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Network Layer, Routing, IP. October 28-30, 2003. Assignments. Homework 4 Project 2 Read Chapter 4 sections 4.1-4.4 for this week. network data link physical. network data link physical. network data link physical. network data link physical. network data link physical. network

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Network Layer, Routing, IP


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    1. Network Layer, Routing, IP October 28-30, 2003

    2. Assignments • Homework 4 • Project 2 • Read Chapter 4 sections 4.1-4.4 for this week

    3. network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical application transport network data link physical Network Layer • Move packet from sender to receiver • Network layer protocols in every host, router • Three Functions: • path determination: route taken by packets from source to dest. Routing algorithms • forwarding: move packets from router’s input to appropriate router output • call setup: some network architectures require router call setup along path before data flows

    4. Service Model • End-to-end transport of data between sending and receiving systems • How is this different than the transport layer services? • Datagram versus Virtual Circuit

    5. Virtual Circuit • Call setup, teardown for each call • 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 allocatedto VC • to get circuit-like performance

    6. application transport network data link physical application transport network data link physical Datagram Networks • Routers: no state about end-to-end connections • Packets forwarded using destination host address • Best-effort service • No guarantees with respect to delay, in-order delivery 1. Send data 2. Receive data

    7. Datagram and the Internet • Why is datagram service okay for the Internet?

    8. A D E B F C Routing • Routing Protocol • Find route from default/first hop/source router to destination router • Job of the algorithm –find a “good path” • Use graph abstraction to represent the network • Where do the numbers come from? 5 3 5 2 2 1 3 1 2 1

    9. Global or decentralized information? Global: all routers have complete topology, link cost info “link state” algorithms Decentralized: router knows physically-connected 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 Load sensitive/insensitive Routing Algorithm Classification

    10. A Link-State Routing Algorithm • Global – every router knows about all others • How does a node find out about all other nodes? • Once a node has the complete topology, it runs Dijkstra’s algorithm to generate the routing table

    11. 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

    12. 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) = infinity 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

    13. A D B E F C Dijkstra’s algorithm: example D(B),p(B) 2,A 2,A 2,A D(D),p(D) 1,A D(C),p(C) 5,A 4,D 3,E 3,E D(E),p(E) infinity 2,D Step 0 1 2 3 4 5 start N A AD ADE ADEB ADEBC ADEBCF D(F),p(F) infinity infinity 4,E 4,E 4,E 5 3 5 2 2 1 3 1 2 1

    14. A A A A D D D D B B B B C C C C Discussion • Algorithm complexity • n nodes • each iteration: need to check all nodes, w, not in N • n*(n+1)/2 comparisons: O(n**2) • more efficient implementations possible: O(nlogn) • Oscillations possible: • e.g., link cost = amount of carried traffic • Solution? 1 1+e 2+e 0 2+e 0 2+e 0 0 0 1 1+e 0 0 1 1+e e 0 0 0 e 1 1+e 0 1 1 e … recompute … recompute routing … recompute initially

    15. distance from X to Y, via Z as next hop X = D (Y,Z) Z c(X,Z) + min {D (Y,w)} = w Distance Vector Routing Algorithm • Iterative, asynchronous, and distributed • Distance table • all nodes have one • row for all destinations and a column for neighbors

    16. cost to destination via E D () A B C D A 1 7 6 4 B 14 8 9 11 D 5 5 4 2 destination A D B E C E E E D (C,D) D (A,D) D (A,B) D D B c(E,D) + min {D (C,w)} c(E,B) + min {D (A,w)} c(E,D) + min {D (A,w)} = = = w w w = = = 2+2 = 4 2+3 = 5 8+6 = 14 Distance Table: Example 1 7 2 8 1 2 loop! loop!

    17. cost to destination via E D () A B C D A 1 7 6 4 B 14 8 9 11 D 5 5 4 2 destination Routing Table Outgoing link to use, cost A B C D A,1 D,5 D,4 D,4 destination Routing table Distance table

    18. 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 DV Overview Each node:

    19. Distance Vector Algorithm At all nodes, X: 1 Initialization: 2 for all adjacent nodes v: 3 D (*,v) = infinity /* the * operator means "for all rows" */ 4 D (v,v) = c(X,v) 5 for all destinations, y 6 send min D (y,w) to each neighbor /* w over all X's neighbors */ X X X w

    20. Distance Vector Algorithm 8 loop 9 wait (until I see a link cost change to neighbor V 10 or until I 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: D (y,V) = D (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 min DV(Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: D (Y,V) = c(X,V) + newval 22 23 if we have a new min D (Y,w)for any destination Y 24 send new value of min D (Y,w) to all neighbors 25 26 forever X X w X X w X w

    21. 2 1 7 Y Z X X c(X,Y) + min {D (Z,w)} c(X,Z) + min {D (Y,w)} D (Y,Z) D (Z,Y) = = w w = = 7+1 = 8 2+1 = 3 X Z Y Example

    22. 2 1 7 X Z Y Example

    23. X Z Y 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) 1 4 1 50 algorithm terminates “good news travels fast”

    24. X Z Y Link Cost Changes • good news travels fast • bad news travels slow - “count to infinity” problem! 60 4 1 50 algorithm continues on!

    25. X Z Y Poisoned Reverse If Z routes through Y to get to X • Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z) • will this completely solve count to infinity problem? 60 4 1 50 algorithm terminates

    26. Comparison of LS and DV • Message Complexity • Speed of Convergence • Robustness

    27. Hierarchical Routing • So far – all routers are equal, network is flat • Why is this view a problem?

    28. Hierarchical Routing • Aggregate routers into autonomous systems ASs • Intra-AS routing • Each AS has a gateway • responsible for inter-AS routing

    29. c b b c a A.c A.a C.b B.a Intra-AS and Inter-AS routing • Gateways: • perform inter-AS routing amongst themselves • perform intra-AS routers with other routers in their AS b a a C B d A network layer inter-AS, intra-AS routing in gateway A.c link layer physical layer

    30. Inter-AS routing between A and B b c a a C b B b c a d Host h1 A A.a A.c C.b B.a Intra-AS and Inter-AS routing Host h2 Intra-AS routing within AS B Intra-AS routing within AS A • We’ll examine specific inter-AS and intra-AS Internet routing protocols shortly

    31. Assignments • Continue work on Project 2 • Finish reading chapter 4 for next week – we will not talk about 4.6 in class

    32. ICMP protocol • error reporting • router “signaling” • IP protocol • addressing conventions • datagram format • packet handling conventions • Routing protocols • path selection • RIP, OSPF, BGP forwarding table Network Layer Transport layer: TCP, UDP Network layer Link layer physical layer

    33. 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 each interface 223.1.1.2 223.1.2.1 223.1.3.27 223.1.3.1 223.1.3.2 223.1.2.2 IP Addressing 223.1.1.1 223.1.2.9 223.1.1.4 223.1.1.3 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1

    34. 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 IP Addressing 223.1.1.1 223.1.2.1 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 223.1.1.3 223.1.3.27 LAN 223.1.3.2 223.1.3.1 network consisting of 3 IP networks (for IP addresses starting with 223, first 24 bits are network address)

    35. multicast address 1110 network host 110 network 10 host Classful Addressing class 1.0.0.0 to 127.255.255.255 A network 0 host 128.0.0.0 to 191.255.255.255 B 192.0.0.0 to 223.255.255.255 C 224.0.0.0 to 239.255.255.255 D 32 bits

    36. host part network part 11001000 0001011100010000 00000000 200.23.16.0/23 CIDR • Classful addressing • inefficient use of address space, address space exhaustion • example? • 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

    37. How do I get an IP address? • Two options – what are they? • Which is used when and why?

    38. What about the network part? 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

    39. 200.23.16.0/23 200.23.18.0/23 200.23.30.0/23 200.23.20.0/23 . . . . . . Route Aggregation Hierarchical addressing allows efficient advertisement of routing information: Organization 0 Organization 1 “Send me anything with addresses beginning 200.23.16.0/20” Organization 2 Fly-By-Night-ISP Internet Organization 7 “Send me anything with addresses beginning 199.31.0.0/16” ISPs-R-Us

    40. source IP addr misc fields dest IP addr data Constructing a Packet

    41. E A B 223.1.1.1 223.1.2.1 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 223.1.1.3 223.1.3.27 Dest. Net. next router Nhops 223.1.1 1 223.1.3.2 223.1.3.1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 Determining the Next Hop misc fields data 223.1.1.1 223.1.1.3

    42. E A B 223.1.1.1 223.1.2.1 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 223.1.1.3 223.1.3.27 Dest. Net. next router Nhops 223.1.1 1 223.1.3.2 223.1.3.1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 Determining the Next Hop misc fields data 223.1.1.1 223.1.2.3

    43. Dest. Net router Nhops interface E A B 223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9 223.1.3 - 1 223.1.3.27 223.1.1.1 223.1.2.1 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 223.1.1.3 223.1.3.27 223.1.3.2 223.1.3.1 Router Forwarding Table misc fields data 223.1.1.1 223.1.2.3

    44. Datagram Format 32 bits type of service head. len ver length fragment offset flgs 16-bit identifier upper layer time to live Internet checksum 32 bit source IP address 32 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment)

    45. 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 IP Fragmentation & Reassembly fragmentation: in: one large datagram out: 3 smaller datagrams reassembly

    46. length =1500 length =1500 length =4000 length =1040 ID =x ID =x ID =x ID =x fragflag =0 fragflag =1 fragflag =0 fragflag =1 offset =0 offset =0 offset =1480 offset =2960 One large datagram becomes several smaller datagrams IP Fragmentation and Reassembly Example • 4000 byte datagram • MTU = 1500 bytes

    47. used by hosts, routers, gateways to communication 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 ICMP: Internet Control Message Protocol TypeCodedescription 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

    48. DHCP: Dynamic Host Configuration Protocol • Goal: allow host to dynamically obtain its IP address from network server when it joins network • Can renew its lease on address in use • Allows reuse of addresses (only hold address while connected an “on” • Support for mobile users who want to join network (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

    49. E B A DHCP client-server scenario 223.1.2.1 DHCP 223.1.1.1 server 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 arriving DHCP client needs address in this network 223.1.1.3 223.1.3.27 223.1.3.2 223.1.3.1

    50. DHCP discover src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 DHCP client-server scenario arriving client DHCP server: 223.1.2.5 DHCP offer src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654 Lifetime: 3600 secs DHCP request src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs time DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs