An RPKI and IPsec-based AS-to-AS Approach for Source Address Validation
draft-xu-ipsecme-risav-00
The information below is for an old version of the document.
| Document | Type |
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|---|---|---|---|
| Authors | Ke Xu , Jianping Wu , Yangfei Guo , Benjamin M. Schwartz , Haiyang Wang | ||
| Last updated | 2023-03-07 | ||
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draft-xu-ipsecme-risav-00
ipsecme K. Xu
Internet-Draft J. Wu
Updates: 4302 (if approved) Tsinghua University
Intended status: Standards Track Y. Guo
Expires: 8 September 2023 Zhongguancun Laboratory
B. M. Schwartz
Google LLC
H. (Henry). Wang
The University of Minnesota at Duluth
7 March 2023
An RPKI and IPsec-based AS-to-AS Approach for Source Address Validation
draft-xu-ipsecme-risav-00
Abstract
This document presents RISAV, a protocol for establishing and using
IPsec security between Autonomous Systems (ASes) using the RPKI
identity system. In this protocol, the originating AS adds
authenticating information to each outgoing packet at its Border
Routers (ASBRs), and the receiving AS verifies and strips this
information at its ASBRs. Packets that fail validation are dropped
by the ASBR. RISAV achieves Source Address Validation among all
participating ASes.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/bemasc/draft-xu-risav.
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
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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."
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This Internet-Draft will expire on 8 September 2023.
Copyright Notice
Copyright (c) 2023 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 (https://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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. What RISAV Is . . . . . . . . . . . . . . . . . . . . . . 5
2.2. How RISAV Works . . . . . . . . . . . . . . . . . . . . . 5
3. Control Plane . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Disabling RISAV . . . . . . . . . . . . . . . . . . . . . 8
3.1.1. Targeted shutdown . . . . . . . . . . . . . . . . . . 8
3.1.2. Total shutdown . . . . . . . . . . . . . . . . . . . 8
3.2. Green Channel . . . . . . . . . . . . . . . . . . . . . . 9
4. Data Plane . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Transport Mode . . . . . . . . . . . . . . . . . . . . . 10
4.1.1. ICMP rewriting . . . . . . . . . . . . . . . . . . . 11
4.2. Tunnel Mode . . . . . . . . . . . . . . . . . . . . . . . 12
5. MTU Handling . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. MTU Enforcement . . . . . . . . . . . . . . . . . . . . . 13
5.2. MTU Estimation . . . . . . . . . . . . . . . . . . . . . 13
5.2.1. Step 1: Initial estimate . . . . . . . . . . . . . . 13
5.2.2. Step 2: MTU monitoring . . . . . . . . . . . . . . . 14
6. Traffic Selectors and Replay Protection in RISAV . . . . . . 15
6.1. Disabling replay protection . . . . . . . . . . . . . . . 15
6.2. Enabling replay protection . . . . . . . . . . . . . . . 15
6.3. Changes to AS IP ranges . . . . . . . . . . . . . . . . . 16
7. Possible Extensions . . . . . . . . . . . . . . . . . . . . . 16
7.1. Header-only authentication . . . . . . . . . . . . . . . 16
7.2. Time-based key rotation . . . . . . . . . . . . . . . . . 16
7.3. Static Negotiation . . . . . . . . . . . . . . . . . . . 17
8. Security Consideration . . . . . . . . . . . . . . . . . . . 18
8.1. Threat models . . . . . . . . . . . . . . . . . . . . . . 18
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8.1.1. Replay attacks . . . . . . . . . . . . . . . . . . . 18
8.1.2. Downgrade attacks . . . . . . . . . . . . . . . . . . 18
8.2. Incremental benefit from partial deployment . . . . . . . 18
8.3. Compatibility . . . . . . . . . . . . . . . . . . . . . . 18
8.3.1. With end-to-end IPsec . . . . . . . . . . . . . . . . 18
8.3.2. With other SAV mechanisms . . . . . . . . . . . . . . 19
9. Operational Considerations . . . . . . . . . . . . . . . . . 19
9.1. Reliability . . . . . . . . . . . . . . . . . . . . . . . 19
9.2. Synchronizing Multiple ASBRs . . . . . . . . . . . . . . 19
9.3. Performance . . . . . . . . . . . . . . . . . . . . . . . 20
9.4. NAT scenario . . . . . . . . . . . . . . . . . . . . . . 20
10. Consistency with Existing Standards . . . . . . . . . . . . . 20
10.1. IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1.1. MTU . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1.2. Header modifications . . . . . . . . . . . . . . . . 21
10.1.3. IP address usage . . . . . . . . . . . . . . . . . . 21
10.2. RPKI Usage . . . . . . . . . . . . . . . . . . . . . . . 21
11. IANA Consideration . . . . . . . . . . . . . . . . . . . . . 22
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1. Normative References . . . . . . . . . . . . . . . . . . 22
12.2. Informative References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
Source address spoofing is the practice of using a source IP address
without proper authorization from its owner. The basic internet
routing architecture does not provide any defense against spoofing,
so any system can send packets that claim any source address. This
practice enables a variety of attacks, most notably volumetric DoS
attacks as discussed in [RFC2827].
There are many possible approaches to preventing address spoofing.
Section 2.1 of [RFC5210] describes three classes of Source Address
Validation (SAV): Access Network, Intra-AS, and Inter-AS. Inter-AS
SAV is the most challenging class, because different ASes have
different policies and operate independently. Inter-AS SAV requires
the different ASes to collaborate to verify the source address.
However, in the absence of total trust between all ASes, Inter-AS SAV
is a prerequisite to defeat source address spoofing.
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Despite years of effort, current Inter-AS SAV protocols are not
widely deployed. An important reason is the difficulty of balancing
the clear security benefits of partial implementations with the
scalability of large-scale deployments. uRPF [RFC5635] [RFC8704], for
example, is a routing-based scheme that filters out spoofed traffic.
In cases where the routing is dynamic or unknown, uRPF deployments
must choose between false negatives (i.e. incomplete SAV) and false
positives (i.e. broken routing).
This document provides an RPKI- [RFC6480] and IPsec-based [RFC4301]
approach to inter-AS source address validation (RISAV). RISAV is a
cryptography-based SAV mechanism to reduce the spoofing of source
addresses. In RISAV, the RPKI database acts as a root of trust for
IPsec between participating ASes. Each pair of ASes uses IKEv2 to
negotiate an IPsec Security Association (SA). Packets between those
ASes are then protected by a modified IPsec Authentication Header
(AH) [RFC4302] or an Encapsulating Security Payload (ESP)[RFC4303].
IPsec authenticates the source address, allowing spoofed packets to
be dropped at the border of the receiving AS.
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
BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
Commonly used terms in this document are described below.
ACS: AS Contact Server, which is the logical representative of one
AS and is responsible for delivering session keys and other
information to ASBR.
Contact IP: The IP address of the ACS.
ASBR: AS border router, which is at the boundary of an AS.
SAV: Source Address Validation, which verifies the source address of
an IP packet and guarantee the source address is valid.
2. Overview
The goal of this section is to provides the high level description of
what RISAV is and how RISAV works.
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2.1. What RISAV Is
RISAV is a cryptographically-based inter-AS source address validation
protocol that provides clear security benefits even at partial
deployment. It aims to prove that each IP datagram was sent from
inside the AS that owns its source address, defeating spoofing and
replay attacks. It is light-weight and efficient, and provides
incremental deployment incentives.
At the source AS Border Router, RISAV adds a MAC to each packet that
proves ownership of the packet's source address. At the recipient's
ASBR, RISAV verifies and removes this MAC, recovering the unmodified
original packet. The MAC is delivered in the Integrity Check Value
(ICV) field of a modified IPsec AH, or as part of the normal IPsec
ESP payload.
2.2. How RISAV Works
RISAV uses IKEv2 to negotiate an IPsec security association (SA)
between any two ASes. RPKI provides the binding relationship between
AS numbers, IP ranges, contact IPs, and public keys. After
negotiation, all packets between these ASes are secured by use of a
modified AH header or a standard ESP payload.
Before deploying RISAV, each AS selects one or more representative
contact IPs, and publishes them in the RPKI database. When
negotiating or consulting with one AS, the peer MUST first
communicate with one of these contact IPs. Each contact IP is used
to enable RISAV only for its own address family (i.e. IPv4 or IPv6),
so ASes wishing to offer RISAV on both IPv4 and IPv6 must publish at
least two contact IPs.
A typical workflow of RISAV is shown in Figure 1.
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+--------------+
| IANA |
+--------------+
|--------------------------+
V |
+--------------+ |
| RIR | |
+--------------+ |
/ \-----------------+-1. Signing CA
V V | Certificate
+--------------+ +--------------+ |
| LIR1 | | LIR2 | |
+--------------+ +--------------+ |
/ \-+
V V
+--------------+ +--------------+
| 3. RISAV |---------+ +------| 3. RISAV |
| Announcement | | 2. Signing EE Certificate| | Announcement |
| | +-------+ +----+ | |
| AS A | | | | AS B |
| contact IP a | V V | contact IP b |
| ####### -------------------------------- ####### |
| # ACS # 4. SA Negotiation and Delivery # ACS # |
| ####### -------------------------------- ####### |
| | | |
| ######## +++++++++++++++++++++++++++++++++ ######## |
| # ASBR # 5. Data Transmission # ASBR # |
| ######## with IPsec AH/ESP ######## |
| | +++++++++++++++++++++++++++++++++ | |
+--------------+ +--------------+
Figure 1: RISAV workflow example.
1. RPKI process. The five Regional Internet Registries (RIR),
authorized by IANA, use their root certificate to sign the
Certificate Authority (CA) certificate of the Local Internet
Registry (LIR), which is used to authorize the Autonomous System
(AS) (sometimes indirectly via the Internet Service Provider
(ISP)). When they obtain their own CA certificate, the AS would
sign an End Entity (EE) certificate with a Route Origin
Authorisation (ROA) which is a cryptographically signed object
that states which AS are authorized to originate a certain
prefix. This authenticated binding of the ASN to its IP prefixes
is published in the RPKI database. This is a prerequisite for
RISAV.
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2. ACS EE certificate provisioning. The ACS would need its own EE
certificate for IKEv2. This EE certificate is REQUIRED like the
BGPsec Router Certificate defined in [RFC8209].
3. RISAV announcement. Each participating AS announces its support
for RISAV in the RPKI database, including the IP address of its
ACS (the "contact IP").
4. SA negotiation and delivery. The ACSes negotiate an SA using
IKEv2. After synchronization, all ASBRs would get the SA,
including the session key and other parameters.
5. IPsec communication. RISAV uses IPsec AH (i.e. "transport mode")
for authentication of the IP source address by default. When an
ASBR in AS A sends a packet to AS B, it uses the established
IPsec channel to add the required AH header. The ASBR in AS B
validates the AH header to ensure that the packet was not
spoofed, and removes the header.
3. Control Plane
The functions of the control plane of RISAV include:
* Announcing that this AS supports RISAV.
* Publishing contact IPs.
* Performing IPsec session initialization (i.e. IKEv2).
These functions are achieved in two steps. First, each participating
AS publishes a Signed Object [RFC6488] in its RPKI Repository
containing a RISAVAnnouncement:
RISAVAnnouncement ::= SEQUENCE {
version [0] INTEGER DEFAULT 0,
asID ASID,
contactIP ipAddress,
testing BOOLEAN }
When a participating AS discovers another participating AS (via its
regular sync of the RPKI database), it initiates an IKEv2 handshake
between its own contact IP and the other AS's contact IP. This
handshake MUST include an IKE_AUTH exchange that authenticates both
ASes with their RPKI ROA certificates.
Once this handshake is complete, each AS MUST activate RISAV on all
outgoing packets, and SHOULD drop all non-RISAV traffic from the
other AS after a reasonable grace period (e.g. 60 seconds).
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The "testing" field indicates whether this contact IP is potentially
unreliable. When this field is set to true, other ASes MUST fall
back to ordinary operation if IKE negotiation fails. Otherwise, the
contact IP is presumed to be fully reliable, and other ASes SHOULD
drop all non-RISAV traffic from this AS if IKE negotiation fails (see
Section 8.1.2).
For more information about RPKI, see [RFC6480].
3.1. Disabling RISAV
3.1.1. Targeted shutdown
IKEv2 SAs can be terminated on demand using the Delete payload
([RFC7296], Section 1.4.1). In ordinary uses of IKEv2, the SAs exist
in inbound-outbound pairs, and deletion of one triggers a response
deleting the other.
In RISAV, SAs do not necessarily exist in pairs. Instead, RISAV's
use of IPsec is strictly unidirectional, so deletion does not trigger
an explicit response. Instead, ASes are permitted to delete both
inbound and outbound SAs, and deletion of an inbound SA SHOULD cause
the other network to retry RISAV negotiation. If this, or any, RISAV
IKEv2 handshake fails with a NO_ADDITIONAL_SAS notification
([RFC7296], Section 1.3), the following convention applies:
* AS $A is said to have signaled a "RISAV shutdown" to $B if it
sends NO_ADDITIONAL_SAS on a handshake with no child SAs.
* In response, $B MUST halt all further RISAV negotiation to $A
until:
- At least one hour has passed, OR
- $A negotiates a new SA from $A to $B.
* After at most 24 hours, $B SHOULD resume its regular negotiation
policy with $A.
This convention enables participating ASes to shut down RISAV with
any other AS, by deleting all SAs and rejecting all new ones. It
also avoids tight retry loops after a shutdown has occurred, but
ensures that RISAV is retried at least once a day.
3.1.2. Total shutdown
To disable RISAV entirely, a participating AS MUST perform the
following steps in order:
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1. Apply a targeted shutdown (Section 3.1.1) to all other networks
and delete all existing SAs. - Note that the shutdown procedure
can fail if another network's ACS is unreachable.
2. Stop requiring RISAV authentication of incoming packets.
3. Remove the RISAVAnnouncement from the RPKI Repository.
4. Wait at least 24 hours.
5. Shut down the contact IP.
Conversely, if any AS no longer publishes a RISAVAnnouncement, other
ASes MUST immediately stop sending RISAV to that AS, but MUST NOT
delete any active Tunnel Mode SAs for at least 24 hours, in order to
continue to process encrypted incoming traffic.
TODO: Discuss changes to the contact IP, check if there are any
race conditions between activation and deactivation, IKEv2
handshakes in progress, SA expiration, etc.
3.2. Green Channel
In the event of a misconfiguration or loss of state, it is possible
that a negotiated SA could become nonfunctional before its expiration
time. For example, if one AS is forced to reset its ACS and ASBRs,
it may lose the private keys for all active RISAV SAs. If RISAV were
applied to the IKEv2 traffic used for bootstrapping, the
participating ASes would be unable to communicate until these broken
SAs expire, likely after multiple hours or days.
To ensure that RISAV participants can rapidly recover from this error
state, RISAV places control plane traffic in a "green channel" that
is exempt from RISAV's protections. This "channel" is defined by two
requirements:
* RISAV senders MUST NOT add RISAV protection to packets to or from
any announced contact IP
* RISAV recipients MUST NOT enforce RISAV validation on packets sent
to or from any announced contact IP.
Although the green channel denies RISAV protection to the ACS, the
additional mitigations described in Section 4 ensure that the ACS has
limited exposure to address-spoofing and DDoS attacks. In addition,
the ACS can use the IKEv2 COOKIE (Section 2.6 of [RFC7296]) and
PUZZLE ([RFC8019]) systems to reject attacks based on source address
spoofing.
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4. Data Plane
All the ASBRs of the AS are REQUIRED to enable RISAV. The
destination ASBR uses the IPsec SPI to locate the correct SA.
As defined in [RFC4301], the Security Association Database (SAD)
stores all the SAs. Each data item in the SAD includes a
cryptographic algorithm (e.g. HMAC-SHA-256), its corresponding key,
and other relevant parameters.
When an outgoing packet arrives at the source ASBR, its treatment
depends on the source and destination address. If the source address
belongs to the AS in which the ASBR is located, and the destination
address is in an AS for which the ASBR has an active RISAV SA, then
the packet needs to be modified for RISAV.
The modification that is applied depends on whether IPsec "transport
mode" or "tunnel mode" is active. RISAV implementations MUST support
transport mode, and MAY support tunnel mode. The initiator chooses
the mode by including or omitting the USE_TRANSPORT_MODE notification
in the IKEv2 handshake, retrying in the other configuration if
necessary.
When a packet arrives at the destination ASBR, it will check the
destination address and the source address. If the destination
belongs to the AS in which the destination ASBR is located, and the
source address is in an AS with which this AS has an active RISAV SA,
then the packet is subject to RISAV processing.
To avoid DoS attacks, participating ASes MUST drop any outgoing
packet to the contact IP of another AS. Only the AS operator's
systems (i.e. the ACS and ASBRs) are permitted to send packets to the
contact IPs of other ASes. ASBRs MAY drop inbound packets to the
contact IP from non-participating ASes.
4.1. Transport Mode
To avoid conflict with other uses of IPsec (Section 8.3.1), RISAV
updates the IPsec Authentication Header (AH) format, converting one
RESERVED octet (which is previously required to always be zero) into
a new "Scope" field. The updated format is shown in Figure 2.
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1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Payload Len | RESERVED | Scope |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Security Parameters Index (SPI) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number Field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Integrity Check Value-ICV (variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Updated AH Format.
The "Scope" field identifies the scope of protection for this
authentication header, i.e. the entities that are expected to produce
and consume it. Two Scope values are defined:
* 0: IP. This is the pre-existing use of the Authentication Header,
to authenticate packets from the source IP to the destination IP.
* 1: AS. This header authenticates the packet from the source AS to
the destination AS.
Other Scope values could be defined in the future.
The AS-scoped AH headers are only for AS-to-AS communication.
Sending ASes MUST NOT add such headers unless the receiving AS has
explicitly opted to receive them. Receiving ASes MUST strip off all
such headers for packets whose destination is inside the AS, even if
the AS is not currently inspecting the ICV values.
Transport mode normally imposes a space overhead of 32 octets.
4.1.1. ICMP rewriting
There are several situations in which an intermediate router on the
path may generate an ICMP response to a packet, such as a Packet Too
Big (PTB) response for Path MTU Discovery, or a Time Exceeded message
for Traceroute. These ICMP responses generally echo a portion of the
original packet in their payload.
An ASBR considers an ICMP payload to match a Transport Mode RISAV SA
if:
1. The payload's source address is in this AS, AND
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2. The payload's destination address is in the other AS, AND
3. The payload contains a RISAV AH header whose SPI matches the
SA's.
When an ASBR observes a matching ICMP response, it MUST forward it to
the intended recipient, with the following modifications:
* The ASBR MUST remove the RISAV AH header from the payload, so that
the echoed payload data matches the packet sent by the original
sender.
* When processing a Packet Too Big message, the ASBR MUST reduce the
indicated MTU value by the total length of the RISAV AH header.
These changes ensure that RISAV remains transparent to the endpoints,
similar to the ICMP rewriting required for Network Address
Translation [RFC5508] (though much simpler).
4.2. Tunnel Mode
In tunnel mode, a RISAV sender ASBR wraps each outgoing packet in an
ESP payload ([RFC4303]) and sends it as directed by the corresponding
SA. This may require the ASBR to set the Contact IP as the source
address, even if it would not otherwise send packets from that
address. (See also "Anycast", Section 9.1).
Tunnel mode imposes a space overhead of 73 octets in IPv6.
5. MTU Handling
Like any IPsec tunnel, RISAV normally reduces the effective IP
Maximum Transmission Unit (MTU) on all paths where RISAV is active.
To ensure standards compliance and avoid operational issues,
participating ASes MUST choose a minimum acceptable "inner MTU", and
reject any RISAV negotiations whose inner MTU would be lower.
There are two ways for a participating AS to compute the inner MTU:
1. *Prior knowledge of the outer MTU*. If a participating AS knows
the minimum outer MTU on all active routes to another AS (e.g.,
from the terms of a transit or peering agreement), it SHOULD use
this information to calculate the inner MTU of a RISAV SA with
that AS.
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2. *Estimation of the outer MTU*. If the outer MTU is not known in
advance, the participating ASes MUST estimate and continuously
monitor the MTU, disabling the SA if the inner MTU falls below
the minimum acceptable value. An acceptable MTU estimation
procedure is described in Section 5.2.
If the minimum acceptable inner MTU is close or equal to a common
outer MTU value (e.g., 1500 octets), RISAV will not be usable in its
baseline configuration. To enable larger inner MTUs, participating
ASes MAY offer support for AGGFRAG [RFC9347] in the IKEv2 handshake
if they are able to deploy it (see Section 6).
5.1. MTU Enforcement
In tunnel mode, RISAV ASBRs MUST treat the tunnel as a single IP hop
whose MTU is given by the current (estimated) inner MTU. Oversize
packets that reach the ASBR SHALL generate Packet Too Big (PTB) ICMP
responses (or be fragmented forward, in IPv4) as usual.
In transport mode, RISAV ASBRs SHOULD NOT enforce the estimated inner
MTU. Instead, ASBRs SHOULD add RISAV headers and attempt to send
packets as normal, regardless of size. (This may cause a PTB ICMP
response at the current router or a later hop, which is modified and
forwarded as described in Section 4.1.1.)
In either mode, the ASBR SHOULD apply TCP MSS clamping [RFC4459],
Section 3.2 to outbound packets based on the current estimated inner
MTU.
5.2. MTU Estimation
This section describes an MTU estimation procedure that is considered
acceptable for deployment of RISAV. Other procedures with similar
performance may also be acceptable.
5.2.1. Step 1: Initial estimate
To compute an initial estimate, the participating ASes use IKEv2 Path
MTU Discovery (PMTUD) [RFC7383], Section 2.5.2 between their ACSes
during the IKEv2 handshake. However, unlike the recommendations in
[RFC7383], the PMTUD process is performed to single-octet
granularity. The IKEv2 handshake only proceeds if the resulting
outer MTU estimate is compatible with the minimum acceptable inner
MTU when using the intended SA parameters.
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5.2.2. Step 2: MTU monitoring
The initial MTU estimate may not be correct indefinitely:
* The Path MTU may change due to a configuration change in either
participating AS.
* The Path MTU may change due to a routing change outside of either
AS.
* The Path MTU may be different for packets to or from different
portions of the participating ASes.
To ensure that the MTU estimate remains acceptable, and allow for
different MTUs across different paths, each ASBR maintains an MTU
estimate for each active SA, and updates its MTU estimate whenever it
observes a PTB message. The ASBR's procedure is as follows:
1. Find the matching SA ({icmp-rewriting}) for this PTB message. If
there is none, abort.
2. Check the SA's current estimated outer MTU against the PTB MTU.
If the current estimate is smaller or equal, abort.
3. Perform an outward Traceroute to the PTB payload's destination
IP, using packets whose size is the current outer MTU estimate,
stopping at the first IP that is equal to the PTB message's
sender IP or is inside the destination AS.
4. If a PTB message is received, reduce the current MTU estimate
accordingly.
5. If the new estimated inner MTU is below the AS's minimum
acceptable MTU, notify the ACS to tear down this SA.
Note that the PTB MTU value is not used, because it could have been
forged by an off-path attacker. To preclude such attacks, all
Traceroute and PMTUD probe packets contain at least 16 bytes of
entropy, which the ASBR checks in the echoed payload.
To prevent wasteful misbehaviors and reflection attacks, this
procedure is rate-limited to some reasonable frequency (e.g., at most
once per minute per SA).
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6. Traffic Selectors and Replay Protection in RISAV
The IKEv2 configuration protocol is highly flexible, allowing
participating ASes to negotiate many different RISAV configurations.
For RISAV, two important IKEv2 parameters are the Traffic Selector
([RFC7296], Section 2.9) and the Replay Status.
TODO: Write draft porting Replay Status from RFC 2407 to IKEv2.
6.1. Disabling replay protection
In the simplest RISAV configuration, the sending AS requests creation
of a single "Child SA" whose Traffic Selector-initiator (TSi) lists
all the IP ranges of the sending AS, and the Traffic Selector-
responder (TSr) lists all the IP ranges of the receiving AS. This
allows a single SA to carry all RISAV traffic from one AS to another.
However, this SA is likely to be shared across many ASBRs, and
potentially many cores within each ASBR, in both participating ASes.
It is difficult or impossible for a multi-sender SA to use monotonic
sequence numbers, as required for anti-replay defense and Extended
Sequence Numbers (ESN) (see [RFC4303], Section 2.2). If the sender
cannot ensure correctly ordered sequence numbers, it MUST set the
REPLAY-STATUS indication to FALSE in the CREATE_CHILD_SA
notification, and MUST delete the SA if the recipient does not
confirm that replay detection is disabled.
6.2. Enabling replay protection
If the sender wishes to allow replay detection, it can create many
Child SAs, one for each of its ASBRs (or each core within an ASBR).
The OPTIONAL CPU_QUEUES IKEv2 notification
[I-D.ietf-ipsecme-multi-sa-performance] may make this process more
efficient. If the sending ASBRs are used for distinct subsets of the
sender's IP addresses, the TSi values SHOULD be narrowed accordingly
to allow routing optimizations by the receiver.
Even if the sender creates many separate SAs, the receiver might not
be able to perform replay detection unless each SA is processed by a
single receiving ASBR. In Tunnel Mode, the receiver can route each
SA to a specific ASBR using IKEv2 Active Session Redirect ([RFC5685],
Section 5).
In Transport Mode, assignment of SAs to receiving ASBRs may be
possible in cases where each ASBR in the receiving AS is responsible
for a distinct subset of its IPs. To support this configuration, the
receiving AS MAY narrow the initial TSr to just the IP ranges for a
single ASBR, returning ADDITIONAL_TS_POSSIBLE. In response, the
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sending AS MUST reissue the CREATE_CHILD_SA request, with TSr
containing the remainder of the IP addresses, allowing the
negotiation of separate SAs for each receiving ASBR.
Future IKEv2 extensions such as Sequence Number Subspaces
[I-D.ponchon-ipsecme-anti-replay-subspaces] or Lightweight SAs
[I-D.mrossberg-ipsecme-multiple-sequence-counters] may enable more
efficient and easily deployed anti-replay configurations for RISAV.
6.3. Changes to AS IP ranges
If the ACS receives a TSi value that includes IP addresses not owned
by the counterpart AS, it MUST reject the SA to prevent IP hijacking.
However, each AS's copy of the RPKI database can be up to 24 hours
out of date. Therefore, when an AS acquires a new IP range, it MUST
wait at least 24 hours before including it in a RISAV TSi.
If a tunnel mode SA is established, the receiving AS MUST drop any
packet from the tunnel whose source address is not within the
tunnel's TSi.
7. Possible Extensions
This section presents potential additions to the design.
TODO: Remove this section once we have consensus on whether these
extensions are worthwhile.
7.1. Header-only authentication
An IPsec Authentication Header authenticates the whole constant part
of a packet, including the entire payload. To improve efficiency, we
could define an IKE parameter to negotiate a header-only variant of
transport mode that only authenticates the IP source address, IP
destination address, etc.
This would likely result in a 10-30x decrease in cryptographic cost
compared to standard IPsec. However, it would also offer no SAV
defense against any attacker who can view legitimate traffic. An
attacker who can read a single authenticated packet could simply
replace the payload, allowing it to issue an unlimited number of
spoofed packets.
7.2. Time-based key rotation
Each IKEv2 handshake negotiates a fixed shared secret, known to both
parties. In some cases, it might be desirable to rotate the shared
secret frequently:
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* In transport mode, frequent rotation would limit how long a single
packet can be replayed by a spoofing attacker.
* If the ASBRs are less secure than the ACS, frequent rotation could
limit the impact of a compromised ASBR.
However, increasing the frequency of IKEv2 handshakes would increase
the burden on the ACS. One alternative possibility is to use a state
machine. The state machine runs and triggers the state transition
when time is up. The tag is generated in the process of state
transition as the side product. The two ACS in peer AS respectively
before data transmission will maintain one state machine pair for
each bound. The state machine runs simultaneously after the initial
state, state transition algorithm, and state transition interval are
negotiated, thus they generate the same tag at the same time. Time
triggers state transition which means the ACS MUST synchronize the
time to the same time base using like NTP defined in [RFC5905].
For the tag generation method, it MUST be to specify the initial
state and initial state length of the state machine, the identifier
of a state machine, state transition interval, length of generated
Tag, and Tag. For the SA, they will transfer all these payloads in a
secure channel between ACS and ASBRs, for instance, in ESP [RFC4303].
It is RECOMMENDED to transfer the tags rather than the SA for
security and efficiency considerations. The initial state and its
length can be specified at the Key Exchange Payload with nothing to
be changed. The state machine identifier is the SPI value as the SPI
value is uniquely in RISAV. The state transition interval and length
of generated Tag should be negotiated by the pair ACS, which will
need to allocate one SA attribute. The generated Tag will be sent
from ACS to ASBR in a secure channel which MAY be, for example, ESP
[RFC4303].
7.3. Static Negotiation
The use of IKEv2 between ASes might be fragile, and creates a number
of potential race conditions (e.g. if the RPKI database contents
change during the handshake). It is also potentially costly to
implement, requiring O(N^2) network activity for N participating
ASes. If these challenges prove significant, one alternative would
be to perform the handshake statically via the RPKI database. For
example, static-static ECDH [RFC6278] would allow ASes to agree on
shared secrets simply by syncing the RPKI database.
Static negotiation makes endpoints nearly stateless, which simplifies
the provisioning of ASBRs. However, it requires inventing a novel
IPsec negotiation system, so it seems best to try a design using
IKEv2 first.
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8. Security Consideration
8.1. Threat models
In general, RISAV seeks to provide a strong defense against arbitrary
active attackers who are external to the source and destination ASes.
However, different RISAV modes and configurations offer different
security properties.
8.1.1. Replay attacks
When replay detection is disabled, off-path attackers cannot spoof
the source IPs of a participating AS, but any attacker with access to
valid traffic can replay it (from anywhere), potentially enabling DoS
attacks by replaying expensive traffic (e.g. TCP SYNs, QUIC
Initials). ASes that wish to have replay defense must enable it
during the IKEv2 handshake (see Section 6).
8.1.2. Downgrade attacks
An on-path attacker between two participating ASes could attempt to
defeat RISAV by blocking IKEv2 handshakes to the Contact IP of a
target AS. If the AS initiating the handshake falls back to non-
RISAV behavior after a handshake failure, this enables the attacker
to remove all RISAV protection.
This vulnerable behavior is required when the "testing" flag is set,
but is otherwise discouraged.
8.2. Incremental benefit from partial deployment
RISAV provides significant security benefits even if it is only
deployed by a fraction of all ASes. This is particularly clear in
the context of reflection attacks. If two networks implement RISAV,
no one in any other network can trigger a reflection attack between
these two networks. Thus, if X% of ASes (selected at random)
implement RISAV, participating ASes should see an X% reduction in
reflection attack traffic volume.
8.3. Compatibility
8.3.1. With end-to-end IPsec
When RISAV is used in transport mode, there is a risk of confusion
between the RISAV AH header and end-to-end AH headers used by
applications. (In tunnel mode, no such confusion is possible.) This
risk is particularly clear during transition periods, when the
recipient is not sure whether the sender is using RISAV or not.
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To prevent any such confusion, RISAV's transport mode uses a
distinctive Scope value in the Authentication Header. The receiving
AS absorbs (and strips) all AH headers with this scope, and ignores
those with any other scope, including ordinary end-to-end AH headers.
8.3.2. With other SAV mechanisms
RISAV is independent from intra-domain SAV and access-layer SAV, such
as [RFC8704] or SAVI [RFC7039]. When these techniques are used
together, intra-domain and access-layer SAV checks MUST be enforced
before applying RISAV.
9. Operational Considerations
9.1. Reliability
The ACS, represented by a contact IP, must be a high-availability,
high-performance service to avoid outages. There are various
strategies to achieve this, including:
* *Election*. This might be achieved by electing one distinguished
ASBR as the ACS. The distinguished ASBR acting as an ACS will
represent the whole AS to communicate with peer AS's ACS. This
election takes place prior to the IKE negotiation. In this
arrangement, an ASBR MUST be a BGP speaker before it is elected as
the distinguished ASBR, and a new election MUST replace the ACS if
it fails.
* *Anycast*. The ACS could be implemented as an anycast service
operated by all the ASBRs. Route flapping can be mitigated using
IKEv2 redirection ([RFC5685], Section 4). Negotiated SAs must be
written into a database that is replicated across all ASBRs.
9.2. Synchronizing Multiple ASBRs
To ensure coherent behavior across the AS, the ACS MUST deliver each
SA to all relevant ASBRs in the AS immediately after it is
negotiated. RISAV does not standardize a mechanism for this update
broadcast.
During the SA broadcast, ASBRs will briefly be out of sync. RISAV
recommends a grace period to prevent outages during the update
process.
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9.3. Performance
RISAV requires participating ASes to perform symmetric cryptography
on every RISAV-protected packet that they originate or terminate.
This will require significant additional compute capacity that may
not be present on existing networks. However, until most ASes
actually implement RISAV, the implementation cost for the few that do
is greatly reduced. For example, if 5% of networks implement RISAV,
then participating networks will only need to apply RISAV to 5% of
their traffic.
Thanks to broad interest in optimization of IPsec, very high
performance implementations are already available. For example, as
of 2021 an IPsec throughput of 1 Terabit per second was achievable
using optimized software on a single server [INTEL].
9.4. NAT scenario
As all the outer IP header should be the unicast IP address, NAT-
traversal mode is not necessary in inter-AS SAV.
10. Consistency with Existing Standards
10.1. IPv6
RISAV modifies the handling of IPv6 packets as they traverse the
network, resulting in novel networking behaviors. This section
describes why those behaviors should not be viewed as violating the
requirements of [RFC8200].
10.1.1. MTU
Section 5 of [RFC8200] says:
IPv6 requires that every link in the Internet have an MTU of 1280
octets or greater. This is known as the IPv6 minimum link MTU.
RISAV adds ~30-80 octets of overhead to each packet, reducing the
effective link MTU. A naive version of RISAV could violate the
1280-octet rule, when running over a (compliant) path with a Path MTU
of 1280 octets.
This violation is avoided by the requirements described in Section 5.
The resulting behavior is fully compliant when the underlying Path
MTU is stable, and should compensate or disable RISAV within a few
seconds if the Path MTU changes.
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10.1.2. Header modifications
Section 4 of [RFC8200] says:
Extension headers (except for the Hop-by-Hop Options header) are
not processed, inserted, or deleted by any node along a packet's
delivery path, until the packet reaches the node (or each of the
set of nodes, in the case of multicast) identified in the
Destination Address field of the IPv6 header.
In "tunnel mode" (Section 4.2), RISAV acts as a classic site-to-site
tunnel, potentially adding its own extension headers. Section 4.1 of
[RFC8200] specifically allows such tunnels, and they are commonly
used.
In "transport mode" (Section 4.1), a RISAV ASBR does insert a new
extension header, which could be viewed as a violation of this
guidance. However, this new extension header is an implementation
detail of a lightweight tunnel: it is only added after confirming
that another router on the path will remove it, so that its presence
is not detectable by either endpoint. (Section 4.1.1 adds further
requirements to ensure that this header cannot be detected in ICMP
responses either.)
10.1.3. IP address usage
In some RISAV configurations, it is expected that many ASBRs will
decrypt and process packets with the destination IP of the ACS and/or
emit packets using the source IP of the ACS. This can be viewed as
replacing the central ACS with an "anycast" service, which is
generally considered permissible.
10.2. RPKI Usage
[RFC9255] describes limits on the use of RPKI certificates for new
purposes, including the following excerpts:
The RPKI was designed and specified to sign certificates for use
within the RPKI itself and to generate Route Origin Authorizations
(ROAs) [RFC6480] for use in routing. Its design intentionally
precluded use for attesting to real-world identity...
RPKI-based credentials of INRs MUST NOT be used to authenticate
real-world documents or transactions.
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When a document is signed with the private key associated with an
RPKI certificate, the signer is speaking for the INRs (the IP
address space and AS numbers) in the certificate. ... If the
signature is valid, the message content comes from a party that is
authorized to speak for that subset of INRs.
RISAV's usage of RPKI key material falls squarely within these
limits. The RPKI signature used in the IKEv2 handshake serves only
to confirm that this party is authorized to originate and terminate
IP packets using the corresponding IP ranges. The "identity" of this
party is not relevant to RISAV.
11. IANA Consideration
TODO: Register RISAVAnnouncement.
12. References
12.1. 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>.
[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/info/rfc2827>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<https://www.rfc-editor.org/info/rfc4302>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
[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/info/rfc5210>.
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[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/info/rfc5635>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC6278] Herzog, J. and R. Khazan, "Use of Static-Static Elliptic
Curve Diffie-Hellman Key Agreement in Cryptographic
Message Syntax", RFC 6278, DOI 10.17487/RFC6278, June
2011, <https://www.rfc-editor.org/info/rfc6278>.
[RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
February 2012, <https://www.rfc-editor.org/info/rfc6480>.
[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,
<https://www.rfc-editor.org/info/rfc7039>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
[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>.
[RFC8209] Reynolds, M., Turner, S., and S. Kent, "A Profile for
BGPsec Router Certificates, Certificate Revocation Lists,
and Certification Requests", RFC 8209,
DOI 10.17487/RFC8209, September 2017,
<https://www.rfc-editor.org/info/rfc8209>.
[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>.
[RFC6488] Lepinski, M., Chi, A., and S. Kent, "Signed Object
Template for the Resource Public Key Infrastructure
(RPKI)", RFC 6488, DOI 10.17487/RFC6488, February 2012,
<https://www.rfc-editor.org/info/rfc6488>.
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[RFC9347] Hopps, C., "Aggregation and Fragmentation Mode for
Encapsulating Security Payload (ESP) and Its Use for IP
Traffic Flow Security (IP-TFS)", RFC 9347,
DOI 10.17487/RFC9347, January 2023,
<https://www.rfc-editor.org/info/rfc9347>.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, DOI 10.17487/RFC4459, April
2006, <https://www.rfc-editor.org/info/rfc4459>.
12.2. Informative References
[INTEL] "Achieving 1 Tbps IPsec with AVX-512", April 2021,
<https://networkbuilders.intel.com/solutionslibrary/3rd-
generation-intel-xeon-scalable-processor-achieving-1-tbps-
ipsec-with-intel-advanced-vector-extensions-512-
technology-guide>.
[RFC8019] Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange
Protocol Version 2 (IKEv2) Implementations from
Distributed Denial-of-Service Attacks", RFC 8019,
DOI 10.17487/RFC8019, November 2016,
<https://www.rfc-editor.org/info/rfc8019>.
[RFC5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
Behavioral Requirements for ICMP", BCP 148, RFC 5508,
DOI 10.17487/RFC5508, April 2009,
<https://www.rfc-editor.org/info/rfc5508>.
[RFC7383] Smyslov, V., "Internet Key Exchange Protocol Version 2
(IKEv2) Message Fragmentation", RFC 7383,
DOI 10.17487/RFC7383, November 2014,
<https://www.rfc-editor.org/info/rfc7383>.
[I-D.ietf-ipsecme-multi-sa-performance]
Antony, A., Brunner, T., Klassert, S., and P. Wouters,
"IKEv2 support for per-queue Child SAs", Work in Progress,
Internet-Draft, draft-ietf-ipsecme-multi-sa-performance-
00, 7 December 2022, <https://www.ietf.org/archive/id/
draft-ietf-ipsecme-multi-sa-performance-00.txt>.
[RFC5685] Devarapalli, V. and K. Weniger, "Redirect Mechanism for
the Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5685, DOI 10.17487/RFC5685, November 2009,
<https://www.rfc-editor.org/info/rfc5685>.
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[I-D.ponchon-ipsecme-anti-replay-subspaces]
Ponchon, P., Shaikh, M., Pfister, P., and G. Solignac,
"IPsec and IKE anti-replay sequence number subspaces for
multi-path tunnels and multi-core processing", Work in
Progress, Internet-Draft, draft-ponchon-ipsecme-anti-
replay-subspaces-00, 24 October 2022,
<https://www.ietf.org/archive/id/draft-ponchon-ipsecme-
anti-replay-subspaces-00.txt>.
[I-D.mrossberg-ipsecme-multiple-sequence-counters]
Rossberg, M., Klassert, S., and M. Pfeiffer, "Problem
statements and uses cases for lightweight Child Security
Associations", Work in Progress, Internet-Draft, draft-
mrossberg-ipsecme-multiple-sequence-counters-00, 27
February 2023, <https://datatracker.ietf.org/doc/html/
draft-mrossberg-ipsecme-multiple-sequence-counters-00>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC9255] Bush, R. and R. Housley, "The 'I' in RPKI Does Not Stand
for Identity", RFC 9255, DOI 10.17487/RFC9255, June 2022,
<https://www.rfc-editor.org/info/rfc9255>.
Authors' Addresses
Ke Xu
Tsinghua University
Beijing
China
Email: xuke@tsinghua.edu.cn
Jianping Wu
Tsinghua University
Beijing
China
Email: jianping@cernet.edu.cn
Yangfei Guo
Zhongguancun Laboratory
Beijing
China
Email: guoyangfei@zgclab.edu.cn
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Benjamin M. Schwartz
Google LLC
Email: ietf@bemasc.net
Haiyang (Henry) Wang
The University of Minnesota at Duluth
Minnesota,
United States of America
Email: haiyang@d.umn.edu
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