ALG0183 Algorithms

NET0183 Networks and Communications by Dr Andy Brooks

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NET0183 Networks and Communications

Lectures 3 and 4ARPANET

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DARPA / ARPA Defense / Advanced Research Project Agency(USA)


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1967• The initial plan for the ARPANET was published in October 1967 in the

Proceedings of the first ACM symposium on Operating System Principles.

• The reasons for establishing such a computer network were:– Load Sharing

• data and program to be sent to a less busy, remote computer

– Message Service• electronic mail

– Data Sharing• remote access to databases

– Program Sharing• data sent to remote computer which ran program

– Remote Service• login to remote computer to access its data and programs

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1967

The initial plan for the ARPANET.

http://portal.acm.org/portal.cfm

Another paper on the packet network concept.


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1967• There was another paper at first ACM symposium on Operating

System Principles on the packet network concept. This paper was by Davies and others from the National Physical Laboratory in the UK.

• Work at MIT (1961-1967), RAND (1962-1965), and at NPL (1964-1967) had proceeded in parallel without the researchers knowing about each others´ work.

• The word “packet” was adopted from the work at NPL.• The proposed line speed in ARPANET of 2.4 kbps was upgraded

to 50 kbps.• Note that the ARPANET was not designed for the purposes of

having a network resistant to nuclear war. This was a false rumor.

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A store and forward network.

• A copy of a packet is stored at a node until it has been received at the following node.– a store and forward network

• A store and forward network must deal with the routing of packets, the queueing of packets, error control, etc.

• To shield host computers from these issues, ARPA chose to use small processors as packet switches called Interface Message Processors or IMPs.

• The work of creating the ARPA network divided into two main parts: (i) the design and implementation of the IMP subnet and (ii) the design and implementation of protocols to allow host computers to make use of the network.

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Heart, F.E., Kahn, R.E., Omstein, S.M., Crowther, W.R., and Walden, D.C. The interface message processor for the Arpa computer network, AFIPS Conf. Proc., Vol. 36, 1970 SJCC, AFIPS Press, Montvale, N.J., 1970, pp. 551-567.


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Host-subnet design principles

1. “The subnet should function as a communications system whose essential task is to transfer bits reliably from a source location to a specified destination. Bit transmission should be sufficiently reliable and error free to obviate the need for special precautions (such as storage for retransmission) on the part of the Hosts;”

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2. “The average transit time through the subnet should be under a half second to provide for convenient interactive use of remote computers;”

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3. “The subnet operation should be completely autonomous. Since the subnet must function as a store and forward system, an IMP must not be dependent upon its local Host.

The IMP must continue to operate whether the Host is functioning properly or not and must not depend upon a Host for buffer storage or other logical assistance such as program reloading.

The Host computer must not in any way be able to change the logical characteristics of the subnet; this restriction avoids the mischievous or inadvertent modification of the communication system by an individual Host user;”

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4. “Establishment of Host-to-Host protocol and the enormous problem of planning to communicate between different computers should be an issue separated from the subnet design.”

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Andy comments: From these design principles it is clear that at least two protocols are required. One protocol governs the movement of packets over the IMP subnet. The other protocol governs how host computers communicate with each other at a message level.


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Message handling

• Hosts communicate via messages.• An IMP takes a message from its Host computer and breaks it

down into packets which it then sends separately over the network. This breaking down of messages into packets during transmission is completely invisible to the Host computers.

• “The transmitting Host attaches an identifying leader to the beginning of each message. The IMP forms a header by adding further information for network use and attaches this header to each packet of the message.”

• The maximum size of a packet is about 1,000 bits. A message from a Host to an IMP is about 8,000 bits.

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Message handling• “Each packet is individually routed from IMP-to-IMP through the

network toward the destination. At each IMP along the way, the transmitting hardware generates initial and terminal framing characters and parity check digits that are shipped with the packet and are used for error detection by the receiving hardware of the next IMP.”

• Packets of a message may arrive out of order. The destination IMP reassembles the message in the right order, stripping headers of each packet and identifying the source Host in the leader.

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Andy comments: Note how some of the subnet protocol is in hardware.


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Message handling

• “Errors in transmission can affect a packet by destroying the framing and/or by modifying the data content. If the framing is disturbed in any way, the packet either will not be recognized or will be rejected by the receiver. In addition, the check digits provide protection against errors that affect only the data.”

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Message handling

• Each IMP stores the packet until it receives a positive acknowledgement from the succeeding IMP.

• An IMP can refuse a packet “by simply not returning a positive acknowledgment”.– The packet may be detected to have an error.– IMP buffer storage may be temporarily full.– Etc.

• If a packet is not acknowledged within a reasonable time (e.g. 100 msec), the transmitting IMP can retransmit, perhaps even along a different route.

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Message handling

• “Acknowledgements themselves are not acknowledged, although they are error checked in the usual fashion.”

• Duplicate packets can arise as a result of a lost acknowledgment. The destination IMP can detect duplication using message and packet numbers in the header.

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An RFNM is also a packet and it is also acknowledged. Note that a later article on ARAPNET design decisions suggests that acknowledgements were not sent separately in order to reduce overheads.

Figure 4 – RFNMs and acknowledgements


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Figure 5 – Format of a packet on phone line

idleidle


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Some ASCII control characters.http://en.wikipedia.org/wiki/ASCII#ASCII_control_characters


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Doubling DLE characters

• IMP hardware doubles any DLE characters within the actual data being sent so that the receiving hardware can distinguish them from the DLEs used to frame packets.

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Andy comments: An alternative design solution could reserve certain characters for control purposes. For example, characters such as DLE could not appear in the data stream itself. Such a solution, however, could be too restrictive and place severe limits on the data that could be sent over the network.


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Routing algorithm• Paths through the network are not determined in advance.• Each IMP decides onto which of its output lines it should

transmit a packet.• For every possible destination, an entry in a lookup table

specifies the appropriate output line.• The system is adaptive. Routing tables are dynamically updated

to reflect changing conditions on the network.– traffic congestion (what is the fastest route?)– breaks in connectivity (what route is currently not available?)

• Neighbouring IMPs exchange minimum delay tables with one another and use this information to construct routing tables.– Estimates of delays are based on queue lengths and recent

performances of connections.8/25/2009



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1968 and 1969

• In 1968 companies were asked to tender bids to build the IMPs (Interface Message Processors).– A request for quotation (RFQ).– A request for proposal (RFP).

• In January 1969, a contract was awarded to Bolt, Beranek and Newman Inc. (of Cambridge, Massachussetts).

• In September 1969, BBN installed the first IMP at UCLA and the first host computer was connected.

• In October 1969, Stanford Research Institute was connected to the ARPANET and the first host-to-host message was sent.

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http://www.nature.com/nature/journal/v461/n7268/full/4611202a.html


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29OCT69“talked to SRI Host to Host”

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http://www.computerhistory.org/internet_history/

Kline began to type "LOG," to log in through telephone wires to the computer more than 300 miles away at Stanford. But after the "L" and the "O," the Stanford computer crashed. LA Daily News 29 October 2009 http://www.dailynews.com/news/ci_13669715


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December, 1969

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University of UtahStanford Research InstituteUniversity of California, Santa Barbara University of California, Los Angeles

http://som.csudh.edu/cis/lpress/history/arpamaps/

Four host computers were now connected together forming the initial ARPANET.


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June, 1970

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April, 1971

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from “The Arpanet and computer networks” by Larry RobertsProceedings of the ACM Conference on The history of personal workstations, 1986


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1970/1971

• Terminals that could be connected directly to the network through a PAD device were designed and built during 1970 and 1971.– PAD Packet Assembler and Disassembler

• The first Terminal Interface Processor (TIP) was added to ARPANET in August, 1971.– Users with no computer of their own could

connect to any computer on the ARPANET.

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1972/1973

• In October 1972, the ARPANET was demonstrated publicly for the first time at the first International Conference on Computer Communications (ICCC) in Washington D.C.– A reliable service was provided to thousands of conference

attendees, proving to all that packet switching really does work.

• In 1972, network traffic rose from 100,000 packets a day to 1,000,000 packets a day.

• In September 1973, network traffic had risen to 2,900,000 packets a day.

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June, 1974

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Satellite links to Hawaii and Norway. Note that the Hawaii site is shown with a TIP and not an IMP plus host computer.


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The Network Control Protocol (NCP)

• In December 1970, the specification of the initial ARPANET Host-to-Host protocol was finished.

• During 1971-1972, ARPANET sites completed the implementation of NCP and network users could then start developing applications.

• In 1972, electronic mail was introduced. Email was the largest network application for over a decade.

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A Brief History of the Internet, Barry M Leiner et al, ACM SIGCOMM Computer Communication Review, Volume 39, Number 5, October 2009, pp 22-31.

Andy comments: other sources suggeststhat NCP really stood for Network Control Program.


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The First Network Email by Ray Tomlinson

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“I used the at sign to indicate that the user was "at" some other host rather than being local.

The first message was sent between two machines that were literally side by side. The only physical connection they had (aside from the floor they sat on) was through the ARPANET. I sent a number of test messages to myself from one machine to the other. The test messages were entirely forgettable and I have, therefore, forgotten them. Most likely the first message was QWERTYUIOP or something similar. When I was satisfied that the program seemed to work, I sent a message to the rest of my group explaining how to send messages over the network. The first use of network email announced its own existence.

These first messages were sent in late 1971.”

http://openmap.bbn.com/~tomlinso/ray/firstemailframe.html

http://openmap.bbn.com/~tomlinso/ray/ka10.html


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The move from NCP to TCP/IP

• NCP could not address networks beyond a destination IMP on the ARPANET.

• “... NCP had no end-end host error control, since the ARPANET was to be the only network in existence and it would be so reliable that no error control would be required on the part of the hosts.”

• Robert Kahn decided to develop a new version of the protocol which would provide an open-architecture network environment. His work with others led to the development of TCP/IP.

Transmission Control Protocol/Internet Protocol8/25/2009



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The move from NCP to TCP/IP

• The ARPANET host protocol moved from NCP to TCP/IP on January 1, 1983.

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Documents

ALG0183 Algorithms