BMP Extension for Path Status TLV

For a given prefix, multiple paths with different path status, e.g., the "best-path", "back-up path" and so on, may co-exist in the BGP RIB after being processed by the local policy and the BGP decision process. The path status information is currently not carried in the BGP Update Message RFC4271 or in the BMP Update Message RFC7854. External systems can use the path status for various applications. The path status is commonly checked by operators when performing troubleshooting. Having such status stored in a centralized system can enable the development of tools that facilitate this process. Optimisation systems can include the path status in their process, and also use the status as a validation source (since it can compare the calculated state to the actual outcome of the network, such as primary and backup path). As a final example, path status information can complement other centralized sources of data, for example, flow collectors. This document defines a so-called Path Status TLV to convey the BGP path status to the BMP server. The BMP Path Status TLV is carried in the BMP Route Monitoring (RM) Message.

This document defines two types of Path Status TLVs: one is the IANA-registered Path Status TLV, and the other is the Enterprise-specific Path Status TLV.

E bit: For an IANA-registered TLV, the E bit MUST be set to 0. Type = TBD2 (15 Bits): indicates that it is the IANA-registered Path Status TLV. Length (2 Octets): indicates the length of the value field of the Path Status TLV. The value field further consists of the Path-Status field and Reason Code field. Index (2 Octets): indicates the encapsulation order of the prefix, in the BGP Update PDU, that the this sub TLV is describing information for. Path Status (4 Octets): indicates the path status of the BGP Update PDU encapsulated in the RM Message. Currently 8 types of path status are defined, as shown in Table 1. All zeros are reserved. Reason Code (variable): indicates the reasons/explanations of the path status indicated in the Path Status field. The reason code field is optional. If no reason code is carried, this field is empty. If a reason code is carried, the reason code is indicated by a 2-byte value, which is defined in Table 2.

The Path Status field contains a bitmap where each bit encodes a specific role of the path. The best-path is defined in RFC4271 and the best-external path is defined in draft-ietf-idr-best-external. An invalid path is a route that does not enter the BGP decision process. A non-selected path is a route that is not selected in the BGP decision process. Back-up routes are considered non-selected, while the best and ECMP routes are not considered as non-selected. A primary path is a recursive or non-recursive path whose nexthop resolution ends with an adjacency draft-ietf-rtgwg-bgp-pic. A prefix can have more than one primary path if multipath is configured draft-lapukhov-bgp-ecmp-considerations. A best-path is also considered as a primary path. A backup path is also installed in the RIB, but it is not used until some or all primary paths become unreachable. Backup paths are used for fast convergence in the event of failures. A non-installed path refers to the route that is not installed into the IP routing table. For the advertisement of multiple paths for the same address prefix without the new paths implicitly replacing any previous ones, the add-path status is applied RFC7911.

E bit: For an Enterprise-specific TLV, the E bit MUST be set to 1. Type = 1 (15 Bits): indicates that it's the Enterprise-specific Path Status TLV. Length (2 Octets): indicates the length of the value field of the Path Status TLV. The value field further consists of the Path-Status field and Reason Code field. Index (2 Octets): indicates the encapsulation order of the prefix, in the BGP Update PDU, that the this sub TLV is describing information for. PEN Number (4 octets): indicates the IANA enterprise number IANA-PEN. Path Status (4 Octets): indicates the enterprise-specific path status. The format is to be determined w.r.t. each PEN number. Reason Code (Variable): indicates the reasons/explanations of the path status indicated in the Path Status field. The format is to be determined w.r.t. each PEN number.