The S (or ???bottom of stack???) bit is set to 1 for the entry in the label stack closest to
the payload and to 0 for all previous labels.
?– There are two basic QoS models in MPLS. The E-LSP model uses the EXP (or
???experimental???) field to infer QoS. The L-LSP model infers QoS from the label and,
optionally, also from the EXP field. Most routers and networks only support the
E-LSP model. There have also been other suggestions as to the use of the EXP field
(such as for congestion notification) but none has been standardised.
?– The outermost label (the ???top??? label in the stack) is looked up at each LSR along
the LSP. The lookup determines the next hop LSR for the packet and also the label
operation to be performed on the packet. The supported operations are
?– Swap Replace the outermost label with a new label. Intermediate routers
along an LSP will always perform a swap operation.
?– Push Add one or more label stack entries to the top of the label stack. At
the ingress, LSR labels are always pushed. At intermediate LSRs along an LSP,
labels may be pushed after first swapping the outermost label.
?– Pop The outermost label stack entry is removed.
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