Computer Networks

1. Computer Networks ●Network Layer ●Transport Layer ●Application Layer Kai Qian

3. Network Layer • The Network Layer is Layer 3 of the seven-layer OSI model of computer networking. • The Network Layer is responsible for routing packets delivery including routing through intermediate routers, whereas the Data Link Layer is responsible for Media Access Control, Flow Control and Error Checking.

4. Network Layer • The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination host via one or more networks while maintaining the quality of service functions.

5. Functions of the Network Layer • Connection model: connectionless communication For example, IP is connectionless, in that a frame can travel from a sender to a recipient without the recipient having to send an acknowledgement. Connection-oriented protocols exist at other higher layers of that model.

6. Functions of the Network Layer • Host addressing Every host in the network needs to have a unique address which determines where it is. This address will normally be assigned from a hierarchical system, so you can be "Fred Murphy" to people in your house, "Fred Murphy, Main Street 1" to Dubliners, or "Fred Murphy, Main Street 1, Dublin" to people in Ireland, or "Fred Murphy, Main Street 1, Dublin, Ireland" to people anywhere in the world. On the Internet, addresses are known as Internet Protocol (IP) addresses.

7. Functions of the Network Layer • Message forwarding • Since many networks are partitioned into sub networks and connect to other networks for wide-area communications, networks use specialized hosts, called gateways or routers to forward packets between networks. This is also of interest to mobile applications, where a user may move from one location to another, and it must be arranged that his messages follow him. Version 4 of the Internet Protocol (IPv4) was not designed with this feature in mind, although mobility extensions exist. IPv6 has a better designed solution.

8. Network Layer • Within the service layering semantics of the OSI network architecture the Network Layer responds to service requests from the Transport Layer and issues service requests to the Data Link Layer.

9. Network Layer: Protocols • IPv4/IPv6, Internet Protocol • DVMRP, Distance Vector Multicast Routing Protocol • ICMP, Internet Control Message Protocol • IGMP, Internet Group Multicast Protocol • PIM-SM, Protocol Independent Multicast Sparse Mode • PIM-DM, Protocol Independent Multicast Dense Mode • IPsec, Internet Protocol Security • IPX, Internetwork Packet Exchange • RIP, Routing Information Protocol • DDP, Datagram Delivery Protocol • BGP, Border Gateway Protocol

10. Relation to TCP/IP model • The TCP/IP model describes the protocol suite of the Internet (RFC 1122). This model has a layer called the Internet Layer, located above the Link Layer. In many text books and other secondary references the Internet Layer is often equated with OSI's Network Layer. However, this is misleading as the allowed characteristics of protocols (e.g., whether they are connection-oriented or connection-less) placed into these layer are different in the two models. The Internet Layer of TCP/IP is in fact only a subset of functionality of the Network Layer. It only describes one type of network architecture, the Internet.

11. Transport Layer • In computer networking, the Transport Layer provides end-to-end communication services for applications[1] within a layered architecture of network components and protocols. The transport layer provides convenient services such as connection-oriented data stream support, reliability, flow control, and multiplexing.

12. Introduction • To eliminate the diversity and unreliability of the network layer, we introduced the concept of transport layer. • The transport layer provides reliable, inexpensive information transferring service from source host to destination host.

13. Transport Layer • Transport layers are contained in both the TCP/IP model (RFC 1122),[2] which is the foundation of the Internet, and the Open Systems Interconnection (OSI) model of general networking. The definitions of the Transport Layer are slightly different in these two models. This article primarily refers to the TCP/IP model, in which TCP is largely for a convenient application programming interface to internet hosts, as opposed to the OSI model definition of the Transport Layer.

14. Transport Layer • The most well-known transport protocol is the Transmission Control Protocol (TCP). It lent its name to the title of the entire Internet Protocol Suite, TCP/IP. It is used for connection-oriented transmissions, whereas the connectionless User Datagram Protocol (UDP) is used for simpler messaging transmissions. TCP is the more complex protocol, due to its stateful design incorporating reliable transmission and data stream services. Other prominent protocols in this group are the Datagram Congestion Control Protocol (DCCP) and the Stream Control Transmission Protocol(SCTP).

15. Transport Layer: Services • There are many services that can be optionally provided by a Transport Layer protocol, and different protocols may or may not implement them.

16. Transport Layer: Services • Connection-oriented communication • Byte orientation • Same order delivery • Reliability • Flow control • Congestion avoidance • Multiplexing

17. Transport Layer • The Transport Layer is responsible for delivering data to the appropriate application process on the host computers. This involves statistical multiplexing of data from different application processes, i.e. forming data packets, and adding source and destination port numbers in the header of each Transport Layer data packet. Together with the source and destination IP address, the port numbers constitutes a network socket, i.e. an identification address of the process-to-process communication. In the OSI model, this function is supported by the Session Layer.

18. Transport Layer • Some Transport Layer protocols, for example TCP, but not UDP, support virtual circuits, i.e. provide connection oriented communication over an underlying packet oriented datagram network. A byte-stream is delivered while hiding the packet mode communication for the application processes. This involves connection establishment, dividing of the data stream into packets called segments, segment numbering and reordering of out-of order data.

19. Transport Layer • Finally, some Transport Layer protocols, for example TCP, but not UDP, provide end-to-end reliable communication, i.e. error recovery by means of error detecting code and automatic repeat request (ARQ) protocol. The ARQ protocol also provides flow control, which may be combined with congestion avoidance.

20. Transport Layer: UDP • UDP is a very simple protocol, and does not provide virtual circuits, nor reliable communication, delegating these functions to the application program. UDP packets are called datagram, rather than segments.

21. Transport Layer: TCP • TCP is used for many protocols, including HTTP web browsing and email transfer. UDP may be used for multicasting and broadcasting, since retransmissions are not possible to a large amount of hosts. UDP typically gives higher throughput and shorter latency, and is therefore often used for real-time multimedia communication where packet loss occasionally can be accepted, for example IP-TV and IP-telephony, and for online computer games.

22. Transport Layer: Protocols • The exact definition of what qualifies as a transport layer protocol is not firm. The following is a short list: • ATP, AppleTalk Transaction Protocol • CUDP, Cyclic UDP • DCCP, Datagram Congestion Control Protocol • FCP, Fiber Channel Protocol • IL, IL Protocol • NBF, NetBIOS Frames protocol • RDP, Reliable Datagram Protocol • SCTP, Stream Control Transmission Protocol • SPX, Sequenced Packet Exchange • SST, Structured Stream Transport • TCP, Transmission Control Protocol • UDP, User Datagram Protocol • UDP Lite • µTP, Micro Transport Protocol

23. Application Layer • The Internet Protocol Suite (TCP/IP) and the Open Systems Interconnection model (OSI model) of computer networking each specify a group of protocols and methods identified by the name Application Layer. • In TCP/IP, the Application Layer contains all protocols and methods that fall into the realm of process-to-process communications across an Internet Protocol (IP) network. Application Layer methods use the underlying Transport Layer protocols to establish host-to-host connections.

24. Application Layer • In the OSI model, the definition of its Application Layer is narrower in scope, explicitly distinguishing additional functionality above the Transport Layer at two additional levels, the Session Layer and the Presentation Layer. OSI specifies strict modular separation of functionality at these layers and provides protocol implementations for each layer.

25. Application Layer: TCP/IP protocols • Remote Login category • Telnet • File Transfer category • FTP • TFTP • Electronic Mail category • SMTP • IMAP • POP • Support Services category • DNS • RARP • BOOTP • SNMP • CMOT

26. Application Layer: TCP/IP • The Internet Protocol Suite is the set of communications protocols used for the Internet and other similar networks. It is commonly also known as TCP/IP named from two of the most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were the first two networking protocols defined in this standard. Modern IP networking represents a synthesis of several developments that began to evolve in the 1960s and 1970s, namely the Internet and local area networks, which emerged during the 1980s, together with the advent of the World Wide Web in the early 1990s.

27. Computer Networks • Ends Reference from Wikipedia.org