X Transport Interface

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Copyright 1993, 1994, 2002 The Open Group

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Designed by Stuart Anderson (NCR) with help from Ralph Mor (X Consortium)

Draft Version 0.6

NOTE: This documentation does not completely match the implementation in R6 (as a result of some late changes made in the code). Specifically, support was added for font server cloning, and conditional compliation was introduced for client vs. server code.

1. Purposes and Goals
The X Transport Interface is intended to combine all system and transport specific code into a single place in the source tree. This API should be used by all libraries, clients and servers of the X Window System. Use of this API should allow the addition of new types of transports and support for new platforms without making any changes to the source except in the X Transport Interface code.
This interface should solve the problem of multiple #ifdef TRANSPORT and #ifdef PLATFORM statements scattered throughout the source tree.
This interface should provide enough functionality to support all types of protocols, including connection oriented protocols such as X11 and FS, and connection-less oriented protocols such as XDMCP.

2. Overview of the interface
The interface provides an API for use by applications. The functions in this API perform work that is common to all transports and systems, such as parsing an address into a host and port number. The functions in this API call transport specific functions that are contained in a table whose contents are defined at compile time. This table contains an entry for each type of transport. Each entry is a record containing mostly pointers to function that implements the interface for the given transport.
This API does not provide an abstraction for select() or poll(). These function are themselves transport independent, so an additional interface is not needed for these functions. It is also unclear how such an interface would affect performance.

3. Definition of Address Specification Format
Addresses are specified in the following syntax,

protocol/host:port

where protocol specifies a protocol family or an alias for a protocol family. A definition of common protocol families is given in a later section.
The host part specifies the name of a host or other transport dependent entity that could be interpreted as a Network Service Access Point (NSAP).
The port part specifies the name of a Transport Service Access Point (TSAP). The format of the TSAP is defined by the underlying transport implementation, but it is represented using a string format when it is part of an address.

4. Internal Data Structures
There are two major data structures associated with the transport independent portion of this interface. Additional data structures may be used internally by each transport.
4.1 Xtransport
Each transport supported has an entry in the transport table. The transport table is an array of Xtransport records. Each record contains all the entry points for a single transport. This record is defined as:

typedef struct _Xtransport {

char *TransName;
int flags;

XtransConnInfo (*OpenCOTSClient)(

);

XtransConnInfo (*OpenCOTSServer)(

);

XtransConnInfo (*OpenCLTSClient)(

);

XtransConnInfo (*OpenCLTSServer)(

);

int

	(*SetOption)(
	XtransConnInfo,
	/* connection */
	int,
	/* option */
	int
	/* arg */

);

int

	(*CreateListener)(
	XtransConnInfo,
	/* connection */
	char *,
	/* port */
	int
	/* flags */

);

int

	(*ResetListener)(
	XtransConnInfo
	/* connection */ ); XtransConnInfo (*Accept)(
	XtransConnInfo
	/* connection */ ); int
	(*Connect)(
	XtransConnInfo,
	/* connection */
	char *,
	/* host */
	char *
	/* port */ ); int
	(*BytesReadable)(
	XtransConnInfo,
	/* connection */
	BytesReadable_t *
	/* pend */ ); int
	(*Read)(
	XtransConnInfo,
	/* connection */
	char *,
	/* buf */
	int
	/* size */ ); int
	(*Write)(
	XtransConnInfo,
	/* connection */
	char *,
	/* buf */
	int
	/* size */ ); int
	(*Readv)(
	XtransConnInfo,
	/* connection */
	struct iovec *,
	/* buf */
	int
	/* size */ ); int
	(*Writev)(
	XtransConnInfo,
	/* connection */
	struct iovec *,
	/* buf */
	int
	/* size */ ); int
	(*Disconnect)(
	XtransConnInfo
	/* connection */

);

int

	(*Close)(
	XtransConnInfo
	/* connection */ );

} Xtransport;

The flags field can contain an OR of the following masks:

TRANS_ALIAS: indicates that this record is providing an alias, and should not be used to create a listner.

TRANS_LOCAL: indicates that this is a LOCALCONN transport.

4.2 XtransConnInfo
Each connection will have an opaque XtransConnInfo transport connection object allocated for it. This record contains information specific to the connection. The record is defined as:

          typedef struct _XtransConnInfo *XtransConnInfo;


          struct _XtransConnInfo {
              struct _Xtransport     *transptr;

char *priv;
int

};

5. Exposed Transport Independent API
This API is included in each library and server that uses it. The API may be used by the library, but it is not added to the public API for that library. This interface is simply an implementation facilitator. This API contains a low level set of core primitives, and a few utility functions that are built on top of the primitives. The utility functions exist to provide a more familiar interface that can be used to port existing code.

A macro is defined in Xtrans.h for TRANS(func) that creates a unique function name depending on where the code is compiled. For example, when built for Xlib, TRANS(OpenCOTSClient) becomes _X11TransOpenCOTSClient.
All failures are considered fatal, and the connection should be closed and re-established if desired. In most cases, however, the value of errno will be available for debugging purposes.
5.1 Core Interface API

• XtransConnInfo TRANS(OpenCOTSClient)(char *address)

This function creates a Connection-Oriented Transport that is suitable for use by a client. The parameter address contains the full address of the server to which this endpoint will be connected. This functions returns an opaque transport connection object on success, or NULL on failure.

• XtransConnInfo TRANS(OpenCOTSServer)(char *address)

• XtransConnInfo TRANS(OpenCLTSClient)(char *address)

• XtransConnInfo TRANS(OpenCLTSServer)(char *address)

• int TRANS(SetOption)(XtransConnInfo connection, int option, int arg)

• int TRANS(CreateListener)(XtransConnInfo connection, char *port, int flags)

that was obtained from TRANS(OpenCOTSServer)() or TRANS(OpenCLTSServer)(). The parameter port specifies the port to which this endpoint should be bound for listening. If port is NULL, then the transport may attempt to allocate any available TSAP for this connection. If the transport cannot support this, then this function will return a failure. The flags parameter can be set to ADDR_IN_USE_ALLOWED to allow the call to the underlying binding function to fail with a EADDRINUSE error without causing the TRANS(CreateListener) function itself to fail. This function return 0 on success and -1 on failure.
• int TRANS(ResetListener)(XtransConnInfo connection)
When a server is restarted, certain listen ports may need to be reset. For example, unix domain needs to check that the file used for communication has not been deleted. If it has, it must be recreated. The parameter connection is an opened and bound endpoint that was obtained from TRANS(OpenCOTSServer)() and passed to TRANS(CreateListener)(). This function will return one of the following values: TRANS_RESET_NOOP, TRANS_RESET_NEW_FD, or TRANS_RESET_FAILURE.
• XtransConnInfo TRANS(Accept)(XtransConnInfo connection)
Once a connection indication is received, this function can be called to accept the connection. The parameter connection is an opened and bound endpoint that was obtained from TRANS(OpenCOTSServer)() and passed to TRANS(CreateListener)(). This function will return a new opaque transport connection object upon success, NULL otherwise.
• int TRANS(Connect)(XtransConnInfo connection, char *address)
This function creates a connection to a server. The parameter connection is an endpoint that was obtained from TRANS(OpenCOTSClient)(). The parameters address specify the TSAP to which this endpoint should connect. If the protocol is included in the address, it will be ignored. This function return 0 on success and -1 on failure.
• int TRANS(BytesReadable)(XtransConnInfo connection, BytesReadable_t *pend);
This function provides the same functionality as the BytesReadable macro.
• int TRANS(Read)(XtransConnInfo connection, char *buf, int size)
This function will return the number of bytes requested on a COTS connection, and will return the minimum of the number bytes requested or the size of the incoming packet on a CLTS connection.
• int TRANS(Write)(XtransConnInfo connection, char *buf, int size)
This function will write the requested number of bytes on a COTS connection, and will send a packet of the requested size on a CLTS connection.

• int TRANS(Readv)(XtransConnInfo connection, struct iovec *buf, int size)
Similar to TRANS(Read)().
• int TRANS(Writev)(XtransConnInfo connection, struct iovec *buf, int size)
Similar to TRANS(Write)().
• int TRANS(Disconnect)(XtransConnInfo connection)
This function is used when an orderly disconnect is desired. This function breaks the connection on the transport. It is similar to the socket function shutdown().
• int TRANS(Close)(XtransConnInfo connection)
This function closes the transport, unbinds it, and frees all resources that was associated with the transport. If a TRANS(Disconnect) call was not made on the connection, a disorderly disconnect may occur.
• int TRANS(IsLocal)(XtransConnInfo connection)
Returns TRUE if it is a local transport.
• int TRANS(GetMyAddr)(XtransConnInfo connection,

This function is similar to getsockname(). This function will allocate space for the address, so it must be freed by the caller. Not all transports will have a valid address until a connection is established. This function should not be used until the connection is established with Connect() or Accept().
• int TRANS(GetPeerAddr)(XtransConnInfo connection,

This function is similar to getpeername(). This function will allocate space for the address, so it must be freed by the caller. Not all transports will have a valid address until a connection is established. This function should not be used until the connection is established with Connect() or Accept().
• int TRANS(GetConnectionNumber)(XtransConnInfo connection)
Returns the file descriptor associated with this transport.
• int TRANS(MakeAllCOTSServerListeners)(

This function should be used by most servers. It will try to establish a COTS server endpoint for each transport listed in the transport table. partial_ret will be set to True if only a partial network could be created. count_ret is the number of transports returns, and connections_ret is the list of transports.
• int TRANS(MakeAllCLTSServerListeners)(

This function should be used by most servers. It will try to establish a CLTS server endpoint for each transport listed in the transport table. partial_ret will be set to True if only a partial network could be

created. count_ret is the number of transports returns, and connections_ret is the list of transports.
5.2 Utility API
This section describes a few useful functions that have been implemented on top of the Core Interface API. These functions are being provided as a convenience.

• int TRANS(ConvertAddress)(int *familyp, int *addrlenp, Xtransaddr *addrp)

This function converts a sockaddr based address to an X authorization based address (ie AF_INET, AF_UNIX to the X protocol definition (ie FamilyInternet, FamilyLocal)).

6. Transport Option Definition
The following options are defined for the TRANS(SetOption)() function. If an OS or transport does not support any of these options, then it will silently ignore the option.

• TRANS_NONBLOCKING

This option controls the blocking mode of the connection. If the argument is set to 1, then the connection will be set to blocking. If the argument is set to 0, then the connection will be set to non-blocking.

• TRANS_CLOSEONEXEC

7. Hidden Transport Dependent API
The hidden transport dependent functions are placed in the Xtransport record. These function are similar to the Exposed Transport Independent API, but some of the parameters and return values are slightly different. Stuff like the #ifdef SUNSYSV should be handled inside these functions.

• XtransConnInfo *OpenCOTSClient (

struct _Xtransport *thistrans, char *protocol, char *host, char *port)

This function creates a Connection-Oriented Transport. The parameter thistrans points to an Xtransport entry in the transport table. The parameters protocol, host, and port point to strings containing the corresponding parts of the address that was passed into TRANS(OpenCOTSClient)(). This function must allocate and initialize the contents of the XtransConnInfo structure that is returned by this function. This function will open the transport, and bind it into the transport namespace if applicable.

The local address portion of the XtransConnInfo structure will also be filled in by this function.
• XtransConnInfo *OpenCOTSServer (

This function creates a Connection-Oriented Transport. The parameter thistrans points to an Xtransport entry in the transport table. The parameters protocol, host, and port point to strings containing the corresponding parts of the address that was passed into TRANS(OpenCOTSClient)().
This function must allocate and initialize the contents of the XtransConnInfo structure that is returned by this function. This function will open the transport.
• XtransConnInfo *OpenCLTSClient (

This function creates a Connection-Less Transport. The parameter thistrans points to an Xtransport entry in the transport table. The parameters protocol, host, and port point to strings containing the corresponding parts of the address that was passed into TRANS(OpenCOTSClient)().
This function must allocate and initialize the contents of the XtransConnInfo structure that is returned by this function. This function will open the transport, and bind it into the transport namespace if applicable. The local address portion of the XtransConnInfo structure will also be filled in by this function.
• XtransConnInfo *OpenCLTSServer (

This function creates a Connection-Less Transport. The parameter thistrans points to an Xtransport entry in the transport table. The parameters protocol, host, and port point to strings containing the corresponding parts of the address that was passed into TRANS(OpenCOTSClient)().
This function must allocate and initialize the contents of the XtransConnInfo structure that is returned by this function. This function will open the transport.
• int SetOption (struct _Xtransport *thistrans, int option, int arg)
This function provides a transport dependent way of implementing the options defined by the X Transport Interface. In the current prototype, this function is not being used, because all of the option defined so far, are transport independent. This function will have to be used if a radically different transport type is added, or a transport dependent option is defined.
• int CreateListener (struct _Xtransport *thistrans, char *port, int flags )
This function takes a transport endpoint opened for a server, and sets it up to listen for incoming

connection requests. The parameter port should contain the port portion of the address that was passed to the Open function.
The parameter flags should be set to ADDR_IN_USE_ALLOWED if the underlying transport endpoint may be already bound and this should not be considered as an error. Otherwise flags sould be set to 0. This is used by IPv6 code, where the same socket can be bound to both an IPv6 address and then to a IPv4 address.
This function will bind the transport into the transport name space if applicable, and fill in the local address portion of the XtransConnInfo structure. The transport endpoint will then be set to listen for incoming connection requests.
• int ResetListener (struct _Xtransport *thistrans)
This function resets the transport for listening.
• XtransConnInfo Accept(struct _Xtransport *thistrans)
This function creates a new transport endpoint as a result of an incoming connection request. The parameter thistrans is the endpoint that was opened for listening by the server. The new endpoint is opened and bound into the transport’s namespace. A XtransConnInfo structure describing the new endpoint is returned from this function
• int Connect(struct _Xtransport *thistrans, char *host, char *port )
This function establishes a connection to a server. The parameters host and port describe the server to which the connection should be established. The connection will be established so that Read() and Write() call can be made.
• int BytesReadable(struct _Xtransport *thistrans, BytesReadable_t *pend )
This function replaces the BytesReadable() macro. This allows each transport to have it’s own mechanism for determining how much data is ready to be read.
• int Read(struct _Xtransport *thistrans, char *buf, int size )
This function reads size bytes into buf from the connection.
• int Write(struct _Xtransport *thistrans, char *buf, int size )
This function writes size bytes from buf to the connection.
• int Readv(struct _Xtransport *thistrans, struct iovec *buf, int size )
This function performs a readv() on the connection.
• int Writev(struct _Xtransport *thistrans, struct iovec *buf, int size )
This function performs a writev() on the connection.
• int Disconnect(struct _Xtransport *thistrans)
This function initiates an orderly shutdown of a connection. If a transport does not distinguish between

orderly and disorderly disconnects, then a call to this function will have no affect.
• int Close(struct _Xtransport *thistrans)
This function will break the connection, and close the endpoint.

8. Configuration
The implementation of each transport can be platform specific. It is expected that existing connection types such as TCPCONN, UNIXCONN, LOCALCONN and STREAMSCONN will be replaced with flags for each possible transport type.
Below are the flags that can be set in ConnectionFlags in the vendor.cf or site.def config files.

          TCPCONN       Enables the INET (IPv4) Domain Socket based transport





          IPv6          Extends TCPCONN to enable IPv6 Socket based transport





          UNIXCONN      Enables the UNIX Domain Sokcet based transport





          STREAMSCONN   Enables the TLI based transports





          LOCALCONN     Enables the SYSV Local connection transports





          DNETCONN      Enables the DECnet transports

9. Transport Specific Definitions

10. Implementation Notes
This section refers to the prototype implementation that is being developed concurrently with this document. This prototype has been able to flush out many details and problems as the specification was being developed.
All of the source code for this interface is located in xc/lib/xtrans.
All functions names in the source are of the format TRANS(func)(). The TRANS() macro is defined as

          #if (__STDC__ && !defined(UNIXCPP)) || defined(ANSICPP)
          #define TRANS(func) _PROTOCOLTrans##func
          #else
          #define TRANS(func) _PROTOCOLTrans/**/func
          #endif


          PROTOCOL will be uniquely defined in  each  directory  wherethis  code  is  compiled. PROTOCOL will be defined to be thename of the protocol that is implemented by the  library  orserver, such as X11, FS, and ICE.          All libraries and servers that use the X Transport Interfaceshould have a new file called transport.c.  This  file  willinclude  the  transports  based  on  the configuration flagsConnectionFlags. Below is an example transport.c.


          #include "Xtransint.h"


          #ifdef DNETCONN
          #include "Xtransdnet.c"
          #endif
          #ifdef LOCALCONN
          #include "Xtranslocal.c"
          #endif
          #ifdef TCPCONN
          #include "Xtranssock.c"
          #endif
          #ifdef STREAMSCONN
          #include "Xtranstli.c"
          #endif
          #include "Xtrans.c"
          #include "Xtransutil.c"

The source files for this interface are listed below.

Xtrans.h
Function prototypes and defines for
the Transport Independent API.

Xtransint.h
Used by the interface implementation only.
Contains the internal data structures.

Xtranssock.c
Socket implementation of the Transport Dependent API.

Xtranstli.c
TLI implementation of the Transport Dependent API.

Xtransdnet.c
DECnet implementation of the Transport Dependent API.

Xtranslocal.c
Implementation of the Transport Dependent API for
SYSV Local connections.

Xtrans.c
Exposed Transport Independent API Functions.

Xtransutil.c
Collection of Utility functions that use the
X Transport Interface.

The file Xtransint.h contains much of the transport related code that previously in Xlibint.h and Xlibnet.h. This will make the definitions available for all transport users. This should also obsolete the equivalent code in other libraries.