Secure Inter-Domain Routing (SIDR) G. Huston
Internet-Draft G. Michaelson
Intended status: Informational APNIC
Expires: May 12, 2011 November 8, 2010
Validation of Route Origination using the Resource Certificate PKI and
ROAs
draft-ietf-sidr-roa-validation-09.txt
Abstract
This document defines the semantics of a Route Origin Authorization
(ROA) in terms of the context of an application of the Resource
Public Key Infrastructure to validate the origination of routes
advertised in the Border Gateway Protocol.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. ROA Validation Outcomes for a Route . . . . . . . . . . . . . . 3
3. Applying Validation Outcomes to Route Selection . . . . . . . . 6
4. Disavowal of Routing Origination . . . . . . . . . . . . . . . 7
5. Route Validation Lifetime . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
This document defines the semantics of a Route Origin Authorization
(ROA) in terms of the context of an application of the Resource
Public Key Infrastructure (RPKI) [I-D.ietf-sidr-arch] to validate the
origination of routes advertised in the Border Gateway Protocol (BGP)
[RFC4271].
The RPKI is based on a hierarchy of Resource Certificates that are
aligned to the Internet number resource allocation structure.
Resource Certificates are X.509 certificates that conform to the PKIX
profile [RFC5280], and to the extensions for IP addresses and AS
identifiers [RFC3779]. A Resource Certificate describes an action by
an issuer that binds a list of IP address blocks and Autonomous
System (AS) numbers to the Subject of a certificate, identified by
the unique association of the Subject's private key with the public
key contained in the Resource Certificate. The RPKI is structured
such that each current Resource Certificate matches a current
resource allocation or assignment. This is further described in
[I-D.ietf-sidr-arch].
ROAs are digitally signed objects that bind an address to an AS
number, signed by the address holder. A ROA provides a means of
verifying that an IP address block holder has authorized a particular
AS to originate routes in the inter-domain routing environment for
that address block. ROAs are described in
[I-D.ietf-sidr-roa-format]. ROAs are intended to fit within the
requirements for adding security to inter-domain routing.
This document describes the semantic interpretation of a ROA, with
particular reference to application in inter-domain routing relating
to the origination of routes, and the intended scope of the authority
that is conveyed in the ROA.
2. ROA Validation Outcomes for a Route
A "route" is unit of information that associates a set of
destinations described by an IP address prefix with a set of
attributes of a path to those destinations, as defined in section 1.1
of [RFC4271].
A route's "origin AS" is defined as follows: If the final path
segment of the AS_PATH is of type AS_SEQUENCE, the "origin AS" is the
first element of the sequence (i.e. the AS in the rightmost position
with respect to the position of octets in the protocol message). If
the final path segment of the AS_PATH is of type AS_SET, indicating
that the route is an aggregate, then the origin AS is taken as the AS
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component of the AGGREGATOR path attribute [RFC4271], if present.
Otherwise the route's origin AS cannot be determined.
In terms of validation of a route in the context of a routing
environment, the address prefix value and the origin AS are used in
the ROA validation operation.
It is assumed here that a Relying Party (RP) has access to a local
cache of the complete set of valid ROAs when performing validation of
a route. (Valid ROAs are defined as ROAs that are determined to be
syntactically correct and are signed using a signature that can be
verified using the RPKI, as described in [I-D.ietf-sidr-roa-format].)
The RP needs to match a route to one or more candidate valid ROAs in
order to determine a validation outcome, which, in turn, can be used
to determine the appropriate local actions to perform on the route.
This approach to route origination validation uses a generic model of
"positive" attestation that has an associated inference that routes
that cannot be validated within the RPKI framework would
conventionally be interpreted by an RP as "invalid". However, the
considerations of accommodating environments of partial adoption of
the use of ROAs, where only a subset of validly advertised address
prefixes have associated published ROAs within the structure of the
RPKI, imply some modification to this model of positive attestation.
In the context of route validation it is assumed that once an address
prefix is described in a ROA, then this ROA specifically encompasses
all address prefixes that are more specific than that described in
the ROA. Thus, any route for a more specific address prefix than
that described by any valid ROA that does not itself have a matching
valid ROA can be considered to be "invalid". However, routes objects
for address prefixes that are not fully described by any single ROA,
i.e., those route objects whose address prefixes may be an aggregate
of address prefixes described in a valid ROA, or have address
prefixes where there is no intersection with any ROA, and are not
matched by any ROA and are not a more specific of any ROA, cannot be
reliably classified as "invalid" in a partial deployment scenario.
Such routes have a validation outcome of "unknown".
An abstract attribute of a route can be determined as the outcome of
this validation procedure, namely a "validity state"
[I-D.ietf-sidr-pfx-validate]. The "validity state" of a route, with
a prefix and an origin AS as defined above, when using single ROA for
determining this validity state is summarized in the following table:
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Route matching non-matching
Prefix AS-> AS AS
V +---------+---------+
Non- | unknown | unknown |
Intersecting | | |
+---------+---------+
Covering | unknown | unknown |
Aggregate | | |
+---------+---------+
match ROA | valid | invalid |
prefix | | |
+---------+---------+
More | | |
Specific | invalid | invalid |
than ROA | | |
+---------+---------+
Route's Validity State
In an environment of a collection of valid ROAs, a route's validity
state is considered to be "valid" if any ROA provides a "valid"
outcome. It's validity state is considered to be "invalid" if one
(or more) ROAs provide an "invalid" outcome and no ROAs provide a
"valid" outcome. Its validity state is considered to be "unknown"
(or, synonymously, "not found" [I-D.ietf-sidr-pfx-validate] when no
valid ROA can produce either a "valid" or an "invalid" validity state
outcome.
A route validity state is defined by the following procedure:
1. Select all valid ROAs that include a ROAIPAddress value that
either matches, or is a covering aggregate of, the address
prefix in the route. This selection forms the set of
"candidate ROAs."
2. If the set of candidate ROAs is empty, then the procedure
stops with an outcome of "unknown" (or, synonymously, "not
found", as used in [I-D.ietf-sidr-pfx-validate]).
3. If the route's origin AS can be determined and any of the set
of candidate ROAs has an asID value that matches the origin AS
in the route, and the route's address prefix matches a
ROAIPAddress in the ROA (where "match" is defined as where the
route's address precisely matches the ROAIPAddress, or where
the ROAIPAddress includes a maxLength element, and the route's
address prefix is a more specific prefix of the ROAIPAddress,
and the route's address prefix length value is less than or
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equal to the ROAIPAddress maxLength value) then the procedure
halts with an outcome of "valid".
4. Otherwise, the procedure halts with an outcome of "invalid".
3. Applying Validation Outcomes to Route Selection
Within the framework of the abstract model of the operation of inter-
domain routing using BGP [RFC4271], a received prefix announcement
from a routing peer is compared to all announcements for this prefix
received from other routing peers and a route selection procedure is
used to select the "best" route from this candidate set.
The route's validity state, described in Section 2, of "valid",
"invalid" or "unknown" may be used as part of the determination of
the local degree of preference, in which case the local order of
preference is as follows:
"valid" is to be preferred over
"unknown", which is to be preferred over
"invalid".
It is a matter of local routing policy as to the actions to be
undertaken by a routing entity in processing those routes with
"unknown" validity states. Due to considerations of partial use of
ROAs in heterogeneous environments, such as in the public Internet,
it is advised that local policy settings should not result in
"unknown" validity state outcomes being considered as sufficient
grounds to reject a route outright from further consideration as a
local "best" route.
It is a matter of local routing policy as to whether routes with an
"invalid" validity state are considered to be ineligible for further
consideration in a route selection process. A possible consideration
here is one of potential circularity of dependence: If the
authoritative publication point of the repository of ROAs, or that of
any certificate used in relation to an address prefix, is located at
an address that lies within the address prefix described in a ROA,
then the repository can only be accessed by the RP once a route for
the prefix has been accepted by the RP's local routing domain. It is
also noted that the propagation time of RPKI objects may be different
to the propagation time of routes, and that routes may be learned by
an RP's routing system before the RP's local RPKI repository cache
picks up the associated ROAs and recognises them as having a validity
state of "valid" within the RPKI.
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4. Disavowal of Routing Origination
A ROA is a positive attestation that a prefix holder has authorized
an AS to originate a route for this prefix into the inter-domain
routing system. It is possible for a prefix holder to construct an
authorization where no valid AS has been granted any such authority
to originate a route for an address prefix. This is achieved by
using a ROA where the ROA's subject AS is one that must not be used
in any routing context. Specifically, AS 0 is reserved by the IANA
such that it may be used to identify non-routed networks
[IANA.AS-Registry].
A ROA with a subject of AS 0 (AS0-ROA) is an attestation by the
holder of a prefix that the prefix described in the ROA, and any more
specific prefix, should not be used in a routing context.
The route validation procedure, described in Section 2, will provide
a "valid" outcome if any ROA matches the address prefix and origin
AS, even if other valid ROAs would provide an "invalid" validation
outcome if used in isolation. Consequently, an AS0-ROA has a lower
relative preference than any other ROA that has a routable AS as its
subject. This allows a prefix holder to use an AS0-ROA to declare a
default condition that any route that is equal to, or more specific
than the prefix to be considered to be invalid, while also allowing
other concurrently issued ROAs to describe valid origination
authorizations for more specific prefixes.
By convention, an AS0-ROA should have a maxLength value of 32 for
IPv4 addresses and a maxlength value of 128 for IPv6 addresses,
although in terms of route validation the same outcome would be
achieved with any valid maxLength value, or even if the maxLength
element were to be omitted from the ROA.
Also by convention, an AS0-ROA should be the only ROA issued for a
given address prefix, although again this is not a strict
requirement. An AS0-ROA MAY coexist with ROAs that have different
subject AS values, although in such cases the presence or otherwise
of the AS0-ROA does not alter the route's validity state in any way.
5. Route Validation Lifetime
The "lifetime" of a validation outcome refers to the time period
during which the original validation outcome can be still applied.
The implicit assumption here is that when the validation lifetime
expires the routing object should be re-tested for validity.
The validation lifetime for a ROA is controlled by the Valid times
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specified in the End Entity (EE) Certificate used to sign the ROA,
and the valid times of those certificates in the certification path
used to validate the EE Certificate. A ROA validation "expires" at
the Validity To field of the signing EE certificate, or at such a
time when there is no certification path that can validate the ROA.
A ROA issuer may elect to prematurely invalidate a ROA by revoking
the EE certificate that was used to sign the ROA.
6. Security Considerations
ROA issuers should be aware of the validation implication in issuing
a ROA, in that a ROA implicitly invalidates all routes that have more
specific prefixes with a prefix length greater than maxLength, and
all originating AS's other than the AS listed in the collection of
ROAs for this prefix.
A conservative operational practice would be to ensure the issuing of
ROAs for all more specific prefixes with distinct origination AS's
prior to the issuing of ROAs for larger encompassing address blocks,
in order to avoid inadvertent invalidation of valid routes during ROA
generation.
ROA issuers should also be aware that if they generate a ROA for one
origin AS, then if the address prefix holder authorises multiple AS's
to originate routes for a given address prefix, then is necessary for
a ROA be generated for every such authorized AS.
7. IANA Considerations
[There are no IANA Considerations.]
8. Acknowledgements
The authors would like to acknowledge the helpful contributions of
John Scudder and Stephen Kent in preparing this document, and also
the contributions of many members of the SIDR Working Group in
response to presentations of this material in SIDR WG sessions. The
authors also acknowledge prior work undertaken by Tony Bates, Randy
Bush, Tony Li, and Yakov Rekhter as the validation outcomes described
here reflect the authentication outcomes and semantics of origin AS
verification described in [exI-D.bates]. A number of validation
concepts relating to a route's "validity state" presented in
[I-D.ietf-sidr-pfx-validate], edited by Pradosh Mohapatra et al, have
be used in this document.
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9. References
9.1. Normative References
[I-D.ietf-sidr-arch]
Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", draft-ietf-sidr-arch (work in
progress), October 2009.
[I-D.ietf-sidr-roa-format]
Lepinski, M., Kent, S., and D. Kong, "An Infrastructure to
Support Secure Internet Routing",
draft-ietf-sidr-roa-format (work in progress),
October 2009.
[RFC3779] Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
Addresses and AS Identifiers", RFC 3779, June 2004.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[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, May 2008.
9.2. Informative References
[I-D.ietf-sidr-pfx-validate]
Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation",
draft-ietf-sidr-pfx-validate-00 (work in progress),
July 2010.
[IANA.AS-Registry]
IANA, "IANA Autonomous System Number Registry",
March 2010.
[exI-D.bates]
Bates, T., Bush, R., Li, T., and Y. Rekhter, "DNS-based
NLRI origin AS verification in BGP",
draft-bates-bgp4-nlri-orig-verif-00.txt (work in
progress), January 1998.
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Authors' Addresses
Geoff Huston
Asia Pacific Network Information Centre
Email: gih@apnic.net
George Michaelson
Asia Pacific Network Information Centre
Email: ggm@apnic.net
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