CSCE Farkas1 CSCE 201 Computer Networks

CSCE Farkas2 Reading Assignment Required: – Security Awareness: Chapter 3 Recommended: – Internet Society (ISOC) homepage, – Computer Network, – Easttom: Chapter 2

CSCE Farkas3 Before Internet Isolated, local packet-switching networks – only nodes on the same network could communicate Each network is autonomous: – different services – different interfaces – different protocols

CSCE Farkas4 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: Univ. of California at LA (UCLA) Stanford Research Institute (SRI) Univ. of California at Santa Barbara (UCSB) Univ. of Utah

CSCE Farkas5 Internet Connect Existing Networks: ARPANET, Packet Radio, and Packet Satellite NCP not sufficient  Develop new protocol 1970s: Transmission Control Protocol (Kahn and Vinton) – Based on packet switching technology – Good for file transfer and remote terminal access 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 DoD standard 1983: ARPANET protocol officially changed from NCP to TCP/IP 1985: Existing Internet technology 1995: U.S. Federal Networking Council (FNC) define the term Internet

CSCE Farkas6 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 Farkas7 Internet Challenge Interconnected networks differ (protocols, interfaces, services, etc.) Solutions: 1. Reengineer and develop one global packet switching network standard: not economically feasible 2. 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 Hosts: one higher-level protocol Network connecting use the same protocol Interface between the new protocol and network

CSCE Farkas8 Layering 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 Farkas9 Without Layering Each application has to be implemented for every network technology! SMTP FTPHTTP Coaxial cable Fiber optic Application Transmission Media

CSCE Farkas10 With Layering Intermediate layer provides a unique abstraction for various network technologies SMTP FTP Coaxial cable Fiber optic Application Transmission Media HTTP Intermediate layer

CSCE Farkas11 Layering 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 Disadvantages – Information hiding – inefficient implementations

CSCE Farkas12 TCP/IP Networking Model TCP/IP has a different layered model Network Access Layer Physical Connection LAN Connection Internetwork Layer (IP) WAN Connectivity Unreliable Datagram Service Transport Layer (TCP) Error Correction Reliable Connection Application Layer

CSCE Farkas13 Network Access Layer Responsible for physical connection – Shape – Size – Voltages Responsible for rules of how to put bits on the “wire” These are the building blocks for the network The goal of the physical layer is to move information across one “hop”

CSCE Farkas14 Internet Layer Transports data from one end-user system to another end-user systems by hopping across as many physical connections as necessary Provides a mechanism to connect many LANs together effectively Connectionless and unreliable datagram protocol Protocols: – Internet Protocol – Routing Protocol – Supporting Protocol

CSCE Farkas15 IP Header Comments – HLen – header length only in 32-bit words (5 <= HLen <= 15) – TOS (Type of Service): now split in Differentiated Service Field (6 bits) 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 VersionHLen TOSLength Identification Fragment offset Flags Source address Destination address TTLProtocolHeader checksum Options (variable) 20 bytes

CSCE Farkas16 IP Addresses 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 address 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 Farkas17 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 Farkas18 Supporting Protocols Handle specific tasks – Address Resolution Protocol (ARP) – Reverse Address Resolution Protocol (RARP) – Internet Control Message Protocol (ICMP) – Internet Group Management Protocol (IGMP)

CSCE Farkas19 The Domain Name System Each system connected to the Internet also has one or more logical addresses. Unlike IP addresses, the domain address have no routing information - they are organized based on administrative units There are no limitations on the mapping from domain addresses to IP addresses

CSCE Farkas20 Domain Name Resolution Domain Name Resolution: looking up a logical name and finding a physical IP address There is a hierarchy of domain name servers Each client system uses one domain name server which in turn queries up and down the hierarchy to find the address If your server does not know the address, it goes up the hierarchy possibly to the top and works its way back down

CSCE Farkas21 Transport Layer (TCP) Present a reliable end-to-end pipe to the application Data either arrives in the proper order or the connection is closed Keeps buffers in the sending and destination system to keep data which has arrived out of order or to retransmit if necessary Provides individual connections between applications

CSCE Farkas22 TCP Connection Establishment Three-way handshake – Goal: agree on a set of parameters: the start sequence number for each side Client (initiator) Server SYN, SeqNum = x SYN and ACK, SeqNum = y and Ack = x + 1 ACK, Ack = y + 1

CSCE Farkas23 Application Layer Uses the reliable TCP connections to accomplish useful work over the network – client-server applications – standard applications telnet (port 23) mail (port 25) finger (port 79) ftp (port 21) Each application uses a “port” and a protocol Each port can have many connections