XTI Overview

Overview of Connection-oriented Mode

• Initialisation/De-initialisation

• Connection Establishment

• Data Transfer

• Connection Release.

A state machine is described in Transport Service Interface Sequence of Functions, and the figure in Example in Connection-oriented Mode, which defines the legal sequence in which functions from each phase may be issued.

In order to establish a transport connection, a user (application) must:

1. supply a transport provider identifier for the appropriate type of transport provider (using t_open()); this establishes a transport endpoint through which the user may communicate with the provider

2. associate (bind) an address with this endpoint (using t_bind())

3. use the appropriate connection functions (using t_connect(), or t_listen() and t_accept()) to establish a transport connection; the set of functions depends on whether the user is an initiator or responder

4. once the connection is established, normal, and if authorised, expedited data can be exchanged; of course, expedited data may be exchanged only if:

   • the provider supports it

   • its use is not precluded by the selection of protocol characteristics; for example, the use of ISO Transport Class 0

   • both the peer tranport providers support the feature, have negotiated it if necessary, and the peer applications have agreed to its use.

   The semantics of expedited data may be quite different for different transport providers. XTI's notion of expedited data has been defined as the lowest reasonable common denominator.

The transport connection can be released at any time by using the disconnection functions. Then the user can either de-initialise the transport endpoint by closing the file descriptor returned by t_open() (thereby freeing the resource for future use), or specify a new local address (after the old one has been unbound) or reuse the same address and establish a new transport connection.

When reusing an endpoint, the user should be aware that the local address may be different from that originally bound (for example, one of the systems' local addresses may be explicitly associated with the endpoint, or the address may have been changed during t_accept()). The value of certain options will also reflect the previous connect and may need to be reset (see the definition for t_optmgmt()) to the providers' default values.

Initialisation/De-initialisation Phase

t_open()

This function creates a transport endpoint and returns protocol-specific information associated with that endpoint. It also returns a file descriptor that serves as the local identifier of the endpoint.

t_bind()

This function associates a protocol address with a given transport endpoint, thereby activating the endpoint. It also directs the transport provider to begin accepting connection indications if so desired.

t_unbind()

This function disables a transport endpoint such that no further request destined for the given endpoint will be accepted by the transport provider.

t_close()

This function informs the transport provider that the user is finished with the transport endpoint, and frees any local resources associated with that endpoint.

t_getprotaddr()

This function returns the addresses (local and remote) associated with the specified transport endpoint.

t_getinfo()

This function returns protocol-specific information associated with the specified transport endpoint.

t_getstate()

This function returns the current state of the transport endpoint.

t_sync()

This function synchronises the data structures managed by the transport library with the transport provider.

t_alloc()

This function allocates storage for the specified library data structure.

t_free()

This function frees storage for a library data structure that was allocated by t_alloc().

t_error()

This function prints out a message describing the last error encountered during a call to a transport library function.

t_look()

This function returns the current event associated with the given transport endpoint.

t_optmgmt()

This function enables the user to get or negotiate protocol options with the transport provider.

t_strerror()

This function maps an XTI error into a language-dependent error message string.

t_sysconf()

This function is used to obtain the values of configurable and implementation-dependent XTI variables.

Overview of Connection Establishment

In connection mode:

• The user has first to establish an endpoint; that is, to open a communications path between the application and the transport provider.

• Once established, an endpoint must be bound to an address and more than one endpoint may be bound to the same address. A transport user can determine the addresses associated with a connection using the t_getprotaddr() function.

• An endpoint can be associated with one, and only one, established transport connection.

• It is possible to use an endpoint to receive and enqueue incoming connection indications (only if the provider is able to accept more than one outstanding connection indication; this mode of operation is declared at the time of calling t_bind() by setting qlen greater than 0). However, if more than one endpoint is bound to the same address, only one of them may be used in this way.

• The t_listen() function is used to look for an enqueued connection indication; if it finds one (at the head of the queue), it returns details of the connection indication, and a local sequence number which uniquely identifies this indication, or it may return a negative value with t_errno set to [TNODATA]. The number of outstanding connection requests to dequeue is limited by the value of the qlen parameter accepted by the transport provider on the t_bind() call.

• If the endpoint has more than one connection indication enqueued, the user should dequeue all connection indications (and disconnection indications) before accepting or rejecting any or all of them. The number of outstanding connection indications is limited by the value of the qlen parameter accepted by the transport provider on the call to t_bind().

• When accepting a connection indication, the transport service user may issue the accept on the same (listening) endpoint or on a different endpoint.

  If the same endpoint is used, the listening endpoint cannot receive or enqueue incoming connect indications for the duration of the connection.

  If a different endpoint is used, the listening endpoint can continue to receive and enqueue incoming connect indications.

• If the user issues a t_connect() on a listening endpoint, again, that endpoint cannot receive or enqueue incoming connect indications for the duration of the connection.

• A connection attempt failure will result in a value -1 returned from either the t_connect() or t_rcvconnect() call, with t_errno set to [TLOOK] indicating that a [T_DISCONNECT] event has arrived. In this case, the reason for the failure may be identified by issuing a t_rcvdis() call.

The functions that support these operations of connection establishment are:

t_connect()

This function requests a connection to the transport user at a specified destination and waits for the remote user's response. This function may be executed in either synchronous or asynchronous mode. In synchronous mode, the function waits for the remote user's response before returning control to the local user. In asynchronous mode, the function initiates connection establishment but returns control to the local user before a response arrives.

t_rcvconnect()

This function enables an active transport user to determine the status of a previously sent connection request. If the request was accepted, the connection establishment phase will be complete on return from this function. This function is used in conjunction with t_connect() to establish a connection in an asynchronous manner, or to continue an interrupted synchronous-mode t_connect() call.

t_listen()

This function enables the passive transport user to receive connection indications from other transport users.

t_accept()

This function is issued by the passive user to accept a particular connection request after an indication has been received.

Overview of Data Transfer

t_snd()

This function enables transport users to send either normal or expedited data over a transport connection.

t_rcv()

This function enables transport users to receive either normal or expedited data on a transport connection.

t_sndv()

This function enables transport users to send either normal or expedited data from non-contiguous buffers over a transport connection.

t_rcvv()

This function enables transport users to receive either normal or expedited data into non-contiguous buffers on a transport connection.

Throughout the rest of this section, all references of calls to t_rcv() include calls to t_rcvv(), and calls to t_snd() include calls to t_sndv().

In data transfer phase, the occurrence of the [T_DISCONNECT] event implies an unsuccessful return from the called function (.Fn t_snd or t_rcv()) with t_errno set to [TLOOK]. The user must then issue a t_look() call to get more details.

Receiving Data

• Asynchronous Mode

  The call returns immediately, indicating failure. The user must continue to "poll" for incoming data, either by issuing repeated call to t_rcv(), or by using the t_look() or the EM interface.

• Synchronous Mode

  The call is blocked until one of the following conditions becomes true:

  • Data (normal or expedited) is received.

  • A disconnection indication is received.

  • A signal has arrived.

  The user may issue a t_look() or use EM calls, to determine if data is available.

If a normal TSDU is to be received in multiple t_rcv() calls, then its delivery may be interrupted at any time by the arrival of expedited data. The application can detect this by checking the flags field on return from a call to t_rcv(); this will be indicated by t_rcv() returning:

• data with T_EXPEDITED flag not set and T_MORE set (this is a fragment of normal data)

• data with T_EXPEDITED set (and T_MORE set or unset); this is an expedited message (whole or part of, depending on the setting of T_MORE). The provider will continue to return the expedited data (on this and subsequent calls to t_rcv()) until the end of the Extended Transport Service Data Unit (ETSDU) is reached, at which time it will continue to return normal data. It is the user's responsibility to remember that the receipt of normal data has been interrupted in this way.

Sending Data

• Asynchronous Mode

  The call returns immediately indicating failure. If the failure was due to flow control restrictions, then it is possible that only part of the data will actually be accepted by the transport provider. In this case t_snd() will return a value that is less than the number of octets requested to be sent. The user may either retry the call to t_snd() or first receive notification of the clearance of the flow control restriction via either t_look() or the EM interface, then retry the call. The user may retry the call with the data remaining from the original call or with more (or less) data, and with the T_MORE flag set appropriately to indicate whether this is now the end of the TSDU.

• Synchronous Mode

  The call is blocked until one of the following conditions becomes true:

  • The flow control restrictions are cleared and the transport provider is able to accept a new data unit. The t_snd() function then returns successfully.

  • A disconnection indication is received. In this case the t_snd() function returns unsuccessfully with t_errno set to [TLOOK]. The user can issue a t_look() function to determine the cause of the error. For this particular case t_look() will return a T_DISCONNECT event. All or part of the data that was being sent might be lost.

  • An internal problem occurs. In this case the t_snd() function returns unsuccessfully with t_errno set to [TSYSERR]. Data that was being sent will be lost.

For some transport providers, normal data and expedited data constitute two distinct flows of data. If either flow is blocked, the user may nevertheless continue using the other one, but in synchronous mode a second process is needed. The user may send expedited data between the fragments of a normal TSDU, that is, a t_snd() call with the T_EXPEDITED flag set may follow a t_snd() with the T_MORE flag set and the T_EXPEDITED flag not set.

Note that XTI supports two modes of sending data, record-oriented and stream-oriented. In the record-oriented mode, the concept of TSDU is supported, that is, message boundaries are preserved. In stream-oriented mode, message boundaries are not preserved and the concept of a TSDU is not supported. A transport user can determine the mode by using the t_getinfo() function, and examining the tsdu field. If tsdu is greater than zero, this indicates that record-oriented mode is supported and the return value indicates the maximum TSDU size. If tsdu is zero, this indicates that stream-oriented transfer is supported. For more details see t_getinfo.

Overview of Connection Release

An abortive release may be invoked from either the connection establishment phase or the data transfer phase. When in the connection establishment phase, a transport user may use the abortive release to reject a connection request. In the data transfer phase, either user may abort a connection at any time. The abortive release is not negotiated by the transport users and it takes effect immediately on request. The user on the other side of the connection is notified when a connection is aborted. The transport provider may also initiate an abortive release, in which case both users are informed that the connection no longer exists. There is no guarantee of delivery of user data once an abortive release has been initiated.

Whatever the state of a transport connection, its user(s) will be informed as soon as possible of the failure of the connection through a disconnection event or an unsuccessful return from a blocking t_snd() or t_rcv() call. If the user wants to prevent loss of data by notifying the remote user of an imminent connection release, it is the user's responsibility to use an upper level mechanism. For example, the user may send specific (expedited) data and wait for the response of the remote user before issuing a disconnection request.

Some transport providers support an orderly release capability (for example, mOSI, NetBIOS, SNA, TCP). If supported by the communications provider, orderly release may be invoked from the data transfer phase to enable two users to gracefully release a connection. The procedure for orderly release prevents the loss of data that may occur during an abortive release.

When supported by the underlying protocol, some communications providers optionally allow applications to send or retrieve user data with an orderly release, through the use of the t_sndreldata() or t_rcvreldata() functions instead of the t_sndrel() or t_rcvrel() functions.

The functions that support connection release are:

t_snddis()

This function can be issued by either transport user to initiate the abortive release of a transport connection. It may also be used to reject a connection request during the connection establishment phase.

t_rcvdis()

This function identifies the reason for the abortive release of a connection, where the connection is released by the transport provider or another transport user.

t_sndrel()

This function can be called by either transport user to initiate an orderly release. The connection remains intact until both users call this function and t_rcvrel().

t_rcvrel()

This function is called when a user is notified of an orderly release request, as a means of informing the transport provider that the user is aware of the remote user's actions.

t_sndreldata()

This function can be used instead of t_sndrel() to send user data with an orderly release.

t_rcvreldata()

This function can be used instead of t_rcvrel() to retrieve orderly release user data.

Overview of Connectionless Mode

In order to permit the transfer of connectionless-mode data, a user (application) must:

1. supply a transport endpoint for the appropriate type of provider (using t_open()); this establishes a transport endpoint through which the user may communicate with the provider

2. associate (bind) an address with this transport endpoint (using t_bind())

The user may then send and/or receive connectionless-mode data, as required, using the functions t_sndudata() and t_rcvudata(). Once the data transfer phase is finished, the application may either directly close the file descriptor returned by t_open() (using t_close()), thereby freeing the resource for future use, or start a new exchange of data after disassociating the old address and binding a new one.

Initialisation/De-initialisation Phase

The functions that support the initialisation/de-initialisation tasks are the same functions used in the connection-mode service.

Overview of Data Transfer

t_sndudata()

This function enables transport users to send a self-contained data unit to the user at the specified protocol address.

t_sndvudata()

This function enables transport users to send a self-contained data unit to the user from one or more non-contiguous buffers at the specified protocol address.

t_rcvudata()

This function enables transport users to receive data units from

t_rcvvudata()

This function enables transport users to receive data units from other users into one or more non-contiguous buffers.

t_rcvuderr()

This function enables transport users to retrieve error information associated with a previously sent data unit.

The only possible events reported to the user are [T_UDERR], [T_DATA] and [T_GODATA]. Expedited data cannot be used with a connectionless-mode transport provider.

Throughout the rest of this section, all references of calls to rcvudata() include calls to rcvvudata(), and calls to t_sndudata() include calls to t_sndvudata().

Receiving Data

• Asynchronous Mode

  The call returns immediately indicating failure. The user must either retry the call repeatedly, or "poll" for incoming data by using the EM interface or the t_look() function so as not to be blocked.

• Synchronous Mode

  The call is blocked until one of the following conditions becomes true:

  • A datagram is received.

  • An error is detected by the transport provider.

  • A signal has arrived.

  The application may use the t_look() function or the EM mechanism to know if data is available instead of issuing a t_rcvudata() call which may be blocking.

Sending Data

• Synchronous Mode

  In order to maintain some flow control, the t_sndudata() function will block until the datagram has been accepted by the provider. The call returns immediately after the datagram has been sent. A process which sends data in synchronous mode may be blocked for some time.

• Asynchronous Mode

  The transport provider may refuse to send a new datagram for flow control restrictions. In this case, the t_sndudata() call fails returning a negative value and setting t_errno to [TFLOW]. The user may retry later or use the t_look() function or EM interface to be informed of the flow control restriction removal.

If t_sndudata() is called before the destination user has activated its transport endpoint, the data unit may be discarded.

XTI Features

The following functions, which correspond to the subset common to connection-mode and connectionless-mode services, are always implemented:

t_bind()
t_close()
t_look()
t_open()
t_sync()
t_unbind()

If a connection-mode Transport Service is provided, then the following functions are always implemented:

t_accept()
t_connect()
t_listen()
t_rcv()
t_rcvv()
t_rcvconnect()
t_rcvdis()
t_snd()
t_sndv()
t_snddis()

If XTI supports the access to the connectionless-mode Transport Service, the following three functions are always implemented:

t_rcvudata()
t_rcvvudata()
t_rcvuderr()
t_sndudata()
t_sndvudata()

Mandatory mechanisms:

• synchronous mode

• asynchronous mode.

Utility functions:

t_alloc()
t_free()
t_error()
t_getprotaddr()
t_getinfo()
t_getstate()
t_optmgmt()
t_strerror()
t_sysconf()

The orderly release mechanism (using t_sndrel(), t_sndreldata(), t_rcvrel() and t_rcvreldata()) is supported only for T_COTS_ORD type providers. Use with other providers will cause the [TNOTSUPPORT] error to be returned.

Optional mechanisms:

• the ability to manage (enqueue) more than one incoming connection indication at any one time

• the address of the caller passed with t_accept() may optionally be checked by an XTI implementation

XTI Functions versus Protocols