<s>
Content	B-Protocol
negotiation	I-Protocol
refers	O
to	O
mechanisms	O
defined	O
as	O
a	O
part	O
of	O
HTTP	B-Protocol
that	O
make	O
it	O
possible	O
to	O
serve	O
different	O
versions	O
of	O
a	O
document	O
(	O
or	O
more	O
generally	O
,	O
representations	O
of	O
a	O
resource	O
)	O
at	O
the	O
same	O
URI	B-Protocol
,	O
so	O
that	O
user	B-Protocol
agents	I-Protocol
can	O
specify	O
which	O
version	O
fits	O
their	O
capabilities	O
the	O
best	O
.	O
</s>
<s>
A	O
resource	O
may	O
be	O
available	O
in	O
several	O
different	O
representations	O
;	O
for	O
example	O
,	O
it	O
might	O
be	O
available	O
in	O
different	O
languages	O
or	O
different	O
media	B-Application
types	I-Application
.	O
</s>
<s>
HTTP	B-Protocol
provides	O
for	O
several	O
different	O
content	B-Protocol
negotiation	I-Protocol
mechanisms	O
including	O
:	O
server-driven	O
(	O
or	O
proactive	O
)	O
,	O
agent-driven	O
(	O
or	O
reactive	O
)	O
,	O
transparent	O
,	O
and/or	O
hybrid	O
combinations	O
thereof	O
.	O
</s>
<s>
Server-driven	O
or	O
proactive	O
content	B-Protocol
negotiation	I-Protocol
is	O
performed	O
by	O
algorithms	O
on	O
the	O
server	O
which	O
choose	O
among	O
the	O
possible	O
variant	O
representations	O
.	O
</s>
<s>
To	O
summarize	O
how	O
this	O
works	O
,	O
when	O
a	O
user	B-Protocol
agent	I-Protocol
submits	O
a	O
request	O
to	O
a	O
server	O
,	O
the	O
user	B-Protocol
agent	I-Protocol
informs	O
the	O
server	O
what	O
media	B-Application
types	I-Application
or	O
other	O
aspects	O
of	O
content	O
presentation	O
it	O
understands	O
with	O
ratings	O
of	O
how	O
well	O
it	O
understands	O
them	O
.	O
</s>
<s>
More	O
precisely	O
,	O
the	O
user	B-Protocol
agent	I-Protocol
provides	O
HTTP	B-Protocol
headers	I-Protocol
that	O
lists	O
acceptable	O
aspects	O
of	O
the	O
resource	O
and	O
quality	O
factors	O
for	O
them	O
.	O
</s>
<s>
The	O
server	O
is	O
then	O
able	O
to	O
supply	O
the	O
version	O
of	O
the	O
resource	O
that	O
best	O
fits	O
the	O
user	B-Protocol
agent	I-Protocol
's	O
needs	O
.	O
</s>
<s>
Multiple	O
HTTP	B-Protocol
headers	I-Protocol
are	O
often	O
supplied	O
together	O
for	O
content	O
format	O
or	O
,	O
specifically	O
media	B-Application
type	I-Application
,	O
language	O
and	O
a	O
few	O
other	O
aspects	O
of	O
a	O
resource	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
commonly	O
used	O
Accept	O
header	O
for	O
Media	B-Application
Type	I-Application
,	O
the	O
Accept-Language	O
header	O
for	O
language	B-Protocol
negotiation	I-Protocol
,	O
RFC	O
7231	O
also	O
describes	O
Accept-Charset	O
&	O
Accept-Encodings	O
for	O
character	O
encodings	O
and	O
content	O
codings	O
(	O
compression	O
)	O
respectively	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
more	O
complex	O
request	O
is	O
where	O
a	O
browser	O
sends	O
headers	O
about	O
language	O
indicating	O
German	O
is	O
preferred	O
but	O
that	O
English	O
is	O
acceptable	O
,	O
as	O
above	O
,	O
and	O
that	O
,	O
regarding	O
formats	O
,	O
HTML	B-Language
(	O
text/html	B-Language
)	O
is	O
preferred	O
over	O
other	O
text	O
types	O
(	O
text/	O
*	O
)	O
,	O
GIF	O
(	O
image/gif	O
)	O
or	O
JPEG	O
(	O
image/jpg	O
)	O
images	O
are	O
preferred	O
over	O
other	O
image	O
formats	O
(	O
image/	O
*	O
)	O
but	O
that	O
any	O
other	O
media	B-Application
type	I-Application
(*/*	O
)	O
is	O
accepted	O
as	O
a	O
last	O
resort	O
:	O
</s>
<s>
In	O
addition	O
to	O
aspects	O
of	O
server-driven	O
content	B-Protocol
negotiation	I-Protocol
by	O
content	B-Application
type	I-Application
and	O
by	O
language	O
specified	O
in	O
RFC	O
7231	O
,	O
there	O
are	O
extensions	O
defining	O
other	O
aspects	O
of	O
content	B-Protocol
negotiation	I-Protocol
,	O
such	O
as	O
Memento	O
which	O
describes	O
use	O
of	O
a	O
Accept-Datetime	O
header	O
to	O
retrieve	O
version	O
of	O
a	O
resource	O
's	O
representation	O
at	O
particular	O
points	O
in	O
time	O
and	O
the	O
IETF/W3C	O
'	O
s	O
Content	B-Protocol
Negotiation	I-Protocol
by	O
Profile	O
which	O
describes	O
use	O
of	O
an	O
Accept-Profile	O
header	O
to	O
retrieve	O
resource	O
representations	O
conforming	O
to	O
data	O
profiles	O
.	O
</s>
<s>
Neither	O
RFC	O
7231	O
nor	O
the	O
more	O
recent	O
related	O
specifications	O
such	O
as	O
Content	B-Protocol
Negotiation	I-Protocol
by	O
Profile	O
specify	O
how	O
to	O
resolve	O
trade-offs	O
in	O
cases	O
where	O
different	O
headers	O
specify	O
conflicting	O
requirements	O
,	O
such	O
as	O
,	O
in	O
the	O
above	O
example	O
,	O
choosing	O
between	O
an	O
HTML	B-Language
page	O
in	O
English	O
and	O
a	O
GIF	O
image	O
in	O
German	O
.	O
</s>
<s>
Agent-driven	O
or	O
reactive	O
content	B-Protocol
negotiation	I-Protocol
is	O
performed	O
by	O
algorithms	O
in	O
the	O
user-agent	B-Protocol
which	O
choose	O
among	O
the	O
possible	O
variant	O
representations	O
.	O
</s>
<s>
To	O
summarize	O
how	O
this	O
works	O
,	O
when	O
a	O
user	B-Protocol
agent	I-Protocol
submits	O
a	O
request	O
to	O
a	O
server	O
,	O
the	O
server	O
informs	O
the	O
user-agent	B-Protocol
which	O
representations	O
it	O
has	O
available	O
as	O
well	O
as	O
any	O
metadata	O
it	O
has	O
about	O
each	O
representation	O
(	O
e.g.	O
,	O
content-type	B-Application
,	O
quality	O
,	O
language	O
,	O
etc	O
.	O
)	O
.	O
</s>
<s>
The	O
user-agent	B-Protocol
then	O
resubmits	O
the	O
request	O
to	O
a	O
specific	O
URL	O
for	O
the	O
chosen	O
representation	O
.	O
</s>
<s>
This	O
can	O
be	O
automatically	O
chosen	O
by	O
the	O
user-agent	B-Protocol
or	O
the	O
user-agent	B-Protocol
can	O
present	O
the	O
user	O
with	O
the	O
choices	O
and	O
the	O
user	O
can	O
directly	O
choose	O
such	O
.	O
</s>
<s>
More	O
precisely	O
,	O
the	O
server	O
responds	O
with	O
either	O
300	O
Multiple	O
Choices	O
or	O
406	O
Not	O
Acceptable	O
(	O
when	O
server-driven	O
,	O
user-agent	B-Protocol
acceptance	O
criteria	O
are	O
provided	O
but	O
the	O
server	O
cannot	O
automatically	O
make	O
a	O
selection	O
)	O
.	O
</s>
<s>
Unfortunately	O
HTTP	B-Protocol
leaves	O
the	O
format	O
of	O
the	O
list	O
of	O
representations	O
and	O
metadata	O
along	O
with	O
selection	O
mechanisms	O
unspecified	O
.	O
</s>
