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OSI Model and Protocols

September 19, 2022 , , , ,

OSI Model and Protocols 

The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes the functions of a network system into seven layers, helping different systems communicate over networks. It’s used for understanding, designing, and troubleshooting networked communications. Evidence supports the layer definitions and purposes from ISO and educational networking texts.

OSI Model and Protocols

OSI Model Layers 

  • Application (Layer 7): Interfaces for end-user network applications and services (e.g., web, email). “The application layer provides services for network applications” [ISO/IEC 7498-1].
  • Presentation (Layer 6): Data format translation, encryption, compression. “The presentation layer … provides independence to application processes from differences in data representation” [ISO/IEC 7498-1].
  • Session (Layer 5): Establishes, manages, and terminates sessions between applications. “The session layer establishes, manages and terminates connections between applications” [ISO/IEC 7498-1].
  • Transport (Layer 4): End-to-end transport and reliability (segmentation, flow control). “Transport layer… provides reliable, transparent data transfer” [RFC 1122].
  • Network (Layer 3): Logical addressing and routing. “Internet Protocol (IP) is the network layer protocol” [RFC 791].
  • Data Link (Layer 2): Framing, MAC addressing, link reliability. “The data link layer provides for the reliable transfer of information across the physical link” [ISO/IEC 7498-1].
  • Physical (Layer 1): Transmission of bits over media. “The physical layer is concerned with the transmission of unstructured bit streams over a physical medium” [ISO/IEC 7498-1].

Protocols and Technologies by OSI Layer

Physical (L1):

USB; DSL; ISDN; T1/E1; Ethernet Cabling; Infrared; Ether loop; Fiber Cables; FDDI (physical signaling/media component); Token Ring (physical signaling/media component). 

Evidence: “The physical layer defines electrical, mechanical, functional, and procedural characteristics” [ISO/IEC 7498-1]. Technologies like DSL, T1/E1, ISDN, and optical fiber specify physical transmission characteristics [ITU-T G.992.x; G.703; I.430/I.431; ITU-T G.652].

Data Link (L2):

  • Ethernet (MAC/LLC); ARP note: ARP straddles L2/L3 but operates over the link to resolve L3-to-L2 addressing. “ARP… resolves IPv4 addresses into MAC addresses” [RFC 826].
  • ATM (data link/Layer 2.5 technology with virtual circuits) [ITU-T I.361].
  • Frame Relay (L2 WAN) [Frame Relay Forum FRF.1].
  • FDDI (MAC protocol portion) [ANSI X3T9.5]; Token Ring (MAC) [IEEE 802.5].
  • L2TP (tunneling protocol operates over L2.5/encapsulation for PPP) [RFC 2661].
  • LLDP (Link Layer Discovery Protocol) [IEEE 802.1AB].
  • PAgP (Cisco) and LACP (IEEE 802.1AX) link aggregation control (L2).
  • STP (Spanning Tree Protocol) [IEEE 802.1D].
  • VTP (VLAN Trunking Protocol, Cisco) and VLAN (IEEE 802.1Q tagging and virtual LAN concept) [IEEE 802.1Q].
  • PPP and related PAP (PAP is an authentication protocol used with PPP; PPP is L2) [RFC 1661; RFC 1334].
  • X.25 (often described at L2/L3; its LAPB is L2) and X.25 PAD (access device/protocol using X.3/X.28/X.29).
  • NBF/NetBEUI (non-routable L2/L3-ish for small LANs; commonly categorized as L2/L3 local transport) [Microsoft KB].
  • STP/LLDP/LACP/PAgP all operate at L2 control plane [IEEE 802.1D/AB/AX].

Network (L3):

  • IP (IPv4/IPv6) [RFC 791; RFC 8200].
  • ICMP (control for IP) [RFC 792; RFC 4443].
  • ARP (address resolution between L3 and L2; specified for IPv4) [RFC 826].
  • IPX (Novell) [IPX Spec]; CLNP (Connectionless Network Protocol, ISO 8473].
  • NAT (address translation for IP) [RFC 3022].
  • AppleTalk DDP (network layer of AppleTalk) [Inside AppleTalk].
  • RIP, OSPF, EIGRP (routing protocols operating at network/control plane) [RFC 2453; RFC 2328; Cisco EIGRP Whitepaper].
  • VRRP and HSRP (first-hop redundancy at L3) [RFC 5798; Cisco HSRP].
  • L2TP is often used over IP/UDP as a tunneling mechanism (encapsulation at L3/4) [RFC 2661].
  • GRE (not listed, noted for context) is also L3 tunneling [RFC 2784].

Transport (L4):

  • TCP [RFC 9293], UDP [RFC 768], SCTP [RFC 4960], DCCP [RFC 4340].
  • SPX (IPX’s transport) [Novell SPX].
  • TUP (Telephone User Part, actually SS7 layer; not OSI L4 for IP networks—application/transport in telephony stack) [ITU-T Q.721–Q.725].
  • ESP and AH are IPsec headers at network/transport boundary; they protect L3 but may be colloquially grouped with transport/security. “AH and ESP provide integrity/confidentiality for IP” [RFC 4302; RFC 4303].
  • RTP Control Protocol (RTCP) accompanies RTP at transport/session boundary [RFC 3550].

Session (L5):

  • PPTP (Point-to-Point Tunneling Protocol; session/tunneling over GRE) [RFC 2637].
  • H.245 (H.323 control channel/session control) [ITU-T H.245].
  • NetBIOS (session services) [RFC 1001/1002].
  • RPC (Remote Procedure Call) provides session-style semantics [RFC 5531].
  • SDP (Session Description Protocol) describes sessions [RFC 8866].
  • SOCKS (proxy protocol, often viewed as session-layer service) [RFC 1928].
  • iSNS (discovery for iSCSI/FC, session/management) [RFC 4171].
  • RTCP also provides session control with RTP [RFC 3550].

Presentation (L6):

  • TLS and SSL (encryption/presentation security) [RFC 5246; RFC 8446].
  • XDR (External Data Representation) [RFC 4506]; NDR (Network Data Representation for DCE/RPC].
  • ASN.1-based encodings used by TCAP, SNMP, etc. (not explicitly listed except TCAP).
  • ToX (peer-to-peer network uses cryptographic presentation and application; categorization varies; mainly application-level but includes its own crypto framing).
  • ZIP (AppleTalk compression/envelope; sometimes seen as presentation/service within AppleTalk).

Application (L7):

  • DHCP [RFC 2131], DNS [RFC 1035], HTTP [RFC 9110/9112], NFS [RFC 8881], POP3 [RFC 1939], SMTP [RFC 5321], SNMP [RFC 1157/3411], FTP [RFC 959], SSH [RFC 4251], IMAP [RFC 9051], Telnet [RFC 854].
  • SMB/CIFS [MS-SMB2], AFP (Apple Filing Protocol) [Apple AFP], NCP (Novell Core Protocol) [Novell NCP], NDR/XDR used within higher-layer RPC.
  • iSCSI [RFC 7143] (runs over TCP; often described as L5–L7 storage protocol).
  • FCP (Fibre Channel Protocol mapping SCSI over FC; more in FC stack than OSI; often placed at application in OSI mapping).
  • AEP (AppleTalk Echo Protocol; application utility) [Inside AppleTalk].
  • H.245 signaling resides at session/application depending on model; H.323 family sits largely at L7.
  • SMPP (Short Message Peer-to-Peer) [SMPP 3.4], SOAP [W3C], SSDP (part of UPnP discovery over HTTPU) [UPnP], TCAP (Transaction Capabilities for SS7 signaling—telecom application layer) [ITU-T Q.771–Q.775].
  • LPP (Location Protocol term is ambiguous; if 3GPP LPP, it’s a telecom application protocol) [3GPP TS 36.355].
  • ICA (Citrix Independent Computing Architecture) application presentation/virtual app remoting [Citrix].
  • PAP (already noted under PPP; authentication sub-protocol at L2 control but sometimes grouped with application auth).
  • NetBEUI/NBF often expose NetBIOS services at application level on small LANs.
  • SDP/RTCP are used alongside RTP-based applications (VoIP, streaming); RTP itself is often classed between L4 and L5 (not explicitly requested).