CS4550:CS4550:

Computer Networks IIComputer Networks II

network layer basics 3network layer basics 3

routing & congestion control routing & congestion control

basic routing techniques basic routing techniques

fixed random flooding adaptive

routing in ARPAnet distance vector link state

optimal

basic routing techniques : fixed 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 basic routing : fixed

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

-224777724

basic routing : flooding 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?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” routingdistance 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” routingdistance 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 exampledistance vector” routing example

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using measured distances shown, show computation of distance vector tables for each node.

““distance vector” routing exampledistance vector” routing example

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3 - -

4 12 4

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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 computationof tables as vectors are exchanged, until stable.

(1) (2)

distance vector routing 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 doesn’t always work

distance vector: count to infinitydistance vector: count to infinity

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Suppose node a goes down. Show updating of the distancevector tables.

distance vector : split horizon distance vector : split horizon

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what happens if the CD link fails? (assume all hops have length of 1)

D

ARPAnet, 2nd version (1979/80) 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 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 pathsrouting 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, linksARPAnet 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

ARPAnet, 3rd versionARPAnet, 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 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 control

congestion: an “excessive” number of packets in the network

termscongestion control

traffic control

flow control : 2 points; link/connection level

network level

congestion - levels 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 - levelscongestion - 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 : <20% 1 moderate : 20-50% 2 heavy : 50-70% 3 total : > 70%

congestion run awaycongestion 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 congestion control : five basic strategies strategies

1 choke packets2 buffer preallocation

3 packet discarding4 isarithmetic control5 flow control (covered elsewhere)

choke packets choke packets

variation of flow control; used only when congestion occurs1. each node monitors utilization U of output lines2. if U becomes heavy, a choke packet sent to hosts sending traffic to that destination3. upon receiving choke packet, the sending host decreases traffic to that destination by X percent

choke packets choke packets 4. host discards other choke packets for that destination for Y time5. 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 preallocationbuffer preallocation

only for VC packet networks1. 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 discardingpacket 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 controlisarithmetic 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 controlisarithmetic 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