| Internet-Draft | BMP path status tlv | September 2023 |
| Cardona, et al. | Expires 14 March 2024 | [Page] |
The BGP Monitoring Protocol (BMP) provides an interface for obtaining BGP Path information. BGP Path Information is conveyed within BMP Route Monitoring (RM) messages. This document proposes an extension to BMP to convey the status of a path after being processed by the BGP process. This extension makes use of the TLV mechanims described in draft-ietf-grow-bmp-tlv [I-D.ietf-grow-bmp-tlv] and draft-ietf-grow-bmp-tlv-ebit [I-D.ietf-grow-bmp-tlv-ebit].¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 RFC 2119 [RFC2119] RFC 8174 [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
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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 [RFC4271] or in the BMP Update Message RFC7854 [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.¶
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
+-------------------------------+-------------------------------+
|E| Type (15 bits) | Length (2 octets) |
+---------------------------------------------------------------+
| Index (2 octets) |
+-------------------------------+-------------------------------+
| Path Status (4 octets) |
+---------------------------------------------------------------+
| Reason Code (2 octets, optional) |
+---------------------------------------------------------------+
Figure 2: Encoding of IANA-Registered Path Status TLV¶
+------------+-----------------------------+ | Value | Path type | +------------------------------------------+ | 0x00000001 | Invalid | | 0x00000002 | Best | | 0x00000004 | Non-selected | | 0x00000008 | Primary | | 0x00000010 | Backup | | 0x00000020 | Non-installed | | 0x00000040 | Best-external | | 0x00000080 | Add-Path | | 0x00000100 | Filtered in inbound policy | | 0x00000200 | Filtered in outbound policy | | 0x00000400 | Invalid ROV | +------------+-----------------------------+ Table 1: IANA-Registered Path Type
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 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.¶
+----------+-----------------------------------------------------+
| Value | Reason code |
+----------------------------------------------------------------+
| [0x0007] | invalid for AS loop |
| [0x0007] | invalid for unresolvable nexthop |
| [0x0016] | not preferred for Local preference |
| [0x0014] | not preferred for AS Path Length |
| [0x0013] | not preferred for origin |
| [0x0012] | not preferred for MED |
| [0x0011] | not preferred for peer type |
| [0x0010] | not preferred for IGP cost |
| [0x000E] | not preferred for router ID |
| [0x000D] | not preferred for peer address |
| [0x0020] | not preferred for AIGP |
+----------+-----------------------------------------------------+
Table 2: IANA-Registered Reason Code
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 +-------------------------------+-------------------------------+ |E| Type (15 bits) | Length (2 octets) | +-------------------------------+-------------------------------+ | PEN number (4 octets) | +---------------------------------------------------------------+ | Index (2 octets) | +---------------------------------------------------------------+ | Path Status (4 octets) | +---------------------------------------------------------------+ | Reason Code (2 octets, optional) | +---------------------------------------------------------------+ Figure 3: Encoding of Enterprise-specific Path Status TLV¶
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 Figure 1 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 Figure 2 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 [I-D.ietf-grow-bmp-tlv-ebit] 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:¶
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 Section 3.4. 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.¶