BMP v4: TLV support for BMP Route Monitoring and Peer Down Messages

BMP v4: TLV support for BMP Route Monitoring and Peer Down Messages NTT

Veemweg 23 Barneveld 3771 MT NL paolo@ntt.net

Huawei

Huawei Bld., No.156 Beiqing Rd. Beijing 100095 China guyunan@huawei.com

General Global Routing Operations BGP BMP tlv Most of the message types defined by the BGP Monitoring Protocol (BMP) make provision for data in TLV format. However, Route Monitoring messages (which provide a snapshot of the monitored Routing Information Base) and Peer Down messages (which indicate that a peering session was terminated) do not. Supporting (optional) data in TLV format across all BMP message types allows for a homogeneous and extensible structure that would be useful for the most different use-cases that need to convey additional data to a BMP station. While it is not intended for this document to cover any specific utilization scenario, it defines a consistent and simple way to support TLV data in all message types.

The BGP Monitoring Protocol (BMP) version 3 is defined in RFC 7854. The Route Monitoring message consists of: Common Header Per-Peer Header BGP Update PDU The Peer Down Notification message consists of: Common Header Per-Peer Header Reason Data (only if Reason code is 1, 2 or 3) TLV (only if Reason code is 6) This means that both Route Monitoring and Peer Down messages have a non-extensible format (except for the specific case of Peer Down Reason Code 6 as ). In the Route Monitoring case, this prevents the transmission of characteristics of transported NLRIs (e.g. to help with stateless parsing), status of a path after being processed by the BGP process or of vendor-specific data. In the Peer Down case, this prevents matching with TLVs previously sent with the Peer Up message. The proposal of this document is to: Bump the BMP version for all message types defined in RFC 7854 for backward compatibility Change the structure of Route Monitoring message type so that the BGP Update PDU is enclosed in a TLV. The BGP Message PDU TLV is mandatory Allow all defined BMP message types to make provision for optional TLV data.

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.

For an exporter to flag a receiver that it does comply with this document, the Version field of the Common Header, documented in Section 4.4 of, MUST be set to 4. This applies to every BMP message type.

The TLV data type is already defined in Section 4.4 of for the Initiation and Peer Up message types. A TLV consists of: 2 octets of TLV Type, 2 octets of TLV Length, 0 or more octets of TLV Value.

TLVs SHOULD be sorted by the sender by their code point. Multiple TLVs of the same type can be repeated as part of the same message, and it is left to the specific use-cases whether all, any, the first or the last TLV should be considered as well as whether ordering matters and repeating is allowed. Route Monitoring messages may require per-NLRI TLVs, that is, there may be a need to map TLVs to NLRIs contained in the BGP Update message, for example, to express additional characteristics of a specific NLRI. For this purpose specifically, TLVs in Route Monitoring messages MUST be indexed, with the index starting at one (1) to refer to the first NLRI. Index zero (0) specifies that a TLV does apply to all NLRIs contained in the BGP Update message. The Index field is 2 bytes long of which the top-most bit, G-bit, is reserved to flag a Group Index (more in ). In general TLVs of the same type and with the same index can be repeated as part of the same message, unless specified otherwise by the definition of the specific TLV. Indexed TLVs are encoded as in the following figure:

Indexed TLVs SHOULD be sorted by the sender by their code point and index value. Also in indexed TLVs, the reported length refers to the total encoded TLV value (ie. it does exclude the length of the index field). A decoder can properly match indexed TLVs to the corresponding NLRI only if - or as long as - NLRIs are decoded successfully. In case of any parsing or error condition that prevents full decoding of the BGP PDU, the decoder MUST stop matching indexed TLVs to NLRIs. Of the BMP message types defined so far, indexed TLVs apply only to Route Monitoring messages and, for example, they do not apply to Route Mirroring messages because the sender may not be aware of the payload of the transported BGP Update message.

defines the Common Header. While the structure remains unaltered, the following two definitions are changed: Version: Indicates the BMP version. This is set to '4' for all message types defined in RFC 7854. Message Length: Total length of the message in bytes (including headers, encapsulated BGP Message PDU TLV and optional TLV data).

The Route Monitoring message type is defined in . The consistency model selected by this document to extend encoding of such message type with TLVs is with the Route Mirroring type defined in where the Per-peer header is being followed by mandatory and optional TLVs. The BGP Update PDU is encoded itself as part of a BGP Message TLV with code point &IANA_RM_CODE_BGP_PDU; and index set to zero. A Route Monitoring message MUST contain one BGP Message TLV which may be preceeded and/or followed by other optional TLV data. Corollary, in BMPv4 the BGP Update PDU is not just encoded as part of the message as it was the case for BMPv3 but it is rather enclosed in a TLV.

In a Route Monitoring message where the BGP Update PDU carries N NLRIs, indexed TLVs do allow to handle the cases of 1:1 and N:1 relationship among TLVs and NLRIs (ie. one TLV applies to one NLRI, N TLVs apply to one same NLRI). The cases of 1:N and M:N relationships (ie. one TLV applies to N NLRIs, M TLVs apply to N NLRIs) can benefit by a form of grouping. This is the context to define a Group TLV to achieve this with the aim to limit both verbosity and repetitions. The TLV value MUST contain: A 2 bytes Group Index where the top-most bit, G-bit or Group Bit MUST be set to one (1). The full 2 bytes value, that is including the G-bit, MUST be unique to the message Two or more 2 bytes NLRI indexes whose values MUST be less or equal to the amount of NLRIs packed in the BGP Update PDU. A NLRI index can be listed as part of multiple Group TLVs within the same message. NLRI indexes within a Group TLV SHOULD be sorted by the sender. A Group Index can not reference an NLRI index 0. A Group TLV MUST NOT include its own or another Group Index. Multiple non-Group TLVs can point to the same Group Index, ie. a group can be reused within the same Route Monitoring message. The Group TLV code point is &IANA_RM_CODE_GROUP;. It is recommended that this TLV is encoded first in order to ease parsing of the Route Monitoring message at the BMP station side.

The Information field contains a UTF-8 string whose value MUST be equal to the value of the VRF or table name (ie. RD instance name) being conveyed. The string size MUST be within the range of 1 to 255 bytes. The VRF/Table Name TLV code point is &IANA_RM_CODE_VRF;

Stateless parsing helps scaling the amount of Route Monitoring messages that can be processed at collection time, avoiding to have to correlate them to BGP capabilities received as part of the Peer Up message, for example. Some BGP capabilities are not per AFI/SAFI, like 4-bytes ASN RFC 6793, and hence these can be part of the BMP Peer flags section of a Route Monitoring message. Those that are, instead, per AFI/SAFI require finer granularity and hence the need to use an indexed TLV. The Stateless Parsing TLV code point is &IANA_RM_CODE_SP; and is organized as a 5-bytes registry, one byte for the type, followed by AFI (2 bytes) and SAFI (1 byte) and finally one byte for the value. The type field encodes a code point from the Stateless Parsing registry. The following code points are defined for this registry: Type = &IANA_RM_SPREG_CODE_PATH_ID;: the BGP Update PDU is encoded with the ADD-PATH capability RFC 7911, the value field MUST be 0 for false and 1 for true. Type = &IANA_RM_SPREG_CODE_MULTI_LABELS;: the BGP Update PDU is encoded with the Multiple Labels capability RFC 8277, the value field MUST match the Count for the relevant AFI/SAFI as it was encoded in the BGP Open PDU. Only a single Stateless Parsing TLV is allowed in a message, this TLV cannot be repeated. Also the index of the TLV MUST be set to zero. If the Stateless Parsing TLV is not present in a Route Monitoring message, the receiver MUST fall back to use capabilities present in the BGP Open PDU contained in the relevant BMP Peer Up message in order to properly parse BGP Update PDUs. It is recommended that the Stateless Parsing TLV is encoded preceeding the BGP Message TLV in order to ease parsing of the Route Monitoring message at the BMP station side.

The diagram in shows an example of a Route Monitoring message carrying a BGP UPDATE containing 10 NLRIs. The TLVs are comprised of: a Group TLV with index 0x000b, pointing to NLRI 1, 2, 3 and 10 a Group TLV with index 0x000c, pointing to NLRI 4, 5 and 6 a Stateless Parsing TLV with Index 0x800b (Group TLV bit set to 1, Index 0x000b) a TLV pertaining to NLRI 7 a TLV pertaining to the NLRIs listed in the Group TLV defined in 1 a TLV pertaining to the NLRIs listed in the Group TLV defined in 2

The Peer Down Notification message type is defined in . The consistency model selected by this document to extend encoding of such message type is with the Peer Up type defined in where optional TLVs are placed at the end of the message. This means for Reason codes 1 or 3, a BGP Notification PDU follows; the PDU MAY be further followed by optional TLV data. For Reason code 2, a 2-byte field follows to provide additional FSM info; this field MAY be followed by optional TLV data. For all other Reason codes, optional TLV data MAY follow the Reason field.

All other message types defined in RFC7854 do already provision for TLV data. It is RECOMMENDED that all future defined BMP message types will also provide for optional TLV data following a consistency model for encoding with existing message types.

It is worth nothing that RFC8654 permits BGP Update and other messages to grow to a length of 65535 octets. This may cause a BMP PDU that attempts to encapsulate such long messages to overflow.

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

In Route Monitoring messages, the number of TLVs can be bound to the amount of NLRIs carried in the BGP Update message. This may degrade the packing of information in such messages and have specific impacts on the memory and CPU used in a BMP implementation. As a result of that it should always be possible to disable such features to mitigate their impact.

This document requests the renaming of the "Peer Up TLVs" registry defined by BMP Peer Up Message Namespace into "Peer Up and Peer Down TLVs" and the definition of one new registry "BMP Route Monitoring TLVs". As part of the "BMP Route Monitoring TLVs" registry, the following new TLV types are defined (): Type = &IANA_RM_CODE_BGP_PDU;: Support for BGP Message TLV. The value field is defined in Type = &IANA_RM_CODE_GROUP;: Support for grouping of TLVs. The value field is defined in . The recommended value for this TLV is 0. Type = &IANA_RM_CODE_VRF;: Support for VRF/Table Name TLV. The value field is defined in Type = &IANA_RM_CODE_SP;: Support for Stateless Parsing TLV. The value field is defined in . The recommended value for this TLV is 1. This document also requests the definition of a "Stateless Parsing TLV" registry seeded as follows: Type = &IANA_RM_SPREG_CODE_PATH_ID;: ADD-PATH capability flag. Set to 1 if the BGP Update PDU enclosed in the Route Monitoring message was encoded according to the capability. The flag is defined in . The recommended value for this flag is 0. Type = &IANA_RM_SPREG_CODE_MULTI_LABELS;: Multiple Labels capability flag. The Set to 1 if the BGP Update PDU enclosed in the Route Monitoring message was encoded according to the capability. The flag is defined in . The recommended value for this flag is 1.

&RFC2119; &RFC8174; &I-D.ietf-grow-bmp-peer-up;

The authors would like to thank Jeff Haas, Camilo Cardona, Thomas Graf, Pierre Francois, Ben Maddison, Tim Evens, Luuk Hendriks, Maxence Younsi, Ahmed Elhassany, Colin Petrie, Dhananjay Pakti and Shunwan Zhuang for their valuable input. The authors would also like to thank Greg Skinner and Zongpeng Du for their review.