Review: Routing algorithms

1

• Review Routing algorithms
• Distance Vector algorithm.
• What information is maintained in each router?
• How to distribute the global network information?
• How to compute the new routing table?
• Distance Vector algorithm
• what is the major problem of this algorithm?
• The count to infinity problem.
• Link State algorithm
• What information is gathered by each router?
• How to distribute the global network information?
• How to compute the new routing table?

2

• Next issue Congestion control
• What is congestion control? Mechanisms to prevent
  hosts from flooding the network.
• Congestion control .vs. flow control?
• Both typically require the sender to slow down.
• Causes are different
• flow control -- link, local
• congestion control -- network, global
• Example
• 50 stations are connected by a 10Mbps Ethernet,
  all stations want to send data at 500Kbps.
• 50 stations are connected by a 10Mbps Ethernet,
  one station wants to send data to another station
  at 500Kbps. The receiving station can only
  process data at 100Kbps.

3

• Congestion control mechanisms
• Open loop (proactive) prevent the congestion
  from happening.
• Closed loop (reactive) detect the congestion and
  do something to fix the problem.
• Traffic shaping
• congestion is typically caused by bursty traffic
• traffic shaping forces the traffic to be not so
  bursty
• Leaky Bucket algorithm.
• each host connected to the network has an
  interface containing a "leaky bucket (a queue)
• Packets can arrive at any rate
• Packets are discarded when the bucket overflows.
• Packets depart at a constant rate.
• Example a burst data of 200Mbps for 40 ms, using
  leaky bucket algorithm with a data rate of 16Mbps?

4

• Congestion control mechanisms
• Traffic shaping
• The leaky bucket algorithm has a constant data
  rate, we want the communication to be a little
  bursty when the computer needs it.
• Token bucket algorithm.
• A packet can go if there is a token. The token is
  destroyed after the packet is gone.
• A token is generated very T second
• Tokens can be accumulated to an upper bound.
• Many variations. A token bucket algorithm is
  typically described using three parameters
  (burst size, average rate, max rate)
• Admission control
• like the telephone system, when there is
  congestion, reject service to avoid making the
  situation worse

5

• Choke packet (used in both VC and datagram)
• Each router monitors the utilization of its
  output lines
• When the utilization is above a threshold, send
  chock packet to the source host
• The source will then slow down sending the
  packets to the same destination.
• The source will avoid listening to the choking
  packet for a while
• Hop by Hop chock packet
• LA to NY at 155Mbps, 30 msec, 4.6Mb in the place
  that causes congestion, choke packets sending to
  the source may take too long to take effect
• Instead, ask every intermediate router to slow
  down.
• quick relief at the point of congestion
• more buffer at upstream routers

6

• load shedding
• I am congested now, let me drop some packets to
  make the network easilier.
• Which packet to drop?
• based on service ftp -- wine policy (older is
  better), real time -- milk policy (newer is
  better)
• indicated by the source which packets that are
  more important. (prioritize packets)

7

• Next issue Internetworking.
• many networks exist ATM, Ethernet, DECNET ...
• These networks have lot of differences
• services connection-oriented and connectionless
• Protocols IP, SNA, IPX
• Addressing Flat and hierachical
• Packet size
• Quality of Service support or not
• Error Handling
• How to send packets from one network to another
  network?

8

• Two types of internetworking.
• Concatenated Virtual Circuits
• A connection to a remote host is set up by
  concatenating virtual circuits in all networks it
  passes by.
• Gateways response for converting packet format
  and maintaining VC.
• Work best when all network have the same
  properties.
• all reliable, all unreliable.
• Can also be done on transport layer for datagram
  subnets.
• Connectionless internetworking
• inject datagrams into subnets and hope for the
  best
• packets may not follow the same route
• also works on VC subnet.

9

• Some problems with internetworking
• format conversion can be hard
• Example IP address is different of OSI address
  (phone number) IP address lt--gt OSI address IP
  address lt--gt IPX address
• fragmentation
• Maximum packet size is different in different
  networks
• Fragmentation deals with the cases when large
  packet sends to the network whose maximum packet
  size is small.
• How to do this?
• solution 1 make it not happening, network
  unreachable.

10

• Solution 2 chop the large packet into small
  fragments and send fragments as individual
  internet packets.
• Problem when to put the packet back together?
• at gateways
• transparent to other networks
• high overhead -- all packet must pass through the
  same exit gateway
• at the destination
• every fragment is treated as an internet packet
• smart end hosts (may not always be true)
• fragments must be numbered
• retransmission overhead (can be complicated).

11

• Tunneling.
• Internetworking for the general case is extremely
  difficult.
• Common case source and destination are on the
  same type of network. Different networks in
  between.
• How it works?
• Source sends packets to an intermediate gateway
• Intermediate gateways put the whole packet into
  the payload field (don't interpret).
• The destination will understand the packet.
• Firewalls
• routers with packet filtering capability
• forward a packet only when the packet passes its
  testing.