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Transcript
Network Layer: Routing Goals: understand principles behind network layer services: routing (path selection) dealing with scale how a router works • Previous two lectures instantiation and implementation in the Internet Overview: network layer services routing principle: path selection hierarchical routing IP Internet routing protocols: intra-domain inter-domain Lecture 6: Network Layer #1 Network Layer Transport packet from source to dest. Network layer in every host, router Basic functions: Data plane: forwarding move packets from router’s input port to router output port Control plane: path determination and call setup determine route taken by packets from source to destination Lecture 6: Network Layer #2 Forwarding: Illustration routing and call setup 3 Lecture 6: Network Layer #3 Network service model 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 network layer: ? ? ? virtual circuit or datagram? Lecture 6: Network Layer #5 Virtual circuits: signaling protocols used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet Cisco’s MPLS application transport 5. Data flow begins network 4. Call connected data link 1. Initiate call physical 6. Receive data application 3. Accept call 2. incoming call transport network data link physical Lecture 6: Network Layer #6 Virtual Circuit: call setup Resource allocation: The call setup msg from source to destination. Path determination: • Source based or network based. Msgs includes the required resoutces: • BW, latency, buffer, etc. A router can either: • accept (and commit) or reject Path accepted if all routers accept. Lecture 6: Network Layer #7 Virtual Circuit: Identifiers Forward call-setup pass: Each router allocate an id for the VC Backward call-setup pass: Each router informs its predecessor its id Runtime: When receiving a packet with an id: • Looks up the output port • Looks up the new id • Send on the required port with new id. Lecture 6: Network Layer #8 Virtual Circuit: identifiers Example setup BW=1Mb 2 1 BW=1Mb 1 2 BW=1Mb In In port port VC VC id id in in Out Out port port VC VC id id out out In In port port VC id in Out port VC id out 11 38 2 22 11 22 2 xx Lecture 6: Network Layer #9 Virtual Circuit: identifiers Example runtime VCid=38 2 1 VCid=22 2 1 VCid=xx In In port port VC VC id id in in Out Out port port VC VC id id out out In In port port VC id in Out port VC id out 11 38 2 22 11 22 2 xx Lecture 6: Network Layer #10 Datagram networks: the Internet model no call setup at network layer routers: no state about end-to-end connections no network-level concept of “connection” packets typically routed using destination host ID packets between same source-dest pair may take different paths application transport network data link 1. Send data physical application transport network 2. Receive data data link physical Lecture 6: Network Layer #14 Forwarding table Destination Address Range 4 billion possible entries Link Interface 11001000 00010111 00010000 00000000 through 11001000 00010111 00010111 11111111 0 11001000 00010111 00011000 00000000 through 11001000 00010111 00011000 11111111 1 11001000 00010111 00011001 00000000 through 11001000 00010111 00011111 11111111 2 otherwise 3 Lecture 6: Network Layer #15 Longest prefix matching Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3 VC implementation Examples DA: 11001000 00010111 00010110 10100001 Which interface? DA: 11001000 00010111 00011000 10101010 Which interface? Lecture 6: Network Layer #16 ATM: overview Asynchronous Transfer Mode Fixed packets size: called cells 53 bytes = 5 header + 48 data All virtual circuit-based Types of virtual circuits Virtual circuits and virtual paths Permanent and switched Architecture is a QoS-based approach Lecture 6: Network Layer #17 Network Layer Quality of Service Network Architecture Internet Service Model Guarantees ? Congestion Bandwidth Loss Order Timing feedback best effort none ATM CBR ATM VBR ATM ABR ATM UBR constant rate guaranteed rate guaranteed minimum none no no no yes yes yes yes yes yes no yes no no (inferred via loss/delay) no congestion no congestion yes no yes no no Internet model being extended: Intserv, Diffserv multimedia networking ATM: Asynchronous Transfer Mode; CBR: Constant Bit Rate; V: Variable; A: available; U: Unspecified Lecture 6: Network Layer #18 Datagram or VC network: why? Internet (Datagram) data exchange among computers “elastic” service, no strict timing req. “smart” end systems (computers) can adapt, perform control, error recovery simple inside network, 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 VC Benefits: Fast forwarding Traffic Engineering. Lecture 6: Network Layer #19 Network Layer: Protocols Network layer functions: Transport layer Routing protocols •path selection •e.g., RIP, OSPF, BGP Control protocols - router “signaling” e.g. RSVP Control protocols •error reporting e.g. ICMP Network layer forwarding Network layer protocol (e.g., IP) •addressing conventions •packet format •packet handling conventions Link layer physical layer Lecture 6: Network Layer #20 Control: ROUTING algorithms Lecture 6: Network Layer #21 Control Plane: Routing Routing Goal: determine “good” paths (sequences of routers) thru network from sources to dest. Graph abstraction for the routing problem: 5 graph nodes are routers graph edges are physical links links have properties: delay, capacity, $ cost, policy A 2 1 B 2 D 3 3 1 C 1 E 5 F 2 Lecture 6: Network Layer #22 Key Desired Properties of a Routing Algorithm Robustness Optimality find good path (for user/provider) Simplicity Lecture 6: Network Layer #23 - Robustness - Optimality - Simplicity Routing Design Space Routing has a large design space who decides routing? • source routing: end hosts make decision • network routing: networks make decision how many paths from source s to destination d? • multi-path routing • single path routing will routing adapt to network traffic demand? • adaptive routing • static routing … Lecture 6: Network Layer #24 Routing Algorithm classification 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 Lecture 6: Network Layer #25 A Link-State Routing 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 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 Lecture 6: Network Layer #26 Dijsktra’s Algorithm 1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A,v) 6 else D(v) = infty 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N: 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N Lecture 6: Network Layer #27 Dijkstra’s algorithm: example Step 0 1 2 3 4 5 start N A AD ADE ADEB ADEBC ADEBCF D(B),p(B) D(C),p(C) D(D),p(D) D(E),p(E) D(F),p(F) 2,A 1,A 5,A infinity infinity 2,A 4,D 2,D infinity 2,A 3,E 4,E 3,E 4,E 4,E 5 2 A B 2 1 D 3 C 3 1 5 F 1 E 2 Lecture 6: Network Layer #28 Dijkstra’s algorithm, discussion Algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n(n+1)/2 comparisons: O(n2) more efficient implementations possible: O(nlogn) Oscillations possible: e.g., link cost = amount of carried traffic D 1 1 0 A 0 0 C e 1+e B e initially 2+e D 0 1 A 1+e 1 C 0 B 0 … recompute routing 0 D 1 A 0 0 2+e B C 1+e … recompute 2+e D 0 A 1+e 1 C 0 B 0 … recompute Lecture 6: Network Layer #29 Distance Vector Routing Algorithm 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 directly- attached 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 = c(X,Z) + min {DZ(Y,w)} w Lecture 6: Network Layer #30 Distance Vector Routing Basis of RIP, IGRP, EIGRP routing protocols Based on the Bellman-Ford algorithm (BFA) Conceptually, runs for each destination separately Lecture 6: Network Layer #31 Distance Vector Routing: Basic Idea At node i, the basic update rule d i min jN ( i ) (d ij d j ) where - di denotes the distance estimation from i to the destination, - N(i) is set of neighbors of node i, and - dij is the distance of the direct link from i to j; assume positive destination j di d ij i dj Lecture 6: Network Layer #32 Distance Table: Example A 7 Below is just one step! The algorithm repeats forever! 10 distance tables dE () computation from neighbors A B D A B D A 0 7 10 15 B 7 0 17 8 C 1 2 D 0 10 8 2 E’s distance table B 1 C 2 8 E D 2 distance table E sends to its neighbors A: 10 A: 10 B: 8 B: 8 9 4 D: 4 C: 4 2 D: 2 D: 2 E: 0 Lecture 6: Network Layer #33 Distance Table: example 7 A B 1 C E cost to destination via D () A B D A 1 14 5 B 7 8 5 C 6 9 4 D 4 11 2 2 8 1 E 2 D E D (C,D) = c(E,D) + min {DD(C,w)} = 2+2 = 4 w E D (A,D) = c(E,D) + min {DD(A,w)} E w = 2+3 = 5 loop! D (A,B) = c(E,B) + min {D B(A,w)} = 8+6 = 14 w (why not 15?) Lecture 6: Network Layer #34 Distance table gives routing table E cost to destination via Outgoing link D () A B D A 1 14 5 A A,1 B 7 8 5 B D,5 C 6 9 4 C D,4 D 4 11 2 D D,2 Distance table to use, cost Routing table Lecture 6: Network Layer #35 Distance Vector Routing: overview 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 of msg from neighbor) recompute distance table if least cost path to any dest has changed, notify neighbors Lecture 6: Network Layer #36 Distance Vector Algorithm: At all nodes, X: 1 Initialization: 2 for all adjacent nodes v: 3 DX(*,v) = infty /* the * operator means "for all rows" */ 4 DX(v,v) = c(X,v) 5 for all destinations, y 6 send minw DX(y,w) to each neighbor /* w over all X's neighbors */ Lecture 6: Network Layer #37 Distance Vector Algorithm (cont.): 8 loop 9 wait (until a link cost change to neighbor V 10 or until receive update from neighbor V) 11 12 if (c(X,V) changes by d) 13 /* change cost to all dest's via neighbor v by d */ 14 /* note: d could be positive or negative */ 15 for all destinations y: DX(y,V) = DX(y,V) + d 16 17 else if (update received from V wrt destination Y) 18 /* shortest path from V to some Y has changed */ 19 /* V has sent a new value for its minw DV(Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: D X(Y,V) = c(X,V) + newval 22 23 if a new minw DX(Y,w) for any destination Y 24 send new value of minw DX(Y,w) to all neighbors 25 Lecture 6: Network Layer 26 forever #38 Distance Vector Algorithm: example X 2 Y 7 1 Z X Z X Y D (Y,Z) = c(X,Z) + minw{D (Y,w)} = 7+1 = 8 D (Z,Y) = c(X,Y) + minw {D (Z,w)} = 2+1 = 3 Lecture 6: Network Layer #39 Distance Vector Algorithm: example X 2 Y 7 1 Z Lecture 6: Network Layer #40 Distance Vector: link cost changes Link cost changes: node detects local link cost change updates distance table (line 15) if cost change in least cost path, notify neighbors (lines 23,24) “good news travels fast” 1 X 4 Y 50 1 Z algorithm terminates Lecture 6: Network Layer #41 Distance Vector: link cost changes Link cost changes: good news travels fast bad news travels slow - “count to infinity” problem! 60 X 4 Y 50 1 Z algorithm continues on! Lecture 6: Network Layer #42 Distance Vector: 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 X 4 Y 50 1 Z algorithm terminates Lecture 6: Network Layer #43 Comparison of LS and DV algorithms Message complexity LS: with n nodes, E links, O(nE) msgs sent DV: exchange between neighbors only larger msgs convergence time varies Speed of Convergence LS: 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 Lecture 6: Network Layer #44 Hierarchical Routing Our routing study thus far - idealization all routers identical network “flat” … not true in practice scale: with 200 million destinations: can’t store all dest’s in routing tables! routing table exchange would swamp links! administrative autonomy internet = network of networks each network admin may want to control routing in its own network Lecture 6: Network Layer #45 Hierarchical Routing aggregate routers into regions, “autonomous systems” (AS) routers in same AS run same routing protocol Gateway router Direct link to router in another AS “intra-AS” routing protocol routers in different AS can run different intraAS routing protocol Lecture 6: Network Layer #46 Interconnected ASes 3c 3a 3b AS3 1a 2a 1c 1d 1b Intra-AS Routing algorithm 2c AS2 AS1 Inter-AS Routing algorithm Forwarding table 2b Forwarding table is 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 Lecture 6: Network Layer #47 Inter-AS tasks AS1 needs: 1. to learn which dests are reachable through AS2 and which through AS3 2. to propagate this reachability info to all routers in AS1 Job of inter-AS routing! Suppose router in AS1 receives datagram for which dest is outside of AS1 Router should forward packet towards on of the gateway routers, but which one? 3c 3b 3a AS3 1a 2a 1c 1d 1b 2c AS2 2b AS1 Lecture 6: Network Layer #48 Example: Setting forwarding table in router 1d Suppose AS1 learns from the inter-AS protocol that subnet x is reachable from AS3 (gateway 1c) but not from AS2. Inter-AS protocol propagates reachability info to all internal routers. Router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c. Puts in forwarding table entry (x,I). Lecture 6: Network Layer #49 Example: Choosing among multiple ASes Now suppose AS1 learns from the inter-AS protocol that subnet x is reachable from AS3 and from AS2. To configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x. This is also the job on inter-AS routing protocol! Hot potato routing: send packet towards closest of two routers. 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 Lecture 6: Network Layer #50 Broadcast and Multicast Routing Lecture 6: Network Layer #51 Broadcast Routing Deliver packets from source to all other nodes Source duplication is inefficient: duplicate duplicate creation/transmission R1 R1 duplicate R2 R2 R3 R4 source duplication R3 R4 in-network duplication Source duplication: how does source determine recipient addresses Lecture 6: Network Layer #52 In-network 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 Lecture 6: Network Layer #53 Spanning Tree First construct a spanning tree Nodes forward copies only along spanning tree A B c F A E B c D F G (a) Broadcast initiated at A E D G (b) Broadcast initiated at D Lecture 6: Network Layer #54 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 A A 3 B c 4 E F 1 2 B c D F 5 E D G G (a) Stepwise construction of spanning tree (b) Constructed spanning tree Lecture 6: Network Layer #55 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 Shared tree Source-based trees Lecture 6: Network Layer #56 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 the basic approaches Lecture 6: Network Layer #57 Shortest Path Tree mcast forwarding tree: tree of shortest path routes from source to all receivers Dijkstra’s algorithm S: source LEGEND R1 1 2 R4 R2 3 R3 router with attached group member 5 4 R6 router with no attached group member R5 6 R7 i link used for forwarding, i indicates order link added by algorithm Lecture 6: Network Layer #58 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 Lecture 6: Network Layer #59 Reverse Path Forwarding: example S: source LEGEND R1 R4 router with attached group member R2 R5 R3 R6 R7 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 Lecture 6: Network Layer #60 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 LEGEND S: source R1 router with attached group member R4 R2 P R5 R3 R6 P R7 P router with no attached group member prune message links with multicast forwarding Lecture 6: Network Layer #61 Shared-Tree: Steiner Tree 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 network needed monolithic: rerun whenever a router needs to join/leave Lecture 6: Network Layer #62 Center-based trees 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 Lecture 6: Network Layer #63 Center-based trees: an example Suppose R6 chosen as center: LEGEND R1 3 R2 router with attached group member R4 2 R5 R3 1 R6 1 router with no attached group member path order in which join messages generated R7 Lecture 6: Network Layer #64 End Part 1 Lecture 6: Network Layer #65 Hierarchical Routing Our routing study thus far - idealization all routers identical network “flat” … not true in practice scale: with 50 million destinations: can’t store all dest’s in routing tables! routing table exchange would swamp links! administrative autonomy internet = network of networks each network admin may want to control routing in its own network Lecture 6: Network Layer #66 Hierarchical Routing aggregate routers into regions, “autonomous systems” (AS) routers in same AS run same routing protocol “intra-AS” routing protocol routers in different AS can run different intraAS routing protocol gateway routers special routers in AS run intra-AS routing protocol with all other routers in AS also responsible for routing to destinations outside AS run inter-AS routing protocol with other gateway routers Lecture 6: Network Layer #67 Intra-AS and Inter-AS routing C.b a C Gateways: B.a A.a b A.c d A a b c a c B b •perform inter-AS routing amongst themselves •perform intra-AS routers with other routers in their AS network layer inter-AS, intra-AS routing in gateway A.c link layer physical layer Lecture 6: Network Layer #68 Intra-AS and Inter-AS routing C.b a Host h1 C b A.a Inter-AS routing between A and B A.c a d c b A Intra-AS routing within AS A B.a a c B Host h2 b Intra-AS routing within AS B We’ll examine specific inter-AS and intra-AS Internet routing protocols shortly Lecture 6: Network Layer #69 AS D Routing: Example E d AS A (OSPF) a2 F No Export to F a1 i AS C AS B i2 (OSPF intra routing) b AS I Lecture 6: Network Layer #71 AS D Routing: Example E d1 d d2 AS A (OSPF) a2 F i AS C a1 How to specify? AS B (OSPF intra routing) b AS I Lecture 6: Network Layer #72 IP Addressing Scheme We need an address to uniquely identify each destination Routing scalability needs flexibility in aggregation of destination addresses we should be able to aggregate a set of destinations as a single routing unit Preview: the unit of routing in the Internet is a network---the destinations in the routing protocols are networks Lecture 6: Network Layer #73 IP Addressing: introduction IP address: 32-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 interface, not host, or router 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 223.1.2.2 223.1.3.2 223.1.3.1 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 Lecture 6: Network Layer 1 #74 IP Addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host, router and physical link 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 223.1.2.2 router’s typically have 223.1.3.2 223.1.3.1 multiple interfaces host may have multiple interfaces IP addresses associated with 132.67.192.133 = 10000100 01000011 11000000 10000101 interface, not host, or 223 67 192 133 router Lecture 6: Network Layer #75 IP Addressing IP address: network part • high order bits host part • low order bits What’s a network ? (from IP address perspective) device interfaces with same network part of IP address can physically reach each other without intervening router 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 223.1.2.2 LAN 223.1.3.1 223.1.3.2 network consisting of 3 IP networks (for IP addresses starting with 223, first 24 bits are network address) Lecture 6: Network Layer #76 IP Addressing How to find the networks? Detach each interface from router, host create “islands of isolated networks 223.1.1.2 223.1.1.1 223.1.1.4 223.1.1.3 223.1.9.2 223.1.7.0 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 Interconnected system consisting of six networks 223.1.2.1 223.1.3.27 223.1.2.2 223.1.3.1 223.1.3.2 Lecture 6: Network Layer #77 IP Addresses given notion of “network”, let’s re-examine IP addresses: “class-full” addressing: class A 0 network B 10 C 110 D 1110 1.0.0.0 to 127.255.255.255 host network 128.0.0.0 to 191.255.255.255 host network multicast address host 192.0.0.0 to 223.255.255.255 224.0.0.0 to 239.255.255.255 32 bits Lecture 6: Network Layer #78 IP addressing: CIDR classful addressing: inefficient use of address space, address space exhaustion e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network CIDR: Classless InterDomain Routing network portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in network portion of address network part host part 11001000 00010111 00010000 00000000 200.23.16.0/23 Lecture 6: Network Layer #79 CIDR Address Aggregation AS D d d1 AS A (OSPF) 130.132.1/24 i i->a1: I can reach 130.132/16; my path: I a2 a1 130.132.2/24 AS I intradomain routing uses /24 130.132.3/24 Lecture 6: Network Layer #80 CIDR Address Aggregation B x00/24: B A x01/24: C C x10/24: E G E x11/24: F F Lecture 6: Network Layer #81 IP addresses: how to get one? Hosts (host portion): hard-coded by system admin in a file DHCP: Dynamic Host Configuration Protocol: dynamically get address: “plug-and-play” 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 The common practice in LAN and home access (why?) Lecture 6: Network Layer #82 IP addresses: how to get one? Network (network portion): get allocated portion of ISP’s address space: ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 ... 11001000 00010111 00010100 00000000 ….. …. 200.23.20.0/23 …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23 Lecture 6: Network Layer #83 Hierarchical addressing: route aggregation Hierarchical addressing allows efficient advertisement of routing information: Organization 0 200.23.16.0/23 Organization 1 200.23.18.0/23 Organization 2 200.23.20.0/23 Organization 7 . . . . . . Fly-By-Night-ISP “Send me anything with addresses beginning 200.23.16.0/20” Internet 200.23.30.0/23 ISPs-R-Us “Send me anything with addresses beginning 199.31.0.0/16” Lecture 6: Network Layer #84 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization 0 200.23.16.0/23 Organization 2 200.23.20.0/23 Organization 7 . . . . . . Fly-By-Night-ISP “Send me anything with addresses beginning 200.23.16.0/20” Internet 200.23.30.0/23 ISPs-R-Us Organization 1 200.23.18.0/23 “Send me anything with addresses beginning 199.31.0.0/16 or 200.23.18.0/23” Lecture 6: Network Layer #85 Network Address Translation: Motivation A local network uses just one public IP address as far as outside world is concerned Each device on the local network is assigned a private IP address rest of Internet local network (e.g., home network) 192.168.1.0/24 192.168.1.1 192.168.1.2 192.168.1.3 138.76.29.7 192.168.1.4 All datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers Datagrams with source or destination in this network have 192.168.1/24 address for source, destination (as usual) Lecture 6: Network Layer #86 NAT: Network Address Translation Implementation: NAT router must: outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #) . . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr. remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table Lecture 6: Network Layer #87 NAT: Network Address Translation NAT translation table WAN side addr LAN side addr 1: host 192.168.1.2 2: NAT router sends datagram to changes datagram 138.76.29.7, 5001 192.168.1.2, 3345 128.119.40.186, 80 source addr from …… …… 192.168.1.2, 3345 to 138.76.29.7, 5001, S: 192.168.1.2, 3345 updates table D: 128.119.40.186, 80 2 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 138.76.29.7 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3: Reply arrives dest. address: 138.76.29.7, 5001 3 192.168.1.2 1 192.168.1.1 192.168.1.3 S: 128.119.40.186, 80 D: 192.168.1.2, 3345 4 192.168.1.4 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 192.168.1.2, 3345 Lecture 6: Network Layer #88 Network Address Translation: Advantages No need to be allocated range of addresses from ISP: - just one public IP address is used for all devices 16-bit port-number field allows 60,000 simultaneous connections with a single LAN-side address ! can change ISP without changing addresses of devices in local network can change addresses of devices in local network without notifying outside world Devices inside local net not explicitly addressable, visible by outside world (a security plus) Lecture 6: Network Layer #89 NAT: Network Address Translation If both hosts are behind NAT, they will have difficulty establishing connection NAT is controversial: routers should process up to only layer 3 violates end-to-end argument • NAT possibility must be taken into account by app designers, e.g., P2P applications address shortage should instead be solved by having more addresses --- IPv6 ! Lecture 6: Network Layer #90 IP addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes Lecture 6: Network Layer #91 Getting a datagram from source to dest. routing table in A Dest. Net. next router Nhops 223.1.1 223.1.2 223.1.3 IP datagram: misc source dest fields IP addr IP addr data A datagram remains unchanged, as it travels source to destination addr fields of interest here mainly dest. IP addr 223.1.1.4 223.1.1.4 1 2 2 223.1.1.1 223.1.2.1 B 223.1.1.2 223.1.1.4 223.1.2.9 223.1.3.27 223.1.1.3 223.1.3.1 223.1.2.2 E 223.1.3.2 Lecture 6: Network Layer #92 Getting a datagram from source to dest. misc data fields 223.1.1.1 223.1.1.3 Dest. Net. next router Nhops 223.1.1 223.1.2 223.1.3 Starting at A, given IP datagram addressed to B: look up net. address of B find B is on same net. as A A 223.1.1.1 223.1.2.1 link layer will send datagram directly to B inside link-layer frame B and A are directly connected 223.1.1.4 223.1.1.4 1 2 2 B 223.1.1.2 223.1.1.4 223.1.2.9 223.1.3.27 223.1.1.3 223.1.3.1 223.1.2.2 E 223.1.3.2 Lecture 6: Network Layer #93 Getting a datagram from source to dest. misc data fields 223.1.1.1 223.1.2.2 Dest. Net. next router Nhops 223.1.1 223.1.2 223.1.3 Starting at A, dest. E: look up network address of E E on different network A, E not directly attached routing table: next hop router to E is 223.1.1.4 link layer sends datagram to router 223.1.1.4 inside linklayer frame datagram arrives at 223.1.1.4 continued….. A 223.1.1.4 223.1.1.4 1 2 2 223.1.1.1 223.1.2.1 B 223.1.1.2 223.1.1.4 223.1.2.9 223.1.3.27 223.1.1.3 223.1.3.1 223.1.2.2 E 223.1.3.2 Lecture 6: Network Layer #94 Getting a datagram from source to dest. misc data fields 223.1.1.1 223.1.2.2 Arriving at 223.1.4, destined for 223.1.2.2 look up network address of E E on same network as router’s interface 223.1.2.9 router, E directly attached link layer sends datagram to 223.1.2.2 inside link-layer frame via interface 223.1.2.9 datagram arrives at 223.1.2.2!!! (hooray!) Dest. next network router Nhops interface 223.1.1 223.1.2 223.1.3 A - 1 1 1 223.1.1.4 223.1.2.9 223.1.3.27 223.1.1.1 223.1.2.1 B 223.1.1.2 223.1.1.4 223.1.2.9 223.1.3.27 223.1.1.3 223.1.3.1 223.1.2.2 E 223.1.3.2 Lecture 6: Network Layer #95 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 32 bits head. type of length len service fragment 16-bit identifier flgs offset time to upper Internet layer live checksum ver total datagram length (bytes) for fragmentation/ reassembly 32 bit source IP address 32 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) E.g. timestamp, record route taken, specify list of routers to visit. Lecture 6: Network Layer #96 IP Fragmentation & Reassembly network 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 fragmentation: in: one large datagram out: 3 smaller datagrams reassembly Network Layer 4-97 IP Fragmentation and Reassembly Example 4000 byte datagram MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 length ID fragflag offset =4000 =x =0 =0 One large datagram becomes several smaller datagrams length ID fragflag offset =1500 =x =1 =0 length ID fragflag offset =1500 =x =1 =185 length ID fragflag offset =1040 =x =0 =370 Network Layer 4-98 Routing in the Internet The Global Internet consists of Autonomous Systems (AS) interconnected with each other: Stub AS: small corporation Multihomed AS: large corporation (no transit) Transit AS: provider Two-level routing: Intra-AS: administrator is responsible for choice Inter-AS: unique standard Lecture 6: Network Layer #99 Internet AS Hierarchy Inter-AS border (exterior gateway) routers Intra-AS interior (gateway) routers Lecture 6: Network Layer #100 Intra-AS Routing Also known as Interior Gateway Protocols (IGP) Most common IGPs: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco propr.) Lecture 6: Network Layer #101 RIP ( Routing Information Protocol) Distance vector algorithm Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops) why? Distance vectors: exchanged every 30 sec via Response Message (also called advertisement) Each advertisement: route to up to 25 destination nets Lecture 6: Network Layer #102 RIP (Routing Information Protocol) z w A x D y B C Destination Network w y z x …. Next Router Num. of hops to dest. …. .... A B B -- 2 2 7 1 Routing table in D Lecture 6: Network Layer #103 RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead routes via neighbor invalidated new advertisements sent to neighbors neighbors in turn send out new advertisements (if tables changed) link failure info quickly propagates to entire net poison reverse used to prevent ping-pong loops (infinite distance = 16 hops) Lecture 6: Network Layer #104 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 router Advertisements disseminated to entire AS (via flooding) Lecture 6: Network Layer #105 OSPF “advanced” features (not in RIP) Security: all OSPF messages authenticated (to prevent malicious intrusion); TCP connections used Multiple same-cost paths allowed only one path in RIP For each link, multiple cost metrics for different ToS (eg, 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. Lecture 6: Network Layer #106 Hierarchical OSPF Lecture 6: Network Layer #107 Hierarchical OSPF Two-level hierarchy: local area, backbone. Link-state advertisements only in area each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers. Backbone routers: run OSPF routing limited to backbone. Boundary routers: connect to other ASs. Lecture 6: Network Layer #108 IGRP (Interior Gateway Routing Protocol) CISCO proprietary; successor of RIP (mid 80s) Distance Vector, like RIP several cost metrics (delay, bandwidth, reliability, load etc) uses TCP to exchange routing updates Loop-free routing via Distributed Updating Alg. (DUAL) based on diffused computation Lecture 6: Network Layer #109 Inter-AS routing Lecture 6: Network Layer #110 Internet inter-AS routing: BGP BGP (Border Gateway Protocol): the de facto standard Path Vector protocol: similar to Distance Vector protocol each Border Gateway broadcast to neighbors (peers) entire path (I.e, sequence of ASs) to destination E.g., Gateway X may send its path to dest. Z: Path (X,Z) = X,Y1,Y2,Y3,…,Z Lecture 6: Network Layer #111 Internet inter-AS routing: BGP 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 AS), 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 its route advertisements to peers: e.g., don’t want to route traffic to Z -> don’t advertise any routes to Z Lecture 6: Network Layer #112 Internet inter-AS routing: BGP 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 Lecture 6: Network Layer #113 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 Lecture 6: Network Layer #114 Extra Lecture 6: Network Layer #115 ICMP: Internet Control Message Protocol used by hosts & routers to communicate network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) network-layer “above” IP: ICMP msgs carried in IP datagrams ICMP message: type, code plus first 8 bytes of IP datagram causing error Type 0 3 3 3 3 3 3 4 Code 0 0 1 2 3 6 7 0 8 9 10 11 12 0 0 0 0 0 description echo reply (ping) dest. network unreachable dest host unreachable dest protocol unreachable dest port unreachable dest network unknown dest host unknown source quench (congestion control - not used) echo request (ping) route advertisement router discovery TTL expired bad IP header Network Layer 4-116 Traceroute and ICMP Source sends series of UDP segments to dest First has TTL =1 Second has TTL=2, etc. Unlikely port number When nth datagram arrives to nth router: Router discards datagram And sends to source an ICMP message (type 11, code 0) Message includes name of router& 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. Network Layer 4-117 IPv6 Initial motivation: 32-bit address space soon to be completely allocated. Additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed Network Layer 4-118 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 4-119 Other Changes from IPv4 Checksum: removed entirely to reduce processing time at each hop Options: allowed, but outside of header, indicated by “Next Header” field ICMPv6: new version of ICMP additional message types, e.g. “Packet Too Big” multicast group management functions Network Layer 4-120 Transition From IPv4 To IPv6 Not all routers can be upgraded simultaneous no “flag days” How will the network operate with mixed IPv4 and IPv6 routers? Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers Network Layer 4-121 Tunneling Logical view: Physical view: A B IPv6 IPv6 A B C IPv6 IPv6 IPv4 Flow: X Src: A Dest: F data A-to-B: IPv6 E F IPv6 IPv6 D E F IPv4 IPv6 IPv6 tunnel Src:B Dest: E Src:B Dest: E Flow: X Src: A Dest: F Flow: X Src: A Dest: F data data B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4 Flow: X Src: A Dest: F data E-to-F: IPv6 Network Layer 4-122