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RFC 1006 ISO Transport Service on top of the TCP

RFC 1006 defines how ISO transport class 0 (TP0) is carried over TCP, so ISO session, presentation and application layers can run on a TCP/IP internetwork without change. Each transport protocol data unit (TPDU) is framed in a TPKT packet consisting of a 4-octet header (version, reserved, packet length) followed by the TPDU defined in ISO 8073. TCP port 102 is reserved for hosts implementing the standard, and TCP connection open/close events are mapped onto the ISO N-CONNECT, N-DATA and N-DISCONNECT primitives.

Source checked15/15 values · 2026-10-25

Specification

1 source
Transportstcp
Architecturerequest response
Encodingbinary
Standard refRFC 1006

Data types

NameSize (bits)Description
vrsn8Version of the protocol; always 3 for the version described in this memo.
packet length16Length of the entire packet in octets including the packet header; min 7, max 65535.
TPDU-NR and EOT8Bit 7 (mask 1000 0000) indicates end of a TSDU; all other bits zero on output and ignored on input.

Default ports

IANA assignments
102TCPISO transport service on top of TCP (TSAP connections)

Study notes

  • The protocol is ISO transport class 0 over TCP, which is stated to achieve functionality identical to transport class 4.
  • Version 3 obsoletes RFC 983; the changes are minor but incompatible.
  • Default TPDU size is 65531 octets instead of the 128 octets of ISO 8073; a smaller size is negotiated with the TPDU Size variable part.
  • Quality of Service parameters of ISO 8072 are not supported and are left for further study.
  • The TSAP ID field format is not specified by ISO; this memo mandates the GOSIP specification (GOSIP86, section 5.2 Upper Layers Addressing).
  • The ISO TSAP is symmetric: there is no underlying client/server model, and TS-users sit in the IDLE state until they generate a REQUEST or accept an INDICATION.
  • Expedited data transfer is negotiated during connection establishment via the Additional Option Selection variable part; the default is not to use it.
  • The non-standard ED TPDU for expedited data is sent on the same TCP connection as all other TPDUs; its only difference from the DT TPDU is the code value ED.
  • Documents referenced but not matching available entries: ISO 8072, ISO 8073, ISO 8327, RFC 791, RFC 793, RFC 983, X.214, X.224, X.225.
  • In most implementations a single server perpetually LISTENs on port 102 and hands off connections as they are made.

Safety

  • Packet length field is 16 bits, minimum 7 and maximum 65535 octets, including the packet header.
  • Maximum TPDU size is 65531 octets, giving a maximum TSDU size of 65524 octets.
  • The TP specification limits the expedited transport service data unit (XSDU) to 16 octets.
  • Only TPDUs formatted for transport class 0 are exchanged.
  • The reserved header field must be handled per the TPKT header definition; the vrsn field is always 3 for this version of the protocol.
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