Queueaka buffer preceding link in buffer has finitecapacity Packet arriving to

Queue(aka buffer) preceding link in buffer has finitecapacity  Packet arriving to full queuedropped (aka lost)  Lost packet may beretransmitted by previous node, by source end system, or not at all A B packet being transmitted packet arriving to full buffer is lost buffer (waiting area)

Overview  Courseoverview  Introduction to theInternet  TheInternet layered architecture

What is layering?

Layering by analogy with postal services

Why layering?  Problem: Without layering, each new application has to be re- implemented for every network technology! Telnet FTP Email Packet radio Coaxial cable Fiber optic Application Transmission Media HTTP

Why layering? (Cont’d)  Solution: introducean intermediatelayer that provides a unique abstraction for various network technologies Telnet FTP Email Packet radio Coaxial cable Fiber optic Application Transmission Media HTTP Intermediate Layer

Layering principles  A layer should becreated wherea different level of abstraction is needed  Each layer should performa well-defined function  Thelayer boundaries should bechosen to minimizeinformation flow across theinterfaces  Number of layers  Largeenough that distinct functions need not be thrown together in the same layer  Small enough that thearchitecture does not become unwieldy

Layering: Pros and Cons  Advantages  Abstract functionality: lower layer can be changed without affecting the upper layer  Reuse: upper layer can reuse the functionality provided by lower layer  Modularity: protocols easier to manage and maintain • Different vendors and organizations responsible for different layers  Disadvantages  Information hiding: inefficient implementations

Protocol hierarchy  Network protocols form a hierarchy of layers  Each protocol falls into a singlelayer  Protocol Interfaces  Serviceinterface: defines theserviceprovided between layers • Layer N uses service provided by layer N-1 • Layer N-1 provides a service to layer N  Peer interface: defines protocol interfacebetween two or morehosts at the samelayer

Protocol interfaces example Node 1 Layer N Layer N-1 Node 2 Layer N Layer N-1 Service Interface Peer Interface

The Internet layered architecture Application Transport Transport Protocol (TCP/UDP) Application Protocols (e.g., Email) Node1 Node2 Router Application Transport Internet/Network Host-to-network IP IP Focus of this class Host-to-network Host-to-network Internet/Network Internet/Network

ISO OSI network architecture Application Presentation Session Transport Network Data Link Physical Node1 Node2 Application Presentation Session Transport Network Data Link Physical Network Data Link Physical Router Host-to-network in Internet arch New layers

Physical layer  Moves bits between two systems connected by a physical link

Data link layer  Provides thenetwork layer with what appears to bean error-free link for data frames transmitted between machines connected by a physical link