Local Networks. Lecture 8 29-Apr-2016

Transcription

1 Local Networks Lecture 8 29-Apr-2016

2 Roadmap of the Course So far Basic telecom concepts General study of LANs Local Networks Ethernet Token bus Token ring ATM LAN Wi-Fi Today LAN performance, Connecting LANs, LAN management

3 LAN Performance

4 LAN performance Theoretical, intricate issue Involves probability and queuing theories as well as stochastic processes We only define some basic notions here We analyze the performance of two (simplified) access methods CSMA/CD Token Ring

5 Parameters Maximum medium length: L medium Medium propagation speed: V Maximum propagation delay: D Capacity: C Average frame length in bits: L frame-bit Average frame transmission time: T frame Bit length of medium: L medium-bit Special ratio parameter: a

6 Maximum medium length L medium Measured in meters Maximum physical length a data frame can travel in a typical transmission Bus topology: bus length Ring topology: ring length Star topology: length between furthest stations

7 L medium illustration in bus

8 L medium illustration in ring

9 L medium illustration in star

10 Medium propagation speed V Propagation speed of the electromagnetic signal representing data in the medium Vacuum: 3*10 8 m/s Air: less than, but about the same Cable: approx. 2*10 8 m/s

11 Maximum propagation delay D Measured in seconds Time needed for the first bit of the frame to travel the maximum length of the medium D=L medium /V

12 Capacity C Measured in bps How many bits can be transmitted per second Also known as data rate LAN Traditional Ethernet Fast Ethernet Gigabit Ethernet Token Ring Wireless Ethernet Capacity 10 Mbps 100 Mbps 1 Gbps 4/16 Mbps 11/54/much more Mbps

13 Average frame length in bits L frame-bit and Average frame transmission time T frame L frame-bit : The average number of bits in a frame, including overhead bits T frame : The time in seconds needed to send the above average frame T frame = L frame-bit / C

14 Bit length of medium L medium-bit Maximum length of the medium in bits Number of bits the medium can hold L medium-bit = D*C = (L medium /V)*C = (C/V)*L medium

15 Special parameter a Ratio of D to T frame : a = D / T frame a = D / T frame = L medium-bit / L frame-bit a = how many frames fit on medium at the same time a proportional to C A network running at 100 Mbps has a bigger a than one running at 10 Mbps 3 cases: a>1, a=1, a<1

16 a>1

17 a=1

18 a<1

19 Efficiency μ of a LAN Defined as the ratio of The time used by a station to send data The time the medium is occupied, directly or indirectly, by this station (cannot be used by other stations) μ = time a station sends data / time medium is occupied for this purpose

20 Example 1 Bus topology, each station can transmit a fixedsize frame Other stations do no transmit until a frame has reached destination Worst case: furthest stations Time used by a station to send frame: T frame Occupied time on medium: T frame + D

21 Example 1, cont μ = T frame / (T frame + D) = 1/ (1+a) Hence When a increases, μ decreases When C increases, μ decreases

22 CSMA/CD efficiency Station sends frame but it s not guaranteed successful transmission due to possible collisions Main issue: how long, on average, it takes a station to send a frame without collision To simplify things, we assume Slotted Aloha No back-off waiting Time slot T s =2*D

23 Some computations Probability that a station finds a time slot with no collision, P success = n*p*(1-p) n-1 When p=1/n => P success = (1-1/n) n-1 (we assumed there are n stations and the probability that a station - for a given slot - has a frame to send is p) P success à 1/e when nà 1/e 0.36 => each station can get a slot to successfully send its frame at least 36% of time

24 Successful slot How many slots will pass, on average, before a station can get a successful slot? Denoted by N slots If a slot is successful with probability p => in average, it is the (1/p) th slot Hence, N slots = (1/P success ) -1 => N slots 1.71

25 Theoretical efficiency μ = time used by a station to send data / time the medium is occupied for this μ = T frame / (T frame + N slots *2*D) => μ = 1 / ( *a)

26 Actual efficiency Has been measured and shown to be less: μ = 1 / (1 + 5*a)

27 Token ring efficiency We assume n stations Each station has data to send When a station gets the token, it holds it and sends data After sending one frame the station releases token μ = time used by a station to send data / time the medium is occupied for this

28 Token ring efficiency 2 μ = T frame / (T frame + D + T token + D/n), i.e., we need the sum of the following T frame = time needed to transmit a frame D = time needed for the first bit of frame to come back T token = time needed to transmit a token The time for the token to reach next station n stations => the latter is D/n

29 Some assumptions Token very small (24 bits) => T token 0 In some token ring networks the sending station releases a token immediately after sending the frame Called early token release Implies the second term 0

30 Final token ring formula μ = T frame / (T frame + D/n), or μ = 1 / (1 + a/n)

31 CSMA/CD and Token ring Token ring has very high efficiency: 100% CSMA/CD has very low efficiency 30-40%

32 Connecting LANs

33 How can we put up a network? Connecting several devices Some possible problems LAN needs to cover bigger distance than media can handle efficiently Number of stations too big for efficient frame delivery (=> network needs to be subdivided) When connecting several networks, some other problems can occur

34 Devices that help

35 Internet An internet: an interconnection of individual network Not to be confused, of course, with Internet Routers and gateways Designed to overcome obstacles to interconnection, without disrupting the independent functioning of the network

36 OSI layers affected by the devices

37 Devices Repeaters: act only upon the electrical components of a signal Active only at the physical layer Bridges: utilize addressing protocols, can affect the flow control of a LAN Most active at data link layer Routers: provide links between separate LANs Most active at network layer Gateways: provide translation services between incompatible LANs or applications Active at all layers Each device also operates in layers below that in which it is most active

38 Repeaters

39 What is a repeater? Electronic device known also as regenerator Installed on a link Receives a signal before it becomes too weak/corrupted and It regenerates the original bit pattern It puts a refreshed copy back on the link Thus extends only the physical length of a network Does not change the functionality of the network in any way

40 Repeater illustration

41 Repeater is not an amplifier Amplifier Cannot discriminate noise from intended signal Amplifies equally whatever fed into it Repeater Does not amplify, buy regenerates the signal A weakened/corrupted signal is copied bit by bit, at original strength

42 Location of repeater Repeater must be placed so that A signal reaches it before any noise changes the meaning A little noise usually alters the precision of the signal s voltage, without destroying its identity The repeater can then read the signal well enough to determine intended voltages

43 Function of repeater

44 Bridges

45 A bridge and segments

46 What is a bridge? Device dividing a large network into smaller segments Also relays frames between two originally separate segments of one type They are repeaters with logic, filters They keep traffic for each segment separate A frame is relayed only to the side of segment containing intended recipient

47 Bridge example

48 Uses of a bridge They filter traffic => useful for Controlling congestion Isolating problem links They partition traffic => useful for security

49 How do bridges function? Do not modify the structure/contents of a frame => can be used only between segments using the same protocol When frame enters bridge Bridge regenerates signal Checks the destination address Forwards new copy only to corresponding segment (uses some tables for this)

50 Types of bridges Simple Most primitive and least expensive Links 2 segments only, has a table containing the addresses of all stations in each segment Addresses have to be entered and removed manually Installation and maintenance costs are time-consuming Multi-port Used to connect more than two LAN segments If N-port, then it connects N segments and thus has N tables Each table contains the physical addresses of stations reachable through corresponding port

51 3-port bridge

52 Transparent bridge Is a learning bridge Builds its tables on its own When first installed, tables are empty When receives a packet, it checks the source and, as it knows the segment from which the packet came, can associate the source address with that segment Eventually completes the table in this way When does not know the destination-segment correspondence it sends packet to all stations Important also as a self-updating mechanism Good for mobile computing

53 Loops in bridged LANs Bridges installed redundantly Two LAN segments may be connected by more than one bridge Loops can occur if bridges are transparent To avoid loops Spanning tree algorithms Source routing The packet source specifies the bridges (and segments) to be used

54 Bridges connecting different LANs Problems to solve Frame format Payload size Data rate Address bit order Other issues: acknowledgements, collisions, priorities that may be different in different LANs

55 Routers

56 What is a router? More sophisticated device (than a repeater or a bridge) Has access to NL addresses Has software enabling it to determine which of the possible paths between the NL addresses is the best for a particular transmission Router relays packets among multiple interconnected networks

57 Routers in an internet

58 Routers and routing Routers act as stations on a network Stations usually belong to one network Routers have addresses on several networks Routers have links to several networks Routing Simplest: get a packet from one connected network and pass it to another connected network If packet destination not on a network to which the router is member, then router can determine which of the connected networks is the next best relay for the packet

59 What does best mean for a router? Least-cost routing: based on efficiency (the cheapest (shortest) path is chosen) Each link assigned a value and the length of a route is the sum of all its component link values Fastest, cheapest, most reliable, most secure, etc Typically a combination of all these Named symbolic link length Smallest number of relays (hops) Hop-count routing Routes need updating only when a link becomes unavailable => link s value becomes

60 Hop-count algorithms Usually limit the routes of a single router to up to 15 hops Special (custom) algorithms for special requirementtransmissions (e.g., military) Some links given value 1, others higher values TCP/IP, OSI Use hop-count routing

61 Symbolic link length E.g., a full-duplex link The length in one direction may be different from the length in the other direction Physical distance is the same Traffic load or quality of cable may differ

62 Types of routing Non-adaptive Once a pathway to a destination is chosen, router sends all packets with that destination along the chosen route Topology or network condition not important Adaptive Router can select a new route for each packet (even packets in same transmission) Topology or network condition considered

63 Packet lifetime Usually number of hops allowed before a packet is considered lost and it s destroyed Used for eventually breaking loops (bouncing) With adaptive routing, a router may update its information before others and thus, loops may result The problem with bouncing is waste of resources

64 Gateways

65 What is a gateway? Protocol converter Usually operates in all 7 OSI layers Can accept a packet formatted for one protocol (e.g. AppleTalk) and convert it for another protocol (e.g. TCP/IP) before forwarding it Generally is software installed within a router

66 What does a gateway modify? In some cases only headers and trailers In some cases also data rate, size, format

67 Other devices Multi-protocol routers Brouters Switches Routing switches

68 Multi-protocol routers Traditional router requires same protocol at network layer for the networks it connects So that routing table uses one single addressing format Multi-protocol router: routes packets belonging to different protocols Sort of a gateway for network layer protocols

69 Traditional vs. multi-protocol routers

70 Brouters Bridge/router Single-protocol (SP) / multi-protocol (MP) router acting as both router and bridge If SP brouter gets a packet of the protocol it uses, it routes the packet; otherwise acts as bridge and passes packet using data link layer addresses If MP brouter gets a packet of one of the protocols it uses, it routes the packet; otherwise acts as bridge and passes packet using data link layer addresses

71 Brouters illustration

72 Switches Devices providing bridging functionality with greater efficiency May act as multi-port bridge to connect devices and segments in a LAN Have buffer for each link they are connected to Store received packet in buffer of receiving link Check address and CRC to find outgoing link If outgoing link is free, frame sent

73 Switch illustration

74 Switching strategies Store-and-forward Frame stored until whole packet arrived Cut-through Frame forwarded when destination is received

75 Routing switches Combining a switch (bridge) and a router Traditional router software Finds the NL address of next station and passes the information to DLL layer to find the output link Routing switches Use the NL destination address to find the output link Faster

76 LAN Management

77 Network management Monitoring, testing, configuring, trouble shooting network components to meet a series of requirements defined by an organization Smooth and efficient operation of the network Provides the predefined quality of service A network management system uses Hardware Software People

78 ISO definition

79 Configuration management (CM) Large networks usually have hundreds of entities physically/logically connected to each other Such entities have an initial configuration when the network is set up The configuration can change over time CM system must know at any time the status and relationships of each entity

80 CM division

81 Reconfiguration Can occur daily in a large network 3 types Hardware reconfiguration Software reconfiguration User-account reconfiguration

82 Hardware reconfiguration Covers all changes to hardware Desktops can be replaced Router may be moved Sub-networks may be added or removed This cannot be automated Is done manually, handled case by case Involves specially trained personnel

83 Software reconfiguration Covers all changes to the software New software may be installed on servers or clients Operating systems may need updating Most software reconfiguration can be automated

84 User-account reconfiguration Not only adding/deleting users on a system User privileges important As individual As member to a group Can be, to some extent, automated

85 Documentation The original network configuration as well as each subsequent change has to be recorded meticulously 3 types Hardware documentation Software documentation User-account documentation

86 Hardware documentation Involves 2 sets of documents Maps Specifications Maps Track each hardware piece and its network connection One general map showing the logical relationship between each 2 sub-networks Possibly another general map showing the physical location of each sub-network For each sub-network: one/more maps showing all pieces of equipment Written in some standard language

87 Hardware documentation 2 Specifications Each hardware piece needs to be documented There is a set of specifications for each hardware piece, connected to the network Information included: hardware type, serial number, vendor s details, time of purchase, warranty information

88 Software documentation All software must be documented Information included Software type Version The installation type License agreement

89 User-account documentation Most operating systems have a utility allowing the documentation of user accounts and their privileges The files containing the above information must be updated and secured Access privileges could be recorded in 2 documents One with all files and access types for each user Another with the list of users having particular access to some files

90 Fault management (FM) Large networks have hundreds/thousands of components Proper operation of the network depends on proper operation of each component Individually In relation to each other FM handles these issues Fault: abnormal condition in the system

91 FM illustration

92 Reactive FM Responsible for detecting, isolating, correcting and recording faults I.e., takes place after a fault occurred Handles short-term solutions to faults

93 Detecting faults Finding the exact location of a fault When a fault occurs The system either stops working properly The system creates excessive errors Example: damaged communication system

94 Isolating and Correcting faults Isolation Useful as it affects only some of the users After isolation, the users are notified and given estimated time of correction Correction Repairing or replacing the faulty component(s)

95 Recording faults Faults are documented Location, possible cause, action(s) taken for correction, cost, time it took for these steps Important because The problem may reoccur Frequency of some type of failure indicates major problem in the system Statistic helpful for performance management

96 Proactive FM Tries to prevent faults from occurring I.e., takes place before faults occur Not always successful Some types of failures may be prevented Replacing an aged component Reconfiguring some frequently faulty component Redundancy is a good strategy

97 Performance management (PM) Closely related to FM Monitors and control the network to ensure it runs as efficiently as possible Quantifies performance using measurable quantities Capacity Throughput Traffic Response time

98 PM criteria

99 Capacity and traffic Capacity Each network has limited capacity PM system ensures the network is not used above this capacity Traffic Can be measured internally and externally Internal: number of packets or bytes traveling inside the network External: exchange of packets or bytes outside the network

100 Throughput and response time Throughput Can be measured of an individual device (e.g., router) or of a part of a network PM system makes sure it is not reduced to unacceptable levels Response time (RT) Time it takes a user to get a service Capacity and traffic can influence RT PM system monitors the average RT and the peak-hour RT Increases in RT indicates network working above its capacity

101 Security management Responsible for controlling access to the network Based on predefined policy Encryption Allows user privacy in exchanging information Authentication Forces the users to identify themselves

102 Accounting management (AM) Controls the users access to the network resources through charges Charged entities for the services received form network Individual users Departments Divisions Projects

103 Accounting management 2 Charge does not have to mean money It may mean debiting the departments or divisions for budgeting purposes Uses of an AM system Prevents users from monopolizing resources Prevents users from using system inefficiently Allows for short/long term planning

104 SNMP Simple network management protocol Used together with TCP/IP protocol suite Provides fundamental operations for monitoring and maintaining a network Application-layer protocol Devices made by different manufacturers and installed on different networks can still be monitored Frees management tasks from physical features of managed devices and from network technology

105 SNMP concept

106 Managers and agents A few manager stations control a set of agents Manager: usually a host Control and monitors a set of agents Runs a SNMP client program Agent: usually a router or host Runs a SNMP server program Management achieved through simple interactions between the two entities

107 Managers and agents 2 Performance information is kept by the agent in a database SNMP management based on 3 ideas Manager checks the agent by requesting information reflecting agent s behavior Manager can force the agent to perform some tasks by resetting values in the database Agent contributes by warning manager of unusual situations

108 Management components

109 Other protocols contributing to Internet management Structure of management information (SMI) Management information base (MIB) SNMP uses SMI and MIB services All three use Abstract syntax notation 1 (ASN.1) Basic encoding rules (BER)

110 SMI functions 1. To name objects 2. To define the types of data to be stored in objects 3. To show how to encode data for transmission over network SMI is a guideline for SNMP, emphasizing 3 attributes for handling an object

111 SMI name SMI requires objects to have unique names Routers Variables in routers Values, etc Object identifier (OI) Used for naming objects globally Hierarchical identifier based on a tree structure

112 OI

113 Data type

114 Simple types Type Size Description Integer 4 bytes Cardinal number between 0 and String Variable 0 or more ASCII characters ObjectIdentifier Variable Object identifier represented in ASCII code IPAddress 4 bytes IP address made of four integers Counter 4 bytes Integer whose value can be incremented from 0 to ; when it reaches max value it wraps back to 0 Gauge 4 bytes Same as counter, but at max value it does not wrap; remains so until it s reset TimeTicks 4 bytes A counting value recording time in ms

115 Structured type sequence and sequence of sequence: combination of simple data types, not necessarily of the same type Analogous to record/struct in C sequence of : combination of simple or sequence data types, all of the same type Analogous to array in C

116 Conceptual data types

117 Encoding method Uses BER to encode data for transmission BER specifies each data piece is encoded in a triplet: (tag, length, value)

118 Tag 1-byte field defining type of data, has 3 subfields: class, format, number class: 2 bit-subfield, defines scope of data Universal: 00, application-wide: 01, context-specific: 10, private: 11 00: data types taken from ASN1 (integer, string, ObjectIdentifier, sequences) 01: data types added by SMI (the other simple 4 types) 10: types with meaning that may change from a protocol to another 11: vendor-specific

119 Tag 2 format: 1-bit subfield, indicates whether the data is simple (0) or structured (1) number: 5-bit subfield, further devides simple/structured data into subgroups

120 Some codes for data types Data type Class Format Number Tag hexa Integer String ObjectIdentifier sequence (of) IPAddress Counter Gauge TimeTicks

121 Length 1 or more bytes If 1 byte: first bit is 0, the rest define the length If more than 1 byte: first bit 1, next 7 bits define how many bytes needed for defining length

122 Some examples

123 MIB Each agent has its own MIB A collection of all objects that manager can manage Objects categorized under 8 different groups System, interface, address translation, ip, icmp, tcp, udp, egp Shown under mib object in the object identifier tree

124 Example: accessing MIB variables in udp group

125 For simple variables To get the instance (contents) we add as suffix a 0: udpindatagrams udpnoports udpinerrors udpoutdatagrams

126 For tables (sequence of) variables When we get to the instance, we need to define which instance At any moment, the table can have several values for the instances To access a certain instance (row) of the table, we need to add the index The indexes are not numeric, but based on one or more fields in the entries Not all tables are indexed the same way

127 udp variables and tables

128 Indexes for udp table

129 SNMP messages

130 Messages GetRequest Sent from manager to agent to retrieve the value of some variable GetNextRequest Same as above Retrieved value is the value of the object following the defined ObjectId in message Mostly used to get table entry values

131 Messages 2 GetNextRequest 2 If manager does not know the entry indexes, cannot access the values It can use this message to find out GetResponse From agent to manager, in response to above messages Contains the values of the variables requested by manager

132 Messages 3 SetRequest From manager to agent to set a value in a variable Trap From agent to manager to report an event Exp: if agent rebooted, reports to manager with time of rebooting

133 Message format

134 Message fields Version: version number -1 Community: defines the password If no password, string public is used Request ID: used to match a request to a response Sequence number used by manager in request message Repeated by agent in response

135 Message fields 2 Error status: integer used only in response messages to show an error occurred Has value 0 in request messages Error index: offset telling the manager which variable caused the error VarBindList: set of variables with corresponding values the manager wants to retrieve or set Null values in GetRequest and GetNextRequest In Trap message it shows the variables and values related to a specific message

136 Message fields 3 Enterprise: defines the ObjectId of the software that generates the Trap Agent address: IP address of agent that created the Trap Trap type: 1-bit field Specific code: if Trap value=6, this field set Time stamp: time elapsed since the event caused a trap

137 Error types Status Name Meaning 0 noerror No error 1 toobig Response too big to fit in one message 2 nosuchname Variable does not exist 3 badvalue The value to be stored is invalid 4 readonly The value cannot be modified 5 generr Other errors

138 Trap types Status Name Meaning 0 coldstart Agent has been booted 1 warmstart Agent has been rebooted 2 linkdown An interface has gone down 3 linkup An interface has come up 4 authenticationfailure Invalid community detected 5 egpneighborloss An EGP router has gone to down state 6 enterprisespecific Other messages

139 Encoding BER standard is used Messages defined using tags: class context sensitive (10), format is structured (1) and numbers 0 4: Data Class Format Number Tag in hexa GetRequest A0 GetNextRequest A1 GetResponse A2 SetRequest A3 Trap A4

140 Encoding SNMP using BER