Tutorial

Advanced MPLS Signaling
(continued)

To help overcome the problems of rerouting congestion, the Internet Engineering Task Force (IETF) and related working groups have looked at several possible solutions.  This problem had to be addressed both in protocols and in the software systems built into the routers.

In order to have full  QoS, a system must be able to mark, classify, and police traffic. From previous articles, we see how MPLS can classify and mark packets with labels, but the policing function has been missing.  Routing and label distribution establishes the Label Switch Paths, but still it does not police traffic and control the load factors on each link.

New software engines, which add management modules between the routing functions and the path selector, allow for the policing and management of bandwidth. These functions, along with the addition of two protocols, allow for traffic policing.

Figure 5: MPLS Routing State Machines

The two protocols that give MPLS the ability to police traffic and control loads are RSVP-TE and CR-LDP.

RSVP-TE

The concept of a call set-up process, wherein resources are reserved before calls are established, goes back to the signaling-theory days of telephony.  This concept was adapted for data networking when QoS became an issue.

An early method designed by the IETF in 1997, called Resource ReSerVation Protocol (RSVP), was designed for this very function.  The protocol was designed to request required bandwidth and traffic conditions on a defined or explained path.  If the bandwidth was available under the stated conditions, then the link would be established.

The link was established with three types of traffic that were similar to first-class, second-class and standby air travel – the paths were called, respectively: guaranteed load, controlled load and best-effort load. 

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