BMP Extension for Path Marking TLV
draft-cppy-grow-bmp-path-marking-tlv-02

Abstract

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 BGP path after being processed by the BGP best-path selection algorithm. This extension makes use of the TLV mechanims described in draft-lucente-bmp-tlv.

Requirements Language

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 RFC 8174 when, and only when, they appear in all capitals, as shown here.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on July 6, 2020.

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Table of Contents

• 1. Introduction
• 2. Path Marking TLV for the RM Message
• 2.1. Path Status
• 2.1.1. IANA-registered Path Status Encoding
• 2.1.2. Enterprise-specific Path Status Encoding
• 2.2. Reason Code
• 2.2.1. IANA-registered Reason Code Encoding
• 2.2.2. Enterprise-specific Reason Code Encoding
• 3. Acknowledgements
• 4. IANA Considerations
• 4.1. Path Marking TLV
• 4.2. Path Marking TLV Reason Code
• 5. Security Considerations
• 6. Normative References
• Authors' Addresses

1. Introduction

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 facilitating 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 Marking TLV to convey the BGP path status information to the BMP server. The BMP Path Marking is defined to be prepended in the BMP Route Monitoring (RM) Message.

2. Path Marking TLV for the RM Message

As per RFC4271, the BMP RM Message consists of the Common Header, Per-Peer Header, and the BGP Update PDU. According to draft-lucente-bmp-tlv , optional trailing data in TLV format is allowed in the BMP RM Message to convey characteristics of transported NLRIs (i.e. to help stateless parsing) or vendor-specific data. Such TLV types are to be defined for each application.

To include the path status along with each BGP path, we define the Path Marking TLV, shown as follows.

 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
+-------------------------------+-------------------------------+
|        Type (2 octets)        |       Length (2 octets)       |
+-------------------------------+-------------------------------+
|E|                    Path Status(variable)                    |
+---------------------------------------------------------------+
|E|                    Reason Code(variable)                    |
+---------------------------------------------------------------+

                     Figure 1: Path Marking TLV 

• Type = TBD1 (2 Octets): indicates that it's the Path Marking TLV.
• Length (2 Octets): indicates the length of the value field of the Path Marking TLV. The value field further consists of the Path-Status field and Reason Code field.
• E bit (1 Bit) for Path Status: indicates if any enterprise-specific path status is used after the IANA-registered path status code.
• Path Status (4 Octets): indicates the path status of the BGP Update PDU encapsulated in the RM Message. Currently 7 types of path status are defined, as shown in Table 1.
• E bit (1 Bit) for Reason Code: indicates if any enterprise-specific reason code is used after the IANA-registered reason code.
• Reason Code (Variable): indicates the reasons/explanations of the path status indicated in the Path Type field. The detailed Reason Code field is defined in Section 2.2.

2.1. Path Status

The Path Status field contains a bit field where each bit encodes a specific role of the path. Multiple bits may be set when multiple path status apply to a path.

Two encoding options for Path Status are described in the following two sections.

2.1.1. IANA-registered Path Status Encoding

 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-------------------------------+
|E| IANA registered path status |
+-------------------------------+

          Figure 2:  IANA-registered encoding of Path Status

• E bit (1 Bit): set to 0, indicating that only IANA-registered path status is used in this TLV.
• IANA-registered Path Status (2 octets): indicates the IANA-registered path status, as specified in Table 1.

+--------+----------------------+
| Value  | Path type            |
+-------------------------------+
| 0x0000 | Unknown              |
| 0x0001 | Invalid              |
| 0x0002 | Best                 |
| 0x0004 | Non-selected         |
| 0x0008 | Primary              |
| 0x0010 | Backup               |
| 0x0020 | Non-installed        |
| 0x0040 | Best external        |
| 0x0080 | Add-Path             |
+--------+----------------------+

     Table 1: Path Type 

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. In other words, Best route 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.

2.1.2. Enterprise-specific Path Status Encoding

 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|            Enterprise-Specific Path Type (4 octets)         |
+---------------------------------------------------------------+
|                    Enterprise Number(4 octets)                |
+---------------------------------------------------------------+

    Figure 3: Enterprise-specific encoding of Path Status

• E bit (1 Bit): set to 1, indicating enterprise-specific path status is used in this TLV.
• Enterprise-specific Path Type (4 octets): indicates enterprise-specific path status, which remains to be defined.
• Enterprise Number (4 octets): indicates the IANA enterprise number IANA-PEN.

2.2. Reason Code

The Reason Code field contains a bit field where each bit encodes a specific reason. Multiple bits may be set when multiple reasons apply to a path.

2.2.1. IANA-registered Reason Code Encoding

 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|           IANA-registered Reason Code(4 octets)             |
+---------------------------------------------------------------+

          Figure 4:  IANA-registered encoding of Reason Code

• E bit (1 Bit): set to 0, indicating that only IANA-registered reason code is used in this TLV. With the E bit set to 0, the Length field of the Path Marking TLV SHOULD be set to 8.
• IANA-registered Reason Code (4 octets): indicates the IANA-registered reason code of the path status.

2.2.2. Enterprise-specific Reason Code Encoding

 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|                  Enterprise Number(4 octets)                |
+---------------------------------------------------------------+
+                  E-specific Reason Code(variable)             +
~                                                               ~
+---------------------------------------------------------------+

    Figure 5: Enterprise-specific encoding of Reason Code

• E bit (1 Bit): set to 1, indicating enterprise-specific reason code is also used in this TLV.
• IANA-registered Reason Code (4 octets): indicates the IANA-registered reason code of the path status.
• Enterprise Number (4 octets): indicates the IANA enterprise number IANA-PEN.
• E-specific Reason Code (Variable): indicates enterprise-specific reason code of the path status.

3. Acknowledgements

We would like to thank Jeff Haas for his valuable comments.

4. IANA Considerations

This document requests that IANA assign the following new parameters to the BMP parameters name space.

4.1. Path Marking TLV

This document defines the Path Marking TLV with Type = TBD1: Path Marking (Section 2).

4.2. Path Marking TLV Reason Code

5. Security Considerations

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

6. Normative References

Authors' Addresses