Frame Relay Local Management Interface Optional Extensions
Optional LMI ExtensionsThe LMI specification also defines several optional extensions:
- Global addressing convention
- Multicast capability
- A simple flow control mechanism
- Ability for the network to communicate a PVC's CIR to the subscriber in a Status message
- A new message type that allows the network to announce PVC status changes without prompting from the subscriber
Global AddressingThe global addressing convention defines a simple commitment from the operator of a network that DLCIs will remain unique throughout the network. In a globally addressed network, each DLCI identifies a subscriber device uniquely.
For a few years Frame Relay networks will remain small enough that they won't need to implement extended addressing to use the global addressing feature. As networks grow and interconnect, any trend toward global addressing will probably require use of extended addresses.
MulticastingThe LMI multicast capability adapts a popular feature from the LAN world. It reserves a block of DLCIs (1019 to 1022) as multicast groups so that a subscriber wishing to transmit a message to all members of the group must transmit the message only once on the multicast DLCI.
The multicasting feature requires a new information element, Multicast Status, in the full LMI Status message. The Multicast Status element is similar in most respects to the PVC Status IE, but it includes a field for the source DLCI transmitting over the multicast group. It also omits the function of the R bit (see below), since a multicast group may use several paths with different congestion conditions.
Flow ControlThe optional LMI flow control capability provides a way for the network to report congestion to the subscriber. The flow control feature uses the optional R bit in the PVC Status information element as a "Receive-Not-Ready" signal for the PVC whose status is being reported. A 1 in the R bit indicates congestion; a 0 indicates no congestion.
On networks where LMI is fully implemented, this feature improves on the ECN bits of the basic Frame Relay protocol because the LMI heartbeat process guarantees that PVC Status elements will reach the subscriber periodically. Of course, according to the laissez faire practice of Frame Relay, the subscriber may or may not have implemented the feature, and may or may not choose to act on the information.
Communicating the Minumum Bandwidth AvailableThe next optional feature uses the three reserved octets at the end of the PVC Status information element to communicate the minimum bandwidth available on the network to the PVC.
In most implementations, this number will be the PVC's CIR. However, clever implementors and operators may begin to use this feature to respond to changing traffic conditions by dynamically increasing or decreasing the bandwidth available to individual PVCs.
The specification neither encourages nor forbids such practices.
Status Update MessageThe final optional feature of LMI allows the network to communicate changes in a PVC's status by means of a message type called Status Update without first receiving a Status Enquiry from the subscriber.
The Status Update contains only PVC Status and Multicast Status information elements, so it cannot function in the heartbeat process. Further, it contains Status elements for only those PVCs and multicast groups whose status has changed.
Changes reported include:
- Deletion of a PVC or multicast group (reported by setting the optional D bit of the Status element)
- Changes in the minimum bandwidth allocated to a PVC
- Activation or deactivation of a PVC (indicated by setting or clearing the A bit)
- Flow control information (changes in congestion status, signalled by setting or resetting the R bit). Besides improving flow control, this feature allows LMI signalling over network-to-network Frame Relay connections where neither partner functions as a subscriber device