TPDU Types

icspacket.proto.cotp.structs.checksum(tpdu_data: Iterable[int], checksum_off: int)

Compute the checksum for a TPDU according to Annex D.3 of X.224.

The algorithm iterates through each octet, maintaining two accumulators C0 and C1. After processing, the two checksum octets X and Y are computed based on the position of the checksum parameter and the total TPDU length.

Algorithm (Annex D.3):

1. Initialize C0 = 0 and C1 = 0.

2. For each octet in the TPDU, add its value to C0 and then add the new C0 to C1.

3. Compute the checksum values: - X = (-C1 + (L - n) * C0) mod 256 - Y = (C1 - (L - n + 1) * C0) mod 256

Parameters:

• tpdu_data (Iterable[int]) – Sequence of TPDU octets.

• checksum_off (int) – Offset where the two checksum octets reside.

Returns:

Two-byte checksum value (X, Y).

Return type:

bytes

icspacket.proto.cotp.structs.verify_checksum(tpdu_data: bytes, checksum_off: int) → bool

Verify the checksum of a TPDU.

The function temporarily zeroes out the checksum field and recomputes it using checksum(). If the recomputed checksum matches the original octets at the checksum offset, the TPDU is considered valid.

Parameters:

• tpdu_data (bytes) – The TPDU octets containing a checksum field.

• checksum_off (int) – Offset to the start of the checksum field.

Returns:

True if checksum matches, False otherwise.

Return type:

bool

class icspacket.proto.cotp.structs.TPDU_Code(*values)

13.2.2.2 TPDU code

ED = 1

EA = 2

RJ = 3

AK = 6

ER = 7

DR = 8

DC = 12

CC = 13

CR = 14

DT = 15

class icspacket.proto.cotp.structs.TPDU_Class(*values)

7. Protocol classes

CLASS0 = 0

8. Simple class (Class 0)

Class 0 provides only the functions needed for connection establishment with negotiation, data transfer with segmenting and protocol error reporting.

CLASS1 = 1

9. Basic error recovery class (Class 1)

Class 1 provides transport connections with flow control based on the network service provided flow control, error recovery, expedited data transfer, disconnection, and also the ability to support consecutive transport connections on a network connection.

CLASS2 = 2

10. Multiplexing class (Class 2)

Class 2 provides transport connections with or without individual flow control; no error detection or error recovery is provided.

CLASS3 = 3

11. Error recovery and multiplexing class

Class 3 provides the functionality of class 2 (with use of explicit flow control) plus the ability to recover after a failure signalled by the Network Layer without involving the TS-user.

CLASS4 = 4

12. Error detection and recovery class (Class 4)

Class 4 provides the functionality of class 3, plus the ability to detect and recover from lost, duplicated, or out of sequence TPDUs without involving the TS-user.

class icspacket.proto.cotp.structs.TPDU_DisconnectReason(*values)

Defines the reason for disconnecting the transport connection.

NORMAL = 128

Normal disconnect initiated by session entity.

REMOTE_CONGEST = 129

Remote transport entity congestion at connect request time.

NEGO_FAILED = 130

Connection negotiation failed [i.e. proposed class(es) not supported].

DUPLICATE_SOURCE = 131

Duplicate source reference detected for the same pair of NSAPs.

MISMATCHED_REFERENCES = 132

Mismatched references.

PROTOCOL_ERROR = 133

Protocol error.

REF_OVERFLOW = 135

Reference overflow.

CONN_REFUSED = 136

Connection request refused on this network connection.

INVALID_LENGTH = 138

Header or parameter length invalid.

UNSPECIFIED = 0

Reason not specified.

TSAP_CONGESTION = 1

Congestion at TSAP.

ENTITIY_NOT_ATTACHED = 2

Session entity not attached to TSAP.

UNKNOWN_ADDRESS = 3

Address unknown.

class icspacket.proto.cotp.structs.TPDU_Size(*values)

Defines the proposed maximum TPDU size (in octets including the header) to be used over the requested transport connection

SIZE_8192 = 13

SIZE_4096 = 12

SIZE_2048 = 11

SIZE_1024 = 10

SIZE_512 = 9

SIZE_256 = 8

SIZE_128 = 7

class icspacket.proto.cotp.structs.TPDU_RejectCause(*values)

Cause of rejection of a connection request

UNSPECIFIED = 0

Cause not specified

INVALID_PARAMETER_CODE = 1

Invalid parameter code

INVALID_PDU_TYPE = 2

Invalid PDU type

INVALID_PARAMETER_VALUE = 3

Invalid parameter value

class icspacket.proto.cotp.structs.TPDU_AdditionalOptions(unused: bool = False, non_blocking: bool = False, use_request_ack: bool = False, use_selective_ack: bool = False, speed_up: bool = False, use_receipt_info: bool = False, use_checksum_16bit: bool = False, use_transport_speed_up: bool = True)

Additional option selection (not used if class 0 is the preferred class)

unused: bool = False

non_blocking: bool = False

Use of non-blocking expedited data in class 4

use_request_ack: bool = False

Use of request acknowledgement in class 1, 3, 4

use_selective_ack: bool = False

Use of selective acknowledgement in class 4

speed_up: bool = False

Use of network expedited in class 1

use_receipt_info: bool = False

• True: Use of receipt confirmation in class 1

• False: Use of explicit AK variant in class 1

use_checksum_16bit: bool = False

16-bit checksum defined in 6.17 shall be used in class 4

use_transport_speed_up: bool = True

Use of transport expedited data transfer service

class icspacket.proto.cotp.structs.TPDU_TransitDelay(calling_target_value: int = 0, calling_maximum_acceptable: int = 0, called_target_value: int = 0, called_maximum_acceptable: int = 0)

Transit delay (not used if class 0 is the preferred class)

calling_target_value: int = 0

target value, calling-called user direction;

calling_maximum_acceptable: int = 0

maximum acceptable, calling-called user direction;

called_target_value: int = 0

target value, called-calling user direction;

called_maximum_acceptable: int = 0

maximum acceptable, called-calling user direction;

class icspacket.proto.cotp.structs.TPDU_ResidualErrorRate(target_value: int = 0, minimum_acceptable: int = 0, tsdu_size_of_interest: int = 0)

Residual error rate (not used if class 0 is the preferred class)

target_value: int = 0

target value, power of 10;

minimum_acceptable: int = 0

minimum acceptable, power of 10;

tsdu_size_of_interest: int = 0

TSDU size of interest, expressed as a power of 2.

class icspacket.proto.cotp.structs.Parameter_Code(*values)

Defines different parameter types used accross TPDUs

CALLED_T_SELECTOR = 193

CALLING_T_SELECTOR = 194

TPDU_SIZE = 192

MAX_TPDU_SIZE = 240

VERSION = 196

PROTECTION = 197

CHECKSUM = 195

ADDITIONAL_OPTS = 198

ALTERNATIVE_CLASSES = 199

ACK_TIME = 133

THROUGHPUT = 137

ERROR_RATE = 134

PRIORITY = 135

TRANSIT_DELAY = 136

REASSIGNMENT_TIME = 139

INACTIVITY = 242

ADDTITIONAL_INFO = 224

SUBSEQUENCE_NUM = 138

FLOW_CONTROL_INFO = 140

ACK_PARAMS = 143

class icspacket.proto.cotp.structs.Parameter(type_id: Parameter_Code | int = 0, value: Any | TPDU_Size | int | bytes | TPDU_AdditionalOptions | list[int] | TPDU_ResidualErrorRate | TPDU_TransitDelay = b'')

13.2.3 Variable part

The variable part is used to define less frequently used parameters. If the variable part is present, it shall contain one or more parameters.

type_id: Parameter_Code | int = 0

The parameter code

value: Any | TPDU_Size | int | bytes | TPDU_AdditionalOptions | list[int] | TPDU_ResidualErrorRate | TPDU_TransitDelay = b''

The parameter length indication indicates the length, in octets, of the parameter value field.

The parameter value field contains the value of the parameter identified in the parameter code field.

class icspacket.proto.cotp.structs.TPDU(li: int = 0, code: int = 0)

Transport Protocol Data Units (TPDUs)

li: int = 0

13.2.1 Length indicator field

The length indicated shall be the header length in octets including parameters, but excluding the length indicator field and user data, if any. The value 255 (1111 1111) is reserved for possible extensions.

code: int = 0

13.2.2.2 TPDU code

This field contains the TPDU code. It is used to define the structure of the remaining header.

property tpdu_code: TPDU_Code

Qualified TPDU code

property code_arg: int

Argument bits of the TPDU code

has_parameters() → bool

Whether the TPDU has parameters

get_parameters() → list[Parameter]

Returns the parameters of the TPDU if present

has_checksum() → bool

Checks whether the TPDU has a checksum parameter

property fixed_size: int

Returns the number of octets that make up the fixed (header) part of this TPDU, excluding the variable part and user data.

This value is normally defined per TPDU type in X.224 and stored as the class attribute TPDU_FIXED_SIZE. If the subclass does not define it, a default of 1 octet is returned.

The fixed size is used when computing the length indicator (LI), and also when locating parameter offsets, such as the position of the checksum parameter.

property first_checksum_octet: int

Returns the zero-based index (offset) of the first checksum octet within the serialized TPDU, or -1 if no parameters are present.

For ease of computation, the checksum parameter MUST appear in the first position of the variable part when present. This means:

• Offset starts at fixed_size (end of fixed header)

• Add 1 byte for the Length Indicator (LI)

• Add 2 bytes for the TLV header (parameter code and length)

get_checksum() → bytes

Returns the current 2-byte checksum value from the variable part, if a checksum parameter is present. If no checksum parameter is found, returns an empty byte string.

The checksum parameter is identified by its type code Parameter_Code.CHECKSUM and should be the first parameter.

set_checksum(value: bytes)

Sets the checksum parameter value in the TPDU.

If a checksum parameter already exists, its value is replaced. If it does not exist, it is inserted into the first position of the parameter list. This ensures the checksum field is correctly located for both building and verification.

is_valid() → bool

Verifies the TPDU checksum if present.

• If no checksum parameter is found, returns True (valid by default).

• If present, recomputes the checksum over the TPDU using the Annex D.3 algorithm, comparing the calculated value to the stored one.

This method does not rebuild the TPDU using build() to avoid unintentional mutations; it uses the raw packed representation for verification.

build(add_checksum: bool = False) → bytes

Serializes the TPDU into its octet representation.

This method constructs a valid TPDU by encoding its fixed and variable parts, and optionally adds a checksum parameter.

Behavior depends on add_checksum:

• False (default): The TPDU is serialized normally without any checksum.

• True:

  1. A placeholder checksum parameter (two zero bytes) is inserted.

  2. The TPDU is packed into octets.

  3. The checksum is recomputed across the entire TPDU (with zeros in the checksum field).

  4. The placeholder checksum is replaced with the computed value and the TPDU is repacked.

Example:

>>> pdu = TPDU_ConnectionRequest()
>>> pdu.build(add_checksum=True)
b'\n\xe0\x00\x00\x00\x00\x00\xc3\x02|\xd5'
>>> parsed = TPDU_ConnectionRequest.from_octets(_)
TPDU_ConnectionRequest(li=10, code=224,...parameters=[Parameter(type_id=<Parameter_Code.CHECKSUM: 195>, value=b'|\xd5')])

Parameters:

add_checksum – Whether to generate and insert a checksum parameter during the build process.

Returns:

Byte string representing the complete TPDU.

classmethod from_octets(octets: bytes)

Deserialize raw octets into a TPDU (or subclass) instance.

This method unpacks the raw TPDU structure into the corresponding class representation.

Parameters:

octets (bytes) – Encoded TPDU octets.

Returns:

TPDU instance populated from octets.

Return type:

TPDU

class icspacket.proto.cotp.structs.TPDU_ClassOption(class_id: TPDU_Class = TPDU_Class.CLASS0, reserved: int = 0, extended_formats: bool = False, explicit_flow_control: bool = False)

CLASS OPTION

Defines the preferred transport protocol class to be operated over the requested transport connection.

class_id: TPDU_Class = 0

reserved: int = 0

extended_formats: bool = False

explicit_flow_control: bool = False

icspacket.proto.cotp.structs.TPDU_VariablePart = Field('<unnamed>', arch='x86_64', order='SysNative', seq=True, struct=Struct(fields=['type_id', 'value']), cond=False, options=False)

13.2.3 Variable part

The variable part is used to define less frequently used parameters. If the variable part is present, it shall contain one or more parameters.

icspacket.proto.cotp.structs.TPDU_UserData = <Bytes>

13.2.4 Data field

This field contains transparent user data

class icspacket.proto.cotp.structs.TPDU_ConnectionRequest(li: int = 0, code: int = 0, dst_ref: int = 0, src_ref: int = 0, class_opt: TPDU_ClassOption = None, parameters: list[Parameter] = None, user_data: bytes = None)

13.3 Connection Request (CR) TPDU

TPDU_FIXED_SIZE = 6

dst_ref: int = 0

3. DST-REF - Set to zero.

src_ref: int = 0

4. SRC-REF

Reference selected by the transport entity initiating the CR-TPDU to identify the requested transport connection.

class_opt: TPDU_ClassOption = None

5. CLASS OPTION

Bits 8 to 5 of octet 7 define the preferred transport protocol class to be operated over the requested transport connection. The CR-TPDU contains the first choice of class in the fixed part. Second and subsequent choices are listed in the variable part if required.

parameters: list[Parameter] = None

13.3.4 Variable part

The following parameters are permitted in the variable part:

• 1. Transport-Selector (T-selector)

• 2. TPDU size

• 3. Preferred maximum TPDU size

• 4. Version number (not used if class 0 is the preferred class)

• 5. Protection parameters (not used is class 0 is the preferred class)

• 6. Checksum (used only if class 4 is the preferred class)

• 8. Alternative protocol class(es) (not used if class 0 is the preferred class or when operating over CLNS) Parameter code:

• 1. Acknowledgement time (used only if class 4 is the preferred class)

• 10. Throughput (not used if class 0 is the preferred class)

• 11. Residual error rate (not used if class 0 is the preferred class)

• 12. Priority (not used if class 0 is the preferred class)

• 13. Transit delay (not used if class 0 is the preferred class)

• 14. Reassignment time (not used if class 0 or 2 is the preferred class; if class 4 is preferred and class 3 is an alternate, it may be used)

• 15. Inactivity timer (used only if class 4 is the preferred or selected class)

user_data: bytes = None

No user data are permitted in class 0, and are optional in other classes.

class icspacket.proto.cotp.structs.TPDU_ConnectionConfirm(li: int = 0, code: int = 0, dst_ref: int = 0, src_ref: int = 0, class_opt: TPDU_ClassOption = None, parameters: list[Parameter] = None, user_data: bytes = b'')

13.4Connection Confirm (CC) TPDU

TPDU_FIXED_SIZE = 6

dst_ref: int = 0

3. DST-REF

Reference identifying the requested transport connection at the remote transport entity.

src_ref: int = 0

4. SRC-REF

Reference selected by the transport entity initiating the CC-TPDU to identify the confirmed transport connection.

class_opt: TPDU_ClassOption = None

5. CLASS OPTION

Defines the selected transport protocol class and option to be operated over the accepted transport connection.

parameters: list[Parameter] = None

Same as in TPDU_ConnectionRequest

user_data: bytes = b''

13.4.5 User Data

No user data are permitted in class 0, and are optional in the other classes.

class icspacket.proto.cotp.structs.TPDU_DisconnectRequest(li: int = 0, code: int = 0, dst_ref: int = 0, src_ref: int = 0, reason: TPDU_DisconnectReason | int = 0, parameters: list[Parameter] = None, user_data: bytes = None)

13.5 Disconnect Request (DR) TPDU

The DR-TPDU is used to terminate a transport connection. It may carry parameters and optional user data.

TPDU_FIXED_SIZE = 6

dst_ref: int = 0

Destination reference — identifies the transport connection to be released.

src_ref: int = 0

Source reference — identifies the transport connection from the sender’s perspective.

reason: TPDU_DisconnectReason | int = 0

Reason code for disconnection (X.224 §13.5.4).

parameters: list[Parameter] = None

Optional parameters (variable part)

Allowed parameters:

• 1. Additional information

• 2. Checksum

user_data: bytes = None

Optional user data — must not exceed 64 octets.

class icspacket.proto.cotp.structs.TPDU_DisconnectConfirm(li: int = 0, code: int = 0, dst_ref: int = 0, src_ref: int = 0, parameters: list[Parameter] = None)

3.6 Disconnect Confirm (DC) TPDU

The DC-TPDU is sent in response to a DR-TPDU to confirm the disconnection of a transport connection.

TPDU_FIXED_SIZE = 5

dst_ref: int = 0

Destination reference — identifies the transport connection being confirmed as disconnected.

src_ref: int = 0

Source reference — identifies the transport connection from the sender’s perspective.

parameters: list[Parameter] = None

Only checksum is allowed as a parameter

class icspacket.proto.cotp.structs.TPDU_Number(eot: bool = False, value: int = 0)

eot: bool = False

4. EOT

When set to ONE, it indicates that the current DT-TPDU is the last data unit of a complete DT-TPDU sequence (end of TSDU).

value: int = 0

5. TPDU-NR

TPDU send sequence number (zero in class 0). May take any value in class 2 without explicit flow control. TPDU-NR is bits 7 to 1 of octet 3 for classes 0 and 1, bits 7 to 1 of octet 5 for classes 2, 3 and 4.

class icspacket.proto.cotp.structs.TPDU_Data(li: int = 0, code: int = 0, nr: TPDU_Number = None, user_data: bytes = b'')

13.7 Data (DT) TPDU

The DA-TPDU is used to transfer data over a transport connection. It may carry parameters and optional user data.

TPDU_FIXED_SIZE = 2

nr: TPDU_Number = None

5. TPDU-NR

user_data: bytes = b''

This field contains data of the TSDU being transmitted.

property tpdu_nr: int

property is_last: bool

class icspacket.proto.cotp.structs.TPDU_ExpeditedData(li: int = 0, code: int = 0, dst_ref: int = 0, ed_nr: TPDU_Number = None, parameters: list[Parameter] = None, user_data: bytes = None)

13.8 Expedited Data (ED) TPDU

The ED-TPDU is used to send expedited (urgent) data across the transport connection.

TPDU_FIXED_SIZE = 4

dst_ref: int = 0

Destination reference — identifies the transport connection to which the expedited data belongs.

ed_nr: TPDU_Number = None

Sequence number for expedited data (X.224 §13.8.4).

parameters: list[Parameter] = None

Only checksum is allowed as a parameter

user_data: bytes = None

The expedited data payload — must not exceed the maximum allowed for expedited service.

class icspacket.proto.cotp.structs.TPDU_DataAcknowledgement(li: int = 0, code: int = 0, dst_ref: int = 0, next_nr: TPDU_Number = None, credit: int = 0, parameters: list[Parameter] = None)

13.9 Data Acknowledgement (AK) TPDU

The AK-TPDU acknowledges receipt of data and communicates flow control information.

TPDU_FIXED_SIZE = 6

dst_ref: int = 0

Destination reference — identifies the transport connection.

next_nr: TPDU_Number = None

Next expected TPDU sequence number.

credit: int = 0

Flow control credit — number of TPDUs the sender is prepared to receive.

parameters: list[Parameter] = None

Optional parameters allowed in AK-TPDU:

• 1. Checksum

• 2. Subsequence number when optionally used under the conditions defined in class 4.

• 3. Flow control confirmation when optionally used under the conditions defined in class 4.

• 4. Selective acknowledgement parameters when optionally used, under conditions defined in class 4.

class icspacket.proto.cotp.structs.TPDU_ExpeditedDataAcknowledgement(li: int = 0, code: int = 0, dst_ref: int = 0, ed_nr: TPDU_Number = None, parameters: list[Parameter] = None)

13.10 Expedited Data Acknowledgement (EA) TPDU

The EA-TPDU is used to acknowledge receipt of expedited data (ED TPDUs).

TPDU_FIXED_SIZE = 4

dst_ref: int = 0

Destination reference — identifies the transport connection.

ed_nr: TPDU_Number = None

Expedited data sequence number being acknowledged.

parameters: list[Parameter] = None

Only checksum is allowed as a parameter

class icspacket.proto.cotp.structs.TPDU_Reject(li: int = 0, code: int = 0, dst_ref: int = 0, y_nr: int = 0)

13.11 Reject (RJ) TPDU

The RJ-TPDU requests retransmission of certain TPDUs due to detected errors.

TPDU_FIXED_SIZE = 5

dst_ref: int = 0

Destination reference — identifies the transport connection.

y_nr: int = 0

Next expected TPDU sequence number (Y(R)).

class icspacket.proto.cotp.structs.ER_RejectCause(*values)

REASON_NOT_SPECIFIED = 0

INVALID_PARAMETER_CODE = 1

INVALID_TPDU_TYPE = 2

INVALID_PARAMETER_VALUE = 3

class icspacket.proto.cotp.structs.TPDU_Error(li: int = 0, code: int = 0, dst_ref: int = 0, reject_cause: ER_RejectCause | int = ER_RejectCause.REASON_NOT_SPECIFIED, parameters: list[Parameter] = None)

13.12 TPDU error (ER) TPDU

No user data.

TPDU_FIXED_SIZE = 4

dst_ref: int = 0

Destination reference (see §13.4.3).

reject_cause: ER_RejectCause | int = 0

Reject cause (§13.12.3).

parameters: list[Parameter] = None

Optional parameters:

• 1. Invalid TPDU

• 2. Checksum

icspacket.proto.cotp.structs.parse_tpdu(octets: bytes) → TPDU | TPDU_ConnectionConfirm | TPDU_ConnectionRequest | TPDU_Data | TPDU_DataAcknowledgement | TPDU_DisconnectConfirm | TPDU_DisconnectRequest | TPDU_Error | TPDU_ExpeditedData | TPDU_ExpeditedDataAcknowledgement | TPDU_Reject

Parse a TPDU (Transport Protocol Data Unit) from raw octets.

First decodes a generic TPDU to extract the TPDU code and uses this to dispatch to the corresponding TPDU subclass implementation defined in TPDU_TYPES.

Example:

>>> tpdu = parse_tpdu(...)
>>> isinstance(tpdu, TPDU_Data)
True
>>> data: bytes = tpdu.user_data

Parameters:

octets (bytes) – Raw TPDU octets to parse.

Raises:

ValueError – If the octet buffer is shorter than two bytes.

Returns:

A parsed TPDU instance corresponding to the TPDU code (e.g., TPDU_ConnectionRequest).

Return type:

_TPDULike