Sunday, May 27, 2007

PPPoE Circuit ID tagging feature in DSLAM

This is the latest Issue I am dealing with - I am suppose test this feature in the DSLAM project I am working in !!!

In an Ethernet access network, there is no unique mapping between the subscriber and the ADSL port. This cause problems in RADIUS access and accouting because the RADIUS server expect the BRAS to send information about the ADSL port it is authenticating and accoring for.

How this work - During the authentication phase the BRAS includes the NAS-Port-Id attribute(Radius attribute 87) in RADIUS authentication and RADIUS Accounting request that identifies the DSL line of the subscriber.

To over come this problem DSL Forum TR-101 proposes that the DSLAM sends the DSL Line-Id in the PPP over Ethernet (PPPoE) discovery phase pkts.

RFC 2516 – Defines A Method for Transmitting PPP Over Ethernet (PPPoE) ,

TR – 101 specifies that the Vender specific TAG 0x0105 in PPPoE Discovery pkts should be added by the DSLAM :

0x0105 Vendor-Specific
This TAG is used to pass vendor proprietary information. The first four octets of the TAG_VALUE contain the vendor id and the remainder is unspecified. The high-order octet of the vendor id is 0 and the low-order 3 octets are the SMI Network Management Private Enterprise Code of the Vendor in network byte order, as defined in the Assigned Numbers RFC [4]. Use of this TAG is NOT RECOMMENDED. To ensure inter-operability, an implementation MAY silently ignore a Vendor-Specific TAG.

Vender specific TAG 0x0105 will look like this after DSLAM TAG the PPPoE pkt with the Circuit-ID

This Tag will be Identified by the BRAS (PPPoE server) and send to the RADIUS server with Authentication request and the Accounting request as RADIUS attribute 87 (in rfc2869)

5.17. NAS-Port-Id Description
This Attribute contains a text string which identifies the port of the NAS which is authenticating the user. It is only used in Access-Request and Accounting-Request packets. Note that this is using "port" in its sense of a physical connection on the NAS, not in the sense of a TCP or UDP port number. Either NAS-Port or NAS-Port-Id SHOULD be present in an Access- Request packet, if the NAS differentiates among its ports. NAS- Port-Id is intended for use by NASes which cannot conveniently number their ports. A summary of the NAS-Port-Id Attribute format is shown below. The fields are transmitted from left to right.
Type 87 for NAS-Port-Id.

to come up with above content I referred the documents given below :

rfc4679 - Vendor-Specific RADIUS Attributes
rfc2865 – RADIUS Authentication
rfc2866 - RADIUS Accounting
rfc2869 - RADIUS Extensions
rfc2516 - A Method for Transmitting PPP Over Ethernet (PPPoE)
TR-101 - Migration to Ethernet-Based DSL Aggregation
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