# RFC 1006 ISO Transport Service on top of the TCP

Protocols · `RFC 1006` · verification: source checked

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.

## Specification

| Property | Value |
|---|---|
| Transports | tcp |
| Architecture | request response |
| Encoding | binary |
| Standard ref | RFC 1006 |

### Default ports

| Port | Transport | Secure | Purpose |
|---|---|---|---|
| 102 | tcp | false | ISO transport service on top of TCP (TSAP connections) |

### Data types

| Name | Size (bits) | Description |
|---|---|---|
| vrsn | 8 | Version of the protocol; always 3 for the version described in this memo. |
| packet length | 16 | Length of the entire packet in octets including the packet header; min 7, max 65535. |
| TPDU-NR and EOT | 8 | Bit 7 (mask 1000 0000) indicates end of a TSDU; all other bits zero on output and ignored on input. |

## 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.

## 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.

## Sources

1. [RFC 1006: ISO Transport Service on top of the TCP Version: 3](https://datatracker.ietf.org/doc/html/rfc1006) (accessed 2026-10-25)

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