<s>
The	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
is	O
a	O
signalling	O
protocol	O
used	O
for	O
controlling	O
communication	O
sessions	O
such	O
as	O
Voice	B-Application
over	I-Application
IP	I-Application
telephone	O
calls	O
.	O
</s>
<s>
SIP	O
is	O
based	O
on	O
request/response	O
transactions	O
,	O
in	O
a	O
similar	O
manner	O
to	O
the	O
Hypertext	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
HTTP	B-Protocol
)	O
.	O
</s>
<s>
SIP	O
requests	O
and	O
responses	O
may	O
be	O
generated	O
by	O
any	O
SIP	O
user	B-Protocol
agent	I-Protocol
;	O
user	B-Protocol
agents	I-Protocol
are	O
divided	O
into	O
clients	O
(	O
UACs	O
)	O
,	O
which	O
initiate	O
requests	O
,	O
and	O
servers	O
(	O
UASes	O
)	O
,	O
which	O
respond	O
to	O
them	O
.	O
</s>
<s>
A	O
single	O
user	B-Protocol
agent	I-Protocol
may	O
act	O
as	O
both	O
UAC	O
and	O
UAS	O
for	O
different	O
transactions	O
:	O
for	O
example	O
,	O
a	O
SIP	B-Device
phone	I-Device
is	O
a	O
user	B-Protocol
agent	I-Protocol
that	O
will	O
be	O
a	O
UAC	O
when	O
making	O
a	O
call	O
,	O
and	O
a	O
UAS	O
when	O
receiving	O
one	O
.	O
</s>
<s>
Additionally	O
,	O
some	O
devices	O
will	O
act	O
as	O
both	O
UAC	O
and	O
UAS	O
for	O
a	O
single	O
transaction	O
;	O
these	O
are	O
called	O
Back-to-Back	O
User	B-Protocol
Agents	I-Protocol
(	O
B2BUAs	O
)	O
.	O
</s>
<s>
SIP	B-Protocol
responses	I-Protocol
specify	O
a	O
three-digit	O
integer	O
response	O
code	O
,	O
which	O
is	O
one	O
of	O
a	O
number	O
of	O
defined	O
codes	O
that	O
detail	O
the	O
status	O
of	O
the	O
request	O
.	O
</s>
<s>
The	O
SIP	O
response	O
codes	O
are	O
consistent	O
with	O
the	O
HTTP	B-Protocol
response	I-Protocol
codes	I-Protocol
,	O
although	O
not	O
all	O
HTTP	B-Protocol
response	I-Protocol
codes	I-Protocol
are	O
valid	O
in	O
SIP	O
.	O
</s>
<s>
SIP	B-Protocol
responses	I-Protocol
also	O
specify	O
a	O
"	O
reason	O
phrase	O
"	O
,	O
and	O
a	O
default	O
reason	O
phrase	O
is	O
defined	O
with	O
each	O
response	O
code	O
.	O
</s>
<s>
SIP	B-Protocol
responses	I-Protocol
may	O
also	O
include	O
an	O
optional	O
Warning	O
header	O
,	O
containing	O
additional	O
details	O
about	O
the	O
response	O
.	O
</s>
<s>
Extended	O
search	O
being	O
performed	O
may	O
take	O
a	O
significant	O
time	O
so	O
a	O
forking	B-Application
proxy	I-Application
must	O
send	O
a	O
100	O
Trying	O
response	O
.	O
</s>
<s>
Destination	O
user	B-Protocol
agent	I-Protocol
received	O
INVITE	O
,	O
and	O
is	O
alerting	O
user	O
of	O
call	O
.	O
</s>
<s>
Can	O
be	O
used	O
by	O
User	B-Application
Agent	I-Application
Server	I-Application
to	O
indicate	O
to	O
upstream	O
SIP	O
entities	O
(	O
including	O
the	O
User	B-Protocol
Agent	I-Protocol
Client	O
(	O
UAC	O
)	O
)	O
that	O
an	O
early	O
dialog	O
has	O
been	O
terminated	O
.	O
</s>
<s>
The	O
Contact	O
field	O
details	O
a	O
proxy	B-Application
that	O
must	O
be	O
used	O
to	O
access	O
the	O
requested	O
destination	O
.	O
</s>
<s>
A	O
specific	O
flow	O
to	O
a	O
user	B-Protocol
agent	I-Protocol
has	O
failed	O
,	O
although	O
other	O
flows	O
may	O
succeed	O
.	O
</s>
<s>
This	O
response	O
is	O
intended	O
for	O
use	O
between	O
proxy	B-Application
devices	O
,	O
and	O
should	O
not	O
be	O
seen	O
by	O
an	O
endpoint	O
(	O
and	O
if	O
it	O
is	O
seen	O
by	O
one	O
,	O
should	O
be	O
treated	O
as	O
a	O
400	O
Bad	O
Request	O
response	O
)	O
.	O
</s>
<s>
The	O
first	O
outbound	O
proxy	B-Application
the	O
user	O
is	O
attempting	O
to	O
register	O
through	O
does	O
not	O
support	O
the	O
"	O
outbound	O
"	O
feature	O
of	O
RFC	O
5626	O
,	O
although	O
the	O
registrar	O
does	O
.	O
</s>
<s>
If	O
a	O
SIP	O
proxy	B-Application
determines	O
a	O
response	O
context	O
has	O
insufficient	O
Incoming	O
Max-Breadth	O
to	O
carry	O
out	O
a	O
desired	O
parallel	O
fork	O
,	O
and	O
the	O
proxy	B-Application
is	O
unwilling/unable	O
to	O
compensate	O
by	O
forking	O
serially	O
or	O
sending	O
a	O
redirect	O
,	O
that	O
proxy	B-Application
MUST	O
return	O
a	O
440	O
response	O
.	O
</s>
<s>
The	O
server	O
has	O
received	O
a	O
request	O
that	O
requires	O
a	O
negotiated	O
security	O
mechanism	O
,	O
and	O
the	O
response	O
contains	O
a	O
list	O
of	O
suitable	O
security	O
mechanisms	O
for	O
the	O
requester	O
to	O
choose	O
between	O
,	O
or	O
a	O
digest	B-Protocol
authentication	I-Protocol
challenge	O
.	O
</s>
<s>
The	O
server	O
is	O
acting	O
as	O
a	O
gateway	O
or	O
proxy	B-Application
,	O
and	O
received	O
an	O
invalid	O
response	O
from	O
a	O
downstream	O
server	O
while	O
attempting	O
to	O
fulfill	O
the	O
request	O
.	O
</s>
