Internet DRAFT - draft-huang-savnet-sav-table
draft-huang-savnet-sav-table
savnet M. Huang
Internet-Draft
Intended status: Standards Track W. Cheng
Expires: 5 September 2024 China Mobile
D. Li
Tsinghua University
N. Geng
M. Liu
Huawei Technologies
L. Chen
Zhongguancun Laboratory
C. Lin
New H3C Technologies
4 March 2024
General Source Address Validation Capabilities
draft-huang-savnet-sav-table-05
Abstract
The SAV rules of existing source address validation (SAV) mechanisms,
are derived from other core data structures, e.g., FIB-based uRPF,
which are not dedicatedly designed for source filtering. Therefore
there are some limitations related to deployable scenarios and
traffic handling policies.
To overcome these limitations, this document introduces the general
SAV capabilities from data plane perspective. How to implement the
capabilities and how to generate SAV rules are not in the scope of
this document.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 5 September 2024.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Validation Modes . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Mode 1: Interface-based prefix allowlist . . . . . . . . 4
2.2. Mode 2: Interface-based prefix blocklist . . . . . . . . 5
2.3. Mode 3: Prefix-based interface list . . . . . . . . . . . 5
2.4. Validation Procedure . . . . . . . . . . . . . . . . . . 6
3. Traffic Handling Policies . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Source address validation (SAV) can detect and prevent source address
spoofing on the SAV-enabled routers. When a packet arrives at an
interface of the router, the source address of the packet will be
validated. The packets with unwanted source addresses or arriving at
unwanted interfaces, will be considered invalid and usually be
conducted elimination actions on. Only validated packets will
continue to be handled or forwarded.
From the perspective of data plane validation, the SAV capabilities
of existing mechanisms have two main limitations. One of them is the
deployable scenario limitation. ACL rules can be configured for
filtering unwanted source addresses at specific interfaces
([RFC3704]). However, ACL is not dedicatedly designed for source
prefix filtering. There exist performance and scalability issues due
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to long-key based searching, and usually expert maintenance efforts
are required. Strict uRPF and loose uRPF are two typical FIB-based
SAV mechanisms ([RFC3704]) and are supported by most commercial
routers/switches. FIB-based validation brings many benefits compared
to ACL-based filtering but also induces some limitations. Strict
uRPF is not applicable for asymmetric routing ([RFC8704]), which
exists in various scenarios such as intra-domain multi-homing access,
inter-domain interconnection, etc. Under asymmetric routing, a
source prefix may have a different incoming interface from the next-
hop interface of the matched entry, or the source prefix does not
exist in the FIB at all. Loose mode can only block unannounced
prefix, which results in massive false negatives. Overall, existing
ACL-based or FIB-based SAVs can only be applied to specific scenarios
and cannot be adaptive to various scenarios (e.g., symmetric vs
asymmetric).
The other limitation is inflexible traffic handling policy. The
current common practice is just to silently drop the spoofed packets.
We don’t know who benefits from this and who is the source. Further
more, the clues of attacks are ignored, which could be very helpful
for dealing with DDoS etc.
The root cause of the above two limitations is that there is no tool
specifically designed for source address filtering. That is, the
capabilities of current tools are derived from other functions, e.g.,
FIB or ACL.
This document describes the general SAV capabilities that the data
plane of SAV-enabled devices should have. Two kinds of capabilities
will be introduced: validation mode and traffic handling policy.
Validation modes describe how to apply validation in different
scenarios. Traffic handling policies are the policies applied on the
validated packets. By implementing the general SAV capabilities, the
above two limitations of existing mechanisms can be overcome.
To achieve accurate and scalable source address validation, a
dedicated SAV table for SAV rules is needed instead of using those
derived from other functions, e.g., FIB or ACL. Note that the
general SAV capabilities described in this document is decoupled with
real implementation. Conforming implementations of this
specification may differ, but the SAV outcomes SHOULD be equivalent
to the described SAV capabilities. How to generate SAV rules is not
the focus of this document.
1.1. Terminology
SAV rule: The entry specifying the valid incoming interfaces of
specific source addresses or source prefixes.
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Validation mode: The mode that describes the typical applications of
SAV in a specific kind of scenarios. Different modes take effect in
different scales and treat the default prefix differently.
Traffic handling policy: The policy taken on the packets validated by
SAV.
1.2. 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. Validation Modes
This section describes validation modes. These modes take effect in
different scales and treat the default prefix differently. By
choosing modes in different scenarios appropriately, the network can
be protected as much as possible while not impacting the forwarding
of legitimate packets.
Validation modes also describe the goal of SAV rule generation. The
modes can be set before generating the rules. By specifying
validation modes, operators can take appropriate SAV mechanisms
matching the modes, and engineers can design new SAV mechanisms to
achieve the goal in challenging scenarios.
2.1. Mode 1: Interface-based prefix allowlist
Mode 1 is an interface-scale mode, i.e., it takes effect on a
specific interface. The interface enabling Mode 1 is maintaining an
interface-based prefix allowlist. Only the source prefixes recorded
in the list will be considered valid, otherwise invalid.
Applying Mode 1 on an interface requires the complete knowledge of
legitimate prefixes connected to the interface. Mode 1 is suitable
to the closed-connected interfaces such as those connecting to a
subnet, a stub AS, or a customer cone. Such a mode can efficiently
prevent the connected network from spoofing source prefixes of other
networks.
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Strict uRPF based on FIB belongs to this mode. However, to overcome
the limitation of asymmetric routing, native source prefix-based SAV
table is suggested. This is essential for new SAV mechanisms/
architectures such as EFP-uRPF [RFC8704], BAR-SAV
[I-D.ietf-sidrops-bar-sav], Intra-domain/Inter-domain SAVNET
([I-D.li-savnet-intra-domain-architecture],
[I-D.wu-savnet-inter-domain-architecture]), etc.
In some cases, it may be difficult for an interface getting all the
legitimate source prefixes. If not all legitimate prefixes are
included in the allowlist, packets with legitimate source addresses
arriving at the interface may be improperly blocked. For example,
the interface with a default route or the interface connecting to the
Internet through a provider AS can hardly promise to know all the
legitimate source prefixes.
2.2. Mode 2: Interface-based prefix blocklist
Mode 2 is also an interface-scale mode, i.e., it takes effect on a
configured interface. An interface cannot enable Mode 1 and Mode 2
at the same time. The interface enabling Mode 2 is maintaining an
interface-based prefix blocklist. The source prefixes recorded in
the list will be considered invalid, otherwise valid.
This mode does not require the complete blocklist. If the packets
with the specific source addresses need to be discarded, Mode 2 can
be taken. Mode 2 is suitable for proactive filtering and reactive
filtering. Usually the source prefixes that are sure to be invalid
will be put into the blocklist, which is proactive filtering.
Reactive filtering rules are usually installed in DDoS elimination
for dropping packets with specific source addresses.
The prefix blocklist can be generated automatically, e.g., one of
Intra-domain SAVNET architecture cases, blocking the incoming traffic
with internal source prefixes. Or operators can configure the
specific source prefixes to block from the interface. This is
similar to ACL-based filtering, but more native SAV rule expression
with better performance and scalability is needed.
2.3. Mode 3: Prefix-based interface list
Mode 3 is a router-scale mode, i.e., it can validate traffic arriving
at the router from all directions. The router enabling Mode 3 will
record the protected source prefixes and maintain an interface list
for each source prefix. The interface list of each source prefix may
be an allowlist or a blocklist.
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If a source prefix has an interface allowlist, the packet whose
source address matches the source prefix is considered valid, only
when its incoming interface is in the allowlist. Otherwise, the
packet is considered invalid.
If a source prefix has an interface blocklist, the packet whose
source address matches the source prefix is considered invalid, only
when its incoming interface is in the blocklist. Otherwise, the
packet is considered valid.
If its source address does not match any recorded source prefix, the
packet is valid by default.
Mode 3 focuses on validating/protecting the interested source
prefixes. Mode 3 is applicable to the scenario where valid incoming
interfaces of a source prefix change dynamically. If some source
prefixes are important, Mode 3 can also provide stricter and more
efficient protection than Mode 1 and Mode 2 for these source
prefixes.
Operators can configure the interface list for a specific source
prefix, to prevent DDoS attack related to this source prefix. Or the
interface list for specific prefixes can be generated automatically,
e.g., one capability defined by Intra-domain and Inter-domain SAVNET
architectures.
2.4. Validation Procedure
Mode 1 and Mode 2 are working on interface-level, while Mode 3 is for
the router-level. Thus, there can be multiple modes configured on
the same router. Mode 1 are most preferred if applicable (with best
pretection effect) and mutual exclusive with the other two modes,
which means while an interface enabled Mode 1, the traffic for this
interface don't need go through Mode 2 or Mode 3 -- while an
interface enabled Mode 2, the traffic still need go through Mode 3.
Figure 1 shows a comparison of different validation modes for dealing
with default prefix.
+-----------------------------------------------------+
+ Mode | Scale | Treatment of the default prefix +
+-----------------------------------------------------+
+ 1 | interface | invalid +
+ 2 | interface | valid +
+ 3 | router | check its incoming interface +
+-----------------------------------------------------+
Figure 1: A comparison of different validation modes
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The validation procedure is shown in Figure 2. Suppose the router
has learned the SAV rules by SAV mechanisms and implemented them in
the data plane. When a packet arrives at the router, the router will
take the source address and the incoming interface of the packet as
the input and look up the SAV rules. The final validity state that
is either "valid" or "invalid" will be returned after the procedure.
Firstly, the packet is validated by the enabled interface-scale mode,
i.e., Mode 1 or Mode 2. If the validity state1 or validity state2 is
"invalid", the final validity state is "invalid" and the packet does
not have to be validated by Mode 3. If the validity state1 or
validity state2 is "valid", the packet still needs to be validated by
the enabled Mode 3 and the validity state3 is the final validity
state.
If a mode is not enabled, then the corresponding list should not be
queried. If Mode 3 is not enabled, either the validity state1 or the
validity state2 is the final validity state.
SAV-enabled router
+--------------------------------------------+
| |
packet1--->Mode1---> validity ----+ |
| state1 | |
| | if the validity state |
packet2--->Mode2---> validity ----| is "valid" and |
| state2 | Mode 3 is enabled |
| | |
| \/ |
packet3----->|---------------->Mode3---> validity |
| state3 |
+--------------------------------------------+
Figure 2: Validation procedure
To achieve accurate and scalable source address validation, a
dedicated SAV table for SAV rules is needed rather than using those
derived from other functions, e.g., FIB or ACL.
3. Traffic Handling Policies
After doing validation, the router gets the validity state of the
incoming packet. For the packet with invalid state, traffic handling
policies should be taken on the packet. Simply forwarding or
silently dropping may not well satisfy the requirements of operators
in different scenarios. This section suggests to provide flexible
traffic handling policies to validated packets just like FlowSpec
([RFC8955], [RFC8956]).
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The followings are the traffic control policies that can be taken.
One and only one of the policies will be chosen for an "invalid"
validation result.
* "Permit": Forward packets normally though the packets are
considered invalid. This policy is useful when operators only
want to monitor the status of source address spoofing in the
network. The "Permit" policy can be taken together with the
"Sample" policy.
* "Discard": Drop packets directly, which is the common choose of
existing SAV mechanisms.
* "Rate limit": Enforce an upper bound of traffic rate (e.g., bps or
pps) for mitigation of source address spoofing attacks. This
policy is helpful while operators want do tentative filtering.
* "Traffic redirect": Redirect the packets to the specified points
(e.g., scrubbing centers) in the network for attack elimination.
There are also traffic monitor policies that are optional. One of
the useful traffic monitor policies is:
* "Sample": Capture the packets with a configurable sampling rate
and reports them to remote servers (e.g., security analysis
center). The sampled packets can be used for threat awareness and
further analysis [I-D.cheng-savnet-proactive-defense-network].
"Sample" can be taken together with any one of the above policies.
Note that, existing techniques like NetStream or NetFlow can be
used for "Sample".
4. Security Considerations
This document focuses on the general SAV capabilities. The
generation of SAV rules is not discussed. There may be some security
considerations for SAV generation, but it is not in the scope of this
document.
The "Sample" policy pushes data to remote servers. This function can
be achieved by existing techniques like NetStream or NetFlow. The
"Sample" policy may induce same security considerations as these
techniques, and the corresponding documents have discussed them.
5. IANA Considerations
This document includes no request to IANA.
6. References
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6.1. Normative References
[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/info/rfc2119>.
[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/info/rfc8174>.
6.2. Informative References
[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/info/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/info/rfc8704>.
[RFC8955] Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M.
Bacher, "Dissemination of Flow Specification Rules",
RFC 8955, DOI 10.17487/RFC8955, December 2020,
<https://www.rfc-editor.org/info/rfc8955>.
[RFC8956] Loibl, C., Ed., Raszuk, R., Ed., and S. Hares, Ed.,
"Dissemination of Flow Specification Rules for IPv6",
RFC 8956, DOI 10.17487/RFC8956, December 2020,
<https://www.rfc-editor.org/info/rfc8956>.
[I-D.cheng-savnet-proactive-defense-network]
Cheng, W., Geng, N., Li, D., and Yue, "Network Proactive
Defense based on Source Address Validation", Work in
Progress, Internet-Draft, draft-cheng-savnet-proactive-
defense-network-01, 18 October 2023,
<https://datatracker.ietf.org/doc/html/draft-cheng-savnet-
proactive-defense-network-01>.
[I-D.ietf-sidrops-bar-sav]
Sriram, K., Lubashev, I., and D. Montgomery, "Source
Address Validation Using BGP UPDATEs, ASPA, and ROA (BAR-
SAV)", Work in Progress, Internet-Draft, draft-ietf-
sidrops-bar-sav-02, 12 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-sidrops-
bar-sav-02>.
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[I-D.li-savnet-intra-domain-architecture]
Li, D., Wu, J., Qin, L., Geng, N., Chen, L., Huang, M.,
and F. Gao, "Intra-domain Source Address Validation
(SAVNET) Architecture", Work in Progress, Internet-Draft,
draft-li-savnet-intra-domain-architecture-06, 21 January
2024, <https://datatracker.ietf.org/doc/html/draft-li-
savnet-intra-domain-architecture-06>.
[I-D.wu-savnet-inter-domain-architecture]
Wu, J., Li, D., Huang, M., Chen, L., Geng, N., Liu, L.,
and L. Qin, "Inter-domain Source Address Validation
(SAVNET) Architecture", Work in Progress, Internet-Draft,
draft-wu-savnet-inter-domain-architecture-06, 5 February
2024, <https://datatracker.ietf.org/doc/html/draft-wu-
savnet-inter-domain-architecture-06>.
Authors' Addresses
Mingqing Huang
Beijing
China
Email: huangmq@vip.sina.com
Weiqiang Cheng
China Mobile
Beijing
China
Email: chengweiqiang@chinamobile.com
Dan Li
Tsinghua University
Beijing
China
Email: tolidan@tsinghua.edu.cn
Nan Geng
Huawei Technologies
Beijing
China
Email: gengnan@huawei.com
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Mingxing Liu
Huawei Technologies
Beijing
China
Email: liumingxing7@huawei.com
Li Chen
Zhongguancun Laboratory
Beijing
China
Email: lichen@zgclab.edu.cn
Changwang Lin
New H3C Technologies
Beijing
China
Email: linchangwang.04414@h3c.com
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