Module A Panko and Panko Business Data Networks and Telecommunications, 8 th Edition © 2011 Pearson Education, Inc. Publishing as Prentice Hall

Module A

Panko and PankoBusiness Data Networks and Telecommunications, 8th Edition© 2011 Pearson Education, Inc. Publishing as Prentice Hall

This module presents additional material about TCP/IP standards.

Most of the material in this module can be read after Chapter 2, but some of it is designed to be covered after Chapter 10.

The material in this module is not designed to be read front-to-back like a regular chapter, although it can be covered this way.

2© 2011 Pearson Education, Inc. Publishing as Prentice Hall


IP packets can carry different things in their data fields.◦ TCP segments

◦ UDP datagrams

◦ ICMP supervisory messages (later)

◦ RIP messages (later)

4

IP Data Field IP Header

© 2011 Pearson Education, Inc. Publishing as Prentice Hall

We say that IP can multiplex (mix) different types of traffic in a stream of IP packets.

5

UDP IP-H TCP IP-H UDP IP-H ICMP IP-H

Stream of Arriving or Outgoing IP Packets

Single IP PacketCarrying UDP Datagram


The IP process must pass contents of arriving IP packets to the correct process for subsequent handling.

6

IP

TCP UDP

ICMPUDP IP-H

IP ProcessArrivingPackets


IP process must also accept messages from multiple processes and multiplex them on an outgoing stream.

7

IP

TCP UDP

ICMPUDPIP-H

IP ProcessOutgoingPackets


Need a way for receiving IP process to know what is in the data field

◦ So it can pass the contents to the appropriate process

8

IP Data Field IP Header


IP Header has an 8-bit Protocol field.

◦ Identifies the contents of the data field

1=ICMP, 8=TCP, 17=UDP, and so on

9

Total Length in Bytes (16)

Time to Live (8)

Version(4)

Hdr Len(4) TOS (8)

Indication (16 bits) Flags (3) Fragment Offset (13)

Source IP Address

Destination IP Address

Header Checksum (16)Protocol (8)


Other Messages Have Analogous Fields◦ Identify contents of data field

TCP and UDP◦ Have Port number fields◦ Identify the application process (80=HTTP)

10

Source Port # (16) Destination Port # (16)

Sequence Number (32 bits)

Acknowledgement Number (32 bits)

Hdr Len(4) Flags (6) Window Size (16)Reserved (6)


Other Messages Have Analogous Fields◦ Identify contents of data field

PPP◦ Protocol field identifies contents of

information field as IP, IPX, a supervisory message, and so on.

11

Flag Addr Ctrl Prot Info CRC Flag



TCP is Reliable.

◦ IP packets carrying TCP segments may arrive out of order.

◦ TCP must put the TCP segments in order.

13

3 4 2 15


TCP is Reliable.

◦ Each correct TCP segment is acknowledged by the receiver.

14

SourceTransportProcess

SourceTransportProcess

DestinationTransportProcess

DestinationTransportProcess

TCP SegmentTCP Segment

ACKACK


Each TCP segment sent by a side must have a sequence number.

◦ Simplest: 1,2,3,4,5,6,7

◦ To detect lost or out-of-sequence messages

◦ TCP uses a more complex approach

15

11 44 22 55

3?


TCP header has a 32-bit sequence number field.

16




Hdr Len(4) Flags (6) Window Size (16)

Options (if any) PAD

Reserved (6)

TCP Checksum (16) Urgent Pointer (16)

Data Field


Initial Sequence Number is randomly selected by the sender; say, 79.

Sent in the sequence number field of the first TCP segment.

17

79

TCP Data Field

TCP Header

Sequence Number Fieldwith Initial Sequence Number (79)


Data octets in data fields of all segments in a connection are viewed as a long string.

TCP Segment 1 79 TCP Segment 2 80

8182

TCP Segment 3 8384

18

3 Octets in Data Field

2 Octets in Data Field

ISN


Supervisory segments, which contain a header but no data, are treated as carrying a single octet of data.

TCP seg 1 898899

TCP seg 2 900 TCP seg 3 901

902…

19

Supervisory Segment

Carries Data

Carries Data


Sequence number field gets the value of the first octet in the data field.

TCP 1 79 TCP 2 80

8182

TCP 3 8384

20

80 is SeqNum Field Value




Acknowledgement must indicate which TCP segment is being acknowledged.

21

SourceTCP

Process

SourceTCP

Process

DestinationTCP

Process

DestinationTCP

Process

TCP SegmentTCP Segment

ACKACK


TCP header contains a 32-bit Acknowledgement Number field to designate the TCP segment being acknowledged.

22



Acknowledgement Number (32 bits)Hdr Len

(4) Flags (6) Window Size (16)


Reserved (6)


Data Field


Acknowledgement Number field contains the next byte expected—the last byte of the segment being acknowledged, plus one.

TCP 1 79

TCP 2 808182

TCP 3 8384

23

83 is AckNum Field Value




Quiz: A TCP segment contains the following data octets:◦ 567, 568, 569, 570, 571, 572, 573, 574

What will be in the sequence number field of the TCP segment delivering the data?

What will be in the acknowledgement number field of the TCP segment acknowledging the TCP segment that delivers these octets?


Flow Control

◦ One TCP process transmits too fast.

◦ Other TCP process is overwhelmed.

◦ Receiver must control transmission rate.

◦ This is flow control.

25

TCP Process TCP Process

Too MuchData

Flow Control Message


A TCP segment has a Window Size field.◦ Used in acknowledgements

26




Hdr Len(4) Flags (6) Window Size (16)


Reserved (6)


Data Field


A TCP segment has a Window Size field.◦ Tell how many more octets the sender can send

beyond the segment being acknowledged

27

TCP Process TCP Process

Data

Acknowledgement with Window Size Field


Example

◦ TCP segment contained octets 45–89

◦ Acknowledgement number for TCP segment acknowledging the segment is 90

◦ If Window Size field value is 50, then

◦ Sender may send through octet 140

◦ Must then stop unless the window has been extended in another acknowledgement


Each Acknowledgement extends the window of octets that may be sent.◦ Called a sliding window protocol

29

1–44 45–79 80–419 420–630

400May send through 480

1–44 45–79 80–419 420–630

500May send through 920


TCP Segments have maximum data field sizes.◦ (Size limit details are discussed later.)◦ What if an application layer message is too large?

30

TCP HeaderTCP Data Field Max

Application Layer Message


Application layer message must be fragmented.◦ Broken into several pieces◦ Delivered in separate TCP segments

31


App Frag 1 App Frag 2 App Frag 3


Note that, in TCP fragmentation, the TCP segment is not fragmented.◦ The application layer message is fragmented.

32


App Frag 1 App Frag 2 App Frag 3


Transport layer process on the source host does the fragmentation.◦ Application layer on the source host is not

involved

◦ Transparent to the application layer

33

Application

Transport

Internet

Application Message

TCP Segment TCP Segment


Transport layer process on the destination host does the reassembly.◦ Application layer on the destination host is

not involved; gets original application layer message

34

Application

Transport

Internet

Application Message

TCP Segment TCP Segment


What is the maximum TCP data field size?◦ Complex

Maximum Segment Size (MSS)◦ Maximum size of a TCP segment’s data field

◦ NOT maximum size of the segment as its name would suggest!!!


MSS Default is 536 octets.

◦ Maximum IP packet size any network must support is 576 octets. Larger IP packets MAY be fragmented

◦ IP and TCP headers are 20 octets each if there are no options.

◦ This gives the default MSS of 536.

◦ Smaller if there are options in the IP or TCP header.


MSS Default is 536 octets.

◦ Suppose the application layer process is 1,000 octets long.

◦ Two TCP segments will be needed to send the data.

◦ The first can send the first 536 octets.

◦ The second can carry the remaining 464 octets of the application layer message.


Each side may announce a larger MSS.

◦ An option usually used in the initial SYN message it sends to the other.

◦ If announces MSS of 2,048, this many octets of data may be sent in each TCP segment.

◦ 536 is only the default—the value to use if no other value is specified by the other side.



Masks were introduced in Chapter 9. IP addresses alone do not tell you the size

of their network or subnet parts. Network Mask

◦ Has 1s in the network part◦ Has 0s in the remaining bits

Subnet Mask◦ Has 1s in the network plus subnet parts◦ Has 0s in the remaining bits


Based on Logical AND◦ Both must be true for the result to be true

Example◦ 1010101010 Data

◦ 1111100000 Mask

◦ 1010100000 Result


Based on Logical AND◦ If mask bit is 1, get back original data◦ If mask bit is 0, bet back zero

Example◦ 1010101010 Data

◦ 1111100000 Mask

◦ 1010100000 Result


IP packet arrives at a router◦ Router sees destination IP address◦ 11111111 01000000 10101010 00000000

Compares to each router forwarding table row◦ Address Part in First Entry

◦ 11111111 01000000 00000000 00000000

◦ Mask in First Entry

◦ 11111111 11100000 00000000 00000000


Mask the IP destination Address◦ 11111111 01000000 10101010 00000000 (IP address)◦ 11111111 11100000 00000000 00000000 (mask)◦ 11111111 01000000 00000000 00000000 (result)

Compare Result with First Entry Address part◦ 11111111 01000000 00000000 00000000 (address part)◦ 11111111 01000000 00000000 00000000 (result)

The Entry is a Match!


Recap◦ Read destination IP address of incoming IP packet.

◦ For each entry in the router forwarding table

Read the mask (prefix).

Mask the incoming IP address.

Compare the result with the entry’s IP address part.

Do they match or not?


Simple for Computers

◦ Computers have circuitry AND two numbers.

◦ Computers have circuitry to COMPARE two numbers to see if they are equal or not.

◦ Very computer-friendly, so used on routers.

Difficult for people, unfortunately



The dominant version of the Internet Protocol is Version 4 (v4).◦ Earlier versions were not implemented

The emerging version is Version 6 (v6).◦ V5 was defined but not implemented

◦ Informally called IPng (Next Generation)

IPv6 is already defined.◦ Continuing improvements in V4 may delay its

adoption


IPv6 raises the size of the Internet address field from 32 bits to 128 bits.

◦ We are running out of IP V4 addresses.

◦ V6 will solve the problem.

◦ But current work-arounds are delaying the need for IPv6 addresses—mostly Network Address Translation.


Improved Security

◦ But, through IPsec, v4 is being upgraded in security as well

Improved Quality of Service (QoS)

◦ But, under IETF Differentiated Services (diffserv) initiative, IPv4 is being upgraded in this area as well


Extension Headers◦ IPv4 headers are complex.

◦ IPv6 basic header is simple.

◦ IPv6 uses extension headers for options.

51

Basic Header

Extension Header 1

Extension Header 2


Extension Headers◦ Basic header has 8-bit Next Header field◦ Identifies first extension header or says

that payload follows

52

Basic Header

Extension Header 1

Extension Header 2

NH


Extension Headers◦ Each extension header also has 8-bit Next

Header field

◦ Identifies next extension header or says that payload follows

53

Basic Header

Extension Header 1

Extension Header 2

NH


Extension Headers◦ Next header field is an elegant way to allow

options◦ Easy to add new extension headers for new

needs

54

Basic Header

Extension Header 1

Extension Header 2

NH



Maximum Transmission Unit (MTU)◦ Largest IP packet a network will accept◦ Arriving IP packet may be larger

56

IP PacketIP Packet

MTU


If IP packet is longer than the MTU, the router breaks packet into smaller packets.◦ Called IP fragments◦ Fragments are still IP packets◦ Earlier in Mod A, fragmentation in TCP

57

IP PacketIP Packet 22 11

IP PacketsFragmentation

MTU

33


What is Fragmented?◦ Only the original data field

◦ New headers are created

58



MTU

33


What Does the Fragmentation?◦ The router◦ Not the subnet

59



MTU

33


Original packet may be fragmented multiple times along its route.

60

DestinationHost

InternetProcess

SourceHost

InternetProcess

Fragmentation


Internet layer process on destination host defragments, restoring the original packet.

IP defragmentation only occurs once.

61

DestinationHost

InternetProcess

Defragmentation

SourceHost

InternetProcess


More Fragments field (1 bit)◦ 1 if more fragments

◦ 0 if not

◦ Source host internet process sets to 0

◦ If router fragments, sets More Fragments field in last fragment to 0

◦ In all other fragments, sets to 1

62

0 0 1 1

Original IP Packet Fragments


IP packet has a 16-bit Identification field.

63


Time to Live (8)

Options (if any)

Version(4)

Hdr Len(4) TOS (8)

Identification(16 bits) Flags (3) Fragment Offset (13)

Source IP Address

Destination IP Address


PAD

Data Field



◦ Source host internet process places a number in the Identification field.

◦ Different for each original (non-fragmented) IP packet.

64


Time to Live (8)

Version(4)

Hdr Len(4) TOS (8)

Identification(16 bits) Flags (3) Fragment Offset (13)




◦ If router fragments a packet, it places the original Identification field value in the Identification field of each fragment.

65

47 47 47 47



Purpose

◦ Allows receiving host’s internet layer process to know what fragments belong to each original packet

◦ Works even if an IP packet is fragmented several times

66

47 47 47 47



Fragment offset field (13 bits) is used to reorder fragments with the same Identification field.

Contains the data field’s starting point (in octets) from the start of the data field in the original IP packet.

67

Total Length in Bytes (16)Version

(4)Hdr Len

(4) TOS (8)

Identification (16 bits) Flags (3) Fragment Offset (13)


Receiving host’s internet layer process assembles fragments in order of increasing fragment offset field value.

This works even if fragments arrive out of order!

It works even if fragmentation occurs multiple times.

68

0212730

Fragment Offset Field


IP Fragmentation

◦ Data field of a large IP packet is fragmented.

◦ The fragments are sent into a series of smaller IP packets fitting a network’s MTU.

◦ Fragmentation is done by routers.

◦ Fragmentation may be done multiple times along the route.


IP Defragmentation

◦ Defragmentation (reassembly) is done once, by destination host’s internet layer process.


All IP packets resulting from the fragmentation of the same original IP packet have the same Identification field value.

Destination host internet process orders all IP packets from the same original on the basis of their Fragment Offset field values.

More Fragments field tells whether there are no more fragments coming.



Why Dynamic Routing Protocols?◦ Each router acts independently, based on

information in its router forwarding table.

◦ Dynamic routing protocols allow routers to share information in their router forwarding tables.

73

RouterForwardingTable Data


Routing Information Protocol (RIP) is the simplest dynamic routing protocol.◦ Each router broadcasts its entire routing table

frequently.

◦ Broadcasting makes RIP unsuitable for large networks.

74

RoutingTable


RIP is the simplest dynamic routing protocol.◦ Broadcasts go to hosts as well as to routers.

◦ RIP interrupts hosts frequently, slowing them down; unsuitable for large networks.

75

RoutingTable


RIP is limited.◦ RIP routing table has a field to indicate the

number of router hops to a distant host.

◦ The RIP maximum is 15 hops.

◦ Farther networks are ignored.

◦ Unsuitable for very large networks.

76

Hop Hop


Is a Distance Vector Protocol◦ “New York” starts, announces itself with a RIP

broadcast.

◦ “Chicago” learns that New York is one hop away.

◦ Passes this on in its broadcasts.

77

New York Chicago Dallas

1 hop

NY is 1


Learning Routing Information◦ “Dallas” receives broadcast from Chicago.

◦ Already knows “Chicago” is one hop from Dallas.

◦ So New York must be two hops from Dallas.

◦ Places this information in its routing table.

78

New York Chicago Dallas

1 hop 1 hop

NY is 1

NY is 2


Slow Convergence

◦ Convergence is getting correct routing tables after a failure in a router or link.

◦ RIP converges very slowly.

◦ May take minutes.

◦ During that time, many packets may be lost.


Encapsulation

◦ Carried in data field of UDP datagram Port number is 520

◦ UDP is unreliable, so RIP messages do not always get through.

◦ A single lost RIP message usually does little or no harm.

80

UDPHeader

UDP Data FieldRIP Message


Link State Protocol◦ Link is a connection between two routers.

◦ OSPF routing table stores more information about each link than just its hop count: cost, reliability, and so on.

◦ Allows OSPF routers to optimize routing based on these variables.

81

Link


Network is Divided into Areas.◦ Each area has a designated router

82

AreaDesignated

Router


When a router senses a link state change◦ Sends this information to the designated router

83

AreaDesignated

Router

Notice ofLink State Change


Designed router notifies all routers◦ Within its area

84

AreaDesignated

Router



Efficient◦ Only routers are informed (not hosts).

◦ Usually only updates are transmitted, not whole tables.

85

AreaDesignated

Router



Fast Convergence

◦ When a failure occurs, a router transmits the notice to the designated router.

◦ Designated router send the information back out to other routers immediately.


Encapsulation

◦ Carried in data field of IP packet Protocol value is 89

◦ IP is unreliable, so OSPF messages do not always get through.

◦ A single lost OSPF message usually does little or no harm.

87

IPHeader

IP Data FieldOSPF Message


Within a network you control, it is your choice.◦ Your network is an autonomous system.

◦ Select RIP or OSPF based on your needs.

◦ Interior routing protocol.


RIP is fine for small networks.◦ Easy to implement

◦ 15 hops is not a problem

◦ Broadcasting, interrupting hosts are not too important


OSPF is scalable.

◦ Works with networks of any size

◦ Management complexities are worth the cost in large networks


To connect different autonomous systems◦ Must standardize cross-system routing

information exchanges

◦ BGP is most popular today

◦ Gateway is the old name for router

◦ Exterior routing protocol

91

AutonomousSystem

AutonomousSystemBGP


Distance vector approach◦ Number of hops to a distant system is stored in

the router forwarding table

Normally only sends updates

92

AutonomousSystem

AutonomousSystemBGP


Encapsulation◦ BGP uses TCP for delivery

◦ Reliable

◦ TCP is only for one-to-one connections

◦ If a border router connects to multiple external routers, must establish a TCP and BGP connection to each

93

AutonomousSystem

AutonomousSystemBGP



Each host and router on a subnet needs a data link layer address to specify its address on the subnet.◦ This address appears in the data link layer

frame sent on a subnet.

◦ For instance, 48-bit 802.3 MAC layer frame addresses for LANs.

95

Subnet DADL Frame for Subnet


Each host and router also needs an IP address at the internet layer to designate its position in the overall Internet.

96

Subnet

Subnet

Subnet128.171.17.13


IP address◦ To guide delivery to destination host across the

Internet (across multiple networks)

Subnet Address◦ To guide delivery between two hosts, two

routers, and a host and router within a single LAN, Frame Relay network, and so on


In a company, each person has a company-wide ID number (like IP address).

In a company, each person also has a local office number in a building.

Paychecks are made out to ID numbers. For delivery, also need to know office

number.


Problem

◦ Router knows that destination host is on its subnet based on the IP address of an arriving packet.

◦ Does not know the destination host’s subnet address, so cannot deliver the packet across the subnet.

99

Subnet128.171.17.13

Subnet Address?

Destination Host


Router creates an ARP Request message to be sent to all hosts on the subnet.

◦ Address resolution protocol message asks “Who has IP address 128.171.17.13?”

◦ Passes ARP request to data link layer process for delivery.

100

Subnet

ARP Request


Data link process of router broadcasts the ARP Request message to all hosts on the subnet.

◦ On a LAN, MAC address of 48 ones tells all stations to pay attention to the frame.

101

Subnet

ARP Request


Host with IP address 128.171.17.13 responds.◦ Internet process creates an ARP Response

message.◦ Contains the destination host’s subnet address

(48-bit MAC address on a LAN).

102

Subnet

ARP Response


Router delivers the IP packet to the destination host.◦ Places the IP packet in the subnet frame

◦ Puts the destination host’s subnet address in the destination address field of the frame

103

Subnet

Deliver IP Packetwithin a Subnet Frame


ARP Requests and Responses are sent between the internet layer processes on the router and the destination host.

104

InternetProcess

Router

InternetProcess

Destination HostARP

Request

ARPResponse


However, the data link processes deliver these ARP packets.◦ Router broadcasts the ARP Request.◦ Destination host sends ARP Response to the

subnet source address found in the broadcast frame.

105

InternetProcess

Router

InternetProcess

Destination Host

Broadcast ARP Request

Direct ARP Response

Data LinkProcess

Data LinkProcess



How large is the network part in an IP address?

Today we use network masks to tell. Originally, IP had address classes with fixed

numbers of bits in the network part.◦ Class A: 8 bits (24 bits in local part)◦ Class B: 16 bits (16 bits in local part)◦ Class C: 24 bits (8 bits in local part)


All Class A IP addresses begin with 0. 7 remaining bits in network part.

◦ Only 128 possible Class A networks.

24 bits in local part.◦ Over 16 million hosts per Class A network!

All Class A network parts are assigned or reserved.


All Class B IP address begin with 10 (1st zero in 2nd position).

14 remaining bits in network part◦ Over 16,000 possible Class B networks

16 bits in local part◦ Over 65,000 possible hosts

A good trade-off between number of networks and hosts per network

Most have been assigned


All Class C IP address begin with 110 (1st zero in 3d position).

21 more bits in network part◦ Over 2 million possible Class C networks!

8 bits in local part◦ Only 256 possible hosts per Class C network!

Unpopular, because large firms must have several


All Class D IP address begin with 1110. Used for multicasting, not defining

networks.

◦ Sending message to group of hosts

◦ Not just to one (unicasting)

◦ Not ALL hosts (broadcasting)

◦ Say, to send a videoconference stream to a group of receivers


All hosts in a multicast group listen for this multicast address as well as for their specific own host IP address.

112

Packets toMulticast Address

Not in GroupReject

In GroupAccept

In GroupAccept


Traditionally, unicasting and broadcasting◦ Unicasting: send to one host

◦ Broadcasting: send to ALL hosts

Multicasting◦ Send to SOME hosts

◦ 500 stations viewing a video course

◦ 50 computers getting software upgrades

◦ Standards exist and are improving

◦ Not widely used yet


Do not need to send an IP packet to each host◦ Single packets go out

◦ Only multiplied when necessary

114

SinglePacket

MultiplePackets



IP addresses are associated with fixed physical locations.

Mobile IP is needed for notebooks, other portable equipment.

Computer still gets a permanent IP address. When travels, also gets a temporary IP

address at its location. This is linked dynamically to its permanent

IP address.


117

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Module A Panko and Panko Business Data Networks and Telecommunications, 8 th Edition © 2011 Pearson Education, Inc. Publishing as Prentice Hall