BMP Extension for Path Status TLV

For a given prefix, multiple paths with different path status, e.g., the "best-path", "back-up path", "invalid", and so on, may co-exist in the BGP RIBs after being processed by 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 prefix that this TLV is describing. Please see for details of the use of the index field to associate the path marking content with one or more NLRIs. Path Status (4 Octets): indicates the path status of the BGP Update PDU encapsulated in the RM Message. Currently 10 types of path status are defined, as shown in Table 1. All zeros are reserved. Reason Code (2 Octets, optional): indicates the reason 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. Multiple bits may be set when multiple path status apply to a 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 . Stale refers to a path which has been declared stale by the BGP Graceful Restart mechanism as described in Section 4.1 of . Suppressed refers to a path which has been declared suppressed by the BGP Route Flap Damping mechanism as described in Section 2.2 of . The path status TLV does not force a BMP client to send any of these paths. It just provides a method to mark the paths that are available with their status.

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 prefix that this TLV is describing. The index is the encapsulation order, starting from 0, of the prefix in the BGP Update PDU. 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 (2 octets, optional): 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.

The BMP path marking TLV remains optional within BMP implementations. An implementation of the BMP path marking TLV may not fully support marking of all status defined in table or any future extensions. Similarly, an implementation may choose to support the inclusion of the reason code (for which support is also optional), without necessarily incorporating any of the reason codes defined in table or future extensions. This document refrains from defining mechanisms for signaling the status or reason codes an implementation supports. This could be established through external means (e.g. documentation) or potentially addressed in a subsequent document. The remainder of this section encompasses additional points related to the implementation of the BMP Path marking TLV.

Implementations supporting the BMP path marking TLV SHOULD provide an option for enabling or disabling the Path Marking TLV over BMP sessions. Furthermore, the configuration options for this TLV SHOULD provide the means to enable and disable the transmission of reason codes, if the reason code are supported by the implementation.

Some BGP routes might not require any type of status or reasons. For example, an unfiltered path obtained via the Adj-RIB-IN may fall under this category since there is really nothing to mark for that path. We suggest a couple of approaches for signaling that a path has no markings: (1) An implicit form of marking, achieved by abstaining from appending any BMP marking TLV pointing toward the route. (2) Alternatively, an explicit marking of the packet through a TLV containing no marked status and no associated reason code.

This document refrains from imposing any implementation to mark specific status from specific RIBs. We recognize the diversity among implementations; some might be able to mark some status over one RIB while other do it on others. For instance, some might be able to mark Adj-RIB-in filtered routes when obtained from the Adj-RIB-IN pre, while other could do it only from the Adj-RIB-IN post. To remove ambiguities in implementations, we recommend the meaning of status (and reason codes) to not depend on the origin RIB of a route.

Implementations introducing their own status and reason codes are advised to adhere to and use ebit and vendor specific status and reasons. Additionally, we recommend all implementations to provide comprehensive documentation for these codes. For scenarios where a path state combines a standard status with an enterprise-specific reason code (or vice versa), the following alternatives are presented:

Replication of the standard definitions within the enterprise-specific space, thus permitting direct marking within the same packet using the ebit.
Assigning two TLVs to the same path(s): one containing the standard part and another housing the vendor-specific part.

We advocate for the employment of TLV grouping wherever feasible. The inclusion of all marking information within a single message is recommended, except on the case described in section . In situations where multiple TLVs are associated with a single route, all markings will be applicable to that route.

We would like to thank Jeff Haas and Maxence Younsi for their valuable comments.

This document requests that IANA assign the following new parameters to the BMP parameters name space. Type = TBD1 (15 Bits): indicates that it is the IANA-registered Path Status TLV.

It is not believed that this document adds any additional security considerations.