resources or otherwise interfere with the operation of the
host.
There is a tendency for abnormal but harmless protocol events
to overflow error logging files; this can be avoided by using a
"circular" log, or by enabling logging only while diagnosing a
known failure. It may be useful to filter and count duplicate
successive messages. One strategy that seems to work well is:
(1) always count abnormalities and make such counts accessible
through the management protocol (see Section 6.3); and (2)
allow the logging of a great variety of events to be
selectively enabled. For example, it might useful to be able
to "log everything" or to "log everything for host X".
Note that different managements may have differing policies
about the amount of error logging that they want normally
enabled in a host. Some will say, "if it doesn't hurt me, I
don't want to know about it", while others will want to take a
more watchful and aggressive attitude about detecting and
removing protocol abnormalities.
1.2.4 Configuration
It would be ideal if a host implementation of the Internet
protocol suite could be entirely self-configuring. This would
allow the whole suite to be implemented in ROM or cast into
silicon, it would simplify diskless workstations, and it would
be an immense boon to harried LAN administrators as well as
system vendors. We have not reached this ideal; in fact, we
are not even close.
At many points in this document, you will find a requirement
that a parameter be a configurable option. There are several
different reasons behind such requirements. In a few cases,
there is current uncertainty or disagreement about the best
value, and it may be necessary to update the recommended value
in the future. In other cases, the value really depends on
external factors -- e.g., the size of the host and the
distribution of its communication load, or the speeds and
topology of nearby networks -- and self-tuning algorithms are
unavailable and may be insufficient. In some cases,
configurability is needed because of administrative
requirements.
Finally, some configuration options are required to communicate
with obsolete or incorrect implementations of the protocols,
distributed without sources, that unfortunately persist in many
parts of the Internet. To make correct systems coexist with
these faulty systems, administrators often have to "mis-
configure" the correct systems. This problem will correct
itself gradually as the faulty systems are retired, but it
cannot be ignored by vendors.
When we say that a parameter must be configurable, we do not
intend to require that its value be explicitly read from a
configuration file at every boot time. We recommend that
implementors set up a default for each parameter, so a
configuration file is only necessary to override those defaults
that are inappropriate in a particular installation. Thus, the
configurability requirement is an assurance that it will be
POSSIBLE to override the default when necessary, even in a
binary-only or ROM-based product.
This document requires a particular value for such defaults in
some cases. The choice of default is a sensitive issue when
the configuration item controls the accommodation to existing
faulty systems. If the Internet is to converge successfully to
complete interoperability, the default values built into
implementations must implement the official protocol, not
"mis-configurations" to accommodate faulty implementations.
Although marketing considerations have led some vendors to
choose mis-configuration defaults, we urge vendors to choose
defaults that will conform to the standard.
Finally, we note that a vendor needs to provide adequate
documentation on all configuration parameters, their limits and
effects.
1.3 Reading this Document
1.3.1 Organization
In general, each major section is organized into the following
subsections:
(1) Introduction
(2) Protocol Walk-Through -- considers the protocol
specification documents section-by-section, correcting
errors, stating requirements that may be ambiguous or
ill-defined, and providing further clarification or
explanation.
(3) Specific Issues -- discusses protocol design and
implementation issues that were not included in the walk-
through.
(4) Interfaces -- discusses the service interface to the next
higher layer.
(5) Summary -- contains a summary of the requirements of the
section.
Under many of the individual topics in this document, there is
parenthetical material labeled "DISCUSSION" or
"IMPLEMENTATION". This material is intended to give
clarification and explanation of the preceding requirements
text. It also includes some suggestions on possible future
directions or developments. The implementation material
contains suggested approaches that an implementor may want to
consider.
The summary sections are intended to be guides and indexes to
the text, but are necessarily cryptic and incomplete. The
summaries should never be used or referenced separately from
the complete RFC.
1.3.2 Requirements
In this document, the words that are used to define the
significance of each particular requirement are capitalized.
These words are:
* "MUST"
This word or the adjective "REQUIRED" means that the item
is an absolute requirement of the specification.
* "SHOULD"
This word or the adjective "RECOMMENDED" means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications should be
understood and the case carefully weighed before choosing
a different course.
* "MAY"
This word or the adjective "OPTIONAL" means that this item
is truly optional. One vendor may choose to include the
item because a particular marketplace requires it or
because it enhances the product, for example; another
vendor may omit the same item.
An implementation is not compliant if it fails to satisfy one
or more of the MUST requirements for the protocols it
implements. An implementation that satisfies all the MUST and
all the SHOULD requirements for its protocols is said to be
"unconditionally compliant"; one that satisfies all the MUST
requirements but not all the SHOULD requirements for its
protocols is said to be "conditionally compliant".
1.3.3 Terminology
This document uses the following technical terms:
Segment
A segment is the unit of end-to-end transmission in the
TCP protocol. A segment consists of a TCP header followed
by application data. A segment is transmitted by
encapsulation in an IP datagram.
Message
This term is used by some application layer protocols
(particularly SMTP) for an application data unit.
Datagram
A [UDP] datagram is the unit of end-to-end transmission in
the UDP protocol.
Multihomed
A host is said to be multihomed if it has multiple IP
addresses to connected networks.
1.4 Acknowledgments
This document incorporates contributions and comments from a large
group of Internet protocol experts, including representatives of
university and research labs, vendors, and government agencies.
It was assembled primarily by the Host Requirements Working Group
of the Internet Engineering Task Force (IETF).
The Editor would especially like to acknowledge the tireless
dedication of the following people, who attended many long
meetings and generated 3 million bytes of electronic mail over the
past 18 months in pursuit of this document: Philip Almquist, Dave
Borman (Cray Research), Noel Chiappa, Dave Crocker (DEC), Steve
Deering (Stanford), Mike Karels (Berkeley), Phil Karn (Bellcore),
John Lekashman (NASA), Charles Lynn (BBN), Keith McCloghrie (TWG),
Paul Mockapetris (ISI), Thomas Narten (Purdue), Craig Partridge
(BBN), Drew Perkins (CMU), and James Van Bokkelen (FTP Software).
In addition, the following people made major contributions to the
effort: Bill Barns (Mitre), Steve Bellovin (AT&T), Mike Brescia
(BBN), Ed Cain (DCA), Annette DeSchon (ISI), Martin Gross (DCA),
Phill Gross (NRI), Charles Hedrick (Rutgers), Van Jacobson (LBL),
John Klensin (MIT), Mark Lottor (SRI), Milo Medin (NASA), Bill
Melohn (Sun Microsystems), Greg Minshall (Kinetics), Jeff Mogul
(DEC), John Mullen (CMC), Jon Postel (ISI), John Romkey (Epilogue
Technology), and Mike StJohns (DCA). The following also made
significant contributions to particular areas: Eric Allman
(Berkeley), Rob Austein (MIT), Art Berggreen (ACC), Keith Bostic
(Berkeley), Vint Cerf (NRI), Wayne Hathaway (NASA), Matt Korn
(IBM), Erik Naggum (Naggum Software, Norway), Robert Ullmann
(Prime Computer), David Waitzman (BBN), Frank Wancho (USA), Arun
Welch (Ohio State), Bill Westfield (Cisco), and Rayan Zachariassen
(Toronto).
We are grateful to all, including any contributors who may have
been inadvertently omitted from this list.
2. GENERAL ISSUES This section contains general requirements that may be applicable to all application-layer protocols. 2.1 Host Names and Numbers The syntax of a legal Internet host name was specified in RFC-952 [DNS:4]. One aspect of host name syntax is hereby changed: the restriction on the first character is relaxed to allow either a letter or a digit. Host software MUST support this more liberal syntax. Host software MUST handle host names of up to 63 characters and SHOULD handle host names of up to 255 characters. Whenever a user inputs the identity of an Internet host, it SHOULD be possible to enter either (1) a host domain name or (2) an IP address in dotted-decimal ("#.#.#.#") form. The host SHOULD check the string syntactically for a dotted-decimal number before looking it up in the Domain Name System. DISCUSSION: This last requirement is not intended to specify the complete syntactic form for entering a dotted-decimal host number; that is considered to be a user-interface issue. For example, a dotted-decimal number must be enclosed within "[ ]" brackets for SMTP mail (see Section 5.2.17). This notation could be made universal within a host system, simplifying the syntactic checking for a dotted-decimal number. If a dotted-decimal number can be entered without such identifying delimiters, then a full syntactic check must be made, because a segment of a host domain name is now allowed to begin with a digit and could legally be entirely numeric (see Section 6.1.2.4). However, a valid host name can never have the dotted-decimal form #.#.#.#, since at least the highest-level component label will be alphabetic. 2.2 Using Domain Name Service Host domain names MUST be translated to IP addresses as described in Section 6.1. Applications using domain name services MUST be able to cope with soft error conditions. Applications MUST wait a reasonable interval between successive retries due to a soft error, and MUST
allow for the possibility that network problems may deny service
for hours or even days.
An application SHOULD NOT rely on the ability to locate a WKS
record containing an accurate listing of all services at a
particular host address, since the WKS RR type is not often used
by Internet sites. To confirm that a service is present, simply
attempt to use it.
2.3 Applications on Multihomed hosts
When the remote host is multihomed, the name-to-address
translation will return a list of alternative IP addresses. As
specified in Section 6.1.3.4, this list should be in order of
decreasing preference. Application protocol implementations
SHOULD be prepared to try multiple addresses from the list until
success is obtained. More specific requirements for SMTP are
given in Section 5.3.4.
When the local host is multihomed, a UDP-based request/response
application SHOULD send the response with an IP source address
that is the same as the specific destination address of the UDP
request datagram. The "specific destination address" is defined
in the "IP Addressing" section of the companion RFC [INTRO:1].
Similarly, a server application that opens multiple TCP
connections to the same client SHOULD use the same local IP
address for all.
2.4 Type-of-Service
Applications MUST select appropriate TOS values when they invoke
transport layer services, and these values MUST be configurable.
Note that a TOS value contains 5 bits, of which only the most-
significant 3 bits are currently defined; the other two bits MUST
be zero.
DISCUSSION:
As gateway algorithms are developed to implement Type-of-
Service, the recommended values for various application
protocols may change. In addition, it is likely that
particular combinations of users and Internet paths will want
non-standard TOS values. For these reasons, the TOS values
must be configurable.
See the latest version of the "Assigned Numbers" RFC
[INTRO:5] for the recommended TOS values for the major
application protocols.
2.5 GENERAL APPLICATION REQUIREMENTS SUMMARY | | | | |S| | | | | | |H| |F | | | | |O|M|o | | |S| |U|U|o | | |H| |L|S|t | |M|O| |D|T|n | |U|U|M| | |o | |S|L|A|N|N|t | |T|D|Y|O|O|t FEATURE |SECTION | | | |T|T|e -----------------------------------------------|----------|-|-|-|-|-|-- | | | | | | | User interfaces: | | | | | | | Allow host name to begin with digit |2.1 |x| | | | | Host names of up to 635 characters |2.1 |x| | | | | Host names of up to 255 characters |2.1 | |x| | | | Support dotted-decimal host numbers |2.1 | |x| | | | Check syntactically for dotted-dec first |2.1 | |x| | | | | | | | | | | Map domain names per Section 6.1 |2.2 |x| | | | | Cope with soft DNS errors |2.2 |x| | | | | Reasonable interval between retries |2.2 |x| | | | | Allow for long outages |2.2 |x| | | | | Expect WKS records to be available |2.2 | | | |x| | | | | | | | | Try multiple addr's for remote multihomed host |2.3 | |x| | | | UDP reply src addr is specific dest of request |2.3 | |x| | | | Use same IP addr for related TCP connections |2.3 | |x| | | | Specify appropriate TOS values |2.4 |x| | | | | TOS values configurable |2.4 |x| | | | | Unused TOS bits zero |2.4 |x| | | | | | | | | | | | | | | | | | |
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