Chapter 4: Network Layer. Chapter 4 Network Layer. Chapter 4: Network Layer. Network layer

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1 Chapter 4 Network Layer Computer Networking: A Top Down Approach 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 009. Chapter 4: Network Layer Chapter goals: understand principles behind layer services: layer service models forwarding versus routing how a works routing (path selection) dealing with scale advanced topics: IPv6, mobility instantiation, implementation in the Network Layer 4- Network Layer 4- Chapter 4: Network Layer 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing Network layer transport segment from sending to receiving host on sending side encapsulates segments into datagrams on rcving side, delivers segments to transport layer layer protocols in every host, examines header fields in all IP datagrams passing through it application transport data link data link data link data link data link data link data link data link data link data link data link data link application transport data link Network Layer 4-3 Network Layer 4-4

2 Two Key Network-Layer Functions Interplay between routing and forwarding forwarding: move packets from s input to appropriate 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 value in arriving packet s header routing algorithm local forwarding table header value output link Network Layer 4-5 Network Layer 4-6 Connection setup 3 rd important function in some architectures: ATM, frame relay, X.5 before datagrams flow, two end hosts and intervening s establish virtual connection s get involved vs transport layer connection service: : between two hosts (may also involve intervening s in case of VCs) transport: between two processes Network service model Q: What service model for channel transporting datagrams from sender to receiver? Example services for individual datagrams: guaranteed delivery guaranteed delivery with less than 40 msec delay Example services for a flow of datagrams: in-order datagram delivery guaranteed minimum bandwidth to flow restrictions on changes in interpacket spacing Network Layer 4-7 Network Layer 4-8

3 Network layer service models: Chapter 4: Network Layer Network Architecture ATM ATM ATM ATM Service Model best effort CBR VBR ABR UBR Bandwidth none Loss no constant yes rate guaranteed yes rate guaranteed no minimum none no Guarantees? Order no yes yes yes yes Timing no yes yes no no Congestion feedback no (inferred via loss) no congestion no congestion yes no 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-9 Network Layer 4-0 Network layer connection and connection-less service datagram provides -layer connectionless service VC provides -layer connection service analogous to the transport-layer services, but: service: host-to-host no choice: provides one or the other implementation: in core Virtual circuits source-to-dest path behaves much like telephone circuit performance-wise 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 on source-dest path maintains state for each passing connection link, resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service) Network Layer 4- Network Layer 4-

4 VC implementation Forwarding table VC number a VC consists of:. path from source to destination. VC numbers, one number for each link along path 3. entries in forwarding tables in s 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 4-3 Forwarding table in northwest : interface number 3 3 Incoming interface Incoming VC # Outgoing interface Outgoing VC # Routers maintain connection state information! Network Layer 4-4 Virtual circuits: signaling protocols used to setup, maintain teardown VC used in ATM, frame-relay, X.5 not used in today s Datagram s no call setup at layer s: no state about end-to-end connections no -level concept of connection packets forwarded using destination host address packets between same source-dest pair may take different paths application transport data link 5. Data flow begins 6. Receive data 4. Call connected 3. Accept call. Initiate call. incoming call application transport data link application transport data link. Send data. Receive data application transport data link Network Layer 4-5 Network Layer 4-6

5 Forwarding table 4 billion possible entries Longest prefix matching Destination Address Range through Link Interface Prefix Match Link Interface otherwise through through otherwise 3 Network Layer 4-7 Examples DA: DA: Which interface? Which interface? Network Layer 4-8 Datagram or VC : why? Chapter 4: Network Layer (datagram) data exchange among computers elastic service, no strict timing req. smart end systems (computers) can adapt, perform control, error recovery simple inside, complexity at edge many link types different characteristics uniform service difficult ATM (VC) evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service dumb end systems telephones complexity inside Network Layer Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-0

6 Router Architecture Overview Input Port Functions Two key functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link Physical layer: bit-level reception Data link layer: e.g., Ethernet see chapter 5 Decentralized switching: given datagram dest., lookup output port using forwarding table in input port memory goal: complete input port processing at line speed queuing: if datagrams arrive faster than forwarding rate into switch fabric Network Layer 4- Network Layer 4- Three types of switching fabrics Switching Via Memory First generation s: traditional computers with switching under direct control of CPU packet copied to system s memory speed limited by memory bandwidth ( bus crossings per datagram) Input Port Memory Output Port System Bus Network Layer 4-3 Network Layer 4-4

7 Switching Via a Bus Switching Via An Interconnection Network datagram from input port memory to output port memory via a shared bus bus contention: switching speed limited by bus bandwidth 3 Gbps bus, Cisco 5600: sufficient speed for access and enterprise s overcome bus bandwidth limitations Banyan s, other interconnection nets initially developed to connect processors in multiprocessor advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. Cisco 000: switches 60 Gbps through the interconnection Network Layer 4-5 Network Layer 4-6 Output Ports Output port queueing Buffering required when datagrams arrive from fabric faster than the transmission rate Scheduling discipline chooses among queued datagrams for transmission Network Layer 4-7 buffering when arrival rate via switch exceeds output line speed queueing (delay) and loss due to output port buffer overflow! Network Layer 4-8

8 How much buffering? RFC 3439 rule of thumb: average buffering equal to typical RTT (say 50 msec) times link capacity C e.g., C = 0 Gps link:.5 Gbit buffer Recent recommendation: with N flows, buffering equal to RTT. C N Input Port Queuing Fabric slower than input ports combined -> queueing may occur at input queues Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward queueing delay and loss due to input buffer overflow! Network Layer 4-9 Network Layer 4-30 Chapter 4: Network Layer 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing The Network layer Host, layer functions: Network layer Routing protocols path selection RIP, OSPF, BGP Transport layer: TCP, UDP forwarding table Link layer layer IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting signaling Network Layer 4-3 Network Layer 4-3

9 Chapter 4: Network Layer 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-33 IP datagram format IP protocol version number header length (bytes) type of data max number remaining hops (decremented at each ) upper layer protocol to deliver payload to how much overhead with TCP? 0 bytes of TCP 0 bytes of IP = 40 bytes + app layer overhead 3 bits head. type of ver length len service fragment 6-bit identifier flgs offset time to upper header live layer checksum 3 bit source IP address 3 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) total datagram length (bytes) for fragmentation/ reassembly E.g. timestamp, record route taken, specify list of s to visit. Network Layer 4-34 IP Fragmentation & Reassembly IP Fragmentation and Reassembly links have MTU (max.transfer size) - largest possible link-level frame. different link types, different MTUs large IP datagram divided ( fragmented ) within net one datagram becomes several datagrams reassembled only at final destination IP header bits used to identify, order related fragments reassembly fragmentation: in: one large datagram out: 3 smaller datagrams Example 4000 byte datagram MTU = 500 bytes 480 bytes in data field offset = 480/8 length =4000 ID =x fragflag =0 offset =0 One large datagram becomes several smaller datagrams length ID fragflag offset =500 =x = =0 length =500 length =040 ID =x ID =x fragflag = fragflag =0 offset =85 offset =370 Network Layer 4-35 Network Layer 4-36

10 Chapter 4: Network Layer IP Addressing: introduction 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing IP address: 3-bit identifier for host, interface interface: connection between host/ and link s typically have multiple interfaces host typically has one interface IP addresses associated with each interface = Network Layer 4-37 Network Layer 4-38 Subnets Subnets 3...0/ /4 IP address: subnet part (high order bits) host part (low order bits) What s a subnet? device interfaces with same subnet part of IP address can ly reach each other without intervening subnet consisting of 3 subnets Recipe To determine the subnets, detach each interface from its host or, creating islands of isolated s. Each isolated is called a subnet /4 Subnet mask: /4 Network Layer 4-39 Network Layer 4-40

11 Subnets How many? 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 host part /3 Network Layer 4-4 Network Layer 4-4 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->->configuration- >tcp/ip->properties UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play Network Layer 4-43 DHCP: Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from server when it joins Can renew its lease on address in use Allows reuse of addresses (only hold address while connected an on ) Support for mobile users who want to join (more shortly) DHCP overview: host broadcasts DHCP discover msg DHCP server responds with DHCP offer msg host requests IP address: DHCP request msg DHCP server sends address: DHCP ack msg Network Layer 4-44

12 DHCP client-server scenario A B 3... DHCP server E arriving DHCP client needs address in this Network Layer 4-45 DHCP client-server scenario DHCP server: time DHCP request DHCP discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 DHCP offer src: , 68 dest:: , 67 yiaddrr: transaction ID: 655 Lifetime: 3600 secs src: 3...5, 67 dest: , 68 yiaddrr: transaction ID: 654 Lifetime: 3600 secs DHCP ACK src: 3...5, 67 dest: , 68 yiaddrr: transaction ID: 655 Lifetime: 3600 secs arriving client Network Layer 4-46 IP addresses: how to get one? Q: How does get subnet part of IP addr? A: gets allocated portion of its provider ISP s address space ISP's block /0 Organization /3 Organization /3 Organization / Organization /3 Hierarchical addressing: route aggregation Hierarchical addressing allows efficient advertisement of routing information: Organization /3 Organization /3 Organization /3 Organization /3.... Fly-By-Night-ISP ISPs-R-Us Send me anything with addresses beginning /0 Send me anything with addresses beginning /6 Network Layer 4-47 Network Layer 4-48

13 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization Organization /3 Organization /3 Organization /3.... Organization /3 Fly-By-Night-ISP ISPs-R-Us Send me anything with addresses beginning /0 Send me anything with addresses beginning /6 or /3 IP addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes Network Layer 4-49 Network Layer 4-50 NAT: Network Address Translation NAT: Network Address Translation rest of All datagrams leaving local have same single source NAT IP address: , different source port numbers local (e.g., home ) 0.0.0/4 Datagrams with source or destination in this have 0.0.0/4 address for source, destination (as usual) Motivation: local 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 without notifying outside world can change ISP without changing addresses of devices in local devices inside local net not explicitly addressable, visible by outside world (a security plus). Network Layer 4-5 Network Layer 4-5

14 NAT: Network Address Translation Implementation: NAT 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 4-53 NAT: Network Address Translation : NAT changes datagram source addr from , 3345 to , 500, updates table NAT translation table WAN side addr LAN side addr , , 3345 S: , 500 D: , 80 S: , 80 D: , : Reply arrives dest. address: , S: , 3345 D: , S: , 80 D: , : host sends datagram to , : NAT changes datagram dest addr from , 500 to , 3345 Network Layer 4-54 NAT: Network Address Translation NAT traversal problem 6-bit port-number field: 60,000 simultaneous connections with a single LAN-side address! NAT is controversial: s should only process up to layer 3 violates end-to-end argument NAT possibility must be taken into account by app designers, eg, PP applications address shortage should instead be solved by IPv6 Network Layer 4-55 client wants to connect to server with address server address local to LAN (client can t use it as destination addr) only one externally visible NATted address: solution : statically configure NAT to forward incoming connection requests at given port to server e.g., ( , port 500) always forwarded to port 5000 Client? NAT Network Layer 4-56

15 NAT traversal problem solution : Universal Plug and Play (UPnP) Gateway Device (IGD) Protocol. Allows NATted host to: learn public IP address ( ) add/remove port mappings (with lease times) i.e., automate static NAT port map configuration NAT IGD NAT traversal problem solution 3: relaying (used in Skype) Client NATed client establishes connection to relay External client connects to relay relay bridges packets between to connections. connection to relay initiated by client 3. relaying established. connection to relay initiated by NATted host NAT Network Layer 4-57 Network Layer 4-58 Chapter 4: Network Layer ICMP: Control Message Protocol 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing used by hosts & s to communicate -level information error reporting: unreachable host,, port, protocol echo request/reply (used by ping) -layer above 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. unreachable 3 dest host unreachable 3 dest protocol unreachable 3 3 dest port unreachable 3 6 dest unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 0 0 discovery 0 TTL expired 0 bad IP header Network Layer 4-59 Network Layer 4-60

16 Traceroute and ICMP Chapter 4: Network Layer Source sends series of UDP segments to dest First has TTL = Second has TTL=, etc. Unlikely port number When nth datagram arrives to nth : Router discards datagram And sends to source an ICMP message (type, code 0) Message includes name of & IP address When ICMP message arrives, source calculates RTT Traceroute does this 3 times Stopping criterion UDP segment eventually arrives at destination host Destination returns ICMP host unreachable packet (type 3, code 3) When source gets this ICMP, stops. 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-6 Network Layer 4-6 IPv6 Initial motivation: 3-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 IPv6 Header (Cont) 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 Network Layer 4-63 Network Layer 4-64

17 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 Transition From IPv4 To IPv6 Not all s can be upgraded simultaneous no flag days How will the operate with mixed IPv4 and IPv6 s? Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 s Network Layer 4-65 Network Layer 4-66 Tunneling Logical view: A B tunnel E F IPv6 IPv6 IPv6 IPv6 Tunneling Logical view: A B tunnel E F IPv6 IPv6 IPv6 IPv6 Physical view: A B E F Physical view: A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 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 Network Layer 4-67 A-to-B: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4 E-to-F: IPv6 Network Layer 4-68

18 Chapter 4: Network Layer Interplay between routing, forwarding 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing value in arriving packet s header routing algorithm local forwarding table header value output link Network Layer 4-69 Network Layer 4-70 Graph abstraction Graph abstraction: costs Graph: G = (N,E) 5 v 3 w 5 u 3 x y N = set of s = { 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) } Remark: Graph abstraction is useful in other contexts Example: PP, where N is set of peers and E is set of TCP connections z u 5 c(x,x ) = cost of link (x,x ) v x 3 w 5 z - e.g., c(w,z) = 5 3 cost could always be, or inversely related to bandwidth, y or inversely related to congestion Cost of path (x, x, x 3,, x p ) = c(x,x ) + c(x,x 3 ) + + c(x p-,x p ) Question: What s the least-cost path between u and z? Routing algorithm: algorithm that finds least-cost path Network Layer 4-7 Network Layer 4-7

19 Routing Algorithm classification Chapter 4: Network Layer Global or decentralized information? Global: all s have complete topology, link cost info link state algorithms Decentralized: knows lyconnected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors distance vector algorithms Static or dynamic? Static: routes change slowly over time Dynamic: routes change more quickly periodic update in response to link cost changes 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-73 Network Layer 4-74 A Link-State Routing Algorithm Dijsktra s Algorithm Dijkstra s algorithm net topology, link costs known to all nodes accomplished via link state broadcast all nodes have same info computes least cost paths from one node ( source ) to all other nodes gives forwarding table for that node iterative: after k iterations, know least cost path to k dest. s Notation: c(x,y): link cost from node x to y; = if not direct neighbors D(v): current value of cost of path from source to dest. v p(v): predecessor node along path from source to v N': set of nodes whose least cost path definitively known Network Layer 4-75 Initialization: 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 0 add w to N' update D(v) for all v adjacent to w and not in N' : D(v) = min( D(v), D(w) + c(w,v) ) 3 /* new cost to v is either old cost to v or known 4 shortest path cost to w plus cost from w to v */ 5 until all nodes in N' Network Layer 4-76

20 Dijkstra s algorithm: example Dijkstra s algorithm: example () Step N' u ux uxy uxyv uxyvw 5 uxyvwz D(v),p(v),u,u,u D(w),p(w) 5,u 4,x 3,y 3,y D(x),p(x),u D(y),p(y),x D(z),p(z) 4,y 4,y 4,y Resulting shortest-path tree from u: u v x w y z Resulting forwarding table in u: u 5 v x 3 3 w y 5 z Network Layer 4-77 destination v x y w z link (u,v) (u,x) (u,x) (u,x) (u,x) Network Layer 4-78 Dijkstra s algorithm, discussion Chapter 4: Network Layer Algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n(n+)/ comparisons: O(n ) more efficient implementations possible: O(nlogn) Oscillations possible: e.g., link cost = amount of carried traffic A +e D B C e e initially A +e 0 D B 0 +e C 0 recompute routing A 0 +e D B 0 0 C +e recompute A +e 0 D B +e 0 C e recompute Network Layer Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-80

21 Distance Vector Algorithm Bellman-Ford Equation (dynamic programming) Define d x (y) := cost of least-cost path from x to y Then d x (y) = min {c(x,v) + d v (y) } v where min is taken over all neighbors v of x Network Layer 4-8 Bellman-Ford example u 5 v x Clearly, d v (z) = 5, d x (z) = 3, d w (z) = 3 3 w 5 z B-F equation says: 3 d u (z) = min { c(u,v) + d v (z), y c(u,x) + d x (z), c(u,w) + d w (z) } = min { + 5, + 3, 5 + 3} = 4 Node that achieves minimum is next hop in shortest path forwarding table Network Layer 4-8 Distance Vector Algorithm D x (y) = estimate of least cost from x to y Node x knows cost to each neighbor v: c(x,v) Node x maintains distance vector D x = [D x (y): y є N ] Node x also maintains its neighbors distance vectors For each neighbor v, x maintains D v = [D v (y): y є N ] Distance vector algorithm (4) Basic idea: From time-to-time, each node sends its own distance vector estimate to neighbors Asynchronous When a node x receives new DV estimate from neighbor, it updates its own DV using B-F equation: D x (y) min v {c(x,v) + D v (y)} for each node y N Under minor, natural conditions, the estimate D x (y) converge to the actual least cost d x (y) Network Layer 4-83 Network Layer 4-84

22 Distance Vector Algorithm (5) Iterative, 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 4-85 D x (y) = min{c(x,y) + D y (y), c(x,z) + D z (y)} = min{+0, 7+} = node x table cost to cost to x y z x y z x 0 7 y z node y table cost to x y z from from from x y z 0 node z table cost to x y z x y z 7 0 from x y z time D x (z) = min{c(x,y) + D y (z), c(x,z) + D z (z)} = min{+, 7+0} = 3 x y 7 z Network Layer 4-86 D x (y) = min{c(x,y) + D y (y), c(x,z) + D z (y)} = min{+0, 7+} = node x table cost to cost to cost to x y z x y z x y z x 0 7 x 0 3 x 0 3 y y 0 y 0 z z 7 0 z 3 0 node y table cost to cost to cost to x y z x y z x y z x x 0 7 x 0 3 y 0 y 0 y 0 z z 7 0 z 3 0 from from node z table cost to x y z from x y z 7 0 from from from x y z cost to x y z from from from x y z cost to x y z time D x (z) = min{c(x,y) + D y (z), c(x,z) + D z (z)} = min{+, 7+0} = 3 x y 7 z Network Layer 4-87 Distance Vector: link cost changes Link cost changes: node detects local link cost change updates routing info, recalculates distance vector if DV changes, notify neighbors good news travels fast 4 x At time t 0, y detects the link-cost change, updates its DV, and informs its neighbors. At time t, z receives the update from y and updates its table. It computes a new least cost to x and sends its neighbors its DV. At time t, y receives z s update and updates its distance table. y s least costs do not change and hence y does not send any message to z. y 50 z Network Layer 4-88

23 Distance Vector: link cost changes Comparison of LS and DV algorithms Link cost changes: good news travels fast bad news travels slow - count to infinity problem! 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? 60 4 x y 50 z Network Layer 4-89 Message complexity LS: with n nodes, E links, O(nE) msgs sent DV: exchange between neighbors only convergence time varies Speed of Convergence LS: O(n ) algorithm requires O(nE) msgs may have oscillations DV: convergence time varies may be routing loops count-to-infinity problem Robustness: what happens if malfunctions? LS: node can advertise incorrect link cost each node computes only its own table DV: DV node can advertise incorrect path cost each node s table used by others error propagate thru Network Layer 4-90 Chapter 4: Network Layer Hierarchical Routing 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing Our routing study thus far - idealization all s identical flat not true in practice scale: with 00 million destinations: can t store all dest s in routing tables! routing table exchange would swamp links! administrative autonomy internet = of s each admin may want to control routing in its own Network Layer 4-9 Network Layer 4-9

24 Hierarchical Routing Interconnected ASes aggregate s into regions, autonomous systems (AS) s in same AS run same routing protocol intra-as routing protocol s in different AS can run different intra- AS routing protocol Gateway Direct link to in another AS 3c 3a 3b AS3 a c d b Intra-AS Routing algorithm AS Forwarding table Inter-AS Routing algorithm a c b AS forwarding table configured by both intra- and inter-as routing algorithm intra-as sets entries for internal dests inter-as & intra-as sets entries for external dests Network Layer 4-93 Network Layer 4-94 Inter-AS tasks suppose in AS receives datagram destined outside of AS: should forward packet to gateway, but which one? AS must:. learn which dests are reachable through AS, which through AS3. propagate this reachability info to all s in AS Job of inter-as routing! Example: Setting forwarding table in d suppose AS learns (via inter-as protocol) that subnet x reachable via AS3 (gateway c) but not via AS. inter-as protocol propagates reachability info to all internal s. d determines from intra-as routing info that its interface I is on the least cost path to c. installs forwarding table entry (x,i) 3c 3a 3b AS3 a c d b AS a c b AS Network Layer c 3a 3b AS3 a c d x b AS a c b AS Network Layer 4-96

25 Example: Choosing among multiple ASes now suppose AS learns from inter-as protocol that subnet x is reachable from AS3 and from AS. to configure forwarding table, d must determine towards which gateway it should forward packets for dest x. this is also job of inter-as routing protocol! 3c 3a 3b AS3 a c d x b AS a c b AS Example: Choosing among multiple ASes now suppose AS learns from inter-as protocol that subnet x is reachable from AS3 and from AS. to configure forwarding table, d 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 s. Learn from inter-as protocol that subnet x is reachable via 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 4-97 Network Layer 4-98 Chapter 4: Network Layer Intra-AS Routing 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast 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 4-99 Network Layer 4-00

26 Chapter 4: Network Layer RIP ( Routing Information Protocol) 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing distance vector algorithm included in BSD-UNIX Distribution in 98 distance metric: # of hops (max = 5 hops) z u A C B D v y w x From A to subnets: destination hops u v w x 3 y 3 z Network Layer 4-0 Network Layer 4-0 RIP advertisements distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement) each advertisement: list of up to 5 destination subnets within AS Network Layer 4-03 RIP: Example w x y A D B C Destination Network Next Router Num. of hops to dest. w A y B z B 7 x Routing/Forwarding table in D z Network Layer 4-04

27 RIP: Example Dest Next hops w - x - z C Advertisement from A to D w x y A D B C Destination Network Next Router Num. of hops to dest. w A y B z B A 7 5 x Routing/Forwarding table in D z Network Layer 4-05 RIP: Link Failure and Recovery If no advertisement heard after 80 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 = 6 hops) Network Layer 4-06 RIP Table processing Chapter 4: Network Layer RIP routing tables managed by application-level process called route-d (daemon) advertisements sent in UDP packets, periodically repeated Transprt (UDP) forwarding (IP) table link routed forwarding table routed Transprt (UDP) (IP) link 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-07 Network Layer 4-08

28 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 disseminated to entire AS (via flooding) carried in OSPF messages directly over IP (rather than TCP or UDP 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; high for real time) integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topology data base as OSPF hierarchical OSPF in large domains. Network Layer 4-09 Network Layer 4-0 Hierarchical OSPF 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 s: summarize distances to nets in own area, advertise to other Area Border s. backbone s: run OSPF routing limited to backbone. boundary s: connect to other AS s. Network Layer 4- Network Layer 4-

29 Chapter 4: Network Layer inter-as routing: BGP 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing BGP (Border Gateway Protocol): the de facto standard BGP provides each AS a means to:. Obtain subnet reachability information from neighboring ASs.. Propagate reachability information to all ASinternal s. 3. Determine good routes to subnets based on reachability information and policy. allows subnet to advertise its existence to rest of : I am here Network Layer 4-3 Network Layer 4-4 BGP basics pairs of s (BGP peers) exchange routing info over semi-permanent TCP connections: BGP sessions BGP sessions need not correspond to links. when AS advertises a prefix to AS: AS promises it will forward datagrams towards that prefix. AS can aggregate prefixes in its advertisement Distributing reachability info using ebgp session between 3a and c, AS3 sends prefix reachability info to AS. c can then use ibgp do distribute new prefix info to all s in AS b can then re-advertise new reachability info to AS over b-to-a ebgp session when learns of new prefix, it creates entry for prefix in its forwarding table. 3c 3a 3b AS3 a AS c d ebgp session ibgp session b a c b AS Network Layer 4-5 3c 3a 3b AS3 a AS c d ebgp session ibgp session b a c b AS Network Layer 4-6

30 Path attributes & BGP routes BGP route selection 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 7 NEXT-HOP: indicates specific internal-as to next-hop AS. (may be multiple links from current AS to next-hop-as) when gateway receives route advertisement, uses import policy to accept/decline. may learn about more than route to some prefix. Router must select route. elimination rules:. local preference value attribute: policy decision. shortest AS-PATH 3. closest NEXT-HOP : hot potato routing 4. additional criteria Network Layer 4-7 Network Layer 4-8 BGP messages BGP messages exchanged using TCP. BGP messages: OPEN: opens TCP connection to peer and authenticates sender UPDATE: advertises new path (or withdraws old) KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request NOTIFICATION: reports errors in previous msg; also used to close connection BGP routing policy W A A,B,C are provider s B C X Y legend: X,W,Y are customer (of provider s) X is dual-homed: attached to two s X does not want to route from B via X to C.. so X will not advertise to B a route to C provider customer : Network Layer 4-9 Network Layer 4-0

31 BGP routing policy () W A A advertises path AW to B B advertises path BAW to X B C X Y legend: Should B advertise path BAW to C? provider customer : No way! B gets no revenue for routing CBAW since neither W nor C are B s customers B wants to force C to route to w via A B wants to route only to/from its customers! Network Layer 4- Why different Intra- and Inter-AS routing? Policy: Inter-AS: admin wants control over how its traffic routed, who routes through its net. Intra-AS: single admin, so no policy decisions needed Scale: hierarchical routing saves table size, reduced update traffic Performance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance Network Layer 4- Chapter 4: Network Layer 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing Broadcast Routing deliver packets from source to all other nodes source duplication is inefficient: duplicate R3 R R source duplication duplicate creation/transmission R4 R3 R R R4 in- duplication duplicate source duplication: how does source determine recipient addresses? Network Layer 4-3 Network Layer 4-4

32 In- duplication flooding: when node receives brdcst pckt, sends copy to all neighbors Problems: cycles & broadcast storm controlled flooding: node only brdcsts pkt if it hasn t brdcst same packet before Node keeps track of pckt ids already brdcsted Or reverse path forwarding (RPF): only forward pckt if it arrived on shortest path between node and source spanning tree No redundant packets received by any node Spanning Tree First construct a spanning tree Nodes forward copies only along spanning tree F c A E B D (a) Broadcast initiated at A G F c A E B D (b) Broadcast initiated at D G Network Layer 4-5 Network Layer 4-6 Spanning Tree: Creation Center node Each node sends unicast join message to center node Message forwarded until it arrives at a node already belonging to spanning tree F c A 4 E B D (a) Stepwise construction of spanning tree 3 5 G F c A E B D (b) Constructed spanning tree G Network Layer 4-7 Multicast Routing: Problem Statement Goal: find a tree (or trees) connecting s having local mcast group members tree: not all paths between s used source-based: different tree from each sender to rcvrs shared-tree: same tree used by all group members Shared tree Source-based trees

33 Approaches for building mcast trees Approaches: source-based tree: one tree per source shortest path trees reverse path forwarding group-shared tree: group uses one tree minimal spanning (Steiner) center-based trees we first look at basic approaches, then specific protocols adopting these approaches Shortest Path Tree mcast forwarding tree: tree of shortest path routes from source to all receivers Dijkstra s algorithm S: source R3 R R 3 4 R6 R4 6 R7 5 R5 LEGEND i with attached group member with no attached group member link used for forwarding, i indicates order link added by algorithm Reverse Path Forwarding rely on s knowledge of unicast shortest path from it to sender each has simple forwarding behavior: if (mcast datagram received on incoming link on shortest path back to center) then flood datagram onto all outgoing links else ignore datagram Reverse Path Forwarding: example S: source R3 R R R6 R4 R7 R5 LEGEND with attached group member with no attached group member datagram will be forwarded datagram will not be forwarded result is a source-specific reverse SPT may be a bad choice with asymmetric links

34 Reverse Path Forwarding: pruning forwarding tree contains subtrees with no mcast group members no need to forward datagrams down subtree prune msgs sent upstream by with no downstream group members S: source R3 R R R6 P R4 P R7 R5 LEGEND P with attached group member with no attached group member prune message links with multicast forwarding Shared-Tree: Steiner Tree Steiner Tree: minimum cost tree connecting all s with attached group members problem is NP-complete excellent heuristics exists not used in practice: computational complexity information about entire needed monolithic: rerun whenever a needs to join/leave Center-based trees single delivery tree shared by all one identified as center of tree to join: edge sends unicast join-msg addressed to center join-msg processed by intermediate s and forwarded towards center join-msg either hits existing tree branch for this center, or arrives at center path taken by join-msg becomes new branch of tree for this Center-based trees: an example Suppose R6 chosen as center: R3 R R 3 R6 R4 R7 R5 LEGEND with attached group member with no attached group member path order in which join messages generated

35 Multicasting Routing: DVMRP DVMRP: distance vector multicast routing protocol, RFC075 flood and prune: reverse path forwarding, source-based tree RPF tree based on DVMRP s own routing tables constructed by communicating DVMRP s no assumptions about underlying unicast initial datagram to mcast group flooded everywhere via RPF s not wanting group: send upstream prune msgs DVMRP: continued soft state: DVMRP periodically ( min.) forgets branches are pruned: mcast data again flows down unpruned branch downstream : reprune or else continue to receive data s can quickly regraft to tree following IGMP join at leaf odds and ends commonly implemented in commercial s Mbone routing done using DVMRP Tunneling Q: How to connect islands of multicast s in a sea of unicast s? PIM: Protocol Independent Multicast not dependent on any specific underlying unicast routing algorithm (works with all) two different multicast distribution scenarios : topology logical topology mcast datagram encapsulated inside normal (non-multicastaddressed) datagram normal IP datagram sent thru tunnel via regular IP unicast to receiving mcast receiving mcast unencapsulates to get mcast datagram Dense: group members densely packed, in close proximity. bandwidth more plentiful Sparse: # s with group members small wrt # interconnected s group members widely dispersed bandwidth not plentiful

36 Consequences of Sparse-Dense Dichotomy: Dense group membership by s assumed until s explicitly prune Sparse: no membership until s explicitly join receiver- driven data-driven construction construction of mcast on mcast tree (e.g., RPF) tree (e.g., center-based) bandwidth and nongroup- bandwidth and non-group- processing processing profligate conservative PIM- Dense Mode flood-and-prune RPF, similar to DVMRP but underlying unicast protocol provides RPF info for incoming datagram less complicated (less efficient) downstream flood than DVMRP reduces reliance on underlying routing algorithm has protocol mechanism for to detect it is a leaf-node PIM - Sparse Mode PIM - Sparse Mode center-based approach sends join msg to rendezvous point (RP) intermediate s update state and forward join after joining via RP, can switch to source-specific tree increased performance: less concentration, shorter paths R3 R R join join all data multicast from rendezvous point R6 join R4 R5 R7 rendezvous point sender(s): unicast data to RP, which distributes down RP-rooted tree RP can extend mcast tree upstream to source RP can send stop msg if no attached receivers no one is listening! R3 R R join join all data multicast from rendezvous point R6 join R4 R5 R7 rendezvous point

37 Chapter 4: summary 4. Introduction 4. Virtual circuit and datagram s 4.3 What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-45