The Resource Public Key Infrastructure (RPKI) to Router Protocol
draft-ietf-sidr-rpki-rtr-rfc6810-bis-01

Abstract

In order to verifiably validate the origin Autonomous Systems of BGP announcements, routers need a simple but reliable mechanism to receive Resource Public Key Infrastructure (RFC 6480) prefix origin data from a trusted cache. This document describes a protocol to deliver validated prefix origin data to routers.

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 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 October 06, 2014.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

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

Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Glossary
• 3. Deployment Structure
• 4. Operational Overview
• 5. Protocol Data Units (PDUs)
• 5.1. Fields of a PDU
• 5.2. Serial Notify
• 5.3. Serial Query
• 5.4. Reset Query
• 5.5. Cache Response
• 5.6. IPv4 Prefix
• 5.7. IPv6 Prefix
• 5.8. End of Data
• 5.9. Cache Reset
• 5.10. Router Key
• 5.11. Error Report
• 6. Protocol Timing Parameters
• 7. Protocol Version Negotiation
• 8. Protocol Sequences
• 8.1. Start or Restart
• 8.2. Typical Exchange
• 8.3. No Incremental Update Available
• 8.4. Cache Has No Data Available
• 9. Transport
• 9.1. SSH Transport
• 9.2. TLS Transport
• 9.3. TCP MD5 Transport
• 9.4. TCP-AO Transport
• 10. Router-Cache Setup
• 11. Deployment Scenarios
• 12. Error Codes
• 13. Security Considerations
• 14. IANA Considerations
• 15. Acknowledgments
• 16. References
• 16.1. Normative References
• 16.2. Informative References
• Authors' Addresses

1. Introduction

In order to verifiably validate the origin Autonomous Systems (ASes) of BGP announcements, routers need a simple but reliable mechanism to receive Resource Public Key Infrastructure (RPKI) [RFC6480] cryptographically validated prefix origin data from a trusted cache. This document describes a protocol to deliver validated prefix origin data to routers. The design is intentionally constrained to be usable on much of the current generation of ISP router platforms.

Section 3 describes the deployment structure, and Section 4 then presents an operational overview. The binary payloads of the protocol are formally described in Section 5, and the expected PDU sequences are described in Section 8. The transport protocol options are described in Section 9. Section 10 details how routers and caches are configured to connect and authenticate. Section 11 describes likely deployment scenarios. The traditional security and IANA considerations end the document.

The protocol is extensible in order to support new PDUs with new semantics, if deployment experience indicates they are needed. PDUs are versioned should deployment experience call for change.

For an implementation (not interoperability) report, see [I-D.ietf-sidr-rpki-rtr-impl]

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119] only when they appear in all upper case. They may also appear in lower or mixed case as English words, without special meaning.

2. Glossary

The following terms are used with special meaning.

Global RPKI:

The authoritative data of the RPKI are published in a distributed set of servers at the IANA, Regional Internet Registries (RIRs), National Internet Registries (NIRs), and ISPs; see [RFC6481].

Cache:

A coalesced copy of the RPKI, which is periodically fetched/refreshed directly or indirectly from the Global RPKI using the [RFC5781] protocol/tools. Relying party software is used to gather and validate the distributed data of the RPKI into a cache. Trusting this cache further is a matter between the provider of the cache and a relying party.

Serial Number:

A 32-bit strictly increasing unsigned integer which wraps from 2^32-1 to 0. It denotes the logical version of a cache. A cache increments the value when it successfully updates its data from a parent cache or from primary RPKI data. As a cache is receiving, new incoming data and implicit deletes are associated with the new serial but MUST NOT be sent until the fetch is complete. A Serial Number is not commensurate between caches, nor need it be maintained across resets of the cache server. See [RFC1982] on DNS Serial Number Arithmetic for too much detail on the topic.

Session ID:

When a cache server is started, it generates a session identifier to uniquely identify the instance of the cache and to bind it to the sequence of Serial Numbers that cache instance will generate. This allows the router to restart a failed session knowing that the Serial Number it is using is commensurate with that of the cache.

3. Deployment Structure

Deployment of the RPKI to reach routers has a three-level structure as follows:

Global RPKI:

The authoritative data of the RPKI are published in a distributed set of servers, RPKI publication repositories, e.g., the IANA, RIRs, NIRs, and ISPs, see [RFC6481].

Local Caches:

A local set of one or more collected and verified caches. A relying party, e.g., router or other client, MUST have a trust relationship with, and a trusted transport channel to, any authoritative cache(s) it uses.

Routers:

A router fetches data from a local cache using the protocol described in this document. It is said to be a client of the cache. There MAY be mechanisms for the router to assure itself of the authenticity of the cache and to authenticate itself to the cache.

4. Operational Overview

A router establishes and keeps open a connection to one or more caches with which it has client/server relationships. It is configured with a semi-ordered list of caches, and establishes a connection to the most preferred cache, or set of caches, which accept the connections.

The router MUST choose the most preferred, by configuration, cache or set of caches so that the operator may control load on their caches and the Global RPKI.

Periodically, the router sends to the cache the Serial Number of the highest numbered data it has received from that cache, i.e., the router's current Serial Number. When a router establishes a new connection to a cache, or wishes to reset a current relationship, it sends a Reset Query.

The Cache responds with all data records which have Serial Numbers greater than that in the router's query. This may be the null set, in which case the End of Data PDU is still sent. Note that 'greater' must take wrap-around into account, see [RFC1982].

When the router has received all data records from the cache, it sets its current Serial Number to that of the Serial Number in the End of Data PDU.

When the cache updates its database, it sends a Notify message to every currently connected router. This is a hint that now would be a good time for the router to poll for an update, but is only a hint. The protocol requires the router to poll for updates periodically in any case.

Strictly speaking, a router could track a cache simply by asking for a complete data set every time it updates, but this would be very inefficient. The Serial Number based incremental update mechanism allows an efficient transfer of just the data records which have changed since last update. As with any update protocol based on incremental transfers, the router must be prepared to fall back to a full transfer if for any reason the cache is unable to provide the necessary incremental data. Unlike some incremental transfer protocols, this protocol requires the router to make an explicit request to start the fallback process; this is deliberate, as the cache has no way of knowing whether the router has also established sessions with other caches that may be able to provide better service.

As a cache server must evaluate certificates and ROAs (Route Origin Attestations; see [RFC6480]), which are time dependent, servers' clocks MUST be correct to a tolerance of approximately an hour.

5. Protocol Data Units (PDUs)

The exchanges between the cache and the router are sequences of exchanges of the following PDUs according to the rules described in Section 8.

Fields with unspecified content MUST be zero on transmission and MAY be ignored on receipt.

5.1. Fields of a PDU

PDUs contain the following data elements:

Protocol Version:

An eight-bit unsigned integer, currently 1, denoting the version of this protocol.

PDU Type:

An eight-bit unsigned integer, denoting the type of the PDU, e.g., IPv4 Prefix, etc.

Serial Number:

The Serial Number of the RPKI Cache when this set of PDUs was received from an upstream cache server or gathered from the Global RPKI. A cache increments its Serial Number when completing a rigorously validated update from a parent cache or the Global RPKI.

Session ID:

When a cache server is started, it generates a Session ID to identify the instance of the cache and to bind it to the sequence of Serial Numbers that cache instance will generate. This allows the router to restart a failed session knowing that the Serial Number it is using is commensurate with that of the cache. If, at any time, either the router or the cache finds the value of the session identifier is not the same as the other's, they MUST completely drop the session and the router MUST flush all data learned from that cache.

Should a cache erroneously reuse a Session ID so that a router does not realize that the session has changed (old session ID and new session ID have same numeric value), the router may become confused as to the content of the cache. The time it takes the router to discover it is confused will depend on whether the Serial Numbers are also reused. If the Serial Numbers in the old and new sessions are different enough, the cache will respond to the router's Serial Query with a Cache Reset, which will solve the problem. If, however, the Serial Numbers are close, the cache may respond with a Cache Response, which may not be enough to bring the router into sync. In such cases, it's likely but not certain that the router will detect some discrepancy between the state that the cache expects and its own state. For example, the Cache Response may tell the router to drop a record which the router does not hold, or may tell the router to add a record which the router already has. In such cases, a router will detect the error and reset the session. The one case in which the router may stay out of sync is when nothing in the Cache Response contradicts any data currently held by the router.

Using persistent storage for the session identifier or a clock-based scheme for generating session identifiers should avoid the risk of session identifier collisions.

The Session ID might be a pseudo-random value, a strictly increasing value if the cache has reliable storage, etc.

Length:

A 32-bit unsigned integer which has as its value the count of the bytes in the entire PDU, including the eight bytes of header which end with the length field.

Flags:

The lowest order bit of the Flags field is 1 for an announcement and 0 for a withdrawal. For a Prefix PDU (IPv4 or IPv6), the flag indicates whether this PDU announces a new right to announce the prefix or withdraws a previously announced right; a withdraw effectively deletes one previously announced Prefix PDU with the exact same Prefix, Length, Max-Len, and Autonomous System Number (ASN). Similarly, for a Router Key PDU, the flag indicates whether this PDU announces a new Router Key or deletes one previously announced Router Key PDU with the exact same AS Number, subjectKeyIdentifier, and subjectPublicKeyInfo.

Prefix Length:

An 8-bit unsigned integer denoting the shortest prefix allowed for the prefix.

Max Length:

An 8-bit unsigned integer denoting the longest prefix allowed by the prefix. This MUST NOT be less than the Prefix Length element.

Prefix:

The IPv4 or IPv6 prefix of the ROA.

Autonomous System Number:

ASN allowed to announce this prefix, a 32-bit unsigned integer.

Zero:

Fields shown as zero or reserved MUST be zero. The value of such a field MUST be ignored on receipt.

5.2. Serial Notify

The cache notifies the router that the cache has new data.

The Session ID reassures the router that the Serial Numbers are commensurate, i.e., the cache session has not been changed.

Upon receipt of a Serial Notify PDU, the router MAY issue an immediate Serial Query or Reset Query without waiting for the Refresh Interval timer to expire.

Serial Notify is the only message that the cache can send that is not in response to a message from the router.

0          8          16         24        31
.-------------------------------------------.
| Protocol |   PDU    |                     |
| Version  |   Type   |     Session ID      |
|    1     |    0     |                     |
+-------------------------------------------+
|                                           |
|                Length=12                  |
|                                           |
+-------------------------------------------+
|                                           |
|               Serial Number               |
|                                           |
`-------------------------------------------'
          

5.3. Serial Query

Serial Query: The router sends Serial Query to ask the cache for all payload PDUs which have Serial Numbers higher than the Serial Number in the Serial Query.

The cache replies to this query with a Cache Response PDU (Section 5.5) if the cache has a, possibly null, record of the changes since the Serial Number specified by the router. If there have been no changes since the router last queried, the cache sends an End Of Data PDU.

If the cache does not have the data needed to update the router, perhaps because its records do not go back to the Serial Number in the Serial Query, then it responds with a Cache Reset PDU (Section 5.9).

The Session ID tells the cache what instance the router expects to ensure that the Serial Numbers are commensurate, i.e., the cache session has not been changed.

0          8          16         24        31
.-------------------------------------------.
| Protocol |   PDU    |                     |
| Version  |   Type   |     Session ID      |
|    1     |    1     |                     |
+-------------------------------------------+
|                                           |
|                 Length=12                 |
|                                           |
+-------------------------------------------+
|                                           |
|               Serial Number               |
|                                           |
`-------------------------------------------'
          

5.4. Reset Query

Reset Query: The router tells the cache that it wants to receive the total active, current, non-withdrawn database. The cache responds with a Cache Response PDU (Section 5.5).

0          8          16         24        31
.-------------------------------------------.
| Protocol |   PDU    |                     |
| Version  |   Type   |    reserved = zero  |
|    1     |    2     |                     |
+-------------------------------------------+
|                                           |
|                 Length=8                  |
|                                           |
`-------------------------------------------'
          

5.5. Cache Response

Cache Response: The cache responds with zero or more payload PDUs. When replying to a Serial Query request (Section 5.3), the cache sends the set of all data records it has with Serial Numbers greater than that sent by the client router. When replying to a Reset Query, the cache sends the set of all data records it has; in this case, the withdraw/announce field in the payload PDUs MUST have the value 1 (announce).

In response to a Reset Query, the new value of the Session ID tells the router the instance of the cache session for future confirmation. In response to a Serial Query, the Session ID being the same reassures the router that the Serial Numbers are commensurate, i.e., the cache session has not changed.

0          8          16         24        31
.-------------------------------------------.
| Protocol |   PDU    |                     |
| Version  |   Type   |     Session ID      |
|    1     |    3     |                     |
+-------------------------------------------+
|                                           |
|                 Length=8                  |
|                                           |
`-------------------------------------------'
          

5.6. IPv4 Prefix

0          8          16         24        31
.-------------------------------------------.
| Protocol |   PDU    |                     |
| Version  |   Type   |    reserved = zero  |
|    1     |    4     |                     |
+-------------------------------------------+
|                                           |
|                 Length=20                 |
|                                           |
+-------------------------------------------+
|          |  Prefix  |   Max    |          |
|  Flags   |  Length  |  Length  |   zero   |
|          |   0..32  |   0..32  |          |
+-------------------------------------------+
|                                           |
|                IPv4 Prefix                |
|                                           |
+-------------------------------------------+
|                                           |
|         Autonomous System Number          |
|                                           |
`-------------------------------------------'
          

The lowest order bit of the Flags field is 1 for an announcement and 0 for a withdrawal.

In the RPKI, nothing prevents a signing certificate from issuing two identical ROAs. In this case, there would be no semantic difference between the objects, merely a process redundancy.

In the RPKI, there is also an actual need for what might appear to a router as identical IPvX PDUs. This can occur when an upstream certificate is being reissued or there is an address ownership transfer up the validation chain. The ROA would be identical in the router sense, i.e., have the same {Prefix, Len, Max-Len, ASN}, but a different validation path in the RPKI. This is important to the RPKI, but not to the router.

The cache server MUST ensure that it has told the router client to have one and only one IPvX PDU for a unique {Prefix, Len, Max-Len, ASN} at any one point in time. Should the router client receive an IPvX PDU with a {Prefix, Len, Max-Len, ASN} identical to one it already has active, it SHOULD raise a Duplicate Announcement Received error.

5.7. IPv6 Prefix

0          8          16         24        31
.-------------------------------------------.
| Protocol |   PDU    |                     |
| Version  |   Type   |    reserved = zero  |
|    1     |    6     |                     |
+-------------------------------------------+
|                                           |
|                 Length=32                 |
|                                           |
+-------------------------------------------+
|          |  Prefix  |   Max    |          |
|  Flags   |  Length  |  Length  |   zero   |
|          |  0..128  |  0..128  |          |
+-------------------------------------------+
|                                           |
+---                                     ---+
|                                           |
+---            IPv6 Prefix              ---+
|                                           |
+---                                     ---+
|                                           |
+-------------------------------------------+
|                                           |
|         Autonomous System Number          |
|                                           |
`-------------------------------------------'
          

Analogous to the IPv4 Prefix PDU, it has 96 more bits and no magic.

5.8. End of Data

End of Data: The cache tells the router it has no more data for the request.

The Session ID MUST be the same as that of the corresponding Cache Response which began the, possibly null, sequence of data PDUs.

0          8          16         24        31
.-------------------------------------------.
| Protocol |   PDU    |                     |
| Version  |   Type   |     Session ID      |
|    1     |    7     |                     |
+-------------------------------------------+
|                                           |
|                 Length=24                 |
|                                           |
+-------------------------------------------+
|                                           |
|               Serial Number               |
|                                           |
+-------------------------------------------+
|                                           |
|              Refresh Interval             |
|                                           |
+-------------------------------------------+
|                                           |
|               Retry Interval              |
|                                           |
+-------------------------------------------+
|                                           |
|              Expire Interval              |
|                                           |
`-------------------------------------------'
          

The Refresh Interval, Retry Interval, and Expire Interval are all 32-bit elapsed times measured in seconds, and express the timing parameters that the cache expects the router to use to decide when next to send the cache another Serial Query or Reset Query PDU. See Section 6 for an explanation of the use and the range of allowed values for these parameters.

5.9. Cache Reset

The cache may respond to a Serial Query informing the router that the cache cannot provide an incremental update starting from the Serial Number specified by the router. The router must decide whether to issue a Reset Query or switch to a different cache.

0          8          16         24        31
.-------------------------------------------.
| Protocol |   PDU    |                     |
| Version  |   Type   |    reserved = zero  |
|    1     |    8     |                     |
+-------------------------------------------+
|                                           |
|                 Length=8                  |
|                                           |
`-------------------------------------------'
          

5.10. Router Key

0          8          16         24        31
.-------------------------------------------.
| Protocol |   PDU    |          |          |
| Version  |   Type   |   Flags  |   zero   |
|     1    |    9     |          |          |
+-------------------------------------------+
|                                           |
|                  Length                   |
|                                           |
+-------------------------------------------+
|                                           |
|          Subject Key Identifier           |
|                 20 octets                 |
|                                           |
+-------------------------------------------+
|                                           |
|                 AS Number                 |
|                                           |
+-------------------------------------------+
|                                           |
|          Subject Public Key Info          |
|                                           |
`-------------------------------------------'
	  

In addition to the normal boilerplate fields of an RPKI-Router PDU, the Router Key PDU has the following fields.

Subject Key Identifier

is the 20-octet subjectKeyIdentifier (SKI) value for the Router Key, as described in [RFC6487].

AS Number

one Autonomous System Number.

Subject Public Key Info

is the Router Key's subjectPublicKeyInfo as described in [I-D.ietf-sidr-bgpsec-algs]. This is the full ASN.1 DER encoding of the subjectPublicKeyInfo, including the ASN.1 tag and length values of the subjectPublicKeyInfo SEQUENCE.

The cache server MUST ensure that it has told the router client to have one and only one Router Key PDU for a unique {SKI, ASN, Subject Public Key} at any one point in time. Should the router client receive a Router Key PDU with a {SKI, ASN, Subject Public Key} identical to one it already has active, it SHOULD raise a Duplicate Announcement Received error.

Note that a particular ASN may appear in multiple Router Key PDUs with different Subject Public Key values, while a particular Subject Public Key value may appear in multiple Router Key PDUs with different ASNs. In the interest of keeping the announcement and withdrawal semantics as simple as possible for the router, this protocol makes no attempt to compress either of these cases.

Also note that it is possible, albeit very unlikely, for multiple distinct Subject Public Key values to hash to the same SKI. For this reason, implementations MUST compare Subject Public Key values as well as SKIs when detecting duplicate PDUs.

5.11. Error Report

This PDU is used by either party to report an error to the other.

Error reports are only sent as responses to other PDUs.

The Error Code is described in Section 12.

If the error is generic (e.g., "Internal Error") and not associated with the PDU to which it is responding, the Erroneous PDU field MUST be empty and the Length of Encapsulated PDU field MUST be zero.

An Error Report PDU MUST NOT be sent for an Error Report PDU. If an erroneous Error Report PDU is received, the session SHOULD be dropped.

If the error is associated with a PDU of excessive length, i.e., too long to be any legal PDU other than another Error Report, or a possibly corrupt length, the Erroneous PDU field MAY be truncated.

The diagnostic text is optional; if not present, the Length of Error Text field MUST be zero. If error text is present, it MUST be a string in UTF-8 encoding (see [RFC3269]).

0          8          16         24        31
.-------------------------------------------.
| Protocol |   PDU    |                     |
| Version  |   Type   |     Error Code      |
|    1     |    10    |                     |
+-------------------------------------------+
|                                           |
|                  Length                   |
|                                           |
+-------------------------------------------+
|                                           |
|       Length of Encapsulated PDU          |
|                                           |
+-------------------------------------------+
|                                           |
~           Copy of Erroneous PDU           ~
|                                           |
+-------------------------------------------+
|                                           |
|           Length of Error Text            |
|                                           |
+-------------------------------------------+
|                                           |
|              Arbitrary Text               |
|                    of                     |
~          Error Diagnostic Message         ~
|                                           |
`-------------------------------------------'
          

6. Protocol Timing Parameters

Since the data the cache distributes via the rpki-rtr protocol are retrieved from the Global RPKI system at intervals which are only known to the cache, only the cache can really know how frequently it makes sense for the router to poll the cache, or how long the data are likely to remain valid (or, at least, unchanged). For this reason, as well as to allow the cache some control over the load placed on it by its client routers, the End Of Data PDU includes three values that allow the router to communicate timing parameters to the router.

Refresh Interval:

This parameter tells the router how long to wait before next attempting to poll the cache, using a Serial Query or Reset Query PDU. Note that receipt of a Serial Notify PDU overrides this interval and allows the router to issue an immediate query without waiting for the Refresh Interval to expire. Countdown for this timer starts upon receipt of the containing End Of Data PDU.

Minimum allowed value:

120 seconds (two minutes).

Maximum allowed value:

86400 seconds (one day).

Recommended default:

3600 seconds (one hour).

Retry Interval:

This parameter tells the router how long to wait before retrying a failed Serial Query or Reset Query. Countdown for this timer starts upon failure of the query, and restarts after each subsequent failure until a query succeeds.

Minimum allowed value:

120 seconds (two minutes).

Maximum allowed value:

7200 seconds (two hours).

Recommended default:

600 seconds (ten minutes).

Expire Interval:

This parameter tells the router how long it can continue to use the current version of the data while unable to perform a sucessful query. Countdown for this timer starts upon receipt of the containing End Of Data PDU.

Minimum allowed value:

600 seconds (ten minutes).

Maximum allowed value:

172800 seconds (two days).

Recommended default:

7200 seconds (two hours).

If the router has never issued a succesful query against a particular cache, it retry periodically using the default Retry Interval, above.

7. Protocol Version Negotiation

A router MUST start each transport session by issuing either a Reset Query or a Serial Query. This query will tell the cache which version of this protocol the router implements.

If a cache which supports version 1 receieves a query from a router which specifies version 0, the cache MUST downgrade to protocol version 0 [RFC6810] or terminate the session.

If a router which supports version 1 sends a query to a cache which only supports version 0, one of two things will happen.

1. The cache may terminate the connection, perhaps with a version 0 Error Report PDU. In this case the router MAY retry the connection using protocol version 0.
2. The cache may reply with a version 0 response. In this case the router MUST either downgrade to version 0 or terminate the connection.

In any of the downgraded combinations above, the new features of version 1 will not be available.

8. Protocol Sequences

The sequences of PDU transmissions fall into three conversations as follows:

8.1. Start or Restart

Cache                         Router
  ~                             ~
  | <----- Reset Query -------- | R requests data (or Serial Query)
  |                             |
  | ----- Cache Response -----> | C confirms request
  | ------- IPvX Prefix ------> | C sends zero or more
  | ------- IPvX Prefix ------> |   IPv4 and IPv6 Prefix
  | ------- IPvX Prefix ------> |   Payload PDUs
  | ------  End of Data ------> | C sends End of Data
  |                             |   and sends new serial
  ~                             ~
          

When a transport session is first established, the router MAY send a Reset Query and the cache responds with a data sequence of all data it contains.

Alternatively, if the router has significant unexpired data from a broken session with the same cache, it MAY start with a Serial Query containing the Session ID from the previous session to ensure the Serial Numbers are commensurate.

This Reset Query sequence is also used when the router receives a Cache Reset, chooses a new cache, or fears that it has otherwise lost its way.

The router MUST send either a Reset Query or a Serial Query when starting a transport session, in order to confirm that router and cache are speaking compatible versions of the protocol. See Section 7 for details on version negotiation.

To limit the length of time a cache must keep the data necessary to generate incremental updates, a router MUST send either a Serial Query or a Reset Query periodically. This also acts as a keep-alive at the application layer. See Section 6 for details on the required polling frequency.

8.2. Typical Exchange

Cache                         Router
  ~                             ~
  | -------- Notify ----------> |  (optional)
  |                             |
  | <----- Serial Query ------- | R requests data
  |                             |
  | ----- Cache Response -----> | C confirms request
  | ------- IPvX Prefix ------> | C sends zero or more
  | ------- IPvX Prefix ------> |   IPv4 and IPv6 Prefix
  | ------- IPvX Prefix ------> |   Payload PDUs
  | ------  End of Data ------> | C sends End of Data
  |                             |   and sends new serial
  ~                             ~
          

The cache server SHOULD send a notify PDU with its current Serial Number when the cache's serial changes, with the expectation that the router MAY then issue a Serial Query earlier than it otherwise might. This is analogous to DNS NOTIFY in [RFC1996]. The cache MUST rate limit Serial Notifies to no more frequently than one per minute.

When the transport layer is up and either a timer has gone off in the router, or the cache has sent a Notify, the router queries for new data by sending a Serial Query, and the cache sends all data newer than the serial in the Serial Query.

To limit the length of time a cache must keep old withdraws, a router MUST send either a Serial Query or a Reset Query periodially. See Section 6 for details on the required polling frequency.

8.3. No Incremental Update Available

Cache                         Router
  ~                             ~
  | <-----  Serial Query ------ | R requests data
  | ------- Cache Reset ------> | C cannot supply update
  |                             |   from specified serial
  | <------ Reset Query ------- | R requests new data
  | ----- Cache Response -----> | C confirms request
  | ------- IPvX Prefix ------> | C sends zero or more
  | ------- IPvX Prefix ------> |   IPv4 and IPv6 Prefix
  | ------- IPvX Prefix ------> |   Payload PDUs
  | ------  End of Data ------> | C sends End of Data
  |                             |   and sends new serial
  ~                             ~
          

The cache may respond to a Serial Query with a Cache Reset, informing the router that the cache cannot supply an incremental update from the Serial Number specified by the router. This might be because the cache has lost state, or because the router has waited too long between polls and the cache has cleaned up old data that it no longer believes it needs, or because the cache has run out of storage space and had to expire some old data early. Regardless of how this state arose, the cache replies with a Cache Reset to tell the router that it cannot honor the request. When a router receives this, the router SHOULD attempt to connect to any more preferred caches in its cache list. If there are no more preferred caches, it MUST issue a Reset Query and get an entire new load from the cache.

8.4. Cache Has No Data Available

Cache                         Router
  ~                             ~
  | <-----  Serial Query ------ | R requests data
  | ---- Error Report PDU ----> | C No Data Available
  ~                             ~

Cache                         Router
  ~                             ~
  | <-----  Reset Query ------- | R requests data
  | ---- Error Report PDU ----> | C No Data Available
  ~                             ~
          

The cache may respond to either a Serial Query or a Reset Query informing the router that the cache cannot supply any update at all. The most likely cause is that the cache has lost state, perhaps due to a restart, and has not yet recovered. While it is possible that a cache might go into such a state without dropping any of its active sessions, a router is more likely to see this behavior when it initially connects and issues a Reset Query while the cache is still rebuilding its database.

When a router receives this kind of error, the router SHOULD attempt to connect to any other caches in its cache list, in preference order. If no other caches are available, the router MUST issue periodic Reset Queries until it gets a new usable load from the cache.

9. Transport

The transport-layer session between a router and a cache carries the binary PDUs in a persistent session.

To prevent cache spoofing and DoS attacks by illegitimate routers, it is highly desirable that the router and the cache be authenticated to each other. Integrity protection for payloads is also desirable to protect against monkey-in-the-middle (MITM) attacks. Unfortunately, there is no protocol to do so on all currently used platforms. Therefore, as of the writing of this document, there is no mandatory-to-implement transport which provides authentication and integrity protection.

To reduce exposure to dropped but non-terminated sessions, both caches and routers SHOULD enable keep-alives when available in the chosen transport protocol.

It is expected that, when the TCP Authentication Option (TCP-AO) [RFC5925] is available on all platforms deployed by operators, it will become the mandatory-to-implement transport.

Caches and routers MUST implement unprotected transport over TCP using a port, rpki-rtr (323); see Section 14. Operators SHOULD use procedural means, e.g., access control lists (ACLs), to reduce the exposure to authentication issues.

Caches and routers SHOULD use TCP-AO, SSHv2, TCP MD5, or IPsec transport.

If unprotected TCP is the transport, the cache and routers MUST be on the same trusted and controlled network.

If available to the operator, caches and routers MUST use one of the following more protected protocols.

Caches and routers SHOULD use TCP-AO transport [RFC5925] over the rpki-rtr port.

Caches and routers MAY use SSHv2 transport [RFC4252] using a the normal SSH port. For an example, see Section 9.1.

Caches and routers MAY use TCP MD5 transport [RFC2385] using the rpki-rtr port. Note that TCP MD5 has been obsoleted by TCP-AO [RFC5925].

Caches and routers MAY use IPsec transport [RFC4301] using the rpki-rtr port.

Caches and routers MAY use TLS transport [RFC5246] using a port, rpki-rtr-tls (324); see Section 14.

9.1. SSH Transport

To run over SSH, the client router first establishes an SSH transport connection using the SSHv2 transport protocol, and the client and server exchange keys for message integrity and encryption. The client then invokes the "ssh-userauth" service to authenticate the application, as described in the SSH authentication protocol [RFC4252]. Once the application has been successfully authenticated, the client invokes the "ssh-connection" service, also known as the SSH connection protocol.

After the ssh-connection service is established, the client opens a channel of type "session", which results in an SSH session.

Once the SSH session has been established, the application invokes the application transport as an SSH subsystem called "rpki-rtr". Subsystem support is a feature of SSH version 2 (SSHv2) and is not included in SSHv1. Running this protocol as an SSH subsystem avoids the need for the application to recognize shell prompts or skip over extraneous information, such as a system message that is sent at shell start-up.

It is assumed that the router and cache have exchanged keys out of band by some reasonably secured means.

Cache servers supporting SSH transport MUST accept RSA and Digital Signature Algorithm (DSA) authentication and SHOULD accept Elliptic Curve Digital Signature Algorithm (ECDSA) authentication. User authentication MUST be supported; host authentication MAY be supported. Implementations MAY support password authentication. Client routers SHOULD verify the public key of the cache to avoid monkey-in-the-middle attacks.

9.2. TLS Transport

Client routers using TLS transport MUST present client-side certificates to authenticate themselves to the cache in order to allow the cache to manage the load by rejecting connections from unauthorized routers. In principle, any type of certificate and certificate authority (CA) may be used; however, in general, cache operators will wish to create their own small-scale CA and issue certificates to each authorized router. This simplifies credential rollover; any unrevoked, unexpired certificate from the proper CA may be used.

Certificates used to authenticate client routers in this protocol MUST include a subjectAltName extension [RFC5280] containing one or more iPAddress identities; when authenticating the router's certificate, the cache MUST check the IP address of the TLS connection against these iPAddress identities and SHOULD reject the connection if none of the iPAddress identities match the connection.

Routers MUST also verify the cache's TLS server certificate, using subjectAltName dNSName identities as described in [RFC6125], to avoid monkey-in-the-middle attacks. The rules and guidelines defined in [RFC6125] apply here, with the following considerations:

• Support for DNS-ID identifier type (that is, the dNSName identity in the subjectAltName extension) is REQUIRED in rpki-rtr server and client implementations which use TLS. Certification authorities which issue rpki-rtr server certificates MUST support the DNS-ID identifier type, and the DNS-ID identifier type MUST be present in rpki-rtr server certificates.
• DNS names in rpki-rtr server certificates SHOULD NOT contain the wildcard character "*".
• rpki-rtr implementations which use TLS MUST NOT use CN-ID identifiers; a CN field may be present in the server certificate's subject name, but MUST NOT be used for authentication within the rules described in [RFC6125].
• The client router MUST set its "reference identifier" to the DNS name of the rpki-rtr cache.

9.3. TCP MD5 Transport

If TCP MD5 is used, implementations MUST support key lengths of at least 80 printable ASCII bytes, per Section 4.5 of [RFC2385]. Implementations MUST also support hexadecimal sequences of at least 32 characters, i.e., 128 bits.

Key rollover with TCP MD5 is problematic. Cache servers SHOULD support [RFC4808].

9.4. TCP-AO Transport

Implementations MUST support key lengths of at least 80 printable ASCII bytes. Implementations MUST also support hexadecimal sequences of at least 32 characters, i.e., 128 bits. Message Authentication Code (MAC) lengths of at least 96 bits MUST be supported, per Section 5.1 of [RFC5925].

The cryptographic algorithms and associcated parameters described in [RFC5926] MUST be supported.

10. Router-Cache Setup

A cache has the public authentication data for each router it is configured to support.

A router may be configured to peer with a selection of caches, and a cache may be configured to support a selection of routers. Each must have the name of, and authentication data for, each peer. In addition, in a router, this list has a non-unique preference value for each server. This preference merely denotes proximity, not trust, preferred belief, etc. The client router attempts to establish a session with each potential serving cache in preference order, and then starts to load data from the most preferred cache to which it can connect and authenticate. The router's list of caches has the following elements:

Preference:

An unsigned integer denoting the router's preference to connect to that cache; the lower the value, the more preferred.

Name:

The IP address or fully qualified domain name of the cache.

Key:

Any needed public key of the cache.

MyKey:

Any needed private key or certificate of this client.

Due to the distributed nature of the RPKI, caches simply cannot be rigorously synchronous. A client may hold data from multiple caches but MUST keep the data marked as to source, as later updates MUST affect the correct data.

Just as there may be more than one covering ROA from a single cache, there may be multiple covering ROAs from multiple caches. The results are as described in [RFC6811].

If data from multiple caches are held, implementations MUST NOT distinguish between data sources when performing validation.

When a more preferred cache becomes available, if resources allow, it would be prudent for the client to start fetching from that cache.

The client SHOULD attempt to maintain at least one set of data, regardless of whether it has chosen a different cache or established a new connection to the previous cache.

A client MAY drop the data from a particular cache when it is fully in sync with one or more other caches.

A client SHOULD delete the data from a cache when it has been unable to refresh from that cache for a configurable timer value. The default for that value is twice the polling period for that cache.

If a client loses connectivity to a cache it is using, or otherwise decides to switch to a new cache, it SHOULD retain the data from the previous cache until it has a full set of data from one or more other caches. Note that this may already be true at the point of connection loss if the client has connections to more than one cache.

11. Deployment Scenarios

For illustration, we present three likely deployment scenarios.

Small End Site:

The small multihomed end site may wish to outsource the RPKI cache to one or more of their upstream ISPs. They would exchange authentication material with the ISP using some out-of-band mechanism, and their router(s) would connect to the cache(s) of one or more upstream ISPs. The ISPs would likely deploy caches intended for customer use separately from the caches with which their own BGP speakers peer.

Large End Site:

A larger multihomed end site might run one or more caches, arranging them in a hierarchy of client caches, each fetching from a serving cache which is closer to the Global RPKI. They might configure fall-back peerings to upstream ISP caches.

ISP Backbone:

A large ISP would likely have one or more redundant caches in each major point of presence (PoP), and these caches would fetch from each other in an ISP-dependent topology so as not to place undue load on the Global RPKI.

Experience with large DNS cache deployments has shown that complex topologies are ill-advised as it is easy to make errors in the graph, e.g., not maintain a loop-free condition.

Of course, these are illustrations and there are other possible deployment strategies. It is expected that minimizing load on the Global RPKI servers will be a major consideration.

To keep load on Global RPKI services from unnecessary peaks, it is recommended that primary caches which load from the distributed Global RPKI not do so all at the same times, e.g., on the hour. Choose a random time, perhaps the ISP's AS number modulo 60 and jitter the inter-fetch timing.

12. Error Codes

This section contains a preliminary list of error codes. The authors expect additions to the list during development of the initial implementations. There is an IANA registry where valid error codes are listed; see Section 14. Errors which are considered fatal SHOULD cause the session to be dropped.

0: Corrupt Data (fatal):

The receiver believes the received PDU to be corrupt in a manner not specified by other error codes.

1: Internal Error (fatal):

The party reporting the error experienced some kind of internal error unrelated to protocol operation (ran out of memory, a coding assertion failed, et cetera).

2: No Data Available:

The cache believes itself to be in good working order, but is unable to answer either a Serial Query or a Reset Query because it has no useful data available at this time. This is likely to be a temporary error, and most likely indicates that the cache has not yet completed pulling down an initial current data set from the Global RPKI system after some kind of event that invalidated whatever data it might have previously held (reboot, network partition, et cetera).

3: Invalid Request (fatal):

The cache server believes the client's request to be invalid.

4: Unsupported Protocol Version (fatal):

The Protocol Version is not known by the receiver of the PDU.

5: Unsupported PDU Type (fatal):

The PDU Type is not known by the receiver of the PDU.

6: Withdrawal of Unknown Record (fatal):

The received PDU has Flag=0 but a matching record ({Prefix, Len, Max-Len, ASN} tuple for an IPvX PDU, {SKI, ASN, Subject Public Key} tuple for a Router Key PDU) does not exist in the receiver's database.

7: Duplicate Announcement Received (fatal):

The received PDU has Flag=1 but a matching record ({Prefix, Len, Max-Len, ASN} tuple for an IPvX PDU, {SKI, ASN, Subject Public Key} tuple for a Router Key PDU) is already active in the router.

13. Security Considerations

As this document describes a security protocol, many aspects of security interest are described in the relevant sections. This section points out issues which may not be obvious in other sections.

Cache Validation:

In order for a collection of caches as described in Section 11 to guarantee a consistent view, they need to be given consistent trust anchors to use in their internal validation process. Distribution of a consistent trust anchor is assumed to be out of band.

Cache Peer Identification:

The router initiates a transport session to a cache, which it identifies by either IP address or fully qualified domain name. Be aware that a DNS or address spoofing attack could make the correct cache unreachable. No session would be established, as the authorization keys would not match.

Transport Security:

The RPKI relies on object, not server or transport, trust. That is, the IANA root trust anchor is distributed to all caches through some out-of-band means, and can then be used by each cache to validate certificates and ROAs all the way down the tree. The inter-cache relationships are based on this object security model; hence, the inter-cache transport can be lightly protected.

But, this protocol document assumes that the routers cannot do the validation cryptography. Hence, the last link, from cache to router, is secured by server authentication and transport-level security. This is dangerous, as server authentication and transport have very different threat models than object security.

So, the strength of the trust relationship and the transport between the router(s) and the cache(s) are critical. You're betting your routing on this.

While we cannot say the cache must be on the same LAN, if only due to the issue of an enterprise wanting to off-load the cache task to their upstream ISP(s), locality, trust, and control are very critical issues here. The cache(s) really SHOULD be as close, in the sense of controlled and protected (against DDoS, MITM) transport, to the router(s) as possible. It also SHOULD be topologically close so that a minimum of validated routing data are needed to bootstrap a router's access to a cache.

The identity of the cache server SHOULD be verified and authenticated by the router client, and vice versa, before any data are exchanged.

Transports which cannot provide the necessary authentication and integrity (see Section 9) must rely on network design and operational controls to provide protection against spoofing/corruption attacks. As pointed out in Section 9, TCP-AO is the long-term plan. Protocols which provide integrity and authenticity SHOULD be used, and if they cannot, i.e., TCP is used as the transport, the router and cache MUST be on the same trusted, controlled network.

14. IANA Considerations

            rpki-rtr
            rpki-rtr-tls
          

IANA has assigned "well-known" TCP Port Numbers to the RPKI-Router Protocol for the following, see Section 9:

           Protocol   PDU
           Version    Type  Description
           --------   ----  ---------------
               0        0   Serial Notify
               0        1   Serial Query
               0        2   Reset Query
               0        3   Cache Response
               0        4   IPv4 Prefix
               0        6   IPv6 Prefix
               0        7   End of Data
               0        8   Cache Reset
               0       10   Error Report
               0      255   Reserved
          

IANA has created a registry for tuples of Protocol Version / PDU Type, each of which may range from 0 to 255. The name of the registry is "rpki-rtr-pdu". The policy for adding to the registry is RFC Required per [RFC5226], either Standards Track or Experimental. The initial entries are as follows:

           Error
           Code    Description
           -----   ----------------
               0   Corrupt Data
               1   Internal Error
               2   No Data Available
               3   Invalid Request
               4   Unsupported Protocol Version
               5   Unsupported PDU Type
               6   Withdrawal of Unknown Record
               7   Duplicate Announcement Received
             255   Reserved
          

IANA has created a registry for Error Codes 0 to 255. The name of the registry is "rpki-rtr-error". The policy for adding to the registry is Expert Review per [RFC5226], where the responsible IESG Area Director should appoint the Expert Reviewer. The initial entries are as follows:

IANA has added an SSH Connection Protocol Subsystem Name, as defined in [RFC4250], of "rpki-rtr".

15. Acknowledgments

The authors wish to thank Steve Bellovin, Tim Bruijnzeels, Rex Fernando, Paul Hoffman, Russ Housley, Pradosh Mohapatra, Keyur Patel, David Mandelberg, Sandy Murphy, Robert Raszuk, John Scudder, Ruediger Volk, Matthias Waehlisch, and David Ward. Particular thanks go to Hannes Gredler for showing us the dangers of unnecessary fields.

16. References

16.1. Normative References

16.2. Informative References

Authors' Addresses