Source-IP-Community Filter for BGP Flow Specification

Abstract

BGP Flowspec mechanism (BGP-FS) propagates both traffic Flow Specifications and Traffic Filtering Actions by making use of the BGP NLRI and the BGP Extended Community encoding formats. This document specifies a new BGP-FS component type to support community-level filtering. The match field is the community of the source IP address that is encoded in the Flowspec NLRI. This function is applied in a single administrative domain.¶

1. Introduction

BGP Flow Specification (BGP-FS) [RFC8955] [RFC8956] defines a new BGP NLRI to distribute traffic flow specification rules via BGP ([RFC4271]). BGP-FS policies have a match condition that may be n-tuple match in a policy, and an action that modifies the packet and forwards/drops the packet. Via BGP, new filter rules can be sent to all BGP peers simultaneously without changing router configuration, and the BGP peer can install these routes in the forwarding table. BGP-FS defines Network Layer Reachability Information (NLRI) format used to distribute traffic flow specification rules. NLRI (AFI=1, SAFI=133) is for IPv4 unicast filtering. NLRI (AFI=1, SAFI=134) is for BGP/MPLS VPN filtering.[I-D.ietf-idr-flowspec-l2vpn] extends the flow-spec rules for layer 2 Ethernet packets.¶

This document specifies a new BGP-FS component type to support community-level filtering. The match field is the community of the source IP address that is encoded in the Flowspec NLRI. This function is applied in a single administrative domain.¶

2. Definitions and Acronyms

• FS: Flow Specification¶

• Source-IP-Community: The community of the source IP address¶

3. The Flow Specification Encoding for Source-IP-Community Filter

This document proposes a new flow specification component type that is encoded in the BGP Flowspec NLRI. The following new component type is defined.¶

• Source-IP-Community¶

Type TBD1 - Source-IP-Community¶

Encoding: <type (1 octet), [op, value]+>¶

Contains a set of {operator, value} pairs that are used to match the Source-IP-Community (i.e. the community of the source IP address).¶

The operator byte is encoded as:¶

    0   1   2   3   4   5   6   7
  +---+---+---+---+---+---+---+---+
  | e | a |  len  | 0 |lt |gt |eq |
  +---+---+---+---+---+---+---+---+

Where:¶

e - end-of-list bit. Set in the last {op, value} pair in the list.¶

a - AND bit. If unset, the previous term is logically ORed with the current one. If set, the operation is a logical AND. It MUST be unset in the Source-IP-Community filter.¶

len - The length of the value field for this operator given as (1 << len). This encodes 1 (len=00), 2 (len=01), 4 (len=10), and 8 (len=11) octets.¶

lt - less than comparison between data and value.¶

gt - greater than comparison between data and value.¶

eq - equality between data and value.¶

The bits lt, gt, and eq can be combined to produce match the Source-IP-Community filter or a range of Source-IP-Community filter(e.g. less than community 1 and greater than community 2).¶

The value field is encoded as:¶

   0                   1                   2                   3
   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
  +---------------------------------------------------------------+
  ~          Source-IP-Community  (4 octets)                      ~
  +---------------------------------------------------------------+

Per section 10 of [RFC8955] , If a receiving BGP speaker cannot support this new Flow Specification component type, it MUST discard the NLRI value field that contains such unknown components. Since the NLRI field encoding (Section 4 of [RFC8955]) is defined in the form of a 2-tuple <length, NLRI value>, message decoding can skip over the unknown NLRI value and continue with subsequent remaining NLRI.¶

4. Use Cases

This section describes how to use this function in a simple scenario. Considering the topology shown in Figure 3 ("Comm" is short for "Community"). In AS64597's R2, if the ISP AS64597 wants to redirect all packets originating from AS64598 to IP Prefix 61:¶

"first go to R3, then forward them to IP Prefix 61", the ISP AS64597 can use the traditional method or the method defining in this draft.¶

                         +---------+
                         | BGP FS  |
                         | Server  |
                         +----|----+
                              |
                              |
                              +
                               \
                ****************\********  IP Prefix 81 with Comm 1:1
                *                \      *  IP Prefix 82 with Comm 1:1
  IP Prefix 61  *        AS64597  \     *  IP Prefix 83 with Comm 1:1
                *                  \    *  IP Prefix 84 with Comm 1:1
   +-------+    *  +---+         +--\+  *   +-------+
   +AS64596+-------+ R1+---------+ R2|------+AS64598+
   +-------+    *  +-+-+\        +---+  */  +-------+
                *        \         |\   /
                *         \        | \ /*  IP Prefix 91 with Comm 1:2
                *          \       |  /\*  IP Prefix 92 with Comm 1:2
                *           \      | /  \  IP Prefix 93 with Comm 1:2
                *            \     |/   *\ IP Prefix 94 with Comm 1:2
                *             \  +-+-+  * \ +-------+
                *              \-+ R3+------+AS64599+
                *                +---+  *   +-------+
                *                       *
                *************************

Using the traditional method, the ISP AS64597 needs to setup multiple "Destination Prefix + Source Prefix" rules in Router R2 as following:¶

    +--------------+--------------+-------------------------+
    | source       | Destination  | Redirect to IP Nexthop  |
    | Prefix       | Prefix       |                         |
    +--------------+--------------+-------------------------+
    | IP Prefix 81 | IP Prefix 61 |           R3            |
    +--------------+--------------+-------------------------+
    | IP Prefix 82 | IP Prefix 61 |           R3            |
    +--------------+--------------+-------------------------+
    | IP Prefix 83 | IP Prefix 61 |           R3            |
    +--------------+--------------+-------------------------+
    | IP Prefix 84 | IP Prefix 61 |           R3            |
    +--------------+--------------+-------------------------+
    |                  More ...                             |
    +--------------+--------------+-------------------------+

Using the method defining in this draft, the ISP AS64597 needs to setup only one "Source Community + Destination Prefix" rule in Router R2 as following:¶

  +--------------+--------------+-------------------------+
  | Source       | Destination  | Redirect to IP Nexthop  |
  | Community    | Prefix       |                         |
  +--------------+--------------+-------------------------+
  |     1::1     | IP Prefix 61 |           R3            |
  +--------------+--------------+-------------------------+

Obviously, the new method defining in this draft saves a lot of entry spaces on the control plane and forwarding plane, and it would greatly simplify the operation of the control plane, and the more source prefixes with the same community has, the more obvious the benefit.¶

5. IANA Considerations

IANA is requested to a new entry in "Flow Spec component types registry" with the following values:¶

   +---------+--------------+---------------------------------+
   |   Type  | RFC or Draft |    Description                  |
   +---------+--------------+---------------------------------+
   |   TBD1  |  This Draft  |    Source-IP-Community          |
   +---------+--------------+---------------------------------+

7. References