VC Merging MPLS OVER ATM

THE LABEL DISTRIBUTION PROTOCOL LDP 151 neighbors are: • lsr170,0 for LSR B, LSR C both interfaces, and LSR F GbE interface • lsr170,1 for LSR D for the ATM interface • lsr170,2 for LSR E for the first ATM interface • lsr170,3 for LSR E for the second ATM interface • lsr170,4 for LSR F for the ATM interface An LDP session is set up between two directly connected LSRs to support the exchange of LDP messages between them. An LDP session between two LSRs is associated with a label space. For the example given above, the following sessions are set up: • A-B: 1 LDP session for lsr170,0 • A-C: 1 LDP session for lsr170,0 • A-D: 1 LDP session for lsr170,1 • A-E: 2 LDP sessions; one for lsr170,2 and one for lsr170,3 • A-F: 2 LDP sessions; one for lsr170,0 and one for lsr170,4 It is also possible to set up an LDP session between two non-directly connected LSRs. This can be useful when two distant LSRs might want to communicate via an LSP. The two LSRs can establish a session in order to communicate a label binding. This label can be pushed down the label stack as in the example discussed 6.2.4. An LDP discovery mechanism enables an LSR to discover potential LDP peers i.e., other LSRs which are directly connected to it. An LSR sends periodically LDP link hellos out of each interface. Hello packets are sent over UDP addressed to a well-known LDP discovery port for the “all routers on this subnet” group multicast address. An LDP link hello sent by an LSR carries the label space id that the LSR wants to use for the interface and possibly additional information. Receipt of an LDP link hello identifies a hello adjacency . For each interface, there is one hello adjacency. An extended discovery mechanism can be used for non-directly connected LSRs. An LSR periodically sends LDP targeted hellos to a specific address over UDP. Receipt of an LDP targeted hello identifies a hello adjacency. The exchange of LDP link hellos between two LSRs triggers the establishment of an LDP session. If there is a single link between two LSRs, then a single hello adjacency and a single LDP session are set up. If there are parallel links with per platform label space, then there are as many hello adjacencies as the number of links, but only one LDP session. If there are parallel links, one with per platform label space and the others per interface label space, then one session per interface and one adjacency per session is set up. For the example in Figure 7.1, the following sessions and hello adjacencies are set up: • A-B: One LDP session with one hello adjacency • A-D: One LDP session with one hello adjacency • A-C: One LDP session with two hello adjacencies • A-E: Two LDP sessions, each associated with one hello adjacency • A-F: Two LDP sessions, each associated with one hello adjacency The establishment of an LDP session involves two steps. First, a TCP session is established. Second, an LDP session is initialized, during which the two LSRs negotiate 152 LABEL DISTRIBUTION PROTOCOLS session parameters such as protocol version, label distribution method, timer values, range of VPIVCI values for ATM, and range of DLCI values for frame relay. An LSR maintains a timer for each hello adjacency, which it restarts each time it receives a hello message. If the timer expires without having received a hello message from the peer LSR, the hello adjacency is deleted. The LDP session is terminated, if all hello adjacencies associated with an LDP session are deleted. An LSR maintains a keepAlive timer for each session. The timer is reset each time it receives any LSP PDU from its LDP peer. If the LDP peer has nothing to send, it sends a keepAlive message.

7.1.2 The LDP PDU Format

An LDP PDU consists of an LDP header followed by one or more LDP messages which might not be related to each other. The LDP PDU format is shown in Figure 7.2. The LDP header consists of the fields: • Version : A 16-bit field that contains the protocol version. • PDU length : A 16-bit field that gives the total length of the LDP PDU in bytes, excluding the version and PDU length fields of the LDP PDU header. • LDP id : A 48-bit field that contain the LDP id that is, the label space id which has the form 32-bit router id, label space number. The LDP message format consists of a header followed by mandatory and optional parameters. The header and the parameters are all encoded using the type-length-value TLV scheme shown in Figure 7.3. The following fields have been defined: LDP header LDP message 1 LDP message N • • • Figure 7.2 The LDP PDU format. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 U F Type Length Value • • • Figure 7.3 The TLV format.