Hello,
I couldn't find the protocol, but the list of domains in the source code:
('162.254.195.44', 27019), ('146.66.152.11', 27017),
('162.254.196.40', 27017), ('146.66.152.11', 27018),
('162.254.197.41', 27017), ('162.254.197.42', 27018),
('146.66.152.10', 27018), ('162.254.196.41', 27020),
('185.25.180.14', 27018), ('162.254.196.43', 27021),
('146.66.155.8', 27018), ('162.254.196.40', 27018),
('162.254.197.41', 27021), ('162.254.197.42', 27017),
('185.25.180.14', 27017), ('185.25.182.10', 27018),
('162.254.196.40', 27021), ('208.78.164.9', 27019),
('208.78.164.12', 27018), ('162.254.196.40', 27019),
('162.254.197.41', 27020), ('208.78.164.14', 27018),
('162.254.195.46', 27019), ('162.254.197.40', 27018),
('155.133.242.8', 27020), ('162.254.196.41', 27018),
('208.78.164.14', 27019), ('208.78.164.12', 27017),
('162.254.196.40', 27020), ('162.254.196.42', 27021)
these are the domains it tries to connect to.
Documentation can be found here: http://steam.readthedocs.io/en/stable/