RFC 8074

Source Address Validation Improvement (SAVI) for Mixed Address Assignment Methods Scenario

📅 published February 2017, from ietf

RFC 8074: Source Address Validation Improvement (SAVI) for Mixed Address Assignment Methods Scenario 🎀

okay so bestie, this is RFC 8074 💖 It's an Internet Standards Track document from February 2017, which means it's an official IETF spec and not just vibes. The authors are J. Bi and G. Yao from Tsinghua University (G. Yao is also at Baidu, iconic 👑), J. Halpern from Ericsson, and E. Levy-Abegnoli, Ed., from Cisco. The ISSN is 2070-1721. The header doesn't list anything this one obsoletes or updates, so it stands on its own ✨

Abstract ✨

bestie, in networks that use more than one way to hand out addresses, you can stop spoofed addresses from each technique with the right SAVI methods 🛡️ This doc goes over how several SAVI methods can live together in a single SAVI device, and what happens when two or more of them find the same binding entry 🫶

Status of This Memo 📜

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8074.

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents 🗂️

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
3.  Problem Scope . . . . . . . . . . . . . . . . . . . . . . . .   3
4.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   4
5.  Recommendations for Assignment Separation . . . . . . . . . .   6
6.  Resolving Binding Collisions  . . . . . . . . . . . . . . . .   6
  6.1.  Same Address on Different Binding Anchors . . . . . . . .   6
    6.1.1.  Basic Preference  . . . . . . . . . . . . . . . . . .   7
    6.1.2.  Exceptions  . . . . . . . . . . . . . . . . . . . . .   7
    6.1.3.  Multiple SAVI Device Scenario . . . . . . . . . . . .   8
  6.2.  Same Address on the Same Binding Anchor . . . . . . . . .   9
7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   9
9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
  10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
  10.2.  Informative References . . . . . . . . . . . . . . . . .  11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1. Introduction 🌸

okay so there are currently several SAVI documents ([RFC6620], [RFC7513], and [RFC7219]) that each describe a different way a switch can discover and record bindings between a node's IP address and a binding anchor, and then use that binding for source address validation 🦋 Each one covers how to learn on-link addresses based on how they were assigned: StateLess Address Autoconfiguration (SLAAC), the Dynamic Host Control Protocol (DHCP), and Secure Neighbor Discovery (SEND), respectively. Each of these documents also explains on its own how its SAVI method handles address collisions (same address but different binding anchor). It's giving separate little silos 😭

While multiple IP assignment techniques can be used in the same layer 2 domain, that means one single SAVI device might have to juggle a mix of SAVI methods 🍓 So the purpose of this document is to give recommendations for avoiding collisions, and to review how collisions get handled when two or more of these methods come up with competing bindings 💜

2. Requirements Language 💖

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

3. Problem Scope 🧚

Three different IP address assignment techniques have been analyzed for SAVI:

  1. StateLess Address Autoconfiguration (SLAAC) -- analyzed in FCFS SAVI (First-Come, First-Served) [RFC6620]
  2. Dynamic Host Control Protocol address assignment (DHCP) -- analyzed in SAVI-DHCP [RFC7513]
  3. Secure Neighbor Discovery (SEND) address assignment -- analyzed in SEND SAVI [RFC7219]

there's also a fourth technique for managing (i.e., creation, management, and deletion) a binding on the switch, called "manual" 🙈 It's based on manual binding configuration. How to manage manual bindings is up to operators, so there is not a new SAVI method for manual addresses.

All combinations of address assignment techniques can coexist within a layer 2 domain. A SAVI device MUST implement the corresponding binding setup methods (referred to as "SAVI methods") for each such technique that is in use if it is to provide source address validation. That's a big deal, bestie 👑

SAVI methods are normally viewed as independent from each other, each one handling its own entries. If multiple methods are used in the same device without coordination, each method will try to reject packets sourced with any addresses that method did not discover. Yikes, chaos 💀 So to keep addresses discovered by one SAVI method from getting filtered out by another, the SAVI binding table SHOULD be shared by all the SAVI methods in use in the device. This can create some conflict when the same entry is discovered by two different methods. The purpose of this document is twofold: to provide recommendations and methods to avoid conflicts, and to resolve conflicts when they happen. Collisions happening within a given method are outside the scope of this document ✨

4. Architecture 🏗️

A SAVI device may implement and use multiple SAVI methods. This mechanism, called "SAVI-MIX", is proposed as an arbiter of the binding generation algorithms from these multiple methods, generating the final binding entries as illustrated in Figure 1. Once a SAVI method generates a candidate binding, it will request that SAVI-MIX set up a corresponding entry in the binding table. Then, SAVI-MIX will check if there is any conflict in the binding table. A new binding will be generated if there is no conflict. If there is a conflict, SAVI-MIX will determine whether to replace the existing binding or reject the candidate binding based on the policies specified in Section 6 💅

As a result of this, packet filtering in the SAVI device will not be performed by each SAVI method separately. Instead, the table resulting from applying SAVI-MIX will be used to perform filtering. Thus, the filtering is based on the combined results of the different SAVI mechanisms. It is beyond the scope of this document to describe the details of the filtering mechanism and its use of the combined SAVI binding table.

+--------------------------------------------------------+
|                                                        |
|                                        SAVI Device     |
|                                                        |
|                                                        |
|     +------+    +------+    +------+                   |
|     | SAVI |    | SAVI |    | SAVI |                   |
|     |      |    |      |    |      |                   |
|     | FCFS |    | DHCP |    | SEND |                   |
|     +------+    +------+    +------+                   |
|        |            |           |   Binding            |
|        |            |           |   setup              |
|        v            v           v   requests           |
|     +------------------------------+                   |
|     |                              |                   |
|     |            SAVI-MIX          |                   |
|     |                              |                   |
|     +------------------------------+                   |
|                     |                                  |
|                     v               Final Binding      |
|             +--------------+                           |
|             |   Binding    |                           |
|             |              |                           |
|             |   Table      |                           |
|             +--------------+                           |
|                                                        |
+--------------------------------------------------------+

                  Figure 1: SAVI-MIX Architecture

Each entry in the binding table will contain the following fields:

  1. IP source address
  2. Binding anchor [RFC7039]
  3. Lifetime
  4. Creation time
  5. Binding methods: the SAVI method used for this entry

5. Recommendations for Assignment Separation 🎀

If each address assignment technique uses a separate portion of the IP address space, collisions won't happen. So using non-overlapping address space across address assignment techniques, and therefore across SAVI methods, is recommended. To that end, one should:

  1. DHCP and SLAAC: use a non-overlapping prefix for DHCP and SLAAC. Set the A bit in the Prefix Information option of the Router Advertisement for the SLAAC prefix, and set the M bit in the Router Advertisement for the DHCP prefix. For detailed explanations of these bits, refer to [RFC4861] and [RFC4862] 🌈
  2. SEND and non-SEND: avoid mixed environments (where SEND and non-SEND nodes are deployed) or separate the prefixes announced to SEND and non-SEND nodes. One way to separate the prefixes is to have the router(s) announcing different (non-overlapping) prefixes to SEND and to non-SEND nodes, using unicast Router Advertisements [RFC6085], in response to SEND/non-SEND Router Solicit. 🫶

6. Resolving Binding Collisions 💥

In situations where collisions cannot be avoided by assignment separation, two cases should be considered:

  1. The same address is bound on two different binding anchors by different SAVI methods.
  2. The same address is bound on the same binding anchor by different SAVI methods.

6.1. Same Address on Different Binding Anchors 🦋

okay so this is what happens when assignment address spaces aren't separated, bestie 😭. say an address gets handed out by SLAAC on node X 🎀. the switch slots it into the binding table using FCFS SAVI and anchors it to "anchor-X" 💅. then later that same address gets assigned by DHCP to node Y 💀. SAVI-DHCP makes a candidate binding entry and anchors it to "anchor-Y" 🙈. chaotic, I know 🌈

6.1.1. Basic Preference ✨

okay so first things first: if there's any manually configured binding, the SAVI device SHOULD choose the manually configured binding anchor 👑. manual always wins, periodt 💖

for an address that isn't covered by any manual binding, the SAVI device MUST decide which binding anchor the address should be bound to (anchor-X or anchor-Y in this example) 🫶. the current standard docs for address assignment methods basically imply a prioritization based on time, aka First-Come, First-Served 🧚. the spots where that's written down are:

  • SLAAC: Section 5.4.5 of [RFC4862] 🌸
  • DHCPv4: Section 3.1, Point 5 of [RFC2131] 🍓
  • DHCPv6: Section 18.1.8 of [RFC3315] 🦋
  • SEND: Section 8 of [RFC3971] 💜

in the absence of any configuration or protocol hint (see Section 6.1.2), the SAVI device SHOULD choose the first-come binding anchor, whether it was learned from SLAAC, SEND, or DHCP ✨. first come, first serve, iykyk 💅

6.1.2. Exceptions 🎀

there are two identified exceptions to the general prioritization model 👀. one is Cryptographically Generated Addresses (CGA) [RFC3971] 🔐, and the other is controlled by the configuration of the switch 🌷.

6.1.2.1. CGA Preference 💖

when CGA addresses are used and a collision is detected, preference SHOULD be given to the anchor that carries the CGA credentials once they're verified, in particular the CGA parameters and the RSA options 🫶. ngl the CGA creds are the main character here 👑. also, if an attacker tried to replay CGA credentials, they'd just be competing on the basis of "First-Come, First-Served" (FCFS) principle, so no free pass for them 💀

6.1.2.2. Configuration Preference 🌈

for configuration-driven exceptions, the SAVI device MAY allow the configuration of a triplet ("prefix", "anchor", "method") or ("address", "anchor", "method") ✨. the "prefix" or "address" is the address or address prefix this preference entry applies to 🦋. the "anchor" is the value of a known binding anchor that this device expects to see using this address or addresses from this prefix 🧚. the "method" is the SAVI method this device expects to use when validating address binding entries from that address or prefix 🎀. at least one of "anchor" and "method" MUST be specified 🙈.

later, if a Duplicate Address Detection (DAD) message [RFC4861] comes in with all of these conditions verified 🔍:

  1. the target in the DAD message does not exist in the binding table 🌸,
  2. the target is within the configured "prefix" (or equal to "address") 🍓,
  3. the anchor bound to the target is different from the configured anchor, when specified, and 💜
  4. the configured method, if any, is different from FCFS SAVI 🫶,

then the switch SHOULD defend the address by responding to the DAD message with a Neighbor Advertisement (NA) message, on behalf of the target node 🛡️✨. it SHOULD NOT install the entry into the binding table 🙅‍♀️. the DAD message SHOULD be discarded and not forwarded 🗑️. forwarding it may cause other SAVI devices to send additional defense NAs, which is so not it 💀. SEND nodes in the network MUST disable the option to ignore unsecured advertisements (see Section 8 of [RFC3971]) 🔒. if that option is enabled, the case is outside the scope of this document 🙃. it is suggested to limit the rate of defense NAs to reduce security threats to the switch 🚦. otherwise, a malicious host could consume the switch's resources heavily with flooding DAD messages 😭.

this will simply stop the node from assigning the address, and it will de facto prioritize the configured anchor 👑. it's especially useful for protecting well-known bindings (like a static address of a server) against any other host, even when the server is down 🖥️💖. it's also a way to give priority to a binding learned from SAVI-DHCP over a binding for the same address learned from FCFS SAVI 🎀.

6.1.3. Multiple SAVI Device Scenario 🌐

a single SAVI device doesn't have the info on all bound addresses on the perimeter 👀, so it's not enough to look up local bindings to spot a collision. BUT, assuming DAD is performed throughout the security perimeter for all addresses regardless of assignment method, the DAD response will inform all SAVI devices about any collision 🔥. in that case, "First-Come, First-Served" applies the same way as in a single switch scenario 🧚. if the admin configured a prefix (or a single static binding) on one of the switches to defend, the DAD response generated by that switch will also stop the binding from being installed on other switches on the perimeter 🛡️. the SAVI-MIX preferences of all the SAVI devices in the same layer 2 domain SHOULD be consistent 🫶. inconsistent configurations may cause network breaks 💀.

6.2. Same Address on the Same Binding Anchor 🦋

a binding may be set up on the same binding anchor by multiple methods, typically FCFS SAVI and SAVI-DHCP 🌸. if the binding lifetimes from the two methods are different, priority SHOULD be given to 1) manual configuration, 2) SAVI-DHCP, and 3) FCFS SAVI as the least authoritative 👑. the binding gets removed when the prioritized lifetime expires, even if a less authoritative method had a longer lifetime ⏳.

7. Security Considerations 🔐

okay this section is serious business, bestie 😭🚨. combining SAVI methods (as in SAVI-MIX) does not improve or eliminate the security considerations of each individual SAVI method 💀. so the security considerations for each enabled SAVI method SHOULD be addressed as described in that method's associated RFC 📚. combining methods (as in SAVI-MIX) has two extra implications for security, so pay attention 👀.

first, it may increase susceptibility to DoS attacks, because the SAVI binding setup rate will be the sum of the rates of all enabled SAVI methods 🔥. implementers MUST take these added resource requirements into account 🧠.

second, because SAVI-MIX supports multiple binding mechanisms, it potentially reduces the security level down to that of the weakest supported method 💀, unless additional steps (e.g., requiring non-overlapping address spaces for different methods) are taken 🫶. the weakest link really is the vibe check here 😭.

8. Privacy Considerations 🙈

when implementing multiple SAVI methods, the privacy considerations of all methods apply cumulatively 🌈.

9. IANA Considerations 🧚

this document does not require any IANA registrations ✨.

10. References 📚

10.1. 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, <http://www.rfc-editor.org/info/rfc2119>.

[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, March 1997, <http://www.rfc-editor.org/info/rfc2131>.

[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 2003, <http://www.rfc-editor.org/info/rfc3315>.

[RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, DOI 10.17487/RFC3971, March 2005, <http://www.rfc-editor.org/info/rfc3971>.

[RFC6085] Gundavelli, S., Townsley, M., Troan, O., and W. Dec, "Address Mapping of IPv6 Multicast Packets on Ethernet", RFC 6085, DOI 10.17487/RFC6085, January 2011, <http://www.rfc-editor.org/info/rfc6085>.

[RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS SAVI: First-Come, First-Served Source Address Validation Improvement for Locally Assigned IPv6 Addresses", RFC 6620, DOI 10.17487/RFC6620, May 2012, <http://www.rfc-editor.org/info/rfc6620>.

[RFC7219] Bagnulo, M. and A. Garcia-Martinez, "SEcure Neighbor Discovery (SEND) Source Address Validation Improvement (SAVI)", RFC 7219, DOI 10.17487/RFC7219, May 2014, <http://www.rfc-editor.org/info/rfc7219>.

[RFC7513] Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address Validation Improvement (SAVI) Solution for DHCP", RFC 7513, DOI 10.17487/RFC7513, May 2015, <http://www.rfc-editor.org/info/rfc7513>.

10.2. Informative References 🌸

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, <http://www.rfc-editor.org/info/rfc4861>.

[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/RFC4862, September 2007, <http://www.rfc-editor.org/info/rfc4862>.

[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, October 2013, <http://www.rfc-editor.org/info/rfc7039>.

Acknowledgments 🫶

thanks to Christian Vogt, Eric Nordmark, Marcelo Bagnulo Braun, David Lamparter, Scott G. Kelly, and Jari Arkko for their valuable contributions 💖✨.

Authors' Addresses 🎀

Jun Bi Tsinghua University Institute for Network Sciences and Cyberspace, Tsinghua University Beijing 100084 China

Email: junbi@tsinghua.edu.cn

Guang Yao Tsinghua University/Baidu Baidu Science and Technology Park, Building 1 Beijing 100193 China

Email: yaoguang.china@gmail.com

Joel M. Halpern Ericsson

Email: joel.halpern@ericsson.com

Eric Levy-Abegnoli (editor) Cisco Systems Village d'Entreprises Green Side - 400, Avenue Roumanille Biot-Sophia Antipolis 06410 France

Email: elevyabe@cisco.com

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