Network Layer 4 1 Introduction 4 2 Virtual

Network Layer • 4. 1 Introduction • 4. 2 Virtual circuit and datagram networks • 4. 3 What’s inside a router • 4. 4 IP: Internet Protocol – – Datagram format IPv 4 addressing ICMP IPv 6 • 4. 5 Routing algorithms – Link state – Distance Vector – Hierarchical routing • 4. 6 Routing in the Internet – RIP – OSPF – BGP • 4. 7 Broadcast and multicast routing Network Layer 4 -1

Network layer • transport segment from sending to receiving host • on sending side encapsulates segments into datagrams • on rcving side, delivers segments to transport layer • network layer protocols in every host, router • Router examines header fields in all IP datagrams passing through it application transport network data link physical Network Layer network data link physical network data link physical application transport network data link physical 4 -2

Key Network-Layer Functions • forwarding: move packets from router’s input to appropriate router output • routing: determine route taken by packets from source to dest. analogy: r routing: process of planning trip from source to dest r forwarding: process of getting through single interchange – Routing algorithms Network Layer 4 -3

Interplay between routing and forwarding routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 value in arriving packet’s header 0111 1 3 2 Network Layer 4 -4

Connection setup • 3 rd important function in some network architectures: – ATM, frame relay, X. 25 • Before datagrams flow, two hosts and intervening routers establish virtual connection – Routers get involved • Network and transport layer cnctn service: – Network: between two hosts – Transport: between two processes Network Layer 4 -5

Network service model Q: What service model for “channel” transporting datagrams from sender to rcvr? Example services for individual datagrams: • guaranteed delivery • Guaranteed delivery bounded delay Example services for a flow of datagrams: • In-order datagram delivery • Guaranteed minimum bandwidth to flow • Guaranteed minimum jitter • Security services Network Layer 4 -6

Network layer service models: Network Architecture Internet Service Model Guarantees ? Congestion Bandwidth Loss Order Timing feedback best effort none ATM CBR ATM VBR ATM ABR ATM UBR constant rate guaranteed minimum none no no no yes yes yes no no (inferred via loss) no congestion yes no no Network Layer 4 -7

Network Layer • 4. 1 Introduction • 4. 2 Virtual circuit and datagram networks • 4. 3 What’s inside a router • 4. 4 IP: Internet Protocol – – Datagram format IPv 4 addressing ICMP IPv 6 • 4. 5 Routing algorithms – Link state – Distance Vector – Hierarchical routing • 4. 6 Routing in the Internet – RIP – OSPF – BGP • 4. 7 Broadcast and multicast routing Network Layer 4 -8

Network layer connection and connection-less service • Datagram network provides network-layer connectionless service • VC network provides network-layer connection service • Analogous to the transport-layer services, but: – Service: host-to-host – No choice: network provides one or the other – Implementation: in the core Network Layer 4 -9

Virtual circuits “source-to-dest path behaves much like telephone circuit” – performance-wise – network actions along source-to-dest path • call setup, teardown for each call before data can flow • each packet carries VC identifier (not destination host address) • every router on source-dest path maintains “state” for each passing connection • link, router resources (bandwidth, buffers) may be allocated to VC Network Layer 4 -10

VC implementation A VC consists of: 1. Path from source to destination 2. VC numbers, one number for each link along path 3. Entries in forwarding tables in routers along path • Packet belonging to VC carries a VC number. • VC number must be changed on each link. – New VC number comes from forwarding table Network Layer 4 -11

Forwarding table. VC number 22 12 1 2 32 3 interface number Incoming interface Incoming VC # 1 2 3 1 … Outgoing interface Outgoing VC # 12 63 7 97 … 2 1 2 3 … 22 18 17 87 … Routers maintain connection state information! Network Layer 4 -12

Virtual circuits: signaling protocols • used to setup, maintain teardown VC • used in ATM, frame-relay, X. 25 • not used in today’s Internet application 5. Data flow begins transport network 4. Call connected 1. Initiate call data link physical application transport 3. Accept call network 2. incoming call data link physical 6. Receive data Network Layer 4 -13

Datagram networks • no call setup at network layer • routers: no state about end-to-end connections – no network-level concept of “connection” • packets forwarded using destination host address – packets between same source-dest pair may take different paths application transport network data link physical application transport network 2. Receive data link physical 1. Send data Network Layer 4 -14

Datagram or VC network: why? Internet ATM • data exchange among computers • evolved from telephony – “elastic” service, no strict • human conversation: timing req. – strict timing, reliability • “smart” end systems (computers) requirements – can adapt, perform control, – need for guaranteed service error recovery • “dumb” end systems – simple inside network, – telephones complexity at “edge” – complexity inside network • many link types – different characteristics – uniform service difficult Network Layer 4 -15

Network Layer • 4. 1 Introduction • 4. 2 Virtual circuit and datagram networks • 4. 3 What’s inside a router • 4. 4 IP: Internet Protocol – – Datagram format IPv 4 addressing ICMP IPv 6 • 4. 5 Routing algorithms – Link state – Distance Vector – Hierarchical routing • 4. 6 Routing in the Internet – RIP – OSPF – BGP • 4. 7 Broadcast and multicast routing Network Layer 4 -16

Router Architecture Overview Two key router functions: • • run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link Network Layer 4 -17

Input Port Functions Physical layer: bit-level reception Data link layer: e. g. , Ethernet Decentralized switching: • given datagram dest. , lookup output port using forwarding table in input port memory • goal: complete input port processing at ‘line speed’ • queuing: if datagrams arrive faster than forwarding rate into switch fabric Network Layer 4 -18

Three types of switching fabrics Network Layer 4 -19

Switching Via Memory First generation routers: • traditional computers with switching under direct control of CPU • packet copied to system’s memory • speed limited by memory bandwidth (2 bus crossings per datagram) Input Port Memory Output Port System Bus Network Layer 4 -20

Switching Via a Bus • datagram from input port memory to output port memory via a shared bus • bus contention: switching speed limited by bus bandwidth • 1 Gbps bus, Cisco 1900: sufficient speed for access and enterprise routers (not regional or backbone) Network Layer 4 -21

Switching Via An Interconnection Network • overcome bus bandwidth limitations • Banyan networks, other interconnection nets initially developed to connect processors in multiprocessor • Advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. • Cisco 12000: switches Gbps through the interconnection network Network Layer 4 -22

Output Ports • • Buffering required when datagrams arrive from fabric faster than the transmission rate Scheduling discipline chooses among queued datagrams for transmission Network Layer 4 -23

Output port queueing • buffering when arrival rate via switch exceeds output line speed • queueing (delay) and loss due to output port buffer overflow! Network Layer 4 -24

Input Port Queuing • Fabric slower than input ports combined -> queueing may occur at input queues • Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward • queueing delay and loss due to input buffer overflow! Network Layer 4 -25

Network Layer • 4. 1 Introduction • 4. 2 Virtual circuit and datagram networks • 4. 3 What’s inside a router • 4. 4 IP: Internet Protocol – – Datagram format IPv 4 addressing ICMP IPv 6 • 4. 5 Routing algorithms – Link state – Distance Vector – Hierarchical routing • 4. 6 Routing in the Internet – RIP – OSPF – BGP • 4. 7 Broadcast and multicast routing Network Layer 4 -26

The Internet Network layer Host, router network layer functions: Transport layer: TCP, UDP Network layer IP protocol • addressing conventions • datagram format • packet handling conventions Routing protocols • path selection • RIP, OSPF, BGP forwarding table ICMP protocol • error reporting • router “signaling” Link layer physical layer Network Layer 4 -27

Network Layer • 4. 1 Introduction • 4. 2 Virtual circuit and datagram networks • 4. 3 What’s inside a router • 4. 4 IP: Internet Protocol – – Datagram format IPv 4 addressing ICMP IPv 6 • 4. 5 Routing algorithms – Link state – Distance Vector – Hierarchical routing • 4. 6 Routing in the Internet – RIP – OSPF – BGP • 4. 7 Broadcast and multicast routing Network Layer 4 -28

IP datagram format IP protocol version number header length (bytes) “type” of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to how much overhead with TCP? r 20 bytes of TCP r 20 bytes of IP r = 40 bytes + app layer overhead 32 bits ver head. type of len service 16 -bit identifier upper time to layer live length fragment flgs offset Internet checksum total datagram length (bytes) for fragmentation/ reassembly 32 bit source IP address 32 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) Network Layer E. g. timestamp, record route taken, specify list of routers to visit. 4 -29

IP Fragmentation & Reassembly • • network links have MTU (max. transfer size) - largest possible link-level frame. – different link types, different MTUs large IP datagram divided (“fragmented”) within net – one datagram becomes several datagrams – “reassembled” only at final destination – IP header bits used to identify, order related fragments fragmentation: in: one large datagram out: 3 smaller datagrams reassembly Network Layer 4 -30

IP Fragmentation and Reassembly Example r 4000 byte datagram r MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 length ID =4000 =x fragflag =0 offset =0 One large datagram becomes several smaller datagrams length ID =1500 =x fragflag =1 offset =0 length ID =1500 =x fragflag =1 offset =185 length ID =1040 =x fragflag =0 offset =370 Network Layer 4 -31

Network Layer • 4. 1 Introduction • 4. 2 Virtual circuit and datagram networks • 4. 3 What’s inside a router • 4. 4 IP: Internet Protocol – – Datagram format IPv 4 addressing ICMP IPv 6 • 4. 5 Routing algorithms – Link state – Distance Vector – Hierarchical routing • 4. 6 Routing in the Internet – RIP – OSPF – BGP • 4. 7 Broadcast and multicast routing Network Layer 4 -32

IP Addressing: introduction • IP address: 32 -bit identifier for host, router interface • interface: connection between host/router and physical link – router’s typically have multiple interfaces – host may have multiple interfaces – IP addresses associated with each interface 223. 1. 1. 1 223. 1. 1. 2 223. 1. 1. 4 223. 1. 1. 3 223. 1. 2. 1 223. 1. 2. 9 223. 1. 3. 27 223. 1. 2. 2 223. 1. 1. 1 = 11011111 00000001 223 Network Layer 1 1 1 4 -33

Subnets 223. 1. 1. 1 • IP address: – subnet part (high order bits) – host part (low order bits) • What’s a subnet ? – device interfaces with same subnet part of IP address – can physically reach other without intervening router 223. 1. 1. 2 223. 1. 1. 4 223. 1. 1. 3 223. 1. 2. 1 223. 1. 2. 9 223. 1. 2. 2 223. 1. 3. 27 LAN 223. 1. 3. 2 network consisting of 3 subnets Network Layer 4 -34

Subnets 223. 1. 1. 0/24 223. 1. 2. 0/24 Recipe • To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. 223. 1. 3. 0/24 Subnet mask: /24 Network Layer 4 -35

Subnets How many? 223. 1. 1. 2 223. 1. 1. 1 223. 1. 1. 4 223. 1. 1. 3 223. 1. 9. 2 223. 1. 7. 0 223. 1. 9. 1 223. 1. 7. 1 223. 1. 8. 0 223. 1. 2. 6 223. 1. 2. 1 223. 1. 3. 27 223. 1. 2. 2 Network Layer 223. 1. 3. 2 4 -36

IP addressing: CIDR: Classless Inter. Domain Routing – subnet portion of address of arbitrary length – address format: a. b. c. d/x, where x is # bits in subnet portion of address host part subnet part 11001000 00010111 00010000 200. 23. 16. 0/23 Network Layer 4 -37

IP addresses: how to get one? Q: How does host get IP address? • • hard-coded by system admin in a file – Wintel: control-panel->network->configuration->tcp/ip>properties – UNIX: /etc/rc. config DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server – “plug-and-play” (more in next chapter) Network Layer 4 -38

IP addresses: how to get one? Q: How does network get subnet part of IP addr? A: gets allocated portion of its provider ISP’s address space ISP's block 11001000 00010111 00010000 200. 23. 16. 0/20 Organization 1 Organization 2. . . 11001000 00010111 00010000 11001000 00010111 00010010 0000 11001000 00010111 00010100 0000 …. 200. 23. 16. 0/23 200. 23. 18. 0/23 200. 23. 20. 0/23 …. Organization 7 11001000 00010111 00011110 0000 200. 23. 30. 0/23 Network Layer 4 -39

Hierarchical addressing: route aggregation Hierarchical addressing allows efficient advertisement of routing information: Organization 0 200. 23. 16. 0/23 Organization 1 200. 23. 18. 0/23 Organization 2 200. 23. 20. 0/23 Organization 7 . . . Fly-By-Night-ISP “Send me anything with addresses beginning 200. 23. 16. 0/20” Internet 200. 23. 30. 0/23 ISPs-R-Us Network Layer “Send me anything with addresses beginning 199. 31. 0. 0/16” 4 -40

Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization 0 200. 23. 16. 0/23 Organization 2 200. 23. 20. 0/23 Organization 7 . . . Fly-By-Night-ISP “Send me anything with addresses beginning 200. 23. 16. 0/20” Internet 200. 23. 30. 0/23 ISPs-R-Us Organization 1 200. 23. 18. 0/23 Network Layer “Send me anything with addresses beginning 199. 31. 0. 0/16 or 200. 23. 18. 0/23” 4 -41

IP addressing: the last word. . . Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers – allocates addresses – manages DNS – assigns domain names, resolves disputes Network Layer 4 -42

NAT: Network Address Translation rest of Internet local network (e. g. , home network) 10. 0. 0/24 10. 0. 0. 1 10. 0. 0. 2 138. 76. 29. 7 10. 0. 0. 3 All datagrams leaving local network have same single source NAT IP address: 138. 76. 29. 7, different source port numbers Datagrams with source or destination in this network have 10. 0. 0/24 address for source, destination (as usual) Network Layer 4 -43

NAT: Network Address Translation • Motivation: local network uses just one IP address as far as outside word is concerned: – no need to be allocated range of addresses from ISP: - just one IP address is used for all devices – can change addresses of devices in local network without notifying outside world – can change ISP without changing addresses of devices in local network – devices inside local net not explicitly addressable, visible by outside world (a security plus). Network Layer 4 -44

NAT: Network Address Translation Implementation: NAT router must: – outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #). . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr. – remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair – incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table Network Layer 4 -45

NAT: Network Address Translation NAT translation table WAN side addr LAN side addr 2: NAT router changes datagram source addr from 10. 0. 0. 1, 3345 to 138. 76. 29. 7, 5001, updates table 1: host 10. 0. 0. 1 sends datagram to 128. 119. 40, 80 138. 76. 29. 7, 5001 10. 0. 0. 1, 3345 …… …… S: 10. 0. 0. 1, 3345 D: 128. 119. 40. 186, 80 2 S: 138. 76. 29. 7, 5001 D: 128. 119. 40. 186, 80 138. 76. 29. 7 S: 128. 119. 40. 186, 80 D: 138. 76. 29. 7, 5001 3: Reply arrives dest. address: 138. 76. 29. 7, 5001 3 1 10. 0. 0. 4 S: 128. 119. 40. 186, 80 D: 10. 0. 0. 1, 3345 10. 0. 0. 1 10. 0. 0. 2 4 10. 0. 0. 3 4: NAT router changes datagram dest addr from 138. 76. 29. 7, 5001 to 10. 0. 0. 1, 3345 Network Layer 4 -46

NAT: Network Address Translation • 16 -bit port-number field: – 60, 000 simultaneous connections with a single LAN-side address! • NAT is controversial: – routers should only process up to layer 3 – violates end-to-end argument • NAT possibility must be taken into account by app designers, eg, P 2 P applications – address shortage should instead be solved by IPv 6 Network Layer 4 -47