a=ice-ufrag:[value]
The "ice-pwd" and "ice-ufrag" attributes can appear at either the
session-level or media-level. When present in both, the value in the
media-level takes precedence. Thus, the value at the session-level is
effectively a default that applies to all media streams, unless overridden by
a media-level value. Whether present at the session or media-level, there
MUST be an ice-pwd and ice-ufrag attribute for each media stream. If two
media streams have identical ice-ufrag's, they MUST have identical ice-pwd's.
The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the beginning
of a session. The ice-ufrag attribute MUST contain at least 24 bits of
randomness, and the ice-pwd attribute MUST contain at least 128 bits of
randomness. This means that the ice-ufrag attribute will be at least 4
characters long, and the ice-pwd at least 22 characters long, since the
grammar for these attributes allows for 6 bits of randomness per character.
The attributes MAY be longer than 4 and 22 characters, respectively, of
course, up to 256 characters. The upper limit allows for buffer sizing in
implementations. Its large upper limit allows for increased amounts of
randomness to be added over time.