Internet-Draft Intra-domain SAVNET Problem Statement November 2022
Li, et al. Expires 11 May 2023 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-li-savnet-intra-domain-problem-statement-03
Published:
Intended Status:
Informational
Expires:
Authors:
D. Li
Tsinghua University
J. Wu
Tsinghua University
L. Qin
Tsinghua University
M. Huang
Huawei
N. Geng
Huawei

Source Address Validation in Intra-domain Networks (Intra-domain SAVNET) Gap Analysis, Problem Statement and Requirements

Abstract

Source Address Validation in Intra-domain Networks (Intra-domain SAVNET) aims to make improvements to existing intra-domain Source Address Validation (SAV). This document provides the gap analysis of existing intra-domain SAV mechanisms, describes the fundamental problems, and defines the requirements for improvements.

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.

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 11 May 2023.

Table of Contents

1. Introduction

Source Address Validation (SAV) is important for defending against source address spoofing attacks and accurately tracing back to the attackers. To be as effective as possible, SAV should be implemented as close to the source as possible. Given numerous access networks managed by different operators, it is difficult to require all access networks to deploy SAV at the source (e.g., SAVI[RFC7039]). When some access networks do not deploy SAV, intra-domain SAV helps filter out spoofed packets as close to the source as possible.

Ingress filtering [RFC2827] [RFC3704] is the current practice of intra-domain SAV. Figure 1 shows the typical adoption scenario of ingress filtering. It is typically deployed at the edge router connected to a subnet to block spoofed traffic from the subnet. A subnet refers to a user network attached to the edge router.

+-------------------------------------------------------------+
|                                                        AS   |
|          +----------+            +----------+               |
|          | Router 5 +------------+ Router 6 |               |
|          +----------+            +----------+               |
|            /      \                       \                 |
|           /        \                       \                |
|          /          \                     +----------+      |
|  +----------+     +----------+            | Router 4 |      |
|  | Router 1 |     | Router 2 |            +--------#-+      |
|  +------#---+     +--#-------+              /      |        |
|          \          /                      /       |        |
|           \        /             +----------+   Subnet3(p3) |
|            \      /              | Router 3 |               |
|             \    /               +-----#----+               |
|           Subnet1(p1)                  |                    |
|                                   Subnet2(p2)               |
|                                                             |
+-------------------------------------------------------------+

Router 1, 2, 3,and 4 implement ingress filtering at interface #
to block spoofed traffic from subnet 1, 2, and 3.

 Figure 1: The typical adoption of ingress filtering.

Static Access Control List (ACL) is a typical implementation of ingress filtering. Operators can configure some matching rules to specify which source addresses are acceptable (or unacceptable). The information of ACL should be updated manually so as to keep consistent with the newest filtering criteria, which inevitably limits the flexibility and accuracy of SAV. Strict unicast Reverse Path Forwarding (uRPF) [RFC3704] is another suitable solution to achieve ingress filtering in intra-domain networks. Routers deploying strict uRPF accept a data packet only when i) the local forwarding information base (FIB) contains a prefix encompassing the packet's source address and ii) the corresponding forwarding next hop for the prefix matches the packet's incoming direction. Otherwise, the packet will be blocked. However, in the scenario where data packets are under asymmetric routing, strict uRPF often improperly blocks legitimate traffic. Feasible uRPF and loose uRPF are two other alternative implementations of ingress filtering, but their filtering rules are very loose and generally permit all (spoofing) packets with source addresses in the local FIB. Therefore, a new intra-domain SAV mechanism is required to improve accuracy upon current ones.

This document provides the gap analysis of existing intra-domain SAV mechanisms, describes their fundamental problems, and defines the requirements for improvements.

2. Terminology

SAV: Source Address Validation, i.e. validating the authenticity of a packet's source IP address.

SAV rule: The rule generated by intra-domain SAV mechanisms that determines valid incoming interfaces for a specific source prefix.

SAV table: The data structure that stores SAV rules on the data plane. The router queries its local SAV table to validate the authenticity of source addresses.

Improper block: The packets with legitimate source IP addresses are blocked improperly due to inaccurate SAV rules.

Improper permit: The packets with spoofed source IP addresses are permitted improperly due to inaccurate SAV rules.

3. Gap Analysis

3.1. Improper Block

Existing intra-domain SAV mechanisms can improperly block traffic with legitimate source addresses due to their technical limitations. For example, figure 2 illustrates an intra-domain scenario of multi-homed subnet.

In this scenario, Subnet 1 is attached to two edge routers, i.e. Router 1 and Router 2. Although Subnet 1 owns prefix 10.0.0.0/15, Subnet 1 expects traffic destined for 10.1.0.0/16 to come only from Router 1 and traffic destined for 10.0.0.0/16 to come only from Router 2, for traffic engineering or load balance purposes. To this end, Router 1 only learns the sub prefix 10.1.0.0/16 from Subnet 1, while Router 2 only learns the other sub prefix 10.0.0.0/16 from Subnet 1. Then, Router 1 and Router 2 advertise the learned sub prefix to other routers in the AS through intra-domain routing protocols such as OSPF or IS-IS. Finally, Router 1 learns the route to 10.0.0.0/16 from Router 5, and Router 2 also learns the route to 10.1.0.0/16 from Router 5. Although Subnet 1 only expects incoming traffic destined for 10.0.0.0/16 to come from Router 2, it still sends outgoing traffic with source addresses of prefix 10.0.0.0/16 to Router 1, resulting in routing asymmetry.

+-------------------------------------------------------------+
|                                                      AS     |
|                        +----------+                         |
|                        | Router 5 |                         |
| FIB for Router 1       +----------+  FIB for Router 2       |
| Dest         Next_hop    /      \    Dest         Next_hop  |
| 10.1.0.0/16  Subnet 1   /        \   10.0.0.0/16  Subnet 1  |
| 10.0.0.0/16  Router 5  /         \/  10.1.0.0/16  Router 5  |
|                +----------+     +----------+                |
|                | Router 1 |     | Router 2 |                |
|                +-----+#+--+     +-+#+------+                |
|                        /\         /                         |
|   Outgoing traffic with \        / Incoming traffic with    |
|   source IP addresses    \      /  destination IP addresses |
|   of 10.0.0.0/16          \    \/  of 10.0.0.0/16           |
|                         +----------+                        |
|                         | Subnet 1 |                        |
|                         +----------+                        |
|                        (10.0.0.0/15 )                       |
|                                                             |
+-------------------------------------------------------------+

If Router 1 and 2 apply ingress filtering at interface #:
   - Strict uRPF has improper block problem;
   - ACL-based SAV requires manual update given prefix or topology
     update in Subnet 1

  Figure 2: Asymmetric routing in multi-homed subnet scenario.

If Router 1 applies strict uRPF at the subnet interface, the SAV rule is that Router 1 only accepts packets with source addresses of 10.1.0.0/16 from Subnet 1. Therefore, when Subnet 1 sends packtes with source addresses of 10.0.0.0/16 to Router 1, strict uRPF at Router 1 will improperly block these legitimate packets. Similarly, when Router 2 with strict uRPF deployed receives packets with source addresses of prefix 10.1.0.0/16 from Subnet 1, it will also improperly block these legitimate packets. If Router 1 and 2 apply ACL-based SAV at interfaces '#', it requires manual update given prefix or topology update in Subnet 1. Once the network operator does not update the ACL in time, resulting in the ACL status is inconsistent with the routing status, it will cause improper block problems as well. Overall, strict uRPF has serious improper block problem in the case of routing asymmetry, and ACL-based SAV needs high operational overhead in dynamic networks.

3.2. Vulnerability in Inbound Direction

As shown in Figure 1, ingress filtering is typically deployed at the edge router connected to a subnet and only works for outbound traffic (traffic from the subnet to other networks) but does not work for inbound traffic (traffic from other networks to the subnet). It prevents subnets from originating spoofed traffic, but does not protect subnets from being the victim of source address spoofing attack.

3.2.1. Spoofing from outside the AS

Figure 3 shows a scenario of source address spoofing from outside AS. Although the AS has applied ingress filtering at all edge routers, the spoofed traffic (even with forged intra-domain source addresses) can easily enter from inbound direction due to the lack of inbound SAV.

               + Spoofed traffic with source addresses
               | of p1, p2, or p3
+--------------|----------------------------------------------+
|              |                                         AS   |
|          +--\/------+            +----------+               |
|          | Router 5 +------------> Router 6 |               |
|          +----------+            +----------+               |
|            /      \                       \                 |
|           /        \                      \/                |
|          \/        \/                     +----------+      |
|  +----------+     +----------+            | Router 4 |      |
|  | Router 1 |     | Router 2 |            +--------#-+      |
|  +------#---+     +--#-------+              /      |        |
|          \          /                      \/      \/       |
|           \        /             +----------+   Subnet3(p3) |
|            \      /              | Router 3 |               |
|            \/    \/              +-----#----+               |
|           Subnet1(p1)                  |                    |
|                                        \/                   |
|                                   Subnet2(p2)               |
+-------------------------------------------------------------+

  Spoofed traffic can easily enter subnets from inbound direction
  without being detected by ingress filtering

        Figure 3: Spoofing from outside AS.

3.2.2. Spoofing from undeployed edge routers

In practice, it is often a challenge to apply intra-domain SAV to all edge routers. For example, the improper block problem described in Section 3.1 prevents ingress filtering from being implemented in some multi-homed scenarios. In addition, there are also some routers that cannot support SAV due to their capabilities, versions, and vendors. However, when ingress filtering is partially deployed, the effectiveness of intra-domain SAV will be significantly degraded.

+-------------------------------------------------------------+
|                                                        AS   |
|          +----------+            +----------+               |
|          | Router 5 +------------> Router 6 |               |
|          +----------+            +----------+               |
|            /      /\                      \                 |
|           /        \                      \/                |
|          /          \                     +----------+      |
|  +----------+     +----------+            | Router 4 |      |
|  | Router 1 |     | Router 2 |            +--------#-+      |
|  +----------+     +----------+              /      |        |
|          \         /\                      /       \/       |
|           \        / Spoofing    +----------+   Subnet3(p3) |
|            \      /  traffic     | Router 3 |      |        |
|             \    /               +-----#----+      +        |
|           Subnet1(p1)                  |        Reflector   |
|               |                        |                    |
|               +                   Subnet2(p2)-+Victim       |
|         Attacker (spoof p2)                                 |
+-------------------------------------------------------------+

    Partial deployment scenario:
        - Router 3 and 4 deploy ingress filtering
        - Router 1 and 2 do not deploy ingress filtering

  Figure 4: Reflection attack in a partial deployment scenario.

Figure 4 describes a reflection attack in a partial deployment scenario. Router 1, 2, 3, and 4 are edge routers, each connected to a subnet. Router 5 and 6 are two core routers that are responsible for transmitting traffic. Assume only Router 3 and 4 apply ingress filtering at subnet interfaces, while Router 1 and 2 do not apply ingress filtering. In this case, although subnets that deploy ingress filtering at the edge router (e.g., Subnet 2 and Subnet 3) cannot forge source addresses of other subnets, they are still vulnerable to reflection attacks from other undeployed subnets (e.g., Subnet 1).

For example, to conduct a reflection attack to the victim in Subnet 2, the attacker in Subnet 1 can send a forged request with victim's source address to the reflector in Subnet 3. Since Router 2 does not apply ingress filtering, the forged request will successfully enter the intra-domain network and be forwarded to the reflector. When receiving the forged request, Router 4 will also permit the request due to the lack of inbound SAV. In the end, the reflector will receive the forged request and generate a large number of responses to the victim, and the reflection attack succeeds.

4. Problem Statement

This section summarizes the fundamental problems existing in current intra-domain SAV from the above gap analysis. The inaccurate validation, limited protection, and high operational overhead of current intra-domain SAV mechanisms are three main factors that hinder the deployment and compromise the effectiveness of intra-domain SAV.

4.1. Inaccurate Validation

ACL-based ingress filtering needs manual configuration and thus faces limitations in flexibility and accuracy in dynamic networks. Strict uRPF-based ingress filtering automatically generates SAV tables, but may improperly block legitimate traffic under asymmetric routing. The root cause is that strict uRPF leverages the local FIB table to determine the incoming interface for source addresses, which may not match the real data-plane forwarding path from the source, due to the existence of asymmetric routes. Hence, it may mistakenly consider a valid incoming interface as invalid, resulting in improper block problems; or consider an invalid incoming interface as valid, resulting in improper permit problems.

4.2. Limited Protection

Currently, ingress filtering is applied at edge routers and only works for traffic from direcly connected subnets, resulting in the inability to block spoofed traffic from inbound direction. Therefore, spoofed traffic with intra-domain source addresses can easily flow to any subnet from outside AS or undeployed edge routers (when intra-domain SAV is partially deployed).

4.3. High Operational Overhead

Since existing intra-domain SAV mechanisms fail to adapt to dynamic or asymmetric routing scenarios, if network operators want to apply intra-domain SAV and avoid improper block , they has to figure out which edge routers have asymmetric routing to the directly connected subnet, and implement ACL-based SAV at those edge routers instead of strict uRPF. In addition, they have to manually update the ACL filtering rules in time when the subnet's prefix or topology changes. Both identifying asymmetric routes and manual update impose significant operational overhead on network operators.

5. Requirements

To make improvements to existing intra-domain SAV mechanisms, a new intra-domain SAV mechanism MUST satisfy the following requirements.

5.1. Accurate SAV

The new intra-domain SAV mechanism MUST ensure accurate SAV and avoid improper block under asymmetric routing. Routers MUST be able to learn the real incoming interfaces for packets originated from the subnet which owns the corresponding source prefix. In other words, accurate SAV MUST match the real data-plane forwarding path from the source. Since this requirement cannot be met by using local FIB information, additional control-plane mechanisms SHOULD be needed to deliver the required information.

5.2. All-direction Protection

The new intra-domain SAV mechanism MUST work for traffic coming from all directions (i.e. for traffic coming from both subnet and neighboring router) and MUST be deployed in more routers to block spoofed traffic (from outside AS and undeployed edge routers) as close to the source as possible. Especially, when partially deployed, it SHOULD be able to limit the malicious behavior of undeployed subnets to some extent. At least, to prevent reflection attacks, it should prevent undeployed subnets from forging source addresses of deployed subnets.

5.3. Acceptable Overhead

The mechanism MUST not induce much overhead. First, it MUST be able to automatically update and adapt to dynamic or asymmetric routing scenarios, instead of relying entirely on manual update. Second, it MUST avoid data-plane packet modification and limit the number of control-plane protocol messages.

6. Intra-domain SAVNET Scope

Intra-domain SAVNET focuses on the same scope corresponding to existing intra-domain SAV mechanisms. Generally, it includes all IP-encapsulated scenarios:

Scope does not include:

7. Security Considerations

Intra-domain SAVNET focuses on routing protocol-based mechanisms. It aims to use or extend existing routing architecture or protocols to implement the SAV function. The intra-domain SAVNET mechanism MUST not introduce additional security vulnerabilities or confusion to the existing intra-domain control-plane protocols. Similar to the security scope of intra-domain routing protocols, intra-domain SAVNET should ensure integrity and authentication of protocol packets that deliver the required SAV information.

Intra-domain SAVNET does not provide protection against compromised or misconfigured routers that poison existing control plane protocols. Such routers can not only disrupt the SAV function, but also affect the entire routing domain.

8. IANA Considerations

This document does not request any IANA allocations.

9. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[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, , <https://www.rfc-editor.org/info/rfc2827>.
[RFC3704]
Baker, F. and P. Savola, "Ingress Filtering for Multihomed Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, , <https://www.rfc-editor.org/info/rfc3704>.
[RFC7039]
Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed., "Source Address Validation Improvement (SAVI) Framework", RFC 7039, DOI 10.17487/RFC7039, , <https://www.rfc-editor.org/info/rfc7039>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.

Authors' Addresses

Dan Li
Tsinghua University
Beijing
China
Jianping Wu
Tsinghua University
Beijing
China
Lancheng Qin
Tsinghua University
Beijing
China
Mingqing Huang
Huawei
Beijing
China
Nan Geng
Huawei
Beijing
China