1. Network Layer Protocols
Internet Protocol Version 4
Service Primitives, Header and Functions
Internet Protocol Version 6
Internet Control Message Protocol
Internet Group Management Protocol
Address Resolution Protocol
Reverse Address Resolution Protocol
Internetwork Packet eXchange (IPX)
X.25 Packet Layer Protocol
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2. Internet Protocol IP is a connectionless network layer protocol
IP is really an internetworking protocol designed to enable datagrams to be forwarded across many types of subnetwork (LANs, MANs and WANs)
A key feature of IP is that it is subnetwork independent. It hides the details of the subnetworks from the transport layer
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3. IP Service Primitives
Because IP is connectionless it only has two primitives
SEND to transmit a packet
DELIVER to receive a packet
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4. IP Service Primitives SEND primitive Source Address
Destination address
Protocol
Type of Service
indicators
Identification
Don’t fragment
indicator
Time to live
Data Length
Option Data
Data
DELIVER primitive
Source Address
Destination address
Protocol
Type of Service
indicators
Data Length
Option Data
Data
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5. IP v4 Header © Tanenbaum, Prentice Hall International 23/11/10
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6. IP v4 Header Version = 4 (or 6)
IHL = Internet Header Length (in 32bit words)
TOS = Type of Service (3 bit Precedent Field of one bit flags to indicate whether delay, throughput or reliability were most important). Mostly ignored by routers, Now used to indicate class of service for Diffserv protocol)
Total Length is of both header and data measured in bytes
Identification = Sequence Number same for all fragments, used to reassemble data
Fragment Offset indicates where in the original datagram a fragment belongs in 8 byte units
DF and MF = Don’t Fragment and More Fragment bits
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7. IP v4 Header
TTL = Time to Live, limits lifetime of a datagram. Each router will decrement TTL by 1
Protocol allows received datagrams to be de-multiplexed to higher layer protocol (TCP/UDP/ICMP)
Header Checksum (standard Internet Checksum) calculated with a header checksum field of 0. It has to be recomputed at every router
Source and Destination addresses – 32 bit IP addresses
Options variable length field up to 40 bytes padded out to a multiple of 4 bytes. One byte option code followed by variable length option. Covers such things as security, source routing, route recording and time-stamping
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8. IP Functions Fragmentation Encapsulation Addressing Multiplexing
Type of Service
Flow Control
Error Control
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9. IP Fragmentation
IP will fragment a datagram when its size is greater than the Maximum Transfer Unit size of a subnetwork
IP will transmit each fragment as a separate packet with a full IP header
All fragments are the same size (the MTU size) except the last one
All fragments have the same identification number
Each fragment has a different fragment offset which is the byte position in the original datagram measured in units of 8 bytes starting at 0
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10. Don’t Fragment Bit
IP cannot fragment a datagram where the DF bit is set.
If a datagram with DF set is too large to transmit over a subnetwork, it must discard it
It must also send an ICMP Destination Unreachable packet back to the source
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11. More Fragments Bit
When IP fragments a datagram it sets the MF bit in all the fragments except the last one.
IP at the destination knows, when it sees datagrams with the MF bit set, that it has to reassemble fragments up until it receives a fragment with the MF bit not set
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12. IP Reassembly
IP reassembles fragments at the host to avoid datagrams being continually fragmented and reassembled on their path
The main disadvantage of this approach is efficiency. Many IP fragments will require a lot of IP protocol header overhead (typically 20 bytes for each fragment)
Also the probability of one of many fragments being discarded is higher than one large datagram being discarded. If one fragment is lost, all fragments have to be re-transmitted which will cause delay
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13. IP Encapsulation © Fitzgerald & Dennis, John Wiley & Sons 23/11/10
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14. IP Addressing © Tanenbaum, Prentice Hall International 23/11/10
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15. IP Address Classes (RFC 1117)
Class ID
1st Octet
Networks
Hosts
Purpose A 01
1-126
126
16,777,214
Large Networks
127
Loopback B 10
16,382
65,534
Medium Networks C
110
2,097,152
254
Small Networks D
1110
Multicast E
1111
Experimental
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16. IP Addressing
Network addresses are hierarchically structured into a network portion and a host portion
Routers look up the network portion of addresses in routing tables
They find the network portion by ANDing the IP address with an address mask, determined by the class of the address
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17. IP Addressing Subnetting
The IP addressing scheme was found to be too inflexible and wasteful of address space. Also routing tables were becoming too big to manage
Subnetting allows bits from the host part of the address to be used to define subnetworks
This means that an organisation does not have to register IP addresses for all its subnetworks on a site
It only has to register its main IP address for the network, which is the only address that needs to appear in routing tables
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18. Classless IP Addresses
Problem with classful addresses
Class B addresses are too popular and were running out
Class C addresses are not popular and were plentiful
Solution
Classless Internet Domain Routing (CIDR) sometimes called supernetting
Allows network addresses to be allocated in variable sized blocks without regard to address classes
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19. IP Multiplexing
IP uses the protocol field to multiplex higher level protocols and determine which process the datagram should be handed to at the receiving end
Common protocols defined in RFC 1700 are:
ICMP
IGMP
6 TCP
17 UDP
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20. IP Flow Control IP does not perform flow control
If IP can’t cope with traffic it must discard datagrams
An ICMP Source Quench message could be sent, but this is rarely used as it just adds to congestion
IP relies on higher layers to perform flow control
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21. IP Error Control
IP offers no error control for data. It relies on higher layer protocols to detect and recover from errors in its data field
IP does detect errors in its headers by means of the Internet checksum.
On detecting an error, it simply discards the datagram
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22. IP Version 4 Problems IP v4 has a number of problems
It is running out of addresses
Routing tables are getting very large
It has a complex header structure
It isn’t very secure
It isn’t good at handling real-time data
It doesn’t easily support mobility
The answer to all these problems and more is IP v6
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23. IP Version 6
16 byte addresses gives a total of 3*1038 addresses (7*1023 per m2 over the earth) which can be provider based or geographical
Simpler header structure. Only 7 fields.
Every header has a next header field which indicates the type of the next header (either an option header or a protocol header such as TCP, UDP or ICMP)
Better support for options
Improved security
Better support for Type of Service and hence real-time applications
Anycasting addresses (like multicasting but with delivery to just one of a group of addresses)
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24. IP Version 6 Header © Tanenbaum, Prentice Hall International 23/11/10
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25. Internet Control Message Protocol
ICMP is a network layer protocol that sits immediately above IP.
The ICMP is encapsulated in IP, so its header comes immediately after the IP header
ICMP is used for reporting errors and running diagnostic tests
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26. Principal ICMP Message Types
© Tanenbaum, Prentice Hall International
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27. Internet Group Management Protocol
Sender
Multicast Router
Host
Multicasting involves a single send operation that results in copies of the sent data being delivered to many receivers
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28. Internet Group Management Protocol
IGMP is a network layer protocol used on the Internet for managing multicast groups
Multicast routers on a LAN must keep track of which addresses on their LAN belong to which groups. This is done by IGMP.
Multicast routers receive one packet with a class D IP address for the group and multicast the packet to all the members of the group on the LAN
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29. Address Resolution Protocol
ARP is a network layer protocol used on the Internet to map an IP address to a physical address used on a LAN
ARP broadcasts an ARP packet on the LAN with the IP address it is trying to resolve
All hosts check to see if it is their IP address
The host with this IP address responds
The sender receives the response and sees which physical address it came from
The sender then caches the mapping in its ARP table for future use
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30. Reverse Address Resolution Protocol
RARP is a network layer protocol used on the Internet by a diskless workstation to discover its IP address from a server on its LAN
It broadcasts a RARP packet with its physical address as the source
The RARP server responds with the station’s IP address
RARP is not used very much these days, as diskless workstations obtain their IP addresses via the BOOTP or Dynamic Host Configuration Protocol that runs above UDP
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31. Internetwork Packet eXchange (IPX)
IPX is a connectionless network layer protocol, similar to IP, developed by Novell for use with their NetWare product
It uses 12 byte addresses which incorporate 6 byte physical addresses, so no mapping is needed between IPX addresses and physical addresses
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32. X.25 Packet Layer Protocol
X.25 is a protocol suite developed by CCITT (now ITU-T) as a standard for public packet-switched data network.
It consists of a physical, data link and network layer.
The network layer, called the PLP, is a connection-oriented network that uses 14 digit (7 byte binary coded decimal) addresses
X.25 is a legacy corporate network technology, but there is some of it still about. It has a good geographical spread as most PTTs/Telcos run an X.25 network
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