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1 Chapter 4 Network Laer A note on the use of these ppt slides: The notes used in this course are substantiall based on powerpoint slides developed and coprighted b J.F. Kurose and K.W. Ross, Computer Networking: A Top Down Approach 4 th edition. Jim Kurose, Keith Ross Addison-Wesle, Jul 7. Chapter 4: Network Laer Chapter goals: Understand principles behind network laer services: Network laer service models Forwarding versus routing How a works Routing (path selection) Dealing with scale Advanced topics: IPv6, mobilit Instantiation, implementation in the Network Laer 4- Network Laer 4- Chapter 4: Network Laer 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Network Laer Transport segment sending to receiving host On sending side encapsulates segments into datagrams On rcving side, delivers segments to transport laer Network laer protocols in ever host, Router eamines header fields in all IP datagrams passing through it application transport network data link phsical network data link phsical network data link phsical network data link network phsical data link phsical network data link phsical network data link phsical network data link phsical network data link application phsical transport network data link phsical Network Laer 4- Network Laer 4-4

2 Two Ke Network-Laer Functions Interpla between Routing and Forwarding Forwarding: move packets s input to appropriate output Routing: determine route taken b packets source to dest Routing algorithms Analog: Routing: process of planning trip 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 Laer 4- Network Laer 4-6 Connection Setup Network Service Model The rd important function in some network architectures: ATM, frame rela, X. Before datagrams flow, two end hosts and intervening s establish virtual connection Routers get involved Network vs. transport laer connection service: Network: between two hosts (ma also involve intervening s in case of VCs) Transport: between two processes Q: What service model for channel transporting packets sender to receiver? Guaranteed bandwidth? Preservation of inter-packet timing (no jitter)? Loss-free deliver? In-order deliver? Congestion feedback to sender? service abstraction The most important abstraction provided b network laer:? or datagram??? virtual circuit Network Laer 4-7 Network Laer 4-8

3 Network Laer Service Models: Chapter 4: Network Laer Network Architecture ATM ATM ATM ATM Service Model best effort CBR VBR ABR UBR Bandwidth none constant rate guaranteed rate guaranteed minimum none Guarantees? Loss Order Timing no es es no no no es es es es no es es no no Congestion feedback no (inferred via loss) no congestion no congestion es no 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Network Laer 4-9 Network Laer 4- Network Laer Connection and Connection-less Service Datagram network provides network-laer connectionless service VC network provides network-laer connection service Analogous to the transport-laer services, but: Service: host-to-host No choice: network provides one or the other Implementation: in network core Virtual Circuits source-to-dest path behaves much like telephone circuit Performance-wise Network actions along source-to-dest path Call setup, teardown for each call before data can flow Each packet carries VC identifier (not destination host address) Ever on source-dest path maintains state for each passing connection Link, resources (bandwidth, buffers) ma be allocated to VC (dedicated resources = predictable service) Network Laer 4- Network Laer 4-

4 VC Implementation Forwarding Table VC number A VC consists of:. Path source to destination. VC numbers, one number for each link along path. 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 forwarding table Network Laer 4- Forwarding table in northwest : interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # Routers maintain connection state information! Network Laer 4-4 Virtual Circuits: Signaling Protocols Used to setup, maintain teardown VC Used in ATM, frame-rela, X. Not used in toda s Datagram Networks No call setup at network laer Routers: no state about end-to-end connections No network-level concept of connection Packets forwarded using destination host address Packets between same source-dest pair ma take different paths application transport network data link phsical. Data flow begins 6. Receive data 4. Call connected. Accept call. Initiate call. incoming call application transport network data link phsical application transport network data link phsical application transport. Send data. Receive data network data link phsical Network Laer 4- Network Laer 4-6 4

5 Forwarding Table 4 billion possible entries Longest Prefi Matching Destination Address Range Link Interface through through through otherwise Prefi Match Link Interface otherwise Eamples DA: DA: Interface Interface Which interface? Which interface? Network Laer 4-7 Network Laer 4-8 Datagram or VC Network: Wh? Chapter 4: Network Laer (datagram) ATM (VC) Data echange among computers Elastic service, no strict timing req. Smart end sstems (computers) Can adapt, perform control, error recover Simple inside network, compleit at edge Man link tpes Different characteristics Uniform service difficult Evolved telephon Human conversation: Strict timing, reliabilit requirements Need for guaranteed service Dumb end sstems Telephones Compleit inside network Network Laer Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Network Laer 4-

6 Router Architecture Overview Input Port Functions Two ke functions: Run routing algorithms/protocol (RIP, OSPF, BGP) Forwarding datagrams incoming to outgoing link Phsical laer: bit-level reception Data link laer: e.g., Ethernet see chapter Decentralied switching: Given datagram dest., lookup output port using forwarding table in input port memor Goal: complete input port processing at line speed Queuing: if datagrams arrive faster than forwarding rate into switch fabric Network Laer 4- Network Laer 4- Three Tpes of Switching Fabrics Output Ports Buffering required when datagrams arrive fabric faster than the transmission rate Scheduling discipline chooses among queued datagrams for transmission Network Laer 4- Network Laer 4-4 6

7 Output Port Queuing Input Port Queuing Fabric slower than input ports combined -> queueing ma occur at input queues Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue moving forward Queuing dela and loss due to input buffer overflow! Buffering when arrival rate via switch eceeds output line speed Queuing (dela) and loss due to output port buffer overflow! Network Laer 4- Network Laer 4-6 Chapter 4: Network Laer 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP The Network Laer Host, network laer functions: Network laer Routing protocols Path selection RIP, OSPF, BGP Transport laer: TCP, UDP forwarding table Link laer phsical laer IP protocol Addressing conventions Datagram format Packet handling conventions ICMP protocol Error reporting Router signaling Network Laer 4-7 Network Laer 4-8 7

8 Chapter 4: Network Laer 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Network Laer 4-9 IP Datagram Format IP protocol version number header length (btes) tpe of data (QoS) ma number remaining hops (decremented at each ) upper laer protocol to deliver paload to How much overhead with TCP? btes of TCP btes of IP = 4 btes + app laer overhead bits ver head. tpe of len service length 6-bit identifier flgs fragment offset time to upper header live laer checksum bit source IP address bit destination IP address Options (if an) data (variable length, tpicall a TCP or UDP segment) total datagram length (btes) for fragmentation/ reassembl E.g. timestamp, record route taken, specif list of s to visit. Network Laer 4- IP Fragmentation & Reassembl IP Fragmentation & Reassembl Network links have MTU (ma. transfer unit/sie) - largest possible link-level frame Different link tpes, different MTUs Large IP datagram divided ( fragmented ) within net One datagram becomes several datagrams Reassembled onl at final destination IP header bits used to identif, order related fragments reassembl fragmentation: in: one large datagram out: smaller datagrams Eample 4 bte datagram MTU = btes 48 btes in data field offset = 48/8 length =4 ID = length = fragflag = ID = ID = offset = One large datagram becomes several smaller datagrams length = length =4 ID = fragflag = fragflag = fragflag = offset = offset =8 offset =7 Network Laer 4- Network Laer 4-8

9 Chapter 4: Network Laer IP Addressing: Introduction 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP IP address: -bit identifier for host, interface Interface: connection between host/ and phsical link Router s tpicall have multiple interfaces Host tpicall has one interface (ma have multiple toda) IP addresses associated with each interface, not with host or = Network Laer 4- Network Laer 4-4 Subnets IP address: Subnet part (high order bits) Host part (low order bits) What s a subnet? Device interfaces with same subnet part of IP address Can phsicall reach each other without intervening subnet... network consisting of subnets (for IP addresses starting with, first 4 bits are network address) IP Addresses Given notion of network, let s re-eamine IP addresses: class-full addressing: class A B C D network host network host network host multicast address bits... to to to to 9... Network Laer 4- Network Laer 4-6 9

10 IP Addressing: CIDR Classful addressing: Inefficient use of address space, address space ehaustion E.g., class B net allocated enough addresses for 6K hosts, even if onl K hosts in that network CIDR: Classless InterDomain Routing Subnet portion of address of arbitrar length Address format: a.b.c.d/, where is # bits in subnet portion of address subnet host part part..6./ Network Laer 4-7 IP Addresses: How to Get One? Q: How does host get IP address? Hard-coded b sstem admin in a file Wintel: control-panel->network- >configuration->tcp/ip->properties UNIX: /etc/rc.config DHCP: Dnamic Host Configuration Protocol: dnamicall get address as server plug-and-pla (more later) Network Laer 4-8 DHCP: Dnamic Host Configuration Protocol Goal: allow host to dnamicall obtain its IP address network server when it joins network Can renew its lease on address in use Allows reuse of addresses (onl hold address while connected an on Support for mobile users who want to join network (more shortl) 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 DHCP Client-Server Scenario A B DHCP server E... arriving DHCP client needs address in this network Network Laer 4-9 Network Laer 4-4

11 DHCP Client-Server Scenario DHCP server:... time DHCP discover src :..., 68 dest.:...,67 iaddr:... transaction ID: 64 DHCP offer src:..., 67 dest:..., 68 iaddrr:...4 transaction ID: 64 Lifetime: 6 secs DHCP request src:..., 68 dest::..., 67 iaddrr:...4 transaction ID: 6 Lifetime: 6 secs DHCP ACK src:..., 67 dest:..., 68 iaddrr:...4 transaction ID: 6 Lifetime: 6 secs arriving client Network Laer 4-4 IP Addresses: How to Get One? Q: How does network get subnet part of IP addr? A: Gets allocated portion of its provider ISP s address space ISP's block..6./ Organiation..6./ Organiation..8./ Organiation.../ Organiation 7.../ Network Laer 4-4 Hierarchical Addressing: Route Aggregation Hierarchical addressing allows efficient advertisement of routing information: Organiation..6./ Organiation..8./ Organiation.../ Organiation 7.../.... Fl-B-Night-ISP ISPs-R-Us Send me anthing with addresses beginning..6./ Send me anthing with addresses beginning 99.../6 Hierarchical Addressing: More Specific Routes ISPs-R-Us has a more specific route to Organiation Organiation..6./ Organiation.../ Organiation 7..../ Organiation..8./.. Fl-B-Night-ISP ISPs-R-Us Send me anthing with addresses beginning..6./ Send me anthing with addresses beginning 99.../6 or..8./ Network Laer 4-4 Network Laer 4-44

12 IP Addressing: the Last Word... NAT: Network Address Translation Q: How does an ISP get block of addresses? rest of local network (e.g., home network)../4... A: ICANN: Corporation for Assigned Names and Numbers Allocates addresses Manages DNS Assigns domain names, resolves disputes All datagrams leaving local network have same single source NAT IP address: , different source port numbers...4 Datagrams with source or destination in this network have../4 address for source, destination (as usual) Network Laer 4-4 Network Laer 4-46 NAT: Network Address Translation Motivation: local network uses just one IP address as far as outside world is concerned: Range of addresses not needed ISP: just one IP address for all devices Can change addresses of devices in local network without notifing outside world Can change ISP without changing addresses of devices in local network Devices inside local net not eplicitl addressable, visible b outside world (a securit plus) NAT: Network Address Translation Implementation: NAT must: Outgoing datagrams: replace (source IP address, port #) of ever 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) ever (source IP address, port #) to (NAT IP address, new port #) translation pair Incoming datagrams: replace (NAT IP address, new port #) in dest fields of ever incoming datagram with corresponding (source IP address, port #) stored in NAT table Network Laer 4-47 Network Laer 4-48

13 NAT: Network Address Translation NAT: Network Address Translation : NAT changes datagram source addr..., 4 to ,, updates table NAT translation table WAN side addr LAN side addr ,..., 4 S: , D: , S: , 8 D: , : Repl arrives dest. address: ,...4 S:..., 4 D: , 8 S: , 8 D:..., 4 4 : host... sends datagram to , : NAT changes datagram dest addr , to..., 4 Network Laer 4-49 Outside node cannot initiate the communication Reserved addresses: / / /6 Network Laer 4- NAT: Network Address Translation 6-bit port-number field: 6, simultaneous connections with a single LAN-side address! NAT is controversial: Routers should onl process up to laer Violates end-to-end argument NAT possibilit must be taken into account b app designers, eg, PP applications Address shortage should instead be solved b IPv6 NAT Traversal Problem Client want to connect to server with address... Server address... local to LAN (client can t use it as destination addr) Onl one eternall visible NATted address: Client NAT Solution : staticall configure NAT to forward incoming connection requests at given port to server e.g., ( , port ) alwas forwarded to... port? Network Laer 4- Network Laer 4-

14 NAT Traversal Problem Solution : Universal Plug and Pla (UPnP) Gatewa Device (IGD) Protocol. Allows NATted host to: Learn public IP address ( ) Enumerate eisting port mappings Add/remove port mappings (with lease times) i.e., automate static NAT port map configuration NAT IGD Network Laer 4- NAT Traversal Problem Solution : relaing (used in Skpe) NATed server establishes connection to rela Eternal client connects to rela Rela bridges packets between to connections Client. connection to rela initiated b client. relaing established. connection to rela initiated b NATted host NAT... Network Laer 4-4 Chapter 4: Network Laer ICMP: Control Message Protocol 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Used b hosts & s to communicate network-level information Error reporting: unreachable host, network, port, protocol Echo request/repl (used b ping) Network-laer above IP: ICMP msgs carried in IP datagrams ICMP message: tpe, code plus first 8 btes of IP datagram causing error Tpe Code Description echo repl (ping) dest. network unreachable dest host unreachable dest protocol unreachable dest port unreachable 6 dest network unknown 7 dest host unknown 4 source quench (congestion control - not used) 8 echo request (ping) 9 route advertisement discover TTL epired bad IP header Network Laer 4- Network Laer 4-6 4

15 Traceroute and ICMP ICMP: Brief Summar Source sends series of UDP segments to dest First has TTL = Second has TTL=, etc. Unlikel port number When nth datagram arrives to nth : Router discards datagram And sends to source an ICMP message (tpe, code ) Message includes name of & IP address When ICMP message arrives, source calculates RTT Traceroute does this times Stopping criterion UDP segment eventuall arrives at destination host Destination returns ICMP host unreachable packet (tpe, code ) When source gets this ICMP, stops ICMP is the control sibling of IP ICMP is used b IP and uses IP as network laer protocol ICMP is used for ping, traceroute, and path MTU discover Ping: Uses ICMP Echo request/repl messages Path MTU Discover Send a large IP datagram with No fragment bit set Reduce sie until success (No ICMP message received) Network Laer 4-7 Network Laer 4-8 Chapter 4: Network Laer IPv6 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Initial motivation: -bit address space soon to be completel allocated Additional motivation: Header format helps speed processing/forwarding Header changes to facilitate QoS IPv6 datagram format: Fied-length 4 bte header No fragmentation allowed Network Laer 4-9 Network Laer 4-6

16 IPv6 Header (Cont.) Priorit: identif priorit among datagrams in flow Flow Label: identif datagrams in same flow (concept of flow not well defined) Net header: identif upper laer protocol for data Other Changes IPv4 Checksum: removed entirel to reduce processing time at each hop Options: allowed, but outside of header, indicated b Net Header field ICMPv6: new version of ICMP Additional message tpes, e.g. Packet Too Big Multicast group management functions Network Laer 4-6 Network Laer 4-6 Transition From IPv4 To IPv6 Not all s can be upgraded simultaneous no flag das How will the network operate with mied IPv4 and IPv6 s? Tunneling: IPv6 carried as paload in IPv4 datagram among IPv4 s Tunneling Logical view: Phsical view: A B E F tunnel IPv6 IPv6 IPv6 IPv6 A B E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 Network Laer 4-6 Network Laer

17 Tunneling Logical view: Phsical view: A B E F tunnel IPv6 IPv6 IPv6 IPv6 A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 Flow: X Src: A Dest: F data A-to-B: IPv6 Src:B Dest: E Flow: X Src: A Dest: F data B-to-C: IPv6 inside IPv4 Src:B Dest: E Flow: X Src: A Dest: F data B-to-C: IPv6 inside IPv4 Flow: X Src: A Dest: F data E-to-F: IPv6 Network Laer 4-6 Chapter 4: Network Laer 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Network Laer 4-66 Interpla between Routing and Forwarding Graph Abstraction routing algorithm local forwarding table header value output link Graph: G = (N,E) u v w value in arriving packet s header N = set of s = { u, v, w,,, } E = set of links ={ (u,v), (u,), (v,), (v,w), (,w), (,), (w,), (w,), (,) } Remark: Graph abstraction is useful in other network contets Eample: PP, where N is set of peers and E is set of TCP connections Network Laer 4-67 Network Laer

18 Graph Abstraction: Costs Routing Algorithm Classification u v w c(, ) = cost of link (, ) - e.g., c(w,) = Cost could alwas be, or inversel related to bandwidth, or inversel related to congestion Cost of path (,,,, p ) = c(, ) + c(, ) + + c( p-, p ) Question: What s the least-cost path between u and? Routing algorithm: algorithm that finds least-cost path Network Laer 4-69 Global or decentralied information? Global: All s have complete topolog, link cost info Link state algorithms Decentralied: Router knows phsicallconnected neighbors, link costs to neighbors Iterative process of computation, echange of info with neighbors Distance vector algorithms Static or dnamic? Static: Routes change slowl over time Dnamic: Routes change more quickl Periodic update In response to link cost changes Network Laer 4-7 Chapter 4: Network Laer A Link-State Routing Algorithm 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Dijkstra s algorithm Net topolog, link costs known to all nodes Accomplished via link state broadcast All nodes have same info Computes least cost paths 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(,): link cost node to ; = if not direct neighbors D(v): current value of cost of path source to dest. v p(v): predecessor node along path source to v N': set of nodes whose least cost path definitivel known Network Laer 4-7 Network Laer 4-7 8

19 Dijkstra s Algorithm Dijkstra s Algorithm: Eample Initialiation: N' = {u} for all nodes v 4 if v adjacent to u 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 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) ) /* new v is either old v or known 4 shortest path w plus cost w to v */ until all nodes in N' Step 4 start N A AD ADE ADEB ADEBC ADEBCF D(B),p(B) D(C),p(C),A,A,A 4,D,A,E,E A B D C E D(D),p(D),A F D(E),p(E) D(F),p(F),D 4,E 4,E 4,E Network Laer 4-7 Network Laer 4-74 Dijkstra s Algorithm, Discussion Chapter 4: Network Laer Algorithm compleit: 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 e C e initiall A +e D B +e C recompute routing A +e D B C +e recompute +e A D +e B C e recompute Network Laer Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Network Laer

20 Distance Vector Algorithm Bellman-Ford Equation (dnamic programming) Define d () := cost of least-cost path to Then d () = min {c(,v) + d v () } v where min is taken over all neighbors v of Bellman-Ford Eample u v w Clearl, d v () =, d () =, d w () = B-F equation sas: d u () = min { c(u,v) + d v (), c(u,) + d (), c(u,w) + d w () } = min { +, +, + } = 4 Node that achieves minimum is net hop in shortest path forwarding table Network Laer 4-77 Network Laer 4-78 Distance Vector Algorithm Bellman-Ford Eample () D () = estimate of least cost to Node knows each neighbor v: c(,v) Node maintains distance vector D = [D (): є N ] Node also maintains its neighbors distance vectors For each neighbor v, maintains D v = [D v (): є N ] u v w Distance vectors stored at node u v w Cost to 4 u v w Network Laer 4-79 Network Laer 4-8

21 Bellman-Ford Eample () Distance Vector Algorithm (4) u v w Cost to u v w Basic idea: Each node periodicall sends its own distance vector estimate to neighbors When a node receives new DV estimate neighbor, it updates its own DV using B-F equation: Routing table at node destination u v w hop, cost,, u, v,, u v w 4 D () min v {c(,v) + D v ()} for each node N Under minor, natural conditions, the estimate D () converge to the actual least cost d () Network Laer 4-8 Network Laer 4-8 Distance Vector Algorithm () Iterative, asnchronous: each local iteration caused b: Local link cost change DV update message neighbor Distributed: Each node notifies neighbors onl when its DV changes Neighbors then notif their neighbors if necessar Each node: wait for (change in local link cost or msg neighbor) recompute estimates if DV to an dest has changed, notif neighbors Network Laer 4-8 node table 7 node table node table 7 time 7 Network Laer 4-84

22 node table 7 node table node table D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+} = 7 7 D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+} = time 7 Network Laer 4-8 node table 7 node table node table D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+} = D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+} = time 7 Network Laer 4-86 node table 7 node table node table D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+} = time D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+} = 7 Network Laer 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, notif neighbors good news travels fast 4 At time t, detects the link-cost change, updates its DV, and informs its neighbors. At time t, receives the update and updates its table. It computes a new least and sends its neighbors its DV. At time t, receives s update and updates its distance table. s least costs do not change and hence does not send an message to. Network Laer 4-88

23 good news travels fast w 7 4 node w table w w 4 6 node table w 4 w 4 6 node table w node table w 6 w 4 6 Initial routing table (before change) Network Laer 4-89 good news travels fast Algorithm converges in steps. w 4 7 Cost of link changes to node w table node table node table node table w w 4 6 w 4 w 4 6 w 4 6 w w 4 6 w w 4 w 4 w w 4 w w w w 4 w w w 4 w w Network Laer 4-9 good news travels fast Algorithm converges in steps. w 4 7 Cost of link changes to node w table node table node table node table w w 4 w w w 4 w w w w w w w w w Network Laer 4-9 bad news travels slow count to infinit problem 4 node table 4 4 node table 4 4 node table 4 4 Initial routing table Network Laer 4-9

24 bad node table node table node table node table news travels slow Algorithm converges in 44 steps. node table node table node table node table node table Cost of link changes to node table 6 7 node table 6 7 node table 8 9 Network Laer 4-9 bad news travels slow Algorithm converges in 44 steps. 6 4 Cost of link changes to 6 node table node table node table node table node table node table node table node table node table node table node table node table Network Laer 4-94 Distance Vector: Link Cost Changes Link cost changes: Good news travels fast Bad news travels slow - count to infinit problem! 44 iterations before algorithm stabilies: see tet 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 completel solve count to infinit problem? 6 4 Real problem is that thinks its shortest path to is through, while thinks its shortest path to is through. The pingpong back and forth with this information. Network Laer node table 4 4 node table 4 4 node table 4 4 poisoned reverse Initial routing table Network Laer

25 node table node table node table node table Algorithm converges node table node table node table node table in steps node table node table node table node table Cost of link 4 changes to 6 Network Laer 4-97 Comparison of LS and DV algorithms Message compleit LS: with n nodes, E links, O(nE) msgs sent DV: echange between neighbors onl Convergence time varies Speed of Convergence LS: O(n ) algorithm requires O(nE) msgs Ma have oscillations DV: convergence time varies Ma be routing loops Count-to-infinit problem Robustness: what happens if malfunctions? LS: Node can advertise incorrect link cost Each node computes onl its own table DV: DV node can advertise incorrect path cost Each node s table used b others Error propagate thru network Network Laer 4-98 Chapter 4: Network Laer Hierarchical Routing 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Our routing stud thus far - idealiation All s identical Network flat nottrue in practice Scale: with million destinations: Can t store all dest s in routing tables! Routing table echange would swamp links! Administrative autonom = network of networks Each network admin ma want to control routing in its own network Network Laer 4-99 Network Laer 4-

26 Hierarchical Routing Interconnected ASes Aggregate s into regions, autonomous sstems (AS) Routers in same AS run same routing protocol Intra-AS routing protocol Routers in different AS can run different intra- AS routing protocol gatewa s Special s in AS Run intra-as routing protocol with all other s in AS Also responsible for routing to destinations outside AS Run inter-as routing protocol with other gatewa s c a b AS a c d b Intra-AS Routing algorithm AS Forwarding table Inter-AS Routing algorithm c a b AS Forwarding table is configured b both intra- and inter-as routing algorithm Intra-AS sets entries for internal dests Inter-AS & Intra-As sets entries for eternal dests Network Laer 4- Network Laer 4- Inter-AS Tasks Suppose in AS receives datagram for which dest is outside of AS Router should forward packet towards one of the gatewa s, but which one? AS needs:. To learn which dests are reachable through AS and which through AS. To propagate this reachabilit info to all s in AS Job of inter-as routing! Eample: Setting Forwarding Table in Router d Suppose AS learns (via inter-as protocol) that subnet is reachable via AS (gatewa c) but not via AS Inter-AS protocol propagates reachabilit info to all internal s Router d determines intra-as routing info that its interface I is on the least cost path to c Puts in forwarding table entr (,I) c a b AS a c d b AS c a b AS Network Laer 4- c a b AS a c d b AS c a b AS Network Laer 4-4 6

27 Eample: Choosing Among Multiple ASes Chapter 4: Network Laer Now suppose AS learns the inter-as protocol that subnet is reachable AS and AS To configure forwarding table, d must determine towards which gatewa it should forward packets for dest This is also the job on inter-as routing protocol! Hot potato routing: send packet towards closest of two s Learn inter-as protocol that subnet is reachable via multiple gatewas Use routing info intra-as protocol to determine costs of least-cost paths to each of the gatewas Hot potato routing: Choose the gatewa that has the smallest least cost Determine forwarding table the interface I that leads to least-cost gatewa. Enter (,I) in forwarding table 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Network Laer 4- Network Laer 4-6 Routing in the Intra-AS Routing The Global consists of Autonomous Sstems (AS) interconnected with each other: Stub AS: small corporation: one connection to other AS s Multihomed AS: large corporation (no transit): multiple connections to other AS s Transit AS: provider, hooking man AS s together Two-level routing: Intra-AS: (within AS) administrator responsible for choice of routing algorithm within network Inter-AS: (between ASes) unique standard for inter-as routing: BGP Also known as Interior Gatewa Protocols (IGP) Most common Intra-AS routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gatewa Routing Protocol (Cisco proprietar) Network Laer 4-7 Network Laer 4-8 7

28 Chapter 4: Network Laer RIP (Routing Information Protocol) 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Distance vector algorithm Included in BSD-UNIX Distribution in 98 Distance metric: # of hops (ma = hops) u A C B D v w From A to subsets: destination hops u v w Network Laer 4-9 Network Laer 4- RIP Advertisements Distance vectors: echanged among neighbors ever sec via Response Message (also called advertisement) Each advertisement: list of up to destination nets within AS Network Laer 4- RIP: Eample w A D B C Destination Network Net Router Num. of hops to dest. w A B B Routing table in D Network Laer 4-8

29 RIP: Eample Dest Net hops w - - C Advertisement A to D w A D B C Destination Network Net Router Num. of hops to dest. w A B B A Routing table in D Network Laer 4- RIP: Link Failure and Recover If no advertisement heard after 8 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 quickl propagates to entire net Poison reverse used to prevent ping-pong loops (infinite distance = 6 hops) Network Laer 4-4 RIP Table Processing Chapter 4: Network Laer RIP routing tables managed b application-level process called route-d (daemon) Advertisements sent in UDP packets, periodicall repeated routed Transport (UDP) network forwarding (IP) table link phsical forwarding table routed Transport (UDP) network (IP) link phsical 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP Network Laer 4- Network Laer 4-6 9

30 OSPF (Open Shortest Path First) OSPF Advanced Features (not in RIP) Open : publicl available Uses Link State algorithm LS packet dissemination Topolog map at each node Route computation using Dijkstra s algorithm OSPF advertisement carries one entr per neighbor Advertisements disseminated to entire AS (via flooding) Carried in OSPF messages directl over IP (rather than TCP or UDP Securit: all OSPF messages authenticated (to prevent malicious intrusion) Multiple same-cost paths allowed (onl 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 topolog data base as OSPF Hierarchical OSPF in large domains Network Laer 4-7 Network Laer 4-8 Hierarchical OSPF Hierarchical OSPF Two-level hierarch: local area, backbone Link-state advertisements onl in area Each nodes has detailed area topolog; onl know direction (shortest path) to nets in other areas Area border s: summarie distances to nets in own area, advertise to other Area Border s Backbone s: run OSPF routing limited to backbone Boundar s: connect to other AS s Network Laer 4-9 Network Laer 4-

31 Chapter 4: Network Laer Inter-AS Routing in the : BGP 4. Introduction 4. Virtual circuit and datagram networks 4. What s inside a 4.4 IP: Protocol Datagram format IPv4 addressing ICMP IPv6 4. Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the RIP OSPF BGP R AS (RIP intra-as routing) R BGP R R BGP AS (OSPF intra-as routing) Figure 4..-new: BGP use for inter-domain routing R4 AS (OSPF intra-as routing) Network Laer 4- Network Laer 4- Inter-AS Routing: BGP BGP (Border Gatewa Protocol): the de facto standard BGP provides each AS a means to:. Obtain subnet reachabilit information neighboring ASs.. Propagate reachabilit information to all ASinternal s.. Determine good routes to subnets based on reachabilit information and polic. Allows subnet to advertise its eistence to rest of : I am here Network Laer 4- In BGP, destination are not individual hosts, the are networks! A network is represented b a CIDR prefi, e.g., /4 If a gatewa broadcasts a BGP message stating that it is /4, it is advertising that it can deliver messages to an host in subnet /4 BGP messages between s in same AS are called (interior) ibgp messages BGP messages between s in diff AS are called (eterior) ebgp messages Network Laer 4-4

32 BGP Basics Pairs of s (BGP peers) echange routing info over semi-permanent TCP connections: BGP sessions BGP sessions need not correspond to phsical links When AS advertises a prefi to AS, AS is promising it will forward an datagrams destined to that prefi towards the prefi AS can aggregate prefies in its advertisement Distributing Reachabilit Info With ebgp session between a and c, AS sends prefi reachabilit info to AS c can then use ibgp do distribute this new prefi reach info to all s in AS b can then re-advertise new reachabilit info to AS over b-to-a ebgp session When learns of new prefi, creates entr for prefi in its forwarding table c a b AS a ebgp session AS ibgp session c d b c a b AS c a b AS a ebgp session AS ibgp session c d b c a b AS Network Laer 4- Network Laer 4-6 Path Attributes & BGP Routes When advertising a prefi, advert includes BGP attributes. prefi + attributes = route Two important attributes: AS-PATH: contains ASs through which prefi advertisement has passed: AS 67 AS 7 NEXT-HOP: Indicates specific internal-as to nethop AS. (There ma be multiple links current AS to net-hop-as.) When gatewa receives route advertisement, uses import polic to accept/decline BGP Route Selection Router ma learn about more than route to some prefi. Router must select route Elimination rules:. Local preference value attribute: polic decision. Shortest AS-PATH. Closest NEXT-HOP : hot potato routing 4. Additional criteria Network Laer 4-7 Network Laer 4-8

33 BGP Messages BGP messages echanged 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 Polic W A B C Figure 4.-BGPnew: a simple BGP scenario A,B,C are provider networks X Y legend: provider network customer network: X,W,Y are customer (of provider networks) X is dual-homed: attached to two networks X does not want to route B via X to C.. so X will not advertise to B a route to C Network Laer 4-9 Network Laer 4- BGP Routing Polic () W A B C Figure 4.-BGPnew: a simple BGP scenario X Y legend: provider network customer network: A advertises to B the path AW B advertises to X the path BAW Should B advertise to C the path BAW? No wa! 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 onl to/ its customers! Network Laer 4- Wh different Intra- and Inter-AS routing? Polic: Inter-AS: admin wants control over how its traffic routed, who routes through its net. Intra-AS: single admin, so no polic decisions needed Scale: Hierarchical routing saves table sie, reduced update traffic Performance: Intra-AS: can focus on performance Inter-AS: polic ma dominate over performance Network Laer 4-

34 Network Laer: Summar What we ve covered: Network laer services Routing principles: link state and distance vector Hierarchical routing IP routing protocols RIP, OSPF, BGP What s inside a? IPv6 Net stop: the Data link laer! Network Laer 4-4

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