s are then encapsulated and specific payloads appended to each LSA. In order to enable advertisement of TE attributes in opaque LSAs, the LSA payload consists of type-length-value (TLV) records. These are information records composed by three fields: two 2-byte fields, the "Type" and "Length" fields, and a variable length field, the "Value" field. Using TLVs, router addresses and TE links can be expressed. In GMPLS, if an advertising router is reachable, a "router address"-TLV can be used to describe a network address at which this router (i.e., GLSR) can always be reached. In turn, LS age Options LS type (10) Type (1) Instance Advertising router LS sequence number FIGURE 8.11 LS checksum Length (variable) Common 20-byte header of OSPF-TE LSA messages. Convergent Network Management and Control Plane TABLE 8.1 Record Containing the Information Needed to the MLCP to Perform Routing Operations Sub-TLV Name Length Value Link local/remote identifiers bytes 2 X 4 bytes local/remote link identifiers Link protection type 4 bytes 1 byte for link protection (3 bytes reserved) Interface switching capability descriptor Variable Minimum 36 bytes for ISCD information Shared risk link group Variable N X 4 bytes for link SRLG identification the "link"-TLV can be used to abstract advertised TE links. Multiple TE attributes can be represented on each link using fields that are related to the "link"-TLV address, which are called sub-TLV. Thus, in the case of a link address, the TLV appears as the first field and pointer of a bigger record containing all the information that are needed to the MLCP about the link in order to perform routing operations. The structure of this bigger record is reported in Table 8.1. Two links attributes of Table 8.1 are worth a comment. The first is the last attribute of the record, advertising that the link is part of a shared risk link group (SRLG). An SRLG is a group of links that will fail contemporary when a certain type of failure happens. As an example, we can look at Figure 8.12, where typical s