introduce re in studiul retelelor
TRANSCRIPT
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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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