CS 164: Computer Networks

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• CS 164 Computer Networks
• Slide Set 10 -- Internetworking (Continued)
• Routing

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In this set ....

• How are link-state algorithms implemented in
  practice ?
• OSPF
• Mobile IP
• ---------------------------------
• Section 4.3 - Global Internet -- Subnetting,
  Classless Inter-domain routing and BGP

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Implementing link state routing

• Each node maintains two lists
• The tentative list and the confirmed list.
• Each list contains entries of the type
  Destination, Cost, Next Hop.
• The two lists are continuously updated as new
  nodes are added to the shortest path tree (as in
  the Dijkstras algorithm).

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The Algorithm

• Step 1 Initialize confirmed list with myself --gt
  cost 0
• Step 2 For the node just added to the confirmed
  list in the previous step, call it node Next,
  select its LSP.
• Step 3 For each neighbor of Next (Neighbor),
  calculate cost to reach this neighbor
• Cost Cost from me to Next Cost of Next to
  Neighbor.
• (a) If Neighbor is currently on neither the
  Tentative or the Confirmed list, add Neighbor,
  Cost NextHop to Tentative list. Note -- here
  NextHop is the one that is used to reach Next.
• (b) If Neighbor is on the Tentative list and the
  cost seen is lt the cost currently listed for
  Neighbor, replace the entry with Neighbor, Cost,
  NextHop. Else, do nothing.
• Step 4 If Tentative list is empty, stop. Else,
  pick entry from the Tentative list with lowest
  cost and move it to Confirmed list. Return to
  Step 2.

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Our Example
We look at D.
Step Confirmed Tentative
5 (D,0,-) (C,2,C) (B,5, C) (A,12,C)
6 (D,0,-) (C,2,C) (B,5, C) (A, 10, C)
7 (D,0,-) (C,2,C) (B,5,C) (A,10,C)
Step Confirmed Tentative
1 (D, 0, -)
2 (D,0,-) (C, 2, C) (B, 11, B)
3 (D,0,-) (C,2,C) (B,11,B)
4 (D,0,-) (C,2,C) (B,5,C) (A,12,C)
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Computational Complexity

• There are n nodes.
• 1st iteration, search through n nodes to
  determine w, the one with the minimum cost.
• 2nd iteration, search through (n-1) nodes and so
  on.
• Total n(n1)/2 O(n2)
• One could use sorting methods (heapsort etc.)
  -could reduce complexity to O(nlogn).

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Comparing Distance Vector and Link State

• Message complexity in link state is O(nE) since
  each node sends an update -- possibly needed for
  each link.
• With distance vector it depends on the rate of
  change of link costs etc.
• Convergence time in link state O(nlogn) In
  distance vector depends on relative path costs --
  count to infinity problem.
• However, link state suffers from oscillations.
• Link state is stable since computation done at
  each node -- distance vector could lead to
  problems if there are malfunctioning routers.

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Open Shortest Path First (OSPF)

• Open standard -- non-proprietary
• Essentially the link-state approach --however
  features are added.
• Authentication of routing messages -- prevent
  misconfigurations-- only sys admin can configure.
• Add a hierarchy -- OSPF runs within a domain or
  administrative region. The region is sub-divided
  into areas.
• Router does not need to know how to get to every
  network within the domain -- enough to know how
  to get to right area.
• Load balancing - if two paths have same cost,
  subdivide traffic among paths.

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OSPF Header

• Nuggets
• Checksum -- same as IP (does not include
  authentication part).

Authentication can be none, password based or
crypto checksum (hash). Types -- HELLO packet,
Request, Send, ACK receipt of link state
messages. Header is common for all OSPF messages.
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Link State Advertisements

• Router uses this to advertise directly connected
  nets and costs of links to other routers.
• Link state age -- similar to TTL (not included in
  checksum).
• Unique IP address for router -- if it has more
  than one, pick lowest.
• TOS -- different routes for different IP packets
  -- not widely used.

Other info on LSA, look up the book.
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Metrics

• How does one assign link costs ?
• Costs distinguish between different physical
  links.
• dynamically, one may consider the load --
  difficult.
• Versions of ARPANET tried to address this.
• One way -- count the number of queued packets
  --does not take bandwidth or latency into
  consideration.
• Another way -- latency on the link.
• Each incoming packet time-stamped. There is an
  associated transmission time and propagation
  latency -- static for the link.
• Delay (Depart time - Arrival time)
  transmission time prop. latency.

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Instability

• Due to improper choice of metrics, instability
  and oscillations could occur.
• Many of links remain idle while others are
  heavily loaded.

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Oscillations in link state
2)
1)
A
A
2 e
1
1 e
D
B
D
B
1e
1
0
e
C
C
A
0
2 e
3)
D
B
1
1e
C
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Avoiding oscillations

• Smoothing variations of the metric in time.
• Choose weights that do not change often --
  averages.
• Impose hard limit on how much metric could
  change from one measurement to next.
• Do not change paths with high frequencies even if
  link weights do change often.

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Mobility

• So far, with IP, implicit assumption that there
  is no mobility.
• Addresses -- network part, host part -- so
  routers determine how to get to correct network.
• If nodes move network may change
• How do we cope with this ?
• Should there be a change in IP addresses with
  mobility ? If so how ?
• Should we use DHCP to assign new addresses ? May
  be adequate in some cases.
• Within the same network no problems -- no need to
  change IP addresses -- link layer delivery.

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Mobile IP

• Problem occurs when user switches between
  networks.
• Applications may keep running and so the remote
  end needs to know how to deliver packets to the
  mobile host.
• Mobile IP
• Need for transparency for the user
• No need to change software of majority of routers
  on the Internet.
• Background compatible

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The Home Agent

• However, it doesnt come for free!
• Some routers need new functionalities.
• Home Agent Permanent IP address somewhere that
  the mobile calls home.
• A router located on the home network of the
  mobile.
• When a node needs to reach the mobile, sends the
  messages to the home address.

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The Foreign Agent

• The Foreign Agent Router located in new network
  to which the mobile attaches itself when it is
  away from the home network.
• Mobile registers with foreign agent and provides
  the address of its home agent.
• Foreign agent contacts the home agent and
  provides a care-of-address --gt IP address of
  the foreign agent.

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Note ....

• The home and foreign agents have to announce
  their presence -- they are specialized routers.
• The attaching mobile may solicit an advertisement
  by sending a request.

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Proxy ARP

• When the mobile is away, the home agent has to
  pick up packets meant for the mobile.
• With Proxy ARP, the home agent (HA) inserts IP
  address of mobile node instead of its own!
• It provides its own hardware address though !
• To invalidate old ARP entries in possible caches,
  as soon as mobile is known to have registered
  with a FA (foreign agent), HA issues an ARP.
• Note that this is not in response to an ARP
  query and hence is called the gratuitous ARP.

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Tunnelling

• Once home agent gets the IP datagram, it tunnels
  the packet to the mobile.
• To recollect, by tunneling, it encapsulates the
  IP packet within another IP packet destined for
  the foreign agent.
• The FA strips the IP wrapper, recognizes that the
  packet was meant for a registered mobile nad uses
  its own ARP entry to send the frame to the
  hardware address of the mobile.

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Other details

• Mobile has to dynamically acquire an IP address
  in the foreign network.
• Packets in the other direction are simple, use
  the sources IP address (a fixed location).
• If the source was a mobile, similar procedures
  could be used.

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Route optimization

• Previous approach sub-optimal. Fixed --gt home --gt
  foreign --gt mobile. This is called the triangle
  routing problem.
• HA will let the sending node know the
  care-of-address of mobile node.
• Sending node creates tunnel to mobile.

HA
S
FA
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Implementation and Other issues

• HA sends a binding update to the source in
  addition to forwarding initial packet.
• Source creates an entry in a binding cache
  which includes mappings of mobile node addresses
  to care-of addresses.
• Entries could become stale -- mobile chooses a
  new FA -- old FA would issue a binding warning.
• Issue Can lead to security problems.
• Mobile IP not widely deployed -- still being
  researched -- Mobile networking in general an
  upcoming research area.

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Routing so far...

• Somewhat scalable.
• Routers need not know of all the hosts that are
  connected to the Internet.
• Enough to know of networks.
• However, in reality, millions of nodes --
  distance vector takes for ever to converge, link
  state too expensive -- both dont scale that much!

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Revisiting the Internet

• Called the customer provider view -- we have
  end-user sites, regional service provider
  networks etc.
• Each unit is independent as far as administration
  goes -- what routing to use, how to assign
  metrics etc. Each unit is called an Autonomous
  System or AS.

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Intra-AS routing

• The routing schemes that we have seen so far are
  used for routing within ASes.
• There are gateway routers that deliver packets to
  outside the AS.
• Internal nodes know which ASes they can reach,
  but not the intricacies.

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Scalability and Addressing

• IP address space is limited -- too many networks.
  
• In addition, the more the networks, the more
  would be the entries in each routing table.
• We need to take care so that address space is not
  used up.
• Note -- 221 Class C addresses but only 214 Class
  B.
• A Class C network can have at most 255 hosts,
  what if we had 257 ? Should we allocate one of
  the fewer Class B addresses?

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Address Efficiency

• In our example, we had 256 hosts. For this, Class
  C is insufficient.
• Class B can accommodate 65535 hosts
  (approximately 64 K).
• If we assign a Class B address to this 256 node
  network, we are wasting the address space -- the
  address efficiency would be 256/65535 0.39 .
• If we construct a separate Class C network with 2
  hosts, we dont do much better -- efficiency
  becomes 2/255 0.78 .

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Sub-Netting

• The solution is to assign a single IP network
  number to a set of several physical networks.
• Each physical network is called a subnetwork or
  subnet for short.
• The requirement is that the subnets have to be
  close to each other --gt they need to look like a
  single network when considered together.
• Each router can have an entry to this
  aggregation of networks.
• Example -- a campus network can be divided into
  sub-networks.

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The Subnet Mask and Subnet Number

• The mechanism by which a single network number
  can be shared among multiple networks involves
  configuring all the nodes on each subnet with a
  subnet mask.
• This enables us to introduce what we call a
  subnet number -- all hosts on the same subnet
  will have the same subnet number.

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Representation

• In this example, we subdivide the network part
  into two sub-parts.
• The first is the network number and the second a
  Subnet ID.

Note With this, the Class B can be divided into
several sub-networks.
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An Example

• We have the Class B network subdivided into three
  subnets.
• Each subnet has a sub-net mask.
• The mask determines how many bits belong to the
  subnet ID.
• Notice that each subnet can have any number of
  bits in its subnet mask.
• Bitwise ANDing of Host IP address and Subnet Mask
  gives the Subnet No.

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Next....

• Given subnets how does one now make the
  forwarding tables ?
• More about Subnets
• Classless Inter Domain Routing.
• BGP