(c) all rights reserved by professor wen-tsuen chen1 ä the network layer deals with the end-to-end...

(C) All rights reserved by Professor (C) All rights reserved by Professor Wen-Tsuen ChenWen-Tsuen Chen

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The Network Layer deals with the end-to-end transmission of packets, The Network Layer deals with the end-to-end transmission of packets, possibly making many hops at intermediate routers along the way.possibly making many hops at intermediate routers along the way.

Types of subnetTypes of subnet Datagram (connectionless)Datagram (connectionless) Virtual circuit (connection-oriented)Virtual circuit (connection-oriented)

Services providedServices provided Connectionless: e.g. UDPConnectionless: e.g. UDP Connection-oriented: e.g. TCPConnection-oriented: e.g. TCP

Chapter 5The Network LayerChapter 5The Network Layer

© All rights reserved. No part of these slides may be reproduced, in any© All rights reserved. No part of these slides may be reproduced, in any form or by any means, without permission in writing from form or by any means, without permission in writing from Professor Wen-Tsuen Chen (email: [email protected]).Professor Wen-Tsuen Chen (email: [email protected]).


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Design Issues:Design Issues:

RoutingRouting Congestion ControlCongestion Control InternetworkingInternetworking Examples:Examples:

The Network Layer in the InternetThe Network Layer in the Internet The Network Layer in ATM networksThe Network Layer in ATM networks


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RoutingRouting

Requirements for routingRequirements for routing CorrectnessCorrectness FairnessFairness SimplicitySimplicity OptimalityOptimality RobustnessRobustness EfficiencyEfficiency StabilityStability


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Types of RoutingTypes of Routing

Static routing:Static routing: routes to destinations is predetermined and is not routes to destinations is predetermined and is not

dependent on the current state (traffic, topology etc.) of dependent on the current state (traffic, topology etc.) of the network.the network.

Dynamic routing (Adaptive routing):Dynamic routing (Adaptive routing): routes being learned via exchange of routing routes being learned via exchange of routing

information to reflect changes in the topology and information to reflect changes in the topology and traffic.traffic.

Default Routing:Default Routing: Traffic to destinations that are unknown to the router is Traffic to destinations that are unknown to the router is

sent to a default “outlet”.sent to a default “outlet”.


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The Optimality PrincipleThe Optimality Principle

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

This implies that the set of optimal routes from all This implies that the set of optimal routes from all sources to a destination form a tree, called a sources to a destination form a tree, called a sink sink treetree, rooted at the destination., rooted at the destination.

The goal of all routing algorithms is to discover The goal of all routing algorithms is to discover and use the sink trees for all routers.and use the sink trees for all routers.


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The goal of all routing algorithms is to discover and use the sinkThe goal of all routing algorithms is to discover and use the sinktrees for all routers.trees for all routers.


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Static Routing AlgorithmsStatic Routing Algorithms

Find the shortest path between a given pair of routFind the shortest path between a given pair of routers.ers.

Cost of a link may be a function of the distance, baCost of a link may be a function of the distance, bandwidth, average traffic, communication cost, meandwidth, average traffic, communication cost, mean queue length, delay. etc.n queue length, delay. etc.

Use Dijkstra’s algorithmUse Dijkstra’s algorithm

Shortest Path RoutingShortest Path Routing


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Dijkstra’s algorithmDijkstra’s algorithmto compute to compute the shortest paththe shortest path


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FloodingFlooding

Every incoming packet is sent out on every Every incoming packet is sent out on every outgoing line except the one it arrived on.outgoing line except the one it arrived on.

Generate vast numbers of duplicate packetsGenerate vast numbers of duplicate packets For robustnessFor robustness Concurrent updates of databasesConcurrent updates of databases Shortest path is always choosedShortest path is always choosed


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Flow-Based RoutingFlow-Based Routing


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Dynamic Routing Algorithms Dynamic Routing Algorithms

Distance Vector RoutingDistance Vector Routing Distributed routing algorithms, first used in APPANET until 1979.Distributed routing algorithms, first used in APPANET until 1979. Used in RIP (Routing Information Protocol) and BGP (Border Used in RIP (Routing Information Protocol) and BGP (Border

Gateway Protocol)Gateway Protocol)

Routing Algorithm:Routing Algorithm: Each router maintains a routing table (i.e, a vector) giving the best Each router maintains a routing table (i.e, a vector) giving the best

known distance (number of hops, delay, queue length) to each known distance (number of hops, delay, queue length) to each destination and which link is used to get there.destination and which link is used to get there.

These tables are updated by exchanging information with the These tables are updated by exchanging information with the (adjacent) neighbors.(adjacent) neighbors.


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To determine the best link from router To determine the best link from router ii to t to the destination he destination jj : : For each adjacent router For each adjacent router kk of router of router ii , compute , compute

XXikik+X+Xkjkj , , where Xwhere Xikik is the distance newly measu is the distance newly measured by router red by router ii and and XXkjkj is the most current distan is the most current distance computed by router ce computed by router kk and sent to router j. and sent to router j.

The best link is (The best link is (ii,,kk) such that X) such that Xikik+X+Xkjkj is minimis minimum among all adjacent routers.um among all adjacent routers.


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The Count-to-Infinity Problem (cont.)The Count-to-Infinity Problem (cont.)


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The distance vector algorithm reacts rapidly to good The distance vector algorithm reacts rapidly to good news, but leisurely to bad news. In Fig. 5-11(a), A is news, but leisurely to bad news. In Fig. 5-11(a), A is down initially and them comes up. In Fig. 5-11(b), A down initially and them comes up. In Fig. 5-11(b), A goes down.goes down.

Due to its slow convergence, it is usually used in Due to its slow convergence, it is usually used in small networks.small networks.

In RIP, the metric of distance is hop counts. A finite In RIP, the metric of distance is hop counts. A finite limit of hops (15) is used, after which a route is limit of hops (15) is used, after which a route is considered unreachable.considered unreachable.

The Count-to-Infinity ProblemThe Count-to-Infinity Problem


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Link State RoutingLink State Routing

First used in ARPANET since 1979.First used in ARPANET since 1979. Used in IS-IS (Intermediate System - Intermediate Used in IS-IS (Intermediate System - Intermediate

System), which was designed for DECnet and lateSystem), which was designed for DECnet and later adopted by ISO for the connectionless network lr adopted by ISO for the connectionless network layer protocol CLNP. IS-IS is also used in IP, CDPayer protocol CLNP. IS-IS is also used in IP, CDPD, IPX.D, IPX.

Also used in OSPF (Open Shortest Path First) inteAlso used in OSPF (Open Shortest Path First) interior routing protocol.rior routing protocol.


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Routing AlgorithmRouting Algorithm

For each router:For each router:

Step 1. Discover its neighbors and learn their network Step 1. Discover its neighbors and learn their network addresses.addresses.

Step 2. Measure the delay or cost to each of its neighbors.Step 2. Measure the delay or cost to each of its neighbors.

Step 3. Construct a link state packet.Step 3. Construct a link state packet.


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Step 4. Broadcast the link state packet to all other routersStep 4. Broadcast the link state packet to all other routers

Sequence number: For discarding duplicatesSequence number: For discarding duplicates Age: Decreased once per second and discarded if the age hits zero. When Age: Decreased once per second and discarded if the age hits zero. When

a router is down, its link state packet will age out.a router is down, its link state packet will age out. Send flags: The packet must be sent on the indicated line.Send flags: The packet must be sent on the indicated line. Acknowledgement flags: It must be acknowledged at the indicated Acknowledgement flags: It must be acknowledged at the indicated

routers.routers.


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Step 5. Construct a new routing tableStep 5. Construct a new routing table

Once the router has a full set of link state packets, Once the router has a full set of link state packets, it knows all the link states in the network.it knows all the link states in the network.

Use Dijkstra’s algorithm to compute the shortest pUse Dijkstra’s algorithm to compute the shortest path to all possible destinations.ath to all possible destinations.

Update the routing table.Update the routing table.


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Hierarchical RoutingHierarchical Routing

To avoid router routing tables grow too large as networks To avoid router routing tables grow too large as networks grow in size.grow in size.


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Autonomous Systems in the InternetAutonomous Systems in the Internet An autonomous system is a set of routers having a single routing policy, rAn autonomous system is a set of routers having a single routing policy, r

unning under a single technical administration.unning under a single technical administration. Interior Gateway Protocol vs.Exterior Gateway Protocol.Interior Gateway Protocol vs.Exterior Gateway Protocol.

BGP4 is the de facto standard for exterior gateway protocol in the Internet.BGP4 is the de facto standard for exterior gateway protocol in the Internet. The main goal of an interior gateway protocol is to route efficiently, while The main goal of an interior gateway protocol is to route efficiently, while

the exterior gateway protocols have to worry about “politics”.the exterior gateway protocols have to worry about “politics”.


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Routing for Mobile HostsRouting for Mobile Hosts

Mobility SupportMobility Support


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Broadcast RoutingBroadcast Routing

FloodingFlooding Multi-destination routingMulti-destination routing

Each packet contains a list of desired destinations.Each packet contains a list of desired destinations. When a packet arrives, the router checks all the When a packet arrives, the router checks all the

destinations to determine the set of output lines for destinations to determine the set of output lines for forwarding the packet. An output line is selected if it is forwarding the packet. An output line is selected if it is the best route to at least one of the destinations.the best route to at least one of the destinations.

The router generates a new copy of the packet for The router generates a new copy of the packet for selected output line, with a set of destinations that are selected output line, with a set of destinations that are to use the line.to use the line.


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Spanning Tree RoutingSpanning Tree Routing

Assume each router has knowledge of a Assume each router has knowledge of a spanning tree (e.q. a sink tree) in the spanning tree (e.q. a sink tree) in the network.network.

Each router copies an incoming broadcast Each router copies an incoming broadcast packet onto all the spanning tree lines packet onto all the spanning tree lines except the one it arrives on.except the one it arrives on.

Use minimum number of packets.Use minimum number of packets.


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Reverse Path ForwardingReverse Path Forwarding

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No knowledge of a spanning tree.No knowledge of a spanning tree. When a broadcast packet arrives at a router, on the When a broadcast packet arrives at a router, on the

line that is normally used for sending packets to line that is normally used for sending packets to the source of the broadcast (It is very likely that the source of the broadcast (It is very likely that this is the first copy to arrive at the router).this is the first copy to arrive at the router).

If so, forward the packet onto all lines except the If so, forward the packet onto all lines except the one it arrived on; otherwise, discard it as a likely one it arrived on; otherwise, discard it as a likely duplicate.duplicate.


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Multicast RoutingMulticast Routing

Each router computes a spanning tree covering all otEach router computes a spanning tree covering all other routers in the subnet.her routers in the subnet.

When a multicast packet for a group arrives, the first When a multicast packet for a group arrives, the first router examines its spanning tree and prunes it, remrouter examines its spanning tree and prunes it, removing all lines that do not lead to hosts in the group.oving all lines that do not lead to hosts in the group.

Multicast packets are forwarded only along the prunMulticast packets are forwarded only along the pruned tree.ed tree.

For a network of n groups, each with an average of For a network of n groups, each with an average of m members, nm trees must be stored.m members, nm trees must be stored.


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Core-base Tree for Multicast RoutingCore-base Tree for Multicast Routing

A spanning tree for a group, with the root ( the A spanning tree for a group, with the root ( the core) near the middle of the group.core) near the middle of the group.

To send a multicast packet, send it to the core, To send a multicast packet, send it to the core, which then does the multicast along the spanning which then does the multicast along the spanning tree.tree.

The tree is not optimal. However only n trees need The tree is not optimal. However only n trees need to be stored.to be stored.

RFC 2189 , 2201.RFC 2189 , 2201.


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Congestion ControlCongestion Control


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Policies that Affect CongestionPolicies that Affect Congestion


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Congestion Control SchemesCongestion Control Schemes

Traffic ShapingTraffic Shaping Forcing the packets to transmitted at a more predicatablForcing the packets to transmitted at a more predicatabl

e rate.e rate.

Admission ControlAdmission Control usually used in virtual circuit subnets, such as ATM netusually used in virtual circuit subnets, such as ATM net

works.works. A virtual circuit is admitted only when it will not cause A virtual circuit is admitted only when it will not cause

congestion.congestion.


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Congestion Control Schemes (cont.)Congestion Control Schemes (cont.)

Choke PacketsChoke Packets If congested, the router sends a choke packets back to If congested, the router sends a choke packets back to

the source, with the packet destination.the source, with the packet destination. When the source gets the choke packet, it is required to When the source gets the choke packet, it is required to

reduce the traffic send to the specified destination by a reduce the traffic send to the specified destination by a certain percent.certain percent.

Load sheddingLoad shedding Drop packets when routers are over drown.Drop packets when routers are over drown.


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InternetworkingInternetworking

RepeatersRepeaters BridgesBridges Multiprotocol RoutersMultiprotocol Routers Transport gatewaysTransport gateways Application gatewaysApplication gateways


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Two Styles of InternetworkingTwo Styles of Internetworking


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Tunneling PacketsTunneling Packets

Using encapsulation of IP packetsUsing encapsulation of IP packets


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Internetwork RoutingInternetwork Routing

Interior gateway protocol vs. Exterior gateway proInterior gateway protocol vs. Exterior gateway protocoltocol


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FragmentationFragmentation

IP protocol uses nontransparent fragmentation scheme.IP protocol uses nontransparent fragmentation scheme.


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FirewallsFirewalls

Packet filter router is a router equipped with some extra functionality that allows Packet filter router is a router equipped with some extra functionality that allows every incoming or outgoing packet to be inspected.every incoming or outgoing packet to be inspected.

Application gateway (e.g.a mail gateway) may examine headers and/or contents of Application gateway (e.g.a mail gateway) may examine headers and/or contents of messages.messages.


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The Network Layer in the InternetThe Network Layer in the Internet


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The IP ProtocolThe IP Protocol

IHL: Header length in 32 bit words.IHL: Header length in 32 bit words. Type of Service: Contains three-bit precedence field (packet priority), three flags, D(delay), Type of Service: Contains three-bit precedence field (packet priority), three flags, D(delay),

T(throughput),and R(reliability), and 2 unused bits.T(throughput),and R(reliability), and 2 unused bits. Total length: Length of header plus data with the maximum length 64K bytes.Total length: Length of header plus data with the maximum length 64K bytes. Identification: To identify a datagram that the fragment belongs to.Identification: To identify a datagram that the fragment belongs to.


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DF: Don’t fragment.DF: Don’t fragment. MF: More fragment.MF: More fragment. Fragment Offset: Position of the fragment in the datagram.All fragments eFragment Offset: Position of the fragment in the datagram.All fragments e

xcept the last one must be a multiple of 8 bytes.xcept the last one must be a multiple of 8 bytes. Time to live: Packet lifetimes in seconds. Decremted on each hop and in quTime to live: Packet lifetimes in seconds. Decremted on each hop and in qu

eue in a router.eue in a router. Protocol: Indicate the transport process that a datagram is given to.Protocol: Indicate the transport process that a datagram is given to. Header checksum: One’s complement computation on the header.Header checksum: One’s complement computation on the header. Source address and Destination address indicate the network number and hSource address and Destination address indicate the network number and h

ost number.ost number.


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OptionsOptions


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IP AddressingIP Addressing

Network numbers are assigned by the NIC (Network Information Center) to avoid conflicts.Network numbers are assigned by the NIC (Network Information Center) to avoid conflicts. NIC: InterNIC in US, RIPE in Europe,and APNIC (in Asia Pacific rim).NIC: InterNIC in US, RIPE in Europe,and APNIC (in Asia Pacific rim). Each router only has to keep track of other network and local hosts , not(network,host) pairs,Each router only has to keep track of other network and local hosts , not(network,host) pairs,

greatly reducing the size of its routing table.greatly reducing the size of its routing table.


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SubnetingSubneting

Splitting a network into several Splitting a network into several subnetssubnets for internal use, but the netwo for internal use, but the network acts as a single network to the outside world.rk acts as a single network to the outside world.

To reduce the size of the routing tables. An entry in a routing table is To reduce the size of the routing tables. An entry in a routing table is of the form(this-network, subnet, 0) and (this-network, this-subnet, hoof the form(this-network, subnet, 0) and (this-network, this-subnet, host).st).


5555

50,00050,000

40,00040,000

30,00030,000

20,00020,000

10,00010,000

00350350

8,5008,500

20,50020,500

34,00034,000

42,00042,000

8888 9292 9494 9595 9696

Routing Table GrowthRouting Table Growth

Num

ber

of r

oute

s in

the

Num

ber

of r

oute

s in

the

inte

rnet

inte

rnet


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CIDR:Classless Inter-Domain RoutingCIDR:Classless Inter-Domain Routing To solve the IP address depletion problem and the routing table To solve the IP address depletion problem and the routing table

explosion problemexplosion problem RFC 1519RFC 1519 The Basic idea behind CIDR is to allocate the remaining class C The Basic idea behind CIDR is to allocate the remaining class C

networks in variable size blocksnetworks in variable size blocks The world was partitioned into zones, each given a portion of the The world was partitioned into zones, each given a portion of the

class C address space:class C address space:

Addresses 194.0.0.0 to 195.255.255.255 for EuropeAddresses 194.0.0.0 to 195.255.255.255 for Europe

Addresses 196.0.0.0 to 197.255.255.255 for OthersAddresses 196.0.0.0 to 197.255.255.255 for Others

Addresses 198.0.0.0 to 199.255.255.255 for North AmericaAddresses 198.0.0.0 to 199.255.255.255 for North America

Addresses 200.0.0.0 to 201.255.255.255 for Central and South AmericaAddresses 200.0.0.0 to 201.255.255.255 for Central and South America

Addresses 202.0.0.0 to 203.255.255.255 for Asia and PacificAddresses 202.0.0.0 to 203.255.255.255 for Asia and Pacific

Addresses 204.0.0.0 to 207.255.255.255 for OthersAddresses 204.0.0.0 to 207.255.255.255 for Others

Addresses 208.0.0.0 to 223.255.255.255 reserved for future useAddresses 208.0.0.0 to 223.255.255.255 reserved for future use


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The address entry in a CIDR routing table contains a base address and The address entry in a CIDR routing table contains a base address and a variable length mask.For example 2048 addresses from 194.24.0.0 ta variable length mask.For example 2048 addresses from 194.24.0.0 to 194.24.7.255o 194.24.7.255

base address:base address:

11000010 00011000 00000000 0000000011000010 00011000 00000000 00000000

mask:mask:

11111111 11111111 11111000 0000000011111111 11111111 11111000 00000000

ie. 194.24.0.0 255.255.248.0 or 194.24.0.0/21ie. 194.24.0.0 255.255.248.0 or 194.24.0.0/21


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Class C: 198.32.1.0 11000110 00100000 00000001 00000000Class C: 198.32.1.0 11000110 00100000 00000001 00000000

Mask 255.255.255.0 11111111 11111111 11111111 00000000Mask 255.255.255.0 11111111 11111111 11111111 00000000Mask 255.255.0.0 11111111 11111111 00000000 00000000Mask 255.255.0.0 11111111 11111111 00000000 00000000

00 88 1616 2424

PrefixPrefixPrefix lengthPrefix length

SupernetSupernet

Natural maskNatural mask

198.32.1.0 255.255.255.0 <==> 198.32.1.0/24198.32.1.0 255.255.255.0 <==> 198.32.1.0/24198.32.0.0 255.255.255.0 <==> 198.32.0.0/16198.32.0.0 255.255.255.0 <==> 198.32.0.0/16

A network is called a supernet when the prefix boundary contains fewer bitsA network is called a supernet when the prefix boundary contains fewer bitsthan the network’s natural mask.than the network’s natural mask.


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IP Address AllocationIP Address Allocation

Class A address allocation is restricted.Class A address allocation is restricted. Class B address are also restricted .They will be allocated Class B address are also restricted .They will be allocated

only if the need for them is justified.only if the need for them is justified. class C addresses are allocated with a contiguous block of class C addresses are allocated with a contiguous block of

addresses which consists of several contiguous class C addaddresses which consists of several contiguous class C addresses.Class C addresses are being distributed to ISPs so thresses.Class C addresses are being distributed to ISPs so that the allocation could last at least two years.at the allocation could last at least two years.

If a subscriber has a requirement for more than 4096 IP adIf a subscriber has a requirement for more than 4096 IP address, a Class B network number may be allocated.dress, a Class B network number may be allocated.

Organizations are encouraged to use Variable Length SubnOrganizations are encouraged to use Variable Length Subnet Mask for efficient use of address space. et Mask for efficient use of address space.


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Internet Control ProtocolsInternet Control Protocols

IICMP(Internet Control Message Protocol)CMP(Internet Control Message Protocol) RFC 792RFC 792

ARP(Address Resolution Protocol)ARP(Address Resolution Protocol) RFC 826RFC 826 For an IP address , find its hardware address.For an IP address , find its hardware address.

RARPRARP RFC 903RFC 903 For a hardware address , find its IP address.For a hardware address , find its IP address. RARP server is needed on each network.RARP server is needed on each network.

BootpBootp RFC 951,1048,1084….RFC 951,1048,1084….


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Internet Control Message ProtocolInternet Control Message Protocol

To report unexpected events or test the InternetTo report unexpected events or test the Internet


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RARP:Reverse Address Resolution ProtocolRARP:Reverse Address Resolution Protocol

Allow a newly-booted (diskless) workstatioAllow a newly-booted (diskless) workstation (with a DLL address) to discover its IP adn (with a DLL address) to discover its IP addressdress

Need a RARP server on each networkNeed a RARP server on each network Bootp:Bootp:

Use UDP messages which are forwarded over rUse UDP messages which are forwarded over routers to find the file server that holds the mappouters to find the file server that holds the mappinging


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ARP: Address Resolution ProtocolARP: Address Resolution Protocol

To map an IP address onto data link layer address , such To map an IP address onto data link layer address , such as Ethernet.as Ethernet.

An IP host runs the ARP protocol to inquiry the unknown An IP host runs the ARP protocol to inquiry the unknown data link layer address of a destination IP address before data link layer address of a destination IP address before a datagram is sent.a datagram is sent.

The ARP of a host may maintain a cache to record known The ARP of a host may maintain a cache to record known IP address and DLL address pairs.IP address and DLL address pairs.

The ARP may broadcast its own mapping when it boots.The ARP may broadcast its own mapping when it boots.

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