<s>
In	O
information	B-General_Concept
technology	I-General_Concept
,	O
header	B-General_Concept
refers	O
to	O
supplemental	O
data	O
placed	O
at	O
the	O
beginning	O
of	O
a	O
block	O
of	O
data	O
being	O
stored	O
or	O
transmitted	O
.	O
</s>
<s>
In	O
data	O
transmission	O
,	O
the	O
data	O
following	O
the	O
header	B-General_Concept
is	O
sometimes	O
called	O
the	O
payload	O
or	O
body	O
.	O
</s>
<s>
It	O
is	O
vital	O
that	O
header	B-General_Concept
composition	O
follows	O
a	O
clear	O
and	O
unambiguous	O
specification	B-Application
or	O
format	O
,	O
to	O
allow	O
for	O
parsing	B-Language
.	O
</s>
<s>
E-mail	O
header	B-General_Concept
:	O
The	O
text	O
(	O
body	O
)	O
is	O
preceded	O
by	O
header	B-General_Concept
lines	O
indicating	O
sender	O
,	O
recipient	O
,	O
subject	O
,	O
sending	O
time	O
stamp	O
,	O
receiving	O
time	O
stamps	O
of	O
all	O
intermediate	O
and	O
the	O
final	O
mail	B-Protocol
transfer	I-Protocol
agents	I-Protocol
,	O
and	O
much	O
more	O
.	O
</s>
<s>
Similar	O
headers	B-General_Concept
are	O
used	O
in	O
Usenet	B-Application
(	O
NNTP	B-Protocol
)	O
messages	O
,	O
and	O
HTTP	B-Protocol
headers	I-Protocol
.	O
</s>
<s>
In	O
a	O
data	B-Protocol
packet	I-Protocol
sent	O
via	O
the	O
Internet	O
,	O
the	O
data	O
(	O
payload	O
)	O
are	O
preceded	O
by	O
header	B-General_Concept
information	O
such	O
as	O
the	O
sender	O
's	O
and	O
the	O
recipient	O
's	O
IP	B-Protocol
addresses	I-Protocol
,	O
the	O
protocol	O
governing	O
the	O
format	O
of	O
the	O
payload	O
and	O
several	O
other	O
formats	O
.	O
</s>
<s>
The	O
header	B-General_Concept
's	O
format	O
is	O
specified	O
in	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
In	O
data	B-Protocol
packets	I-Protocol
sent	O
by	O
wireless	O
communication	O
,	O
and	O
in	O
sectors	O
of	O
data	O
stored	O
on	O
magnetic	B-Architecture
media	I-Architecture
,	O
typically	O
the	O
header	B-General_Concept
begins	O
with	O
a	O
syncword	O
to	O
allow	O
the	O
receiver	O
to	O
adapt	O
to	O
analog	O
amplitude	O
and	O
speed	O
variations	O
and	O
for	O
frame	O
synchronization	O
.	O
</s>
<s>
In	O
graphics	O
file	O
formats	O
,	O
the	O
header	B-General_Concept
might	O
give	O
information	O
about	O
an	O
image	O
's	O
size	O
,	O
resolution	O
,	O
number	O
of	O
colors	O
,	O
and	O
the	O
like	O
.	O
</s>
<s>
In	O
archive	O
file	O
formats	O
,	O
the	O
file	B-General_Concept
header	I-General_Concept
might	O
serve	O
as	O
a	O
fingerprint	B-Algorithm
or	O
signature	O
to	O
identify	O
the	O
specific	O
file	O
format	O
and	O
corresponding	O
software	O
utility	O
.	O
</s>
<s>
In	O
some	O
programming	O
languages	O
(	O
for	O
example	O
C	B-Language
and	O
C++	B-Language
)	O
the	O
functions	O
are	O
declared	O
in	O
header	B-Language
files	I-Language
.	O
</s>
