Problem Statement, Gap Analysis, and Requirements for Intra-Domain Source Address Validation

Internet Engineering Task Force (IETF)                            L. Qin
Request for Comments: 10078                      Zhongguancun Laboratory
Category: Informational                                            D. Li
ISSN: 2070-1721                                                    J. Wu
                                                     Tsinghua University
                                                                M. Huang
                                                 Zhongguancun Laboratory
                                                                 N. Geng
                                                                  Huawei
                                                            October 2026


   Problem Statement, Gap Analysis, and Requirements for Intra-Domain
                       Source Address Validation

Abstract

   Source Address Validation (SAV) is an important means to mitigate IP
   source address spoofing (RFC 2827).  This document analyzes the gaps
   in current operational mechanisms for intra-domain SAV.  It also
   identifies the properties that new intra-domain SAV mechanisms are
   expected to provide.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc10078.

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Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  Problem Statement
   3.  Current Operational Intra-Domain SAV Mechanisms
   4.  Gap Analysis
     4.1.  Asymmetric Routing Scenario
     4.2.  Hidden Prefix Scenario
   5.  Requirements for New SAV Mechanisms
     5.1.  Accurate Validation
     5.2.  Automatic Updates
     5.3.  Incremental Deployment Support
     5.4.  No Adverse Impact on Routing Convergence and Fast Reroute
     5.5.  Authentication of Information Used for SAV
   6.  Security Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Source Address Validation (SAV) defends against IP source address
   spoofing [RFC2827].  Network operators can enforce SAV at the
   following levels (see [RFC5210]):

   *  IP SAV in the access network

   *  IP SAV at intra-AS/ingress point

   *  IP SAV in the inter-AS case (neighboring Autonomous System (AS))

   Some access networks have already deployed SAV mechanisms.  These
   mechanisms are typically deployed on switches in the access network
   and prevent hosts connected to those switches from using the source
   address of another host on the Internet [RFC5210].  Mechanisms
   include:

   *  Source Address Validation Improvement (SAVI) Solution for DHCP
      [RFC7513]

   *  IP Source Guard (IPSG) based on DHCP snooping [IPSG]

   *  Cable Source-Verify [cable-verify]

   However, access-network SAV mechanisms are not universally deployed
   [CAIDA-spoofer].  Therefore, intra-domain (i.e., intra-AS) SAV and
   inter-domain (i.e., inter-AS) SAV are required [RFC5210].  For the
   purpose of this document, intra-domain SAV is defined as follows:

      The AS validates the source addresses of data traffic that it
      originates directly or indirectly.  Intra-domain SAV is applied at
      external interfaces (on routers) facing entities that are not
      deployed as neighboring ASes and are therefore not covered by
      inter-domain SAV.  For example, as illustrated in Figure 1, an
      entity can be a single host, a set of hosts, or a customer network
      with no AS that manages one or more IP prefixes.  The entity may
      source traffic using prefixes assigned by the AS or its own Bring
      Your Own IP (BYOIP) prefixes.  From the perspective of other ASes,
      such traffic is originated by the AS.

   SAV on traffic received on external interfaces facing a neighboring
   AS is considered inter-domain SAV, regardless of whether the
   neighboring AS uses a public Autonomous System Number (ASN) or a
   private ASN.  SAV on internal interfaces (e.g., interfaces between
   Router 1 and Router 3 in Figure 1) is also outside the scope of this
   document.  This is because routers inside the same AS are generally
   assumed to be trusted, so SAV on internal interfaces provides limited
   additional benefit when SAV is already applied at external
   interfaces.  In addition, techniques such as fast reroute can make
   SAV at internal interfaces technically challenging.

       +--------------------+
       |  A neighboring AS  |
       +--------------------+
                 |
                 |
                 |
   +-------------|-------------------------------------+
   |Domain       |                                     |
   |         +----------+               +----------+   |
   |         | Router 3 +---------------+ Router 4 |   |
   |         +----------+               +----------+   |
   |          /        \                     |         |
   |         /          \                    |         |
   |        /            \                   |         |
   | +----------+     +----------+      +----------+   |
   | | Router 1 |     | Router 2 +------+ Router 5 |   |
   | +------*---+     +--*-------+      +----X-----+   |
   |        /\           /\                  /\        |
   |         \           /                   |         |
   +----------\---------/--------------------|---------+
          +----------------+         +---------------+
          | A customer     |         | A single host |
          | network with   |         | or a set of   |
          | no AS          |         | hosts         |
          +----------------+         +---------------+

             Figure 1: Deployment Locations of Intra-Domain SAV

   Intra-domain SAV is applied at interfaces "*" and "X".

   This document analyzes the gaps in current operational mechanisms for
   intra-domain SAV.  It also identifies the properties that new intra-
   domain SAV mechanisms are expected to provide.

1.1.  Terminology

   SAV Rule:
      The rule that indicates the validity of a specific source IP
      address or source IP prefix per router interface.  It is used by a
      router to make SAV decisions.

   Improper Block:
      The validation results in packets with legitimate source addresses
      being blocked improperly due to inaccurate SAV rules.  (The terms
      "improper block" and "false positive" are used synonymously.)

   Improper Permit:
      The validation results in packets with spoofed source addresses
      being permitted improperly due to inaccurate SAV rules.  (The
      terms "improper permit" and "false negative" are used
      synonymously.)

   Proper Block:
      The validation results in packets with spoofed source addresses
      being blocked.

   Proper Permit:
      The validation results in packets with legitimate source addresses
      being permitted.

   SAV-specific Information:
      The information specialized for SAV rule generation.

   Direct Server Return (DSR):
      A traffic delivery model commonly used by Content Delivery
      Networks (CDNs) that use anycast service addresses while
      delivering data from edge locations that do not announce those
      addresses.  In such deployments, a request is received by the
      anycast server or location, but the response is sent directly by
      another server (i.e., the edge location) with the anycast service
      address as the source address, rather than the address used to
      reach the edge server.  This can create a legitimate hidden-prefix
      scenario.

2.  Problem Statement

   The problems of existing intra-domain SAV mechanisms can be
   characterized along three dimensions: improper block, improper
   permit, and operational overhead:

   Improper block:  Existing intra-domain SAV mechanisms may block data
      packets using legitimate source addresses when the applied SAV
      rules are inaccurate.

   Improper permit:  Existing intra-domain SAV mechanisms may permit
      data packets using spoofed source addresses when the applied SAV
      rules are inaccurate.

   Operational overhead:  Existing intra-domain SAV mechanisms may
      require operator involvement to determine and update SAV rules.
      This overhead depends on how much manual effort is needed to keep
      the SAV rules up to date.

   In this document, these three dimensions are used to analyze the gaps
   in existing intra-domain SAV mechanisms.

3.  Current Operational Intra-Domain SAV Mechanisms

   Although [RFC2827] and [RFC3704] specify several ingress filtering
   methods primarily intended for inter-domain SAV, some of these
   methods have also been applied to intra-domain SAV in operational
   practice.  This section introduces the mechanisms currently used to
   implement intra-domain SAV.

   *  Access Control Lists (ACLs) can be used as SAV filters [RFC2827]
      to check the source address of each packet against a set of
      permitted or prohibited prefixes.  When applied on a router
      interface, each Access Control Entry (ACE) used for SAV filtering
      specifies both matching conditions (i.e., prefixes) and the
      corresponding action (e.g., permit or deny), and packets are
      processed accordingly.

   *  Strict unicast Reverse Path Forwarding (uRPF) [RFC3704] provides
      an automated SAV filter by validating the source address of each
      packet against the router's local Forwarding Information Base
      (FIB).  A packet is accepted only if (i) the FIB contains a prefix
      covering the source address and (ii) the FIB entry's outgoing
      interface matches the packet's incoming interface.  Otherwise, the
      packet is discarded.

   *  Loose uRPF [RFC3704] also relies on the local FIB for validation
      but only checks for the presence of a covering prefix.  A packet
      is accepted if the FIB contains a prefix that covers the source
      address, regardless of the incoming interface.

4.  Gap Analysis

   This section analyzes the gaps of the current operational intra-
   domain SAV mechanisms.

   ACLs can be used on interfaces facing a customer network with no AS
   or a set of hosts to permit only packets whose source addresses
   belong to specific prefixes.  To ensure correct filtering behavior,
   the ACLs used for SAV filtering need to be updated when the permitted
   prefixes change; otherwise, packets may be improperly permitted or
   blocked.  In ACL-based SAV deployments, keeping these ACLs up to date
   can introduce operational challenges when operators need to detect
   prefix changes and determine and apply the corresponding ACL updates.

   As described in Section 3 and also noted in [RFC3704], loose uRPF
   sacrifices directionality when validating source addresses of data
   packets.  Since its rules are overly permissive, any spoofed packet
   with a source address present in the FIB may be permitted by loose
   uRPF (i.e., an improper permit problem).

   Strict uRPF may block legitimate traffic in the asymmetric routing or
   hidden prefix scenarios (see Sections 4.1 and 4.2).  It may
   mistakenly consider a valid incoming interface as invalid, resulting
   in legitimate packets being blocked (i.e., an improper block
   problem).

   The following subsections describe two specific gap scenarios for
   intra-domain SAV.

4.1.  Asymmetric Routing Scenario

   Asymmetric routing means a packet traverses from a source to a
   destination in one path and takes a different path when it returns to
   the source.  Asymmetric routing can occur due to routing policy,
   traffic engineering, etc.

   For example, a customer network with no AS connected to multiple
   routers of the AS may need to perform load balancing on incoming
   traffic, thereby resulting in asymmetric routing.  Figure 2
   illustrates an example of asymmetric routing.  The customer network
   owns prefix 2001:db8::/55 and connects to two routers of the AS,
   Router 1 and Router 2.  Router 1, Router 2, and Router 3 exchange
   routing information via the intra-domain routing protocol.  To
   achieve load balancing for inbound traffic, the customer network
   expects traffic destined for 2001:db8:0::/56 to enter through Router
   1 and traffic destined for 2001:db8:0:100::/56 to enter through
   Router 2.  To this end, Router 1 advertises 2001:db8:0::/56 and
   Router 2 advertises 2001:db8:0:100::/56 through the intra-domain
   routing protocol.  Figure 2 also shows the corresponding FIB entries
   of Router 1 and Router 2 for the two prefixes.

    +----------------------------------------------------------+
    |Domain                                                    |
    |                      +----------+                        |
    |                      | Router 3 |                        |
    |                      +----------+                        |
    |                       /       \                          |
    |                      /         \                         |
    |                     /           \                        |
    |            +----------+       +----------+               |
    |            | Router 1 |       | Router 2 |               |
    |            +-----*----+       +----------+               |
    |                  /\               /                      |
    |                   \              /                       |
    +--------------------\------------/------------------------+
     Traffic with         \          / Traffic with
     source IP addresses   \        /  destination IP addresses
     of 2001:db8:0:100::/56 \      \/  of 2001:db8:0:100::/56
                       +----------------+
                       |Customer network|
                       |with no AS      |
                       |(2001:db8::/55) |
                       +----------------+

    FIB of Router 1                FIB of Router 2
    Dest                Next_hop   Dest                Next_hop
    2001:db8:0::/56     Customer   2001:db8:0:100::/56 Customer
                        Network                        Network
    2001:db8:0:100::/56 Router 3   2001:db8:0::/56     Router 3

                 Figure 2: An Example of Asymmetric Routing

   The legitimate traffic originated from the customer network with
   source addresses in 2001:db8:0:100::/56 will be improperly blocked by
   strict uRPF on Router 1.

   Although the customer network does not expect to receive inbound
   traffic for 2001:db8:0:100::/56 via Router 1, it can send outbound
   traffic with source addresses in that prefix through Router 1.  As a
   result, data packets between the customer network and Router 1 may
   follow asymmetric paths.  Arrows in Figure 2 indicate the direction
   of traffic flow.

   If Router 1 enforces strict uRPF by checking the FIB entry for the
   prefix 2001:db8:0:100::/56, the corresponding SAV rule would only
   allow packets with a source address from 2001:db8:0:100::/56 that
   arrive via Router 3.  Consequently, when the customer network sends
   packets with a source address in 2001:db8:0:100::/56 to Router 1,
   strict uRPF would improperly drop these legitimate packets.
   Similarly, if Router 2 enforces strict uRPF, it would improperly
   block legitimate packets from the customer network that use source
   addresses within the prefix 2001:db8:0::/56.

4.2.  Hidden Prefix Scenario

   The intra-domain hidden prefix scenario refers to situations in which
   a host or a customer network with no AS legitimately originates
   traffic using source addresses that are not visible to the intra-
   domain routing protocol within the domain.

   *  A host (for example, a cloud server instance operated by a tenant)
      may originate traffic using a source address not allocated by the
      AS operator.  This can occur in deployments such as DSR, where
      return traffic is sent directly from the server using a service IP
      address that is not part of the operator's internal routing view.

   *  A customer network with no AS may originate traffic using source
      addresses that are not advertised to the AS operator.  This can
      occur in scenarios such as DSR deployments or when the customer
      network uses address space assigned by another provider (e.g., in
      multi-homing or hybrid connectivity scenarios), and such prefixes
      are not propagated within the operator's intra-domain routing
      system.

   For ACL-based SAV, enforcing correct filtering in these scenarios
   requires authoritative information that explicitly specifies which
   source addresses the host or the customer network is authorized to
   use.  In practice, such authoritative information is often missing.
   Strict uRPF and loose uRPF also fail in hidden prefix scenarios.
   They will drop packets from hidden prefixes because the source
   addresses are absent from the router's FIB or are received from
   unexpected interfaces.

5.  Requirements for New SAV Mechanisms

   This section identifies five requirements that can inform the design
   of new intra-domain SAV mechanisms.  These requirements describe the
   properties that new mechanisms are expected to provide in order to
   improve upon existing mechanisms but do not make assumptions about
   how those properties are achieved.  They do not mandate or justify
   any specific extension to routing or other protocols and therefore
   cannot be used to directly initiate standards-track protocol changes.

   Existing intra-domain SAV mechanisms have problems in terms of
   validation accuracy and operational overhead.  Current uRPF-based
   mechanisms derive SAV decisions solely from routing or forwarding
   state, which is intended to express reachability rather than
   authorization of source address usage.  More generally, current
   mechanisms lack authoritative information specifically intended for
   SAV that can be consistently and automatically consumed by SAV
   mechanisms.  As a result, uRPF-based mechanisms may not provide
   accurate validation in scenarios such as asymmetric routing or hidden
   prefixes (Section 4).  Existing ACL-based SAV deployments may have
   limited applicability in dynamic environments when they rely on
   operator-driven ACL maintenance.  These problems motivate the first
   two requirements below (in Sections 5.1 and 5.2).  The remaining
   three requirements (in Sections 5.3, 5.4, and 5.5) are motivated by
   deployment and operational considerations.

5.1.  Accurate Validation

   Any new intra-domain SAV mechanism must improve SAV accuracy over
   existing intra-domain SAV mechanisms.  This improvement can be
   reflected in reduced improper blocks (false positives), reduced
   improper permits (false negatives), or both.  Furthermore, it must
   seek to mitigate improper blocks and improve the ability to reject
   spoofed traffic in the gap scenarios described in Section 4.  To
   support this, additional information beyond the local FIB, such as
   SAV-specific information, may be needed to make validation decisions
   and generate accurate SAV rules.

5.2.  Automatic Updates

   Any new intra-domain SAV mechanism must be capable of automatically
   collecting and processing relevant information and updating the
   corresponding SAV rules in response to relevant information changes.
   Automation helps reduce operational complexity and maintenance
   overhead while allowing some initial configuration to improve SAV
   accuracy.  This ensures the mechanism is deployable in practical
   networks without introducing an excessive management burden.

5.3.  Incremental Deployment Support

   Any new intra-domain SAV mechanism must support incremental
   deployment and provide measurable benefits even when deployed on only
   a subset of external interfaces facing hosts or customer networks
   with no AS.

5.4.  No Adverse Impact on Routing Convergence and Fast Reroute

   If any new intra-domain SAV mechanism requires disseminating SAV-
   specific information among intra-domain routers via a protocol, it
   must not adversely affect the convergence of existing routing
   protocols or the operation of fast-reroute mechanisms.

5.5.  Authentication of Information Used for SAV

   Any new intra-domain SAV mechanism must use information that is
   authenticated or trusted, either through verification of its
   integrity and authenticity or via an established trust relationship
   with the information source.

6.  Security Considerations

   This document discusses the problems with existing intra-domain SAV
   practices and identifies informational requirements for new intra-
   domain SAV mechanisms.  As it does not specify any new protocol/
   mechanism or protocol extension, it does not introduce new security
   considerations.

7.  IANA Considerations

   This document has no IANA actions.

8.  References

8.1.  Normative References

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <https://www.rfc-editor.org/info/rfc2827>.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
              2004, <https://www.rfc-editor.org/info/rfc3704>.

   [RFC5210]  Wu, J., Bi, J., Li, X., Ren, G., Xu, K., and M. Williams,
              "A Source Address Validation Architecture (SAVA) Testbed
              and Deployment Experience", RFC 5210,
              DOI 10.17487/RFC5210, June 2008,
              <https://www.rfc-editor.org/info/rfc5210>.

   [RFC7513]  Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address
              Validation Improvement (SAVI) Solution for DHCP",
              RFC 7513, DOI 10.17487/RFC7513, May 2015,
              <https://www.rfc-editor.org/info/rfc7513>.

8.2.  Informative References

   [cable-verify]
              Cisco, "Cable Source-Verify and IP Address Security",
              January 2021, <https://www.cisco.com/c/en/us/support/docs/
              broadband-cable/cable-security/20691-source-verify.html>.

   [CAIDA-spoofer]
              CAIDA, "State of IP Spoofing",
              <https://spoofer.caida.org/summary.php?>.

   [IPSG]     Cisco, "Configuring DHCP Features and IP Source Guard",
              August 2008,
              <https://www.cisco.com/c/en/us/td/docs/switches/lan/
              catalyst2960/software/release/12-
              2_53_se/configuration/guide/2960scg/swdhcp82.html>.

Acknowledgements

   The authors thank Jared Mauch, Joel Halpern, Aijun Wang, Michael
   Richardson, Gert Doering, Tony Przygienda, Yingzhen Qu, James
   Guichard, Ron Bonica, Xueyan Song, and others for their valuable
   comments.  The authors also thank Kotikalapudi Sriram for his
   suggestions on the definition of "intra-domain SAV".  The authors
   thank the IETF directorates and the IESG for their reviews and
   comments, which helped improve the clarity of this document.

Authors' Addresses

   Lancheng Qin
   Zhongguancun Laboratory
   Beijing
   China
   Email: qinlc@mail.zgclab.edu.cn


   Dan Li
   Tsinghua University
   Beijing
   China
   Email: tolidan@tsinghua.edu.cn


   Jianping Wu
   Tsinghua University
   Beijing
   China
   Email: jianping@cernet.edu.cn


   Mingqing Huang
   Zhongguancun Laboratory
   Beijing
   China
   Email: huangmq@mail.zgclab.edu.cn


   Nan Geng
   Huawei
   Beijing
   China
   Email: gengnan@huawei.com