| Network Working Group | R.G.M. Gagliano |
| Internet-Draft | Cisco Systems |
| Intended status: Standards Track | S.K. Kent |
| Expires: May 02, 2012 | BBN Technologies |
| S.T. Turner | |
| IECA, Inc. | |
| November 2011 |
Algorithm Agility Procedure for RPKI.
draft-ietf-sidr-algorithm-agility-04
This document specifies the process that Certification Authorities (CAs) and Relying Parties (RP) participating in the Resource Public Key Infrastructure (RPKI) will need to follow to transition to a new (and probably cryptographically stronger) algorithm set. The process is expected to be completed in a time scale of months or years. Consequently, no emergency transition is specified. The transition procedure defined in this document supports only a top-down migration (parent migrates before children).
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 http:/⁠/⁠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 May 02, 2012.
Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http:/⁠/⁠trustee.ietf.org/⁠license-⁠info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The RPKI must accommodate transitions between the public keys used by CAs. Transitions of this sort are usually termed “key rollover”. Planned key rollover will occur at regular intervals throughout the life of the RPKI, as each CA changes its public keys, in a non-coordinated fashion. (By non-coordinated we mean that the time at which each CA elects to change its keys is locally determined, not coordinated across the RPKI.) Moreover, because a key change might be necessitated by suspected private key compromise, one can never assume coordination of these events among all of the CAs in the RPKI. In an emergency key rollover, the old certificate is revoked and a new certificate with a new key is issued. The mechanisms to perform a key rollover in RPKI (either planned or in an emergency), while maintaining the same algorithm suite, are covered in [I-D.ietf-sidr-keyroll].
This document describes the mechanism to perform a key rollover in RPKI due to the migration to a new signature algorithm suite. A signature algorithm suite encompasses both a signature algorithm (with a specified key size range) and a one-way hash algorithm. It is anticipated that the RPKI will require the adoption of updated key sizes and/or different algorithm suites over time. This document treats the adoption of a new hash algorithm while retaining the current signature algorithm as equivalent to an algorithm migration, and requires the CA to change its key. Migration to a new algorithm suite will be required in order to maintain an acceptable level of cryptographic security and protect the integrity of certificates, CRLs and signed objects in the RPKI. All of the data structures in the RPKI explicitly identify the signature and hash algorithms being used. However, experience has demonstrated that the ability to represent algorithm IDs is not sufficient to enable migration to new algorithm suites (algorithm agility). One also must ensure that protocols, infrastructure elements, and operational procedures also accommodate migration from one algorithm suite to another. Algorithm migration is expected to be very infrequent, but it also will require support of a “current” and “next” suite for a prolonged interval, probably several years.
This document defines how entities in the RPKI execute (planned) CA key rollover when the algorithm suite changes. The description covers actions by CAs, repository operators, and RPs. It describes the behavior required of both CAs and RPs to make such key changes work in the RPKI context, including how the RPKI repository system is used to support key rollover.
This document does not specify any algorithm suite.
This document does not specify any algorithm suite per se. The RPKI Certificate Policy (CP) [I-D.ietf-sidr-cp] mandates the use of the algorithms defined in [I-D.ietf-sidr-rpki-algs] by CAs and RPs. When an algorithm transition is initiated, [I-D.ietf-sidr-rpki-algs] will be updated (as defined in Section 4.1 of this document) redefining the required algorithm(s) for compliant RPKI CAs and RPs under the CP. The CP will not change as a side effect of algorithm transition (and thus the policy OID in RPKI certificates will not change.)
An additional document, the algorithm transition timetable, will be published (as a BCP) to define the dates for each milestone defined in this document. It will define dates for the phase transitions, consistent with the descriptions provided in Section 4. It also will describe how the RPKI community will measure the readiness of CAs and RPs to transition to each phase. CAs publish certificates, CRLs, and other signed objects under the new algorithm suite as the transition progresses. This provides visibility into the deployment of the new algorithm suite, enabling the community to evaluate deployment progress. The transition procedure allows CAs to remove old certificates, CRLs, and signed products, during the twilight phase. This provides an ability to observe and measure the withdrawal of the old algorithm suite. Thus the phases defined in this document enable the community to evaluate the progress of the transition. The timetable document will also describe procedures to amend the timetable if problems arise in implementing later phases of the transition. It is RECOMMENDED that the timetable document be developed by representatives of the RPKI community, e.g., IANA, Internet Registries, and network operators.
This document assumes that the reader is familiar with the terms and concepts described in "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile" [RFC5280], "X.509 Extensions for IP Addresses and AS Identifiers" [RFC3779], and “A Profile for Resource Certificate Repository Structure” [I-D.ietf-sidr-repos-struct]. Additional terms and conventions use in examples are provided below.
The “current” RPKI algorithm suite (Suite A) is defined in the RPKI CP document, by reference to [I-D.ietf-sidr-rpki-algs]. When a migration of the RPKI algorithm suite is needed, the first step MUST be an update of the [I-D.ietf-sidr-rpki-algs] document to define the new algorithm suite. The algorithm transition timeline document MUST also be published, to inform the community of the dates selected for milestones in the transition process, as described in Section 4.3.
The migration process described in this document involves a series of steps that MUST be executed in chronological order by CAs and RPs. The only milestone at which both CAs and RPs take action at the same time is the "EOL Algorithm A" date. Due to the decentralized nature of the RPKI infrastructure, it is expected that an algorithm transition will span several years.
In order to facilitate the transition, CAs will start issuing certificates using the Algorithm B in a hierarchical top-down fashion. In our example, CA Y will issue certificates using the Algorithm B suite only after CA X has started to do so (CA Y Ready Algorithm B Date > CA X Ready Algorithm B Date). This ordered transition avoids issuance of "mixed" suite certificates, e.g., a CA certificate signed using Suite A, containing a key from Suite B. In the RPKI, a CA MUST NOT sign a CA certificate carrying a subject key that corresponds to an algorithm suite that differs from the one used to sign the certificate.(X.509 accommodates such mixed algorithm certificates, but this process avoids using that capability. A not top-down transition approach would require use of such mixed mode certificates, but this would lead to exponential growth of the RPKI repository. Also, because the RPKI CP mandates “proof of possession” for certificate requests, it is not possible for a CA to request a certificate for Algorithm Suite B, until its parent CA supports that Suite. See Section 5 for more details.)
The algorithm agility model described here does not prohibit a CA from issuing an EE certificate with a subject public key from a different algorithm suite, if that certificate is not used to verify repository objects. This exception to the mixed algorithm suite certificate rule is allowed because an EE certificate that is not used to verify repository objects does not interfere with the ability of RPs to download and verify repository content. As noted above, every CA in the RPKI is required to perform a Proof of Possession (PoP) check for the subject public key when issuing a certificate. In general a subject cannot assume that a CA is capable of supporting a different algorithm. However, if the subject is closely affiliated with the CA, it is reasonable to assume that there are ways for the subject to know whether the CA can support a request to issue an EE certificate containing a specific, different public key algorithm. This document does not specify how a subject can determine whether a CA is capable of issuing a mixed suite EE certificate, because it anticipates that such certificates will be issued only in contexts where the subject and CA are sufficiently closely affiliated (for example, an ISP issuing certificates to devices that it manages).
The following figure gives an overview of the process:
Process for RPKI CAs:
Phase 0 Phase 1 Phase 2 Phase 4 Phase 0
-----------x---------x-------------------x--------x-----------
^ ^ ^ ^ ^
| | | | |
(1) (2) (3) (5) (6)
Process for RPKI RPs:
Phase 0 Phase 3 Phase 4 Phase 0
-------------------------------x---------x--------x-----
^ ^ ^ ^
| | | |
(1) (4) (5) (6)
(1) RPKI algorithm document is updated and the algorithm transition timeline document is issued
(2) CA Ready Algorithm B Date
(3) CA Go Algorithm B Date
(4) RP Ready Algorithm B Date
(5) Twilight Date
(6) End Of Live (EOL) Date
Each of these milestones is discussed in the next section as part of describing each phase of the transition process.
Two situations have been identified that motivate pausing or rolling back the transition process. The first situation arises if the RPKI community is not ready to make the transition. For example, many CAs might not prepared to issue signed products under Suite B, or many RPs might be ready to process Suite B product sets. Under these circumstances, the timetable MUST be reissued, postponing the date for the phase in question, and pushing back the dates for later phases. The other situation arises if, during the transition, serious concerns arise about the security of the Suite B algorithms. Such concerns would motivate terminating the transition and rolling back signed products, i.e., reverting to Suite A. In this case the timetable MUST be republished, and the RPKI algorithm document MUST be superseded. The phase descriptions below allude to these two situations, as appropriate.
Phase 0 is the initial phase of the process, throughout this phase the algorithm suite A is the only supported algorithm suite in RPKI. This is also the steady state for the RPKI.
The following figure illustrates the format used to describe signed objects in the repository. It reflects the algorithm suites in use, and shows the relationship between three CAs (X, Y, and Z) that form a chain.
During Phase 0, CAs X, Y and Z are required to generate signed product sets using only the Algorithm Suite A. Also, RPs are required to validate signed product sets issued using only Algorithm Suite A.
CA X-Certificate-Algorithm-Suite-A (Cert-XA) | |-> CA-Y-Certificate-Algorithm-Suite-A (Cert-YA) |-> CA-Z-Certificate-Algorithm-Suite-A (Cert-ZA) |-> CA-Z-CRL-Algorithm-Suite-A (CRL-ZA) |-> CA-Z-Signed-Objects-Algorithm-Suite-A |-> CA-Y-CRL-Algorithm-Suite-A (CRL-YA) |-> CA-Y-Signed-Objects-Algorithm-Suite-A |-> CA-X-CRL-Algorithm-Suite-A (CRL-XA) |-> CA-X-Signed-Objects-Algorithm-Suite-A
Note: Cert-XA represent the certificate for CA X, that is signed using the algorithm suite A.
The first milestone initiates the migration process. It updates the [I-D.ietf-sidr-rpki-algs] document with the following definitions for the RPKI:
Additionally, the new algorithm transition timeline document will be published with the following information:
All Dates MUST be represented using the local UTC date-time format specified in [RFC3339].
Phase 1 starts at the CA Ready Algorithm B Date. During Phase 1, all (non-leaf) CAs MUST be ready to process a request from a child CA to issue or revoke a certificate using the Algorithm B suite. If it is determined that a substantial number of CAs are not ready, the algorithm transition timeline document will be reissued, as noted in Section 4.2. However, CAs that are capable of issuing Suite B certificates may continue to do so, if requested by their child CAs. Since this phase does not require any RPs to process signed objects under Suite B, and since Suite B product SHOULD be stored at independent publication points, there is no adverse impact on RPs. If the Suite B algorithm is deemed unsuitable, the algorithm transition timeline and the algorithm specification documents MUST be replaced. CAs MUST cease accepting requests for certificates under Suite B, and Suite B certificates that have been issued MUST be revoked.
As the transition will happen using a (hierarchic) top-down model, a child CA will be able to issue certificates using the Algorithm B suite only after its parent CA has issued its own. The RPKI provisioning protocol can identify if a parent CA is capable of issuing certificates using the Algorithm Suite B, and can identify the corresponding algorithm suite in each Certificate Signing Request (see Section 5).
The following figure shows the status of repository entries for the three example CAs during this Phase. Two distinct certificate chains are maintained and CA Z has not yet requested any material using the Algorithm B suite.
CA X-Certificate-Algorithm-Suite-A (Cert-XA) | |-> CA-Y-Certificate-Algorithm-Suite-A (Cert-YA) |-> CA-Z-Certificate-Algorithm-Suite-A (Cert-ZA) |-> CA-Z-CRL-Algorithm-Suite-A (CRL-ZA) |-> CA-Z-Signed-Objects-Algorithm-Suite-A |-> CA-Y-CRL-Algorithm-Suite-A (CRL-YA) |-> CA-Y-Signed-Objects-Algorithm-Suite-A |-> CA-X-CRL-Algorithm-Suite-A (CRL-XA) |-> CA-X-Signed-Objects-Algorithm-Suite-A CA X-Certificate-Algorithm-Suite-B (Cert-XB) | |-> CA-Y-Certificate-Algorithm-Suite-B (Cert-YB) |-> CA-Y-CRL-Algorithm-Suite-B (CRL-YB) |-> CA-Y-Signed-Objects-Algorithm-Suite-B |-> CA-X-CRL-Algorithm-Suite-B (CRL-XB) |-> CA-X-Signed-Objects-Algorithm-Suite-B
Phase 2 starts at the CA Go Algorithm B Date. At the start of this phase, each signed product sets MUST be available using both Algorithm Suite A and Algorithm Suite B. During this phase, RPs MUST be prepared to validate sets issued using Algorithm Suite A and MAY be prepared to validate sets issued using the Algorithm Suite B.
If it is determined that a substantial number of CAs are not ready, the algorithm transition timeline document will be reissued, as noted in Section 4.2.. (Since the processing requirement for RPs here is a MAY, if RPs have problems with Suite B products this does not require pushing back the Phase 2 milestone, but it does motivate delaying the start of Phase 3.) CAs that are capable of publishing products under Suite B MAY continue to do so. Phase 2, like Phase 1, does not require any RPs to process signed objects under Suite B. Also, Suite B product SHOULD be stored at independent publication points, so there is no adverse impact on RPs. If the Suite B algorithm is deemed unsuitable, the algorithm transition timeline and the algorithm specification documents MUST be replaced. CAs MUST cease accepting requests for certificates under Suite B, and Suite B certificates that have been issued MUST be revoked.
An RP that validates all signed product sets using both Algorithm Suite A or Algorithm Suite B, SHOULD expect the same results. However, an object that validates using either Algorithm Suite A or Algorithm Suite B MUST be considered valid. A detailed analysis on the validation of multiple instance of signed objects is included in Section 6.
The following figure shows the status of the repository entries for the three example CAs throughout this phase, where all signed objects are available using both algorithm suites.
CA X-Certificate-Algorithm-Suite-A (Cert-XA) | |-> CA-Y-Certificate-Algorithm-Suite-A (Cert-YA) |-> CA-Z-Certificate-Algorithm-Suite-A (Cert-ZA) |-> CA-Z-CRL-Algorithm-Suite-A (CRL-ZA) |-> CA-Z-Signed-Objects-Algorithm-Suite-A |-> CA-Y-CRL-Algorithm-Suite-A (CRL-YA) |-> CA-Y-Signed-Objects-Algorithm-Suite-A |-> CA-X-CRL-Algorithm-Suite-A (CRL-XA) |-> CA-X-Signed-Objects-Algorithm-Suite-A CA X-Certificate-Algorithm-Suite-B (Cert-XB) | |-> CA-Y-Certificate-Algorithm-Suite-B (Cert-YB) |-> CA-Z-Certificate-Algorithm-Suite-B (Cert-ZB) |-> CA-Z-CRL-Algorithm-Suite-B (CRL-ZB) |-> CA-Z-Signed-Objects-Algorithm-Suite-B |-> CA-Y-CRL-Algorithm-Suite-B (CRL-YB) |-> CA-Y-Signed-Objects-Algorithm-Suite-B |-> CA-X-CRL-Algorithm-Suite-B (CRL-XB) |-> CA-X-Signed-Objects-Algorithm-Suite-B
Phase 3 starts at the RP Ready Algorithm B Date. During this phase, all signed product sets are available using both algorithm suites and all RPs MUST be able to validate them using either suite. An object that validates using either Algorithm Suite A or Algorithm Suite B MUST be considered valid. It is RECOMMENDED that, in preparation for Phase 4, RPs utilize Suite B as the first and preferred option for validation throughout this phase. Thus, for example, an RP SHOULD try to validate the sets of signed products retrieved from the Algorithm Suite B repository first. If this effort fails (relative to the local validation policy), the RP SHOULD revert to using the Algorithm Suite A repository.
If a substantial number of RPs are unable to process product sets signed with Suite B, the algorithm transition timeline document MUST be reissued, pushing back the date for this and later milestones, as discussed in Section 4.2. Since the Suite B products SHOULD be published at distinct publication points, RPs that cannot process Suite B products can be expected to revert to the Suite A products that still exist. If the Suite B algorithm is deemed unsuitable, the algorithm transition timeline and the algorithm specification documents MUST be replaced. CAs MUST cease accepting requests for certificates under Suite B, and Suite B certificates that have been issued MUST be revoked.
There are no changes to the CA behavior throughout this phase.
Phase 4 starts at the Algorithm A Twilight Date. At that date, the Algorithm A is labeled as “old” and the Algorithm B is labeled as “current”:
Before Twilight --> After Twilight
Algorithm Suite A ("current") --> Algorithm Suite C ("old")
Algorithm Suite B ("new") --> Algorithm Suite A ("current")
During this phase, all signed product sets MUST be issued using Algorithm Suite A (formerly B) and MAY be issued using Algorithm Suite C (formerly A). All signed products sets issued using Suite A MUST be published at their corresponding publication points, but signed products sets issued using Suite C MAY be published at their corresponding publication points. Also, every RP MUST validate signed product sets using Suite A but also MAY validate signed product sets using Suite C. However, RPs SHOULD NOT assume the Suite C repository is complete.
If it is determined that many RPs are not capable of processing the new algorithm suite, the algorithm transition timeline document MUST be reissued pushing back the date for this and the next milestone. The document MUST requires CA to not remove Suite C (old Suite A) product sets if this phase is delayed. If the Suite B algorithm is deemed unsuitable, the algorithm transition timeline and the algorithm specification documents MUST be replaced. CAs MUST cease accepting requests for certificates under Suite B, and Suite B certificates that have been issued MUST be revoked. At this stage, RPs are still capable of processing old Suite A signed products.
The following figure describes a possible status for the repositories of the example CAs. In this case, CA Z no longer issues signed products using the Algorithm Suite C.
CA X-Certificate-Algorithm-Suite-C (Cert-XC) | |-> CA-Y-Certificate-Algorithm-Suite-C (Cert-YC) |-> CA-Y-CRL-Algorithm-Suite-C (CRL-YC) |-> CA-Y-Signed-Objects-Algorithm-Suite-C |-> CA-X-CRL-Algorithm-Suite-C (CRL-XC) |-> CA-X-Signed-Objects-Algorithm-Suite-C CA X-Certificate-Algorithm-Suite-A (Cert-XA) | |-> CA-Y-Certificate-Algorithm-Suite-A (Cert-YA) |-> CA-Z-Certificate-Algorithm-Suite-A (Cert-ZA) |-> CA-Z-CRL-Algorithm-Suite-A (CRL-ZA) |-> CA-Z-Signed-Objects-Algorithm-Suite-A |-> CA-Y-CRL-Algorithm-Suite-A (CRL-YA) |-> CA-Y-Signed-Objects-Algorithm-Suite-A |-> CA-X-CRL-Algorithm-Suite-A (CRL-XA) |-> CA-X-Signed-Objects-Algorithm-Suite-A
The EOL Algorithm Date triggers a return to Phase 0 (steady state). At this phase, ALL signed product sets using Algorithm Suite C MUST be considered invalid. CAs MUST neither issue nor publish signed products using Algorithm Suite C.
This phase closes the loop as Algorithm Suite A is the only required algorithm suite in RPKI.
If it is determined that many RPs are not capable of processing the new algorithm suite, the algorithm transition timeline document MUST be reissued pushing back the date for this milestone. CAs MUST NOT remove Suite C (old Suite A) product sets if this phase is delayed.
The migration described in this document is a top-down process, where two synchronization issues need to be solved between child and parent CAs:
The RPKI provisioning protocol [I-D.ietf-sidr-rescerts-provisioning] supports multiple algorithms suites by implementing different resource classes for each suite. Several different resource classes also may use the same algorithm suite for different resource sets.
A child CA that wants to identify which algorithm suites are supported by its parent CA MUST perform the following tasks:
A child CA that wants to specify an Algorithm Suite to its parent CA (e.g., in a certificate request) MUST perform the following tasks:
Upon receipt of a certificate request from a child CA, a parent CA will verify the PoP of the private key. If a child CA requests issuing a certificate using an algorithm suite that does not match a resource class, the PoP validation will fail and the request will not be performed.
During Phases 1,2,3 and 4, two algorithm suites will be valid simultaneously in RPKI. In this section, we describe the RP behavior when validating instances of the same signed product but signed with different algorithm suites. As a general rule, the validation of signed products using different algorithm suites are independent and the RP MUST NOT keep any relationship between the different hierarchies.
During Phase 1 two (corresponding) files for an object MAY be available for each signed product, one signed under Algorithm Suite A and one under Algorithm Suite B. When an RP validates these signed products, if either instance of an object validates, the product is accepted. A failure to validate one instance of a product, under either algorithm Suite MUST NOT cause the RP to reject the other instance of the product. Because both instances of such products MUST contain the same resources, relying on either instance will yield the same outcome.
During Phases 2 and 3 of this process, two (corresponding) instances of all signed products MUST be available to RPs. As in Phase 1, when an RP validates these signed products, if either instance validates, the product is accepted. A failure to validate one instance of a product, under either algorithm Suite MUST NOT cause the RP to reject the other instance of the product. Also, as above, if only one instance of a signed product can be validated, subordinate products issued under the other (non-validated) algorithm suite cannot be used, and thus SHOULD NOT be processed (or even retrieved).
During Phase 4 two (corresponding) files for an object MAY be available for each signed product, one signed under Algorithm Suite A and one under Algorithm Suite C. When an RP validates these signed products, if either instance of an object validates, the product is accepted. A failure to validate one instance of a product, under either algorithm Suite MUST NOT cause the RP to reject the other instance of the product. Because both instances of such products MUST contain the same resources, relying on either instance will yield the same outcome.
As the algorithm migration process mandates the maintenance of two parallel certificate hierarchies, revocations requests for each algorithm suite MUST be handled independently. A Child CA MUST request revocation of a certificate relative to a specific algorithm suite.
During phase 2 and phase 3, the two parallel certificate hierarchies are designed to carry identical information. Consequently, a child CA requesting the revocation of a certificate during these two phases MUST perform that request for both algorithm suites (A and B). A non-leaf CA is NOT required to verify that its child CAs comply with this requirement.
Key rollover (without algorithm changes) is effected independently for each algorithm suite and MUST follow the process described in [I-D.ietf-sidr-keyroll].
The two parallel hierarchies that will exist during the transition process SHOULD have independent publications points. The repository structures for each algorithm suite are described in [I-D.ietf-sidr-repos-struct].
No IANA requirements
An algorithm transition in RPKI should be a very infrequent event and it requires wide community consensus. The events that may lead to an algorithm transition may be related to a weakness of the cryptographic strength of the algorithm suite in use by RPKI, which is normal to happen over time. The procedure described in this document will take years to complete an algorithm transition. During that time, the RPKI system will be vulnerable to any cryptographic weakness that may have triggered this procedure (i.e. downgrade attack).
This document does not describe an emergency mechanism for algorithm migration. Due to the distributed nature of RPKI, and the very large number of CAs and RPs, the authors do not believe it is feasible to effect an emergency algorithm migration procedure.
If a CA does not complete its migration to the new algorithm suite as described in this document (after the EOL of the "old" algorithm suite), its signed product set will no longer be valid. Consequently, the RPKI may, at the end of Phase 4, have a smaller number of valid signed products than before starting the process. Conversely, a RP that does not follow this process will lose the ability to validate signed products issued under the new algorithm suite. The resulting incomplete view of routing info from the RPKI (as a result of a failure by CAs or RPs to complete the transition) could degrade routing in the public Internet.
The authors would like to acknowledge the work of the SIDR working group co-chairs (Sandra Murphy and Chris Morrow) as well as the contributions given by Geoff Huston, Arturo Servin, Brian Weis, Terry Manderson, Brian Dickson and Danny McPherson.
| [RFC5781] | Weiler, S., Ward, D. and R. Housley, "The rsync URI Scheme", RFC 5781, February 2010. |
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