CS 164 -- Internetworking

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CS 164 -- Internetworking

• Slide Set 8

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In this set...

• Addressing
• Datagram forwarding

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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.

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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
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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.

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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.

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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.

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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.

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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.

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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.

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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 --gt router
• Consult what is called the forwarding table that
  contains entries that look like lt Network Number,
  Next Hopgt.
• Also a default router (possible only default
  exists).

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Our example network

• H1 --gt H2, same network number in IP address --
  deliver via Ethernet.
• H1 --gt H8. How ?

• H1 --gt R1 default router over Ethernet.
• R1 knows it cannot deliver directly.
• R1 has to deliver it to a default router -- R2.

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Example Continued

• Let us look at R2s forwarding table.

Network Number Next Hop
2 R1
1 R3

• Thus, R2 --gt R3 via PPP and then, finally, R3
  --gt H8 via Ethernet.

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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

Net Num Next Hop
1 R3
2 R1
3 Int 2
4 Int 4
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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.

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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.

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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.

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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.

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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.

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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.

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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!

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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.
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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.

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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.

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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

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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 !

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Representing a virtual interface
Network Number Next Hop
1 Interface 0
2 Virtual Interface 0
Default Interface 0

• Router R1 will have a forwarding table that
  looks like -gt

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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.

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Where are we ?

• We are done with Section 4.1
• We move onto Section 4.2 -- on Routing.

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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.

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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.

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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 ?

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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.

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Two popular approaches

• Routing Information Protocol (RIP) based on
  Distributed Bellman Ford or Distance Vector
  Routing
• OSPF based on Link State Routing or Dijkstras
  shortest path algorithm.

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Next....

• Different routing approaches.