IETF RFC 7274 PDF

Some MPLS labels have been allocated for specific purposes. A block of labels (0-15) has been set aside to this end; these labels are commonly called "reserved labels". They will be called "specialpurpose labels" in this document.

As there are only 16 of these special-purpose labels, caution is needed in the allocation of new special-purpose labels; yet, at the same time, forward progress should be allowed when one is called for.

This memo defines new procedures for the allocation and retirement of special-purpose labels, as well as a method to extend the specialpurpose label space and a description of how to handle extended special-purpose labels in the data plane. Finally, this memo renames the IANA registry for special-purpose labels to "Special-Purpose MPLS Label Values" and creates a new registry called the "Extended Special-Purpose MPLS Label Values" registry.

This document updates a number of previous RFCs that use the term "reserved label". Specifically, this document updates RFCs 3032, 3038, 3209, 3811, 4182, 4928, 5331, 5586, 5921, 5960, 6391, 6478, and 6790.