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Network Layer in the Internet
- In the network layer, the Internet can be viewed as a collection of networks or ASes ( Autonomous Systems ) that are interconnected.
- There is no real structure, but several major backbones exist. These are constructed from high-bandwidth lines and fast routers. Attached to the backbones are regional (midlevel) networks, and attached to these regional networks are the LANs at many universities, companies, and Internet service providers.
- A sketch of this quasi-hierarchical organization is given
- The glue that holds the whole Internet together is the network layer protocol, IP
( Internet Protocol ). Unlike most older network layer protocols, it was designed from the beginning with internetworking in mind.
- The network layer job is to provide a best-efforts (i.e., not guaranteed) way to transport datagram from source to destination, without regard to whether these machines are on the same network or whether there are other networks in between them.
- The transport layer takes data streams and breaks them up into datagram’s.
- The Time to live field is a counter used to limit packet lifetimes. It is supposed to count time in seconds, allowing a maximum lifetime of 255 sec.
- When the network layer has assembled a complete datagram, it needs to know what to do with it. The Protocol field tells it which transport process to give it to. TCP is one possibility, but so are UDP and some others. The numbering of protocols is global across the entire Internet.
- The Header checksum verifies the header only.
- The Source address and Destination address indicate the network number and host number.
- The Options field was designed to provide an escape to allow subsequent versions of the protocol to include information not present in the original design, to permit experimenters to try out new ideas, and to avoid allocating header bits to information that is rarely needed. The options are variable length. - The Security option tells how secret the information is. - (^) The Strict source routing option gives the complete path from source to destination as a sequence of IP addresses. The datagram is required to follow that exact route. - The Loose source routing option requires the packet to traverse the list of routers specified, in the order specified, but it is allowed to pass through other routers on the way. - The Record route option tells each router along the path to append its IP address to the Options field. This allows system managers to track down bugs in the routing algorithms. - Finally, the Timestamp option is like the Record route option, except that in addition to recording its 32-bit IP address, each router also records a 32-bit timestamp. This option, too, is mostly useful for network measurement. IP ADDRESS
- Every host and router on the Internet has an IP address, which encodes its network number and host number.
- IP Addressing is one of the major responsibilities of the network layer.
- The network portion has the same value for all hosts on a single network, such as an Ethernet LAN. This means that a network corresponds to a contiguous block of IP address space. This block is called a prefix.
- Each IP address is 32 bits long, and they are represented in the form of " dotted decimal notation " where each byte is written in the decimal form, and they are separated by the period. An IP address would look like 193.32.216.9 where 193 represents the decimal notation of first 8 bits of an address, 32 represents the decimal notation of second 8 bits of an address.
- In the above figure, a router has three interfaces labeled as 1, 2 & 3 and each router interface contains its own IP address.
- Each host contains its own interface and IP address.
- All the interfaces attached to the LAN 1 is having an IP address in the form of 223.1.1.xxx, and the interfaces attached to the LAN 2 and LAN 3 have an IP address in the form of 223.1.2.xxx and 223.1.3.xxx respectively.
- Each IP address consists of two parts. The first part (first three bytes in IP address) specifies the network and second part (last byte of an IP address) specifies the host in the network.
- IP addresses were divided into the five categories Classful Addressing An IP address is 32-bit long. An IP address is divided into sub-classes:
- Class A
- Class B
- (^) Class C
- Class D
- Class E An ip address is divided into two parts:
- Network ID: It represents the number of networks.
- Host ID: It represents the number of hosts.
- The Network ID is 24 bits long.
- The host ID is 8 bits long.
- In Class C, the higher order bits of the first octet is always set to 110, and the remaining 21 bits determine the network ID. The 8 bits of the host ID determine the host in a network.
- The total number of networks = 2^21 = 2097152 network address
- The total number of hosts = 2^8 - 2 = 254 host address
192.0.0.0 to 223.255.255. Class D In Class D, an IP address is reserved for multicast addresses. It does not possess subnetting. The higher order bits of the first octet is always set to 1110, and the remaining bits determines the host ID in any network.
224.0.0.0 to 239.255.255.
Class E In Class E, an IP address is used for the future use or for the research and development purposes. It does not possess any subnetting. The higher order bits of the first octet is always set to 1111, and the remaining bits determines the host ID in any network.
240.0.0.0 to 255.255.255. Rules for assigning Host ID: The Host ID is used to determine the host within any network. The Host ID is assigned based on the following rules:
- The Host ID must be unique within any network.
- The Host ID in which all the bits are set to 0 cannot be assigned as it is used to represent the network ID of the IP address.
- The Host ID in which all the bits are set to 1 cannot be assigned as it is reserved for the multicast address. Rules for assigning Network ID: If the hosts are located within the same local network, then they are assigned with the same network ID. The following are the rules for assigning Network ID:
- The network ID cannot start with 127 as 127 is used by Class A.
- The Network ID in which all the bits are set to 0 cannot be assigned as it is used to specify a particular host on the local network.
- The Network ID in which all the bits are set to 1 cannot be assigned as it is reserved for the multicast address. SUBNET
- (^) All the hosts in a network must have the same network number. This property of IP addressing can cause problems as networks grow.
- The problem is the rule that a single class A, B, or C address refers to one network, not to a collection of LANs.
- The solution is to allow a network to be split into several parts for internal use but still act like a single network to the outside world. This is called subnetting and the networks (such as Ethernet LANs) that result from dividing up a larger network are called subnets.
- To implement subnetting, the main router needs a subnet mask that indicates the split between network + subnet number and host.
- For example, if the university has a B address (130.50.0.0) and 35 departments, it could use a 6-bit subnet number and a 10-bit host number, allowing for up to 64 Ethernets, each with a maximum of 1022 hosts.
- The subnet mask can be written as 255.255.252.0. An alternative notation is /22 to indicate that the subnet mask is 22 bits long.
Internet Protocols Internet Control Message Protocol (ICMP) The Internet Control Message Protocol (ICMP) is an error reporting protocol that is an integral part of the IP protocol. ICMP communicate control data, information data, and error recovery data across the network. Problems that are less severe than transmission errors result in error conditions that can be reported. For example, suppose some of the physical paths in Internet fail causing the Internet to be partitioned into two sets of networks with no path between the sets. A datagram sent from a host in one set to a host in other cannot be delivered. The TCP/IP suite includes a protocol called ICMP that IP uses to send error messages when condition such as the one described above arises. The protocol is required for a standard implementation of IP. We will see that the two protocols are co-dependent. IP uses ICMP when it sends an error message, and ICMP uses IP to transport messages. Following is a brief description of some of the error messages defined by ICMP protocol:
- Source Quench: A router or host whose receive communication buffers are nearly full normally triggers this message. A source quench message is sent to the sending host, the
Steps taken by ARP protocol
If a device wants to communicate with another device, the following steps are taken by the device:
- The device will first look at its internet list, called the ARP cache to check whether an IP address contains a matching MAC address or not. It will check the ARP cache.
- If ARP cache is empty, then device broadcast the message to the entire network asking each device for a matching MAC address.
- The device that has the matching IP address will then respond back to the sender with its MAC address
- Once the MAC address is received by the device, then the communication can take place between two devices.
- If the device receives the MAC address, then the MAC address gets stored in the ARP cache.
RARP
It stands for Reverse Address Resolution Protocol. It is TCP/IP protocol which is used for converting physical address to IP address. RARP is used to assign IP address to the host who previously don't have any IP address. Reverse Address Resolution Protocol is a protocol using which a physical machine in a LAN could request to find its IP address from ARP table or cache from a gateway server. It was defined for booting diskless workstations. When a new machine is setup or any machine which don’t have memory to store IP address, needs an IP address for its own use. So the machine sends a RARP broadcast packet which contains its own MAC address in both sender and receiver hardware address field. Problems with RARP:
- It works on top of Ethernet so cannot be used for same subnet.
- It is used to assign IP address but not default gateway.
- It is not simple to implement.
