591 lines
30 KiB
Plaintext
591 lines
30 KiB
Plaintext
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_______________________________________________________________________________
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An Introduction to Packet Switched Networks Part I
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Written by Blade Runner on 08/20/88
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A Telecom Computer Security Bulletin File
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_______________________________________________________________________________
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1. GENERALITIES
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The growth of data transmission services present more problems due to
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several types of protocols, either in asyncronous or syncronous ways, and
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it needs higher speeds that can allow a faster service.
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To guarantee a service with these characteristics it is helpful to work
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on lines, making them less afflicted from line noise, and on data, with the
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adding of redundancy codes for data correction procedures; we also have to
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find a solution that can allow the use of all protocols, either asyncronous
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or syncronous, on the same physical media in order to use the same line for
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both means.
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At that, we must allow the capability to optimize data transmission
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systems, by arise the connections wires between two points, so the safety
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of the system, in it's globality from computer to terminals, will be
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augmented and made as best is possible.
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The International Standard Organization (ISO) has studied a way that
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works with all connection needs between computers and terminals, building
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a model to refer that can be used as common mind to develop branch reagles.
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The ISO's model is OSI Architecture (Open System Interconnection),
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where "Open" means that system is open to other systems that have well
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fixed standards.
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In that architecture a data transmission system, meant as globality from
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computer to transmission line, is sub-divided into seven levels, each one
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is doing a colloquial protocol with an analogous level that look for
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interconnection between the two, following a logical wire equal to this
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which is shown in the following scheme:
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A B
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+----------+ +----------+
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| 7 |< ---------------------- >| 7 |
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|----------| |----------|
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| 6 |< ---------------------- >| 6 |
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|----------| |----------|
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| 5 |< ---------------------- >| 5 |
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|----------| |----------|
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| 4 |< ----------------------->| 4 |
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|----------| +----------+ |----------|
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| 3 |< --- >| |< --- >| 3 |
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|----------| |----------| |----------|
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| 2 |< --- >| |< --- >| 2 |
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|----------| |----------| |----------|
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| 1 |< --- >| |< --- >| 1 |
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+----------+ +----------+ +----------+
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V
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Physical device of interconnection
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The seven levels are called:
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Application Level - 7
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Greeting Level - 6
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Session Level - 5
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Carrying Level - 4
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Network Level - 3
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Line Level - 2
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Physical Level - 1
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The higher levels are strictly wired to the computer world and intellig-
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ent terminals (levels 5,6 and 7) while lower levels are wired to network
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interconnection problems, about transmitting devices and error correction.
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The physical device of interconnection between two OSI systems will
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provide a three-level structure, making able the structurization of a net,
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e.g., that of following figure:
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users users
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+-----+ | | | | | |
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| OP1 | | | | | | |
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+-----+ +------+ +------+
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|____| city |<----------------------->| city |
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____| A | | B |____
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/ +------+ +------+ \
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| | X | |
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| +------+ +------+ |
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| | city |<----------------------->| city | |
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| | C | | D | |
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| +------+ +------+ |
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| \ / |
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| \ / |
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| \ / |
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| +------+ |
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\--------------------->| city |<-----------------/
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| E |----\
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+------+ +------+
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| | | | OP2 |
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| | | +------+
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users
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We can make the following assumptions about the above structure:
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a) The net is composed by a certain number of devices (in table: A, B,
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C, D, E), called nodes, connected between themselves at high speed so that
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operator "1" can connect to operator "2" by several ways, following more
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than 1 directive (e.g. "A, C, E" or "A, C, D, E" or "A, B, D, E") and
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following traditional system he can use only one way, the "A,E" connection;
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b) The user doesn't need more than one physical line in his "possession"
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when connecting (e.g. OP1 with OP2), but only of links to network (in table
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1 links to the "A and E" nodes) with short wiring, that are possible with a
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modem at base band then at low prices.
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The structure that is nearest to the OSI model is called "PSS" (Packet
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Switching Network) and was analyzed from CCITT (International Consultative
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Commitee for Telephonist and Telegraphy) with the X25 recommendation. This
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recommendation defines the interface between DTE (Data Terminal Equipment)
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and DCE (Data Circuit Terminating Equipment) for terminals that work with
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the PSS net.
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The DTE is strictly known as the source (or the receiver) of data pack-
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ets to (or from) the network and physically can be a Host computer, a Front
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End Processor or an Intelligent terminal.
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The DCE, strictly talking, as common consent, is the device that maybe
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converts these packet-signals received from DTE in a way that might be
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transmitted on lines (e.g. the modem, the TDM, the line couplers), but in
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the sense used by X25, it can be assumed the means of node access or swit-
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ching node to which DTE is connected.
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The X25 recomendation is therefore the local interface between a DTE
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and a DCE (see next table).
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+----------------+ +------------------+ +-----------------+
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| Computer |<-->| network |<-->| Intelligent |
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| | | | | terminal |
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| X.25 | X.75 | X.25 |
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The above table is composed by several blocks, which we can observe as:
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| Host | FEP |
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|<----------->|<---------->| .
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+-------------+------------+ . +---------------------------------+
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| | | . | modem --------|
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| characters | X.25 |====.======|----> -----z----<---| |
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| generator | generator | . | | node |
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| | | . | |_______|
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|-------------+------------+ . +---------------------------------|
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| . |
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| DTE . |
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|<--------------->.<--------------------------------->|
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|<--------------------------------------------------->|
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We can see in "table 2" wires in the network domain are marked as X.75.
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The CCITT has issued this X.75 recomendation (control procedures for
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transit calls or terminals and data transfer systems on international
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calls between PSS networks) that usually is used only for international
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calls but can be used in country calls for interconnection between nodes.
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The X.25 recomendation is not valid to simple terminals (start-stop)
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and ACPs (Packet Adapter-Concentrator or PAD) that can be connected to
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public data networks. Standards for these devices are regulated by X.3
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recommendation (Protocol converters/adapters), X.28 (DTE/DCE Interface for
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start-stop terminals who logon to ACP on a structured network in national
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places) and X.29 (exchange procedure for information control and data
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between ACP and X.25 terminal or other ACP).
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2. X.25 STRUCTURE
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In the "X.25 Interface", three levels are defined. Each level procedure
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uses functions offered by the other level as soon under, but they dont care
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how the lowest level is to be implemented. This is so that a particular
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can be implemented in any of several levels, so long as it will obtain the
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final results.
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In addition, the X.25 recommendation will specify protocols and rules
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that will decide the information exchange between simiar levels in DTE and
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DCE, which can be understood from the next table:
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| DTE |
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|<------------------------>|
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--------+-------+-------+-------+ | +-------+-------+-------+
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. | level | level | level | | | level | level | level |
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. | 3 | 2 | 1 |--|->==<---| 1 | 2 | 3 |
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--------+-------+-------+-------+ | +-------+-------+-------+
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| | | | X.21 | | |
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| | | | bis | | |
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| | | |<---->| | |
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| | |<--------------------------------->| |
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| | HLDC Connection |
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| Packet level X.25
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|<------------------------------------------------------------...
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Talking protocol
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Every level will accumulate information from the lower level and it will
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add a header with eventual redundancy codes before to make the information
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transmitted through present the interface from the lowest level using a
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step by step structure as shown in the next table (block 1 is inserted into
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block 1 and so on).
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LEVEL 1
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Physical Interface: this defines the electric and physical
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characteristics of the interface going on the used line or switched line
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into network. Voltage tensions, connectors used, and transmission methods
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are defined in level 1. The most important characteristic of this level is
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that it supplies a point-to-point transmission, full-duplex, for digital
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transmission.
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LEVEL 2
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Access to connection procedure: (LAP = Link Access Procedure or LAPB =
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Line Access Procedure Balanced). This level will specify a control
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procedure on data to correct mistakes due to physical level. It includes
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control methods of Network Congestions during the DTE and DCE exchange.
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This uses the media known as HDLC protocol (High Level Data Link Control
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defined from ISO as the header building as an activation procedure of the
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connection.
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+---------------+
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| message with |
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| destination |
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+---------------+
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| packet level
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V
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+-------+----------------+
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| packet| |
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| header| |
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| start | |
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+-------+----------------+
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| connection level
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V
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+--------+------------------------+---------+
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| HLDC | . CRC |
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| header | information . |
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| start | . |
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+--------+------------------------+---------+
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V
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+--------+-------------------------------------------+------+........
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| | | | next
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| flag | | flag | header
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+--------+-------------------------------------------+------+........
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V
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+-------------------------------------------------------------------+
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| bit string |
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+-------------------------------------------------------------------+
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LEVEL 3
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Packet Level: this level is the higest and specifies the way that
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information are packet structured and the procedure in which to proceed
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with connections. It has the function to Concentrator because it can mult-
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iplex a number of logical channels into a unique physical channel, mixing
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packets coming from differents channels. Each logical channel has an
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independent control regarding packets and has a CRC for each channel.
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Virtual circuits
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The Third level has virtual channels, that are bi-directional associat-
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ions between two DTE; via these associations packets are exchanged.
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It is like, via the several nodes in the network, a dedicated link betw-
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ween the two DTE. These virtual circuits maybe temporary, and in this case
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they are called "switched Virtual Circuits" (SVC) or fixed, called
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"Permanent Switched Circuits" (PVC).
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3. LEVEL 1 - Physical interfacing
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This level is specified from physical characteristics of CCITT X.21
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recommendation (physical interface between DTE and DCE; for asychronous
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operations on data) and X.21bis (data network usage for designed terminals
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to interface with syncronous modems of series V) used in a provisory way
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to afford to use modems actually on market. Upon mentioned recomandations
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are not depending on the transmission device as they provide that DCE
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(modem or line coupler) will be the part that takes care of the line
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technology.
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The X.21 CCITT recomandation declares:
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- Physical characteristics about the interface, the type of connectors
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and the wire assignment (X.24, 8 ways, 15 pin);
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- electrical characteristics of signals (X.26 and X.27 CCITT
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characteristics as EIA RS 423 and RS 422 respectively);
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- the serial asyncronous transmission;
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- wires that must be point-to-point, working in full-duplex (from that
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we can understand we cannot work in a multi-point structure);
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- the necessary procedures to afford a switched connection;
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- the necessary procedures to afford a dedicated connection.
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The level 1 will consider only first 4 points suggested from X.21,
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all others are of level 3.
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Table 6 shows the circuitry, for functions exchange, provided from the
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X.24 recommendation of CCITT.
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This interface is absolutely transparent to data transfers thanks to
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special C and I lines that are used to determine if the data on T and R
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lines are controls signals or data signals.
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The X.21 recommendation is supplied for interfacing devices at digital
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level, so it is difficult to use for moment, the temporarly is used the
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X.21bis recommendation that is compatible with actual series V modems.
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Electrical characteristics about iterfacing circuitery for speeds less
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than 20 kbit/s are conform to V.28 recomandation of CCITT that use a 25
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pins connector with pins as standard from ISO with scheme # 2110 or with
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X.26 recomandation that provide a 37 pins connector with ISO standard
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scheme at # 4902. It is up to the local administrators to choose the con-
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nector types and the interfacing type to offer as part of their service.
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user Network
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|<--------------------------------------->|<--------------->
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+-------------+ +--------------+ | +---------------+
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| |<-(T) xmit data->| | | | |
|
|||
|
|
| |<-(C) control--->| |-|->| |
|
|||
|
|
| DTE |<-(R) rec'd data>| DCE | | | node |
|
|||
|
|
| |<-(I) info------>| | | | |
|
|||
|
|
| |<-(S) time base->| |<|--| |
|
|||
|
|
| | | | | | |
|
|||
|
|
+-------------+ | +--------------+ | +---------------+
|
|||
|
|
|
|
|||
|
|
---------------------
|
|||
|
|
Interfacing point
|
|||
|
|
|
|||
|
|
To obtain speeds in exceess of 20 kbit/s the electrical characteristics
|
|||
|
|
are following what is provided by the V.35 recommendation that uses 34 pin
|
|||
|
|
connectors as ISO standard draw #2593. The table's indicating interfacing
|
|||
|
|
circuits considered by X.21bis recommendation.
|
|||
|
|
|
|||
|
|
|
|||
|
|
Interface Circuit Description
|
|||
|
|
|
|||
|
|
102 Signal ground
|
|||
|
|
103 Send data
|
|||
|
|
104 Received data
|
|||
|
|
105 Transmission request
|
|||
|
|
106 Ready to transmit
|
|||
|
|
107 DCE Ready
|
|||
|
|
108/2 DTE Ready
|
|||
|
|
109 Carrier detector
|
|||
|
|
114 Time base for transmission
|
|||
|
|
115 Time base for receiving
|
|||
|
|
140 Loop remote probe
|
|||
|
|
141 Local loop probe
|
|||
|
|
142 Running test
|
|||
|
|
|
|||
|
|
4. LEVEL 2 - Link procedures
|
|||
|
|
|
|||
|
|
This level is a "point-to-point" link, and is normally known as the
|
|||
|
|
"frame level" or "header level".
|
|||
|
|
|
|||
|
|
It follows terminologies and is under options specified from ISO HLDC
|
|||
|
|
protocol.
|
|||
|
|
|
|||
|
|
4.1 Level 2 functions
|
|||
|
|
|
|||
|
|
Level 2 transforms to a physical circuit than can be affected by errors
|
|||
|
|
in a logical connection between DTE and the network, a link that can be
|
|||
|
|
understood as released from an error happening: this defines a correction
|
|||
|
|
level based on automatic request about echoing as data is not considered
|
|||
|
|
as transmitted since an error is received or a receive confirm has
|
|||
|
|
been received. Only fully completed data are accepted from receiver.
|
|||
|
|
|
|||
|
|
In addition, this level will provide the ways for the recognizing of a
|
|||
|
|
start and end header, the error recognizes about a bit (via a CRC
|
|||
|
|
computation) and the loss of header (by count headers).
|
|||
|
|
|
|||
|
|
Basic directives of the system will provide:
|
|||
|
|
|
|||
|
|
- the "bit oriented" and no "char oriented" structure: this means that
|
|||
|
|
information may be contained also in only one bit, and we are released from
|
|||
|
|
a certain bit multiple as in the "character oriented way" in which the
|
|||
|
|
information (character is linked to a table (e.g. ASCII 7 bit).
|
|||
|
|
|
|||
|
|
- the existence of CRC ad each end of header and sequential numbering
|
|||
|
|
of headers.
|
|||
|
|
|
|||
|
|
- the correction of error by the re-transmition of data.
|
|||
|
|
|
|||
|
|
- the primary and secondary station definition without any particular
|
|||
|
|
priority of the start of transmission.
|
|||
|
|
|
|||
|
|
- complete full duplex.
|
|||
|
|
|
|||
|
|
The wire specific at level 2 are the point-longs :
|
|||
|
|
|
|||
|
|
1) the structure of header: meant as format of header, then as length,
|
|||
|
|
as CRC computation point, as sincronicity character;
|
|||
|
|
|
|||
|
|
2) procedure elements: allowed commands, answers and actions that must
|
|||
|
|
be taken following the cases: these operations follow the HDLC;
|
|||
|
|
|
|||
|
|
3) class of long procedure: the HDLC will provide a certain number of
|
|||
|
|
cases about the classes and procedures of link following the configuration
|
|||
|
|
type and operating way; the X.25 recomandation uses two classes of link
|
|||
|
|
procedures (see table 7):
|
|||
|
|
|
|||
|
|
- simmetric, usually called LAP (link access procedure),
|
|||
|
|
|
|||
|
|
- balanced, usually called LAPB (link access procedure balanced).
|
|||
|
|
|
|||
|
|
+----------------+ +-------------------+
|
|||
|
|
| primary source | | receiver |
|
|||
|
|
| A |-->OO OO-->| A |
|
|||
|
|
+----------------+< >OO----------------OO +-------------------+
|
|||
|
|
X \ /
|
|||
|
|
/ \ X
|
|||
|
|
+----------------+ OO----------------OO< >+-------------------+
|
|||
|
|
| receiver | OO OO<_ | primary source |
|
|||
|
|
| B |<_/ \| B |
|
|||
|
|
+----------------+ +-------------------+
|
|||
|
|
|
|||
|
|
Simmetric LAP configuration
|
|||
|
|
|
|||
|
|
|
|||
|
|
DTE DCE
|
|||
|
|
+----------------------------+ +-----------------------------+
|
|||
|
|
| source | | | | receiver |
|
|||
|
|
| | primary or | | primary or | |
|
|||
|
|
| | secondary |====>| secondary | |
|
|||
|
|
|-------------| combinator | | combinator|---------------|
|
|||
|
|
| receiver | | | | source |
|
|||
|
|
| | |<====| | |
|
|||
|
|
+----------------------------+ +-----------------------------+
|
|||
|
|
|
|||
|
|
Balanced LAPB configuration
|
|||
|
|
|
|||
|
|
In the first case the running can be compared with half-duplex running,
|
|||
|
|
in the mean that initialization is done before in a way and then in the
|
|||
|
|
other, before an error the channel can be re-initialized without other
|
|||
|
|
aid. This can cause, in some operating conditions, malfunctioning phenomena
|
|||
|
|
(see table 8).
|
|||
|
|
|
|||
|
|
The LAPB procedure will have none of these malfunctions because only
|
|||
|
|
with a command will these do the re-initialization to both sides.
|
|||
|
|
|
|||
|
|
The B station will re-initialize, but primary station A can not perceive
|
|||
|
|
therefore it has not requested no one correct recognizing of secondary
|
|||
|
|
station. In this way we obtain a reset of counters only in one way.
|
|||
|
|
|
|||
|
|
Instant 1,2 Normal Running
|
|||
|
|
/---------- ---------------\
|
|||
|
|
| |
|
|||
|
|
| +-----------+ Information +------------+ |
|
|||
|
|
| | Primary A |---------------------------------->| Secondary | |
|
|||
|
|
|--| |<----------------------------------| A |--|
|
|||
|
|
| +-----------+ Correct receiving +------------+ |
|
|||
|
|
| |
|
|||
|
|
| |
|
|||
|
|
| |
|
|||
|
|
| |
|
|||
|
|
| +-----------+ Information +------------+ |
|
|||
|
|
| | Secondary |---------------------------------->| Primary B | |
|
|||
|
|
\--| B |<----------------------------------| |--/
|
|||
|
|
+-----------+ Correct receiving +------------+
|
|||
|
|
|
|||
|
|
|
|||
|
|
|
|||
|
|
Instant 3,4 Abnormal Running
|
|||
|
|
/---------- -----------------\
|
|||
|
|
| |
|
|||
|
|
| +-----------+ Don't transmit +------------+ |
|
|||
|
|
| | Primary A |---------------------------------->| Secondary | |
|
|||
|
|
|--| |<----------------------------------| A |--|
|
|||
|
|
| +-----------+ +------------+ |
|
|||
|
|
| |
|
|||
|
|
| |
|
|||
|
|
| |
|
|||
|
|
| |
|
|||
|
|
| +-----------+ Reset (due to line error) +------------+ |
|
|||
|
|
| | Secondary |---------------------------------->| Primary B | |
|
|||
|
|
\--| B |<----------------------------------| |--/
|
|||
|
|
+-----------+ Confirm +------------+
|
|||
|
|
|
|||
|
|
|
|||
|
|
4.2 Header Structure
|
|||
|
|
|
|||
|
|
In the next table his supplies the Header structure.
|
|||
|
|
|
|||
|
|
|
|||
|
|
8 bit 8 bit 8 bit variable => 0 16 bit 8 bit
|
|||
|
|
+----------+----------+----------+-------......---+-----------+----------+
|
|||
|
|
| Flag | Address | Control | Information | CRC Code | Flag |
|
|||
|
|
| 01111110 | | | (data or ctrl) | | 01111110 |
|
|||
|
|
+----------+----------+----------+-------......---+-----------+----------+
|
|||
|
|
| Header start | |
|
|||
|
|
|<------------------->| |
|
|||
|
|
| |
|
|||
|
|
| Stored bits |
|
|||
|
|
|<------------------------------------------------>|
|
|||
|
|
| |
|
|||
|
|
|
|||
|
|
The above table shows that the information field provides a variable
|
|||
|
|
length but that length can't be a 8 multiple.
|
|||
|
|
|
|||
|
|
The Flag sequence (01111110) defines header boundaries and it can be
|
|||
|
|
used to close a header and open another.
|
|||
|
|
|
|||
|
|
The same is also used as a syncro character and can be put a on line
|
|||
|
|
when no one information header is yet present.
|
|||
|
|
|
|||
|
|
A header is not recognized it it does not have at its start and at its
|
|||
|
|
end flag sequence, and if within there are not at least 32 bits present (8
|
|||
|
|
for address, 8 for control and 16 as CRC).
|
|||
|
|
|
|||
|
|
The address was originally used from HLDC as an addressing function in
|
|||
|
|
case of "multiple-point" wiring. The X.25 recomandation will provide that
|
|||
|
|
the address function is used only to be able to distinguish commands and
|
|||
|
|
replies in both ways. Its function is rendondancy, because there exists
|
|||
|
|
some control bits to that specific function, but it can be used for addit-
|
|||
|
|
ional researching of errors. Therefore it can distinguish data flow in
|
|||
|
|
both ways and it can then recognize immediately some line loops.
|
|||
|
|
|
|||
|
|
Two are recognized addresses
|
|||
|
|
|
|||
|
|
A = 00000011
|
|||
|
|
|
|||
|
|
will determine commands header from DCE to DTE and answer headers from
|
|||
|
|
DTE to DCE.
|
|||
|
|
|
|||
|
|
B = 00000001
|
|||
|
|
|
|||
|
|
will determine commands header from DTE to DCE and answer headers from
|
|||
|
|
DCE to DTE.
|
|||
|
|
|
|||
|
|
The CONTROL field will identify headers and contain the count of them.
|
|||
|
|
Three header types can be sended in line:
|
|||
|
|
|
|||
|
|
1) Information headers (I): are there who contains usefull data;
|
|||
|
|
|
|||
|
|
2) Supervision headers (S): are there only for control, used e.g. to
|
|||
|
|
confirm a right receive, or for temporary hold of transmission;
|
|||
|
|
|
|||
|
|
3) Numbered headers (N): used e.g. as initialization of connection or
|
|||
|
|
as closer ot connection: they have not CRC sequences, because they
|
|||
|
|
transfer a know information, and did not provide neither an header count.
|
|||
|
|
|
|||
|
|
The format about control field will identify these three headers as
|
|||
|
|
shown in next table:
|
|||
|
|
|
|||
|
|
|
|||
|
|
Thus concludes the Part 1 of the TCSB Introduction to Packet Switched
|
|||
|
|
Networks. Now go grab a hold of Part 2 and learn something.
|
|||
|
|
|
|||
|
|
_______________________________________________________________________________
|
|||
|
|
$
|