BCP 128

This document describes the Equal Cost Multipath (ECMP) behavior of currently deployed MPLS networks. This document makes best practice recommendations for anyone defining an application to run over an MPLS network that wishes to avoid the reordering that can result from transmission of different packets from the same flow over multiple different equal cost paths. These recommendations rely on inspection of the IP version number field in packets. Despite the heuristic nature of the recommendations, they provide a relatively safe way to operate MPLS networks, even if future allocations of IP version numbers were made for some purpose. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.