Side_1_360

• S – 1 bit indicates end of label stacking as sev-
  eral labels may be stacked;


• TTL – 8 bit giving the Time To Live informa-
  tion.

When an MPLS packet enters a Label Switching

Router(LSR) a table containing information,

Label Information Base(LIB) on further treat-

ment of the packet is looked up. This is illus-

trated in Figure 23. This base may also be re-

ferred to as the Next Hop Label Forwarding

Entry(NHLFE), which typically contains

the following information (ref. [RFC3031]):

• Next hop of the packet;


• Operation to perform on the packet’s label
  stack (replace the label at the top with another
  label, pop the label stack, or replace the label
  at the top of the stack with a new label and at
  the same time push one or more new labels
  onto the stack);


• Data link encapsulation to use when transmit-
  ting the packet;


• Way of encoding the label stack when trans-
  mitting the packet;


• Other information relevant to forwarding
  treatment.

In a given LSR, the “next hop LSR” may be

the same LSR, implying that the top level label

should be popped and the packet “forwarded” to

itself, allowing or more forwarding decisions.

At the ingress of an MPLS domain an FEC-to-

NHLFE mapping is needed, that is when packets

arrive without an MPLS label.

Within an MPLS domain an incoming label

mapping is executed, mapping the packet onto

a set of NHLFEs.

MPLS can operate on a label stack. Operations

on this stack are push, pop and swap. This can

be used to merge and split traffic streams. The

push operation adds a new label at the top of the

stack and the pop operation removes one label

from the stack. The MPLS stack functionality

can be used to aggregate traffic trunks. A com-

mon label is added to the stack of labels. The

result is an aggregated trunk. When this MPLS

path is terminated the result will be a splitting

(de-aggregation) of the aggregated trunk into

its individual components. Two trunks can be

aggregated in this way if they share a portion of

their path. Hence, MPLS can provide hier-

archical forwarding, which may become an

important feature. A consequence may be that

the transit provider need not carry global routing

information, thus making the MPLS network

more stable and scalable than a full-blown

routed network.

To limit the number of MPLS paths, merging

can be utilised. Then two paths in the same

direction and with common requirements are

placed together in a common LSP on the out-

going side resulting in a many-to-one mapping

of labels.

Figure 23 Illustration of

information attached to an

LSP in an LSR

Ingress LSR

packet

Transit LSR Egress LSR

IP packet header

MPLS header

Outgoing:

• interface


• label
  Label Information Base

Forwarding

Packet ForwardEquivalence Class Traffic trunk LabelSwitched Path Physicalnetwork

Forward

Equivalence

Class

Incoming:

• interface


• label