LIS508 lecture 10: Ethernet and TCP/IP

1. LIS508 lecture 10:Ethernet and TCP/IP Thomas Krichel 2002-11-24

2. Structure • General things about networks • History of Internet • LANs and Ethernet • IP • TCP • More general things • the TCP/IP model • discussion mostly based on Tanenbaum’s “computer networks” classic book….

3. Origins of TCP/IP • 1957: USSR launches the Sputnik • US worried about command and control structure after a nuclear attack • Early 60s Paul Baran promotes packet switching rather than circuit switching. • Mid 60s: Pentagon says that it wants this, gives grants to ARPA.

4. ARPAnet original design • Network nodes have host computers and router computer • Hosts sends messages smaller than 8061 bits • routers breaks it up into packets smaller than 1008 bits • all router are connected by transmission lines • each router connected to 2 others • store and forward principle

5. implementation • 12k times 16 bit words memory minicomputer without hard disk as routers connected by 56kbps leased phone lines • life with four hosts in 1969–12 • over 30 hosts in 1972–09 • protocol research leads to TCP/IP in 1974 • integrated into Berkeley UNIX, freely available • Internet research group developing protocols • 1979 Internet research group reorganized to Internet Control and Configuration Board

6. More history • 1980: MILNET split off the ARPAnet • CSNET set up • links researchers at non-ARPA contract institutions to the ARPAnet • ran on a single box with dial-up lines since late 70s • 1984 NSF links adds a router to it supercomputers, sets up NSFnet, • the first purely TCP/IP network, on 56kbps. • additional funding for (eventually 20) regional networks connect to the backbone • 1990 ANS (MERIT,MCI,IBM) take over NSFnet

7. LAN • A LAN generally operates a broadcasting network. • Something that is transmitted by one station is heard by all other stations • An individual message is called a frame. • This creates a media access control (mac) problem. When one station issues a frame, it destroys a frame from another station, through frame collision.

8. Ethernet • comes from the term "luminiferous ether". • Today the name for a family of LAN protocols implementing CSMA/CD protocol for the mac problem. • Initially only referred to IEEE 802.3 standard • Can run over a variety of physical devices.

9. 10baseT • This is most widely used physical infrastructure today. • It runs over twisted-pair cable and RJ-45 jack. • Cables are cheap, transmission is fast • Maximum length of cable about 150m, thus not good for bus architecture.

10. other devices • “repeater” is a device that amplifies an electric signal so that it can travel over some more wire. • “hub” is a repeater with a lot of outgoing wires. Hubs are dump, or smart. A smart hub can be inspected by a monitoring device to see what the traffic is doing there. • A “bridge” or “switch” is a device that links different LANs together. In normal “promiscuous” mode, it receives signals from all LANs. It will know which MAC address is on which LAN and transmit echo signal to the right LAN.

11. MAC address • A physical address burned onto each network interface card used on a LAN. • Forms an address for your computer on a LAN • It is 6 bytes long, written in hex, usually written with each byte separated by “:”. • 2 power 48 addresses, that is about 11000 addresses for every inhabitant of the planet.

12. Internet Protocol IP • LAN protocols are not part of the Internet. • IP operates on top of LANs to make them interoperable. • IP sets up a set of logical addresses that locate a physical device over the whole range of locations over the Internet. • It provides for the means to send datagrams from one address to the other… • Such datagrams are called packets. The IP service is not reliable!

13. in the words of rfc791 • “The Internet protocol is designed for use in interconnected systems of packet-switched computer connection networks. The Internet protocol provides for transmitting blocks of data called datagrams from sources to destinations. The Internet protocol also provides for fragmentation and reassembly of long datagram, if necessary, for transmission through 'small packet' networks.” • “There are no mechanisms to augment end-to-end data reliability, flow control, sequencing, or other services commonly found in host-to-host protocols.”

14. IP address • 32 bit address, usually written as a sequence of four decimal numbers between 0 and 255. • Contains a network part and a host part • dependent on the network mask • depending on the class of the network • All traffic that is not on the local network is sent to a gateway

15. finding the IP address • A host is on the Internet when it has an IP address that others can reach. Usually that implies knowing • IP address of the machine • network mask • IP address of the gateway • There can be written into a file on the machine. This is usually referred to as a static address.

16. static and dynamic addresses • Static IP addresses have problems • waste a lot of address space on machines that are switched off • have to be rewritten when device moves • dynamic addressing has become more common • the most widely used protocol is the dynamic host configuration protocol

17. dhcp • When connected to a network a client software on a host machine sends out a request on the LAN it is attached to. • A dhcp server will answer and lease an IP address to the host, sending it the network mask and the gateway address as well. • Internet providers work like that to save address space. • Problem: some services require IP addresses to be stable. They can not be run in such a setting.

18. routing • using IP, all datagrams travel individually. • they may travel through many networks in order to get to their destination • networks are interconnected through routers who make the decision where to send the packet to next. • Such decision are based on routing protocols such as OSPF or BGP etc. This is a very complicated piece of engineering.

19. frame / packet / segment • The Ethernet frame on the LAN contains the IP packet. • A gateway between two networks takes the packet out of the Ethernet frame of one network, and “wraps” it in a frame used on another network. • Inside the IP packet, there is a TCP segment. The TCP segment tells the destination machine what to do with the data.

20. transmission control protocol • provides a reliable service of communication • stream orientation: bytes come out from the sender, arrive in the same order at the receiver • packet buffering: fast arrived data is stored at the destination until it can be processed • full duplex: communication in two ways. • if a packet has gone astray, TCP retransmit it. • uses port numbers as addresses to tell the host what to do with arriving packets.

21. applications and ports • TCP uses port numbers to detect which application protocol to send the data to. • Some ports are well-known • 80 for http -- 20 for ssh • 23 for smtp -- 53 for dns • firewalls can block traffic for specific ports on specific machines and therefore allow or prevent services.

22. summary : TCP/IP “model” • Application layer • http, smtp, ftp, dns, ntp • Transport layer • tcp, udp • Network layer • IP • Host-to-host layer • LAN and WAN protocols

23. Critique of tcp/ip • Not a model but a protocol stack • No proper distinction between • Service • Protocol • Interface • Ad hoc set of application protocols

24. http://openlib.org/home/krichel Thank you for your attention!