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Chapter 4 Network Layer Computer Networking A Top-Down Approach 6th edition These slides are based on the slides made available by Kurose and Ross. Jim Kurose, Keith Ross © All material copyright 1996-2012 Addison-Wesley J.F Kurose and K.W. Ross, All Rights Reserved March 2012 Network Layer 4-1 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-2 Network Layer Functions network layer protocol in every host and router Consider transporting a segment from sender to receiver sending side: encapsulates segments into datagrams receiving side: delivers segments to transport layer application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical Path Determination: sum of routes chosen by routers to deliver packets from source to destination. Forwarding: move packets from router’s input to appropriate router’s output network data link physical network data link physical network data link physical network data link physical network data link physical Network Layer application transport network data link physical 4-3 Routing and Forwarding routing algorithm routing algorithm determines path through network local forwarding table header value output link forwarding table determines local forwarding at this router 0100 0101 0111 1001 3 2 2 1 value in arriving packet’s header 0111 1 3 2 router examines header fields in all datagrams passing through it Network Layer 4-4 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-5 Network Service Model What service model can be considered for a network transporting packets from sender to receiver? example services for individual datagrams: example services for a flow of packets: “best effort” delivery in-order delivery No constraints on delay or guaranteed minimum bandwidth bandwidth to flow restrictions on changes in inter-packet timespacing Network Layer 4-6 Connection-oriented & connectionless Virtual Circuit-network provides link or network-layer connection-oriented service. Datagram-based network provides networklayer connectionless service. Analogous to the transport-layer services but: Service: host-to-host packet delivery Implementation: every router in the network Network Layer 4-7 Virtual Circuit: VC source-to-destination path behaves much like telephone “circuit” Performance-wise (but it is virtual circuit) Network actions along the source-to-destination path Setup: for each connection before data packets can flow Each packet carries VC identifier (not destination address) Every router on the path maintains “state” for each passing connection. Benefit: Link & router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service) Network Layer 4-8 VC: Signaling Protocols used to setup, maintain and teardown VC used in ATM, Frame-Relay, X.25 not used in today’s Internet on network layer application transport network data link physical 5. data flow begins 4. call connected 1. initiate call application transport 3. accept call network 2. incoming call data link physical 6. receive data Network Layer 4-9 VC: Forwarding Table VC identifier 22 12 1 Forwarding table in northwest router: 2 32 3 interface number Incoming interface 1 2 3 1 … Incoming VC # 12 63 7 97 … Outgoing interface 3 1 2 3 Outgoing VC # 22 18 17 87 … … Routers maintain connection state information! Network Layer 4-10 Datagram Networks (Internet) no call setup to establish path through network routers: no state about end-to-end connections no network-level concept of “connection” packets forwarded using destination host address packets between same source-destination pair may take different paths application transport network 1. send datagrams data link physical application transport 2. receive datagrams network data link physical Network Layer 4-11 Datagram: Forwarding Table routing algorithm local forwarding table dest. address output link address-range 1 address-range 2 address-range 3 address-range 4 4 billion IP addresses, so rather than list individual destination address list range of addresses (aggregate table entries) 3 2 2 1 IP destination address in arriving packet’s header 1 3 2 Network Layer 4-12 Datagram or VC network: why? Internet (datagram) ATM (VC) data exchange among more complicated computers evolved from telephony “elastic” service, no strict human conversation: timing requirements. strict timing, reliability “smart” end systems requirements (computers) need for guaranteed can adapt, perform control, service error recovery “dumb” end systems simple inside network, telephones complexity at “edge” moves complexity to many link types inside network different characteristics uniform service difficult Network Layer 4-13 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-14 Router Architecture: Overview Two key router functions: run routing algorithms/protocols (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link forwarding tables computed, pushed to input ports routing processor routing, management control plane (software) forwarding packets plane (hardware) high-seed switching fabric router input ports router output ports Network Layer 4-15 Input Port Functions Physical layer: bit-level reception Data link layer: e.g., Ethernet Decentralized switching: given datagram destination address, 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 4-16 Three types of switching fabrics transfer packet from input buffer to appropriate output buffer switching rate: rate at which packets can be transferred from inputs to outputs often measured as multiple of input/output line rates N inputs: switching rate N times line rate is desirable Network Layer 4-17 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 (e.g., Ethernet) memory output port (e.g., Ethernet) system bus Network Layer 4-18 Switching via Bus datagram from input port memory bus to output port memory via a shared bus, one packet at a time bus contention: switching speed limited by bus bandwidth 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers Network Layer 4-19 Switching via Interconnection Network overcome bus bandwidth limitations crossbar banyan networks, crossbar, other interconnection networks initially developed to connect processors in multiprocessor advanced design: fragmenting datagram into fixed length cells, tag and switch cells through the fabric. Cisco 12000: switches 60 Gbps through the interconnection network Network Layer 4-20 Output Ports switch fabric datagram buffer queueing link layer protocol (send) line termination Buffering required when datagrams arrive from fabric faster than the transmission rate of the outgoing link Scheduling discipline chooses among queued datagrams for transmission Network Layer 4-21 Output Port Queueing switch fabric at t, packets more from input to output switch fabric one packet time later buffering when arrival rate via switch exceeds output line speed delay due to queueing and loss due to output port buffer overflow! Network Layer 4-22 Input Port Queuing fabric slower (seldom!) than input ports combined → queueing may occur at input port queueing delay and loss due to input buffer overflow! Head-Of-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward switch fabric output port contention: only one red datagram can be transferred. lower red packet is blocked switch fabric one packet time later: green packet experiences HOL blocking Network Layer 4-23 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-24 The Internet Network Layer Host, router network layer functions: Transport layer: TCP, UDP Network layer Routing protocols • path selection • RIP/UDP, OSPF/IP, BGP/TCP forwarding table ICMP protocol • error reporting • router “signaling” IP protocol • addressing conventions • datagram format • packet handling conventions Link layer Physical layer Network Layer 4-25 IP datagram format 32 bits IP protocol version = 4 header length 32-bits blocks, 5 standard) TOS (priority) TTL: max number of remaining hops (decremented by one at each router) Upper layer protocol to deliver payload to 6 for TCP 17 for UDP how much overhead? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead ver head. type of length service total datagram length (bytes) length fragment 16-bit identifier flags offset time to Protocol Header Checksum live 32-bit source IP address 32-bit destination IP address Options (if any) Data Field Variable length (typically a TCP or UDP segment) for fragmentation/ reassembly Flags (3 bits): Reserved (0) DF= don’t frag. MF= more frag. e.g. timestamp, security label, record route taken, specify list of routers to visit, etc Network Layer 4-26 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-27 IP Addressing: Introduction 223.1.1.1 interface: connection between host/router and physical link 223.1.2.1 223.1.1.2 routers typically have multiple active interfaces hosts typically have one active interface (either wired Ethernet or wireless 802.11) IP address associated with each interface IP address: 32-bit identifier for host, router interface 223.1.1.4 223.1.2.9 223.1.3.27 223.1.1.3 223.1.2.2 223.1.3.1 223.1.3.2 223.1.3.1 = 11011111 00000001 00000011 00000001 Dotted Decimal Notation 223 1 3 1 Network Layer 4-28 Subnets 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 Contains hosts that can physically reach each other without intervening router All other hosts are reached by sending datagrams to router interface that works as “default gateway” 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.3.27 223.1.1.3 223.1.2.2 subnet 223.1.3.1 223.1.3.2 network consisting of 3 subnets Network Layer 4-29 Subnets Subnet 1 223.1.1.0/24 How long should the Subnet 2 223.1.2.0/24 network prefix be? Depends on number of hosts on subnet All hosts in subnet have same subnetwork part of the address. Subnet 3 Typical info given to a host: 223.1.3.0/24 Your address is 223.1.3.1/24 Default route via 223.1.3.27 Subnet mask: /24 24 bits belong to the network (called length of “CIDR” prefix) Network Layer 4-30 Subnets 223.1.1.2 How many? 223.1.1.1 223.1.1.4 223.1.1.3 223.1.9.2 223.1.7.0 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 223.1.2.1 223.1.3.27 223.1.2.2 223.1.3.1 223.1.3.2 Network Layer 4-31 IP Addressing: CIDR 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 subnet part host part 11001000 00010111 00010000 00000000 200.23.16.0/23 Network Layer 4-32 Subnets, masks, calculations Example subnet: 192.168.5.0/24 Binary form Dot-decimal notation IP address 11000000.10101000.00000101.10000010 192.168.5.130 Subnet mask 11111111.11111111.11111111.00000000 24 higher order bits set to 1 255.255.255.0 Network prefix: (bitwise AND 11000000.10101000.00000101.00000000 192.168.5.0 of address, mask) Host part (similar calculation, with eg a ”wild card” where the 32 – 24 lower order bits set to 1) 00000000.00000000.00000000.10000010 0.0.0.130 Network Layer 4-33 IP Addressing: Q: How does an ISP get block of addresses? A: ICANN: http://www.icann.org/ Internet Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes These services were originally performed under U.S. Government contract by the Internet Assigned Numbers Authority (IANA) and other entities. The IANA now is part of ICANN. Network Layer 4-34 IP Address Allocation: ICANN is responsible for global coordination of the Internet Protocol addressing systems and other naming and numbering standards. Users are assigned IP addresses by Internet Service Providers (ISPs). ISPs obtain allocations of IP addresses from a Local Internet Registry (LIR) or National Internet Registry (NIR), or from their appropriate Regional Internet Registry (RIR). There are five RIRs : AfriNIC, Africa APNIC, Asia Pacific ARIN, Canada, United States, Caribbean and North Atlantic Islands LACNIC, Latin America and parts of the Caribbean region RIPE NCC, Europe, Russia, Middle East, and Parts of Central Asia (NIC Network Information Center) Network NetworkLayer Layer 4-35 IP addresses: How to get one? Network (subnet) addresses are allocated from a portion of its provider ISP’s address space. 20 3 9 bits bits bits ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 Organization 1 Organization 2 ... 11001000 00010111 00010000 00000000 11001000 00010111 00010010 00000000 11001000 00010111 00010100 00000000 ….. …. 200.23.16.0/23 200.23.18.0/23 200.23.20.0/23 …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23 Network Layer 4-36 Hierarchical Addressing: Route Aggregation Hierarchical addressing allows efficient advertisement of routing information The “outside” does not need to know about subnets. Organization 0 200.23.16.0/23 Organization 1 200.23.18.0/23 Organization 2 200.23.20.0/23 Organization 7 . . . . . . ISP #1 “Send me anything with addresses beginning 200.23.16.0/20” Internet 200.23.30.0/23 ISP #2 “Send me anything with addresses beginning 199.31.0.0/16” Network Layer 4-37 Classless Address: example An ISP has an address block 122.211.0.0/16 A customer needs max. 6 host addresses, ISP can e.g. allocate: 122.211.176.208/29 3 bits enough for host part subnet mask 255.255.255.248 Dotted Decimal Last 8 bits Network 122.211.176.208 11010000 1st address 122.211.176.209 11010001 …………. ………………… ………… 6th address 122.211.176.214 11010110 Broadcast 122.211.176.215 11010111 2013 Ali Salehson, Chalmers, CSE Networks and Systems Reserved Network Layer 4-38 CIDR Address Mask CIDR Notation Dotted Decimal CIDR Notation Dotted Decimal /1 /2 /3 /4 /5 /6 /7 /8 /9 /10 /11 /12 /13 /14 /15 /16 128.0.0.0 192.0.0.0 224.0.0.0 240.0.0.0 248.0.0.0 252.0.0.0 254.0.0.0 255.0.0.0 255.128.0.0 255.192.0.0 255.224.0.0 255.240.0.0 255.248.0.0 255.252.0.0 255.254.0.0 255.255.0.0 /17 /18 /19 /20 /21 /22 /23 /24 /25 /26 /27 /28 /29 /30 /31 /32 255.255.128.0 255.255.192.0 255.255.224.0 255.255.240.0 255.255.248.0 255.255.252.0 255.255.254.0 255.255.255.0 255.255.255.128 255.255.255.192 255.255.255.224 255.255.255.240 255.255.255.248 255.255.255.252 255.255.255.254 255.255.255.255 2013 Ali Salehson, Chalmers, CSE Networks and Systems Network Layer 4-39 Special IP Addresses Localhost and local loopback 127.0.0.1 of the reserved 127.0.0.0 (127.0.0.0/8) Private IP-addresses 10.0.0.0 – 10.255.255.255 172.16.0.0 – 172.31.255.255 192.168.0.0 – 192.168.255.255 (10.0.0.0/8) (172.16.0.0/12) (192.168.0.0/16) Link-local Addresses (stateless autoconfig) 169.254.0.0 – 169.254.255.255 (169.254.0.0/16) Network Layer 4-40 IP addresses: how to get one? Q: How does host get IP address? manually hard-coded by system admin in a file Windows: Control Panel Network Connections Local Area Connection Properties Internet Protocol (TCP/IP) Properties UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol (RFC 2131) dynamically gets address from a DHCP server Network Layer 4-41 Dynamic Host Configuration Protocol Goal: allows host to dynamically obtain its IP address from network server when it joins network. Host can renew its lease on address in use Allows reuse of addresses (only hold address while connected) Support for nomad users who want to join network (short time) DHCP overview: host broadcasts “DHCP discover” message DHCP server responds with “DHCP offer” message host requests IP address: “DHCP request” message DHCP server sends address: “DHCP ACK” message Network Layer 4-42 DHCP client-server scenario DHCP server 223.1.1.0/24 223.1.2.1 223.1.1.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 223.1.2.2 arriving DHCP client needs address in this network 223.1.2.0/24 223.1.3.2 223.1.3.1 223.1.3.0/24 Network Layer 4-43 DHCP client-server scenario DHCP server: 223.1.2.5 arriving client DHCP discover src : 0.0.0.0, port 68 dest: 255.255.255.255, port 67 Your IPaddr: 0.0.0.0 transaction ID: 654 DHCP offer src: 223.1.2.5, port 67 dest: 255.255.255.255, port 68 Your IPaddr: 223.1.2.4 transaction ID: 654 Lease time: 3600 secs DHCP request src: 0.0.0.0, port 68 dest: 255.255.255.255, port 67 Req. IPaddr: 223.1.2.4 transaction ID: 654 DHCP ACK time src: 223.1.2.5, port 67 dest: 255.255.255.255, port 68 Your IPaddr: 223.1.2.4 transaction ID: 654 Lease time: 3600 secs Network Layer 4-44 DHCP: more than an IP address DHCP can return more than just allocated IP address on subnet: address of first-hop router (default gateway) name and IP address of DNS sever network mask (indicating network portion of address) Network Layer 4-45 DHCP: example Connecting laptop needs: its IP address, subnetmask address of first-hop router address of DNS server DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy 168.1.1.1 router with built-in DHCP server DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.3 Ethernet MAC frame Ethernet frame broadcast (FFFFFFFFFFFF) on LAN, received at router running DHCP server Network Layer 4-46 DHCP: example DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, IP address of DNS server DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy router with built-in DHCP server encapsulation of DHCP server, frame forwarded to client client now knows its IP address, IP address of DNS server, IP address of its first-hop router Network Layer 4-47 NAT: Network Address Translation Router with NAT can translate network addresses Many internal (private) addresses translated to one (or few) external (global) addresses. Gives freedom when configuring internal network fewer addresses needed from ISP or just one IP global address 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 can hide internal structure (devices not visible by outside world, a security plus) Internal network should use non-routable (private) addresses reserved for this purpose (RFC 1918) 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16 Network Layer 4-48 NAT: Network Address Translation rest of Internet local network (e.g., home network) 10.0.0.0/24 10.0.0.4 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 or destination (as usual) Network Layer 4-49 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 address 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 4-50 NAT: Network Address Translation 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table NAT translation table WAN side addr LAN side addr 1: host 10.0.0.1 sends datagram to 128.119.40.186, 80 138.76.29.7, 5001 10.0.0.1, 3345 …… …… S: 10.0.0.1, 3345 D: 128.119.40.186, 80 10.0.0.1 1 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 10.0.0.4 S: 128.119.40.186, 80 D: 10.0.0.1, 3345 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 4-51 NAT: Network Address Translation 16-bit port-number field: 65,000 simultaneous connections with a single WAN-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 application designers, e.g., P2P applications address shortage should instead be solved by IPv6 …. Network Layer 4-52 NAT: Traversal Problem client wants to connect to server server with address 10.0.0.1 server address 10.0.0.1 local to LAN (client can’t use it as destination addr) only one externally visible NATed address: 138.76.29.7 solution1: statically configure 10.0.0.1 client ? 10.0.0.4 138.76.29.7 NAT router NAT to forward incoming connection requests at given port to server e.g., (123.76.29.7, port 2500) always forwarded to 10.0.0.1 port 2500 Network Layer 4-53 NAT: Traversal Problem solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATed host to: learn public IP address (138.76.29.7) add/remove port mappings (with lease times) 10.0.0.1 IGD 138.76.29.7 NAT router i.e., automate static NAT port map configuration Network Layer 4-54 NAT: Traversal Problem solution 3: relaying (used in p2p) NATed host establishes connection to relay external client connects to relay relay bridges packets between two connections 2. connection to relay initiated by client client 3. relaying established 1. connection to relay initiated by NATed host 138.76.29.7 10.0.0.1 NAT router Network Layer 4-55 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-56 ICMP: Internet Control Message Protocol Control and error messages from network layer. All IP implementations must have ICMP support. ICMP messages carried in IP datagrams used by hosts & routers to communicate network-level control information and error reporting Error reporting: e.g., unreachable network, host, .. Example: (used by ping command) • Sends ICMP echo request • Receives ICMP echo reply Any ICMP error message may never generate a new one. Network Layer 4-57 ICMP: message format ICMP message: type field: 1 byte code field: 1 byte Checksum: 2 bytes 0s, (ID + Seq. #) or other fields: 4 bytes Optional data or when error reporting message always include header of IP datagram causing error plus first 8 bytes of its payload Type Code description 0 0 echo reply (ping) 3 3 3 3 3 3 0 1 2 3 6 7 dest. network unreachable dest. host unreachable dest. protocol unreachable dest. port unreachable dest. network unknown dest. host unknown 4 0 source quench 8 0 echo request (ping) 9 10 11 12 0 0 0 0 route advertisement router discovery TTL expired bad IP header Network Layer 4-58 Traceroute and ICMP Source sends series of UDP segments to destination First has TTL =1 Second has TTL=2, etc. Unlikely port number When datagram sent with TTL = n arrives to n:th router: TTL becomes 0 Router discards datagram Router sends to source an ICMP message “TTL expired” (type 11, code 0) Message is carried in IP datagram with the router IP address as source When ICMP message arrives, source measures RTT Traceroute does this 3 times Stop criteria UDP segment eventually arrives at destination host Destination returns ICMP message “destination port unreachable” (type 3, code 3) When source gets this ICMP 3 times, traceroute stops. Network Layer 4-59 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-60 IPv6: motivation initial motivation: 32-bit address space was about to be completely allocated. additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed 128-bit addresses (2128 = 1038 numbers) Standard subnet size: 64 bits Network Layer 4-61 IPv6 datagram format priority: identify priority among datagrams in flow flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined). ver pri flow label payload length next header hop limit source address (128 bits) destination address (128 bits) data 32 bits Network Layer 4-62 Other changes from IPv4 checksum: removed entirely to reduce processing time at each hop options: allowed, but outside of header, indicated by “Next Header” field ICMPv6: new version of ICMP additional message types, e.g. “Packet Too Big” Neighbor and router discovery multicast group management functions Network Layer 4-63 More slides IPv4 Fragmentation Datagram Forwarding Table Getting a datagram from source to destination IPv6-IPv4 Tunneling Network Layer 4-64 IP Fragmentation & Reassembly fragmentation: MTU (Maximum Transmission Unit) reassembly … • More Fragments bit • Datagram ID • Fragment Offset (in 8-byte units) in: one large datagram out: 3 smaller datagrams … largest possible data amount carried by link-level frame. different link types, different MTUs large IP datagrams will be divided (“fragmented”) by host or router one datagram becomes several datagrams “reassembled” only at final destination IP header fields used to identify, order related fragments Network Layer 4-65 IP Fragmentation Example 4000 bytes datagram MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 length ID fragflag =4000 =x =0 offset =0 One large datagram becomes several smaller datagrams length ID fragflag =1500 =x =1 offset =0 length ID fragflag =1500 =x =1 offset =185 length ID fragflag =1040 =x =0 offset =370 Network Layer 4-66 Datagram forwarding table Destination Address Range Link Interface 11001000 00010111 00010000 00000000 through 11001000 00010111 00010111 11111111 0 11001000 00010111 00011000 00000000 through 11001000 00010111 00011000 11111111 1 11001000 00010111 00011001 00000000 through 11001000 00010111 00011111 11111111 2 otherwise 3 Q: but what happens if ranges don’t divide up nicely? Network Layer 4-67 Longest prefix matching longest prefix matching when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address (more on this coming soon) Destination Address Range Link interface 11001000 00010111 00010*** ********* 0 11001000 00010111 00011000 ********* 1 11001000 00010111 00011*** ********* 2 otherwise 3 examples: DA: 11001000 00010111 00010110 10100001 DA: 11001000 00010111 00011000 10101010 which interface? which interface? Network Layer 4-68 Getting a datagram from source to dest. forwarding table in A Dest. Net. next router Nhops 223.1.1 223.1.2 223.1.3 IP datagram: misc source dest fields IP addr IP addr data A 223.1.1.4 223.1.1.4 223.1.1.1 Payload in datagram remains unchanged, as it travels source to destination addr fields of interest here 1 2 2 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 B 223.1.1.3 223.1.3.1 223.1.3.27 223.1.2.2 E 223.1.3.2 Network Layer 4-69 Getting a datagram from source to dest. Dest. Net. next router Nhops misc data fields 223.1.1.1 223.1.1.3 223.1.1 223.1.2 223.1.3 Starting at A, given IP datagram addressed to B: look up net. address of B A 223.1.1.4 223.1.1.4 223.1.1.1 find B is on same net. as A (B and 223.1.2.1 A are directly connected) 223.1.1.2 223.1.1.4 link layer will send datagram directly to B (inside link-layer frame) 1 2 2 223.1.2.9 B 223.1.1.3 223.1.3.1 223.1.3.27 223.1.2.2 E 223.1.3.2 Network Layer 4-70 Getting a datagram from source to dest. Dest. Net. next router Nhops misc data fields 223.1.1.1 223.1.2.3 223.1.1 223.1.2 223.1.3 Starting at A, dest. E: look up network address of E E on different network A 223.1.1.4 223.1.1.4 223.1.1.1 routing table: next hop router to E is 223.1.1.4 link layer is asked to send datagram to router 223.1.1.4 (inside link-layer frame) datagram arrives at 223.1.1.4 continued….. 1 2 2 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 B 223.1.1.3 223.1.3.1 223.1.3.27 223.1.2.2 E 223.1.3.2 Network Layer 4-71 Getting a datagram from source to dest. misc data fields 223.1.1.1 223.1.2.3 Arriving at 223.1.4, destined for 223.1.2.2 look up network address of E Dest. next network router Nhops interface 223.1.1 223.1.2 223.1.3 A E on same network as router’s interface 223.1.2.9 router, E directly attached link layer sends datagram to 223.1.2.2 (inside link-layer frame) via interface 223.1.2.9 datagram arrives at 223.1.2.2!!! (hooray!) - 1 1 1 223.1.1.4 223.1.2.9 223.1.3.27 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 B 223.1.1.3 223.1.3.1 223.1.3.27 223.1.2.2 E 223.1.3.2 Network Layer 4-72 Transition from IPv4 to IPv6 not all routers can be upgraded simultaneously “flag days” how will network operate with mixed IPv4 and IPv6 routers? tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers no IPv4 header fields IPv4 source, dest addr IPv6 header fields IPv6 source dest addr IPv4 payload UDP/TCP payload IPv6 datagram IPv4 datagram Network Layer 4-73 Tunneling (6in4 – static tunnel) IPv4 tunnel connecting IPv6 routers A B IPv6 IPv6 A B C IPv6 IPv6 IPv4 logical view: E F IPv6 IPv6 D E F IPv4 IPv6 IPv6 physical view: flow: X src: A dest: F data A-to-B: IPv6 src:B dest: E src:B dest: E Flow: X Src: A Dest: F Flow: X Src: A Dest: F data data B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4 flow: X src: A dest: F data E-to-F: IPv6 Network Layer 4-74 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-75 Interplay between routing and forwarding routing algorithm determines end-end path through network routing algorithm forwarding table determines local forwarding at this router local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 IP destination address in arriving packet’s header 0111 1 3 2 Network Layer 4-76 Graph abstraction: costs 5 2 Graph: G = (N,E) N = set of “Nodes” routers = { u, v, w, x, y, z } u v 2 1 x 3 w 3 1 5 z 1 y 2 E = set of “Edges” links = { (u,v), (u,x), (u,w), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } Cost of link xw is c(x,w) = 3 Cost of link could be always 1 hop, or related directly to delay or inversely to bandwidth, or any other metric Question: What is the least-cost path between u and z? Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp) Routing algorithm: is algorithm that finds least-cost path Network Layer 4-77 Routing Algorithm Classification Global or decentralized? Static or dynamic routing? Global: all routers have complete and global knowledge about topology, and all link-costs “link state” algorithms Static: routes change slowly over time, manually configured Decentralized: router knows physicallyconnected neighbors, link costs to neighbors exchange of info with neighbors Iteratively calculate the leastcost paths to other routers “distance vector” algorithms Dynamic: routes change more quickly periodic update, or in response to link-cost changes Network Layer 4-78 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-79 A Link-State (LS) Routing Algorithm Dijkstra’s algorithm link costs known to all nodes Each node sends out “link state multicasts” with costs to its neighbors all nodes get same info Each node computes least cost paths from itself to all other nodes gives forwarding table for that node iterative: after k iterations, knows least cost path to k destinations Notation: c(x,y): link cost from node x to y. Initially cost(x,y) = ∞ if not direct neighbors D(v): Distance; current value of cost of path from source to destination v p(v): predecessor node, i.e. previous node that is neighbor of v along current path from the source to node v N': set of Nodes whose least cost path definitively known Network Layer 4-80 Dijsktra’s Algorithm at node u 1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent (directly attached neighbor) to u 5 then D(v) = c(u,v) 6 else D(v) = 7 8 Loop 9 find node w not in N' such that D(w) is a minimum 10 add node w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min{ D(v), D(w) + c(w,v) } 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes N in N' Network Layer 4-81 Dijkstra’s algorithm: example D(v) D(w) D(x) D(y) D(z) Step 0 1 2 3 4 5 N' p(v) p(w) p(x) u uw uwx uwxv uwxvy uwxvyz 7,u 6,w 6,w 3,u ∞ ∞ 5,u ∞ 5,u 11,w 11,w 14,x 10,v 14,x 12,y p(y) p(z) x 9 notes: construct shortest path tree by tracing predecessor nodes ties can exist (can be broken arbitrarily) 5 7 4 8 u 3 w y 2 z 3 4 7 v Network Layer 4-82 Dijkstra’s algorithm: example node u v Step 0 1 2 3 4 5 N' u ux uxv uxvy uxvyw uxvywz w D(v),p(v) D(w),p(w) 2,u 5,u 2,u 4,x 4,x 3,y x y z D(x),p(x) 1,u D(y),p(y) 2,x D(z),p(z) 4,y 4,y 2,x 5 2 u v 2 1 x 3 w 3 1 5 z 1 y 2 Network Layer 4-83 Dijkstra’s algorithm: forwarding table Resulting shortest-path tree from node u as root: v w u Root z x y Resulting forwarding table in u: destination via link cost v x (u,v) (u,x) 2 y (u,x) 2 w (u,x) 3 z (u,x) 4 1 Network Layer 4-84 Dijkstra’s algorithm, discussion Algorithm complexity: n nodes (root node is excluded) each iteration: need to check all nodes, not in N’ n(n+1)/2 comparisons: Order of (n2) Oscillations possible: e.g., if link cost = delay-based or traffic-based, dynamically variable metric must avoid these metrics But: Good for small networks Link-cost changes are not frequent, more stable network Faster to converge when changes in link-costs Network Layer 4-85 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-86 Distance Vector (DV) Algorithm Bellman-Ford Equation: Define dx(y) := cost of least-cost path from x to y If v is any neighbor to x with link cost c(x,v) and has dv(y) as leastcost path to y Then the DV estimate: dx(y) = min { c(x,v) + dv(y) } cost from neighbor v to destination y cost to neighbor v Network Layer 4-87 min taken over all neighbors v of x Distance vector (DV) algorithm Basic idea: distributed, asynchronous, iterative from time-to-time, each node sends to neighbors only its own distance vector DV estimate When a node x receives new DV estimate from neighbor v, it updates its own DV using Bellman-Ford equation: dx(y) ← minv {c(x,v) + dv(y)} for each node y ∊ N Under normal conditions, when information comes in about new link costs: The estimate dx(y) converge to the actual least cost Routing table recalculated New results sent out to all neighbors Network Layer 4-88 Bellman-Ford: example Nodes v, x & w are the neighbors of u 5 2 u v 2 1 x 3 w 3 1 Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3 5 z 1 y 2 BF equation says: du(z) = min { c(u,v) + dv(z), c(u,x) + dx(z), c(u,w) + dw(z) } = min {2 + 5, 1 + 3, 5 + 3} = 4 Node x that achieves minimum is the next hop in least-cost path to z ➜ forwarding table Network Layer 4-89 Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2 node x table cost to x y z cost to x y z from from x 0 2 7 y ∞∞ ∞ z ∞∞ ∞ node y table cost to x y z Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3 x 0 2 3 y 2 0 1 z 7 1 0 2 x ∞ ∞ ∞ y 2 0 1 z ∞∞ ∞ node z table cost to x y z from from x x ∞∞ ∞ y ∞∞ ∞ z 7 1 0 y 7 1 z time Network Layer 4-90 Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2 x ∞∞ ∞ y ∞∞ ∞ z 7 1 0 x 0 2 3 y 2 0 1 z 7 1 0 x 0 2 3 y 2 0 1 z 3 1 0 from cost to x y z cost to x y z cost to x y z x 0 2 7 y 2 0 1 z 7 1 0 x 0 2 3 y 2 0 1 z 3 1 0 from from cost to x y z cost to x y z x 0 2 7 y 2 0 1 z 3 1 0 2 x y 7 1 z cost to x y z from from from x ∞ ∞ ∞ y 2 0 1 z ∞∞ ∞ node z table cost to x y z from from x 0 2 7 y ∞∞ ∞ z ∞∞ ∞ node y table cost to x y z from node x table cost to x y z Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3 x 0 2 3 y 2 0 1 z 3 1 0 time Network Layer 4-91 DV: link cost changes (good news) Link cost changes: decreased cost node detects local link cost change 1 4 updates routing info, recalculates distance vector if DV changes, notify neighbors x y 50 1 z At time t0, y detects the link-cost change, updates its DV, and informs its neighbors. “good news travels fast” At time t1, z receives the update from y and updates its table. It computes a new least cost to x and sends its neighbors its DV. At time t2, y receives z’s update and checks its distance table. y’s least costs do not change and hence y does not send any update to z. Network Layer 4-92 DV: link cost changes (bad news) Link cost changes: increased cost 60 bad news travels slow - “count to infinity” problem! 44 iterations before algorithm stabilizes! x y 50 1 z y already knows z has cost 5 to reach x y therefore announces cost 6 to reach x z announces cost is now 7, etc.. Poisoned reverse: If z routes through y to get to x: 4 “bad news travels slow” z tells y its (z’s) distance to x is infinite (so y won’t route to x via z) will this completely solve count to infinity problem? Network Layer 4-93 Comparison of LS and DV algorithms Message complexity LS: with n nodes, E links, O(nE) messages sent DV: exchange between neighbors only Convergence due changes LS: may have oscillations fast convergence DV: may be routing loops count-to-infinity problem slow convergence Robustness: what happens if router malfunctions? LS: node can advertise incorrect link cost each node computes only its own table limited damage DV: node can advertise incorrect path cost each node’s table used by others • error propagates through network Network Layer 4-94 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-95 Hierarchical Routing Our routing study thus far - idealization all routers identical network “flat” … not true in practice scale: millions of destinations! administrative autonomy can’t store all destinations in Internet = network of networks routing tables! LS routing info exchange would swamp links! DV would never terminate each network administrator may want to control routing in its own network Network Layer 4-96 Hierarchical Routing aggregate routers into regions, “autonomous systems” (AS) Internet: > 39,000 AS routers in same AS run same routing protocol “intra-AS” routing protocol routers in another AS can run different intra-AS routing protocol border router at “edge” of its own AS with direct link to router in another AS Network Layer 4-97 Interconnected ASs 3c 3b AS3 3a 2a 1c AS1 1a 2b AS2 1b 1d Forwarding table configured by both intra- and inter-AS routing algorithms 2c Intra-AS Routing algorithm Inter-AS Routing algorithm Forwarding table intra-AS sets entries for internal destinations inter-AS sets entries for external destinations Network Layer 4-98 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-99 Intra-AS Routing also known as Interior Gateway Protocols (IGP) most common Intra-AS routing protocols: RIP: Routing Information Protocol [DV] OSPF: Open Shortest Path First [LS] IS-IS: Intermediate System-Intermediate System [LS] EIGRP: Enhanced Interior Gateway Routing Protocol (Cisco proprietary) Network Layer 4-100 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-101 RIP ( Routing Information Protocol) distance vector algorithm included in BSD-UNIX Distribution 4.3 in 1982 distance metric: number of hops (max = 15 hops) Version 1 classful and version 2 classless From router A to subnets: u v A z C B w x D destination hops u 0 v 1 w 1 x 2 y 2 z 1 y Network Layer 4-102 RIP advertisements distance vectors are exchanged among neighbors every 30 sec via Response Message (also called advertisement) each advertisement: list of up to 25 destination subnets within AS If no advertisement heard after 180 sec neighbor or link declared dead (unreachable). Routes via neighbor invalidated New advertisements sent to other neighbors Link failure info propagates to entire network Poisoned reverse used with max hop count 15 Infinite distance is 16 hops RIP v.2 also supports route aggregation (1998) Network Layer 4-103 RIP Table processing RIP routing tables managed by application-level process called routed (route daemon) advertisements periodically sent in UDP packets (port 520) using broadcast (or multicast, RIP v.2) routed routed transport (UDP) network (IP) link physical transport (UDP) forwarding table forwarding table network (IP) link physical Network Layer 4-104 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-105 OSPF (Open Shortest Path First) “open”: just means publicly available (RFC 2328) uses Link State algorithm complete topology map built at each node route computation using Dijkstra’s algorithm works in larger networks (hierarchical structure with areas) OSPF advertisements sent within area via flooding. carried in OSPF messages directly over IP with protocol number 89 (no UDP- or TCP-transport) sent at least every 30 minutes Network Layer 4-106 OSPF “advanced” features security: all OSPF messages can be authenticated (to prevent malicious intrusion) multiple same-cost paths allowed Send HELLO messages to establish adjacencies with neighbors to check operational links hierarchical OSPF in large domains. Network Layer 4-107 Hierarchical OSPF boundary router backbone router backbone area border routers Area 3 internal routers Area 1 Area 2 Network Layer 4-108 Hierarchical OSPF two-level hierarchy: local areas, one backbone (area 0). Link-state advertisements only in area each node has detailed area topology; only knows direction (shortest path) to subnets in other areas. area border routers: “summarize” subnets in own area, advertise to other area border routers. backbone routers: run OSPF routing limited to backbone. boundary routers: connect to other AS’s. Network Layer 4-109 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-110 Internet inter-AS routing: BGP BGP (Border Gateway Protocol) the de facto standard routing protocol on the Internet Complex protocol Communicates over TCP port 179 with authentication BGP provides each AS a means to: o Obtain prefix reachability information from neighboring ASs. o Propagate reachability information to all ASinternal routers. o Determine “good” routes to prefixes based on reachability information and policy. Network Layer 4-111 BGP basics pairs of routers (BGP peers) exchange routing info over semi-permanent TCP connections: BGP sessions BGP sessions need not correspond to physical links. advertising paths to different destination network prefixes (“path vector” protocol) when AS3 advertises a prefix to AS1: AS3 promises it will forward datagrams towards that prefix. AS3 can aggregate prefixes in its advertisement 3c 3b other networks 3a BGP message AS3 2c 1c 1a AS1 1d 2a 1b 2b other networks AS2 Network Layer 4-112 Distributing Reachability Info With “external BGP” eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. 1c can then use “internal BGP” iBGP to distribute new prefix info to all routers in AS1 1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session when router learns of new prefix, it creates entry for prefix in its forwarding table. eBGP session 3c 3b other networks iBGP session 3a BGP message AS3 2c 1c 1a AS1 1d 2a 1b 2b other networks AS2 Network Layer 4-113 Path attributes & BGP routes advertised prefix includes BGP attributes. prefix + attributes = “route” two important attributes: AS-PATH: contains ASs through which prefix can be reached: e.g., AS 67, AS 17 NEXT-HOP: indicates specific AS router to next-hop AS. when gateway router receives route advertisement, uses import policy to accept or decline. May or may not accept/announce everything to/from peers Router may learn about more than 1 route to some prefix. Router must select route based on: Policy decision Shortest AS_PATH Closest NEXT_HOP router Network Layer 4-114 BGP messages BGP messages exchanged using TCP. BGP messages: OPEN: opens TCP connection to peer and authenticates sender UPDATE: advertises new path (or withdraws old) KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs OPEN request NOTIFICATION: reports errors in previous message; also used to close connection Network Layer 4-115 BGP routing policy (1) legend: B W provider network X A customer network: C Y A,B,C are provider networks x,w,y are customers (of provider networks) x is dual-homed: attached to two networks x does not want to route from B via x to C .. so x will not advertise to B a route to C Network Layer 4-116 BGP routing policy (2) legend: B W provider network X A customer network: C Y A advertises to B path A-w B advertises to x path B-A-w Should B advertise path B-A-w to C? No way! B gets no “revenue” for routing C-B-A-w since neither w nor C are B’s customers B wants to force C to route to w via A B wants to route only to/from its customers! Network Layer 4-117 Why different Intra- & Inter-AS routing? Policy: Inter-AS: admin wants control over how its traffic routed, who routes through its net. Intra-AS: single admin, so no policy decisions needed Scale: hierarchical routing saves routing table size, reduced update traffic Performance: Inter-AS: policy may dominate over performance Intra-AS: can focus on performance Network Layer 4-118 Growth of the BGP table: 1994 to 2011 http://bgp.potaroo.net Network Layer 4-119 Growth of the BGP table: 1994 to Present http://bgp.potaroo.net Network Layer 4-120 Chapter 4: 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 IPv4 addressing ICMP IPv6 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 4-121