<s>
Protocol	B-Protocol
ossification	I-Protocol
is	O
the	O
loss	O
of	O
flexibility	O
,	O
extensibility	B-Architecture
and	O
evolvability	O
of	O
network	O
protocols	O
.	O
</s>
<s>
This	O
is	O
a	O
violation	O
of	O
the	O
end-to-end	B-Application
principle	I-Application
.	O
</s>
<s>
Ossification	O
is	O
a	O
major	O
issue	O
in	O
Internet	O
protocol	O
design	O
and	O
deployment	O
,	O
as	O
it	O
can	O
prevent	O
new	O
protocols	O
or	O
extensions	O
from	O
being	O
deployed	O
on	O
the	O
Internet	O
,	O
or	O
place	O
strictures	O
on	O
the	O
design	O
of	O
new	O
protocols	O
;	O
new	O
protocols	O
may	O
have	O
to	O
be	O
encapsulated	B-Protocol
in	O
an	O
already-deployed	O
protocol	O
or	O
mimic	O
the	O
wire	O
image	O
of	O
another	O
protocol	O
.	O
</s>
<s>
Because	O
of	O
ossification	O
,	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
and	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
are	O
the	O
only	O
practical	O
choices	O
for	O
transport	B-Architecture
protocols	I-Architecture
on	O
the	O
Internet	O
,	O
and	O
TCP	O
itself	O
has	O
significantly	O
ossified	O
,	O
making	O
extension	O
or	O
modification	O
of	O
the	O
protocol	O
difficult	O
.	O
</s>
<s>
QUIC	B-Protocol
is	O
the	O
first	O
IETF	O
transport	B-Architecture
protocol	I-Architecture
to	O
have	O
been	O
designed	O
with	O
deliberate	O
anti-ossification	O
properties	O
.	O
</s>
<s>
Multipath	B-Protocol
TCP	I-Protocol
was	O
the	O
first	O
extension	O
to	O
a	O
core	O
Internet	O
protocol	O
to	O
deeply	O
confront	O
protocol	B-Protocol
ossification	I-Protocol
during	O
its	O
design	O
.	O
</s>
<s>
It	O
has	O
a	O
mandate	O
to	O
mitigate	O
ossification	O
at	O
the	O
transport	B-Architecture
protocol	I-Architecture
layer	O
.	O
</s>
<s>
QUIC	B-Protocol
is	O
the	O
first	O
IETF	O
transport	B-Architecture
protocol	I-Architecture
to	O
deliberately	O
minimise	O
its	O
wire	O
image	O
to	O
avoid	O
ossification	O
.	O
</s>
<s>
The	O
primary	O
cause	O
of	O
protocol	B-Protocol
ossification	I-Protocol
is	O
middlebox	O
interference	O
,	O
invalidating	O
the	O
end-to-end	B-Application
principle	I-Application
.	O
</s>
<s>
Middleboxes	O
are	O
deployed	O
by	O
network	O
operators	O
unilaterally	O
to	O
solve	O
specific	O
problems	O
,	O
including	O
performance	O
optimisation	O
,	O
security	O
requirements	O
(	O
e.g.	O
,	O
firewalls	O
)	O
,	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
or	O
enhancing	O
control	O
of	O
networks	O
.	O
</s>
<s>
Operating	B-Operating_System
system	I-Operating_System
kernels	I-Operating_System
are	O
slow	O
to	O
change	O
and	O
deploy	O
,	O
and	O
protocols	O
implemented	O
in	O
hardware	O
can	O
also	O
inappropriately	O
fix	O
protocol	O
details	O
.	O
</s>
<s>
A	O
widely-used	O
API	B-Application
that	O
makes	O
assumptions	O
about	O
the	O
operation	O
of	O
underlying	O
protocols	O
can	O
hinder	O
the	O
deployment	O
of	O
protocols	O
that	O
do	O
not	O
share	O
those	O
assumptions	O
.	O
</s>
<s>
The	O
Internet	O
Architecture	O
Board	O
recommended	O
in	O
2019	O
that	O
implicit	O
signals	O
to	O
observers	O
should	O
be	O
replaced	O
with	O
signals	O
deliberately	O
intended	O
for	O
the	O
consumption	O
of	O
those	O
observers	O
,	O
and	O
signals	O
not	O
intended	O
for	O
their	O
consumption	O
should	O
be	O
not	O
be	O
available	O
to	O
them	O
(	O
e.g.	O
,	O
by	O
encryption	O
)	O
;	O
and	O
also	O
that	O
the	O
protocol	O
metadata	O
should	O
be	O
integrity	B-Error_Name
protected	I-Error_Name
so	O
that	O
it	O
cannot	O
be	O
modified	O
by	O
middleboxes	O
.	O
</s>
<s>
chaos	B-Application
engineering	I-Application
)	O
.	O
</s>
<s>
HTTP	B-Protocol
headers	I-Protocol
are	O
an	O
example	O
of	O
an	O
extension	O
point	O
that	O
has	O
successfully	O
avoided	O
significant	O
ossification	O
,	O
as	O
participants	O
will	O
generally	O
ignore	O
unrecognised	O
headers	O
.	O
</s>
<s>
A	O
new	O
protocol	O
may	O
be	O
designed	O
to	O
mimic	O
the	O
wire	O
image	O
of	O
an	O
existing	O
ossified	O
protocol	O
;	O
alternatively	O
,	O
a	O
new	O
protocol	O
may	O
be	O
to	O
encapsulated	B-Protocol
within	O
an	O
existing	O
,	O
tolerated	O
protocol	O
.	O
</s>
<s>
A	O
disadvantage	O
of	O
encapsulation	B-Protocol
is	O
that	O
there	O
is	O
typically	O
overhead	O
and	O
redundant	O
work	O
(	O
e.g.	O
,	O
outer	O
checksums	O
made	O
redundant	O
by	O
inner	O
integrity	O
checks	O
)	O
.	O
</s>
<s>
User-space	B-Operating_System
implementation	O
of	O
protocols	O
can	O
lead	O
to	O
more	O
rapid	O
evolution	O
.	O
</s>
<s>
If	O
the	O
new	O
protocol	O
is	O
encapsulated	B-Protocol
in	O
UDP	O
,	O
then	O
user-space	B-Operating_System
implementation	O
is	O
possible	O
.	O
</s>
<s>
A	O
flag	B-General_Concept
day	I-General_Concept
,	O
where	O
protocol	O
participants	O
make	O
changes	O
in	O
concert	O
,	O
can	O
break	O
the	O
vicious	O
cycle	O
and	O
establish	O
active	O
use	O
.	O
</s>
<s>
This	O
approach	O
was	O
used	O
to	O
deploy	O
EDNS	B-Protocol
,	O
which	O
had	O
formerly	O
not	O
been	O
tolerated	O
by	O
servers	O
.	O
</s>
<s>
The	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
has	O
suffered	O
from	O
ossification	O
.	O
</s>
<s>
Extensions	O
to	O
TCP	O
have	O
been	O
affected	O
:	O
the	O
design	O
of	O
MPTCP	B-Protocol
was	O
constrained	O
by	O
middlebox	O
behaviour	O
,	O
and	O
the	O
deployment	O
of	O
TCP	B-Protocol
Fast	I-Protocol
Open	I-Protocol
has	O
been	O
likewise	O
hindered	O
.	O
</s>
<s>
The	O
Stream	B-Protocol
Control	I-Protocol
Transmission	I-Protocol
Protocol	I-Protocol
has	O
been	O
little-deployed	O
on	O
the	O
Internet	O
due	O
to	O
intolerance	O
from	O
middleboxes	O
,	O
and	O
also	O
due	O
to	O
the	O
very	O
widespread	O
BSD	B-Protocol
sockets	I-Protocol
API	I-Protocol
ill-fitting	O
its	O
capabilities	O
.	O
</s>
<s>
In	O
practice	O
,	O
TCP	O
and	O
UDP	O
are	O
the	O
only	O
usable	O
Internet	O
transport	B-Architecture
protocols	I-Architecture
.	O
</s>
<s>
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	O
)	O
has	O
experienced	O
ossification	O
.	O
</s>
<s>
This	O
was	O
discovered	O
late	O
in	O
the	O
protocol	O
design	O
process	O
,	O
during	O
experimental	O
deployments	O
by	O
web	B-Application
browsers	I-Application
.	O
</s>
<s>
QUIC	B-Protocol
has	O
been	O
specifically	O
designed	O
to	O
be	O
deployable	O
,	O
evolvable	O
and	O
to	O
have	O
anti-ossification	O
properties	O
;	O
it	O
is	O
the	O
first	O
IETF	O
transport	B-Architecture
protocol	I-Architecture
to	O
deliberately	O
minimise	O
its	O
wire	O
image	O
for	O
these	O
ends	O
.	O
</s>
<s>
It	O
is	O
greased	O
,	O
it	O
has	O
protocol	O
invariants	O
explicitly	O
specified	O
,	O
it	O
is	O
encapsulated	B-Protocol
in	O
UDP	O
,	O
and	O
its	O
protocol	O
metadata	O
is	O
encrypted	O
.	O
</s>
<s>
Still	O
,	O
applications	O
using	O
QUIC	B-Protocol
must	O
be	O
prepared	O
to	O
fall	O
back	O
to	O
other	O
protocols	O
,	O
as	O
UDP	O
is	O
blocked	O
by	O
some	O
middleboxes	O
.	O
</s>
