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CR-LDP depends on the following minimal LDP functionality:
Basic and/or extended discovery mechanism
Label request message for downstream on demand with ordered control
Label mapping message for downstream on demand with ordered control
Label withdraw and release messages
Loop detection for loosely routed segments
7.2.1 CR-LSP Setup Procedure
A CR-LSP is set up using downstream on demand allocation with ordered control. Recall that in the downstream on demand allocation scheme, each LSR binds an incoming label to a FEC and creates an appropriate entry in its LFIB. However, it does not advertise its label mapping to its neighbors as in the unsolicited downstream allocation scheme. Instead, an upstream LSR obtains the label mapping by issuing a request. In the ordered control scheme, the allocation of labels proceeds backwards from the egress LSR towards the ingress LSR. Specifically, an LSR only binds a label to a FEC if it is the egress LSR for that FEC, or if it has already received a label binding for that FEC from its next hop LSR.
An example of how a CR-LSP is set up is shown in Figure 7.10. Let us assume that LSR A has been requested to establish a CR-LSP to LSR E. A request to set up a CR-LSP to LSR E might originate from a management system or an application. LSR A calculates the explicit route using information provided by the management system, or the application, or from a routing table, and creates the label request message. The explicit
THE CONSTRAINED-BASED ROUTING LABEL DISTRIBUTION PROTOCOL 159
A B C D E
Label request message
Label - mapping
Figure 7.10 An example of a CR-LSP setup.
route in this case is given by the series of LSRs B, C, and D. It is carried in a special TLV in the label request message called the explicit route TLV (ER-TLV).
The ingress LSR A sends the label request message to LSR B, the first LSR indicated in the ER-TLV, requesting a label mapping for the FEC associated with the CR-LSP. Because of the ordered control scheme, LSR B cannot create a label mapping to the FEC until it has received a label mapping from its next hop LSR C. Also, because of the downstream on demand allocation scheme, LSR C does not advertise its label mappings to its neighbors. In view of this, LSR B forwards the label request message to LSR C requesting a label mapping for the FEC. LSR C forwards the label mapping request to LSR D for the same reasons, which then forwards the label request message to the egress LSR E. The egress LSR E is now allowed to create a label mapping to the FEC. It does so, and it responds to LSR Ds label request message with a label mapping message that contains the allocated label. When LSR D receives the label mapping message form LSR E it responds to LSR Cs label request message with a label mapping message that contains its own incoming label, and so on, until LSR A receives a label mapping message form LSR B. At that time, the CR-LSP has been set up.
Next, the label request message and the label mapping message are described.
7.2.2 The Label Request Message
The label request message is shown in Figure 7.11. The U bit is set to 0, and the message type set to label request (0x0401). The FEC TLV (see Figure 7.7) must be included in the label request message, and it contains a single new FEC element, named CR-LSP.
The LSPID TLV is required and is used to give a unique identifier of a CR-LSP. It is composed of the ingress LSR router id (or any of its IPv4 addresses) and a CR-LSP id which is locally unique to that LSR. The LSPID is useful in network management, in CR-LSP repair, and in using an already established CR-LSP as a hop in an ER-TLV.
The explicit-route TLV (ER-TLV), shown in Figure 7.12, is used to specify the path to be taken by the LSP being established. It is composed of one or more explicit route hop TLVs (ER-hop TLV) which have the format shown in Figure 7.13. The type field indicates the type of the ER-hop contents, and it can take one of the following values: IPv4 prefix, IPv6 prefix, autonomous system number, LSPID. If an LSR receives a label
LABEL DISTRIBUTION PROTOCOLS
0 12 3
Label request (0x0401)
LSPID TLV (mandatory)
Traffic parameters TLV (optional)
Pinning TLV (optional)
Resource class TLV (optional)
Preemption TLV (optional)
Figure 7.11 The CR-LDP label request message.
Figure 7.12 The ER-TLV.
0 1 2 3
0 0 Type Length
L Content //
Figure 7.13 The ER-hop TLV.