page 1 comp210 network layer. page 2 the network layer the network layer is responsible for...

COMP210

Network layer

The Network Layer

The network layer is responsible for establishing, maintaining and terminating connections

The network layer provides:Routing Flow and congestion control Internetworking

Routing

Packets originating from a source must be delivered to the destination by routing them through several intermediate nodes

Routing involves the selection of the paths for the packets

When a stream of packets need to be delivered, the network may handle them in two ways:

Virtual circuit Datagram

In this approach a preplanned route is established before any packets are sent

Packets are still buffered at each node and queue up to use an outgoing link

Virtual Circuit

Since all packets follow the same route, sequencing of packets is maintained

Virtual circuit is useful when two stations exchange data over an extended period of time

Virtual Circuit

Implementation of Connection-Oriented Service

Routing within a virtual-circuit subnet.

The call setup phase is avoided It is more flexible and can respond to congestion Packet sequencing may not be maintained Datagram is useful when only a few packets are exchanged

Datagram Each packet is routed independently

Implementation of Connectionless Service

Routing within a diagram subnet.

Comparison of Virtual-Circuit and Datagram

5-4

Routing Strategies

The decision time for a route may be made either at the packet level (datagram) or at the virtual circuit establishment time

The routing decisions may be either centralized or distributed

Routing decisions are usually based on topology, traffic load and cost

Fixed Routing

Each node needs to store only a single row of the routing directory to decide the next node to take for each destination

This strategy is simple, and works well in a reliable network, but the method is not flexible and cannot adapt to network load

Shortest Path Routing

Adaptive Routing

These are dynamic algorithms and react to changes in the network conditions

The routing decisions are more complex and require more processing at the nodes

The network status information needs to be exchanged among the nodes, imposing further traffic among the nodes

Adaptive Routing Methods

Distance Vector Routing each router exchanges routing

tables with its neighbours every x seconds

creates heavy network traffic can be out of date

Link State Routing each router broadcasts its

entire routing table at startup routers broadcast subsequent

updates

Distance Vector Routing

Distance Vector Routing (2)

The count-to-infinity problem.

Link State Routing

Each router must do the following:

Discover its neighbors, learn their network address.

Measure the delay or cost to each of its neighbors.

Construct a packet telling all it has just learned.

Send this packet to all other routers.

Compute the shortest path to every other router.

Learning about the Neighbors

Measuring Line Cost

Building Link State Packets

(a) A subnet. (b) The link state packets for this subnet.

Distributing the Link State Packets

The packet buffer for router B in the previous slide

(Fig. 5-13).

Hierarchical Routing

Hierarchical routing.

Routing for Mobile Hosts

A WAN to which LANs, MANs, and wireless cells

are attached.

Q of S Requirements

How stringent the quality-of-service requirements are.

5-30

How Networks Differ

Some of the many ways networks can differ.

5-43

Routers

Use Network and Physical (NIC card) addressing

Maintain routing tables to direct packets to correct network

Function at the network layer

Routers

Routing terms hop count

number of routers a message passes to get to its destination

tick count time required to deliver a

message. One tick is 1/8 sec TTL Time To Live

how many routers will a message pass until it is undeliverable

Gateways

Protocol translator Connecting two dissimilar

networks functions at the network

layer and above

IP addressing-History

1969 ARPANET used by U.S. Department of Defence

1970+TCP/IP developed and used as the common protocol

Developed to link educational an governmental agencies (military)

IP addressing-History (cont)

1984 National Science Foundation (NSF) decided to build "network of networks" (Internet) based on TCP/IP

1997 5 Million + internet users

Made up of non proprietary protocols

changed by RFC (request for comment)

OSI Model versus TCP/IP

Protocol layers

Internet Protocol (IP)

defining the datagram defining the internet

addressing scheme moving data between

the Network Access Layer and the Transport Layer

routing datagrams to remote hosts

IP (cont)

performing fragmentation and re-assembly of datagrams

connectionless protocol does not exchange control information (handshake) to establish end to end connection before transmitting data

IP (cont)

relies on other protocols to establish connection if required

unreliable contains no error detection, does not check if message is delivered (Time To Live)

relies on other protocols for this

Collection of Subnetworks

OSPF – The Interior Gateway Routing Protocol

(a) Autonomous system. (b) Graph representation of (a)

The relation between ASes, backbones, and areas in OSPF.

The five types of OSPF messages.

5-66

BGP – The Exterior Gateway Routing Protocol

(a) A set of BGP routers. (b) Information

sent to F.

The IP Protocol

The IPv4 (Internet Protocol) header.

The IP Protocol

Some of the IP options.

5-54

IP address

The IP address is a 32 bit address identifies the network and the

host on a given network divided into two parts first part

identifies the network, second part identifies the host on the network

the format is not the same for each address

IP Address

the 32 bit number is represented in the following format

xxx.xxx.xxx.xxx where xxx is the

decimal representation of the binary bit string

Example: 142.110.3.4 10001110 01101110 00000011

00000100

Classes of IP Addresses

IP address formats.

Classes of IP addresses

Class A used for small number of

networks and large number of hosts

first byte (8 bits) represent the network address

last 3 bytes (24 bits) represent the host address

class A address have a first bit of 0

class A network addresses range from 0 to 127


Class B Provide an equal number of

networks and hosts first two bytes are network

address and last two bytes are host addresses

first two bits of a class B address are 10

network addresses range from 128 to 191


Class C greater number of network

addresses fewer host addresses first three bits are 110 network addresses range form

192-223


Class D used for special multicast

addresses first four bits 1110


Class E used for experimental purposes first four bits 1111

Special IP addresses.

Subnets

subnets are used to divide a large network into smaller networks

each address allows for one network address and many hosts (ie all hosts are on the same network)

subnet masks are used to create many subnets within the same network address

Subnet masks

a bit string applied to an address

if the bit is on the corresponding bit in the address is considered to be a network bit

the network mask is known locally only

Subnets

A class B network subnetted into 64 subnets.

Subnet Mask for Class B address

Subnet Example #1

IP Address 130.97.16.132 Subnet Mask 255.255.255.192 11111111 11111111 11111111

11000000 10000010 01100001 00010000

10000100

Network 10000010 01100001 00010000

10000000

130.97.16.128 Host 00000000 00000000 00000000

00000100

4

Subnet Example #2

IP Address 130.97.17.132 Subnet Mask 255.255.254.0 11111111 11111111 11111110

00000000 10000010 01100001 00010001

10000100

Network 10000010 01100001 00010000

00000000

130.97.16. Host 00000000 00000000 00000001

10000100

1.132

P a g e 5 8

ROUTING EXAMPLE

Script started on Mon Mar 11 15:46:32 2002[root@clash ijirasek]# netstat -i

eth0 1500 0 1282464 0 0 0 309442 0 0 0 BRUeth1 1500 0 11233 0 0 0 13268 0 0 0 BRUlo 16436 0 16545 0 0 0 16545 0 0 0 LRU

[root@clash ijirasek]# ifconfig eth0

eth0 Link encap:Ethernet HWaddr 00:10:5A:98:02:F5 inet addr:136.159.6.32 Bcast:136.159.6.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:1282496 errors:0 dropped:0 overruns:0 frame:0 TX packets:309466 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:100 Interrupt:10 Base address:0x6800

ROUTING EXAMPLE - CONT

netstat -r

Kernel IP routing tablesentinel * 255.255.255.255 UH 40 0 0 eth1136.159.6.0 * 255.255.255.0 U 40 0 0 eth0192.168.66.0 * 255.255.255.0 U 40 0 0 eth1127.0.0.0 * 255.0.0.0 U 40 0 0 lodefault 136.159.6.1 0.0.0.0 UG 40 0 0 eth0

netstat -rn

192.168.66.1 0.0.0.0 255.255.255.255 UH 40 0 0 eth1136.159.6.0 0.0.0.0 255.255.255.0 U 40 0 0 eth0192.168.66.0 0.0.0.0 255.255.255.0 U 40 0 0 eth1127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo0.0.0.0 136.159.6.1 0.0.0.0 UG 40 0 0 eth0

Script done on Mon Mar 11 15:47:48 2002

Classless InterDomain Routing - CIDR

CIDR Block Prefix # Equivalent Class C # of Host Addresses /27 1/8th of a Class C 32 hosts /26 1/4th of a Class C 64 hosts /25 1/2 of a Class C 128 hosts /24 1 Class C 256 hosts /23 2 Class C 512 hosts /22 4 Class C 1,024 hosts /21 8 Class C 2,048 hosts /20 16 Class C 4,096 hosts . . . . . .

. . . /13 2,048 Class C 524,288 hosts Example: Consider the following: Sidte ID: 194.24.16.0 11000010 00011000 00010000 00000000CIDR mask: /20 11111111 11111111 11110000 00000000Netmask: 255.255.255.192 11111111 11111111 11111111 11000000 The highest IP 194.24.31.255 11000010 00011000 00011111 11111111# of subnets: 2**6 -2# of hosts: 2**6 - 2

CIDR examples

5-59

Network Address Translation (NAT)

Method to connect multiple computers to Internet via one IP address

Private network ---------- NAT router ----- Internet (client) (server)

|Source Address| Source Port | Dest. Address | Dest. Port| -

When the packet arrives from the Private Network to NAT router NAT router will:

1. Insert |Source Address| Source Port| into a table 2. Change Source address to NAT router address 3. Change Source Port to the table offset 4. Send the modified packet to destination

When the response comes back NAT router replaces the modified Source info with the original source info and sends it to the client.

Outside node cannot initiate the communication

Reserved addresses:10.0.0.0 - 10.255.255.255/8 172.16.0.0 – 172.31.255.255/12

192.168.0.0 – 192.168.255.255/16

NAT example

PROBLEMS WITH NAT

1. If NAT box fails all the connections are lost

2. Violates the OSI layers independency

3. Some applications insert IP address as a part of the message. Those applications will fail

4. NAT changes the content of the IP datagram.This in incompatible with the secure data communication

NIC card address IP Address translation

1. IP address -> NIC card address Address Resolution Protocol (ARP)

Each node broadcasts its IP address and NIC card address in the boot time. Other nodes on the subnet store the info. Alternatively, a node can broadcast the question: “What NIC card has IP address .

2. NIC card address -> IP address Reverse Address Resolution Protocol (RARP)

Used for diskless workstation. Diskless workstation has to be booted from remote file server. After the boot the workstation will broadcast the message My NIC card address is Who knows my IP address? RARP server will know the answer

3. BOOTP: Similar to RARP. Uses UDP protocol

4. DHCP: Extension of BOOTP. Current technology

BOOTP, DHCP functionality

1 BOOTP server will provide the client with the following information:

IP address and netmaskIP address of default routerIP address of DNS server

The assignment between IP address and NIC card address is static (manual)

2. DHCP (Dynamic Host Configuration Protocol)

Same functionality as BOOTP plus dynamic IP addresses assignment. DHCP server is given a block of IP addresses to choose from.

BOOTP, DHCP functionality - cont

Work station (client) creates a following UDP message:

Source IP address: 0.0.0.0

Destination IP address: 255.255.255.255

Source Port #: 68

Destination Port #: 67

Message:”Here is my NIC address. What is my IP address?”

Server answers with a following UDP message:

Source IP address: Its own address

Destination IP address: 255.255.255.255

Source port #: 67

Destination port #: 68

Message: “Machine with NIC card address ….! Your IP is …..”

Routers know about it and make exception in limited broadcast forwarding

Dynamic Host Configuration Protocol(DHCP)

Operation of DHCP.

The Main IPv6 Header

Internet Control Message Protocol (ICMP)

Used to: 1. Transmit error messages (type 3 message)2. Pass router info (type 9, 10 message)

ICMP is also used to facilitate mobile networking

Mobile IP

Design criteria:

1. Each mobile host must be able to use itshome IP address anywhere

2. Software changes to fixed hosts not permitted

3. Changes to router software not permitted

4. Most packets for mobile hosts should notmake detour.

5. No overheads when mobile host is at home

Mobile IP

Purpose: To provide routing for nodes which move between radio ranges of routers.Components of mobile network:

Mobile node: Node on the moveHome network: Network to which Mobile

node belongs toHome agent: Router in Home Network

which provides mobility management

Foreign network: Network where Mobile node resides

Foreign agent: Router in the Foreign network which provides mobility management


A WAN to which LANs, MANs, and wireless cells

are attached.

Mobile IP - cont

Permanent Address: IP address permanentlyassigned to the Mobile node

Care-of address: Temporary IP address assigned to Mobile node by Foreign agent

Correspondent: Node wishing to communicatewith Mobile node

Mobile IP - cont

Functionality:

Correspondent sends a message addressed to the Permanent address. This message will arrive to the Home agent. Home agent forwardsthe message to Foreign agent. Foreign agentforwards it to Mobile node. Mobile node replies directly to Correspondent.

Mobile IP - cont

Additional protocols required:

Agent discovery: Mobile node finds theForeign agent or Home agent

Registration with Home agent: Foreignagent sends Home agent Care-of address

Indirect routing of Correspondent’s datagrams: Rules about encapsulatingdatagrams forwarded to Mobile node byHome agent

page 1 comp210 network layer. page 2 the network layer the network layer is responsible for...

Documents

routing flow

fixed routing

routing strategies

routing directory

intermediate nodes routing

distributed routing

entire routing table

virtual circuit datagram