Internet Engineering Task Force D. Li
Internet-Draft J. Wu
Intended status: Informational Tsinghua University
Expires: 4 September 2025 L. Liu
Zhongguancun Laboratory
M. Huang
Huawei
K. Sriram
USA NIST
3 March 2025
Source Address Validation in Inter-domain Networks Gap Analysis, Problem
Statement, and Requirements
draft-ietf-savnet-inter-domain-problem-statement-07
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Existing Inter-domain SAV Mechanisms . . . . . . . . . . . . 5
4. Gap Analysis . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. SAV at Customer Interfaces . . . . . . . . . . . . . . . 7
4.1.1. Limited Propagation of Prefixes . . . . . . . . . . . 9
4.1.2. Hidden Prefixes . . . . . . . . . . . . . . . . . . . 10
4.1.3. Source Address Spoofing within a Customer Cone . . . 12
4.2. SAV at Provider/Peer Interfaces . . . . . . . . . . . . . 13
4.2.1. Source Address Spoofing from Provider/Peer AS . . . . 14
5. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 16
6. Requirements for New Inter-domain SAV Mechanisms . . . . . . 18
6.1. Accurate Validation . . . . . . . . . . . . . . . . . . . 18
6.1.1. Improving Validation Accuracy over Existing
Mechanisms . . . . . . . . . . . . . . . . . . . . . 18
6.1.2. Working in Incremental/Partial Deployment . . . . . . 20
6.1.3. Providing Necessary Security Guarantee . . . . . . . 21
6.2. Automatic Update . . . . . . . . . . . . . . . . . . . . 21
6.2.1. Reducing Operational Overhead . . . . . . . . . . . . 21
6.2.2. Guaranteeing Convergence . . . . . . . . . . . . . . 21
7. Inter-domain SAV Scope . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 22
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . 23
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
Source address validation (SAV) is crucial for protecting networks
from source address (SA) spoofing attacks [RFC2827] [RFC3704]
[RFC8704]. The MANRS initiative advocates deploying SAV as close to
the source as possible [manrs], and access networks are the first
line of defense against source address spoofing. However, access
networks face various challenges in deploying SAV mechanisms due to
different network environments, router vendors, and operational
preferences. Hence, SAV may not be deployed ubiquitously in access
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networks. In addition, SA spoofing may also originate in ISP
networks at higher levels of heirarchy in the Internet. So,
deployment of SAV mechanisms in the edge routers of enterprises as
well as the ISP networks (at different heirarchal levels or tiers) is
needed to prevent source address spoofing along the data forwarding
paths. [RFC5210] highlighted the importance of SAV at various
network locations: access, intra-domain, and inter-domain. This
document focuses on providing gap analysis and describing the problem
space of existing inter-domain SAV solutions, and defining the
requirements for a new solution of these problems. Access Control
List (ACL) and unicast Reverse Path Forwarding (uRPF) techniques are
currently utilized for inter-domain SAV [RFC3704] [RFC8704]. Here
only existing IETF RFCs are considered as the state of the art (BCP
38 [RFC2827] and BCP 84 [RFC3704] [RFC8704]); IETF works-in-progress
are not included in that.
There are several existing mechanisms for inter-domain SAV. This
document analyzes them and attempts to answer: i) what are the
technical gaps (Section 4), ii) what are the fundamental problems
(Section 5), and iii) what are the practical requirements for the
solution of 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 in two inter-domain scenarios: imited propagation
of prefixes and hidden prefixes.
* Improper permit: Existing uRPF-based mechanisms exhibit improper
permit in scenarios involving source address spoofing within a
customer cone or from a provider/peer AS.
* High operational overhead: 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.
To address these problems, in Section 6, this document outlines the
following technical requirements for a new solution:
* Improving validation accuracy over existing mechanisms: A new
solution MUST avoid improper block and minimize improper permit.
* Reducing operational overhead: A new solution MUST have less
operational overhead than ACL-based ingress SAV filtering.
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In addition, this document defines three more requirements to ensure
practicality:
* Working in incremental/partial deployment: A new solution MUST NOT
assume pervasive adoption and SHOULD provide effective protection
for source addresses when it is partially deployed in the
Internet.
* Providing necessary security guarantee: A new solution SHOULD
secure the communicated information between ASes if it requires
exchanging specific information between ASes.
* Guaranteeing convergence: A new solution SHOULD achieve accurate
SAV rule convergence in response to prefix or routing changes.
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.
2. Terminology
SAV Rule:
The rule that indicates the validity of a specific source IP
address or source IP prefix.
Improper Block:
The validation results that the packets with legitimate source
addresses are blocked improperly due to inaccurate SAV rules.
Improper Permit:
The validation results that the packets with spoofed source
addresses are permitted improperly due to inaccurate SAV rules.
Real forwarding paths:
The paths that the legitimate traffic goes through in the data
plane.
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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.
* 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. It can be applied at
provider interfaces, peer interfaces, or customer interfaces of an
AS, and is recommended for deployment at provider interfaces
[manrs]. At the provider interfaces, ACL-based ingress SAV
filtering can block source prefixes that are clearly invalid in
the inter-domain routing context, such as IANA special purpose or
unallocated IPv4/IPv6 prefixes and the AS's internal-only
prefixes. 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. It is also impractical to store a
very large and dynamically varying unallocated IPv6 prefixes. At
the customer interfaces, ACL-based ingress filtering is less
desirable. Other techniques (as described below) are more
effective for ingress SAV filtering on customer interfaces. ACL-
based ingress SAV filtering has applicability for broadband cable
or digital subscriber access loop (DSL) access networks where the
service provider has clear knwoledge of IP address prefixes it has
allocated to manage those services.
* uRPF-based mechanisms: A class of SAV mechanisms are based on
Unicast Reverse Path Forwarding (uRPF) [RFC3704]. 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 holds in
practice, and to address cases where it does not hold, many
enhancements and modes of uRPF are proposed. Different modes of
uRPF have different levels of strictness and flexibility, and
network operators can choose from them to suit particular network
scenarios. We describe these modes as follows:
- Strict uRPF [RFC3704]: Strict uRPF is the most stringent mode,
and it only permits packets that have a source address that is
covered by a prefix in the FIB, and that the next hop for that
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prefix is the same as the incoming interface. This mode is
recommended for deployment at customer interfaces that directly
connect to an AS with suballocated address space, as it can
prevent spoofing attacks from that AS or its downstream ASes
[nist].
- Loose uRPF [RFC3704]: Loose uRPF verifies that the source
address of the packet is routable in the Internet by matching
it with one or more prefixes in the FIB, regardless of which
interface the packet arrives at. 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 that are obviously disallowed,
such as non-global prefixes (e.g., private addresses, multicast
addresses, etc.) or the prefixes that belong to the customer AS
itself [nist].
- FP-uRPF [RFC3704]: FP-uRPF maintains a reverse path forwarding
(RPF) list, which contains the prefixes and all their
permissible routes including the optimal and alternative ones.
It permits an incoming packet only if the packet's source
address is encompassed in the prefixes of the RPF list and its
incoming interface is included in the permissible routes of the
corresponding prefix. FP-uRPF is recommended to be deployed at
customer interfaces or peer interfaces, especially those that
are connected to multi-homed customer ASes [nist].
- 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 table.
- EFP-uRPF [RFC8704]: EFP-uRPF consists of two algorithms,
algorithm A and algorithm B. EFP-uRPF is based on the idea
that an AS can receive BGP updates for multiple prefixes that
have the same origin AS at different interfaces. For example,
this can happen when the origin AS is multi-homed and
advertises the same prefixes to different providers. In this
case, EFP-uRPF allows an incoming packet with a source address
in any of those prefixes to pass on any of those interfaces.
This way, EFP-uRPF can handle asymmetric routing scenarios
where the incoming and outgoing interfaces for a packet are
different. EFP-uRPF has not been implemented in practical
networks yet, but BCP84 [RFC3704] [RFC8704] suggests using EFP-
uRPF with algorithm B at customer interfaces of an AS. EFP-
uRPF can also be used at peer interfaces of an AS.
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* Carrier Grade NAT (CGN): CGN is a network technology used by
service providers to translate between private and public IPv4
addresses within their network. CGN enables service providers to
assign private IPv4 addresses to their customer ASes instead of
public, globally unique IPv4 addresses. The private side of the
CGN faces the customer ASes, and when an incoming packet is
received from a customer AS, CGN checks its source address. If
the source address is included in the address list of the CGN's
private side, CGN performs address translation. Otherwise, it
forwards the packet without translation. However, since CGN
cannot determine whether the source address of an incoming packet
is spoofed or not, additional SAV mechanisms need to be
implemented to prevent source address spoofing [manrs].
* BGP origin validation (BGP-OV) [RFC6811]: Attackers can bypass
uRPF-based SAV mechanisms by using prefix hijacking in combination
with source address spoofing. By announcing a less-specific
prefix that does not have a legitimate announcement, the attacker
can deceive existing uRPF-based SAV mechanisms and successfully
perform address spoofing. To protect against this type of attack,
a combination of BGP-OV and uRPF-based mechanisms like FP-uRPF or
EFP-uRPF is recommended [nist]. BGP routers can use ROA
information, which is a validated list of {prefix, maximum length,
origin AS}, to mitigate the risk of prefix hijacks in advertised
routes.
4. Gap Analysis
Inter-domain SAV is essential in preventing source address spoofing
traffic across all AS interfaces, including those of customers,
providers, and peers. An ideal inter-domain SAV mechanism MUST block
all spoofing traffic while permitting legitimate traffic in all
scenarios. 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 used in the corresponding interfaces of these scenarios to
identify their technical limitations.
4.1. SAV at Customer Interfaces
SAV is used at customer interfaces to validate traffic from the
customer cone, including both legitimate traffic and spoofing
traffic. To prevent the source address spoofing, operators can
enable ACL-based ingress filtering and/or uRPF-based mechanisms at
customer interfaces, namely Strict uRPF, FP-uRPF, or EFP-uRPF.
However, uRPF-based mechanisms may cause improper block problems in
two inter-domain scenarios: limited propagation of prefixes and
hidden prefixes, or may cause improper permit problems in the
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scenarios of source address spoofing within a customer cone, while
ACL-based SAV ingress filtering needs to update SAV rules in a timely
manner and lead to high operational overhead.
+--------------------+------------+-----------+-------+--------+
|Traffic & Scenarios | ACL |Strict uRPF|FP-uRPF|EFP-uRPF|
+----------+---------+------------+-----------+-------+--------+
|Legitimate| LPP | | |
|Traffic +---------+ | Improper Block |
| | HP | High | |
+----------+---------+Operational +-------------------+--------+
|Spoofing |Spoofing | Overhead | |Improper|
|Traffic | within | | Functioning as |Permit |
| | a CC | | Expected | |
+----------+---------+------------+-------------------+--------+
"LPP" represents a class of scenario called limited propagation of
prefixes.
"HP" represents a class of scenario called hidden prefixes.
"Spoofing within a CC" represents a class of scenario where
spoofing traffic occurs within a customer cone (CC) and the spoofed
source addresses belong to this customer cone.
"Functioning as Expected" represents the inter-domain SAV mechanism
does not cause improper block for legitimate traffic or improper
permit for spoofing traffic in the corresponding scenarios, and has
low operational overhead.
Figure 1: The gaps of ACL-based ingress filtering, Strict uRPF,
FP-uRPF, and EFP-uRPF in the corresponding scenarios.
Figure 1 provides an overview of the gaps associated with ACL-based
ingress filtering, Strict uRPF, FP-uRPF, and EFP-uRPF for SAV at
customer interfaces in the corresponding scenarios. ACL-based
ingress filtering has high operational overhead as performing SAV at
customer interfaces. Strict uRPF, FP-uRPF, and EFP-uRPF, on the
other hand, may incorrectly block legitimate traffic in the scenarios
of limited propagation of prefixes or hidden prefixes. Furthermore,
in the scenarios of source address spoofing within a customer cone,
EFP-uRPF with algorithm B may inadvertently permit the spoofing
traffic.
In the following, we analyze the gaps of Strict uRPF, FP-uRPF, and
EFP-uRPF for SAV at customer interfaces in scenarios of limited
propagation of prefixes, hidden prefixes, and source address spoofing
within a customer cone, respectively.
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4.1.1. Limited Propagation of Prefixes
In inter-domain networks, some prefixes may not be propagated to all
domains due to various factors, such as NO_EXPORT or NO_ADVERTISE
communities or other route filtering policies. This may cause
asymmetric routing in the inter-domain context, which may lead to
improper block when performing SAV with existing mechanisms. These
mechanisms include EFP-uRPF, which we focus on in the following
analysis, as well as 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] \ \
\ | NO_EXPORT \ \
P1[AS 1] \ | \ \
NO_EXPORT \ | \ \
\ (C2P) | (C2P/P2P) (C2P) \ (C2P) \
+----------------+ +----------------+
| AS 1(P1, P6) | | AS 5(P5) |
+----------------+ +----------------+
Figure 2: Limited propagation of prefixes caused by NO_EXPORT.
Figure 2 presents a scenario where the limited propagation of
prefixes occurs due to the NO_EXPORT community attribute. In this
scenario, AS 1 is a customer of AS 2, AS 2 is a customer of AS 4, AS
4 is a customer of AS 3, and AS 5 is a customer of both AS 3 and AS
4. The relationship between AS 1 and AS 4 can be either customer-to-
provider (C2P) or peer-to-peer (P2P). AS 1 advertises prefixes P1 to
AS 2 and adds the NO_EXPORT community attribute to the BGP
advertisement sent to AS 2, preventing AS 2 from further propagating
the route for prefix P1 to AS 4. Similarly, AS 1 adds the NO_EXPORT
community attribute to the BGP advertisement sent to AS 4, resulting
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in AS 4 not propagating the route for prefix P6 to AS 3.
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.
Assuming that AS 1 is the customer of AS 4, if AS 4 deploys EFP-uRPF
with algorithm A at customer interfaces, it will require packets with
source addresses in P1 to only arrive from AS 1. When AS 1 sends
legitimate packets with source addresses in P1 to AS 4 through AS 2,
AS 4 improperly blocks these packets. The same problem applies to
Strict uRPF and FP-uRPF. Although EFP-uRPF with algorithm B can
avoid improper block in this case, network operators need to first
determine whether limited prefix propagation exists before choosing
the suitable EFP-uRPF algorithms, which adds more complexity and
overhead to network operators. Furthermore, EFP-uRPF with algorithm
B is not without its problems. For example, if AS 1 is the peer of
AS 4, AS 4 will not learn the route of P1 from its customer
interfaces. In such case, both EFP-uRPF with algorithm A and
algorithm B have improper block problems.
4.1.2. Hidden Prefixes
Some servers' source addresses are not advertised through BGP to
other ASes. These addresses are unknown to the inter-domain routing
system and are called hidden prefixes. Legitimate traffic with these
hidden prefixes may be dropped by existing inter-domain SAV
mechanisms, such as Strict uRPF, FP-uRPF, or EFP-uRPF, because they
do not match any known prefix.
For example, Content Delivery Networks (CDN) use anycast [RFC4786]
[RFC7094] to improve the quality of service by bringing content
closer to users. An anycast IP address is assigned to devices in
different locations, and incoming requests are routed to the closest
location. Usually, only locations with multiple connectivity
announce the anycast IP address through BGP. The CDN server receives
requests from users and creates tunnels to the edge locations, where
content is sent directly to users using direct server return (DSR).
DSR requires servers in the edge locations to use the anycast IP
address as the source address in response packets. However, these
edge locations do not announce the anycast prefixes through BGP, so
an intermediate AS with existing inter-domain SAV mechanisms may
improperly block these response packets.
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+----------------+
Anycast Server+-+ AS 3(P3) |
+-+/\----+/\+----+
/ \
P3[AS 3] / \ P3[AS 3]
/ \
/ (C2P) \
+----------------+ \
| AS 4(P4) | \
++/\+--+/\+--+/\++ \
P6[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] \ | NO_EXPORT \ \
P1[AS 1] \ | \ \
NO_EXPORT \ | \ \
\ (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.
Figure 3 illustrates a DSR scenario where the anycast IP prefix P3 is
only 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 1 and AS 4 have deployed
inter-domain SAV, while other ASes have not. When users in AS 2 send
requests to the anycast destination IP, the forwarding path is AS
2->AS 4->AS 3. The anycast servers in AS 3 receive the requests and
tunnel them to the edge servers in AS 1. Finally, the edge servers
send the content to the users with source addresses in prefix P3.
The reverse forwarding path is AS 1->AS 4->AS 2. Since AS 4 does not
receive routing information for prefix P3 from AS 1, EFP-uRPF with
algorithm A/B, and all other existing uRPF-based mechanisms at the
customer interface of AS 4 facing AS 1 will improperly block the
legitimate response packets from AS 1.
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Moreover, EFP-uRPF with algorithm B may also permit spoofing traffic
improperly in scenarios where source address spoofing within a
customer cone occur. We provide illustrations of these scenarios
using an example in the following. The source address spoofing
within a customer cone represents a class of scenario where spoofing
traffic comes from a customer AS within a customer cone and the
spoofed source addresses belong to this customer cone.
4.1.3. Source Address Spoofing within a Customer Cone
Figure 4 portrays a scenario of source address spoofing within a
customer cone and is used to analyze the gaps of uRPF-based
mechanisms below.
+----------------+
| AS 3(P3) |
+-+/\----+/\+----+
/ \
/ \
/ \
/ (C2P) \
+----------------+ \
| AS 4(P4) | \
++/\+--+/\+--+/\++ \
P6[AS 1, AS 2] / | \ \
P1[AS 1, AS 2] / | \ \
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
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3. The arrows in Figure 4 illustrate the commercial relationships
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 with algorithm 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, when the spoofer which is inside
of AS 2 or connected to AS 2 through other ASes sends spoofing
packets with spoofed source addresses in P5 to AS 3, AS 4 will
improperly permit these packets, thus enabling the spoofing traffic
to propagate.
In scenarios like these, Strict uRPF, FP-uRPF, VRF uRPF, and EFP-uRPF
with algorithm A do not suffer from improper permit problems. This
is because these mechanisms enforce strict filtering rules that
ensure packets with source addresses in P5 are only permitted to
arrive at AS 4's customer interfaces facing AS 5.
4.2. SAV at Provider/Peer Interfaces
SAV is used at provider/peer interfaces to validate traffic entering
the customer cone, including both legitimate and spoofing traffic.
To prevent packets with spoofed source addresses from the provider/
peer AS, ACL-based ingress filtering and/or Loose uRPF can be
deployed [nist].
+------------------------+------------+---------------+
| Traffic & Scenarios | ACL | Loose uRPF |
+----------+-------------+------------+---------------+
|Legitimate| Any | | Functioning |
|Traffic | Scenarios | High | as Expected |
+----------+-------------+Operational +---------------+
|Spoofing | Spoofing | Overhead | |
|Traffic | from | |Improper Permit|
| |Provider/Peer| | |
| | AS | | |
+----------+-------------+------------+---------------+
"Spoofing from provider/peer AS" represents a class of scenario where
source address spoofing traffic from provider/peer AS occurs and the
spoofed source addresses belong to the customer cone which the
spoofing traffic enters.
"Functioning as Expected" represents the inter-domain SAV mechanism
does not cause improper block for legitimate traffic or improper
permit for spoofing traffic in the corresponding scenarios, and has
low operational overhead.
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Figure 5: The gaps of ACL-based ingress filtering, and Loose uRPF
in the corresponding scenarios.
Figure 5 summarizes the gaps of ACL-based ingress filtering and Loose
uRPF for SAV at provider/peer interfaces in the corresponding
scenarios. ACL-based ingress filtering effectively blocks spoofing
traffic from provider/peer AS, while appropriately allowing
legitimate traffic. However, these methods may come with high
operational overhead. On the other hand, Loose uRPF correctly
permits legitimate traffic, but it can also mistakenly allow spoofing
traffic to pass through.
In the following, we expose the limitations of ACL-based ingress
filtering and Loose uRPF for SAV at provider/peer interfaces in
scenarios of source address spoofing from provider/peer AS. The
source address spoofing from provider/peer AS represents a class of
scenario where spoofing traffic comes from a provider/peer AS and the
spoofed source addresses belong to the customer cone which the
spoofing traffic enters.
4.2.1. Source Address Spoofing from Provider/Peer AS
Figure 6 depicts the scenario of source address spoofing from
provider/peer AS and is used to analyze the gaps of ACL-based ingress
filtering and Loose uRPF below.
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+----------------+
Spoofer(P1')+-+ AS 3(P3) |
+-+/\----+/\+----+
/ \
/ \
/ \
/ (C2P/P2P) \
+----------------+ \
| AS 4(P4) | \
++/\+--+/\+--+/\++ \
P6[AS 1, AS 2] / | \ \
P1[AS 1, AS 2] / | \ \
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/
peer AS.
In the case of 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.
The arrows in Figure 6 represent the commercial relationships between
ASes. AS 3 acts as the provider or lateral peer of AS 4 and the
provider for AS 5, while AS 4 serves 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.
By applying ACL-based ingress filtering at the provider/peer
interface of AS 4, the ACL rules can block any packets with spoofed
source addresses from AS 3 in P1. However, this approach incurs
heavy operational overhead, as it requires network operators to
update the ACL rules promptly based on changes in prefixes or
topology of AS 4's customer cone. Otherwise, it may cause improper
block of legitimate traffic or improper permit of spoofing traffic.
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Loose uRPF can greatly reduce the operational overhead because it
uses the local FIB as information source, and can adapt to changes in
the network. However, it would improperly permit spoofed packets.
In Figure 6, Loose uRPF is enabled at AS 4's provider/peer interface,
while EFP-uRPF is enabled at AS 4's customer interfaces. A spoofer
inside AS 3 or connected to it through other ASes may send packets
with source addresses spoofing P1 to AS 2. As AS 3 lacks deployment
of inter-domain SAV, the spoofing packets will reach AS 4's provider/
peer interface. With Loose uRPF, AS 4 cannot block them at its
provider/peer interface facing AS 3, and thus resulting in improper
permit.
5. Problem Statement
+--------+----------+---------+----------+-------+----------+
|Problems| ACL | Strict | Loose |FP-uRPF|EFP-uRPF |
| | | uRPF | uRPF | | |
+--------+----------+---------+----------+-------+----------+
|Improper|Not Exist | Exist |Not Exist | Exist |
|Block | |(LPP, HP)| | (LPP, HP) |
+--------+----------+---------+----------+-------+----------+
|Improper| Not Exist | Exist |Not | Exist |
|Permit | | (SPP) |Exist | (SCC) |
+--------+----------+---------+----------+-------+----------+
| | Exist | |
| HOO | (Any | Not Exist |
| |Scenarios)| |
+--------+----------+---------------------------------------+
HOO: High Operational Overhead.
"LPP" represents a class of scenario called limited propagation of
prefixes.
"HP" represents a class of scenario called hidden prefixes.
"SPP" represents a class of scenario called source address spoofing
from provider/peer AS.
"SCC" represents a class of scenario called source address spoofing
within a customer cone.
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.
Based on the analysis above, we conclude that existing inter-domain
SAV mechanisms exhibit limitations in asymmetric routing scenarios,
leading to potential issues of improper block or improper permit.
Additionally, these mechanisms can result in high operational
overhead, especially when network routing undergoes dynamic changes.
Figure 7 provides a comprehensive summary of scenarios where existing
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inter-domain SAV mechanisms may encounter issues, including instances
of improper blocking of legitimate traffic, improper permitting of
spoofing traffic, or high operational overhead.
For ACL-based ingress filtering, network operators need to manually
update ACL rules to adapt to network changes. Otherwise, they may
cause improper block or improper permit issues. Manual updates
induce high operational overhead, especially in networks with
frequent policy and route changes.
Strict uRPF and Loose uRPF are automatic SAV mechanisms, thus they do
not need any manual effort to adapt to network changes. However,
they have issues in scenarios with asymmetric routing. Strict uRPF
may cause improper block problems when an AS is multi-homed and
routes are not symmetrically announced to all its providers. This is
because the local FIB may not include the asymmetric routes of the
legitimate packets, and Strict uRPF only uses the local FIB to check
the source addresses and incoming interfaces of packets. Loose uRPF
may cause improper permit problems and fail to prevent source address
spoofing. This is because it is oblivious to the incoming interfaces
of packets.
FP-uRPF improve Strict uRPF in multi-homing scenarios. However, they
still have improper block issues in asymmetric routing scenarios.
For example, they may not handle the cases of limited propagation of
prefixes. These mechanisms use the local RIB to learn the source
prefixes and their valid incoming interfaces. But the RIB may not
have all the prefixes with limited propagation and their permissible
incoming interfaces.
EFP-uRPF allows the prefixes from the same customer cone at all
customer interfaces. This solves the improper block problems of FP-
uRPF in multi-homing scenarios. However, this approach also
compromises partial protection against spoofing from the customer
cone. EFP-uRPF may still have improper block problems when it does
not learn legitimate source prefixes. For example, hidden prefixes
are not learned by EFP-uRPF.
Finally, existing inter-domain SAV mechanisms cannot work in all
directions (i.e. interfaces) of ASes to achieve effective SAV.
Network operators need to carefully analyze the network environment
and choose appropriate SAV mechanism for each interface. This leads
to additional operational and cognitive overhead, which hinders the
rate of adoption of inter-domain SAV.
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6. Requirements for New Inter-domain SAV Mechanisms
This section lists the requirements which can help bridge the
technical gaps of existing inter-domain SAV mechanisms. These
requirements serve as the practical guidelines that can be met, in
part or in full, by proposing new techniques.
6.1. Accurate Validation
The new inter-domain SAV mechanism MUST improve the validation
accuracy in all directions of ASes over existing inter-domain SAV
mechanisms, while working in incremental/partial deployment and
providing necessary security guarantee.
6.1.1. Improving Validation Accuracy over Existing Mechanisms
It MUST avoid improper block and permit less spoofing traffic than
existing inter-domain SAV mechanisms. To avoid improper block, ASes
that deploy the new inter-domain SAV mechanism SHOULD be able to
acquire all the real data plane forwarding paths, which are the paths
that the legitimate traffic goes through in the data plane.
However, it may be hard to learn the real forwarding paths of
prefixes exactly under some scenarios, such as asymmetric routing
scenario and DSR scenario. For such scenarios, it is crucial to
minimize the set of acceptable paths while ensuring the inclusion of
all real forwarding paths, thereby preventing improper block and
minimizing improper permit. Note that the acceptable paths are all
the possible paths that the legitimate traffic may go through in the
data plane, cover all the links at each level of customer-provider
hierarchy, and MUST include all the real forwarding paths. Reducing
the set of acceptable paths means eliminating the paths that are not
the real forwarding paths of the prefixes from the set.
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+----------------+
| AS 3(P3) |
+-+/\----+/\+----+
/ \
/ \
/ \
/ (C2P) \
+----------------+ \
| AS 4(P4) | \
++/\+--+/\+--+/\++ \
P6[AS 1, AS 2] / | \ \
P2[AS 2] / | \ \
/ | \ \
/ (C2P) | \ P5[AS 5] \ P5[AS 5]
+----------------+ | \ \
| AS 2(P2) | | P1[AS 1] \ \
+----------+/\+--+ | P6[AS 1] \ \
P6[AS 1] \ | NO_EXPORT \ \
P1[AS 1] \ | \ \
NO_EXPORT \ | \ \
\ (C2P) | (C2P) (C2P) \ (C2P) \
+----------------+ +----------------+
| AS 1(P1, P6) | | AS 5(P5) |
+----------------+ +----------------+
Figure 8: An example to illustrate accurate validation in all
directions of an AS.
Multiple sources of SAV-related information, such as RPKI ROA objects
and ASPA objects, and SAV-specific information from other ASes, can
assist in reducing the set of acceptable paths. Figure 8 is used as
an example to illustrate how to avoid improper block and minimize
improper permit in all directions of an AS based on different SAV
information sources. AS 3 is the provider of AS 4 and AS 5, while AS
4 is the provider of AS 1, AS 2, and AS 5, and AS 2 is the provider
of AS 1. Assuming prefixes P1, P2, P3, P4, P5, and P6 are all the
prefixes in the network. Inter-domain SAV has been deployed by AS 1
and AS 4, but not by other ASes. Here, the focus is on how to
conduct SAV in all directions of AS 4 when different SAV information
sources are available to use.
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Since the source prefix ranges of the traffic entering the customer
cone of AS 4 are not fully learned in the partial deployment
scenario, SAV at provider/peer interfaces can use a blocklist. For
example, as shown in Figure 8, the traffic with source addresses in
P5 may come from AS 5 or AS 3. In contrast, SAV at customer
interfaces for traffic going out of the customer cone can use an
allowlist to allow the known prefixes of the customer cone at the
corresponding customer interfaces and other unknown prefixes at all
the customer interfaces.
The followings show how to generate SAV rules based on the SAV-
related information from different SAV information sources to avoid
improper block and reduce as much improper permit as possible.
* If only the RIB is available, AS 4 can conduct SAV towards its
neighboring ASes as follows like [RFC8704]: SAV towards AS 1
permits the prefixes P1 and P6, SAV towards AS 2 permits the
prefixes P1, P2, and P6, SAV towards AS 5 permits the prefix P5,
and SAV towards AS 3 does not block any prefix.
* When both RPKI ROA objects and ASPA objects are deployed by AS 1
and AS 4, AS 4 can conduct SAV towards its neighboring ASes as
follows like [bar-sav]: SAV towards AS 1 permits the prefixes P1
and P6, SAV towards AS 2 permits the prefixes P1, P2, and P6, SAV
towards AS 5 permits the prefix P5, and SAV towards AS 3 blocks
the prefixes P1, P2, and P6.
* Moreover, if SAV-specific information that exactly contains all
the real data plane forwarding paths of prefixes is accessible,
SAV rules can be refined. AS 4 can conduct SAV towards its
neighboring ASes as follows: SAV towards AS 1 permits only P1.
SAV towards AS 2 permits the prefixes P2 and P6, while SAV towards
AS 5 permits the prefix P5 and SAV towards AS 3 blocks the
prefixes P1, P2, and P6.
It is evident that, in a partial deployment scenario, more accurate
SAV-related information can effectively achieve 0% improper block and
significantly minimize improper permit.
6.1.2. Working in Incremental/Partial Deployment
The new inter-domain SAV mechanism MUST NOT assume pervasive adoption
and SHOULD provide effective protection for source addresses when it
is partially deployed in the Internet. Not all AS border routers can
support the new SAV mechanism at once, due to various constraints
such as capabilities, versions, or vendors. The new SAV mechanism
should not be less effective in protecting all directions of ASes
under partial deployment than existing mechanisms.
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6.1.3. Providing Necessary Security Guarantee
The new inter-domain SAV mechanism SHOULD secure the communicated
SAV-specific information between ASes and prevent malicious ASes from
generating forged information.
6.2. Automatic Update
The new inter-domain SAV mechanism SHOULD update SAV rules and detect
the changes of SAV-specific information automatically while
guaranteeing convergence.
6.2.1. Reducing Operational Overhead
The new inter-domain SAV mechanism MUST be able to adapt to dynamic
networks and asymmetric routing scenarios automatically, instead of
relying on manual update. At least, it MUST have less operational
overhead than ACL-based ingress filtering.
6.2.2. Guaranteeing Convergence
The new inter-domain SAV mechanism SHOULD promptly detect the network
changes and launch the convergence process quickly. It is essential
that the new inter-domain SAV mechanism converges towards accurate
SAV rules in a proper manner, effectively reducing improper block and
improper permit throughout the whole convergence process.
7. Inter-domain SAV Scope
The new inter-domain SAV mechanisms should work in the same scenarios
as existing ones. Generally, it includes all IP-encapsulated
scenarios:
* Native IP forwarding: This includes both global routing table
forwarding and CE site forwarding of VPN.
* IP-encapsulated Tunnel (IPsec, GRE, SRv6, etc.): In this scenario,
we focus on the validation of the outer layer IP address.
* Both IPv4 and IPv6 addresses.
Scope does not include:
* Non-IP packets: This includes MPLS label-based forwarding and
other non-IP-based forwarding.
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In addition, the new inter-domain SAV mechanisms should not modify
data plane packets. Existing architectures or protocols or
mechanisms can be inherited by the new SAV mechanism to achieve
better SAV effectiveness.
8. Security Considerations
SAV rules can be generated based on route information (FIB/RIB) or
non-route information. If the information is poisoned by attackers,
the SAV rules will be false. Legitimate packets may be dropped
improperly or malicious traffic with spoofed source addresses may be
permitted improperly. Route security should be considered by routing
protocols. Non-route information, such as RPKI ASPA objects, should
also be protected by corresponding mechanisms or infrastructure. If
SAV mechanisms or protocols require exchanging specific information
between ASes, some considerations on the avoidance of message
alteration or message injection are needed to propose.
The SAV procedure referred in this document modifies no field of
packets. So, security considerations on the data plane are not in
the scope of this document.
9. IANA Considerations
This document does not request any IANA allocations.
10. Contributors
Lancheng Qin
Zhongguancun Laboratory
Beijing, China
Email: qinlc@zgclab.edu.cn
Nan Geng
Huawei
Beijing, China
Email: gengnan@huawei.com
11. References
11.1. Normative References
[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>.
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[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>.
[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>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/rfc/rfc6811>.
[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>.
[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>.
11.2. Informative References
[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>.
[manrs] MANRS, "MANRS Implementation Guide", 2023,
<https://www.manrs.org/netops/guide/antispoofing/>.
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[isoc] Internet Society, "Addressing the challenge of IP
spoofing", 2015,
<https://www.internetsociety.org/resources/doc/2015/
addressing-the-challenge-of-ip-spoofing/>.
[nist] NIST, "Border Gateway Protocol Security and Resilience",
2025, <https://doi.org/10.6028/NIST.SP.800-189r1.ipd>.
[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>.
[bar-sav] NIST, Akamai, "Source Address Validation Using BGP
UPDATEs, ASPA, and ROA (BAR-SAV)", 2024,
<https://datatracker.ietf.org/doc/draft-ietf-sidrops-bar-
sav/>.
Acknowledgements
Many thanks to Jared Mauch, Barry Greene, Fang Gao, Anthony Somerset,
Yuanyuan Zhang, Igor Lubashev, Alvaro Retana, Joel Halpern, Aijun
Wang, Michael Richardson, Li Chen, Gert Doering, Mingxing Liu, John
O'Brien, Roland Dobbins, etc. for their valuable comments on this
document.
Authors' Addresses
Dan Li
Tsinghua University
Beijing
China
Email: tolidan@tsinghua.edu.cn
Jianping Wu
Tsinghua University
Beijing
China
Email: jianping@cernet.edu.cn
Libin Liu
Zhongguancun Laboratory
Beijing
China
Email: liulb@zgclab.edu.cn
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Mingqing Huang
Huawei
Beijing
China
Email: huangmingqing@huawei.com
Kotikalapudi Sriram
USA National Institute of Standards and Technology
Gaithersburg, MD
United States of America
Email: ksriram@nist.gov
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