Since the identifiers are chosen randomly, it is possible that two or
more sources will choose the same number. Collision occurs with the
highest probability when all sources are started simultaneously, for
example when triggered automatically by some session management
event. If N is the number of sources and L the length of the
identifier (here, 32 bits), the probability that two sources
Schulzrinne, et al Standards Track [Page 44]
RFC 1889 RTP January 1996
independently pick the same value can be approximated for large N
[20] as 1 - exp(-N**2 / 2**(L+1)). For N=1000, the probability is
roughly 10**-4.
The typical collision probability is much lower than the worst-case
above. When one new source joins an RTP session in which all the
other sources already have unique identifiers, the probability of
collision is just the fraction of numbers used out of the space.
Again, if N is the number of sources and L the length of the
identifier, the probability of collision is N / 2**L. For N=1000, the
probability is roughly 2*10**-7.
The probability of collision is further reduced by the opportunity
for a new source to receive packets from other participants before
sending its first packet (either data or control). If the new source
keeps track of the other participants (by SSRC identifier), then
before transmitting its first packet the new source can verify that
its identifier does not conflict with any that have been received, or
else choose again.
8.2 Collision Resolution and Loop Detection
Although the probability of SSRC identifier collision is low, all RTP
implementations must be prepared to detect collisions and take the
appropriate actions to resolve them. If a source discovers at any
time that another source is using the same SSRC identifier as its
own, it must send an RTCP BYE packet for the old identifier and
choose another random one. If a receiver discovers that two other
sources are colliding, it may keep the packets from one and discard
the packets from the other when this can be detected by different
source transport addresses or CNAMEs. The two sources are expected to
resolve the collision so that the situation doesn't last.
Because the random identifiers are kept globally unique for each RTP
session, they can also be used to detect loops that may be introduced
by mixers or translators. A loop causes duplication of data and
control information, either unmodified or possibly mixed, as in the
following examples:
o A translator may incorrectly forward a packet to the same
multicast group from which it has received the packet, either
directly or through a chain of translators. In that case, the
same packet appears several times, originating from different
network sources.
o Two translators incorrectly set up in parallel, i.e., with the
same multicast groups on both sides, would both forward packets
from one multicast group to the other. Unidirectional
