<s>
It	O
evolved	O
from	O
the	O
earlier	O
RADIUS	B-Protocol
protocol	O
.	O
</s>
<s>
It	O
belongs	O
to	O
the	O
application	B-Application
layer	I-Application
protocols	O
in	O
the	O
internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
.	O
</s>
<s>
Diameter	O
Applications	O
extend	O
the	O
base	O
protocol	O
by	O
adding	O
new	O
commands	O
and/or	O
attributes	O
,	O
such	O
as	O
those	O
for	O
use	O
with	O
the	O
Extensible	B-Protocol
Authentication	I-Protocol
Protocol	I-Protocol
(	O
EAP	O
)	O
.	O
</s>
<s>
The	O
name	O
is	O
a	O
play	O
on	O
words	O
,	O
derived	O
from	O
the	O
RADIUS	B-Protocol
protocol	O
,	O
which	O
is	O
the	O
predecessor	O
(	O
a	O
diameter	O
is	O
twice	O
the	O
radius	B-Protocol
)	O
.	O
</s>
<s>
Diameter	O
is	O
not	O
directly	O
backward	B-General_Concept
compatible	I-General_Concept
but	O
provides	O
an	O
upgrade	O
path	O
for	O
RADIUS	B-Protocol
.	O
</s>
<s>
The	O
main	O
features	O
provided	O
by	O
Diameter	O
but	O
lacking	O
in	O
RADIUS	B-Protocol
are	O
:	O
</s>
<s>
Like	O
RADIUS	B-Protocol
,	O
it	O
is	O
intended	O
to	O
work	O
in	O
both	O
local	O
and	O
roaming	O
AAA	O
situations	O
.	O
</s>
<s>
It	O
uses	O
TCP	B-Protocol
or	O
SCTP	B-Protocol
,	O
unlike	O
RADIUS	B-Protocol
which	O
uses	O
UDP	B-Protocol
.	O
</s>
<s>
Unlike	O
RADIUS	B-Protocol
it	O
includes	O
no	O
encryption	O
but	O
can	O
be	O
protected	O
by	O
transport-level	O
security	O
(	O
IPSEC	B-Protocol
or	O
TLS	B-Protocol
)	O
.	O
</s>
<s>
The	O
base	O
size	O
of	O
the	O
AV	O
identifier	O
is	O
32	O
bit	O
unlike	O
RADIUS	B-Protocol
which	O
uses	O
8	O
bit	O
as	O
the	O
base	O
AV	O
identifier	O
size	O
.	O
</s>
<s>
Like	O
RADIUS	B-Protocol
,	O
it	O
supports	O
stateless	O
as	O
well	O
as	O
stateful	O
modes	O
.	O
</s>
<s>
Like	O
RADIUS	B-Protocol
,	O
it	O
supports	O
application-layer	B-Application
acknowledgment	O
and	O
defines	O
failover	B-General_Concept
.	O
</s>
<s>
A	O
Diameter	O
Application	O
is	O
not	O
a	O
software	B-Application
application	I-Application
but	O
is	O
a	O
protocol	O
based	O
on	O
the	O
Diameter	O
base	O
protocol	O
defined	O
in	O
RFC	O
6733	O
and	O
RFC	O
7075	O
(	O
Obsoletes	O
:	O
RFC	O
3588	O
)	O
.	O
</s>
<s>
Each	O
application	O
is	O
defined	O
by	O
an	O
application	O
identifier	O
and	O
can	O
add	O
new	O
command	O
codes	O
and/or	O
new	O
mandatory	O
AVPs	O
(	O
Attribute-Value	B-Application
Pair	I-Application
)	O
.	O
</s>
<s>
The	O
Diameter	O
protocol	O
was	O
initially	O
developed	O
by	O
Pat	O
R	O
.	O
Calhoun	O
,	O
Glen	O
Zorn	O
,	O
and	O
Ping	O
Pan	O
in	O
1998	O
to	O
provide	O
a	O
framework	O
for	O
authentication	O
,	O
authorization	O
,	O
and	O
accounting	O
(	O
AAA	O
)	O
that	O
could	O
overcome	O
the	O
limitations	O
of	O
RADIUS	B-Protocol
.	O
</s>
<s>
RADIUS	B-Protocol
had	O
issues	O
with	O
reliability	O
,	O
scalability	O
,	O
security	O
,	O
and	O
flexibility	O
.	O
</s>
<s>
RADIUS	B-Protocol
cannot	O
deal	O
effectively	O
with	O
remote	O
access	O
,	O
IP	O
mobility	O
,	O
and	O
policy	O
control	O
.	O
</s>
<s>
Like	O
RADIUS	B-Protocol
,	O
Diameter	O
provides	O
AAA	O
functionality	O
,	O
but	O
uses	O
TCP	B-Protocol
and	O
SCTP	B-Protocol
instead	O
of	O
UDP	B-Protocol
,	O
therefore	O
delegating	O
detection	O
and	O
handling	O
of	O
communication	O
problems	O
to	O
those	O
protocols	O
.	O
</s>
<s>
The	O
Diameter	O
protocol	O
is	O
enhanced	O
further	O
by	O
the	O
development	O
of	O
the	O
3rd	O
Generation	O
Partnership	O
Project	O
(	O
3GPP	O
)	O
IP	B-Protocol
Multimedia	I-Protocol
Subsystem	I-Protocol
(	O
IMS	O
)	O
.	O
</s>
<s>
Diameter	O
security	O
is	O
provided	O
by	O
IPsec	B-Protocol
or	O
TLS	B-Protocol
.	O
</s>
<s>
The	O
IANA	O
has	O
assigned	O
TCP	B-Protocol
and	O
SCTP	B-Protocol
port	O
number	O
3868	O
to	O
Diameter	O
,	O
as	O
stated	O
in	O
section	O
11.4	O
of	O
RFC	O
6733	O
.	O
</s>
<s>
The	O
packet	O
consists	O
of	O
a	O
Diameter	O
header	O
and	O
a	O
variable	O
number	O
of	O
Attribute	B-Application
–	I-Application
Value	I-Application
Pairs	I-Application
,	O
or	O
AVPs	O
,	O
for	O
encapsulating	O
information	O
relevant	O
to	O
the	O
Diameter	O
message	O
.	O
</s>
<s>
The	O
"	O
T	O
"	O
(	O
Potentially	O
re-transmitted	O
message	O
)	O
bit	O
–	O
This	O
flag	O
is	O
set	O
after	O
a	O
link	O
failover	B-General_Concept
procedure	O
,	O
to	O
aid	O
the	O
removal	O
of	O
duplicate	O
requests	O
.	O
</s>
<s>
The	O
values	O
0-255	O
are	O
reserved	O
for	O
RADIUS	B-Protocol
backward	B-General_Concept
compatibility	I-General_Concept
.	O
</s>
<s>
For	O
instance	O
,	O
the	O
value	O
of	O
the	O
Application-ID	O
and	O
of	O
the	O
Auth-Application-Id	O
Attribute	O
in	O
the	O
Credit-Control-Request	O
(	O
CCR	O
)	O
and	O
Credit-Control-Answer	O
(	O
CCA	O
)	O
Command	O
for	O
the	O
Diameter	B-Protocol
Credit-Control	I-Protocol
Application	I-Protocol
is	O
4	O
.	O
</s>
<s>
The	O
Diameter	O
protocol	O
requires	O
that	O
relaying	O
and	O
proxying	O
agents	O
maintain	O
transaction	O
state	O
,	O
which	O
is	O
used	O
for	O
failover	B-General_Concept
purposes	O
.	O
</s>
<s>
The	O
communication	O
between	O
two	O
diameter	O
peers	O
starts	O
with	O
the	O
establishment	O
of	O
a	O
transport	O
connection	O
(	O
TCP	B-Protocol
or	O
SCTP	B-Protocol
)	O
.	O
</s>
<s>
For	O
RFC3588	O
compliant	O
peers	O
TLS	B-Protocol
(	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
)	O
may	O
optionally	O
be	O
negotiated	O
.	O
</s>
<s>
For	O
RFC6733	O
compliant	O
peers	O
TLS	B-Protocol
negotiation	O
may	O
optionally	O
happen	O
before	O
the	O
CER/CEA	O
.	O
</s>
<s>
August	O
2005	O
RFC	O
7155	O
RFC	O
4006Diameter	O
Credit-Control	O
Application.August	O
2005Diameter	O
Credit-Control	O
Application	O
RFC	O
8506	O
RFC	O
4072	O
Diameter	O
Extensible	B-Protocol
Authentication	I-Protocol
Protocol	I-Protocol
(	O
EAP	O
)	O
Application	O
.	O
</s>
