Computer Networks Dr. Jorge A. Cobb Internetworking (Pet. and Davie, chapter 4)2 Internetworking ATM Ethernet WIFI Host Router3 Basics of Internetworking l What is an internetwork? • Gives an illusion of a single (direct link) network • Built on a set of distributed heterogeneous networks • Abstraction typically supported by software l Internetwork properties • Supports heterogeneity: Hardware, OS, network type, and topology independent • Scales to global connectivity l Network (ATM, Ethernet, etc) properties • Must be able to transfer messages between any two nodes in the network. • Preferably must support broadcast l The Internet is the specific global internetwork that grew out of ARPANET4 Internet Protocol (IP) l Network-level protocol for the Internet l Operates on all hosts and routers l Protocol stack has an “hourglass” shape Optical Link ATM WiFi Ethernet FTP TFTP NV HTTP TCP UDP IP5 H1 R1 R2 R3 H2 Internetworking with IP IP IP IP IP IP TCP TCP ETH WiFi Wifi PPP PPP ETH ETH ETH APPL APPL m th,m ih,th,m eh,ih,th,m m th,m ih,th,m eh,ih,th,m wifih,ih,th,m ppph,ih,th,m Each layer adds its own header as messages go down Each layer removes its header as messages go up TCP – End-to-end issues, provides service to APPL IP – mostly addressing/routing, provides service to TCP ETH, WiFi, PPP – local networks (perhaps multi-hop)6 Internet Protocol (IP) l What Services does IP provide? • Defines a global name (and address) space • Provides service to the transport layer (TCP, UDP) • Host-to-host connectivity (connectionless) • Best-effort packet delivery l Not in IP service model • Delivery guarantees on bandwidth, delay or loss l Delivery failure modes • Packet delayed for a very long time • Packet loss • Packets delivered out of order • Packet delivered more than once7 Simple Internetworking with IPv4 l Host addressing l Forwarding l Fragmentation and reassembly l Error reporting/control messages8 IPv4 Address Model (the first try …) l Properties • 32-bit address • Hierarchical • (Network, host) hierarchy • Each network has a unique id in the globe • Each host within each network has a unique id within the network. • Maps to logically unique network adaptor • Hosts have (typically) one IP address • Routers have multiple IP addresses, one per attached network9 Internetworking Network 3 Network 1 Network 2 (1,6) (2,1) (1,3) (3,5) (1,2) (1,5) (1,1) (3,2) (3,1) (3,6) (3,4) (1,4) (2,2) (3,3) (2,3) (2,4) (2,5) Routers Hosts10 Three Classes of Networks (and IP addresses within them) 0 Network (7 bits) Network (14 bits) 1 1 0 1 0 Network (21 bits) Host (24 bits) Host (16 bits) Host (8 bits) Class A: Class B: Class C: Host bits are set to zero if we are referring to the network itself In my early slides, (1,3) is actually a 32 bit number, how many bits for the network and how many for the host depends on if it is type A, B, or C11 IPv4 Address Model Class Network ID Host ID # of Addresses in each network # of possible Networks A “0” + 7 bits 24 bit 224 - 2 128 B “10” + 14 bits 16 bit 65,536 - 2 214 C “110” + 21 bits 8 bit 256 - 2 221 D “1110” + Multicast Address IP Multicast E Future Use12 IPv4 Address Model l IP addresses • Usually represented using decimal-dot notation (instead of hexadecimal, don’t ask me why) • Host in class A network • 56.0.78.100 www.usps.gov (what is the netw #??) • Host in class B network • 128.174.252.1 www.cs.uiuc.edu (what is the netw #??) • Host in class C network • 198.182.196.56 www.linux.org (what is the netw #??) l Internet domain names • ASCII strings separated by periods • Provides some administrative hierarchy • host.subdomain.domain.domain_type (com, edu, gov, org, …) • host.domain.country (us, de, jp, …)13 IPv4 Translation support l Internet domain name to IP address (and vice-versa) • Assume your application knows the domain name of the destination • Your application must obtain the IP address of the destination before it can send data to it. • You call upon the Domain Name Service (DNS) • A hierarchy of servers. • Give your DNS server a domain name, and it returns to you the IP address (and vice-versa). l What about physical addresses?Translation to Physical Addresses l Problem • An IP route can pass through many physical networks • E.g., Ethernet (recall that IP is just software) • Physical network source and destination addresses are needed at each hop along the route l Router (1,2) wants to send an IP message to (1,4) • It needs the Ethernet (i.e., physical) address of (1,4) (how to get it?) 14 Ethernet (1,0) (1,2) (1,5) (1,1) (1,4)Solution – Address Translation l Solution • Translate from IP address to physical address • This is done via the Address Resolution Protocol (ARP) • IP asks ARP to translate the IP address into a physical address • This is done as needed at each hop. l Example … 1516 Example l Router (1,2) wants to send an IP message to (1,4) • It first finds out the Ethernet (i.e., physical) address of (1,4) (via ARP, more on ARP later) l Router (1,2) gives the message and the Ethernet destination address to the Ethernet software (driver). l The Ethernet software encapsulates the IP message (turns it into an Ethernet message) and sends it over the Ethernet hardware. l The Ethernet hardware at the other router receives the message. l The Ethernet software decapsulates the message, and gives it to the IP software. Ethernet (1,0) (1,2) (1,5) (1,1) (1,4)17 IP to Physical Address Translation l Hard-coded: Encode physical address in IP address • E.g, map Ethernet addresses to IP addresses • Makes it impossible to associate IP address with topology (routing becomes too difficult) l Centralized • Maintain a central repository and distribute to hosts • Bottleneck for queries and updates l Automatically generated table • Use ARP to build table at each host as needed. • Take advantage of broadcast. • Use timeouts to clean up table (remove old unused entries)18 ARP in the Protocol “Stack” Physical
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