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Chapter 15&16 Internetworking • Internetwork Structure & Terms • Internetworking Architecture Features Connection/Connectionless Architecture Fragmentation & Reassembly Internet Protocol & Services IP Addressing Subnetting • Routing Protocols in IP Spring, 2003 EE 4272 Internetworking Terms • An internet Collection of communications networks interconnected by bridges and/or routers • The Internet - note upper case I • • • • • The global collection of thousands of individual machines and networks Intranet: Corporate internet operating within the organization Isolated or may have links to Internet End System (ES): Device attached to one of the networks of an internet Supports end-user applications or services Intermediate System (IS): Device used to connect two networks Permits communication between end systems attached to different networks Bridge: IS used to connect two or more LANs using similar LAN protocols Address filter passing on packets to the required network only Operated at OSI layer 2 (Data Link) Router: Connects two or more (possibly dissimilar) networks Uses internet protocol present in each router and end system Operated at OSI Layer 3 (Network) Spring, 2003 EE 4272 Internet Structure Recent Past (1990) End user NSFNET backbone Stanford ISU BARRNET MidNet regional regional Westnet regional Berkeley PARC UNM NCAR UNL KU UA Service Provider AS (autonomous system): each with its own idea of routing and metrics defining. An AS is administered independently. Spring, 2003 EE 4272 Internet Structure Today Service provider networks Large corporation “Consumer ” ISP Peering point Backbone service provider “ Consumer ” ISP Large corporation Small corporation Spring, 2003 Peering point EE 4272 “Consumer ”ISP Internetworking Protocols in TCP/IP Suite • Requirements of Internetworking Link between networks: Minimum physical and link layer Routing and delivery of data between processes on different networks Accounting services and Independent of Spring, 2003 status info constituting network architectures EE 4272 Internetworking Architecture Features • Accommodate difference among networks Addressing: global network addressing must be provided Packet size -> fragmentation Timeouts: longer timeout for delivery across multiple networks Error recovery: independent to individual network error rec. cap. Status reporting Routing Connection based or connectionless Spring, 2003 EE 4272 Architectural Approaches • Connection oriented: Assume that each network is connection oriented IS connect two or more networks: IS appear as DTE to each network Logical connection set up between DTEs (Data Terminal Equipment) Concatenation of logical connections across networks Individual network virtual circuits joined by IS May require enhancement of local network services (e.g. 802 or FDDI) IS performs Relaying & Routing functions • Connectionless Corresponds to datagram mechanism in packet switched network Each PDU treated separately Network layer protocol common to all DTEs and routers Internet Protocol (RFC 791 -> IETF) Known generically as the internet protocol One such internet protocol developed for ARPANET Lower layer protocol needed to access particular network Spring, 2003 EE 4272 Connectionless Internetworking • Advantages Flexibility Robust No unnecessary overhead • Unreliable Not guaranteed delivery Not guaranteed order of delivery: Packets can take different routes Reliability is responsibility of next layer up (e.g. TCP) • Design Issues Routing Datagram lifetime Fragmentation & re-assembly Error control Flow control Spring, 2003 EE 4272 Routing • End systems & routers maintain routing tables to indicate next router to which datagram should be sent Static: May contain alternative routes Dynamic: Flexible response to congestion and errors • Source routing Source specifies route as sequential list of routers to be followed Spring, 2003 EE 4272 Datagram Lifetime • Datagrams could loop indefinitely Consumes resources Transport protocol may need upper bound on datagram life • Datagram marked with lifetime Time-To-Live (TTL) field in IP Once lifetime expires, datagram discarded (not forwarded) Hop count: a simple way to implement TTL Decrement TTL on passing through at each router True time count: global clocking mechanism needed Spring, 2003 Need to know how long since last router EE 4272 Fragmentation and Reassembly • Each network has some MTU (Maximum Transmission Unit) e.g., Ethernet:1500B; FDDI:4500B, IP: 65,535B • When to re-assemble At destination (preferred) Results in packets getting smaller as data traverses internet Intermediate re-assembly Need large buffers at routers Buffers may fill with fragments All fragments must go through same router Inhibits dynamic routing H1 H8 TCP R1 IP IP ETH Spring, 2003 R2 ETH R3 IP FDDI FDDI IP PPP EE 4272 PPP TCP IP ETH ETH Example Start of header Ident= x Offset= 0 0 Rest of header H1 R1 R2 R3 H8 1400 data bytes Start of header ETH IP (1400) FDDI IP (1400) PPP IP (512) ETH IP (512) PPP IP (512) ETH IP (512) PPP IP (376) ETH IP (376) Ident= x 1 Offset= 0 Rest of header 512 data bytes Start of header Ident= x 1 Offset= 512 Rest of header Note: Offset field counts 8-byte units of data, not individual bytes 512 data bytes Start of header Ident= x 0 Offset= 1024 Rest of header 376 data bytes Spring, 2003 EE 4272 Error & Flow Control • Error Control Not guaranteed delivery Router should attempt to inform source if packet discarded Source may modify transmission strategy after the discard May inform high layer protocol Datagram identification needed • Flow Control (? Congestion Control) Allows routers and/or stations to limit rate of incoming data The mechanism is limited in connectionless systems Send flow control packets: Requesting reduced flow Spring, 2003 EE 4272 Internet Protocol (IP) • Part of TCP/IP: Used by the Internet Specifies interface with higher layer: e.g. TCP Specifies protocol format and mechanisms • IP Services can be described by Primitives to specify functions to be performed: Implementation dependent Send: Request transmission of data unit Deliver: Notify user of arrival of data unit Parameters: Used to pass data and control info Source/Destination address Protocol: Recipient e.g. TCP Type of Service (TOS): Specify QoS of data unit during transmission through networks Identification: combined with source, destination address and user protocol Uniquely identifies PDU Needed for re-assembly and error reporting Spring, 2003 EE 4272 IP Services Parameters (Con’t) • Time to live (TTL): Send only • Data length • Option data : options requested by the IP user Security Source routing Route recording Stream identification Timestamping • User data Carries user data from next layer up Integer multiple of 8 bits long (octet) Max length of datagram (header plus data) 65,535 octets Spring, 2003 EE 4272 IP Header • Version: Currently 4 • Internet header length (IHL): In 32 bit words • • • IP v6 – next generation Including options Type of service (TOS) Total length : Of datagram, in octets Identification: Sequence number • Used with addresses and user protocol to identify datagram uniquely Flags: More bit Don’t fragment Fragmentation offset • • • Time to live (TTL) Protocol: Next higher layer to receive data field at destination Header checksum • • • • Reverified and recomputed at each router 16 bit ones complement sum of all 16 bit words in header Set to zero during calculation Source/Destination address Options Padding: To fill to multiple of 32 bits long Spring, 2003 EE 4272 Global IP Addresses • Properties globally unique hierarchical: network + host A: B: • Dot Notation 0 7 24 Network Host 1 0 14 16 Network Host 21 8 Network Host 10.3.2.4 128.96.33.81 192.12.69.77 C: 1 1 0 Class D (start 1110) address specify a multicast group Class E (start 1111): reserved for future use Network 1 (Ethernet) Note: It is more precise to think of IP address as belonging to interfaces than to hosts H7 H2 H1 Network 2 (Ethernet) H3 R1 Network 3 (FDDI) H5 Spring, 2003 EE 4272 H8 Network 4 (point-to-point) R2 H4 R3 H6 Subnetting & Subnet Mask • Problem: Assigning one network # per physical network, not only used up the IP address space very fast, but also increase the burden of routing. • Solution: Add another level to address/routing hierarchy: subnet assign a single IP network # and allocate the IP addresses with that network # to several physical networks • Subnet masks define variable partition of host part Network number Host number Bitwise AND Class B address 111111111111111111111111 00000000 Subnet mask (255.255.255.0) Network number Subnet ID Subnetted address Spring, 2003 EE 4272 Host ID Subnet Example Subnet mask: 255.255.255.128 Subnet number: 128.96.34.0 128.96.34.15 A host connected to this subnetwork could have an IP address between 128.96.34.1 and 128.96.34.127 128.96.34.1 H1 R1 Subnet mask: 255.255.255.128 Subnet number: 128.96.34.128 128.96.34.130 128.96.34.139 128.96.34.129 H2 R2 H3 128.96.33.14 A host connected to this subnetwork could have an IP address between 128.96.34.129 and 128.96.34.255 128.96.33.1 Subnet mask: 255.255.255.0 Subnet number: 128.96.33.0 Bitwise AND of the host IP address & subnet mask = subnet number A host connected to this subnetwork could have an IP address between 128.96.33.1 and 128.96.33.255 A single class B (128.96.*.*) address shared by several physical network Spring, 2003 EE 4272 IP Versions • • • • IP v 1-3 defined and replaced IP v4 - current version IP v5 - streams protocol IP v6 - replacement for IP v4 Under development it is called IPng (Next Generation) • Why IP v6 Address space exhaustion Two level addressing (network and host) wastes space Growth of networks and the Internet Single address per host Requirements for new types of service Spring, 2003 EE 4272 Autonomous Systems (AS) • Set of routers and networks managed by single organization • Group of routers exchange information • Each AS with its own idea of routing and metrics defining. An AS is administered independently. Spring, 2003 EE 4272 Routing Protocols • Routing Information About topology and delays in the internet • Routing Algorithm Used to make routing decisions based on information • Interior Router Protocol: Passes routing information between routers within AS Routing algorithms and tables may differ between different AS IRP needs detailed model e.g., RIP (using Bellman-Ford algorithm) e.g., OSPF ( using Dijkstra’s algorithm) • Exterior router protocol (ERP): Routers need some info about networks outside their AS: e.g. BGP in Internet supports summary information on reachability Spring, 2003 EE 4272