Common BMP Route-Monitoring Messages for Routes Unchanged by Policy

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Table of Contents

1. Introduction

[RFC7854] defined Pre-Policy and Post-Policy Adj-RIB-In Route-Monitoring messages, whereas [RFC8671] defined Pre-Policy and Post-Policy Adj-RIB-Out Route-Monitoring messages. If both Pre-Policy and Post-Policy Route-Monitoring modes are enabled on a device for a RIB (Adj-RIB-In or Adj-RIB-Out), the routes are included in both Pre-Policy and Post-Policy Route-Monitoring messages, even if the routes remains unmodified as a result of the application of policy.¶

The optimization proposed in this document will help reduce the number of Route-Monitoring messages sent up to 2 times in the best case when policy does not modify any of the routes. The exact reduction in number of messages depends on the policy being used. The higher number of Route-monitoring messages impacts the performance/convergence of BMP and increases the load on the transport layers and the network connecting to the BMP server. Reducing this will aid in improving the BGP, BMP and network performance and scalability.¶

To avoid sending duplicate unmodified routes in the Post-Policy Route-Monitoring messages, we introduce in this document two alternate methods based on:¶

• Common Update Flag, or C flag, in the Per-Peer header¶
• Common Update TLV based on [I-D.ietf-grow-bmp-tlv]¶

When the Monitored Router does not have TLV support as defined by [I-D.ietf-grow-bmp-tlv], the method using the Common Update Flag MAY be used. When the Monitored Router does support TLV, either of the two methods MAY be used. However, it is to be noted that the two methods are mutually exclusive.¶

2. Common Update Flag in Peer Flags

The per-peer header has the same structure and flags as defined in Section 4 of [RFC8671] with the addition of the C flag in the Peer Flags in the Per-Peer Header as shown here.¶

     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |V|L|A|O|C| Resv|
    +-+-+-+-+-+-+-+-+

• The C flag, when set to 0, indicates that the BGP Update PDU that follows the common BMP header and per-peer header in the Route-Monitoring message reflects either the Pre-Policy or the Post-Policy view as indicated by the L flag.¶
• The C flag, when set to 1, indicates that the BGP Update PDU that follows the common BMP header and per-peer header in the Route-Monitoring message is the same for Pre-Policy and Post-Policy views. The C flag and the L flag are mutually exclusive. When the C flag is set to 1, the L flag has no significance and MUST be transmitted as 0 and its value MUST be ignored on receipt.¶
• If the C flag is set to 1 and the O flag is set to 0, then it indicates that the BGP Update PDU reflects the Adj-RIB-In route which is the same for both Pre-Policy and Post-Policy views.¶
• If the C flag is set to 1 and the O flag is set to 1, then it indicates that the BGP Update PDU reflects the Adj-RIB-Out route which is the same for both Pre-Policy and Post-Policy views.¶

Note that the C flag can be used only if the BGP Update PDU is common between Pre-Policy and Post-Policy Adj-RIB-In or Pre-Policy and Post-Policy Adj-RIB-Out. It does not allow for indicating a BGP Update PDU that is common between Adj-RIB-In and Adj-RIB-Out.¶

3. Common Update TLV

Here we define a new TLV named Common Update TLV using the TLV construct defined in Section 4.2 of [I-D.ietf-grow-bmp-tlv] as an alternative to C flag method proposed above. In addition to allowing sharing a common BGP Update PDU between pre-policy and post-policy modes of Adj-RIB-In and the same for Adj-RIB-Out like the Common Update Flag method, this method is extensible in allowing sharing across Adj-RIB-In and Adj-R-RIB-Out views, though we see it being used rarely.¶

The TLV has Index zero (0) which specifies that a TLV applies to all NLRIs contained in the BGP Update PDU. The value of the TLC defines flags that are described below.¶

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Type=TBD2            |          Length = 3         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Index=0             |I|J|O|P|  Resv |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

• When I flag is set, it indicates that the BGP Update PDU reflects the Pre-Policy Adj-RIB-In view of all contained NLRIs¶
• When J flag is set, it indicates that the BGP Update PDU reflects the Post-Policy Adj-RIB-In view of all contained NLRIs¶
• When O flag is set, it indicates that the BGP Update PDU reflects the Pre-Policy Adj-RIB-Out view of all contained NLRIs¶
• When P flag is set, it indicates that the BGP Update PDU reflects the Post-Policy Adj-RIB-Out view of all contained NLRIs¶
• The remaining bits are reserved for future use. They MUST be transmitted as 0 and their values MUST be ignored on receipt.¶

4. BMP Messages

Since the C flag is used in the context of BGP Update PDU, it has no significance for Peer-Up, Peer-Down, Initiation, Termination and Statistics Report messages. Though the Route Mirroring message contains a BGP Update PDU, as there is no policy execution involved in its transmission the C flag has no significance. In all messages except the Route-Monitoring message, the C flag MUST be set to 0 in the per-peer header during transmission and MUST be ignored on reception.¶

Similarly, the Common Update TLV MUST not be included in the above listed BMP messages during transmission and MUST be ignored if found on reception.¶

          +-+-+-+-+-+-+-+-+
          |I|J|O|P|  Resv |
          +-+-+-+-+-+-+-+-+

5. IANA Considerations

IANA needs to assign the following new parameters to the "BGP Monitoring Protocol (BMP) Parameters" registry.¶

6. Security Considerations

This document does not add any additional security considerations. The considerations in Section 11 of [RFC7854] apply to this document.¶

7. Acknowledgements

TBD¶

Authors' Addresses