Advanced MPLS Signaling

Rick Gallaher is course director for CCI, President of Telecommunications Technical Services Inc., and author of  Rick Gallaher's MPLS Training Guide

December 10, 2001

In previous tutorials, we talked about data flow (Tutorial #1) and label distribution (Tutorial #2).  This article discusses MPLS signaling and the ongoing conversations regarding signaling choices.

Vocabulary

• Soft State – A link, path, or call that needs to be refreshed to stay alive.

• Hard State – A link, path, or call that will stay alive until it is specifically shut down.

• Explicit Route – A path across the Internet wherein all routers are specified. Packets must follow this route, and they cannot detour.

• CR-LDP – Constraint-based Routing over Label-Distribution Protocol.

• RSVP-TE – The Resource ReSerVation Protocol (RSVP), modified to handle MPLS traffic-engineering requirements.

• IntServ – Integrated Service; allows traffic to be classified into three groups: guaranteed, controlled load, and best effort.  IntServ works together with RSVP protocol.

Your commute to work every day is a long one, but with all the congestion it seems to take forever. New lanes have been added to the highway, but they are reserved as express lanes – sure, they will cut your travel time in half, but you will have to carry extra passengers in order to use them. You decide, finally, to try it; you decide to carry four additional passengers in order to use the express lane.  You are permitted to pass through the express-lane gate and scurry on your way to and from work.

The four passengers do not cost much more to transport than yourself alone, and they really allow you to increase the speed and lower the rate of interference from the unpredictable and impossible-to-correct behavior of the routine traffic. (Figure 1)

Figure 1: Backed Up Express Lane

One day you enter the express lanes and find that they are all in a state of bumper-to-bumper congestion. You look around and find routine traffic in the express lanes. You are angry, of course, because you had guaranteed express lanes, and the routine traffic is required to stay off the express lanes unless they are carrying extra passengers.   As you slowly progress down your road, you see that construction has closed down the routine lanes and diverted the traffic to your express lanes.  So, what good is it to be special if regular traffic is diverted to your express lanes?

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