<s>
The	O
X/Open	B-Application
Transport	I-Application
Interface	I-Application
(	O
XTI	O
)	O
is	O
an	O
Open	B-Application
Group	I-Application
specification	O
for	O
network	O
application	O
programming	O
present	O
in	O
UNIX	B-Operating_System
System	I-Operating_System
V	I-Operating_System
operating	O
systems	O
.	O
</s>
<s>
It	O
provides	O
OSI	O
transport	B-Architecture
layer	I-Architecture
services	O
with	O
protocol	O
independence	O
.	O
</s>
<s>
Although	O
Open	B-Application
Group	I-Application
considers	O
this	O
specification	O
withdrawn	O
,	O
an	O
implementation	O
is	O
part	O
of	O
the	O
standard	O
programming	B-Application
interfaces	I-Application
on	O
modern	O
UNIX	B-Operating_System
System	I-Operating_System
V	I-Operating_System
operating	O
systems	O
where	O
it	O
is	O
implemented	O
using	O
the	O
STREAMS	B-Operating_System
character	O
input/output	O
mechanism	O
.	O
</s>
<s>
The	O
X/Open	B-Application
Transport	I-Application
Interface	I-Application
is	O
a	O
specification	O
that	O
defines	O
an	O
independent	O
transport-service	O
interface	O
for	O
network	O
applications	O
.	O
</s>
<s>
Programs	O
using	O
XTI	O
can	O
be	O
run	O
over	O
a	O
variety	O
of	O
transport	O
providers	O
,	O
such	O
as	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
,	O
Xerox	B-Protocol
Network	I-Protocol
Systems	I-Protocol
(	O
XNS	O
)	O
,	O
Systems	B-Architecture
Network	I-Architecture
Architecture	I-Architecture
(	O
SNA	O
)	O
,	O
X.25	B-Protocol
,	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
or	O
any	O
other	O
transport	B-Architecture
layer	I-Architecture
provider	O
that	O
function	O
at	O
Layer	B-Architecture
4	I-Architecture
of	O
the	O
OSI	O
Model	O
.	O
</s>
<s>
XTI	O
provides	O
similar	O
functionality	O
as	O
the	O
Berkeley	B-Protocol
sockets	I-Protocol
interface	O
,	O
however	O
it	O
is	O
protocol	O
independent	O
in	O
contrast	O
to	O
the	O
socket	O
interface	O
which	O
is	O
heavily	O
biased	O
toward	O
the	O
Internet	B-Protocol
Protocols	I-Protocol
.	O
</s>
<s>
It	O
is	O
based	O
on	O
the	O
UNIX	B-Operating_System
System	I-Operating_System
V	I-Operating_System
Transport	B-Application
Layer	I-Application
Interface	I-Application
(	O
TLI	O
)	O
.	O
,	O
which	O
is	O
a	O
transport	O
service	O
definition	O
adhering	O
to	O
the	O
Open	O
Systems	O
Interconnection	O
(	O
OSI	O
)	O
model	O
.	O
</s>
<s>
XTI	O
consists	O
of	O
a	O
cooperating	O
STREAMS	B-Operating_System
module	O
,	O
link	O
library	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
,	O
header	O
files	O
,	O
and	O
the	O
rules	O
and	O
restrictions	O
elaborating	O
how	O
XTI	O
processes	O
work	O
and	O
interact	O
.	O
</s>
<s>
Applications	O
can	O
achieve	O
this	O
transport	B-Architecture
protocol	I-Architecture
independence	O
using	O
the	O
Network	O
Selection	O
Facilities	O
also	O
provided	O
by	O
the	O
XTI/TLI	O
library	O
(	O
libnsl	O
)	O
.	O
</s>
<s>
Both	O
,	O
the	O
TLI	O
and	O
BSD	B-Protocol
sockets	I-Protocol
interfaces	O
in	O
UNIX	O
SVR	O
3	O
and	O
SVR	O
4	O
operating	O
systems	O
releases	O
are	O
implemented	O
using	O
the	O
same	O
underlying	O
STREAMS	B-Operating_System
Transport	O
Service	O
Interface	O
.	O
</s>
<s>
To	O
ascertain	O
the	O
proper	O
sequence	O
of	O
library	O
function	O
calls	O
,	O
XTI/TLI	O
uses	O
state	O
indicators	O
,	O
in	O
a	O
similar	O
manner	O
to	O
the	O
sockets	O
API	B-Application
.	O
</s>
<s>
However	O
,	O
some	O
socket	O
API	B-Application
functions	I-Application
can	O
be	O
called	O
from	O
multiple	O
states	O
(	O
e.g.	O
</s>
<s>
sendmsg	O
from	O
unbound	O
state	O
on	O
connectionless	O
socket	O
,	O
connect	O
from	O
unbound	O
state	O
on	O
connection-oriented	O
socket	O
)	O
;	O
whereas	O
XTI	O
API	B-Application
functions	I-Application
can	O
only	O
be	O
issued	O
from	O
a	O
specific	O
and	O
corresponding	O
state	O
(	O
e.g.	O
</s>
<s>
The	O
XTI	O
library	O
is	O
found	O
in	O
UNIX	B-Operating_System
System	I-Operating_System
V	I-Operating_System
but	O
also	O
has	O
ports	O
for	O
other	O
systems	O
,	O
such	O
as	O
OpenSS7	O
for	O
Linux	O
.	O
</s>
