Networks
Until ten years ago, the telecommunications and computer
industries were almost entirely separate. Shortly they will be almost
completely fused. Most of today's hackers operate largely in
ignorance of what goes on in the lines and switching centres between
the computer they own and the computer they wish to access.
Increasingly, dedicated hackers are having to acquire knowledge and
experience of data networks, a task made more interesting, but not
easier, by the fact that the world's leading telecommunications
organisations are pushing through an unprecedented rate of
innovation, both technical and commercial. Apart from purely local
lowspeed working, computer communications are now almost
exclusively found on separate high-speed data networks, separate that
is from the two traditional telecommunications systems telegraphy and
telephone. Telex lines operate typically at 50 or 75 baud with an
upper limit of 110 baud.
The highest efficient speed for telephone-line-based data is 1200
baud. All of these are pitifully slow compared with the internal
speed of even the most sluggish computer. When system designers first
came to evaluate what sort of facilities and performance would be
needed for data communications, it became obvious that relatively few
lessons would be drawn from the solutions already worked out in voice
communications.
Analogue Networks
In voicegrade networks, the challenge had been to squeeze as many
analogue signals down limited-size cables as possible. One of the
earlier solutions, still very widely used, is frequency division
multiplexing (FDM): each of the original speech paths is modulated
onto one of a specific series of radio frequency carrier waves; each
such rf wave is then suppressed at the transmitting source and
reinserted close to the receiving position so that only one of the
sidebands (the lower), the part that actually contains the
intelligence of the transmission, is actually sent over the main data
path. This is similar to ssb transmission in radio.
The entire series of suppressed carrier waves are then modulated onto
a further carrier wave, which then becomes the main vehicle for
taking the bundle of channels from one end of a line to the other.
** Page 69
Typically, a small coaxial cable can handle 60 to 120 channels in
this way, but large cables (the type dropped on the beds of oceans
and employing several stages of modulation) can carry 2700 analogue
channels. Changing audio channels (as they leave the telephone
instrument and enter the local exchange) into rf channels, as well as
making frequency division multiplexing possible, also brings benefits
in that over long circuits it is easier to amplify rf signals to
overcome losses in the cable.
Just before World War II, the first theoretical work was carried
out to find further ways of economising on cable usage; what was then
developed is called Pulse Code Modulation (PCM).
There are several stages. In the first, an analogue signal is
sampled at specific intervals to produce a series of pulses; this is
called Pulse Amplitude Modulation, and takes advantage of the
characteristic of the human ear that if such pulses are sent down a
line with only a very small interval between them, the brain smoothes
over the gaps and reconstitutes the entire original signal.
In the second stage, the levels of amplitude are sampled and
translated into a binary code. The process of dividing an analogue
signal into digital form and then reassembling it in analogue form is
called quantization. Most PCM systems use 128 quantizing levels, each
pulse being coded into 7 binary digits, with an eighth added for
supervisory purposes.
OPERATION OF A CHARACTER TDM
+-----+-----+-----+-----+-----+-----+-----+--
<------| SYN | CH1 | CH2 | CH3 | CH4 | SYN | CH1 |
+-----+-----+-----+-----+-----+-----+-----+--
+-----------------+ +-----------------+
1 | | | |1
--+ | +---+ +---+ | +--
2 | | | | | | | |2
--+ MULTIPLEXER |==+ M +--\/\/--+ M +==--+ MULTIPLEXER +--
3 | | | | | | | |3
--+ | +---+ +---+ | +--
4 | | | |4
--+-----------------+ +-----------------+--
--+-----+-----+-----+-----+-----+-----+----+
| CH1 | SYN | CH4 | CH3 | CH2 | CH1 |SYN |------->
--+-----+-----+-----+-----+-----+-----+----+
<---------------------------->
ONE DATA FRAME
** Page 70
By interleaving coded characters in a highspeed digital stream it
is possible to send several separate voice channels along one
physical link. This process is called Time Division Multiplexing
(TDM) and together with FDM still forms the basis of most of the
globe's voicegrade communications.
Digital Networks
Elegant though these solutions are, though, they are rapidly being
replaced by totally digital schemes. Analogue systems would be very
wasteful when all that is being transmitted are the discrete audio
tones of the output of a modem. In a speech circuit, the technology
has to be able to 'hear', receive, digitize and reassemble the entire
audio spectrum between 100 Hz and 3000 Hz, which is the usual
passband of what we have come to expect from the audio quality of the
telephone. Moreover, the technology must be sensitive to a wide range
of amplitude; speech is made up of pitch and associated loudness. In
a digital network, however, all one really wants to transmit are the
digits, and it doesn't matter whether they are signified by audio
tones, radio frequency values, voltage conditions or light pulses,
just so long as there is circuitry at either end which can encode and
decode.
There are other problems with voice transmission: once two parties
have made a connection with each other (by the one dialling a number
and the other lifting a handset), good sense has suggested that it
was desirable to keep a total physical path open between them, it not
being practical to close down the path during silences and re-open it
when someone speaks. In any case the electromechanical nature of most
of today's phone exchanges would make such turning off and on very
cumbersome and noisy.
But with a purely digital transmission, routing of a 'call'
doesn't have to be physical--individual blocks merely have to bear an
electronic label of their originating and destination addresses, such
addresses being 'read' in digital switching exchanges using chips,
rather than electromechanical ones. Two benefits are thus
simultaneously obtained: the valuable physical path (the cable or
satellite link) is only in use when some intelligence is actually
being transmitted and is not in use during 'silence'; secondly,
switching can be much faster and more reliable.
Packet Switching
These ideas were synthesised into creating what has now become
packet switching. The methods were first described in the mid-1960's
but it was not until a decade later that suitable cheap technology
existed to create a viable commercial service.
** Page 71
The British Telecom product is called Packet SwitchStream (PSS) and
notable comparable US services are Compuserve, Telenet and Tymnet.
Many other countries have their own services and international packet
switching is entirely possible--the UK service is called,
unsurprisingly, IPSS.
International Packet Switched Services and DNICs
INTERNATIONAL NETWORKS
Datacalls can be made to hosts on any listed International Networks.
The NIC (Data Network Identification Code) must precede the
international host's NUA. Charges quoted are for duration (per hour)
and volume (per Ksegment) and are raised in steps of 1 minute and 10
segments respectively.
Country Network DNIC
Australia Midas 5053
8elgium Euronet 2062
Belgium Euronet 2063
Canada Datapac 3020
Canada Globedat 3025
Canada Infoswitch 3029
Denmark Euronet 2383
France Transpac 2080
French Antilles Euronet 3400
Germany (FDR) Datex P 2624
Germany (FDR) Euronet 2623
Hong Kong IDAS 4542
Irish Republic Euronet 2723
Italy Euronet 2223
Japan DDX-P 4401
Japan Venus-P 4408
Luxembourg Euronet 2703
** Page 72
Netherlands Euronet 2043
Country Network DNIC
Norway Norpak 2422
Portugal N/A 2682
Singapore Telepac 5252
South Africa Saponet 6550
Spain TIDA 2141
Sweden Telepak 2405
Switzerland Datalink 2289
Switzerland Euronet 2283
U.S.A. Autonet 3126
U.S.A. Compuserve 3132
U.S.A. ITT (UDTS) 3103
U.S.A. RCA (LSDS) 3113
U.S.A. Telenet 3110
U.S.A. Tymnet 3106
U.S.A. Uninet 3125
U.S.A. WUI (DBS) 3104
Additionally, Datacalls to the U.K. may be initiated from:
Bahrain, Barbados, Bermuda, Israel, New Zealand and the United Arabs
Emirates.
Up to date Information can be obtained from IPSS Marketing on
01-9362743
In essence, the service operates at 48kbits/sec full duplex (both
directions simultaneously) and uses an extension of time division
multiplexing Transmission streams are separated in convenient- sized
blocks or packets, each one of which contains a head and tail
signifying origination and destination. The packets are assembled
either by the originating computer or by a special facility supplied
by the packet switch system. The packets in a single transmission
stream may all follow the same physical path or may use alternate
routes depending on congestion. The packets from one 'conversation'
are very likely to be interleaved with packets from many Other
'conversations'. The originating and receiving computers see none of
this. At the receiving end, the various packets are stripped of their
routing information, and re-assembled in the correct order before
presentation to the computer's VDU or applications program.
** Page 73
PACKET ASSEMBLY/DISASSEMBLY
+-------------------------
|
| PSS
+-----+
o> o> o> o> o> o> o> o> o> o> | | O> O> O>
Terminal D================================-+ PAD +-==========
+-----+
|
|
+-------------------------
Key:
o> CHARACTERS O> PACKETS
All public data networks using packet switching seek to be
compatible with each other, at least to a considerable degree. The
international standard they have to implement is called CCITT X.25.
This is a multi-layered protocol covering (potentially) everything
from electrical connections to the user interface.
The levels work like this:
7 APPLICATION User interface
6 PRESENTATION Data formatting & code conversion
5 SESSION Co-ordination between processes
4 TRANSPORT Control of quality service
3 NETWORK Set up and maintenance of connections
2 DATA LINK Reliable transfer between terminal and network
PHYSICAL Transfer of bitstream between terminal and network
** Page 74
At the moment international agreement has only been reached on the
lowest three levels, Physical, Data Link and Network. Above that,
there is a battle in progress between IBM, which has solutions to the
problems under the name SNA (Systems Network Architecture) and most
of the remainder of the principal main- frame manufacturers, whose
solution is called OSI (Open Systems Interconnection).
Packet Switching and the Single User
So much for the background explanation. How does this affect the
user? Single users can access packet switching in one of two
principal ways. They can use special terminals able to create the
data packets in an appropriate form--called Packet Terminals, in the
(In the original book there is a diagram showing Dial-up termials and
single users connecting to a PAD system and Packet Terminals directly
connected to the PSS. Note added by Electronic Images)
** Page 75
jargon--and these sit on the packet switch circuit, accessing it via
the nearest PSS exchange using a permanent dataline and modems
operating at speeds of 2400, 4800, 9600 or 48K baud, depending on
level of traffic. Alternatively, the customer can use an ordinary
asynchronous terminal without packet-creating capabilities, and
connect into a special PSS facility which handles the packet assembly
for him. Such devices are called Packet Assembler/ Disassemblers, or
PADs. In the jargon, such users are said to have Character Terminals.
PADs are accessed either via leased line at 300 or 1200, or via
dial-up at those speeds, but also at 110 and 1200/75.
Most readers of this book, if they have used packet switching at
all, will have done so using their own computers as character
terminals and by dialling into a PAD. The phone numbers of UK PADs
can be found in the PSS directory, published by Telecom National
Networks. In order to use PSS, you as an individual need a Network
User Identity (NUI), which is registered at your local Packet Switch
Exchange (PSE). The PAD at the PSE will throw you off if you don't
give it a recognisable NUI. PADs are extremely flexible devices; they
will configure their ports to suit your equipment, both as to speed
and screen addressing, rather like a bulletin board (though to be
accurate, it is the bulletin board which mimics the PAD).
Phone numbers to access PSS PADs
Terminal operating speed:
PSE (STD) 110 OR 300 1200/75 1200 Duplex
Aberdeen (0224) 642242 642484 642644
Birmingham (021) 2145139 2146191 241 3061
Bristol (0272) 216411 216511 216611
Cambridge (0223) 82511 82411 82111
Edinburgh (031) 337 9141 337 9121 337 9393
Glasgow (041) 204 2011 204 2031 204 2051
Leeds (0532) 470711 470611 470811
Liverpool (051) 211 0000 212 5127 213 6327
London (01) 825 9421 407 8344 928 2333
or (01) 928 9111 928 3399 928 1737
Luton (0582) 8181 8191 8101
Manchester (061) 833 0242 833 0091 833 0631
Newcastle/Tyne (0632) 314171 314181 314161
Nottingham (0602) 881311 881411 881511
Portsmouth (0705) 53011 53911 53811
Reading (0734) 389111 380111 384111
(*)Slough (0753) 6141 6131 6171
(*)Local area code access to Slough is not available.
Switch the modem/dataphone to 'data' on receipt of data tone.
** Page 76
Next, you need the Network User Address (NUA) of the host you are
calling. These are also available from the same directory: Cambridge
University Computing Services's NUA is 234 222339399, BLAISE is 234
219200222, Istel is 234 252724241, and so on. The first four numbers
are known as the DNIC (Data Network Identification Code); of these
the first three are the country ('234' is the UK identifier), and the
last one the specific service in that country, '2' signifying PSS.
You can also get into Prestel via PSS, though for UK purposes it is
an academic exercise: A9 234 1100 2018 gives you Prestel without the
graphics (A9 indicates to the system that you have a teletype
terminal).
Once you have been routed to the host computer of your choice,
then it is exactly if you were entering by direct dial; your password
and so on will be requested. Costs of using PSS are governed by the
number of packets exchanged, rather than the distance between two
computers or the actual time of the call. A typical PSS session will
thus contain the following running costs: local phone call to PAD (on
regular phone bill, time-related), PSS charges (dependent on number
of packets sent) and host computer bills (which could be time-related
or be per record accessed or on fixed subscription).
Packet switching techniques are not confined to public data
networks Prestel uses them for its own mini-network between the
various Retrieval Computers (the ones the public dial into) and the
Update and Mailbox Computers, and also to handle Gateway connections.
Most newer private networks are packet switched.
** Page 77
Valued Added Networks (VANs) are basic telecoms networks or
facilities to which some additional service--data processing or
hosting of publishing ventures, for example--has been added.
Public Packet Switching, by offering easier and cheaper access, is
a boon to the hacker. No longer does the hacker have to worry about
the protocols that the host computer normally expects to see from its
users. The X.25 protocol and the adaptability of the PAD mean that
the hacker with even lowest quality asynchronous comms can talk to
anything on the network. The tariff structure, favouring packets
exchanged and not distance, means that any computer anywhere in the
world can be a target.
Austin and Poulsen, the ARPAnet hackers, made dramatic use of a
private packet-switched net; the Milwaukee 414s ran around GTE's
Telenet service, one of the biggest public systems in the US. Their
self-adopted name comes from the telephone area code for Milwaukee, a
city chiefly known hitherto as a centre of the American beer
industry. During the Spring and Summer of 1983, using publicly
published directories, and the usual guessing games about
pass-numbers and pass-words, the 414s dropped into the Security
Pacific Bank in Los Angeles, the Sloan-Kettering Cancer Clinic in New
York (it is still not clear to me if they actually altered patients
records or merely looked at them), a Canadian cement company and the
Los Alamos research laboratory in New Mexico, home of the atomic
bomb, and where work on nuclear weapons continues to this day. It is
believed that they saw there 'sensitive' but not 'classified' files.
Commenting about their activities, one prominent computer security
consultant, Joesph Coates, said: 'The Milwaukee babies are great, the
kind of kids anyone would like their own to - ~be...There's nothing
wrong with those kids. The problem is with the idiots who sold the
system and the ignorant people who bought it. Nobody should buy a
computer without knowing how much ~ . security is built in....You
have the timid dealing with the foolish.'
During the first couple of months of 1984, British hackers carried
out a thorough exploration of SERCNET, the private packet-switched
network sponsored by the Science and Engineering Research Council and
centred on the Rutherford Appleton Laboratory in Cambridge. It links
together all the science and technology universities and polytechnics
in the United Kingdom and has gateways to PSS and CERN (European
Nuclear Research).
** Page 78
Almost every type of mainframe and large mini-computer can be
discovered hanging on to the system, IBM 3032 and 370 at Rutherford
itself, Prime 400s, 550s and 750s all over the place, VAX 11/780s at
Oxford, Daresbury, other VAXs at Durham, Cambridge, York, East Anglia
and Newcastle, large numbers of GEC 4000 family members, and the odd
PDP11 running Unix.
Penetration was first achieved when a telephone number appeared on
a popular hobbyist bulletin board, together with the suggestion that
the instruction 'CALL 40' might give results. It was soon discovered
that if the hacker typed DEMO when asked for name and establishment,
things started to happen. For several days hackers left each other
messages on the hobbyist bulletin board, reporting progress, or the
lack of it. Eventually, it became obvious that DEMO was supposed, as
its name suggests, to be a limited facilities demonstration for
casual users, but that it had been insecurely set up.
I can remember the night I pulled down the system manual, which
had been left in an electronic file, watching page after page scroll
down my VDU at 300 baud. All I had had to do was type the word
'GUIDE'. I remember also fetching down lists of addresses and
mnemonics of SERCNET members. Included in the manual were extensive
descriptions of the network protocols and their relation to
'standard' PSS-style networks.
As I complete this chapter I know that certain forms of access to
SERCNET have been shut off, but that hacker exploration appears to
continue. Some of the best hacker stories do not have a definite
ending. I offer some brief extracts from captured SERCNET sessions.
03EOEHaae NODE 3.
Which Service?
PAD
COM
FAD>CALL 40
Welcome to SERCNET-PSS Gateway. Type HELP for help.
Gatew::~cInkging in
user HELP
ID last used Wednesday, 18 January 1984 16:53
Started - Wed 18 Jan 19a4 17:07:55
Please enter your name and establishment DEMO
Due to a local FTP problem messages entered via the HELP system
during the last month have been lost. Please resubmit if
problem/question is still outstanding 9/1/84
No authorisation is required for calls which do not incur charges at
the Gateway. There is now special support for TELEX. A TELEX service
may be announced shortlY.
Copies of the PSS Guide issue 4 are available on request to Program
Advisory Office at RAL, telephone 0235 44 6111 (direct dial in) or
0235 21900 Ext 6111. Requests for copies should no longer be placed
in this help system.
The following options are available:
** Page 79
NOTES GUIDE TITLES ERRORS EXAMPLES HELP QUIT
Which option do you require? GUIDE
The program 'VIEW' is used to display the Gateway guide
Commands available are:
p previous page
n list page n
+n or -n go forward or back n pages
S first page
E last page
L/string find line Containing string
F/string find line beginning string
Q exit from VIEW
VIEW Vn 6> Q
The following options are available:
NOTES GUIDE TITLES ERRORS EXAMPLES HELP OUIT
Which option do you require? HELP
NOTES replies to user queries & other notes
GUIDE Is the complete Gateway user guide (including the Appendices)
TITLES 1- a list of SERCNET L PSS addresses & mnemonics (Guide
Appendix 1)
ERRORS List of error codes you may receive EXAMPLES are ome examples
of use of the Gateway (Guide Appendix 2)
QUIT exits from this session
The following options are available:
NOTES GUIDE TITLES ERRORS EXAMPLES HELP QUIT
Which option do you require? TITLES
VIEW Vn o>
If you have any comments, please type them now, terminate with E
on a line on its own. Otherwise just type
CPU used: 2 ieu, Elapsed: 14 mins, IO: 2380 units, Break: 114
Budgets: this period = 32.000 AUs, used = 0.015 AU, left - 29.161 AUs
User HELP terminal 2 logged out Wed 18 Jan 1984 17:21:59
84/04/18. 18.47.00.
I.C.C.C. NETWORK OPERATING SYSTEM. NOS 1.1-430.20A
USER NUMBER:
PASSWORD:
IMPROPER LOG IN, TRY AGAIN.
USER NUMBER:
PASSWORD:
>SCIENCE AND ENGINEERING RESEARCH COUNCIL
>RUTHERFORD APPLETON LABORATORY
COMPUTING DIVISION
>
> ThE SERCNET - PSS Gateway
> User's Guide
A S Dunn
>Issue 4 16 February 1983
>Introduction
** Page 80
Frm 1; Next>
The SERCNET-PSS Gateway provides access from SERCNET to PSS and PSS
to SERCNET. It functions as a 'straight through' connection between
the networks, ie it is protocol transparant. It operates as a
Transport Level gateway, in accordance with the 'Yellow book'
Transport Service. However the present implementation does not have a
full Transport Service. and therefore there are some limitations in
the service provided. For X29 which is incompatible with the Yellow
book Transport Service. special facilities are provided for the input
of user identification and addresses.
No protocol conversion facilities are provided by the Gateway -
protocol conversion facilities (eg X29 - TS29) can be provided by
calling through a third party machine (usually on SERCNET).
The Transport Service addressing has been extended to include
authorisation fields, so that users can be billed for any charges
they incur.
The Gateway also provides facilities for users to inspect their
accounts and change their passwords, and also a limited HELP
facility.
User Interface
The interface which the user sees will depend on the local equipment
to
Frm 2; Next>
which he is attached. This may be a PAD in which case he will
probably be using the X29 protocol, or a HOST (DTE) in which case he
might be using FTP for example. The local equipment must have some
way of generating a Transport Service Called Address for the Gateway,
which also includes an authorisation field - the format of this is
described below. The documentation for the local system must
therefore be consulted in order to find out how to generate the
Transport Service Called Address. Some examples given in Appendix 2.
A facility is provided for the benefit of users without access to the
'Fast Select' facility, eg BT PAD users (but available to all X29
terminal users) whereby either a minimal address can be included in
the Call User Data Field or an X25 subaddress can be used and the
Call User Data Field left absent.
The authorisation and address can then be entered when prompted by
the Gateway.
Unauthorised Use
Frm 5: Next>
No unauthorised use of the Gateway is allowed regardless of whether
charges are Incurred at the Gateway or not.
However, there is an account DEMO (password will be supplied on
request) With a small allocation which is available for users to try
out the Gateway but it should be noted that excessive use of this
account will soon exhaust the allocation thus depriving others of its
use.
Prospective users of the Gateway should first contact User Interface
Group In the Computing Division of the Rutherford Appleton
Laboratory.
Addressing
To connect a call through the Gateway the following information is
required in the Transport Service Called Address:
1) The name of the called network
2) Authorisation. consisting of a USERID, PASSWORD and ACCOUNT, and
optionally, a reverse charging request
3) The address of the target host on the called network
The format is as follows:
1)
** Page 81
SERCNET to connect to the SERC network
PSS to connect to PSS
S an alias for SERCNET
69 another alias for SERCNET
2)
parameters or booleans as follows:
keyword Meaning
US User identifier
PW User's password
AC the account - not used at present - talen to be same as US
RF 'reply paid' request (see below)
R reverse charging indicator (boolean)
keywords are separated from their values by '='.
keyword-value pairs positional parameters and booleans are separated
from each other by ','. The whole string is enclosed in parentheses:
().
Examples:
(FRED.XYZ R)
(US=FRED,PW=XYZ,R)
(R,PW=XYZ,US=FRED)
All the above have exactly the same meaning. The first form is the
most usual.
When using positionals, the order is: US,PW,AC,RP,R
3)
target network. It may be a compound address, giving the service
within the target machine to be used. It may begin with a mnemonic
instead of a full DTE address. A list of current mnemonics for both
SERCNET and PSS is given in Appendix 1.
A restriction of using the Gateway is that where a Transport Service
address (service name) is required by the target machine to identify
the service to be used, then this must be included explicitly by the
user in the Transport Service Called Address, and not assumed from
the mnemonic, since the Gateway cannot Inow from the mnemonic. which
protocol is being used.
Examples:
RLGS.FTP
4.FTP
Both the above would refer to the FTP service on the GEC 'B' machine
at Rutherford.
RLGB alone would in fact connect to the X29 server, since no service
name is Frm 7; Next>
required for X29.
In order to enable subaddresses to be entered more easily with PSS
addresses, the delimiter '-' can be used to delimit a mnemonic. When
the mnemonic is translated to an address the delimiting '-' is
deleted so that the following string is combined with the address.
Eg:
SERC-99 is translated to 23422351919199
Putting the abovementioned three components together, a full
Transport Service Called Address might look like:
S(FRED,XYZ,R).RLGS.FTF
** Page 82
Of course a request for reverse charging on SERCNET is meaningless,
but not illegal.
Reply Paid Facility (Omit at first reading)
In many circumstances it is necessary for temporary authorisation to
be passed to a third party. For example, the recipient of network
MAIL may not himself be authorised to use the Gateway, and therefore
the sender may wish to grant him temporary authorisation in order to
reply. With the Job Transfer and maniplulation protocol, there is a
requirement to return output documents from jobs which have been
executed on a remote site.
The reply paid facility is involved by including the RP keyword in the
authorisation. It can be used either as a boolean or as a
keyword-value pair. When used as a boolean, a default value of I is
assumed.
The value of the RP parameter indicates the number of reply paid
calls which are to be authorised. All calls which use the reply paid
authorisation will be charged to the account of the user who
initiated the reply paid authorisation.
Frm 9; Next:
The reply paid authorisation parameters are transmitted to the
destination address of a call as a temporary user name and password
in the Transport Service Calling Address. The temporary user name and
password are in a form available for use by automatic systems in
setting up a reply to the address which initiated the original call.
Each time a successful call is completed using the temporary user
name and password, the number of reply paid authorisations is reduced
by 1, until there are none left, when no further replies are allowed.
In addition there is an expiry date of I week, after which the
authorisations are cancelled.
In the event of call failures and error situations, it is important
that the effects are clearly defined. In the following definitions,
the term 'fail' is used to refer to any call which terminates with
either a non-zero clearing cause or diagnostic code or both,
regardless of whether data has been communicated or not. The rules
are defined as follows:
1) If a call which has requested reply paid authorisation fails for
any reason, then the reply paid authorisation is not set up.
2) If the Gateway is unable to set up the reply paid authorisation
for any reason (eg insufficient space), then the call requesting the
authorisation will be refused.
3) A call which is using reply paid authorisation may not create
another reply paid authorisation.
4) If a call which is using reply paid authorisation fails due to a
network error (clearing cause non zero) then the reply paid count is
not reduced.
5) If a call which is using reply paid authorisation fails due to a
host clearing (clearing cause zero, diagnostic code non-zero) then
the reply paid count is reduced, except where the total number of
segments transferred on the call is zero (ie call setup was never
completed).
Frm 11; Next?
X29 Terminal Protocol
There is a problem in that X29 is incompatible with the Transport
Service. For this reason, it is possible that some PAD
implementations will be unable to generate the Transport Service
Called Address. Also some PAD's, eg the British Telecom PAD, may be
unable to generate Fast Select calls - this means that the Call User
Data Field is only 12 bytes long - insufficient to hold the Transport
Service Address.
If a PAD is able to insert a text string into the Call User Data Field
beginning at the fifth byte, but is restricted to 12 characters
because of inability to generate Fast Select calls, then a partial
address can be included consisting of either the network name being
called, or the network name plus authorisation.
** Page 83
The first character is treated as a delimiter, and should be entered
as the character '7'. This is followed by the name of the called
network - SERCNET.
Alternatively, if the PAD is incapable of generating a Call User Data
Field, then the network name can be entered as an X25 subaddress. The
mechanism employed by the Gateway is to transcribe the X25 subaddress
to the beginning of the Transport Service Called Address, converting
the digits of the subaddress into ASCII characters in the process.
Note that this means only SERCNET can be called with this method at
present by using subaddress 69.
The response from the Gateway will be the following message:
Please enter your authorisation and address required in form:
(user,password).address
Reply with the appropriate response eg:
(FRED,XYZ).RLGB
There is a timeout of between 3 and 4 minutes for this response.
after which the call will be cleared. There is no limit to the number
of attempts which may be made within this time limit - if the
authorisation or address entered is invalid, the Gateway will request
it again. To abandon the attempt. the call should be cleared from the
local PAD.
A restriction of this method of use of the Gateway is that a call
must be correctly authorised by the Gateway before charging can
begin, thus reverse charge calls from PSS which do not contain
authorisation in the Call Request packet will be refused. However it
is possible to include the authorisation but not the address in the
Call Request packet. The authorisation must then be entered again
together with the address when requested by the Gateway.
The above also applies when using a subaddress to identify the called
network. In this case the Call User Data Field will contain only the
authorisation in parentheses (preceded by the delimiter '@')
- 5 -
Due to the lack of a Transport Service ACCEPT primitive in X29 it will be
found, on some PADs, that a 'call connected' message will appear on the
terminal as soon as the call has been connected to the Gateway. The 'call
connected' message should not be taken to imply that contact has been made
With the ultimate destination. The Gateway will output a message 'Call
connected to remote address' when the connection has been established.
Frm 14; Next
ITP Terminal Protocol
The terminal protocol ITP is used extensively on SERCNET and some
hosts support only this terminal protocol. Thus it will not be
possible to make calls directly between these hosts on SERCNET and
addresses on PSS which support only X29 or TS29. In these cases it
will be necessary to go through an intermediate machine on SERCNET
which supports both x29 and ITP or TS29 and ITP, such as a GEC ITP.
This is done by first making a call to the GEC MUM, and then making
an outgoing call from there to the desired destination.
PTS29 Terminal Protocol
This is the ideal protocol to use through the Gateway. since there
should be no problem about entering the Transport Service address.
However, it is divisable first to ascertain that the machine to be
called will support
When using this protocol, the service name of the TS29 server should be
entered explicitly, eg:
** Page 84
S(FRED,XYZ).RLGB.TS29
Restrictions
Due to the present lack of a full Transport Service in the Gateway,
some primitives are not fully supported.
In particular, the ADRESS, DISCONNECT and RESET primitives are not
fully supported. Howerver this should not present serious problems,
since the ADDRESS and REASET primitives are not widely used, and the
DISCONNECT primitive can be carried in a Clear Request packet.
IPSS
Access to IPSS is through PSS. Just enter the IPSS address in place
of the PSS address.
................ and on and on for 17 pages
.bmp)
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