CS4550: Computer Networks II network layer basics 3 routing

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CS4550Computer Networks IInetwork layer
basics 3routing congestion control

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basic routing techniques

• fixed
• random
• flooding
• adaptive
• routing in ARPAnet
• distance vector
• link state
• optimal

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basic routing techniques fixed

• routes between node pairs determined in advance
  by network control center or administrator
• routing tables loaded into nodes, and are not
  changed dynamically
• simple, may be best technique for small networks
  which do not change often (topology, traffic
  patterns
• routing more easily controlled
• not responsive to changes in traffic

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basic routing fixed
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5
9
3
2
7
1
8
4
10
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routing table for node 1
next node
dest
1 2 3 4 5 6 7 8 9 10
- 2 2 4 7 7 7 7 2 4
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basic routing flooding

• send packet to all neighbors, except the one from
  which received the packet
• should assign packet or ID, to avoid needless
  redundant transmissions
• should assign hop count/lifetime to packet to
  limit needless congestion, distance traveled
• advantages
• disadvantages

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routing in ARPAnet 1969-90?

• 1st version
• -distributed Bellman-Ford SP algorithm also
  known as distance vector routing
• -each node exchanged delay info each 128 ms with
  neighbor
• -delay info based solely on queue lengths
• -problems excessive overhead, unstable network,
  inaccurate delay estimates responded too quickly
  to some problems, too slowly to others

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distance vector routing

• -each node measures distance to adjacent
  neighbors
• -these distances put into a message
  (vector) and sent to neighbors
• -distances ( route) to other nodes (not
  neighbors) computed by adding the measured
  distance to those in received distance vectors
• - if dont know, distance is infinity

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distance vector routing

• key assumption that each node knows (can
  measure) distance to its neighbors
• each node maintains a routing table has 1 entry
  for all nodes in the network

dest node next hop distance
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distance vector routing example
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2
3
10
5
1
18
12
5
4
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using measured distances shown, show computation
of distance vector tables for each node.
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distance vector routing example
(1)
(2)
2 10 2 3 - - 4
12 4 5 - -
1 10 1 3 12 3 4
- - 5 18 5
initial tables (vectors) for nodes 1 (left) and
2 give initial tables for 3,4,5 then show
computation of tables as vectors are exchanged,
until stable.
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distance vector routing

• works reacts quickly to good news (a new,
  shorter route), but reacts slowly to bad news
  (router or link down)
• count to infinity problem
• split horizon solution, and why it doesnt always
  work

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distance vector count to infinity
a
c
e
1
1
2
d
2
b
Suppose node a goes down. Show updating of the
distance vector tables.
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distance vector split horizon
B
A
C
D
what happens if the CD link fails? (assume all
hops have length of 1)
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ARPAnet, 2nd version (1979/80)

• delay measurements much better measured,
  averaged actual delay of all packets on the link
• updates transmitted only every 10 seconds much
  improved stability
• switched to link-state routing, and used
  Dijkstras SP algorithm
• much improved but later, as use of network
  approached capacity, more problems surfaced

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Iink state routing

• -each node measures distance to neighbors
• -sends this info to ALL other nodes in the
  network (flooding)
• -each node constructs a weighted graph of the
  network, and
• -used Dijkstras SP algorithm to compute
  shortest path

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routing metrics, shortest paths

• routing metrics vary widely
• -instantaneous queue length
• -measured time
• -hop count
• -distance
• -capacity (bandwidth), data rate
• getting the shortest path may not really matter
  to end user. (How many people count time in
  milliseconds?)

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ARPAnet data rates, links

• initially, all ARPAnet links were 56Kbps
  telephone lines (digital)
• later, higher capacity lines added (T1 lines,
  satellite links)
• computers faster than networks, then
• data link protocol very close to HDLC

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ARPAnet, 3rd version

• increased use of network led to new problems
• north-south oscillation problem
• true solution -
• multipath routing
• increased capacity
• fix implemented
• damping algorithm
• increase update interval to 17 seconds

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optimal routing

• defined mathematically as routing for which
  average delay (distance, or other metric) for
  all packets is minimized.
• characterized mathematically as a network flow
  problem
• solution computable, but not efficient algorithm
  not useful. However,
• light load single SP routing is optimal
• heavier load use more routes to spread out
  traffic

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congestion control

• congestion an excessive number of packets in
  the network
• terms
• congestion control
• traffic control
• flow control 2 points link/connection level

network level
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congestion - levels

• need to define levels of congestion. Below is a
  very rough rule of thumb.
• 0 none - no delay experienced
• 1 moderate - experience minimal delay
• 2 heavy - significant delays, but still able to
  get packets through
• 3 total (deadlocked) - so clogged, cant get
  packets through in any reasonable time

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congestion - levels

• - general guidelines only percentages vary
  according to the network protocols, topologies,
  and actual distributions of the traffic on the
  network
• reduction from Max throughput
• 0 none lt20
• 1 moderate 20-50
• 2 heavy 50-70
• 3 total gt 70

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congestion run away

• in packet switched networks, this problem is
  exacerbated, because as delay increases, timeouts
  occur, causing packet retransmissions, which
  cause increased congestion
• care must be taken first to avoid congestion,
  second, to reduce it when it occurs packet
  retransmissions may have the opposite result, of
  actually making things worse

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congestion control five basic strategies

• 1 choke packets
• 2 buffer preallocation
• 3 packet discarding
• 4 isarithmetic control
• 5 flow control (covered elsewhere)

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choke packets

• variation of flow control used only when
  congestion occurs
• 1. each node monitors utilization U of output
  lines
• 2. if U becomes heavy, a choke packet sent to
  hosts sending traffic to that destination
• 3. upon receiving choke packet, the sending host
  decreases traffic to that destination by X
  percent

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choke packets

• 4. host discards other choke packets for that
  destination for Y time
• 5. After some time, host continues watching for
  choke packets if no more come OK, if they do,
  reduces output again.
• 6. After longer time, if no more choke packets
  arrive, again slowly may increase the flow.
  MDAI
• adds packets to a congested network

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buffer preallocation

• only for VC packet networks
• 1. as call request made, buffer reserved in each
  node for the connection if not available, must
  find another or refuse.
• 2. as data packets transmitted, acks returned
  new packets only sent after receiving acks (which
  are not sent until packet is in output queue)
• completely solves congestion ... at what price?

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packet discarding

• no buffers reserved in advance
• when packets arrive, if no buffer space exists,
  discard them
• Irlands strategy
• refinements
• discard older packets
• lower priority packets
• packets on more congested links
• check packet for acks before discarding

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isarithmetic control

• keeps total number of packets in network below a
  specified limit
• whenever a node transmits a packet into the
  network, must get a permit for the packet
• similar to token for LANs
• several problems

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isarithmetic control

• 1. guarantees network not overloaded but a part
  or node could be
• 2. how many permits to use? not easy to decide
• 3. what if permits lost? how can we tell?
• 4. in some networks, some packets much larger
  than others all use same permit?
• 5. not clear that it is fair
• 6. difficult to respond to changing conditions