Gap Analysis, Problem Statement, and Requirements for Inter-Domain SAV
draft-ietf-savnet-inter-domain-problem-statement-14
| Document | Type | Active Internet-Draft (savnet WG) | |
|---|---|---|---|
| Authors | Dan Li , Lancheng Qin , Libin Liu , Mingqing(Michael) Huang , Kotikalapudi Sriram | ||
| Last updated | 2026-02-14 | ||
| Replaces | draft-wu-savnet-inter-domain-problem-statement | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
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| Send notices to | (None) |
draft-ietf-savnet-inter-domain-problem-statement-14
Internet Engineering Task Force D. Li
Internet-Draft Tsinghua University
Intended status: Informational L. Qin
Expires: 19 August 2026 L. Liu
Zhongguancun Laboratory
M. Huang
Huawei
K. Sriram
USA NIST
15 February 2026
Gap Analysis, Problem Statement, and Requirements for Inter-Domain SAV
draft-ietf-savnet-inter-domain-problem-statement-14
Abstract
This document provides a gap analysis of existing inter-domain source
address validation mechanisms, describes the problem space, and
defines the requirements for technical improvements.
Status of This Memo
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This Internet-Draft will expire on 19 August 2026.
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Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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extracted from this document must include Revised BSD License text as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Existing Inter-domain SAV Mechanisms . . . . . . . . . . . . 5
4. Gap Analysis . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. SAV at Customer Interfaces . . . . . . . . . . . . . . . 8
4.1.1. Limited Propagation of a Prefix (LPP) Scenario . . . 10
4.1.2. Hidden Prefix (HP) Scenario . . . . . . . . . . . . . 11
4.1.3. Source Address Spoofing within a Customer Cone (SCC)
Scenario . . . . . . . . . . . . . . . . . . . . . . 12
4.2. SAV at Peer Interfaces . . . . . . . . . . . . . . . . . 14
4.3. SAV at Provider Interfaces . . . . . . . . . . . . . . . 14
5. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 17
6. Requirements for New Inter-domain SAV Mechanisms . . . . . . 18
6.1. Accurate Validation . . . . . . . . . . . . . . . . . . . 19
6.2. Reducing Operational Overhead . . . . . . . . . . . . . . 19
6.3. Early Adopters Benefit in Incremental/Partial
Deployment . . . . . . . . . . . . . . . . . . . . . . . 19
6.4. Providing Necessary Security Guarantee . . . . . . . . . 19
6.5. Automatic Updates to the SAV List and Efficient
Convergence . . . . . . . . . . . . . . . . . . . . . . . 19
7. Inter-domain SAV Scope . . . . . . . . . . . . . . . . . . . 20
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . 21
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
Source Address Validation (SAV) is a fundamental mechanism for
detecting and mitigating source address spoofing attacks [RFC2827]
[RFC5210] [RFC3704] [RFC8704]. This document provides a gap analysis
of existing inter-domain SAV mechanisms, describes the problem space,
and defines the requirements for technical improvements. The
corresponding work related to intra-domain SAV is documented in
[I-D.ietf-savnet-intra-domain-problem-statement].
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In this document, inter-domain SAV refers to SAV on AS-to-AS
interfaces that carry external BGP (eBGP) sessions. The eBGP
sessions include Customer-to-Provider (C2P), Provider-to-Customer
(P2C), lateral peering (p2p), and Route Server to RS-client
connections. The terms customer, provider (transit provider), and
lateral peer (non-transit peer; peer (for simplicity)) used in this
document are consistent with those defined in [RFC7908] [RFC9234].
Further, [RFC9234] mentions Route Server (RS) and RS-client. An RS-
to-RS-client interface is akin to the customer interface. For the
purposes of SAV, an RS-client-to-RS interface may be treated (1) like
a provider interface for simplicity, or (2) like a union of lateral
peers considering all the ASes the RS-client chose to peer with at
the IXP RS.
Access Control List (ACL) and unicast Reverse Path Forwarding (uRPF)
based techniques are currently utilized to some extent for inter-
domain SAV. In this document, the inter-domain SAV methods from only
the existing IETF RFCs (BCP 38 [RFC2827] and BCP 84 [RFC3704]
[RFC8704]) are considered for the gap analysis; IETF work-in-progress
documents are not considered. This document analyzes the available
methods and attempts to answer: (1) what are the technical gaps
(Section 4), (2) what are the outstanding problems (problem
statement) (Section 5), and (3) what are the practical requirements
for the solutions to these problems (Section 6).
The following summarizes the fundamental problems with existing SAV
mechanisms, as analyzed in Section 4 and Section 5:
* Improper block: Existing uRPF-based mechanisms suffer from
improper block (false positives) in two inter-domain scenarios:
limited propagation of a prefix and hidden prefix.
* Improper permit: With some existing uRPF-based SAV mechanisms,
improper permit (false negatives) can happen on any type of
interface (customer, lateral peer, or provider). Specifically, if
the method relaxes the directionality constraint [RFC3704]
[RFC8704]} to try to achieve zero improper blocking, the
possibility of improper permit increases. (Note: It is recognized
that unless there is full adoption of SAV in the customer cone
(CC) of the interface in consideration, improper permit is not
fully preventable in scenarios where source address spoofing
occurs from within the CC, i.e., a prefix at one Autonomous System
(AS) in the CC is spoofed from another AS in the same CC.)
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* High operational overhead (HOO): ACL-based ingress SAV filtering
introduces significant operational overhead, as it needs to update
ACL rules manually to adapt to prefix or routing changes in a
timely manner. The HOO issue does not pertain to existing uRPF-
based mechanisms.
To address these problems, this document specifies (Section 6) the
following key technical requirements for any new solution:
* Improved SAV accuracy over existing mechanisms: Any new inter-
domain SAV mechanism MUST avoid improper blocking and have
superior directionality property (reject more spoofed traffic)
than existing inter-domain SAV mechanisms.
* Reduced operational overhead: Any new inter-domain SAV mechanism
MUST be able to automatically adapt to network dynamics and
asymmetric routing scenarios. Any such mechanism MUST have less
operational overhead than ACL-based ingress SAV filtering.
* Benefit in incremental/partial deployment: Any new solution SHOULD
NOT assume pervasive adoption of the SAV method or the SAV-related
information (e.g., Resource Public Key Infrastructure (RPKI)
object registrations). It SHOULD benefit early adopters by
providing effective protection from spoofing of source addresses
even in partial deployment.
* Automatic updates to the SAV list and efficient convergence: Any
new inter-domain SAV mechanism SHOULD be responsive to changes in
the BGP (FIB/RIB) data, the SAV-related information (Section 2),
or the SAV-specific information (Section 2). It SHOULD
automatically update the SAV list while achieving efficient re-
convergence of the same.
* Providing necessary security guarantee: If any proposed new SAV
method requires exchanging SAV-related or SAV-specific information
between ASes, security mechanisms SHOULD exist to assure
trustworthiness of the information.
1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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2. Terminology
SAV List:
The table of prefixes that indicates the validity of a specific
source IP address or source IP prefix per interface. Sometimes
the terms 'RPF (Reverse Path Forwarding) list' or 'SAV rules' are
used interchangeably with 'SAV list'.
Improper Block:
The validation results in packets with legitimate source addresses
being blocked improperly due to an inaccurate SAV list.
Improper Permit:
The validation results in packets with spoofed source addresses
being permitted improperly due to an inaccurate SAV list.
Customer Cone:
The Customer Cone (CC) of a given AS, denoted as AS-A, includes:
(1) AS-A itself, (2) AS-A's direct customers (ASes), (3) The
customers of AS-A's direct customers (indirect customers), (4) And
so on, recursively, following all chains of provider-to-customer
(P2C) links down the hierarchy.
Customer Cone Prefixes (CC Prefixes):
IP prefixes permitted by their owners to be originated by, or used
as source addresses for data traffic originated from, one or more
Autonomous Systems (ASes) within the CC.
SAV-related Information:
Objects registered using Resource Public Key Infrastructure
(RPKI). This can include existing RPKI object types (e.g., ROAs
and ASPAs) or any new type(s) that may be proposed.
SAV-specific Information:
Information dedicated to SAV, which may be defined and exchanged
between ASes using a potentially new inter-AS communication
protocol. The information may also be in the form of new RPKI
object type(s) meant to assist SAV.
3. Existing Inter-domain SAV Mechanisms
Inter-domain SAV is typically performed at the AS level (on a per
neighbor-AS-interface basis) and can be deployed at AS border routers
(ASBRs) to prevent source address spoofing. There are various
mechanisms available to implement inter-domain SAV for anti-spoofing
ingress filtering [nist] [manrs] [isoc], which are reviewed in this
section.
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* ACL-based ingress filtering [RFC3704]: ACL-based ingress SAV
filtering is a technique that relies on ACL rules to filter
packets based on their source addresses. However, ACL-based
ingress SAV filtering introduces significant operational overhead,
as ACL rules need to be updated in a timely manner to reflect
prefix or routing changes in the inter-domain routing system. One
may think of using ACL as a disallow list on a provider interface
to block source prefixes that are clearly invalid in the inter-
domain routing context, such as IANA special purpose or
unallocated IPv4/IPv6 prefixes, etc. But it is impractical to
store and maintain a very large and dynamically varying set of
unallocated IPv6 prefixes. Also, for the customer interfaces, the
ACL method is impractical while other techniques (as described
below) are more effective. ACL-based ingress SAV filtering has
applicability for broadband cable or digital subscriber access
loop (DSL) access networks where the service provider has clear
knowledge of IP address prefixes it has allocated to manage those
services. Here ACL can be used in an allow-list form.
* uRPF-based mechanisms: A class of SAV mechanisms are based on
Unicast Reverse Path Forwarding (uRPF) [RFC3704] [RFC8704]. The
core idea of uRPF for SAV is to exploit the symmetry of inter-
domain routing: in many cases, the best next hop for a destination
is also the best previous hop for the source. In other words, if
a packet arrives from a certain interface, the source address of
that packet should be reachable via the same interface, according
to the FIB. However, symmetry in routing does not always hold in
practice, and to address cases where it does not hold, many
enhancements and modes of uRPF have evolved. Different modes of
uRPF have different levels of strictness and flexibility, and
network operators can choose from them to suit particular network
scenarios. We briefly describe these modes as follows:
- Strict uRPF [RFC3704]: Strict uRPF is the most stringent mode.
It permits a packet only if it has a source address that is
covered by a prefix in the FIB, and the next hop for that
prefix is the same interface that the packet arrived on. This
mode can be deployed at customer interfaces in some scenarios,
e.g., a directly connected single-homed stub customer AS
[nist].
- Loose uRPF [RFC3704]: Loose uRPF verifies that the source
address of a packet is routable on the internet by matching it
with one or more prefixes in the FIB, regardless of the
interface on which the packet arrives. If the source address
is not routable, Loose uRPF discards the packet. Loose uRPF is
typically deployed at the provider interfaces of an AS to block
packets with source addresses from prefixes that are not routed
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on the global internet (e.g., IANA-allocated private-use
addresses, unallocated IPv4/IPv6 addresses, multicast
addresses, etc.).
- Feasible Path uRPF (FP-uRPF) [RFC3704]: Unlike Strict uRPF,
which requires the packet to arrive on the exact best return
path, FP-uRPF allows a packet to pass as long as the router
could reach that source address through the interface it
arrived on (based on the feasible routes in the Adj-RIBs-In
[RFC4271]), even if the route isn't the primary route (per best
path selection). This makes it more effective in multi-homed
environments where asymmetric routing is common, as it prevents
legitimate traffic from being dropped simply because it didn't
take the "best" path back to the sender.
- Enhanced Feasible Path uRPF with Algorithm A (EFP-uRPF Alg-A)
[RFC8704]: EFP-uRPF Alg-A expands the list of valid source
addresses for a specific interface by including all prefixes
associated with any Origin AS that is reachable through that
interface. Instead of only accepting prefixes directly
advertised on a link, the router identifies all the origin ASes
present in the BGP updates received on that interface and then
permits any prefix from those same ASes that it sees elsewhere
in its Adj-RIBs-In (associated with all neighbors — customers,
providers, peers). This "Origin AS-based" approach provides
significantly more flexibility than strict or traditional FP-
uRPF, as it accounts for cases where an AS in the CC may send
traffic for one of its prefixes over a link where it only
advertised a different prefix (multi-homing and asymmetric
routing scenarios).
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- Enhanced Feasible Path uRPF with Algorithm B (EFP-uRPF Alg-B)
[RFC8704]: EFP-uRPF Alg-B provides even greater flexibility
(compared to EFP-uRPF Alg-A) by aggregating all customer
interfaces into a single "customer group" for validation
purposes. The router first identifies all unique prefixes and
origin ASes associated with all directly connected customer
interfaces using only the Adj-RIBs-In associated with them. It
then constructs a comprehensive RPF list that includes every
prefix originated by those ASes, regardless of whether those
prefixes were learned via customer, peer, or transit provider
links. This list is applied uniformly across all customer-
facing interfaces, attempting to ensure that legitimate traffic
from a multihomed AS in the CC is never dropped, even if the
traffic arrives on a different customer-facing port than the
one where the specific prefix was advertised. In comparison to
EFP-uRPF Alg-A, this method (Alg-B) reduces the possibility of
improper block but at the expense of increased possibility of
improper permit, i.e., reduced directionality.
- Virtual Routing and Forwarding (VRF) uRPF [RFC4364] [urpf]
[manrs]: VRF uRPF uses a separate VRF table for each external
BGP peer and is only a way of implementation for a SAV list.
4. Gap Analysis
Inter-domain SAV is essential for preventing source address spoofing
traffic at all AS interfaces — customers, providers, and lateral
peers. An ideal inter-domain SAV mechanism must block all spoofing
traffic while permitting legitimate traffic in all scenarios of
interest. However, in some cases, existing SAV mechanisms may
unintentionally block legitimate traffic or permit spoofing traffic.
This section aims to conduct a gap analysis of existing SAV
mechanisms for different types of interfaces under various scenarios
to identify their technical limitations.
4.1. SAV at Customer Interfaces
To prevent source address spoofing on customer interfaces, operators
can enable ACL-based ingress filtering, or uRPF-based mechanisms such
as Strict uRPF, FP-uRPF, or EFP-uRPF. However, the ACL method
typically has high operational overhead. The uRPF-based mechanisms
may cause improper block in two inter-domain scenarios: Limited
Propagation of a Prefix (LPP) and Hidden Prefix (HP). They may also
cause improper permit in the scenarios of source address Spoofing
within a Customer Cone (SCC). The LPP scenario occurs when an AS
applies traffic engineering (TE) using a no-export policy. One
example is when an AS attaches NO_EXPORT BGP Community to some
prefixes (routes) forwarded to some upstream providers (in multi-
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homing scenarios) (see Section 4.1.1). Sometimes this type of TE is
done without attaching the NO_EXPORT, i.e., by selectively
propagating different sets of prefixes to different upstream
providers. The Hidden Prefix (HP) scenario is typically associated
with the Direct Server Return (DSR) scenario; anycast prefix in a
Content Delivery Network (CDN) application is not announced by the AS
where the DSR (edge server) is located (see Section 4.1.2). Source
address Spoofing within a Customer Cone (SCC) scenario arises when a
prefix at one Autonomous System (AS) in the CC is spoofed from
another AS in the same CC Section 4.1.3. It is recognized that
unless there is full adoption of SAV in the customer cone (CC) of the
interface in consideration, improper permit is not fully preventable
in the SCC scenario.
Figure 1 provides an overview of the gaps associated with the ACL
method, Strict uRPF, FP-uRPF, and EFP-uRPF for SAV at customer
interfaces in the LPP, HP, and SCC scenarios mentioned above.
Illustrations and analyses of these gaps are provided in
Section 4.1.1, Section 4.1.2, and Section 4.1.3, respectively.
+--------------------+------------+-----------+-------+--------+
|Traffic & Scenarios | ACL |Strict uRPF|FP-uRPF|EFP-uRPF|
+----------+---------+------------+-----------+-------+--------+
|Legitimate| LPP | | |
|Traffic +---------+ | Improper Block |
| | HP | High | possible |
+----------+---------+Operational-+-------------------+--------+
| | | Overhead | |Improper|
|Spoofed | no SCC | (HOO) | |Permit |
|Traffic | | | Functions as |only for|
| | | | Expected |EFP-uRPF|
| | | | |Alg-B |
|+---------+---------+ +-------------------+--------|
|Spoofed | SCC | | |
|Traffic | | | Improper Permit |
| | | | (in partial deployment) |
+----------+---------+------------+----------------------------+
LPP = Limited Propagation of a Prefix
HP = Hidden Prefix
SCC = Spoofing within a CC
'Functions as Expected' connotes the absence of improper permit.
It also connotes low operational overhead.
Figure 1: The gaps of ACL-based ingress filtering, Strict uRPF,
FP-uRPF, and EFP-uRPF for customer interfaces for the scenarios
of interest.
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4.1.1. Limited Propagation of a Prefix (LPP) Scenario
In inter-domain networks, some prefixes may not propagate from a
customer to all its providers and/or may not propagate transitively
from the providers to all their providers due to various factors,
such as the use of NO_EXPORT or NO_ADVERTISE Communities, or some
other selective-export policies meant for traffic engineering. In
these cases, it is possible that a prefix (route) announcement in the
CC associated with a customer interface has limited propagation in
the CC and is not received on that interface. Then the prefix is
invisible in BGP at that interface but the traffic with source
address in that prefix may still be received on that interface. This
can give rise to improper block when performing SAV with existing
mechanisms. These mechanisms include EFP-uRPF Alg-A, which is the
focus on in the following analysis, while it also applies to Strict
uRPF and FP-uRPF. All these mechanisms suffer from the same problem
of improper block in this scenario.
+----------------+
| AS 3(P3) |
+-+/\------+/\+--+
/ \
/ \
/ \
/ (C2P) \
+------------------+ \
| AS 4(P4) | \
++/\+--+/\+----+/\++ \
/ | \ \
P2[AS 2] / | \ \
/ | \ \
/ (C2P) | \ P5[AS 5] \ P5[AS 5]
+----------------+ | \ \
| AS 2(P2) | | P1[AS 1] \ \
+----------+/\+--+ | P6[AS 1] \ \
\ | \ \
P1[AS 1] \ | \ \
NO_EXPORT \ | \ \
\(C2P) |(C2P) (C2P)\ (C2P)\
+----------------+ +----------------+
| AS 1(P1, P6) | | AS 5(P5) |
+----------------+ +----------------+
Figure 2: Limited propagation of a prefix caused by NO_EXPORT.
In the scenario of Figure 2, AS 1 is a customer of AS 2; AS 1 and AS
2 are customers of AS 4; AS 4 is a customer of AS 3; and AS 5 is a
customer of both AS 3 and AS 4. AS 1 advertises prefixes P1 to AS 2
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with the NO_EXPORT community attribute attached, preventing AS 2 from
further propagating the route for prefix P1 to AS 4. Consequently,
AS 4 only learns the route for prefix P1 from AS 1 in this scenario.
Suppose AS 1 and AS 4 have deployed inter-domain SAV while other ASes
have not, and AS 4 has deployed EFP-uRPF at its customer interfaces.
If AS 4 deploys EFP-uRPF Alg-A at customer interfaces, it will
require packets with source addresses in P1 or P6 to only arrive on
the interface with AS 1. When AS 1 sends legitimate packets with
source addresses in P1 or P6 to AS 4 via AS 2, AS 4 improperly blocks
these packets. The same improper block problem occurs with the use
of Strict uRPF or FP-uRPF. EFP-uRPF with Alg-B can avoid the
improper block in this specific scenario, but even this SAV method
would have the improper block if the TE at AS 1 is such that none of
the customer interfaces at AS 4 receives a route for P1 (or P6).
4.1.2. Hidden Prefix (HP) Scenario
CDNs use the concepts of anycast [RFC4786][RFC7094] and DSR to
improve the quality of service by placing edge servers with content
closer to users. An anycast IP address is assigned to devices in
different locations, and incoming requests are routed to the closest
edge server (DSR) location. Usually, only locations with rich
connectivity announce the anycast IP address through BGP. The CDN
server receives requests from users and creates tunnels to the edge
locations, from where content is sent directly to users. DSR
requires servers in the edge locations to use the anycast IP address
as the source address in response packets. However, the ASes serving
the edge servers do not announce the anycast prefixes through BGP, so
the anycast prefix is hidden (invisible in BGP) on the customer
interface side at intermediate ASes which — with existing inter-
domain SAV mechanisms — would improperly block the response packets.
Figure 3 illustrates a DSR scenario where the anycast IP prefix P3 is
advertised by AS 3 through BGP. In this example, AS 3 is the
provider of AS 4 and AS 5; AS 4 is the provider of AS 1, AS 2, and AS
5; and AS 2 is the provider of AS 1. AS 2 and AS 4 have deployed
inter-domain SAV. When a user at AS 2 sends a request to the anycast
destination IP, the forwarding path is AS 2->AS 4->AS 3. The anycast
server in AS 3 receives the request and tunnels it to the edge
servers in AS 1. Finally, the edge server sends the content packets
to the user with source addresses in prefix P3. Let us say, the
forwarding path for the content packets is AS 1-> AS 4->AS 2. Since
AS 4 does not receive routing information for prefix P3 from AS 1,
EFP-uRPF Alg-A or EFP-uRPF Alg-B (or any other existing uRPF-based
mechanism) at the customer interface of AS 4 facing AS 1 will
improperly block the response packets from AS 1.
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+----------------+
Anycast Server+-+ AS 3(P3) |
+-+/\----+/\+----+
/ \
P3[AS 3] / \
/ \
/ (C2P) \
+----------------+ \
| AS 4(P4) | \
++/\+--+/\+--+/\++ \
P6[AS 2, AS 1] / | \ \
P1[AS 2, AS 1] / | \ \
P2[AS 2] / | \ \
/ (C2P) | \ P5[AS 5] \ P5[AS 5]
+----------------+ | \ \
User+-+ AS 2(P2) | | P1[AS 1] \ \
+----------+/\+--+ | P6[AS 1] \ \
\ | \ \
P6[AS 1] \ | \ \
P1[AS 1] \ | \ \
\(C2P) |(C2P) (C2P)\ (C2P)\
+---------------+ +----------------+
Edge Server+-+ AS 1(P1, P6) | | AS 5(P5) |
+----------------+ +----------------+
P3 is the anycast prefix and is only advertised by AS 3 through BGP.
Figure 3: A Direct Server Return (DSR) scenario.
Further, there are cases of specific prefixes that may be exclusively
used as source addresses (legitimately) without being advertised via
BGP by any AS. While different from DSR scenarios, these cases
similarly result in existing inter-domain SAV mechanisms improperly
blocking legitimate traffic originating from such prefixes.
4.1.3. Source Address Spoofing within a Customer Cone (SCC) Scenario
In general, improper permit of spoofed packets in SCC scenarios is
unavoidable for various uRPF-based methods in partial deployment.
For example, consider a topology in which AS 1 and AS 2 are customers
of AS 3; and AS 3 is a customer of AS 4. AS 1 and AS 2 originate
prefixes P1 and P2, respectively. AS 4 performs SAV on its customer
interface with AS 3. P1 and P2 are announced from AS 3 to AS 4 and
they would be included in the SAV list (allowlist) of AS 4 with any
SAV mechanism. Assume AS 3 doesn't do SAV. Now as an example of
SCC, if AS 2 spoofs AS 1's prefix P1 and sends the spoofed packets to
AS 4 (via AS 3), there is no way for AS 4 to detect the spoofed
traffic. AS 4's SAV cannot differentiate between the spoofed and the
legitimate packets that have source address in P1. In an SCC
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scenario of this nature, the only recourse for blocking the spoofed
traffic is for AS 3 also to be upgraded to do SAV, i.e., deployment
of SAV closer to the source of spoofing.
Another scenario is highlighted in Figure 4 while using EFP-uRPF
Alg-B method on customer interfaces. This scenario is non-SCC from
the perspective of each individual customer interfaces of AS 4, but
it is SCC from the perspective of AS 4 as a whole. EFP-uRPF Alg-B
relaxes directionality to reduce (or eliminate) false positives and
that makes it more susceptible to SCC (per the latter perspective).
This is expected because EFP-uRPF Alg-B somewhat conservatively
applies the same relaxed SAV list across all customer interfaces.
+----------------+
| AS 3(P3) |
+-+/\----+/\+----+
/ \
/ \
/ \
/ (C2P) \
+----------------+ \
| AS 4(P4) | \
++/\+--+/\+--+/\++ \
P6[AS 2, AS 1] / | \ \
P1[AS 2, AS 1] / | \ \
P2[AS 2] / | \ \
/ (C2P) | \ P5[AS 5] \ P5[AS 5]
+----------------+ | \ \
Spoofer(P5')-+ AS 2(P2) | | P1[AS 1] \ \
+----------+/\+--+ | P6[AS 1] \ \
\ | \ \
P6[AS 1] \ | \ \
P1[AS 1] \ | \ \
\(C2P) |(C2P) (C2P)\ (C2P)\
+----------------+ +----------------+
| AS 1(P1, P6) | | AS 5(P5) |
+----------------+ +----------------+
P5' is the spoofed source prefix P5 by the spoofer which is inside of
AS 2 or connected to AS 2 through other ASes.
Figure 4: A scenario of source address spoofing within a customer
cone.
In Figure 4, the source address spoofing takes place within AS 4's
customer cone, where the spoofer, which is inside of AS 2 or
connected to AS 2 through other ASes, sends spoofing traffic with
spoofed source addresses in P5 to AS 3 along the path AS 2->AS 4-> AS
3. The arrows in Figure 4 illustrate the commercial relationships
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between ASes. AS 3 serves as the provider for AS 4 and AS 5, while
AS 4 acts as the provider for AS 1, AS 2, and AS 5. Additionally, AS
2 is the provider for AS 1. Suppose AS 1 and AS 4 have deployed
inter-domain SAV, while the other ASes have not.
If AS 4 deploys EFP-uRPF Alg-B at its customer interfaces, it will
allow packets with source addresses in P5 to originate from AS 1, AS
2, and AS 5. Consequently, AS 4 will improperly permit the spoofed
packets from AS 2, enabling them to propagate further.
In the scenario of Figure 4, Strict uRPF, FP-uRPF, and EFP-uRPF Alg-A
— applied on the customer interfaces — work effectively to block the
spoofed packets from AS 2. This is because these mechanisms have
stronger directionality property than EFP-uRPF Alg-B.
4.2. SAV at Peer Interfaces
SAV is used at peer interfaces for validating the traffic entering
the validating AS and destined for the AS's customer cone. The data
packets received from a customer or lateral peer AS must have source
addresses belonging only to the prefixes in the customer cone (CC) of
that AS. In both cases, the focus is on discovering all prefixes in
the CC of the neighbor AS. So, in principle, the SAV techniques
suitable on a customer interface may also be used on a peer
interface, especially EFP-uRPF Alg-A or Alg-B, which are more
accommodative of asymmetric routing. Indeed, asymmetric routing is
thought to be prevalent for peer interfaces. If SAV techniques
suitable for customer interfaces are considered for peer interfaces,
then the gap analysis of Section 4.1 would also be applicable to the
SAV for the peer interfaces. However, due to increased concern about
asymmetric routing, network operators may conservatively use the same
relaxed SAV techniques for peer interfaces as those for provider
interfaces, e.g., Loose uRPF Section 4.3. In that case, the gap
analysis of Section 4.3 would also be applicable to the SAV for peer
interfaces.
4.3. SAV at Provider Interfaces
SAV is used at provider interfaces for validating the traffic
entering the AS and destined for the AS's customer cone. Figure 5
summarizes the gaps of ACL-based ingress filtering and Loose uRPF for
SAV at provider interfaces in the scenarios of interest. ACL-based
ingress filtering may effectively block spoofing traffic from
provider AS, while appropriately allowing legitimate traffic, but it
has high operational overhead. On the other hand, Loose uRPF
correctly permits legitimate traffic, but it can also mistakenly
allow spoofing traffic to pass through.
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In Figure 5, Spoofed from Provider Tree (SPT) is a scenario where the
spoofed traffic comes from the provider tree, i.e., the providers in
the transitive hierarchy above the validating AS. The spoofed prefix
may belong to (originated by) any AS in the Internet other than the
spoofing AS; it may even belong to an AS in the customer cone of the
validating AS (example below).
+------------------------+------------+---------------+
| Traffic & Scenarios | ACL | Loose uRPF |
+----------+-------------+------------+---------------+
|Legitimate| | | Functions |
|Traffic | -- | High | as Expected |
+----------+-------------+Operational +---------------+
|Spoofed | Spoofed | Overhead | |
|Traffic | from | (HOO) |Improper Permit|
| | Provider | | |
| | Tree (SPT) | | |
+----------+-------------+------------+---------------+
'Functions as Expected' connotes the absence of improper block.
It also connotes low operational overhead.
Figure 5: The gaps of ACL-based ingress filtering and Loose uRPF
at provider interfaces in the scenarios of interest.
Figure 6 illustrates a scenario of SPT and is used to analyze the
gaps of ACL-based ingress filtering and Loose uRPF.
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+----------------+
Spoofer(P1')+-+ AS 3(P3) |
+-+/\----+/\+----+
/ \
/ \
/ \
/ (C2P) \
+----------------+ \
| AS 4(P4) | \
++/\+--+/\+--+/\++ \
P6[AS 2, AS 1] / | \ \
P1[AS 2, AS 1] / | \ \
P2[AS 2] / | \ \
/ (C2P) | \ P5[AS 5] \ P5[AS 5]
+----------------+ | \ \
| AS 2(P2) | | P1[AS 1] \ \
+----------+/\+--+ | P6[AS 1] \ \
\ | \ \
P6[AS 1] \ | \ \
P1[AS 1] \ | \ \
\ (C2P) | (C2P) (C2P) \ (C2P) \
+----------------+ +----------------+
| AS 1(P1, P6) | | AS 5(P5) |
+----------------+ +----------------+
P1' is the spoofed source prefix P1 by the spoofer which is inside of
AS 3 or connected to AS 3 through other ASes.
Figure 6: A scenario of source address spoofing from provider AS.
In Figure 6, the spoofer which is inside of AS 3 or connected to AS 3
through other ASes forges the source addresses in P1 and sends the
spoofing traffic to the destination addresses in P2 at AS 2. AS 1 is
a customer of AS 2; AS 1 and AS 2 are customers of AS 4; AS 4 is a
customer of AS 3; and AS 5 is a customer of both AS 3 and AS 4.
Suppose AS 4 and AS 1 have deployed inter-domain SAV, while the other
ASes have not.
Using the ACL method in the form of a disallow (deny) list at the
provider interface of AS 4 (facing AS 3) incurs a very high
operational overhead. As mentioned before (Section 3), it is
impractical to store and maintain a very large and dynamically
varying set of unallocated IPv6 prefixes in the ACL.
Applying Loose uRPF at the provider interface of AS 4 (facing AS 3)
can greatly reduce the operational overhead because it uses the FIB
as the information source for allowed prefixes, and can adapt to
changes in the network to prevent false positives (improper
blocking). However, using Loose uRPF at AS 4 will naturally permit
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packets with source addresses in P1 (since P1 is present in the FIB)
and hence will not prevent the improper permit of the spoofed packets
from AS 3 Figure 6. This is an expected limitation of Loose uRPF.
5. Problem Statement
Figure 7 provides a comprehensive summary of the gap analysis in
Section 4. It highlights the scenarios where existing inter-domain
SAV mechanisms may encounter issues, including instances of improper
blocking of legitimate traffic, improper permitting of spoofing
traffic, or high operational overhead. The various entries in the
table in Section 4 can be traced back to the terminology and analyses
presented in Section 4.
+--------+----------+-----------+----------+-------+--------+
|Problems| ACL | Strict | Loose |FP-uRPF|EFP-uRPF|
| | | uRPF | uRPF | | |
| |(CI or PI)| (CI) | (PI) | (CI) | (CI) |
+--------+----------+-----------+----------+-------+--------+
|Improper| YES/NO | YES | NO** | YES |
|Block |(manual | (LPP, HP) | | (LPP, HP) |
| |operator | | | |
| |diligence)| | | |
+--------+----------+-----------+----------+-------+--------+
|Improper| YES/NO |NO (no SCC)| YES | NO (no SCC) |
|Permit |(manual |YES (SCC) | (SPT) | YES (SCC) |
| |operator | | | |
| |diligence)| | | |
+--------+----------+-----------+----------+-------+--------+
| | YES | |
| HOO | (Any | NO |
| |Scenarios)| |
+--------+----------+---------------------------------------+
CI = Customer Interface
PI = Provider Interface
HOO = High Operational Overhead
LPP = Limited Propagation of a Prefix
HP = Hidden Prefix
SCC = Spoofing within a CC
SPT = Spoofing from Provider Tree
** Typically, an HP (like DSR prefixes) is hidden on the CIs
but received on a provider or peer interface;
hence included in the FIB and that helps avoid
improper block for Loose uRPF.
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Figure 7: The scenarios where existing inter-domain SAV
mechanisms may have improper block problem for legitimate
traffic, improper permit problem for spoofing traffic, or high
operational overhead.
The problem statement that results from the gap analysis can be
expressed as follows. New proposals for SAV should aim to fill in
the following problem areas (gaps) found in the currently
standardized SAV methods (found in IETF RFCs):
* Improper block: Existing uRPF-based mechanisms suffer from
improper block (false positives) in two inter-domain scenarios:
limited propagation of a prefix (e.g., NO_EXPORT and some other
traffic engineering (TE) scenarios) and hidden prefix (e.g., CDN/
DSR scenario).
* Improper permit: With some existing uRPF-based SAV mechanisms,
improper permit (false negatives) can happen on any type of
interface (customer, lateral peer, or provider). Specifically, if
the method relaxes the directionality constraint [RFC3704]
[RFC8704]} to try to achieve zero improper blocking, the
possibility of improper permit increases. (Note: It is recognized
that unless there is full adoption of SAV in the customer cone
(CC) of the interface in consideration, improper permit is not
fully preventable in scenarios where source address spoofing
occurs from within the CC, i.e., a prefix at one Autonomous System
(AS) in the CC is spoofed from another AS in the same CC.)
* High operational overhead (HOO): ACL-based ingress SAV filtering
introduces significant operational overhead, as it needs to update
ACL rules manually to adapt to prefix or routing changes in a
timely manner. The HOO issue does not pertain to existing uRPF-
based mechanisms.
The limitations of existing uRPF-based mechanisms are due to their
exclusive reliance on BGP data. Although the algorithms themselves
have evolved (e.g., [RFC8704]), the underlying input has remained
unchanged, inherently constraining their accuracy in scenarios such
as LPP and HP. With the availability of authoritative SAV-related
information, plus the potential SAV-specific information (Section 4),
it would be possible to develop comprehensive new SAV algorithms or
mechanisms to overcome the existing gaps.
6. Requirements for New Inter-domain SAV Mechanisms
This section lists the requirements for any new inter-domain SAV
mechanisms which may be proposed to bridge the technical gaps of
existing mechanisms.
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6.1. Accurate Validation
Any new inter-domain SAV mechanism MUST avoid improper blocking and
have superior directionality property (reject more spoofed traffic)
than existing inter-domain SAV mechanisms. The requirement applies
for all directions of AS peering (customer, provider, and peer).
6.2. Reducing Operational Overhead
Any new inter-domain SAV mechanism MUST be able to automatically
adapt to network dynamics and asymmetric routing scenarios. Any such
solution MUST have less operational overhead than ACL-based ingress
SAV filtering.
6.3. Early Adopters Benefit in Incremental/Partial Deployment
Any new solution SHOULD NOT assume pervasive adoption of the SAV
method or the SAV-related information (e.g., Resource Public Key
Infrastructure (RPKI) objects such as ROAs and ASPAs). It SHOULD
benefit early adopters by providing effective protection from
spoofing of source addresses even in partial deployment.
6.4. Providing Necessary Security Guarantee
SAV-related information, such as RPKI objects, may be used for
designing a more accurate SAV. Such information must be protected at
their repositories and during communication to the relying parties
(the BGP security community is already diligent about this). If any
proposed SAV method requires exchanging SAV-specific information
between ASes, security mechanisms must exist to assure
trustworthiness of the information. The idea is to prevent malicious
injection or alteration of the SAV-specific information.
6.5. Automatic Updates to the SAV List and Efficient Convergence
Any new inter-domain SAV mechanism SHOULD be responsive to changes in
the BGP (FIB/RIB) data, the SAV-related information (Section 2), or
the SAV-specific information (Section 2). It SHOULD automatically
update the SAV list while achieving efficient re-convergence of the
same. In this context, convergence refers to the stabilization of
the SAV lists on the AS-to-AS interfaces performing SAV. It is
essential that any new SAV mechanism converges to the correct updated
SAV list in a proper manner, minimizing both improper block and
improper permit during the process.
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7. Inter-domain SAV Scope
Any new inter-domain SAV mechanisms should work in the same Internet
Protocol (IP) address scenarios as existing SAV methods do.
Generally, it includes all IP-encapsulated scenarios:
* Native IP forwarding: This includes both the global routing table
based forwarding and Customer Edge (CE) site forwarding of VPN
traffic.
* IP-encapsulated Tunnel (IPsec, GRE, SRv6, etc.): In this scenario,
the focus is on the validation of the outer layer IP source
address.
* Both IPv4 and IPv6 addresses.
The scope does not include:
* Non-IP packets: This includes MPLS label-based forwarding and
other non-IP-based forwarding.
In addition, any new inter-domain SAV mechanisms MUST NOT modify the
data plane packets. Existing architectures or protocols or
mechanisms can be inherited by any such mechanism to achieve better
SAV effectiveness.
8. Security Considerations
The SAV list will be generated based on routing information from BGP
(FIB/RIB), SAV-related information, and/or SAV-specific information.
If the information is poisoned by attackers, the SAV list will be
inaccurate. Legitimate packets may be dropped improperly or
malicious traffic with spoofed source addresses may be permitted
improperly. BGP routing security using available methods for the
prevention, detection, and mitigation of route hijacks, route leaks,
and AS_PATH manipulations should be deployed which leads to greater
accuracy of the BGP (FIB/RIB) information used for computing SAV
lists.
9. IANA Considerations
This document does not request any IANA allocations.
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10. Contributors
Nan Geng
Huawei
Beijing, China
Email: gengnan@huawei.com
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
11.2. Informative References
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/rfc/rfc4271>.
[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/rfc/rfc5210>.
[RFC7908] Sriram, K., Montgomery, D., McPherson, D., Osterweil, E.,
and B. Dickson, "Problem Definition and Classification of
BGP Route Leaks", RFC 7908, DOI 10.17487/RFC7908, June
2016, <https://www.rfc-editor.org/rfc/rfc7908>.
[RFC9234] Azimov, A., Bogomazov, E., Bush, R., Patel, K., and K.
Sriram, "Route Leak Prevention and Detection Using Roles
in UPDATE and OPEN Messages", RFC 9234,
DOI 10.17487/RFC9234, May 2022,
<https://www.rfc-editor.org/rfc/rfc9234>.
[RFC7094] McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
"Architectural Considerations of IP Anycast", RFC 7094,
DOI 10.17487/RFC7094, January 2014,
<https://www.rfc-editor.org/rfc/rfc7094>.
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[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/rfc/rfc3704>.
[RFC8704] Sriram, K., Montgomery, D., and J. Haas, "Enhanced
Feasible-Path Unicast Reverse Path Forwarding", BCP 84,
RFC 8704, DOI 10.17487/RFC8704, February 2020,
<https://www.rfc-editor.org/rfc/rfc8704>.
[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/rfc/rfc2827>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/rfc/rfc4364>.
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
December 2006, <https://www.rfc-editor.org/rfc/rfc4786>.
[I-D.ietf-savnet-intra-domain-problem-statement]
Li, D., Wu, J., Qin, L., Huang, M., and N. Geng, "Source
Address Validation in Intra-domain Networks Gap Analysis,
Problem Statement, and Requirements", Work in Progress,
Internet-Draft, draft-ietf-savnet-intra-domain-problem-
statement-21, 18 January 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-savnet-
intra-domain-problem-statement-21>.
[manrs] MANRS, "Anti-Spoofing - Preventing traffic with spoofed
source IP addresses (Module 5)",
<https://manrs.org/resources/training/tutorials/anti-
spoofing/>.
[isoc] Internet Society, "Addressing the challenge of IP
spoofing", 2015,
<https://www.internetsociety.org/resources/doc/2015/
addressing-the-challenge-of-ip-spoofing/>.
[nist] Sriram, K. and D. Montgomery, "Border Gateway Protocol
Security and Resilience", NIST SP 800-189r1 , 2025,
<https://doi.org/10.6028/NIST.SP.800-189r1.ipd>.
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[urpf] Cisco Systems, Inc., "Unicast Reverse Path Forwarding
Enhancements for the Internet Service Provider-Internet
Service Provider Network Edge", 2005,
<https://www.cisco.com/c/dam/en_us/about/security/
intelligence/urpf.pdf>.
Acknowledgements
Many thanks to Jared Mauch, Barry Greene, Fang Gao, Anthony Somerset,
Yuanyuan Zhang, Igor Lubashev, Alvaro Retana, Joel Halpern, Ron
Bonica, Aijun Wang, Michael Richardson, Li Chen, Gert Doering,
Mingxing Liu, John O'Brien, and Roland Dobbins for their reviews,
comments, and suggestions. Apologies to any others whose names the
authors may have inadvertently missed mentioning.
Authors' Addresses
Dan Li
Tsinghua University
Beijing
China
Email: tolidan@tsinghua.edu.cn
Lancheng Qin
Zhongguancun Laboratory
Beijing
China
Email: qinlc@zgclab.edu.cn
Libin Liu
Zhongguancun Laboratory
Beijing
China
Email: liulb@zgclab.edu.cn
Mingqing Huang
Huawei
Beijing
China
Email: huangmingqing@huawei.com
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Kotikalapudi Sriram
USA National Institute of Standards and Technology
Gaithersburg, MD
United States of America
Email: sriram.ietf@gmail.com
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