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Module 5.0: Internetworking & Network Layer
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Basic concepts
Congestion Control
Routing Protocols
– Flooding
– Source routing
– Distance vector routing
– Link state routing
– Routing for VC
Routers vs. Switches
Chapters: 19,20,21,22
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Concepts
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Internetworking refers to a collection of interconnected networks that function as a
singe network.
Internetwork is abbreviated as internet. However, Internet refers to the world’s
largest internetwork.
Network layer performs:
– Provides services to Layer 4 (Transport) including fragmentation and reassembly
– Performs congestion control
– End-to-end routing
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Congestion Control
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Congestion occurs when the number of
packets being transmitted through the
network approaches the packet handling
capacity of the network
Congestion control aims to keep number of
packets below level at which performance
falls off dramatically
Data network is a network of queues
Generally 80% utilization is critical
Finite queues mean data may be lost
Packets arriving are stored at input buffers
Routing decision made
Packet moves to output buffer
Packets queued for output transmitted as fast
as possible
– Statistical time division multiplexing
If packets arrive too fast to be routed, or to be
output, buffers will fill. And packets will be
discarded
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Congestion Control
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When packets are discarded, the sources must retransmit these packets,
in addition to a new packets. This will increase the load on the network
and more buffers become saturated. Even if a packet makes it through,
by that time, the upper layer times out. Under these circumstances, the
throughput becomes zero.
Not the same as flow control. Flow control applies only to point-to-point
traffic. Flow control affects congestion. Congestion has a global scope.
Mechanisms of congestion control:
– Backpressue
 Send choke packet hop-by-hop
– Choke packet
 Send choke packet to source
– Implicit congestion signaling
 Source slows down if packets are discarded or delayed
– Explicit congestion signaling
 Binary based: A bit set in a packet indicates congestion
 Credit based: Indicates how many packets source may send
 Rate based: Supply explicit data rate limit, e.g. ATM.
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Routing Protocols
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Routing protocols perform two primary functions:
– Determine the “best” path
– Maintain a routing table
Routing algorithms are used to calculate the least-cost path from source
to destination.
– Common cost metrics
 Hops (the number of routers in the path)
 Propagation delay
 Bandwidth
 Time
 Channel utilization
– Two general algorithms
 Distance Vector Routing
 Link State Routing
Other routing protocols
– Flooding
– Source Routing
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Flooding
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The principle says a router forward
an incoming packet to all ports
except the one the packet came
through.
Effective method:
– At startup to build routing table
– Survivability (military networks).
Can drag down the network:
– TTL
– Each switch adds its id to
packet before it floods it
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Source Routing
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Does not require intermediate node routing
Sender must specify the entire route
Sender uses router discovery at initialization. Intermediate nodes use
flooding.
At intermediate nodes, the header is examined, strips off the label
identifying the node, and forward to the next node.
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Distance Vector Routing
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Each router periodically shares its knowledge about the entire network
with its neighbors.
Knowledge about the whole network: Each router sends its
accumulated knowledge about the entire network to its neighbors. This
knowledge is sparse at the beginning.
Routing only to neighbors: Each router sends periodically
knowledge to those routers that has directed links. Neighboring
routers use this information to update their own knowledge.
Information sharing at regular intervals: Every 30 seconds, Each
router sends its knowledge to neighboring routers, regardless of any
changes.
Distance vector algorithm adapts to changes in network topology
gradually as the information on the changes percolates through the
network.
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Example
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Routing Table
Original routing tables. Next hop field is empty initially.
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Updating routing table for router A
Final routing tables
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•An Example of updating routing table
•Distance vector is based on “Bellman-Ford”
algorithm to find the next hop that yields the
shortest distance to destination.
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Link State Routing
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Each router shares its knowledge of its neighborhood with all routers in
the internetwork.
Knowledge about the neighborhood: Router sends information about
the neighborhood only, not the entire table.
To all routers: Such information propagates from router to router, using
flooding. Eventually every router receives a copy of the same
information.
Information sharing when there is a change: Each router sends out
information about the neighbors when there is a change.
In link state routing, eventually each router is able to construct a map of
the entire network and from this map the best route is found.
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Link state packet (LSP)
•Cost is an outbound cost and set by the router.
•A router gets information about its neighbors by periodically sending
them a short greeting message, thus determining the cost and
Neighbor for the LSP.
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Link state database
Flooding of A’s LSP
•Because every router receives the same LSPs, every router builds the
same database.
•This database is used to calculate the routing table, by applying
Dijkstra algorithm.
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The Dijkastra Shortest Path First (SPF) Algorithm
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The algorithm uses the “closest nodes”
concept and is based on the following
principle:
“Give a source node, n, the
shortest path from n to the next
closest node, s, either (a) is a
path that directly connects n to s
or (b) includes a path containing n
and any of the previously found
intermediate closest nodes plus a
direct link from the last
intermediate closest node of this
path to s”.
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Routing table for router A
Routing for VC
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Virtual Circuit network with 3 VCs
– A to B with VCI 1
– A to D with VCI 5
– C to B with VCI 6
Using local VCI over global VCI
– Searching for available VCI is not simple
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•Routing tables for the previous network at intermediate nodes
•Used in ATM networks. ATM uses link-state routing to find the
best route to construct its routing table.
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Most Popular Routing Protocols
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OSPF* (Open Shortest Path First)
– Link-state, interior, 50 routers per area, about 100 areas,
RIP v1* (Routing Information Protocol, version 1)
– Distance-vector, interior, 15 hops
RIP v2* (Routing Information Protocol, version 2)
– Distance-vector, interior, 15 hops
BGP* (Border Gateway Protocol)
– Path-vector, exterior, thousands of routers
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EIGRP+ (Enhanced Interior Gateway Routing Protocol)
– Advanced distance-vector, interior, thousands of routers
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IGRP+ (Interior Gateway Routing Protocol)
– Distance-vector, interior, 255 hops
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IS-ISx (Intermediate System to Intermediate System)
– Link-state, interior, thousands of routers
* Defined by IETF
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Defined by Cisco
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Defined y OSI
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Routers vs. Switches
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The primary difference is one semantics. Switches historically infer CO
links; routers use CL links. Traditionally, routers have performed router
table lookups and packet forwarding in software.
Layer-2 Switches start having routing functionality, and Layer-3
routers start having ASIC (Application Specific Integrated Circuit)
switching technology for packets.
Layers 2 and 3 are merging and it is becoming difficult to distinguish
between switches and routers.
Layer 3 or IP switching: routing IP packets in ASIC, e.g, MPLS.
Layer 4/7 switching is a new and emerging area, called information
content switching.
– Layer 4: direct all traffic based on TCP destination port.
 All traffic with destination TCP port 80, is directed to a
switch port where a web cache resides.
– Layer 7: direct traffic based on information used in the payload.
 Examine URL GET request. If request for image, direct it
request to an optimized image server port.
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