How Packet Data Travels through the Networks - Case Study Example

The network and protocol processes that occur in the network when you access X-Stream from a remote location Introduction In a computer network, thecomputer system relies on a set of protocols namely TCP/IP protocol suite for information exchange, storage, and communication. A network protocol can be considered as a set of rules which specifies the meaning and format of data exchanged between computers. While accessing an X-stream server from a remote location like home, several network and protocol processes take place in the network. This report is intends to provide an insight on how packet data travels through the networks, the role of various protocols, and about various processes that occur within the network. Internet Access Nowadays, high speed broadband connection using DSL (digital subscriber line) or cables provides internet access in homes. In home networks, Ethernet is one of most widely used internet access technologies (Kurose & Ross, 2005, p. 16). In Ethernet, twisted pair copper wires are used for connecting to the Ethernet switch. These Ethernet switches are then used to connect to the larger internet. Wireless LAN technologies are also used in home networks. In this case, a wireless LAN user send or receives packets to or from the access pointes which are connected to enterprise network(wired Ethernet) and these networks again gets connected to the larger wired internet (Ibid). The data or bits travels from one end system to other through physical medium. There are several types of physical media like coaxial cable, twisted pair copper wire, optic fiber, terrestrial radio spectrum etc. The physical media is normally categorized as guided or unguided media, where in guided media data signals are transmitted through solid medium where as in unguided media, signals propagate through atmosphere or outer space. Moreover, signals are also transmitted through a hybrid media which involves both guided as well as unguided mediums. TCP/IP protocol stack In order to organize the networking architecture, the network protocols as well as the software and hardware which implement these protocols are organized into several layers called as protocol stack (Oracle System Administration Guide: IP Services, 2011.). Each of the layer has got specific purpose and each of them exist in both senders and receivers system. A particular layer in one system works parallel with the same layer on the other system and they are independent from the tasks of other layers above or below them. TCP/IP architectural model combines some layers in OSI model and has got five different layers which include application layer, transport layer, network layer, link layer and physical layer (Ibid). Application layer: Application layer is the visible layer in which applications as well as application protocol resides. Users interact in the network using the applications in this layer. This layer includes various protocols like HTTP (for request and transfer of web documents), SMTP (for email transfer), FTP (for file transfer) and DNS (for translating URL to 32 bit network address), Simple network management protocol (SNMP), Trivial file transfer protocol (TFTP) etc. The protocols in the application layer is distributed in multiple end systems, where protocols used by the application in one end system exchanges data packets with application within another end system. The data packets in application layer are referred as messages. Transport layer: The function of the transport layer is to transfer application layer messages between the application end points. This layer uses two protocols namely TCP and UDP. TCP is connection oriented service which guarantees delivery of application layer messages whereas UDP is connectionless service which provides no reliability or flow control. Transport layer packets can be called as segments. Network layer: The transport layer protocols namely TCP/UDP passes a segment with the destination address to the network layer which delivers this segment to the destination host’s transport layer. Network layer packets can be called as datagram’s. It includes the IP protocol which defines the datagram fields and routing protocols. Link layer: The network layer transfers datagram’s through several routers to reach the source or destination with the help of link layer. At each node, network layer passes the datagram to the link layer and the link layer delivers the datagram’s to the next node in the route. Services offered by the link layer depend on the specific protocol used. Different link layer protocols are Ethernet, wifi, PPP etc. link layer packets are also known as frames. Physical layer: The physical layer moves individual bits in the frame from one node to other. The protocols used within this layer depend on the link layer and the transmission medium used in the link (eg: Ethernet). How packet travels through the TCP/IP stack and network? The source end system sends messages to destination end system by breaking the messages into small data chunks encapsulated in packets. These packets then travel through communication links and packet switches which include routers as well as link layer switches. A packet is made of payload which is the raw data chunks and has a header which holds the address of source or destination end system (Oracle System Administration guide, 2011). When a packet travels through different layers, some header information are added or removed from the packet header. The process of adding data to the packet header of the sending system by protocols can be called as data encapsulation. When user initiates a command through the application layer using application layer protocols like HTTP or FTP, several processes are initiated (Ibid). Hypertext Transfer Protocol HTTP helps to establish connection between client and server and distributes information in World Wide Web. It uses port number 80. It defines the structure of http messages and how these messages are exchanged between client and server. An HTTP client can be called as web browser (firefox, internet explorer.etc) and HTTP server can be Internet Information Server (IIS), Apache Web Server etc. When user request a webpage through browser, http request messages are send to the server and when server receives the message it respond with the specified webpage along with HTTP response messages. File transfer Protocol FTP is used to transfer files to and from remote computer. During an FTP session, FTP client initiate a TCP connection with the remote host server through port number 21. FTP client then sends user identification and password to the remote host. When the server side authorizes the ftp client, the user can transfer one or more files to the remote file system. What happens in application layer? The messages from application layer travels through different TCP/IP protocol stacks on the local system, which then passes through the network media and by passing through the different protocol stacks reaches receiving end system. In the application layer, user issues a command or sends a message to the remote server. The message is encapsulated with application layer headers and passed to transport layer for transport layer protocols like TCP or UDP to handle. What happens in transport layer? The transport layer on receiving the message, encapsulate the data in a TCP or UDP header and makes it a segment and add source and destination UDP or TCP port to the segment. TCP connection TCP is connection oriented protocol and hence it makes sure that data is delivered correctly at the receiving end (Bonaventure, 2012, p.89). TCP adds header to the segment received from the transport layer which includes source and destination ports, sequence number (32 bits), acknowledgement number (32bits), window fields, flags and checksum field. The checksum field is used to know whether data is transferred without error. Using segment TCP make sure that the receiving end system is ready for data receiving. For the TCP at the sending end system to establish connection, a segment called SYN is send to the TCP protocol at the receiving end .on receiving this segment, the receiving end TCP protocol send a segment called ACK to acknowledge that segment is received. After this, the TCP protocol at sending end again sends a segment called ACK and begins data sending. This kind of establishing connection and exchanging control information is generally called as ‘three way handshakes’ (Ibid, p. 90). UDP Connection UDP is connectionless protocol and will not ensure the successful delivery of data at the receiving end. Here application layer messages are made as UDP packets by attaching a header to it. The header has source and destination port, 16 bit length field and 16 bit checksum. UDP protocol at the sending end sends this packet to the UDP protocol at the receiving end. The sending of acknowledgement by receiving end depends on the application layer protocol. What happens in network layer? The segments from the transport layer are received by network layer for making packets ready for delivery. The segments on reaching the network layer are handled by IP (internet protocol). The segments are encapsulated by IP header and made in to units called datagram. Internet protocol and IP datagram IP protocol is the most significant one in the TCP/IP protocols suite. It performs functions like IP addressing, host to host communication, packet formatting and fragmentation. The IP protocol encapsulate the segments in transport layer with an IP header which includes IP address of sending and receiving end system, length of datagram and its sequence order .if a datagram is large, the link and physical layers may not be able to handle these data chunks. In this case, large datagrams are broken into packed sized chunks which link or physical layer can handle. This process is called Fragmentation. Domain name services IP addresses are obtained from domain names using Domain name service (DNS). DNS translates the hostnames or domain names to corresponding IP address through a DNS database which has a record that maps user-friendly hostnames to the IP address and has an application protocol which assist host to query this database.DNS works between application layer and network layer. Thus an IP datagram with source and destination IP addresses and other fields is then routed through the link layer. What happens in link layer? In link layer, protocols like PPP and Ethernet encapsulate the IP datagram with third header and trailer to make it a frame. Frame has got a source and destination MAC address and a frame header which includes a field namely cyclic redundancy check (CRC). CRC field is used to detect errors when the frame travels through the network. MAC or Media access control layer address are the unique identifier of the network interfaces. It is also called as physical address or hardware address. Here the MAC address is obtained from the IP address using Address resolution protocol (ARP). Address Resolution Protocol The link layer uses ARP protocol for translation of IP addresses to MAC layer addresses. In link layer delivery of frames from source to source, or router to router, or router to source is through local or physical address called MAC layer address. Similarly, RARP (Reverse Address resolution Protocol) is used for getting IP addresses for machine within a LAN from gateway servers ARP table (ARP and RARP, Chapter 7). What happens in Physical layer? Frames from the link layer are then transferred to the physical layer. In the physical layer frames are converted into signals appropriate to the type of media used and send through the network media. Processes that occur in the end system The data packets reaches the receiving end and they travels through the protocol stack in the reverse order of sending. At each layer header information is de-encoded from the data packet. At the physical layer of receiving end system, data packets are received in the frame form. The physical layer then calculates the cyclic redundancy check (CRC) of the frame and forwards the frames to the link layer. In the link layer of receiving end system, the CRC is checked for errors and frame header as well as CRC is removed by the link layer and the frame is forwarded to the upper network layer. The network layer on receiving the frame checks its source and destination IP address and make sure that Destination address is same as that of the receiving end system. The network layer also checks whether the packet is fragmented. If it is fragmented, the internet protocol assembles these fragments into datagram. Internet protocol then removes the IP header from the datagram and transfers the datagram to the transport layer. In the transport layer, header information of the datagram is removed and transport layer protocols analyze the segments and based on port number and determines which transport layer protocol (TCP or UDP) needs to handle the message. The message is then send to the receiving ends application layer service. The application layer receives the message and process the request of the sending end system. Conclusion In this report the TCP/IP protocol stack architecture, the various processes that takes place in TCP/IP protocol layers and the changes that occurs for the packet headers as they travel through the different TCP/IP layers and network are studied. The role of important protocols like TCP, UDP, IP, DNS, and ARP etc in handling the data and the various processes that occur in the end system and network is also discussed. References ARP and RARP. Chapter 7. (n.d.). [online] available at: http://medusa.sdsu.edu/network/CS576/Lectures/ch07_ARP.pdf [accessed 14 May 2014]. Bonaventure, O. 2012. Computer Networking : Principles, Protocols and Practice. Google Books. Kurose, J. F & Ross, K. W. 2005. Computer networking: a top-down approach featuring the internet. New Delhi: Pearson Education India. Oracle System Administration Guide: IP Services. 2011. [online] available at: http://docs.oracle.com/cd/E18752_01/pdf/816-4554.pdf [accessed 14 May 2014]. Read More