Ch 4. The Network Layer Myungchul Kim

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

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5 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

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7 Virtual circuit and datagram networks o 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

8 o Virtual circuit networks – ATM and frame relay – Virtual circuit  A path  VC numbers  Entries in the forwarding table  Page 345

9 Forwarding table VC number interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # … … Forwarding table in northwest router: Routers maintain connection state information!

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

11 o 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

12 Forwarding table Destination Address Range Link Interface through through through otherwise 3 4 billion possible entries

13 What’s inside a router

14 o 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.

15 o Switching fabric

16 o Output ports

17 o 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

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20 IP: forwarding and addressing in the Internet

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22 o 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

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24 o IPv4 addressing – 32 bits long (4 bytes) – Dotted-decimal notation – Globally unique – subnet

25 – 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 =

26 o 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 o 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

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29 o 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.

30 – 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

31 o Internet Control Message Protocol (ICMP) – Error reporting – Ping program – Source quench message – Tracerout

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33 o 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

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35 – 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) – Difficult to change network-layer protocols

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38 o IP security – IPsec – Virtual Private Networks (VPN)  Cryptographic agreement on algorithms and keys  Encryption of IP datagram payload  Data integrity  Origin authentication

39 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

40 o 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.

41 Routing in the Internet o RIP (routing information protocol) – DV protocol – Hop count as a cost metric (max 15) – Routing updates every 30 seconds

42 o 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

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44 o 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.

45 BGP routing policy o A,B,C are provider networks o X,W,Y are customer (of provider networks) o 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 A B C W X Y legend : customer network: provider network

46 BGP routing policy (2) o A advertises path AW to B o B advertises path BAW to X o 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! A B C W X Y legend : customer network: provider network

47 Broadcast and Multicast Routing o Broadcast routing algorithms – N-way unicast – Uncontrolled flooding -> broadcast storm

48 – Controlled flooding  Sequence-number-controlled flooding  Reverse path forwarding (RPF)

49 – Spanning-tree broadcast

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51 o Multicast – To a subset of network nodes – Class D multicast IP address for multicast group – Internet Group Management Protocol and mulcast routing protocols

52 – Multicast routing algorithms  Multicast routing using a group-shared tree  Multicast routing using a source-based tree with pruning

53 – Multicast routing in the Internet  Distance Vector Multicast Routing Protocol (DVMRP)  Protocol-Independent Multicast (PIM) routing protocol