© 2009 pearson education, inc. publishing as prentice hall more on tcp/ip module a updated january...

© 2009 Pearson Education, Inc. Publishing as Prentice Hall

More on TCP/IP

Module AUpdated January 2009

Raymond Panko’sBusiness Data Networks and Telecommunications, 7th edition

May only be used by adopters of the book


Multiplexing

© 2009 Pearson Education, Inc. Publishing as Prentice HallA-3

Multiplexing

• IP packets can carry different things in their data fields

– TCP segments

– UDP datagrams

– ICMP supervisory messages (later)

– RIP messages (later)

IP Data Field IP Header


Multiplexing

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

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

Stream of Arriving or Outgoing IP Packets

Single IP PacketCarrying UDP

Datagram


Multiplexing

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

IP

TCP UDP

ICMPUDP IP-H

IP ProcessArrivingPackets


Multiplexing

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

IP

TCP UDP

ICMPUDPIP-H

IP ProcessOutgoingPackets


Multiplexing

• 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

IP Data Field IP Header


Multiplexing

• IP Header has an 8-bit Protocol field

– Identifies the contents of the data field

• 1=ICMP, 8=TCP, 17=UDP, etc.

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)


Multiplexing

• Other Messages Have Analogous Fields– Identify contents of data field

• TCP and UDP– Have Port number fields

– Identify the application process (80=HTTP)

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

Sequence Number (32 bits)

Acknowledgement Number (32 bits)

Hdr Len(4)

Flags (6) Window Size (16)Reserved

(6)


Multiplexing

• Other Messages Have Analogous Fields

– Identify contents of data field

• PPP

– Protocol field identifies contents of information field as IP, IPX, a supervisory message, etc.

Flag Addr Ctrl Prot Info CRC Flag


More on TCP Acknowledgements

Sequence Number Field

Initial Sequence Number

Acknowledgement Number Field


TCP

• TCP is Reliable

– IP packets carrying TCP segments may arrive out of order

– TCP must put the TCP segments in order

3 4 2 15


TCP

• TCP is Reliable

– Each correct TCP segment is acknowledged by the receiver

SourceTransportProcess

SourceTransportProcess

DestinationTransportProcess

DestinationTransportProcess

TCP SegmentTCP Segment

ACKACK


TCP Segment

• 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

11 44 22 55

3?


TCP Sequence Numbers

• TCP header has a 32-bit sequence number field




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

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



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 901902

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 808182

• TCP 3 8384

80 is SeqNum Field Value




TCP Acknowledgements

• Acknowledgement must indicate which TCP segment is being acknowledged

SourceTCP

Process

SourceTCP

Process

DestinationTCP

Process

DestinationTCP

Process

TCP SegmentTCP Segment

ACKACK


TCP Acknowledgements

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



Acknowledgement Number (32 bits)Hdr Len

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


Reserved (6)


Data Field


TCP Acknowledgment Numbers

• 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

82+1=83 is AckNum Value

84+1=85 is AckNum Value

79+1=80 is AckNumField Value


TCP Acknowledgement Number

• 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?


TCP Flow Control

• Flow Control

– One TCP process transmits too fast

– Other TCP process is overwhelmed

– Receiver must control transmission rate

– This is flow control

TCP Process TCP Process

Too MuchData

Flow Control Message


TCP Flow Control

• A TCP segment has a Window Size field

– Used in acknowledgements




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


Reserved (6)


Data Field


TCP Flow Control

• A TCP segment has a Window Size field

– Tell how many more octets the sender can send beyond the segment being acknowledged

TCP Process TCP Process

Data

Acknowledgement with Window Size Field


TCP Flow Control

• 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


TCP Flow Control

• Each Acknowledgement extends the window of octets that may be sent

– Called a sliding window protocol

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 Fragmentation

• TCP Segments have maximum data field sizes

– (Size limit details are discussed later)

– What if an application layer message is too large?

TCP HeaderTCP Data Field Max

Application Layer Message


TCP Fragmentation

• Application layer message must be fragmented

– Broken into several pieces

– Delivered in separate TCP segments


App Frag 1 App Frag 2 App Frag 3


TCP Fragmentation

• Note that, in TCP fragmentation, the TCP segment is NOT fragmented

– The application layer message is fragmented


App Frag 1 App Frag 2 App Frag 3


TCP Fragmentation

• Transport layer process on the source host does the fragmentation

– Application layer on the source host is not involved

– Transparent to the application layer

Application

Transport

Internet

Application Message

TCP Segment TCP Segment


TCP Fragmentation

• Transport layer process on the destination host does the reassembly

– Application layer on the destination host is not involved; Gets original application layer message

Application

Transport

Internet

Application Message

TCP Segment TCP Segment


TCP Fragmentation

• 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!!!


TCP Fragmentation

• 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


TCP Fragmentation

• 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


TCP Fragmentation

• 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 segments

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


More on Internet Layer Processes


Mask Operations

• Masks were introduced in Chapter 3

• 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


Mask Operations

• Based on Logical AND

– Both must be true for the result to be true

• Example

– 1010101010 Data

– 1111100000 Mask

– 1010100000 Result


Mask Operations

• Based on Logical AND

– If mask bit is 1, get back original data

– If mask bit is 0, get back zero

• Example

– 1010101010 Data

– 1111100000 Mask

– 1010100000 Result


Mask Operations

• 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 Operations

• 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!


Mask Operations

• 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?


Mask Operations

• Simple for Computers

– Computers have circuitry to AND to 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


IPv6

• Current version of the Internet Protocol is Version 4 (v4)– Earlier versions were not implemented

• The next version will be Version 6 (v6)– No v5 was implemented– Informally called IPng (Next Generation)

• IPv6 is Already Defined– Continuing improvements in v4 may delay its adoption


IPv6

• IPv6 will raise the size of the internet address from 32 bits to 128 bits

– Now running out of IP addresses

– Will solve the problem

– But current work-arounds are delaying the need for IPv6 addresses


IPv6

• 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


IPv6

• Extension Headers– IPv4 Headers are complex

– IPv6 basic header is simple

• Extension headers for options

Basic Header

Extension Header 1

Extension Header 2


IPv6

• Extension Headers

– Basic header has 8-bit Next Header field

– Identifies first extension header or says that payload follows

Basic Header

Extension Header 1

Extension Header 2

NH


IPv6


– Each extension header also has 8-bit Next Header field

– Identifies next extension header or says that payload follows

Basic Header

Extension Header 1

Extension Header 2

NH


IPv6


– Next header field is an elegant way to allow options

– Easy to add new extension headers for new needs

Basic Header

Extension Header 1

Extension Header 2

NH


IP Fragmentation


MTU

• Maximum Transmission Unit (MTU)

– Largest IP packet a network will accept

– Arriving IP packet may be larger

IP Packet

MTU


IP Fragmentation

• 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

IP Packet 2 1

IP PacketsFragmentation

MTU

3


IP Fragmentation

• What Is Fragmented?

– Only the original data field

– New headers are created

IP Packet 2 1


MTU

3


IP Fragmentation

• What Does the Fragmentation?

– The router

– Not the subnet

IP Packet 2 1


MTU

3


Multiple Fragmentations

• Original packet may be fragmented multiple times along its route

DestinationHost

InternetProcess

SourceHost

InternetProcess

Fragmentation


Defragmentation

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

• IP Defragmentation only occurs once

DestinationHost

InternetProcess

Defragmentation

SourceHost

InternetProcess


Fragmentation and IP Fields

• 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

0 0 1 1

Original IP Packet Fragments


Identification Field

• IP packet has a 16-bit Identification field

– Source host internet process places a number in the Identification field

– Different for each original IP packet

Total Length in bytes (16)

Time to Live (8)

Version(4)

Hdr Len(4) TOS (8)

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

Header Checksum (16)Protocol (8)



• IP packet has a 16-bit Identification field

– If router fragments, places the original Identification field value in the Identification field of each fragment

47 47 47 47




• Purpose

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

– Works even if an IP packet is fragmented several times

47 47 47 47



Fragment Offset Field

• 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

Total Length in bytes (16)Version

(4)Hdr Len

(4) TOS (8)

Indication (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!

• Works even if fragmentation occurs multiple times

0212730



Fragmentation: Recap

• 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


Defragmentation: Recap

• IP Defragmentation

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


Defragmentation: Recap

• 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


Dynamic Routing Protocols

• 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

RouterForwardingTable Data


Routing Information Protocol (RIP)

• 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

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

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

Hop Hop


Routing Information Protocol

• Is a Distance Vector Protocol

– “New York” starts, announces itself with an RIP broadcast

– “Chicago” learns that New York is one hop away

– Passes this on in its broadcasts

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

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 does little or no harm

UDPHeader

UDP Data FieldRIP Message


OSPF Routing Protocol

• Link State Protocol

– Link is connection between two routers

– OSPF routing table stores more information about each link than just its hop count: cost, reliability, etc.

– Each router knows the state of every link

– Allows OSPF routers to optimize routing based on these variables

Link


OSPF Routers

• Network Is Divided into Areas

– Each area has a designated router

AreaDesignated

Router


OSPF Routers

• When a router senses a link state change

– Sends this information to the designated router

AreaDesignated

Router

Notice ofLink State Change


OSPF Routers

• Designed Router Notifies all Routers

– Within its area

AreaDesignated

Router



OSPF Routers

• Efficient

– Only routers are informed (not hosts)

– Usually only updates are transmitted, not whole tables

AreaDesignated

Router



OSPF

• Fast Convergence

– When a failure occurs, a router transmits the notice to the designated router

– Designated router sends the information back out to other routers immediately


OSPF

• 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 does little or no harm

IPHeader

IP Data FieldOSPF Message


Selecting RIP or OSPF

• 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


Border Gateway Protocol (BGP)

• 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

AutonomousSystem

AutonomousSystemBGP



• Distance vector approach

– Number of hops to a distant system is stored in the router forwarding table

• Normally only sends updates

AutonomousSystem

AutonomousSystemBGP



• Encapsulation

– BGP uses TCP for delivery

– Reliable

– TCP is only for one-to-one connections

– If have several external routers, must establish a TCP and BGP connection to each

BGP MessageTCP

HeaderIP

Header


Address Resolution Protocol (ARP)


Internet and Data Link Layer Addresses

• Each host and router on a subnet needs a data link layer address to specify its address on its switched network

– 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

Subnet DADL Frame for Subnet


Addresses

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

– Switched networks in an internet are called subnets

Subnet

Subnet

Subnet128.171.17.13


Internet and Data Link Addresses Serve Different Purposes

• IP address

– To guide delivery to destination host across the Internet (across multiple networks)

• Switched Network Address

– To guide delivery between two hosts, two routers, and a host and router within a single switched network

– Same LAN, Frame Relay network, etc.


Analogy

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

• In company, 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


Address Resolution

• 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

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

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

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)

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

Subnet

Deliver IP Packetwithin a subnet frame


Address Resolution Protocol

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

InternetProcess

Router

InternetProcess

Destination HostARP

Request

ARPResponse


Address Resolution Protocol

• 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

InternetProcess

Router

InternetProcess

Destination Host

Broadcast ARP Request

Direct ARP Response

Data LinkProcess

Data LinkProcess


IP Address Classes

• 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)


Class A IP Address

• IP address begins 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


Class B IP Address

• IP address begins 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


Class C IP Address

• IP address begins 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


Class D IP Address

• IP address begins 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


Class D IP Address

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

Packets toMulticast Address

Not in GroupReject

In GroupAccept

In GroupAccept


Multicasting

• Traditionally, unicasting and broadcasting– Unicasting: send to one host– Broadcasting: send to ALL hosts

• Multicasting– Send to SOME hosts– Hosts on a list– 500 stations viewing a video course– 50 computers getting software upgrades– Standards exist and are improving– Not widely implemented yet


Why Multicasting

• Do not need to send an IP packet to each host

– Routers split when needed

– Reduces traffic

SinglePacket

MultiplePackets


Mobile IP

• 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


MultiProtocol Label Switching (MPLS)

• When a packet arrives, routers must consider all possible routes, then select the best one

• This is extremely expensive

• MPLS adds a tag to each packet

• An MPLS router examines the tag and passes the packet back out

• This is fast and inexpensive



• This is similar to virtual circuits in ATM

• In addition, packets with similar priority or other characteristics can be given the same label and so be handled in the same way



110

TaggedPacket

MPLS Table for Q

Label110…

Port1…

NHRR…

Q

R

S

RoutingDecision

© 2009 Pearson Education, Inc. Publishing as Prentice Hall A-114

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic,

mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Printed in the United States of America.

Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Publishing as Prentice HallPublishing as Prentice Hall

© 2009 pearson education, inc. publishing as prentice hall more on tcp/ip module a updated january...

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