**Connected: An Internet Encyclopedia**

*8.1 Probability of Collision*

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8. SSRC Identifier Allocation and Use

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**Next:** 8.2 Collision Resolution and Loop Detection

### 8.1 Probability of Collision

8.1 Probability of Collision
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
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.

**Next:** 8.2 Collision Resolution and Loop Detection

**Connected: An Internet Encyclopedia**

*8.1 Probability of Collision*