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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

Network Working Group                                   M. Lepinski, Ed.
Internet-Draft                                                       BBN
Intended status: Standards Track                           July 16, 2012
Expires: January 17, 2013


                     BGPSEC Protocol Specification
                   draft-ietf-sidr-bgpsec-protocol-04

Abstract

   This document describes BGPSEC, an extension to the Border Gateway
   Protocol (BGP) that provides security for the AS-PATH attribute in
   BGP update messages.  BGPSEC is implemented via a new optional non-
   transitive BGP path attribute that carries a digital signature
   produced by each autonomous system on the AS-PATH.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [8].

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 January 17, 2013.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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   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
   3.  The BGPSEC_Path_Signatures Attribute . . . . . . . . . . . . .  6
     3.1.  Secure_Path  . . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Additional_Info  . . . . . . . . . . . . . . . . . . . . .  9
     3.3.  Signature_Block  . . . . . . . . . . . . . . . . . . . . . 11
   4.  Generating a BGPSEC Update . . . . . . . . . . . . . . . . . . 12
     4.1.  Originating a New BGPSEC Update  . . . . . . . . . . . . . 13
     4.2.  Propagating a Route Advertisement  . . . . . . . . . . . . 16
     4.3.  Reconstructing the AS_Path Attribute . . . . . . . . . . . 19
     4.4.  Processing Instructions for Confederation Members  . . . . 19
   5.  Processing a Received BGPSEC Update  . . . . . . . . . . . . . 21
     5.1.  Validation Algorithm . . . . . . . . . . . . . . . . . . . 22
   6.  Algorithms and Extensibility . . . . . . . . . . . . . . . . . 26
     6.1.  Algorithm Suite Considerations . . . . . . . . . . . . . . 26
     6.2.  Extensibility Considerations . . . . . . . . . . . . . . . 27
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 31
     8.1.  Authors  . . . . . . . . . . . . . . . . . . . . . . . . . 31
     8.2.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . 32
   9.  Normative References . . . . . . . . . . . . . . . . . . . . . 32
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 33



















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1.  Introduction

   This document describes BGPSEC, a mechanism for providing path
   security for Border Gateway Protocol (BGP) [1] route advertisements.
   That is, a BGP speaker who receives a valid BGPSEC update has
   cryptographic assurance that the advertised route has the following
   two properties:

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

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

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

   BGPSEC relies on the Resource Public Key Infrastructure (RPKI)
   certificates that attest to the allocation of AS number and IP
   address resources.  (For more information on the RPKI, see [6] and
   the documents referenced therein.)  Any BGPSEC speaker who wishes to
   send BGP update messages to external peers (eBGP) containing the
   BGPSEC_Path_Signatures must have an RPKI end-entity certificate (as
   well as the associated private signing key) corresponding to the
   BGPSEC speaker's AS number.  Note, however, that a BGPSEC speaker
   does not require such a certificate in order to validate update
   messages containing the BGPSEC_Path_Signatures attribute.


2.  BGPSEC Negotiation

   This document defines a new BGP capability [4]that allows a BGP
   speaker to advertise to its neighbors the ability to send and/or
   receive BGPSEC update messages (i.e., update messages containing the
   BGPSEC_Path_Signatures attribute).

   This capability has capability code : TBD

   The capability length for this capability MUST be set to 5.

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





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                             Capability Value:

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


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

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

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

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

   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



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   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.

   The fourth octet in the capability is reserved.  It is anticipated
   that this octet will not be used until such a time as the reserved
   octet in the Multi-protocol extensions capability advertisement [2]
   is specified for use.  The reserved octet should be set to zero by
   the sender and ignored by the receiver.

   The fifth octet in the capability contains the 8-bit Subsequent
   Address Family Identifier (SAFI).  This value is encoded as in the
   BGP multiprotocol extensions [2].

   Note that if the BGPSEC speaker wishes to use BGPSEC with two
   different address families (i.e., IPv4 and IPv6) over the same BGP
   session, then the speaker must include two instances of this
   capability (one for each address family) in the BGP OPEN message.  A
   BGPSEC speaker SHOULD NOT advertise the capability of BGPSEC support
   for any <AFI, SAFI> combination unless it has also advertises the
   multiprotocol extension capability for the same <AFI, SAFI>
   combination [2].

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

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

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

   Note that BGPSEC update messages can be quite large, therefore any
   BGPSEC speaker announcing the capability to receive BGPSEC messages
   SHOULD also announce support for the capability to receive BGP
   extended messages [9].

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

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

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






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3.  The BGPSEC_Path_Signatures Attribute

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

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

   The BGPSEC_Path_Signatures algorithm carries the secured AS Path
   information, including the digital signatures that protect this AS
   Path information.  We refer to those update messages that contain the
   BGPSEC_Path_Signatures attribute as "BGPSEC Update messages".  The
   BGPSEC_Path_Signatures attribute replaces the AS_PATH attribute, in a
   BGPSEC update message.  That is, update messages that contain the
   BGPSEC_Path_Signatures attribute MUST NOT contain the AS_PATH
   attribute.

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































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        High-Level Diagram of the BGPSEC_Path_Signatures Attribute
                          BGPSEC_Path_Signatures
        +---------------------------------------------------------+
        |     +-----------------+                                 |
        |     |   Secure Path   |        +-----------------+      |
        |     +-----------------+        | Additional Info |      |
        |     |    AS X         |        +-----------------+      |
        |     |    pCount X     |        |  Info Type      |      |
        |     |    Flags X      |        |  Info Length    |      |
        |     |    AS Y         |        |  Info Value     |      |
        |     |    pCount Y     |        +-----------------+      |
        |     |    Flags Y      |                                 |
        |     |      ...        |                                 |
        |     +-----------------+                                 |
        |                                                         |
        |     +-----------------+       +-----------------+       |
        |     | Sig Block 1     |       |  Sig Block 2    |       |
        |     +-----------------+       +-----------------+       |
        |     | Alg Suite 1     |       |  Alg Suite 2    |       |
        |     | SKI X           |       |  SKI X          |       |
        |     | Sig Length X    |       |  Sig Length X   |       |
        |     | Signature X     |       |  Signature X    |       |
        |     | SKI Length Y    |       |  SKI Length Y   |       |
        |     | SKI Y           |       |  SKI Y          |       |
        |     | Sig Length Y    |       |  Sig Length Y   |       |
        |     | Signature Y     |       |  Signature Y    |       |
        |     |      ...        |       |      ....       |       |
        |     +-----------------+       +-----------------+       |
        |                                                         |
        +---------------------------------------------------------+


   The following is a more detailed explanation of the format of the
   BGPSEC_Path_Signatures attribute.

                     BGPSEC_Path_Signatures Attribute

         +-------------------------------------------------------+
         | Secure_Path                             (variable)    |
         +-------------------------------------------------------+
         | Additional_Info                         (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 would



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   be contained in the AS_PATH attribute.  A BGPSEC update message
   containing the BGPSEC_PATH_SIGNATURES attribute MUST NOT contain the
   AS_PATH attribute.  The format of the Secure_Path is described below
   in Section 3.1.

   The Additional_Info contains additional signed information about the
   update message.  Additional_Info is specified as a type-length-value
   field for future extensibility.  However, this specification defines
   only a single (null) type of Additional Info which has zero length.
   It is anticipated that future specifications may specify semantics
   for Info Types other than zero.  See Section 3.2 below for more
   detail.

   The BGPSEC_Path_Signatures 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_Signatures 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, 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 3.3.

3.1.  Secure_Path

   Here we provide a detailed description of the Secure_Path information
   in the BGPSEC_Path_Signatures 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
   variable-length sequence of Secure_Path Segments.  As explained
   below, each Secure_Path segment is six octets long.  Note that this
   means the Secure_Path Length is 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 NLRI specified in the update



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   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_Signatures 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 adding 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 Entering_Confed flag.  The
   Entering_Confed flag is set to one in the Secure_Path Segment
   corresponding to the first Autonomous System in a confederation [3].
   (That is, the Secure_Path Segment corresponding to the AS that would
   otherwise have created an AS_Path segment of type AS_Confed_Sequence
   in a non-BGSPEC update message.)  In all other cases the
   Entering_Confed flag is set to zero.

   The remaining seven bits of the Flags field are reserved for future
   use.  These bits MUST be set to zero by the sender.  The receiver
   uses the entire Flags octet to verify the digital signature
   (regardless of what value the reserved bits contain), but otherwise
   ignores the reserved flags (see Section 4 for sender instructions and
   Section 5 for receiver validation instructions).

   EDITOR'S NOTE: The unused portion of the signed flags field provides
   the possibility of adding in the future (in a backwards compatible
   fashion) a new feature that requires some per-AS signed bits.  For
   example, one could use a couple bits from this flag field to mark
   some property of the connection between two ASes.

3.2.  Additional_Info

   Here we provide a detailed description of the Additional_Info in the
   BGPSEC_Path_Signatures attribute.



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                              Additional_Info

              +---------------------------------------------+
              | Info Type                      (1 octet)    |
              +---------------------------------------------+
              | Info Length                    (1 octet)    |
              +---------------------------------------------+
              | Info Value                     (variable)   |
              +---------------------------------------------+


   The Info Type field is a one-octet value that identifies the type of
   additional information included in the Info Value field.  This
   specification defines a single (null) type of Additional_Info.  The
   Info Type for this null type is zero.

   The Info Length field contains the length in octets of the Info Value
   field.  For the (null) Info Type zero specified in this document, the
   Info Length MUST be zero.

   The syntax and semantics contained in the Info Value field depends on
   the type contained in the Info Type field.  For the (null) Info Type
   zero specified in this document, the Info Value field is empty (since
   the Info Length field must be zero).

   Implementations compliant with this specification MUST set the Info
   Type to zero in BGPSEC update messages for route advertisements that
   they originate (see Section 4.1 for more details).  When an
   implementation compliant with this specification receives a BGPSEC
   update message with an Info Type field that it does not understand
   (i.e., an Info Type other than zero), the implementation MUST use the
   Additional_Info when it verifies digital signatures (as per Section
   5.1).  However, other than signature verification, the implementation
   MUST ignore the Info Value field when it does not understand the Info
   Type.

   EDITOR'S NOTE: In a previous version of this document there was an
   Expire Time that was used to provide protection against replay of old
   (stale) digital signatures or failure to propagate a withdrawal
   message.  This mechanism was removed from the current version of the
   document.  Please see the SIDR mailing list for discussions related
   to protection against replay attacks.  Depending on the result of
   discussions within the SIDR working group this Additional Info field
   could at some future point be used to re-introduce Expire Time, or
   some other octets used in a future replay protection mechanism.  The
   authors believe that the current instructions whereby the sender uses
   a null Additional_Info type and the receiver ignores Additional_Info
   types that it does not understand provides an opportunity to use



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   these octets in the future in a backwards-compatible fashion.

3.3.  Signature_Block

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

                              Signature_Block

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


   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[12].

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

   A Signature_Block has exactly one Signature Segment for each
   Secure_Path Segment in the Secure_Path portion of the
   BGPSEC_Path_Signatures 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 end-entity certificate that is used
   to verify the signature (see Section 5 for details on validity of
   BGPSEC update messages).




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   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_Signatures attribute (see Sections 4 and 5 for
   details on generating and verifying this signature, respectively).


4.  Generating a BGPSEC Update

   Sections 4.1 and 4.2 cover two cases in which a BGPSEC speaker may
   generate an update message containing the BGPSEC_Path_Signatures
   attribute.  The first case is that in which the BGPSEC speaker
   originates a new route advertisement (Section 4.1).  That is, the
   BGPSEC speaker is constructing an update message in which the only AS
   to appear in the BGPSEC_Path_Signatures 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_Signatures attribute will contain AS number(s) other than
   the speaker's own AS.

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

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

   A BGPSEC update message MUST advertise a route to only a single NLRI.
   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



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   update message for each NLRI.

   Note that 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 key in a valid Resource PKI end-entity certificate whose
   AS number resource extension includes the BGPSEC speaker's AS number
   [11].  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_Signatures 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 the AS number in
   the AS number resource extension field of the Resource PKI end-entity
   certificate(s) that will be used to verify the digital signature(s)
   constructed by this BGPSEC speaker.

   Note that the BGPSEC_Path_Signatures attribute and the AS4_Path
   attribute are mutually exclusive.  That is, any update message
   containing the BGPSEC_Path_Signatures attribute MUST NOT contain the
   AS4_Path attribute nor the AS_Path attribute.  The information that
   would be contained in the AS4_Path (or AS_Path) attribute is instead
   conveyed in the Secure_Path portion of the BGPSEC_Path_Signatures
   attribute.

   Note that 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]).  Note that
   validation of a BGPSEC update message will fail (i.e., the validation
   algorithm, specified in Section 5.1, returns 'Not Good') unless there
   exists a valid ROA authorizing the first AS in the Secure_Path
   portion of the BGPSEC_Path_Signatures 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



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   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
   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 [3], 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.)

   The BGPSEC speaker next constructs the Additional_Info portion of the
   BGPSEC_Path_Signatures attribute.  The Info Type MUST be set to zero
   and the Info Length MUST also be set to zero.  The Info Value field
   is empty (has length zero).  It is anticipated that future
   specifications may specify values of Info Type other than zero.
   Therefore, BGPSEC receivers compliant with this specification must be
   able to accept Additional_Info fields with non-zero Info Type.  Such
   receivers will use the Additional_Field to verify digital signatures
   (see Section 5) but will otherwise ignore Additional_Field non-zero
   Info Fields.

   Typically, a BGPSEC speaker will use only a single algorithm suite,
   and thus create only a single Signature_Block in the
   BGPSEC_Path_Signatures 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 end-entity certificate used by the BGPSEC speaker.  This
   Subject Key Identifier will be used by recipients of the route
   advertisement to identify the proper certificate to use in verifying
   the signature.




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

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

                      Sequence of Octets to be Signed
      +------------------------------------+
      | Target AS Number      (4 octets)   |
      +------------------------------------+
      | Origin AS Number      (4 octets)   |  ---\
      +------------------------------------+      \
      | pCount                (1 octet)    |       >  Secure_Path
      +------------------------------------+      /
      | Flags                 (1 octet)    |  ---/
      +------------------------------------+
      | Info Type             (1 octet)    |  ---\
      +------------------------------------+      \
      | Info Length           (1 octet)    |       >  Additional_Info
      +------------------------------------+      /
      | Info Value            (variable)   |  ---/
      +------------------------------------+
      | 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.




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4.2.  Propagating a Route Advertisement

   When a BGPSEC speaker receives a BGPSEC update message containing a
   BGPSEC_Path_Signatures 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 non-BGPSEC update messages
   (without the BGPSEC_Path_Signatures 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_Signatures 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.)

   Conversely, if a BGPSEC router has received a BGPSEC update message
   (with the BGPSEC_Path_Signatures 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_Signatures attribute.  However, the BGPSEC
   speaker MAY propagate the route as a (unsigned) BGP update message
   without the BGPSEC_Path_Signatures attribute.

   Note that removing BGPSEC signatures (i.e., propagating a route
   advertisement without the BGPSEC_Path_Signatures 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_Signatures attribute is NOT RECOMMENDED.  Furthermore,
   note that when a BGPSEC speaker propagates a route advertisement with
   the BGPSEC_Path_Signatures 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_Signatures attribute.  In such a case, the
   BGPSEC speaker must remove any existing BGPSEC_Path_Signatures in the
   received advertisement(s) for this prefix and produce a standard
   (non-BGPSEC) update message.  It should be noted that BCP 172 [5]
   recommends against the use of AS_SET and AS_CONFED_SET in AS_PATH in



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   BGP updates.

   To generate the BGPSEC_Path_Signatures 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 end-entity
   certificate(s) that will be used to verify the digital signature(s)
   constructed by this BGPSEC speaker.

   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
   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_Signatures 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 [3], 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.)

   The BGPSEC speaker next copies the Additional_Info portion of the
   BGPSEC_Path_Signatures directly from the received update message to
   the new update message (that it is constructing).  Note that the
   BGPSEC speaker MUST NOT change the Additional_Info as any change to
   Additional_Info will cause the new BGPSEC update message to fail
   validation (see Section 5).

   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



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   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_Signatures attribute (i.e., propagate it 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 5.1) deems a BGPSEC update message to be
   'Good' if there is at least one supported algorithm suite (and
   corresponding Signature_Block) that is deemed 'Good'.  This means
   that a 'Good' BGPSEC update message may contain a Signature_Block
   which is not deemed 'Good' (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 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_Signatures attribute.  The BGPSEC speaker
   populates the fields of this new signature segment as follows.

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

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

   o  Construct a sequence of octets by concatenating the 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



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      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
      signature.

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

4.3.  Reconstructing the AS_Path Attribute

   EDITOR'S NOTE: This is a place-holder section.  Given that BGPSEC
   update messages do not contain the AS_Path attribute, this document
   needs to include a clearly specified algorithm for reconstructing the
   AS_Path attribute from the data in the BGPSEC_Path_Signatures
   attribute.  (For example, when propagating a path received via BGPSEC
   to a non-BGPSEC peer.)  This algorithm for reconstructing the AS_Path
   will appear in the next version of this document.  In essence, the
   algorithm is: For each Secure_Path Segment put into the AS_Path
   pCount copies of the AS number field of the segment --- and if you
   see the Entering_Confed flag is set to one, then add an
   AS_Confed_Sequence to the AS_Path.

4.4.  Processing Instructions for Confederation Members

   Members of autonomous system confederations [3] 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



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   who 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 sending a BGPSEC update message to a peer who is a
   member of the same confederation, the first confederation member to
   add a Secure_Path Segment to a BGPSEC update message sets the
   Entering_Cofed flag in the Flags field to be one in the Secure_Path
   Segment that it produces.  In the case where the route advertisement
   originates within a confederation, the member AS that originates the
   route and sends it to a peer is the member AS that sets its
   Entering_Confed flag to one.  In the case where the route
   advertisement is received from outside the confederation, it is the
   member AS that receives the route advertisement from a peer outside
   the confederation and propagates it to a peer inside the
   confederation that sets its Entering_Confed flag to one.  Note that
   this is the same of the confederation member who would have added a
   new AS_Path segment of type AS_Confed_Sequence to the AS_Path
   attribute in a non-BGPSEC update message.

   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 must 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, search through the Secure_Path, going from most recently
      added segment to least recently added segment, and find first
      Secure_Path Segment with the Entering_Confed flag to set to one.
      (That is, of all the Secure_Path Segments with the Entering_Confed
      flag set to one, find the one that was most recently added.)

   o  Second, remove the Secure_Path Segment found in previous step
      along with all more recently added Secure_Path Segments.  Keep a
      count of the number of segments removed in this fashion.

   o  Third, 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



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      Sections 4.1 and 4.2.

   When validating a received BGPSEC update message, confederation
   members must make the following adjustment to the algorithm presented
   in Section 5.1.  That is, when a confederation member is processing
   (validating) a Signature Segment and its corresponding Secure_Path
   Segment, the confederation member must note that when a signature is
   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 processing a current Secure_Path Segment, if
   the next most recently added Secure_Path segment has the
   Entering_Confed flag set then ,when computing the digest for the
   current Secure_Path segment, take the Target AS Number to be the AS
   Confederation Identifier of the validating BGPSEC speaker's own
   confederation.  (Note that the algorithm in Section 5.1 processes
   Secure_Path Segments in order from most recently added to least
   recently added, therefore the algorithm encounters the
   Entering_Confed flag immediately before it encounters the Secure_Path
   segment that requires using the AS Confederation Identifier to
   validate.)


5.  Processing a Received BGPSEC Update

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

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

   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.  (The latter
   case in analogous to the use of the RPKI-RTR protocol [13] for origin
   validation.)

   To validate a BGPSEC update message containing the
   BGPSEC_Path_Signatures attribute, the recipient performs the



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   validation steps specified in Section 5.1.  The validation procedure
   results in one of two states: 'Good' and 'Not Good'.

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

   BGPSEC validation 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.  Local
   policy in the AS determines the specific means for conveying the
   validation status through various pre-existing mechanisms (e.g.,
   modifying an attribute).  As discussed in Section 4, when a BGPSEC
   speaker chooses to forward a (syntactically correct) BGPSEC update
   message, it SHOULD be forwarded with its BGPSEC_Path_Signatures
   attribute intact (regardless of the validation state of the update
   message).  Based entirely on local policy settings, an egress router
   MAY trust the validation status conveyed by an ingress router or it
   MAY perform its own validation.

   Upon receiving a BGPSEC update message, a BGPSEC speaker SHOULD sum
   the pCount values within BGPSEC_Path_Signatures attribute to
   determine the effective length of the AS Path.  The BGPSEC speaker
   SHOULD use this sum of pCount values in precisely the same way as it
   uses the length of the AS Path in non-BGPSEC update messages.

5.1.  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 external
   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:

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



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      document).

   o  Check that each Signature_Block contains one Signature segment for
      each Secure_Path segment in the Secure_Path portion of the
      BGPSEC_Path_Signatures 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.)

   If there are two Signature_Blocks within the BGPSEC_Path_Signatures
   attribute and one of them is poorly formed (or contains the wrong
   number of Signature segments) , then the recipient should log that an
   error occurred, strip off that particular Signature_Block and process
   the update message as though it arrived with a single
   Signature_Block.  If the BGPSEC_Path_Signatures attribute contains a
   syntax error that is not local to one of two Signature_Blocks, then
   the recipient should log that an error occurred and drop the update
   message containing the error.  Similarly, if an update message
   contains both the BGPSEC_Path_Signatures attribute and either an
   AS_Path or AS4_Path attribute, then the recipient should log that an
   error occurred and drop the update message containing the error.

   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
   BGPSEC_Path_Signatures 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 Good' and the
   validation algorithm terminates.

   Finally, the BGPSEC speaker examines the Signature_Blocks in the
   BGPSEC_Path_Signatures 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_Signatures 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_Signatures attribute.  The following steps



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   make use of this correspondence.

   o  (Step I): Locate the public key needed to verify the signature (in
      the current Signature segment).  To do this, consult the valid
      RPKI end-entity certificate data and look 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-List Block as 'Not Good' and proceed to
      the next Signature-List Block.

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

                      Sequence of Octets to be Hashed

     +-------------------------------------------+
     | 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.)




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   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:

                      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)   |  ---/
      +------------------------------------+
      | Info Type              (1 octet)   |  ---\
      +------------------------------------+      \
      | Info Length            (1 octet)   |       >  Additional_Info
      +------------------------------------+      /
      | Info Value             (variable)  |  ---/
      +------------------------------------+
      | Algorithm Suite Id.    (1 octet)   |
      +------------------------------------+
      | NLRI Length            (1 octet)   |
      +------------------------------------+
      | NLRI Prefix            (variable)  |
      +------------------------------------+

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





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   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-List Block as
      'Not Good' 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-List Block).

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

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


6.  Algorithms and Extensibility

6.1.  Algorithm Suite Considerations

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

   To this end, a mandatory algorithm suites document will be created
   which specifies a mandatory-to-use 'current' algorithm suite for use
   by all BGPSEC speakers [12].  Additionally, the document specifies an
   additional 'new' algorithm suite that is recommended to implement.

   It is anticipated that in the future the mandatory algorithm suites
   document will be updated to specify a transition from the 'current'
   algorithm suite to the 'new' algorithm suite.  During the period of



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   transition (likely a small number of years), all BGPSEC update
   messages SHOULD simultaneously use both the 'current' algorithm suite
   and the 'new' algorithm suite.  (Note that Sections 3 and 4 specify
   how the BGPSEC_Path_Signatures attribute can contain signatures, in
   parallel, for two algorithm suites.)  Once the transition is
   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_Signatures
   and thus necessitate a new version of BGPSEC.  Examples of such
   changes include:

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

   o  A new type of signature algorithm for which the number of
      signatures in the Signature_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_SIG_TWO, which is designed to accommodate
   the desired changes to BGPSEC.  In such a case, the mandatory
   algorithm suites document would be updated to specify algorithm
   suites appropriate for the new version of BGPSEC.

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





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7.  Security Considerations

   For discussion of the BGPSEC threat model and related security
   considerations, please see [10].

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

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

   o  For each AS number in the AS 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
      next AS in the Secure_Path.

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

   Note that 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 'Good' (as
   per the validation algorithm in Section 5.1) a BGPSEC update message
   that contains both a 'Good' and a 'Not Good' 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



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   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 'Good' and the 'Not Good'
   set of signatures (from its own vantage point).

   To understand the reason for such a design decision consider the case
   where the BGPSEC speaker receives an update message with both a set
   of algorithm A signatures which are 'Good' and a set of algorithm B
   signatures which are 'Not Good'.  In such a case it is possible
   (perhaps even quite likely) that some of the BGPSEC speaker's peers
   (or other entities further 'downstream' in the BGP topology) do not
   support algorithm A. Therefore, if the BGPSEC speaker were to remove
   the 'Not Good' set of signatures corresponding to algorithm B, such
   entities would treat the message as though it were unsigned.  By
   including the 'Not Good' set of signatures when propagating a route
   advertisement, the BGPSEC speaker ensures that 'downstream' entities
   have as much information as possible to make an informed opinion
   about the validation status of a BGPSEC update.

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

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



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   vantage points in the network, a BGPSEC update that was deemed 'Not
   Good' at an upstream BGPSEC speaker may indeed be deemed 'Good' at
   another BGP speaker downstream.

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

   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
   expensive checks (e.g., signature verification) after performing less
   expensive checks (e.g., syntax checks).  The validation algorithm
   specified in Section 5.1 was chosen so as to perform checks which are
   likely to be expensive after checks that are likely to be
   inexpensive.  However, the relative cost of performing required
   validation steps may vary between implementations, and thus the
   algorithm specified in Section 5.1 may not provide the best denial of
   service protection for all implementations.

   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 all 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



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   peer to tear-down the BGP session.  Therefore, BGPSEC implementations
   MUST support appropriate transport security mechanisms.

   EDITOR'S NOTE: Do we want to mandate a specific transport security
   mechanism (e.g., TCP-AO)?


8.  Contributors

8.1.  Authors

   Rob Austein
   Dragon Research Labs
   sra@hactrn.net


   Steven Bellovin
   Columbia University
   smb@cs.columbia.edu


   Randy Bush
   Internet Initiative Japan
   randy@psg.com


   Russ Housley
   Vigil Security
   housley@vigilsec.com


   Matt Lepinski
   BBN Technologies
   lepinski@bbn.com



   Stephen Kent
   BBN Technologies
   kent@bbn.com


   Warren Kumari
   Google
   warren@kumari.net


   Doug Montgomery



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   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
   Cobham
   weiler+ietf@watson.org

8.2.  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.


9.  Normative References

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

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

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

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

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

   [6]   Lepinski, M. and S. Kent, "Recommendation for Not Using AS_SET
         and AS_CONFED_SET in BGP", RFC 6480, February 2012.

   [7]   Lepinski, M., Kent, S., and D. Kong, "Recommendation for Not
         Using AS_SET and AS_CONFED_SET in BGP", RFC 6482,
         February 2012.

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



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   [9]   Patel, K., Ward, D., and R. Bush, "Extended Message support for
         BGP", March 2011.

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

   [11]  Reynolds, M., Turner, S., and S. Kent, "A Profile for BGPSEC
         Router Certificates, Certificate Revocation Lists, and
         Certification Requests", December 2011.

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

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


Author's Address

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

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

























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