Chapter 2 Network Architectures A global view

Transcription

1 Chapter 2 Architectures A global view Professor Dongning uo Architecture Architecture, n. 2a: formation or construction as or as if as the result of conscious act, 2b: a unifying or coherent form or structure. (Merriam-Webster) Modern communication networks are complex Can be enormous in size Variety of applications The key for dealing with complexity is by using functional modularity. To break complex problem into simpler sub-problems. To be able to use black box abstraction of sub-problems. Anaolgy: Computer architecture. CPU + Cache + Memory + Bus + Card + ing, n. 2a : one thickness, course, or fold laid or lying over or under another 2b : STRATUM 2c : HORIZON Partitioning related communications functions into manageable groups on top of each other The transport of data from a process in machine A to machine B The routing and forwarding of data via multiple hops The transport of data through a link Each layer operates according to a protocol Each layer provides a service to the layer above Protocol makes calls for services from layer below 1

2 Protocol Protocol, n. 3a : a code prescribing strict adherence to correct etiquette and precedence 3b : a set of conventions governing the treatment and especially the formatting of data in an electronic communications system Here, a protocol is a set of rules that governs how two or more communicating entities in a layer are to interact Messages that can be sent and received Actions that are to be taken when a certain event occurs, e.g. Sending or receiving messages, expiry of timers Why ing? Monolithic non-layered architectures are costly, inflexible, and soon obsolete ing simplifies design, implementation, and testing Protocol in each layer can be designed separately from those in other layers ing provides flexibility for modifying and evolving protocols and services without having to change layers below Standardization A standard protocol for peer processes A standard interface (hardware or software) Example of s layer: communications functions that are used by application programs HTTP, DNS, SMTP ( ) layer: end-to-end communications between two processes in two machines TCP (Transmission Control Protocol) provides reliable byte stream service across the layer: node-to-node communications between two machines IP ( Protocol) 2

3 Example: HTTP HTTP (Hypertext Transfer Protocol) is an application layer protocol It retrieves documents on behalf of a browser It specifies fields in request and response messages Request types; Response codes Content type, options, cookies, It specifies actions to be taken upon receipt of certain messages HTTP Client ET Response HTTP Server HTTP uses service of TCP HTTP client ET HTTP server Response Port 1127 Port 80 ET Response TCP80, , 80TCP ET bytes Response Open Systems Interconnection (OSI) architecture Definition of all the layers Design of protocols for every layer Proprietary architecture By the 1970s every computer vendor had developed its own architecture. Could not network computers from different vendors. OSI By the International Organization for Standardization (ISO) To enable multivendor computer interconnection. 3

4 OSI Reference Model A 7-layer abstract reference model for a network architecture. Purposes: To provide a framework for the development of protocols Detailed standards were developed for each layer, but most of these are not in use The success of TCP/IP preempted deployment of OSI protocols Nevertheless, the OSI Reference model provides a unified view which is still in use in the development of new protocols Presentation Session 7- OSI Reference Model Presentation Session End-to-End Protocols Presentation Session Communicating End Systems One or More Nodes Transfers bits across link Definition & specification of the physical aspects of a link Mechanical: cable, plugs, pins... Electrical/optical: modulation, signal strength, voltage levels, bit times, functional/procedural: how to activate, maintain, and deactivate physical links Ethernet, DSL, cable modem, telephone modems Media: Twisted-pair cable, coaxial cable optical fiber, radio, infrared, 4

5 Transfers frames across direct connections Functions: roups bits into frames Detection of bit errors; Retransmission of frames Activation, maintenance, & deactivation of data link connections Medium access control for local area networks Flow control frames bits Transfers packets across multiple links and/or multiple networks Issues: Addressing must scale to large networks Nodes jointly execute routing algorithm to determine paths across the network Forwarding transfers packet across a node Congestion control to deal with traffic surges Connection setup, maintenance, and teardown when connection-based working: transfer of packets across multiple possibly dissimilar networks working Ethernet LAN Switch HSwitch Switch H H Net Net 3 Net Net 1 Net 2 Net 4 Switch Net Net 5 H = gateway H = host 5

6 Transfers data end-to-end from process in a machine to process in another machine Functions: Reliable stream transfer or quick-and-simple single-block transfer Message segmentation and reassembly Connection setup, maintenance, and release Communication & Upper s : Provides services that are frequently required by applications:web acess, file transfer, Presentation : machineindependent representation of data Session : dialog management, recovery from errors, Incorporated into Presentation Session TCP/IP TCP provides reliable stream service IP provides best-effort connectionless packet transfer 6

7 Protocol Approach IP packets transfer information across Host A IP router router router Host B IP IP layer in each router determines next hop (router) interfaces transfer IP packets across networks Host A Router Net Net 51 Router Router Net Net 54 Net Net 52 Net Net 53 Host B TCP/IP Protocol Suite HTTP SMTP DNS RTP TCP UDP User datagram service IP interface 1 interface 2 interface 3 Diverse network technologies Headers & Trailers Each protocol uses a that carries addresses, sequence numbers, flag bits, length indicators, etc CRC check bits may be appended for error detection APP DATA AH APP DATA TH AH APP DATA NH TH AH APP DATA DH NH TH AH APP DATA CRC bits 7

8 Encapsulation TCP Header contains source & destination port numbers IP Header contains source and destination IP addresses; transport protocol type Ethernet Header contains source & destination MAC addresses; network protocol type IP TCP TCP HTTP Request HTTP Request HTTP Request Ethernet IP TCP HTTP Request FCS 8