Overview of SQL Server Reporting Services 2012 Architecture, Features, and Tools - Learning SQL Server Reporting Services 2012 - page 159

Authentication types
All users who want to access Report Server need authentication. The following table
from Microsoft shows the authentication types available:

Authentication type
name

HTTP
authentication layer
value

Used
by
default

Description

RSWindowsNegotiate Negotiate Yes RSWindowsNegotiate

directs the Report Server
to handle authentication
requests that specify
Negotiate. Negotiate
attempts Kerberos
authentication first, but
falls back to NTLM if
Active Directory cannot
grant a ticket for the client
request to the Report Server.
Negotiate will only fall
back to NTLM if the ticket
is not available. If the first
attempt results in an error
rather than a missing ticket,
the Report Server does not
make a second.

RSWindowsNTLM NTLM Yes NTLM authenticates a user

through an exchange of
private data described as
challenge-response.
The credentials will
not be delegated or
impersonated on other
requests. Subsequent
requests will follow a
new challenge-response
sequence. Depending on
network security settings,
a user might be prompted
for credentials or the
authentication request will
be handled transparently.