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Section B
Q. Explain the TCP/IP Reference Model in detail along with functions of its each layer.
The TCP/IP reference model was developed prior to OSI model. The major design goals of this model were,
- To connect multiple networks together so that they appear as a single network.
- To provide a flexible architecture.
- To survive after partial subnet hardware failures.
Unlike OSI reference model, TCP/IP reference model has only 4 layers. They are,
- Host-to-Network Layer
- Internet Layer
- Transport Layer
- Application Layer
Host-to-Network Layer: The TCP/IP reference model does not really say much about what happens here, except to point out that the host has to connect to the network using some protocol so it can send IP packets to it. This protocol is not defined and varies from host to host and network to network. Internet Layer: This layer, called the internet layer, is the linchpin that holds the whole architecture together. Its job is to permit hosts to inject packets into any network and have they travel independently to the destination (potentially on a different network). They may even arrive in a different order than they were sent, in which case it is the job of higher layers to rearrange them, if in-order delivery is desired. Note that ''internet'' is used here in a generic sense, even though this layer is present in the Internet. The internet layer defines an official packet format and protocol called IP (Internet Protocol). The job of the internet layer is to deliver IP packets where they are supposed to go. Packet routing is clearly the major issue here, as is avoiding congestion. For these reasons, it is reasonable to say that the TCP/IP internet layer is similar in functionality to the OSI network layer. Fig. shows this correspondence.
The Transport Layer: The layer above the internet layer in the TCP/IP model is now usually called the transport layer. It is designed to allow peer entities on the source and destination hosts to carry on a conversation, just as in the OSI transport layer. Two end-to-end transport protocols have been defined here. The first one, TCP (Transmission Control Protocol), is a reliable connection- oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine in the internet. It fragments the incoming byte stream into discrete messages and passes each one on to the internet layer. At the destination, the receiving TCP process reassembles the received messages into the output stream. TCP also
handles flow control to make sure a fast sender cannot swamp a slow receiver with more messages than it can handle.
Fig. The TCP/IP reference model.
The second protocol in this layer, UDP (User Datagram Protocol), is an unreliable, connectionless protocol for applications that do not want TCP's sequencing or flow control and wish to provide their own. It is also widely used for one-shot, client-server-type request-reply queries and applications in which prompt delivery is more important than accurate delivery, such as transmitting speech or video. The relation of IP, TCP, and UDP. Since the model was developed, IP has been implemented on many other networks.
The OSI and TCP/IP reference models have much in common. Both are based on the concept of a stack of independent protocols. Also, the functionality of the layers is roughly similar. For example, in both models the layers up through and including the transport layer are there to provide an end-to-end, networkindependent transport service to processes wishing to communicate. These layers form the transport provider. Again in both models, the layers above transport are application-oriented users of the transport service. Despite these fundamental similarities, the two models also have many differences Three concepts are central to the OSI model:
- Services.
- Interfaces.
- Protocols.
Probably the biggest contribution of the OSI model is to make the distinction between these three concepts explicit. Each layer performs some services for the layer above it. The service definition tells what the layer does, not how entities above it access it or how the layer works. It defines the layer's semantics. A layer's interface tells the processes above it how to access it. It specifies what the parameters are and what results to expect. It, too, says nothing about how the layer works inside.
Finally, the peer protocols used in a layer are the layer's own business. It can use any protocols it wants to, as long as it gets the job done (i.e., provides the offered services). It can also change them at will without affecting software in higher layers. The TCP/IP model did not originally clearly distinguish between service, interface, and protocol, although people have tried to retrofit it after the fact to make it more OSI-like. For example, the only real services offered by the internet layer are SEND IP PACKET and RECEIVE IP PACKET.
As a consequence, the protocols in the OSI model are better hidden than in the TCP/IP model and can be replaced relatively easily as the technology changes. Being able to make such changes is one of the main purposes of having layered protocols in the first place. The OSI reference model was devised before the corresponding protocols were invented. This ordering means that the model was not biased toward one particular set of protocols, a fact that made it quite general. The downside of this ordering is that the designers did not have much experience with the subject and did not have a good idea of which functionality to put in which layer. Another difference is in the area of connectionless versus connection-oriented communication. The OSI model supports both connectionless and connection-oriented communication in the network layer, but only connection-oriented communication in the transport layer, where it counts (because the transport service is visible to the users). The TCP/IP model has only one mode in the network layer (connectionless) but supports both modes in the transport layer, giving the users a choice. This choice is especially important for simple request-response protocols.
Q. What do you understand by the term “internet”
The Internet
What “internet” means is interconnected networks, but what happens if you join up a few thousand ethernets, point to point links, star networks (like ATM), etc.? Nothing, they all have different packet formats, addresses, protocols and capabilities, so they cannot exchange data. It is necessary to have software on every machine (hosts on networks and on machines that join networks) that can make them work together—this software is IP. It is the network layer protocol IP that is the Internet. How it works:
- every network has a unique address, every machine on each network has a unique address. These two addresses are combined together as the IP address,
- all machines that will use the network have the IP protocol software installed,
- data is sent it a fixed format “packet” known as an IP datagram,
- each separate network is joined to one or more other networks by one or more routers that know how to reach any network on the Internet,
- when an ordinary host sends a packet to an IP address the IP protocol software consults its local forwarding table that tells it whether to send it direct to a machine on the local network, or to send it to a router.
Q. Explain the concept of IP addresses and IP address classes.
IP addresses
Every host connected to an internet must have a unique IP address on that network. The address in IPv4 is a 32 bit number. It is usually represented as 4, 8 bit numbers separated by dots, for example: 147.197.205.211 In order to address different networks on an internet the address is structured into a net- work part and a host part. So the University of Hertfordshire network address is 147.197 and one host on it is 205.211. Not all networks have a 16 bit address. The NIC allocates network addresses to organisations which in turn are responsible for allocating their own host addresses.
Type A: If the first bit is 0 (the first 8 bit field is less than 127) then that’s the network address and the host address is 24 bits, there are only just over 100 of these and each can have over 16 million hosts on their nets,
Type B: If the first two bits are “10” then the network address is the next 14 bits that means there are about 16000 of these networks, each with upto 65000 hosts,
Type C: For smaller organisations if the first 2 bits are “110” than the network address is the following 22 bits and there is only an 8 bit host number.
stream, in this context, means connection- oriented: A connection must be established between both ends of a transmission before either can transmit data. At the sending end of each transmission, TCP divides a stream of data into smaller units called segments. Each segment includes a sequence number for reordering after receipt, together with an acknowledgment number for the segments received. Segments are carried across the internet inside of IP datagrams. At the receiving end, TCP collects each datagram as it comes in and reorders the transmission based on sequence numbers.
Stream Control Transmission Protocol: The Stream Control Transmission Protocol (SCTP) provides support for newer applications such as voice over the Internet. It is a transport layer protocol that combines the best features of UDP and TCP.
Application Layer: The application layer in TCP/IP is equivalent to the combined session, presentation, and application layers in the OSI model. Many protocols are defined at this layer.
Q. What are FTP and TFTP?
FTP and TFTP
FTP or File Transfer Protocol and TFTP or Trivial File Transfer Protocol are both optimized for downloading and uploading data. FTP uses TCP as its transport protocol on port 20 for data transfer and on port 21 for control (commands). TFTP uses UDP on port 69. One of the main differences between FTP and TFTP is speed. Because UDP does not perform error checking or correction, TFTP is faster than the FTP. FTP provides a generic method of transferring files. It can include security through usernames and passwords, and it allows file transfer between dissimilar computer systems. FTP can transfer both binary and text files, including HTML, to another host. FTP URLs are preceded by ftp:// followed by the DNS name of the FTP server. To log in to an FTP server, use:ftp://username@servername. TFTP is similar to FTP. It lets us transfer files between a host and an FTP server. However, it provides no user authentication and no error detection. TFTP is often used when transferring files such as video, audio, or images. Because it does not perform error detection, TFTP is faster than FTP, but might be subject to file errors. One other file transfer protocol that we need to know about is SFTP. The S in this case stands for Secure. So we have Secure File Transfer Protocol. SFTP protocol uses another protocol called SSH to provide encryption of data and user authentication. SSH ensures that SFTP transmissions use encrypted commands and data which prevent data from being transmitted over the network in clear text. SFTP uses TCP as the transport protocol on port 115.
Q. What is the use of SMTP, POP and IMAP protocols?
SMTP, POP and IMAP:
Another set of protocol that we’ll talk about that are email transfer protocols. The first is the Simple Mail Transport Protocol or SMTP. SMTP is used between email servers and clients on each end that need to send mail. SMTP is used by email clients to send mail to the mail server. Then it’s used between mail servers to send mail from one server to the next. SMTP uses TCP
transport protocol on port 25. On the end, a different protocol may be used by the client to download or receive the mail. For example, client can use a protocol called Post Office Protocol or POP, or POP3 which is the third version of POP. By some e-mail client programs, such as Microsoft Outlook, SMTP can be used for receiving mail from an Exchange server. POP3 is used to retrieve e-mail from a remote server to a local client over a TCP/IP connection. With POP3, e-mail messages are downloaded to the client. Remember, an e-mail client that uses POP3 for receiving mail, uses SMTP for sending mail. POP3 uses TCP as the transport protocol on port
- Another protocol used by clients to download email is IMAP. There are various versions of IMAP as well, IMAP4 being the latest one. IMAP supports both on-line and off-line modes of operation. E-mail clients using IMAP generally leave messages on the server until the user explicitly deletes them. This and other characteristics of IMAP operation allow multiple e-mail clients to manage the same mailbox. IMAP offers access to the mail store which means that with IMAP the mail stays on the server, so as we read the mail it’s not copied to the local system. Clients may store local copies of the messages, but these are considered to be a temporary cache. Remember, an e-mail client that uses IMAP for receiving mail uses SMTP for sending mail. IMAP uses TCP as the transport protocol on port 143.
DNS
Another great protocol is DNS or the Domain Name System. DNS is a system that is distributed throughout the internetwork to provide address to name resolution. DNS allows us to use host names to identify computers instead of using an IP address. For instance, www.google.com is a logical name that identifies a web server. When we use this name we initiate the HTTP protocol to read the information that’s on that server. DNS allows us to use logical names like this instead of remembering an IP address for the computer that we need to get to. DNS uses TCP and UDP as transport layers, on port 53.
NNTP
Another protocol is NNTP or Network News Transport Protocol. Network news transport protocol is used for reading news messages on the Internet. It’s often used with usenet news groups. NNTP uses TCP as the transport protocol on port 119.
SNMP
These protocols are used for network management. SNMP or Simple Network Management Protocol is a protocol designed for managing complex networks. SNMP lets network hosts exchange configuration and status information. Information can be gathered by management software and used to monitor and manage the network. SNMP allows us to take a central computer and communicate with other devices on the network to find out information about those devices. We can send commands to other devices, tell them to do things like to start-up and to shutdown, or we can just monitor those computers. SNMP is the protocol that enables the communication and also the transfer of information from those other computers. SNMP uses UDP as the transport protocol on port 161.
