Egress Control was an early GMPLS requirement. As GMPLS evolved, the ability to control a label on an egress interface was generalized to support any interface (RFC 3471 Section 6, RFC 3473 Section 5.1), but the original egress-specific intent became unclear, motivating this clarification document.

ERO-based egress control is needed beyond simple IP addressing when the outgoing interface is a component link of a link bundle, or when 'splicing' two LSPs together (tail of first LSP into head of second), which is more common in non-PSC link classes.

This document complements but does not modify the procedures in Sections 5.1.1 and 5.2.1 of RFC 3473. It only clarifies procedures related to the label used on an egress output/downstream interface.

Any errors encountered while processing the ERO, including if the listed label(s) are not acceptable or cannot be supported in forwarding, SHOULD result in the generation of a PathErr message with error code 'Routing Error' and error value of 'Bad Explicit Route Object'.

If an ERO is used to specify outgoing interface information at the egress and label recording is indicated for the LSP, the egress should include the specified interface information and the specified label or labels in the corresponding RRO.

When the U-bit of the ERO subobject being examined is clear (0), the value of the label MUST be used for transmitting traffic associated with the LSP on the indicated outgoing/downstream interface.

Egress Control occurs when the node processing an ERO is the egress and the ERO contains one or more subobjects related to the output/downstream interface, which is indicated as the last listed local interface. The interface may be numbered or unnumbered.

The Egress Label subobject may appear only as the last subobject in the ERO/ER. Its appearance anywhere else in the ERO/ER is treated as a 'Bad strict node' error.

The U-bit in an ERO label subobject distinguishes downstream (U=0) from upstream (U=1) label assignment for bidirectional LSPs. For unidirectional LSPs, one subobject is examined; for bidirectional LSPs, two subobjects are examined.

When a U-bit=1 subobject is examined, the label value is used for upstream traffic of a bidirectional LSP — specifically, traffic that will be received on the indicated outgoing/downstream interface.

This document clarifies procedures defined in RFC 3473 but defines no new procedures and therefore introduces no new security considerations.

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