Computer Networks(Routing)Frm Net

8/13/2019 Computer Networks(Routing)Frm Net

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Computer Networks

Section1. Routing Algorithms andNetwork Layer Protocol

Presenter: Shu-Ping Lin


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Outline

Routing Algorithms

The Network Layer in The Internet


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Outline

Routing Algorithms



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Store-and-Forward Packet Switching


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Routing Algorithm

Routing algorithm

Datagram

Virtual circuit

Differences between routing and forwarding

Routing versus forwarding


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Routing Algorithm (contd)

Requirement of routing algorithm

Correctness and Simplicity

Robustness

Stability

Fairness

Optimality


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Datagram Routing


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Virtual-Circuit Routing


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Comparison


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Optimality Principle

If routerJis on the optimal path from routerIto routerK, the the optimal path fromJto

Kalso falls along the same route. Sink tree

The set of optimal routes from all sources to agiven destination

The goal of all routing algorithms is todiscover and use the sink trees for allrouters.


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Sink Tree


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Shortest Path Routing

Link metric

Dijkstra algorithm


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Flooding

Every incoming packet is sent out on everyoutgoing link except the one it arrived on.

Termination of flooding processHop counter

Record of packet which has been flooded

Applications of floodingMilitary application

Comparison


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Distance Vector Routing (contd)


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Distance Vector Routing (contd)

Converge to the correct answer, but do so

slowly.

It reacts rapidly to good news, but leisurelyto bad news.

The count-to-infinity problem


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Count-to-Infinity Problem


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Problem of DVR

Does not take line bandwidth into account.

Take too long to converge.


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Link State Routing

Learning about the neighbors

Sending a special HELLO packet on each

point-to-point line

Measuring line cost to each of its neighbors

Round-trip time

Traffic loadLine bandwidth


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Link State Routing (contd)

Building link state packet


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Link State Routing (contd)

Distributing the link state packets

Flooding is used to distribute the link state

packet.Each packet contains a sequence number that is

incremented for each new packet sent.

Computing the new routes Dijkstras algorithm can be run locally to

construct the shortest path to all destinations.


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Hierarchical Routing

As networks grow in size, the router routing

tables grow proportionally.

Each router has responsible for its regionand knows nothing about the internal

structure of other router.


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Hierarchical Routing (contd)


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Hierarchical Routing (contd)

Penalty of increased path length

The optimal number of levels for an N

router subnets is ln N.


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Broadcasting Routing

The source simply send a distinct packet to

each destination.

Waste bandwidth

Have to know complete list of all destinations

Flooding

Generate too many packets

Consume too much bandwidth


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Broadcasting Routing (contd)

Multidestination routing

Each packet contains a list of destinations.

The destination set is partitioned among theoutput lines.

Using spanning tree

Routers must know which of its lines belong tothe spanning tree in advance.

Copy an incoming packet onto all the spanninglines except the on it arrived on.


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Broadcasting Routing (contd)

Reverse path forwarding

Router checks to see if the packet arrived on the

line that is normally used for sending packets tothe source of the broadcast.


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Multicast Routing

Each router computes a spanning tree

covering all other routers.

When a process sends a multicast packet toa group

Examining the spanning tree of this group

Removing all lines that do not lead to hosts thatare members of group


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Multicast Routing (contd)


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Pruning spanning tree

Reverse path forwarding

Router with no hosts interested in a particulargroup sends a PRUNE message.

The subnet is recursively pruned.


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Disadvantage of algorithm, suppose thatnetwork has ngroups, each with an average

of mmembers.For each group, mpruned spanning trees must

be stored

Total of mntrees

Core-based treeA host wanting to multicast sends packets to the

core, which does the multicast along thespanning tree.


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Routing for Mobile Hosts


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Routing for Mobile Hosts (contd)

Mobile host

Migratory hosts

Roaming hosts All hosts are assumed to have a permanent

home locationthat never changes.

Hosts also have a permanent home addressthat can be used to determine their homelocation.


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Each area has a home agentwhich keeps

track of hosts whose home is in the area, but

who are currently visiting another area. Each area has foreign agentwhich keeps

track of all mobile hosts visiting this area.


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Registration procedure

Foreign agent searching

Registration

Foreign agent contacts the mobile hosts home

agent.

Verification of mobile hosts home agent Acknowledgement from the mobile hosts

home agent


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Routing in Ad Hoc Networks

AODV (Ad hoc On-demand Distance

Vector)

Distant relative of the Bellman-Ford distancevector algorithm

Considering the limited bandwidth and low

battery life

On-demand algorithm


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Routing in Ad Hoc Networks (contd)

AODV algorithm maintains a table at each

node, keyed by destination and which

neighbor to send packets in order to reachdestination.

Route request packet


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Destination constructs a ROUTE REPLYpacket which follows the reverse path to

source. Each intermediate node enters this packet

into local routing table when

No route to destination is known.

Sequence number for destination in REPLY isgreater than the value in the routing table.

Sequence number is equal but the new route isshorter.


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Problem of mobility

Route maintenance

Periodically, each node broadcasts aHello

message.

If no response is returned, router prune this link.

Active neighbor


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Critical difference between AODV and

DVR

Nodes do not send out periodic broadcastscontaining their entire routing table.


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Node Lookup in Peer-to-Peer Networks

(contd) Finger table

Start field = k +2i(modulo 2m)

If keyfalls between kandsuccessort (k),thenthe node holding information about keyis

successor (k).

Otherwise, the entry whosestart field is the

closest predecessor of key is tried.


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(contd)


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(contd) Noderjoining

New node raskssuccessor (r) for its predecessor.

Insert rin between successor and predecessor. Successor should hand over those keys in the range

predecessor (r)-r, which now belong to r.

Every node runs a background process that

periodically recomputes each finger by callingsuccessor.


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Outline

Routing Algorithm



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Principles for network design

Make sure it works.

Keep it simple.Make clear choices.

Exploit modularity.

Expect heterogeneity.

Avoid static options and parameters.

Look for a good design; it need not be perfect.


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(contd)Think about scalability.

Consider performance and cost.

Network layer provides best-efforts way totransport datagrams from source to

destination.


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The IP Protocol

The IPv4 header


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IP Address

IP address formats


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IP Address (contd)

Special IP address


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Subnet

Split a network into several parts for

internal use.


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Subnet (contd)

Some bits are taken away from the hostnumber to create a subnet number.

For example, a university can use a 6-bitsubnet number and a 10-bit host number,allowing fro up to 64 subnets.

Outside the network, the subnet is notvisible, so allocating a new subnet does notrequire contacting ICANN.


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Subnet (contd)

Subnet mask indicates the split between

network + subnet number and host.


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Network Address Translation (contd)


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Use TCP port to distinguish the traffic.

TCPsource portfield is replaced by an

index into the NAT boxs translation table.

Each entry contains original IP address and

original source port.


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Violation of the architectural model of IP

Every IP address uniquely identifies a machine.

NAT changes the Internet from aconnectionless network to a kind ofconnection-oriented network.

Violation of protocol layering

layer kmay not make any assumptions aboutwhat layer k+1has put into the payload field.


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Internet Control Protocols

Internet Control Message Protocol (ICMP)


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Address Resolution Protocol

How do IP address get mapped onto data

link layer addresses, such as Ethernet?

Configuration file ARP

Broadcast a packet onto the Ethernet asking:

Who owns IP address xxx.xxx.xxx.xxx.Correspondent will reply with its Ethernet

address


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Address Resolution Protocol (contd)


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RARP, BOOTP, and DHCP

When a computer is booted how does it get

the IP address.

RARPBroadcast a packet to ask RARP server.

Packet cannot be forwarded by routers, so

RARP server is needed on each network.


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RARP, BOOTP, and DHCP (contd)

DHCP


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OSPFThe Interior Gateway Routing

Protocol

Routing algorithm within as autonomous

system (AS) is called an interior gateway

protocol. The original Internet interior gateway

protocol was distance vector protocol (RIP).

Link state protocol replaced RIP in 1979. OSPF began work in 1988.


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Protocol (contd)

Internet is made up of a large number ofautonomous systems.

Each AS is operated by a differentorganization and can use its own routingalgorithm.

Every AS has a backbonearea.

All areas are connected to the backbone andcan communicate each other via backbone.


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Protocol (contd)

Using flooding, each router informs all theother router in its area of its neighbors andcosts.

This information allows each router toconstruct the graph for its area and computethe shortest path.

Backbone routers accept information fromthe area border routers in order to computethe best route from each backbone router toevery other router.


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BGPThe Exterior Gateway Routing

Protocol

All interior gateway protocol has to do is to move

packets as efficiently as possible without worrying

about politics.

Exterior gateway protocol routers have to worry

about politics.

No transit traffic through certain ASes.

Traffic starting or ending at IBM should not transitMicrosoft.

Policies are typically manually configured into

each BGP router and are not part of protocol.


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Protocol (contd)

Stub networks

Has only one connection to BGP graph andcannot be used for transit traffic.

Multiconnected networks

Could be used for transit traffic, except thatthey refuse

Transit networksWilling to handle third-party packet


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Protocol (contd)

Border Gateway Protocol (BGP)

Fundamentally a distance vector protocol, but

each BGP router keeps track of the path used.


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Internet Multicasting

IP uses class D address to support multicast.

Two kinds of group address

Permanent address

Temporary address


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Internet Multicasting (contd)

224.0.0.1 All systems on a LAN

224.0.0.2 All routers on a LAN

224.0.0.5 All OSPF routers on a LAN

224.0.0.6 All designated OSPF routers on a

LAN

Temporary groups must be created before

they can be used.


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Internet Multicasting (contd)

Multicasting routing is done using spanning

tree.

Each multicast router exchangesinformation with its neighbors, using a

modified distance vector protocol.


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Mobile IP

Increment of portable computers.

Major goals

Mobile host must be able to use its home IPaddress anywhere.

Changes to software and router are not

permitted.

No overhead should be incurred when a mobile

host is at home.


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IPv6

While NAT may buy a few more yearstime, IP in its current form (IPv4) isnumbered.

IETF issued a call for proposal anddiscussion in RFC 1550.

IPv6 is not compatible with IPv4, but it is

compatible with other auxiliary Internetprotocol.


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IPv6 (contd)

Main features

Longer addresses

Simplification of the headerBetter support for options

Security

Quality of service


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IPv6 (contd)

The IPv6 fixed header


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IPv6 (contd)

New notation of 16-byte address

8000:0000:0000:0000:0123:4567:89AB:CDEF

8000::123:4567:89AB:CDEF IPv4 can be written as ::192.168.20.46

Vanishment of IPv4 headers

IHL

Protocol

Checksum


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IPv6 (contd)

Extension header is encoded as (Type,

Length, Value) tuple.

Type is a 1-byte field telling which option thisis.

Length is a 1-byte field telling how long the

value is.

Value is any information required.


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IPv6 (contd)

IPv6 extension header


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IPv6 (contd)

Controversies

Hop limit field length

Maximum packet sizeChecksum

Mobile issue


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Section2. The Internet Transport Protocols:TCP


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Introduction to TCP

Function of providing reliable end-to-end

byte stream over an unreliable internetwork.

Lack of IPNo guarantee that packets will be delivered

properly.

Packets may arrive in the wrong order.


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TCP Service Model

TCP service is obtained by both sender and

receiver creating sockets.

Each socket has IP address and a 16-bitnumber called port.

Port number below 1024 are called well-

known portsand reserved for standardservice.


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TCP Service Model (contd)

Well-Known Ports


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Characteristics of TCP connections

Full duplex

Point-to-pointByte stream, not a message stream


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A key feature of TCP is that every packet

on a TCP connection has its own 32-bit

sequence number. TCP segmentconsisting of fixed 20-byte

header is used for exchanging data between

sender and receiver.


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Segment size issue

Fit in the 65515-byte IP paylo.ad

Cant exceed maximum transfer unit (MTU).

MTU is generally 1500 bytes.

The basic protocol used by TCP is thesliding window protocol.

Timer

Acknowledgement number


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TCP Segment Header


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TCP Connection Establishment

Three-way handshake.


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TCP Connection Release

To release a connection , either party can

send a TCP segment with theFINbit set. When the FINis acknowledged, that

direction is shut down for new data.

Normally, four TCP segments are required

to release a connection.


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TCP Transmission Policy


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TCP Transmission Policy (contd)

When the window size is 0, the sender stop

sending data to receiver except

Urgent data may be sent.Send a 1-byte segment to make the receiver

reannounce the next byte expected and window

size.


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Consider the worst case in performance

issue.

For receiversDelay acknowledgements and window updates

for 500 msec in the hope of acquiring some

data.

For senders

Nagles algorithm


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Silly window syndrome

Data are passed to the sending TCP entity in

large blocks, but an interactive application onthe receiving side reads data 1 byte at a time.


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Clarks algorithm

It forces window update to wait until it has a

decent amount of space available. Combination of Nagles and Clarks

algorithm.


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TCP Congestion Control

When the load offered to any network ismore than it can handle, congestion buildsup.

TCP achieves congestion control bydynamically manipulating the window size.

First step of congestion control: detection.

All the Internet TCP algorithms assume thattimeouts are caused by congestion.


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TCP Congestion Control (contd)


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Two potential problemsnetwork capacity

and receiver capacity.

Each sender maintains two windowsreceiver window and congestion window.

The number of bytes that may be sent is the

minimum of the two windows.


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Slow start


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TCP Timer Management

Retransmission timer

Difficulties of setting retransmission timer


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TCP Timer Management (contd)

Jacobson proposed proposed to use mean

deviationas a cheap estimator of the

standard deviation. ||)1( MRTTDD

DRTTTimeout 4


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TCP Timer Management (contd)

Potential problem

When the ack of retransmission packet comes

in, it is unclear whether the ack refers to thefirst transmission or a later one.

Karns algorithm

Persistence timer is design to prevent the

deadlock.

Keepalive timer.


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Wireless TCP and UDP

Problem due to congestion control.

TCP assumes that timeouts are caused by

congestion, not by lost packets. A packet is lost on a wired network, the

sender should slow down.

When one is lost on a wireless network, thesender should try harder.


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Wireless TCP and UDP (contd)

Bakne and Badrinath propose indirect TCP

which split the TCP connection into two

separate connections.


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Wireless TCP and UDP (contd)

The advantage of this scheme is that

bothconnections are now homogeneous.

Timeouts on the first connection can slowthe sender down, whereas timeouts on the

second one can speed it up.z

The disadvantage of the scheme is that itviolates the semantics of TCP.


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Transactional TCP

Efficiency problem of TCP

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