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Ch 4. The Network Layer

Myungchul Kim mckim@icu.ac.kr. Ch 4. The Network Layer. Datagram Routers do not run application- and transport- layer protocols Forwarding vs routing Forwarding: router-local action of transfering a packet from an input link to the appropriate out link

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Ch 4. The Network Layer

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  1. Myungchul Kim mckim@icu.ac.kr Ch 4. The Network Layer

  2. Datagram • Routers do not run application- and transport- layer protocols • Forwarding vs routing • Forwarding: router-local action of transfering a packet from an input link to the appropriate out link • Routing: network-wide process determining the end-to-end paths that packets take from source to destination • Thr routing algorithm determines the values that are inserted into the routers’ forwarding table. • Packet switch • Link-layer switch • Router

  3. Network service model • Defines the characteristics of end-to-end transport of packets between sending and receiving end systems. • Guaranteed delivery • Guaranteed delivery with bounded delay • In-order packet delivery • Guranteed minimal bandwidth • Guaranteed maximum jitter • Security service • Best-effort service: no service at all • ATM service model • Constant bit rate (CBR): as if a dedicated fixed-bandwidth transmission link • Available bit rate (ABR): cells cannot be reordered and a min cell transmission rate is guaranteed

  4. Virtual circuit and datagram networks • Network layer vs transport layer • Host-to-host services, process-to-process services • Network layer: host-to-host connectionless service (datagram networks), host-to-host connection service (virtual-circuit networks) • End systems for transport layer vs routers and end systems for network layers

  5. Virtual circuit networks • ATM and frame relay • Virtual circuit • A path • VC numbers • Entries in the forwarding table • Page 345

  6. VC number 22 32 12 3 1 2 interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # 1 12 3 22 2 63 1 18 3 7 2 17 1 97 3 87 … … … … Forwarding table Forwarding table in northwest router: Routers maintain connection state information!

  7. VC setup -> Data transfer -> VC tear down by ATM’s Q.2931 signaling protocol • Connection set up at the transport layer?

  8. Datagram networks • Prefix match at the page 348 • Longest prefix matching rule • Forwarding tables can be modifed at any time -> packets go different paths and arrive out of order

  9. Forwarding table 4 billion possible entries Destination Address RangeLink Interface 11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111 11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111 11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111 otherwise 3

  10. What’s inside a router

  11. Input ports • A shadow copy of the forwarding table is typically stored at ech input port and updated by the routing processor • Increase lookup speeds: content addressable memories (CAM) allows a 32-bit IP address to be presented to the CAM, which returns the content of the forwarding table entry for that address in essentially constant time.

  12. Switching fabric

  13. Output ports

  14. Where does queueing occur • Packet queue can form at both the input ports and the output ports • Packet loss • Packet scheduler at the output port must choose one packet among those queued for transmission • First-come-first-served • Weighted fair queueing • For quality-of-service guarantees

  15. IP: forwarding and addressing in the Internet

  16. IP datagram fragmentation • Maximum transmission unit: a hard limit on the length of an IP datagram • Jolt2 attack: none of fragments has an offset of zero or overlapping IP fragments

  17. IPv4 addressing • 32 bits long (4 bytes) • Dotted-decimal notation • Globally unique • subnet

  18. Classless interdomain routing (CIDR) • a.b.c.d/x network portion of th IP address = prefix • Classfule addressing: C(/24) = 254 hosts, B(/16) = 65,634 hosts, broadcast = 255.255.255.255

  19. Obtaining a block of addresses • Internet Corporation for Assigned Name and Numbers (ICANN) • Allocate IP addresses to regional Internet registries • Manage the DNS root servers • Obtaining a host address: the Dynamic Host Configuration Protocol (DHCP) • DHCP server discovery • DHCP server offer(s) • DHCP request • DHCP ACK • A TCP connection maintanence problem for a mobile node

  20. Network address translation (NAT) • Private addresses have meaning within that network • The NAT router behaves to the outside world as a single device with a single IP address.

  21. Arguments on Network address translation (NAT) • Prot numbers for addressing processes not for addressing hosts • Routers are supposed to process packets only up to layer 3 • Violates the end-to-end arguments • IPv6

  22. Internet Control Message Protocol (ICMP) • Error reporting • Ping program • Source quench message • Tracerout

  23. IPv6 • IPv5 (ST-2 similar to RSVP) • Datagram format • Expanded addressing capabilities: unicast, multicast, anycast address • A streamlined 40-byte header • Flow labeling and priority • IPv4 vs IPv6 • Fragmentation/reassembly • Header checksum • Options

  24. Transition from IPv4 to IPv6 • A flag day • Dual-stack approach • Tunneling • The US Office of Management and Budget (OMB): to IPv6 by June 2008 • Europe’s Third Generation Partnership Program (3GPP) 2007. • Difficult to change network-layer protocols

  25. IP security • IPsec • Virtual Private Networks (VPN) • Cryptographic agreement on algorithms and keys • Encryption of IP datagram payload • Data integrity • Origin authentication

  26. Routing algoritms • Default router: the first-hop router • The least cost path • Global routing algorithm: link-state (LS) algorithms • Decentralized routing algorithm: distance-vector (DV) algorithms • Static routing algorithms vs dynamic routing • Load-sensitive algorithms vs load-insensitive

  27. Hierarchical routing • Autonomous systems (ASs) • Gateway routers • Within an AS, all routers run the same intra-AS routing protocol. • The ASs run the same inter-AS routing protocol.

  28. Routing in the Internet • RIP (routing information protocol) • DV protocol • Hop count as a cost metric (max 15) • Routing updates every 30 seconds

  29. OSPF(open shortest path first) • LS protocol • Link’s state updates every 30 minutes • Advantages: • Security: MD5 • Multiple same-cost paths • Integrated support for unicast and multicast routing • Support for hierarchy within a single routing domain

  30. BGP (Border Gateway Protocol) • Obtain subnet reachablility information from neighboring ASs • Propagate the reachablility information to all routers interanl to the AS • Determine “good” routes to subnets based on the reachability information on AS policy.

  31. legend: provider B network X W A customer network: C Y BGP routing policy • A,B,C are provider networks • X,W,Y are customer (of provider networks) • X is dual-homed: attached to two networks • X does not want to route from B via X to C • .. so X will not advertise to B a route to C

  32. legend: provider B network X W A customer network: C Y BGP routing policy (2) • A advertises path AW to B • B advertises path BAW to X • Should B advertise path BAW to C? • No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers • B wants to force C to route to w via A • B wants to route only to/from its customers!

  33. Broadcast and Multicast Routing • Broadcast routing algorithms • N-way unicast • Uncontrolled flooding -> broadcast storm

  34. Controlled flooding • Sequence-number-controlled flooding • Reverse path forwarding (RPF)

  35. Spanning-tree broadcast

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