Chapter 4 Network Layer PDF

Chapter 4 Network Layer A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). Computer They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your...

Chapter 4 Network Layer A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). Computer They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. Networking: A Top They obviously represent a lot of work on our part. In return for use, we only ask the following: Down Approach  If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!) 6th edition  If you post any slides on a www site, that you note that they are adapted Jim Kurose, Keith Ross from (or perhaps identical to) our slides, and note our copyright of this material. Addison-Wesley March 2012 Thanks and enjoy! JFK/KWR All material copyright 1996-2013 J.F Kurose and K.W. Ross, All Rights Reserved Network Layer 4-1 Chapter 4: network layer chapter goals:  understand principles behind network layer services:  network layer service models  forwarding versus routing  how a router works  routing (path selection)  broadcast, multicast  instantiation, implementation in the Internet Network Layer 4-2 Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and  link state datagram networks  distance vector 4.3 what’s inside a router  hierarchical routing 4.4 IP: Internet Protocol 4.6 routing in the Internet  datagram format  RIP  IPv4 addressing  OSPF  ICMP  BGP  IPv6 4.7 broadcast and multicast routing Network Layer 4-3 Network layer application  transport segment from transport network sending to receiving host data link physical network network  on sending side network data link data link physical data link physical encapsulates segments physical network data link network data link into datagrams physical physical  on receiving side, delivers network data link network data link segments to transport physical physical network data link layer network physical application transport  network layer protocols network data link physical network data link network data link in every host, router data link physical physical physical  router examines header fields in all IP datagrams passing through it Network Layer 4-4 Two key network-layer functions  forwarding: move packets analogy: from router’s input to appropriate router  routing: process of output planning trip from source to dest  routing: determine route taken by packets from  forwarding: process of source to dest. getting through single interchange  routing algorithms Network Layer 4-5 Interplay between routing and forwarding routing algorithm routing algorithm determines end-end-path through network local forwarding table forwarding table determines header value output link local forwarding at this router 0100 3 0101 2 0111 2 1001 1 value in arriving packet’s header 0111 1 3 2 Network Layer 4-6 Connection setup  3rd important function in some network architectures:  ATM, frame relay, X.25  before datagrams flow, two end hosts and intervening routers establish virtual connection  routers get involved  network vs transport layer connection service:  network: between two hosts (may also involve intervening routers in case of VCs)  transport: between two processes Network Layer 4-7 Network service model Q: What service model for “channel” transporting datagrams from sender to receiver? example services for example services for a flow individual datagrams: of datagrams:  guaranteed delivery  in-order datagram  guaranteed delivery with delivery less than 40 msec delay  guaranteed minimum bandwidth to flow  restrictions on changes in inter-packet spacing Network Layer 4-8  Network layer service models: Guarantees ? Network Service Congestion Architecture Model Bandwidth Loss Order Timing feedback Internet best effort none no no no no (inferred via loss) ATM CBR constant yes yes yes no rate congestion ATM VBR guaranteed yes yes yes no rate congestion ATM ABR guaranteed no yes no yes minimum ATM UBR none no yes no no Network Layer 4-9 Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and  link state datagram networks  distance vector 4.3 what’s inside a router  hierarchical routing 4.4 IP: Internet Protocol 4.6 routing in the Internet  datagram format  RIP  IPv4 addressing  OSPF  ICMP  BGP  IPv6 4.7 broadcast and multicast routing Network Layer 4-10 Connection, connection-less service  datagram network provides network-layer connectionless service  virtual-circuit network provides network-layer connection service  analogous to TCP/UDP connecton-oriented / connectionless transport-layer services, but:  service: host-to-host  no choice: network provides one or the other  implementation: in network core Network Layer 4-11 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 (dedicated resources = predictable service) Network Layer 4-12 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 VC number (rather than dest address)  VC number can be changed on each link.  new VC number comes from forwarding table Network Layer 4-13  VC forwarding table 12 22 32 1 3 2 VC number interface forwarding table in number northwest router: Incoming interface Incoming VC # Outgoing interface Outgoing VC # 1 12 3 22 2 63 1 18 3 7 2 17 1 97 3 87 … … … … VC routers maintain connection state information! Network Layer 4-14 Virtual circuits: signaling protocols  used to setup, maintain teardown VC  used in ATM, frame-relay, X.25  not used in today’s Internet application application 5. data flow begins 6. receive data transport transport network 4. call connected 3. accept call 1. initiate call network data link 2. incoming call data link physical physical Network Layer 4-15 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 application application transport transport network 1. send datagrams 2. receive datagrams network data link data link physical physical Network Layer 4-16 Datagram forwarding table 4 billion IP addresses, so routing algorithm rather than list individual destination address local forwarding table list range of addresses dest address output link (aggregate table entries) address-range 1 3 address-range 2 2 address-range 3 2 address-range 4 1 IP destination address in arriving packet’s header 1 3 2 Network Layer 4-17 Datagram forwarding table Destination Address Range Link Interface 11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111 11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111 11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111 otherwise 3 Q: but what happens if ranges don’t divide up so nicely? Network Layer 4-18 Longest prefix matching longest prefix matching when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address. 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 which interface? DA: 11001000 00010111 00011000 10101010 which interface? Network Layer 4-19 Datagram or VC network: why? Internet (datagram) ATM (VC)  data exchange among  evolved from telephony computers  human conversation:  “elastic” service, no strict  strict timing, reliability timing req. requirements  need for guaranteed service  many link types  “dumb” end systems  different characteristics  telephones  uniform service difficult  complexity inside  “smart” end systems network (computers)  can adapt, perform control, error recovery  simple inside network, complexity at “edge” Network Layer 4-20 Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and  link state datagram networks  distance vector 4.3 what’s inside a router  hierarchical routing 4.4 IP: Internet Protocol 4.6 routing in the Internet  datagram format  RIP  IPv4 addressing  OSPF  ICMP  BGP  IPv6 4.7 broadcast and multicast routing Network Layer 4-21 Router architecture overview two key router functions:  run routing algorithms/protocol (RIP, OSPF, BGP)  forwarding datagrams from incoming to outgoing link forwarding tables computed, routing pushed to input ports routing, management processor control plane (software) forwarding data plane (hardware) high-seed switching fabric router input ports router output ports Network Layer 4-22  Input port functions lookup, link forwarding line layer switch termination protocol fabric (receive) queueing physical layer: bit-level reception data link layer: decentralized switching: e.g., Ethernet  given datagram dest., lookup output port see chapter 5 using forwarding table in input port memory (“match plus action”)  goal: complete input port processing at ‘line speed’  queuing: if datagrams arrive faster than forwarding rate into switch fabric Network Layer 4-23 Switching fabrics  transfer packet from input buffer to appropriate output buffer  switching rate: rate at which packets can be transfer from inputs to outputs  often measured as multiple of input/output line rate  N inputs: switching rate N times line rate desirable  three types of switching fabrics memory memory bus crossbar Network Layer 4-24 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 output port memory port (e.g., (e.g., Ethernet) Ethernet) system bus Network Layer 4-25 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  32 Gbps bus, Cisco 5600: sufficient bus speed for access and enterprise routers Network Layer 4-26 Switching via interconnection network  overcome bus bandwidth limitations  banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor  advanced design: fragmenting datagram into fixed length cells, crossbar switch cells through the fabric.  Cisco 12000: switches 60 Gbps through the interconnection network Network Layer 4-27 Output ports This slide in HUGELY important! datagram switch buffer link fabric layer line protocol termination queueing (send)  buffering required when datagrams Datagram arrive (packets) can be lost from fabric faster than the due to transmission congestion, lack of buffers rate  scheduling discipline chooses Priority among scheduling – who queued gets best datagrams for transmission performance, network neutrality Network Layer 4-28 Output port queueing switch switch fabric fabric at t, packets more one packet time later from input to output  buffering when arrival rate via switch exceeds output line speed  queueing (delay) and loss due to output port buffer overflow! Network Layer 4-29 How much buffering?  RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) times link capacity C  e.g., C = 10 Gpbs link: 2.5 Gbit buffer  recent recommendation: with N flows, buffering equal to RTT. C N Network Layer 4-30 Input port queuing  fabric slower than input ports combined -> queueing may occur at input queues  queueing delay and loss due to input buffer overflow!  Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward switch switch fabric fabric output port contention: one packet time later: only one red datagram can be green packet transferred. experiences HOL lower red packet is blocked blocking Network Layer 4-31 Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and  link state datagram networks  distance vector 4.3 what’s inside a router  hierarchical routing 4.4 IP: Internet Protocol 4.6 routing in the Internet  datagram format  RIP  IPv4 addressing  OSPF  ICMP  BGP  IPv6 4.7 broadcast and multicast routing Network Layer 4-32 The Internet network layer host, router network layer functions: transport layer: TCP, UDP routing protocols IP protocol path selection addressing conventions RIP, OSPF, BGP datagram format network packet handling conventions layer forwarding table ICMP protocol error reporting router “signaling” link layer physical layer Network Layer 4-33  IP datagram format IP protocol version 32 bits number total datagram header length length (bytes) ver head. type of length (bytes) len service for “type” of data fragment fragmentation/ 16-bit identifier flgs offset reassembly max number time to upper header remaining hops live layer checksum (decremented at 32 bit source IP address each router) 32 bit destination IP address upper layer protocol to deliver payload to options (if any) e.g. timestamp, record route how much overhead? data taken, specify (variable length, list of routers  20 bytes of TCP typically a TCP to visit.  20 bytes of IP or UDP segment)  = 40 bytes + app layer overhead Network Layer 4-34  IP fragmentation, reassembly  network links have MTU (max.transfer size) - largest possible link-level fragmentation: frame … in: one large datagram  different link types, out: 3 smaller datagrams different MTUs  large IP datagram divided (“fragmented”) within net reassembly  one datagram becomes several datagrams  “reassembled” only at … final destination  IP header bits used to identify, order related fragments Network Layer 4-35  IP fragmentation, reassembly length ID fragflag offset example: =4000 =x =0 =0  4000 byte datagram one large datagram becomes  MTU = 1500 bytes several smaller datagrams 1480 bytes in length ID fragflag offset data field =1500 =x =1 =0 offset = length ID fragflag offset 1480/8 =1500 =x =1 =185 length ID fragflag offset =1040 =x =0 =370 Network Layer 4-36 Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and  link state datagram networks  distance vector 4.3 what’s inside a router  hierarchical routing 4.4 IP: Internet Protocol 4.6 routing in the Internet  datagram format  RIP  IPv4 addressing  OSPF  ICMP  BGP  IPv6 4.7 broadcast and multicast routing Network Layer 4-37 IP addressing: introduction 223.1.1.1  IP address: 32-bit identifier for host, router 223.1.2.1 interface 223.1.1.2 223.1.1.4 223.1.2.9  interface: connection between host/router and 223.1.3.27 physical link 223.1.1.3 223.1.2.2  router’s typically have multiple interfaces  host typically has one or two interfaces (e.g., wired 223.1.3.1 223.1.3.2 Ethernet, wireless 802.11)  IP addresses associated with each interface 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1 Network Layer 4-38 IP addressing: introduction 223.1.1.1 Q: how are interfaces actually connected? 223.1.2.1 A: we’ll learn about that 223.1.1.2 223.1.1.4 223.1.2.9 in chapter 5, 6. 223.1.3.27 223.1.1.3 223.1.2.2 A: wired Ethernet interfaces connected by Ethernet switches 223.1.3.1 223.1.3.2 For now: don’t need to worry about how one interface is connected to another (with no A: wireless WiFi interfaces intervening router) connected by WiFi base station Network Layer 4-39 Subnets  IP address: 223.1.1.1 subnet part - high order bits 223.1.1.2 223.1.2.1 223.1.1.4 223.1.2.9 host part - low order bits 223.1.2.2  what ’s a subnet ? 223.1.1.3 223.1.3.27 device interfaces with subnet same subnet part of IP address 223.1.3.1 223.1.3.2 can physically reach each other without intervening router network consisting of 3 subnets Network Layer 4-40 Subnets 223.1.1.0/24 223.1.2.0/24 recipe 223.1.1.1  to determine the 223.1.1.2 223.1.2.1 subnets, detach each 223.1.1.4 223.1.2.9 interface from its host 223.1.2.2 or router, creating 223.1.1.3 223.1.3.27 islands of isolated subnet networks  each isolated network 223.1.3.1 223.1.3.2 is called a subnet 223.1.3.0/24 subnet mask: /24 Network Layer 4-41 Subnets 223.1.1.2 how many? 223.1.1.1 223.1.1.4 223.1.1.3 223.1.9.2 223.1.7.0 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 223.1.3.27 223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2 Network Layer 4-42 IP addressing: CIDR CIDR: Classless InterDomain 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 host part part 11001000 00010111 00010000 00000000 200.23.16.0/23 Network Layer 4-43 IP addresses: how to get one? Q: How does a host get IP address?  hard-coded by system admin in a file  Windows: 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” Network Layer 4-44 DHCP: Dynamic Host Configuration Protocol goal: allow host to dynamically obtain its IP address from network server when it joins network  can renew its lease on address in use  allows reuse of addresses (only hold address while connected/“on”)  support for mobile users who want to join network (more shortly) DHCP overview:  host broadcasts “DHCP discover” msg [optional]  DHCP server responds with “DHCP offer” msg [optional]  host requests IP address: “DHCP request” msg  DHCP server sends address: “DHCP ack” msg Network Layer 4-45 DHCP client-server scenario DHCP 223.1.1.0/24 server 223.1.1.1 223.1.2.1 223.1.1.2 arriving DHCP 223.1.1.4 223.1.2.9 client needs address in this 223.1.3.27 223.1.2.2 network 223.1.1.3 223.1.2.0/24 223.1.3.1 223.1.3.2 223.1.3.0/24 Network Layer 4-46 DHCP client-server scenario DHCP server: 223.1.2.5 DHCP discover arriving client src : 0.0.0.0, 68 Broadcast: is there a dest.: 255.255.255.255,67 DHCPyiaddr: server0.0.0.0 out there? transaction ID: 654 DHCP offer src: 223.1.2.5, 67 Broadcast: I’m a DHCP dest: 255.255.255.255, 68 server! Here’s an IP yiaddrr: 223.1.2.4 address youID:can transaction 654 use lifetime: 3600 secs DHCP request src: 0.0.0.0, 68 Broadcast: OK. I’ll take dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 that IP address! transaction ID: 655 lifetime: 3600 secs DHCP ACK src: 223.1.2.5, 67 Broadcast: OK. You’ve dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 got that IPID: transaction address! 655 lifetime: 3600 secs Network Layer 4-47 DHCP: more than IP addresses DHCP can return more than just allocated IP address on subnet:  address of first-hop router for client  name and IP address of DNS sever  network mask (indicating network versus host portion of address) Network Layer 4-48 DHCP: example DHCP DHCP  connecting laptop needs DHCP UDP its IP address, addr of IP first-hop router, addr of DHCP DHCP Eth Phy DNS server: use DHCP DHCP request encapsulated DHCP  in UDP, encapsulated in IP, DHCP DHCP 168.1.1.1 encapsulated in 802.1 DHCP UDP Ethernet IP Ethernet frame broadcast DHCP  DHCP Eth router with DHCP Phy server built into (dest: FFFFFFFFFFFF) on LAN, router received at router running DHCP server  Ethernet demuxed to IP demuxed, UDP demuxed to DHCP Network Layer 4-49 DHCP: example DHCP DHCP  DCP server formulates DHCP UDP DHCP ACK containing DHCP IP client’s IP address, IP DHCP Eth address of first-hop Phy router for client, name & IP address of DNS server  encapsulation of DHCP DHCP DHCP server, frame forwarded DHCP UDP to client, demuxing up to DHCP IP DHCP at client DHCP Eth router with DHCP DHCP Phy server built into  client now knows its IP router address, name and IP address of DSN server, IP address of its first-hop router Network Layer 4-50  DHCP: Wireshark Message type: Boot Reply (2) reply output (home LAN) Hardware type: Ethernet Hardware address length: 6 Hops: 0 Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Message type: Boot Request (1) Bootp flags: 0x0000 (Unicast) Hardware type: Ethernet Client IP address: 192.168.1.101 (192.168.1.101) Hardware address length: 6 Your (client) IP address: 0.0.0.0 (0.0.0.0) Hops: 0 Transaction ID: 0x6b3a11b7 request Next server IP address: 192.168.1.1 (192.168.1.1) Relay agent IP address: 0.0.0.0 (0.0.0.0) Seconds elapsed: 0 Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Bootp flags: 0x0000 (Unicast) Server host name not given Client IP address: 0.0.0.0 (0.0.0.0) Boot file name not given Your (client) IP address: 0.0.0.0 (0.0.0.0) Magic cookie: (OK) Next server IP address: 0.0.0.0 (0.0.0.0) Option: (t=53,l=1) DHCP Message Type = DHCP ACK Relay agent IP address: 0.0.0.0 (0.0.0.0) Option: (t=54,l=4) Server Identifier = 192.168.1.1 Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Option: (t=1,l=4) Subnet Mask = 255.255.255.0 Server host name not given Option: (t=3,l=4) Router = 192.168.1.1 Boot file name not given Option: (6) Domain Name Server Magic cookie: (OK) Length: 12; Value: 445747E2445749F244574092; Option: (t=53,l=1) DHCP Message Type = DHCP Request IP Address: 68.87.71.226; Option: (61) Client identifier IP Address: 68.87.73.242; Length: 7; Value: 010016D323688A; IP Address: 68.87.64.146 Hardware type: Ethernet Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net." Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Option: (t=50,l=4) Requested IP Address = 192.168.1.101 Option: (t=12,l=5) Host Name = "nomad" Option: (55) Parameter Request List Length: 11; Value: 010F03062C2E2F1F21F92B 1 = Subnet Mask; 15 = Domain Name 3 = Router; 6 = Domain Name Server 44 = NetBIOS over TCP/IP Name Server …… Network Layer 4-51 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 00000000 200.23.16.0/20 Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23... ….. …. …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23 Network Layer 4-52 Hierarchical addressing: route aggregation hierarchical addressing allows efficient advertisement of routing information: Organization 0 200.23.16.0/23 Organization 1 “Send me anything 200.23.18.0/23 with addresses Organization 2 beginning 200.23.20.0/23. Fly-By-Night-ISP 200.23.16.0/20”... Internet. Organization 7. 200.23.30.0/23 “Send me anything ISPs-R-Us with addresses beginning 199.31.0.0/16” Network Layer 4-53 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization 0 200.23.16.0/23 “Send me anything with addresses Organization 2 beginning 200.23.20.0/23. Fly-By-Night-ISP 200.23.16.0/20”... Internet. Organization 7. 200.23.30.0/23 “Send me anything ISPs-R-Us with addresses Organization 1 beginning 199.31.0.0/16 or 200.23.18.0/23” 200.23.18.0/23 Network Layer 4-54 IP addressing: the last word... Q: how does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers http://www.icann.org/  allocates addresses  manages DNS  assigns domain names, resolves disputes Network Layer 4-55  NAT: network address translation rest of local network Internet (e.g., home network) 10.0.0/24 10.0.0.1 10.0.0.4 10.0.0.2 138.76.29.7 10.0.0.3 all datagrams leaving local datagrams with source or network have same single destination in this network source NAT IP address: have 10.0.0/24 address for 138.76.29.7,different source source, destination (as usual) port numbers Network Layer 4-56  NAT: network address translation motivation: local network uses just one IP address as far as outside world is concerned:  range of addresses not needed from ISP: just one IP 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  devices inside local net not explicitly addressable, visible by outside world (a security plus) Network Layer 4-57 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 4-58  NAT: network address translation NAT translation table 1: host 10.0.0.1 2: NAT router WAN side addr LAN side addr changes datagram sends datagram to source addr from 138.76.29.7, 5001 10.0.0.1, 3345 128.119.40.186, 80 10.0.0.1, 3345 to …… …… 138.76.29.7, 5001, updates table S: 10.0.0.1, 3345 D: 128.119.40.186, 80 10.0.0.1 1 S: 138.76.29.7, 5001 2 D: 128.119.40.186, 80 10.0.0.4 10.0.0.2 138.76.29.7 S: 128.119.40.186, 80 D: 10.0.0.1, 3345 4 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3 10.0.0.3 4: NAT router 3: reply arrives changes datagram dest. address: dest addr from 138.76.29.7, 5001 138.76.29.7, 5001 to 10.0.0.1, 3345 Network Layer 4-59 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, e.g., P2P applications  address shortage should instead be solved by IPv6 Network Layer 4-60 NAT traversal problem  client wants to connect to server with address 10.0.0.1  server address 10.0.0.1 local to 10.0.0.1 client LAN (client can’t use it as destination addr) ?  only one externally visible NATed 10.0.0.4 address: 138.76.29.7  solution1: statically configure 138.76.29.7 NAT NAT to forward incoming router connection requests at given port to server  e.g., (123.76.29.7, port 2500) always forwarded to 10.0.0.1 port 25000 Network Layer 4-61 NAT traversal problem  solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATed 10.0.0.1 host to: IGD  learn public IP address (138.76.29.7)  add/remove port mappings (with lease times) NAT router i.e., automate static NAT port map configuration Network Layer 4-62  NAT traversal problem  solution 3: relaying (used in Skype)  NATed client establishes connection to relay  external client connects to relay  relay bridges packets between to connections 2. connection to relay initiated 1. connection to 10.0.0.1 by client relay initiated by NATed host 3. relaying client established 138.76.29.7 NAT router Network Layer 4-63 Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and  link state datagram networks  distance vector 4.3 what’s inside a router  hierarchical routing 4.4 IP: Internet Protocol 4.6 routing in the Internet  datagram format  RIP  IPv4 addressing  OSPF  ICMP  BGP  IPv6 4.7 broadcast and multicast routing Network Layer 4-64 ICMP: internet control message protocol  used by hosts & routers to communicate network- Type Code description 0 0 echo reply (ping) level information 3 0 dest. network unreachable  error reporting: 3 1 dest host unreachable unreachable host, network, 3 2 dest protocol unreachable port, protocol 3 3 dest port unreachable  echo request/reply (used by 3 6 dest network unknown ping) 3 7 dest host unknown  network-layer “above” IP: 4 0 source quench (congestion  ICMP msgs carried in IP control - not used) datagrams 8 0 echo request (ping) 9 0 route advertisement  ICMP message: type, code 10 0 router discovery plus first 8 bytes of IP 11 0 TTL expired datagram causing error 12 0 bad IP header Network Layer 4-65 Traceroute and ICMP  source sends series of  when ICMP messages UDP segments to dest arrives, source records  first set has TTL =1 RTTs  second set has TTL=2, etc.  unlikely port number stopping criteria:  when nth set of datagrams  UDP segment eventually arrives to nth router: arrives at destination host  router discards datagrams  destination returns ICMP  and sends source ICMP “port unreachable” messages (type 11, code 0) message (type 3, code 3)  ICMP messages includes  source stops name of router & IP address 3 probes 3 probes 3 probes Network Layer 4-66  IPv6: motivation  initial motivation: 32-bit address space soon 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 Network Layer 4-67 IPv6 datagram format priority: identify priority among datagrams in flow flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined). next header: identify upper layer protocol for data ver pri flow label payload len next hdr hop limit source address (128 bits) destination address (128 bits) data 32 bits Network Layer 4-68 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”  multicast group management functions Network Layer 4-69 Transition from IPv4 to IPv6  not all routers can be upgraded simultaneously  no “flag days”  how will network operate with mixed IPv4 and IPv6 routers?  tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers IPv4 header fields IPv6 header fields IPv4 payload IPv4 source, dest addr IPv6 source dest addr UDP/TCP payload IPv6 datagram IPv4 datagram Network Layer 4-70 Tunneling A B IPv4 tunnel E F connecting IPv6 routers logical view: IPv6 IPv6 IPv6 IPv6 A B C D E F physical view: IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 Network Layer 4-71 Tunneling A B IPv4 tunnel E F connecting IPv6 routers logical view: IPv6 IPv6 IPv6 IPv6 A B C D E F physical view: IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 flow: X src:B src:B flow: X src: A dest: E src: A dest: F dest: E dest: F Flow: X Flow: X Src: A Src: A data Dest: F Dest: F data data data A-to-B: E-to-F: IPv6 B-to-C: B-to-C: IPv6 IPv6 inside IPv6 inside IPv4 IPv4 Network Layer 4-72 IPv6: adoption  US National Institutes of Standards estimate :  ~3% of industry IP routers  ~11% of US gov’t routers  Long (long!) time for deployment, use  20 years and counting!  think of application-level changes in last 20 years: WWW, Facebook, …  Why? Network Layer 4-73 Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and  link state datagram networks  distance vector 4.3 what’s inside a router  hierarchical routing 4.4 IP: Internet Protocol 4.6 routing in the Internet  datagram format  RIP  IPv4 addressing  OSPF  ICMP  BGP  IPv6 4.7 broadcast and multicast routing Network Layer 4-74 Interplay between routing, forwarding routing algorithm determines routing algorithm end-end-path through network forwarding table determines local forwarding table local forwarding at this router dest address output link address-range 1 3 address-range 2 2 address-range 3 2 address-range 4 1 IP destination address in arriving packet’s header 1 3 2 Network Layer 4-75 Graph abstraction 5 v 3 w 2 5 u 2 1 z 3 1 2 x 1 y graph: G = (N,E) N = set of routers = { u, v, w, x, y, z } E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } aside: graph abstraction is useful in other network contexts, e.g., P2P, where N is set of peers and E is set of TCP connections Network Layer 4-76 Graph abstraction: costs 5 c(x,x’) = cost of link (x,x’) 3 e.g., c(w,z) = 5 v w 5 2 u cost could always be 1, or 2 3 1 z inversely related to bandwidth, 1 2 or inversely related to x 1 y congestion cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp) key question: what is the least-cost path between u and z ? routing algorithm: algorithm that finds that least cost path Network Layer 4-77 Routing algorithm classification Q: global or decentralized Q: static or dynamic? information? static: global:  routes change slowly over  all routers have complete time topology, link cost info dynamic:  “link state” algorithms  routes change more decentralized: quickly  router knows physically-  periodic update connected neighbors, link  in response to link costs to neighbors cost changes  iterative process of computation, exchange of info with neighbors  “distance vector” algorithms Network Layer 4-78 Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and  link state datagram networks  distance vector 4.3 what’s inside a router  hierarchical routing 4.4 IP: Internet Protocol 4.6 routing in the Internet  datagram format  RIP  IPv4 addressing  OSPF  ICMP  BGP  IPv6 4.7 broadcast and multicast routing Network Layer 4-79 A Link-State Routing Algorithm Dijkstra’s algorithm notation:  net topology, link costs  c(x,y): link cost from known to all nodes node x to y; = ∞ if not  accomplished via “link state direct neighbors broadcast”  D(v): current value of  all nodes have same info cost of path from source  computes least cost paths to dest. v from one node (‘source”)  p(v): predecessor node to all other nodes along path from source to  gives forwarding table for v that node  N': set of nodes whose  iterative: after k least cost path definitively iterations, know least cost known path to k dest.’s Network Layer 4-80 Dijsktra’s Algorithm 1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 add 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 15 until all nodes in N' Network Layer 4-81 Dijkstra’s algorithm: example D(v) D(w) D(x) D(y) D(z) Step N' p(v) p(w) p(x) p(y) p(z) 0 u 7,u 3,u 5,u ∞ ∞ 1 uw 6,w 5,u 11,w ∞ 2 uwx 6,w 11,w 14,x 3 uwxv 10,v 14,x 4 uwxvy 12,y 5 uwxvyz x 9 notes: 5 7 4  construct shortest path tree by tracing predecessor nodes 8  ties can exist (can be broken u 3 w y z arbitrarily) 2 3 7 4 v Network Layer 4-82  Dijkstra’s algorithm: another example Step N' D(v),p(v) D(w),p(w) D(x),p(x) D(y),p(y) D(z),p(z) 0 u 2,u 5,u 1,u ∞ ∞ 1 ux 2,u 4,x 2,x ∞ 2 uxy 2,u 3,y 4,y 3 uxyv 3,y 4,y 4 uxyvw 4,y 5 uxyvwz 5 v 3 w 2 5 u 2 1 z 3 1 2 x 1 y Network Layer 4-83 Dijkstra’s algorithm: example (2) resulting shortest-path tree from u: v w u z x y resulting forwarding table in u: destination link v (u,v) x (u,x) y (u,x) w (u,x) z (u,x) Network Layer 4-84  Dijkstra’s algorithm, discussion algorithm complexity: n nodes  each iteration: need to check all nodes, w, not in N  n(n+1)/2 comparisons: O(n2)  more efficient implementations possible: O(nlogn) oscillations possible:  e.g., support link cost equals amount of carried traffic: 1 A 1+e A A A 2+e 0 0 2+e 2+e 0 D 0 0 B D 1+e 1 B D B D 1+e 1 B 0 0 0 e 0 0 C 0 1 1+e 0 1 C C C 1 e given these costs, given these costs, given these costs, initially find new routing…. find new routing…. find new routing…. resulting in new costs resulting in new costs resulting in new costs Network Layer 4-85 Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and  link state datagram networks  distance vector 4.3 what’s inside a router  hierarchical routing 4.4 IP: Internet Protocol 4.6 routing in the Internet  datagram format  RIP  IPv4 addressing  OSPF  ICMP  BGP  IPv6 4.7 broadcast and multicast routing Network Layer 4-86 Distance vector algorithm Bellman-Ford equation (dynamic programming) let dx(y) := cost of least-cost path from x to y then dx(y) = min v {c(x,v) + dv (y) } cost from neighbor v to destination y cost to neighbor v min taken over all neighbors v of x Network Layer 4-87 Bellman-Ford example 5 3 clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3 v w 5 2 u 2 1 z B-F equation says: 3 1 2 du(z) = min { c(u,v) + dv(z), x y 1 c(u,x) + dx(z), c(u,w) + dw(z) } = min {2 + 5, 1 + 3, 5 + 3} = 4 node achieving minimum is next hop in shortest path, used in forwarding table Network Layer 4-88 Distance vector algorithm  Dx(y) = estimate of least cost from x to y  x maintains distance vector Dx = [Dx(y): y є N ]  node x:  knows cost to each neighbor v: c(x,v)  maintains its neighbors’ distance vectors. For each neighbor v, x maintains Dv = [Dv(y): y є N ] Network Layer 4-89 Distance vector algorithm key idea:  from time-to-time, each node sends its own distance vector estimate to neighbors  when x receives new DV estimate from neighbor, it updates its own DV using B-F equation: Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N  under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y) Network Layer 4-90 Distance vector algorithm iterative, asynchronous: each node: each local iteration caused by:  local link cost change wait for (change in local link cost or msg from neighbor)  DV update message from neighbor distributed: recompute estimates  each node notifies neighbors only when its DV changes if DV to any dest has  neighbors then notify their changed, notify neighbors neighbors if necessary Network Layer 4-91  Dx(z) = min{c(x,y) + Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2 Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3 node x cost to cost to table x y z x y z x 0 2 7 x 0 2 3 from from y ∞∞ ∞ y 2 0 1 z ∞∞ ∞ z 7 1 0 node y cost to table x y z y 2 1 x ∞ ∞ ∞ x z from y 2 0 1 7 z ∞∞ ∞ node z cost to table x y z x ∞∞ ∞ from y ∞∞ ∞ z 7 1 0 time Network Layer 4-92  Dx(z) = min{c(x,y) + Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2 Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3 node x cost to cost to cost to table x y z x y z x y z x 0 2 7 x 0 2 3 x 0 2 3 from from y ∞∞ ∞ y 2 0 1 from y 2 0 1 z ∞∞ ∞ z 7 1 0 z 3 1 0 node y cost to cost to cost to table x y z x y z x y z y 2 1 x ∞ ∞ ∞ x 0 2 7 x 0 2 3 x z from from y 2 0 1 y 2 0 1 7 from y 2 0 1 z ∞∞ ∞ z 7 1 0 z 3 1 0 node z cost to cost to cost to table x y z x y z x y z x ∞∞ ∞ x 0 2 7 x 0 2 3 from from y 2 0 1 y 2 0 1 from y ∞∞ ∞ z 7 1 0 z 3 1 0 z 3 1 0 time Network Layer 4-93  Distance vector: link cost changes link cost changes: 1  node detects local link cost change 4 y 1  updates routing info, recalculates x z distance vector 50  if DV changes, notify neighbors “good t0 : y detects link-cost change, updates its DV, informs its news neighbors. travels t1 : z receives update from y, updates its table, computes new fast” least cost to x , sends its neighbors its DV. t2 : y receives z’s update, updates its distance table. y’s least costs do not change, so y does not send a message to z. Network Layer 4-94 Distance vector: link cost changes link cost changes: 60  node detects local link cost change 4 y 1  bad news travels slow - “count to x z infinity” problem! 50  44 iterations before algorithm stabilizes: see text poisoned reverse:  If Z routes through Y to get to X :  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 4-95 Comparison of LS and DV algorithms message complexity robustness: what happens if  LS: with n nodes, E links, O(nE) router malfunctions? msgs sent LS:  DV: exchange between neighbors  node can advertise incorrect only link cost  convergence time varies  each node computes only its own table speed of convergence DV:  LS:O(n2) algorithm requires O(nE) msgs  DV node can advertise incorrect path cost  may have oscillations  each node’s table used by  DV: convergence time varies others  may be routing loops error propagate thru  count-to-infinity problem network Network Layer 4-96 Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and  link state datagram networks  distance vector 4.3 what’s inside a router  hierarchical routing 4.4 IP: Internet Protocol 4.6 routing in the Internet  datagram format  RIP  IPv4 addressing  OSPF  ICMP  BGP  IPv6 4.7 broadcast and multicast routing Network Layer 4-97 Hierarchical routing our routing study thus far - idealization  all routers identical  network “flat” … not true in practice scale: with 600 million administrative autonomy destinations:  internet = network of  can’t store all dest’s in networks routing tables!  each network admin may  routing table exchange want to control routing in would swamp links! its own network Network Layer 4-98 Hierarchical routing  aggregate routers into gateway router: regions, “autonomous  at “edge” of its own AS systems” (AS)  has link to router in  routers in same AS another AS run same routing protocol  “intra-AS” routing protocol  routers in different AS can run different intra- AS routing protocol Network Layer 4-99 Interconnected ASes 3c 3a 2c 3b 2a AS3 2b 1c AS2 1a 1b AS1 1d  forwarding table configured by both intra- and inter-AS routing Intra-AS Inter-AS algorithm Routing Routing algorithm algorithm  intra-AS sets entries Forwarding for internal dests table  inter-AS & intra-AS sets entries for external dests Network Layer 4-100  Inter-AS tasks  suppose router in AS1 AS1 must: receives datagram 1. learn which dests are destined outside of AS1: reachable through AS2,  router should forward which through AS3 packet to gateway 2. propagate this router, but which one? reachability info to all routers in AS1 job of inter-AS routing! 3c 3a 3b AS3 2c other 1c 2a networks other 1a 2b networks 1b AS2 AS1 1d Network Layer 4-101  Example: setting forwarding table in router 1d  suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c), but not via AS2  inter-AS protocol propagates reachability info to all internal routers  router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c  installs forwarding table entry (x,I) 3c x 3a 3b AS3 2c other 1c 2a networks other 1a 2b networks 1b AS2 AS1 1d Network Layer 4-102 Example: choosing among multiple ASes  now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.  to configure forwarding table, router 1d must determine which gateway it should forward packets towards for dest x  this is also job of inter-AS routing protocol! 3c x 3a 3b AS3 2c other 1c 2a networks other 1a 2b networks 1b AS2 AS1 1d ? Network Layer 4-103  Example: choosing among multiple ASes  now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.  to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x  this is also job of inter-AS routing protocol!  hot potato routing: send packet towards closest of two routers. use routing info determine from learn from inter-AS hot potato routing: forwarding table the from intra-AS protocol that subnet choose the gateway interface I that leads protocol to determine x is reachable via that has the to least-cost gateway. costs of least-cost multiple gateways smallest least cost Enter (x,I) in paths to each of the gateways forwarding table Network Layer 4-104 Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and  link state datagram networks  distance vector 4.3 what’s inside a router  hierarchical routing 4.4 IP: Internet Protocol 4.6 routing in the Internet  datagram format  RIP  IPv4 addressing  OSPF  ICMP  BGP  IPv6 4.7 broadcast and multicast routing Network Layer 4-105 Intra-AS Routing  also known as interior gateway protocols (IGP)  most common intra-AS routing protocols:  RIP: Routing Information Protocol  OSPF: Open Shortest Path First  IGRP: Interior Gateway Routing Protocol (Cisco proprietary) Network Layer 4-106  RIP ( Routing Information Protocol)  included in BSD-UNIX distribution in 1982  distance vector algorithm  distance metric: # hops (max = 15 hops), each link has cost 1  DVs exchanged with neighbors every 30 sec in response message (aka advertisement)  each advertisement: list of up to 25 destination subnets (in IP addressing sense) from router A to destination subnets: u v subnet hops w u 1 A B v 2 w 2 x x 3 z C D y 3 y z 2 Network Layer 4-107 RIP: example z w x y A D B C routing table in router D destination subnet next router # hops to dest w A 2 y B 2 z B 7 x -- 1 …. …..... Network Layer 4-108 RIP: example A-to-D advertisement dest next hops w - 1 x - 1 z C 4 …. …... z w x y A D B C routing table in router D destination subnet next router # hops to dest w A 2 y B 2 A 5 z B 7 x -- 1 …. …..... Network Layer 4-109 RIP: link failure, recovery if no advertisement heard after 180 sec --> neighbor/link declared dead  routes via neighbor invalidated  new advertisements sent to neighbors  neighbors in turn send out new advertisements (if tables changed)  link failure info quickly (?) propagates to entire net  poison reverse used to prevent ping-pong loops (infinite distance = 16 hops) Network Layer 4-110 RIP table processing  RIP routing tables managed by application-level process called route-d (daemon)  advertisements sent in UDP packets, periodically repeated routed routed transport transprt (UDP) (UDP) network forwarding forwarding network (IP) table table (IP) link link physical physical Network Layer 4-111 OSPF (Open Shortest Path First)  “open”: publicly available  uses link state algorithm  LS packet dissemination  topology map at each node  route computation using Dijkstra’s algorithm  OSPF advertisement carries one entry per neighbor  advertisements flooded to entire AS  carried in OSPF messages directly over IP (rather than TCP or UDP  IS-IS routing protocol: nearly identical to OSPF Network Layer 4-112 OSPF “advanced” features (not in RIP)  security: all OSPF messages authenticated (to prevent malicious intrusion)  multiple same-cost paths allowed (only one path in RIP)  for each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort ToS; high for real time ToS)  integrated uni- and multicast support:  Multicast OSPF (MOSPF) uses same topology data base as OSPF  hierarchical OSPF in large domains. Network Layer 4-113 Hierarchical OSPF boundary router backbone router backbone area border routers area 3 internal routers area 1 area 2 Network Layer 4-114 Hierarchical OSPF  two-level hierarchy: local area, backbone.  link-state advertisements only in area  each nodes has detailed area topology; only know direction (shortest path) to nets in other areas.  area border routers: “summarize” distances to nets 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 4-115 Internet inter-AS routing: BGP  BGP (Border Gateway Protocol): the de facto inter-domain routing protocol  “glue that holds the Internet together”  BGP provides each AS a means to:  eBGP: obtain subnet reachability information from neighboring ASs.  iBGP: propagate reachability information to all AS- internal routers.  determine “good” routes to other networks based on reachability information and policy.  allows subnet to advertise its existence to rest of Internet: “I am here” Network Layer 4-116  BGP basics  BGP session: two BGP routers (“peers”) exchange BGP messages:  advertising paths to different destination network prefixes (“path vector” protocol)  exchanged over semi-permanent TCP connections  when AS3 advertises a prefix to AS1:  AS3 promises it will forward datagrams towards that prefix  AS3 can aggregate prefixes in its advertisement 3c BGP 3a message 3b AS3 2c other 1c 2a networks other 1a 2b networks 1b AS2 AS1 1d Network Layer 4-117 BGP basics: distributing path information  using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1.  1c can then use iBGP do 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 3a iBGP session 3b AS3 2c other 1c 2a networks other 1a 2b networks 1b AS2 AS1 1d Network Layer 4-118 Path attributes and BGP routes  advertised prefix includes BGP attributes  prefix + attributes = “route”  two important attributes:  AS-PATH: contains ASs through which prefix advertisement has passed: e.g., AS 67, AS 17  NEXT-HOP: indicates specific internal-AS router to next- hop AS. (may be multiple links from current AS to next- hop-AS)  gateway router receiving route advertisement uses import policy to accept/decline  e.g., never route through AS x  policy-based routing Network Layer 4-119 BGP route selection  router may learn about more than 1 route to destination AS, selects route based on: 1. local preference value attribute: policy decision 2. shortest AS-PATH 3. closest NEXT-HOP router: hot potato routing 4. additional criteria Network Layer 4-120 BGP messages  BGP messages exchanged between peers over TCP connection  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 msg; also used to close connection Network Layer 4-121 Putting it Altogether: How Does an Entry Get Into a Router’s Forwarding Table?  Answer is complicated!  Ties together hierarchical routing (Section 4.5.3) with BGP (4.6.3) and OSPF (4.6.2).  Provides nice overview of BGP! How does entry get in forwarding table? routing algorithms Assume prefix is local forwarding table in another AS. entry prefix output port 138.16.64/22 3 124.12/16 2 212/8 4 ………….. … Dest IP 1 3 2 How does entry get in forwarding table? High-level overview 1. Router becomes aware of prefix 2. Router determines output port for prefix 3. Router enters prefix-port in forwarding table Router becomes aware of prefix 3c BGP 3a message 3b AS3 2c other 1c 2a networks other 1a 2b networks 1b AS2 AS1 1d  BGP message contains “routes”  “route” is a prefix and attributes: AS-PATH, NEXT- HOP,…  Example: route:  Prefix:138.16.64/22 ; AS-PATH: AS3 AS131 ; NEXT-HOP: 201.44.13.125 Router may receive multiple routes 3c BGP 3a message 3b AS3 2c other 1c 2a networks other 1a 2b networks 1b AS2 AS1 1d  Router may receive multiple routes for same prefix  Has to select one route Select best BGP route to prefix  Router selects route based on shortest AS-PATH  Example: select  AS2 AS17 to 138.16.64/22  AS3 AS131 AS201 to 138.16.64/22  What if there is a tie? We’ll come back to that! Find best intra-route to BGP route  Use selected route’s NEXT-HOP attribute  Route’s NEXT-HOP attribute is the IP address of the router interface that begins the AS PATH.  Example:  AS-PATH: AS2 AS17 ; NEXT-HOP: 111.99.86.55  Router uses OSPF to find shortest path from 1c to 111.99.86.55 3c 3a 111.99.86.55 3b AS3 2c other 1c 2a networks other 1a 2b networks 1b AS2 AS1 1d Router identifies port for route  Identifies port along the OSPF shortest path  Adds prefix-port entry to its forwarding table:  (138.16.64/22 , port 4) 3c router 3a port 3b AS3 1 2c other 1c 4 2a networks 2 3 other 1a 2b networks 1b AS2 AS1 1d  Hot Potato Routing  Suppose there two or more best inter-routes.  Then choose route with closest NEXT-HOP  Use OSPF to determine which gateway is closest  Q: From 1c, chose AS3 AS131 or AS2 AS17?  A: route AS3 AS201 since it is closer 3c 3a 3b AS3 2c other 1c 2a networks other 1a