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An Introduction to Packet Switched Networks Part II
Written by Blade Runner on 08/20/88
A Telecom Computer Security Bulletin File
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This is Part II of the TCSB "Introduction to Packet Switched Networks".
Without Part I you are going to be very lost. So, if you are without it
you had best go find it and download it. Now on with Blade Runner's tab-
le of the control field headers:
+----------------------------------------+
| bits |
+------+-------------+-----+-------------+
Control Field | 1st | 2nd 3rd 4th | 5th | 6th 7th 8th |
+-----------------------+------+-------------+-----+-------------|
| Header I | 0 | N(S) | P/F | N(R) |
|-----------------------+------+-------------+-----+-------------|
| Header S | 1 | 0 S | P/F | N(R) |
|-----------------------+------+-------------+-----+-------------|
| Header U | 1 | 1 M | P/F | M |
+-----------------------+------+-------------+-----+-------------+
N(S): number of sended sequence
N(R): number of sequence we looking for receive
S : supervisioning commands
M : not numbered sequence commands
P : Poll bit (sended as command)
F : Final bit (sended as answer)
From what we can observe, N(R) and N(S) will take at maximum the value
of 7 (starting from 0) therefore they are composed at maximum with 3 bits
2nd 3rd 4th N(S)
6th 7th 8th N(R)
Therefore a counter exits a loop, before the sending of the 4th header
will reset the N(S) and N(R).
Supervision commands (S) occupy the 3rd and 4th position of control
string and we can therefore assume that the 4 header is coupled to 2 bit
(the C.25 will consider only 3):
RR = Receive Ready (00)
REJ = Reject (01)
RNR = Receive Not Ready (10)
The RR header can be used to indicate that a station is ready to
receive an information header or to give the confirmation that it has
received a number of headers thru N(R) - 1 (where N(R) is the header we
are looking to receive).
The REJ header can be sent as request for the retransmission of a header
starting from header N(R) and until we have understood transmission can
not be resent with the same header.
The RNR header can be used to mark a busy case due (e.g. from a
temporary impossibility to receive data).
The non-numbered headers don't have, as already stated, transmitting or
receiving counters and will therefore provide a 5 bit (called modifiers)
that allow 32 particular functions.
Between these we can consider some examples:
SARM header (Set Asyncronous Responde Mode)
in which a syncronization between two running stations is requested in
ARM (Asyncronous Response Mode), typical of LAP procedures; this command
will reset all counters.
SABM header (Set Asyncronous Balanced Mode)
that is a synchronicity request between two running stations running in
ABM (Asyncronous Balanced Mode), typical of LAPB procedures. The receiving
of this command will reset all counters.
The remote station answers these commands with UA header (Un-numbered
Acknowledgement) to confirm the right synchronicity.
The DM header (Disconnect Mode) is also used in ABM mode. It is used to
mark the station that should answer is not connected.
The FMRM commands (Frame-reject response) or CRMR (Command-reject
Response) are sent when a correct CRC header is received, but with an
unrecoverable error with a repeated header (e.g. not a sequenced header).
The receiving station must re-initialize the connection procedure.
The following table fully describes the level 2 functions:
----------------------------------------------------------------------
| | |
Header type | Commands | Answers | bits of control field
------------+--------------+--------------+---------------------------
Information | I | | 0 N(S) | P | N(R)
------------+--------------+--------------+---------+-----+-----------
| RR | RR | 1 0 0 0 | P/F | N(R)
Supervising | RNR | RNR | 1 0 1 0 | P/F | N(R)
| REJ | REJ | 1 0 0 1 | P/F | N(R)
------------+--------------+--------------+---------+-----+-----------
| SARM | DM | 1 1 1 1 | P/F | 0 0 0
|--------------+--------------+---------+-----+-----------
| SABM | | 1 1 1 1 | P | 1 0 0
Not |--------------+--------------+---------+-----+-----------
Numbered | DISC | | 1 1 0 0 | P | 0 1 0
|--------------+--------------+---------+-----+-----------
| | UA | 1 1 0 0 | F | 1 1 0
|--------------+--------------+---------+-----+-----------
| | CMDR | 1 1 1 0 | F | 0 0 1
| | FMRM | | |
----------------------------------------------------------------------
The very last important function of level 2 is the frame window: this
will indicate the number of headers that can be sent without recieving
confirmation from the remote station. It assumes a value from 1 to 7.
Every time a sequence is sent a time-out starts that is reset every
time an acknowledgement is received (basically, a successful recieve). If
this acknlowledgment is not received by the end of counting, an automatic
of the message with the (P) poll but taken to logical level 1 starts. This
is to request an immediate response from the remote station. The reply
will be followed by the (F) final bit raised to logical level 1 and the
adequate answer to the command was recieved (e.g., RR with F bit set to 2
if regarding answer with I header with P bit to 1).
5. LEVEL 3 - Packet level
The level 3 or X.25 recomendation allows the multiplexing timing
functions because it transforms the single connection or logical channel
provided from level 2 in a greather number of logical channels. It enables
independant control of the data flow of each channel. Some error recovery,
re-initialization or others, may be recovered on the logical channel or
single or in a more complete manner on all channels. The packet level
also provides the capability to insert some interrupt procedures that allow
the DTE to send information that allows exit from the normal data flow and
has high priority on the flow.
The features of this level can be listed in the following groups:
1) The multiplexing of n logical channels on one logical channel of
connection;
2) The control of the flow locally routed into DTE and DCE
interfaces (not between DTE and DTE);
3) Guarantee the sequence of packet numbering;
4) The capability to send interrupt requests;
5) Recovery from errors (as bad counts) by reinitialization or reject
of packets;
6) Virtual switched circuits between two DTE (that are equal to
switched lines);
7) Virtuals permanent circuits between two DTE (that are equal to
dedicated lines);
The info field at level 3 is in the header at level 2 into the field
we have defines as information.
Each information header will contain a single packet, that is
structured as is in following table.
Information field (level 2)
__________________________/\________________________________
/ \
+---------------------+----------+-----------+----------+......
| : : | | | |
| Q : format: logical | logical | packet | info |
| : : group | channel | identif. | field |
| : : | | | |
+---------------------+----------+-----------+----------+......
: : : :
:<------------------->:<-------->:<--------->:<----------------
: 8 bit : 8 bit : 8 bit : => 0
Note that the information is divided into octettes taking therefore a
structure as shown above:
bit -> | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | octetts
+-----+-----+-----+-----+-----+-----+-----+-----+ |
| | | | V
| Q D | 0 1 | LCG | 1
|-----------+-----------+-----------------------|
| |
| LC | 2
|-----------------------------------------------|
| |
| Packet identifier | 3
|-----------------------------------------------|
| length of address of | length of address of |
| calling DTE | called DTE | 4
|-----------------------------------------------|
| |
| adresses of DTE calling and called |
| |
|...............................................|
Level 3 allows 16 groups (LCG=Logical channel group) of 256 logical
channels (LC=logical channels) between each DCE and DTE. That information
is contained at the start of the packet in a 4 bit + 8 bit field (called
LCG and LC into upon table), that allow to obtain thru 4,096 logical
channels (from 0 to 4,095).
DCE and DTE use the same logical channel to understand a connect.
Locigal channels are used to provide bi-directional associations between
two DTE: these associations are called virtual circuits and can have a
different numeration from the logical channel, therefore, as we can observe
from the next table, X.25 newtork nodes can be busy from any of several
things than a specific logical channel that is on one of both ends of a
network, between two DTE, can be already busy from another wire. From
here the presence of a unique virtual circuit between A and B with two
different logical channels.
A is simultaneously connected with B and C.
+-------+ Lc 5 Lc 5 +---------+
| DTE B |----------- --------------| DTE C |
+-------+ | | +---------+
| |
| |
| |
| |
+-------------------------------+
| | DCE Y| | DCE Z| |
| |______| |______| |
| |
| X.25 |
| |
| |
| ____________________ |
| | | |
| | DCE X | |
+-------------------------------+
| |
+-------+ Lc 4 A,B | | Lc 5 +---------+
| DTE A |----------- --------------| DTE D |
+-------+ Lc 5 A,C +---------+
The virtual circuit between A and B uses Lc4 between A and X and Lc5
between Y and B; logical channel 5 between A and X was already busy from
DTE C who has requested the line before B did.
DCE X can use a Lc5 on DTE A wire and Lc5 on DTE D wire.
THIS WOULD SIGNIFY THAT LOGICAL CHANNELS MUST ME KNOWN AS DEDICATED
LINES BETWEEN DTE AND DCE, WHILE VIRTUAL CIRCUITS AS EQUIVALENTS TO
DEDICATED LINES BETWEEN TWO DTE.
Packets function and formats
There are several types of packet identifiers, as shown in the next
table. The relative format to those is shown in next tables.
-------------------------------------------------------------------------
packet type and his | third byte bit
identificator +--------------------+---------------------------
| DCE to DTE | DTE to DCE | 87654321
------------------------+--------------------+-----------------+---------
| incoming call | call request | 00001011
|--------------------+-----------------|
| completed call | accepted call | 00001111
call and interrupt |--------------------+-----------------|
packets | disconnect indicat | disconnect req | 00010111
------------------------+--------------------+-----------------|
| data from DCE | data from DTE | xxxxxxx0
data packets and |--------------------+-----------------|
interrupt | Interrupt from DCE | interr from DTE | 00100011
|--------------------+-----------------|
| interrupt confirm | interr confirm | 00101111
------------------------+--------------------+-----------------|
| ready to receive | Ready to receive| xxx00001
| (RR) | (RR) |
|--------------------+-----------------|
| not ready to rec. | not ready to rec| xxx00101
data flow packet and | (RNR) | (RNR) |
reset |--------------------+-----------------|
| | re xmit req | xxx01001
| | (RJR) |
|--------------------+-----------------|
| reset indication | reset confirm | 00011011
|--------------------+-----------------|
| reset confirm | reset confirm | 00011111
------------------------+--------------------+-----------------|
| restart indication | restart req | 11111011
re-initialization |--------------------+-----------------|
packets | restart confirm | restart confirm | 11111111
----------------------------------------------------------------
bit -> | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | octets
+-----+-----+-----+-----+-----+-----+-----+-----+
| | | | | |
| 0 | X | 0 | 1 | LCG |
|-----------+-----------+-----------------------|
| |
| LC |
|-----------------------------------------------|
| |
| Packet identifier |
| 0 | 0 | 0 | 0 | 1 | 0 | 1 | 1 |
+-----+-----+-----+-----+-----+-----+-----+-----+
| length of address of | length of address of |
| calling DTE | called DTE |
|-----------------------------------------------|
| |
: adresses field :
: ________________________:
| | | | | |
| | 0 | 0 | 0 | 0 |
| | | | | |
|-----------------------------------------------|
| | | |
| 0 | 0 | lenght of optional feautures |
| | | field |
|-----------------------------------------------|
| |
: optional features field :
: :
|-----------------------------------------------|
| |
| user data field |
| |
+-----------------------------------------------+
bit -> | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | octets
+-----+-----+-----+-----+-----+-----+-----+-----+ |
| | | V
| Q D 0 1 | LCG | 1
|-----------------------+-----------------------|
| |
| LC | 2
|-----------------------------------------------|
| | | | |
| P(R) | M | P(S) | 0 | 3
|-----------------------------------------------|
| |
: User data :
: :
+-----------------------------------------------+
bit -> | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | octets
+-----+-----+-----+-----+-----+-----+-----+-----+ |
| | | V
| Q D 0 1 | LCG | 1
|-----------------------+-----------------------|
| |
| LC | 2
|-----------------------------------------------|
| | |
| P(R) | 0 0 0 0 1 | 3
+-----------------------------------------------+
Connect and deconnect procedure
When a user (DTE A) asks for a virtual connection to other DTE B end
then the deconnection, it will follow these next phases:
DTE A DCE DTE B
call request -------> incoming call ---------------------->
call connected <----- call accepted <----------------------
data ----------------------------------------------------->
data ----------------\ /-------------------------- data
\ /
X
/ \
<---------------/ \------------------------------>
--- clear indication <----- clear request <-------------------
| |
| |------------------> clear confirmation
|
|
--- clear confirmation --------------------------------------->
Previously we showed a table is not written, but is obvious as told in
before paragraph, that the appearing information, from A to B, will travel
on the same virtual circuit but can also have different logical channels.
The data packet identification field is recognized thanks to the first
bit of octet is a zero, in opposition to other identificators, that has a
bit set to 1 as first octet bit.
Next octed bit, seen in next table, will take following mean:
bit -> | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
+-----+-----+-----+-----+-----+-----+-----+-----+
| | | | |
| P(R) | M | P(S) | 0 |
+-----------------------------------------------+
P(S) (bit 2, 3, 4) is the packet number in transmission (0 < P(S) < 7)
P(R) (bit 6, 7, 8) is the packet number that terminal is waiting for
(0 < P(R) < 7 .
M (bit 5) (More bit) will signal if packet contains complete
information (M=0) or an information that need more packets to be completed
to be ended (M=1).
The number of the packet terminal is waiting for is also contained in
the identifier of the control packet of flow control (bit 8, 7, 6).
As in levels 2 level 3 also provide a transmission window known as the
maximum number of packets that can be sendt without receiving the good
receive signal. This window is decided at the time of wiring as function
of speed and can assume values from 1 to 7.
The other two bits are Q and D.
The Q bit, as 8th bit of first octet of data exchange packet, can be
considered as the second logical channel generator within the same channel,
in the way of, when it is equal to zero, it points to a certain data
flow, when it is set to one, it will individualize another totally indep-
endent flow of data, in the same logical channel.
The X.29 recomendation uses this bit to send "information" between two
devices that are making the usual data flow exchange between two users.
The D bit is used when a particular network is so complex that we want
to have the confirmation about the receiving of a message from a remote
terminal to which we are logically connected, before erase from our memory
the sent information.
6. X RECOMENDATION APPLICATIONS
The X.25 Recomendation, as told at start, considers the interface
between DTE and DCE. Actual users have a start-stop terminal usage or
synchronous type that is not appropriate to be directly connected to the
PSS World, but we have do to some protocol convertions done by PADs (Packet
Assembly-Disassembly).
By the use of these devices, it is possible build networks as shown:
+----------+ +----------+
| CPU X.25 |-------- --------------| DTE X.25 |
+----------+ | | +----------+
| |
| |
| |
| |
+-------------------------------+
| |
| |
| |
| X.25 |
| |
| |
| |
| |
| |
+-------------------------------+
| |
| |
/ \ / \
/ \ / \
/_PAD_\ /_PAD_\
| | | | | |
| | | | | |
start/stop +----------+
users | not X.25 |
| CPU |
+----------+
In the table shown above we can observe as the X.25 type terminals and
the X.25 terminals connect directly to the X.25 world.
CCITT has provided some recomendations for these kinds of devices:
X.3 recomandation - Features for PAD interfacing with a public network.
X.28 recomandation - DTE/DCE interface for start/stop terminals
connected to a PAD which is connected to a PSS in same country.
X.29 recomandation - Procedures to control the information exchange
between a PAD and a DTE X.25 or another PAD.
Those three recomandations and X.25 can be showed as follows:
: X.29 : :
:<-------------------------->:<------------------>:
: : :
: : :
|\ : : +---------+
| \ +-----+ +----------------+ RS 232 | |
| | X.3 | | | PAD|----------| terminal|
| P | | | X.25 +----|....... +---------+
| | par |\__________| | !
| A | ame |/ | network | !
| | ter | | | ! X.29
| D | s | | | !
| / +-----+ +----------------+ !
|/ : | !
: | ..........!
: | :
: | :
: | : /|
: | +-----+ / |
: | | X.3 | |
: | | | P | +---------+
: |__/| par | | RS 232 | |
: \| ame | A |---------| terminal|
: | ter | | +---------+
: | s | D | :
: +-----+ \ | :
: : \| :
: X.29 : X.28 :
:<-------------------------->:<-------------------->:
Thus concludes the TCSB "Introduction to Packet Switched Networks". We do
hope that you enjoyed this file. Look out for more PSS Network files soon.
_______________________________________________________________________________
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