6.3.2. RLOC-Probing Algorithm

6.3.2. RLOC-Probing Algorithm

RLOC-Probing is a method that an ITR or PITR can use to determine the reachability status of one or more Locators that it has cached in a Map-Cache entry. The probe-bit of the Map-Request and Map-Reply messages is used for RLOC-Probing.

RLOC-Probing is done in the control plane on a timer basis, where an ITR or PITR will originate a Map-Request destined to a locator address from one of its own locator addresses. A Map-Request used as an RLOC-probe is NOT encapsulated and NOT sent to a Map-Server or to the mapping database system as one would when soliciting mapping data. The EID record encoded in the Map-Request is the EID-Prefix of the Map-Cache entry cached by the ITR or PITR. The ITR may include a mapping data record for its own database mapping information that contains the local EID-Prefixes and RLOCs for its site. RLOC-probes are sent periodically using a jittered timer interval.

When an ETR receives a Map-Request message with the probe-bit set, it returns a Map-Reply with the probe-bit set. The source address of the Map-Reply is set according to the procedure described in Section 6.1.5. The Map-Reply SHOULD contain mapping data for the EID-Prefix contained in the Map-Request. This provides the opportunity for the ITR or PITR that sent the RLOC-probe to get mapping updates if there were changes to the ETR's database mapping entries.

There are advantages and disadvantages of RLOC-Probing. The greatest benefit of RLOC-Probing is that it can handle many failure scenarios allowing the ITR to determine when the path to a specific Locator is reachable or has become unreachable, thus providing a robust mechanism for switching to using another Locator from the cached Locator. RLOC-Probing can also provide rough Round-Trip Time (RTT) estimates between a pair of Locators, which can be useful for network management purposes as well as for selecting low delay paths. The major disadvantage of RLOC-Probing is in the number of control messages required and the amount of bandwidth used to obtain those benefits, especially if the requirement for failure detection times is very small.

Continued research and testing will attempt to characterize the tradeoffs of failure detection times versus message overhead.