RFC 1122
Requirements for Internet Hosts - Communication Layers
Pages: 116
Internet Standard: 3
→ Errata
STD 3 is also: 1123
Updates: 0793
Updated by: 1349 4379 5884 6093 6298 6633 6864 8029 9293
Internet Standard: 3
→ Errata
STD 3 is also: 1123
Updates: 0793
Updated by: 1349 4379 5884 6093 6298 6633 6864 8029 9293
Part 1 of 5 – Pages 1 to 20
Network Working Group Internet Engineering Task Force Request for Comments: 1122 R. Braden, Editor October 1989 Requirements for Internet Hosts -- Communication Layers 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 is one RFC of a pair that defines and discusses the requirements for Internet host software. This RFC covers the communications protocol layers: link layer, IP layer, and transport layer; its companion RFC-1123 covers the application and support protocols. Table of Contents 1. INTRODUCTION ............................................... 5 1.1 The Internet Architecture .............................. 6 1.1.1 Internet Hosts .................................... 6 1.1.2 Architectural Assumptions ......................... 7 1.1.3 Internet Protocol Suite ........................... 8 1.1.4 Embedded Gateway Code ............................. 10 1.2 General Considerations ................................. 12 1.2.1 Continuing Internet Evolution ..................... 12 1.2.2 Robustness Principle .............................. 12 1.2.3 Error Logging ..................................... 13 1.2.4 Configuration ..................................... 14 1.3 Reading this Document .................................. 15 1.3.1 Organization ...................................... 15 1.3.2 Requirements ...................................... 16 1.3.3 Terminology ....................................... 17 1.4 Acknowledgments ........................................ 20 2. LINK LAYER .................................................. 21 2.1 INTRODUCTION ........................................... 21
2.2 PROTOCOL WALK-THROUGH .................................. 21
2.3 SPECIFIC ISSUES ........................................ 21
2.3.1 Trailer Protocol Negotiation ...................... 21
2.3.2 Address Resolution Protocol -- ARP ................ 22
2.3.2.1 ARP Cache Validation ......................... 22
2.3.2.2 ARP Packet Queue ............................. 24
2.3.3 Ethernet and IEEE 802 Encapsulation ............... 24
2.4 LINK/INTERNET LAYER INTERFACE .......................... 25
2.5 LINK LAYER REQUIREMENTS SUMMARY ........................ 26
3. INTERNET LAYER PROTOCOLS .................................... 27
3.1 INTRODUCTION ............................................ 27
3.2 PROTOCOL WALK-THROUGH .................................. 29
3.2.1 Internet Protocol -- IP ............................ 29
3.2.1.1 Version Number ............................... 29
3.2.1.2 Checksum ..................................... 29
3.2.1.3 Addressing ................................... 29
3.2.1.4 Fragmentation and Reassembly ................. 32
3.2.1.5 Identification ............................... 32
3.2.1.6 Type-of-Service .............................. 33
3.2.1.7 Time-to-Live ................................. 34
3.2.1.8 Options ...................................... 35
3.2.2 Internet Control Message Protocol -- ICMP .......... 38
3.2.2.1 Destination Unreachable ...................... 39
3.2.2.2 Redirect ..................................... 40
3.2.2.3 Source Quench ................................ 41
3.2.2.4 Time Exceeded ................................ 41
3.2.2.5 Parameter Problem ............................ 42
3.2.2.6 Echo Request/Reply ........................... 42
3.2.2.7 Information Request/Reply .................... 43
3.2.2.8 Timestamp and Timestamp Reply ................ 43
3.2.2.9 Address Mask Request/Reply ................... 45
3.2.3 Internet Group Management Protocol IGMP ........... 47
3.3 SPECIFIC ISSUES ........................................ 47
3.3.1 Routing Outbound Datagrams ........................ 47
3.3.1.1 Local/Remote Decision ........................ 47
3.3.1.2 Gateway Selection ............................ 48
3.3.1.3 Route Cache .................................. 49
3.3.1.4 Dead Gateway Detection ....................... 51
3.3.1.5 New Gateway Selection ........................ 55
3.3.1.6 Initialization ............................... 56
3.3.2 Reassembly ........................................ 56
3.3.3 Fragmentation ..................................... 58
3.3.4 Local Multihoming ................................. 60
3.3.4.1 Introduction ................................. 60
3.3.4.2 Multihoming Requirements ..................... 61
3.3.4.3 Choosing a Source Address .................... 64
3.3.5 Source Route Forwarding ........................... 65
3.3.6 Broadcasts ........................................ 66
3.3.7 IP Multicasting ................................... 67
3.3.8 Error Reporting ................................... 69
3.4 INTERNET/TRANSPORT LAYER INTERFACE ..................... 69
3.5 INTERNET LAYER REQUIREMENTS SUMMARY .................... 72
4. TRANSPORT PROTOCOLS ......................................... 77
4.1 USER DATAGRAM PROTOCOL -- UDP .......................... 77
4.1.1 INTRODUCTION ...................................... 77
4.1.2 PROTOCOL WALK-THROUGH ............................. 77
4.1.3 SPECIFIC ISSUES ................................... 77
4.1.3.1 Ports ........................................ 77
4.1.3.2 IP Options ................................... 77
4.1.3.3 ICMP Messages ................................ 78
4.1.3.4 UDP Checksums ................................ 78
4.1.3.5 UDP Multihoming .............................. 79
4.1.3.6 Invalid Addresses ............................ 79
4.1.4 UDP/APPLICATION LAYER INTERFACE ................... 79
4.1.5 UDP REQUIREMENTS SUMMARY .......................... 80
4.2 TRANSMISSION CONTROL PROTOCOL -- TCP ................... 82
4.2.1 INTRODUCTION ...................................... 82
4.2.2 PROTOCOL WALK-THROUGH ............................. 82
4.2.2.1 Well-Known Ports ............................. 82
4.2.2.2 Use of Push .................................. 82
4.2.2.3 Window Size .................................. 83
4.2.2.4 Urgent Pointer ............................... 84
4.2.2.5 TCP Options .................................. 85
4.2.2.6 Maximum Segment Size Option .................. 85
4.2.2.7 TCP Checksum ................................. 86
4.2.2.8 TCP Connection State Diagram ................. 86
4.2.2.9 Initial Sequence Number Selection ............ 87
4.2.2.10 Simultaneous Open Attempts .................. 87
4.2.2.11 Recovery from Old Duplicate SYN ............. 87
4.2.2.12 RST Segment ................................. 87
4.2.2.13 Closing a Connection ........................ 87
4.2.2.14 Data Communication .......................... 89
4.2.2.15 Retransmission Timeout ...................... 90
4.2.2.16 Managing the Window ......................... 91
4.2.2.17 Probing Zero Windows ........................ 92
4.2.2.18 Passive OPEN Calls .......................... 92
4.2.2.19 Time to Live ................................ 93
4.2.2.20 Event Processing ............................ 93
4.2.2.21 Acknowledging Queued Segments ............... 94
4.2.3 SPECIFIC ISSUES ................................... 95
4.2.3.1 Retransmission Timeout Calculation ........... 95
4.2.3.2 When to Send an ACK Segment .................. 96
4.2.3.3 When to Send a Window Update ................. 97
4.2.3.4 When to Send Data ............................ 98
4.2.3.5 TCP Connection Failures ...................... 100
4.2.3.6 TCP Keep-Alives .............................. 101
4.2.3.7 TCP Multihoming .............................. 103
4.2.3.8 IP Options ................................... 103
4.2.3.9 ICMP Messages ................................ 103
4.2.3.10 Remote Address Validation ................... 104
4.2.3.11 TCP Traffic Patterns ........................ 104
4.2.3.12 Efficiency .................................. 105
4.2.4 TCP/APPLICATION LAYER INTERFACE ................... 106
4.2.4.1 Asynchronous Reports ......................... 106
4.2.4.2 Type-of-Service .............................. 107
4.2.4.3 Flush Call ................................... 107
4.2.4.4 Multihoming .................................. 108
4.2.5 TCP REQUIREMENT SUMMARY ........................... 108
5. REFERENCES ................................................. 112
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 communication protocol layers: link layer, IP layer, and transport layer. Its companion RFC, "Requirements for Internet Hosts -- Application and Support" [INTRO:1], covers the application layer protocols. This document should also be read in conjunction with "Requirements for Internet Gateways" [INTRO:2]. 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 the 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 a brief overview of the
Internet architecture as it relates to hosts, and then gives some
general advice to host software vendors. Finally, there is some
guidance on reading the rest of the document and some terminology.
1.1 The Internet Architecture
General background and discussion on the Internet architecture and
supporting protocol suite can be found in the DDN Protocol
Handbook [INTRO:3]; for background see for example [INTRO:9],
[INTRO:10], and [INTRO:11]. Reference [INTRO:5] describes the
procedure for obtaining Internet protocol documents, while
[INTRO:6] contains a list of the numbers assigned within Internet
protocols.
1.1.1 Internet Hosts
A host computer, or simply "host," is the ultimate consumer of
communication services. A host generally executes application
programs on behalf of user(s), employing network and/or
Internet communication services in support of this function.
An Internet host corresponds to the concept of an "End-System"
used in the OSI protocol suite [INTRO:13].
An Internet communication system consists of interconnected
packet networks supporting communication among host computers
using the Internet protocols. The networks are interconnected
using packet-switching computers called "gateways" or "IP
routers" by the Internet community, and "Intermediate Systems"
by the OSI world [INTRO:13]. The RFC "Requirements for
Internet Gateways" [INTRO:2] contains the official
specifications for Internet gateways. That RFC together with
the present document and its companion [INTRO:1] define the
rules for the current realization of the Internet architecture.
Internet hosts span a wide range of size, speed, and function.
They range in size from small microprocessors through
workstations to mainframes and supercomputers. In function,
they range from single-purpose hosts (such as terminal servers)
to full-service hosts that support a variety of online network
services, typically including remote login, file transfer, and
electronic mail.
A host is generally said to be multihomed if it has more than
one interface to the same or to different networks. See
Section 1.1.3 on "Terminology".
1.1.2 Architectural Assumptions
The current Internet architecture is based on a set of
assumptions about the communication system. The assumptions
most relevant to hosts are as follows:
(a) The Internet is a network of networks.
Each host is directly connected to some particular
network(s); its connection to the Internet is only
conceptual. Two hosts on the same network communicate
with each other using the same set of protocols that they
would use to communicate with hosts on distant networks.
(b) Gateways don't keep connection state information.
To improve robustness of the communication system,
gateways are designed to be stateless, forwarding each IP
datagram independently of other datagrams. As a result,
redundant paths can be exploited to provide robust service
in spite of failures of intervening gateways and networks.
All state information required for end-to-end flow control
and reliability is implemented in the hosts, in the
transport layer or in application programs. All
connection control information is thus co-located with the
end points of the communication, so it will be lost only
if an end point fails.
(c) Routing complexity should be in the gateways.
Routing is a complex and difficult problem, and ought to
be performed by the gateways, not the hosts. An important
objective is to insulate host software from changes caused
by the inevitable evolution of the Internet routing
architecture.
(d) The System must tolerate wide network variation.
A basic objective of the Internet design is to tolerate a
wide range of network characteristics -- e.g., bandwidth,
delay, packet loss, packet reordering, and maximum packet
size. Another objective is robustness against failure of
individual networks, gateways, and hosts, using whatever
bandwidth is still available. Finally, the goal is full
"open system interconnection": an Internet host must be
able to interoperate robustly and effectively with any
other Internet host, across diverse Internet paths.
Sometimes host implementors have designed for less
ambitious goals. For example, the LAN environment is
typically much more benign than the Internet as a whole;
LANs have low packet loss and delay and do not reorder
packets. Some vendors have fielded host implementations
that are adequate for a simple LAN environment, but work
badly for general interoperation. The vendor justifies
such a product as being economical within the restricted
LAN market. However, isolated LANs seldom stay isolated
for long; they are soon gatewayed to each other, to
organization-wide internets, and eventually to the global
Internet system. In the end, neither the customer nor the
vendor is served by incomplete or substandard Internet
host software.
The requirements spelled out in this document are designed
for a full-function Internet host, capable of full
interoperation over an arbitrary Internet path.
1.1.3 Internet Protocol Suite
To communicate using the Internet system, a host must implement
the layered set of protocols comprising the Internet protocol
suite. A host typically must implement at least one protocol
from each layer.
The protocol layers used in the Internet architecture are as
follows [INTRO:4]:
o Application Layer
The application layer is the top layer of the Internet
protocol suite. The Internet suite does not further
subdivide the application layer, although some of the
Internet application layer protocols do contain some
internal sub-layering. The application layer of the
Internet suite essentially combines the functions of the
top two layers -- Presentation and Application -- of the
OSI reference model.
We distinguish two categories of application layer
protocols: user protocols that provide service directly
to users, and support protocols that provide common system
functions. Requirements for user and support protocols
will be found in the companion RFC [INTRO:1].
The most common Internet user protocols are:
o Telnet (remote login)
o FTP (file transfer)
o SMTP (electronic mail delivery)
There are a number of other standardized user protocols
[INTRO:4] and many private user protocols.
Support protocols, used for host name mapping, booting,
and management, include SNMP, BOOTP, RARP, and the Domain
Name System (DNS) protocols.
o Transport Layer
The transport layer provides end-to-end communication
services for applications. There are two primary
transport layer protocols at present:
o Transmission Control Protocol (TCP)
o User Datagram Protocol (UDP)
TCP is a reliable connection-oriented transport service
that provides end-to-end reliability, resequencing, and
flow control. UDP is a connectionless ("datagram")
transport service.
Other transport protocols have been developed by the
research community, and the set of official Internet
transport protocols may be expanded in the future.
Transport layer protocols are discussed in Chapter 4.
o Internet Layer
All Internet transport protocols use the Internet Protocol
(IP) to carry data from source host to destination host.
IP is a connectionless or datagram internetwork service,
providing no end-to-end delivery guarantees. Thus, IP
datagrams may arrive at the destination host damaged,
duplicated, out of order, or not at all. The layers above
IP are responsible for reliable delivery service when it
is required. The IP protocol includes provision for
addressing, type-of-service specification, fragmentation
and reassembly, and security information.
The datagram or connectionless nature of the IP protocol
is a fundamental and characteristic feature of the
Internet architecture. Internet IP was the model for the
OSI Connectionless Network Protocol [INTRO:12].
ICMP is a control protocol that is considered to be an
integral part of IP, although it is architecturally
layered upon IP, i.e., it uses IP to carry its data end-
to-end just as a transport protocol like TCP or UDP does.
ICMP provides error reporting, congestion reporting, and
first-hop gateway redirection.
IGMP is an Internet layer protocol used for establishing
dynamic host groups for IP multicasting.
The Internet layer protocols IP, ICMP, and IGMP are
discussed in Chapter 3.
o Link Layer
To communicate on its directly-connected network, a host
must implement the communication protocol used to
interface to that network. We call this a link layer or
media-access layer protocol.
There is a wide variety of link layer protocols,
corresponding to the many different types of networks.
See Chapter 2.
1.1.4 Embedded Gateway Code
Some Internet host software includes embedded gateway
functionality, so that these hosts can forward packets as a
gateway would, while still performing the application layer
functions of a host.
Such dual-purpose systems must follow the Gateway Requirements
RFC [INTRO:2] with respect to their gateway functions, and
must follow the present document with respect to their host
functions. In all overlapping cases, the two specifications
should be in agreement.
There are varying opinions in the Internet community about
embedded gateway functionality. The main arguments are as
follows:
o Pro: in a local network environment where networking is
informal, or in isolated internets, it may be convenient
and economical to use existing host systems as gateways.
There is also an architectural argument for embedded
gateway functionality: multihoming is much more common
than originally foreseen, and multihoming forces a host to
make routing decisions as if it were a gateway. If the
multihomed host contains an embedded gateway, it will
have full routing knowledge and as a result will be able
to make more optimal routing decisions.
o Con: Gateway algorithms and protocols are still changing,
and they will continue to change as the Internet system
grows larger. Attempting to include a general gateway
function within the host IP layer will force host system
maintainers to track these (more frequent) changes. Also,
a larger pool of gateway implementations will make
coordinating the changes more difficult. Finally, the
complexity of a gateway IP layer is somewhat greater than
that of a host, making the implementation and operation
tasks more complex.
In addition, the style of operation of some hosts is not
appropriate for providing stable and robust gateway
service.
There is considerable merit in both of these viewpoints. One
conclusion can be drawn: an host administrator must have
conscious control over whether or not a given host acts as a
gateway. See Section 3.1 for the detailed requirements.
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 [IP:1]: "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
Internet 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 [INTRO:1]); 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
Protocol layering, which is generally used as an organizing
principle in implementing network software, has also been used
to organize this document. In describing the rules, we assume
that an implementation does strictly mirror the layering of the
protocols. Thus, the following three major sections specify
the requirements for the link layer, the internet layer, and
the transport layer, respectively. A companion RFC [INTRO:1]
covers application level software. This layerist organization
was chosen for simplicity and clarity.
However, strict layering is an imperfect model, both for the
protocol suite and for recommended implementation approaches.
Protocols in different layers interact in complex and sometimes
subtle ways, and particular functions often involve multiple
layers. There are many design choices in an implementation,
many of which involve creative "breaking" of strict layering.
Every implementor is urged to read references [INTRO:7] and
[INTRO:8].
This document describes the conceptual service interface
between layers using a functional ("procedure call") notation,
like that used in the TCP specification [TCP:1]. A host
implementation must support the logical information flow
implied by these calls, but need not literally implement the
calls themselves. For example, many implementations reflect
the coupling between the transport layer and the IP layer by
giving them shared access to common data structures. These
data structures, rather than explicit procedure calls, are then
the agency for passing much of the information that is
required.
In general, each major section of this document 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 inside an IP datagram.
Message
In this description of the lower-layer protocols, a
message is the unit of transmission in a transport layer
protocol. In particular, a TCP segment is a message. A
message consists of a transport protocol header followed
by application protocol data. To be transmitted end-to-
end through the Internet, a message must be encapsulated
inside a datagram.
IP Datagram
An IP datagram is the unit of end-to-end transmission in
the IP protocol. An IP datagram consists of an IP header
followed by transport layer data, i.e., of an IP header
followed by a message.
In the description of the internet layer (Section 3), the
unqualified term "datagram" should be understood to refer
to an IP datagram.
Packet
A packet is the unit of data passed across the interface
between the internet layer and the link layer. It
includes an IP header and data. A packet may be a
complete IP datagram or a fragment of an IP datagram.
Frame
A frame is the unit of transmission in a link layer
protocol, and consists of a link-layer header followed by
a packet.
Connected Network
A network to which a host is interfaced is often known as
the "local network" or the "subnetwork" relative to that
host. However, these terms can cause confusion, and
therefore we use the term "connected network" in this
document.
Multihomed
A host is said to be multihomed if it has multiple IP
addresses. For a discussion of multihoming, see Section
3.3.4 below.
Physical network interface
This is a physical interface to a connected network and
has a (possibly unique) link-layer address. Multiple
physical network interfaces on a single host may share the
same link-layer address, but the address must be unique
for different hosts on the same physical network.
Logical [network] interface
We define a logical [network] interface to be a logical
path, distinguished by a unique IP address, to a connected
network. See Section 3.3.4.
Specific-destination address
This is the effective destination address of a datagram,
even if it is broadcast or multicast; see Section 3.2.1.3.
Path
At a given moment, all the IP datagrams from a particular
source host to a particular destination host will
typically traverse the same sequence of gateways. We use
the term "path" for this sequence. Note that a path is
uni-directional; it is not unusual to have different paths
in the two directions between a given host pair.
MTU
The maximum transmission unit, i.e., the size of the
largest packet that can be transmitted.
The terms frame, packet, datagram, message, and segment are
illustrated by the following schematic diagrams:
A. Transmission on connected network:
_______________________________________________
| LL hdr | IP hdr | (data) |
|________|________|_____________________________|
<---------- Frame ----------------------------->
<----------Packet -------------------->
B. Before IP fragmentation or after IP reassembly:
______________________________________
| IP hdr | transport| Application Data |
|________|____hdr___|__________________|
<-------- Datagram ------------------>
<-------- Message ----------->
or, for TCP:
______________________________________
| IP hdr | TCP hdr | Application Data |
|________|__________|__________________|
<-------- Datagram ------------------>
<-------- Segment ----------->
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.
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