Types of connections -Peer to peer connection
- 1. COC3130: Computer Networks Ash Mohammad Abbas Lecture 11 Chapter 2: Application Layer
- 2. Peer-to-Peer (P2P) Architecture • In a P2P architecture, there is minimal (or no) reliance on dedicated servers in data centers. • Instead the application exploits direct communication between pairs of intermittently connected hosts, called peers. • The peers are not owned by the service provider, but are instead desktops and laptops controlled by users, with most of the peers residing in homes, universities, and offices. • Because the peers communicate without passing through a dedicated server, the architecture is called peer-to-peer (P2P). • Many of today’s most popular and traffic-intensive applications are based on P2P architectures. • These applications include file sharing (e.g., BitTorrent), peer- assisted download acceleration (e.g., Xunlei), and Internet telephony and video conference (e.g., Skype).
- 3. Application Layer 2-3 P2P architecture • no always-on server • arbitrary end systems directly communicate • peers request service from other peers, provide service in return to other peers • self scalability – new peers bring new service capacity, as well as new service demands • peers are intermittently connected and change IP addresses • complex management peer-peer
- 4. Hybrid of Client-and-Server and P2P • Some applications have hybrid architectures, combining both client-server and P2P elements. • For example, for many instant messaging applications, servers are used to track the IP addresses of users, but user-to-user messages are sent directly between user hosts (without passing through intermediate servers). • One of the most compelling features of P2P architectures is their self-scalability. • For example, in a P2P file-sharing application, although each peer generates workload by requesting files, each peer also adds service capacity to the system by distributing files to other peers. • P2P architectures are also cost effective, since they normally don’t require significant server infrastructure and server bandwidth (in contrast with clients-server designs with data centers). • However, P2P applications face challenges of security, performance, and reliability due to their highly decentralized structure.
- 5. Process Communication • Before building your network application, you also need a basic understanding of how the programs, running in multiple end systems, communicate with each other. • In the jargon of operating systems, it is not actually programs but processes that communicate. • A process can be thought of as a program that is running within an end system. • When processes are running on the same end system, they can communicate with each other with inter-process communication, using rules that are governed by the end system’s operating system. • But in this course we are not particularly interested in how processes in the same host communicate, but instead in how processes running on different hosts (with potentially different operating systems) communicate. • Processes on two different end systems communicate with each other by exchanging messages across the computer network. • A sending process creates and sends messages into the network; a receiving process receives these messages and possibly responds by sending messages back.
- 6. Client and Server Processes • A network application consists of pairs of processes that send messages to each other over a network. • For each pair of communicating processes, we typically label one of the two processes as the client and the other process as the server. • For example, in the Web application a client browser process exchanges messages with a Web server process. • With the Web, a browser is a client process and a Web server is a server process. • In a P2P file-sharing system, a file is transferred from a process in one peer to a process in another peer. • With P2P file sharing, the peer that is downloading the file is labeled as the client, and the peer that is uploading the file is labeled as the server.
- 7. Client and Server Processes • In some applications, such as in P2P file sharing, a process can be both a client and a server. • Indeed, a process in a P2P file-sharing system can both upload and download files. • Nevertheless, in the context of any given communication session between a pair of processes, we can still label one process as the client and the other process as the server. • We define the client and server processes as follows: • In the context of a communication session between a pair of processes, the process that initiates the communication (that is, initially contacts the other process at the beginning of the session) is labelled as the client. • The process that waits to be contacted to begin the session is the server.
- 8. Client and Server Processes • In the Web, a browser process initializes contact with a Web server process; hence the browser process is the client and the Web server process is the server. • In P2P file sharing, when Peer A asks Peer B to send a specific file, Peer A is the client and Peer B is the server in the context of this specific communication session. • When there’s no confusion, we’ll sometimes also use the terminology “client side and server side of an application.”
- 9. Interface Between Process and Network • Most of the applications consist of pairs of communicating processes, with the two processes in each pair sending messages to each other. • Any message sent from one process to another must go through the underlying network. • A process sends messages into, and receives messages from, the network through a software interface called a socket. • A process is analogous to a house and its socket is analogous to its door. • When a process wants to send a message to another process on another host, it shoves the message out its door (socket). • This sending process assumes that there is a transportation infrastructure on the other side of its door that will transport the message to the door of the destination process. • Once the message arrives at the destination host, the message passes through the receiving process’s door (socket), and the receiving process then acts on the message.
- 10. Application Layer 2-10 Sockets • process sends/receives messages to/from its socket • socket analogous to door • sending process shoves message out door • sending process relies on transport infrastructure on other side of door to deliver message to socket at receiving process Internet controlled by OS controlled by app developer transport application physical link network process transport application physical link network process socket
- 11. Socket • A socket is the interface between the application layer and the transport layer within a host. • It is also referred to as the Application Programming Interface (API) between the application and the network, since the socket is the programming interface with which network applications are built. • The application developer has control of everything on the application layer side of the socket but has little control of the transport-layer side of the socket. • The only control that the application developer has on the transport-layer side is (1) the choice of transport protocol and (2) perhaps the ability to fix a few transport-layer parameters such as maximum buffer and maximum segment sizes. • Once the application developer chooses a transport protocol (if a choice is available), the application is built using the transport-layer services provided by that protocol.
- 12. Addressing Processes • In order for a process running on one host to send packets to a process running on another host, the receiving process needs to have an address. • To identify the receiving process, two pieces of information need to be specified: (1) the address of the host, and (2) an identifier that specifies the receiving process in the destination host. • In the Internet, the host is identified by its IP address. • In addition to knowing the address of the host to which a message is destined, the sending process must also identify the receiving process (more specifically, the receiving socket) running in the host. • This information is needed because in general a host could be running many network applications. • A destination port number serves this purpose. • Popular applications have been assigned specific port numbers. For example, a Web server is identified by port number 80. A mail server process (using the SMTP protocol) is identified by port number 25.