Datacommchapter 6 Part 1

Chapter 6 Internetworking Protocol

1 Main reference: Copyright © The McGraw-Hill Companies.

Part 1 6.1 6.2 6.3

Internetworking Internet Protocol IPv4 vs.IPv6

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6.1: INTERNETWORKING

Internetworking is connecting networks together to make an internetwork or an internet.

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Links between two hosts

Physical & data link layers operate locally

Problem: When data arrive at the interface f1 of S1, how does S1 know that Interface f3 is the outgoing interface? To solve this problem, the network layer was designed. 4

Network layer in an internetwork

The network layer is responsible for host-to-host delivery and for routing the packets through the routers or switches. 5

Network layer at the source, router, and destination


Creating a packet from the data coming
Address verification: ensure the destination address on the packet is the same in the host. from another protocol.
Checking its routing to find the routing
Fragmentation: waits for all fragments, information. reassembles them and then delivers the reassembled packet to the transport layer. 6

Network layer at the source, router, and destination (continued)


Responsible for routing the packet The internet, at the network layer is a switched-packet network. 7

Note

Switching at the network layer in the Internet uses the datagram approach to packet switching.

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Note Communication at the network layer in the Internet is connectionless.

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6.2: Internet Protocol The ISO adopted a connectionless internet service which is based on the internet protocol developed for ARPANET, which grows steady into its existing shape today: Internet. The internet protocol is one of the protocol associated with the complete protocol suite used with the Internet :TCP/IP, which includes transport and application protocols used for commercial and research works. IP provides a connectionless, or datagram service between end systems. 10

The Internet Protocol version 4 (IPv4) is the delivery mechanism used by the TCP/IP protocols.

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Position of IPv4 in TCP/IP protocol suite

Unreliable – a best-effort delivery service: provides no error control or flow control. If reliability if important, it must be paired with a reliable protocol s.a. TCP. Connectionless protocol for packet switching network that uses datagram approach. Each datagram is handled independently and it can follow different route to a destination. 12

IP Addressing: IPv4 An IP address is a numeric identifier that uniquely defines a host on the Internet. It consists of 32 bits [in IPv4], normally written as 4 octets [dotted decimal format], and composed of a network identifier [netid] and a host identifier [hostid].

A router is connected to more than one network, hence it needs one IP address for each interface, as shown below:

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Network Address Classes

There are 5 different IP address classes: A, B, C, D and E as shown below

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Network Classes

Address is coded to allow a variable allocation of bits to specify network and host. This allows flexibility in assigning addresses to hosts and allows a mix of network sizes on an internet. Three principles network classes: Class A: Few network, each with many hosts Class B: Medium number of networks, each with medium no. of hosts Class C: Many networks, each with few slots It is possible to mix all three classes of addresses on the same internetwork.

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Class A

The first bit of zero defining the class as A. The first byte is the netid, defining 27 - 2 reserved addresses = 126 networks [network addresses with a first byte of 0 (binary 00000000) and 127 (01111111) are reserved]. The other 3 bytes [24 bits] gives the hostid. Hence, each network can have up to 224 - 2 reserved addresses = 16,777,214 hosts ARPANET and only few large commercial networks are given these Class A addresses. For normal organization, Class B addresses are used.

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Class B

The two leftmost bits are 10 to identify the class as B. The first 2 bytes define the netid and the remaining 2 bytes for hostid. Since class B network addresses begin with a binary 10, the range of The 1st byte number is 128 to 191 [binary 10000000 to 10111111]. Combining with the 2nd byte number, there are a total of 214 = 16,384 possible Class B addresses. Each network can have up to 216 - 2 reserved addresses = 65,534 hosts

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Class C

The first three bits of 110 indicates the address class as C. First three octets are assigned for netid and the last octet for hostid. The first decimal number ranges from 192 to 223 [binary 11000000 to 11011111]. The total number of Class C network is thus 221 = 2,097,152. However, each of the network can only have a maximum of 28 – 2 reserved address = 254 host addresses.

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Class D and E

There is no netid or hostid in both classes. Class D is used for multicasting. Class E is reserved by Internet for special use.

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Network Classes

Classes can also be distinguished in decimal notation by observing the first byte: 1 to 126 Class A address 128 to 191 Class B address 192 to 223 Class C address 224 to 239 Class D address 240 to 255 Class E address Generally, addresses should never begin with 0, 127 or any number above 223 (reserved). Addresses violating these rules are sometimes known as Martian Address, since these addresses are not used on earth.

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Special Addresses

Some addresses are reserved for special purposes. A network defines itself by a network address in which the hostid is set to all zeroes. The IP address of 0.0.0.0 is used for default network. IP address with netid of 127 is used as the loopback address [used by the host computer to send a message back to itself]. The hostid with all 1s is used to broadcast message to all hosts on a specific network. In addition, the IP address of 255.255.255.255 can also be used for broadcasting on the local network.

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Special Addresses (cont …)

These special addresses are summarized in the table below:

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Subnetting

Subnetting is a process of dividing a large network into smaller subnetworks or subnets connected together by routers.

If an organization has a large number of computers, it would be more convenient to implement subnetting. 23

Subnetting (cont …)

Benefits
Reduced network traffic: most traffic will stay on the local network, only packets destined for other networks will pass through routers; this will optimize the network performance.
Simplified management: it is easier to identify and isolate network problems in a group of smaller networks than within a large one. Example: A network with Class B address of 158.108.X.X

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Subnetting (cont …) An organization has been given a Class C address of 198.228.120.X Part of the fourth octet can be used to indicate the subnetwork and the host

This division has no significance outside of the organization; an outsider would treat all datagrams addressed to 198.228.120.X the same way. Outside networks will not look at the fourth octet of the address. 25

Subnetting (cont …)

There is only one gateway attached to the local site network that performs internet-wide routing (communicate with outside networks). For a single netid with a number of associated subnetworks, the hostid part consists of 2 subfieds: subnetid and hostid, In subnetting, bits are stolen from the hostid to create the subnetid, as shown below.

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Subnetting (cont …) In the example, 6 bits are stolen from the hostid for the subnetid
Netid - defines a site
Subnetid - defines a subnetwork within a site
Hostid - identifies a host in the subnetwork

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Masking Masking is a process of extracting subnetwork address from an IP address using a 32-bit mask. The part of the mask containing 1s represents the netid and subnetid
The part of the mask containing 0s identifies the hostid.
To obtain the subnet address, a bitwise-AND operation on the IP address and the subnet mask is performed.
The effect of the subnet mask is to erase the portion of the host field that refers to an actual host on the subnet.
What remains is the network number and the subnet number.

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Masking (cont ..)

Consider a local complex consisting of 3 LANs and 2 routers To the rest of the internet, this complex is a single network with Class C address of 192.228.17.X

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Masking (cont ..)

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Masking (cont ..) Both routers R1 and R2 are configured with a subnet mask of 255.255.255.224. If a datagram with the destination address 192.228.17.57 arrives at R1, R1 applies the subnet mask to determine the destination subnet address and the host:

Hence, the datagram will be routed to LAN X which has subnet no. 1 (192.228.17.32) and received by station B (host number 25). 31

Masking (cont ..)

Maximum number of subnets in a site-wide network: 2 no. of masked bits corresponding to the subnetid - 2 special address Maximum number of hosts per subnet: 2 no. of unmasked bits - 2 special addresses For each of the subnetid and hostid, all zeroes and all ones is not allowed [reserved]

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Masking (cont ..) Subnet interpretation:

IP address Subnet mask

130.122.34.3 255.255.255.192

10000010.01111010.00100010.00000011 11111111.11111111.11111111.11000000

Bitwise AND of IP address and subnet mask (resultant network / subnet number )

10000010.01111010.00100010.00000000

Subnet number Host number

= 130.122.34.0 10000010.01111010.00100010.00000000 00000000.00000000.00000000.00000011

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IPv4 datagram format Packets in the IPv4 layer are called datagrams.

A variable length packet consists of header and data. The header contains information essential to routing and delivery. It is a customary in TCP/IP to show the header in 4-byte sections.

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Service type or differentiated services

1) Precedence



A 3-bit subfield (0-7) Defines the priority of the datagram in issues such as congestion

2) TOS bits



1) Codepoint





When the 3 rightmost bits are 0s, the 3 leftmost are interpreted as the precedence bits. When the 3 rightmost bits are not all 0s, the 6 bits define 64 services based on the priority assignment by the Internet or local authorities.

A 4-bit subfield with each bit having a special meaning. Only 1 bit is set at a time 35

Note The precedence subfield was part of version 4, but never used.

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Types of service

Bit patterns and their interpretations for 5 different type of services

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Default types of service

Application programs can request a specific type of service.

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Values for codepoints

Category

Codepoint

Assigning Authority

1

XXXXX0

Internet

2

XXXX11

Local

3

XXXX01

Temporary or experimental

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Note The total length field defines the total length of the datagram including the header.

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Encapsulation of a small datagram in an Ethernet frame There are occasions in which the datagram is not the only thing encapsulated in a frame; it may be that padding has been added.

E.g.:

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Protocol field and encapsulated data The 8-bit field defined the higher-level protocol that uses the services of the IPv4 layer. A IPv4 datagram can encapsulate data from several higher-level protocol s.a TCP, UDP etc.

Since the IPv4 protocol carriers data from different other protocols, the value of this field helps in receiving network layer know to which protocol the data belong. 42

Protocol values

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Example 1 An IPv4 packet has arrived with the first 8 bits as shown: 01000010 The receiver discards the packet. Why?

Solution There is an error in this packet. The 4 leftmost bits (0100) show the version, which is correct. The next 4 bits (0010) show an invalid header length (2 × 4 = 8). The minimum number of bytes in the header must be 20. The packet has been corrupted in transmission. 44

Example 2 In an IPv4 packet, the value of HLEN is 1000 in binary. How many bytes of options are being carried by this packet?

Solution The HLEN value is 8, which means the total number of bytes in the header is 8 × 4, or 32 bytes. The first 20 bytes are the base header, the next 12 bytes are the options.

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Example 3 In an IPv4 packet, the value of HLEN is 5, and the value of the total length field is 0x0028. How many bytes of data are being carried by this packet?

Solution The HLEN value is 5, which means the total number of bytes in the header is 5 × 4, or 20 bytes (no options). The total length is 40 bytes, which means the packet is carrying 20 bytes of data (40 − 20). Note: Length of data = total length - header length 46

Example 4 An IPv4 packet has arrived with the first few hexadecimal digits as shown. 0x45000028000100000102 . . . How many hops can this packet travel before being dropped? The data belong to what upper-layer protocol?

Solution To find the time-to-live field, we skip 8 bytes. The time-to-live field is the ninth byte, which is 01. This means the packet can travel only one hop. The protocol field is the next byte (02), which means that the upper-layer protocol is IGMP. 47

Fragmentation

A datagram can travel through different networks. Each router decapsulates the IPv4 datagram from the frame it receives, process it and then encapsulates it in another frame. Maximum Transfer Unit (MTU) is the maximum number of bytes that a data link protocol can encapsulate. MTUs vary from protocol to protocol. Fragmentation is the division of a datagram into smaller units to accommodate the MTU of a data link protocol.

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Maximum transfer unit (MTU)

Maximum Transfer Unit (MTU) is the maximum number of bytes that a data link protocol can encapsulate.

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MTUs for some networks

MTUs vary from protocol to protocol. 50

Fields related to fragmentation


Identification: identifies a datagram originating from the source host.
Fragmentation offset: shows the relative position of this fragment with respect to the whole datagram.
Flags 51

Flags used in fragmentation

1st bit : reserved bit D or 2nd bit: do not fragment bit M or 3rd bit: the more fragment bit

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Fragmentation example

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Detailed fragmentation example

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Example 5 A packet has arrived with an M bit value of 0. Is this the first fragment, the last fragment, or a middle fragment? Do we know if the packet was fragmented?

Solution If the M bit is 0, it means that there are no more fragments; the fragment is the last one. However, we cannot say if the original packet was fragmented or not. A non-fragmented packet is considered the last fragment. 55

Example 6 A packet has arrived with an M bit value of 1. Is this the first fragment, the last fragment, or a middle fragment? Do we know if the packet was fragmented?

Solution If the M bit is 1, it means that there is at least one more fragment. This fragment can be the first one or a middle one, but not the last one. We don’t know if it is the first one or a middle one; we need more information (the value of the fragmentation offset). 56

Example 7 A packet has arrived with an M bit value of 1 and a fragmentation offset value of 0. Is this the first fragment, the last fragment, or a middle fragment?

Solution Because the M bit is 1, it is either the first fragment or a middle one. Because the offset value is 0, it is the first fragment.

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Example 8 A packet has arrived in which the offset value is 100. What is the number of the first byte? Do we know the number of the last byte?

Solution To find the number of the first byte, we multiply the offset value by 8. This means that the first byte number is 800. We cannot determine the number of the last byte unless we know the length.

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Example 9 A packet has arrived in which the offset value is 100, the value of HLEN is 5, and the value of the total length field is 100. What are the numbers of the first byte and the last byte?

Solution The first byte number is 100 × 8 = 800. The total length is 100 bytes, and the header length is 20 bytes (5 × 4), which means that there are 80 bytes in this datagram. If the first byte number is 800, the last byte number must be 879. 59

6.3: IPv6 vs IPv4

The network layer protocol in the TCP/IP protocol suite is currently IPv4. Although IPv4 is well designed, data communication has evolved since the inception of IPv4 in the 1970s.

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IPv4 vs. IPv6 IPv4 has some deficiencies that make it unsuitable for the fast-growing Internet:
Long term problem in the Internet: address depletion


The Internet must accommodate real-time audio and video transmission. It requires minimum delay strategies and reservation of resources not provided in the IPv4 design.




The Internet must accommodate encryption and authentication of data for some application. No encryption or authentication is provided by IPv4.

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IPv4 vs. IPv6

IPv6 has some advantages over IPv4:
Larger address space
Better header format
New options
Allowance for extension
Support for resource allocation.
Support for more security

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Transition From IPv4 to IPv6

Because of the huge number of systems on the Internet, the transition from IPv4 to IPv6 cannot happen suddenly. It takes a considerable amount of time before every system in the Internet can move from IPv4 to IPv6. The transition must be smooth to prevent any problems between IPv4 and IPv6 systems.

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