ISO RTSE: Reliable Transfer Service Element Protocol (X.228, X.218, ISO 9066)

ISO RTSE: Reliable Transfer Service Element Protocol

Reliable Transfer Service Element (RTSE), an ISO application layer protocol, provides for the reliable transfer of bulk data by transforming the data into a string of octets, then breaking the string into segments and handing each segment to the Presentation Layer for delivery. Checkpoints are established between segments. Through the services of the Presentation Layer, RTSE uses the activity management services of the Session Layer to manage the transfer of the collection of segments that makes up the bulk data. Activity and minor synchronization facilities of the Session Layer support interruption and possible resumption of data transfer if the underlying network connection is lost.

RTSE is used in the X.400 Message Handling Service (MHS) and is available for use by ROSE when remote operations require reliable transfer. Because of its use in X.400, RTSE is widely available.

Typically the Transport layer is supposed to ensure reliable delivery but this is insufficient for two reasons: 1) No class of transport protocol will recover from a failure of the underlying network, which results in the required QOS (Quality of Service) not being met. Under these circumstances, the underlying connection will be lost. 2) For historical reasons, MHS (X.400) was designed to operate over TP0 which provides no recovery at all from signalled errors (including X.25 resets). In the event of either an X.25 reset or a disconnect, TP0 would terminate the underlying connection

RTSE is required to re-establish the underlying failed connection and to repeat the transmission attempt, transparently to the user. However, RTSE cannot guarantee delivery if success cannot be achieved within a given time, RTSE will report failure. This may occur if there is a catastrophic failure either of the underlying network or of the peer application, which clearly neither RTSE nor any other ASE(Application Service Element) can do anything positive about.

RTSE is not viable on its own. RTSE has no knowledge of the context of the PDU which it is attempting to deliver, nor indeed would it have anything to deliver. There must be a 'RTSE user' which understands what is being used for, typically a MHS service element using ROSE .

RTSE uses Session Layer Activities for the following reasons: Each PDU (e.g. message) and the response confirming (or otherwise) its successful delivery are encapsulated within dialogue units (major synchronization points). RTSE may also inset minor synchronization points at suitable intervals during the activity, as it sees fit. An activity may be interrupted in the event of minor errors occurring, and can be resumed later. In the case of more severe errors, such as loss of the application association itself, the activity may need to be discarded and in this case the transaction will start again from scratch at a later time, in a new activity.

Protocol Structure - ISO RTSE: Reliable Transfer Service Element Protocol

RTSE Service Summary

Service	Type
RT-OPEN	Confirmed
RT-CLOSE	Confirmed
RT-TRANSFER	Confirmed
RT-TURN-PLEASE	Non-confirmed
RT-TURN-GIVE	Non-confirmed
RT-P-ABORT	Provider-initiated
RT-U-ABORT	Non-confirmed

Related Protocols
ISO Session Protocol, ISO Presentation Protocol , ACSE, ROSE , X.400 , X.25 , TP

Sponsor Source

The RTSE protocol is defined in ISO (www.iso.org ) documents 9066 and ITU (www.itu.org ) documents X.218 and X.228.

Reference

http://www.doc.ua.pt/arch/itu/rec/product/X.htm :
X.218 : Reliable Transfer: Model and service definition
X.228 : Reliable Transfer: Protocol specification