Internet Service Providers
126
20.1.4. Broadband
In a typical broadband network we don't have dialup modems in the same. The modems are jumpered to the
phone line at the exchanged and are part of an Access Node, usually called a DSLAM or MSAN. This then
passes PPP packets on to a Remote Access Server, usually called a BRAS. The link from DSLAM to BRAS is
typically PPPoE. The BRAS acts as the LAC and connects to an ISPs LNSs.
PPPoE is PPP over Ethernet. Some access networks use DSL to carry PPP packets directly (PPPoA), and some
use the ADSL as an Ethernet Bridge (PPPoE). There are access networks which provide Ethernet by some
means to the end user equipment which then commincations via PPPoE to the BRAS. All of these work in
much the same way at the BRAS as it sees PPPoE connections.
Typically the BRAS provides the initial proxy negotiation and then establishes an L2TP connection, after which
it is no longer involved in any negotiation, but just passes on PPP packets each way.
20.1.5. RADIUS
Remote Access Dial Up Server is a system that allows the authentication decisions and allocation of IP addresses
to be passed on to separate servers rather than being configured in to the various equipment. RADIUS uses
UDP to send a request to a server and send a reply back.
RADIUS is used within carrier networks so that the BRAS can check to where it is to send an L2TP connection.
The RADIUS response can contain the tunnel details it needs, including the authentication within L2TP.
RADIUS is also used between carrier and an ISP. The carrier will send a RADIUS request to the ISP asking
the ISP for details of the LNS to which the connection is to be sent. This allows the ISP to steer sessions as
they need.
Once the LNS gets the L2TP connection, RADIUS is used to obtain the IP address details to be assigned to
the specific connection.
RADIUS is also used for accounting, to provide details of connections in progress and volumes of data
transferred.
Appendix F provides details of the specific AVPs used with RADIUS for L2TP.
20.1.6. BGP
Once a connection is made to an LNS, the end user is assigned IP addresses. Obviously there is a need to ensure
that the IP addresses are routed within the ISPs network to the correct LNS. OSPF and BGP are the main routing
protocols used for this (though, back in dialup days, RIP and RIP2 were often used, and a bits slow). OSPF is
not ideal for this as it means the whole OSPF network tracking every connection of every user. The FireBrick
supports use of BGP to announce connected IP addresses in to an ISPs internal network as connections are
made via L2TP.
20.2. Incoming L2TP connections
To allow a connection to the FireBrick you have to decide on a
hostname
. This is not a DNS hostname and
is more like a login or username. It can be anything you like. You can pre-agree with your carrier the hostname
they will use and the IP address of your LNS. When the connection arrives the protocol includes the hostname
and a
secret
(i.e. a password. The hostname allows the FireBrick to check which connection details apply,
and the password confirms that the connection is authentic.
The FireBrick can be configured with many hostnames, which would typically be used for different carriers
to connect. You can also use the hostname to separate different types of connection - for example, in the UK,
BT have 20CN IPStream, and 21CN WBC connections which typically need separate monitoring and traffic