Chapter 35. ITOT: ISO Transport Service on top of TCP

Protocol Description ISO Transport Service on top of TCP (ITOT) is a mechanism that enables ISO applications to be ported to a TCP/IP network. There are two basic approaches which can be taken when “porting” ISO applications to TCP/IP (and IPv6) environments. One approach is to port each individual application separately, developing local protocols on top of TCP. A second approach is based on the notion of layering the ISO Transport Service over TCP/IP. This approach solves the problem for all applications which use the ISO Transport Service.

ITOT is a Transport Service which is identical to the Services and Interfaces offered by the ISO Transport Service Definition [ISO8072], but which will in fact implement the ISO Transport Protocol [ISO8073] on top of TCP/IP (IPv4 or IPv6), rather than the ISO Network Service [ISO8348]. The ‘well known’ TCP port 102 is reserved for hosts which implement the ITOT Protocol.

Two variants of the ITOT protocol are defined, “Class 0 over TCP” and “Class 2 over TCP”, which are based closely on the ISO Transport Class 0 and 2 Protocol. Class 0 provides the functions needed for connection establishment with negotiation, data transfer with segmentation, and protocol error reporting. It provides Transport Connection with flow control based on that of the NS-provider (TCP). It provides Transport Disconnection based on the NS-provider Disconnection. Class 0 is suitable for data transfer with no Explicit Transport Disconnection.

Class 2 provides the functions needed for connection establishment with negotiation, data transfer with segmentation and protocol error reporting. It provides Transport Connection with flow control based on that of the NS-provider TCP. It provides Explicit Transport Disconnection. Class 2 is suitable when independence of Normal and Expedited Data channels is required or when Explicit Transport Disconnection is needed.

Protocol Structure

Table 36.

8	6	32bit	Variable
Version	Reserved	Packet Length	TPDU
Message Length

• Protocol Version: Value: 3

• Reserved - Value: 0

• Packet Length - Value: Length of the entire TPKT in octets, including Packet Header

• TPDU - ISO Transport TPDU as defined in ISO 8073.

Mapping parameters between the TCP service and the ISO 8348 CONS service is done as follow:

Table 37.

ISO Network Service	TCP
CONNECTION ESTABLISHMENT
Called address	Server’s IPv4 or IPv6 address and TCP port number.
Calling address	Client’s IPv4 or IPv6 address
All other parameters	Ignored
DATA TRANSFER
NS User Data (NSDU)	DATA
CONNECTION RELEASE
All parameters	Ignored