Download - The Internet Protocol - 2
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The Internet Protocol - 2
Professor Richard Harris
School of Engineering and AdvancedTechnology (SEAT)
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Presentation Outline
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Learning Objectives
You will be able to:
Discuss the format of IP packets
Apply internet addressing to sample problems
Perform allocation of hosts and subnets to meetrequirements for IP network design.
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References
Tanenbaum, “Computer Networks”, 4th Edition
Forouzan, “Data Communications and Networking”, 4th
Edition
Cisco CCNA1 Module 10 - part 1
Stallings, William 2000 ‘Data and ComputerCommunications’, Prentice Hall, Sixth Edition
Russell, Travis 1997 ‘Telecommunications Protocols’,McGraw Hill
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Special Purpose Addresses - 2
The special address 255.255.255.255 is known asthe limited broadcast address
This address can only be used as a destination address
Send a packet to all the hosts on the local subnet.
Packets cannot be relayed to any other network outside ofthe local one.
The special addresses A.255.255.255 orB.B.255.255 or C.C.C.255 are referred to asdirected broadcast message addresses.
Packets sent to these addresses are intended to reachall hosts within the Class A, B or C networksrespectively.
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Special Purpose Addresses - 3
A consequence of this special purpose addressing is thatno subnet is allowed to have a null number, or a numberthat is expressed entirely in binary ones.
If we use 3 bits for the subnet number, for example, then wecan use only the values from 1 - 6.
In addition, we cannot have a subnet with a number that isonly one bit long!!!
In addition to the above special addresses, Internetnumbering authorities have reserved other special“multicast” addresses as well.
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Special Purpose Addresses - 4
Multicast addresses224.0.0.1 = All systems on this subnet
224.0.0.2 = All routers on this subnet
Examples
Some unknown host (source)Any host (destination)Host 3 in Class B network 129.34Some host in network 129.34 (source)Any host in 129.34 (destination)Host number 3 on this network (source)This host (local loop)
Addresses Description
0.0.0.0255.255.255.255129.34.0.3129.34.0.0129.34.255.2550.0.0.3127.0.0.1
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Internet Protocol - Introduction
The Internet Protocol will now be presented in moredetail.
The components of interest are:Formats
Procedures
Service types
Fragmentation
Reassembly
Options
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Internet Header - 1
012345678901234567890123456789011 2 3
Version IHL Type of service Total Length
Identification
Protocol
Flags
Header Checksum
Source Address
Destination Address
PaddingOptions
Fragment Offset
Time to live
The following diagram shows the Internet Headerdetails:
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Internet Header - 2
The header must carry source and destination information,parameters for use by the routers in the network and optiondetails.
We review the first 32 bits of the header below:
IHL Type of service Total LengthVersion
Version: Currently is version ‘4’
IHL: Internet header length, expressed in 32-bit words(Varies from 5 to 15 depending on options present)
Type of Service: Packet’s precedence and desired routing.
Total Length: Number of octets contained in the packet(including the packet header). As this field is 16 bits longthis limits the packet size to 65,535 octets.
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Internet Header - 3
The identification, flags and fragment offset will beconsidered in more detail a little later.
The second 32 bit section is presented below:
Identification Flags Fragment Offset
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Internet Header - 4
The third 32 bit word of the header:
Time to live Protocol Header Checksum
Time to live: This was supposed to be measured in seconds andrepresented the maximum lifetime in the network.Decremented by one after passing through a router. Ifzero, the packet should be destroyed. More detail later.
Protocol: When the packet reaches its destination, this field isused to determine which program should be passedthe packet. (See later for possible programs.)
Header Checksum:This is computed as the 16-bit one’s complement sumof all 16-bit words in the header after zeroing thechecksum field itself.
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Internet Header - 5
The fourth and fifth 32 bit words from the header are thesource and destination addresses
Source Address
Destination Address
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Internet Header - 6
The Sixth 32 bit word from the header
Options Padding
Options: This field was defined for the specific purpose of requestingparticular routing for some packets. A packet can carry severaloptional parameters - all concatenated together.
Padding: This field simply contains “No Op” fields to follow the optionsinformation where required.
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Protocol Numbers
As mentioned above, the protocol field is used to identify theprogram to run when the packet is received at the destination.
The following table lists some of these programs:
01234568
172938808889
255
ProtocolKeywordDecimal
ICMPIGMPGGPIPSTTCPEGPUDPISO-TP4IDPR-CMTPISO-IPIGRPOSPF
ReservedInternet Control MessageInternet Group ManagementGateway to GatewayIP in IP (encapsulation)StreamTransmission ControlExterior Gateway ProtocolUser DatagramISO-Transport Protocol Class 4IDPR-Control Messager Transport ProtocolISO- Internet Protocol (CLNP)IGRPOpen Shortest Path FirstReserved
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Precedence and Type of Service Field
The type of service field actually carries two sub-fieldsPrecedence
An indication of priority
Type of service
An indication for routing
Precedence Type of service
D T R C
0 1 2 3 4 5 6 7
D - Delay; T - Throughput; R - Reliability; C - Cost
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Precedence
The precedence indicator affects queueing.
If several packets are awaiting transmission on the samechannel, the one with the highest precedence issupposed to be sent first.
There are eight preference values, viz:
111 - Network Control110 - Inter-network Control101 - CRITIC-ECP100 - Flash Override011 - Flash010 - Immediate001 - Priority000 - Routine
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Type of Service
Often there is more than one route to adestination. These routes may have quitedifferent characteristics. Eg telephone circuits,satellite links, radio channels,…
We want to indicate to the routing protocols howwe would like the packet to be routed.
As indicated above, the four types of service areD, T, R and C:
D - Delay: This is a mechanism for requesting low delays (avoidsatellite links)
T - Throughput: Select path with highest throughput (avoid telephonelinks)
R - Reliability: Highest reliability (avoid radio channels)
C - Cost: Cheapest route
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Fragmentation and Reassembly - 1
Internetworking programs need to pass packets from one networkto another.
Sometimes this means that the packets are the wrong size andthey would be rejected or broken into more manageable pieces.
The sizes used may be related to the maximum size but it is morecommon to have them related to the data rate of the network or tothe expected/known error rate.
The IP Header contains specific fields to manage thefragmentation and reassembly process involved in interworkingbetween networks.
Version IHL Type of service Total Length
Identification Flags Fragment Offset
Network
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Fragmentation and Reassembly - 2
The flags field has three bits.
The first bit is reserved for future use and must be set tozero.
The DF bit means don’t fragment. If this bit is set, thenthe router should discard the packet if it is too large. (AnICMP error message needs to be returned to the sourceaddress - if possible.)
The MF bit means more fragments to follow.
Flags0
DF
MF
0 1 2
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Options and Header Processing
Options are not used very often in modern internets andintranets
There is an overhead penalty in processing these options in theheader.
Alternative approaches have been found - particularly for highspeed networks.