Internet DRAFT - draft-wu-savnet-inter-domain-problem-statement
draft-wu-savnet-inter-domain-problem-statement
Internet Engineering Task Force J. Wu
Internet-Draft D. Li
Intended status: Informational Tsinghua University
Expires: 26 September 2023 L. Liu
Zhongguancun Laboratory
M. Huang
Huawei
L. Qin
Tsinghua University
N. Geng
Huawei
25 March 2023
Source Address Validation in Inter-domain Networks Gap Analysis, Problem
Statement, and Requirements
draft-wu-savnet-inter-domain-problem-statement-07
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
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This Internet-Draft will expire on 26 September 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Existing Inter-domain SAV Mechanisms . . . . . . . . . . . . 4
4. Gap Analysis . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. SAV at Provider Interface . . . . . . . . . . . . . . . . 8
4.2. SAV at Customer Interface . . . . . . . . . . . . . . . . 9
4.2.1. Limited Propagation of Prefixes . . . . . . . . . . . 9
4.2.2. Hidden Prefixes . . . . . . . . . . . . . . . . . . . 11
4.3. SAV at Peer Interface . . . . . . . . . . . . . . . . . . 12
5. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 13
6. Requirements for New Inter-domain SAV Mechanisms . . . . . . 14
6.1. Automatic Update . . . . . . . . . . . . . . . . . . . . 14
6.2. Accurate Validation . . . . . . . . . . . . . . . . . . . 14
6.3. Working in Incremental/Partial Deployment . . . . . . . . 14
7. Inter-domain SAV Scope . . . . . . . . . . . . . . . . . . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Normative References . . . . . . . . . . . . . . . . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . 17
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
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,
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]
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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], 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
complementrary, 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.
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 address, and the spoofing packets can thus be blocked by AS 2
since they come from the incorrect interface.
+------------+
| 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
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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.
3. Existing Inter-domain SAV Mechanisms
Inter-domain SAV is typically performed at the AS level and can be
deployed at AS border routers (ASBR) 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, customer interfaces, or peer interfaces of an
AS, and is recommended to deploy at provider interfaces or
customer interfaces [manrs] [nist]. At the provider interface,
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 interface, 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
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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
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].
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- 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
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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* 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.
4. Gap Analysis
Inter-domain SAV protects against source address spoofing attacks
across all AS interfaces, including provider, customer, and peer
interfaces. This section performs a gap analysis of existing SAV
mechanisms at these interfaces to identify their technical
shortcomings.
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4.1. SAV at Provider Interface
SAV at provider interface is recommended to deploy ACL-based ingress
filtering and/or Loose uRPF to prevent spoofing source addresses from
provider AS [nist] [RFC3704], and can utilize source-based RTBH
filtering to configure SAV rules remotely. In the following, we
expose the problems with existing inter-domain SAV mechanisms at the
provider interface with a reflection attack scenario.
+-----------+
Attacker(P1') +-+ AS 3(P3) |
+---+/\+----+
|
|
| (C2P)
+-----------+
| AS 4(P4) |
+/\+-----+/\+
/ \
/ \
(C2P) / \ (C2P)
+-----------+ +-----------+
Victim+-+ AS 1(P1) | | AS 2(P2) +-+Server
+-----------+ +-----------+
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 2: A scenario of the reflection attack from provider AS
As depicted in Figure 2, a reflection attack is a type of cyber
attack in which the attacker spoofs the victim's IP address (P1) and
sends requests to server's IP address (P2) that respond to that type
of request. The servers then send responses back to the victim,
overwhelming its network resources. The arrows represent the
commercial relationship between ASes. AS 3 is the provider of AS 4,
and AS 4 is the provider of AS 1 and AS 2. 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 interface of
AS 4, the ACL rules can block any packets with spoofed source
addresses from AS 3 in P1 and P2, thus stopping the attack. 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 or improper permit of legitimate traffic.
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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 2, Loose uRPF is enabled at AS 4's provider 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
interface. With Loose uRPF, AS 4 cannot block the attack packets at
its provider interface, and thus resulting in improper permit.
4.2. SAV at Customer Interface
To prevent the spoofing of source addresses within a customer cone,
operators can enable ACL-based ingress filtering, source-based RTBH
filtering, and/or uRPF-based mechanisms at the customer interface,
namely Strict uRPF, FP-uRPF, VRF uRPF, or EFP-uRPF. However, ACL-
based ingress filtering and source-based RTBH filtering need to
update SAV rules in a timely manner and have the same operational
overhead as performing SAV at provider interfaces, while uRPF-based
mechanisms may cause improper block problems in two inter-domain
scenarios: limited propagation of prefixes and hidden prefixes. In
the following, we show how uRPF-based mechanisms may block legitimate
traffic.
4.2.1. 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 trict uRPF, FP-uRPF, and VRF uRPF. All these
mechanisms suffer from the same problem of improper block in this
scenario.
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+-----------------+
| AS 4 |
+-+/\+-------+/\+-+
/ \
/ \
P1[AS 1] / \
/ \
/ (C2P/P2P) (C2P) \
+----------------+ +----------------+
| AS 3 | | AS 2 |
+-------+/\+-----+ +------+/\+------+
\ /
P1[AS 1] \ / P1[AS 1]
\ (C2P) (C2P) / NO_EXPORT
+------------------+
| AS 1(P1) +---P1
+------------------+
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. AS 1 is
the customer of both AS 2 and AS 3, and AS 4 is the provider of AS 2.
The relationship between AS 3 and AS 4 can be either customer-to-
provider (C2P) or peer-to-peer (P2P). AS 1 advertises prefix P1 to
its provider, AS 2 and AS 3. Upon receiving the route for prefix P1
from AS 1, AS 3 propagates it to AS 4. However, AS 2 does not
propagate the route for prefix P1 to AS 4 because AS 1 adds the
NO_EXPORT community attribute in the BGP advertisement sent to AS 2.
In this scenario, AS 4 learns the route for prefix P1 only from AS 3.
Suppose AS 1 and AS 4 have deployed inter-domain SAV while other ASes
have not, and AS 4 have deployed EFP-uRPF at the customer interface.
Assuming that AS 3 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 3. 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 3 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.
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4.2.2. 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.
+----------+
Anycast Server+-+ AS 3(P3) |
+--+/\+----+
|
|
| (C2P)
+----------+
| AS 4 |
+/\+----+/\+
/ \
/ \
(C2P) / \ (C2P)
+-----------+ +-----------+
User+-+ AS 1 | | AS 2 +-+Edge Server
+-----------+ +-----------+
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 4 is the provider of AS 1 and AS 2. AS 1
and AS 4 have deployed inter-domain SAV, while other ASes have not.
When users in AS 1 send requests to the anycast destination IP, the
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forwarding path is AS 1->AS 4->AS 3. The anycast servers in AS 3
receive the requests and tunnel them to the edge servers in AS 2.
Finally, the edge servers send the content to the users with source
addresses in prefix P3. The reverse forwarding path is AS 2->AS
4->AS 1. Since AS 4 does not receive routing information for prefix
P3 from AS 2, EFP-uRPF with algorithm A/B, and all other existing
uRPF-based mechanisms at the customer interface of AS 4 facing AS 2
will improperly block the legitimate response packets from AS 2.
4.3. SAV at Peer Interface
To prevent spoofing traffic from peer ASes, SAV at peer interfaces
can enable ACL-based ingress filtering, source-based RTBH filtering,
and/or employ one of the following uRPF-based SAV mechanisms: FP-
uRPF, VRF uRPF, or EFP-uRPF. ACL-based ingress filtering and source-
based RTBH filtering have high operational overhead and uRPF-based
SAV mechanisms share the same improper block problems with the inter-
domain SAV mechanisms when applied at provider interfaces or customer
interfaces. The improper block problems occur in cases of limited
propagation of prefixes and hidden prefixes. For brevity, we do not
analyze these problems again here. Moreover, if we apply EFP-uRPF
with algorithm B at peer or customer interfaces, we may encounter
improper permit problems, as explained below.
+-----------+ (P2P) +-----------+
| AS 3(P3) +-------------+ AS 4(P4) |
+-----+-----+ +/\+-----+/\+
| / \
+ / \
Attacker(P1') (C2P) / \ (C2P)
+-----------+ +-----------+
Victim+-+ AS 1(P1) | | AS 2(P2) +-+Server
+-----------+ +-----------+
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 5: A scenario of the reflection attack from peer AS
Figure 5 depicts a scenario of a reflection attack originating from a
peer AS. The direction of the commercial relationship between ASes
is indicated by arrows. AS 3 is the lateral peer of AS 4, which is
the provider of AS 1 and AS 2. Assuming that AS 1 and AS 4 have
deployed inter-domain SAV and that EFP-uRPF with algorithm B is
enabled at AS 4's peer and customer interfaces, a reflection attacker
located in AS 3 or connected to it through other ASes can launch an
attack on a victim in AS 1 by sending packets with spoofed source
addresses in P1 to a server in AS 2. Since AS 3 does not perform
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SAV, the spoofed attack packets will arrive at the peer interface of
AS 4, EFP-uRPF with algorithm B cannot block them since it allows
packets with source addresses in prefix P1 on any of AS 4's peer
interfaces.
5. Problem Statement
Based on the above analysis, we conclude that existing inter-domain
SAV mechanisms have limitations in asymmetric routing scenarios, and
they may cause improper block or improper permit issues. They also
incur high operational overhead when network routing changes
dynamically.
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.
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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.
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. Automatic Update
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.
6.2. Accurate Validation
The new inter-domain SAV mechanism SHOULD improve the validation
accuracy in all directions of ASes over existing inter-domain SAV
mechanisms. It SHOULD avoid improper block and minimize improper
permit so that the legitimate traffic from an AS will not be blocked
and the spoofed traffic will be reduced as much as possible. 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.
In cases where it is difficult to acquire all the real forwarding
paths exactly, it is essential to obtain a minimal set of acceptable
paths that cover the real forwarding paths to avoid improper block
and minimize improper permit. Additionally, multiple sources of SAV-
related information, such as RPKI ROA objects, ASPA objects, and
collaborative advertisements of other ASes, can help reduce the set
of acceptable paths and improve the validation accuracy.
6.3. 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.
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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.
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. References
10.1. Normative References
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[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>.
[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>.
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[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>.
10.2. Informative References
[intra-domain]
"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>.
Acknowledgements
Many thanks to Jared Mauch, Barry Greene, Fang Gao, Anthony Somerset,
Kotikalapudi Sriram, 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
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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
Lancheng Qin
Tsinghua University
Beijing
China
Email: qlc19@mails.tsinghua.edu.cn
Nan Geng
Huawei
Beijing
China
Email: gengnan@huawei.com
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