<s>
The	O
Stream	B-Protocol
Control	I-Protocol
Transmission	I-Protocol
Protocol	I-Protocol
(	O
SCTP	B-Protocol
)	O
is	O
a	O
computer	B-Architecture
networking	I-Architecture
communications	O
protocol	O
in	O
the	O
transport	B-Architecture
layer	I-Architecture
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
.	O
</s>
<s>
Originally	O
intended	O
for	O
Signaling	O
System	O
7	O
(	O
SS7	O
)	O
message	O
transport	O
in	O
telecommunication	O
,	O
the	O
protocol	O
provides	O
the	O
message-oriented	O
feature	O
of	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
,	O
while	O
ensuring	O
reliable	O
,	O
in-sequence	O
transport	O
of	O
messages	O
with	O
congestion	O
control	O
like	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
.	O
</s>
<s>
Unlike	O
UDP	O
and	O
TCP	O
,	O
the	O
protocol	O
supports	O
multihoming	B-Protocol
and	O
redundant	O
paths	O
to	O
increase	O
resilience	O
and	O
reliability	O
.	O
</s>
<s>
SCTP	B-Protocol
is	O
standardized	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
in	O
.	O
</s>
<s>
The	O
SCTP	B-Protocol
reference	O
implementation	O
was	O
released	O
as	O
part	O
of	O
FreeBSD	B-Operating_System
version	O
7	O
,	O
and	O
has	O
since	O
been	O
widely	O
ported	O
to	O
other	O
platforms	O
.	O
</s>
<s>
SCTP	B-Protocol
applications	O
submit	O
data	O
for	O
transmission	O
in	O
messages	O
(	O
groups	O
of	O
bytes	O
)	O
to	O
the	O
SCTP	B-Protocol
transport	B-Architecture
layer	I-Architecture
.	O
</s>
<s>
SCTP	B-Protocol
places	O
messages	O
and	O
control	O
information	O
into	O
separate	O
chunks	O
(	O
data	O
chunks	O
and	O
control	O
chunks	O
)	O
,	O
each	O
identified	O
by	O
a	O
chunk	O
header	O
.	O
</s>
<s>
SCTP	B-Protocol
bundles	O
the	O
chunks	O
into	O
SCTP	B-Protocol
packets	O
.	O
</s>
<s>
The	O
SCTP	B-Protocol
packet	O
,	O
which	O
is	O
submitted	O
to	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
,	O
consists	O
of	O
a	O
packet	O
header	O
,	O
SCTP	B-Protocol
control	O
chunks	O
(	O
when	O
necessary	O
)	O
,	O
followed	O
by	O
SCTP	B-Protocol
data	O
chunks	O
(	O
when	O
available	O
)	O
.	O
</s>
<s>
SCTP	B-Protocol
may	O
be	O
characterized	O
as	O
message-oriented	O
,	O
meaning	O
it	O
transports	O
a	O
sequence	O
of	O
messages	O
(	O
each	O
being	O
a	O
group	O
of	O
bytes	O
)	O
,	O
rather	O
than	O
transporting	O
an	O
unbroken	O
stream	O
of	O
bytes	O
as	O
in	O
TCP	O
.	O
</s>
<s>
As	O
in	O
UDP	O
,	O
in	O
SCTP	B-Protocol
a	O
sender	O
sends	O
a	O
message	O
in	O
one	O
operation	O
,	O
and	O
that	O
exact	O
message	O
is	O
passed	O
to	O
the	O
receiving	O
application	O
process	O
in	O
one	O
operation	O
.	O
</s>
<s>
The	O
term	O
multi-streaming	O
refers	O
to	O
the	O
capability	O
of	O
SCTP	B-Protocol
to	O
transmit	O
several	O
independent	O
streams	O
of	O
chunks	O
in	O
parallel	O
,	O
for	O
example	O
transmitting	O
web	O
page	O
images	O
simultaneously	O
with	O
the	O
web	O
page	O
text	O
.	O
</s>
<s>
In	O
essence	O
,	O
it	O
involves	O
bundling	O
several	O
connections	O
into	O
a	O
single	O
SCTP	B-Protocol
association	O
,	O
operating	O
on	O
messages	O
(	O
or	O
chunks	O
)	O
rather	O
than	O
bytes	O
.	O
</s>
<s>
TCP	O
preserves	O
byte	O
order	O
in	O
the	O
stream	O
by	O
including	O
a	O
byte	O
sequence	B-Protocol
number	I-Protocol
with	O
each	O
segment	O
.	O
</s>
<s>
SCTP	B-Protocol
,	O
on	O
the	O
other	O
hand	O
,	O
assigns	O
a	O
sequence	B-Protocol
number	I-Protocol
or	O
a	O
message-id	O
to	O
each	O
message	O
sent	O
in	O
a	O
stream	O
.	O
</s>
<s>
However	O
,	O
message	O
ordering	O
is	O
optional	O
in	O
SCTP	B-Protocol
;	O
a	O
receiving	O
application	O
may	O
choose	O
to	O
process	O
messages	O
in	O
the	O
order	O
of	O
receipt	O
instead	O
of	O
in	O
the	O
order	O
of	O
sending	O
.	O
</s>
<s>
Features	O
of	O
SCTP	B-Protocol
include	O
:	O
</s>
<s>
Delivery	O
of	O
chunks	O
within	O
independent	O
streams	O
eliminates	O
unnecessary	O
head-of-line	B-Protocol
blocking	I-Protocol
,	O
as	O
opposed	O
to	O
TCP	O
byte-stream	O
delivery	O
.	O
</s>
<s>
The	O
designers	O
of	O
SCTP	B-Protocol
originally	O
intended	O
it	O
for	O
the	O
transport	O
of	O
telephony	O
(	O
i.e.	O
</s>
<s>
Signaling	O
System	O
7	O
)	O
over	O
Internet	B-Protocol
Protocol	I-Protocol
,	O
with	O
the	O
goal	O
of	O
duplicating	O
some	O
of	O
the	O
reliability	O
attributes	O
of	O
the	O
SS7	O
signaling	O
network	O
in	O
IP	O
.	O
</s>
<s>
In	O
the	O
meantime	O
,	O
other	O
uses	O
have	O
been	O
proposed	O
,	O
for	O
example	O
,	O
the	O
Diameter	B-Protocol
protocol	O
and	O
Reliable	B-Protocol
Server	I-Protocol
Pooling	I-Protocol
(	O
RSerPool	B-Protocol
)	O
.	O
</s>
<s>
In	O
both	O
of	O
these	O
cases	O
,	O
the	O
head-of-line	B-Protocol
blocking	I-Protocol
property	O
of	O
TCP	O
causes	O
unnecessary	O
delay	O
.	O
</s>
<s>
In	O
order	O
to	O
avoid	O
sending	O
many	O
small	O
IP	O
packets	O
where	O
one	O
single	O
larger	O
packet	O
would	O
have	O
sufficed	O
,	O
the	O
TCP	O
implementation	O
may	O
delay	O
transmitting	O
data	O
while	O
waiting	O
for	O
possibly	O
more	O
data	O
being	O
queued	O
by	O
the	O
application	O
(	O
Nagle	B-Protocol
's	I-Protocol
algorithm	I-Protocol
)	O
.	O
</s>
<s>
SCTP	B-Protocol
on	O
the	O
other	O
hand	O
allows	O
undelayed	O
transmission	O
to	O
be	O
configured	O
as	O
a	O
default	O
for	O
an	O
association	O
,	O
eliminating	O
any	O
undesired	O
delays	O
,	O
but	O
at	O
the	O
cost	O
of	O
higher	O
transfer	O
overhead	O
.	O
</s>
<s>
The	O
limited	O
scope	O
of	O
TCP	O
sockets	O
complicates	O
the	O
task	O
of	O
providing	O
highly-available	O
data	O
transfer	O
capability	O
using	O
multihomed	B-Protocol
hosts	O
.	O
</s>
<s>
Adoption	O
has	O
been	O
slowed	O
by	O
lack	O
of	O
awareness	O
,	O
lack	O
of	O
implementations	O
(	O
particularly	O
in	O
Microsoft	B-Application
Windows	I-Application
)	O
,	O
lack	O
of	O
application	O
support	O
and	O
lack	O
of	O
network	O
support	O
.	O
</s>
<s>
SCTP	B-Protocol
provides	O
redundant	O
paths	O
to	O
increase	O
reliability	O
.	O
</s>
<s>
Each	O
SCTP	B-Protocol
end	O
point	O
needs	O
to	O
check	O
reachability	O
of	O
the	O
primary	O
and	O
redundant	O
addresses	O
of	O
the	O
remote	O
end	O
point	O
using	O
a	O
heartbeat	B-Operating_System
.	O
</s>
<s>
Each	O
SCTP	B-Protocol
end	O
point	O
needs	O
to	O
acknowledge	O
the	O
heartbeats	B-Operating_System
it	O
receives	O
from	O
the	O
remote	O
end	O
point	O
.	O
</s>
<s>
When	O
SCTP	B-Protocol
sends	O
a	O
message	O
to	O
a	O
remote	O
address	O
,	O
the	O
source	O
interface	O
will	O
only	O
be	O
decided	O
by	O
the	O
routing	O
table	O
of	O
the	O
host	O
(	O
and	O
not	O
by	O
SCTP	B-Protocol
)	O
.	O
</s>
<s>
In	O
asymmetric	O
multihoming	B-Protocol
,	O
one	O
of	O
the	O
two	O
endpoints	O
does	O
not	O
support	O
multihoming	B-Protocol
.	O
</s>
<s>
In	O
local	O
multihoming	B-Protocol
and	O
remote	O
single	O
homing	O
,	O
if	O
the	O
remote	O
primary	O
address	O
is	O
not	O
reachable	O
,	O
the	O
SCTP	B-Protocol
association	O
fails	O
even	O
if	O
an	O
alternate	O
path	O
is	O
possible	O
.	O
</s>
<s>
An	O
SCTP	B-Protocol
packet	O
consists	O
of	O
two	O
basic	O
sections	O
:	O
</s>
<s>
Although	O
encryption	O
was	O
not	O
part	O
of	O
the	O
original	O
SCTP	B-Protocol
design	O
,	O
SCTP	B-Protocol
was	O
designed	O
with	O
features	O
for	O
improved	O
security	O
,	O
such	O
as	O
4-way	O
handshake	B-Protocol
(	O
compared	O
to	O
TCP	O
3-way	O
handshake	B-Protocol
)	O
to	O
protect	O
against	O
SYN	O
flooding	O
attacks	O
,	O
and	O
large	O
"	O
cookies	O
"	O
for	O
association	O
verification	O
and	O
authenticity	O
.	O
</s>
<s>
Reliability	O
was	O
also	O
a	O
key	O
part	O
of	O
the	O
security	O
design	O
of	O
SCTP	B-Protocol
.	O
</s>
<s>
Multihoming	B-Protocol
enables	O
an	O
association	O
to	O
stay	O
open	O
even	O
when	O
some	O
routes	O
and	O
interfaces	O
are	O
down	O
.	O
</s>
<s>
This	O
is	O
of	O
particular	O
importance	O
for	O
SIGTRAN	O
as	O
it	O
carries	O
SS7	O
over	O
an	O
IP	B-Protocol
network	I-Protocol
using	O
SCTP	B-Protocol
,	O
and	O
requires	O
strong	O
resilience	O
during	O
link	O
outages	O
to	O
maintain	O
telecommunication	O
service	O
even	O
when	O
enduring	O
network	O
anomalies	O
.	O
</s>
<s>
SCTP	B-Protocol
is	O
sometimes	O
a	O
good	O
fingerprinting	B-Protocol
candidate	O
.	O
</s>
<s>
Some	O
operating	B-General_Concept
systems	I-General_Concept
ship	O
with	O
SCTP	B-Protocol
support	O
enabled	O
,	O
and	O
,	O
as	O
it	O
is	O
not	O
as	O
well	O
known	O
as	O
TCP	O
or	O
UDP	O
,	O
it	O
is	O
sometimes	O
overlooked	O
in	O
firewall	O
and	O
intrusion	O
detection	O
configurations	O
,	O
thus	O
often	O
permitting	O
probing	O
traffic	O
.	O
</s>
<s>
The	O
SCTP	B-Protocol
reference	O
implementation	O
runs	O
on	O
FreeBSD	B-Operating_System
,	O
Mac	B-Application
OS	I-Application
X	O
,	O
Microsoft	B-Application
Windows	I-Application
,	O
and	O
Linux	B-Application
.	O
</s>
<s>
The	O
following	O
operating	B-General_Concept
systems	I-General_Concept
implement	O
SCTP	B-Protocol
:	O
</s>
<s>
Microsoft	B-Application
Windows	I-Application
:	O
</s>
<s>
MacOS	B-Application
:	O
</s>
<s>
Userspace	B-Operating_System
library	O
:	O
</s>
<s>
The	O
following	O
applications	O
implement	O
SCTP	B-Protocol
:	O
</s>
<s>
In	O
the	O
absence	O
of	O
native	O
SCTP	B-Protocol
support	O
in	O
operating	B-General_Concept
systems	I-General_Concept
,	O
it	O
is	O
possible	O
to	O
tunnel	B-Protocol
SCTP	B-Protocol
over	O
UDP	O
,	O
as	O
well	O
as	O
to	O
map	O
TCP	O
API	O
calls	O
to	O
SCTP	B-Protocol
calls	O
so	O
existing	O
applications	O
can	O
use	O
SCTP	B-Protocol
without	O
modification	O
.	O
</s>
