CCIE Routing and Switching Exam Certification Guide P2

Network Layer (OSI Layer 3)The network layer is concerned with the routing of information and methods to determine paths to a destination. Information at this layer is called packets. Specifications include routing protocols, logical network addressing, and packet fragmentation. Routers operate in this layer. The CCIE lives and dies in this layer. Examples of network layer specifications are the following:
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CCIE Routing and Switching Exam Certification Guide P2 The OSI Reference Model 21 Examples of data-link layer technologies are the following: • Frame Relay • Asynchronous Transport Mode (ATM) • Synchronous Data Link Control (SDLC) • High-level data-link control (HDLC) • IEEE 802.3z and IEEE 802.3ab (Gigabit Ethernet) • IEEE 802.3u (Fast Ethernet) • Ethernet version 2 • Integrated Services Digital Network (ISDN) • Point-to-Point Protocol (PPP) • Token Ring • Spanning-Tree Protocol (STP)Network Layer (OSI Layer 3) The network layer is concerned with the routing of information and methods to determine paths to a destination. Information at this layer is called packets. Specifications include routing protocols, logical network addressing, and packet fragmentation. Routers operate in this layer. The CCIE lives and dies in this layer. Examples of network layer specifications are the following: • Internet Protocol (IP) • Routing Information Protocol (RIP) • Open Shortest Path First (OSPF) • Enhanced Interior Gateway Routing Protocol (EIGRP) • Internetwork Packet Exchange (IPX) • Connectionless Network Protocol (CLNP)Transport Layer (OSI Layer 4) The transport layer provides reliable, transparent transport of data segments from upper layers. It provides end-to-end error checking and recovery, multiplexing, virtual circuit management, and flow control. Messages are assigned a sequence number at the transmission end. At the receiving end the packets are reassembled, checked for errors, and acknowledged. Flow control22 Chapter 2: Networking Concepts Review manages the data transmission to ensure that the transmitting device does not send more data than the receiving device can process. Examples of transport layer specifications are the following: • Transmission Control Protocol (TCP) • Real-Time Transport Protocol (RTP) • Sequenced Packet Exchange (SPX) • AppleTalk’s Transaction Protocol (ATP) • User Datagram Protocol (UDP) (provides unreliable transport at this layer with less overhead than TCP)Session Layer (OSI Layer 5) The session layer provides a control structure for communication between applications. It establishes, manages, and terminates communication connections called sessions. Communi- cation sessions consist of service requests and responses that occur between applications on dif- ferent devices. The management of sessions involves the synchronization of dialog control by using checkpoints in the data stream. Examples of specifications that operate at the session layer are the following: • NetBIOS • Real-Time Control Protocol (RTCP) • Session Control Protocol (SCP) • AppleTalk’s Zone Information Protocol (ZIP) • DECnet’s Session Control Protocol (SCP) • H.323, H.245, H.225 • Real-Time Control Protocol (RTCP)Presentation Layer (OSI Layer 6) The presentation layer provides data representation with a variety of coding and conversion functions. These functions ensure that data sent from a sending application on one system is readable by the application layer on another system. This layer provides the conversion of character representation formats, data compression schemes, and encryption schemes. Voice coding schemes are specified at this layer. The OSI Reference Model 23 Examples of specifications that operate at the presentation layer are the following: • Abstract Syntax Notation 1 (ASN.1) • ASCII • EBCDIC • Motion Picture Experts Group (MPEG) • Graphics Interchange Format (GIF) • Joint Photographic Experts Group (JPEG) • Tagged Image File Format (TIFF) • G.711, G.729a, G.726, G.728Application Layer (OSI Layer 7) The application layer provides the user or operating system access to the network services. It interacts with software applications by identifying communication resources, determining network availability, and distributing information services. It also provides synchronization between the peer applications that reside on separate systems. Examples of application layer specifications are the following: • Telnet • File Transfer Protocol (FTP) • Simple Mail Transfer Protocol (SMTP) • Simple Network Management Protocol (SNMP) • Network File System (NFS) • Association Control Service Element (ACSE) • Remote Operations Service Element (ROSE)Example of Layered Communication If you use a Telnet application, Telnet maps into the top three layers of the OSI model. Figure 2-3 shows that a user on Host 1 enables the Telnet application to access a remote host (Host 2). The Telnet application provides a user interface (application layer) to network services. As defined in Request For Comments (RFC) 854, ASCII is the default code format used (presen- tation layer). There is no session layer defined for Telnet; it is not an OSI protocol. Per the RFC, Telnet uses TCP for connectivity (transport layer). The TCP segment gets placed into an IP packet (network layer) with a destination ...