Path Computation Element Communication Protocol for Source Address Validation

Introduction Source Address Validation (SAV) is a critical security mechanism designed to mitigate IPv4 and IPv6 source address spoofing attacks by validating the legitimacy of source prefixes against their ingress interfaces. Traditional methods like ACL-based ingress filtering, strict uRPF and loose uRPF mechanisms have some issues (e.g., high operational overhead, improper block or permit of traffic) as described in and . The new inter-domain SAV mechanism is required to have accurate source address validation to avoid improper block and minimize improper permit of traffic, and to support automatic update of SAV rules. The PCE architecture, defined in , provides centralized control for path computation in networks. This document presents a PCE-based solution for SAVNET to provide dynamic policy enforcement in networks. By extending the PCEP protocol, PCE can efficiently manage SAV policies, validate source prefixes, and enforce traffic filtering actions to mitigate the threats of source address spoofing.

Conventions

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

Terminology This document uses the following terms defined in : PCC, PCE, PCEP Peer, and PCEP speaker. This document uses the following terms defined in : stateful PCE. This document uses the following terms defined in : SAV, SAV rule, SAV Information Base.

PCE for SAV Support

PCE Integration to SANET Architecture SAV Information Base (see ) that can be a table or data structure in a router which stores SAV-specific information and local routing information. SAV Information Base synchronizes SAV rules to PCE via BGP-LS or NETCONF/YANG interfaces, enabling PCE to compute validation paths. The PCE sends requests to PCC, which receives the SAV rules distributed by PCE and enforces SAV rules on data plane. For example, in an enterprise AS, PCE collects SAV specific routes from edge routers or border routers. It computes that source prefix (for example, 2001:db8:1::/48) is only valid on interfaces connected to the data center subnet. Any traffic with this prefix arriving at an AS border router is dropped unless it originates from the designated interfaces. The PCE-SAVNET integration proposed here supports both single-PCE and multi-PCE coorperative environments. It is applicable to single-domain and multi-domain AS scenarios, leveraging PCE for cross-domain policy coordination. For example, if the attacker switches the entry from Eth1/0 of R1 in AS60001 to Eth5/0 of R5 in AS60004, the PCE controller needs to synchronize the SAV rules and enforce policies for source address validation across domains.

PCE as SAV Policy Controller The example of process for PCE using PCEP to install SAV policies on edge routers (i.e., PCC role) in inter-domain networks is showed below: PCE as SAV controller collects SAV-specific information for SAV policy generation of mapping valid interfaces with prefix (e.g., 2001:db8::/32) to have the capability of global SAV policy visibility for single or multiple domains policy enforcement and coordination. PCE sends PCEP protocol messages (see ) to instal SAV policies, dynamic SAV policy updates. PCC deploys SAV policies which stores in SAV Information Database for mapping of source address prefix with valid ingress interfaces for ingress traffic filtering.

Requirements When the PCE speakers supporting SAV, the PCEP is required to support the following functionalities. The PCEP MUST support SAV-specific information and local routing information collection for cross-domain coordination and policy enforcement. The PCEP MUST support SAV capability advertisement in single and multi-domains. The PCEP MUST support dynamic updates of SAV policies for network changes (e.g., link failures, prefix additions). The PCEP MUST support backward compatibility with existing SAVNET mechanisms (e.g., BAR-SAV). The PCEP sessions for SAV MUST be secured against tampering and unauthorized access.

PCEP Extenstions

SAV capability advertisement The open message is used to establish a PCEP session between PCEP speakers. To support SAV functionality, a new flag is required for SAV capability advertisement which is introduced in this document. The SAV-CAPABILITY TLV is an optional TLV for use in the OPEN object for SAV capability advertisement. The format for SAV-CAPABILITY TLV refers to figure 9 in . A new value for Flags field is TBD for the SAV capability advertisement.

SAV-POLICY Object To support SAV functions, a new optional SAV object (class=TBD) for SAV information and its related object body formatted as TLV introduced in this document. A SAV-POLICY object is used to carry information of SAV policy within a PCEP update message, which may carry a set of SAV policies delivery and updates.

SAV-POLICY Object Format SAV-POLICY object class value is TBD. The TLV format for SAV-POLICY object consists of IPv4/IPv6 prefix and incoming-interface list (legitimate or illegitimate interfaces). The information for SAV-POLICY object is carried as TLV format, showed as the following figure:

SAV-CAPABILITY TLV Format The type (8 bits) of the TLV is TBD. The length field is 16 bits long, indicates the total length of the TLV in octects. The value contains the following fields: L: 1 bit long, identifies IP source prefix types, including IPv4 and IPv6 types. Flag: 8 bits long, identifies the validation modes used in network nodes. The validation modes include 4 modes: interface-based prefix allowlist, interface-based prefix blocklist, prefix-based interface allowlist, prefix-based interface blocklist. By selecting modes in different scenarios, the network can be secured to mitigate spoofing attacks, as introduced in . Interface List: contains a list of interfaces. If it is a whitelist, it represents a list of interfaces allowed to access; if it is a blacklist, it represents a list of interfaces not allowed to access. The interface list can be expressed in the form of interface ID, interface name, or index. IP source prefix: contains the source address prefix information.

Mechanism for Updates The PCE needs send PCUpd message for triggering mechanism when PCE makes actively update to the SAV policies, the possible trigger conditions may involve: topology changes (e.g., interface status modified), policy updates (e.g., the new added IP source prefix affiliated interfaces), and attack response from external threats.

IANA Considerations This document requests one new value for SAV capability advertisement, one new value for class of SAV policy object introduced in this document. IANA is requested to allocate TLV values for SAV-CAPABILITY TLV.

Security Considerations Security considerations for PCE are covered in the PCE Architecture , PCEP and stateful PCE . PCEP sessions for SAV policy distribution MUST use TLS 1.3 to prevent tampering. SAVNET security considerations covered in and are also applicable to the SAVNET procedures through PCEP defined in this document.

Acknowledgements The authors would like to acknowledge Haisheng Wu and Zhenghai Wang for their helpful comments.