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Versions: (draft-bgpsec-spec) 00 draft-ietf-sidr-bgpsec-protocol

Network Working Group                                   M. Lepinski, Ed.
Internet-Draft                                                       BBN
Intended status: Standards Track                           March 7, 2011
Expires: September 8, 2011


                     BGPSEC Protocol Specification
                 draft-lepinski-bgpsec-protocol-00.txt

Abstract

   This document describes BGPSEC, a mechanism for providing path
   security for BGP route advertisements.  BGPSEC is implemented via a
   new optional non-transitive BGP path attribute.

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 [4].

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 September 8, 2011.

Copyright Notice

   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



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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  BGPSEC Negotiation . . . . . . . . . . . . . . . . . . . . . .  3
   3.  The BGPSEC_Path_Signatures Attribute . . . . . . . . . . . . .  5
   4.  Generating a BGPSEC Update . . . . . . . . . . . . . . . . . .  7
     4.1.  Originating a New BGPSEC Update  . . . . . . . . . . . . .  8
     4.2.  Propagating a Route Advertisement  . . . . . . . . . . . . 11
   5.  Validating a BGPSEC Update . . . . . . . . . . . . . . . . . . 13
     5.1.  Validation Algorithm . . . . . . . . . . . . . . . . . . . 14
   6.  Algorithms and Extensibility . . . . . . . . . . . . . . . . . 18
     6.1.  Algorithm Suite Considerations . . . . . . . . . . . . . . 18
     6.2.  Extensibility Considerations . . . . . . . . . . . . . . . 19
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 22
     8.1.  Authors  . . . . . . . . . . . . . . . . . . . . . . . . . 22
     8.2.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . 23
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24


























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1.  Introduction

   This document describes BGPSEC, a mechanism for providing path
   security for BGP route advertisements.  That is, a BGP speaker who
   receives a valid BGPSEC update has cryptographic assurance that the
   advertised route has the following two properties:

   1.  The route was originated by an AS that has been explicitly
       authorized by the holder of the IP address prefix to originate
       route advertisements for that prefix.

   2.  Every AS listed in the AS_Path attribute of the update explicitly
       authorized the advertisement of the route to the subsequent AS in
       the AS_Path.

   This document specifies a new optional (non-transitive) BGP path
   attribute, BGPSEC_Path_Signatures.  It also describes how a BGPSEC-
   compliant BGP speaker (referred to hereafter as a BGPSEC speaker) can
   generate, propagate, and validate BGP update messages containing this
   attribute to obtain the above assurances.

   BGPSEC relies on the Resource Public Key Infrastructure (RPKI)
   certificates that attest to the allocation of AS number and IP
   address resources.  (For more information on the RPKI, see [7] and
   the documents referenced therein.)  Any BGPSEC speaker who wishes to
   send BGP update messages to external peers (eBGP) containing the
   BGPSEC_Path_Signatures must have an RPKI end-entity certificate (as
   well as the associated private signing key) corresponding to the
   BGPSEC speaker's AS number.  Note, however, that a BGPSEC speaker
   does not require such a certificate in order to validate update
   messages containing the BGPSEC_Path_Signatures attribute.


2.  BGPSEC Negotiation

   This document defines a new BGP capability [3]that allows a BGP
   speaker to advertise to its neighbors the ability to send and/or
   receive BGPSEC update messages (i.e., update messages containing the
   BGPSEC_Path_Signatures attribute).

   This capability has capability code : TBD

   The capability length for this capability MUST be set to 3.

   The three octets of the capability value are specified as follows.






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                             Capability Value:

                    0        1        2  3       4 5 6 7
                 +---------------------------------------+
                 | Send | Receive | Reserved  |  Version |
                 +---------------------------------------+
                 |               AFI                     |
                 +---------------------------------------+
                 |                                       |
                 +---------------------------------------+

   The high order bit (bit 0) of the first octet is set to 1 to indicate
   that the sender is able to send BGPSEC update messages, and is set to
   zero otherwise.  The next highest order bit (bit 1) of this octet is
   set to 1 to indicate that the sender is able to receive BGPSEC update
   messages, and is set to zero otherwise.  The next two bits of the
   capability value (bits 2 and 3) are reserved for future use.

   The four low order bits (4, 5, 6 and 7) of the first octet indicate
   the version of BGPSEC for which the BGP speaker is advertising
   support.  This document defines only BGPSEC version 0 (all four bits
   set to zero).  Other versions of BGPSEC may be defined in future
   documents.  A BGPSEC speaker MAY advertise support for multiple
   versions of BGPSEC by including multiple versions of the BGPSEC
   capability in its BGP OPEN message.

   If there does not exist at least one version of BGPSEC that is
   supported by both peers in a BGP session, then the use of BGPSEC has
   not been negotiated.  (That is, in such a case, messages containing
   the BGPSEC_Path_Signatures MUST not be sent.)

   If version 0 is the only version of BGPSEC for which both peers (in a
   BGP session) advertise support, then the use of BGPSEC has been
   negotiated and the BGPSEC peers MUST adhere to the specification of
   BGPSEC provided in this document.  (If there are multiple versions of
   BGPSEC which are supported by both peer, then the behavior of those
   peers is outside the scope of this document.)

   The second two octets contain the 16-bit Address Family Identifier
   (AFI) which indicates the address family for which the BGPSEC speaker
   is advertising support for BGPSEC.  This document only specifies
   BGPSEC for use with two address families, IPv4 and IPv6.  BGPSEC for
   use with other address families may be specified in future documents.
   Note that if the BGPSEC speaker wishes to use BGPSEC with two
   different address families (i.e., IPv4 and IPv6) over the same BGP
   session, then the speaker must include two instances of this
   capability (one for each address family) in the BGP OPEN message.
   Also note that a BGPSEC speaker SHOULD NOT advertise the capability



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   of BGPSEC support for IPv6 unless it has also advertised support for
   IPv6 [2].

   By indicating support for receiving BGPSEC update messages, a BGP
   speaker is, in particular, indicating that the following are true:

   o  The BGP speaker understands the BGPSEC_Path_Signatures attribute
      (see Section 3).

   o  The BGP speaker supports 4-byte AS numbers (see RFC 4893).

   Note that BGPSEC update messages can be quite large, therefore any
   BGPSEC speaker announcing the capability to receive BGPSEC messages
   SHOULD also announce support for the capability to receive BGP
   extended messages [5].

   A BGP speaker MUST NOT send an update message containing the
   BGPSEC_Path_Signatures attribute within a given BGP session unless
   both of the following are true:

   o  The BGP speaker indicated support for sending BGPSEC update
      messages in its open message.

   o  The peer of the BGP speaker indicated support for receiving BGPSEC
      update messages in its open message.


3.  The BGPSEC_Path_Signatures Attribute

   The BGPSEC_Path_Signatures attribute is a new optional (non-
   transitive) BGP path attribute.

   This document registers a new attribute type code for this attribute
   : TBD

   The BGPSEC_Path_Signatures attribute has the following structure:

                     BGPSEC_Path_Signatures Attribute

        +---------------------------------------------------------+
        | Expire Time  (8 octets)                                 |
        +---------------------------------------------------------+
        | Sequence of one or two Signature-List Blocks (variable) |
        +---------------------------------------------------------+


   Expire Time contains a binary representation of a time as an unsigned
   integer number of (non-leap) seconds that have elapsed since midnight



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   UTC January 1, 1970.  The Expire Time indicates the latest point in
   time that the route advertised in the update message can possibly be
   considered valid (see Section 5 for details on validity of BGPSEC
   update messages).

   The BGPSEC_Path_Signatures attribute will contain one or two
   Signature-List Blocks, each of which corresponds to a different
   algorithm suite.  Each of the Signature-List Blocks will contain a
   signature segment for each AS in the AS Path attribute.  In the most
   common case, the BGPSEC_Path_Signatures attribute will contain only a
   single Signature-List Block.  However, in order to enable a
   transition from an old algorithm suite to a new algorithm suite, it
   will be necessary to include two Signature-List Blocks (one for the
   old algorithm suite and one for the new algorithm suite) during the
   transition period.

                           Signature-List Block

              +---------------------------------------------+
              | Algorithm Suite Identifier    (1 octet)     |
              +---------------------------------------------+
              | Signature-List Block Length   (2 octets)    |
              +---------------------------------------------+
              | Sequence of Signature-Segments (variable)   |
              +---------------------------------------------+


   An algorithm suite consists of a digest algorithm and a signature
   algorithm.  This version of BGPSEC only supports signature algorithms
   that produce a signatures of fixed length.  This specification
   creates an IANA registry of one-octet BGPSEC algorithm suite
   identifiers.  Additionally, this document registers a single
   algorithm suite which uses the digest algorithm SHA-256 and the
   signature algorithm RSA with 2048-bit keys [1].  The signatures
   produced by this algorithm suite have a length of 256 octets.  Future
   registrations of algorithm suites for BGPSEC must specify the length
   of signatures produced by the algorithm suite.

                          BGPSEC Algorithm Suites

      Algorithm Suite      Digest        Signature      Specification
      Identifier         Algorithm       Algorithm         Pointer
    +-----------------+--------------+----------------+---------------+
    |   TBA           |   SHA-256    |     RSA 2048   |    RFC 3447   |
    +-----------------+--------------+----------------+---------------+

   The Signature-List Block Length is the total number of octets in all
   Signature-Segments (i.e., the total size of the variable-length



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   portion of the Signature-List block.)

   A Signature-Segment has the following structure:

                            Signature Segments

              +-------------------------------------------- +
              | Subject Key Identifier Length  (1 octet)    |
              +---------------------------------------------+
              | Subject Key Identifier        (variable)    |
              +---------------------------------------------+
              | Signature     (fixed by algorithm suite)    |
              +---------------------------------------------+


   The Subject Key Identifier Length contains the size (in octets) of
   the value in the Subject Key Identifier field of the Signature-
   Segment.  The Subject Key Identifier contains the value in the
   Subject Key Identifier extension of the RPKI end-entity certificate
   that is used to verify the signature (see Section 5 for details on
   validity of BGPSEC update messages).

   The Signature contains a digital signature that protects the NLRI,
   the AS_Path and the BGPSEC_Path_Signatures attribute (see Sections 4
   and 5 for details on generating and verifying this signature,
   respectively).  The length of the Signature field is a function of
   the algorithm suite for a given Signature-List Block.  The
   specification for each BGPSEC algorithm suite must provide the length
   of signatures constructed using the given algorithm suite.


4.  Generating a BGPSEC Update

   Sections 4.1 and 4.2 cover two cases in which a BGPSEC speaker may
   generate an update message containing the BGPSEC_Path_Signatures
   attribute.  The first case is that in which the BGPSEC speaker
   originates a new route advertisement (Section 4.1).  That is, the
   BGPSEC speaker is constructing an update message in which the only AS
   to appear in the AS Path attribute is the speaker's own AS (normally
   appears once but may appear multiple times if AS prepending is
   applied).  The second case is that in which the BGPSEC speaker
   receives a route advertisement from a peer and then decides to
   propagate the route advertisement to an external (eBGP) peer (Section
   4.2).  That is, the BGPSEC speaker has received a BGPSEC update
   message and is constructing a new update message for the same NLRI in
   which the AS Path attribute will contain AS number(s) other than the
   speaker's own AS.




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   In the remaining case where the BGPSEC speaker is sending the update
   message to an internal (iBGP) peer, the BGPSEC speaker populates the
   BGPSEC_Path_Signatures attribute by copying the
   BGPSEC_Path_Signatures attribute from the received update message.
   That is, the BGPSEC_Path_Signatures attribute is copied verbatim.
   Note that in the case that a BGPSEC speaker chooses to forward to an
   iBGP peer a BGPSEC update message that has not been successfully
   validated (see Section 5), the BGPSEC_Path_Signatures attribute
   SHOULD NOT be removed.  (See Section 7 for the security ramifications
   of removing BGPSEC signatures.)

   The information protected by the signature on a BGPSEC update message
   includes the AS number of the peer to whom the update message is
   being sent.  Therefore, if a BGPSEC speaker wishes to send a BGPSEC
   update to multiple BGP peers, it MUST generate a separate BGPSEC
   update message for each unique peer AS to which the update message is
   sent.

   A BGPSEC update message MUST advertise a route to only a single NLRI.
   If a BGPSEC speaker wishes to advertise routes to multiple NLRI, then
   it MUST generate a separate BGPSEC update message for each NLRI.

   Note that in order to create or add a new signature to a Signature-
   List Block for a given algorithm suite, the BGPSEC speaker must
   possess a private key suitable for generating signatures for this
   algorithm suite.  Additionally, this private key must correspond to
   the public key in a valid Resource PKI end-entity certificate whose
   AS number resource extension includes the BGPSEC speaker's AS number.
   Note also new signatures are only added to a BGPSEC update message
   when a BGPSEC speaker is generating an update message to send to an
   external peer (i.e., when the AS number of the peer is not equal to
   the BGPSEC speaker's own AS number).  Therefore, a BGPSEC speaker who
   only sends BGPSEC update messages to peers within its own AS, it does
   not need to possess any private signature keys.

4.1.  Originating a New BGPSEC Update

   In an update message that originates a new route advertisement (i.e.,
   an update whose AS_Path contains, possibly multiple occurrences of, a
   single AS number), the BGPSEC speaker creates one Signature-List
   Block for each algorithm suite that will be used.  Typically, a
   BGPSEC speaker will use only a single algorithm suite.  However, to
   ensure backwards compatibility during a period of transition from a
   'current' algorithm suite to a 'new' algorithm suite, it will be
   necessary to originate update messages containing Signature-List
   Blocks for both the 'current' and the 'new' algorithm suites (see
   Section 6.1).




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   The Resource PKI enables the legitimate holder of IP address
   prefix(es) to issue a signed object, called a Route Origination
   Authorization (ROA), that authorizes a given AS to originate routes
   to a given set of prefixes (see [6]).Note that validation of a BGPSEC
   update message will fail (i.e., the validation algorithm, specified
   in Section 5.1, returns 'Not Good') unless there exists a valid ROA
   authorizing the first AS in the AS PATH attribute to originate routes
   to the prefix being advertised.  Therefore, a BGPSEC speaker SHOULD
   NOT originate a BGPSEC update advertising a route for a given prefix
   unless there exists a valid ROA authorizing the BGPSEC speaker's AS
   to originate routes to this prefix.

   The Expire Time field is set to specify a time at which the route
   advertisement specified in the update message will cease to be valid.
   Once the Expire Time has been reached, all BGPSEC speakers who have
   received the advertisement will treat it as invalid.  The purpose of
   this field is to protect the BGPSEC speaker against attacks in which
   the BGPSEC speaker wishes to withdraw the route, but intermediate
   (malicious) BGP speakers fail to propagate the withdrawal to their
   peers.

   It is therefore necessary for the originating BGPSEC speaker to issue
   a new BGPSEC update prior to reaching the Expire Time.  It is
   RECOMMENDED that a BGPSEC speaker originate a new route advertisement
   for a given NLRI at intervals equal to roughly one-third the validity
   period of the route advertisement.  (Note that it is necessary to add
   some small amount of random jitter to the interval to avoid
   synchronization effects.)  For instance, if a BGPSEC speaker is
   originating route advertisements that are valid for one day (i.e.,
   the Expire Time is 24 hours after the generation of the update
   message), then it is recommended that the BGPSEC speaker re-issue new
   a new BGPSEC update message for advertising the given prefix roughly
   once every 8 hours (plus or minus a small random value).

   (Editor's Note: The parameter recommendations in the previous
   paragraph are preliminary and may need to be updated based on further
   implementation and deployment experience.)

   There is a natural trade-off in setting the Expire Time.  Setting a
   later Expire Time increases the amount of time by which a malicious
   intermediate can delay a future route withdrawal.  Similarly, setting
   a later Expire Time also increases the window of opportunity for
   malicious replay attacks in which a previous BGPSEC announcement is
   replayed while suppressing a more recent withdrawal for the same
   prefix.  However, setting a sooner Expire Timed increases the
   frequency with which the BGPSEC speaker needs to send new
   announcements for the given prefix.




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   When originating a new route advertisement, each Signature-List Block
   MUST consist of a single Signature-Segment.  The following describes
   how the BGPSEC speaker populates the fields of the Signature-List
   Block (see Section 3 for more information on the syntax of Signature-
   List Blocks).

   The Subject Key Identifier field (see Section 3) is populated with
   the identifier contained in the Subject Key Identifier extension of
   the RPKI end-entity certificate used by the BGPSEC speaker.  This
   Subject Key Identifier will be used by recipients of the route
   advertisement to identify the proper certificate to use in verifying
   the signature.

   The Subject Key Identifier Length field is populated with the length
   (in octets) of the Subject Key Identifier.

   The Signature field contains a digital signature that binds the NLRI,
   AS_Path attribute and BGPSEC_Path_Signatures attribute to the RPKI
   end-entity certificate used by the BGPSEC speaker.  The digital
   signature is computed as follows:

   o  Construct a sequence of octets by concatenating the Expire Time,
      Target AS Number, Origin AS Number, Algorithm Suite Identifier,
      and NLRI.  The Target AS Number is the AS to whom the BGPSEC
      speaker intends to send the update message.  (Note that the Target
      AS number is the AS number announced by the peer in the OPEN
      message of the BGP session within which the update is sent.)  The
      Origin AS number prepend to this sequence the Target AS (the AS to
      whom the BGPSEC speaker intends to send the update message) and
      the Origin AS Number refers to the AS of the BGPSEC speaker who is
      originating the route advertisement.




















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                      Sequence of Octets to be Signed
                 +---------------------------------------+
                 | Expire Time (8 octets)                |
                 +---------------------------------------+
                 | Target AS Number (4 octets)           |
                 +---------------------------------------+
                 | Origin AS Number (4 octets)           |
                 +---------------------------------------+
                 | Algorithm Suite Identifier  (1 octet) |
                 +---------------------------------------+
                 | NLRI Length  (1 octet)                |
                 +---------------------------------------+
                 | NLRI Prefix  (variable)               |
                 +---------------------------------------+

   o  Apply to this octet sequence the digest algorithm (for the
      algorithm suite of this Signature-List) to obtain a digest value.

   o  Apply to this digest value the signature algorithm, (for the
      algorithm suite of this Signature-List) to obtain the digital
      signature.  Then populate the Signature Field with this digital
      signature.

4.2.  Propagating a Route Advertisement

   When a BGPSEC speaker receives a BGPSEC update message containing a
   BGPSEC_Path_Signatures algorithm (with one or more signatures) from a
   (internal or external) peer, it may choose to propagate the route
   advertisement by sending to its (internal or external) peers by
   creating a new BGPSEC advertisement for the same prefix.

   A BGPSEC speaker MUST NOT generate an update message containing the
   BGPSEC_Path_Signatures attribute unless it has selected, as the best
   route to the given prefix, a route that it received in an update
   message containing the BGPSEC_Path_Signatures attribute.  In
   particular, this means that whenever a BGPSEC speaker generates an
   update message with a BGPSEC_Path_Signatures attribute that it will
   possess a received update message for the same prefix that also
   contains a BGPSEC_Path_Signatures attribute.

   Additionally, whenever a BGPSEC speaker selects as the best route to
   a given prefix a route that it received in an update message
   containing the BGPSEC_Path_Signatures attribute, it is RECOMMENDED
   that if the BGPSEC speaker chooses to propagate the route that it
   generate an update message containing the BGPSEC_Path_Signatures
   attribute.  However, a BGPSEC speaker MAY propagate a route
   advertisement by generating a (non-BGPSEC) update message that does
   not contain the BGPSEC_Path_Signatures attribute.  (See Section 7 for



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   discussion of the security ramifications of removing BGPSEC
   signatures.)

   If the BGPSEC speaker is producing an update message which contains
   an AS-SET (e.g., the BGPSEC speaker is performing proxy aggregation),
   then the BGPSEC speaker MUST not include the BGPSEC_Path_Signatures
   attribute.  In such a case, the BGPSEC speaker must remove any
   existing BGPSEC_Path_Signatures in the received advertisement(s) for
   this prefix and produce a standard (non-BGPSEC) update message.

   To generate the BGPSEC_Path_Signatures attribute on the outgoing
   update message, the BGPSEC first copies the Expire Time directly from
   the received update message to the new update message (that it is
   constructing).  Note that the BGPSEC speaker MUST NOT change the
   Expire Time as any change to Expire Time will cause the new BGPSEC
   update message to fail validation (see Section 5).

   The BGPSEC speaker next removes from the BGPSEC_Path_Signatures
   attribute any Signature-List Blocks corresponding to algorithm suites
   that it does not support.  The BGPSEC_Path_Signatures attribute for
   the new update message SHOULD contain a Signature-List Block for
   every algorithm suite that is both present in the received update
   message and which is supported by the BGPSEC speaker.

   Note that the validation algorithm (see Section 5.1) deems a BGPSEC
   update message to be 'Good' if there is at least one supported
   algorithm suite (and corresponding Signature-List Block) that is
   deemed 'Good'.  This means that a 'Good' BGPSEC update message may
   contain Signature-List Blocks which are deemed 'Not Good' (e.g.,
   contain signatures that the BGPSEC is unable to verify).
   Nonetheless, such Signature-List Blocks MUST NOT be removed.  (See
   Section 7 for a discussion of the security ramifications of this
   design choice.)

   For each Signature-List Block corresponding to an algorithm suite
   that the BGPSEC speaker does support, the BGPSEC speaker then adds a
   new Signature-Segment to the Signature-List Block.  This Signature-
   Segment is prepended to the list of Signature-Segments (placed in the
   first position) so that the list of Signature-Segments appears in the
   same order as the corresponding AS numbers in the AS-Path attribute.
   The BGPSEC speaker populates the fields of this new signature-segment
   as follows.

   The Subject Key Identifier field in the new segment is populated with
   the identifier contained in the Subject Key Identifier extension of
   the RPKI end-entity certificate used by the BGPSEC speaker.  This
   Subject Key Identifier will be used by recipients of the route
   advertisement to identify the proper certificate to use in verifying



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   the signature.

   The Subject Key Identifier Length field is populated with the length
   (in octets) of the Subject Key Identifier.

   The Signature field in the new segment contains a digital signature
   that binds the NLRI, AS_Path attribute and BGPSEC_Path_Signatures
   attribute to the RPKI end-entity certificate used by the BGPSEC
   speaker.  The digital signature is computed as follows:

   o  Construct a sequence of octets by concatenating the signature
      field of the most recent Signature-Segment (the one corresponding
      to AS from whom the BGPSEC speaker's AS received the announcement)
      with the Target AS (the AS to whom the BGPSEC speaker intends to
      send the update message).  Note that the Target AS number is the
      AS number announced by the peer in the OPEN message of the BGP
      session within which the BGPSEC update message is sent.

                      Sequence of Octets to be Signed

       +-----------------------------------------------------------+
       | Most Recent Signature Field   (fixed by algorithm suite)  |
       ------------------------------------------------------------+
       | Target AS Number              (4 octets)                  |
       +-----------------------------------------------------------+

   o  Apply to this octet sequence the digest algorithm (for the
      algorithm suite of this Signature-List) to obtain a digest value.

   o  Apply to this digest value the signature algorithm, (for the
      algorithm suite of this Signature-List) to obtain the digital
      signature.  Then populate the Signature Field with this digital
      signature.


5.  Validating a BGPSEC Update

   Validation of a BGPSEC update messages makes use of data from RPKI
   certificates and signed Route Origination Authorizations (ROA).  In
   particular, to validate update messages containing the
   BGPSEC_Path_Signatures attribute, it is necessary that the recipient
   have access to the following data obtained from valid RPKI
   certificates and ROAs:

   o  For each valid RPKI end-entity certificate containing an AS Number
      extension, the AS Number, Public Key and Subject Key Identifier
      are required




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   o  For each valid ROA, the AS Number and the list of IP address
      prefixes

   Note that the BGPSEC speaker could perform the validation of RPKI
   certificates and ROAs on its own and extract the required data, or it
   could receive the same data from a trusted cache that performs RPKI
   validation on behalf of (some set of) BGPSEC speakers.

   To validate a BGPSEC update message containing the
   BGPSEC_Path_Signatures attribute, the recipient performs the
   validation steps specified in Section 5.1.  The validation procedure
   results in one of two states: 'Good' and 'Not Good'.

   It is expected that the output of the validation procedure will be
   used as an input to BGP route selection.  However, BGP route
   selection and thus the handling of the two validation states is a
   matter of local policy, and shall be handled using existing local
   policy mechanisms.  It is expected that BGP peers will generally
   prefer routes received via 'Good' BGPSEC update messages over routes
   received via 'Not Good' BGPSEC update messages as well as routes
   received via update messages that do not contain the
   BGPSEC_Path_Signatures attribute.  However, BGPSEC specifies no
   changes to the BGP decision process and leaves to the operator the
   selection of an appropriate policy mechanism to achieve the
   operator's desired results within the BGP decision process.

   BGPSEC validation need only be performed at eBGP edge.  The
   validation status of a BGP signed/unsigned update MAY be conveyed via
   iBGP from an ingress edge router to an egress edge router.  Local
   policy in the AS determines the specific means for conveying the
   validation status through various pre-existing mechanisms such as
   setting a BGP community, or modifying a metric value such as
   Local_Pref or MED.  As discussed in Section 4, when a BGPSEC speaker
   chooses to forward a (syntactically correct) BGPSEC update message,
   it SHOULD be forwarded with its BGPSEC_Path_Signatures attribute
   intact (regardless of the validation state of the update message).
   Based entirely on local policy settings, an egress router MAY trust
   the validation status conveyed by an ingress router or it MAY perform
   its own validation.

5.1.  Validation Algorithm

   This section specifies an algorithm for validation of BGPSEC update
   messages.  A conformant implementation MUST include an BGPSEC update
   validation algorithm that is functionally equivalent to the external
   behavior of this algorithm.

   First, the recipient of a BGPSEC update message performs a check to



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   ensure that the message is properly formed.  Specifically, the
   recipient performs the following checks:

   o  Check to ensure that the entire BGPSEC_Path_Signatures attribute
      is syntactically correct (conforms to the specification in this
      document).

   o  Check to ensure that the AS-Path attribute contains no AS-Set
      segments.

   o  Check that each Signature-List Block contains one Signature-
      Segment for each AS in the AS-Path attribute.  (Note that the
      entirety of each Signature-List Block must be checked to ensure
      that it is well formed, even though the validation process may
      terminate before all signatures are cryptographically verified.)

   If there are two Signature-List Blocks within the
   BGPSEC_Path_Signatures attribute and one of them is poorly formed (or
   contains the wrong number of Signature-Segments) , then the recipient
   should log that an error occurred, strip off that particular
   Signature-List Block and process the update message as though it
   arrived with a single Signature-List Block.  If the
   BGPSEC_Path_Signatures attribute contains a syntax error which is not
   local to a single Signature-List Block, or if the AS-Path attribute
   contains an AS-Set segment, then the recipient should log that an
   error occurred, strip off the BGPSEC_Path_Signatures attribute and
   process the update message as though it arrived without a
   BGPSEC_Path_Signatures attribute.

   Second, the BGPSEC speaker verifies that the update message has not
   yet expired.  To do this, locate the Expire Time field in the
   BGPSEC_Path_Signatures attribute, and compare it with the current
   time.  If the current time is later than the Expire Time, the BGPSEC
   update is 'Not Good' and the validation algorithm terminates.

   Third, the BGPSEC speaker verifies that the origin AS is authorized
   to advertise the prefix in question.  To do this, consult the valid
   ROA data to obtain a list of AS numbers that are associated with the
   given IP address prefix in the update message.  Then locate the last
   (least recently added) AS number in the AS-Path.  If the origin AS in
   the AS-Path is not in the set of AS numbers associated with the given
   prefix, then BGPSEC update message is 'Not Good' and the validation
   algorithm terminates.

   Finally, the BGPSEC speaker examines the Signature-List Blocks in the
   BGPSEC_Path_Signatures attribute.  Any Signature-List Block
   corresponding to an algorithm suite that the BGPSEC speaker does not
   support MUST be discarded.  If all Signature-List Blocks are



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   discarded in this manner then the BGPSEC speaker MUST treat the
   update message as though it arrived without a BGPSEC_Path_Signatures
   attribute.

   For each remaining Signature-List Block (corresponding to an
   algorithm suite supported by the BGPSEC speaker), the BGPSEC speaker
   iterates through the Signature-Segments in the Signature-List block,
   starting with the most recently added segment (and concluding with
   the least recently added segment).  Note that there is a one-to-one
   correspondence between Signature-Segments and AS numbers in the AS-
   Path attribute, and the following steps make use of this
   correspondence.

   o  (Step I): Locate the public key needed to verify the signature (in
      the current Signature-Segment).  To do this, consult the valid
      RPKI end-entity certificate data and look for an SKI that matches
      the value in the SKI field of the Signature-Segment.  If no such
      SKI value is found in the valid RPKI data then mark the entire
      Signature-List Block as 'Not Good' and proceed to the next
      Signature-List Block.  Similarly, if the SKI exists but the AS
      Number associated with the SKI does NOT match the AS Number (in
      the AS-Path attribute) which corresponds to the current Signature-
      Segment, then mark the entire Signature-List Block as 'Not Good'
      and proceed to the next Signature-List Block.

   o  (Step II): Compute the digest function (for the given algorithm
      suite) on the appropriate data.  If the segment is not the (least
      recently added) segment corresponding to the origin AS, then the
      digest function should be computed on the following sequence of
      octets:

                      Sequence of Octets to be Hashed

            +-------------------------------------------------+
            | Signature Field in the Next Segment  (variable) |
            --------------------------------------------------+
            | AS Number of Subsequent AS           (4 octets) |
            +-------------------------------------------------+

   The 'Signature Field in the Next Segment' is the Signature field
   found in the Signature-Segment that is next to be processed (that is,
   the next most recently added Signature- Segment).

   For the first segment to be processed (the most recently added
   segment), the 'AS Number of Subsequent AS' is the AS number of the
   BGPSEC speaker validating the update message.  Note that if a BGPSEC
   speaker uses multiple AS Numbers (e.g., the BGPSEC speaker is a
   member of a confederation), the AS number used here MUST be the AS



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   number announced in the OPEN message for the BGP session over which
   the BGPSEC update was received.

   For each other Signature-Segment, the 'AS Number of Subsequent AS' is
   the AS that corresponds to the Signature-Segment added immediately
   after the one being processed.  (That is, find the AS number
   corresponding to the Signature-Segment currently being processed and
   the 'AS Number of Subsequent AS' is the next AS number that was added
   to the AS-Path attribute.)

   Alternatively, if the segment being processed corresponds to the
   origin AS, then the digest function should be computed on the
   following sequence of octets:

                      Sequence of Octets to be Hashed

                 +----------------------------------------+
                 | Expire Time  (8 octets)                |
                 -----------------------------------------+
                 | AS Number of Subsequent AS  (4 octets) |
                 +----------------------------------------+
                 | Origin AS Number            (4 octets) |
                 +----------------------------------------+
                 | Algorithm Suite Identifier  (1 octet)  |
                 +----------------------------------------+
                 | NLRI Length  (1 octet)                 |
                 +----------------------------------------+
                 | NLRI Prefix  (variable)                |
                 +----------------------------------------+

   The NLRI Length, NLRI Prefix, Expire Time, and Algorithm Suite
   Identifier are all obtained in a straight forward manner from the
   NLRI of the update message or the BGPSEC_Path_Signatures attribute
   being validated.

   The Origin AS Number is the same Origin AS Number that was located in
   Step I above.  (That is, the AS number corresponding to the least
   recently added Signature-Segment.)

   The 'AS Number of Subsequent AS' is the AS Number added to the AS-
   Path immediately after the Origin AS Number.  (That is, the second AS
   Number that was added to the AS Path.)

   o  (Step III): Use the signature validation algorithm (for the given
      algorithm suite) to verify the signature in the current segment.
      That is, invoke the signature validation algorithm on the
      following three inputs: the value of the Signature field in the
      current segment; the digest value computed in Step II above; and



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      the public key obtained from the valid RPKI data in Step I above.
      If the signature validation algorithm determines that the
      signature is invalid, then mark the entire Signature-List Block as
      'Not Good' and proceed to the next Signature-List Block.  If the
      signature validation algorithm determines that the signature is
      valid, then continue processing Signature-Segments (within the
      current Signature-List Block).

   If all Signature-Segments within a Signature-List Block pass
   validation (i.e., all segments are processed and the Signature-List
   Block has not yet been marked 'Not Good'), then the Signature-List
   Block is marked as 'Good'.

   If at least one Signature-List Block is marked as 'Good', then the
   validation algorithm terminates and the BGPSEC update message is
   deemed to be 'Good'.  (That is, if a BGPSEC update message contains
   two Signature-List Blocks then the update message is deemed 'Good' if
   the first Signature-List block is marked 'Good' OR the second
   Signature-List block is marked 'Good'.)


6.  Algorithms and Extensibility

6.1.  Algorithm Suite Considerations

   Note that there is currently no support for bilateral negotiation
   between BGPSEC peers to use of a particular (digest and signature)
   algorithm suite using BGP capabilities.  This is because the
   algorithm suite used by the sender of a BGPSEC update message must be
   understood not only by the peer to whom he is directly sending the
   message, but also by all BGPSEC speakers to whom the route
   advertisement is eventually propagated.  Therefore, selection of an
   algorithm suite cannot be a local matter negotiated by BGP peers, but
   instead must be coordinated throughout the Internet.

   To this end, a mandatory algorithm suites document will be created
   which specifies a mandatory-to-use 'current' algorithm suite for use
   by all BGPSEC speakers.  Additionally, the document specifies an
   additional 'new' algorithm suite that is recommended to implement.

   It is anticipated that in the future the mandatory algorithm suites
   document will be updated to specify a transition from the 'current'
   algorithm suite to the 'new' algorithm suite.  During the period of
   transition (likely a small number of years), all BGPSEC update
   messages SHOULD simultaneously use both the 'current' algorithm suite
   and the 'new' algorithm suite.  (Note that Sections 3 and 4 specify
   how the BGPSEC_Path_Signatures attribute can contain signatures, in
   parallel, for two algorithm suites.)  Once the transition is



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   complete, use of the old 'current' algorithm will be deprecated, use
   of the 'new' algorithm will be mandatory, and a subsequent 'even
   newer' algorithm suite may be specified as recommend to implement.
   Once the transition has successfully been completed in this manner,
   BGPSEC speakers SHOULD include only a single Signature-List Block
   (corresponding to the 'new' algorithm).

6.2.  Extensibility Considerations

   This section discusses potential changes to BGPSEC that would require
   substantial changes to the processing of the BGPSEC_Path_Signatures
   and thus necessitate a new version of BGPSEC.  Examples of such
   changes include:

   o  A new type of signature algorithm that produces signatures of
      variable length

   o  A new type of signature algorithm for which the number of
      signatures in the Signature-List Block is not equal to the number
      of ASes in the AS-PATH (e.g., aggregate signatures)

   o  Changes to the data that is protected by the BGPSEC signatures
      (e.g., protection of attributes other than AS-PATH)

   In the case that such a change to BGPSEC were deemed desirable, it is
   expected that a subsequent version of BGPSEC would be created and
   that this version of BGPSEC would specify a new BGP Path Attribute,
   let's call it BGPSEC_PATH_SIG_TWO, which is designed to accommodate
   the desired changes to BGPSEC.  In such a case, the mandatory
   algorithm suites document would be updated to specify algorithm
   suites appropriate for the new version of BGPSEC.

   At this point a transition would begin which is analogous to the
   algorithm transition discussed in Section 6.2.  During the transition
   period all BGPSEC speakers SHOULD simultaneously include both the
   BGPSEC_PATH_SIGNATURES attribute and the new BGPSEC_PATH_SIG_TWO
   attribute.  Once the transition is complete, the use of
   BGPSEC_PATH_SIGNATURES could then be deprecated, at which point
   BGPSEC speakers SHOULD include only the new BGPSEC_PATH_SIG_TWO
   attribute.  Such a process could facilitate a transition to a new
   BGPSEC semantics in a backwards compatible fashion.


7.  Security Considerations

   For discussion of the BGPSEC threat model and related security
   considerations, please see [8].




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   A BGPSEC speaker who receives a valid BGPSEC update message,
   containing a route advertisement for a given prefix, is provided with
   the following security guarantees:

   o  The origin AS number corresponds to an autonomous system that has
      been authorized by the IP address space holder to originate route
      advertisements for the given prefix.

   o  For each subsequent AS number in the AS-Path, a BGPSEC speaker
      authorized by the holder of the AS number selected the given route
      as the best route to the given prefix.

   o  For each AS number in the AS Path, a BGPSEC speaker authorized by
      the holder of the AS number intentionally propagated the route
      advertisement to the next AS in the AS-Path.

   That is, the recipient of a valid BGPSEC Update message is assured
   that the AS-Path corresponds to a sequence of autonomous systems who
   have all agreed in principle to forward packets to the given prefix
   along the indicated path.  (It should be noted BGPSEC does not offer
   a precise guarantee that the data packets would propagate along the
   indicated path; it only guarantees that the BGP update conveying the
   path indeed propagated along the indicated path.)  Furthermore, the
   recipient is assured that this path terminates in an autonomous
   system that has been authorized by the IP address space holder as a
   legitimate destination for traffic to the given prefix.

   Note that there may be cases where a BGPSEC speaker deems 'Good' (as
   per the validation algorithm in Section 5.1) a BGPSEC update message
   that contains both a 'Good' and a 'Not Good' Signature-List Block.
   That is, the update message contains two sets of signatures
   corresponding to two algorithm suites, and one set of signatures
   verifies correctly and the other set of signatures fails to verify.
   In this case, the protocol specifies that if the BGPSEC speaker
   propagates the route advertisement received in such an update message
   then the BGPSEC speaker SHOULD add its signature to each of the
   Signature-List Blocks using both the corresponding algorithm suite.
   Thus the BGPSEC speaker creates a signature using both algorithm
   suites and creates a new update message that contains both the 'Good'
   and the 'Not Good' set of signatures (from its own vantage point).

   To understand the reason for such a design decision consider the case
   where the BGPSEC speaker receives an update message with both a set
   of algorithm A signatures which are 'Good' and a set of algorithm B
   signatures which are 'Not Good'.  In such a case it is possible
   (perhaps even quite likely) that some of the BGPSEC speaker's peers
   (or other entities further 'downstream' in the BGP topology) do not
   support algorithm A. Therefore, if the BGPSEC speaker were to remove



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   the 'Not Good' set of signatures corresponding to algorithm B, such
   entities would treat the message as though it were unsigned.  By
   including the 'Not Good' set of signatures when propagating a route
   advertisement, the BGPSEC speaker ensures that 'downstream' entities
   have as much information as possible to make an informed opinion
   about the validation status of a BGPSEC update.

   Note also that during a period of partial BGPSEC deployment, a
   'downstream' entity might reasonably treat unsigned messages
   different from BGPSEC updates that contain a single set of 'Not Good'
   signatures.  That is, by removing the set of 'Not Good' signatures
   the BGPSEC speaker might actually cause a downstream entity to
   'upgrade' the status of a route advertisement from 'Not Good' to
   unsigned.  Finally, note that in the above scenario, the BGPSEC
   speaker might have deemed algorithm A signatures 'Good' only because
   of some issue with RPKI state local to his AS (for example, his AS
   might not yet have obtained a CRL indicating that a key used to
   verify an algorithm A signature belongs to a newly revoked
   certificate).  In such a case, it is highly desirable for a
   downstream entity to treat the update as 'Not Good' (due to the
   revocation) and not as 'unsigned' (which would happen if the 'Not
   Good' Signature-List Blocks were removed).

   A similar argument applies to the case where a BGPSEC speaker (for
   some reason such as lack of viable alternatives) selects as his best
   route to a given prefix a route obtained via a 'Not Good' BGPSEC
   update message.  (That is, a BGPSEC update containing only 'Not Good'
   Signature-List Blocks.)  In such a case, the BGPSEC speaker should
   propagate a signed BGPSEC update message, adding his signature to the
   'Not Good' signatures that already exist.  Again, this is to ensure
   that 'downstream' entities are able to make an informed decision and
   not erroneously treat the route as unsigned.  It may also be noted
   here that due to possible differences in RPKI data at different
   vantage points in the network, a BGPSEC update that was deemed 'Not
   Good' at an upstream BGPSEC speaker may indeed be deemed 'Good' at
   another BGP speaker downstream.

   Therefore, it is important to note that when a BGPSEC speaker signs
   an outgoing update message, it is not attesting to a belief that all
   signatures prior to its are valid.  Instead it is merely asserting
   that:

   1.  The BGPSEC speaker received the given route advertisement with
       the indicated NLRI and AS Path;

   2.  The BGPSEC speaker selected this route as the best route to the
       given prefix; and




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   3.  The BGPSEC speaker chose to propagate an advertisement for this
       route to the peer (implicitly) indicated by the 'Target AS'

   The BGPSEC update validation procedure is a potential target for
   denial of service attacks against a BGPSEC speaker.  To mitigate the
   effectiveness of such denial of service attacks, BGPSEC speakers
   should implement an update validation algorithm that performs
   expensive checks (e.g., signature verification) after less expensive
   checks (e.g., syntax checks).  The validation algorithm specified in
   Section 5.1 was chosen so as to perform checks which are likely to be
   expensive after checks that are likely to be inexpensive.  However,
   the relative cost of performing required validation steps may vary
   between implementations, and thus the algorithm specified in Section
   5.1 may not provide the best denial of service protection for all
   implementations.


8.  Contributors

8.1.  Authors

   Rob Austein
   Internet Systems Consortium
   sra@hactrn.net


   Steven Bellovin
   Columbia University
   smb@cs.columbia.edu


   Randy Bush
   Internet Initiative Japan
   randy@psg.com


   Russ Housley
   Vigil Security
   housley@vigilsec.com


   Stephen Kent
   BBN Technologies
   kent@bbn.com


   Warren Kumari
   Google



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   warren@kumari.net


   Doug Montgomery
   USA National Institute of Standards and Technology
   dougm@nist.gov


   Kotikalapudi Sriram
   USA National Institute of Standards and Technology
   kotikalapudi.sriram@nist.gov


   Samuel Weiler
   weiler@watson.org
   Cobham

8.2.  Acknowledgements

   The authors would like to thank Sharon Goldberg, Ed Kern, Chris
   Morrow, Sandy Murphy, Mark Reynolds, Heather Schiller, Jason
   Schiller, John Scudder, and David Ward for their valuable input and
   review.


9.  References

   [1]  Jonsson, J. and B. Kaliski, "PKCS #1", RFC 3447, February 2003.

   [2]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol
        Extensions for BGP-4", RFC 4760, January 2007.

   [3]  Scudder, J. and R. Chandra, "Capabilities Advertisement with
        BGP-4", RFC 4760, February 2009.

   [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [5]  Patel, K., Ward, D., and R. Bush, "Extended Message support for
        BGP", draft-ymbk-bgp-extended-messages, March 2011.

   [6]  Lepinski, M., Kent, S., and D. Kong, "A Profile for Route Origin
        Authorizations", draft-ietf-sidr-roa-format, February 2011.

   [7]  Lepinski, M. and S. Kent, "An Infrastructure to Support Secure
        Internet Routing", draft-ietf-sidr-arch, February 2011.

   [8]  Kent, S., "Threat Model for BGP Path Security",
        draft-kent-bgpsec-threats, February 2011.


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Author's Address

   (Editor) Matthew Lepinski
   BBN
   10 Moulton St
   Cambridge, MA  55409

   Phone: +1-617-873-5939
   Email: mlepinski@bbn.com









































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