Source-IP-Origin-AS Filter for BGP Flow Specification

BGP Flow Specification (FlowSpec), defined in and , allows for the dissemination of traffic filtering rules. Current FlowSpec components support filtering by destination prefix, source prefix, and various Layer 4 parameters. In certain DDoS mitigation scenarios, an operator may need to apply rate-limiting or filtering to all traffic sourced from a particular network (Autonomous System), even when the specific souce IP prefixes within that AS are numerous or rapidly changing. Manually updating hundreds of prefix-based FlowSpec rules is inefficient. This document introduces a new FlowSpec component that allows operators to use the Source Origin AS as a matching criterion.

FS: Flow Specification Source-IP-Origin-AS: The origin AS number of the source IP address

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-Origin-AS Type TBD1 - Source-IP-Origin-AS Encoding: <type (1 octet), [op, value]+> Contains a set of {operator, value} pairs that are used to match the Source-IP-Origin-AS (i.e. the origin AS number of the source IP address). The operator byte is encoded as:

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-Origin-AS 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-Origin-AS filter or a range of Source-IP-Origin-AS filter(e.g. less than AS1 and greater than AS2). The value field is encoded as:

Per section 10 of , 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 ) 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. In cases of multi-homed prefixes with multiple Origin ASes, the match succeeds if any of the valid Origin ASes match the filter.

When a BGP speaker receives a FlowSpec update containing the Source-IP-Origin-AS component: It MUST determine the Origin AS of the souce IP of the transit packet by performing a lookup in its local BGP RIB. If the packet's source IP matches a prefix whose BGP path has an AS_PATH where the rightmost AS (the origin) matches the value in the FlowSpec rule, the action (e.g., rate-limit, discard) MUST be applied.

This section describes how to use this function in a simple scenario. Considering the topology shown in Figure 3. 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.

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

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

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 an AS has, the more obvious the benefit.

In addition to the security considerations in [RFC8955], operators must be aware of: Routing Inconsistency: If different routers in the network have different views of the BGP table, the "Origin AS" for a given IP may differ, leading to inconsistent filter application. AS_PATH Manipulation: An attacker could potentially spoof or prepend ASes to bypass filters if the local BGP table is compromised. Validation: Implementations SHOULD ensure that FlowSpec rules are validated against the originating peer to prevent unauthorized AS-based filtering across administrative boundaries.

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

TBD