Internetworking Terms. Internet Structure. Internet Structure. Chapter 15&16 Internetworking. Internetwork Structure & Terms

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Eugene Jackson
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1 Chapter 15&16 Internetworking Internetwork Structure & Terms Internetworking Architecture Features Connection/Connectionless Architecture Fragmentation & Reassembly Internet Protocol & Services Addressing Subnetting Routing Protocols in An internet Internetworking Terms 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) Internet Structure Internet Structure Recent Past (1990) End user Today Service provider networks Stanford NSFNET backbone ISU Large corporation Consumer ISP BARRNET regional Berkeley PARC NCAR Westnet regional UNM UNL MidNet regional KU Peering point Consumer ISP Backbone service provider Peering point Service Provider UA AS (autonomous system): each with its own idea of routing and metrics defining. An AS is administered independently. Small corporation Large corporation Consumer ISP

2 Internetworking Protocols in TCP/ Suite Requirements of Internetworking Link between networks: Minimum physical and link layer 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 Routing and delivery of data between processes on different networks Accounting services and status info Independent of constituting network architectures Architectural Approaches Connectionless Internetworking 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 Known generically as the internet protocol Internet Protocol (RFC 791 -> IETF) One such internet protocol developed for ARPANET Lower layer protocol needed to access particular network 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

3 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 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 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 Need to know how long since last router Fragmentation and Reassembly Example Each network has some MTU (Maximum Transmission Unit) e.g., Ethernet:1500B; FDDI:4500B, : 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 R1 R2 R3 H8 (1400) FDD(1400) PPP(512) PPP(512) (512) (512) PPP(376) (376) Ident= x 0 Offset= data bytes Ident= x 1 Offset= data bytes H1 H8 Ident= x 1 Offset= 512 TCP R1 R2 R3 TCP Note: Offset field counts 8-byte units of data, not individual bytes 512 data bytes FDDI FDDI PPP PPP Ident= x 0 Offset= data bytes

Error Control Not guaranteed delivery Error & Flow Control Router should attempt to inform source if packet discarded Source may modify transmission strategy after the discard May inform high layer

4 Error Control Not guaranteed delivery Error & Flow Control 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 Internet Protocol () Part of TCP/: Used by the Internet Specifies interface with higher layer: e.g. TCP Specifies protocol format and mechanisms 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 Services Parameters (Con t) Header Time to live (TTL): Send only Data length Option data : options requested by the 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 Version: Currently 4 v6 next generation Internet header length (IHL): In 32 bit words 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

5 Properties globally unique hierarchical: network + host Dot Notation Global Addresses Note: It is more precise to think of address as belonging to interfaces than to hosts A: B: C: H Network Network Host Network Host 21 8 Host Class D (start 1110) address specify a multicast group Class E (start 1111): reserved for future use H1 H4 H2 Network 2 (Ethernet) R1 H3 Network 3 (FDDI) H6 R2 Network 1 (Ethernet) H7 R3 H8 Network 4 (point-to-point) Problem: Assigning one network # per physical network, not only used up the address space very fast, but also increase the burden of routing. Solution: Add another level to address/routing hierarchy: subnet assign a single network # and allocate the addresses with that network # to several physical networks Subnet masks define variable partition of host part Subnetting & Subnet Mask Network number Class B address Host number Subnet mask ( ) Network number Subnet ID Subnetted address Host ID Bitwise AND Subnet mask: Subnet number: H1 R Subnet Example Subnet mask: Subnet number: A host connected to this subnetwork H2 could have an address between R and H Bitwise AND of the host address & subnet mask = subnet number Subnet mask: Subnet number: A host connected to this subnetwork could have an address between and A host connected to this subnetwork could have an address between and A single class B ( *.*) address shared by several physical network Versions v 1-3 defined and replaced v4 - current version v5 - streams protocol v6 - replacement for v4 Under development it is called ng (Next Generation) Why 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

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.

6 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. 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., R (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

Internetwork Protocols

Internetwork Protocols Background to IP IP, and related protocols Internetworking Terms (1) Communications Network Facility that provides data transfer service An internet Collection of communications