rfc892.txt

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        ED
        - ED TPDU-NR (7 or 31 bits)

        Description:

        After the reset of an underlying network connection,
the resynchronization procedures below are carried out by both
transport entities.  

        After a network connection failure, the reassignment after
failure function is invoked and then the resynchronization function.  
The sequence of events at the two transport entities is the following:

        Events at the transport entity initiating reassignment:
        (the transport entity immediately commences resynchronization
         by sending a TPDU)

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        o       if a CR is retained then retransmit it.

        o       if a DR is retained then retransmit it.

        o       otherwise, resynchronize data:

                -       send RJ TPDU with 'YR-TU-NR' field set to
                        the 'TPDU-NR' of the first unreceived DT
                        TPDU

                -       when RJ TPDU has been received retransmit any
                        ED TPDUs then DT TPDUs which are unacknowledged

                -       any ED TPDUs received which are duplicates shall
                        be acknowledged (by EA TPDUs) and discarded.  

        Events at the other transport entity:

        The transport entity shall not send any TPDUs until after 
receipt of the TPDU which commenced resynchronization.  This TPDU
therefore serves two purposes, namely indication of re-assignment
and commencement of resynchronization.  

        o       if the first received TPDU os a DR, then transmit
                a DC TPDU.

        o       if the first received TPDU is a CR and the transport
                connection is not idle, this means that a CC TPDU is
                retained:  then retransmit it followed by any ED TPDU 
                and then DT TPDUs which are outstanding (that may or
                may not have been transmitted previously).  

        NOTE:  no TPDUs can be transmitted using network expedited until 
CC becomes acknowledged, to prevent the network expedited overtaking the 
CC.  

        o       if the first received TPDU is a RJ, then act as follows:

                -       if a DR TPDU is retained, then retransmit it

                -       if a CC TPDU remains unacknowledged, then carry
                        out the data resynchronization procedure described
                        below

                -       otherwise resynchronize data:

                        -       send RJ TPDU with 'YR-TU-NR' field set to
                                the 'TPDU-NR' of the first unreceived DT
                                TPDU


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                        -       retransmit any ED TPDUs then DT TPDUs which
                                are unacknowledged

                        -       any ED TPDUs received which are duplicates 
                                should be acknowledged (by EA TPDUs) and 
                                discarded.  

        NOTE:  It is possible for a transport entity using the Class 1
protocol to decide on a local basis to issue an N-RESET Request.  The effect
of this request at the remote transport entity is to force it to perform
the resynchronization mechanism.  This possibility may be used to remove 
congestion within the network connection.  

6.16    Multiplexing and Demultiplexing

        Purpose:  Concurrent sharing of a network connection by several
transport connections.

        TPDUs and Fields Used:

        CC, DR, DC, DT, AK, ED, EA, RJ, ERR
        - destination reference

        Description:

        This function is pervasive.  

        When this function is in operation, more than one transport 
connection can be simultaneously assigned to the same network connection.

        Every TPDU (including DT TPDUs) must carry the destination 
reference, to identify the transport connection to which it refers.  

6.17    Explicit Flow Control

        Purpose:  Regulation of flow of DT TPDUs independently of 
the flow control in the other layers.  

        TPDUs and Fields Used:

        CR, CC, AK, RJ
        - CDT (4 or 16 bits)

        DT
        - TPDU-NR (7 or 31 bits)

        AK, RJ
        - YR-TU-NR (7 or 31 bits)
        - subsequence number (optional)
        - flow control confirmation (optional)

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        Description:

        The mechanism depends on the class.  Thus the description can
be found in the section describing the class. 

6.18    Checksum

        Purpose:  To detect corruption of TPDUs by the network service
provider.  

        TPDUs and Fields Used:

        All TPDUs
        - checksum (16 bits - 32 bits)

        Description:

        When a TPDU is to be transmited for a TC which has selected the
checksum option, the sending transport entity must generate a checksum
for the TPDU and store it in the checksum parameter in the variable
part of the TPDU header.  The checksum must be generated as follows:

        1.      Set up the complete TPDU, including the header and 
user data (if any).  The header must include the checksum parameter in
its variable part.  The value field of the checksum parameter must be
set to zero at this point.  

        2.      Initialize two variables to zero.  Let these variables 
be called C0 and C1.  

        3.      For each octet of the TPDU, including the header, 
variable part of the header and the user data, add the octet value to 
C0, and then add the value of C0 to C1.  Octets should be processed
sequentially, starting with the first octet (the Length Indicator) and
proceeding through the TPDU.  All addition is to be performed modulo 255.

        4.      Calculate the value field of the checksum parameter as
follows.  Let the offset into the TPDU of the first octet of the value 
field be 'n' (where the first octet of the TPDU, the Length Indicator
of the header, is considered to be at offset 1).  Let the length 
of the TPDU, i.e. the number of times the above operation was repeated,
be 'L'.  Let the first octet of the checksum value, i.e., the one at offset
'n' be called 'X', and the second octet, at offset 'n+1', be called 'Y'.  
Then:  

        X = (((L - n) *  C0) - C1) modulo 255
        Y = (((L - n + 1) * (-C0)) + C1) modulo 255

        5.      Place the computed values of X and Y in the appropriate
octets of the TPDU.  

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                                NOTE

        An implementation may use one's complete arithmetic as an
        alternative to modulo 255 arithmetic.  However, if either
        of the checksum octets X and Y has the value minus zero
        (i.e., 255) then it must be converted to plus zero 
        (i.e., 0) before being stored.  

        When a TPDU is received for a TC for which the checksum option
has been selected, the TPDU must be verified to ensure that it has been
received correctly.  This is done by computing the checksum, using the
same algorithm by which it was generated.  The nature of the checksum
algorithm is such that it is not necessary to compare explicitly the stored
checksum bytes.  The procedure described below may be used to verify that 
a TPDU has been correctly received.  

        1.      Initialize two variable to zero.  Let these variables
be called C0 and C1.  

        2.      For each octet in the received TPDU, add the value of
the octet to C0 and then add the value of C0 to C1, starting with the
first octet and proceeding sequentially through the TPDU.  All 
addition is to be performed modulo 255.  

        3.      When all octets have been sequentially processed, the
values of C0 and C1 should be zero.  If either or both of them is 
non-zero, the TPDU has been received incorrectly and the verification
has failed.  Otherwise, the TPDU has been received correctly and the 
TPDU should be processed normally.  

                                NOTE

        An implementation may use one's complement arithmetic as an
        alternative to modulo 255 arithmetic.  In this case, if either
        C0 or C1 has the value minus zero (i.e., 255) it is to be 
        regarded as though it was plus zero (i.e., 0)

        If a checksum verification failure occurs, it is not possible
to determine the TC that the TPDU relates to, since the Reference field
of the TPDU may have been received incorrectly.  Therefore, all TCs
multiplexed onto the same NC must be treated as though a network signalled
error has occurred.  

6.19    Frozen References

        Purpose:  To prevent re-use of a reference while TPDUs associated
with the old use of the reference may still exist.  

        Description:  When a transport entity determines that a particular
connection has terminated, the reference shall be placed in a frozen state 

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during which time it will not be reused.  The circumstances under which
this is done, and the period of time for which the reference remains
frozen depends on the class.   

6.20    Retransmission on Timeout

        Purpose:  To cope with unsignalled loss of TPDUs by the network
service provider.  

        TPDUs and Fields Used:

        CR, CC, DR, DT, ED, AK

        Description:  

        The description is given in the section related to class 4.  

6.21    Resequencing 

        Purpose:  To cope with misordering of TPDUs by the network
service provider.  

        TPDUs and Field Used:

        DT
        - TPDU NR

        ED
        - ED TPDU NR

        Description:

        The description is given in the section related to class 4.  

6.22    Inactivity Control

        Purpose:  To cope with unsignalled termination of a network
connection.  

        TPDUs and Fields Used:

        AK

        Description:

        The description is given in the section related to class 4.  

6.23    Treatment of Protocol Errors

        Purpose:  To deal with invalid TPDUs.

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        TPDUs and Fields Used:

        ERR
        - reject cause
        - TPDU in error (string of octets)

        DR
        - reason code

        Description:

        This function is inherent.  

        Any received TPDU which is invalid or which cannot be dealt with by
any operative function, or which is regarded as a violation of the protocol
rules of the class in use (e.g., receipt in a wrong state, window error,
sequencing error, TPDU with incorrect format), shall be considered as a 
protocol error.  Such an error shall be signalled to the transport entity
responsible by the sending of an TPDU Error (ERR) TPDU or by initiating a 
release.  The ERR TPDU conveys the octets of the offending TPDU up to
and including the octet where the error was detected.  

        In general, no further action is defined for the sender of 
ERR TPDU, since it is expected that the offender will either correct 
the error, or close the connection.  

        Action to be done by the receiver depends on local implementation
decision; e.g., freeze the connection, report to management, disconnect.  

NOTES:  

        1.      Further action is a local implementation issue.  Care
should be taken by the transport entity receiving several invalid TPDUs
or ERR TPDUs to avoid looping if the error is repeatedly generated.  

        2.      There are two cases in which specific action is defined
for the receiver of the ERR TPDU (see Sections 6.6 and 7.0.7).  

6.24    Splitting and Recombining

        Purpose:  To allow a transport connection to make use of 
multiple network connections to provide additional resilience against
network failure, to increase throughput, or for other reasons.  

        Description:

        This function is available only in Class 4.  

        When this function is being used, a transport connection is 
assigned (see Section 6.1) to multiple network connections. TPDUs for the 

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connection may be sent over any