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The Network Layer

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Introduction

Route packets through subnet So, it must know the topology of the subnet It should also be able to choose path from it It should do load balancing between routers

and communication lines When source and destination are in different

networks, it solves the differences

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Store-and-Forward Packet Switching

The packet is stored at router until it has fully arrived so the checksum can be verified. Then it is forwarded to the next router

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Services Provided to the Transport Layer

Connectionless Service packets are injected into the subnet individually

and routed independently of each other Internet

Connection-Oriented Service a path from the source router to the destination

router must be established before any data packets can be sent. This connection is called a VC (virtual circuit) ATM networks

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Connectionless Service

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Connection-Oriented Service

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Comparison of datagram and virtual-circuit subnets

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Routing Algorithms

deciding which output line an incoming packet should be transmitted on

datagram : decision must be made anew for every arriving data

packet virtual circuits :

decisions are made only when a new virtual circuit is being set up

Also called session routing Forwarding and Routing

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Conflict between fairness and optimality.

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Nonadaptive algorithms choice of the route to use to get from I to J is

computed in advance, off-line, and downloaded to the routers when the network is booted.

This procedure is sometimes called static routing. Adaptive algorithms

choice of the route to use to get from I to J is computed during transmission.

This procedure is sometimes called dynamic routing.

Routing Algorithms

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The Optimality Principle

If router J is on the optimal path from router I to router K, then the optimal path from router J to router K also falls along it

The set of optimal routes from all sources to a given destination forms a tree

This tree has destination as a root and is known as sink tree

All routing algo tries to discover it

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A Subnet And A Sink Tree for Router B

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Shortest Path Routing

Represent Subnet as a graph where routers are nodes and com lines are edges or links

The metric or weight of an edge is imp No. of Hops, Physical distance, time to reach

the destination are the examples It can be bandwidth, average traffic, com

cost, mean delay, or any combination of it

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Shortest Path From A to D

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Dijkstra's Algorithm

Each node is labeled from the source node Label can be temporary or permanent The starting node is permanent, nearest node

is made permanent & is the next node All other neighbors are temp’ly labeled

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Flooding

Every IC packet is sent to all OG lines Generates vast numbers of duplicate packets It is highly robust, useful in military apps Selective flooding for better throughput

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Distance Vector Routing

It operates by maintaining best known distance to each destination and an OG line

These tables are updated by exchanging info with its neighbors

It is also known as distributed Bellman-Ford or Ford-Fulkerson algo

It’s original Arpanet routing algo

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A Subnet

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Inputs from A I H K and New RT

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The Count-to-Infinity Problem

Consider a linear subnet having A, B, C, D, E nodes, with metric is no of hops

The figure shows two cases, When A is up when A is down

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Count to Infinity Problem

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Count to Infinity Problem

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Link State Routing DV used delay metric not considering line BW

The algo take very long time to converge

Each router using LSR must do Discover the neighbors and their NW addresses Measure the delay or cost to them Construct a packet Send to all neighbors Compute the shortest path to every other router

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1) Learning About The Neighbors

Sending special HELLO packet to each PtoP lines when booted

Names of routers must be globally unique

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Learning About The Neighbors

(a) Nine routers and a LAN. (b) A graph model of (a).

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2) Measuring Line Cost

Delay is used as a metric, so special ECHO packet is sent the RTT is calculated

RTT (round-trip time) is divided by 2 to get the estimate

Whether to consider load or not is crucial Load Considered : RTT to start when Echo

packet is queued Load Not Considered : when it comes to the front

of queue

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The Two Line Problem

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3) Building LS Packets

The packet contains, ID of sender, seq, age, and list of neighbors with respective delay

One possibility is to do it periodically Another is to do when some significant like

router coming up or going down happens

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Link State Packets for a Subnet

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The Age field

Age is included in every packets and is decremented once per second

When age hits zero, info from that router is discarded

It ensures that no packet gets lost and live forever

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4) Distributing Link State Packets

Flooding is used for distributing,seq no is used for checking

Routers keep track of all the source, seq pairs they see

If seq is new it is forwarded

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The Packet Buffer for Router B--- Refinements

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5) Computing New Routes

Once router receives all LSPs, it can construct the whole graph for the subnet

Dijkstra's algorithm now can run locally to construct shortest paths to all possible dest

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Routing to Mobile Hosts

Users who never move are known as stationary, they use copper or fiber to communicate

Migratory users are stationary but moving from one fixed site to another

Roaming users are connected to the internet wherever they are

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Routing for Mobile Hosts

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The Registration Process

Periodically each agent broadcasts a packet Packet contains it’s address

MH registers with FA, giving HA, current DLL address, + some security info

FA contacts MH’s HA and convince him FA gets ack and inform MH about

registration, and add info in routing table

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Packet Routing for Mobile Hosts

Routing in AdHoc Networks

Hosts are mobile and also routers are mobile in Adhoc networks

Also known as MANET(Mobile AdHoc Networks)

No Fixed topologies, no fixed neighbors etc AODV(Ad hoc Distance Vector) Routing

Algorithm

Route Discovery

Route Discovery

Route Maintenance

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Congestion Control Algorithms

Defn:- When too many packets are present in (a part of)

the subnet, performance degrades. This situation is called congestion

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General Principles of CC

Open Loop attempts to solve it by good design Once the system is up and running, midcourse

corrections are not made. Closed Loop based on the concept of

feedback Monitor the system to detect congestion Pass the information elsewhere to take action

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General Principles of CC

In Open, act at source or at a destination In Closed, explicit vs. implicit feedback

In explicit feedback case, the router sends the packets back

In implicit feed back the source deduce it by making local observations

Delay in receiving ack

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Open Loop Solution

Minimize congestion in the first place, rather than letting it happen and reacting after the fact

Goal is achieved by using appropriate policies at various layers

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Congestion Prevention Policies

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Closed loop Solutions

Let Congestion occur first and than control it Congestion Control in Virtual-Circuit Subnets Congestion Control in Datagram Subnets Load Shedding Jitter Control

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Congestion Control in Virtual-Circuit Subnets

Admission control (Don't allow new connection after congestion detection) once congestion has been signaled, no more

virtual circuits are set up until the problem has gone away

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A Congested Subnet

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Congestion Control in Datagram Subnets

The Warning Bit Choke Packets Hop-by-Hop Choke Packets

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The Warning Bit

setting a special bit in the packet's header into the next acknowledgement sent back to the

source. The source then cut back on traffic. As long as the warning bits continued to flow

in, the source continued to decrease its transmission rate

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Choke Packets

the router sends a choke packet back to the source host

the host should ignore choke packets referring to that destination for a fixed time interval

After that period has expired, the host listens for more choke packets for another interval.

If one arrives, the line is still congested, so the host reduces the flow still more and begins ignoring choke packets again.

If no choke packets arrive during the listening period, the host may increase the flow again

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Hop by Hop Chock Packets

At long lines, chock packet will take little long to reach source

Instead, at each hop the traffic is slowed down when choke packet passes thru

Intermediate buffers are needed to be increased

Immediate relief to the congested area

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Load Shedding

when routers are being inundated by packets that they cannot handle, they just throw them away

Router can do it randomly picking packets Dropping can be done better dependent on app In some app old packet is more important than new

one and vice versa Still improvement : (intelligent discard policy)

applications must mark their packets in priority classes to indicate how important they are (VERY IMPORTANT— NEVER, EVER DISCARD )

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Random Early Detection

idea of discarding packets before all the buffer space is really exhausted

When the average queue length on some line exceeds a threshold, the line is said to be congested and action is taken.

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Jitter Control

The variation (i.e., standard deviation) in the packet arrival times is called jitter. High jitter, for example, having some packets taking 20 msec and others taking 30 msec to arrive

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Jitter Control