<s>
VoIP	B-Protocol
spam	I-Protocol
or	O
SPIT	B-Protocol
(	O
spam	B-Protocol
over	I-Protocol
Internet	I-Protocol
telephony	I-Protocol
)	O
is	O
unsolicited	O
,	O
automatically	O
dialed	O
telephone	O
calls	O
,	O
typically	O
using	O
voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
(	O
VoIP	B-Application
)	O
technology	O
.	O
</s>
<s>
VoIP	B-Application
systems	O
,	O
like	O
e-mail	O
and	O
other	O
Internet	O
applications	O
,	O
are	O
susceptible	O
to	O
abuse	O
by	O
malicious	O
parties	O
who	O
initiate	O
unsolicited	O
and	O
unwanted	O
communications	O
,	O
such	O
as	O
telemarketers	O
and	O
prank	O
callers	O
.	O
</s>
<s>
VoIP	B-Application
calling	O
rates	O
are	O
cheap	O
,	O
and	O
the	O
technology	O
provides	O
convenient	O
,	O
often	O
free	O
tools	O
,	O
such	O
as	O
Asterisk	B-Language
and	O
other	O
applications	O
.	O
</s>
<s>
The	O
primary	O
underlying	O
technology	O
driving	O
this	O
threat	O
is	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
,	O
which	O
is	O
a	O
standard	O
for	O
VoIP	B-Application
telecommunications	O
.	O
</s>
<s>
VoIP	B-Protocol
spam	I-Protocol
is	O
characterized	O
as	O
unsolicited	O
calls	O
initiated	O
by	O
voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
systems	O
.	O
</s>
<s>
Robocalls	O
can	O
be	O
delivered	O
automatically	O
using	O
telephony	O
software	O
,	O
such	O
as	O
Asterisk	B-Language
.	O
</s>
<s>
A	O
strong	O
identification	O
of	O
the	O
caller	O
,	O
for	O
example	O
as	O
described	O
in	O
RFC	O
4474	O
,	O
helps	O
to	O
mitigate	O
SPIT	B-Protocol
.	O
</s>
<s>
Various	O
SPIT	B-Protocol
mitigation	O
methods	O
and	O
frameworks	O
have	O
been	O
proposed	O
.	O
</s>
<s>
A	O
comprehensive	O
survey	O
of	O
Voice	B-Application
over	I-Application
IP	I-Application
Security	O
Research	O
(	O
Chapter	O
IV	O
b	O
)	O
provides	O
an	O
overview	O
.	O
</s>
<s>
A	O
semi-supervised	B-General_Concept
machine	O
learning	O
tool	O
creates	O
clusters	O
of	O
similar	O
calls	O
and	O
a	O
human	O
operator	O
can	O
flag	O
any	O
given	O
cluster	O
as	O
being	O
spam	O
.	O
</s>
<s>
The	O
SPIDER	O
project	O
proposes	O
a	O
SPIT	B-Protocol
mitigation	O
architecture	O
,	O
which	O
uses	O
a	O
detection	O
layer	O
consisting	O
of	O
various	O
modules	O
and	O
a	O
decision	O
layer	O
.	O
</s>
<s>
The	O
VoIP	B-Application
SEAL	O
system	O
uses	O
different	O
stages	O
.	O
</s>
<s>
SymRank	O
adapts	O
of	O
the	O
PageRank	B-Algorithm
algorithm	I-Algorithm
and	O
computes	O
the	O
reputation	O
of	O
subscribers	O
based	O
on	O
both	O
incoming	O
and	O
outgoing	O
calls	O
.	O
</s>
<s>
SPIT	B-Protocol
detection	O
can	O
make	O
use	O
of	O
sophisticated	O
machine	O
learning	O
algorithms	O
,	O
including	O
semi-supervised	B-General_Concept
machine	O
learning	O
algorithms	O
.	O
</s>
<s>
It	O
builds	O
on	O
the	O
notion	O
of	O
clustering	O
whereby	O
calls	O
with	O
similar	O
features	O
are	O
placed	O
in	O
a	O
cluster	O
for	O
SPIT	B-Protocol
or	O
legitimate	O
calls	O
and	O
human	O
input	O
is	O
used	O
to	O
mark	O
which	O
cluster	O
corresponds	O
to	O
SPIT	B-Protocol
.	O
</s>
<s>
SPIT	B-Protocol
detection	O
and	O
mitigation	O
can	O
also	O
be	O
based	O
solely	O
on	O
the	O
caller	O
's	O
audio	O
data	O
.	O
</s>
<s>
This	O
approach	O
uses	O
audio	O
identification	O
techniques	O
(	O
similar	O
to	O
music	B-Algorithm
identification	I-Algorithm
)	O
to	O
detect	O
calls	O
with	O
identical	O
audio	O
data	O
including	O
certain	O
degradations	O
(	O
e.g.	O
,	O
noise	O
and	O
different	O
audio	O
codecs	O
)	O
.	O
</s>
<s>
A	O
robust	O
Acoustic	B-Algorithm
fingerprint	I-Algorithm
(	O
perceptual	B-Application
hashing	I-Application
)	O
is	O
derived	O
from	O
spectral	O
parameters	O
of	O
the	O
audio	O
data	O
and	O
replayed	O
calls	O
are	O
identified	O
by	O
a	O
comparison	O
of	O
fingerprints	O
.	O
</s>
<s>
Little	O
information	O
is	O
available	O
about	O
implementations	O
of	O
SPIT	B-Protocol
mitigation	O
measures	O
by	O
telephone	O
companies	O
.	O
</s>
<s>
Some	O
recent	O
smartphone	O
vendors	O
are	O
incorporating	O
notification	O
of	O
possible	O
spam	O
for	O
incoming	O
calls	O
,	O
such	O
as	O
Google	B-Application
in	O
its	O
Nexus	B-Device
Android	I-Device
devices	O
and	O
Apple	O
in	O
its	O
iOS	B-Operating_System
10	I-Operating_System
release	O
.	O
</s>
<s>
SPIT	B-Protocol
is	O
generally	O
not	O
yet	O
considered	O
to	O
be	O
a	O
problem	O
as	O
critical	O
as	O
email	O
spam	O
.	O
</s>
<s>
An	O
automated	O
analysis	O
of	O
the	O
call	O
signaling	O
flow	O
can	O
help	O
to	O
discover	O
SPIT	B-Protocol
.	O
</s>
<s>
Commercial	O
VoIP	B-Application
software	O
for	O
communication	O
service	O
providers	O
may	O
include	O
a	O
behavioral	O
analysis	O
,	O
e.g.	O
</s>
<s>
Relevant	O
parameters	O
and	O
indications	O
of	O
SPIT	B-Protocol
are	O
,	O
for	O
example	O
,	O
a	O
high	O
call	O
attempt	O
frequency	O
,	O
concurrent	O
calls	O
,	O
low	O
call	O
completion	O
and	O
low	O
call	O
duration	O
average	O
.	O
</s>
