55:134/22C:134 Summer, Chapter 4: Network Layer. Chapter 4 Network Layer. Chapter 4 roadmap. Jon Kuhl. Network layer functions

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1 Chapter 4 Network Layer ll material copyright J.F Kurose and K.W. Ross, ll Rights Reserved Computer Networking: Top Down pproach Featuring the Internet, nd edition. Jim Kurose, Keith Ross ddison-wesley, July 00. Chapter 4: Network Layer Chapter goals: understand principles behind layer services: routing (path selection) dealing with scale how a router works advanced topics: IPv6, mobility instantiation and implementation in the Internet Overview: layer services routing principles: path selection hierarchical routing IP Internet routing protocols intra-domain inter-domain what s inside a router? IPv6 mobility Network Layer 4- Network Layer 4- Chapter 4 roadmap 4. Introduction and Network Service Models 4. Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet 4.6 What s Inside a Router 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility Network Layer 4-3 Network layer functions deliver packets between hosts residing at edge layer protocols in every host, router three important functions: routing: determine path taken by packets from source to dest. Routing algorithms bueffering & forwarding: move packets from router s input to appropriate router output call setup: some architectures require router call setup along path before data flows application transport data link physical data link physical data link physical data link physical data link physical data link physical data link physical data link physical data link application physical transport data link physical Network Layer 4-4

2 Network service model Virtual circuits service abstraction Q: What service model for channel transporting packets from sender to receiver? guaranteed bandwidth? preservation of inter-packet timing (no jitter)? loss-free delivery? in-order delivery? congestion feedback to sender? The most important abstraction provided by layer:? or datagram??? virtual circuit Network Layer 4-5 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 ID) every router on source-dest path maintains state for each passing connection transport-layer connection only involved two end systems link, router resources (bandwidth, buffers) may be allocated to VC to get circuit-like perf. Network Layer 4-6 Virtual circuits: signaling protocols used to setup, maintain teardown VC used in TM, frame-relay, X.5 not used in today s Internet application transport data link physical 5. Data flow begins 6. Receive data 4. Call connected 3. ccept call. Initiate call. incoming call application transport data link physical Datagram s: the Internet model no call setup at layer routers: 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 physical application transport. Send data. Receive data data link physical Network Layer 4-7 Network Layer 4-8

3 Network layer service models: Datagram or VC : why? Network rchitecture Internet TM TM TM TM Service Model best effort CR VR R UR andwidth none constant rate guaranteed rate guaranteed minimum none Guarantees? Loss Order Timing no yes yes no no no yes yes yes yes no yes yes Internet model being extended: Intserv, Diffserv Chapter 6 no no Congestion feedback no (inferred via loss) no congestion no congestion yes no Internet 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 TM evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service dumb end systems telephones complexity inside Network Layer 4-9 Network Layer 4-0 Chapter 4 roadmap 4. Introduction and Network Service Models 4. Routing Principles Link state routing Distance vector routing 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet 4.6 What s Inside a Router 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility Routing Routing protocol Goal: determine good path (sequence of routers) thru from source to dest. Graph abstraction for routing algorithms: graph nodes are routers graph edges are physical links link cost: delay, $ cost, or congestion level 5 D 3 3 C E 5 F good path: typically means minimum cost path other def s possible Network Layer 4- Network Layer 4-

4 Routing lgorithm classification Link-State Routing lgorithm Global or decentralized information? Global: all routers have complete topology, link cost info link state algorithms Decentralized: router knows physicallyconnected 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 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 routing table for that node iterative: after k iterations, know least cost path to k dest. s Notation: c(i,j): link cost from node i to j. cost infinite 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, that is next v N: set of nodes whose least cost path definitively known Network Layer 4-3 Network Layer 4-4 Dijsktra s lgorithm Dijkstra s algorithm: example Initialization: N = {} 3 for all nodes v 4 if v adjacent to 5 then D(v) = c(,v) 6 else D(v) = infinity 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 4-5 Step start N D DE DE DEC DECF D(),p(),,, D(C),p(C) 5, 4,D 3,E 3,E 5 D 3 3 C E D(D),p(D), 5 F D(E),p(E) infinity,d D(F),p(F) infinity infinity 4,E 4,E 4,E Network Layer 4-6

5 Dijkstra s algorithm, discussion Distance Vector Routing lgorithm lgorithm 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 +e D e C e initially +e 0 D +e 0 C 0 recompute routing 0 +e D 0 0 C +e recompute +e 0 D 0 +e e C recompute Network Layer 4-7 iterative: continues until no nodes exchange info. self-terminating: no signal to stop asynchronous: nodes need not exchange info/iterate in lock step! distributed: each node communicates only with directly-attached neighbors Distance Table data structure each node has its own row for each possible destination column for each directlyattached neighbor to node example: in node X, for dest. Y via neighbor Z: X D (Y,Z) = = distance from X to Y, via Z as next hop Z c(x,z) + min {D (Y,w)} w Network Layer 4-8 Distance Table: example Distance table gives routing table 7 E D (C,D) E D (,D) E D (,) 8 E C D D c(e,d) + min {D (C,w)} w = = + = 4 D c(e,d) + min {D (,w)} w = = +3 = 5 loop! = c(e,) + min {D (,w)} w = 8+6 = 4 loop! cost to destination via E D () D destination C D Network Layer 4-9 cost to destination via E D () D destination C D Distance table destination C D Outgoing link to use, cost, D,5 D,4 D,4 Routing table Network Layer 4-0

6 Distance Vector Routing: overview Distance Vector lgorithm: Iterative, asynchronous: each local iteration caused by: local link cost change message from neighbor: its least cost path change from neighbor Distributed: each node notifies neighbors only when its least cost path to any destination changes neighbors then notify their neighbors if necessary Each node: wait for (change in local link cost or msg from neighbor) recompute distance table if least cost path to any dest has changed, notify neighbors Network Layer 4- t all nodes, X: Initialization: for all adjacent nodes v: 3 D X(*,v) = infinity /* the * operator means "for all rows" */ X 4 D (v,v) = c(x,v) 5 for all destinations, y X 6 send min D (y,w) to each neighbor /* w over all X's neighbors */ w Network Layer 4- Distance Vector lgorithm (cont.): 8 loop 9 wait (until I see a link cost change to neighbor V 0 or until I receive update from neighbor V) if (c(x,v) changes by d) 3 /* change cost to all dest's via neighbor v by d */ 4 /* note: d could be positive or negative */ 5 for all destinations y: D X(y,V) = D X(y,V) + d 6 7 else if (update received from V wrt destination Y) 8 /* shortest path from V to some Y has changed */ 9 /* V has sent a new value for its min w DV(Y,w) */ 0 /* call this received new value is "newval" */ for the single destination y: D X (Y,V) = c(x,v) + newval X 3 if we have a new min w D (Y,w)for any destination Y X 4 send new value of min D (Y,w) to all neighbors w 5 6 forever Network Layer 4-3 Distance Vector lgorithm: example X Y 7 Z Network Layer 4-4

7 Distance Vector lgorithm: example Distance Vector: link cost changes X Y 7 Z X Z D (Y,Z) = c(x,z) + min {D (Y,w)} w = 7+ = 8 X Y D (Z,Y) = c(x,y) + min {D (Z,w)} w = + = 3 Link cost changes: node detects local link cost change updates distance table (line 5) if cost change in least cost path, notify neighbors (lines 3,4) good news travels fast 4 X Y 50 Z algorithm terminates Network Layer 4-5 Network Layer 4-6 Distance Vector: link cost changes Distance Vector: poisoned reverse Link cost changes: good news travels fast bad news travels slow - count to infinity problem! 60 4 X Y 50 Z 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 algorithm continues on! algorithm terminates Network Layer 4-7 Network Layer 4-8

8 Comparison of LS and DV algorithms Chapter 4 roadmap Message complexity LS: with n nodes, E links, O(nE) msgs sent each 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 router 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 Introduction and Network Service Models 4. Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet 4.6 What s Inside a Router 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility Network Layer 4-30 Hierarchical Routing Our routing study thus far - idealization all routers identical flat nottrue 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 Hierarchical Routing aggregate routers into regions, autonomous systems (S) routers in same S run same routing protocol intra-s routing protocol routers in different S can run different intra- S routing protocol gateway routers special routers in S run intra-s routing protocol with all other routers in S also responsible for routing to destinations outside S run inter-s routing protocol with other gateway routers Network Layer 4-3 Network Layer 4-3

9 Intra-S and Inter-S routing Intra-S and Inter-S routing a C C.b b d.a a b inter-s, intra-s routing in gateway.c.c c.a a c Gateways: perform inter-s routing amongst themselves b perform intra-s routers with other routers in their S layer link layer physical layer a Host h Inter-S C.b routing between.a and b.c C a d b c Intra-S routing within S.a Host c h a b Intra-S routing within S We ll examine specific inter-s and intra-s Internet routing protocols shortly Network Layer 4-33 Network Layer 4-34 Chapter 4 roadmap 4. Introduction and Network Service Models 4. Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.4. IPv4 addressing 4.4. Moving a datagram from source to destination Datagram format IP fragmentation ICMP: Internet Control Message Protocol DHCP: Dynamic Host Configuration Protocol NT: Network ddress Translation 4.5 Routing in the Internet 4.6 What s Inside a Router 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility Network Layer 4-35 The Internet Network layer Host, router layer functions: Network layer Routing protocols path selection RIP, OSPF, GP Transport layer: TCP, UDP forwarding table Link layer physical layer IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router signaling Network Layer 4-36

10 IP ddressing: introduction IP address: 3-bit identifier for host, router interface interface: connection between host/router and physical link router s typically have multiple interfaces host may have multiple interfaces IP addresses associated with each interface = Network Layer 4-37 IP ddressing IP address: part (high order bits) host part (low order bits) What s a? (from IP address perspective) device interfaces with same part of IP address can physically reach each other without intervening router LN consisting of 3 IP s (for IP addresses starting with 3, first 4 bits are address) Network Layer 4-38 IP ddressing How to find the s? Detach each interface from router, host create islands of isolated s Interconnected system consisting of six s Network Layer 4-39 IP ddresses given notion of, let s re-examine IP addresses: class-full addressing: class C D 0 host 0 host 0 host 0 multicast address 3 bits to to to to Network Layer 4-40

11 IP addressing: CIDR Classful addressing: inefficient use of address space, address space exhaustion e.g., class net allocated enough addresses for 65K hosts, even if only K hosts in that CIDR: Classless InterDomain Routing portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in portion of address host part part /3 Network Layer 4-4 IP addresses: how to get one? Q: How does host get IP address? hard-coded by system admin in a file Wintel: control-panel->->configuration- >tcp/ip->properties UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play (more shortly) Network Layer 4-4 IP addresses: how to get one? Q: How does get part of IP addr? : 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-y-Night-ISP ISPs-R-Us Send me anything with addresses beginning /0 Send me anything with addresses beginning /6 Internet Network Layer 4-43 Network Layer 4-44

12 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization Organization /3 Organization /3 Organization /3.... Organization /3 Fly-y-Night-ISP ISPs-R-Us Send me anything with addresses beginning /0 Send me anything with addresses beginning /6 or /3 Internet IP addressing: the last word... Q: How does an ISP get block of addresses? : ICNN: Internet Corporation for ssigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes Visit the ICNN web site ( for a plethora of information regarding the complex and ever-evolving mechanisms for management of domain names and IP address space Network Layer 4-45 Network Layer 4-46 Getting a datagram from source to dest. IP datagram: misc fields source IP addr dest IP addr data datagram remains unchanged*, as it travels source to destination addr fields of interest here * with the exception of a few fields in the header forwarding table in Dest. Net. next router Nhops E Network Layer 4-47 Getting a datagram from source to dest. misc fields data Starting at, send IP datagram addressed to : look up net. address of in forwarding table find is on same net. as link layer will send datagram directly to inside link-layer frame and are directly connected forwarding table in Dest. Net. next router Nhops E Network Layer 4-48

13 Getting a datagram from source to dest. misc fields data Starting at, dest. E: look up address of E in forwarding table E on different, E not directly attached routing table: next hop router to E is link layer sends datagram to router inside linklayer frame datagram arrives at continued.. forwarding table in Dest. Net. next router Nhops E Network Layer 4-49 Getting a datagram from source to dest. forwarding table in router misc fields data Dest. Net router Nhops interface rriving at 3..4, destined for 3... look up address of E in router s forwarding table E on same as router s interface router, E directly attached link layer sends datagram to 3... inside link-layer frame via interface datagram arrives at 3...!!! (hooray!) E Network Layer 4-50 IP datagram format IP protocol version number header length (bytes) type of data max number remaining hops (decremented at each router) 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 ver head. type of len service length 6-bit identifier flgs fragment offset time to upper Internet 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 routers to visit. Network Layer 4-5 IP Fragmentation & 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 Network Layer 4-5

14 IP Fragmentation and Reassembly ICMP: Internet Control Message Protocol Example 4000 byte datagram MTU = 500 bytes length =4000 ID =x length =500 length =500 length =040 fragflag =0 ID =x ID =x ID =x offset =0 One large datagram becomes several smaller datagrams fragflag = offset =0 fragflag offset = =480 fragflag offset =0 =960 used by hosts, routers, gateways to communication -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 router discovery 0 TTL expired 0 bad IP header Network Layer 4-53 Network Layer 4-54 DHCP: Dynamic Host Configuration Protocol DHCP client-server scenario Goal: allow host to dynamically obtain its IP address from server when it joins Can renew its lease on address in use llows 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 DHCP server E arriving DHCP client needs address in this Network Layer 4-56

15 DHCP client-server scenario DHCP server: DHCP discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 DHCP offer src: 3...5, 67 dest: , 68 yiaddrr: transaction ID: 654 Lifetime: 3600 secs DHCP request src: , 68 dest:: , 67 yiaddrr: transaction ID: 655 time Lifetime: 3600 secs DHCP CK src: 3...5, 67 dest: , 68 yiaddrr: transaction ID: 655 Lifetime: 3600 secs arriving client Network Layer 4-57 NT: Network ddress Translation rest of Internet ll datagrams leaving local have same single source NT 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) Network Layer 4-58 NT: Network ddress Translation Motivation: local uses just one IP address as far as outside word is concerned: no need to be allocated range of addresses from ISP: - just one IP address is used 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 4-59 NT: Network ddress Translation Implementation: NT router must: outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NT IP address, new port #)... remote clients/servers will respond using (NT IP address, new port #) as destination addr. remember (in NT translation table) every (source IP address, port #) to (NT IP address, new port #) translation pair incoming datagrams: replace (NT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NT table Network Layer 4-60

16 NT: Network ddress Translation NT: Network ddress Translation : NT router changes datagram source addr from , 3345 to , 500, updates table NT translation table WN side addr LN side addr , , 3345 S: , 500 D: , S: , 80 D: , : Reply arrives dest. address: , S: , 3345 D: , 80 S: , 80 D: , : host sends datagram to , : NT router changes datagram dest addr from , 500 to , 3345 Network Layer bit port-number field: 60,000 simultaneous connections with a single LN-side address! NT is controversial: routers should only process up to layer 3 violates end-to-end argument NT possibility must be taken into account by app designers, eg, PP applications address shortage should instead be solved by IPv6 Network Layer 4-6 Chapter 4 roadmap 4. Introduction and Network Service Models 4. Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet 4.5. Intra-S routing: RIP and OSPF 4.5. Inter-S routing: GP 4.6 What s Inside a Router? 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility Network Layer 4-63 Routing in the Internet The Global Internet consists of utonomous Systems (S) interconnected with each other: Stub S: small corporation: one connection to other S s Multihomed S: large corporation (no transit): multiple connections to other S s Transit S: provider, hooking many S s together Two-level routing: Intra-S: administrator responsible for choice of routing algorithm within Inter-S: unique standard for inter-s routing: GP Network Layer 4-64

17 Internet S Hierarchy Intra-S border (exterior gateway) routers Intra-S Routing lso known as Interior Gateway Protocols (IGP) Most common Intra-S routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco proprietary) Inter-S interior (gateway) routers Network Layer 4-65 Network Layer 4-66 RIP ( Routing Information Protocol) Distance vector algorithm Included in SD-UNIX Distribution in 98 Distance metric: # of hops (max = 5 hops) Can you guess why? Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement) Each advertisement: list of up to 5 destination nets within S Network Layer 4-67 RIP: Example w x y D C Destination Network Next Router Num. of hops to dest. w y z 7 x Routing table in D z Network Layer 4-68

18 RIP: Example Dest Next hops w - - x - - z C dvertisement from to D w x y D C Destination Network Next Router Num. of hops to dest. w y z 7 5 x Routing table in D z Network Layer 4-69 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 4-70 RIP Table processing RIP routing tables managed by application-level process called route-d (daemon) advertisements sent in UDP packets, periodically repeated routed routed RIP Table example (continued) Router: giroflee.eurocom.fr Destination Gateway Flags Ref Use Interface UH lo U 3 fa U le U 5 qaa U 3 0 le0 default UG Transprt (UDP) (IP) link physical forwarding table forwarding table Transprt (UDP) (IP) link physical Three attached class C s (LNs) Router only knows routes to attached LNs Default router used to go up Route multicast address: Loopback interface (for debugging) Network Layer 4-7 Network Layer 4-7

19 OSPF (Open Shortest Path First) OSPF advanced features (not in RIP) 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 router dvertisements disseminated to entire S (via flooding) Carried in OSPF messages directly over IP (rather than TCP or UDP 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 4-73 Network Layer 4-74 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. rea border routers: summarize distances to nets in own area, advertise to other rea order routers. ackbone routers: run OSPF routing limited to backbone. oundary routers: connect to other S s. Network Layer 4-75 Network Layer 4-76

20 Inter-S routing in the Internet: GP Internet inter-s routing: GP R5 S (RIP intra-s routing) R GP R R3 GP S (OSPF intra-s routing) Figure 4.5.-new: GP use for inter-domain routing R4 S3 (OSPF intra-s routing) GP (order Gateway Protocol): the de facto standard Path Vector protocol: similar to Distance Vector protocol each order Gateway broadcast to neighbors (peers) entire path (i.e., sequence of S s) to destination GP routes to s (Ss), not individual hosts E.g., Gateway X may send its path to dest. Z: Path (X,Z) = X,Y,Y,Y3,,Z Network Layer 4-77 Network Layer 4-78 Internet inter-s routing: GP Suppose: gateway X send its path to peer gateway W W may or may not select path offered by X cost, policy (don t route via competitors S), loop prevention reasons. If W selects path advertised by X, then: Path (W,Z) = w, Path (X,Z) Note: X can control incoming traffic by controlling it route advertisements to peers: e.g., don t want to route traffic to Z -> don t advertise any routes to Z GP: controlling who routes to you W Figure 4.5-GPnew: a simple GP scenario,,c are provider s C X Y legend: provider customer : X,W,Y are customer (of provider s) X is dual-homed: attached to two s X does not want to route from via X to C.. so X will not advertise to a route to C Network Layer 4-79 Network Layer 4-80

21 GP: controlling who routes to you W C Figure 4.5-GPnew: a simple GP scenario X Y legend: provider customer : advertises to the path W advertises to X the path W Should advertise to C the path W? No way! gets no revenue for routing CW since neither W nor C are s customers wants to force C to route to w via wants to route only to/from its customers! Network Layer 4-8 GP operation Q: What does a GP router do? Receiving and filtering route advertisements from directly attached neighbor(s). Route selection. To route to destination X, which path )of several advertised) will be taken? Sending route advertisements to neighbors. Network Layer 4-8 GP messages GP messages exchanged using TCP. GP messages: OPEN: opens TCP connection to peer and authenticates sender UPDTE: advertises new path (or withdraws old) KEEPLIVE keeps connection alive in absence of UPDTES; also CKs OPEN request NOTIFICTION: reports errors in previous msg; also used to close connection Why different Intra- and Inter-S routing? Policy: Inter-S: admin wants control over how its traffic routed, who routes through its net. Intra-S: single admin, so no policy decisions needed Scale: hierarchical routing saves table size, reduced update traffic Performance: Intra-S: can focus on performance Inter-S: policy may dominate over performance Network Layer 4-83 Network Layer 4-84

22 Chapter 4 roadmap 4. Introduction and Network Service Models 4. Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet 4.6 What s Inside a Router? 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility Router rchitecture Overview Two key router functions: run routing algorithms/protocol (RIP, OSPF, GP) switching datagrams from incoming to outgoing link Network Layer 4-85 Network Layer 4-86 Input Port Functions Physical layer: bit-level reception Data link layer: e.g., Ethernet see chapter 5 Decentralized switching: given datagram dest., lookup output port using routing table in input port memory goal: complete input port processing at line speed queuing: if datagrams arrive faster than forwarding rate into switch fabric Input Port Queuing Fabric slower that 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 4-87 Network Layer 4-88

23 Three types of switching fabrics Switching Via Memory First generation routers: packet copied by system s (single) CPU speed limited by memory bandwidth ( bus crossings per datagram) Input Port Memory Output Port System us Network Layer 4-89 Modern routers: input port processor performs lookup, copy into memory Cisco Catalyst 8500 Network Layer 4-90 Switching Via a us Switching Via n Interconnection Network datagram from input port memory to output port memory via a shared bus bus contention: switching speed limited by bus bandwidth Gbps bus, Cisco 900: sufficient speed for access and enterprise routers (not regional or backbone) overcome bus bandwidth limitations anyan s, other interconnection nets initially developed to connect processors in multiprocessor dvanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. Cisco 000: switches Gbps through the interconnection Network Layer 4-9 Network Layer 4-9

24 Output Ports Output port queueing uffering required when datagrams arrive from fabric faster than the transmission rate Scheduling discipline chooses among queued datagrams for transmission Network Layer 4-93 buffering when arrival rate via switch exceeds output line speed queueing (delay) and loss due to output port buffer overflow! Network Layer 4-94 Chapter 4 roadmap 4. Introduction and Network Service Models 4. Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet 4.6 What s Inside a Router? 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility IPv6 Initial motivation: 3-bit address space completely allocated by 008. dditional motivation: header format helps speed processing/forwarding header changes to facilitate QoS new anycast address: route to best of several replicated servers IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed Network Layer 4-95 Network Layer 4-96

25 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 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 ig multicast group management functions Network Layer 4-97 Network Layer 4-98 Transition From IPv4 To IPv6 Dual Stack pproach Not all routers can be upgraded simultaneous no flag days How will the operate with mixed IPv4 and IPv6 routers? Two proposed approaches: Dual Stack: some routers with dual stack (v6, v4) can translate between formats Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 Flow: X Src: Dest: F data -to-: IPv6 Src: Dest: F data -to-c: IPv4 Src: Dest: F data -to-c: IPv4 Flow:?? Src: Dest: F data -to-c: IPv6 Network Layer 4-99 Network Layer 4-00

26 Tunneling Logical view: Physical view: E F tunnel IPv6 IPv6 IPv6 IPv6 C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 Flow: X Src: Dest: F data -to-: IPv6 Src: Dest: E Flow: X Src: Dest: F data -to-c: IPv6 inside IPv4 Src: Dest: E Flow: X Src: Dest: F data -to-c: IPv6 inside IPv4 Flow: X Src: Dest: F data E-to-F: IPv6 Network Layer 4-0 Chapter 4 roadmap 4. Introduction and Network Service Models 4. Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet 4.6 What s Inside a Router? 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility Network Layer 4-0 Multicast: one sender to many receivers Multicast: act of sending datagram to multiple receivers with single transmit operation analogy: one teacher to many students Question: how to achieve multicast Multicast: one sender to many receivers Multicast: act of sending datagram to multiple receivers with single transmit operation analogy: one teacher to many students Question: how to achieve multicast routers forward unicast datagrams Multicast via unicast source sends N unicast datagrams, one addressed to each of N receivers multicast receiver (red) not a multicast receiver (red) Network Layer 4-03 Multicast routers (red) duplicate and forward multicast datagrams Network multicast Router actively participate in multicast, making copies of packets as needed and forwarding towards multicast receivers Network Layer 4-04

27 Multicast: one sender to many receivers Multicast: act of sending datagram to multiple receivers with single transmit operation analogy: one teacher to many students Question: how to achieve multicast pplication-layer multicast end systems involved in multicast copy and forward unicast datagrams among themselves Internet Multicast Service Model multicast group multicast group concept: use of indirection hosts addresses IP datagram to multicast group routers forward multicast datagrams to hosts that have joined that multicast group Network Layer 4-05 Network Layer 4-06 Multicast groups class D Internet addresses reserved for multicast: host group semantics: o anyone can join (receive) multicast group o anyone can send to multicast group o no -layer identification to hosts of members needed: infrastructure to deliver mcast-addressed datagrams to all hosts that have joined that multicast group Joining a mcast group: two-step process local: host informs local mcast router of desire to join group: IGMP (Internet Group Management Protocol) wide area: local router interacts with other routers to receive mcast datagram flow many protocols (e.g., DVMRP, MOSPF, PIM) IGMP wide-area multicast routing IGMP Network Layer 4-07 IGMP Network Layer 4-08

28 IGMP: Internet Group Management Protocol host: sends IGMP report when application joins mcast group IP_DD_MEMERSHIP socket option host need not explicitly unjoin group when leaving router: sends IGMP query at regular intervals host belonging to a mcast group must reply to query query report Network Layer 4-09 IGMP IGMP version router: Host Membership Query msg broadcast on LN to all hosts host: Host Membership Report msg to indicate group membership randomized delay before responding implicit leave via no reply to Query RFC IGMP v: additions include group-specific Query Leave Group msg last host replying to Query can send explicit Leave Group msg router performs groupspecific query to see if any hosts left in group RFC 36 IGMP v3: under development as Internet draft Network Layer 4-0 Multicast Routing: Problem Statement Goal: find a tree (or trees) connecting routers having local mcast group members tree: not all paths between routers used source-based: different tree from each sender to rcvrs shared-tree: same tree used by all group members pproaches for building mcast trees pproaches: 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 Shared tree Source-based trees

29 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 router with attached group member router with no attached group member link used for forwarding, i indicates order link added by algorithm Reverse Path Forwarding rely on router s knowledge of unicast shortest path from it to sender each router 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 R R3 R R6 R4 R7 R5 LEGEND router with attached group member router 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 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 router with no downstream group members S: source R R3 R R6 P R4 P R7 R5 LEGEND P router with attached group member router with no attached group member prune message links with multicast forwarding

30 Shared-Tree: Steiner Tree Center-based trees Steiner Tree: minimum cost tree connecting all routers 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 router needs to join/leave single delivery tree shared by all one router identified as center of tree to join: edge router sends unicast join-msg addressed to center router join-msg processed by intermediate routers 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 router Center-based trees: an example Suppose R6 chosen as center: R3 R R 3 R6 R4 R7 R5 LEGEND router with attached group member router with no attached group member path order in which join messages generated Internet 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 routers no assumptions about underlying unicast initial datagram to mcast group flooded everywhere via RPF routers not wanting group: send upstream prune msgs

31 DVMRP: continued soft state: DVMRP router periodically ( min.) forgets branches are pruned: mcast data again flows down unpruned branch downstream router: reprune or else continue to receive data routers can quickly regraft to tree following IGMP join at leaf odds and ends commonly implemented in commercial routers Mbone routing done using DVMRP Tunneling Q: How to connect islands of multicast routers in a sea of unicast routers? physical topology logical topology mcast datagram encapsulated inside normal (non-multicastaddressed) datagram normal IP datagram sent thru tunnel via regular IP unicast to receiving mcast router receiving mcast router unencapsulates to get mcast datagram PIM: Protocol Independent Multicast not dependent on any specific underlying unicast routing algorithm (works with all) two different multicast distribution scenarios : 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 Consequences of Sparse-Dense Dichotomy: Dense Sparse: group membership by no membership until routers assumed until routers explicitly join routers explicitly prune receiver- driven data-driven construction construction of mcast on mcast tree (e.g., RPF) tree (e.g., center-based) bandwidth and nongroup-router processing router processing bandwidth and non-group- profligate conservative

32 PIM- Dense Mode PIM - Sparse 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 router to detect it is a leaf-node router center-based approach router sends join msg to rendezvous point (RP) intermediate routers update state and forward join after joining via RP, router 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 PIM - Sparse Mode Chapter 4 roadmap 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 4. Introduction and Network Service Models 4. Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet 4.6 What s Inside a Router? 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility Network Layer 4-8

33 What is mobility? Mobility: Vocabulary spectrum of mobility, from the perspective: no mobility high mobility home : permanent home of mobile (e.g., /4) home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote mobile user, using same access point mobile user, connecting/ disconnecting from using DHCP. mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone) Permanent address: address in home, can always be used to reach mobile e.g., wide area correspondent Network Layer 4-9 Network Layer 4-30 Mobility: more vocabulary How do you contact a mobile friend: Permanent address: remains constant (e.g., ) Care-of-address: address in visited. (e.g., 79,9.3.) wide area visited : in which mobile currently resides (e.g., /4) Consider friend frequently changing addresses, how do you find her? search all phone books? call her parents? expect her to let you know where he/she is? I wonder where lice moved to? correspondent: wants to communicate with mobile foreign agent: entity in visited that performs mobility functions on behalf of mobile. Network Layer 4-3 Network Layer 4-3

34 Mobility: approaches Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. routing tables indicate where each mobile located no changes to end-systems Let end-systems handle it: indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote direct routing: correspondent gets foreign address of mobile, sends directly to mobile Mobility: approaches Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence not via usual routing table exchange. scalable to millions of routing tables indicate mobileswhere each mobile located no changes to end-systems let end-systems handle it: indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote direct routing: correspondent gets foreign address of mobile, sends directly to mobile Network Layer 4-33 Network Layer 4-34 Mobility: registration home wide area foreign agent contacts home agent home: this mobile is resident in my End result: Foreign agent knows about mobile Home agent knows location of mobile visited mobile contacts foreign agent on entering visited Network Layer 4-35 Mobility via Indirect Routing home correspondent addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent wide area foreign agent receives packets, forwards to mobile 4 3 visited mobile replies directly to correspondent Network Layer 4-36

35 Indirect Routing: comments Forwarding datagrams to remote mobile Mobile uses two addresses: permanent address: used by correspondent (hence mobile location is transparent to correspondent) care-of-address: used by home agent to forward datagrams to mobile foreign agent functions may be done by mobile itself triangle routing: correspondent-home-mobile inefficient when correspondent, mobile are in same packet sent by home agent to foreign agent: a packet within a packet dest: dest: Permanent address: dest: packet sent by correspondent foreign-agent-to-mobile packet dest: Care-of address: Network Layer 4-37 Network Layer 4-38 Indirect Routing: moving between s suppose mobile user moves to another registers with new foreign agent new foreign agent registers with home agent home agent update care-of-address for mobile packets continue to be forwarded to mobile (but with new care-of-address) Mobility, changing foreign s transparent: on going connections can be maintained! Mobility via Direct Routing home correspondent requests, receives foreign address of mobile correspondent forwards to foreign agent wide area foreign agent receives packets, forwards to mobile visited mobile replies directly to correspondent Network Layer 4-39 Network Layer 4-40

36 Mobility via Direct Routing: comments overcome triangle routing problem non-transparent to correspondent: correspondent must get care-of-address from home agent What happens if mobile changes s? Mobile IP RFC 30 has many features we ve seen: home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet) three components to standard: agent discovery registration with home agent indirect routing of datagrams Network Layer 4-4 Network Layer 4-4 Mobile IP: agent discovery Mobile IP: registration example agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9) H,F bits: home and/or foreign agent R bit: registration required type = 9 code = 0 checksum router address type = 6 length sequence # RHFMGV registration lifetime reserved bits 0 or more care-ofaddresses standard ICMP fields mobility agent advertisement extension home agent H: time foreign agent CO: registration req. CO: H: M: Lifetime: 9999 identification: 74 encapsulation format. registration reply H: M: Lifetime: 4999 Identification: 74 encapsulation format. ICMP agent adv. CO: registration req. CO: H: M: Lifetime: 9999 identification:74. registration reply H: M: Lifetime: 4999 Identification: 74. visited : /4 Mobile agent M: Network Layer 4-43 Network Layer 4-44

37 Network Layer: summary What we ve covered: layer services routing principles: link state and distance vector hierarchical routing IP Internet routing protocols RIP, OSPF, GP what s inside a router? IPv6 mobility Next stop: the Data link layer! Network Layer 4-45