OSI Architecture

• Internet has multiple protocol layers
• Each layer implements a service, relying on services provided by layers below
• Enables encapsulation

Example of layered network system:

1. Application programs (Application Layer)
2. Process-to-process channels (Transport Layer)
3. Host-to-host connectivity (Network Layer and Link Layer (?))
4. Hardware (Physical Layer)

Internet protocol stack (OSI 7-layer model)

• The bottom 3 layers are implemented on all nodes
  • Network, data link, physical
• The top layers typically run only on end-to-end hosts rather than intermediate switches and routers
  • Application, presentation, session, transport

Application layer

Application layer

• Standardize common types of exchanges
• e.g. FTP, SMTP, HTTP

Addressing

• In previous layers, we used IP addresses
• Now, we use URLs and email addresses (provided by human)
• So, need a database mapping URLs to IP addresses
  • Distributed
  • Hierarchical
  • At the application layer
• DNS!

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Presentation layer

Presentation layer

• Concerned with format of data exchanged between peers
  • Includes data encryption

Considered part of Transport Layer

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Session layer

Session layer

• Provides namespace to tie together the potentially different transport streams that are part of a single application

Considered part of Transport Layer

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Transport layer

Transport-layer

Network layer

Network layer

Routing of packets from source to destination

• IP, routing protocols
• Ensures data delivery
• Unit of data exchanged between nodes in this layer is packet

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Link layer

Link layer

• Data transfer between neighboring network elements
• e.g. Ethernet, 802.11 (WiFi), P2P/PPP
• Collects a stream of bits into a larger aggregate called a frame
• Network adapters, along with device drivers in OS implement the protocol in this layer
• Frames are actually delivered to hosts

Tip

At the link layer, everything is a broadcast, and unicast is only an illusion.

Link Layer Responsibilities

• Single-hop addressing (e.g. Ethernet addresses)
• Media access control: Multi-Access Protocol
  • Link-layer congestion control
  • Collision detection/collision avoidance
• Single-hop acknowledgments
  • Automatic Repeat Query (ARQ)
  • Flow control
  • Not implemented in wired LAN (e.g. Ethernet)

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Physical layer

Physical-layer