Information Technology Reference
In-Depth Information
S field —When the Bottom of Stack (S) bit is set to 1, the current label is on the bottom
of the stack. This allows multiple labels to be encoded into the same packet to form a
label stack. With this definition, an ordinary IP packet can be thought of as a labeled
packet with zero depth of label stack.
TTL field —The 8-bit Time to Live (TTL) field is used to encode a time-to-live
value. If a labeled packet's outgoing TTL is 0, the packet's lifetime in the network is
considered to have expired. The packet should not be forwarded further either labeled
or unlabeled.
To create a labeled frame, a label is inserted into a section of the frame header that imme-
diately precedes the Layer 3 header. This inserted label is often called the shim header . The
shim header is used for links such as PPP, Packet over SONET (POS), Ethernet, and packet-
based Asynchronous Transfer Mode (ATM) links.
To indicate a shim MPLS header, a new protocol number has been defined for MPLS. The
MPLS protocol ID for PPP is 0x0281 for unicast and 0x0283 for multicast. The MPLS
EtherType is 0x8847 for unicast and 0x8848 for multicast.
In a cell-based network such as ATM, cell headers are used to carry label information.
Specifically, they are virtual path identifier (VPI) and virtual circuit identifier (VCI). An
ATM LSR is typically a conventional ATM switch appended with a Layer 3 router.
When MPLS is applied to non-packet-based networks, such as optical networks, the
definition of label is generalized. A generalized label can be a physical port, a wavelength,
or a SONET or Synchronous Digital Hierarchy (SDH) circuit. This form of MPLS is now
called Generalized MPLS (GMPLS).
In all three networks, there always exists a control channel between LSRs to initialize LSP
setup. This channel is in the form of unlabeled packets in frame-based networks, a control
virtual circuit (VC) in cell-based networks, or a special circuit in non-packet-based
networks.
Label Exchange and LSP Setup
The labeled packets are forwarded along preestablished LSPs. An LSP is a forwarding path
from the ingress LER to the egress LER that associates labels with destination prefixes and
appropriate encapsulation information. To set up an LSP, label binding information must be
exchanged among LSRs using the control channel. Here, label binding or mapping means
the mapping of labels to prefixes and neighbors that advertised the labels.
There are several ways to exchange labels for unicast prefixes. For IGP prefixes, there is
the standard Label Distribution Protocol (LDP) as well as Cisco's Tag Distribution Protocol
(TDP). Multiprotocol BGP can carry label information for BGP prefixes. Resource
Reservation Protocol (RSVP) is extended to exchange label information for MPLS traffic
engineering tunnels.
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