<s>
A	O
datagram	B-Protocol
is	O
a	O
basic	O
transfer	O
unit	O
associated	O
with	O
a	O
packet-switched	B-Protocol
network	I-Protocol
.	O
</s>
<s>
Datagrams	B-Protocol
are	O
typically	O
structured	O
in	O
header	B-General_Concept
and	O
payload	O
sections	O
.	O
</s>
<s>
Datagrams	B-Protocol
provide	O
a	O
connectionless	B-Protocol
communication	I-Protocol
service	O
across	O
a	O
packet-switched	B-Protocol
network	I-Protocol
.	O
</s>
<s>
The	O
delivery	O
,	O
arrival	O
time	O
,	O
and	O
order	O
of	O
arrival	O
of	O
datagrams	B-Protocol
need	O
not	O
be	O
guaranteed	O
by	O
the	O
network	O
.	O
</s>
<s>
In	O
the	O
early	O
1970s	O
,	O
the	O
term	O
datagram	B-Protocol
was	O
created	O
by	O
combining	O
the	O
words	O
data	O
and	O
telegram	O
by	O
the	O
CCITT	O
rapporteur	O
on	O
packet	B-Protocol
switching	I-Protocol
,	O
Halvor	O
Bothner-By	O
.	O
</s>
<s>
Small	O
standardized	O
"	O
message	O
blocks	O
"	O
,	O
bearing	O
source	O
and	O
destination	O
addresses	O
,	O
were	O
stored	B-Protocol
and	I-Protocol
forwarded	I-Protocol
in	O
computer	O
nodes	O
of	O
a	O
highly	O
redundant	O
meshed	O
computer	O
network	O
.	O
</s>
<s>
In	O
1967	O
,	O
Donald	O
Davies	O
published	O
a	O
seminal	O
article	O
in	O
which	O
he	O
introduced	O
the	O
now	O
largely	O
used	O
words	O
packet	B-Protocol
and	O
packet	B-Protocol
switching	I-Protocol
.	O
</s>
<s>
To	O
deal	O
with	O
datagram	B-Protocol
permutations	O
(	O
due	O
to	O
dynamically	O
updated	O
routing	O
preferences	O
)	O
and	O
to	O
datagram	B-Protocol
losses	O
(	O
unavoidable	O
when	O
fast	O
sources	O
send	O
to	O
a	O
slow	O
destinations	O
)	O
,	O
he	O
assumes	O
that	O
"	O
all	O
users	O
of	O
the	O
network	O
will	O
provide	O
themselves	O
with	O
some	O
kind	O
of	O
error	O
control	O
"	O
(	O
what	O
will	O
be	O
called	O
later	O
on	O
a	O
pure	O
datagram	B-Protocol
service	I-Protocol
)	O
.	O
</s>
<s>
His	O
target	O
is	O
,	O
for	O
the	O
first	O
time	O
in	O
packet	B-Protocol
switching	I-Protocol
,	O
a	O
"	O
common-carrier	O
communication	O
network	O
"	O
.	O
</s>
<s>
To	O
support	O
remote	O
access	O
to	O
computer	O
services	O
by	O
user	O
terminals	O
,	O
which	O
at	O
that	O
time	O
transmitted	O
in	O
general	O
character	O
by	O
character	O
,	O
he	O
included	O
at	O
the	O
network	O
periphery	O
interface	O
computers	O
that	O
convert	O
character	O
flows	O
into	O
packet	B-Protocol
flows	O
and	O
conversely	O
.	O
</s>
<s>
In	O
1970	O
,	O
Lawrence	O
Roberts	O
and	O
Barry	O
D	O
.	O
Wessler	O
published	O
an	O
article	O
about	O
ARPANET	O
,	O
the	O
first	O
multi-node	O
packet-switching	B-Protocol
network	O
.	O
</s>
<s>
An	O
accompanying	O
paper	O
described	O
its	O
switching	O
nodes	O
(	O
the	O
IMPs	O
)	O
and	O
its	O
packet	B-Protocol
formats	O
.	O
</s>
<s>
The	O
network	O
core	O
performed	O
datagram	B-Protocol
switching	O
as	O
in	O
Baran	O
's	O
and	O
Davies	O
 '	O
model	O
,	O
but	O
provision	O
was	O
added	O
within	O
the	O
network	O
,	O
at	O
its	O
periphery	O
,	O
to	O
deal	O
with	O
datagram	B-Protocol
losses	O
and	O
permutations	O
.	O
</s>
<s>
In	O
1973	O
,	O
Louis	O
Pouzin	O
presented	O
his	O
design	O
for	O
Cyclades	O
,	O
the	O
first	O
real	O
size	O
network	O
implementing	O
the	O
pure	O
datagram	B-Protocol
model	O
of	O
Donald	O
Davies	O
.	O
</s>
<s>
The	O
Cyclades	O
team	O
has	O
thus	O
been	O
first	O
to	O
tackle	O
the	O
highly	O
complex	O
problem	O
of	O
providing	O
to	O
user	O
applications	O
a	O
reliable	O
virtual	O
circuit	O
service	O
(	O
the	O
equivalent	O
of	O
an	O
Internet	O
TCP	B-Protocol
connection	O
)	O
while	O
using	O
an	O
end	B-Application
to	I-Application
end	I-Application
network	O
service	O
known	O
to	O
possibly	O
produce	O
non	O
negligible	O
datagram	B-Protocol
losses	O
and	O
permutations	O
.	O
</s>
<s>
Although	O
Pouzin	O
's	O
concern	O
"	O
in	O
a	O
first	O
stage	O
is	O
not	O
to	O
make	O
breakthrough	O
in	O
packet	B-Protocol
switching	I-Protocol
technology	O
,	O
but	O
to	O
build	O
a	O
reliable	B-Protocol
communications	I-Protocol
tool	O
for	O
Cyclades	O
"	O
,	O
two	O
members	O
of	O
his	O
team	O
,	O
Hubert	O
Zimmerman	O
and	O
Gérard	O
Le	O
Lann	O
,	O
made	O
significant	O
contributions	O
to	O
the	O
design	O
of	O
Internet	O
's	O
TCP	B-Protocol
that	O
Vint	O
Cerf	O
,	O
its	O
main	O
designer	O
,	O
acknowledged	O
.	O
</s>
<s>
In	O
1981	O
,	O
the	O
Defense	O
Advanced	O
Research	O
Projects	O
Agency	O
(	O
DARPA	O
)	O
issued	O
the	O
first	O
specification	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
.	O
</s>
<s>
It	O
introduced	O
a	O
major	O
evolution	O
of	O
the	O
datagram	B-Protocol
concept	O
:	O
fragmentation	B-Protocol
.	O
</s>
<s>
With	O
fragmentation	B-Protocol
,	O
some	O
parts	O
of	O
the	O
global	O
network	O
may	O
use	O
large	O
packet	B-Protocol
size	O
(	O
typically	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
for	O
processing	O
power	O
minimization	O
)	O
,	O
while	O
some	O
others	O
may	O
impose	O
smaller	O
packet	B-Protocol
sizes	O
(	O
typically	O
wide	O
area	O
networks	O
for	O
response	O
time	O
minimization	O
)	O
.	O
</s>
<s>
Network	O
nodes	O
may	O
split	O
a	O
packet	B-Protocol
of	O
a	O
datagram	B-Protocol
into	O
several	O
smaller	O
packets	B-Protocol
of	O
the	O
same	O
datagram	B-Protocol
.	O
</s>
<s>
With	O
it	O
,	O
the	O
forthcoming	O
Internet	B-Protocol
Address	I-Protocol
exhaustion	I-Protocol
was	O
delayed	O
,	O
leaving	O
enough	O
time	O
to	O
introduce	O
IPv6	B-Protocol
,	O
the	O
new	O
generation	O
of	O
Internet	O
packets	B-Protocol
supporting	O
longer	O
addresses	O
.	O
</s>
<s>
The	O
initial	O
principle	O
of	O
full	O
end	B-Application
to	I-Application
end	I-Application
network	O
transparency	O
to	O
datagrams	B-Protocol
was	O
for	O
this	O
relaxed	O
:	O
NAT	O
nodes	O
had	O
to	O
manage	O
per-connection	O
states	O
,	O
making	O
them	O
in	O
part	O
connection	B-Protocol
oriented	I-Protocol
.	O
</s>
<s>
In	O
2015	O
,	O
the	O
IETF	O
upgraded	O
its	O
weak	O
"	O
informational	O
"	O
recommendation	O
of	O
1998	O
,	O
that	O
datagram	B-Protocol
switching	O
nodes	O
perform	O
active	B-Algorithm
queue	I-Algorithm
management	I-Algorithm
(	O
AQM	O
)	O
,	O
to	O
make	O
it	O
a	O
stronger	O
and	O
more	O
detailed	O
"	O
best	O
current	O
practice	O
"	O
recommendation	O
.	O
</s>
<s>
While	O
the	O
initial	O
datagram	B-Protocol
queueing	O
model	O
was	O
simple	O
to	O
implement	O
and	O
needed	O
no	O
more	O
tuning	O
than	O
queue	O
lengths	O
,	O
support	O
of	O
more	O
sophisticated	O
and	O
parametrized	O
mechanisms	O
were	O
found	O
necessary	O
"	O
to	O
improve	O
and	O
preserve	O
Internet	O
performance	O
"	O
(	O
RED	B-Algorithm
,	O
ECN	O
etc	O
.	O
)	O
.	O
</s>
<s>
The	O
term	O
datagram	B-Protocol
is	O
defined	O
as	O
follows	O
:	O
</s>
<s>
A	O
datagram	B-Protocol
needs	O
to	O
be	O
self-contained	O
without	O
reliance	O
on	O
earlier	O
exchanges	O
because	O
there	O
is	O
no	O
connection	O
of	O
fixed	O
duration	O
between	O
the	O
two	O
communicating	O
points	O
as	O
there	O
is	O
,	O
for	O
example	O
,	O
in	O
most	O
voice	O
telephone	O
conversations	O
.	O
</s>
<s>
Datagram	B-Protocol
service	I-Protocol
is	O
often	O
compared	O
to	O
a	O
mail	O
delivery	O
service	O
;	O
the	O
user	O
only	O
provides	O
the	O
destination	O
address	O
,	O
but	O
receives	O
no	O
guarantee	O
of	O
delivery	O
,	O
and	O
no	O
confirmation	O
upon	O
successful	O
delivery	O
.	O
</s>
<s>
Datagram	B-Protocol
service	I-Protocol
is	O
therefore	O
considered	O
unreliable	B-Protocol
.	O
</s>
<s>
Datagram	B-Protocol
service	I-Protocol
routes	O
datagrams	B-Protocol
without	O
first	O
creating	O
a	O
predetermined	O
path	O
.	O
</s>
<s>
Datagram	B-Protocol
service	I-Protocol
is	O
therefore	O
considered	O
connectionless	B-Protocol
.	O
</s>
<s>
There	O
is	O
also	O
no	O
consideration	O
given	O
to	O
the	O
order	O
in	O
which	O
it	O
and	O
other	O
datagrams	B-Protocol
are	O
sent	O
or	O
received	O
.	O
</s>
<s>
In	O
fact	O
,	O
many	O
datagrams	B-Protocol
in	O
the	O
same	O
group	O
can	O
travel	O
along	O
different	O
paths	O
before	O
reaching	O
the	O
same	O
destination	O
.	O
</s>
<s>
Each	O
datagram	B-Protocol
has	O
two	O
components	O
,	O
a	O
header	B-General_Concept
and	O
a	O
data	O
payload	O
.	O
</s>
<s>
The	O
header	B-General_Concept
contains	O
all	O
the	O
information	O
sufficient	O
for	O
routing	O
from	O
the	O
originating	O
equipment	O
to	O
the	O
destination	O
without	O
relying	O
on	O
prior	O
exchanges	O
between	O
the	O
equipment	O
and	O
the	O
network	O
.	O
</s>
<s>
Headers	B-General_Concept
may	O
include	O
source	O
and	O
destination	O
addresses	O
as	O
well	O
as	O
a	O
type	O
field	O
.	O
</s>
<s>
This	O
process	O
of	O
nesting	O
data	O
payloads	O
in	O
a	O
tagged	O
header	B-General_Concept
is	O
called	O
encapsulation	B-Protocol
.	O
</s>
<s>
The	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
defines	O
standards	O
for	O
several	O
types	O
of	O
datagrams	B-Protocol
.	O
</s>
<s>
The	O
internet	B-Protocol
layer	I-Protocol
is	O
a	O
datagram	B-Protocol
service	I-Protocol
provided	O
by	O
an	O
IP	O
.	O
</s>
<s>
For	O
example	O
,	O
UDP	B-Protocol
is	O
run	O
by	O
a	O
datagram	B-Protocol
service	I-Protocol
on	O
the	O
internet	B-Protocol
layer	I-Protocol
.	O
</s>
<s>
IP	O
is	O
an	O
entirely	O
connectionless	B-Protocol
,	O
best	O
effort	O
,	O
unreliable	B-Protocol
,	O
message	O
delivery	O
service	O
.	O
</s>
<s>
TCP	B-Protocol
is	O
a	O
higher	O
level	O
protocol	O
running	O
on	O
top	O
of	O
IP	O
that	O
provides	O
a	O
reliable	O
connection-oriented	B-Protocol
service	I-Protocol
.	O
</s>
