Internetwork protocol
Encyclopedia
In networking
, a communications protocol
or network protocol is the specification of a set of rules for a particular type of communication
.
Multiple protocols often describe different aspects of a single communication. A group of protocols designed to work together are known as a protocol suite; when implemented in software they are a protocol stack
. The terms are often intermingled; people may use the term "protocol" to refer to a software implementation, or use "protocol stack" to refer to the specification.
Most recent protocols are assigned by the IETF for Internet
communications, and the IEEE, or the ISO
organizations for other types. The ITU-T
handles telecommunications protocols and formats for the PSTN. As the PSTN and Internet converge, the two sets of standards are also being driven towards convergence.
principles have been applied to create a set of common network protocol design principles.
For example, one layer might describe how to encode text (with ASCII
, say), while another describes how to inquire for messages (with the Internet's simple mail transfer protocol
, for example), while another may detect and retry errors (with the Internet's transmission control protocol
), another handles addressing (say with IP, the Internet Protocol
), another handles the encapsulation of that data into a stream of bits (for example, with the point-to-point protocol
), and another handles the electrical encoding of the bits, (with a V.42 modem
, for example).
Layering allows the parts of a protocol to be designed and tested without a combinatorial explosion
of cases, keeping each design relatively simple. Layering also permits familiar protocols to be adapted to unusual circumstances. For example, the mail protocol above can be adapted to send messages to aircraft. Just change the V.42 modem protocol to the INMARS LAPD data protocol used by the international marine radio satellites.
The reference model usually used for layering is the OSI seven layer model, which can be applied to any protocol, not just the OSI protocols. In particular, the Internet Protocol can be analysed using the OSI model
.
that communication media are always faulty. The conventional measure of quality is the number of failed bits per bit transmitted. This has the wonderful feature of being a dimensionless figure of merit that can be compared across any speed or type of communication media.
In telephony
, failure rates of 10-4 bit per bit are faulty (they interfere with telephone
conversations), while 10-5 bit per bit or more should be dealt with by routine maintenance (they can be heard).
Communication systems correct errors by selectively resending bad parts of a message
. For example, in TCP (the internet's Transmission Control Protocol), messages are divided into packets, each of which has a checksum. When a checksum
is bad (meaning the checksum on the receiver does not match the checksum on the sender), the packet is discarded. When a packet is lost, the receiver acknowledges all of the packets up to, but not including the failed packet. Eventually, the sender sees that too much time has elapsed without an acknowledgement, so it resends all of the packets that have not been acknowledged. At the same time, the sender backs off its rate of sending, in case the packet loss was caused by saturation of the path between sender and receiver. (Note: this is an over-simplification: see TCP
and congestion collapse for more detail)
In general, the performance of TCP is severely degraded in conditions of high packet loss (more than 0.1%), due to the need to resend packets repeatedly. For this reason, TCP/IP connections are typically either run on highly reliable fiber networks, or over a lower-level protocol with added error-detection and correction features (such as modem links with ARQ
). These connections typically have uncorrected bit error rates of 10-9 to 10-12, ensuring high TCP/IP performance.
In packet switched networks, the equivalent functions are performed using router update messages to detect loss of connectivity.
s or families:
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
, a communications protocol
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
or network protocol is the specification of a set of rules for a particular type of communication
Communication
Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...
.
Multiple protocols often describe different aspects of a single communication. A group of protocols designed to work together are known as a protocol suite; when implemented in software they are a protocol stack
Protocol stack
The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....
. The terms are often intermingled; people may use the term "protocol" to refer to a software implementation, or use "protocol stack" to refer to the specification.
Most recent protocols are assigned by the IETF for Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
communications, and the IEEE, or the ISO
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
organizations for other types. The ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
handles telecommunications protocols and formats for the PSTN. As the PSTN and Internet converge, the two sets of standards are also being driven towards convergence.
Network protocol design principles
Systems engineeringSystems engineering
Systems engineering is an interdisciplinary field of engineering that focuses on how complex engineering projects should be designed and managed over the life cycle of the project. Issues such as logistics, the coordination of different teams, and automatic control of machinery become more...
principles have been applied to create a set of common network protocol design principles.
Protocol layering
In modern protocol design, protocols are "layered". Layering is a design principle which divides the protocol design into a number of smaller parts, each of which accomplishes a particular sub-task, and interacts with the other parts of the protocol only in a small number of well-defined ways.For example, one layer might describe how to encode text (with ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
, say), while another describes how to inquire for messages (with the Internet's simple mail transfer protocol
Simple Mail Transfer Protocol
Simple Mail Transfer Protocol is an Internet standard for electronic mail transmission across Internet Protocol networks. SMTP was first defined by RFC 821 , and last updated by RFC 5321 which includes the extended SMTP additions, and is the protocol in widespread use today...
, for example), while another may detect and retry errors (with the Internet's transmission control protocol
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
), another handles addressing (say with IP, the Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
), another handles the encapsulation of that data into a stream of bits (for example, with the point-to-point protocol
Point-to-Point Protocol
In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
), and another handles the electrical encoding of the bits, (with a V.42 modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
, for example).
Layering allows the parts of a protocol to be designed and tested without a combinatorial explosion
Combinatorial explosion
In administration and computing, a combinatorial explosion is the rapidly accelerating increase in lines of communication as organizations are added in a process...
of cases, keeping each design relatively simple. Layering also permits familiar protocols to be adapted to unusual circumstances. For example, the mail protocol above can be adapted to send messages to aircraft. Just change the V.42 modem protocol to the INMARS LAPD data protocol used by the international marine radio satellites.
The reference model usually used for layering is the OSI seven layer model, which can be applied to any protocol, not just the OSI protocols. In particular, the Internet Protocol can be analysed using the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
.
Error detection and correction
It is a truismTruism
A truism is a claim that is so obvious or self-evident as to be hardly worth mentioning, except as a reminder or as a rhetorical or literary device and is the opposite of falsism....
that communication media are always faulty. The conventional measure of quality is the number of failed bits per bit transmitted. This has the wonderful feature of being a dimensionless figure of merit that can be compared across any speed or type of communication media.
In telephony
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
, failure rates of 10-4 bit per bit are faulty (they interfere with telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
conversations), while 10-5 bit per bit or more should be dealt with by routine maintenance (they can be heard).
Communication systems correct errors by selectively resending bad parts of a message
Message
A message in its most general meaning is an object of communication. It is a vessel which provides information. Yet, it can also be this information. Therefore, its meaning is dependent upon the context in which it is used; the term may apply to both the information and its form...
. For example, in TCP (the internet's Transmission Control Protocol), messages are divided into packets, each of which has a checksum. When a checksum
Checksum
A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and...
is bad (meaning the checksum on the receiver does not match the checksum on the sender), the packet is discarded. When a packet is lost, the receiver acknowledges all of the packets up to, but not including the failed packet. Eventually, the sender sees that too much time has elapsed without an acknowledgement, so it resends all of the packets that have not been acknowledged. At the same time, the sender backs off its rate of sending, in case the packet loss was caused by saturation of the path between sender and receiver. (Note: this is an over-simplification: see TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
and congestion collapse for more detail)
In general, the performance of TCP is severely degraded in conditions of high packet loss (more than 0.1%), due to the need to resend packets repeatedly. For this reason, TCP/IP connections are typically either run on highly reliable fiber networks, or over a lower-level protocol with added error-detection and correction features (such as modem links with ARQ
ARQ
Automatic Repeat reQuest , also known as Automatic Repeat Query, is an error-control method for data transmission that uses acknowledgements and timeouts to achieve reliable data...
). These connections typically have uncorrected bit error rates of 10-9 to 10-12, ensuring high TCP/IP performance.
Resiliency
Another form of network failure is topological failure, in which a communications link is cut, or degrades below usable quality. Most modern communication protocols periodically send messages to test a link. In phones, a framing bit is sent every 24 bits on T1 lines. In phone systems, when "sync is lost", fail-safe mechanisms reroute the signals around the failing equipment.In packet switched networks, the equivalent functions are performed using router update messages to detect loss of connectivity.
Protocol families
Various protocol stackProtocol stack
The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....
s or families:
- PUPPARC Universal PacketThe PARC Universal Packet was one of the two earliest internetwork protocol suites; it was created by researchers at Xerox PARC in the mid-1970s...
- Internet protocol suiteInternet protocol suiteThe Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
- AppleTalkAppleTalkAppleTalk is a proprietary suite of protocols developed by Apple Inc. for networking computers. It was included in the original Macintosh released in 1984, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
- DECnetDECnetDECnet is a suite of network protocols created by Digital Equipment Corporation, originally released in 1975 in order to connect two PDP-11 minicomputers. It evolved into one of the first peer-to-peer network architectures, thus transforming DEC into a networking powerhouse in the 1980s...
- IPX/SPXIPX/SPXIPX/SPX stands for Internetwork Packet Exchange/Sequenced Packet Exchange. IPX and SPX are networking protocols used primarily on networks using the Novell NetWare operating systems.-Protocol Layers:...
- Open Systems InterconnectionOpen Systems InterconnectionOpen Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
(OSI) - Systems Network ArchitectureSystems Network ArchitectureSystems Network Architecture is IBM's proprietary networking architecture created in 1974. It is a complete protocol stack for interconnecting computers and their resources. SNA describes the protocol and is, in itself, not actually a program...
(SNA)
See also
- Protocol (computing)
- List of network protocols
- Network architectureNetwork architectureNetwork architecture is the design of a communications network. It is a framework for the specification of a network's physical components and their functional organization and configuration, its operational principles and procedures, as well as data formats used in its operation.In...
- Congestion collapse
- Tunneling protocolTunneling protocolComputer networks use a tunneling protocol when one network protocol encapsulates a different payload protocol...
Further reading
- Radia PerlmanRadia PerlmanRadia Joy Perlman is a software designer and network engineer sometimes referred to as the "Mother of the Internet." She is most famous for her invention of the spanning-tree protocol , which is fundamental to the operation of network bridges, while working for Digital Equipment Corporation...
, Interconnections: Bridges, Routers, Switches, and Internetworking Protocols (2nd Edition). Addison-Wesley 1999. ISBN 0-201-63448-1. In particular Ch. 18 on "network design folklore". - Gerard J. HolzmannGerard J. HolzmannGerard J. Holzmann is an American computer scientist, best known as the developer of the SPIN model checker. Holzmann was born in Amsterdam, Netherlands and received an Engineer's degree in Electrical Engineering from the Delft University of Technology in 1976...
, Design and Validation of Computer Protocols. Prentice Hall, 1991. ISBN 0-13-539925-4. Also available online at http://spinroot.com/spin/Doc/Book91.html