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Versions: (draft-lepinski-bgpsec-protocol) 00 01 02 03 04 05 06 07 08 09

Network Working Group                                   M. Lepinski, Ed.
Internet-Draft                                                       BBN
Intended status: Standards Track                       February 25, 2013
Expires: August 25, 2013


                     BGPSEC Protocol Specification
                   draft-ietf-sidr-bgpsec-protocol-07

Abstract

   This document describes BGPSEC, an extension to the Border Gateway
   Protocol (BGP) that provides security for the path of autonomous
   systems through which a BGP update message passes.  BGPSEC is
   implemented via a new optional non-transitive BGP path attribute that
   carries a digital signature produced by each autonomous system that
   propagates the update message.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" are to be interpreted as described in RFC 2119 [1] only
   when they appear in all upper case.  They may also appear in lower or
   mixed case as English words, without normative meaning

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 April 25, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  BGPSEC Negotiation . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  BGPSEC Send Capability . . . . . . . . . . . . . . . . . .  3
     2.2.  BGPSEC Receive Capability  . . . . . . . . . . . . . . . .  4
     2.3.  Negotiating BGPSEC Support . . . . . . . . . . . . . . . .  5
   3.  The BGPSEC_Path Attribute  . . . . . . . . . . . . . . . . . .  6
     3.1.  Secure_Path  . . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Signature_Block  . . . . . . . . . . . . . . . . . . . . .  9
   4.  Generating a BGPSEC Update . . . . . . . . . . . . . . . . . . 11
     4.1.  Originating a New BGPSEC Update  . . . . . . . . . . . . . 12
     4.2.  Propagating a Route Advertisement  . . . . . . . . . . . . 14
     4.3.  Processing Instructions for Confederation Members  . . . . 18
     4.4.  Reconstructing the AS_PATH Attribute . . . . . . . . . . . 20
   5.  Processing a Received BGPSEC Update  . . . . . . . . . . . . . 21
     5.1.  Overview of BGPSEC Validation  . . . . . . . . . . . . . . 23
     5.2.  Validation Algorithm . . . . . . . . . . . . . . . . . . . 24
   6.  Algorithms and Extensibility . . . . . . . . . . . . . . . . . 28
     6.1.  Algorithm Suite Considerations . . . . . . . . . . . . . . 28
     6.2.  Extensibility Considerations . . . . . . . . . . . . . . . 28
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 29
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 32
   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 32
     9.1.  Authors  . . . . . . . . . . . . . . . . . . . . . . . . . 32
     9.2.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . 34
   10. Normative References . . . . . . . . . . . . . . . . . . . . . 34
   11. Informative References . . . . . . . . . . . . . . . . . . . . 35
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 35











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1.  Introduction

   This document describes BGPSEC, a mechanism for providing path
   security for Border Gateway Protocol (BGP) [2] 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 on the path of ASes through which the update message
       passes has explicitly authorized the advertisement of the route
       to the subsequent AS in the path.

   This document specifies a new optional (non-transitive) BGP path
   attribute, BGPSEC_Path.  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 needs to have the private key associated with an RPKI
   router certificate [10] that corresponds to the BGPSEC speaker's AS
   number.  Note, however, that a BGPSEC speaker does not need such a
   certificate in order to validate update messages containing the
   BGPSEC_Path attribute.


2.  BGPSEC Negotiation

   This document defines a new BGP capability [6] that allows a BGP
   speaker to advertise to a neighbor the ability to send or to receive
   BGPSEC update messages (i.e., update messages containing the
   BGPSEC_Path attribute).

2.1.  The BGPSEC Capability

   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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                       BGPSEC Send Capability Value:

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

   The first four bits 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.

   The fifth bit of the first octet is a direction bit which indicates
   whether the BGP speaker is advertising the capability to send BGPSEC
   update message or receive BGPSEC update messages. The BGP speaker
   sets this bit to 0 to indicate the capability to receive BGPSEC
   update messages. The BGP speaker sets this bit to 1 to indicate the
   capability to send BGPSEC update messages.

   The remaining three bits of the first octet are reserved for future
   use.  These bits are set to zero by the sender of the capability and
   ignored by the receiver of the capability.

   The second and third 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,
   AFI values 1 and 2 respectively.  BGPSEC for use with other address
   families may be specified in future documents.

2.2.  Negotiating BGPSEC Support

   In order to indicate that a BGP speaker is willing to send BGPSEC
   update messages (for a particular address family), a BGP speaker
   sends the BGPSEC Capability (see Section 2.1) with the Direction bit
   (the fifth bit of the first octet) set to 1. In order to indicate
   that the speaker is willing to receive BGP update messages containing
   the BGPSEC_Path attribute (for a particular address family), a BGP
   speaker sends the BGPSEC capability with the Direction bit set to 0.
   In order to advertise the capability to both send and receive BGPSEC
   update messages, the BGP speaker sends two copies of the BGPSEC



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   capability (one with the direction bit set to 0 and one with the
   direction bit set to 1).

   Similarly, if a BGP speaker wishes to use BGPSEC with two different
   address families (i.e., IPv4 and IPv6) over the same BGP session,
   then the speaker includes two instances of this capability (one for
   each address family) in the BGP OPEN message.  A BGP speaker SHOULD
   NOT advertise the capability of BGPSEC support for a particular AFI
   unless it has also advertised the multiprotocol extension capability
   for the same AFI combination [3].

   In a session where BGP session, a peer is permitted to send update
   messages containing the BGPSEC_Path attribute if, and only if:

   o  The given peer has sent the BGPSEC capability for a particular
      version of BGPSEC and a particular address family with the
      Direction bit set to 1; and

   o  The other peer has sent the BGPSEC capability for the same version
      of BGPSEC and the same address family with the Direction bit set
      to 0.

   In such a session, we say that the use of (the particular version of)
   BGPSEC has been negotiated (for a particular address family).  BGP
   update messages without the BGPSEC_PATH attribute MAY be sent within
   a session regardless of whether or not the use of BGPSEC is
   successfully negotiated.  However, if BGPSEC is not successfully
   negotiated, then BGP update messages containing the BGPSEC_PATH
   attribute MUST NOT be sent.

   This document defines the behavior of implementations in the case
   where BGPSEC version zero is the only version that has been
   successfully negotiated.  If there exist multiple versions have
   BGPSEC that are negotiated for a particular session, the behavior of
   the peers (e.g., which version of BGPSEC shall actually be used) will
   be specified in a future document.

   BGPSEC cannot provide meaningful security guarantees without support
   for four-byte AS numbers.  Therefore, any BGP speaker that announces
   the BGPSEC capability, MUST also announce the capability for four-
   byte AS support [4].  If a BGP speaker sends the BGPSEC capability
   but not the four-byte AS support capability then BGPSEC has not been
   successfully negotiated, and update messages containing the
   BGPSEC_Path attribute MUST NOT be sent within such a session.

   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



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   extended messages [9].


3.  The BGPSEC_Path Attribute

   The BGPSEC_Path 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 attribute carries the secured information regarding
   the path of ASes through which an update message passes.  This
   includes the digital signatures used to protect this information.  We
   refer to those update messages that contain the BGPSEC_Path attribute
   as "BGPSEC Update messages".  The BGPSEC_Path attribute replaces the
   AS_PATH attribute in a BGPSEC update message.  That is, update
   messages that contain the BGPSEC_Path attribute MUST NOT contain the
   AS_PATH attribute, and vice versa.

   The BGPSEC_Path attribute is made up of several parts.  The following
   high-level diagram provides an overview of the structure of the
   BGPSEC_Path attribute:

              High-Level Diagram of the BGPSEC_Path Attribute
        +---------------------------------------------------------+
        |     +-----------------+                                 |
        |     |   Secure Path   |                                 |
        |     +-----------------+                                 |
        |     |    AS X         |                                 |
        |     |    pCount X     |                                 |
        |     |    Flags X      |                                 |
        |     |    AS Y         |                                 |
        |     |    pCount Y     |                                 |
        |     |    Flags Y      |                                 |
        |     |      ...        |                                 |
        |     +-----------------+                                 |
        |                                                         |
        |     +-----------------+       +-----------------+       |
        |     | Sig Block 1     |       |  Sig Block 2    |       |
        |     +-----------------+       +-----------------+       |
        |     | Alg Suite 1     |       |  Alg Suite 2    |       |
        |     | SKI X1          |       |  SKI X1         |       |
        |     | Signature X1    |       |  Signature X1   |       |
        |     | SKI Y1          |       |  SKI Y1         |       |
        |     | Signature Y1    |       |  Signature Y1   |       |
        |     |      ...        |       |      ....       |       |
        |     +-----------------+       +-----------------+       |



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        |                                                         |
        +---------------------------------------------------------+


   The following is the specification of the format for the BGPSEC_Path
   attribute.













































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                           BGPSEC_Path Attribute

         +-------------------------------------------------------+
         | Secure_Path                             (variable)    |
         +-------------------------------------------------------+
         | Sequence of one or two Signature_Blocks (variable)    |
         +-------------------------------------------------------+


   The Secure_Path contains AS path information for the BGPSEC update
   message.  This is logically equivalent to the information that is
   contained in a non-BGPSEC AS_PATH attribute.  A BGPSEC update message
   containing the BGPSEC_PATH attribute MUST NOT contain the AS_PATH
   attribute.  The Secure_Path is used by BGPSEC speakers in the same
   way that information from the AS_PATH is used by non-BGPSEC speakers.
   The format of the Secure_Path is described below in Section 3.1.

   The BGPSEC_Path attribute will contain one or two Signature_Blocks,
   each of which corresponds to a different algorithm suite.  Each of
   the Signature_Blocks will contain a signature segment for one AS
   number (i.e, secure path segment) in the Secure_Path.  In the most
   common case, the BGPSEC_Path attribute will contain only a single
   Signature_Block.  However, in order to enable a transition from an
   old algorithm suite to a new algorithm suite (without a flag day), it
   will be necessary to include two Signature_Blocks (one for the old
   algorithm suite and one for the new algorithm suite) during the
   transition period.  (See Section 6.1 for more discussion of algorithm
   transitions.)  The format of the Signature_Blocks is described below
   in Section 3.2.

3.1.  Secure_Path

   Here we provide a detailed description of the Secure_Path information
   in the BGPSEC_Path attribute.

                                Secure_Path

             +-----------------------------------------------+
             | Secure_Path Length                 (2 octets) |
             +-----------------------------------------------+
             | One or More Secure_Path Segments   (variable) |
             +-----------------------------------------------+


   The Secure_Path Length contains the length (in octets) of the entire
   Secure_Path (including the two octets used to express this length
   field).  As explained below, each Secure_Path segment is six octets
   long.  Note that this means the Secure_Path Length is two greater



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   than six times the number Secure_Path Segments (i.e., the number of
   AS numbers in the path).

   The Secure_Path contains one Secure_Path Segment for each (distinct)
   Autonomous System in the path to the originating AS of the NLRI
   specified in the update message.

                            Secure_Path Segment

                       +----------------------------+
                       | AS Number      (4 octets)  |
                       +----------------------------+
                       | pCount         (1 octet)   |
                       +----------------------------+
                       | Flags          (1 octet)   |
                       +----------------------------+


   The AS Number is the AS number of the BGP speaker that added this
   Secure_Path segment to the BGPSEC_Path attribute.  (See Section 4 for
   more information on populating this field.)

   The pCount field contains the number of repetitions of the associated
   autonomous system number that the signature covers.  This field
   enables a BGPSEC speaker to mimic the semantics of prepending
   multiple copies of their AS to the AS_PATH without requiring the
   speaker to generate multiple signatures.

   The first bit of the Flags field is the Confed_Segment flag.  The
   Confed_Segment flag is set to one to indicate that the BGPSEC speaker
   that constructed this Secure_Path segment is sending the update
   message to a peer AS within the same Autonomous System confederation
   [5].  (That is, the Confed_Segment flag is set in a BGPSEC update
   message whenever in a non-BGPSEC update message the BGP speaker's AS
   would appear in a AS_PATH segment of type AS_CONFED_SEQUENCE.)  In
   all other cases the Confed_Segment flag is set to zero.

   The remaining seven bits of the Flags MUST be set to zero by the
   sender, and ignored by the receiver.  Note, however, that the
   signature is computed over all eight bits of the flags field.

3.2.  Signature_Block

   Here we provide a detailed description of the Signature_Blocks in the
   BGPSEC_Path attribute.






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                              Signature_Block

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


   The Signature_Block Length is the total number of octets in the
   Signature_Block (including the two octets used to express this length
   field).

   The Algorithm Suite Identifier is a one-octet identifier specifying
   the digest algorithm and digital signature algorithm used to produce
   the digital signature in each Signature Segment.  An IANA registry of
   algorithm identifiers for use in BGPSEC is created in the BGPSEC
   algorithms document[11].

   A Signature_Block has exactly one Signature Segment for each
   Secure_Path Segment in the Secure_Path portion of the BGPSEC_Path
   Attribute.  (That is, one Signature Segment for each distinct AS on
   the path for the NLRI in the Update message.)

                            Signature Segments
              +---------------------------------------------+
              | Subject Key Identifier        (20 octets)   |
              +---------------------------------------------+
              | Signature Length              (2 octets)    |
              +---------------------------------------------+
              | Signature                     (variable)    |
              +---------------------------------------------+


   The Subject Key Identifier contains the value in the Subject Key
   Identifier extension of the RPKI router certificate [10] that is used
   to verify the signature (see Section 5 for details on validity of
   BGPSEC update messages).

   The Signature Length field contains the size (in octets) of the value
   in the Signature field of the Signature Segment.

   The Signature contains a digital signature that protects the NLRI and
   the BGPSEC_Path attribute (see Sections 4 and 5 for details on
   signature generation and validation, respectively).




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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 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
   BGPSEC_Path is the speaker's own AS.  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 BGPSEC_Path attribute will contain AS
   number(s) other than the speaker's own AS.

   The remaining case is where the BGPSEC speaker sends the update
   message to an internal (iBGP) peer.  When originating a new route
   advertisement and sending it to an internal peer, the BGPSEC speaker
   creates a new BGPSEC_Path attribute with zero Secure_Path segments
   and zero Signature Segments.  When propagating a received route
   advertisement to an internal peer, the BGPSEC speaker populates the
   BGPSEC_Path attribute by copying the BGPSEC_Path attribute from the
   received update message.  That is, the BGPSEC_Path 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 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.
   This is because a BGPSEC speaker receiving an update message with
   multiple NLRI would be unable to construct a valid BGPSEC update
   message (i.e., valid path signatures) containing a subset of the NLRI
   in the received update.  If a BGPSEC speaker wishes to advertise
   routes to multiple NLRI, then it MUST generate a separate BGPSEC
   update message for each NLRI.

   In order to create or add a new signature to a BGPSEC update message
   with 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



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   key in a valid Resource PKI end-entity certificate whose AS number
   resource extension includes the BGPSEC speaker's AS number [10]. Note
   also that 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 path will contain only a single AS number), when
   sending the route advertisement to an external, BGPSEC-speaking peer,
   the BGPSEC speaker creates a new BGPSEC_Path attribute as follows.

   First, the BGPSEC speaker constructs the Secure_Path with a single
   Secure_Path Segment.  The AS in this path is the BGPSEC speaker's own
   AS number.  In particular, this AS number MUST match an AS number in
   the AS number resource extension field of the Resource PKI router
   certificate(s) [10] that will be used to verify the digital
   signature(s) constructed by this BGPSEC speaker.

   The BGPSEC_Path attribute and the AS_Path attribute are mutually
   exclusive.  That is, any update message containing the BGPSEC_Path
   attribute MUST NOT contain the AS_Path attribute.  The information
   that would be contained in the AS_Path attribute is instead conveyed
   in the Secure_Path portion of the BGPSEC_Path attribute.

   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 [8]).  Note that validation of a
   BGPSEC update message will fail (i.e., the validation algorithm,
   specified in Section 5.2, returns 'Not Valid') unless there exists a
   valid ROA authorizing the first AS in the Secure_Path portion of the
   BGPSEC_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 pCount field of the Secure_Path Segment is typically set to the
   value 1.  However, a BGPSEC speaker may set the pCount field to a
   value greater than 1.  Setting the pCount field to a value greater
   than one has the same semantics as repeating an AS number multiple
   times in the AS_PATH of a non-BGPSEC update message (e.g., for
   traffic engineering purposes).  Setting the pCount field to a value



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   greater than one permits this repetition without requiring a separate
   digital signature for each repetition.

   If the BGPSEC speaker is not a member of an autonomous system
   confederation [5], then the Flags field of the Secure_Path Segment
   MUST be set to zero.  (Members of a confederation should follow the
   special processing instructions for confederation members in Section
   4.4.)

   Typically, a BGPSEC speaker will use only a single algorithm suite,
   and thus create only a single Signature_Block in the BGPSEC_Path
   attribute.  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
   that contain a Signature_Block for both the 'current' and the 'new'
   algorithm suites (see Section 6.1).

   When originating a new route advertisement, each Signature_Block MUST
   consist of a single Signature Segment.  The following describes how
   the BGPSEC speaker populates the fields of the Signature_Block.

   The Subject Key Identifier field (see Section 3) is populated with
   the identifier contained in the Subject Key Identifier extension of
   the RPKI router certificate corresponding to the BGPSEC speaker[10].
   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 Signature field contains a digital signature that binds the NLRI
   and BGPSEC_Path attribute to the RPKI router corresponding to the
   BGPSEC speaker.  The digital signature is computed as follows:

   o  Construct a sequence of octets by concatenating the Target AS
      Number, the Secure_Path (Origin AS, pCount, and Flags), 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.)












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                      Sequence of Octets to be Signed
        +------------------------------------+
        | Target AS Number      (4 octets)   |
        +------------------------------------+
        | Origin AS Number      (4 octets)   |  ---\
        +------------------------------------+      \
        | pCount                (1 octet)    |       >  Secure_Path
        +------------------------------------+      /
        | Flags                 (1 octet)    |  ---/
        +------------------------------------+
        | Algorithm Suite Id.   (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_Block) to obtain a digest value.

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

   The Signature Length field is populated with the length (in octets)
   of the Signature field.

4.2.  Propagating a Route Advertisement

   When a BGPSEC speaker receives a BGPSEC update message containing a
   BGPSEC_Path attribute (with one or more signatures) from an (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.

   If a BGPSEC router has received only a non-BGPSEC update message
   (without the BGPSEC_Path attribute), containing the AS_Path
   attribute, from a peer for a given prefix and if it chooses to
   propagate that peer's route for the prefix, then it MUST NOT attach
   any BGPSEC_Path attribute to the corresponding update being
   propagated.  (Note that a BGPSEC router may also receive a non-BGPSEC
   update message from an internal peer without the AS_Path attribute,
   i.e., with just the NLRI in it.  In that case, the prefix is
   originating from that AS and hence the BGPSEC speaker SHOULD sign and
   forward the update to its external peers, as specified in Section
   4.1.)




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   Conversely, if a BGPSEC router has received a BGPSEC update message
   (with the BGPSEC_Path attribute) from a peer for a given prefix and
   it chooses to propagate that peer's route for the prefix, then it
   SHOULD propagate the route as a BGPSEC update message containing the
   BGPSEC_Path attribute.  However, the BGPSEC speaker MAY propagate the
   route as a (unsigned) BGP update message without the BGPSEC_Path
   attribute.

   Note that removing BGPSEC signatures (i.e., propagating a route
   advertisement without the BGPSEC_Path attribute) has significant
   security ramifications.  (See Section 7 for discussion of the
   security ramifications of removing BGPSEC signatures.)  Therefore,
   when a route advertisement is received via a BGPSEC update message,
   propagating the route advertisement without the BGPSEC_Path attribute
   is NOT RECOMMENDED, unless the message is sent to a peer that did not
   advertise the capability to receive BGPSEC update messages (see
   Section 4.4).

   Furthermore, note that when a BGPSEC speaker propagates a route
   advertisement with the BGPSEC_Path attribute it is not attesting to
   the validation state of the update message it received.  (See Section
   7 for more discussion of the security semantics of BGPSEC
   signatures.)

   If the BGPSEC speaker is producing an update message which would, in
   the absence of BGPSEC, contain an AS_SET (e.g., the BGPSEC speaker is
   performing proxy aggregation), then the BGPSEC speaker MUST NOT
   include the BGPSEC_Path attribute.  In such a case, the BGPSEC
   speaker must remove any existing BGPSEC_Path in the received
   advertisement(s) for this prefix and produce a standard (non-BGPSEC)
   update message.  It should be noted that BCP 172 [12] recommends
   against the use of AS_SET and AS_CONFED_SET in AS_PATH in BGP
   updates.

   To generate the BGPSEC_Path attribute on the outgoing update message,
   the BGPSEC speaker first prepends a new Secure_Path Segment (places
   in first position) to the Secure_Path.  The AS number in this
   Secure_Path segment MUST match the AS number in the AS number
   resource extension field of the Resource PKI router certificate(s)
   that will be used to verify the digital signature(s) constructed by
   this BGPSEC speaker[10].

   The pCount is typically set to the value 1.  A BGPSEC speaker may set
   the pCount field to a value greater than 1.  (See Section 4.1 for a
   discussion of setting pCount to a value greater than 1.)  A route
   server that participates in the BGP control path, but does not act as
   a transit AS in the data plane, may choose to set pCount to 0.  This
   option enables the route server to participate in BGPSEC and obtain



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   the associated security guarantees without increasing the effective
   length of the AS path.  (Note that BGPSEC speakers compute the
   effective length of the AS path by summing the pCount values in the
   BGPSEC_Path attribute, see Section 5.)  However, when a route server
   sets the pCount value to 0, it still inserts its AS number into the
   Secure_Path segment, as this information is needed to validate the
   signature added by the route server.  Note that the option of setting
   pCount to 0 is intended only for use by route servers that desire not
   to increase the effective AS-PATH length of routes they advertise.
   The pCount field SHOULD NOT be set to 0 in other circumstances.
   BGPSEC speakers SHOULD drop incoming update messages with pCount set
   to zero in cases where the BGPSEC speaker does not expect its peer to
   set pCount to zero (i.e., cases where the peer is not acting as a
   route server).

   If the BGPSEC speaker is not a member of an autonomous system
   confederation [5], then the Confed_Segment bit of the Flags field of
   the Secure_Path Segment MUST be set to zero.  (Members of a
   confederation should follow the special processing instructions for
   confederation members in Section 4.3.)

   If the received BGPSEC update message contains two Signature_ Blocks
   and the BGPSEC speaker supports both of the corresponding algorithms
   suites, then the new update message generated by the BGPSEC speaker
   SHOULD include both of the Signature_Blocks.  If the received BGPSEC
   update message contains two Signature_Blocks and the BGPSEC speaker
   only supports one of the two corresponding algorithm suites, then the
   BGPSEC speaker MUST remove the Signature_Block corresponding to the
   algorithm suite that it does not understand.  If the BGPSEC speaker
   does not support the algorithm suites in any of the Signature_Blocks
   contained in the received update message, then the BGPSEC speaker
   MUST NOT propagate the route advertisement with the BGPSEC_Path
   attribute.  (That is, if it chooses to propagate this route
   advertisement at all, it must do so as an unsigned BGP update
   message).

   Note that in the case where there are two Signature_Blocks
   (corresponding to different algorithm suites) that the validation
   algorithm (see Section 5.2) deems a BGPSEC update message to be
   'Valid' if there is at least one supported algorithm suite (and
   corresponding Signature_Block) that is deemed 'Valid'.  This means
   that a 'Valid' BGPSEC update message may contain a Signature_Block
   which is not deemed 'Valid' (e.g., contains signatures that the
   BGPSEC does not successfully verify).  Nonetheless, such
   Signature_Blocks MUST NOT be removed.  (See Section 7 for a
   discussion of the security ramifications of this design choice.)

   For each Signature_Block corresponding to an algorithm suite that the



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   BGPSEC speaker does support, the BGPSEC speaker then adds a new
   Signature Segment to the Signature_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 Secure_Path segments in the Secure_Path
   portion of the BGPSEC_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 router corresponding to the BGPSEC speaker[10].  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 Signature field in the new segment contains a digital signature
   that binds the NLRI and BGPSEC_Path attribute to the RPKI router
   certificate corresponding to the BGPSEC speaker.  The digital
   signature is computed as follows:

   o  Construct a sequence of octets by concatenating the Target AS
      number, the Secure_Path segment that is being added by the BGPSEC
      speaker constructing the signature, and the signature field of the
      most recent Signature Segment (the one corresponding to AS from
      whom the BGPSEC speaker's AS received the announcement).  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
       +--------------------------------------+
       | Target AS Number        (4 octets)   |
       +--------------------------------------+
       | Signer's AS Number      (4 octets)   |  ---\
       +--------------------------------------+      \
       | pCount                  (1 octet)    |       >  Secure_Path
       +--------- ----------------------------+      /
       | Flags                   (1 octet)    |  ---/
       +--------------------------------------+
       | Most Recent Sig Field   (variable)   |
       +--------------------------------------+

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

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



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      signature.

   The Signature Length field is populated with the length (in octets)
   of the Signature field.

4.3.  Processing Instructions for Confederation Members

   Members of autonomous system confederations [5] MUST additionally
   follow the instructions in this section for processing BGPSEC update
   messages.

   When a confederation member sends a BGPSEC update message to a peer
   that is a member of the same confederation, the confederation member
   puts its (private) Member-AS Number (as opposed to the public AS
   Confederation Identifier) in the AS Number field of the Secure_Path
   Segment that it adds to the BGPSEC update message.  Furthermore, when
   a confederation member sends a BGPSEC update message to a peer that
   is a member of the same confederation, the BGPSEC speaker that
   generates the Secure_Path Segment sets the Confed_Segment flag to
   one.  Note that this means that in a BGPSEC update message, an AS
   number appears in a Secure_Path Segment with the Confed_Segment flag
   set to one, in precisely those circumstances where the AS number
   would appear in a segment of type AS_CONFED_SEQUENCE in a non-BGPSEC
   update message.

   Within a confederation, the verification of BGPSEC signatures added
   by other members of the confederation is optional.  If a
   confederation chooses to have its members not verify signatures added
   by other confederation members, then when sending a BGPSEC update
   message to a peer that is a member of the same confederation, the
   confederation MAY set the Signature field within the
   Signature_Segment that it generates to be zero (in lieu of
   calculating the correct digital signature as described in Sections
   4.1 and 4.2).  Note that if a confederation chooses not to verify
   digital signatures within the confederation, then BGPSEC is able to
   provide no assurances about the integrity of the (private) Member-AS
   Numbers placed in Secure_Path segments where the Confed_Segment flag
   is set to one.

   When a confederation member receives a BGPSEC update message from a
   peer within the confederation and propagates it to a peer outside the
   confederation, it needs to remove all of the Secure_Path Segments
   added by confederation members as well as the corresponding Signature
   Segments.  To do this, the confederation member propagating the route
   outside the confederation does the following:

   o  First, starting with the most recently added Secure_Path segments,
      remove all of the consecutive Secure_Path segments that have the



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      Confed_Segment flag set to one.  Stop this process once a
      Scure_Path segment is reached which has its Confed_Segment flag
      set to zero.  Keep a count of the number of segments removed in
      this fashion.

   o  Second, starting with the most recently added Signature Segment,
      remove a number of Signature Segments equal to the number of
      Secure_Path Segments removed in the previous step.  (That is,
      remove the K most recently added signature segments, where K is
      the number of Secure_Path Segments removed in the previous step.)

   o  Finally, add a Secure_Path Segment containing, in the AS field,
      the AS Confederation Identifier (the public AS number of the
      confederation) as well as a corresponding Signature Segment.  Note
      that all fields other that the AS field are populated as per
      Sections 4.1 and 4.2.

   When validating a received BGPSEC update message, confederation
   members need to make the following adjustment to the algorithm
   presented in Section 5.2.  When a confederation member processes
   (validates) a Signature Segment and its corresponding Secure_Path
   Segment, the confederation member must note that for a signature
   produced by a BGPSEC speaker outside of a confederation, the Target
   AS will always be the AS Confederation Identifier (the public AS
   number of the confederation) as opposed to the Member-AS Number.

   To handle this case, when a BGPSEC speaker (that is a confederation
   member) processes a current Secure_Path Segment that has the
   Confed_Segment flag set to zero, if the next most recently added
   Secure_Path segment has the Confed_Segment flag set to one then, when
   computing the digest for the current Secure_Path segment, the BGPSEC
   speaker takes the Target AS Number to be the AS Confederation
   Identifier of the validating BGPSEC speaker's own confederation.
   (Note that the algorithm in Section 5.2 processes Secure_Path
   Segments in order from most recently added to least recently added,
   therefore this special case will apply to the first Secure_Path
   segment that the algorithm encounters that has the Confed_Segment
   flag set to zero.)

   Finally, as discussed above, an AS confederation may optionally
   decide that its members will not verify digital signatures added by
   members.  In such a federation, when a confederation member runs the
   algorithm in Section 5.2, when processing a Signature_Segment, the
   confederation member first checks whether the Confed_Sequence flag in
   the corresponding Secure_Path segment is set to one.  If the
   Confed_Sequence flag is set to one in the corresponding Secure_Path
   segment, the confederation member does not perform any further checks
   on the Signature_Segment and immediately moves on to the next



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   Signature_Segment (and checks its corresponding Secure_Path segment).
   Note that as specified in Section 5.2, it is an error for a BGPSEC
   speaker to receive a BGPSEC update messages containing a Secure_Path
   segment with the Confed_Sequence flag set to one from a peer who is
   not a member of the same AS confederation.  (Such an error is treated
   in exactly the same way as receipt of a non-BGPSEC update message
   containing an AS_CONFED_SEQUENCE from a peer that is not a member of
   the same AS confederation.)

4.4.  Reconstructing the AS_PATH Attribute

   BGPSEC update messages do not contain the AS_PATH attribute.  Note,
   however, that the AS_PATH attribute can be reconstructed from the
   BGPSEC_Path attribute.  This is necessary in the case where a route
   advertisement is received via a BGPSEC update message and then
   propagated to a peer via a non-BGPSEC update message.  There may be
   additional cases where an implementation finds it useful to perform
   this reconstruction.

   The AS_PATH attribute can be constructed from the BGPSEC_Path
   attribute as follows.  Starting with an empty AS_PATH attribute,
   process the Secure_Path segments in order from least-recently added
   (corresponding to the origin) to most-recently added.  For each
   Secure_Path segment perform the following steps:

   1.  If the Confed_Segment flag in the Secure_Path segment is set to
       one, then look at the most-recently added segment in the AS_PATH.

       *  In the case where the AS_PATH is empty or in the case where
          the most-recently added segment is of type AS_SEQUENCE then
          add (prepend to the AS_PATH) a new AS_PATH segment of type
          AS_CONFED_SEQUENCE.  This segment of type AS_CONFED_SEQUENCE
          shall contain a number of elements equal to the pCount field
          in the current Secure_Path segment.  Each of these elements
          shall be the AS number contained in the current Secure_Path
          segment.  (That is, if the pCount field is X, then the segment
          of type AS_CONFED_SEQUENCE contains X copies of the
          Secure_Path segment's AS Number field.)

       *  In the case where the most-recently added segment in the
          AS_PATH is of type AS_CONFED_SEQUENCE then add (prepend to the
          segment) a number of elements equal to the pCount field in the
          current Secure_Path segment.  The value of each of these
          elements shall be the AS number contained in the current
          Secure_Path segment.  (That is, if the pCount field is X, then
          add X copies of the Secure_Path segment's AS Number field to
          the existing AS_CONFED_SEQUENCE.)




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   2.  If the Confed_Segment flag in the Secure_Path segment is set to
       zero, then look at the most-recently added segment in the
       AS_PATH.

       *  In the case where the AS_PATH is empty, and the pCount field
          in the Secure_Path segment is greater than zero, add (prepend
          to the AS_PATH) a new AS_PATH segment of type AS_SEQUENCE.
          This segment of type AS_SEQUENCE shall contain a number of
          elements equal to the pCount field in the current Secure_Path
          segment.  Each of these elements shall be the AS number
          contained in the current Secure_Path segment.  (That is, if
          the pCount field is X, then the segment of type AS_SEQUENCE
          contains X copies of the Secure_Path segment's AS Number
          field.)

       *  In the case where the most recently added segment in the
          AS_PATH is of type AS_SEQUENCE then add (prepend to the
          segment) a number of elements equal to the pCount field in the
          current Secure_Path segment.  The value of each of these
          elements shall be the AS number contained in the current
          Secure_Path segment.  (That is, if the pCount field is X, then
          add X copies of the Secure_Path segment's AS Number field to
          the existing AS_SEQUENCE.)


5.  Processing a Received BGPSEC Update

   Upon receiving a BGPSEC update message from an external (eBGP) peer,
   a BGPSEC speaker SHOULD validate the message to determine the
   authenticity of the path information contained in the BGPSEC_Path
   attribute.  Section 5.1 provides an overview of BGPSEC validation and
   Section 5.2 provides a specific algorithm for performing such
   validation.  (Note that an implementation need not follow the
   specific algorithm in Section 5.2 as long as the input/output
   behavior of the validation is identical to that of the algorithm in
   Section 5.2.)  During exceptional conditions (e.g., the BGPSEC
   speaker receives an incredibly large number of update messages at
   once) a BGPSEC speaker MAY temporarily defer validation of incoming
   BGPSEC update messages.  The treatment of such BGPSEC update
   messages, whose validation has been deferred, is a matter of local
   policy.

   The validity of BGPSEC update messages is a function of the current
   RPKI state.  When a BGPSEC speaker learns that RPKI state has changed
   (e.g., from an RPKI validating cache via the RTR protocol), the
   BGPSEC speaker MUST re-run validation on all affected update messages
   stored in its ADJ-RIB-IN.  That is, when a given RPKI certificate
   ceases to be valid (e.g., it expires or revoked), all update messages



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   containing a signature whose SKI matches the SKI in the given
   certificate must be re-assessed to determine if they are still valid.
   Note that this reassessment determines that the validity state of an
   update has changed then, depending on local policy, it may be
   necessary to re-run best path selection.

   BGPSEC update messages do not contain an AS_PATH attribute.
   Therefore, a BGPSEC speaker MUST utilize the AS path information in
   the BGPSEC_Path attribute in all cases where it would otherwise use
   the AS path information in the AS_PATH attribute.  The only exception
   to this rule is when AS path information must be updated in order to
   propagate a route to a peer (in which case the BGPSEC speaker follows
   the instructions in Section 4).  Section 4.4 provides an algorithm
   for constructing an AS_PATH attribute from a BGPSEC_Path attribute.
   Whenever the use of AS path information is called for (e.g., loop
   detection, or use of AS path length in best path selection) the
   externally visible behavior of the implementation shall be the same
   as if the implementation had run the algorithm in Section 4.4 and
   used the resulting AS_PATH attribute as it would for a non-BGPSEC
   update message.

   Many signature algorithms are non-deterministic.  That is, many
   signature algorithms will produce different signatures each time they
   are run (even when they are signing the same data with the same key).
   Therefore, if an implementation receives a BGPSEC update from a peer
   and later receives a second BGPSEC update message from the same peer,
   the implementation SHOULD treat the second message as a duplicate
   update message if it differs from the first update message only in
   the Signature fields (within the BGPSEC_Path attribute).  That is, if
   all the fields in the second update are identical to the fields in
   the first update message, except for the Signature fields, then the
   second update message should be treated as a duplicate of the first
   update message.  Note that if other fields (e.g., the Subject Key
   Identifier field) within a Signature segment differ between two
   update messages then the two updates are not duplicates.

   With regards to the processing of duplicate update messages, if the
   first update message is valid, then an implementation SHOULD NOT run
   the validation procedure on the second, duplicate update message
   (even if the bits of the signature field are different).  If the
   first update message is not valid, then an implementation SHOULD run
   the validation procedure on the second duplicate update message (as
   the signatures in the second update may be valid even though the
   first contained a signature that was invalid).







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5.1.  Overview of BGPSEC Validation

   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
   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 router certificate containing an AS Number
      extension, the AS Number, Public Key and Subject Key Identifier
      are required,

   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.  (For example,
   the trusted cache could deliver the necessary validity information to
   the BGPSEC speaker using the router key PDU [15]for the RTR protocol
   [14].)

   To validate a BGPSEC update message containing the BGPSEC_Path
   attribute, the recipient performs the validation steps specified in
   Section 5.2.  The validation procedure results in one of two states:
   'Valid' and 'Not Valid'.

   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 local policy
   mechanisms.  It is expected that BGP peers will generally prefer
   routes received via 'Valid' BGPSEC update messages over routes
   received via 'Not Valid' BGPSEC update messages as well as routes
   received via update messages that do not contain the BGPSEC_Path
   attribute.  However, BGPSEC specifies no changes to the BGP decision
   process.  (See [16] for related operational considerations.)

   BGPSEC validation needs 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 via some
   mechanism, according to local policy within an AS.  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 attribute intact (regardless of the validation state of
   the update message).  Based entirely on local policy, an egress
   router receiving a BGPSEC update message from within its own AS MAY



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   choose to perform its own validation.

5.2.  Validation Algorithm

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

   First, the recipient of a BGPSEC update message performs a check to
   ensure that the message is properly formed.  Specifically, the
   recipient performs the following checks:

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

   2.  Check that each Signature_Block contains one Signature segment
       for each Secure_Path segment in the Secure_Path portion of the
       BGPSEC_Path attribute.  (Note that the entirety of each
       Signature_Block must be checked to ensure that it is well formed,
       even though the validation process may terminate before all
       signatures are cryptographically verified.)

   3.  Check that the update message does not contain an AS_PATH
       attribute.

   4.  If the update message was received from a peer that is not a
       member of the BGPSEC speaker's AS confederation, check to ensure
       that none of the Secure_Path segments contain a Flags field with
       the Confed_Sequence flag set to one.

   5.  If the update message was received from a peer that is not
       expected to set pCount equal to zero (see Section 4.2) then check
       to ensure that the pCount field in the most-recently added
       Secure_Path segment is not equal to zero.

   If any of these checks identify an error in the BGPSEC_Path
   attribute, then the implementation should notify the operator that an
   error has occurred and treat the update in a manner consistent with
   other BGP errors (i.e., following RFC 4271[2] or any future updates
   to that document).

   Next, 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 Secure_Path portion of the



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   BGPSEC_Path attribute.  If the origin AS in the Secure_Path is not in
   the set of AS numbers associated with the given prefix, then the
   BGPSEC update message is 'Not Valid' and the validation algorithm
   terminates.

   Finally, the BGPSEC speaker examines the Signature_Blocks in the
   BGPSEC_Path attribute.  A Signature_Block corresponding to an
   algorithm suite that the BGPSEC speaker does not support is not
   considered in validation.  If there does not exist a Signature_Block
   corresponding to an algorithm suite that the BGPSEC speaker supports,
   then the BGPSEC speaker MUST treat the update message in the same
   manner that the BGPSEC speaker would treat an (unsigned) update
   message that arrived without a BGPSEC_Path attribute.

   For each remaining Signature_Block (corresponding to an algorithm
   suite supported by the BGPSEC speaker), the BGPSEC speaker iterates
   through the Signature segments in the Signature_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 Secure_Path segments
   within the BGPSEC_Path attribute.  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 router certificate data and look up all valid (AS, SKI,
      Public Key) triples in which the AS matches the AS number in the
      corresponding Secure_Path segment.  Of these triples that match
      the AS number, check whether there is an SKI that matches the
      value in the Subject Key Identifier field of the Signature
      segment.  If this check finds no such matching SKI value, then
      mark the entire Signature_Block as 'Not Valid' and proceed to the
      next Signature_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:












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                      Sequence of Octets to be Hashed

     +-------------------------------------------+
     | AS Number of Target AS         (4 octets) |
     +-------------------------------------------+
     | AS Number                      (4 octets) |  ---\
     +-------------------------------------------+      \
     | pCount                         (1 octet)  |       >  Secure_Path
     +-------------------------------------------+      /
     | Flags                          (1 octet)  |  ---/
     +-------------------------------------------+
     | Sig Field in the Next Segment  (variable) |
     +-------------- ----------------------------+


   For the first segment to be processed (the most recently added
   segment), the 'AS Number of Target 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 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 Target AS' is the
   AS number in the Secure_Path segment that corresponds to the
   Signature Segment added immediately after the one being processed.
   (That is, in the Secure_Path segment that corresponds to the
   Signature segment that the validator just finished processing.)

   The AS Number, pCount and Flags fields are taken from the Secure_Path
   segment that corresponds to the Signature segment currently being
   processed.  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).

   Alternatively, if the segment being processed corresponds to the
   origin AS (i.e., if it is the least recently added segment), then the
   digest function should be computed on the following sequence of
   octets:












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                      Sequence of Octets to be Hashed
        +------------------------------------+
        | AS Number of Target AS (4 octets)  |
        +------------------------------------+
        | Origin AS Number       (4 octets)  |  ---\
        +------------------------------------+      \
        | pCount                 (1 octet)   |       >  Secure_Path
        +------------------------------------+      /
        | Flags                  (1 octet)   |  ---/
        +------------------------------------+
        | Algorithm Suite Id.    (1 octet)   |
        +------------------------------------+
        | NLRI Length            (1 octet)   |
        +------------------------------------+
        | NLRI Prefix            (variable)  |
        +------------------------------------+

   The NLRI Length, NLRI Prefix, and Algorithm Suite Identifier are all
   obtained in a straight forward manner from the NLRI of the update
   message or the BGPSEC_Path attribute being validated.  The Origin AS
   Number, pCount, and Flags fields are taken from the Secure_Path
   segment corresponding to the Signature Segment currently being
   processed.

   The 'AS Number of Target AS' is the AS Number from the Secure_Path
   segment that was added immediately after the Secure_Path segment
   containing the Origin AS Number.  (That is, the Secure_Path segment
   corresponding to the Signature segment that the receiver just
   finished processing prior to the current Signature segment.)

   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
      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_Block as 'Not
      Valid' and proceed to the next Signature_Block.  If the signature
      validation algorithm determines that the signature is valid, then
      continue processing Signature Segments (within the current
      Signature_Block).

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




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   If at least one Signature_Block is marked as 'Valid', then the
   validation algorithm terminates and the BGPSEC update message is
   deemed to be 'Valid'.  (That is, if a BGPSEC update message contains
   two Signature_Blocks then the update message is deemed 'Valid' if the
   first Signature_Block is marked 'Valid' OR the second Signature_Block
   is marked 'Valid'.)


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 [11].

   It is anticipated that in the future mandatory algorithm suites
   document will be updated to specify a transition from the 'current'
   algorithm suite to a '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 attribute can contain signatures, in parallel,
   for two algorithm suites.)  Once the transition is 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_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 and thus
   necessitate a new version of BGPSEC.  Examples of such changes
   include:




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   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_Block is not equal to the number of
      ASes in the Secure_Path (e.g., aggregate signatures)

   o  Changes to the data that is protected by the BGPSEC signatures
      (e.g., attributes other than the 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_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 6.1.  During the transition
   period all BGPSEC speakers SHOULD simultaneously include both the
   BGPSEC_PATH attribute and the new BGPSEC_PATH_TWO attribute.  Once
   the transition is complete, the use of BGPSEC_PATH could then be
   deprecated, at which point BGPSEC speakers SHOULD include only the
   new BGPSEC_PATH_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 [13].

   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, in the RPKI, by the IP address space holder to
      originate route advertisements for the given prefix.

   o  For each AS in the path, a BGPSEC speaker authorized by the holder
      of the AS number intentionally chose (in accordance with local
      policy) to propagate the route advertisement to the subsequent AS
      in the path.

   That is, the recipient of a valid BGPSEC Update message is assured



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   that the Secure_Path portion of the BGPSEC_Path attribute 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 that BGPSEC does not offer any 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 although BGPSEC provides a mechanism for an AS to validate
   that a received update message has certain security properties, the
   use of such a mechanism to influence route selection is completely a
   matter of local policy.  Therefore, a BGPSEC speaker can make no
   assumptions about the validity of a route received from an external
   BGPSEC peer.  That is, a compliant BGPSEC peer may (depending on the
   local policy of the peer) send update messages that fail the validity
   test in Section 5.  Thus, a BGPSEC speaker MUST completely validate
   all BGPSEC update messages received from external peers.  (Validation
   of update messages received from internal peers is a matter of local
   policy, see Section 5).

   Note that there may be cases where a BGPSEC speaker deems 'Valid' (as
   per the validation algorithm in Section 5.2) a BGPSEC update message
   that contains both a 'Valid' and a 'Not Valid' Signature_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_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 'Valid' and the 'Not Valid'
   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 'Valid' and a set of algorithm B
   signatures which are 'Not Valid'.  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
   the 'Not Valid' set of signatures corresponding to algorithm B, such
   entities would treat the message as though it were unsigned.  By
   including the 'Not Valid' set of signatures when propagating a route
   advertisement, the BGPSEC speaker ensures that 'downstream' entities



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   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
   Valid' signatures.  That is, by removing the set of 'Not Valid'
   signatures the BGPSEC speaker might actually cause a downstream
   entity to 'upgrade' the status of a route advertisement from 'Not
   Valid' to unsigned.  Finally, note that in the above scenario, the
   BGPSEC speaker might have deemed algorithm A signatures 'Valid' 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 Valid' (due to the
   revocation) and not as 'unsigned' (which would happen if the 'Not
   Valid' Signature_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 Valid' BGPSEC
   update message.  (That is, a BGPSEC update containing only 'Not
   Valid' Signature_Blocks.)  In such a case, the BGPSEC speaker should
   propagate a signed BGPSEC update message, adding his signature to the
   'Not Valid' 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
   Valid' at an upstream BGPSEC speaker may indeed be deemed 'Valid' 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:

   o  The BGPSEC speaker received the given route advertisement with the
      indicated NLRI and Secure_Path; and

   o  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



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   expensive checks (e.g., signature verification) after performing less
   expensive checks (e.g., syntax checks).  The validation algorithm
   specified in Section 5.2 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 5.2 may not provide the best denial of
   service protection for all implementations.

   The mechanism of setting the pCount field to zero is included in this
   specification to enable route servers in the control path to
   participate in BGPSEC without increasing the effective length of the
   AS-PATH.  However, entities other than route servers could
   conceivably use this mechanism (set the pCount to zero) to attract
   traffic (by reducing the effective length of the AS-PATH)
   illegitimately.  This risk is largely mitigated if every BGPSEC
   speaker drops incoming update messages that set pCount to zero but
   come from a peer that is not a route server.  However, note that a
   recipient of a BGPSEC update message in which an upstream entity that
   is two or more hops away set pCount to zero is unable to verify for
   themselves whether pCount was set to zero legitimately.

   Finally, BGPSEC does not provide protection against attacks at the
   transport layer.  An adversary on the path between a BGPSEC speaker
   and its peer is able to perform attacks such as modifying valid
   BGPSEC updates to cause them to fail validation, injecting (unsigned)
   BGP update messages without BGPSEC_Path_Signature attributes, or
   injecting BGPSEC update messages with BGPSEC_Path_Signature
   attributes that fail validation, or causing the peer to tear-down the
   BGP session.  Therefore, BGPSEC sessions SHOULD be protected by
   appropriate transport security mechanisms.


8.  IANA Considerations

   TBD: Need IANA to assign numbers for the two capabilities and the
   BGPSEC_PATH attribute.

   This document does not create any new IANA registries.


9.  Contributors

9.1.  Authors

   Rob Austein
   Dragon Research Labs
   sra@hactrn.net



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   Steven Bellovin
   Columbia University
   smb@cs.columbia.edu


   Randy Bush
   Internet Initiative Japan
   randy@psg.com


   Russ Housley
   Vigil Security
   housley@vigilsec.com


   Matt Lepinski
   BBN Technologies
   lepinski@bbn.com



   Stephen Kent
   BBN Technologies
   kent@bbn.com


   Warren Kumari
   Google
   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
   Sparta
   weiler+ietf@watson.org







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9.2.  Acknowledgements

   The authors would like to thank Luke Berndt, Sharon Goldberg, Ed
   Kern, Chris Morrow, Doug Maughan, Pradosh Mohapatra, Russ Mundy,
   Sandy Murphy, Keyur Patel, Mark Reynolds, Heather Schiller, Jason
   Schiller, John Scudder, Ruediger Volk and David Ward for their
   valuable input and review.


10.  Normative References

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

   [2]   Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border
         Gateway Protocol 4", RFC 4271, January 2006.

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

   [4]   Vohra, Q. and E. Chen, "BGP Support for Four-octet AS Number
         Space", RFC 4893, May 2007.

   [5]   Traina, P., McPherson, D., and J. Scudder, "Autonomous System
         Confederations for BGP", RFC 5065, August 2007.

   [6]   Scudder, J. and R. Chandra, "Capabilities Advertisement with
         BGP-4", RFC 5492, February 2009.

   [7]   Lepinski, M. and S. Kent, "An Infrastructure to Support Secure
         Internet Routing", RFC 6480, February 2012.

   [8]   Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
         Origin Authorizations (ROAs)", RFC 6482, February 2012.

   [9]   Patel, K., Ward, D., and R. Bush, "Extended Message support for
         BGP", July 2012.

   [10]  Reynolds, M., Turner, S., and S. Kent, "A Profile for BGPSEC
         Router Certificates, Certificate Revocation Lists, and
         Certification Requests", April 2012.

   [11]  Turner, S., "BGP Algorithms, Key Formats, & Signature Formats",
         March 2012.

11.  Informative References

   [12]  Kumari, W. and K. Sriram, "Recommendation for Not Using AS_SET



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         and AS_CONFED_SET in BGP", RFC 6472, December 2011.


   [13]  Kent, S., "Threat Model for BGP Path Security", February 2012.

   [14]  Bush, R. and R. Austein, "The RPKI/Router Protocol",
         February 2012.

   [15]  Bush, R., Patel, K., and S. Turner, "Router Key PDU for RPKI-
         Router Protocol", October 2012.

   [16]  Bush, R., "BGPsec Operational Considerations", May 2012.


Author's Address

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

   Phone: +1 617 873 5939
   Email: mlepinski.ietf@gmail.com



























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