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Networking Laboratory 1/52

Sungkyunkwan University

Copyright 2000-2014 Networking Laboratory 2018-Fall Computer Networks Copyright 2000-2018 Networking Laboratory

Chapter 4

Network Layer

Prepared by H. Choo

2018-Fall Computer Networks Networking Laboratory 2/237

Presentation Outline

4.1 Introduction

4.2 Network-Layer Protocols

4.3 Unicast Routing

4.4 Multicast Routing

4.5 IPv6


4.1 Introduction (1/3)

Video Content

► The video lets you have a view with a route of data packet on the Internet

One of trillions involved in the trillions of Internet interactions that happen every

second

Look deep beneath the surface of the most basic Internet transaction, and follow

the packet as it flows from your fingertips, through circuits, wires, and cables, to

a host server, and then back again, all in less than a second

Link: https://www.youtube.com/watch?v=ewrBalT_eBM

introduction.mp4



https://www.youtube.com/watch?v=ewrBalT_eBM

introduction.mp4



[Fig 4.1 Communication at the network layer ]


Encapsulating the payload (data received from upper layer) in a

network-layer packet at the source

► If the content of the payload is too large, it needs to be fragmented

Decapsulating the payload from the network layer packet at the

destination

► If the packet is fragmented at the source or at routers along the path, the

network layer is responsible for waiting until all fragments arrive,

reassembling them, and delivering to the upper-layer

4.1 Introduction Network-layer Services: Packetizing


Routing

► Since a physical network is a combination of networks (LANs and WANs),

therefore, there is more than one route from the source to the destination

► We have to find the best one among these possible routes

Forwarding

► Move packets from router’s input to appropriate router output

4.1 Introduction Network-Layer Services

Forwarding

value

B Data

Send the packet

out of interface 2

B Data

[ Fig 4.2 Forwarding process ]


Error control

► The packet in the network layer maybe fragmented at each router, so error

checking at this layer is inefficient. However, a checksum field is added to

control any corruption in the header

Flow control

► This service is provided for most of the upper-layer protocols because of

the reasons as follows:

No error control in this layer

Upper-layer can use buffers to receive data

Flow control makes the network-layer more complicated and

the whole system less efficient



Congestion control

► Congestion in the network-layer is a situation in which too many

datagrams are present in an area of the Internet

► No congestion control is implemented at the Network layer in the Internet

Quality of Service

► The provisions are mostly implemented in the upper layer

Security

► The network layer was designed with no security provision



Data communication switching techniques are divided into

two broad categories: circuit switching and packet switching

Packet switching, however, is only used at network-layer

A message from upper layer is divided into manageable

packets and each packet is sent through the network

Today, a packet-switched network can use two different

approaches:

► Datagram approach: Connectionless Service

► Virtual circuit approach: Connection-Oriented Service

4.1 Introduction Packet Switching


Connectionless Service

► Each packet has no relationship to any other packet

[Fig 4.3 A connectionless packet-switched network]

4.1 Introduction Packet Switching: Datagram approach (1/2)


► Each packet is routed based on the information contained in its header

[ Fig 4.4 Forwarding process in a router when used in a connectionless network]

SA DA Data SA DA Data

4.1 Introduction Packet Switching: Datagram approach (2/2)


Connection-Oriented Service

► A virtual connection should be set up to define the path before sending

datagrams

[ Fig 4.5 A virtual-circuit packet-switched network ]

4.1 Introduction Packet Switching: Virtual-Circuit Approach (1/5)


► Each packet is forwarded based on the label in the packet

[ Fig 4.6 Forwarding process in a router when used in a virtual-circuit network ]



► Set up phase: request packet and acknowledge packet need to be

exchanged between the sender and the receiver

[ Fig 4.7 Sending request packet in a virtual-circuit network ]

A to B

A to B

A to B A to B



[ Fig 4.8 Sending acknowledgments in a virtual-circuit network ]



► Data-Transfer Phase


[ Fig 4.9 Flow of one packet in an established virtual circuit ]


The upper-layer protocols that use the service of the network layer

expect to receive an ideal service, but the network layer is not perfect

The performance of a network can be measured in terms of

► Delay

► Throughput

► Packet loss

4.1 Introduction Network-Layer Performance


Transmission delay

► Delaytr = (Packet length) / (Transmission rate)

Propagation delay

► Delaypg = (Distance) / (Propagation speed)

Processing Delay

► Delaypr = Time required process a packet in a router or a destination

Queuing delay

► Delayqu = The time a packet waits in input and output queues

Total delay

► n: the number of routers between two end hosts

► Delaytotal = (n+1)(Delaytr+Delaypg+Delaypr) + (n)(Delayqu)

4.1 Introduction Network-Layer Performance: Delay


Throughput at any point in a network is defined as the number of bits

passing through the point in a second, transmission rate

► Throughput = minimum{TR1, TR2,…, TRn}

[ Fig 4.10 Throughput in a path with three links in a series ]

4.1 Introduction Network-Layer Performance: Throughput (1/3)


The Internet backbone has a very high transmission rate, in the

range of gigabits per second

The throughput, therefore, is normally defined as the minimum

transmission rate of the two access links that connect to the source

and destination


[ Figure 4.11 A path through the Internet backbone ]


Besides, the link between two routers is not always dedicated to one

flow. A router may collect the flow from several sources or distribute

the flow between several sources. In this case the transmission rate

is actually shared between the flows

[ Fig 4.12 Effect of throughput is shared links ]



When a router receives a packet while processing another packet, the

received packet needs to be stored in the input buffer. A router,

however, has an input buffer with a limited size

A time may come when the buffer is full and the next packet needs to

be dropped

The effect of packet loss on the Internet network layer is that the

packet needs to be resent, which in turn may create overflow and

cause more packet loss

4.1 Introduction Network-Layer Performance: Packet loss


The study of congestion in this layer may only help us to better

understand cause of congestion at the transport layer and find possible

remedies to be used in network layer

There are two issues related: Packet delay and throughput

[ Fig 4.13 Packet delay and throughput as functions of load ]

4.1 Introduction Network-Layer Congestion


Open-loop congestion control: prevents congestion before it

happens.

► Retransmission policy: retransmission is sometimes unavoidable.

Retransmission in general may increase congestion in the network.

However, a good retransmission policy can prevent congestion

► Window policy: the type of window at the sender which consists of Go-

Back-N window and Selective Repeat window also effect congestion.

Go-Back-N window: when the timer for a packet time out, several packets

may be resent, even some may have arrived safe

Selective Repeat window: tries to send the specific packets that have been

lost or corrupted

4.1 Introduction Network-Layer Congestion: Congestion control (1/4)


Open-loop congestion control:

► Acknowledgement policy: imposed by the receiver also effect

congestion. If the receiver does not acknowledge every packet it

receives, it may slow down the sender and help prevent congestion

► Discard policy: operated by routers may prevent congestion and not

harm the integrity of the transmission

► Admission policy: is a quality-of-service mechanism (discussed in

chapter 8), can also prevent congestion in virtual-circuit networks



Closed-loop congestion control: tries to alleviate congestion after it

happens

► Backpressure

► Choke packet

[ Fig 4.14 Backpressure method for alleviating congestion ]

[ Fig 4.15 Choke packet ]



Closed-loop congestion control:

► Implicit Signaling: there is no communication between the congested

node or nodes and the source. The source guesses that there is

congestion somewhere in the network from other symptoms

► Explicit Signaling: Explicit signaling can occur in either the forward or the

backward direction which can be seen in an ATM network. It will be

discussed in Chapter 5



Practice Problem (1/4)

Consider the following network path with 3 links and two store and

forward packet switches

► Note that each switch has 1.5 MB of memory for doing store and forward.

Assume that the processing delay at switches is 0 sec. The transmission

rate and the propagation delays of the links are as indicated in the picture



What’s the maximum size of data this network path can carry at any

moment?


Practice Problem (3/4) (cont.)

A MP3 file is roughly 4MB, how many MP3 files the path can carry at

any moment?



Calculate the total time required to transfer a 1.5 MB file in the following

case, assuming RTT of 80 ms, and an initial handshaking before the file

is sent


A router has four components: input ports, output ports, the routing

processor, and the switching fabric

[ Fig 4.16 Router components ]

4.1 Introduction Structure of A Router: Components (1/6)


Input ports

Output ports

[ Fig 4.17 Input port ]

[ Fig 4.18 Output port ]




Routing Processor

► Performing the function of the network layer

► Finding the address of the next hop and, at the same time, the

output port number from which packet is sent out

► This activity is sometimes referred to as table lookup because the

routing processor searches the forwarding table


Switching Fabrics

[ Fig 4.20 Banyan switch ] [ Fig 4.19 Crossbar switch ]




Switching Fabrics

► Banyan switch

[ Figure 4.21: Examples of routing in a banyan switch ]


Batcher-Banyan switch: design a switch that comes before the banyan

switch and sorts incoming packets according to their destination

[ Fig 4.22 Batcher-Banyan switch ]



4.2 Network-Layer Protocol

In this section, we show how the network layer is implemented in the

TCP/IP protocol suite

The protocols in the network layer have gone through several

versions

We concentrate on the current version (4), in the last section of this

chapter, we briefly discuss version 6, which is on the horizon


4.2 Network-Layer Protocol

In this section, we show how the network layer is implemented in the

TCP/IP protocol suite

The network layer in version 4 can be thought of as one main protocol

and three auxiliary protocols:

► Internet Protocol ver. 4 (IPv4), main protocol, is responsible for packetizing,

forwarding and delivery of packet

► Internet Control Message Protocol ver. 4 (ICMPv4) helps IPv4 to handle

some errors in the network-layer delivery

► Internet Group Management Protocol (IGMP) helps IPv4 in multicasting

► Address Resolution Protocol (ARP) glues the network and data-link layer


4.2 Network-Layer Protocol Network layer protocol in TCP/IP protocol suite

[ Fig 4.23 Position of IP and other network-layer protocols in TCP/IP protocol suite ]


4.2 Network-Layer Protocol IP Datagram Format (1/11)

Packets used by the IP are called IP datagrams

[ Fig 4.24 IP datagram ]



Version (VER)

► 4-bit field which defines the version of the IP protocol

► Currently the version is 4

► Version 6 may totally replace version 4 in the future

► If the machine is using some other version of IP, the datagram is discarded

Header length (HLEN)

► 4-bit field which defines the total length of the datagram header

in 4-byte words

► This field is needed because the head length is variable

Service type

► 8-bit field which was referred to as type of service (TOS)



Total length

► 16-bit field which defines the total length of the IP datagram in bytes

► Length of data = total length – header length

► This field is necessary in case that the padding is added in the frame

Identification: 16-bit field which is used in fragmentation

L2 header

L2 Trailer

Data < 46 bytes Padding

Length: Minimum 46 bytes

[ Encapsulation of a small datagram in an Ethernet frame ]



Flags

► 3-bit field which is used in fragmentation

Time to live

► A datagram has a limited lifetime in its travel through an internet

► This field is originally designed to hold a timestamp, which was

decremented by each visited router

► This field is mostly used to control the maximum number of hops visited by

the datagram. This value is approximately two times the maximum number

of routers between any two hosts

► If the value is zero, the router discards the datagram



Protocol: 8-bit field which defines the higher level protocol that uses

the services of the IP layer

[ Figure 4.25: Multiplexing and demultiplexing using the value of the protocol field ]



Checksum

► 16-bit field used for error check using checksum

Source address

► 32-bit field which defines the IP address of the source

Destination address

► 32-bit field which defines the IP address of the destination



Maximum Transfer Unit: when a datagram is encapsulated in a

frame, the total size of the datagram must be less than MTU

[ Figure 4.26: Maximum transfer unit (MTU) ]



Fields Related to Fragmentation:

► Fragmentation is to divide the datagram to make it possible to pass

through a network

► The source usually does not fragment the IP packet

► When a datagram is fragmented, each fragment has its own header with

most of the fields repeated, but some changed

► The reassembly of the datagram is done only by the destination host



Fields Related to Fragmentation

► Identification (16-bit)

This field identifies a datagram originating from the source host

The combination of the identification and source IP address must uniquely define

a datagram as it leaves the source host

The identification number helps the destination in reassembling the datagram

► Flags (3-bit)

The first bit is reserved

The second bit is called the do not fragment bit

The third bit is called the more fragment bit

D M

D: Do not fragment

M: more fragment



Fields Related to Fragmentation

► Fragmentation offset (13-bit)

This is offset of the data in the original datagram measured in units of 8 bytes

[ Fig 4.27 Fragmentation examples ]



An example of fragmentation

[ Fig 4.28 Detailed fragmentation example ]


Practice Problem

An IP message 12,000 bytes wide (including the 20-byte IP header) that

needs to be sent over a link with an MTU of 3,300 bytes.


4.2 Network-Layer Protocol IP Datagram Format: Security of IPv4 (1/2)

In this section, we only give a brief idea about the security issues in IP

protocol and the solution. There are 3 issues:

► Packet Sniffing: a passive attack, in which the attacker does not change

the contents of data packet

► Packet Modification: the attacker intercepts the packet, changes its

contents, and sends the new packet to the receiver

► IP Spoofing: An attacker can masquerade as somebody else and create

an IP packet that carries the source address of another computer

► IPSec: used in conjunction with the IP protocol, creates a connection-

oriented service between two entities in which they can exchange IP

packets


4.2 Network-Layer Protocol IP Datagram Format: Security of IPv4 (2/2)

► IPSec: used in conjunction with the IP protocol, creates a connection-

oriented service between two entities in which they can exchange IP

packets

Defining Algorithm and Keys: The two entities that want to create a secure

channel between themselves can agree on some available algorithms and keys

to be used for security purposes

Packet Encryption: The packets exchanged between two parties can be

encrypted for privacy using one of the encryption algorithms and a shared key

agreed upon in the first step

Data Integrity: Data integrity guarantees that the packet is not modified during

the transmission

Origin Authentication: IPSec can authenticate the origin of the packet to be

sure that the packet is not created by an imposter


4.2 Network-Layer Protocol IPv4 address

An IP address is a 32-bit address

The IP addresses are unique

The address space of IPv4 is 232 or 4, 294, 967, 296

Notation

► Binary Notation : In binary notation, the IP address is displayed as 32

bits

► Dotted-Decimal Notation : Internet addresses are usually written in

decimal form with a decimal point separating the bytes

[ Fig 4.29 Three different notations in IPv4 addressing ]


4.2 Network-Layer Protocol IPv4 address: Hierarchy in addressing

A 32-bit IPv4 address is hierarchical, but divided only into two parts

The first part of the address called the prefix, defines the network

The second part of the address called the suffix, defined the node

[ Fig 4.30 Hierarchy in addressing ]


4.2 Network-Layer Protocol IPv4 address: Classful addressing

In classful addressing, the IP address space is divided into five classes:

A, B, C, D, E

[ Fig 4.31 Occupation of the address space in classful addressing ]


4.2 Network-Layer Protocol IPv4 address: Address Depletion

The addresses being rapidly used up. So, no more addresses

available for organizations or individuals that needed to be connected

to the Internet

Class A with 16,777,216 IPs. Since there may be only a few

organizations that are this large, most of the addresses in this class

were wasted

Class B addresses were designed for midsize organization, but many

of the addresses also remained unused

Class C has only 256 IPs, it was so small that most companies were

not comfortable using a block in this address

Class E were almost never used, wasting the whole class


4.2 Network-Layer Protocol IPv4 address: Subnetting and Supernetting

Subnetting and Supernetting are proposed to alleviate

address depletion

► In Subnetting, Class A or B is divided into several subnets. Each

subnet has a larger prefix length than the original network. In other

words, we divide a large block into smaller block

This idea did not work because most large organizations were not

happy about dividing the block and giving some of the unused

addresses to smaller organizations

► In Supernetting, we combine several class C blocks into a larger

block to be attractive to organizations that need more than 256 IPs

This idea did not work either because it makes the routing of packets more

difficult


4.2 Network-Layer Protocol IPv4 address: Classless addressing (1/7)

In classless addressing, variable-length blocks are used that belong to

no classes. We can have a block of 1 address, 2 addresses, 4

addresses, 128 addresses, and so on

The prefix in an address defines the block (network); the suffix defines

the node (device). Therefore, we can have a block of 20, 21, 22,…, 232

addresses

► A small prefix means a larger network; a large prefix means a smaller

network

[ Figure 4.32: Variable-length blocks in classless addressing ]



Restrictions

► Number of Addresses in a Block

The number of addresses in a block must be a power of two

► First Address

The first address must be evenly divisible by the number of addresses

► Mask

In classless addressing, the address must be accompanied by the mask

The mask is given in classless inter-domain routing or CIDR notation with the

number of 1s

Format of classless addressing address is X.Y.Z.t/n, the n after the slash

defines the number of bits that are the same in every address in the block



Mask

► Prefix is another name for the common part of the address range, similar to

the netid in classful addressing

► Prefix length is the length of the prefix (n in the CIDR notation)

► Classful addressing is a special case of classless addressing

/n Mask /n Mask /n Mask /n Mask

/1 128.0.0.0 /9 255.128.0.0 /17 255.255.128.0 /25 255.255.255.128

/2 192.0.0.0 /10 255.192.0.0 /18 255.255.192.0 /26 255.255.255.192

/3 224.0.0.0 /11 255.224.0.0 /19 255.255.224.0 /27 255.255.255.224

/4 240.0.0.0 /12 255.240.0.0 /20 255.255.240.0 /28 255.255.255.240

/5 248.0.0.0 /13 255.248.0.0 /21 255.255.248.0 /29 255.255.255.248

/6 252.0.0.0 /14 255.252.0.0 /22 255.255.252.0 /30 255.255.255.252

/7 254.0.0.0 /15 255.254.0.0 /23 255.255.254.0 /31 255.255.255.254

/8 255.0.0.0 /16 255.255.0.0 /24 255.255.255.0 /32 255.255.255.255



Mask

► The suffix is the varying part, similar to the hostid

► The suffix length is the length of the suffix (32-n) in CIDR notation

[ Fig 4.34 Information extraction in classless addressing ]



Example 4.1: A classless address is given as 167.199.170.82/27. We

can find the above three pieces of information as follows

► The number of addresses in the network is 232− n= 25 = 32 addresses.

► The first address can be found by keeping the first 27 bits and changing the

rest of the bits to 0s

► The last address can be found by keeping the first 27 bits and changing the

rest of the bits to 1s



Example 4.2: We repeat Example 4.1 using the mask. The mask in

dotted-decimal notation is 255.255.255.224. The AND, OR, and NOT

operations can be applied to individual bytes using calculators and

applets at the book website



Example 4.3: In classless addressing, an address cannot per se define

the block the address belongs to. For example, the address 230.8.24.56

can belong to many blocks. Some of them are shown below with the

value of the prefix associated with that block


4.2 Network-Layer Protocol IPv4 address: Network address (1/6)

The first address of the address block, network address, is particularly

important because it is used in routing a packet to its destination

network

First address = (prefix in decimal) * 232-n = (prefix in decimal) * N

[ Fig 4.35 Network address ]



An ISP has requested a block of 1000 addresses. Since 1000 is not a

power of 2, 1024 addresses are granted

► The prefix length is calculated as n = 32 − log21024 = 22. An available

block, 18.14.12.0/22, is granted to the ISP



An organization is granted a block of addresses with the beginning

address 14.24.74.0/24. The organization needs to have 3 subblocks

of addresses to use in its three subnets: one subblock of 10

addresses, one subblock of 60 addresses, and one subblock of 120

addresses. Design the subblocks

► There are 232– 24 = 256 addresses in this block

The first address is 14.24.74.0/24

The last address is 14.24.74.255/24

► To satisfy the third requirement, we assign addresses to subblocks,

starting with the largest and ending with the smallest one



► The number of addresses in the largest subblock, which

requires 120 addresses, is not a power of 2

We allocate 128 addresses

The subnet mask for this subnet can be found as n1= 32 − log2 128 = 25

The first address in this block is 14.24.74.0/25; the last address is

14.24.74.127/25

► The number of addresses in the second largest subblock,

which requires 60 addresses, is not a power of 2 either


The subnet mask for this subnet can be found as n2 = 32 − log2 64 = 26


14.24.74.191/26



► The number of addresses in the largest subblock, which requires 10

addresses, is not a power of 2


The subnet mask for this subnet can be found as n1= 32 − log2 16 = 28.


14.24.74.207/28

► If we add all addresses in the previous subblocks, the result is 208

addresses, which means 48 addresses are left in reserve.

The first address in this range is 14.24.74.208.

The last address is 14.24.74.255.

We don’t know about the prefix length yet. Figure 4.36 shows the

configuration of blocks. We have shown the first address in each block.



[ Figure 4.36: Solution to Example 5 ]

14.24.74.192/28


4.2 Network-Layer Protocol IPv4 address: Address aggregation

One of the advantages of the CIDR strategy is address aggregation.

When blocks of addresses are combined to create a larger block,

routing can be done based on prefix of the larger block

[ Fig 4.37 Example of address aggregation ]


4.2 Network-Layer Protocol IPv4 address: Special addresses

This-host address: It is used whenever a host needs to send an IP

datagram but it does not know its own address to use as the source

address: 0.0.0.0/32

Limited-broadcast address: when router want to send data to all

devices: 255.255.255.255/32

Loopback address: one of the addresses is always as a destination

address: 127.0.0.0/8

Private addresses:

► 10.0.0.0/8

► 172.16.0.0/12

► 192.168.0.0/16

► 169.254.0.0/16 We will see the application of these addresses when we

discuss NAT later

Multicast addresses: is reserved for multicast addresses: 224.0.0.0/4


Practice Problem

For the IP address 109.190.10.164/16,

► Find the First useable IP address

► Find the Broadcast address

► Find the Last useable IP address


4.2 Network-Layer Protocol IPv4 address: DHCP (1/8)

Dynamic Host Configuration Protocol (DHCP)

► A large organization or an ISP can receive a block of addresses directly

from ICANN (Internet Corporation for Assigned Names and Numbers)

► A small organization can receive a block of addresses from an ISP

► Address assignment in an organization can be done automatically using the

Dynamic Host Configuration Protocol (DHCP)

► Permanent or temporary IP addresses are assigned to hosts



DHCP Message Format

[ Fig 4.38 DHCP message format ]

[ Fig 4.39 Option format ]



The joining host creates a DHCPDISCOVER message where only the

transaction-ID field is set to a random number

The DHCP server or servers responds with a DHCPOFFER message in

which the your-IP-address field defines the offered IP address for the

joining host and the server-IP-address includes the IP address of the

server

The joining host receives one or more offers and selects the best of

them. The joining host then sends a DHCPREQUEST message to the

server that has given the best offer

The selected server responds with an DHCPACK message



DHCP Operation

[ Fig 4.40 Operation of DHCP ]



Video Content

► How a computer gets its IP address

► An explanation of DHCP and how it works.

► Link: https://www.youtube.com/watch?v=RUZohsAxPxQ

Automatic IP Address Assignment How DHCP Works - YouTube.mp4



DHCP Operation

https://www.youtube.com/watch?v=RUZohsAxPxQ

Automatic IP Address Assignment How DHCP Works - YouTube.mp4



Two Well-known Ports

► DHCP uses two well-known ports: 68 and 67

Using FTP

► The server does not send all information that a client may need for joining

the network

► In DHCPACK message, the server defines the pathname of a file in which

the client can find the complete information

Error Control

► DHCP uses the service of UDP, which is not reliable

► To provide error control, DHCP uses two strategies:

Requires that UDP uses the checksum

Requires DHCP client uses timers and a retransmission policy if it

does not receive DHCP reply to a request



To provide dynamic address allocation, the DHCP client acts as state

machine that performs transitions from one state to another

[ Fig 4.41 FSM for the DHCP client ]


4.2 Network-Layer Protocol IPv4 address: NAT (1/4)

Network Address Translation(NAT)

► This technology can provide the mapping between the private and universal

addresses and at the same time support virtual private networks

[ Figure 4.42: NAT ]



[ Figure 4.43: Address translation ]



[ Fig 4.44 Translation ]



Using both IP addresses and port addresses

► To allow a many-to-many relationship between private-network hosts and

external server programs

► The translation table includes source and destination port addresses and

the transport layer protocol

[ Table 4.1 Five-column translation table ]


4.2 Network-Layer Protocol Forwarding of IP Packets: Forwarding Techniques (1/4)

Next-Hop Method: the routing table holds only the address of the next

hop

Destination Next Hop

Host B R1


Host B R2


Host B -

Network Network Network R1 R2

Host A Host B

Destination Route

Host B R1, R2, Host B

Destination Route

Host B R2, Host

Destination Route

Host B Host B

Routing tables based on route

[ Routing tables based on next hop ]



Network-Specific Method: only one entry that defines the address of the

destination network itself

N1 N2

R1

S

A B C

D


A B C D

R1 R1 R1 R1


N2 R1

Routing table for host S based

on host-specific method

Routing table for host S based

on network-specific method



Host-Specific Method:

► The destination host address is given in the routing table

► Efficiency is sacrificed

N1


Host B N2 N3 …

R1 R1 R3 …

N2 N3

R1 R3

R2

Host A

Routing table for host A

Host B



Default Method

N1


N2 …

Default

R1 … R2

Host A

N2

R1

R2 Default

router

Routing table for host A

Rest of the Internet


4.2 Network-Layer Protocol Forwarding of IP Packets: with Classless Addressing (1/7)

One column for the mask is needed to find the network address in a

routing table

At least four columns are needed in a routing table

Address Aggregation

► To reduce the number of entries in a routing table

► In this method, the blocks of addresses for several organizations are

aggregated into one larger block

Extract destination

address

Search table

Packet

Forwarding module

Next-hop address

and

interface number

To ARP

Mask (/n)

Network address

Next-hop

address

Interface number

… …

… …

… …

… …

[ Fig 4.45 Simplified forwarding

module in classless address ]



Address Aggregation

140.24.7.0/26

140.24.7.64/26

140.24.7.128/26

140.24.7.192/26

Organization 1

Organization 2

Organization 3

Organization 4

m0 m4 m1

m2 m3

m0 m1

Mask Network address

Next-hop address

Interface

/26 /26 /26 /26 /0

140.24.7.0 140.24.7.64 140.24.7.128 140.24.7.192

0.0.0.0

- - - -

Default router

m0 m1 m2 m3 m4

Mask Network address

Next-hop address

Interface

/24 /0

140.24.7.0 0.0.0.0

- Default router

m0 m1

Routing table for R1

Routing table for R2

R1 R2

[ Fig 4.47 Address aggregation ]



Longest Mask Matching

► In case that some organizations are not geographically close to the others

► The routing table is sorted from the longest mask to the shortest mask



Longest Mask Matching

► Suppose a packet arrives at router R2 for organization 4 with destination address

140.24.7.200. The first mask at router R2 is applied, which gives the network address

140.24.7.192. The packet is routed correctly from interface m1 and reaches

organization 4

[ Fig 4.48 Longest mask matching ]



Hierarchical Routing

► If the routing table has a sense of hierarchy like the Internet architecture,

the routing table can decrease in size

► If a block assigned to the local ISP starts with a.b.c.d/n, the ISP can create

blocks starting with e.f.g.h/m, m is greater than n

► The rest of the Internet does not have to know this division


4.2 Network-Layer Protocol Forwarding of IP Packets: Forwarding with Classless

Addressing (6/7)


[ Fig 4.49 Hierarchical routing with ISPs ]



Geographical Routing

► Extension of hierarchical routing

Routing Table Search Algorithms

► Searching in Classful Addressing

The routing table can be divided into three tables for efficiency

The default mask is applied to find the corresponding bucket

► Searching in Classless Addressing

The longest match can be used

Instead of list, other data structure such as a tree or a binary tree can be used


4.2 Network-Layer Protocol Forwarding of IP Packets: based on label (1/3)

In a connection-oriented network, a switch forwards a packet based on

the label attached to the packet

[ Fig 4.51 Forwarding based on label ]



Multi-Protocol Label Switching (MPLS)

► When behaving like a router, MPLS can forward the packet based on the

destination address

► When behaving like a switch, it can forward a packet based on the label

► The MPLS header is actually a stack of sub headers that is used for

multilevel hierarchical switching

[ Fig 4.52 MPLS header added to an IP packet ]



Multi-Protocol Label Switching (MPLS)

► The MPLS header is actually a stack of subheaders that is used for

multilevel hierarchical switching

► Label: the label used to index the forwarding table in the router

► Exp: reserved

► S: if this is 1, the header is last on in the stack

► TTL: when it reaches zero, the packet is discarded

[ Fig 4.53 MPLS header made of a stack of labels ]


4.2 Network-Layer Protocol ICMPv4

The Internet Control Message Protocol (ICMP) has been designed to

compensate the following two deficiencies:

► The IP protocol has no error-reporting or error-correcting mechanism

► The IP protocol has no built-in mechanism to notify the original host

► ICMP itself is a network layer protocol

► ICMP messages are first encapsulated inside IP datagrams before going to

the lower layer

Frame header

Frame data Trailer

IP header IP data

ICMP message


4.2 Network-Layer Protocol ICMPv4: ICMP messages (1/6)

ICMP messages are first encapsulated inside IP datagrams before

going to the lower layer

Frame header

Frame data Trailer

IP header IP data

ICMP message



ICMP messages are divided into two broad categories

► The error-reporting messages report problems that a router or a

host(destination) may encounter when it processes an IP packet

► The query messages help a host or a network manager get specific

information from a router or another host

ICMP message

Error-reporting Query



ICMP messages

Category Type Message

Error-reporting messages

3 Destination unreachable

4 Source quench

11 Time exceeded

12 Parameter problem

5 Redirection

Query messages

8 or 0 Echo request or reply

13 or 14 Timestamp request or reply

17 or 18 Address mask request or reply

10 or 9 Router solicitation or advertisement



An ICMP message has an 8 bytes header and a variable size data

section

► In error-reporting messages, the data section carries information for finding

the original packet that had the error

► In query messages, the data section carries extra information based on the

type of the query

Type Code Checksum

Rest of header

Data section

8 bits 8 bits 8 bits 8 bits



Error-reporting messages

► ICMP always reports error messages to the original source

► No ICMP error messages will be generated in response to a datagram

carrying an ICMP error messages

► No ICMP error messages will be generated for a fragmented datagram that

is not the first fragment

► No ICMP error messages will be generated for a datagram having a

multicast address

► No ICMP error message will be generated for a datagram having a special

address such as 127.0.0.0 or 0.0.0.0

Error reports

Time exceeded

Source quench

Destination unreachable

Redirection Parameter problem


4.2 Network-Layer Protocol ICMPv4: Destination Unreachable

When a router cannot route a datagram or a host cannot deliver a

datagram, the datagram is discarded

The router or the host sends a destination-unreachable message

back to the source host

Type : 3 Code: 0 to 15 Checksum

Unused (All 0s)

Part of the received IP datagram including IP header plus the first 8 bytes of the datagram data


4.2 Network-Layer Protocol ICMPv4: Source Quench

The source-quench message in ICMP was designed to add a kind of

flow control to the IP

When a router or host discards a datagram due to congestion, it sends

a source-quench message to the sender of the datagram

► It informs the source that the datagram has been discarded

► It warns the source that there is congestion somewhere in the path and that

the source should slow down the sending process

One source-quench message is sent for each datagram that is

discarded due to congestion

Type : 4 Code : 0 Checksum

Unused (All 0s)



4.2 Network-Layer Protocol ICMPv4: Time Exceeded (1/3)

Whenever a router decrements a datagram with a time-to-live value to

zero, it discards the datagram and sends a time-exceeded message to

the original source (Code 0)

When the final destination does not receive all of the fragments in a set

time, it discards the received fragments and sends a time-exceeded

message to the original source (Code 1)

Type : 11 Code : 0 or 1 Checksum

Unused (All 0s)




The traceroute program can be used to trace the route of a packet from

the source to the destination

The traceroute program uses the ICMP messages and the TTL field in

the IP packet to find the route



traceroute

[ Fig 4.55 Example of traceroute program ]


4.2 Network-Layer Protocol ICMPv4: Parameter Problem

A parameter-problem message can be created by a router or the

destination host

► Code 0

There is an error ambiguity in one of the header fields

The value in the pointer field points to the byte with the problem

► Code 1

The required part of an option is missing

Type : 12 Code : 0 or 1 Checksum

Unused (All 0s)


Pointer


4.2 Network-Layer Protocol ICMPv4: Redirection (1/2)

A host usually starts with a small routing table that is gradually

augmented and updated

To update the routing table of the host, routers send a redirection

message to the host

LAN LAN

IP packet

IP packet IP packet

Redirection

message

R1 R2 B

A


4.2 Network-Layer Protocol ICMPv4: Redirection (2/2)

Code 0

► Redirection for a network-specific route

Code 1

► Redirection for a host-specific route

Code 2

► Redirection for a network-specific route based on a specified type of service

Code 3

► Redirection for a host-specific route based on a specified type of service

Type : 5 Code : 0 to 3 Checksum

IP address of the target router



4.2 Network-Layer Protocol ICMPv4: Query Message

In this type of ICMP message, a node sends a message that is

answered in a specific format by the destination node

Query

Timestamp request and reply

Address-mask request and reply

Echo request and reply

Router solicitation and advertisement


Practice Problem

How many DHCP packets are exchanged between a client and a server

before the client receives an IP address?


4.2 Network-Layer Protocol ICMPv4: Echo Request and Reply (1/2)

An echo-request message can be sent by a host or router

An echo-reply message is sent by the host or router which receives an

echo-request message

Echo-request and echo-reply messages can be used by network

managers to check the operation of the IP protocol

Echo-request and echo-reply messages can test the reachability of a

host

Type : 8 or 0 Code : 0 Checksum

Optional data Sent by the request message; repeated by the reply

message

Sequence number Identifier


4.2 Network-Layer Protocol ICMPv4: Echo Request and Reply (2/2)

The ping program is used to find if a host is alive and responding

► Ping approximate round trip times in milli-seconds

► The initial TTL packet value for an IP packet is 255 and then it is

decremented by 1 each time it encounters a router


4.2 Network-Layer Protocol ICMPv4: Timestamp Request and Reply

Hosts and routers can use the timestamp-request and timestamp-reply

messages to determine the round-trip time needed for an IP datagram

to travel between them

Each timestamp field can hold a number representing time measured in

milliseconds from midnight in Universal Time


Original timestamp


Receive timestamp

Transmit timestamp


4.2 Network-Layer Protocol ICMPv4: Address-Mask Request and Reply

To obtain its mask, a host sends an address-mask-request message

to a router on the LAN

The router receiving the address-mask-request message responds with

an address-mask-reply message

The address-mask field is filled with zeros in the request message


Address mask



4.3 Unicast routing

In an internet, the goal of the network layer is to deliver a datagram

from its source to its destination or destinations.

If a datagram is destined for only one destination (one-to-one delivery),

we have unicast routing.

In this section and the next, we discuss only unicast routing; multicast

and broadcast routing will be discussed later in the chapter.


4.3 Unicast routing General Idea: An Internet as a Graph

To find the best route, an internet can be modeled as a graph

We can think of each router as a node and each network between a

pair of routers as an edge

[ Fig 4.56 An internet and its graphical representation ]


4.3 Unicast routing General Idea: Least-cost routing

When an internet is modeled as a weighted graph, one of the ways to

interpret the best route from the source router to the destination router

is to find the least cost between the two

If there are N routers in an internet, there are (N-1) least-cost paths

from each router to any other router

We need N*(N-1) least-cost paths for the whole internet


4.3 Unicast routing General Idea: Least-cost-trees

A least-cost tree is a tree with the source router as the root that spans

the whole graph (visits all other nodes)

[ Fig 4.57 Least-cost trees for nodes in the internet ]


4.3 Unicast routing Routing Algorithm: Distance vector routing(1/9)

In distance vector routing, the least cost route between any two nodes

is the route with minimum distance

[ Fig 4.58 The distance vector corresponding to a tree ]



Bellman-Ford equation is used to find the least cost (shortest

distance) between a source node x, and a destination node y, through

some intermediary nodes (a, b, c,…)

[ Fig 4.59 Graphical idea behind Bellman-Ford equation ]



Initialization: each node can know only the distance between itself and

its immediate neighbors

[ Fig 4.60 The first distance vector for an internet ]



Sharing:

► A node is not aware of a neighbor’s table

► The best solution for each node is to send its entire table to the neighbor

and let the neighbor decide what part to use and what part to discard

► In distance vector routing, each node shares its routing table with its

immediate neighbors periodically and when there is a change



Updating:

[ Fig 4.61 Updating distance vectors ]



[ Table 4.4 Distance vector routing algorithm for a node ]



Two-Node Loop Instability: A problem with distance vector routing is

instability

X A B

X 2 - X 6 A

2 4 Before failure

After failure

After A receives

update from B

Finally

X

X ∞ X 6 A

2 4 B A

X

X 10 B X 6 A

2 4 B A

X

X ∞ X ∞

2 4 B A

[ Fig 4.62 Two-node instability ]



Solutions of two-node loop instability:

► Defining Infinity

Most implementations of the distance vector protocol define the distance

between each node to be 1 and define 16 as infinity

► Split Horizon

In this strategy, instead of flooding the table through each interface, each node

sends only part of its table through each interface

• Node B eliminates the last line of its forwarding table before it sends to A

► Poison Reverse

Node B can still advertise the value for X, but if the source information is A, it can

replace the distance with infinity as a warning: “Do not use this value; what I

know about this come from you”



X A B

C

X 2 - X 6 A

X 5 A

X A B

C

X ∞ - X 8 C

X 5 A

X A B

C

X 12 B X 8 C

X 5 A

X A B

C

X ∞ - X ∞

X 5 A

4

3 3

4

3 3

4

3 3

4

3 3

1. Before failure 2. After A sends the route to B

and C, but the packet to C is lost

3. After C sends the route to B 4. After B sends the route to A

Three-Node Instability


4.3 Unicast Routing Routing Algorithms: Link-State Routing (1/7)

Link-state (LS) routing is a routing algorithm for creating least-cost trees

and forwarding tables

Link-state database (LSDB)

► To create a least cost tree with this method, each node needs to have a

complete map of the network, which means it needs to know the state of

each link

► The collection of states for all links is called the link-state database.

[Fig 4.63 Example of a link-state database]



Building LSDB

► This can be done by a process called flooding.

1. Each node can send some greeting messages to all its immediate neighbors

(those nodes to which it is connected directly) to collect two pieces of

information for each neighboring node: the identity of the node and the cost of

the link.

2. The combination of these two pieces of information is called the LS packet

(LSP). LSP also includes a sequence number, which increases when a new

version of the LSP is created.

3. Each node creates the LSP and sends out of each interface.

• When a node receives an LSP, it compares the LSP with the copy it may already

have by checking the sequence number.

• Then, it discards the old LSP and keeps the new one and then sends a copy of it

out of each interface except the one from which the packet arrived.

4. After receiving all new LSPs, each node creates the comprehensive LSDB as

shown in Figure 4.64



Building LSDB

[Fig 4.64 LSPs created and sent out by each node to build LSDB]



Formation of Least-Cost Trees

► After receiving all LSPs, each node will have a copy of the whole topology

► A shortest path tree is needed

► The Dijkstra algorithm creates a shortest path tree from a graph


4.3 Unicast Routing Routing Algorithms: Link-state routing (5/7)

Dijkstra’s algorithm Start

Set root to local node and

move it to permanent list

Tentative list

is empty?

Among nodes in tentative list, move the one

with the shortest path to permanent list

Add each unprocessed neighbor of last moved

node to tentative list if not already there.

If neighbor is in the tentative list with larger

cumulative cost, replace it with new one

Stop

No

Yes



► Dijkstra’s algorithm

[Fig 4.65 Least-cost tree]



► Dijkstra’s algorithm

[Table 4.5 Dijkstra’s algorithm]


4.3 Unicast Routing Routing Algorithms: Path-Vector Routing (1/5)

Path Vector Routing

► Path vector routing is not based on least-cost routing

► Path vector routing is mostly designed to route a packet

between ISPs

Spanning trees

► The path from a source to all destinations is also determined

by the best spanning tree

► If there is more than one route to a destination, the source can choose the

route that meets its policy best

► A source may apply several policies at the same time. One of the common

policies uses the minimum number of nodes to be visited.



Spanning trees:

► Each source has created its own spanning tree that meets its policy

► The policy is to use the minimum number of nodes to reach a destination

[Fig 4.66 Spanning trees in path-vector routing]



Creation of spanning trees

► When a node is booted, it creates a path vector based on the information it

can obtain about its immediate neighbor

► A node sends greeting messages to its immediate neighbors to collect

these pieces of information

► Each node, after the creation of the initial path vector, sends it to all its

immediate neighbors.

► Each node, when it receives a path vector from a neighbor, updates its path

vector the equation:

Path(x, y) = best {path(x, y), [(x+path(v,y)]} for all v’s in the internet



Creation of spanning trees

[ Fig 4.67 path vectors made at booting time ]

[ Fig 4.68 Updating path vectors ]



Path-vector algorithm

[ Table 4.6 Path-vector algorithm for a node ]


Practice Problem

Dijkstra shortest path algorithm example

A

D E

B C

D

1


Practice Problem

What is the significance of name “Distance vector routing”


4.3 Unicast Routing Unicast Routing Protocols: Introduction

Introduction

► Three common protocols used in the Internet are introduced

Routing Information Protocol (RIP): based on the distance-vector

algorithm

Open Shortest Path First (OSPF): based on the link-state algorithm

Border Gateway Protocol (BGP): based on the path-vector algorithm


4.3 Unicast Routing Unicast Routing Protocols: Internet structure(1/2)

Internet structure

► Backbones: provide global connectivity

► Provider network: at a lower level, use the backbones for global connectivity

► Customer networks: use the services provided by the provider networks

► Any of these three entities can be called an Internet Service Provider or ISP,

but at different level

[ Fig 4.69 Internet structure ]


4.3 Unicast Routing Unicast Routing Protocols: Internet structure(2/2)


► Hierarchical routing means considering each ISP as an

autonomous system (AS).

► Routing inside an AS is referred to as intradomain routing

► Global routing to glue all ASs together is referred to as

interdomain routing

Autonomous

system

Autonomous

system

Autonomous

system

Autonomous

system


4.3 Unicast Routing Unicast Routing Protocols: RIP (1/10)

RIP

► The Routing Information Protocol (RIP) is an intradomain routing

protocol used inside an autonomous system

Hop Count

► RIP basically implements distance vector routing algorithm with some

considerations

The cost is defined as the number of hops, which means the

number of networks (subnets) a packet needs to travel through

from the source router to the final destination host

In RIP, the maximumcost of a path can be 15, which means

16 is considered as infinity (no connection)



Forwarding Tables

3 hops (N2, N3, N4)

2 hops (N3, N4)

1 hop (N4)

[Fig 4.70 Hop counts in RIP]

[Fig 4.71 Forwarding tables]



RIP Implementation

► RIP runs at the application layer, but creates forwarding tables for

IP at the network layer

► RIP uses UDP port 520 for route updates

► RIP has two versions RIP-1 and RIP-2. We discuss only RIP-2.



RIP messages

► RIP has two types of messages: request and response

Command Version Reserved

Tag Family

Network address

Subnet mask

Next-hop address

Distance

Repeate

d

[Fig 4.72 RIP message format]



RIP messages

► Request

A request message is sent by a router that has just come up or by a router that

has some time-out entries

► Response (or update)

A response can be either solicited or unsolicited

A solicited response is sent only in answer to a request

An unsolicited response is sent periodically, every 30s or when

there is a change in the routing table



RIP algorithm

► RIP implements the same algorithm as the distance-vector routing

algorithm, but has some changes

A router needs to send the whole contents of its forwarding table in a response

message

The receiver adds one hop to each cost and changes the next router field to the

address of the sending router. Each route in the modified forwarding table is

called the received route and each route in the old forwarding table is called the

old route. The received router selects the old routes as the new ones except in

the following three cases:



1. If the received route does not exist in the old forwarding table, it should be

added to the route.

2. If the cost of the received route is lower than the cost of the old one, the

received route should be selected as the new one.

3. If the cost of the received route is higher than the cost of the old

one, but the value of the next router is the same in both routes, the received

route should be selected as the new one.

• This is the case where the route was actually advertised by the same router in the

past, but now the situation has been changed.

• For example, suppose a neighbor has previously advertised a route to a destination

with cost 3, but now there is no path between this neighbor and that destination. The

neighbor advertises this destination with cost value infinity (16 in RIP)



RIP algorithm

[ Fig 4.73 Example of an autonomous system using RIP ]



Timer in RIP

► Periodic Timer

It controls the advertising of regular update messages

The working model uses a random number between 25 and 35s

It counts down; when zero is reached, the update message is sent

Timers

Expiration 180 s

Periodic 25~35 s

Garbage collection 120 s



Timer in RIP (cont’d)

► Expiration Timer

It governs the validity of a route

If there is a problem on an internet and no update is received within the allotted

180s, the route is considered expired and the hop count of the route is set to 16

► Garbage Collection Timer

When the information about a route becomes invalid, the router does not

immediately purge that route from its table

It continues to advertise the route with a metric value of 16

At the same time, a timer called the garbage collection timer is set to 120s

When the count reaches zero, the route is purged


4.3 Unicast Routing Unicast Routing Protocols: OSPF(1/15)

OSPF (Open Shortest Path First)

► OSPF is an intradomain routing protocol like RIP

► It is based on the link-state routing protocol

Metric

► Each link can be assigned a weight based on the throughput, RTT (round

trip time), reliability, hop count, and so on.

Total cost: 12

Total cost: 7

Total cost: 4

[ Fig 4.74 Metric in OSPF ]



Forwarding tables

► Each OSPF router can create a forwarding table after finding the shortest-

path tree between itself and the destination using Dijkstra’s algorithm

[ Fig 4.75 Forwarding tables in OSPF ]



Areas

► OSPF divides an autonomous system into areas

► An area is a collection of networks, hosts, and routers all contained within

an autonomous system

► Area border routers summarize the information about the area and send it

to other areas

► All of the areas inside an autonomous system must be connected to a

special area called backbone

► If the connectivity between a backbone and an area is broken, a virtual link

between routers must be created



Areas

[ Fig 4.76 Areas in an autonomous system ]



Link-state advertisement

► Router link

A router link advertises the existence of a router as a node

A point-to-point link connects two routers without any other host or router in

between

A transient link is a network with several routers attached to it

A stub link is a network that is connected to only one router

[ Fig 4.77 Router link]




► Network link

A network link advertises the network as a node

Since a network cannot do announcements itself, one of the routers is assigned

as the designated router and does the advertising

[ Fig 4.78 Network link]




► Summary link to network

This is done by an area border router

It advertises the summary of links collected by the backbone to an area or the

summary of links collected by the area to the backbone

► Summary link to AS

This is done by an AS router

It advertises the summary links from other AS to the backbone area

► External link

This is done by an AS router to announce the existence of a single network

outside the AS




[ Fig 4.79 Five different LSPs ]



OSPF Implementation

► OSPF is implemented as a program in the network layer that uses

the service of the IP for propagation.

► An IP datagram that carries a message from OSPF sets the value of the

protocol field to 89.

► There are two versions: version 1 and version 2. Most

implementations use version 2.



OSPF Messages

► Two message headers used in OSPF:

OSPF common header: is used in all messages.

The link-state general header: is used in some messages.

► Five message types used in OSPF:

Hello message: Used by a router to introduce itself to the neighbors and

announces all neighbors that it already knows

Database description message: Sent in response to the hello message to allow a

newly joined router to acquire the full LSDB

Link-state request message: Sent by a router that needs information about a

specific link state

Link-state update message: Used for building the LSDB

Link-state acknowledgment: Used to create reliability in OSPF



OSPF Messages

► Hello message

Used by a router to introduce itself to the neighbors and announces all neighbors

that it already knows

► Database description message

Sent in response to the hello message to allow a newly joined router to acquire

the full LSDB

► Link-state request message

Sent by a router that needs information about a specific link state



OSPF Messages

► Link-state update message

Used for building the LSDB

► Link-state acknowledgment

Used to create reliability in OSPF



OSPF Messages

[ Fig 4.80 OSPF message formats ]



OSPF Messages

[ Fig 4.81 OSPF message formats ]



OSPF Algorithm

► OSPF implements the link-state routing algorithm with some considerations:

After each router has created the shortest-path tree, the algorithm

needs to use it to create the corresponding routing algorithm

The algorithm needs to be augmented to handle sending and

receiving all five types of messages


4.3 Unicast Routing Unicast Routing Protocols: Practice Problem

What is the difference between OSPF and RIP?


4.3 Unicast Routing Unicast Routing Protocols: BGP4 (1/13)

BGP4: Border Gateway Protocol Version 4

► is the only interdomain routing protocol used in the Internet today

► BGP4 is based on the path-vector algorithm

► To enable each router to route a packet to any network in the internet,

external BGP4 (eBGP) is installed on each border router

► internal BGP (iBGP) is installed on all routers

[ Fig 4.82 A sample internet with four ASs ]



Operation of external BGP

► Two routers, which eBGP is installed on, try to create a TCP connection

using the well-known port 179 as eBGP session.

The circled number defines the sending router in each case. For example,

message number 1 is sent by router R1 and tells router R5 that N1, N2, N3,and

N4 can be reached through router R1.

[ Fig 4.83 eBGP operation ]



Operation of internal BGP

► It also uses the service of TCP on the well-known port 179, but it creates a

session between any possible pair of routers inside an autonomous system

► An AS router combines the reachability information about another AS with

the reachability information it already knows about the AS it belongs to

► Then the AS router sends a new update message to another AS

► In this way, all the network devices can transmit data to another in another

AS



► The first message (numbered1) is sent by R1 announcing that networks N8 and N9

are reachable through the path AS1-AS2 to R2, R3, and R4. Routers R2, R4, and

R6 do the same thing, like R1, but send different messages to different destinations.

► When R1 receives the update message from R2, it combines the reachability

information about AS3 with the reachability information it already knows about AS1

and sends a new update message to R5. The process continues when R1 receives

the update message from R4.

[ Fig 4.84 Combination of eBGP and iBGP sessions in our Internet ]



Operation of internal BGP

[ Fig 4.85 Finalized BGP path tables]



Injection of Information into Intradomain Routing

► The role of BGP is to help the routers inside the AS to augment their

routing information. In other words, it is injected into intradomain

forwarding tables (RIP or OSPF). Fig 4.83 assumes that all ASs are using

RIP as the intradomain routing protocol.

[ Fig 4.86 Forwarding tables after injection from BGP]



Path attributes

► Interdomain routing needs more information about how to reach the final

destination compared to intradomain routing

► These pieces(information) are called path attributes

[ Fig 4.87 Format of path attribute ]



Path attributes

► ORIGIN (type 1)

A well-known mandatory attribute

Defines the source of the routing information

(1: RIP or OSPF, 2:BGP, 3: unknown source)

► AS-PATH (type 2)


Defines the list of autonomous systems through which the destination can be

reached

► NEXT-HOP


Defines the next router to which the data packet should be forwarded



Path attributes

► MULTI-EXIT-DISC (type 4)

An optional non-transitive attribute

Discriminates among multiple exit paths to a destination

► LOCAL-PREF (type 5)

A well-known discretionary attribute

It is normally set by the administrator, based on the organization policy

The routes the administrator prefers are given a higher local preference



Path attributes

► ATOMIC-AGGREGATE (type 6)

A well-known discretionary attribute

Defines the destination prefix; only single destination network

► AGGREGATOR (type7)

Optional transitive attribute

Emphasizes that the destination prefix is an aggregate



Route Selection

► In the case where multiple routes are received to a destination, BGP needs

to select one among them.

[ Fig 4.88 Flow diagram for route selection]



Messages

► Open message

To create a neighborhood relationship, a router running BGP opens a

TCP connection with a neighbor and sends an open message

► Update message

It is used by a router to withdraw destinations that have been advertised

previously, to announce a route to a new destination

► Keepalive message

BGP peers exchange keepalive messages regularly to tell each other they are

alive

► Notification

is sent when an error condition is detected or a router wants to close session



Messages

[ Fig 4.89 BGP Messages]


4.4 Multicasting Routing Introduction: Unicasting

Unicasting

► There is one source and one destination network

[ Fig 4.90 Unicasting ]


4.4 Multicasting Routing Introduction: Multicasting (1/2)

Multicasting

► There is one source and a group of destinations

[ Fig 4.91 Multicasting ]


4.4 Multicasting Routing Introduction: Multicasting (2/2)

Multicasting versus multiple unicasting

► Multicasting starts with a single packet from the source that is duplicated by

the routers

► In multiple unicasting, several packets start from the source

[ Fig 4.92 Multicasting versus multiple unicasting ]


4.4 Multicasting Routing Introduction: Multicast Applications

Multicast applications

► Access to distributed databases

Most of the large databases today are distributed. A user’s request is multicast to

all the database locations, and the location that has the information responds

► Information dissemination

For example, a software update can be sent to all purchasers of a particular

software package. News can be easily disseminated through multicasting

► Teleconferencing

The individuals attending a teleconference all need to receive the same

information at the same time.

► Distance learning

Lessons taught by one professor can be received by a specific group of

students.


4.4 Multicasting Routing Introduction: Broadcasting

Broadcasting

► One-to-all communication

Broadcasting means one-to-all communication: A host sends a packet to all

hosts in an internet

Broadcasting in this sense is not provided at the Internet level for the obvious

reason that it may create a huge volume of traffic and use a huge amount of

bandwidth

Partial broadcasting, however, is done in the Internet. For example, some peer-

to-peer applications may use broadcasting to access all peers.


4.4 Multicasting Routing Multicast Basics: Multicast Addresses (1/3)

Multicast addresses

► In multicast communication, the sender is only one, but the receiver is many

► For this reason, we need multicast addresses

► A multicast address defines a group of recipients

[ Fig 4.93 Needs for multicast addresses ]



Multicast addresses in IPv4

► A router or a destination host needs to distinguish between a unicast and a

multicast datagram. IPv4 assigns a block of addresses for this purpose.

► In addressing, all of class D was used for multicast addresses as the block

224.0.0.0/4 (from 224.0.0.0 to 239.255.255.255).

[ Fig 4.94 A multicast address in binary ]



Multicast addresses in IPv4

► The multicast block (class D) is divided into several common subblocks

Local network control block (224.0.0.0/24)

• Assigned to a multicast routing protocol to be used inside a network

Internetwork control block (224.0.1.0/24)

• Assigned to a multicast routing protocol to used in the whole Internet

Source-specific multicast block (232.0.0.0/8)

• It is used for source specific multicast routing in IGMP protocol

GLOP Block (233.0.0.0/8)

• This block defines a range of addresses that can be used inside an autonomous

system (AS)

Administratively scoped block (239.0.0.0/8)

• The addresses in this block are used in a particular area of the Internet


4.4 Multicasting Routing Multicast Basics: Collecting groups information(1/3)

IGMP: Internet Group Management Protocol

► The protocol that is used today for collecting information about group

membership

► IGMP is a protocol defined at the network layer

► IGMP messages are encapsulated in an IP datagram

► There are only two types of messages in IGMP version3

Query and report messages

[ Fig 4.95 IGMP operation ]



IGMP

► Query message:

A query message is periodically sent by a router to all hosts attached to it to ask

them to report their interests about membership in groups. It can take one of

three forms:

1. A general query message is sent about membership in any group

• It is encapsulated in a datagram with the destination address 224.0.0.1

• All routers attached to the same network receive this message to inform them that this

message is already sent and that they should refrain from resending it

2. A group-specific query message is sent from a router to ask about the

membership related to a specific group

• This is sent when a router does not receive a response about a specific group and

wants to be sure that there is no active member of that group in the network



IGMP

► Query message

3. A source-and-group-specific query message is sent from a router to ask about

the membership related to a specific group when the message comes from a

specific source or sources

► Report message

A report message is sent by a host as a response to a query message

The message contains a list of records in which each record gives the identifier

of the corresponding group and the addresses of all sources that the host is

interested in receiving messages from

The message is encapsulated in a datagram with the multicast address

224.0.0.22


4.4 Multicasting Routing Multicast Basics: Multicast forwarding (1/2)

Multicast forwarding

► In unicast communication, the destination address of the packet defines one

single destination

► In multicast communication, the destination of the packet defines one

group, but that group may have more than one member in the internet

[ Fig 4.96 Destination in unicasting and multicasting ]


4.4 Multicasting Routing Multicast Basics: Multicast forwarding (2/2)

Multicast forwarding

► Forwarding in unicast communication depends only on the destination

address of the packet

► Forwarding decisions in multicast communication depend on both the

destination and the source address of the packet

Forwarding is based on where the packet should go and where the packet has

come from

[ Fig 4.97 Forwarding depends on the destination and the source ]


Practice Problem

Difference between multicast and unicast forwarding


4.4 Multicasting Routing Multicast Basics: Two approaches to multicasting

Two approaches to multicasting

► Source-based tree: one tree per source

shortest path trees

reverse path forwarding

► Group-shared tree: group uses one tree

minimal spanning (Steiner)

center-based trees


4.4 Multicasting Routing Intradomain Routing Protocol: DVMRP (1/3)

Multicast Distance Vector (Distance Vector Multicast Routing Protocol -

DVMRP) is the extension of the RIP which is used in unicast routing. It

uses the source-based tree approach to multicasting in three steps:

► The router uses an algorithm called Reverse Path Forwarding (RPF)

► The router uses an algorithm called Reverse Path Broadcasting (RPB)

► The router uses an algorithm called Reverse Path Multicasting (RPM)


4.4 Multicasting Routing Intradomain Routing Protocol: DVMRP (2/3)

Reverse Path Forwarding (RPF)

► It relies on router’s knowledge of unicast shortest path from the source

to the router

Reverse Path Broadcasting (RPB)

► RPB actually creates a broadcast tree from the graph that has been created

by the RPF algorithm

[ Fig 4.98 RPF versus RPB ]


4.4 Multicasting Routing Intradomain Routing Protocol: MOSPF (1/2)

Multicast Link State (Multicast Open Shortest Path First -MOSPF)

► It is the extension of the OSPF protocol, which is used in unicasting routing

The router uses the Dijkstra algorithm to create a shortest-path tree with S as the

root and all destination in the internet as the leaves

The router finds itself in the shortest-path tree created in the first step

The shortest-path subtree is actually a broadcast subtree with the router as the

root and all networks as the leaves

The router can forward the received packet out of only those interfaces that

correspond to the branches of the multicast tree


4.4 Multicasting Routing Intradomain Routing Protocol: MOSPF (2/2)

Multicast Open Shortest Path First (MOSPF)

[ Fig 4.99 Example of tree formation in MOSPF ]


4.4 Multicasting Routing Intradomain Routing Protocol: PIM (1/4)

Protocol Independent Multicast (PIM)

► Not dependent on any specific underlying unicast routing algorithm (works

with all)

► Two different multicast distribution scenarios

Protocol Independent Multicast-Dense Mode (PIM-DM)

• Group members densely packed, in “close” proximity.

• Bandwidth more plentiful

Protocol Independent Multicast-Sparse Mode (PIM-SM)

• Group members “widely dispersed”

• Bandwidth not plentiful




► PIM-DM: The first packet is broadcast to all networks, which have or do not

have members. After a prune message arrives from a router with no

member, the second packet is only multicast.

[ Fig 4.100 Idea behind PIM-DM ]




► PIM-SM: The core router in PIM-SM is called the rendezvous point (RP).

Multicast communication is achieved in two steps:

Any router that has a multicast packet to send to a group of

destinations first encapsulates the multicast packet in a unicast packet

(tunneling) and sends it to the RP.

The RP then decapsulates the unicast packet and sends the multicastpacket to

its destination.




[ Fig 4.101 Idea behind PIM-SM ]


4.5 Internet Protocol version 6 Introduction: Internet Protocol version 6 (IPv6)

An IPv6 address is 128 bits long. Compared with the 32-bit address of

IPv4, this is a huge (296 times) increase in the address space

The main changes in the IPv6 protocol:

► Larger address space.

► Better header format: IPv6 uses a new header format in which options are

separated from the base header and inserted. This simplifies and speeds up

the routing process because most of the options do not need to be checked

by routers

► New options: IPv6 has new options to allow for additional functionalities

► Allowance for extension: to allow the extension of the protocol if required by

new technologies or applications

► Support for resource allocation: two new fields, traffic class and flow label,

have been added to enable the source to request special handling of the

packet. E.g. support traffic such as real-time audio and video

► Support for more security: providing the encryption and authentication options


4.5 Internet Protocol version 6 Introduction: Video (1/2)

Video Content

► When the Internet was launched in 1983, no one ever dreamed that there

might be billions of devices and users trying to get online

► But like a telephone network that is running out of phone numbers, the

current Internet is running out of IP addresses

► And if we don't roll out Internet Protocol v6 (IPv6), we won't have the room

we need to grow and the Internet would become tangled, unsafe and

unsustainable

► Link: https://www.youtube.com/watch?v=-Uwjt32NvVA

https://www.youtube.com/watch?v=-Uwjt32NvVA




4.5 Internet Protocol version 6 Introduction: Video (1/2)


ipv6.mp4

ipv6.mp4

ipv6.mp4


4.5 Internet Protocol version 6 Packet Format (1/4)

[ Fig 4.101 IPv6 datagram ]



Version: For IPv6, the value is 6

Traffic class: Identify priority among datagrams in flow. It replaces the

type-of-service field in IPv4.

Flow Label: Identify datagrams in same “flow”

Payload length: 2-byte payload length field defines the length of the IP

datagram excluding the header

Next header: Identify upper layer protocol or the type of first

extensionheader

Hop limit: The 8-bit hop limit field serves the same purpose as the TTL

field in IPv4

Source and destination address: These are 16-byte field which defines

Internet address of the source and destination

Payload: the payload field in IPv6 has a different format and meaning



Payload: the payload field in IPv6 has a different format and meaning

[ Fig 4.102 Payload in an IPv6 datagram ]



Concept of flow and priority in IPv6

► In IPv6, the flow label has been directly added to the format of the IPv6

datagram to allow us to use IPv6 as a connection-oriented protocol

Fragmentation and reassembly

► IPv6 datagrams can be fragmented only by the source, not by the routers

► The reassembly takes place at the destination

Extension headers

► Extension headers play a necessary and important part in IPv6. In

particular, three extension headers - fragmentation, authentication, and

extended security payload - are present in some packets.


4.5 Internet Protocol version 6 IPv6 Addressing

An IPv6 address is 128 bits or 16 bytes long, four times the address

length in IPv4

Binary : 1111111011110110 ……... 1111111100000000

Colon hexadecimal : FEF6:BA98:7654:3210:ADEF:BBFF:2922:FF00

The leading zeros of a section can be omitted

0074 74, 000F F

Zero compression

FDEC:0:0:0:0:BBFF:0:FFFF FEDC::BBFF:0:FFFF


4.5 Internet Protocol version 6 IPv6 Addressing

IPv6 uses hierarchical addressing

IPv6 allows slash or CIDR notation

Address space

► The address space of IPv6 contains 2128 addresses

► This address space is 296 times the IPv4 address

Three address types

► Unicast

► Anycast

► Multicast


4.5 Internet Protocol version 6 IPv6 Addressing: Address space allocation(1/3)

The address space of IPv6 is divided into several blocks of varying size

and block is allocated for a special purpose

[ Table 4.7 Prefixes for assigned IPv6 addresses ]



Global unicast addresses

[ Fig 4.103 Global unicast address ]



The block in global unicast addresses is used for unicast

communication between two hosts in the Internet

The global routing prefix is used to route the packet through the Internet

to the organization site, such as the ISP that owns the block

The last q bits define the interface identifier


4.5 Internet Protocol version 6 IPv6 Addressing: Special addresses (1/4)

The unspecified address is a subblock containing only one single

address, which is used during bootstrap when a host does not know its

own address and wants to send inquiry to find it

The loopback address consists of one single address



A compatible address is used when a computer using IPv6 wants to

send a message to another computer using IPv6

A mapped address is used when a computer already migrated to

version 6 wants to send an address to a computer still using version 4



Other Assigned Blocks

► IPv6 uses two large blocks for private addressing and one large block for

multicasting

► Unique local block

The packet carrying this type of address as the destination address is not

expected to be routed

► Link local block

It is designed for private addresses



Other Assigned Blocks

► Multicast block

It is used to define a group of hosts instead of just one


4.5 Internet Protocol version 6 Transition from IPv4 to IPv6: Dual stack

It is recommended that all hosts before migrating completely to version

6, have a dual stack of protocols during the transition

[ Fig 4.106 Dual stack ]


4.5 Internet Protocol version 6 Transition from IPv4 to IPv6: Tunneling

Tunneling is a strategy used when two computers using IPv6 want to

communicate with each other and the packet must pass through a

region that used IPv4

[ Fig 4.107 Tunneling strategy ]


4.5 Internet Protocol version 6 Transition from IPv4 to IPv6: Header Translation

Header translation is necessary when the majority of the Internet has

moved to IPv6 but some systems still use IPv4

[ Fig 4.108 Header translation strategy ]


Practice Problem

Why we are moving to IPv6


4.5 Internet Protocol version 6 ICMPv6

ICMPv6 (Internet Control Message Protocol version 6)

► This new version follows the same strategy and purposes of version 4

► It is more complicated than ICMPv4

Some protocols that were independent in version 4 are now part of ICMPv6

Some new messages have been added to make it more useful

[ Fig 4.109 Comparison of network layer in version 4 and version 6 ]


4.5 Internet Protocol version 6 ICMPv6: Types of ICMPv6 (1/2)

[ Fig 4.110 ICMPv6 messages (part 1) ]


[ Fig 4.110 ICMPv6 messages (part 2) ]

4.5 Internet Protocol version 6 ICMPv6: Types of ICMPv6 (2/2)

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