BGP AS_PATH Verification Based on Autonomous System Provider Authorization (ASPA) Objects
draft-ietf-sidrops-aspa-verification-16
| Document | Type | Active Internet-Draft (sidrops WG) | |
|---|---|---|---|
| Authors | Alexander Azimov , Eugene Bogomazov , Randy Bush , Keyur Patel , Job Snijders , Kotikalapudi Sriram | ||
| Last updated | 2023-08-29 | ||
| Replaces | draft-azimov-sidrops-aspa-verification | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | In WG Last Call | |
| Document shepherd | Chris Morrow | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | morrowc@ops-netman.net |
draft-ietf-sidrops-aspa-verification-16
Network Working Group A. Azimov
Internet-Draft Yandex
Intended status: Standards Track E. Bogomazov
Expires: 1 March 2024 Qrator Labs
R. Bush
IIJ & Arrcus
K. Patel
Arrcus
J. Snijders
Fastly
K. Sriram
USA NIST
29 August 2023
BGP AS_PATH Verification Based on Autonomous System Provider
Authorization (ASPA) Objects
draft-ietf-sidrops-aspa-verification-16
Abstract
This document describes procedures that make use of Autonomous System
Provider Authorization (ASPA) objects in the Resource Public Key
Infrastructure (RPKI) to verify the Border Gateway Protocol (BGP)
AS_PATH attribute of advertised routes. This type of AS_PATH
verification provides detection and mitigation of route leaks and
improbable AS paths. It also provides protection, to some degree,
against prefix hijacks with forged-origin or forged-path-segment.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 1 March 2024.
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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
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Anomaly Propagation . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. BGP Roles . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Autonomous System Provider Authorization . . . . . . . . . . 5
4. ASPA Registration Recommendations . . . . . . . . . . . . . . 5
5. Hop-Check Function . . . . . . . . . . . . . . . . . . . . . 6
6. AS_PATH Verification . . . . . . . . . . . . . . . . . . . . 7
6.1. Algorithm for Upstream Paths . . . . . . . . . . . . . . 8
6.2. Algorithm for Downstream Paths . . . . . . . . . . . . . 9
6.2.1. Principles for Determination of Invalid, Valid, and
Unknown in Downstream Path Verification (for N >= 3) 9
6.2.2. Formal Procedure for Verification of Downstream
Paths . . . . . . . . . . . . . . . . . . . . . . . . 11
7. AS_PATH Verification and Anomaly Mitigation
Recommendations . . . . . . . . . . . . . . . . . . . . . 12
7.1. Verification and Mitigation at Ingress eBGP Router . . . 12
7.2. Only to Customer (OTC) Attribute . . . . . . . . . . . . 13
8. Properties of ASPA-based Path Verification . . . . . . . . . 13
9. Operational Considerations . . . . . . . . . . . . . . . . . 15
9.1. 4-Byte AS Number Requirement . . . . . . . . . . . . . . 15
9.2. Correctness of the ASPA . . . . . . . . . . . . . . . . . 15
9.3. Make Before Break . . . . . . . . . . . . . . . . . . . . 15
9.4. DoS/DDoS Mitigation Service Provider . . . . . . . . . . 16
10. Comparison to Other Technologies . . . . . . . . . . . . . . 16
10.1. BGPsec . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.2. Peerlock . . . . . . . . . . . . . . . . . . . . . . . . 16
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
12. Security Considerations . . . . . . . . . . . . . . . . . . . 17
13. Implementation Status . . . . . . . . . . . . . . . . . . . . 18
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
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14.1. Normative References . . . . . . . . . . . . . . . . . . 19
14.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 22
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
The Border Gateway Protocol (BGP) as originally designed is known to
be vulnerable to prefix (or route) hijacks and BGP route leaks
[RFC7908]. Some existing BGP extensions are able to partially solve
these problems. For example, Resource Public Key Infrastructure
(RPKI) based route origin validation (RPKI-ROV) [RFC6480] [RFC6482]
[RFC6811] [RFC9319] can be used to detect and filter accidental mis-
originations. [RFC9234] or
[I-D.ietf-grow-route-leak-detection-mitigation] can be used to detect
and mitigate accidental route leaks. While RPKI-ROV can prevent
accidental prefix hijacks, malicious forged-origin prefix hijacks can
still occur [RFC9319]. RFC9319 includes some recommendations for
reducing the attack surface for forged-origin prefix hijacks.
This document describes procedures that make use of Autonomous System
Provider Authorization (ASPA) objects [I-D.ietf-sidrops-aspa-profile]
in the RPKI to verify properties of the BGP AS_PATH attribute of
advertised routes. ASPA-based AS_PATH verification provides
detection and mitigation of route leaks and improbable AS paths. It
also provides protection, to some degree, against prefix hijacks with
forged-origin or forged-path-segment (Section 8). These new ASPA-
based procedures automatically detect such anomalous AS_PATHs in
announcements that are received from customers, lateral peers
(defined in [RFC7908]), transit providers, IXP Route Servers (RS),
RS-clients, and mutual-transits. The protections provided by these
procedures (together with RPKI-ROV) are based on cryptographic
techniques, and they are effective against many accidental and
malicious actions.
ASPA objects are cryptographically signed registrations of customer-
to-provider relationships and stored in a distributed database
[I-D.ietf-sidrops-aspa-profile]. ASPA-based path verification is an
incrementally deployable technique and provides benefits to early
adopters in the context of limited deployment.
The procedures described in this document are applicable only for BGP
routes with {AFI, SAFI} combinations {AFI 1 (IPv4), SAFI 1} and {AFI
2 (IPv6), SAFI 1} [IANA-AF]. SAFI 1 represents NLRI used for unicast
forwarding [IANA-SAF].
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1.1. Anomaly Propagation
Both route leaks and hijacks have similar effects on ISP operations -
they redirect traffic and can result in denial of service (DoS),
eavesdropping, increased latency, and packet loss. The level of
risk, however, depends significantly on the extent of propagation of
the anomalies. For example, a route leak or hijack that is
propagated only to customers may cause bottlenecking within a
particular ISP's customer cone, but if the anomaly propagates through
lateral (i.e., non-transit) peers and transit providers, or reaches
global distribution through transit-free networks, then the ill
effects will likely be amplified and experienced across continents.
The ability to constrain the propagation of BGP anomalies to transit
providers and lateral peers - without requiring support from the
source of the anomaly (which is critical if the source has malicious
intent) - should significantly improve the robustness of the global
inter-domain routing system.
1.2. Terminology
The use of the term "route is ineligible" in this document has the
same meaning as in [RFC4271], i.e., "route is ineligible to be
installed in Loc-RIB and will be excluded from the next phase of
route selection."
For brevity, the term "provider" is often used instead of "transit
provider" in this document; they mean the same.
1.3. 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. BGP Roles
For path verification purposes in this document, the BGP roles an AS
can have in relation to a neighbor AS are customer, provider, lateral
peer, Route Server (RS), RS-client, and mutual-transit. These
relationships, except mutual-transit, are defined in [RFC9234].
Mutual-transit ASes MAY export everything (both customer and non-
customer routes) to each other, i.e., consider each other as a
customer. For mutual-transit ASes, the customer-to-provider
relationship applies in each direction.
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All roles are configured locally and used for the registration of
ASPA objects (Section 3, Section 4) and/or for path verification
(Section 6). The procedures for local BGP Role announcement in the
BGP OPEN message and neighbor role cross-check specified in [RFC9234]
are RECOMMENDED. Those procedures are not applied for cross-checking
a mutual-transit role since this role is not specified in [RFC9234].
3. Autonomous System Provider Authorization
An ASPA record is a digitally signed object that binds a set of
Provider AS numbers to a Customer AS (CAS) number (in terms of BGP
announcements) and is signed by the CAS
[I-D.ietf-sidrops-aspa-profile]. The ASPA attests that the CAS
indicated a Set of Provider ASes (SPAS), which applies only to the
IPv4 and IPv6 address families (i.e., AFI = 1 and AFI = 2) and only
to Network Layer Reachability Information used for unicast forwarding
(SAFI = 1). The definition of Provider AS is given in Section 1 of
the ASPA profile object document [I-D.ietf-sidrops-aspa-profile]. A
function of a Provider AS is to propagate a CAS's route announcements
onward, i.e., to the Provider's upstream providers, lateral peers, or
customers. Another function is to offer outbound (customer to
Internet) data traffic connectivity to the CAS.
The notation (AS x, {AS y1, AS y2, ...}), is used to represent an
ASPA object for a CAS denoted as AS x. In this notation, the set {AS
y1, AS y2, ...} represents the Set of Provider ASes (SPAS) of the CAS
(AS x). A CAS is expected to register a single ASPA listing all its
Provider ASes (see Section 4). If a CAS has a single ASPA, then the
SPAS for the CAS is the set of Provider ASes listed in that ASPA. In
case a CAS has multiple ASPAs, then the SPAS is the union of the
Provider ASes listed in all ASPAs of the CAS.
Verified ASPA Payload (VAP) refers to the payload in a
cryptographically verified (i.e., X.509 valid [RFC3779] [RFC5280])
ASPA object. In the procedures for the AS path verification
described in this document (Section 5, Section 6), VAP-SPAS refers to
the set of provider ASes derived from the VAP(s) of the CAS in
consideration.
4. ASPA Registration Recommendations
An ASPA object showing only AS 0 as a provider AS is referred to as
an AS0 ASPA. A non-transparent Route Server AS (RS AS) is one that
includes its AS number in the AS_PATH. Registering as AS0 ASPA is a
statement by the registering AS that it has no transit providers, and
it is also not an RS-client at a non-transparent RS AS. If that
statement is true, then the AS MUST register an AS 0 ASPA.
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Normally, the Provider ASes of a CAS would be congruent for the
address family combinations {AFI 1 (IPv4), SAFI 1} and {AFI 2 (IPv6),
SAFI 1}. Exceptions to this are expected to be rare. In any case,
the CAS MUST list the union of all Provider ASes applicable to the
address family combinations stated above in the SPAS and MUST also
include any non-transparent RS AS(es) at which it is an RS-client.
In the procedures for the AS path verification described in this
document (Section 5, Section 6), the SPAS is always considered to be
uniformly applicable to {AFI 1 (IPv4), SAFI 1} and {AFI 2 (IPv6),
SAFI 1}.
A compliant AS, including a Route Server AS (RS AS), MUST have an
ASPA. An AS SHOULD NOT have more than one ASPA. An RS AS SHOULD
register an AS 0 ASPA.
As mentioned in Section 3, the set of provider ASes contained in the
VAP(s) is referred to as the VAP-SPAS of the AS registering the
ASPA(s). Normally, a VAP-SPAS is not expected to contain both an AS
0 and other Provider ASes, but an unexpected presence of AS 0 has no
influence on the AS path verification procedures (see Section 5,
Section 6).
Each of the two ASes in a mutual-transit pair MUST register its ASPA
including the other AS in its SPAS. If one of the ASes in the pair
does this registration but the other does not, it increases the risk
of incorrect AS path verification results for routes that include the
pair.
The ASes on the boundary of an AS Confederation MUST register ASPAs
using the Confederation's global ASN as the CAS.
As specified earlier, a compliant AS should maintain a single ASPA
object that includes all its provider ASes, including any non-
transparent RS ASes. Such a practice helps prevent race conditions
during ASPA updates that might affect prefix propagation. The
software that provides hosting for ASPA records SHOULD support
enforcement of this practice. During a transition process between
different certificate authority (CA) registries, the ASPA records
SHOULD be kept identical in all relevant registries.
5. Hop-Check Function
Let AS(i) and AS(j) represent adjacent unique ASes in an AS_PATH, and
thus (AS(i), AS(j)) represents an AS hop. A hop-check function,
hop(AS(i), AS(j)), checks if the ordered pair of ASNs, (AS(i),
AS(j)), has the property that AS(j) is an attested provider of AS(i)
per VAP-SPAS of AS(i). The VAP-SPAS table is assumed to be organized
in such a way that it can be queried to check (1) if a specified CAS
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= AS(i) has an entry (i.e., SPAS listed), or (2) if for a given
(AS(i), AS(j)) tuple, AS(j) is listed in the VAP-SPAS as a provider
associated with CAS = AS(i). A provider AS ID included in the SPAS
can correspond to a Provider, a non-transparent RS, or a mutual-
transit neighbor. A non-transparent RS is effectively a Provider to
its RS-client. Mutual-transit neighbors regard each other as a
Provider (see Section 4). The term "Provider+" in the definition of
the hop-check function is meant to encompass all three possibilities:
Provider, non-transparent RS, or mutual-transit neighbor. This
function is specified as follows:
/
| "No Attestation" if there is no entry
| in VAP-SPAS table for CAS = AS(i)
|
hop(AS(i), AS(j)) = / Else, "Provider+" if the VAP-SPAS entry
\ for CAS = AS(i) includes AS(j)
|
| Else, "Not Provider+"
\
Figure 1: Hop-check function.
To be clear, this function checks if AS(j) is included in the VAP-
SPAS of AS(i), and in doing so it does not need to distinguish
between Provider, non-transparent RS, and mutual-transit neighbor.
The "No Attestation" result is returned only when the CAS = AS(i) has
no entry in the VAP-SPAS table, which occurs when no ASPA is
registered for the CAS or none of its ASPAs are cryptographically
valid. The hop-check function is used in the ASPA-based AS_PATH
verification algorithms described in Section 6.1 and Section 6.2.
6. AS_PATH Verification
The procedures described in this document are applicable only to
four-octet AS number compatible BGP speakers [RFC6793]. If such a
BGP speaker receives both AS_PATH and AS4_PATH attributes in an
UPDATE, then the procedures are applied on the reconstructed AS path
(Section 4.2.3 of [RFC6793]). So, the term AS_PATH is used in this
document to refer to the usual AS_PATH [RFC4271] as well as the
reconstructed AS path.
If an attacker creates a route leak intentionally, they may try to
strip their AS from the AS_PATH. To partly guard against that, a
check is necessary to match the most recently added AS in the AS_PATH
to the BGP neighbor's ASN. This check MUST be performed as specified
in Section 6.3 of [RFC4271]. If the check fails, then the AS_PATH is
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considered a Malformed AS_PATH and the UPDATE is considered to be in
error (Section 6.3 of [RFC4271]). The case of transparent RS MUST
also be appropriately taken care of (e.g., by suspending the neighbor
ASN check). The check fails also when the AS_PATH is empty (zero
length) and such UPDATEs will also be considered to be in error.
[I-D.ietf-idr-deprecate-as-set-confed-set] specifies that "treat-as-
withdraw" error handling [RFC7606] SHOULD be applied to routes with
AS_SET in the AS_PATH. In the current document, routes with AS_SET
are given Invalid evaluation in the AS_PATH verification procedures
(Section 6.1 and Section 6.2). See Section 7 for how routes with
Invalid AS_PATH are handled.
In Section 6.1 and Section 6.2 below, the terms "upstream path" and
"downstream path" generally refer to AS paths received in the
upstream direction (from a customer or a lateral peer) and in the
downstream direction (from a provider or a mutual-transit neighbor),
respectively. An RS-client receiving a route from its RS is a
special case where the algorithm for upstream paths is applied
(Section 6.1).
6.1. Algorithm for Upstream Paths
The upstream verification algorithm described here is applied when a
route is received from a customer or lateral peer, or is received by
an RS from an RS-client, or is received by an RS-client from an RS.
In all these cases, the receiving/validating eBGP router expects the
AS_PATH to consist of only customer-to-provider hops successively
from the origin AS to the neighbor AS (most recently added).
The basic principles of the upstream verification algorithm are
stated here. Let the sequence {AS(N), AS(N-1),..., AS(2), AS(1)}
represent the AS_PATH in terms of unique ASNs, where AS(1) is the
origin AS and AS(N) is the most recently added AS and neighbor of the
receiving/validating AS. For each hop AS(i-1) to AS(i) in this
sequence, the hop-check function, hop(AS(i-1), AS(i)), must equal
"Provider+" (Section 5) for the AS_PATH to be Valid. If the hop-
check function for at least one of those hops is "Not Provider+",
then the AS_PATH is deemed Invalid. If the AS_PATH verification
outcome is neither Valid nor Invalid (per the above principles), then
it is evaluated as Unknown.
The upstream path verification procedure is specified as follows:
1. If the AS_PATH has an AS_SET, then the procedure halts with the
outcome "Invalid".
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2. Collapse prepends in the AS_SEQUENCE(s) in the AS_PATH (i.e.,
keep only the unique AS numbers). Let the resulting ordered
sequence be represented by {AS(N), AS(N-1), ..., AS(2), AS(1)},
where AS(1) is the first-added (i.e., origin) AS and AS(N) is the
last-added AS and neighbor to the receiving/validating AS.
3. If N = 1, then the procedure halts with the outcome "Valid".
Else, continue.
4. At this step, N ≥ 2. If there is an i such that 2 ≤ i ≤ N and
hop(AS(i-1), AS(i)) = "Not Provider+", then the procedure halts
with the outcome "Invalid". Else, continue.
5. If there is an i such that 2 ≤ i ≤ N and hop(AS(i-1), AS(i)) =
"No Attestation", then the procedure halts with the outcome
"Unknown". Else, the procedure halts with the outcome "Valid".
6.2. Algorithm for Downstream Paths
The downstream verification algorithm described here is applied when
a route is received from a transit provider or mutual-transit
neighbor. As described in Section 4, a sending mutual-transit AS
acts towards its receiving mutual-transit AS in a manner similar to
that of a provider towards its customer.
It is not essential, but the reader may take a look at the
illustrations and formal proof in [sriram1] to develop a clearer
understanding of the algorithm described here.
Here again (as in Section 6.1), let the AS_PATH be simplified and
represented by the ordered sequence of unique ASNs as {AS(N),
AS(N-1),..., AS(2), AS(1)}.
If 1 <= N <= 2, then the AS_PATH is trivially Valid.
Section 6.2.1 below assumes that the AS_PATH contains 3 or more
unique ASNs (N >= 3).
6.2.1. Principles for Determination of Invalid, Valid, and Unknown in
Downstream Path Verification (for N >= 3)
*Determination of Invalid AS_PATH:*
Given the above-mentioned ordered sequence, if there exist indices u
and v such that (1) u <= v, (2) hop(AS(u-1), AS(u)) = "Not
Provider+", and (3) hop(AS(v+1), AS(v)) = "Not Provider+", then the
AS_PATH is Invalid.
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------------
*Determination of Valid AS_PATH:*
As shown in Figure 2, assume that the ASes in the AS_PATH are in the
same physical (locational) order as in the sequence representation
{AS(N), AS(N-1),..., AS(2), AS(1)}, i.e., AS(N) is the left-most and
AS(1) the right-most.
AS(L) ............. AS(K)
/ \
AS(L+1) AS(K-1)
. .
. .
(down-ramp) . . (up-ramp)
. .
. .
AS(N-1) AS(2)
/ \
AS(N) AS(1)
/ (Origin AS)
Receiving & Validating AS
Each ramp has consecutive ASPA-attested
customer-to-provider hops in the bottom-to-top direction
Figure 2: Illustration of up-ramp and down-ramp.
Looking at Figure 2, the UPDATE is received from a provider or a
mutual-transit neighbor (i.e., AS(N) has that role in relation to the
receiver). The AS_PATH may have both an up-ramp (on the right
starting at AS(1)) and a down-ramp (on the left starting at AS(N)).
The ramps are described as a sequence of ASes that consists of
consecutive customer-to-provider hops. The up-ramp starts at AS(1)
and each AS hop, (AS(i), AS(i+1)), in it has the property that
hop(AS(i), AS(i+1)) = "Provider+" for i = 1, 2,... , K-1. If such a
K does not exist, then K is set to 1. The up-ramp ends (reaches its
apex) at AS(K) because hop(AS(K), AS(K+1)) = "Not Provider+" or "No
Attestation". The down-ramp runs backward from AS(N) to AS(L). Each
AS hop, (AS(j), AS(j-1)), in it has the property that hop(AS(j),
AS(j-1)) = "Provider+" for j = N, N-1,... , L+1. If such an L does
not exist, then L is set to N. The down-ramp ends at AS(L) because
hop(AS(L), AS(L-1)) = "Not Provider+" or "No Attestation". Thus, the
apex of the down-ramp is AS(L).
If there is an up-ramp that runs across all ASes in the AS_PATH
(i.e., K = N), then clearly the AS_PATH is Valid. Similarly, if
there is a down-ramp that runs across all ASes in the AS_PATH (i.e.,
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L = 1), then also the AS_PATH is Valid. However, if both ramps exist
in an AS_PATH with K < N and L > 1, then the AS_PATH is Valid if and
only if L-K <= 1. Note that K could be greater than L (i.e., L-K has
a negative value), which means that the up-ramp and down-ramp
overlap, and that could happen when some adjacent ASes in the AS_PATH
have mutual-transit relationship between them (i.e., include each
other in their respective SPAS) (see Section 4). If L-K = 0, it
means that the apexes of the up-ramp and down-ramp are at the same
AS. If L-K = 1, it means that the apexes are at adjacent ASes. In
summary, the AS_PATH is Valid if L-K is 0 or 1 or has a negative
value.
------------
*Determination of Unknown AS_PATH:*
If L-K >= 2, then the AS_PATH is either Invalid (route leak) or
Unknown (see illustrations and proof in [sriram1]). However, if L-K
>= 2 and an Invalid outcome was not found by the process described
earlier in this section, then the AS_PATH is determined to be
Unknown.
6.2.2. Formal Procedure for Verification of Downstream Paths
The downstream path verification procedure is formally specified as
follows:
1. If the AS_PATH has an AS_SET, then the procedure halts with the
outcome "Invalid".
2. Collapse prepends in the AS_SEQUENCE(s) in the AS_PATH (i.e.,
keep only the unique AS numbers). Let the resulting ordered
sequence be represented by {AS(N), AS(N-1), ..., AS(2), AS(1)},
where AS(1) is the first-added (i.e., origin) AS and AS(N) is the
last-added AS and neighbor to the receiving/validating AS.
3. If 1 ≤ N ≤ 2, then the procedure halts with the outcome "Valid".
Else, continue.
4. At this step, N ≥ 3. Given the above-mentioned ordered sequence,
find the lowest value of u (2 ≤ u ≤ N) for which hop(AS(u-1),
AS(u)) = "Not Provider+". Call it u_min. If no such u_min
exists, set u_min = N+1. Find the highest value of v (N-1 ≥ v ≥
1) for which hop(AS(v+1), AS(v)) = "Not Provider+". Call it
v_max. If no such v_max exists, then set v_max = 0. If u_min ≤
v_max, then the procedure halts with the outcome "Invalid".
Else, continue.
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5. Up-ramp: For 2 ≤ i ≤ N, determine the largest K such that
hop(AS(i-1), AS(i)) = "Provider+" for each i in the range 2 ≤ i ≤
K. If such a largest K does not exist, then set K = 1.
6. Down-ramp: For N-1 ≥ j ≥ 1, determine the smallest L such that
hop(AS(j+1), AS(j)) = "Provider+" for each j in the range N-1 ≥ j
≥ L. If such smallest L does not exist, then set L = N.
7. If L-K ≤ 1, then the procedure halts with the outcome "Valid".
Else, the procedure halts with the outcome "Unknown".
In the above procedure, the computations in Steps 4, 5, and 6 can be
done at the same time.
7. AS_PATH Verification and Anomaly Mitigation Recommendations
AS_PATH verification and anomaly mitigation recommendations for eBGP
routers are specified in this section. The recommendations apply to
eBGP routers in general, including those on the boundary of an AS
Confederation facing external ASes. However, the procedures for
ASPA-based AS_PATH verification in this document are NOT RECOMMENDED
for use on eBGP links internal to the Confederation.
The verification procedures described in this document MUST be
applied to BGP routes with {AFI, SAFI} combinations {AFI 1 (IPv4),
SAFI 1} and {AFI 2 (IPv6), SAFI 1} [IANA-AF]. The procedures MUST
NOT be applied to other address families by default.
7.1. Verification and Mitigation at Ingress eBGP Router
*Verification:* Conforming implementations of this specification are
not required to implement the AS_PATH verification procedures (step-
wise lists) exactly as described in Section 6.1 and Section 6.2 but
MUST provide functionality equivalent to the external behavior
resulting from those procedures. In other words, the algorithms used
in a specific implementation may differ, for example, for
computational efficiency purposes, but the AS_PATH verification
outcomes MUST be identical to those obtained by the procedures
described in Section 6.1 and Section 6.2.
*Mitigation:* Mitigation recommendations are provided here with the
understanding that the deployed mitigation policy is set by network
operator discretion. If the AS_PATH is determined to be Invalid,
then the route SHOULD be considered ineligible for route selection
(see Section 1.2) and MUST be kept in the Adj-RIB-In for potential
future re-evaluation (see [RFC9324]). Also, for any route with an
Invalid AS_PATH, the cause of the Invalid state SHOULD be logged for
monitoring and diagnostic purposes. The cause of the Invalid state
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can be in the form of listing the AS hops which were evaluated by the
hop-check function to be "Not Provider+". For any route with an
Unknown AS_PATH, the cause of the Unknown state SHOULD be logged for
monitoring and diagnostic purposes. The cause of the Unknown state
can be in the form of listing the AS hops which were evaluated by the
hop-check function to be "No Attestation" or "Not Provider+".
7.2. Only to Customer (OTC) Attribute
While the ASPA-based AS_PATH verification method (Section 7.1)
detects and mitigates route leaks that were created by preceding ASes
listed in the AS_PATH, it lacks the ability to prevent route leaks
from occuring at the local AS. The use of the Only to Customer (OTC)
Attribute [RFC9234] fills in that gap. The procedures utilizing the
OTC Attribute set out in [RFC9234] complement those described in this
document. Implementation of those procedures in addition to ASPA-
based AS_PATH verification is encouraged.
8. Properties of ASPA-based Path Verification
The ASPA-based path verification procedures are able to check routes
received from customers, lateral peers, transit providers, RSes, RS-
clients, and mutual-transits. These procedures combined with BGP
Roles and the OTC Attribute [RFC9234] and RPKI-ROV [RFC6811]
[RFC9319] can provide a fully automated solution to detect and filter
many of the ordinary prefix hijacks, route leaks, and prefix hijacks
with forged-origin or forged-path-segment (see Property 3 below).
The ASPA-based path verification at ingress eBGP routers (Section 6,
Section 7.1) has the following properties (detection capabilities):
Property 1: Let AS A and AS B be any two ASes in the Internet
doing ASPA (registration and path verification) and no assumption
is made about the ASPA deployment status of other ASes. Consider
a route propagated from AS A to a customer or lateral peer. The
route is subsequently leaked by an offending AS in the AS path
before being received at AS B on a customer or lateral peer
interface. The ASPA-based path verification at AS B always
detects such a route leak though it may not be able to identify
the AS that originated the leak. This assertion is true even when
the sender AS A (or receiver AS B) is an RS AS and the neighbor AS
that AS A sent to (or AS B received from) is an RS-client.
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Property 2: Again, let AS A and AS B be any two ASes in the
Internet doing ASPA (registration and path verification) and no
assumption is made about the ASPA deployment status of other ASes.
Consider a route received at AS B on a customer or lateral peer
interface that is a forged-origin prefix hijack involving AS A as
the forged-origin. The ASPA-based path verification at AS B
always detects such a forged-origin prefix hijack.
Property 3: This is an extension of Property 2 above to the case
of prefix hijacking with a forged-path-segment. Such hijacking
refers to the forging of multiple contiguous ASes in an AS path
beginning with the origin AS. Again, let AS A and AS B be any two
ASes in the Internet doing ASPA (registration and path
verification). Let AS A's providers, AS P and AS Q, also be
registering ASPA. No assumption is made about the ASPA deployment
status of any other ASes in the Internet. Consider a route
received at AS B on a customer or lateral peer interface that is a
prefix hijack with a forged-path-segment {AS P, AS A} or {AS Q, AS
A}. That is, the hijacker attaches this path-segment at the
beginning of their route announcement. The ASPA-based path
verification at AS B always detects such a forged-path-segment
prefix hijack. For a chance to be successful (remain undetected
by AS B), the hijacker may resort to a forged-path-segment with
three ASes including a provider AS of AS P (or AS Q). But even
that can be foiled (detected) if the providers of AS P and AS Q
also register ASPA. Having to use a longer forged-path-segment to
avoid detection by AS B diminishes the ability of the hijacked
route to compete with the corresponding legitimate route in route
selection.
Property 4: Let AS A, AS B, and AS C be any three ASes in the
Internet doing ASPA (registration and path verification).
Consider a route propagated from AS A in any direction (i.e., to a
neighbor AS with any of the BGP roles described in Section 2).
Let the route be received at AS B from any direction and detected
to be a route leak (facilitated due to a sufficient set of ASes
doing ASPA in the AS path from AS A to AS B). Assume that AS B's
local policy is such that it only lowers the route's LOCAL_PREF
[RFC4271]. Let such a route, selected and forwarded by AS B, be
subsequently received at AS C. No assumption is made about the
ASPA compliance of the ASes in the intervening path from AS B to
AS C. The ASPA-based path verification at AS C always detects
such received route as a leak regardless of the direction (type of
peer) it was received from.
In the description of the properties listed above, the term
"customer" can be replaced with "RS-client".
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An observation that follows from Property #1 above is that if any two
ISP ASes register ASPAs and implement the detection and mitigation
procedures, then any route received from one of them and leaked to
the other by a common customer AS (ASPA compliant or not) will be
automatically detected and mitigated. In effect, if most major ISPs
are compliant, the propagation of route leaks in the Internet will be
severely limited.
The above properties show that ASPA-based path verification offers
significant benefits to early adopters. Limitations of the method
with regard to some forms of malicious AS path manipulations are
discussed in Section 12.
9. Operational Considerations
9.1. 4-Byte AS Number Requirement
The procedures specified in this document are compatible only with
BGP implementations that support 4-byte ASNs in the AS_PATH. This
limitation should not have a real effect on operations since legacy
BGP routers are rare, and it is highly unlikely that they support
integration with the RPKI.
9.2. Correctness of the ASPA
ASPA issuers should be aware of the implications of ASPA-based AS
path verification. Network operators must keep their ASPA objects
correct and up to date. Otherwise, for example, if a provider AS is
left out of the Set of Provider ASes (SPAS) in the ASPA, then routes
containing the CAS (in the ASPA) and said provider AS may be
incorrectly labeled as route leaks and considered ineligible for
route selection (see Section 7.1).
9.3. Make Before Break
ASPA issuers SHOULD apply the make-before-break principle while
updating an ASPA registration. For example, when adding new Provider
AS(es) in the SPAS, if the new ASPA is meant to replace a previously
created ASPA, the latter SHOULD be decommissioned only after allowing
sufficient time for the new ASPA to propagate to Relying Parties (RP)
through the global RPKI system.
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9.4. DoS/DDoS Mitigation Service Provider
An AS may have a mitigation service provider (MSP) for protection
from Denial of Service (DoS)/Distributed DoS (DDoS) attacks targeting
servers with IP addresses in the prefixes the AS originates. Such an
AS MAY include the MSP's AS in the SPAS of its ASPA. With such an
ASPA in place, in the event of an attack, the AS (customer of the
MSP) can announce more specific prefixes (over a BGP session) to the
MSP's AS for mitigation purposes, and such announcements would be
able to pass the ASPA-based path verification. It is assumed that
appropriate ROAs are registered in advance so that the announcements
can pass RPKI-ROV as well.
10. Comparison to Other Technologies
10.1. BGPsec
BGPsec [RFC8205] was designed to solve the problem of AS_PATH
verification by including cryptographic signatures in BGP Update
messages. It offers protection against unauthorized path
modifications and assures that the BGPsec Update actually traveled
the path shown in the BGPsec_PATH Attribute. However, it does not
detect route leaks (valley-free violations). In comparison, the
ASPA-based path verification described in this document detects if
the AS path is improbable and focuses on detecting route leaks
(including malicious cases) and forged-origin hijacks.
BGPsec and ASPA are complementary technologies.
10.2. Peerlock
The Peerlock mechanism [Peerlock] [Flexsealing] has a similar
objective as the APSA-based route leak protection mechanism described
in this document. It is commonly deployed by large Internet carriers
to protect each other from route leaks. Peerlock depends on a
laborious manual process in which operators coordinate the
distribution of unstructured Provider Authorizations through out-of-
band means in a many-to-many fashion. On the other hand, ASPA's use
of the RPKI allows for automated, scalable, and ubiquitous
deployment, making the protection mechanism available to a wider
range of network operators.
The ASPA mechanism implemented in router code (in contrast to
Peerlock's AS_PATH regular expressions) also provides a way to detect
anomalies propagated from transit providers and IX route servers.
ASPA is intended to be a complete solution and replacement for
existing Peerlock deployments.
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11. IANA Considerations
This document includes no request to IANA.
12. Security Considerations
While the ASPA-based mechanism is able to detect and mitigate the
majority of mistakes and malicious activity affecting routes, it
might fail to detect some malicious path modifications, especially
for routes that are received from transit providers.
Since an upstream provider becomes a trusted point, in theory, it
might be able to propagate some instances of hijacked prefixes with
forged-origin or forged-path-segment or even routes with manipulated
AS_PATHs, and such attacks might go undetected by its customers.
This can be illustrated with some examples. In Figure 3, normally
the receiving/validating AS located at the lower left side should
receive a route with AS_PATH {AS(5), AS(4), AS(3), AS(2), AS(1)} and
it would be Valid (Section 6.2) given all the ASPAs that are shown in
the figure. However, if AS(5) which is a transit provider to the
validating AS acts maliciously and sends the route with a shortened
AS_PATH such as {AS(5), AS(3), AS(2), AS(1)} or {AS(5), AS(2),
AS(1)}, such path manipulation would be undetectable (i.e., the
AS_PATH would be considered Valid). Also, if AS(5) were to perform a
forged-origin hijack by inserting an AS_PATH {AS(5), AS(1)}, that
would also be undetectable.
AS(4) - AS(3)
/ \
(down-ramp) / \ (up-ramp)
AS(5) AS(2)
/ \
/ AS(1)
/ (Origin AS)
Receiving & Validating AS
ASPAs: {AS(1), [AS(2)]}, {AS(2), [AS(3)]}, {AS(5), [AS(4)]},
{AS(3), [AS 0]}, {AS(4), [AS 0]}
Figure 3: Illustration for discussion of undetectable AS_PATH
manipulations.
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While attacks like the examples above may happen, it does not seem to
be a realistic scenario. Normally a customer and their transit
provider would have a signed agreement, and a policy violation (of
the above kind) should have legal consequences or the customer can
just drop the relationship with such a provider and remove the
corresponding ASPA record.
The key properties or strengths of the ASPA method were described in
Section 8. If detection of any and all kinds of path manipulation
attacks is the goal, then BGPsec [RFC8205] would need to be deployed
complementary to the ASPA method. It may be noted that BGPsec in its
current form lacks route leak detection capabilities.
13. Implementation Status
This section is to be removed before publishing as an RFC.
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft. The inclusion of this section here follows the
process described in [RFC7942]. The description of implementations
in this section is intended to assist the IETF in its decision
processes in progressing drafts to RFCs. Please note that the
listing of any individual implementation here does not imply
endorsement by the IETF. Furthermore, no effort has been spent to
verify the information presented here that was supplied by IETF
contributors. This is not intended as, and must not be construed to
be, a catalog of available implementations or their features.
Readers are advised to note that other implementations may exist.
According to [RFC7942], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
* A BGP implementation OpenBGPD [bgpd] (version 7.8 and higher),
written in C, was provided by Claudio Jeker, Theo Buehler, and Job
Snijders.
* The implementation NIST-BGP-SRx [BGP-SRx] is a software suite that
provides a validation engine (BGP-SRx) and a Quagga-based BGP
router (Quagga-SRx). It includes unit test cases for testing the
ASPA-based path verification. It was provided by Oliver Borchert,
Kyehwan Lee, and their colleagues at US NIST. It requires some
additional work to incorporate the latest changes in the draft
specifications related to IXP RS AS and RS-client.
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14. References
14.1. Normative References
[I-D.ietf-sidrops-aspa-profile]
Azimov, A., Uskov, E., Bush, R., Snijders, J., Housley,
R., and B. Maddison, "A Profile for Autonomous System
Provider Authorization", Work in Progress, Internet-Draft,
draft-ietf-sidrops-aspa-profile-16, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-sidrops-
aspa-profile-16>.
[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>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[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>.
[RFC6482] Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
Origin Authorizations (ROAs)", RFC 6482,
DOI 10.17487/RFC6482, February 2012,
<https://www.rfc-editor.org/info/rfc6482>.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012,
<https://www.rfc-editor.org/info/rfc6793>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
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[RFC7908] Sriram, K., Montgomery, D., McPherson, D., Osterweil, E.,
and B. Dickson, "Problem Definition and Classification of
BGP Route Leaks", RFC 7908, DOI 10.17487/RFC7908, June
2016, <https://www.rfc-editor.org/info/rfc7908>.
[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>.
[RFC8481] Bush, R., "Clarifications to BGP Origin Validation Based
on Resource Public Key Infrastructure (RPKI)", RFC 8481,
DOI 10.17487/RFC8481, September 2018,
<https://www.rfc-editor.org/info/rfc8481>.
[RFC8893] Bush, R., Volk, R., and J. Heitz, "Resource Public Key
Infrastructure (RPKI) Origin Validation for BGP Export",
RFC 8893, DOI 10.17487/RFC8893, September 2020,
<https://www.rfc-editor.org/info/rfc8893>.
[RFC9234] Azimov, A., Bogomazov, E., Bush, R., Patel, K., and K.
Sriram, "Route Leak Prevention and Detection Using Roles
in UPDATE and OPEN Messages", RFC 9234,
DOI 10.17487/RFC9234, May 2022,
<https://www.rfc-editor.org/info/rfc9234>.
[RFC9324] Bush, R., Patel, K., Smith, P., and M. Tinka, "Policy
Based on the Resource Public Key Infrastructure (RPKI)
without Route Refresh", RFC 9324, DOI 10.17487/RFC9324,
December 2022, <https://www.rfc-editor.org/info/rfc9324>.
14.2. Informative References
[BGP-SRx] NIST, "BGP Secure Routing Extension (BGP-SRx) Software
Suite", NIST Open-Source Software , <https://www.nist.gov/
services-resources/software/bgp-secure-routing-extension-
bgp-srx-software-suite>.
[bgpd] Jeker, C., "OpenBGPD", <http://www.openbgpd.org/>.
[Flexsealing]
McDaniel, T., Smith, J., and M. Schuchard, "Flexsealing
BGP Against Route Leaks: Peerlock Active Measurement and
Analysis", November 2020,
<https://arxiv.org/pdf/2006.06576.pdf>.
[I-D.ietf-grow-route-leak-detection-mitigation]
Sriram, K. and A. Azimov, "Methods for Detection and
Mitigation of BGP Route Leaks", Work in Progress,
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Internet-Draft, draft-ietf-grow-route-leak-detection-
mitigation-09, 8 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-grow-
route-leak-detection-mitigation-09>.
[I-D.ietf-idr-deprecate-as-set-confed-set]
Kumari, W. A., Sriram, K., Hannachi, L., and J. Haas,
"Deprecation of AS_SET and AS_CONFED_SET in BGP", Work in
Progress, Internet-Draft, draft-ietf-idr-deprecate-as-set-
confed-set-11, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
deprecate-as-set-confed-set-11>.
[IANA-AF] IANA, "Address Family Numbers",
<https://www.iana.org/assignments/address-family-numbers/
address-family-numbers.xhtml>.
[IANA-SAF] IANA, "Subsequent Address Family Identifiers (SAFI)
Parameters", <https://www.iana.org/assignments/safi-
namespace/safi-namespace.xhtml>.
[Peerlock] Snijders, J., "Peerlock", June 2016,
<https://www.nanog.org/sites/default/files/
Snijders_Everyday_Practical_Bgp.pdf>.
[RFC3779] Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
Addresses and AS Identifiers", RFC 3779,
DOI 10.17487/RFC3779, June 2004,
<https://www.rfc-editor.org/info/rfc3779>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
Specification", RFC 8205, DOI 10.17487/RFC8205, September
2017, <https://www.rfc-editor.org/info/rfc8205>.
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[RFC9319] Gilad, Y., Goldberg, S., Sriram, K., Snijders, J., and B.
Maddison, "The Use of maxLength in the Resource Public Key
Infrastructure (RPKI)", BCP 185, RFC 9319,
DOI 10.17487/RFC9319, October 2022,
<https://www.rfc-editor.org/info/rfc9319>.
[sriram1] Sriram, K. and J. Heitz, "On the Accuracy of Algorithms
for ASPA Based Route Leak Detection", IETF SIDROPS
Meeting, Proceedings of the IETF 110, March 2021,
<https://datatracker.ietf.org/meeting/110/materials/
slides-110-sidrops-sriram-aspa-alg-accuracy-01>.
Appendix A. Acknowledgments
The authors wish to thank Jakob Heitz, Amir Herzberg, Igor Lubashev,
Ben Maddison, Russ Housley, Jeff Haas, Nan Geng, Nick Hilliard,
Shunwan Zhuang, Yangyang Wang, Martin Hoffmann, Jay Borkenhagen,
Amreesh Phokeer, Aftab Siddiqui, Dai Zhibin, Doug Montgomery, Rich
Compton, Andrei Robachevsky, and Iljitsch van Beijnum for comments,
suggestions, and discussion on the path verification procedures or
the text in the document. For the implementation and testing of the
procedures in the document, the authors wish to thank Claudio Jeker
and Theo Buehler [bgpd] as well as Kyehwan Lee and Oliver Borchert
[BGP-SRx].
Contributors
The following people made significant contributions to this document
and should be considered co-authors:
Claudio Jeker
OpenBSD
Email: cjeker@diehard.n-r-g.com
Authors' Addresses
Alexander Azimov
Yandex
Ulitsa Lva Tolstogo 16
Moscow
119021
Russian Federation
Email: a.e.azimov@gmail.com
Eugene Bogomazov
Qrator Labs
1-y Magistralnyy tupik 5A
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Moscow
123290
Russian Federation
Email: eb@qrator.net
Randy Bush
Internet Initiative Japan & Arrcus, Inc.
5147 Crystal Springs
Bainbridge Island, Washington 98110
United States of America
Email: randy@psg.com
Keyur Patel
Arrcus
2077 Gateway Place
Suite #400
San Jose, CA 95119
United States of America
Email: keyur@arrcus.com
Job Snijders
Fastly
Amsterdam
Netherlands
Email: job@fastly.com
Kotikalapudi Sriram
USA National Institute of Standards and Technology
100 Bureau Drive
Gaithersburg, MD 20899
United States of America
Email: ksriram@nist.gov
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