Lecture 2 Overview

Multiaccess vs. Point-to-point Multi-access means shared medium many end-systems share the same physical communication resources (wire, frequency, ...) There must be some arbitration mechanism. Point-to-point only 2 systems involved no doubt about where data came from ! Internet

The Internet millions of connected computing devices: hosts = end systems running network apps communication links fiber, copper, radio, satellite routers: forward packets (chunks of data) Home network Institutional network Mobile network Global ISP Regional ISP PC server wireless laptop cellular handheld wired links access points router Internet

A Network of Networks roughly hierarchical Tier-1 ISPs provide national, international coverage Tier-2 ISPs provide regional coverage Tier-3 and lower levels provide local coverage any tier may sell to business and residential customers any ISP may have a link to any other ISP not strictly hierarchical Internet

Internet Design Goals primary goal: interoperability among existing networks a network of networks obey administrative boundaries secondary goals fault tolerance multiple transport protocols support a variety of networks distributed management cost effective, low effort for host attachment, accountability first three were more important, so remaining four did not receive as much attention no mention of security Internet

Internet Design Principles minimal assumptions about services network should support ability to send packets no reliability or security end-to-end principle keep the core of the network as simple as possible, put complex functionality at the edges exception: significant performance improvement Internet

Layering Divide a task into pieces and then solve each piece independently (or nearly so) Establishing a well defined interface between layers makes porting easier Major Advantages: Code Reuse Extensibility Internet

The Internet Hourglass

The Internet at each Hop

CPE 401 / 601 Computer Network Systems Lecture 3 Protocols and Layering Lecture 1 Internet CPE 401 / 601 Computer Network Systems slides are modified from Dave Hollinger

What’s a protocol? human protocols: network protocols: “what’s the time?” “I have a question” introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols: machines rather than humans all communication activity in Internet governed by protocols Internet

What’s a protocol? a human protocol and a computer network protocol: Hi TCP connection request Hi TCP connection response Got the time? Get http://www.awl.com/kurose-ross 2:00 <file> time Q: Other human protocols? Internet

Protocol An agreed upon convention for communication. both endpoints need to understand the protocol. Protocols must be formally defined and unambiguous! Protocols define format, order of msgs sent and received among network entities, actions taken on msg transmission, receipt We will study lots of existing protocols and perhaps develop a few of our own. Internet

Programs & Processes A program is an executable file. A process or task is an instance of a program that is being executed. A single program can generate multiple processes. Internet

Client - Server A server is a process - not a machine ! A server waits for a request from a client. A client is a process that sends a request to an existing server and (usually) waits for a reply. Internet

Client - Server Examples Server returns the time-of-day. Server returns a document. Server prints a file for client. Server does a disk read or write. Server records a transaction. Internet

Servers Servers are generally more complex (more interesting). Basic types of servers: Iterative - server handles one client at a time. Concurrent - server handles many clients at a time. We will study the differences later. Internet

Layering Internet

OSI Reference Model The International Standards Organization (ISO) proposal for the standardization of various protocols used in computer networks is called the Open Systems Interconnection Reference Model. Although the OSI model is a just a model (not a specification), it is generally regarded as the most complete model. Protocol Layers

OSI 7 Layer Model: 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data-Link 1 Physical High level protocols TCP/IP Model Low level protocols Protocol Layers

Simplified Network Model Application Application Interface Protocols Transport Transport Peer-to-peer Protocols Network Network Data Link Data Link Protocol Layers

The Physical Layer Responsibility: Issues: transmission of raw bits over a communication channel Issues: mechanical and electrical interfaces time per bit distances Protocol Layers

The Data Link Layer Responsibility: Issues: provide an error-free communication link Issues: framing (dividing data into chunks) header & trailer bits addressing 10110110101 01100010011 10110000001 Protocol Layers

The Data Link Layer Data Link Control sublayer Medium Access Control sublayer needed by mutiaccess networks. MAC provides Data Link Control with “virtual wires” on multiaccess networks. Protocol Layers

The Network Layer Responsibilities: Issues: path selection between end-systems (routing). flow control. fragmentation & reassembly translation between different network types. Issues: packet headers virtual circuits Protocol Layers

The Transport Layer Responsibilities: Issues: provides virtual end-to-end links between peer processes. end-to-end flow control Issues: headers error detection reliable communication Protocol Layers

The Session Layer Responsibilities: establishes, manages, and terminates sessions between applications. service location lookup Many protocol suites do not include a session layer. Not in TCP/IP model Protocol Layers

The Presentation Layer Responsibilities: data encryption data compression data conversion Many protocol suites do not include a Presentation Layer. Not in TCP/IP model Protocol Layers

The Application Layer Responsibilities: Issues: anything not provided by any of the other layers TCP/IP model Session and Presentation Layer functions Issues: application level protocols appropriate selection of “type of service” Protocol Layers

Layering & Headers Each layer needs to add some control information to the data in order to do it’s job. This information is typically prepended to the data before being given to the lower layer. Once the lower layers deliver the data and control information - the peer layer uses the control information. Protocol Layers

Headers DATA Application Application Transport H DATA Transport Network H H DATA Network Data Link H H H DATA Data Link Protocol Layers

What are the headers? Physical: Data Link: no header - just a bunch of bits Data Link: address of the receiving endpoints address of the sending endpoint length of the data checksum Protocol Layers

What are the headers? Network: Protocol Protocol version type of service packet identifier time to live source network address destination network address length of the data fragment number header checksum Protocol Layers

Layers Summary Data-Link: communication between machines on the same network. Network: communication between machines on possibly different networks. Transport: communication between processes (running on machines on possibly different networks). Protocol Layers