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Versions: (draft-lepinski-bgpsec-protocol) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 RFC 8205

Network Working Group                                   M. Lepinski, Ed.
Internet-Draft                                                       NCF
Intended status: Standards Track                          K. Sriram, Ed.
Expires: December 21, 2016                                          NIST
                                                           June 21, 2016


                     BGPsec Protocol Specification
                   draft-ietf-sidr-bgpsec-protocol-16

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 December 21, 2016.

Copyright Notice

   Copyright (c) 2016 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 . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  BGPsec Negotiation . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  The BGPsec Capability  . . . . . . . . . . . . . . . . . .  3
     2.2.  Negotiating BGPsec Support . . . . . . . . . . . . . . . .  4
   3.  The BGPsec_Path Attribute  . . . . . . . . . . . . . . . . . .  6
     3.1.  Secure_Path  . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Signature_Block  . . . . . . . . . . . . . . . . . . . . .  8
   4.  BGPsec Update Messages . . . . . . . . . . . . . . . . . . . . 10
     4.1.  General Guidance . . . . . . . . . . . . . . . . . . . . . 10
     4.2.  Constructing the BGPsec_Path Attribute . . . . . . . . . . 12
     4.3.  Processing Instructions for Confederation Members  . . . . 16
     4.4.  Reconstructing the AS_PATH Attribute . . . . . . . . . . . 18
   5.  Processing a Received BGPsec Update  . . . . . . . . . . . . . 19
     5.1.  Overview of BGPsec Validation  . . . . . . . . . . . . . . 21
     5.2.  Validation Algorithm . . . . . . . . . . . . . . . . . . . 22
   6.  Algorithms and Extensibility . . . . . . . . . . . . . . . . . 25
     6.1.  Algorithm Suite Considerations . . . . . . . . . . . . . . 25
     6.2.  Extensibility Considerations . . . . . . . . . . . . . . . 26
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
     7.1 Security Guarantees  . . . . . . . . . . . . . . . . . . . . 27
     7.2 On the Removal of BGPsec Signatures  . . . . . . . . . . . . 27
     7.3 Mitigation of Denial of Service Attacks  . . . . . . . . . . 29
     7.4 Additional Security Considerations . . . . . . . . . . . . . 29
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 30
   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 30
     9.1.  Authors  . . . . . . . . . . . . . . . . . . . . . . . . . 30
     9.2.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . 31
   10.  Normative References  . . . . . . . . . . . . . . . . . . . . 31
   11.  Informative References  . . . . . . . . . . . . . . . . . . . 32
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 34



1.  Introduction




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   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
   property: Every AS on the path of ASes listed in the update message
   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 is intended to be used to supplement BGP Origin Validation
   [19][20] and when used in conjunction with origin validation, it is
   possible to prevent a wide variety of route hijacking attacks against
   BGP.

   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 [12] and
   the documents referenced therein.)  Any BGPsec speaker who wishes to
   send, to external (eBGP) peers, BGP update messages containing the
   BGPsec_Path needs to possess a private key associated with an RPKI
   router certificate [9] 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 received update messages containing
   the BGPsec_Path attribute (see Section 5.2).

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.


                       BGPsec Send Capability Value:




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



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   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 MUST
   support the BGP multiprotocol extension [3]. Additionally, a BGP
   speaker MUST NOT advertise the capability of BGPsec support for a
   particular AFI unless it has also advertised the multiprotocol
   extension capability for the same AFI [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 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 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.  Any future document which specifies
   additional versions of BGPsec will need to specify behavior in the
   case that support for multiple versions is negotiated.

   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
   extended messages [8].





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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 the path 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.

                           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. The information in
   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 each 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



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   field).  As explained below, each Secure_Path segment is six octets
   long.  Note that this means the Secure_Path Length is two greater
   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

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


   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 pCount field is also
   useful in managing route servers (see Section 4.2) and AS Number
   migrations, see [18] for details.

   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



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   BGPsec_Path attribute.

                              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 specified in the BGPsec
   algorithms document [10].

   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 [9] 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



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   signature generation and validation, respectively).

4.  BGPsec Update Messages

   Section 4.1 provides general guidance on the creation of BGPsec
   Update Messages -- that is, update messages containing the
   BGPsec_Path attribute.

   Section 4.2 specifies how a BGPsec speaker generates the BGPsec_Path
   attribute to include in a BGPsec Update message.

   Section 4.3 contains special processing instructions for members of
   an autonomous system confederation [5]. A BGPsec speaker that is not
   a member of such a confederation MUST set the Flags field of the
   Secure_Path Segment to zero in all BGPsec update messages it sends.

   Section 4.4 contains instructions for reconstructing the AS_PATH
   attribute in cases where a BGPsec speaker receives an update message
   with a BGPsec_Path attribute and wishes to propagate the update
   message to a peer who does not support BGPsec.

4.1.  General Guidance

   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 whom 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. Additionally, a BGPsec update message
   MUST use the MP_REACH_NLRI [3] attribute to encode the NLRI.

   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.

   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



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   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 [9]. 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.

   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 [7]). It is expected that most
   relying parties will utilize BGPsec in tandem with origin validation
   (see [19] and [20]). Therefore, it is RECOMMENDED that a BGPsec
   speaker only originate a BGPsec update advertising a route for a
   given prefix if there exists a valid ROA authorizing the BGPsec
   speaker's AS to originate routes to this 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 then it MUST NOT attach a
   BGPsec_Path attribute when it propagates the update message.  (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 BGPsec-speaking peers.)

   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.

   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



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   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 traditional (non-
   BGPsec) update message.  It should be noted that BCP 172 [13]
   recommends against the use of AS_SET and AS_CONFED_SET in the AS_PATH
   of BGP updates.

   The case where the BGPsec speaker sends a BGPsec update message to an
   internal (iBGP) peer is quite simple.  When originating a new route
   advertisement and sending it to an internal peer, the BGPsec speaker
   omits the BGPsec_Path attribute.  When propagating a received route
   advertisement to an internal peer, the BGPsec speaker typically
   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. However, in the case that the BGPsec
   speaker is performing an AS Migration, the BGPsec speaker may add an
   additional signature on ingress before copying the BGPsec_Path
   attribute (see [18] for more details). Note that when a BGPsec
   speaker chooses to forward a BGPsec update message to an iBGP peer,
   the BGPsec attribute SHOULD NOT be removed, unless the peer doesn't
   support BGPsec. In particular, the BGPsec attribute SHOULD NOT be
   removed even in the case where the BGPsec update message has not been
   successfully validated. (See Section 5 for more information on
   validation, and Section 7 for the security ramifications of removing
   BGPsec signatures.)


4.2.  Constructing the BGPsec_Path Attribute

   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. Similarly, when sending a
   new route advertisement to an external, BGPsec-speaking peer, the
   BGPsec speaker may send a BGPsec Update message by generating a new
   BGPsec_Path attribute.

   To generate the BGPsec_Path attribute on the outgoing update message,
   the BGPsec speaker first generates a new Secure_Path Segment. Note
   that if the BGPsec speaker is not the origin AS and there is an



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   existing BGPsec_Path attribute, then the BGPsec speaker prepends its
   new Secure_Path Segment (places in first position) onto the existing
   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 [9].

   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).

   To prevent unnecessary processing load in the validation of BGPsec
   signatures, a BGPsec speaker SHOULD NOT produce multiple consecutive
   Secure_Path Segments with the same AS number. This means that to
   achieve the semantics of prepending the same AS number k times, a
   BGPsec speaker SHOULD produce a single Secure_Path Segment -- with
   pCount of k -- and a single corresponding Signature Segment.

   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 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. (See [18] for a
   discussion of setting pCount to 0 to facilitate AS Number Migration.)
   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. (That is, pCount is only to be set to zero in
   cases such as route servers or AS Number Migration where the BGPsec
   speaker's peer expects pCount to be set to zero.)

   Next, the BGPsec speaker generates one or two Signature_Blocks.
   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).



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   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. See Section 4.4 for more information on converting to an
   unsigned BGP message.)

   Note that in the case where the BGPsec_Path has two Signature_Blocks
   (corresponding to different algorithm suites), 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
   BGPsec speaker does support, the BGPsec speaker 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 appear in the same order as the
   corresponding Secure_Path segments.  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 certificate corresponding to the BGPsec speaker [9].
   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  For clarity, let us number the Secure_Path and corresponding



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      Signature Segments from 1 to N as follows. Let Secure_Path Segment
      1 and Signature Segment 1 be the  segments produced by the origin
      AS. Let Secure_Path Segment 2 and Signature Segment 2 be the
      segments added by the next AS after the origin. Continue this
      method of numbering and ultimately let Secure_Path Segment N be
      the Secure_Path segment that is being added by the current AS.

   o  In order to construct the digital signature for Signature Segment
      N (the signature segment being produced by the current AS), first
      construct the following sequence of octets to be hashed.

            Sequence of Octets to be Hashed
         +------------------------------------+
         | Target AS Number                   |
         +------------------------------------+ -\
         | Signature Segment   : N-1          |   \
         +------------------------------------+   |
         | Secure_Path Segment : N            |   |
         +------------------------------------+   \
                ...                                > For N Hops
         +------------------------------------+   /
         | Signature Segment   : 1            |   |
         +------------------------------------+   |
         | Secure_Path Segment : 2            |   /
         +------------------------------------+ -/
         | Secure_Path Segment : 1            |
         +------------------------------------+
         | Algorithm Suite Identifier         |
         +------------------------------------+
         | AFI                                |
         +------------------------------------+
         | SAFI                               |
         +------------------------------------+
         | NLRI                               |
         +------------------------------------+

      In this sequence, 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 Secure_Path and Signature Segments (1 through N-1) are
      obtained from the BGPsec_Path attribute. Finally, the Address
      Family Identifier (AFI), Subsequent Address Family Identifier
      (SAFI), and Network Layer Reachability Information (NLRI) fields
      are obtained from the MP_REACH_NLRI attribute. Additionally, in
      the Prefix field of the NLRI (from MP_REACH_NLRI), all of the
      trailing bits MUST be set to zero when constructing this
      sequence.



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   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 value in 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.  Additionally, in
   this case, the confederation member that generates the Secure_Path
   Segment sets the Confed_Segment flag to one.  This means that in a
   BGPsec update message, an AS number appears in a Secure_Path Segment
   with the Confed_Segment flag set whenever, in a non-BGPsec update
   message, the AS number would appear in a segment of type
   AS_CONFED_SEQUENCE.

   Within a confederation, the verification of BGPsec signatures added
   by other members of the confederation is optional.  If a
   confederation chooses not to have its members 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 members 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 Section
   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



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   outside the confederation does the following:

   o  First, starting with the most recently added Secure_Path segment,
      remove all of the consecutive Secure_Path segments that have the
      Confed_Segment flag set to one.  Stop this process once a
      Secure_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
      Section 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 the following. For a
   signature produced by a peer 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



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   algorithm in Section 5.2, the confederation member, during processing
   of a Signature Segment, 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
   Signature Segment (and checks its corresponding Secure_Path segment).
   Note that as specified in Section 5.2, it is an error when a BGPsec
   speaker receives from a peer, who is not in the same AS
   confederation, a BGPsec update containing a Confed_Sequence flag set
   to one.  (As discussed in Section 5.2, any error in the BGPsec_Path
   attribute MUST be handled using the "treat-as-withdraw", approach as
   defined in RFC 7606 [11].)

4.4.  Reconstructing the AS_PATH Attribute

   BGPsec update messages do not contain the AS_PATH attribute. However,
   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 (e.g., because the latter peer does
   not support BGPsec). Note that there may be additional cases where an
   implementation finds it useful to perform this reconstruction. Before
   attempting to reconstruct an AS_PATH for the purpose of forwarding an
   unsigned (non-BGPsec) update to a peer, a BGPsec speaker MUST perform
   the basic integrity checks listed in Section 5.2 to ensure that the
   received BGPsec update is properly formed.

   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.)




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       *  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.)

   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. Typically, a BGPsec speaker will also wish to perform
   origin validation (see [19] and [20]) on an incoming BGPsec update
   message, but such validation is independent of the validation
   described in this section.

   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



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   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. However, an
   implementation SHOULD ensure that deferment of validation and status
   of deferred messages is visible to the operator.

   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 RPKI-to-Router protocol
   [15]), 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 is revoked), all
   update messages containing a signature whose SKI matches the SKI in
   the given certificate must be re-assessed to determine if they are
   still valid. If 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



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   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).

5.1.  Overview of BGPsec Validation

   Validation of a BGPsec update messages makes use of data from RPKI
   certificates.  In particular, it is necessary that the recipient have
   access to the following data obtained from valid RPKI certificates:
   the AS Number, Public Key and Subject Key Identifier from each valid
   RPKI router certificate.

   Note that the BGPsec speaker could perform the validation of RPKI
   certificates 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 [16] for the RTR protocol
   [15].)

   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.  That said, BGP route
   selection, and thus the handling of the validation states is a matter
   of local policy, and is handled using local policy mechanisms.
   Implementations SHOULD enable operators to set such local policy on a
   per-session basis. (That is, we expect some operators will choose to
   treat BGPsec validation status differently for update messages
   received over different BGP sessions.)

   It is expected that BGP peers will generally prefer routes received
   via 'Valid' BGPsec update messages over both routes received via 'Not
   Valid' BGPsec update messages and routes received via update messages
   that do not contain the BGPsec_Path attribute.  However, BGPsec
   specifies no changes to the BGP decision process.  (See [17] for
   related operational considerations.)



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   BGPsec validation needs only be performed at the 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
   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 fail, it is an error in the BGPsec_Path
   attribute. Any of these errors in the BGPsec_Path attribute are



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   handled as per RFC 7606 [11]. BGPsec speakers MUST handle these
   errors using the "treat-as-withdraw" approach as defined in RFC 7606
   [11].

   Next, 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 is no 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 0): For clarity, let us number the Secure_Path and
      corresponding Signature Segments from 1 to N as follows. Let
      Secure_Path Segment 1 and Signature Segment 1 be the  segments
      produced by the origin AS. Let Secure_Path Segment 2 and Signature
      Segment 2 be the segments added by the next AS after the origin.
      Continue this method of numbering and ultimately let Signature
      Segment N be the Signature Segment that is currently being
      verified and let Secure_Path Segment N be the corresponding
      Secure_Path Segment.

   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.

      In order to verify the digital signature in Signature Segment N,
      construct the following sequence of octets to be hashed.



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            Sequence of Octets to be Hashed
         +------------------------------------+
         | Target AS Number                   |
         +------------------------------------+ -\
         | Signature Segment   : N-1          |   \
         +------------------------------------+   |
         | Secure_Path Segment : N            |   |
         +------------------------------------+   \
                ...                                > For N Hops
         +------------------------------------+   /
         | Signature Segment   : 1            |   |
         +------------------------------------+   |
         | Secure_Path Segment : 2            |   /
         +------------------------------------+ -/
         | Secure_Path Segment : 1            |
         +------------------------------------+
         | Algorithm Suite Identifier         |
         +------------------------------------+
         | AFI                                |
         +------------------------------------+
         | SAFI                               |
         +------------------------------------+
         | NLRI                               |
         +------------------------------------+

      For the first segment to be processed (the most recently added
      segment), the 'Target AS Number' 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 'Target AS Number' 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.)

      Additionally, the Secure_Path and Signature Segment are obtained
      from the BGPsec_Path attribute. The Address Family Identifier
      (AFI), Subsequent Address Family Identifier (SAFI), and Network
      Layer Reachability Information (NLRI) fields are obtained from the
      MP_REACH_NLRI attribute. Additionally, in the Prefix field of the
      NLRI (from MP_REACH_NLRI), all of the trailing bits MUST be set to
      zero when constructing this sequence.

   o  (Step III): Use the signature validation algorithm (for the given



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      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'.

   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
   (using BGP capabilities) between BGPsec peers to use of a particular
   (digest and signature) algorithm suite. 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 exists which
   specifies a mandatory-to-use 'current' algorithm suite for use by all
   BGPsec speakers [10].

   We anticipate that, in the future, the 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



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   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:


   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



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   For a discussion of the BGPsec threat model and related security
   considerations, please see [14].

7.1 Security Guarantees

   When used in conjunction with Origin Validation (see [19] and [20]),
   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
   that the update propagated via the sequences ASes listed in the
   Secure_Path portion of the BGPsec_Path attribute. (It should be noted
   that BGPsec does not offer any guarantee that the data packets would
   flow 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).

7.2 On the Removal of BGPsec Signatures

   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



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   and the other set of signatures fails to verify.  In this case, the
   protocol specifies that a BGPsec speaker choosing to propagate the
   route advertisement in such an update message SHOULD add its
   signature to each of the Signature_Blocks. 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 likely, depending on the state of the algorithm
   transition) 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 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
   differently 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
   path (to a given prefix) a route obtained via a 'Not Valid' BGPsec
   update message. 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 should also be noted
   that due to possible differences in RPKI data observed at different



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   vantage points in the network, a BGPsec update deemed 'Not Valid' at
   an upstream BGPsec speaker may be deemed 'Valid' by another BGP
   speaker downstream.

   Indeed, 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'

7.3 Mitigation of Denial of Service Attacks

   The BGPsec update validation procedure is a potential target for
   denial of service attacks against a BGPsec speaker. Here we consider
   the mitigation only of denial of service attacks that are specific to
   BGPsec.

   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
   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.

   Additionally, sending update messages with very long AS paths (and
   hence a large number of signatures) is a potential mechanism to
   conduct denial of service attacks. For this reason, it is important
   that an implementation of the validation algorithm stops attempting
   to verify signatures as soon as an invalid signature is found. (This
   ensures that long sequences of invalid signatures cannot be used for
   denial of service attacks.) Furthermore, implementations can mitigate
   such attacks by only performing validation on update messages that,
   if valid, would be selected as the best path. That is, if an update
   message contains a route that would lose out in best path selection
   for other reasons (e.g., a very long AS path) then it is not
   necessary to determine the BGPsec-validity status of the route.

7.4 Additional Security Considerations

   The mechanism of setting the pCount field to zero is included in this



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   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 within which an upstream entity
   two or more hops away has set pCount to zero is unable to verify for
   themselves whether pCount was set to zero legitimately.

   BGPsec does not provide protection against attacks at the transport
   layer.  As with any BGP session, 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, injecting BGPsec update messages
   with BGPsec_Path_Signature attributes that fail validation, or
   causing the peer to tear-down the BGP session. The use of BGPsec does
   nothing to increase the power of an on-path adversary -- in
   particular, even an on-path adversary cannot cause a BGPsec speaker
   to believe a BGPsec-invalid route is valid. However, as with any BGP
   session, BGPsec sessions SHOULD be protected by appropriate transport
   security mechanisms.

8.  IANA Considerations

   This document registers a new capability in the registry of BGP
   Capabilities. The description for the new capability is "BGPsec
   Capability". The reference for the new capability is this document
   (i.e., the RFC that replaces draft-ietf-sidr-bgpsec-protocol), see
   Section 2.1.

   This document registers a new path attribute in the registry of BGP
   Path Attributes. The code for this new attribute is "BGPsec_PATH".
   The reference for the new capability is this document (i.e., the RFC
   that replaces draft-ietf-sidr-bgpsec-protocol), see Section 3.

   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
   New College of Florida
   mlepinski@ncf.edu

   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
   Parsons
   weiler+ietf@watson.org

9.2.  Acknowledgements

   The authors would like to thank Michael Baer, Luke Berndt, Oliver
   Borchert, Wes George, Jeff Haas, Sharon Goldberg, Ed Kern, David
   Mandelberg, Doug Maughan, Pradosh Mohapatra, Chris Morrow, 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




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   [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 6793, December 2012.

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

   [8]   Patel, K., Ward, D., and R. Bush, "Extended Message support for
         BGP", draft-ietf-idr-bgp-extended-messages (work in progress),
         May 2016.

   [9]  Reynolds, M., Turner, S., and S. Kent, "A Profile for BGPsec
         Router Certificates, Certificate Revocation Lists, and
         Certification Requests", draft-ietf-sidr-bgpsec-pki-profiles
         (work in progress), June 2016.

   [10]  Turner, S., "BGP Algorithms, Key Formats, & Signature Formats",
         draft-ietf-sidr-bgpsec-algs (work in progress), April 2016.

   [11]  Chen, E., Scudder, J., Mohapatra, P., and K. Patel, "Revised
         Error Handling for BGP UPDATE Messages", RFC 7606, August 2015.

11.  Informative References

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

   [13]  Kumari, W. and K. Sriram, "Recommendation for Not Using AS_SET
         and AS_CONFED_SET in BGP", RFC 6472, December 2011.

   [14]  Kent, S. and A. Chi, "Threat Model for BGP Path Security", RFC
         7132, February 2014.

   [15]  Bush, R. and R. Austein, "The Resource Public Key
         Infrastructure (RPKI) to Router Protocol", draft-ietf-sidr-



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         rpki-rtr-rfc6810-bis (work in progress), March 2016.

   [16]  Bush, R., Turner, S., and K. Patel, "Router Keying for BGPsec",
         draft-ietf-sidr-rtr-keying (work in progress), June 2016.

   [17]  Bush, R., "BGPsec Operational Considerations", draft-ietf-sidr-
         bgpsec-ops (work in progress), June 2016.

   [18]  George, W. and S. Murphy, "BGPsec Considerations for AS
         Migration", draft-ietf-sidr-as-migration (work in progress),
         April 2016.

   [19] Huston, G. and G. Michaelson, "Validation of Route Origination
         Using the Resource Certificate Public Key Infrastructure (PKI)
         and Route Origin Authorizations (ROAs)", RFC 6483, February
         2013.

   [20] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R. Austein,
         "BGP Prefix Origin Validation", RFC 6811, January 2013.
































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Author's Address

   Matthew Lepinski (editor)
   New College of Florida
   5800 Bay Shore Road
   Sarasota, FL 34243
   USA

   Email: mlepinski@ncf.edu

   Kotikalapudi Sriram (editor)
   National Institute of Standards and Technology
   100 Bureau Drive
   Gaithersburg, MD 20899
   USA

   Email: kotikalapudi.sriram@nist.gov


































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