RADIUS

Name

Port-Limit

Synopsis

Attribute Number	62
Length	6
Value	INTEGER
Allowed in	Access-Accept, Access-Request
Prohibited in	Access-Reject, Access-Challenge
Presence in Packet	Not required
Maximum Iterations	1

The value of this attribute dictates the upper limit on the number of ports that NAS is authorized to give to the client. In practice, the use of this packet is most often found in support for bonding channels together with ISDN or for multilink point-to-point (MLPPP) protocol, which allows a user to aggregate two modems and phone lines into one IP channel.

There are a couple of caveats to the implementation of the Port-Limit attribute. The problem lies squarely in the fact that the enforcement of this attribute is done at the NAS machine, not at the RADIUS server. In implementations where there is more than one NAS machine, the effective port-limit would be the number of NAS machines present multiplied by the value in Port-Limit. Realistically, some sort of mechanism is needed to keep track of active logins over the entire network lest the efficacy of the Port-Limit attribute be reduced to zero. This exemplifies the need for some sort of third-party session management software, especially in large, distributed networks.