<s>
Multipath	B-Protocol
TCP	I-Protocol
(	O
MPTCP	B-Protocol
)	O
is	O
an	O
ongoing	O
effort	O
of	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
's	O
(	O
IETF	O
)	O
Multipath	B-Protocol
TCP	I-Protocol
working	O
group	O
,	O
that	O
aims	O
at	O
allowing	O
a	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	B-Protocol
)	O
connection	O
to	O
use	O
multiple	O
paths	O
to	O
maximize	O
throughput	O
and	O
increase	O
redundancy	O
.	O
</s>
<s>
It	O
was	O
replaced	O
in	O
March	O
2020	O
by	O
the	O
Multipath	B-Protocol
TCP	I-Protocol
v1	O
specification	O
in	O
.	O
</s>
<s>
The	O
redundancy	O
offered	O
by	O
Multipath	B-Protocol
TCP	I-Protocol
enables	O
inverse	B-Protocol
multiplexing	I-Protocol
of	O
resources	O
,	O
and	O
thus	O
increases	O
TCP	B-Protocol
throughput	O
to	O
the	O
sum	O
of	O
all	O
available	O
link-level	B-Protocol
channels	O
instead	O
of	O
using	O
a	O
single	O
one	O
as	O
required	O
by	O
standard	O
TCP	B-Protocol
.	O
</s>
<s>
Multipath	B-Protocol
TCP	I-Protocol
is	O
backward	O
compatible	O
with	O
standard	O
TCP	B-Protocol
.	O
</s>
<s>
Multipath	B-Protocol
TCP	I-Protocol
is	O
particularly	O
useful	O
in	O
the	O
context	O
of	O
wireless	O
networks	O
;	O
using	O
both	O
Wi-Fi	O
and	O
a	O
mobile	O
network	B-Application
is	O
a	O
typical	O
use	B-Architecture
case	I-Architecture
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
gains	O
in	O
throughput	O
from	O
inverse	B-Protocol
multiplexing	I-Protocol
,	O
links	O
may	O
be	O
added	O
or	O
dropped	O
as	O
the	O
user	O
moves	O
in	O
or	O
out	O
of	O
coverage	O
without	O
disrupting	O
the	O
end-to-end	O
TCP	B-Protocol
connection	O
.	O
</s>
<s>
The	O
problem	O
of	O
link	O
handover	B-Protocol
is	O
thus	O
solved	O
by	O
abstraction	O
in	O
the	O
transport	B-Architecture
layer	I-Architecture
,	O
without	O
any	O
special	O
mechanisms	O
at	O
the	O
network	B-Application
or	O
link	B-Protocol
layers	I-Protocol
.	O
</s>
<s>
Handover	B-Protocol
functionality	O
can	O
then	O
be	O
implemented	O
at	O
the	O
endpoints	O
without	O
requiring	O
special	O
functionality	O
in	O
the	O
subnetworks	B-Protocol
-	O
in	O
accordance	O
to	O
the	O
Internet	O
's	O
end-to-end	B-Application
principle	I-Application
.	O
</s>
<s>
Multipath	B-Protocol
TCP	I-Protocol
also	O
brings	O
performance	O
benefits	O
in	O
datacenter	B-Operating_System
environments	O
.	O
</s>
<s>
In	O
contrast	O
to	O
Ethernet	O
channel	O
bonding	O
using	O
802.3ad	O
link	O
aggregation	O
,	O
Multipath	B-Protocol
TCP	I-Protocol
can	O
balance	O
a	O
single	O
TCP	B-Protocol
connection	O
across	O
multiple	O
interfaces	O
and	O
reach	O
very	O
high	O
throughput	O
.	O
</s>
<s>
Multipath	B-Protocol
TCP	I-Protocol
causes	O
a	O
number	O
of	O
new	O
issues	O
.	O
</s>
<s>
From	O
a	O
network	B-Application
security	O
perspective	O
,	O
multipath	O
routing	O
causes	O
cross-path	O
data	O
fragmentation	O
that	O
results	O
in	O
firewalls	O
and	O
malware	O
scanners	O
becoming	O
inefficient	O
when	O
they	O
only	O
see	O
one	O
path	O
's	O
traffic	O
.	O
</s>
<s>
In	O
order	O
to	O
facilitate	O
its	O
deployment	O
,	O
Multipath	B-Protocol
TCP	I-Protocol
presents	O
the	O
same	O
socket	O
interface	O
as	O
TCP	B-Protocol
.	O
</s>
<s>
This	O
implies	O
that	O
any	O
standard	O
TCP	B-Protocol
application	O
can	O
be	O
used	O
above	O
Multipath	B-Protocol
TCP	I-Protocol
while	O
in	O
fact	O
spreading	O
data	O
across	O
several	O
subflows	O
.	O
</s>
<s>
Some	O
applications	O
could	O
benefit	O
from	O
an	O
enhanced	O
API	O
to	O
control	O
the	O
underlying	O
Multipath	B-Protocol
TCP	I-Protocol
stack	O
.	O
</s>
<s>
Two	O
different	O
APIs	O
have	O
been	O
proposed	O
to	O
expose	O
some	O
of	O
features	O
of	O
the	O
Multipath	B-Protocol
TCP	I-Protocol
stack	O
to	O
applications	O
:	O
an	O
API	O
that	O
extends	O
Netlink	B-Application
on	O
Linux	B-Operating_System
and	O
an	O
enhanced	O
socket	O
API	O
.	O
</s>
<s>
In	O
July	O
2013	O
,	O
the	O
MPTCP	B-Protocol
working	O
group	O
reported	O
five	O
independent	O
implementations	O
of	O
Multipath	B-Protocol
TCP	I-Protocol
,	O
including	O
the	O
reference	O
implementation	O
in	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
Linux	B-Operating_System
kernel	I-Operating_System
(	O
reference	O
implementation	O
)	O
from	O
Université	O
catholique	O
de	O
Louvain	O
researchers	O
and	O
other	O
collaborators	O
,	O
</s>
<s>
FreeBSD	B-Operating_System
(	O
IPv4	O
only	O
)	O
from	O
Swinburne	O
University	O
of	O
Technology	O
,	O
</s>
<s>
Citrix	B-Application
Netscaler	O
,	O
</s>
<s>
Apple	O
iOS	B-Operating_System
7	I-Operating_System
,	O
released	O
on	O
September	O
18	O
,	O
2013	O
is	O
the	O
first	O
large	O
scale	O
commercial	O
deployment	O
of	O
Multipath	B-Protocol
TCP	I-Protocol
.	O
</s>
<s>
Since	O
iOS	B-Operating_System
7	I-Operating_System
,	O
any	O
application	O
can	O
use	O
Multipath	B-Protocol
TCP	I-Protocol
.	O
</s>
<s>
Apple	O
Mac	B-Device
OS	I-Device
X	I-Device
10.10	I-Device
,	O
released	O
on	O
October	O
16	O
,	O
2014	O
.	O
</s>
<s>
Alcatel-Lucent	O
released	O
MPTCP	B-Protocol
proxy	O
version	O
0.9	O
source	O
code	O
on	O
October	O
26	O
,	O
2012	O
.	O
</s>
<s>
In	O
July	O
2014	O
,	O
Oracle	O
reported	O
that	O
an	O
implementation	O
on	O
Solaris	B-Application
was	O
being	O
developed	O
.	O
</s>
<s>
During	O
the	O
MPTCP	B-Protocol
WG	O
meeting	O
at	O
IETF	O
93	O
,	O
SungHoon	O
Seo	O
announced	O
that	O
KT	O
had	O
deployed	O
since	O
mid	O
June	O
a	O
commercial	O
service	O
that	O
allows	O
smartphone	O
users	O
to	O
reach	O
1	O
Gbit/s	O
using	O
a	O
MPTCP	B-Protocol
proxy	O
service	O
.	O
</s>
<s>
There	O
is	O
an	O
ongoing	O
effort	O
to	O
push	O
a	O
new	O
Multipath	B-Protocol
TCP	I-Protocol
implementation	O
in	O
the	O
mainline	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
</s>
<s>
Multipath	B-Protocol
TCP	I-Protocol
was	O
designed	O
to	O
be	O
backward	O
compatible	O
with	O
regular	O
TCP	B-Protocol
.	O
</s>
<s>
Apple	O
uses	O
Multipath	B-Protocol
TCP	I-Protocol
to	O
support	O
the	O
Siri	B-Application
application	O
on	O
iPhone	B-Device
.	O
</s>
<s>
Siri	B-Application
sends	O
voice	O
samples	O
over	O
an	O
HTTPS	B-Protocol
session	O
to	O
Apple	O
servers	O
.	O
</s>
<s>
According	O
to	O
Apple	O
engineers	O
,	O
the	O
main	O
benefits	O
of	O
Multipath	B-Protocol
TCP	I-Protocol
with	O
this	O
application	O
are	O
:	O
</s>
<s>
Other	O
deployment	O
use	O
Multipath	B-Protocol
TCP	I-Protocol
to	O
aggregate	O
the	O
bandwidth	O
of	O
different	O
networks	O
.	O
</s>
<s>
For	O
example	O
,	O
several	O
types	O
of	O
smartphones	O
,	O
notably	O
in	O
Korea	O
,	O
use	O
Multipath	B-Protocol
TCP	I-Protocol
to	O
bond	O
WiFi	O
and	O
4G	O
through	O
SOCKS	O
proxies	O
.	O
</s>
<s>
Another	O
example	O
are	O
the	O
Hybrid	O
Access	O
Networks	O
that	O
are	O
deployed	O
by	O
network	B-Application
operators	O
willing	O
to	O
combine	O
xDSL	O
and	O
LTE	O
networks	O
.	O
</s>
<s>
In	O
this	O
deployment	O
,	O
Multipath	B-Protocol
TCP	I-Protocol
is	O
used	O
to	O
efficiently	O
balance	O
the	O
traffic	O
over	O
the	O
xDSL	O
and	O
the	O
LTE	O
network	B-Application
.	O
</s>
<s>
In	O
standardisation	O
of	O
converged	O
fixed	O
and	O
mobile	O
communication	O
networks	O
3GPP	O
and	O
BBF	O
are	O
interoperating	O
to	O
provide	O
an	O
ATSSS	O
(	O
Access	O
Traffic	O
Selection	O
,	O
Switching	O
,	O
Splitting	O
)	O
feature	O
to	O
support	O
multipath	O
sessions	O
,	O
e.g	O
,	O
by	O
applying	O
Multipath	B-Protocol
TCP	I-Protocol
both	O
in	O
the	O
User	O
Equipment	O
(	O
UE	O
)	O
or	O
Residential	O
Gateway	O
(	O
RG	O
)	O
and	O
on	O
the	O
network	B-Application
side	O
.	O
</s>
<s>
Multipath	B-Protocol
TCP	I-Protocol
uses	O
options	O
that	O
are	O
described	O
in	O
detail	O
in	O
RFC	O
6824	O
.	O
</s>
<s>
All	O
Multipath	B-Protocol
TCP	I-Protocol
options	O
are	O
encoded	O
as	O
TCP	B-Protocol
options	O
with	O
Option	O
Kind	O
is	O
30	O
,	O
as	O
reserved	O
by	O
IANA	O
.	O
</s>
<s>
The	O
Multipath	B-Protocol
TCP	I-Protocol
option	O
has	O
the	O
Kind	O
(	O
30	O
)	O
,	O
length	O
(	O
variable	O
)	O
and	O
the	O
remainder	O
of	O
the	O
content	O
begins	O
with	O
a	O
4-bit	O
subtype	O
field	O
,	O
for	O
which	O
IANA	O
has	O
created	O
and	O
will	O
maintain	O
a	O
sub-registry	O
entitled	O
"	O
MPTCP	B-Protocol
Option	O
Subtypes	O
"	O
under	O
the	O
"	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	B-Protocol
)	O
Parameters	O
"	O
registry	O
.	O
</s>
<s>
The	O
core	O
idea	O
of	O
multipath	B-Protocol
TCP	I-Protocol
is	O
to	O
define	O
a	O
way	O
to	O
build	O
a	O
connection	O
between	O
two	O
hosts	O
and	O
not	O
between	O
two	O
interfaces	O
(	O
as	O
standard	O
TCP	B-Protocol
does	O
)	O
.	O
</s>
<s>
In	O
standard	O
TCP	B-Protocol
,	O
the	O
connection	O
should	O
be	O
established	O
between	O
two	O
IP	O
addresses	O
.	O
</s>
<s>
Each	O
TCP	B-Protocol
connection	O
is	O
identified	O
by	O
a	O
four-tuple	O
(	O
source	O
and	O
destination	O
addresses	O
and	O
ports	O
)	O
.	O
</s>
<s>
Given	O
this	O
restriction	O
,	O
an	O
application	O
can	O
only	O
create	O
one	O
TCP	B-Protocol
connection	O
through	O
a	O
single	O
link	O
.	O
</s>
<s>
Multipath	B-Protocol
TCP	I-Protocol
allows	O
the	O
connection	O
to	O
use	O
several	O
paths	O
simultaneously	O
.	O
</s>
<s>
For	O
this	O
,	O
Multipath	B-Protocol
TCP	I-Protocol
creates	O
one	O
TCP	B-Protocol
connection	O
,	O
called	O
subflow	O
,	O
over	O
each	O
path	O
that	O
needs	O
to	O
be	O
used	O
.	O
</s>
<s>
Multipath	B-Protocol
TCP	I-Protocol
adds	O
new	O
mechanisms	O
to	O
TCP	B-Protocol
transmissions	O
:	O
</s>
<s>
The	O
subflow	O
system	O
,	O
used	O
to	O
gather	O
multiple	O
standard	O
TCP	B-Protocol
connections	O
(	O
the	O
paths	O
from	O
one	O
host	O
to	O
another	O
)	O
.	O
</s>
<s>
Subflows	O
are	O
identified	O
during	O
the	O
TCP	B-Protocol
three-way	O
handshake	O
.	O
</s>
<s>
The	O
MPTCP	B-Protocol
DSS	O
option	O
contains	O
a	O
data	O
sequence	B-Protocol
number	I-Protocol
and	O
an	O
acknowledgement	O
number	O
.	O
</s>
<s>
Several	O
congestion	O
control	O
mechanisms	O
have	O
been	O
defined	O
for	O
Multipath	B-Protocol
TCP	I-Protocol
.	O
</s>
<s>
Their	O
main	O
difference	O
with	O
classical	O
TCP	B-Protocol
congestion	O
control	O
schemes	O
is	O
that	O
they	O
need	O
to	O
react	O
to	O
congestion	O
on	O
the	O
different	O
paths	O
without	O
being	O
unfair	O
with	O
single	O
path	O
TCP	B-Protocol
sources	O
that	O
could	O
compete	O
with	O
them	O
on	O
one	O
of	O
the	O
paths	O
.	O
</s>
<s>
Four	O
Multipath	B-Protocol
TCP	I-Protocol
congestion	O
control	O
schemes	O
are	O
currently	O
supported	O
by	O
the	O
Multipath	B-Protocol
TCP	I-Protocol
implementation	O
in	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
Stream	B-Protocol
Control	I-Protocol
Transmission	I-Protocol
Protocol	I-Protocol
(	O
SCTP	B-Protocol
)	O
is	O
a	O
reliable	O
in-order	O
datagram	B-Protocol
stream	O
transport	B-Architecture
protocol	I-Architecture
originally	O
intended	O
for	O
telecommunication	O
signaling	O
.	O
</s>
<s>
It	O
supports	O
concurrent	O
use	O
of	O
multiple	O
access	O
links	O
and	O
allows	O
the	O
application	O
to	O
influence	O
the	O
access	O
interface	O
selections	O
on	O
a	O
datagram	B-Protocol
stream	O
basis	O
.	O
</s>
<s>
Hence	O
,	O
SCTP	B-Protocol
is	O
also	O
a	O
transport	B-Architecture
layer	I-Architecture
solution	O
.	O
</s>
<s>
It	O
offers	O
type	O
3	O
flow	O
granularity	O
with	O
concurrency	O
,	O
but	O
with	O
more	O
flow	O
scheduling	O
control	O
than	O
Multipath	B-Protocol
TCP	I-Protocol
.	O
</s>
<s>
It	O
also	O
fully	O
supports	O
mobility	O
in	O
a	O
fashion	O
similar	O
to	O
Multipath	B-Protocol
TCP	I-Protocol
.	O
</s>
<s>
Within	O
the	O
IP	B-Protocol
Multimedia	I-Protocol
Subsystem	I-Protocol
(	O
IMS	O
)	O
architecture	O
,	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
can	O
support	O
the	O
concurrent	O
use	O
of	O
multiple	O
contact	O
IP	O
addresses	O
for	O
the	O
registration	O
of	O
one	O
or	O
more	O
IMS	O
user	O
agents	O
.	O
</s>
<s>
On	O
these	O
signaling	O
paths	O
,	O
signaling	O
messages	O
carry	O
Session	B-Language
Description	I-Language
Protocol	I-Language
(	O
SDP	O
)	O
messaging	O
to	O
negotiate	O
media	O
streams	O
.	O
</s>
<s>
A	O
multipath	O
extension	O
to	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
has	O
been	O
under	O
discussion	O
within	O
the	O
IETF	O
.	O
</s>
<s>
Very	O
recently	O
in	O
addition	O
a	O
proposal	O
to	O
extend	O
also	O
DCCP	O
(	O
Datagram	B-Protocol
Congestion	O
Control	O
Protocol	O
)	O
by	O
a	O
multipath	O
feature	O
is	O
discussed	O
at	O
IETF	O
in	O
TSVWG	O
(	O
Transport	O
Area	O
Working	O
Group	O
)	O
dubbed	O
as	O
MP-DCCP	O
.	O
</s>
<s>
The	O
IETF	O
is	O
currently	O
developing	O
the	O
QUIC	B-Protocol
protocol	I-Protocol
that	O
integrates	O
the	O
features	O
that	O
are	O
traditionally	O
found	O
in	O
the	O
TCP	B-Protocol
,	O
TLS	B-Protocol
and	O
HTTP	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
Thanks	O
to	O
the	O
flexibility	O
and	O
extensibility	O
of	O
QUIC	B-Protocol
,	O
it	O
is	O
possible	O
to	O
extend	O
it	O
to	O
support	O
multiple	O
paths	O
and	O
address	O
the	O
same	O
use	B-Architecture
cases	I-Architecture
as	O
Multipath	B-Protocol
TCP	I-Protocol
.	O
</s>
<s>
A	O
first	O
design	O
for	O
Multipath	O
QUIC	B-Protocol
has	O
been	O
proposed	O
,	O
implemented	O
and	O
evaluated	O
.	O
</s>
<s>
Parallel	O
access	O
schemes	O
used	O
to	O
accelerate	O
transfers	O
by	O
taking	O
advantage	O
of	O
HTTP	B-Protocol
range	I-Protocol
requests	I-Protocol
to	O
initiate	O
connections	O
to	O
multiple	O
servers	O
of	O
a	O
replicated	O
content	O
,	O
are	O
not	O
equivalent	O
to	O
Multipath	B-Protocol
TCP	I-Protocol
as	O
they	O
involve	O
the	O
application	O
layer	O
and	O
are	O
limited	O
to	O
content	O
of	O
known	O
size	O
.	O
</s>
