Telecommunication Protocols Laboratory Course

Transcription

1 Telecommunication Protocols Laboratory Course Lecture 1 G721 (KTF) March 4,

2 Behind the name Telecommunication: the science and technology of transmitting information (words, sounds, images) over great distances, in the form of electromagnetic signals (telegraph, telephone, radio, TV) In short: communication at a distance Protocol: a set of conventions governing the treatment and especially the formatting of data in an electronic communications system => underlying framework : COMPUTER NETWORKS March 4,

3 Preliminaries 2 credits, 20h course, weeks 10-19, Thu from 8:30 to 10 AM Goal of the course: to understand the specifics of telecommunication protocols How to get there Simulate protocols Teams formed, of 2-4 persons March 4,

4 Preliminaries 2 How to get to the goal Each team chooses their protocols and simulate them in whatever framework they agree upon The result has to work User interface not essential but not neglected either March 4,

5 Other details Lectures 1-6: teaching, forming groups, choosing protocols, start implementing Remaining time: implementation and feedback Individuals are graded based on the log There is no exam March 4,

6 Networks Definition autonomous computing systems (NODES) are interconnected Result resources and information are accessible, independently of their physical location Advantages resource sharing and redundancy parallel processing improved reliability, availability, and performance Tradeoff increased complexity March 4,

7 Network taxonomy Criterion: transmission technology Broadcast links Point-to-point links Criterion: scale (geographical extent) Very local area networks (personal area network) Local area networks LAN Metropolitan area networks MAN Wide area networks WAN Internetworks Criterion: organization Private networks Public networks Other criterions Wireless networks Home networks March 4,

8 Networks for Communication Basic form of communications shared memory message exchange Networks are loosely coupled, they are links w/o memory there is no shared memory applications residing on different nodes cooperate ONLY by passing messages to each other March 4,

9 Communication Application Application Network March 4,

10 Network Layers Networks organized as stacks of layers Each layer is built upon the one below it offers certain services to the higher layers shields higher layers from the implementation of the services The number, contents, functions and names of layers depends on the network Layer n on one machine communicates with layer n on another machine Rules and conventions used in this: layer n protocol Entities comprising the corresponding layers on different machines: peers (processes, hardware devices, humans) March 4,

11 Interfaces Between each pair of adjacent layers Define which primitive operations and services the lower layer makes available to the upper one Clean interfaces between layers -> an important issue for network designers each layer performs a specific collection of well-understood functions Clear-cut interfaces Make it simpler to replace the implementation of one layer with a completely different implementation Minimize the amount of information that must be passed between layers March 4,

12 Layers, protocols, and interfaces March 4,

13 Network architecture Is a set of layers and protocols The spec of the architecture should contain enough information for building layers that obbey the protocols Interfaces and details of implementation are not part of the architecture Protocol stack list of protocols, one per layer, used by a certain system March 4,

14 Protocol Stack 1 Location A Location B 3 I like rabbits Message Philosopher J'aime bien les lapins 3 2 L: Dutch Ik vind konijnen leuk Information for the remote translator Translator L: Dutch Ik vind konijnen leuk 2 1 Fax #--- L: Dutch Ik vind konijnen leuk Information for the remote secretary Secretary Fax #--- L: Dutch Ik vind konijnen leuk 1 The philosopher-translator-secretary architecture.

15 Protocol Stack 2 Example information flow supporting virtual communication in layer 5.

16 Design Issues for the Layers Addressing Layers need to identify senders and receivers Data Transfer Rules Directions of data travel, logical channels Error Control Many error-detecting and error-controlling codes exist The peers have to agree on which ones are used The receiver has to inform the sender about correctly received messages Flow Control Keep a faster sender from swamping a slow receiver Arbitrarily long messages Multiplexing and demultiplexing Using the same connection for multiple, unrelated communications Routing If multiple paths between source and destination exist, a route must be chosen

17 Types of services Layers offer 2 types of services to the layers above them Connection-oriented Connectionless Connection-oriented network service Service user establishes connection, uses it, then releases it Connection tube Negotiation between sender, receiver, subnet on parameters such as: maximum message size, quality of service, etc Connectionless network service Each message carries the full destination address and is routed through the system independently of all the others March 4,

18 Types of services 2 March 4,

19 Quality of service Usually refers to reliability of services (not losing data) implemented by having the receiver acknowledge the receipt of each message so the sender is sure it arrived acknowledgement process introduces overhead and delays March 4,

20 Service primitives A service is formally specified by a set of primitives (operations) available to a user process to access the service (Primitives for implementing a simple connection-oriented service) March 4,

21 Service primitives 2 Packets sent in a simple client-server interaction on a connection-oriented network. March 4,

22 Services and protocols Service Set of primitives that a layer provides to the layer above it Defines what operations the layer is prepared to perform on behalf of its users Relates to an interface between 2 layers: service provider and service user Protocol Set of rules governing the format and meaning of packets (messages) that are exchanged by peer entities within a layer Implements service definitions March 4,

23 A layer with services and protocols March 4,

24 Reference models Network architectures OSI OSI reference model TCP/IP reference model The associated protocols are not really used The model is quite general and still valid Features discussed at each layer are still important TCP/IP The model is not of much use The protocols are widely used March 4,

25 OSI/ISO reference model Based on a proposal of ISO (International Standards Organization) Intended as a first step toward international standardization of protocols used in the various layers (1983, revised 1995) Called ISO/OSI (Open Systems Interconnection) It deals with connecting open systems (systems open for communication with other systems) March 4,

26 OSI model March 4,

27 OSI model principles A layer should be created where a different abstraction is needed Each layer should perform a well-defined function The function of each layer should be chosen so that internationally standardized protocols can be then defined The layer boundaries should be chosen to minimize the information flow across interfaces The number of layers should be large enough that distinct functions are not thrown together in the same layer and small enough so that architecture does not become unhandy March 4,

28 Physical layer Concerned with transmitting raw bits over a communication channel When a 1-bit is sent, a 1-bit has to be received, not a 0-bit Typical questions How many volts used to represent a 1, how many for a 0 How many ns a bit lasts Whether transmission can proceed simultaneously in both directions How the initial connection is established and how is it then torn down How many pins the network connector has and what are their uses Design issues deal with Mechanical, electrical and timing interfaces Physical transmission medium March 4,

29 Data Link Layer Transforms a raw transmission facility into a line that appears free of undetected transmission errors to the above network layer Obliges the sender to break up input data into data frames (few hundred or thousands bytes each) and transmit frames sequentially If service reliable => receiver confirms correct receipt of each frame by sending back an acknowledgement frame Flow control regulations and error handling are integrated Medium access control sublayer Controls access to a shared channel (for broadcast networks) March 4,

30 Network layer Controls the operation of a subnet Key design issue: how are packets routed from source to destination Routes: static / determined at intialization / highly dynamic Controls the bottlenecks in the subnet Handles quality of service issues: delay, transit time, jitter, etc Compatibility among different networks (addressing, max message sizes, different protocols) Broadcast networks: this layer is very thin March 4,

31 Transport layer Accepts data from the above layers and manipulates it, ensuring that it arrives correctly at the destination Efficiency is important Hiding the hardware from the above layers also important When a connection is established, the type of service is determined, e.g. Error-free, point-to-point channel Transporting isolated messages Broadcasting True end-to-end layer a program on the source machine carries on a conversation with a similar program on the destination machine using message headers and control messages In contrast, layers 1-3 are chained

32 Session layer Allows users on different machines to establish sessions between them Session services Dialog control: keeping track of whose turn it is to transmit Token management: preventing 2 parties from attempting the same critical operation at the same time Synchronization: check-pointing long transmissions to allow them to continue from where they were after a crash March 4,

33 Presentation layer Concerned with the syntax and semantics of the transmitted information Data structures to be exchanged can be defined in an abstract way, along with a standard encoding to be used on the wire Useful for computers with different data representation Manages these abstract data structures and allows higher-level data structures to be defined and exchanged E.g. banking records March 4,

34 Application layer Contains a variety of protocols commonly needed by users E.g. HTTP basis for WWW File transfer Electronic mail Network news March 4,

35 TCP/IP reference model US Department of Defense sponsored a research network called ARPANET (1960s) that needed ultimately to interconnect with satellites and radio networks a reference architecture was needed Major goal: the ability to connect multiple networks in a seamless way Another major goal: the ability to survive loss of subnet hardware with existing conversations not being broken off Also: flexible architecture a packet-switching network, based on a connectionless internetwork layer was conceived March 4,

36 TCP/IP model March 4,

37 Internet layer Connectionless internetwork layer that holds TCP/IP architecture together It permits hosts to inject packets into any network and have them travel independently to the destination (in possibly another network) Defines an official packet format and protocol called IP (internet protocol) It delivers IP packets to their destination Major issues: packet routing and avoiding congestion Similar in functionality to OSI network layer March 4,

38 Transport layer Allows peer entities on the source and destination hosts to carry on a conversation Similar to the OSI transport layer 2 end-to-end transport protocols are defined here: TCP (transmission control protocol) UDP (user datagram protocol) March 4,

39 TCP Reliable connection oriented protocol A byte stream originating on one machine is delivered w/o error on any other machine in the internet It fragments the byte stream into discrete messages and passes each one to the internet layer At destination the receiving TCP process reassembles the received messages into the output stream Also handles flow control March 4,

40 UDP Unreliable, connectionless protocol Used for applications not desiring TCP s sequencing or flow control but wishing to provide their own Widely used also for one-shot, client-server type requestreply queries and applications Prompt delivery more important than accurate delivery March 4,

41 TCP/IP Other Layers Application layer - contains all the higher level protocols TELNET, FTP, SMTP, DNS, NNTP, HTTP Host-to-network layer Not really specified The host should connect to the network (with some protocol) so that it could send IP packets to it March 4,

42 TCP/IP Protocols and Networks (initially) March 4,

43 Similarities between OSI and TCP/IP reference models Both are based on the concept of a stack of independent protocols Functionality of layers is roughly similar Layers up through and including the transport layer provide an endto-end, network-independent transport service to processes wishing to communicate These layers are application-oriented users of the transport service March 4,

44 Central concepts to OSI Service: what the layer does; it defines the layer s semantics Interface: tells the processes above it how to access it It specifies the parameters and expected results Protocol: implements the services Peer protocols used in a layer are the layer s own business The layer can use any protocols as long as it gets the job done (provides the offered services) The layer can also change the protocols at will without affecting software in higher layers March 4,

45 Protocols and models OSI model was devised before the corresponding protocols The model was not biased toward one set of protocols Designers did not have experience and did not know well what to put in which layer TCP/IP model was devised just as a description of the existing protocols The protocols hence fit perfectly into the layers The model does not fit any other protocol stack => not useful for describing other, non-tcp/ip networks March 4,

46 Other differences Except for the (inter)network, transport and application layers, other layers are different in OSI and TCP/IP Connection vs connectionless services OSI supports both in the network layer but only connectionoriented in the transport layer TCP/IP has only connectionless services in the internet layer but supports both in the transport layer March 4,

47 A Critique of the OSI Model and Protocols Why OSI did not take over the world Bad timing Bad technology Bad implementations Bad politics March 4,

48 Bad Timing March 4,

49 A Critique of the TCP/IP Reference Model Problems: Service, interface, and protocol not distinguished Not a general model Host-to-network layer not really a layer No mention of physical and data link layers Minor protocols deeply entrenched, hard to replace March 4,

50 Hybrid Model March 4,