LM CCN Section 1.2: Layered Network Models
Computer and Communication Networks
Layered Network Models John Easton
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Session Overview
o Layered network models – OSI
– TCP/IP
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Layered Architectures
o Communications between machines across networks is complex
o Layering puts similar communications functionality into manageable groups
– Layers provide services to the layers above them
– Layers operate according to a particular protocol
– Each layer adds checksums and parity bits to allow recovery from failure
o As you’ve probably already guessed, the Internet relies on a layered model
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LM CCN Section 1.2: Layered Network Models
The OSI Reference Model
o A network architecture requires:
– Definition of layers
– Selection of protocols at each layer
o By 1970 manufacturers all had proprietary layered network models
– No cross-compatibility
o Open Systems Interconnection (OSI) model was an effort by ISO to enable multivendor communication
o Seven-layer, abstract model with specified protocols at each layer
o Published in 1984 as standard ISO 7498
Image by Pieter Brueghel the Elder [Public domain], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/e/e1/Brueghel-tower-of-babel.jpg
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The OSI Reference Model (Cont.)
The OSI Layers
o Application layer: e-mail, file transfer, network management, etc.
o Presentation layer: independent representation of data
o Session layer: dialogue control
o Transport layer: end-to-end transfer of data – error control, flow control, connection setup (TCP segment / UDP datagram)
o Network layer: transfer and routing of packets (IP)
o Data link layer: transfer of frames between network nodes
(Ethernet)
o Physical layer: transfer of bits over a channel, setup / release of physical connection
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LM CCN Section 1.2: Layered Network Models
Peer-to-Peer Communication Terminology
o Processes at layer n (of the 7) are called layer n entities
o Layer n+1 entities make use of layer n services below them via
the layer’s Service Access Point (SAP)
o Entities exchange Protocol Data Units (PDUs)
Service Data Unit (SDU), which is the information to communicate
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Layer Services
o The layer n SDU is the layer n+1 PDU, and is encapsulated in the layer n PDU
o The service provided by a layer can be connection-oriented or connectionless
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Headers and Trailers
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LM CCN Section 1.2: Layered Network Models
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Segmentation and Reassembly
o Layers may impose limits on the size of data blocks
– AnSDUfromlayernmaybe too large to handle as a single unit at layer n-1
o Sender side: SDU segmented over multiple PDUs
o Receiver side: SDU is reassembled from a sequence of PDUs
TCP/IP Network Architecture
o Standards development takes a long time
– Standards normally follow implementations in modern
software
o While OSI was being developed, TCP/IP emerged as an
alternative
o Distributed freely as part of Berkeley (BSD) Unix
– Numerous applications developed for free at Universities
– Led to TCP/IP dominance of the Internet
o Fewer layers, and does not enforce strict layers
o Heavily dependent on IP – the “narrow waist” of the Internet
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The “Narrow Waist”
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LM CCN Section 1.2: Layered Network Models
TCP/IP Network Architecture (Cont.)
o Although the “pure” TCP/IP model is fully layered, TCP/IP does not enforce this
o Applications can run directly on the Internet layer
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How the TCP/IP Layers Interact
*FCS – Frame Check Sequence (checksum)
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Basics of IP Packet Transfer
o IP packets transfer information between hosts
o Host A IP -> router(1) ->-> router(n) -> Host B IP
o IP layer in router determines next hop based on routing tables o Network interfaces transfer the packets across networks
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LM CCN Section 1.2: Layered Network Models
Example IP Utilities
o PING: Determine if a host is reachable – uses Internet Control Message Protocol (ICMP), designed to inform sending hosts of errors on route
o Traceroute: Determines the (possible) route a packet will follow to a target IP
o Netstat: Provides information about network status
o TCPdump: Capture and observe packet exchanges on a link
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Summary
o Layered network models – OSI
– TCP/IP
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