<s>
Robust	B-General_Concept
Header	I-General_Concept
Compression	I-General_Concept
(	O
ROHC	B-General_Concept
)	O
is	O
a	O
standardized	O
method	O
to	O
compress	O
the	O
IP	B-Protocol
,	O
UDP	B-Protocol
,	O
UDP-Lite	B-Protocol
,	O
RTP	B-Protocol
,	O
and	O
TCP	B-Protocol
headers	I-Protocol
of	O
Internet	O
packets	O
.	O
</s>
<s>
In	O
streaming	O
applications	O
,	O
the	O
overhead	O
of	O
IP	B-Protocol
,	O
UDP	B-Protocol
,	O
and	O
RTP	B-Protocol
is	O
40	O
bytes	B-Application
for	O
IPv4	B-Protocol
,	O
or	O
60	O
bytes	B-Application
for	O
IPv6	B-Protocol
.	O
</s>
<s>
For	O
VoIP	B-Application
,	O
this	O
corresponds	O
to	O
around	O
60%	O
of	O
the	O
total	O
amount	O
of	O
data	O
sent	O
.	O
</s>
<s>
ROHC	B-General_Concept
compresses	O
these	O
40	O
bytes	B-Application
or	O
60	O
bytes	B-Application
of	O
overhead	O
typically	O
into	O
only	O
one	O
or	O
three	O
bytes	B-Application
,	O
by	O
placing	O
a	O
compressor	O
before	O
the	O
link	O
that	O
has	O
limited	O
capacity	O
,	O
and	O
a	O
decompressor	O
after	O
that	O
link	O
.	O
</s>
<s>
The	O
compressor	O
converts	O
the	O
large	O
overhead	O
to	O
only	O
a	O
few	O
bytes	B-Application
,	O
while	O
the	O
decompressor	O
does	O
the	O
opposite	O
.	O
</s>
<s>
The	O
ROHC	B-General_Concept
compression	O
scheme	O
differs	O
from	O
other	O
compression	O
schemes	O
,	O
such	O
as	O
IETF	O
and	O
,	O
by	O
the	O
fact	O
that	O
it	O
performs	O
well	O
over	O
links	O
where	O
the	O
packet	O
loss	O
rate	O
is	O
high	O
,	O
such	O
as	O
wireless	O
links	O
.	O
</s>
<s>
The	O
ROHC	B-General_Concept
protocol	O
takes	O
advantage	O
of	O
information	O
redundancy	O
in	O
the	O
headers	O
of	O
the	O
following	O
:	O
</s>
<s>
identifiers	O
or	O
sequence	B-Protocol
numbers	I-Protocol
.	O
</s>
<s>
The	O
classification	O
algorithm	O
is	O
not	O
defined	O
by	O
the	O
ROHC	B-General_Concept
protocol	O
itself	O
but	O
left	O
to	O
the	O
equipment	O
vendor	O
's	O
implementation	O
.	O
</s>
<s>
According	O
to	O
RFC	O
3095	O
,	O
the	O
ROHC	B-General_Concept
scheme	O
has	O
three	O
modes	O
of	O
operation	O
,	O
as	O
follows	O
:	O
</s>
<s>
This	O
mode	O
therefore	O
makes	O
ROHC	B-General_Concept
usable	O
over	O
links	O
where	O
a	O
return	O
path	O
from	O
decompressor	O
to	O
compressor	O
is	O
unavailable	O
or	O
undesirable	O
.	O
</s>
<s>
The	O
ROHC	B-General_Concept
algorithm	O
is	O
similar	O
to	O
video	O
compression	O
,	O
in	O
that	O
a	O
base	O
frame	O
and	O
then	O
several	O
difference	O
frames	O
are	O
sent	O
to	O
represent	O
an	O
IP	B-Protocol
packet	O
flow	O
.	O
</s>
<s>
This	O
has	O
the	O
advantage	O
of	O
allowing	O
ROHC	B-General_Concept
to	O
survive	O
many	O
packet	O
losses	O
in	O
its	O
highest	O
compression	O
state	O
,	O
as	O
long	O
as	O
the	O
base	O
frames	O
are	O
not	O
lost	O
.	O
</s>
<s>
In	O
First-Order	O
(	O
FO	O
)	O
state	O
,	O
the	O
compressor	O
has	O
detected	O
and	O
stored	O
the	O
static	O
fields	O
(	O
such	O
as	O
IP	B-Protocol
addresses	O
and	O
port	O
numbers	O
)	O
on	O
both	O
sides	O
of	O
the	O
connection	O
.	O
</s>
<s>
In	O
Second-Order	O
(	O
SO	O
)	O
state	O
,	O
the	O
compressor	O
is	O
suppressing	O
all	O
dynamic	O
fields	O
such	O
as	O
RTP	B-Protocol
sequence	B-Protocol
numbers	I-Protocol
,	O
and	O
sending	O
only	O
a	O
logical	O
sequence	B-Protocol
number	I-Protocol
and	O
partial	O
checksum	O
to	O
cause	O
the	O
other	O
side	O
to	O
predictively	O
generate	O
and	O
verify	O
the	O
headers	O
of	O
the	O
next	O
expected	O
packet	O
.	O
</s>
<s>
SO	O
state	O
is	O
compressing	O
all	O
dynamic	O
fields	O
predictively	O
using	O
a	O
sequence	B-Protocol
number	I-Protocol
and	O
checksum	O
.	O
</s>
<s>
A	O
typical	O
ROHC	B-General_Concept
implementation	O
will	O
aim	O
to	O
get	O
the	O
terminal	O
into	O
Second-Order	O
state	O
,	O
where	O
a	O
1-byte	O
ROHC	B-General_Concept
header	O
can	O
be	O
substituted	O
for	O
the	O
40-byte	O
IPv4/UDP/RTP	O
or	O
the	O
60-byte	O
IPv6/UDP/RTP	O
(	O
i.e.	O
</s>
<s>
VoIP	B-Application
)	O
header	O
.	O
</s>
<s>
In	O
this	O
state	O
,	O
the	O
8-bit	O
ROHC	B-General_Concept
header	O
contains	O
three	O
fields	O
:	O
</s>
<s>
The	O
size	O
of	O
the	O
sequence	B-Protocol
number	I-Protocol
(	O
SN	O
)	O
field	O
governs	O
the	O
number	O
of	O
packets	O
that	O
ROHC	B-General_Concept
can	O
lose	O
before	O
the	O
compressor	O
must	O
be	O
reset	O
to	O
continue	O
.	O
</s>
<s>
The	O
size	O
of	O
the	O
sequence	B-Protocol
number	I-Protocol
in	O
1	O
and	O
2	O
byte	B-Application
ROHC	B-General_Concept
packets	O
is	O
either	O
4	O
bits	O
(	O
−	O
1/	O
+14	O
frame	O
offset	O
)	O
,	O
or	O
6	O
bits	O
(	O
−	O
1/	O
+62	O
frame	O
offset	O
)	O
,	O
respectively	O
,	O
so	O
ROHC	B-General_Concept
can	O
tolerate	O
at	O
most	O
62	O
lost	O
frames	O
with	O
a	O
1-2	O
byte	B-Application
header	O
.	O
</s>
<s>
The	O
defines	O
a	O
compression	O
profile	O
for	O
IP	B-Protocol
headers	O
or	O
IP	B-Protocol
tunnels	O
.	O
</s>
<s>
The	O
defines	O
a	O
compression	O
profile	O
for	O
UDP-Lite/IP	O
and	O
RTP/UDP-Lite/IP	O
headers	O
.	O
</s>
<s>
The	O
defines	O
a	O
compression	O
profile	O
for	O
TCP/IP	O
headers	O
.	O
</s>
<s>
There	O
have	O
been	O
two	O
new	O
RFCs	O
published	O
and	O
to	O
address	O
the	O
confusion	O
some	O
have	O
encountered	O
when	O
attempting	O
to	O
interpret	O
and	O
implement	O
ROHC	B-General_Concept
.	O
</s>
<s>
The	O
first	O
document	O
defines	O
a	O
ROHC	B-General_Concept
framework	O
,	O
while	O
the	O
second	O
defines	O
newer	O
versions	O
of	O
the	O
established	O
ROHC	B-General_Concept
profiles	O
.	O
</s>
