MPLS-Enabled Applications: Emerging Developments and New Technologies

3.3. FAILURE DETECTION

The ability to detect that a failure has happened is the first step towards providing recovery and therefore is an essential building block for providing traffic protection. Some transmission media provide hardware indications of connectivity loss. One example is packet-over-SONET/SDH (synchronous digital hierarchy), which is widely used in the network backbones and where a break in the link is detected within milliseconds at the physical layer. Other transmission media do not have this capability, e.g. Ethernet, which is commonly used in PoPs.^[]

^[] The fast detection capability has been added for optical Ethernet.

When failure detection is not provided in the hardware, this task can be accomplished by an entity at a higher layer in the network. Let us take a look at the disadvantages of doing so, using IGP hellos as an example. The IGPs send periodic hello packets to ensure connectivity to their neighbors. When the packets stop arriving, a failure is assumed. There are two reasons why hello-based failure detection using IGP hellos cannot provide fast detection times:

1. The architectural limits of IGP hello-based failure detection are 3 seconds for OSPF and 1 second for ISIS. In common configurations, the detection times range from 5 to 40 seconds.

2. Handling IGP hellos is relatively complex, so raising the frequency of the hellos places a considerable burden on the CPU.

The heart of the matter is the lack of a hello protocol to detect the failure at a lower layer. ...