Chapter 4 Network Layer

Transcription

1 Chapter 4 Network Layer Modified form the following All material copyright J.F Kurose and K.W. Ross, All Rights Resered Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 Network Layer 4-1

2 Chapter 4: network layer chapter goals understand principles behind network layer serices network layer serice models forwarding ersus routing how a router works routing (path selection) broadcast, multicast instantiation, implementation in the Internet Network Layer 4-2

3 Chapter 4: outline 4.1 introduction 4.2 irtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IP4 addressing ICMP IP6 4.5 routing algorithms link state distance ector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-3

4 Network layer transport segment from sending to receiing host on sending side encapsulates segments into datagrams on receiing side deliers segments to transport layer network layer protocols in eery host, router router examines header fields in all IP datagrams passing through it 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 network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical Network Layer 4-4

5 Two key network-layer functions forwarding moe packets from router s input to appropriate router output routing determine route taken by packets from source to dest routing algorithms Analogy routing process of planning trip from source to dest forwarding process of getting through single interchange Network Layer 4-5

6 Interplay between routing and forwarding routing algorithm local forwarding table header alue output link routing algorithm determines end-end-path through network forwarding table determines local forwarding at this router dest. IP address alue in arriing packet s header Network Layer 4-6

7 Connection setup important function in some (not all) network architectures (nor Internet) ATM, frame relay, X.25 before datagrams flow, two end hosts and interening routers establish irtual connection routers get inoled network s. transport layer connection serice network: between two hosts (may also inole interening routers in case of VCs) transport: between two processes Network Layer 4-7

8 Network serice model Q: What serice model for channel transporting datagrams from sender to receier? example serices for indiidual datagrams guaranteed deliery guaranteed deliery with less than 40 msec delay example serices for a flow of datagrams in-order datagram deliery guaranteed minimum bandwidth to flow restrictions on changes in inter-packet spacing (delay jitter) Network Layer 4-8

9 Network layer serice models Network Architecture Serice Model Bandwidth Guarantees? Loss Order Timing Congestion feedback Internet ATM ATM ATM ATM best effort CBR VBR ABR UBR none constant rate guaranteed rate guaranteed minimum none no yes yes no no no yes yes yes yes no yes yes no no no (inferred ia loss) no congestion no congestion yes CBR: Constant Bit Rate (e.g. lease line, expensie) VBR: Variable Bit Rate ABR: Aailable Bit Rate (ABR is allocated after CBR, VBR are allocated) UBR: Unspecified Bit Rate (UBR is allocated after CBR, VBR, ABR are allocated) ATM is ery good but too expensie no (RM cell)

10 Chapter 4: outline 4.1 introduction 4.2 irtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IP4 addressing ICMP IP6 4.5 routing algorithms link state distance ector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-10

11 Connection, connection-less serice datagram network proides network-layer connectionless serice irtual-circuit network proides network-layer connection serice analogous to TCP/UDP connection-oriented / connectionless transport-layer serices, but serice: host-to-host no choice: network proides one or the other (either one of datagram and VC) implementation: in network core Network Layer 4-11

12 Virtual circuits source-to-dest path behaes 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) eery router on source-dest path maintains state for each passing connection link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable serice) Network Layer 4-12

13 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 (each entry for each connection) 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

14 VC forwarding table forwarding table in northwest router: VC number interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # VC routers maintain connection state information! Network Layer 4-14

15 Virtual circuits: signaling protocols used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today s Internet application transport network data link physical 5. data flow begins 6. receie data 4. call connected 3. accept call 1. initiate call 2. incoming call application transport network data link physical Network Layer 4-15

16 Datagram networks no call setup at network layer routers: no state about end-to-end connections no network-leel concept of connection packets forwarded using destination host address packets between same source-dest pair may take different paths application transport network data link physical 1. send datagrams 2. receie datagrams application transport network data link physical Network Layer 4-16

17 Datagram forwarding table routing algorithm local forwarding table dest address output link address-range 1 address-range 2 address-range 3 address-range billion IP addresses, so rather than list indiidual destination address list range of addresses (aggregate table entries) IP destination address in arriing packet s header Network Layer 4-17

18 Datagram forwarding table Destination Address Range through don t care through don t care through don t care otherwise Link Interface (router port) Network Layer 4-18

19 Longest prefix matching longest prefix matching when looking for forwarding table entry for gien destination address, use longest address prefix that matches destination address. examples: Destination Address Range *** ********* ********* *** ********* otherwise DA: DA: Link interface which interface? (port#0) which interface? (port#1) Network Layer 4-19

20 Datagram or VC network: why? Internet (datagram) data exchange among computers elastic serice (elastic bandwidth), no strict timing req. many link types (WiFi, WiMax, Ethernet, etc.) different characteristics (speed, delay, protocol) uniform serice is difficult smart end systems (computers) can adapt, perform control, error recoery simple inside network (router), complexity at edge (client) ATM (VC) eoled from telephony human conersation strict timing, reliability requirements need for guaranteed serice dumb end systems telephones (simple phones) complexity inside network Network Layer 4-20

21 Chapter 4: outline 4.1 introduction 4.2 irtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IP4 addressing ICMP IP6 4.5 routing algorithms link state distance ector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-21

22 Router architecture oeriew two key router functions run routing algorithms/protocol (RIP, OSPF, BGP) to create routing tables forwarding datagrams from incoming to outgoing link forwarding tables computed, pushed to input ports routing processor routing, management control plane (software) forwarding data plane (hardware) high-seed switching fabric (IC) router input ports router output ports Network Layer 4-22

23 Input port functions wired or wireless links line termination link layer protocol (receie) lookup, forwarding queueing switch fabric physical layer: bit-leel reception data link layer: *error checking *e.g., Ethernet decentralized switching gien datagram dest., lookup output port using forwarding table in input port memory ( match plus action ) goal: complete input port processing at line (link) speed (no delay) [e.g. 168,000 pkts/s I/O for 100Mbps Ethernet, 1,680,000 pkts/s I/O for 1Gbps Ethernet] queuing: if datagrams arrie faster than forwarding rate into switch fabric Network Layer 4-23

24 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 rate N inputs: switching rate N times line rate desirable three types of switching fabrics memory memory bus crossbar Network Layer 4-24

25 Switching ia 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 I/O memory memory I/O output port (e.g., Ethernet) system bus (memory bus) Network Layer 4-25

26 Switching ia a bus second generation routers datagram from input port memory to output port memory ia a shared bus bus contention: switching speed limited by bus bandwidth 32 Gbps bus, Cisco 5600: sufficient speed for access an enterprise routers broadcast datagram bus Network Layer 4-26

27 Switching ia interconnection network third generation routers oercome bus bandwidth limitations Banyan networks, crossbar, other interconnection nets initially deeloped to connect processors in multiprocessor adanced design: fragmenting datagram into fixed length cells, switch cells through the fabric Cisco 12000: switches 60 Gbps through the interconnection network crossbar Network Layer 4-27

28 Output ports switch fabric datagram buffer queueing link layer protocol (send) line termination buffering required when datagrams arrie from fabric faster than the transmission rate scheduling discipline chooses among queued datagrams for transmission e.g. FIFO, Priority Queue, Round Robin, Weighted Fair Queue Network Layer 4-28

29 Output port queueing switch fabric switch fabric At time t, packets more from input to output one packet time later buffering when arrial rate ia switch exceeds output line speed queueing (delay) and loss due to output port buffer oerflow! Network Layer 4-29

30 How much buffering? RFC 3439 rule of thumb: aerage buffering equal to typical RTT (say 250 msec) times link capacity C e.g., if C = 10 Gpbs link, then 2.5 Gbit buffer recent recommendation: with N flows, buffering equal to RTT. C N Network Layer 4-30

31 Input port queuing fabric slower than input ports combined à queuing may occur at input queues queuing delay and loss due to input buffer oerflow! Head-of-the-Line (HOL) blocking: queued datagram at front of queue (input port buffer) preents others in queue from moing forward switch fabric switch fabric output port contention: only one red datagram can be transferred. lower red packet is blocked one packet time later: green packet experiences HOL blocking Network Layer 4-31

32 Chapter 4: outline 4.1 introduction 4.2 irtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IP4 addressing ICMP IP6 4.5 routing algorithms link state distance ector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-32

33 The Internet network layer (IP4) host, router network layer functions transport layer: TCP, UDP network layer routing protocols path selection RIP, OSPF, BGP forwarding table IP protocol addressing conentions datagram format packet handling conentions ICMP protocol error reporting router signaling link layer physical layer Network Layer 4-33

34 IP datagram format IP protocol ersion number header length (bytes) type of data (priority) max number remaining hops (decremented at each router) upper layer protocol to delier payload to how much oerhead? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer oerhead er head. len 16-bit identifier time to lie type of serice upper layer 32 bits flgs length fragment offset header checksum 32 bit source IP address 32 bit destination IP address options (if any) data (ariable length, typically a TCP or UDP segment) total datagram length (bytes) for fragmentation/ reassembly e.g. timestamp, record route taken, specify list of routers to isit Network Layer 4-34

35 IP fragmentation, reassembly network links hae MTU (max transfer unit) - largest possible link-leel frame different link types, different MTUs e.g. max Ethernet MTU = 1518 bytes, FDDI MTU = 4500 bytes large IP datagram diided ( fragmented ) within net one datagram becomes seeral datagrams reassembled only at final destination IP header bits used to identify, order related fragments reassembly fragmentation: in: one large datagram out: 3 smaller datagrams Network Layer 4-35

36 IP fragmentation, reassembly example: 4000 byte datagram MTU = 1500 bytes length =4000 ID =x fragflag =0 offset =0 one large datagram becomes seeral smaller datagrams 1480 bytes in data field length =1500 ID =x fragflag =1 offset =0 offset = 1480/8 length =1500 ID =x fragflag =1 offset =185 length =1060 ID =x fragflag =0 offset = = 1500 (MTU) - 20 (IP header) 4000 = offset = 1 means 8 bytes Network Layer 4-36

37 Chapter 4: outline 4.1 introduction 4.2 irtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IP4 addressing ICMP IP6 4.5 routing algorithms link state distance ector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-37

38 IP addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link routers typically hae multiple interfaces host typically has one or two interfaces (e.g., wired Ethernet, wireless ) IP addresses associated with each interface = Network Layer 4-38

39 IP addressing: introduction Q: how are interfaces actually connected? A: wired Ethernet interfaces connected by Ethernet switches A: wireless WiFi interfaces connected by WiFi base station Network Layer 4-39

40 Subnets IP address subnet part - high order bits host part - low order bits what s a subnet? deice interfaces with same subnet part of IP address can physically reach each other without interening router class A: subnet mask: /8 class B: subnet mask: /16 class C: subnet mask: / / / subnet network consisting of 3 subnets /24 Network Layer 4-40

41 Subnets how many? /24 mask Network Layer 4-41

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 part /23 mask host part class A: subnet mask: /8 class B: subnet mask: /16 class C: subnet mask: /24 Network Layer 4-42

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 à network à configuration à tcp/ip à properties UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol: dynamically get address from a serer plug-and-play Network Layer 4-43

44 DHCP: Dynamic Host Configuration Protocol goal: allow host to dynamically obtain its IP address from network serer when it joins network can renew its lease on address in use allows reuse of addresses (only hold address while connected on ) [oerbooking] support for mobile users who want to join network (more shortly) DHCP oeriew host broadcasts DHCP discoer msg [optional] DHCP serer responds with DHCP offer msg [optional] host requests IP address: DHCP request msg DHCP serer sends address: DHCP ack msg Network Layer 4-44

45 DHCP client-serer scenario / DHCP serer arriing DHCP client needs address in this network / /24 Network Layer 4-45

46 DHCP client-serer scenario DHCP serer: DHCP discoer src : , 68 dest.: ,67 yiaddr: transaction ID: 654 arriing client DHCP request DHCP offer src: , 68 dest:: , 67 yiaddrr: transaction ID: 655 lifetime: 3600 secs src: , 67 dest: , 68 yiaddrr: transaction ID: 654 lifetime: 3600 secs DHCP ACK src: , 67 dest: , 68 yiaddrr: transaction ID: 655 lifetime: 3600 secs YIADDR:Your IP Address (DHCP) Network Layer 4-46

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 seer network mask (indicating network ersus host portion of address) Network Layer 4-47

48 DHCP: example DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy DHCP DHCP UDP IP Eth Phy router with DHCP serer built into router connecting laptop needs its IP address, addr of first-hop router, addr of DNS serer: use DHCP DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in Ethernet Ethernet frame broadcast (dest: FFFFFFFFFFFF[ ]) on LAN, receied at router running DHCP serer Ethernet demuxed to IP demuxed, UDP demuxed to DHCP Network Layer 4-48

49 DHCP: example DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy DHCP UDP IP Eth Phy router with DHCP serer built into router DHCP serer formulates DHCP ACK containing client s IP address, IP address of first-hop router for client, name & IP address of DNS serer encapsulation of DHCP serer, frame forwarded to client, demuxing up to DHCP at client client now knows its IP address, name and IP address of DSN serer, IP address of its first-hop router Network Layer 4-49

50 DHCP: Wireshark output (home LAN) Message type: Boot Request (1) Hardware type: Ethernet Hardware address length: 6 Hops: 0 request Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Bootp flags: 0x0000 (Unicast) Client IP address: ( ) Your (client) IP address: ( ) Next serer IP address: ( ) Relay agent IP address: ( ) Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Serer host name not gien Boot file name not gien Magic cookie: (OK) Option: (t=53,l=1) DHCP Message Type = DHCP Request Option: (61) Client identifier Length: 7; Value: D323688A; Hardware type: Ethernet Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Option: (t=50,l=4) Requested IP Address = 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 Serer 44 = NetBIOS oer TCP/IP Name Serer Message type: Boot Reply (2) Hardware type: Ethernet reply Hardware address length: 6 Hops: 0 Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Bootp flags: 0x0000 (Unicast) Client IP address: ( ) Your (client) IP address: ( ) Next serer IP address: ( ) Relay agent IP address: ( ) Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Serer host name not gien Boot file name not gien Magic cookie: (OK) Option: (t=53,l=1) DHCP Message Type = DHCP ACK Option: (t=54,l=4) Serer Identifier = Option: (t=1,l=4) Subnet Mask = Option: (t=3,l=4) Router = Option: (6) Domain Name Serer Length: 12; Value: E F ; IP Address: ; IP Address: ; IP Address: Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net." Network Layer 4-50

51 IP addresses: how to get one? Q: how does network get subnet part of IP addr? A: gets allocated portion of its ISP s address space ISP's block /20 Organization /23 Organization /23 Organization / Organization /23 Network Layer 4-51

52 Hierarchical addressing: route aggregation hierarchical addressing allows efficient adertisement of routing information: Organization /23 Organization /23 Organization /23 Organization Fly-By-Night-ISP Send me anything with addresses beginning /20 Internet /23 ISPs-R-Us Send me anything with addresses beginning /16 Network Layer 4-52

53 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization /23 Organization /23 Organization /23 Organization / Fly-By-Night-ISP ISPs-R-Us Send me anything with addresses beginning /20 Send me anything with addresses beginning /16 or /23 (Longest prefix matching) Internet Network Layer 4-53

54 IP addressing: the last word... Q: how does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resoles disputes Network Layer 4-54

55 IP4 address 枯竭時間 IANA 進入無 IP4 位址可核發的階段 RIRs 進入無 IP4 位址可核發的階段 IANA:Internet Assigned Numbers Authority ( 國際 IP 位址管理機構 ) RIR:Regional Internet Registry Network Layer 4-55

56 NAT: network address translation rest of Internet local network (e.g., home network) / all datagrams leaing local network hae same single source NAT IP address: , different source port numbers datagrams with source or destination in this network hae /24 address for source, destination (as usual) Network Layer 4-56

57 NAT: network address translation motiation: 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 deices can change addresses of deices in local network without notifying outside world can change ISP without changing addresses of deices in local network deices inside local net not explicitly addressable, isible by outside world (a security plus) Network Layer 4-57

58 NAT: network address translation implementation: NAT router must outgoing datagrams: replace (source IP address, port #, e.g ) of eery outgoing datagram to (NAT IP address, new port #, e.g )... remote clients/serers will respond using (NAT IP address, new port #, e.g ) as destination addr remember (in NAT translation table) eery (source IP address, port #) to (NAT IP address, new port #) translation pair incoming datagrams: replace (NAT IP address, new port #) in dest fields of eery incoming datagram with corresponding (source IP address, port #) stored in NAT table Network Layer 4-58

59 NAT: network address translation 2: NAT router changes datagram source addr from , 3345 to , 5001, updates table 2 NAT translation table WAN side addr LAN side addr , , 3345 S: , 5001 D: , S: , 3345 D: , : host sends datagram to , S: , 80 D: , : reply arries dest. address: , 5001 S: , 80 D: , : NAT router changes datagram dest. addr from , 5001 to , Network Layer 4-59

60 NAT: network address translation 16-bit port-number field 65,536 simultaneous connections with a single LAN-side address! NAT is controersial routers should only process up to layer 3 (howeer, port is of layer 4) iolates end-to-end argument NAT possibility must be taken into account by app designers, e.g., P2P applications address shortage should instead be soled by IP6 Network Layer 4-60

61 NAT traersal problem client wants to connect to serer with address serer address local to LAN (client can t use it as destination addr.) only one externally isible NATed address: solution1: statically configure NAT to forward incoming connection requests at gien port to serer e.g., ( , port 2500) always forwarded to port client? NAT router Network Layer 4-61

62 NAT traersal problem solution 2: Uniersal Plug and Play (UPnP) Internet Gateway Deice (IGD) Protocol. Allows NATed host to: learn public IP address ( ) add/remoe port mappings (with lease times) NAT router IGD i.e., automate static NAT port map configuration Network Layer 4-62

63 NAT traersal problem solution 3: relaying (used in Skype) NATed client establishes connection to relay external client connects to relay relay bridges packets between to connections client 2. connection to relay initiated by client 3. relaying established 1. connection to relay initiated by NATed host NAT router Network Layer 4-63

64 Chapter 4: outline 4.1 introduction 4.2 irtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IP4 addressing ICMP IP6 4.5 routing algorithms link state distance ector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-64

65 ICMP: internet control message protocol used by hosts & routers to communicate network-leel information error reporting unreachable host, network, port, protocol echo request/reply (used by ping) network-layer aboe IP ICMP msgs carried in IP datagrams ICMP message type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route adertisement 10 0 router discoery 11 0 TTL expired 12 0 bad IP header Network Layer 4-65

66 Traceroute and ICMP source sends series of UDP segments to dest first set has TTL =1 second set has TTL=2, etc. unlikely port number (not exist port number) When the nth set of datagrams arries to the nth router router discards datagrams and sends source ICMP messages (type 11, code 0) ICMP messages includes name of router & IP address when ICMP messages arries, source records RTTs traceroute does this 3 times stopping criteria UDP segment eentually arries at destination host destination returns ICMP port unreachable message (type 3, code 3) source stops 3 probes 3 probes 3 probes Network Layer 4-66

67 IP6: motiation initial motiation 32-bit address space soon to be completely allocated additional motiation header format helps speed processing / forwarding header changes to facilitate QoS IP6 datagram format fixed-length 40 byte header (easy processed by router) no fragmentation allowed fragmentation is done at source, not router Network Layer 4-67

68 IP6 datagram format priority: identify priority among datagrams in flow flow label: identify datagrams in same flow (sametcp connection) (concept of flow not well defined) next header: identify upper layer protocol for data, e.g. fragmentation er pri flow label payload len next hdr hop limit source address (128 bits) destination address (128 bits) data 32 bits Network Layer 4-68

69 Network Layer 4-69

70 Network Layer 4-70

71 Network Layer 4-71

72 Network Layer 4-72

73 Other changes from IP4 checksum: remoed entirely to reduce processing time at each hop (leae error check to Layer 2) options: allowed, but outside of header, indicated by next header field (e.g. track the routers isited, RTT, etc.) ICMP6: new ersion of ICMP additional message types, e.g. Packet Too Big (fragmentation is done at source, not router) multicast group management functions Network Layer 4-73

74 Anycast is a network addressing and routing method in which datagrams from a single sender are routed to the topologically nearest node in a group of potential receiers, though it may be sent to seeral nodes, all identified by the same destination address. Network Layer 4-74

75 Transition from IP4 to IP6 not all routers can be upgraded simultaneously no flag days how will network operate with mixed IP4 and IP6 routers? tunneling: IP6 datagram carried as payload in IP4 datagram among IP4 routers IP4 header fields IP4 source, dest addr IP6 header fields IP6 source dest addr UDP/TCP payload IP4 payload IP4 datagram IP6 datagram Network Layer 4-75

76 Tunneling logical iew: A IP6 B IP6 IP4 tunnel connecting IP6 routers E IP6 F IP6 physical iew: A B C D E F IP6 IP6 IP4 IP4 IP6 IP6 Network Layer 4-76

77 Tunneling logical iew: A IP6 B IP6 IP4 tunnel connecting IP6 routers E IP6 F IP6 physical iew: A B C D E F IP6 IP6 IP4 IP4 IP6 IP6 flow: X src: A dest: F data src:b dest: E flow: X src: A dest: F src:b dest: E flow: X src: A dest: F flow: X src: A dest: F data data data A-to-B: IP6 B-to-E: IP6 inside IP4 B-to-E: IP6 inside IP4 E-to-F: IP6 Network Layer 4-77

78 Chapter 4: outline 4.1 introduction 4.2 irtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IP4 addressing ICMP IP6 4.5 routing algorithms link state distance ector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-78

79 Interplay between routing, forwarding routing algorithm local forwarding table dest address output link address-range 1 address-range 2 address-range 3 address-range routing algorithm determines end-end-path through network forwarding table determines local forwarding at this router IP destination address in arriing packet s header Network Layer 4-79

80 Graph abstraction 5 graph: G = (N,E) u 1 2 x w y z N = set of routers = { u,, w, x, y, z } E = set of links ={(u,), (u,x), (,x), (,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 4-80

81 Graph abstraction: costs u c(x,x ) = cost of link (x,x ) e.g., c(w,z) = 5 3 w 5 cost could always be 1, or z inersely related to bandwidth (more bandwidth less cost), or related to 2 x y congestion (more congestion 1 more cost) cost of path (x 1, x 2, x 3,, x p ) = c(x 1,x 2 ) + c(x 2,x 3 ) + + c(x p-1,x p ) key question: what is the least-cost path between u and z? (u à x à y à z) routing algorithm: algorithm that finds the least cost path Network Layer 4-81

82 Routing algorithm classification Q: global or decentralized information? global all routers hae complete topology, link cost info link state algorithms decentralized router knows physicallyconnected neighbors, link costs to neighbors iteratie process of computation, exchange of info with neighbors distance ector algorithms Q: static or dynamic? static routes change slowly oer time dynamic routes change more quickly periodic update in response to link cost changes Network Layer 4-82

83 Chapter 4: outline 4.1 introduction 4.2 irtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IP4 addressing ICMP IP6 4.5 routing algorithms link state distance ector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-83

84 A Link-State Routing Algorithm Dijkstra s algorithm net topology, link costs known to all nodes accomplished ia link state broadcast all nodes hae same info computes least cost paths from one node ( source ) to all other nodes gies forwarding table for that node iteratie: after k iterations, know least cost path to k dests notation: c(x,y): link cost from node x to y; = if not direct neighbors D(): current alue of cost of path from source to dest. p(): predecessor node along path from source to N': set of nodes whose least cost path definitiely known Network Layer 4-84

85 Dijsktra s Algorithm 1 Initialization 2 N' = {u} 3 for all nodes 4 if adjacent to u 5 then D() = c(u,) 6 else D() = 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 add w to N' 11 update D() for all adjacent to w and not in N' : 12 D() = min( D(), D(w) + c(w,) ) 13 /* new cost to is either old cost to or known 14 shortest path cost to w plus cost from w to */ 15 until all nodes in N' Network Layer 4-85

86 Dijkstra s algorithm: example Step N' D() p() D(w) p(w) D(x) p(x) D(y) p(y) D(z) p(z) u 7,u 3,u 5,u uw 6,w 5,u 11,w uwx 6,w 11,w 14,x uwx 10, 14,x uwxy 12,y uwxyz notes: construct shortest path tree by tracing predecessor nodes u 5 3 x w y 9 2 z Network Layer 4-86

87 Network Layer 4-87 Dijkstra s algorithm: another example Step N' u ux uxy uxy uxyw uxywz D(),p() 2,u 2,u 2,u D(w),p(w) 5,u 4,x 3,y 3,y D(x),p(x) 1,u D(y),p(y) 2,x D(z),p(z) 4,y 4,y 4,y u y x w z

88 Dijkstra s algorithm: example (2) resulting shortest-path tree from u w u z x y resulting forwarding table in u destination x y w z link (u,) (u,x) (u,x) (u,x) (u,x) Network Layer 4-88

89 Dijkstra s algorithm, discussion 1 D algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n(n+1)/2 comparisons: O(n 2 ) more efficient implementations possible: O(nlogn) e.g. quick sort, merge sort, heap sort, etc. oscillations possible e.g., suppose link cost equals amount of carried traffic A 1 1+e C e initially e B D A 2+e e 1 C B 1 1 e gien these costs, find new routing. resulting in new costs 0 D A 0 2+e C 1+e B e gien these costs, find new routing. resulting in new costs D A 2+e e C B e gien these costs, find new routing. resulting in new costs 0

90 Chapter 4: outline 4.1 introduction 4.2 irtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IP4 addressing ICMP IP6 4.5 routing algorithms link state distance ector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-90

91 Distance ector algorithm Bellman-Ford equation (dynamic programming) let d x (y) := cost of least-cost path from x to y then d x (y) = min {c(x,) + d (y) } cost from neighbor to destination y cost to neighbor min is taken oer all neighbors of x Network Layer 4-91

92 Bellman-Ford example u x u à z 3 w y z clearly, d (z) = 5, d x (z) = 3, d w (z) = 3 B-F equation says d u (z) = min {c(u,) + d (z), c(u,x) + d x (z), c(u,w) + d w (z)} = min {2 + 5, 1 + 3, 5 + 3} = 4 node achieing minimum is next hop in shortest path, used in forwarding table Network Layer 4-92

93 Distance ector algorithm D x (y) = estimate of least cost from x to y node x knows cost to each neighbor : c(x,) maintains its own distance ector D x = [D x (y): y є N ] maintains its neighbors distance ectors for each neighbor, x maintains D = [D (y): y є N ] Network Layer 4-93

94 Distance ector algorithm key idea from time-to-time, each node sends its own distance ector estimate to neighbors asynchronous message exchange with neighbors when x receies new DV estimate from neighbor, it updates its own DV using B-F equation D x (y) min {c(x,) + D (y)} for each node y N under natural conditions, the estimate D x (y) conerge to the actual least cost d x (y) Network Layer 4-94

95 Distance ector algorithm iteratie, asynchronous: each local iteration caused by local link cost change DV update message from neighbor distributed each node notifies neighbors only when its DV changes neighbors then notify their neighbors if necessary each node wait for (change in local link cost or msg from neighbor) recompute estimates if DV to any dest has changed, notify neighbors Network Layer 4-95

96 node x table from x y z cost to x y z D x (y) = min{c(x,y) + D y (y), c(x,z) + D z (y)} = min{2+0, 7+1} = 2 from x y z x y z 0 cost to D x (z) = min{c(x,y) + D y (z), c(x,z) + D z (z)} = min{2+1, 7+0} = 3 node y table from x y z cost to x y z x 2 y 7 1 z node z table from cost to x y z x y z time Network Layer 4-96

97 node x table from x y z node y table from x y z cost to x y z x y z D x (y) = min{c(x,y) + D y (y), c(x,z) + D z (y)} = min{2+0, 7+1} = 2 cost to from from x y z x y z x y z 0 cost to 2 cost to x y z from from x y z x y z cost to x y z cost to x y z D x (z) = min{c(x,y) + D y (z), c(x,z) + D z (z)} = min{2+1, 7+0} = 3 x 2 y 7 1 z node z table from cost to x y z x y z from x y z cost to x y z x y z x y z Stable time from cost to Network Layer 4-97

98 Distance ector: link cost changes link cost changes good news traels fast node detects local link cost change (e.g. 4 à 1) updates routing info, recalculates distance ector if DV changes, notify neighbors t 0 : y detects link-cost change, updates its DV, informs its neighbors t 1 : z receies update from y, updates its table, computes new least cost to x, sends its neighbors its DV t 2 : y receies z s update, updates its distance table. y s least costs do not change, so y does not send a message to z x 1 4 y 50 1 z Network Layer 4-98

99 Distance ector: link cost changes link cost changes node detects local link cost change bad news traels slow - count to infinity problem! (e.g. 4 à 60) 44 iterations before algorithm stabilizes: see text poisoned reerse 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 ia Z) will this completely sole count to infinity problem? (No) 60 x 4 y 50 1 z Network Layer 4-99

100 44 iterations!

101 Comparison of LS and DV algorithms message complexity LS: with n nodes, E links, O(nE) msgs sent DV: exchange between neighbors only conergence time aries (e.g. good news fast, bad news slow) speed of conergence LS: O(n 2 ) algorithm (Dijkstra) requires O(nE) msgs may hae oscillations DV: conergence time aries may be routing loops count-to-infinity problem (bad news) robustness: what happens if router malfunctions? LS: (more robust) node can adertise incorrect link cost each node computes only its own table DV: (less robust) DV node can adertise incorrect path cost (e.g. D x (y)) each node s table used by others error propagate thru network Network Layer 4-101

102 Chapter 4: outline 4.1 introduction 4.2 irtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IP4 addressing ICMP IP6 4.5 routing algorithms link state distance ector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-102

103 Hierarchical routing our routing study thus far - idealization all routers identical network flat not true in practice scale: with 600 million destinations can t store all dest s in routing tables! routing table exchange would swamp links! administratie autonomy internet = network of networks each network admin may want to control routing in its own network Network Layer 4-103

104 Hierarchical routing aggregate routers into regions, autonomous systems (AS) routers in same AS run same routing protocol (e.g. all LS or all DV) intra-as routing protocol, e.g. CS & Math of NCCU routers in different AS (intra-as) can run different intra-as routing protocol, e.g. NCCU & NTU gateway router at edge of its own AS has link to router in another AS Network Layer 4-104

105 Interconnected ASes 3c 3a 3b AS3 1a 1c 1d 1b Intra-AS routing algorithm AS1 Forwarding table Inter-AS routing algorithm 2a 2c AS2 2b forwarding table configured by both intraand inter-as routing algorithm intra-as sets entries for internal dests inter-as & intra-as sets entries for external dests Network Layer 4-105

106 Inter-AS tasks suppose router in AS1 receies datagram destined outside of AS1 router should forward packet to gateway router, but which one? AS1 must 1. learn which dests are reachable through AS2, which through AS3 2. propagate this reachability info to all routers in AS1 job of inter-as routing! 3c other networks 3b 3a AS3 1a AS1 1c 1d 1b 2a AS2 2c 2b other networks Network Layer 4-106

107 Example: setting forwarding table in router 1d suppose AS1 learns (ia inter-as protocol) that subnet x is reachable ia AS3 (gateway 1c), but not ia 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) other networks 3c 3a 3b AS3 1a AS1 1c 1d x 1b 2a AS2 2c 2b other networks Network Layer 4-107

108 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! other networks 3c 3a 3b AS3 1a AS1 1c 1d x? 1b 2a AS2 2c 2b other networks Network Layer 4-108

109 Example: choosing among multiple ASes hot potato routing: send packet towards closest of two routers learn from inter-as protocol that subnet x is reachable ia multiple gateways use routing info from intra-as protocol to determine costs of least-cost paths to each of the gateways hot potato routing: choose the gateway that has the smallest least cost determine from forwarding table the interface I that leads to least-cost gateway. enter (x,i) in forwarding table Network Layer 4-109

110 Chapter 4: outline 4.1 introduction 4.2 irtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IP4 addressing ICMP IP6 4.5 routing algorithms link state distance ector hierarchical routing 4.6 routing in the Internet RIP (Intra AS, DV) OSPF (Intra AS, LS) BGP (Inter AS) 4.7 broadcast and multicast routing Network Layer 4-110

111 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 4-111

112 RIP (Routing Information Protocol) included in BSD-UNIX distribution in 1982 distance ector algorithm distance metric: # hops (max = 15 hops), each link has cost 1 DVs exchanged with neighbors eery 30 sec in response message (aka adertisement) each adertisement: list of up to 25 destination subnets (in IP addressing sense) [e.g. use two adertisements if, say, 35 destinations] from router A to destination subnets z u A C B D y w x subnet hops u 1 2 w 2 x 3 y 3 z 2 Network Layer 4-112

113 RIP: example w x y A D B z C routing table in router D destination subnet next router # hops to dest w A 2 y B 2 z B 7 x Network Layer 4-113

114 RIP: example A-to-D adertisement dest next hops w - 1 x - 1 z C w x y A D B z C routing table in router D destination subnet next router # hops to dest w A 2 y B 2 A z B 7 5 x Network Layer 4-114

115 RIP: link failure, recoery if no adertisement heard after 180 sec --> neighbor/link declared dead routes ia neighbor inalidated new adertisements sent to neighbors neighbors in turn send out new adertisements (if tables changed) link failure info quickly (??? 180sec) propagates to entire net poison reerse used to preent ping-pong loops (infinite distance = 16 hops (normally 15 hops)) Network Layer 4-115

116 RIP table processing RIP routing tables managed by application-leel process called route-d (daemon) adertisements sent in UDP packets, periodically repeated route-d route-d transport (UDP) transprt (UDP) network (IP) forwarding table forwarding table network (IP) link link physical physical Network Layer 4-116

117 OSPF (Open Shortest Path First) open : publicly aailable uses link state algorithm LS packet dissemination topology map at each node route computation using Dijkstra s algorithm OSPF adertisement carries one entry per neighbor adertisements flooded to entire AS carried in OSPF messages directly oer IP (rather than TCP or UDP Network Layer 4-117

118 OSPF adanced features (not in RIP) security: all OSPF messages authenticated (to preent 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 (e.g. RIP is unicast) Multicast OSPF (MOSPF) uses same topology data base as OSPF hierarchical OSPF in large domains Network Layer 4-118

119 Hierarchical OSPF boundary router backbone router area border routers backbone area 3 area 1 area 2 internal routers Network Layer 4-119

120 Hierarchical OSPF two-leel hierarchy: local area, backbone link-state adertisements only in area each node has detailed area topology; only know direction (shortest path) to nets in other areas area border routers: summarize distances to nets in own area, adertise to other area border routers backbone routers: run OSPF routing limited to backbone boundary routers: connect to other AS s Network Layer 4-120

121 Internet inter-as routing: BGP BGP (Border Gateway Protocol): the de facto inter-domain routing protocol glue that holds the Internet together BGP proides each AS a means to ebgp: obtain subnet reachability information from neighboring ASs ibgp: propagate reachability information to all ASinternal routers determine good routes to other networks based on reachability information and policy allows subnet to adertise its existence to rest of Internet: I am here Network Layer 4-121

122 BGP basics BGP session: two BGP routers ( peers ) exchange BGP messages adertising paths to different destination network prefixes ( path ector protocol) exchanged oer semi-permanent TCP connections (BGP sessions) when AS3 adertises a prefix to AS1 AS3 promises it will forward datagrams towards that prefix AS3 can aggregate prefixes in its adertisement other networks 3b 3c AS3 3a 1a AS1 BGP message 1c 1d 1b 2a AS2 2c 2b other networks Network Layer 4-122

123 BGP basics: distributing path information using ebgp session between 3a and 1c, AS3 sends prefix reachability info to AS1 1c can then use ibgp to distribute new prefix info to all routers in AS1 1b can then re-adertise new reachability info to AS2 oer 1b-to- 2a ebgp session when router learns new prefix, it creates entry for prefix in its forwarding table other networks 3b 3a AS3 1a AS1 1c 1d ebgp session ibgp session 1b 2a AS2 2c 2b other networks Network Layer 4-123

124 Path attributes and BGP routes adertised prefix includes BGP attributes prefix + attributes = route two important attributes AS-PATH: contains ASs through which prefix adertisement has passed: e.g., AS 67, AS 17, AS 53, NEXT-HOP: indicates specific internal-as router to nexthop AS (may be multiple links from current AS to nexthop-as) gateway router receiing route adertisement uses import policy to accept / decline e.g., neer route through AS x policy-based routing Network Layer 4-124

125 BGP route selection router may learn about more than 1 route to destination AS, selects route based on 1. local preference alue attribute: policy decision 2. shortest AS-PATH 3. closest NEXT-HOP router: hot potato routing 4. additional criteria Network Layer 4-125

126 BGP messages BGP messages exchanged between peers oer TCP connection BGP messages OPEN: opens TCP connection to peer and authenticates sender UPDATE: adertises new path (or withdraws old) KEEPALIVE: keeps connection alie (heart beat) in absence of UPDATES; also ACKs OPEN request NOTIFICATION: reports errors in preious msg; also used to close connection Network Layer 4-126

127 BGP routing policy W A B C X legend: proider network customer network: Y A, B, C are proider networks x, w, y are customer (of proider networks) x is dual-homed: attached to two networks x does not want to route traffic from B ia x to C so x will not adertise to B a route to C Network Layer 4-127

128 BGP routing policy (2) W A B C X legend: proider network customer network: A adertises path Aw to B B adertises path BAw to x Should B adertise path BAw to C? No way! B gets no reenue for routing CBAw since neither w nor C are B s customers B wants to force C to route w ia A B wants to route only to/from its customers! Y Network Layer 4-128

129 Why different Intra-, Inter-AS routing? policy inter-as: admin wants control oer how its traffic routed, who routes through its net intra-as: single admin, so no policy decisions needed scale hierarchical routing saes table size, reduced update traffic performance intra-as: can focus on performance (speed, conergence rate, etc.) inter-as: policy may dominate oer performance Network Layer 4-129