Schemes for Setting up an LSP

150 LABEL DISTRIBUTION PROTOCOLS LDP runs on top of TCP for reliability, with the exception of the LDP discovery messages that run over UDP. Before we proceed to describe the LDP messages and their format, we divert to discuss in the following section several LDP concepts, such as the per platform and per interface label space, LDP session , and hello adjacencies. The following section can be skipped in the first reading.

7.1.1 Label Spaces, LDP Sessions, and Hello Adjacencies

LDP makes use of the concept of the label space, which is the set of all labels. Two types of label space have been defined: per interface label space and per platform label space . The per interface label space is a set of labels which are specific to a particular interface. For instance, an ATM interface uses VPIVCI numbers which are specific to the interface. Likewise, a frame relay interface uses DLCI values which are also specific to the interface. The per platform label space is a set of labels shared by all interfaces other than ATM and frame relay interfaces, such as packet-over-SONET PoS and gigabit Ethernet GbE . When transmitting packets over these links, the MPLS labels are carried in the special label encapsulation see Figure 6.3. These labels are processed by the same MPLS software, and they all belong to the same per-platform label space. An LSR label space is identified by a 6-byte value. The first four bytes carry a globally unique value identifying the LSR, such as a 32-bit router id that has been assigned to the LSR by the autonomous system administrator. The last two bytes identify the label space. If the label space is per platform, then the last two bytes are set to 0. A label space id, formally referred to as an LDP id, is expressed as LSRDid, label space number. An example of label space ids is given in Figure 7.1. The LSR id number for LSR A is lsr170 . LSR A is connected to LSRs B, C, D, E, and F. It connects to B via the A GbE interface. It connects to C via a GbE interface, and also via a separate PoS interface. It connects to D via an ATM interface. It connects to E via two separate ATM interfaces. It connects to F via a GbE interface and a separate ATM interface. LSR A advertises the per-platform label space id lsr170,0 to B and C. It advertises the non-zero label space lsr170,1 to D. It advertises the non-zero label space id lsr170,2 over the first ATM link and lsr170,3 over the second ATM link to E. It advertises the non-zero label space lsr170,4 over the ATM link and the per-platform label space id lsr170,0 over the GbE interface to F. In summary, the label space ids that A advertises to its B A E C F D GbE ATM GbE PoS ATM ATM GbE ATM Figure 7.1 An example of label space ids.