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Page 2 of 15 Open System Interconnect ModelOSI model has seven layers with different protocols defining different network functions at each layer.
- Protocol A network protocol is a standard set of rules that determines how systems will communicate across networks. Two different systems that use the same protocol can communicate and understand each other despite their differences, similar to how two people can communicate and understand each other by using the same language.
Application layer
Application layer protocols interact with actual programs, e.g. web browsers, and ftp clients. These protocols or services communicate with actual applications using standard APIs. Protocols include like SMTP, SNMP, HTTP, LPD, FTP, WWW, Telnet, and TFTP.
Some well known application ports are Telnet port 23, SMTP port 25, HTTP port 80, SNMP ports 161 and 162, FTP ports 21 and 20.
The application layer in the TCP/IP architecture model is equivalent to a combination of the application, presentation, and session layers in the OSI model.
Applications send requests to an API, which is the interface to the supporting protocol.
Figure 1Interaction of Application programs with Protocol through API
Presentation layer
It provides a common means of representing data in a structure that can be properly processed by the end system. Handles functions like data compression and encryption. Example includes graphic formats like TIFF, GIF, JPEG and ASCII, Unicode encodings and MIME. Session layer
Only used in a client/server model. It is involved in establishing client/server connection and subsequently maintaining its state. Protocols include SSL, NFS, NetBIOS, SQL and RPC.
The session layer protocol can enable communication between two applications to happen in three different modes: - Simplex Communication takes place in one direction.
- Half-duplex Communication takes place in both directions, but only one application can send information at a time.
- Full-duplex Communication takes place in both directions, and both applications can send information at the same time.
Session layer protocols control application-to-application communication, whereas the transport layer protocols handle computer-to-computer communication.
Different references can place specific protocols at different layers. For example, many references place the Secure Sockets Layer (SSL) protocol in the session layer, while other references place it in the transport layer. It is not that one is right or wrong. The OSI model tries to draw boxes around reality, but some protocols straddle the different layers. SSL is made up of two protocols - one works in the lower portion of the session layer and the other works in the transport layer. For purposes of the CISSP exam, SSL resides in the transport layer. Transport layer
Provides end-to-end (host-to-host) data transport services and establishes the logical connection between two communicating computers. Protocols include TCP, UDP, SPX, and SSL (some literature also say it is at session layer).
Transport layer formats /multiplex data from multiple applications into a stream to be prepared for transmission. - TCP It is a reliable and connection-oriented protocol, which ensures that packets are delivered to the destination computer. If a packet is lost during transmission, TCP has the capability to resend it. Provides reliability and ensures that the packets are delivered. There is more overhead in a TCP packet. TCP communication also include following TCP Handshake:
- Host sends a SYN packet
- Receiver answers with a SYN/ACK packet
- Host sends an ACK packet
- UDP It is a best-effort and connectionless protocol at transport layer. It does not have packet sequencing, flow and congestion control and the destination does not acknowledge every packet it receives. There is less overhead in UDP packet.
Network layer
Insert information into the packet’s header so that it can be properly routed. The main task is to support internet work addressing, packet forwarding and routing. Network layer protocols include IP, ICMP, RIP, OSPF, BGP and IGMP.
Protocols that work at this layer do not ensure the delivery of the packets. Data Link layer
It is only one that actually understands what kind of network or environment you are communicating. It format frames for proper technology.
The operating system format the data frame to properly transmit over networks (Token Ring, Ethernet, ATM or FDDI). It has two sub layers, LLC and MAC. Actual framing takes place at MAC layer, the lower sub layer.
Data link layer protocols include ARP, SLIP, PPP, RARP, L2F, L2TP, FDDI and ISDN. Each network technology has defined electronic signaling and bit patterns. Physical layer
This layer converts bits into some kind of electricity for transmission. At physical layer different interfaces are defined. Some of the standard interfaces include HSSI, X.21, EIA/TIA-232 and EIA/TIA-449.
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