CS 164 -- Internetworking

1. CS 164 -- Internetworking Slide Set 8

2. In this set... • Addressing • Datagram forwarding

3. Requirements for Addressing • Uniqueness -- each host needs to have a unique address. • A global addressing scheme/policy is needed. • Why can we not use underlying Ethernet/MAC layer addresses ? • Unique but there is a “flat” structure -- no hierarchy. • Provides no clues as to how data is to be routed.

4. IP addressing • IP addressing is hierarchical. IP Address Uniquely identifies network to which host is attached Network Part Host part Identifies host uniquely given the network Note: Hosts on the same physical network can communicate using frames

5. Addresses and Interfaces • Each host that is attached to the same network has the same “network” part of the IP address. • If routers are attached to multiple networks then, they need to have an address for each network. • Address assigned to the interface on the network. • Appropriate to think of IP addresses as being associated with interfaces.

6. IP address classes • Hierarchical structure not same for all addresses. • Division into classes, A, B, C, D and E. • D -- multicast, E -- unused. • We are mainly concerned with types A, B and C. • All IP addresses are 32 bits long.

7. Classes A, B and C • Class A : 7 Network bits, 24 host bits. • Class B: 14 Network bits and 16 host bits. • Class C: 21 Network bits and 8 host bits. • Of approximately 4 billion IP addresses, 1/2 belong to Class A, 1/4 belong to Class B and 1/8 to Class C.

8. Specifically... • Number of Class A networks = 27 = 128. But on each Class A Network, one can have 224 -2 hosts. • For class C, larger number of networks but each network can have at most 28 = 256 hosts.

9. IP Address Notation • Dotted Decimal (for IPv4) -- W.X.Y.Z -- each represents each of the four bytes. • Example 171.45.210.4 • Remember -- the source and destination addresses are in the IP header.

10. Forwarding versus Routing • Forwarding is the process of taking a packet from the input and sending it on the appropriate output. • Routing -- in contrast -- is the process of building tables that allow the determination of the correct output.

11. Datagram forwarding A node that gets a datagram first tries to establish whether the destination is on the same physical network. • Compare network part of the destination address with the network part of its own interfaces. • If they are the same, destination is on the same physical network. • If yes, deliver packet. • If no, choose the appropriate router to forward packet. • Next Hop --> router • Consult what is called the forwarding table that contains entries that look like < Network Number, Next Hop>. • Also a default router (possible only default exists).

12. Our example network • H1 --> H2, same network number in IP address -- deliver via Ethernet. • H1 --> H8. How ? • H1 --> R1 default router over Ethernet. • R1 knows it cannot deliver directly. • R1 has to deliver it to a default router -- R2.

13. Example Continued • Let us look at R2’s forwarding table. • Thus, R2 --> R3 via PPP and then, finally, R3 --> H8 via Ethernet.

14. Directly Connected Nets • It is possible to include information with regard to the directly connected networks in forwarding table. • As an example, let PPP interface of R2 be Int 1 and let the FDDI interface be Int 2. Then, the table looks like:

15. Address Resolution • Physical interface hardware understands only the “link addresses” of the particular network. • Thus, IP addresses have to be translated into a link layer address prior to sending a datagram to a destination or an intermediate router. • Remember Ethernet address == 48 bits -- one way is to encode the host physical address in host part of IP address. • This is however not scalable -- not always possible. • A second way is to maintain a static table that maps an IP address to a physical address -- maintained by our sys admin. The table is copied onto every host.

16. Dynamic address resolution using ARP • Dynamic resolution is possible using the Address Resolution Protocol or ARP. • Protects against the possibility that Ethernet cards may be replaced. • ARP requires that a dynamic table that maps IP addresses onto physical addresses is refreshed every 15 minutes or so. • It takes advantage of the “broadcast” nature of the link.

17. ARP Mechanics • When a destination PHY address is to be found, an ARP query is broadcasted. • Query includes destination IP address and link layer address of sending host. • Each host checks for match with indicated IP address. • If match, it sends a response to originator of query with link layer or PHY address. • Originator adds this information into its ARP table. • TTL for each entry in ARP table is 20 minutes. • Just a reminder -- note that a broadcast address consists of all 1s.

18. 0 8 16 31 Hardware type = 1 ProtocolType = 0x0800 HLen = 48 PLen = 32 Operation SourceHardwareAddr (bytes 0 ― 3) ― 5) ― 1) SourceHardwareAddr (bytes 4 SourceProtocolAddr (bytes 0 SourceProtocolAddr (bytes 2 TargetHardwareAddr (bytes 0 ― 3) ― 1) ― 5) TargetHardwareAddr (bytes 2 TargetProtocolAddr (bytes 0 ― 3) ARP Message • Important nuggets : Hardware type specified type of physical network -- Ethernet/FDDI • Protocol Type -- typically IP (higher layer) • Operation -- specified whether query or response.

19. DHCP • IP addresses not only need to be unique but they need to reflect some structure. • IP address space is limited -- IP addresses cannot be hard configured. • Reconfigurability • In addition to its own address, typically, node needs address of default router. • Manual configuration difficult -- especially in terms of ensuring uniqueness. • Automated configuration is done via DHCP -- Dynamic Host Configuration Protocol.

20. How does DHCP work ? • DHCP server-- responsible for providing configuration information. • Each host, upon being booted or connected to the network, obtains configuration info. from DHCP. • Note -- admin still picks the IP addresses but now stores them at the DHCP server. • Configuration info stored in a table that is indexed by some unique identifer -- typically the hardware address.

21. Increasing flexibility • On demand allocation possible with DHCP. • Only a pool of IP addresses specified. • All of these have same network number. • When a host needs an address an unused address from this pool is assigned to the host. • Leasing: When DHCP assigns an address, hosts cannot hold onto address for too long -- lease has to be renewed!

22. Particulars • To contact the DHCP server, host sends a DHCPDISCOVER message to the broadcast address (255.255.255.255). • DHCP server responds. • Note that a single DHCP server for a plurality of networks (via DHCP relays) • DHCP relay knows DHCP server address. Self Study: DHCP Packet Formats etc.

23. Error Reporting and ICMP • When a router is unable to process IP datagrams correctly, a collection of error messages sent back to host. • Use of Internet Control Message Protocol or ICMP. • Examples -- host is unreachable, Reassembly process failed, TTL =0, IP header checksum failed etc.

24. ICMP • Architecturally above IP -- ICMP messages are carried in IP packets and are demultiplexed at receiver. • Examples are ping, traceroute etc. • ICMP-redirect -- ICMP can suggest a better route --default router sends the better route so that host can add new route to its routing table.

25. Virtual Private Networks • Virtual Private Networks or VPNs: Private networks -- connections among a set of sites. • Private networks have to have their own links but in the shared world ... • One possibility -- Virtual Circuits

26. IP Tunnels • A virtual point to point link between a pair of nodes that are in fact separated by an arbitrary number of networks. • An IP packet encapsulated within another !

27. Representing a virtual interface • Router R1 will have a forwarding table that looks like ->

28. Why IP tunnels ? • Security -- IPSEC -- internal IP packet encrypted. • Specific services -- R1 and R2 may have specific capabilities such as multicast routing. • Other protocols. • Why not ? -- downside is larger IP packets can deteriorate router performance.

29. Where are we ? • We are done with Section 4.1 • We move onto Section 4.2 -- on Routing.

30. Routing Tables • Routing is the process by which forwarding tables are built. • A routing table is a precursor to building a forwarding table. • It contains mappings from network numbers to next hops -- which is the next hop for a given network number ? • There may be information as to how this info was got. Can help router decide on when to discard information. • Mainly for calculating changes to topology.

31. To remind ourselves... • The forwarding table is a mapping between the network number and an outgoing interface. • Can contain some MAC (link layer) info such as the Ethernet address of the next hop.

32. Network as a graph • We can visualize the network as a graph. • Nodes represent hosts, routers or even networks. • Each edge has an associated cost metric -- how desirable is it to send data on that link ?

33. The Problem • Find the minimum cost path among any two nodes in the graph. • Cost of the path = Sum of the costs of edges that make up the path. • Process -- Calculate the shortest paths and store in some nonvolatile storage. • We need completely distributed routing policies • centralized approaches not scalable.

34. Two popular approaches • Routing Information Protocol (RIP) based on Distributed Bellman Ford or Distance Vector Routing • OSPF based on Link State Routing or Dijkstra’s shortest path algorithm.

35. Next.... • Different routing approaches.