CISSP: OSI Model and Its Protocols

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.”