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  1. EEC-484/584Computer Networks Lecture 10 Wenbing Zhao wenbingz@gmail.com (Part of the slides are based on Drs. Kurose & Ross’s slides for their Computer Networking book)

  2. Outline Routing algorithms Link state routing Distance vector routing Internet protocol v4 Header Fragmentation Internet Protocol v6 EEC-484/584: Computer Networks

  3. Link State Routing Basic idea Assumes 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, using Dijkstra’s Algorithm Gives forwarding table for that node EEC-484/584: Computer Networks

  4. Dijkstra’s Algorithm Each node labeled with distance from source node along best known path Initially, no paths known so all nodes labeled with infinity As algorithm proceeds, labels may change reflecting shortest path Label may be tentative or permanent, initially, all tentative When label represents shortest path from source to node, label becomes permanent EEC-484/584: Computer Networks

  5. Compute Shortest Path from A to D Start with node A as the initial working node Examine each of the nodes adjacent to A, i.e., B and G, relabeling them with the distance to A Examine all the tentatively labeled nodes in the whole graph and make the one with the smallest label permanent, i.e., B. B is the new working node EEC-484/584: Computer Networks

  6. Compute Shortest Path from A to D EEC-484/584: Computer Networks

  7. EEC-484/584: Computer Networks

  8. Computation Results C B E F A D H G Destination link Routing Table in A (A,B) (A,B) (A,B) (A,B) (A,B) (A,B) (A,B) B C D E F G H EEC-484/584: Computer Networks

  9. Distance Vector Routing Also called Bellman-Ford or Ford-Fulkerson Each router maintains a table, giving best known distance to each destination and which line to use to get there Table is updated by exchanging info with neighbors Table contains one entry for each router in network with Preferred outgoing line to that destination Estimate of time or distance to that destination Once every T msec, router sends to each neighbor a list of estimated delays to each destination and receives same from those neighbors EEC-484/584: Computer Networks

  10. Distance Vector Routing:How each entry is updated A d(A,Y) Y At router A, for Z Compute d(A,X) + d(X,Z) and d(A,Y) + d(Y,Z), take minimum d(Y,Z) d(A,X) Z X d(X,Z) d(A,Z) = min {d(A,v) + d(v,Z) } where min is taken over all neighbors v of A EEC-484/584: Computer Networks

  11. cost to x y z x 0 2 7 y from ∞ ∞ ∞ z ∞ ∞ ∞ 2 1 7 z x y d(x,z) = min{d(x,y) + d(y,z), d(x,z) + d(z,z)} = min{2+1 , 7+0} = 3 d(x,y) = min{d(x,y) + d(y,y), d(x,z) + d(z,y)} = min{2+0 , 7+1} = 2 node x table cost to x y z x 0 2 3 y from 2 0 1 z 7 1 0 node y table cost to x y z x ∞ ∞ ∞ 2 0 1 y from z ∞ ∞ ∞ • Each node keeps track of the following info: • Its own distance vector: least-cost to each of other routers • Each of its neighbor’s distance vector received most recently • If there is a change in distance vector, a node sends the update to all its neighbors node z table cost to x y z x ∞ ∞ ∞ y from ∞ ∞ ∞ z 7 1 0 time EEC-484/584: Computer Networks

  12. cost to x y z x 0 2 7 y from ∞ ∞ ∞ z ∞ ∞ ∞ 2 1 7 z x y d(x,z) = min{d(x,y) + d(y,z), d(x,z) + d(z,z)} = min{2+1 , 7+0} = 3 d(x,y) = min{d(x,y) + d(y,y), d(x,z) + d(z,y)} = min{2+0 , 7+1} = 2 node x table cost to cost to x y z x y z x 0 2 3 x 0 2 3 y from 2 0 1 y from 2 0 1 z 7 1 0 z 3 1 0 node y table cost to cost to cost to x y z x y z x y z x ∞ ∞ x 0 2 7 ∞ 2 0 1 x 0 2 3 y y from 2 0 1 y from from 2 0 1 z z ∞ ∞ ∞ 7 1 0 z 3 1 0 node z table cost to cost to cost to x y z x y z x y z x 0 2 7 x 0 2 3 x ∞ ∞ ∞ y y 2 0 1 from from y 2 0 1 from ∞ ∞ ∞ z z z 3 1 0 3 1 0 7 1 0 time EEC-484/584: Computer Networks

  13. Distance Vector Routing Distance from A to B 12ms, to C 25ms, to D 40ms, to G 18ms Distance from J to A 8ms, to I 10ms, to H 12ms, to K 6ms Distance from J to A to G 8+18 = 26msto I to G 10+31 = 41ms to H to G 12+6=18ms to K to G 6+31=37ms EEC-484/584: Computer Networks

  14. Distance Vector Routing Good news travels fast Bad news travels slow Count to infinity problem: Takes too long to converge upon router failure × Routers’ knowledge about the cost to A EEC-484/584: Computer Networks

  15. The Network Layer in Internet Host, router network layer functions: • ICMP protocol • error reporting • router “signaling” • IP protocol • addressing conventions • datagram format • packet handling conventions • Routing protocols • path selection • RIP, OSPF, BGP forwarding table Transport layer: TCP, UDP Network layer Link layer physical layer EEC-484/584: Computer Networks

  16. IPv4 Datagram Format IP protocol version number 32 bits total datagram length (bytes) header length (bytes) Total length type of service IHL ver for fragmentation/ reassembly fragment offset “type” of data flgs 16-bit identifier max number remaining hops (decremented at each router) time to live header checksum protocol 32 bit source IP address 32 bit destination IP address upper layer protocol to deliver payload to E.g. timestamp, record route taken, specify list of routers to visit. Options (if any) • How much overhead with TCP? • 20 bytes of TCP • 20 bytes of IP • = 40 bytes + app layer overhead data (variable length, typically a TCP or UDP segment) EEC-484/584: Computer Networks

  17. The IPv4 Header Version – 4 IHL – length of header in 32-bit words Min 5, max 15 – i.e., 60 bytes Type of service - to distinguish different classes of service To accommodate differentiated services (which class this packet belongs to) Total length – header and data  65,535 (216-1) bytes Identification – allows destination to determine which datagram a fragment belongs to EEC-484/584: Computer Networks

  18. The IPv4 Header Time to live – counter to limit packet lifetimes Max lifetime 255sec Packet is destroyed when counter becomes 0 Protocol – which transport layer protocols being used Header checksum – verifies header EEC-484/584: Computer Networks

  19. The IPv4 Header Options – security, error reporting, etc. Some of the IP options EEC-484/584: Computer Networks

  20. IPv4 Fragmentation Fragmentation Flags DF – tells routers “Don’t Fragment” MF – More Fragments. All fragments except last have this set. Used as check against total length Fragment offset – where in datagram this fragment belongs All fragments (payload in the IP packet) except last must be multiples of 8 bytes The number of 8 byte blocks is called Number of Fragment Blocks (NFB) The unit of the offset is NFB EEC-484/584: Computer Networks

  21. IPv4 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 EEC-484/584: Computer Networks

  22. IPv4 Fragmentation and Reassembly length =1040 length =4000 length =1500 length =1500 ID =x ID =x ID =x ID =x MF =0 MF =0 MF =1 MF =1 offset =370 offset =185 offset =0 offset =0 • Example • 4000 byte datagram • MTU = 1500 bytes One large datagram becomes several smaller datagrams 1480 bytes in data field offset = 1480/8 Fragment should be as large as possible EEC-484/584: Computer Networks

  23. IPv6: motivation • 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

  24. IPv6 datagram format 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 pri ver flow label hop limit payload len next hdr source address (128 bits) destination address (128 bits) data 32 bits Network Layer

  25. 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

  26. IPv6 header fields IPv6 source dest addr UDP/TCP payload IPv4 payload Transition from IPv4 to IPv6 • not all routers can be upgraded simultaneously • no “flag days” • how will network operate with mixed IPv4 and IPv6 routers? • tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers IPv4 header fields IPv4 source, dest addr IPv6 datagram IPv4 datagram Network Layer

  27. IPv4 tunnel connecting IPv6 routers logical view: A A E E B B F F IPv6 IPv6 IPv6 IPv6 IPv6 IPv6 IPv6 IPv6 Tunneling C D physical view: IPv4 IPv4 Network Layer

  28. Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data IPv4 tunnel connecting IPv6 routers logical view: A A E E B B F F IPv6 IPv6 IPv6 IPv6 IPv6 IPv6 IPv6 IPv6 src:B dest: E src:B dest: E flow: X src: A dest: F data flow: X src: A dest: F data A-to-B: IPv6 E-to-F: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4 Tunneling C D physical view: IPv4 IPv4 Network Layer

  29. Dijkstra’s Algorithm: Exercise Given the subnet shown below, using the Dijkstra’s Algorithm, determine the shortest path tree from node u and its routing table 5 3 5 2 2 1 3 1 2 1 x z w y u v EEC-484/584: Computer Networks

  30. Distance Vector Routing: Exercise Consider the subnet shown below. Distance vector routing is used, and the following vectors have just come in to router C: from B: (5, 0, 8, 12, 6, 2); from D: (16, 12, 6, 0, 9, 10); and from E: (7, 6, 3, 9, 0, 4). The measured delays to B, D, and E, are 6, 3, and 5, respectively. What is C's new routing table? Give both the outgoing line to use and the expected delay. EEC-484/584: Computer Networks

  31. Exercise: IP Fragmentation Suppose that host A is connected to a router R 1, R 1 is connected to another router, R 2, and R 2 is connected to host B. Suppose that a TCP message that contains 900 bytes of data and 20 bytes of TCP header is passed to the IP code at host A for delivery to B. Show the Total length, Identification, DF, MF, and Fragment offset fields of the IP header in each packet transmitted over the three links. Assume that link A-R1 can support a maximum frame size of 1024 bytes including a 14-byte frame header, link R1-R2 can support a maximum frame size of 512 bytes, including an 8-byte frame header, and link R2-B can support a maximum frame size of 512 bytes including a 12-byte frame header. EEC-484/584: Computer Networks