OSI Model
• Communications over networks are made possible by protocols
• A protocol is a set of rules and restrictions that define how data is transmitted over a network
Layer 1: Physical Layer
• Controls throughput rates, handles synchronization, manages line noise and medium access, and determines whether to use digital/analog signals or light pulses
• Electrical specifications, protocols, and interface standards includes
• EIA/TIA-232 and EIA/TIA-449, X.21, High-Speed Serial Interface (HSSI), SONET, V.24 and V.35, FHSS, DSSS, PSK, OFDM, QAM
• NICs, hubs, repeaters, concentrators, radios, antenna, and amplifiers
Layer 2: Data Link Layer
• Ethernet (IEEE 802.3), Token Ring (IEEE 802.5), ATM, FDDI, and CDDI
• Media Access Methods happen here: CSMA, Token Passing, Polling
• Some protocols and devices
– SLIP, PPP, ARP, L2F, L2TP, PPTP, ISDN, X.25
– Switches, bridge, Wireless access point, DTEs/DCEs
· 3-bytes in MAC address denote vendor; known as Organizationally Unique Identifier (OUI)
Layer 3: Network Layer
• Adds routing and addressing information to the data and packet includes source and destination IP
• ICMP, RIP, OSPF, BGP, IGMP, IP, IPSec, IPX, NAT, SKIP
• Manages error detection and node data traffic (traffic control)
• Routers and brouters
Non-IP Protocols
• Alternative to IP at the layer 3
• Internetwork Packet Exchange (IPX), AppleTalk, and NetBIOS Extended User Interface (NetBEUI)
• Potential security risk firewalls are unable to perform content filtering, must either block all or allow
Routing Protocols
· Distance vector routing protocols maintain a list of destination networks along with metrics of direction and distance as measured in hops. RIP, IGRP
· Link state routing protocols maintain a topography map of all connected networks and use this map to determine the shortest path to the destination. OSPF, IS-IS
Layer 4: Transport Layer
• Manages integrity of connection & controls session. Accepts a PDU (data passed between network)
• Controls addressing/referencing, establishes connections between nodes & defines session rules
• Session rules: data flow, integrity verification, and determine data loss
• Session rules established by handshaking process
• Includes mechanisms for segmentation, sequencing, error checking, controlling the flow of data, error correction, multiplexing, and network service optimization
• TCP, UDP, Sequenced Packet Exchange (SPX), SSL, TLS
Layer 5: Session Layer
• Responsible for establishing, maintaining, and terminating communication sessions
• It manages dialogue discipline or dialogue control (simplex, half-duplex, full-duplex), establishes checkpoints for grouping and recovery, and retransmits PDUs that have failed or been lost since the last verified checkpoint
• NFS, SQL, RPC, NetBIOS, PAP
Layer 6: Presentation Layer
• Responsible for encryption/decryption and compression/decompression
• Most file/data formats operate within this layer.
• ASCII, EBCDICM, TIFF, JPEG, MPEG, MIDI
Layer 7: Application Layer
• HTTP, FTP, LPD, SMTP, Telnet, TFTP, EDI, POP3, IMAP, SNMP, Network News Transport Protocol (NNTP), S-RPC, Secure Electronic Transaction (SET)
• Network device: gateway.
– IP-to-IPX gateway takes inbound communications and translates to IPX/SPX for outbound
Reference: Mike Chapple. “(ISC)2 CISSP Certified Information Systems Security Professional Official Study Guide.”
