Special-Purpose Label Terminology
updates
Extracted elements (13)
The ELI rule in RFC 7274 was revised to remove the special carve-out that allowed ELI (label 7) to appear after an XL. The original carve-out existed for backward compatibility with deployed hardware that detects ELI without checking the preceding label, but this conflicted with the general rule that 0–15 are invalid after XL and caused implementer confusion.
The term 'Reserved Labels' was deprecated by RFC 7274 because 'Reserved' means 'set aside, not to be used', but those labels were actually available for allocation. 'Special-Purpose Labels' (SPLs) was introduced as a replacement term.
The terminology hierarchy — SPL (collective), bSPL (0–15), eSPL (16–1048575), xSPL (value 15 used as extension), and cSPL (XL + eSPL) — was introduced to provide consistent, unambiguous naming for specifications and documentation of MPLS special-purpose labels.
A preexisting implementation searching for the Entropy Label Indicator (ELI, label 7) may fail to inspect the previous label and not detect that it is an XL. Such systems can continue processing entropy information and forward the packet without immediate harm, but the packet will be dropped when the XL reaches the top of the stack at another LSR.
An LSR processing a packet with an XL at the top of the label stack immediately followed by a label with a value in the range 0–15 MUST drop the packet.
Values 0–15 of the Extended Special-Purpose MPLS Label Values registry are reserved and MUST NOT be used as eSPLs. An implementation MUST NOT place a label with a value in the range 0–15 in the label stack immediately following an XL.
Base Special-Purpose Labels (bSPLs) occupy label values 0–15 as defined in RFC 3032. They are one class of MPLS Special-Purpose Labels.
Extended Special-Purpose Labels (eSPLs) occupy the range 0–1048575 of the Extended SPL space. Values 0–15 are reserved and never allocated; 16–239 are available for allocation; 240–255 are for experimental use; 256–1048575 are currently unavailable without a new Standards Track RFC.
The Extension Label (XL) has value 15 and is both a bSPL and is also called the xSPL. The combination of XL and an eSPL is called a Composite Special-Purpose Label (cSPL).
IANA renamed the registry previously called 'Special-Purpose MPLS Label Values' to 'Base Special-Purpose MPLS Label Values' to align with the bSPL terminology introduced by this document.
IANA updated the Base Special-Purpose MPLS Label Values registry: the description for value 15 was changed from 'Extension Label' to 'Extension Label (XL)', and RFC 9017 was added as an additional reference alongside RFC 7274.
RFC 9017 only changes terminology and clarifies label handling rules. It does not affect MPLS data-plane forwarding, Label Switched Path establishment by control planes or centralized controllers, and does not introduce or describe new SPL vulnerabilities.
A bSPL occupies a single MPLS label stack entry (32 bits). A cSPL occupies two consecutive label stack entries: the XL (value 15) followed immediately by the eSPL value.