Routing Metrics

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Routing Metrics

• The ARPANET Experience

2
History

• The original ARPAnet was actually a terminal
  concentrator network so lots of dumb terminals
  could use a few big, expensive machines
• In the early Internet, the ARPAnet became an
  access network for little IP/TCP clients to use a
  few big, expensive IP/TCP servers
• In the adolescent Internet, the ARPAnet became a
  transit network for widely distributed IP/TCP
  local area networks
• In the mature Internet, the ARPAnet faded to the
  museums, but MILnet and clones remain for IP/TCP
  and ITU-T legacy stuff

3
Importance of cost metric

• Choice of link cost defines traffic load
• low cost high probability link belongs to SPT
  and will attract traffic, which increases cost.
• Main problem convergence
• avoid oscillations
• achieve good network utilization

4
Metric choices

• Static metrics (e.g., hop count)
• good only if links are homogeneous
• definitely not the case in the Internet
• Static metrics do not take into account
• link delay
• link capacity
• link load (hard to measure)

5
Original routing algorithm

• Circa 1969
• Distance vector algorithm
• Routing tables exchanged every 2/3 seconds

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Original ARPANET metric

• Cost proportional to queue size
• instantaneous queue length as delay estimator
• Problems
• did not take into account link speed
• poor indicator of expected delay due to rapid
  fluctuations
• delay may be longer even if queue size is small
  due to contention for other resources

7
New algorithm

• Link state algorithm
• D-SPF (delay shortest path tree)
• Only link cost disseminated into the network
  (standard LS approach), not routes

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New metric

• Delay (depart time - arrival time)
  transmission time link propagation delay
• (depart time - arrival time) captures queuing
• transmission time captures link capacity
• link propagation delay captures the physical
  length of the link
• Measurements averaged over 10 seconds
• Update sent if difference gt threshold, or every
  50 seconds

9
Performance of new metric

• Works well for light to moderate load
• static values dominate
• Oscillates under heavy load
• queuing dominates
• Reason there is no correlation between original
  and new values of delay after re-routing!

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Specific problems

• Range is too wide
• 9.6 Kbps highly loaded link can appear 127 times
  costlier than 56 Kbps lightly loaded link
• can make a 127-hop path look better than 1-hop
• No limit in reported delay variation
• All nodes calculate routes simultaneously
• triggered by link update

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Example
A
Net X
Net Y
B
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..example
After everyone re-calculates routes
A
Net X
Net Y
B
.. Oscillations!
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Consequences

• Low network utilization (50 in example)
• Congestion can spread elsewhere
• Routes could oscillate between short and long
  paths
• Large swings lead to frequent route updates
• more messages
• frequent SPT re-calculation

14
Revised link metric

• Better metric packet delay f(queueing,
  transmission, propagation).
• When lightly loaded, transmission and propagation
  are good predictors
• When heavily loaded queuing delay is dominant and
  so transmission and propagation are bad predictors

15
Revised Metric

• Avg utilization measurements limit range of
  change.5sample .5last average
• Normalize according to link type (e.g., satellite
  should look good when queuing on other links
  increases)
• Max change allowed is link type specific
• change per update cannot be more than 1/2 of that
  hops delay value (e.g. if max is 90 and min is
  30, worst case is only 2 hops worse than best)

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Routing metric v.s. link utilization
225
New metric (routing units)
9.6 satellite
140
90
9.6 terrestrial
75
56 satellite
60
56 terrestrial
30
0
50
100
25
75
Utilization
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Observations

• Cost of highly loaded link never more than 3cost
  when idle
• Most expensive link is 7 least expensive link
• High-speed satellite link is more attractive than
  low-speed terrestrial link

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..observations

• Cost f(link utilization) only at moderate to
  high loads
• Allows routes to be gradually shed from link
• also takes into account link characteristics

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Routing Characteristics

• Vern Paxson used traceroute to study 40,000
  routes
• Probability of encountering serious route failure
  1/30 with problem lasting 30 seconds
• 2/3 of routes persist for days or weeks
• 1/3 of route use different path in each
  direction.
• Routes becoming less predictable

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Mobile IP
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Architecture Entities

• Mobile Node
• Home Agent
• where the mobile node is registered permanently
• Foreign Agent
• where the mobile node visits currently

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Protocol Overview

• Agent Discovery
• Home agents and foreign agents may advertise
  their availability.
• On the contrary, a newly arrived mobile node can
  send a solicitation to learn if any prospective
  agents are present.
• Registration
• When the mobile node is away from home, it
  registers its care-of address with its home agent.

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Routing for Mobile Hosts
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Mobile Routers

• A mobile node can also be a foreign agent
  (similar to a router) of other mobile nodes.
• Ex servers in airplane, train, car, etc.

home agent of x
Airplane
home agent of y
Airport
x
y
Internet
foreign agent of y
foreign agent of x
a sender to y
Ground Station
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Proxy ARP

• Proxy ARP (RFC 925)
• an ARP Reply sent by one node on behalf of
  another node which is either unable or unwilling
  to answer its own ARP Requests
• The ARP Proxy supplies ARP Reply with
• its own link-layer address
• IP of the proxyee (the mobile node).
• The receiver of the Reply then associates the
  proxys link-layer address with the proxyees IP
  address, causing future datagrams being directed
  to the proxy.

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Handoff
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Handoff
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Handoff