Internet DRAFT - draft-ietf-savnet-inter-domain-problem-statement
draft-ietf-savnet-inter-domain-problem-statement
Internet Engineering Task Force J. Wu
Internet-Draft D. Li
Intended status: Informational Tsinghua University
Expires: 21 August 2024 L. Liu
Zhongguancun Laboratory
M. Huang
Huawei
K. Sriram
USA NIST
18 February 2024
Source Address Validation in Inter-domain Networks Gap Analysis, Problem
Statement, and Requirements
draft-ietf-savnet-inter-domain-problem-statement-03
Abstract
This document provides the gap analysis of existing inter-domain
source address validation mechanisms, describes the fundamental
problems, and defines the requirements for technical improvements.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 21 August 2024.
Copyright Notice
Copyright (c) 2024 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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
Wu, et al. Expires 21 August 2024 [Page 1]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Existing Inter-domain SAV Mechanisms . . . . . . . . . . . . 5
4. Gap Analysis . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. SAV at Customer Interfaces . . . . . . . . . . . . . . . 8
4.1.1. Limited Propagation of Prefixes . . . . . . . . . . . 10
4.1.2. Hidden Prefixes . . . . . . . . . . . . . . . . . . . 11
4.1.3. Reflection Attacks . . . . . . . . . . . . . . . . . 13
4.1.4. Direct Attacks . . . . . . . . . . . . . . . . . . . 14
4.2. SAV at Provider/Peer Interfaces . . . . . . . . . . . . . 16
4.2.1. Reflection Attacks . . . . . . . . . . . . . . . . . 17
4.2.2. Direct Attacks . . . . . . . . . . . . . . . . . . . 18
5. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 20
6. Requirements for New Inter-domain SAV Mechanisms . . . . . . 22
6.1. Accurate Validation . . . . . . . . . . . . . . . . . . . 22
6.1.1. Improving Validation Accuracy over Existing
Mechanisms . . . . . . . . . . . . . . . . . . . . . 22
6.1.2. Working in Incremental/Partial Deployment . . . . . . 24
6.1.3. Providing Necessary Security Guarantee . . . . . . . 25
6.2. Automatic Update . . . . . . . . . . . . . . . . . . . . 25
6.2.1. Reducing Operational Overhead . . . . . . . . . . . . 25
6.2.2. Guaranteeing Convergence . . . . . . . . . . . . . . 25
7. Inter-domain SAV Scope . . . . . . . . . . . . . . . . . . . 25
8. Security Considerations . . . . . . . . . . . . . . . . . . . 26
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 26
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
11.1. Normative References . . . . . . . . . . . . . . . . . . 26
11.2. Informative References . . . . . . . . . . . . . . . . . 28
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
Source address validation (SAV) is crucial for protecting networks
from source address spoofing attacks. 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,
Wu, et al. Expires 21 August 2024 [Page 2]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
router vendors, and operational preferences. Hence, it is not
feasible to deploy SAV at every network edge. Additional SAV
mechanisms are needed at other levels of the network to prevent
source address spoofing along the forwarding paths of the spoofed
packets. The Source Address Validation Architecture (SAVA) [RFC5210]
proposes a multi-fence approach that implements SAV at three levels
of the network: access, intra-domain, and inter-domain.
If a spoofing packet is not blocked at the originating access
network, intra-domain and inter-domain SAV mechanisms can help block
the packet along the forwarding path of the packet. As analyzed in
[intra-domain-ps], intra-domain SAV for an AS can prevent a subnet of
the AS from spoofing the addresses of other subnets as well as
prevent incoming traffic to the AS from spoofing the addresses of the
AS, without relying on the collaboration of other ASes. As
complementary, in scenarios where intra-domain SAV cannot work,
inter-domain SAV leverages the collaboration among ASes to help block
incoming spoofing packets in an AS which spoof the source addresses
of other ASes. It is noteworthy that scenarios where intra-domain
SAV cannot work may consist of three cases: (1) the AS whose prefixes
are being spoofed does not have intra-domain SAV deployed, (2) an AS
requires the ASes near the spoofing attack source to filter the
spoofed traffic, and (3) an AS whose source prefixes are spoofed may
not be in the path of the spoofing traffic.
This document provides an analysis of inter-domain SAV. Figure 1
illustrates an example for inter-domain SAV. P1 is the source prefix
of AS 1, and AS 4 sends spoofing packets with P1 as source addresses
to AS 3 through AS 2. Assume AS 4 does not deploy intra-domain SAV,
these spoofing packets cannot be blocked by AS 4. Although AS 1 can
deploy intra-domain SAV to block incoming packets which spoof the
addresses of AS 1, these spoofing traffic from AS 4 to AS 3 do not go
through AS 1, so they cannot be blocked by AS 1. Inter-domain SAV
can help in this scenario. If AS 1 and AS 2 deploy inter-domain SAV,
AS 2 knows the correct incoming interface of packets with P1 as
source addresses, and the spoofing packets can thus be blocked by AS
2 since they come from the incorrect interface.
Wu, et al. Expires 21 August 2024 [Page 3]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
+------------+
| AS 1(P1) #
+------------+ \
\ Spoofed Packets
+-+#+--------+ with Source Addresses in P1 +------------+
| AS 2 #-----------------------------# AS 4 |
+-+#+--------+ +------------+
/
+------------+ /
| AS 3 #
+------------+
AS 4 sends spoofed packets with source addresses in P1 to AS 3
through AS 2.
If AS 1 and AS 2 deploy inter-domain SAV, the spoofed packets
can be blocked at AS 2.
Figure 1: An example for illustrating inter-domain SAV.
There are many 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).
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 forwading paths:
Wu, et al. Expires 21 August 2024 [Page 4]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
The paths that the legitimate traffic goes through in the data
plane.
3. Existing Inter-domain SAV Mechanisms
Inter-domain SAV is typically performed at the AS level 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 [manrs] [isoc], which
are reviewed in this section.
* ACL-based ingress filtering [RFC2827] [RFC3704]: ACL-based ingress
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 to deploy at provider interfaces or
customer interfaces [manrs] [nist]. At the provider interfaces,
ACL-based ingress filtering can block source prefixes that are
clearly invalid in the inter-domain routing context, such as
suballocated or internal-only prefixes of customer ASes [nist].
At the customer interfaces, ACL-based ingress filtering can
prevent customer ASes from spoofing source addresses of other ASes
that are not reachable via the provider AS. It can be implemented
at border routers or aggregation routers if border ACLs are not
feasible [manrs]. However, ACL-based ingress 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, which requires manual configuration
to avoid improper block or improper permit.
* uRPF-based machanisms: 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
prefix is the same as the incoming interface. This mode is
Wu, et al. Expires 21 August 2024 [Page 5]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
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]: VRF
uRPF uses a separate VRF table for each external BGP peer. A
VRF table is a table that contains the prefixes and the routes
that are advertised by a specific peer. VRF uRPF checks the
source address of an incoming packet from an external BGP peer
against the VRF table for that peer. If the source address
matches one of the prefixes in the VRF table, VRF uRPF allows
the packet to pass. Otherwise, it drops the packet. VRF uRPF
can also be used as a way to implement BCP38 [RFC2827], which
is a set of recommendations to prevent IP spoofing. However,
the operational feasibility of VRF uRPF as BCP38 has not been
proven [manrs].
- 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
Wu, et al. Expires 21 August 2024 [Page 6]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
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.
* Source-based remote triggered black hole (RTBH) filtering
[RFC5635]: Source-based RTBH filtering enables the targeted
dropping of traffic by specifying particular source addresses or
address ranges. Source-based RTBH filtering uses uRPF, usually
Loose uRPF, to check the source address of an incoming packet
against the FIB. If the source address of the packet does not
match or is not covered by any prefix in the FIB, or if the route
for that prefix points to a black hole (i.e., Null0), Loose uRPF
discards the packet. This way, source-based RTBH filtering can
filter out attack traffic at specific devices (e.g., ASBR) in an
AS based on source addresses, and improve the security of the
network.
* 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.
Wu, et al. Expires 21 August 2024 [Page 7]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
4. Gap Analysis
Inter-domain SAV is essential in preventing source address spoofing
attacks across all AS interfaces, including those of customers,
providers, and peers. An ideal inter-domain SAV mechanism SHOULD
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, source-based RTBH filtering, and/
or uRPF-based mechanisms at customer interfaces, namely Strict uRPF,
FP-uRPF, VRF 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 scenarios of source address spoofing attacks
within a customer cone, while ACL-based ingress filtering and source-
based RTBH filtering need to update SAV rules in a timely manner and
lead to high operational overhead. For brevity, we will analyze ACL-
based ingress filtering and source-based RTBH filtering in detail
using the concrete cases in Section 4.2.
Wu, et al. Expires 21 August 2024 [Page 8]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
+-------------------+------------+-----------+-------+--------+--------+
|Traffic & Scenarios|ACL & S/RTBH|Strict uRPF|FP-uRPF|VRF uRPF|EFP-uRPF|
+----------+--------+------------+-----------+-------+--------+--------+
|Legitimate| LPP | | |
|Traffic +--------+ | Improper Block |
| | HP | High | |
+----------+--------+Operational +----------------------------+--------+
|Spoofing | RAC | Overhead | |Improper|
|Traffic +--------+ | Functioning as Expected |Permit |
| | DAC | | | |
+----------+--------+------------+----------------------------+--------+
S/RTBH: Source-based RTBH filtering.
"LPP" represents a class of scenario called limited propagation of
prefixes.
"HP" represents a calss of scenario called hidden prefixes.
"RAC" represents a class of scenario called reflection attacks by source
address spoofing within a customer cone.
"DAC" represents a class of scenario called direct attacks by source
address spoofing within a 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 2: The gaps of ACL-based ingress filtering, source-based
RTBH filtering, Strict uRPF, FP-uRPF, VRF uRPF, and EFP-uRPF in
the corrresponding scenarios.
Figure 2 provides an overview of the gaps associated with ACL-based
ingress filtering, source-based RTBH filtering, Strict uRPF, FP-uRPF,
VRF uRPF, and EFP-uRPF for SAV at customer interfaces in the
corresponding scenarios. Both ACL-based ingress filtering and
source-based RTBH filtering have high operational overhead as
performing SAV at provider/peer interfaces. Strict uRPF, FP-uRPF,
VRF 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 scenarios of source
address spoofing attacks within a customer cone, EFP-uRPF may
inadvertently permit spoofing traffic.
In the following, we analyze the gaps of Strict uRPF, FP-uRPF, VRF
uRPF, and EFP-uRPF for SAV at customer interfaces in scenarios of
limited propagation of prefixes and hidden prefixes, respectively.
Wu, et al. Expires 21 August 2024 [Page 9]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
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
false positives when performing SAV with existing mechanisms. These
mechanisms include EFP-uRPF, which we focus on in the following
analysis, as well as Strict uRPF, FP-uRPF, and VRF 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 3: Limited propagation of prefixes caused by NO_EXPORT.
Figure 3 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
Wu, et al. Expires 21 August 2024 [Page 10]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
community attribute to the BGP advertisement sent to AS 4, resulting
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, FP-uRPF, and VRF 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, VRF 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.
Wu, et al. Expires 21 August 2024 [Page 11]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
+----------------+
Anycast Server+-+ AS 3(P3) |
+-+/\----+/\+----+
/ \
P3[AS 3] / \ P3[AS 3]
/ \
/ (C2P) \
+----------------+ \
| AS 4(P4) | \
++/\+--+/\+--+/\++ \
P6[AS 1, AS 2] / | \ \
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 4: A Direct Server Return (DSR) scenario.
Figure 4 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.
Wu, et al. Expires 21 August 2024 [Page 12]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
Moreover, it is worth mentioning that EFP-uRPF with algorithm A/B may
also permit spoofing traffic improperly in scenarios where source
address spoofing attacks within a customer cone occur. We provide
illustrations of these scenarios in the following. The source
address spoofing attacks within a customer cone include reflection
attacks within a customer cone (Section 4.1.3) and direct attacks
within a customer cone (Section 4.1.4).
4.1.3. Reflection Attacks
Figure 5 depicts the scenario of reflection attacks by 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] / | \ \
P2[AS 2] / | \ \
/ | \ \
/ (C2P) | \ P5[AS 5] \ P5[AS 5]
+----------------+ | \ \
Attacker(P1')-+ AS 2(P2) | | P1[AS 1] \ \
+----------+/\+--+ | P6[AS 1] \ \
P6[AS 1] \ | NO_EXPORT \ \
P1[AS 1] \ | \ \
NO_EXPORT \ | \ \
\ (C2P) | (C2P) (C2P) \ (C2P) \
+----------------+ +----------------+
Victim+-+ AS 1(P1, P6) | Server+-+ AS 5(P5) |
+----------------+ +----------------+
P1' is the spoofed source prefix P1 by the attacker which is inside of
AS 2 or connected to AS 2 through other ASes.
Figure 5: A scenario of reflection attacks by source address
spoofing within a customer cone.
In Figure 5, the reflection attack by source address spoofing takes
place within AS 4's customer cone, where the attacker spoofs the
victim's IP address (P1) and sends requests to servers' IP address
(P5) that are designed to respond to such requests. As a result, the
Wu, et al. Expires 21 August 2024 [Page 13]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
server sends overwhelming responses back to the victim, thereby
exhausting its network resources. The arrows in Figure 5 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 A/B at its customer
interfaces, it will allow packets with source addresses in P1 to
originate from AS 1 and AS 2. Consequently, when AS 2 sends spoofing
packets with source addresses in P1 to AS 4, AS 4 will improperly
permit these packets, thus enabling the spoofing attacks to
propagate.
In scenarios like these, Strict uRPF, FP-uRPF, and VRF uRPF do not
suffer from improper permit problems. This is because these
mechanisms enforce strict filtering rules that ensure packets with
source addresses in P1 are only allowed to arrive from AS 1.
4.1.4. Direct Attacks
Figure 6 portrays a scenario of direct attacks by source address
spoofing within a customer cone and is used to analyze the gaps of
uRPF-based mechanisms below.
Wu, et al. Expires 21 August 2024 [Page 14]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
+----------------+
| AS 3(P3) |
+-+/\----+/\+----+
/ \
/ \
/ \
/ (C2P) \
+----------------+ \
| AS 4(P4) | \
++/\+--+/\+--+/\++ \
P6[AS 1, AS 2] / | \ \
P2[AS 2] / | \ \
/ | \ \
/ (C2P) | \ P5[AS 5] \ P5[AS 5]
+----------------+ | \ \
Attacker(P5')-+ AS 2(P2) | | P1[AS 1] \ \
+----------+/\+--+ | P6[AS 1] \ \
P6[AS 1] \ | NO_EXPORT \ \
P1[AS 1] \ | \ \
NO_EXPORT \ | \ \
\ (C2P) | (C2P) (C2P) \ (C2P) \
+----------------+ +----------------+
Victim+-+ AS 1(P1, P6) | | AS 5(P5) |
+----------------+ +----------------+
P5' is the spoofed source prefix P5 by the attacker which is inside of
AS 2 or connected to AS 2 through other ASes.
Figure 6: A scenario of the direct attacks by source address
spoofing within a customer cone.
In Figure 6, the direct attack by source address spoofing takes place
within AS 4's customer cone, where the attacker spoofs a source
address (P5) and directly targets the victim's IP address (P1),
overwhelming its network resources. The arrows in Figure 6
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 A/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 AS 2 sends
spoofing packets with source addresses in P5 to AS 4, AS 4 will
improperly permit these packets, thus enabling the spoofing attacks
to propagate.
Wu, et al. Expires 21 August 2024 [Page 15]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
In scenarios like these, Strict uRPF, FP-uRPF, and VRF uRPF 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 from 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]. In addition, source-based RTBH filtering can be
used to remotely configure SAV rules.
+--------------------+------------+---------------+
|Traffic & Scenarios |ACL & S/RTBH| Loose uRPF |
+----------+---------+------------+---------------+
|Legitimate| Any | | Functioning |
|Traffic |Scenarios| High | as Expected |
+----------+---------+Operational +---------------+
|Spoofing | RAP | Overhead | |
|Traffic +---------+ |Improper Permit|
| | DAP | | |
+----------+---------+------------+---------------+
S/RTBH: Source-based RTBH filtering.
"RAP" represents a class of scenario called reflection attacks by
source address spoofing from provider/peer AS.
"DAP" represents a class of scenario called direct attacks by source
address spoofing from provider/peer AS.
"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 7: The gaps of ACL-based ingress filtering, source-based
RTBH filtering, and Loose uRPF in the corresponding scenarios.
Figure 7 summarizes the gaps of ACL-based ingress filtering, source-
based RTBH filtering, and Loose uRPF for SAV at provider/peer
interfaces in the corresponding scenarios. ACL-based ingress
filtering and source-based RTBH filtering effectively block spoofing
traffic from the reflection attacks or direct attacks originating
from provider/peer AS, while appropriately allowing legitimate
traffic. However, these methods may come with a high operational
overhead. On the other hand, Loose uRPF correctly permits legitimate
traffic, but it can also mistakenly allow spoofing traffic to pass
through.
Wu, et al. Expires 21 August 2024 [Page 16]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
In the following, we expose the limitations of ACL-based ingress
filtering, source-based RTBH filtering, and Loose uRPF for SAV at
provider/peer interfaces in scenarios of source address spoofing
attacks from provider/peer AS. The source address spoofing attacks
from provider/peer AS include reflection attacks from provider/peer
AS (Section 4.2.1) and direct attacks from provider/peer AS
(Section 4.2.2).
4.2.1. Reflection Attacks
Figure 8 depicts the scenario of reflection attacks by source address
spoofing from provider/peer AS and is used to analyze the gaps of
ACL-based ingress filtering, source-based RTBH filtering, and Loose
uRPF below.
+----------------+
Attacker(P1')+-+ AS 3(P3) |
+-+/\----+/\+----+
/ \
/ \
/ \
/ (C2P/P2P) \
+----------------+ \
| AS 4(P4) | \
++/\+--+/\+--+/\++ \
P6[AS 1, AS 2] / | \ \
P2[AS 2] / | \ \
/ | \ \
/ (C2P) | \ P5[AS 5] \ P5[AS 5]
+----------------+ | \ \
Server+-+ AS 2(P2) | | P1[AS 1] \ \
+----------+/\+--+ | P6[AS 1] \ \
P6[AS 1] \ | NO_EXPORT \ \
P1[AS 1] \ | \ \
NO_EXPORT \ | \ \
\ (C2P) | (C2P) (C2P) \ (C2P) \
+----------------+ +----------------+
Victim+-+ AS 1(P1, P6) | | AS 5(P5) |
+----------------+ +----------------+
P1' is the spoofed source prefix P1 by the attacker which is inside of
AS 3 or connected to AS 3 through other ASes.
Figure 8: A scenario of reflection attacks by source address
spoofing from provider/peer AS.
In the case of Figure 8, the attacker spoofs the victim's IP address
(P1) and sends requests to servers' IP address (P2) that respond to
such requests. The servers then send overwhelming responses back to
Wu, et al. Expires 21 August 2024 [Page 17]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
the victim, exhausting its network resources. The arrows in Figure 8
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, effectively preventing reflection
attacks. 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 1 and AS 2.
Otherwise, it may cause improper block of legitimate traffic or
improper permit of spoofing traffic.
Source-based RTBH filtering allows for the deployment of SAV rules on
AS 1 and AS 4 remotely. However, in order to avoid improper block or
improper permit, the specified source addresses need to be updated in
a timely manner, which incurs additional operational overhead.
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 can improperly permit spoofed packets. In
Figure 8, Loose uRPF is enabled at AS 4's provider/peer interface,
while EFP-uRPF is enabled at AS 4's customer interfaces, following
[RFC3704]. An attacker inside AS 3 or connected to it through other
ASes may send packets with source addresses spoofing P1 to a server
in AS 2 to attack the victim in AS 1. As AS 3 lacks deployment of
inter-domain SAV, the attack packets will reach AS 4's provider/peer
interface. With Loose uRPF, AS 4 cannot block the reflection attack
packets at its provider/peer interface, and thus resulting in
improper permit.
4.2.2. Direct Attacks
Conversely, Figure 9 showcases a scenario of direct attack by source
address spoofing from provider/peer AS and is used to analyze the
gaps of ACL-based ingress filtering, source-based RTBH filtering, and
Loose uRPF below.
Wu, et al. Expires 21 August 2024 [Page 18]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
+----------------+
Attacker(P2')+-+ AS 3(P3) |
+-+/\----+/\+----+
/ \
/ \
/ \
/ (C2P/P2P) \
+----------------+ \
| 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) \
+----------------+ +----------------+
Victim+-+ AS 1(P1, P6) | | AS 5(P5) |
+----------------+ +----------------+
P2' is the spoofed source prefix P2 by the attacker which is inside of
AS 3 or connected to AS 3 through other ASes.
Figure 9: A scenario of direct attacks by source address spoofing
from provider/peer AS.
In the case of Figure 9, the attacker spoofs another source address
(P2) and directly targets the victim's IP address (P1), overwhelming
its network resources. The arrows in Figure 9 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.
Similar to the scenario of reflection attacks by source address
spoofing depicted in Figure 8, both ACL-based ingress filtering and
Source-based RTBH filtering have high operational overhead for the
scenario of direct attacks by source address spoofing from provider/
peer AS shown in Figure 9. Otherwise, they may cause improper block
of legitimate traffic or improper permit of spoofing traffic.
Wu, et al. Expires 21 August 2024 [Page 19]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
Also, Loose uRPF can improperly permit spoofed packets. In Figure 9,
Loose uRPF is enabled at AS 4's provider/peer interface, while EFP-
uRPF is enabled at AS 4's customer interfaces, following [RFC3704].
An attacker inside AS 3 or connected to it through other ASes may
send packets with source addresses spoofing P2 and destimation
addresses in P1 to directly attack the victim in AS 1. As AS 3 lacks
deployment of inter-domain SAV, the attack packets will reach AS 4's
provider/peer interface. With Loose uRPF, AS 4 cannot block the
direct attack packets at its provider/peer interface, and thus
resulting in improper permit.
5. Problem Statement
+--------+----------+---------+----------+-------+--------+----------+
|Problems| ACL | Strict | Loose |FP-uRPF|VRF uRPF|EFP-uRPF |
| | & S/RTBH | uRPF | uRPF | | | |
+--------+----------+---------+----------+-------+--------+----------+
|Improper|Not Exist | Exist |Not Exist | Exist |
|Block | |(LPP, HP)| | (LPP, HP) |
+--------+----------+---------+----------+-------+--------+----------+
|Improper| Not Exist | Exist | Not Exist | Exist |
|Permit | |(RAP, DAP)| |(RAC, DAC)|
+--------+----------+---------+----------+-------+--------+----------+
| | Exist | |
| HOO | (Any | Not Exist |
| |Scenarios)| |
+--------+----------+------------------------------------------------+
S/RTBH: Source-based RTBH filtering, HOO: High Operational Overhead.
"LPP" represents a class of scenario called limited propagation of
prefixes.
"HP" represents a class of scenario called hidden prefixes.
"RAP" represents a class of scenario called reflection attacks by
source address spoofing from provider/peer AS.
"DAP" represents a class of scenario called direct attacks by source
address spoofing from provider/peer AS.
"RAC" represents a class of scenario called reflection attacks by
source address spoofing within a customer cone.
"DAC" represents a class of scenario called direct attacks by source
address spoofing within a customer cone.
Figure 10: 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.
Wu, et al. Expires 21 August 2024 [Page 20]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
Additionally, these mechanisms can result in high operational
overhead, especially when network routing undergoes dynamic changes.
Figure 10 provides a comprehensive summary of 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.
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. Source-based RTBH filtering has
the similar problem as ACL-based ingress filtering.
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 and VRF 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 and VRF 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 approapriate SAV mechansim for each interface. This leads
to additional operational and cognitive overhead, which can hinder
the rate of adoption of inter-domain SAV.
Wu, et al. Expires 21 August 2024 [Page 21]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
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 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 SHOULD 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 SHOULD avoid improper block and permit less spoofed 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 and
DSR. 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.
Wu, et al. Expires 21 August 2024 [Page 22]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
+----------------+
| 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 11: An example to illustrate accurate validation in all
directions of an AS.
Multiple sources of SAV-related information, such as RPKI ROA
objects, ASPA objects, and SAV-specific information from other ASes,
can assist in reducing the set of acceptable paths. Figure 11 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 used.
Wu, et al. Expires 21 August 2024 [Page 23]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
Additionally, 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 block
list. For example, as shown in Figure 11, 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 a permit list 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 and ASPA 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 SHOULD 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.
Wu, et al. Expires 21 August 2024 [Page 24]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
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 SHOULD have less operational
overhead than ACL-based ingress filtering and Source-based RTBH
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 convergence process.
7. Inter-domain SAV Scope
The new inter-domain SAV mechanism should work in the same scenarios
as existing inter-domain SAV mechanisms. 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.
Wu, et al. Expires 21 August 2024 [Page 25]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
In addition, the new inter-domain SAV mechanism 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 ASPA, should also be
protected by corresponding mechanisms or infrastructure. If SAV
mechanisms or protocols require information exchange, there should be
some considerations on the avoidance of message alteration or message
injection.
The SAV procedure referred in this document modifies no field of
packets. So, security considerations on 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
Tsinghua University
Beijing, China
Email: qlc19@mails.tsinghua.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>.
Wu, et al. Expires 21 August 2024 [Page 26]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
[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>.
[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>.
[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>.
[RFC5635] Kumari, W. and D. McPherson, "Remote Triggered Black Hole
Filtering with Unicast Reverse Path Forwarding (uRPF)",
RFC 5635, DOI 10.17487/RFC5635, August 2009,
<https://www.rfc-editor.org/rfc/rfc5635>.
[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>.
Wu, et al. Expires 21 August 2024 [Page 27]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
11.2. Informative References
[intra-domain-ps]
"Source Address Validation in Intra-domain Networks Gap
Analysis, Problem Statement, and Requirements", 2023,
<https://datatracker.ietf.org/doc/draft-li-savnet-intra-
domain-problem-statement/>.
[manrs] MANRS, "MANRS Implementation Guide", 2023,
<https://www.manrs.org/netops/guide/antispoofing/>.
[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, "Resilient Interdomain Traffic Exchange: BGP
Security and DDos Mitigation", 2019,
<https://www.nist.gov/publications/resilient-interdomain-
traffic-exchange-bgp-security-and-ddos-mitigation>.
[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)", 2023,
<https://datatracker.ietf.org/doc/draft-sriram-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
Jianping Wu
Tsinghua University
Beijing
China
Email: jianping@cernet.edu.cn
Wu, et al. Expires 21 August 2024 [Page 28]
Internet-Draft Inter-domain SAVNET Problem Statement February 2024
Dan Li
Tsinghua University
Beijing
China
Email: tolidan@tsinghua.edu.cn
Libin Liu
Zhongguancun Laboratory
Beijing
China
Email: liulb@zgclab.edu.cn
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
Wu, et al. Expires 21 August 2024 [Page 29]
