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    Retele de calculatoare Lectia 1 Introducere in studiul retelelor

    Page 1 | 2011, Copyright Alex Averian

    Reele de calculatoare

    Obiective:

    -Modele de referin OSI si TCP/IP

    -Nivelul legaturii de date - tipuri de retele

    -Nivelul de retea/rutare, algoritmi de dirijare

    -Nivelul aplicaie modelul client-server

    -Programarea aplicaiilor de reea

    -Servicii i protocoale la nivel de aplicaie

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    Modele de referin

    Arhitecturile de reea cele mai cunoscute sunt:

    -modelul de referina OSI (Open System Interconnection

    interconectarea sistemelor deschise)

    -modelul TCP/IP (Transmission Control Protocol / InternetProtocol).

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    Modelul ISO/OSIUn nivel OSI are un set bine definit de funcii de reea, iar

    funciile fiecrui nivel comunic i colaboreaz cu funciile

    nivelurilor aflate imediat deasupra i dedesubtul nivelului

    respectiv.

    Fiecare nivel asigur anumite servicii sau aciuni carepregtesc datele pentru a fi transmise n reea ctre un alt

    calculator. Toate cererile sunt transmise de la un nivel la

    altul prin intermediul interfeelor. Fiecare nivel se bazeaza

    pe activittile si serviciile nivelului ierarhic inferior.

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    Nivelul 8

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    Nivelul fizic1. Nivelul fizic are rolul de a transmite datele de la uncalculator la altul prin intermediul unui mediu de

    comunicaie. Datele sunt vzute la acest nivel ca un ir de

    bii. Problemele tipice sunt de natur electric: nivelele de

    tensiune corespunztoare unui bit 1 sau 0, durataimpulsurilor de tensiune, cum se iniiaz i cum se oprete

    transmiterea semnalelor electrice, asigurarea pstrrii

    formei semnalului propagat. Mediul de comunicaie nu

    face parte din nivelul fizic.

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    Medii fizice de transmisie

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    Thephysical layeris concerned with transmitting raw bits over a communication channel. The design issues

    have to do with making sure that when one side sends a 1 bit, the other side as a 1 bit, not as a 0 bit

    receives it. Typical questions here are how many volts should be used to represent a 1 and how many for

    a 0, how many microseconds a bit lasts, whether transmission may proceed simultaneously in both

    directions, how the initial connection is established and how it is torn down when both sides are finished,

    and how many pins the network connector has and what each pin is used for. The design issues here deal

    largely with mechanical, electrical, and procedural interfaces, and the physical transmission medium,

    which lies below thephysical layer. Physical layerdesign can properly be considered to be within the domain of

    the electrical engineer.

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    Nivelul legturii de date2. Nivelul legturii de date corecteaz erorile detransmitere aprute la nivelul fizic, realiznd o comunicare

    corect ntre dou noduri adiacente ale reelei.

    Mecanismul utilizat n acest scop este mprirea fluxului

    de bii n cadre ( frame), crora le sunt adugate informaiide control. Cadrele sunt transmise individual, putnd fi

    verificate i confirmate de ctre receptor. Alte funcii ale

    nivelului se refer la fluxul de date (astfel nct

    transmitorul s nu furnizeze date mai rapid dect le

    poate accepta receptorul) i la gestiunea legturii

    (stabilirea conexiunii, controlul schimbului de date i

    nchiderea conexiunii). Folosete adresare fizic.

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    Protocoale la nivelul legturii de dateARP/RARP Address Resolution Protocol/Reverse AddressDCAP Data Link Switching Client Access Protocol

    HDLC High-Level Data Link Control

    PPP Point-to-Point Protocol

    FDDI Fiber Distributed Data Interface

    Ethernet

    Token Ring

    Frame Relay

    ATM Asynchronous Transfer Mode

    STP Spanning tree protocol

    IEEE 802.11 wireless LAN

    LLC Logical Link Control

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    Servicii oferite de nivelul legturii de date

    Encapsulation of network layer data packets into frames

    Frame synchronization

    Logical link control (LLC) sublayer:

    Error control (automatic repeat request, ARQ), in addition to ARQ provided by

    some Transport layer protocols, to forward error correction (FEC) techniques

    provided on the Physical Layer, and to error-detection and packet canceling

    provided at all layers, including the network layer. Data link layer error control

    (i.e. retransmission of erroneous packets) is provided in wireless networks and

    V.42 telephone network modems, but not in LAN protocols such as Ethernet,

    since bit errors are so uncommon in short wires. In that case, only error

    detection and canceling of erroneous packets are provided.

    Flow control, in addition to the one provided on the Transport layer. Data link

    layer error control is not used in LAN protocols such as Ethernet, but in modems

    and wireless networks.

    Media access control (MAC) sublayer:

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    Multiple access protocols for channel-access control, for example CSMA/CD

    protocols for collision detection and retransmission in Ethernet bus networks and

    hub networks, or the CSMA/CA protocol for collision avoidance in wirelessnetworks.

    Physical addressing (MAC addressing)

    LAN switching (packet switching) including MAC filtering and spanning tree

    protocol

    Data packet queueing or scheduling

    Store-and-forward switching or cut-through switching

    Quality of Service (QoS) control

    Virtual LANs (VLAN)

    The main task of the data link layer is to take a raw transmission facility andtransform it into a line that appears free of transmission errors in the network layer.

    It accomplishes this task by having the sender break the input data up into data

    frames (typically a few hundred bytes), transmit the frames sequentially, and

    process the acknowledgment frames sent back by the receiver. Since the physical

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    layer merely accepts and transmits a stream of bits without any regard to meaning

    of structure, it is up to the data link layer to create and recognize frame boundaries.

    This can be accomplished by attaching special bit patterns to the beginning and endof the frame. If there is a chance that these bit patterns might occur in the data,

    special care must be taken to avoid confusion. The data link layer should provide

    error control between adjacent nodes.

    A noise burst on the line can destroy a frame completely. In this case, the data link

    layer software on the source machine must retransmit the frame. However, multiple

    transmissions of the same frame introduce the possibility of duplicate frames. A

    duplicate frame could be sent, for example, if the acknowledgment frame from the

    receiver back to the sender was destroyed. It is up to this layer to solve the

    problems caused by damaged, list, and duplicate frames. The data link layer may

    offer several different service classes to the network layer, each of a different quality

    and with a different price.

    Another issue that arises in the data link layer (and most of the higher layers as

    well) is how to keep a fast transmitter from drowning a slow receiver in data. Some

    traffic regulation mechanism must be employed in order to let the transmitter know

    how much buffer space the receiver has at the moment. Frequently, flow regulation

    and error handling are integrated, for convenience.

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    If the line can be used to transmit data in both directions, this introduces a new

    complication that the data link layer software must deal with. The problem is that the

    acknowledgment frames for A to B traffic competes for the use of the line with dataframes for the B to A traffic. A clever solution piggybacking has been devised.

    In most practical situations, there is a need for transmitting data in both directions.

    One way of achieving full-duplex data transmission would be to have two separate

    communication channels, and use each one for simplex data traffic (in different

    directions). If this were done, we would have two separate physical circuits, each

    with a "forward" channel (for data) and a "reverse" channel (for acknowledgment).

    In both cases the bandwidth of the reverse channel would be almost entirely wasted.

    In effect, the user would be paying the cost of two circuits but only using the

    capacity of one.

    A better idea is to use the same circuit for data in both directions. In this model the

    data frames from A to B are intermixed with the acknowledgment frames from A to

    B. By looking at the "kind" field in the header of an incoming frame, the receiver can

    tell whether the frame is data or acknowledgment.

    Although interweaving data and control frames on the same circuit is an

    improvement over having two separate physical circuits, yet another improvement is

    possible. When a data frame arrives, instead of immediately sending a separate

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    Nivelul reea3. Nivelul reea asigur dirijarea unitilor de date ntrenodurile surs i destinaie, trecnd eventual prin noduri

    intermediare (routing ). Este foarte important ca fluxul de

    date s fie astfel dirijat nct s se evite aglomerarea

    anumitor zone ale reelei (congestionare). Interconectareareelelor cu arhitecturi diferite este o funcie a nivelului

    reea. Folosete adresare logic.

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    Protocoale la nivelul de reea

    DHCP Dynamic Host Configuration Protocol

    ICMP/ICMPv6 Internet Control Message Protocol

    IGMP Internet Group Management Protocol

    IP Internet Protocol version 4

    IPv6 Internet Protocol version 6

    RIP2 Routing Information Protocol

    RIP for IPv6 Routing Information Protocol for IPv6

    This layer provides switching and routing technologies, creating logical paths, known

    as virtual circuits for transmitting data from node. Routing and forwarding are

    functions of this layer, as well as addressing, internetworking error handling,

    congestion control and packet sequencing.

    The network layer is concerned with controlling the operation of the subnet. A key

    design issue is determining how packets are routed from source to destination.

    Routes could be based on static tables that are "wired into" thenetwork and rarely

    changed. They could also be determined at the start of each conversation, for

    example a terminal session. Finally, they could be highly dynamic, being determined

    anew for each packet, to reflect the current network load.

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    If too many packets are present in the subnet at the same time, they will get in each

    other's way, forming bottlenecks. The control of such congestion also belongs to the

    network layer.Since the operators of the subnet may well expect remuneration for their efforts,

    there is often some accounting function built into the network layer. At the very

    least, the software must count how many packets or characters or each customer

    sends bits, to produce billing information. When a packet crosses a national border,

    with different rates on each side, the accounting can become complicated.

    When a packet has to travel from one network to another to get to its destination,

    many problems can arise. The addressing used by the second network may be

    different from the first one. The second one may not accept the packet at all because

    it is too large. The protocols may differ, and so on. It is up to the network layer to

    overcome all these problems to allow heterogeneous networks to be interconnected.

    In broadcast networks, the routing problem is simple, so the network layer is often

    thin or even nonexistent.

    NFS uses Internetwork Protocol (IP) as its network layer interface. IP is responsible

    for routing, directing datagrams from one network to another. The network layer

    may have to break large datagrams, larger than MTU, into smaller packets and host

    receiving the packet will have to reassemble the fragmented datagram. The

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    Internetwork Protocol identifies each host with a 32-bit IP address. IP addresses are

    written as four dot-separated decimal numbers between 0 and 255, e.g.,

    199.89.60.14. The leading 1-3 bytes of the IP identify the network and theremaining bytes identify the host on that network. The network portion of the IP is

    assigned by InterNIC Registration Services, under the contract to the National

    Science Foundation, and the local network administrators assign the host portion of

    the IP, locally by [email protected]. For large sites, usually subnetted like ours,

    the first two bytes represent the network portion of the IP, and the third and fourth

    bytes identify the subnet and host respectively. Even though IP packets areaddressed using IP addresses, hardware addresses must be used to actually

    transport data from one host to another. The Address Resolution Protocol (ARP) is

    used to map the IP address to it hardware.

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    Nivelul transport4. Nivelul transport realizeaz o conexiune ntre doucalculatoare gazda (host) detectnd i corectnd erorile pe

    care nivelul reea nu le trateaz. Este nivelul aflat n

    mijlocul ierarhiei, asigurnd nivelelor superioare o

    interfa independent de tipul reelei utilizate. Funciileprincipale sunt: stabilirea unei conexiuni sigure ntre dou

    maini gazd, iniierea transferului, controlul fluxului de

    date i nchiderea conexiunii.

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    Transport Layer

    Mobile IP Mobile IP Protocol

    RUDP Reliable UDP

    TCP Transmission Control Protocol

    UDP User Datagram Protocol

    XOT X.25 over TCP

    This layer provides transparent transfer of data between end systems, or hosts, and

    is responsible for end-to-end error recovery and flow control. It ensures complete

    data transfer.

    The basic function of the transport layer is to accept data from the session

    layer, split it up into smaller units if need be, pass these to the network layer, and

    ensure that the pieces all arrive correctly at the other end. Furthermore, all this must

    be done efficiently, and in a way that isolates the session layer from the inevitable

    changes in the hardware technology.

    Under normal conditions, the transport layer creates a distinct network connection

    for each transport connection required by the session layer. If the transport

    connection requires a high throughput, however, the transport layer might create

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    multiple network connections, dividing the data among the network connections to

    improve throughput. On the other hand, if creating or maintaining a network

    connection is expensive, the transport layer might multiplex several transportconnections onto the same network connection to reduce the cost. In all cases, the

    transport layer is required to make the multiplexing transparent to the session layer.

    The transport layer also determines what type of service to provide to the session

    layer, and ultimately, the users of the network. The most popular type of transport

    connection is an error-free point-to-point channel that delivers messages in the order

    in which they were sent. However, other possible kinds of transport, service andtransport isolated messages with no guarantee about the order of delivery, and

    broadcasting of messages to multiple destinations. The type of service is determined

    when the connection is established.

    The transport layer is a true source-to-destination or end-to-end layer. In other

    words, a program on the source machine carries on a conversation with a similar

    program on the destination machine, using the message headers and control

    messages.

    Many hosts are multi-programmed, which implies that multiple connections will be

    entering and leaving each host. Their needs to be some way to tell which message

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    belong to which connection. The transport header is one place this information could

    be put.

    In addition to multiplexing several message streams onto one channel, the transportlayer musk takes care of establishing and deleting connections across the network.

    This requires some kind of naming mechanism, so that process on one machine has

    a way of describing with whom it wishes to converse. There must also be a

    mechanism to regulate the flow of information, so that a fast host cannot overrun a

    slow one. Flow control between hosts is distinct from flow control between switches,

    although similar principles apply to both.

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    Nivelul sesiune5. Nivelul sesiune stabilete i ntreine conexiuni (sesiuni)

    ntre procesele aplicaie, rolul su fiind acela de a permite

    proceselor s stabileasc "de comun acord" caracteristicile

    dialogului si sa sincronizeze acest dialog.

    This layer establishes, manages and terminates connections betweenapplications. The session layer sets up, coordinates, and terminates conversations,

    exchanges, and dialogues between the applications at each end. It deals with session

    and connection coordination.

    The session layer allows users on different machines to establish sessions between

    them. A session allows ordinary data transport, as does the transport layer, but it

    also provides some enhanced services useful in some applications. A session might

    be used to allow a user to log into a remote time-sharing system or to transfer a file

    between two machines.

    One of the services of the session layer is to manage dialogue control. Sessions can

    allow traffic to go in both directions at the same time, or in only one direction at a

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    time. If traffic can only go one way at a time, the session layer can help keep track

    of whose turn it is.

    A related session service is token management. For some protocols, it is essentialthat both sides do not attempt the same operation at the same time. To manage

    these activities, the session layer provides tokens that can be exchanged. Only the

    side holding the token may perform the critical operation.

    Another session service is synchronization. Consider the problems that might occur

    when trying to do a two-hour file transfer between two machines on a network with a

    1-hour mean time between crashes. After each transfer was aborted, the whole

    transfer would have to start over again, and would probably fail again with the next

    network crash. To eliminate this problem, the session layer provides a way to insert

    checkpoints into the data stream, so that after a crash, only the data after the last

    checkpoint has to be repeated.

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    Protocoale la nivelul sesiune

    BGMP Border Gateway Multicast Protocol

    DIS Distributed Interactive Simulation

    DNS Domain Name Service

    ISAKMP/IKE Internet Security Association and Key Management Protocol and Internet Key Exchange

    Protocol

    LDAP Lightweight Directory Access Protocol

    NetBIOS/IP NetBIOS/IP for TCP/IP Environment

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    Nivelul prezentare

    6. Nivelul prezentare realizeaz operaii de transformare adatelor n formate nelese de entitile ce intervin intr-o

    conexiune. Transferul de date ntre maini de tipuri diferite

    (Unix-DOS, de exemplu) necesit i codificarea datelor n

    funcie de caracteristicile acestora. Nivelul prezentare artrebui s ofere i servicii de criptare/decriptare a datelor,

    n vederea asigurrii securitii comunicaiei n reea.

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    This layer provides independence from differences in data representation (e.g.,

    encryption by translating from application to network format, and vice versa. The

    presentation layer works to transform data into the form that the application layer

    can accept. This layer formats and encrypts data to be sent across a network,

    providing freedom from compatibility problems. It is sometimes called the syntax

    layer.

    The presentation layer performs certain functions that are requested sufficiently

    often to warrant finding a general solution for them, rather than letting each user

    solve the problems. In particular, unlike all the lower layers, which are justinterested in moving bits reliably from here to there, the presentation layer is

    concerned with the syntax and semantics of the information transmitted.

    A typical example of a presentation service is encoding data in a standard, agreed

    upon way. Most user programs do not exchange random binary bit strings. They

    exchange things such as people's names, dates, amounts of money, and invoices.

    These items are represented as character strings, integers, floating point numbers,

    and data structures composed of several simpler items.

    Different computers have different codes for representing character strings, integers

    and so on. In order to make it possible for computers with different representation to

    communicate, the data structures to be exchanged can be defined in an abstract

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    way, along with a standard encoding to be used "on the wire". The presentation

    layerhandles the job of managing these abstract data structures and converting from

    the representation used inside the computer to the network standard representation.

    The presentation layer is also concerned with other aspects of information

    representation. For example, data compression can be used here to reduce the

    number of bits that have to be transmitted and cryptography is frequently required

    for privacy and authentication.

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    Nivelul aplicaie

    7. Nivelul aplicaie are rolul de "fereastra" de comunicaie ntre utilizatori, acetia fiind reprezentai de entitile

    aplicaie (programele). Nivelul aplicaie nu comunic cu

    aplicaiile ci controleaz mediul n care se execut

    aplicaiile, punndu-le la dispoziie servicii de comunicaie.

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    Protocoale la nivelul aplicaieFANP Flow Attribute Notification Protocol

    Finger User Information Protocol

    FTP File Transfer Protocol

    HTTP Hypertext Transfer Protocol

    IMAP4 Internet Message Access Protocol rev 4

    IMPPpre/IMPPmes Instant Messaging and Presence Protocols

    IPDC IP Device Control

    IRC Internet Relay Chat Protocol

    NTP Network Time Protocol

    POP3 Post Office Protocol version 3

    Radius Remote Authentication Dial In User Service

    RLOGIN Remote Login

    RTSP Real-time Streaming Protocol

    SCTP Stream Control Transmision Protocol

    S-HTTP Secure Hypertext Transfer Protocol

    SLP Service Location Protocol

    SMTP Simple Mail Transfer Protocol

    SNMP Simple Network Management Protocol

    SOCKS Socket Secure (Server)

    TELNET TCP/IP Terminal Emulation ProtocolTFTP Trivial File Transfer Protocol

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    Funciile nivelului aplicaie

    Printre funciile nivelului aplicaie se afl:

    identificarea partenerilor de comunicaie,

    determinarea disponibilitii acestora i autentificarea

    lor;

    sincronizarea aplicaiilor cooperante i selectareamodului de dialog;

    stabilirea responsabilitilor pentru tratarea erorilor;

    identificarea constrngerilor asupra reprezentrii

    datelor; transferul informaiei.

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    This layer supports application and end-user processes. Communication partners are

    identified, quality of service is identified, user authentication and privacy are

    considered, and any constraints on data syntax are identified. Everything at this

    layer is application-specific. This layer provides application services for file transfers,

    e-mail and other network software services. Telnet and FTP are applications that

    exist entirely in the application level. Tiered application architectures are part of this

    layer.

    The application layer contains a variety of protocols that are commonly needed. For

    example, there are hundreds of incompatible terminal types in the world. Considerthe plight of a full screen editor that is supposed to work over a network with many

    different terminal types, each with different screen layouts, escape sequences for

    inserting and deleting text, moving the cursor, etc.

    One way to solve this problem is to define an abstract network virtual terminal for

    which editors and other programs can be written to deal with. To handle each

    terminal type, a piece of software must be written to map the functions of the

    network virtual terminal onto the real terminal. For example, when the editor moves

    the virtual terminal's cursor to the upper left-hand corner of the screen, this software

    must issue the proper command sequence to the real terminal to get its cursor there

    too. All the virtual terminal software is in the application layer.

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    Another application layer function is file transfer. Different file systems have different

    file naming conventions, different ways of representing text lines, and so on.

    Transferring a file between two different systems requires handling these and other

    incompatibilities. This work, too, belongs to the application layer, as do electronic

    mail, remote job entry, directory lookup, and various other general-purpose and

    special-purpose facilities.

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    RezumatOSI 7 Layer Model

    7. Application Layer - DHCP, DNS, FTP, HTTP, IMAP4, NNTP, POP3, SMTP, SNMP, SSH, TELNET and NTP and

    more .

    6. Presentation layer SSL, WEP, WPA, Kerberos,

    5. Session layer Logical Ports 21, 22, 23, 80 etc

    4. Transport - TCP, SPX and UDP

    3. Network - IPv4, IPV6, IPX, OSPF, ICMP, IGMP and ARP

    2. Data Link- 802.11abgn ( Wi-Fi), - 802.16(WiMAX), ATM, Ethernet(802.3), Token Ring(802.5), Frame Relay,

    PPTP, L2TP and ISDN

    1. Physical -Hubs, Repeaters, Cables, Optical Fiber, Coaxial Cable, Twisted Pair Cable and Connectors

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    ncapsulare - protocol data unit1. Layer 1 (Physical Layer) PDU is the bit.

    2. Layer 2 (Data Link Layer) PDU is the frame.

    3. Layer 3 (Network Layer) PDU is the packet.

    4. Layer 4 (Transport Layer) PDU is the segment.

    5. Layer 5 and above are referred to as data.

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    Modelul TCP/IP

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    Bibliografie Tanenbaum A. S., Reele de calculatoare, Editura Agora, Trgu Mure, 1997

    TCP/IP Illustrated: the protocols, ISBN 0-201-63346-9, W. Richard Stevens, 1994

    William Stallings, Data and Computer Communications, Prentice Hall 2006, ISBN 0-13-

    243310-9

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    Intrebri ?