| Internet-Draft | Intra-domain SAVNET Problem Statement | February 2023 |
| Li, et al. | Expires 27 August 2023 | [Page] |
This document provides the gap analysis of existing intra-domain source address validation mechanisms, describes the fundamental problems, and defines the requirements for technical improvements.¶
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Source Address Validation (SAV) is important for defending against source address spoofing attacks and allowing accurate traceback. A multi-fence architecture called Source Address Validation Architecture (SAVA) [RFC5210] was proposed to validate source addresses at three levels: access network SAV, intra-domain SAV, and inter-domain SAV. When SAV is not fully enabled at the edge of the Internet, the multi-fence architecture can help enhance the validation across the whole Internet and thus reduce the opportunities of launching source address spoofing attacks.¶
Particularly, access network SAV ensures a host uses a valid address assigned to the host statically or dynamically. The host cannot use the source address of another host. There are many mechanisms for SAV in access networks. Static ACL rules can be statically configured for validation by specifying which source addresses are acceptable or unacceptable. Dynamic ACL is another efficient mechanism which is associated with authentication servers (e.g., RADIUS and DIAMETER). The servers receive access requests and then install or enable ACL rules on the device to permit particular users' packets. SAVI [RFC7039] represents a kind of mechanism enforcing that the legitimate IP address of a host matches the link-layer property of the host's network attachment. For example, SAVI solution for DHCP [RFC7513] creates a binding between a DHCPv4/DHCPv6-assigned IP address and a link-layer property (like MAC address or switch port) on a SAVI device. IP Source Guard (IPSG) [IPSG] combined with DHCP snooping is an implementation of SAVI solution for DHCP. Cable Source-Verify [cable-verify] also shares some features of SAVI and is used in cable modem networks. Cable modem termination system (CMTS) devices with Cable Source-Verify will maintain the bindings of the CPE's IP address, the CPE's MAC address, and the corresponding cable modem identifier. When receiving packets, the device will check the validity of the packets according to the bindings.¶
Given numerous access networks managed by different operators throughout the world, it is difficult to require all access networks to effectively deploy SAV. Therefore, behind access network SAV, intra-domain SAV and inter-domain SAV are needed to provide the second- and third-level protection, respectively. Both intra-domain SAV and inter-domain SAV usually perform validation at the granularity of IP prefixes, which is coarser than access network SAV as they covers a range of IP addresses.¶
This document focuses on the analysis of intra-domain SAV. In contrast to inter-domain SAV, intra-domain SAV does not require collaboration between networks (e.g., ASes) for SAV. The SAV rules can be generate by the network itself. Consider an AS which provides its own subnets with the connectivity to other ASes or the Internet. The intra-domain SAV for the network has two goals: i) blocking the spoofed packets originated by the subnets from entering the network and ii) blocking the packets with the AS's prefixes as source addresses from other networks or the Internet.¶
Figure 1 shows two cases for showing the effectiveness of intra-domain SAV. In case i, AS X sends the packets spoofing the source addresses of AS Z to AS Y. If AS X deploys intra-domain SAV at '#', the spoofed packets can be blocked (i.e., Goal i). In case ii, If AS Z deploys intra-domain SAV at '#', the spoofed packets will be blocked (i.e., Goal ii).¶
Case i: AS X sends the packets spoofing the
source addresses of AS Z to AS Y
Goal i: If AS X deploys intra-domain SAV at '#',
the spoofed packets can be blocked
+------+ Spoofed packets +------+
| AS X #------------------>| AS Y |
+------+ +------+
Case ii: AS Z receives the packets spoofing
Z's source addresses from AS X
Goal ii: If AS Z deploys intra-domain SAV at
'#', the spoofed packets can be blocked
+------+ Spoofed packets +------+
| AS X |------------------># AS Z |
+------+ +------+
There are many mechanisms for intra-domain SAV. This document provides the gap analysis of existing intra-domain SAV mechanisms. According to the gap analysis, the document concludes the main problems of existing mechanisms and describes the requirements for future intra-domain SAV mechanisms.¶
SAV Rule: The rule that indicates the validity of a specific source IP address or source IP prefix.¶
SAV Table: The table or data structure that implements the SAV rules and is used for source address validation in the data plane.¶
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.¶
Ingress filtering [RFC2827][RFC3704] is the current practice of intra-domain SAV. This section briefly introduces the existing intra-domain SAV mechanisms.¶
Existing intra-domain SAV mechanisms may improperly block the traffic with legitimate source addresses (i.e., improper block) or improperly permit the traffic with spoofed source addresses (i.e., improper permit) due to their technical limitations. Significant operational attention is required to keep intra-domain SAV accurate.¶
Outbound traffic validation is used at downstream interfaces of routers to validate the packets from the connected subnets. As described previously, ACL rules can be configured at downstream interfaces for ingress filtering. These rules need to be updated when the prefixes or topologies of subnets change. If rules updates are not conducted in time, resulting in the discrepancy between ACL configurations and routing status, ACL-based ingress filtering will cause improper block problems or improper permit problems. However, high operational overhead will be induced to achieve timely updates of ACL configurations.¶
Strict uRPF can also be used for outbound traffic validation, but there are improper block problems in the multi-homing scenario. Figure 2 shows the intra-domain scenario of a multi-homed subnet. In the figure, Subnet 1 owns prefix 10.0.0.0/15 and is attached to two edge routers, i.e., Router 1 and Router 2. For the load balance purposes of inbound traffic, Subnet 1 expects the inbound traffic destined for 10.1.0.0/16 to come only from Router 1 and the inbound traffic destined for 10.0.0.0/16 to come only from Router 2. To this end, Router 1 only learns the route to sub prefix 10.1.0.0/16 from Subnet 1, while Router 2 only learns the route to the other sub prefix 10.0.0.0/16 from Subnet 1. Then, Router 1 and Router 2 advertise the learned sub prefix to the 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 3, and Router 2 learns the route to 10.1.0.0/16 from Router 3. The FIBs on Router 1 and Router 2 are shown in the figure. Although Subnet 1 only expects inbound traffic destined for 10.0.0.0/16 to come from Router 2, it may send outbound traffic with source addresses of prefix 10.0.0.0/16 to Router 1 for outbound traffic load balancing. Similarly, Subnet 1 may send outbound traffic with source addresses of prefix 10.1.0.0/16 to Router 2. As a result, asymmetric routing is created.¶
+-------------------------------------------------------------+ | AS | | +----------+ | | | Router 3 | | | FIB on Router 1 +----------+ FIB on 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 3 / \/ 10.1.0.0/16 Router 3 | | +----------+ +----------+ | | | Router 1 | | Router 2 | | | +-----+#+--+ +-+#+------+ | | /\ / | | Outbound traffic with \ / Inbound 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 ) | | | +-------------------------------------------------------------+
Strict uRPF takes the entries in FIB for SAV. It can improperly block the packets with legitimate source prefixes when asymmetric routing exists. In the figure, if Router 1 applies strict uRPF at 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 packets 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. So, under asymmetric routing, strict uRPF cannot be applied directly.¶
Inbound traffic validation is performed at upstream interfaces of routers to validate the packets from other networks or the Internet. Figure 3 shows an example of inbound SAV. In the figure, Router 3 and Router 4 deploy SAV mechanisms at interface '#' for validating external packets. That is, there are multiple points for inbound traffic validation.¶
ACL-based ingress filtering is usually used for validating inbound traffic. By configuring specified ACL rules, disallowed source prefixes, such as non-global addresses and the internal source prefixes, can be blocked. As analyzed previously, ACL-based ingress filtering requires timely updates especially for possibly dynamic internal source prefixes. When the rules are not updated in time, there may be improper block or improper permit problems. The ACL rules on routers for diversified purposes (not only for filtering) also induce operation challenges. Besides, the operational overhead will increase significantly when there are multiple inbound validation points as shown in Figure 3.¶
Source-based RTBH filtering improves the automation of installing SAV rules on multiple edge routers. However, the configurations on the trigger router still needs to be updated in time, which is similar to ACL-based ingress filtering. Otherwise, there will be improper block problems.¶
Loose uRPF considers FIB as a SAV table. It is more adaptive than ACL-based ingress filtering. However, loose uRPF has limited blocking capability, e.g., cannot "generate" SAV rules for blocking internal prefixes. Some source address spoofing packets may be improperly permitted.¶
+ Packets with + Packets with
| spoofed p1/p2 | spoofed p1/p2
+--------------|---------------------------|----------+
| AS \/ \/ |
| +--+#+-----+ +---+#+----+ |
| | Router 3 +-------------->| Router 4 | |
| +----------+ +----------+ |
| / \ | |
| / \ | |
| \/ \/ \/ |
| +----------+ +----------+ +----------+ |
| | Router 1 | | Router 2 | | Router 5 | |
| +----------+ +----------+ +----------+ |
| \ / | |
| \ / | |
| \ / \/ |
| Subnet1(p1) Subnet2(p2) |
+-----------------------------------------------------+
Accurate validation and low operational overhead are two important design goals of intra-domain SAV mechanisms. As analyzed above, asymmetric routing and dynamic networks are two challenging scenarios for the two goals. In these scenarios, existing SAV mechanisms have problems of inaccurate validation and high operational overhead.¶
ACL-based ingress filtering relies on configurations and thus faces limitations in efficiency and accuracy in dynamic networks. The limitations mainly come from the lack of automation. Operators have to update the ACL-based filtering rules in time when the subnet's prefix or topology changes. Otherwise, improper block or improper permit problems may appear. Source-based RTBH filtering has the similar gap to ACL-based ingress filtering.¶
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 it may consider an invalid incoming interface as valid, resulting in improper permit problems.¶
Loose uRPF is also an automated mechanism, but it has improper permit problems. The limitation comes from that the validation of loose uRPF depends on FIB, and FIB is not enough for achieving accurate validation.¶
Existing intra-domain SAV mechanisms are lacking in adaptiveness in dynamic or asymmetric routing scenarios. If network operators want to apply intra-domain SAV and avoid improper block, they have to figure out which edge routers have asymmetric routing to the directly connected subnet, and implement ACL-based ingress filtering at those edge routers instead of strict uRPF. Identifying asymmetric routes also imposes operational overhead on network operators.¶
This section lists the requirements which can be a guidance for narrowing the gaps. The requirements are some practical points that can be fully or partially fulfilled by proposing new mechanisms.¶
The new intra-domain SAV mechanism needs to improve the accuracy upon existing intra-domain SAV mechanisms. It SHOULD 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 SHOULD match the real data-plane forwarding path from the source and adapt to routing changes automatically. Specially, in the case of asymmetric routing, it MUST avoid improper block problems of strict uRPF and have no more improper permit problems than existing uRPF-based mechanisms (e.g., strict uRPF and loose uRPF).¶
There are cases where it is impossible or hard to guarantee that every learned path is a real forwarding path. In such cases, the learned paths MUST cover the real forwarding paths so as to avoid improper block. By finding the least number of paths while covering all the real forwarding paths, improper permit can be minimized.¶
The new intra-domain SAV mechanism MUST be able to adapt to dynamic scenarios automatically, instead of entirely relying on manual update. At least, it MUST require less operational overhead than existing ACL-based ingress filtering.¶
The new intra-domain SAV mechanism SHOULD NOT assume pervasive adoption. Some routers may not be able to be easily upgraded for supporting the new SAV mechanism due to their limitations of capabilities, versions, or vendors. The mechanism SHOULD be able to provide protection even when it is partially deployed. The effectiveness of protection under partial deployment SHOULD be no worse than automated SAV mechanisms of strict uRPF and loose uRPF.¶
The new intra-domain SAV mechanism should work in the same scenarios as existing intra-domain SAV mechanisms. Generally, it includes all IP-encapsulated scenarios:¶
Scope does not include:¶
In addition, the new intra-domain SAV mechanism should avoid data-plane packet modification. Existing architectures or protocols or mechanisms can be used in the new SAV mechanism to achieve better SAV function.¶
The new intra-domain SAV mechanism MUST NOT introduce additional security vulnerabilities or confusion to the existing intra-domain architectures or control or management plane protocols. Similar to the security scope of intra-domain routing protocols, intra-domain SAV mechanism should ensure integrity and authentication of protocol packets that deliver the required SAV information.¶
The new intra-domain SAV mechanism 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.¶
This document does not request any IANA allocations.¶
Many thanks to the valuable comments from: 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, Libin Liu, John O'Brien, Roland Dobbins, etc.¶