CSCE 201 Computer Networks CSCE 201 Farkas 1

CSCE 201 Computer Networks CSCE 201 - Farkas 1

Reading Assignment Required: – Security Awareness: Chapter 3 Recommended: – Internet Society (ISOC) homepage, http: //www. isoc. org – Computer Network, http: //en. wikipedia. org/wiki/Computer_network – Easttom: Chapter 2 CSCE 201 - Farkas 2

Before Internet l Isolated, local packet-switching networks – only nodes on the same network could communicate l Each network is autonomous: – different services – different interfaces – different protocols CSCE 201 - Farkas 3

Before Internet (cont) ARPANET: sponsored by Defense Advanced Research Projects Agency (DARPA): • 1969: interconnected 4 hosts • 1970: host-to-host protocol: Network Control Protocol (NCP) • 1972: first application: e-mail Stanford Research Institute (SRI) Univ. of California at Santa Barbara (UCSB) Univ. of California at LA (UCLA) Univ. of Utah CSCE 201 - Farkas 4

Internet Connect Existing Networks: ARPANET, Packet Radio, and Packet Satellite l NCP not sufficient Develop new protocol l l 1970 s: Transmission Control Protocol (Kahn and Vinton) – Based on packet switching technology – Good for file transfer and remote terminal access l Divide TCP into 2 protocols – Internet Protocol (IP): addressing and forwarding of packets – Transmission Control Protocol (TCP): sophisticated services, e. g. flow control, recovery 1980: TCP/IP adopted as a Do. D standard l 1983: ARPANET protocol officially changed from NCP to TCP/IP l 1985: Existing Internet technology l 1995: U. S. Federal Networking Council (FNC) define the term l Internet CSCE 201 - Farkas 5

Goals (Clark’ 88) Connect existing networks 1. Survivability 2. Support multiple types of services 3. Must accommodate a variety of networks 4. Allow distributed management 5. Allow host attachment with a low level of effort 6. Be cost effective 7. Allow resource accountability CSCE 201 - Farkas 6

Internet Challenge Interconnected networks differ (protocols, interfaces, services, etc. ) l Solutions: l 1. 2. Reengineer and develop one global packet switching network standard: not economically feasible Have every host implement the protocols of any network it wants to communicate with: too complex, very high engineering cost 3. Add an extra layer: internetworking layer l l l Hosts: one higher-level protocol Network connecting use the same protocol Interface between the new protocol and network CSCE 201 - Farkas 7

Layering l Organize a network system into logically distinct entities – the service provided by one layer is based only on the service provided by the lower level entity CSCE 201 - Farkas 8

Without Layering Application Transmission Media SMTP FTP Coaxial cable HTTP Fiber optic l Each application has to be implemented for every network technology! CSCE 201 - Farkas 9

With Layering l Intermediate layer provides a unique abstraction for various network technologies Application SMTP FTP HTTP Intermediate layer Transmission Media Coaxial cable Fiber optic CSCE 201 - Farkas 10

Layering l Advantages – Modularity – protocols easier to manage and maintain – Abstract functionality –lower layers can be changed without affecting the upper layers – Reuse – upper layers can reuse the functionality provided by lower layers l Disadvantages – Information hiding – inefficient implementations CSCE 201 - Farkas 11

TCP/IP Networking Model l TCP/IP has a different layered model Application Layer Transport Layer (TCP) Error Correction Reliable Connection Internetwork Layer (IP) WAN Connectivity Unreliable Datagram Service Network Access Layer Physical Connection LAN Connection CSCE 201 - Farkas 12

Network Access Layer Responsible for physical connection – Shape – Size – Voltages l Responsible for rules of how to put bits on the “wire” l These are the building blocks for the network l The goal of the physical layer is to move information across one “hop” l CSCE 201 - Farkas 13

Internet Layer Transports data from one end-user system to another end-user systems by hopping across as many physical connections as necessary l Provides a mechanism to connect many LANs together effectively l Connectionless and unreliable datagram protocol l Protocols: l – Internet Protocol – Routing Protocol – Supporting Protocol CSCE 201 - Farkas 14

IP Header 0 4 8 Version HLen TOS Identification TTL 16 19 Flags 31 Length Fragment offset Protocol Header checksum Source address Destination address 20 bytes Options (variable) l Comments – HLen – header length only in 32 -bit words (5 <= HLen <= 15) – TOS (Type of Service): now split in Differentiated Service Field (6 bits) l remaining two bits used by ECN (Early Congestion Notification) Length – the length of the entire datagram/segment; header + data Flags: Don’t Fragment (DF) and More Fragments (MF) Fragment offset – all fragments excepting last one contain multiples of 8 bytes Header checksum - uses 1’s complement l – – CSCE 201 - Farkas 15

IP Addresses l l l IP provides logical address space and a corresponding addressing schema IP address is a globally unique or private number associated with a host network interface Every system which will send packets directly out across the Internet must have a unique IP addresses are based on where station is connected IP addresses are controlled by a single organization address ranges are assigned They are running out of space! CSCE 201 - Farkas 16

Routing Protocols • Enable routing decisions to be made • Manage and periodically update routing tables, stored at each router • Autonomous collection of routers: • Under single administration • Use same routing protocol: Interior Gateway Protocol (IGP) • Use Exterior Gateway Protocol (EGP) to communicate other systems • Router : “which way” to send the packet closer. (Keep routing table small and allow to handle unlimited number of systems. ) • Protocol types: • Reachability • Distance vector CSCE 201 - Farkas 17

Supporting Protocols l Handle specific tasks – Address Resolution Protocol (ARP) – Reverse Address Resolution Protocol (RARP) – Internet Control Message Protocol (ICMP) – Internet Group Management Protocol (IGMP) CSCE 201 - Farkas 18

The Domain Name System Each system connected to the Internet also has one or more logical addresses. l Unlike IP addresses, the domain address have no routing information - they are organized based on administrative units l There are no limitations on the mapping from domain addresses to IP addresses l CSCE 201 - Farkas 19

Domain Name Resolution: looking up a logical name and finding a physical IP address l There is a hierarchy of domain name servers l Each client system uses one domain name server which in turn queries up and down the hierarchy to find the address l If your server does not know the address, it goes up the hierarchy possibly to the top and works its way back down l CSCE 201 - Farkas 20

Transport Layer (TCP) Present a reliable end-to-end pipe to the application l Data either arrives in the proper order or the connection is closed l Keeps buffers in the sending and destination system to keep data which has arrived out of order or to retransmit if necessary l Provides individual connections between applications l CSCE 201 - Farkas 21

TCP Connection Establishment l Three-way handshake – Goal: agree on a set of parameters: the start sequence number for each side Server Client (initiator) SYN, Seq Num = x Ack = d n a y = m K, Seq. Nu C A d n a N x+1 SY ACK, Ack =y+1 CSCE 201 - Farkas 22

Application Layer l Uses the reliable TCP connections to accomplish useful work over the network – client-server applications – standard applications l l telnet (port 23) mail (port 25) finger (port 79) ftp (port 21) Each application uses a “port” and a protocol l Each port can have many connections l CSCE 201 - Farkas 23