CS4550:
Computer Networks II
network layer basics 3
routing & congestion control
basic routing techniques
 fixed
 random
 flooding
 adaptive
 routing
in ARPAnet
 distance
 link
vector
state
 optimal
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
basic routing : fixed
6
5
9
3
2
7
1
8
4
10
routing table for node 1
dest
1
2
3
4
5
6
7
8
9
10
next node
2
2
4
7
7
7
7
2
4
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
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
“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 don’t know, distance is “infinity”
“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
“distance vector” routing example
2
12
10
1
18
3
5
12
4
20
5
using measured distances shown, show computation
of distance vector tables for each node.
“distance vector” routing example
(1)
(2)
2
10
2
1
10
1
3
-
-
3
12
3
4
12
4
4
-
-
5
-
-
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.
distance vector routing
 works
; reacts quickly to “good news” (a new,
shorter route), but reacts slowly to “bad” news
(router or link down)
 count
 split
to infinity problem
horizon solution, and why it doesn’t
always work
distance vector: count to infinity
a
c
1
b
2
1
e
d
2
Suppose node a goes down. Show updating of the distance
vector tables.
distance vector : split horizon
B
A
C
D
what happens if the CD link fails? (assume all
hops have length of 1)
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
Dijkstra’s SP algorithm
 much improved; but later, as use of network
approached capacity, more problems surfaced
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 Dijkstra’s SP algorithm to compute
shortest path
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?)
ARPAnet data rates, links
 initially,
all ARPAnet links were 56Kbps
telephone lines (digital)
 later,
higher capacity lines added (T1 lines,
satellite links)
 computers
 data
faster than networks, then
link protocol very close to HDLC
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
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
congestion control
 congestion:
an “excessive” number of packets
in the network
 terms
congestion control
network level
traffic control
flow control : 2 points; link/connection level
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, can’t get
packets through in any reasonable time
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 :
 1 moderate :
 2 heavy :
 3 total :
<20%
20-50%
50-70%
> 70%
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
congestion control : five basic strategies
1 choke packets
2 buffer preallocation
3 packet discarding
4 isarithmetic control
5 flow control (covered elsewhere)
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
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
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?
packet discarding
 no
buffers reserved in advance
 when packets arrive, if no buffer space exists,
discard them
 Irland’s strategy
 refinements
 discard
older packets
 lower priority packets
 packets on more congested links
 check packet for acks before discarding
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
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