





Independent Submission                                    K. Sriram, Ed.
Request for Comments: 8374                                      USA NIST
Category: Informational                                       April 2018
ISSN: 2070-1721


      BGPsec Design Choices and Summary of Supporting Discussions

Abstract

   This document captures the design rationale of the initial draft
   version of what became RFC 8205 (the BGPsec protocol specification).
   The designers needed to balance many competing factors, and this
   document lists the decisions that were made in favor of or against
   each design choice.  This document also presents brief summaries of
   the arguments that aided the decision process.  Where appropriate,
   this document also provides brief notes on design decisions that
   changed as the specification was reviewed and updated by the IETF
   SIDR Working Group and that resulted in RFC 8205.  These notes
   highlight the differences and provide pointers to details and
   rationale regarding those design changes.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8374.














Sriram                        Informational                     [Page 1]

RFC 8374                  BGPsec Design Choices               April 2018


Copyright Notice

   Copyright (c) 2018 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
   (https://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.

Table of Contents

   1. Introduction ....................................................4
   2. Creating Signatures and the Structure of BGPsec Update
      Messages ........................................................5
      2.1. Origin Validation Using ROAs ...............................5
      2.2. Attributes Signed by an Originating AS .....................6
      2.3. Attributes Signed by an Upstream AS ........................7
      2.4. Attributes That Are Not Signed .............................8
      2.5. Receiving Router Actions ...................................9
      2.6. Prepending of ASes in AS Path .............................10
      2.7. RPKI Data That Needs to Be Included in Updates ............10
   3. Withdrawal Protection ..........................................11
      3.1. Withdrawals Not Signed ....................................11
      3.2. Signature Expire Time for Withdrawal Protection
           (a.k.a. Mitigation of Replay Attacks) .....................12
      3.3. Should Route Expire Time be Communicated in a
           Separate Message? .........................................13
      3.4. Effect of Expire Time Updates in BGPsec on RFD ............14
   4. Signature Algorithms and Router Keys ...........................16
      4.1. Signature Algorithms ......................................16
      4.2. Agility of Signature Algorithms ...........................17
      4.3. Sequential Aggregate Signatures ...........................18
      4.4. Protocol Extensibility ....................................19
      4.5. Key per Router (Rogue Router Problem) .....................20
      4.6. Router ID .................................................20
   5. Optimizations and Resource Sizing ..............................21
      5.1. Update Packing and Repacking ..............................21
      5.2. Signature per Prefix vs. Signature per Update .............22
      5.3. Maximum BGPsec Update PDU Size ............................22
      5.4. Temporary Suspension of Attestations and Validations ......23








Sriram                        Informational                     [Page 2]

RFC 8374                  BGPsec Design Choices               April 2018


   6. Incremental Deployment and Negotiation of BGPsec ...............24
      6.1. Downgrade Attacks .........................................24
      6.2. Inclusion of Address Family in Capability Advertisement ...24
      6.3. Incremental Deployment: Capability Negotiation ............25
      6.4. Partial Path Signing ......................................25
      6.5. Consideration of Stub ASes with Resource
           Constraints: Encouraging Early Adoption ...................26
      6.6. Proxy Signing .............................................27
      6.7. Multiple Peering Sessions between ASes ....................28
   7. Interaction of BGPsec with Common BGP Features .................29
      7.1. Peer Groups ...............................................29
      7.2. Communities ...............................................29
      7.3. Consideration of iBGP Speakers and Confederations .........30
      7.4. Consideration of Route Servers in IXPs ....................31
      7.5. Proxy Aggregation (a.k.a. AS_SETs) ........................32
      7.6. 4-Byte AS Numbers .........................................32
   8. BGPsec Validation ..............................................33
      8.1. Sequence of BGPsec Validation Processing in a Receiver ....33
      8.2. Signing and Forwarding Updates when Signatures
           Failed Validation .........................................34
      8.3. Enumeration of Error Conditions ...........................35
      8.4. Procedure for Processing Unsigned Updates .................36
      8.5. Response to Syntactic Errors in Signatures and
           Recommendations for How to React to Them ..................36
      8.6. Enumeration of Validation States ..........................37
      8.7. Mechanism for Transporting Validation State through iBGP ..39
   9. Operational Considerations .....................................41
      9.1. Interworking with BGP Graceful Restart ....................41
      9.2. BCP Recommendations for Minimizing Churn:
           Certificate Expiry/Revocation and Signature Expire Time ...42
      9.3. Outsourcing Update Validation .............................42
      9.4. New Hardware Capability ...................................43
      9.5. Signed Peering Registrations ..............................44
   10. Security Considerations .......................................44
   11. IANA Considerations ...........................................44
   12. Informative References ........................................44
   Acknowledgements ..................................................49
   Contributors ......................................................49
   Author's Address ..................................................50












Sriram                        Informational                     [Page 3]

RFC 8374                  BGPsec Design Choices               April 2018


1.  Introduction

   The goal of the BGPsec effort is to enhance the security of BGP by
   enabling full Autonomous System (AS) path validation based on
   cryptographic principles.  Standards work on route origin validation
   based on a Resource PKI (RPKI) is already completed or nearing
   completion in the IETF SIDR WG [RFC6480] [RFC6482] [RFC6483]
   [RFC6487] [RFC6811].  The BGPsec effort is aimed at taking advantage
   of the same RPKI infrastructure developed in the SIDR WG to add
   cryptographic signatures to BGP updates, so that routers can perform
   full AS path validation [RFC7132] [RFC7353] [RFC8205].  The BGPsec
   protocol specification, [RFC8205], was published recently.  The key
   high-level design goals of the BGPsec protocol are as follows
   [RFC7353]:

   o  Rigorous path validation for all announced prefixes -- not merely
      showing that a path is not impossible.

   o  Incremental deployment capability -- no flag-day requirement for
      global deployment.

   o  Protection of AS paths only in inter-domain routing (External BGP
      (eBGP)) -- not applicable to Internal BGP (iBGP) (or to IGPs).

   o  Aiming for no increase in a provider's data exposure (e.g., not
      requiring any disclosure of peering relations).

   This document provides design justifications for the initial draft
   version of the BGPsec protocol specification [BGPsec-Initial].  The
   designers needed to balance many competing factors, and this document
   lists the decisions that were made in favor of or against each design
   choice.  This document also presents brief summaries of the
   discussions that weighed in on the pros and cons and aided the
   decision process.  Where appropriate, this document provides brief
   notes (starting with "Note:") on design decisions that changed from
   the approach taken in the initial draft version of the BGPsec
   protocol specification as the specification was reviewed and updated
   by the IETF SIDR WG.  (These design decisions resulted in RFC 8205
   [RFC8205].)  The notes provide pointers to the details and/or
   discussions about the design changes.











Sriram                        Informational                     [Page 4]

RFC 8374                  BGPsec Design Choices               April 2018


   The design choices and discussions are presented in the following
   sections (under the following eight broad categories, with many
   subtopics within each category):

   o  Section 2 ("Creating Signatures and the Structure of BGPsec Update
      Messages")

   o  Section 3 ("Withdrawal Protection")

   o  Section 4 ("Signature Algorithms and Router Keys")

   o  Section 5 ("Optimizations and Resource Sizing")

   o  Section 6 ("Incremental Deployment and Negotiation of BGPsec")

   o  Section 7 ("Interaction of BGPsec with Common BGP Features")

   o  Section 8 ("BGPsec Validation")

   o  Section 9 ("Operational Considerations")

2.  Creating Signatures and the Structure of BGPsec Update Messages

2.1.  Origin Validation Using ROAs

2.1.1.  Decision

   Route origin validation using Route Origin Authorizations (ROAs)
   [RFC6482] [RFC6811] is necessary and complements AS path attestation
   based on signed updates.  Thus, the BGPsec design makes use of the
   origin validation capability facilitated by the ROAs in the RPKI.

   Note: In the finalized BGPsec protocol specification [RFC8205],
   BGPsec is synonymous with cryptographic AS path attestation.  Origin
   validation and BGPsec (path signatures) are the two key pieces of the
   SIDR WG solution for BGP security.

2.1.2.  Discussion

   Route origin validation using RPKI constructs, as developed in the
   IETF SIDR WG, is a necessary component of BGP security.  It provides
   cryptographic validation that the first-hop AS is authorized to
   originate a route for the prefix in question.








Sriram                        Informational                     [Page 5]

RFC 8374                  BGPsec Design Choices               April 2018


2.2.  Attributes Signed by an Originating AS

2.2.1.  Decision

   An originating AS will sign over the Network Layer Reachability
   Information (NLRI) length, NLRI prefix, its own AS number (ASN), the
   next ASN, the signature algorithm suite ID, and a signature
   Expire Time (see Section 3.2) for the update.  The update signatures
   will be carried in a new optional, non-transitive BGP attribute.

   Note: The finalized BGPsec protocol specification [RFC8205] differs
   from the above.  There is no mention in RFC 8205 of a signature
   Expire Time field in the BGPsec update.  Further, there are some
   additional details concerning attributes signed by the origin AS that
   can be found in Figure 8 in Section 4.2 of RFC 8205 [RFC8205].  In
   particular, the signed data also includes the Address Family
   Identifier (AFI) as described in RFC 8205.  By adding the AFI in the
   data covered by a signature, a specific security concern was
   alleviated; see [Mandelberg1] (post to the SIDR WG Mailing List) and
   the discussion thread that followed on the topic.  The AFI is
   obtained from the MP_REACH_NLRI attribute in the BGPsec update.  As
   stated in Section 4.1 of RFC 8205, a BGPsec update message "MUST use
   the MP_REACH_NLRI attribute [RFC4760] to encode the prefix."

2.2.2.  Discussion

   The next-hop ASN is included in the data covered by the signature.
   Without this inclusion, the AS path cannot be secured; for example,
   the path can be shortened (by a MITM (man in the middle)) without
   being detected.

   It was decided that only the originating AS needs to insert a
   signature Expire Time in the update, as it is the originator of the
   route.  The origin AS also will re-originate, i.e., beacon, the
   update prior to the Expire Time of the advertisement (see
   Section 3.2).  (For an explanation of why upstream ASes do not insert
   their respective signature Expire Times, please see Section 3.2.2.)

   Note: Expire Time and beaconing were eventually replaced by router
   key rollover.  The BGPsec protocol [RFC8205] is expected to make use
   of router key rollover to mitigate replay attacks and withdrawal
   suppression [BGPsec-Rollover] [Replay-Protection].

   It was decided that each signed update would include only one NLRI
   prefix.  If more than one NLRI prefix were included and an upstream
   AS elected to propagate the advertisement for a subset of the
   prefixes, then the signature(s) on the update would break (see
   Sections 5.1 and 5.2).  If a mechanism were employed to preserve



Sriram                        Informational                     [Page 6]

RFC 8374                  BGPsec Design Choices               April 2018


   prefixes that were dropped, this would reveal information to
   subsequent ASes that is not revealed in normal BGP operation.  Thus,
   a trade-off was made to preserve the level of route information
   exposure that is intrinsic to BGP over the performance hit implied by
   limiting each update to carry only one prefix.

   The signature data is carried in an optional, non-transitive BGP
   attribute.  The attribute is optional because this is the standard
   mechanism available in BGP to propagate new types of data.  It was
   decided that the attribute should be non-transitive because of
   concern about the impact of sending the (potentially large)
   signatures to routers that don't understand them.  Also, if a router
   that does not understand BGPsec somehow gets an update message with
   path signatures (i.e., the update includes the BGPsec_PATH attribute
   (see Section 3 of RFC 8205)), then it would be undesirable for that
   router to forward the update to all of its neighbors, especially
   those who do not understand BGPsec and may choke if they receive many
   updates with large optional BGP attributes.  It is envisioned that
   BGPsec and traditional BGP will coexist while BGPsec is deployed
   incrementally.

2.3.  Attributes Signed by an Upstream AS

   In the context of BGPsec and throughout this document, an "upstream
   AS" simply refers to an AS that is further along in an AS path (the
   origin AS being the nearest to a prefix).  In principle, an AS that
   is upstream from an originating AS would digitally sign the combined
   information, including the NLRI length, NLRI prefix, AS path, next
   ASN, signature algorithm suite ID, and Expire Time.  There are
   multiple choices regarding what is signed by an upstream AS, as
   follows:

   o  Method 1: The signature protects the combination of the NLRI
      length, NLRI prefix, AS path, next ASN, signature algorithm suite
      ID, and Expire Time,

   o  Method 2: The signature protects just the combination of the
      previous signature (i.e., the signature of the neighbor AS who
      forwarded the update) and the next ASN, or

   o  Method 3: The signature protects everything that was received from
      the preceding AS plus the next (i.e., target) ASN; thus, ASi signs
      over the NLRI length, NLRI prefix, signature algorithm suite ID,
      Expire Time, {ASi, AS(i-1), AS(i-2), ..., AS2, AS1}, AS(i+1)
      (i.e., the next ASN), and {Sig(i-1), Sig(i-2), ..., Sig2, Sig1}.






Sriram                        Informational                     [Page 7]

RFC 8374                  BGPsec Design Choices               April 2018


   Note: Please see the notes in Sections 2.2.1 and 2.2.2 regarding the
   elimination of the Expire Time field in the finalized BGPsec protocol
   specification [RFC8205].

2.3.1.  Decision

   It was decided that Method 2 will be used.  Please see
   [BGPsec-Initial] for additional protocol details and syntax.

   Note: The finalized BGPsec protocol specification [RFC8205]
   essentially uses Method 3 (except for Expire Time).  Additional
   details concerning attributes signed by an upstream AS can be found
   in Figure 8 in Section 4.2 of RFC 8205 [RFC8205].  The decision to go
   with Method 3 (with suitable additions to the data signed) was
   motivated by a security concern that was associated with Method 2;
   see [Mandelberg2] (post to the SIDR WG Mailing List) and the
   discussion thread that followed on the topic.  Also, there is a
   strong rationale for the sequence of octets to be hashed (as shown in
   Figure 8 in Section 4.2 of RFC 8205); this sequencing of data is
   motivated by implementation efficiency considerations.  See
   [Borchert] (post to the SIDR WG Mailing List) for an explanation.

2.3.2.  Discussion

   The rationale for this choice (Method 2) was as follows.  Signatures
   are performed over hash blocks.  When the number of bytes to be
   signed exceeds one hash block, the remaining bytes will overflow into
   a second hash block, resulting in a performance penalty.  So, it is
   advantageous to minimize the number of bytes being hashed.  Also, an
   analysis of the three options noted above did not identify any
   vulnerabilities associated with this approach.

2.4.  Attributes That Are Not Signed

2.4.1.  Decision

   Any attributes other than those identified in Sections 2.2 and 2.3
   are not signed.  Examples of such attributes include the community
   attribute, the NO-EXPORT attribute, and Local_Pref.












Sriram                        Informational                     [Page 8]

RFC 8374                  BGPsec Design Choices               April 2018


2.4.2.  Discussion

   Any of the above-mentioned attributes that are not signed are viewed
   as local (e.g., do not need to propagate beyond the next hop) or lack
   clear security needs.  NO-EXPORT is sent over a secured next hop and
   does not need signing.  The BGPsec design should work with any
   transport-layer protections.  It is well understood that the
   transport layer must be protected hop by hop (if only to prevent
   malicious session termination).

2.5.  Receiving Router Actions

2.5.1.  Decision

   The following example describes the expected router actions on
   receipt of a signed update.  Consider an update that was originated
   by AS1 with NLRI prefix p and has traversed the AS path [AS(i-1)
   AS(i-2) ... AS2 AS1] before arriving at ASi.  Let the Expire Time
   (inserted by AS1) for the signature in this update be denoted as Te.
   Let AlgID represent the ID of the signature algorithm suite that is
   in use.  The update is to be processed at ASi and possibly forwarded
   to AS(i+1).  Let the attestations (signatures) inserted by each
   router in the AS path be denoted by Sig1, Sig2, ..., Sig(i-2), and
   Sig(i-1) corresponding to AS1, AS2, ..., AS(i-2), and AS(i-1),
   respectively.

   The method (Method 2 in Section 2.3) selected for signing requires a
   receiving router in ASi to perform the following actions:

   o  Validate the route origin pair (p, AS1) by performing a ROA match.

   o  Verify that Te is greater than the clock time at the router
      performing these checks.

   o  Check Sig1 with inputs {NLRI length, p, AlgID, Te, AS1, AS2}.

   o  Check Sig2 with inputs {Sig1, AS3}.

   o  Check Sig3 with inputs {Sig2, AS4}.

   o  ...

   o  ...

   o  Check Sig(i-2) with inputs {Sig(i-3), AS(i-1)}.






Sriram                        Informational                     [Page 9]

RFC 8374                  BGPsec Design Choices               April 2018


   o  Check Sig(i-1) with inputs {Sig(i-2), ASi}.

   o  If the route that has been verified is selected as the best path
      (for prefix p), then generate Sig(i) with inputs {Sig(i-1),
      AS(i+1)}, and generate an update including Sig(i) to AS(i+1).

   Note: The above description of BGPsec update validation and
   forwarding differs in its details from the published BGPsec protocol
   specification [RFC8205].  Please see Sections 4 and 5 of [RFC8205].

2.5.2.  Discussion

   See Section 8.1 for suggestions regarding efficient sequencing of
   BGPsec validation processing in a receiving router.  Some or all of
   the validation actions may be performed by an off-board server (see
   Section 9.3).

2.6.  Prepending of ASes in AS Path

2.6.1.  Decision

   Prepending will be allowed.  Prepending is defined as including more
   than one instance of the AS number (ASN) of the router that is
   signing the update.

   Note: The finalized BGPsec protocol specification [RFC8205] uses a
   pCount field associated with each AS in the path to indicate the
   number of prepends for that AS (see Figure 5 in Section 3.1 of
   [RFC8205]).

2.6.2.  Discussion

   The initial version [BGPsec-Initial] of the BGPsec specification
   calls for a signature to be associated with each prepended AS.  The
   optimization of having just one signature for multiple prepended ASes
   was pursued later.  The pCount field is now used to represent AS
   prepends; see Section 3.1 in RFC 8205.

2.7.  RPKI Data That Needs to Be Included in Updates

2.7.1.  Decision

   Concerning the inclusion of RPKI data in an update, it was decided
   that only the Subject Key Identifier (SKI) of the router certificate
   must be included in a signed update.  This information identifies the
   router certificate, based on the SKI generation criteria defined in
   [RFC6487].




Sriram                        Informational                    [Page 10]

RFC 8374                  BGPsec Design Choices               April 2018


2.7.2.  Discussion

   Whether or not each router public key certificate should be included
   in a signed update was discussed.  Inclusion of this information
   might be helpful for routers that do not have access to RPKI servers
   or temporarily lose connectivity to them.  It is safe to assume that
   in the majority of network environments, intermittent connectivity
   would not be a problem.  So, it is best to avoid this complexity,
   because the majority of the use environments do not have connectivity
   constraints.  Because the SKI of a router certificate is a hash of
   the public key of that certificate, it suffices to select the public
   key from that certificate.  This design assumes that each BGPsec
   router has access to a cache containing the relevant data from
   (validated) router certificates.

3.  Withdrawal Protection

3.1.  Withdrawals Not Signed

3.1.1.  Decision

   Withdrawals are not signed.

3.1.2.  Discussion

   In the current BGP protocol, any AS can withdraw, at any time, any
   prefix it previously announced.  The rationale for not signing
   withdrawals is that BGPsec assumes the use of transport security
   between neighboring BGPsec routers.  Thus, no external entity can
   inject an update that withdraws a route or replay a previously
   transmitted update containing a withdrawal.  Because the rationale
   for withdrawing a route is not visible to a neighboring BGPsec
   router, there are residual vulnerabilities associated with
   withdrawals.  For example, a router that advertised a (valid) route
   may fail to withdraw that route when it is no longer viable.  A
   router also might re-advertise a route that it previously withdrew,
   before the route is again viable.  This latter vulnerability is
   mitigated by the Expire Time associated with the origin AS's
   signature (see Section 3.2).

   Repeated withdrawals and announcements for a prefix can run up the
   BGP Route Flap Damping (RFD) penalty [RFC2439] and may result in
   unreachability for that prefix at upstream routers.  But what can the
   attacker gain from doing so?  This phenomenon is intrinsic to the
   design and operation of RFD.






Sriram                        Informational                    [Page 11]

RFC 8374                  BGPsec Design Choices               April 2018


3.2.  Signature Expire Time for Withdrawal Protection (a.k.a.
      Mitigation of Replay Attacks)

3.2.1.  Decision

   Note: As mentioned earlier (Section 2.2.2), the Expire Time approach
   to mitigation of replay attacks and withdrawal suppression was
   subsequently changed to an approach based on router key rollover
   [BGPsec-Rollover] [Replay-Protection].

   Only the originating AS inserts a signature Expire Time in the
   update; all other ASes along an AS path do not insert Expire Times
   associated with their respective signatures.  Further, the
   originating AS will re-originate a route sufficiently in advance of
   the Expire Time of its signature so that other ASes along an AS path
   will typically receive the re-originated route well ahead of the
   current Expire Time for that route.

   It is recommended that the duration of the signature Expire Time be
   on the order of days (preferably), but it may be on the order of
   hours (about 4 to 8 hours) in some cases on the basis of perceived
   need for extra protection from replay attacks (i.e., where extra
   replay protection is perceived to be critical).

   Each AS should stagger the Expire Time values in the routes it
   originates.  Re-origination will be done, say, at time Tb after
   origination or the last re-origination, where Tb will equal a certain
   percentage of the Expire Time, Te (for example, Tb = 0.75 x Te).  The
   percentage will be configurable.  Additional guidance can be provided
   via an operational considerations document later.  Further, the
   actual re-origination time should be jittered with a uniform random
   distribution over a short interval {Tb1, Tb2} centered at Tb.

   It is also recommended that a receiving BGPsec router detect that the
   only attribute change in an announcement (relative to the current
   best path) is the Expire Time (besides, of course, the signatures).
   In that case, assuming that the update is found valid, the route
   processor should not re-announce the route to non-BGPsec peers.  (It
   should sign and re-announce the route to BGPsec speakers only.)  This
   procedure will reduce BGP chattiness for the non-BGPsec border
   routers.










Sriram                        Informational                    [Page 12]

RFC 8374                  BGPsec Design Choices               April 2018


3.2.2.  Discussion

   Mitigation of BGPsec update replay attacks can be thought of as
   protection against malicious re-advertisements of withdrawn routes.
   If each AS along a path were to insert its own signature Expire Time,
   then there would be much additional BGP chattiness and an increase in
   BGP processing load due to the need to detect and react to multiple
   (possibly redundant) signature Expire Times.  Furthermore, there
   would be no extra benefit from the point of view of mitigation of
   replay attacks as compared to having a single Expire Time
   corresponding to the signature of the originating AS.

   As noted in Section 3.2.1, the recommended Expire Time value is on
   the order of days, but 4 to 8 hours may be used in some cases on the
   basis of perceived need for extra protection from replay attacks.
   Thus, different ASes may choose different values based on the
   perceived need to protect against malicious route replays.  (A
   shorter Expire Time reduces the window during which an AS can
   maliciously replay the route.  However, shorter Expire Time values
   cause routes to be refreshed more often, thus causing more BGP
   chatter.)  Even a 4-hour duration seems long enough to keep the
   re-origination workload manageable.  For example, if 500K routes are
   re-originated every 4 hours, it amounts to an increase in BGP update
   load of 35 updates per second; this can be considered reasonable.
   However, further analysis is needed to confirm these recommendations.

   As stated in Section 3.2.1, the originating AS will re-originate a
   route sufficiently in advance of its Expire Time.  What is considered
   "sufficiently in advance"?  To answer this question, modeling should
   be performed to determine the 95th-percentile convergence time of
   update propagation in a BGPsec-enabled Internet.

   Each BGPsec router should stagger the Expire Time values in the
   updates it originates, especially during table dumps to a neighbor or
   during its own recovery from a BGP session failure.  By doing this,
   the re-origination (i.e., beaconing) workload at the router will be
   dispersed.

3.3.  Should Route Expire Time be Communicated in a Separate Message?

3.3.1.  Decision

   The idea of sending a new signature Expire Time in a special message
   (rather than retransmitting the entire update with signatures) was
   considered.  However, the decision was made to not do this.
   Re-origination to communicate a new signature Expire Time will be
   done by propagating a normal update message; no special type of
   message will be required.



Sriram                        Informational                    [Page 13]

RFC 8374                  BGPsec Design Choices               April 2018


3.3.2.  Discussion

   It was suggested that if the re-beaconing of the signature
   Expire Time is carried in a separate special message, then any
   processing load related to the update may be reduced.  But it was
   recognized that such a re-beaconing message by necessity entails AS
   path and prefix information and, hence, cannot be separated from the
   update.

   It was observed that at the edge of the Internet, there are frequent
   updates that may result from such simple situations as a BGP session
   being switched from one interface to another (e.g., from primary to
   backup) between two peering ASes (e.g., customer and provider).  With
   traditional BGP, these updates do not propagate beyond the two ASes
   involved.  But with BGPsec, the customer AS will put in a new
   signature Expire Time each time such an event happens; hence, the
   update will need to propagate throughout the Internet (limited only
   by the process of best-path selection).  It was accepted that this
   cost of added churn will be unavoidable.

3.4.  Effect of Expire Time Updates in BGPsec on RFD

3.4.1.  Decision

   With regard to the RFD protocol [RFC2439] [JunOS] [CiscoIOS], no
   differential treatment is required for Expire-Time-triggered
   (re-beaconed) BGPsec updates.

   However, it was noted that it would be preferable if these updates
   did not cause route churn (and perhaps did not even require any
   RFD-related processing), since they are identical except for the
   change in the Expire Time value.  This can be accomplished by not
   assigning an RFD penalty to Expire-Time-triggered updates.  If the
   community agrees, this could be accommodated, but a change to the
   BGP-RFD protocol will be required.
















Sriram                        Informational                    [Page 14]

RFC 8374                  BGPsec Design Choices               April 2018


3.4.2.  Discussion

   To summarize, this decision is supported by the following
   observations:

   1.  Expire-Time-triggered updates are generally not preceded by
       withdrawals; hence, the path hunting and associated RFD
       exacerbation [Mao02] [RIPE580] problems are not anticipated.

   2.  Such updates would not normally change the best path (unless
       another concurrent event impacts the best path).

   3.  Expire-Time-triggered updates would have a negligible impact on
       RFD penalty accumulation because the re-advertisement interval is
       much longer relative to the half-time of RFD penalty decay.

   Elaborating further on the third observation above, it may be noted
   that the re-advertisements (i.e., beacons) of a route for a given
   address prefix from a given peer will be received at intervals of
   several hours (see Section 3.2).  During that time period, any
   incremental contribution to the RFD penalty due to an Expire-Time-
   triggered update would decay sufficiently to have negligible (if any)
   impact on damping the address prefix in question.

   Additional details regarding this analysis and justification are as
   follows:

   The frequency with which RFD penalty increments may be triggered for
   a given prefix from a given peer is the same as the re-beaconing
   frequency for that prefix from its origin AS.  The re-beaconing
   frequency is on the order of once every several hours (see
   Section 3.2).  The incremental RFD penalty assigned to a prefix due
   to a re-beaconed update varies, depending on the implementation.  For
   example, it appears that the JunOS implementation [JunOS] would
   assign a penalty of 1000 or 500, depending on whether the re-beaconed
   update is regarded as a re-advertisement or an attribute change,
   respectively.  Normally, a re-beaconed update would be treated as an
   attribute change.  On the other hand, the Cisco implementation
   [CiscoIOS] assigns an RFD penalty only in the case of an actual flap
   (i.e., a route is available, then unavailable, or vice versa).  So,
   it appears that Cisco's implementation of RFD would not assign any
   penalty for a re-beaconed update (i.e., a route was already
   advertised previously and was not withdrawn, and the re-beaconed
   update is merely updating the Expire Time attribute).  Even if one
   assumes that an RFD penalty of 500 is assigned (corresponding to an
   attribute change according to the JunOS RFD implementation), it can
   be illustrated that the incremental effect it would have on damping
   the prefix in question would be negligible: the half-time of RFD



Sriram                        Informational                    [Page 15]

RFC 8374                  BGPsec Design Choices               April 2018


   penalty decay is normally set to 15 minutes, whereas the re-beaconing
   frequency is on the order of once every several hours.  An
   incremental penalty of 500 would decay to 31.25 in 1 hour, 0.12 in
   2 hours, and 3x10^(-5) in 3 hours.  It may also be noted that the
   threshold for route suppression is 3000 in JunOS and 2000 in
   Cisco IOS.  Based on the foregoing analysis, it may be concluded that
   routine re-beaconing by itself would not result in RFD suppression of
   routes in the BGPsec protocol.

4.  Signature Algorithms and Router Keys

4.1.  Signature Algorithms

4.1.1.  Decision

   Initially, the Elliptic Curve Digital Signature Algorithm (ECDSA)
   with curve P-256 and SHA-256 will be used for generating BGPsec path
   signatures.  One other signature algorithm, e.g., RSA-2048, will also
   be used during prototyping and testing.  The use of a second
   signature algorithm is needed to verify the ability of the BGPsec
   implementations to change from a current algorithm to the next
   algorithm.

   Note: The BGPsec cryptographic algorithms document [RFC8208]
   specifies only the ECDSA with curve P-256 and SHA-256.

4.1.2.  Discussion

   Initially, the RSA-2048 algorithm for BGPsec update signatures was
   considered as a choice because it is being used ubiquitously in the
   RPKI system.  However, the use of ECDSA P-256 was decided upon
   because it yields a smaller signature size; hence, the update size
   and (in turn) the RIB size needed in BGPsec routers would be much
   smaller [RIB_size].

   Using two different signature algorithms (e.g., ECDSA P-256 and
   RSA-2048) to test the transition from one algorithm to the other will
   increase confidence in prototype implementations.

   Optimizations and specialized algorithms (e.g., for speedups) built
   on Elliptic Curve Cryptography (ECC) algorithms may have active IPR
   (intellectual property rights), but at the time of publication of
   this document no IPR had been disclosed to the IETF for the basic
   (unoptimized) algorithms.  (To understand this better, [RFC6090] can
   be useful as a starting point.)






Sriram                        Informational                    [Page 16]

RFC 8374                  BGPsec Design Choices               April 2018


   Note: Recently, even open-source implementations have incorporated
   certain cryptographic optimizations and demonstrated significant
   performance speedup [Gueron].  Researchers continue to devote
   significant effort toward demonstrating substantial speedup for the
   ECDSA as part of BGPsec implementations [Mehmet1] [Mehmet2].

4.2.  Agility of Signature Algorithms

4.2.1.  Decision

   During the transition period from one algorithm (i.e., the current
   algorithm) to the next (new) algorithm, the updates will carry two
   sets of signatures (i.e., two Signature_Blocks), one corresponding to
   each algorithm.  Each Signature_Block will be preceded by its
   type-length field and an algorithm suite identifier.  A BGPsec
   speaker that has been upgraded to handle the new algorithm should
   validate both Signature_Blocks and then add its corresponding
   signature to each Signature_Block for forwarding the update to the
   next AS.  A BGPsec speaker that has not been upgraded to handle the
   new algorithm will strip off the Signature_Block of the new algorithm
   and then will forward the update after adding its own signature to
   the Signature_Block of the current algorithm.

   It was decided that there will be at most two Signature_Blocks per
   update.

   Note: BGPsec path signatures are carried in the Signature_Block,
   which is an attribute contained in the BGPsec_PATH attribute (see
   Section 3.2 in [RFC8205]).  The algorithm agility scheme described in
   the published BGPsec protocol specification is consistent with the
   above; see Section 6.1 of [RFC8205].

4.2.2.  Discussion

   A length field in the Signature_Block allows for delineation of the
   two signature blocks.  Hence, a BGPsec router that doesn't know about
   a particular algorithm suite (and, hence, doesn't know how long
   signatures were for that algorithm suite) could still skip over the
   corresponding Signature_Block when parsing the message.

   The overlap period between the two algorithms is expected to last
   2 to 4 years.  The RIB memory and cryptographic processing capacity
   will have to be sized to cope with such overlap periods when updates
   would contain two sets of signatures [RIB_size].







Sriram                        Informational                    [Page 17]

RFC 8374                  BGPsec Design Choices               April 2018


   The lifetime of a signature algorithm is anticipated to be much
   longer than the duration of a transition period from the current
   algorithm to a new algorithm.  It is fully expected that all ASes
   will have converted to the required new algorithm within a certain
   amount of time that is much shorter than the interval in which a
   subsequent newer algorithm may be investigated and standardized for
   BGPsec.  Hence, the need for more than two Signature_Blocks per
   update is not envisioned.

4.3.  Sequential Aggregate Signatures

4.3.1.  Decision

   There is currently weak or no support for the Sequential Aggregate
   Signature (SAS) approach.  Please see Section 4.3.2 for a brief
   description of what the SAS is and what its pros and cons are.

4.3.2.  Discussion

   In the SAS method, there would be only one (aggregated) signature per
   signature block, irrespective of the number of AS hops.  For example,
   ASn (the nth AS) takes as input the signatures of all previous ASes
   [AS1, ..., AS(n-1)] and produces a single composite signature.  This
   composite signature has the property that a recipient who has the
   public keys for AS1, ..., ASn can verify (using only the single
   composite signature) that all of the ASes actually signed the
   message.  The SAS could potentially result in savings in bandwidth
   and in Protocol Data Unit (PDU) size, and maybe in RIB size, but the
   signature generation and validation costs will be higher as compared
   to one signature per AS hop.

   SAS schemes exist in the literature, typically based on RSA or its
   equivalent.  For a SAS with RSA and for the cryptographic strength
   needed for BGPsec signatures, a 2048-bit signature size (RSA-2048)
   would be required.  However, without a SAS, the ECDSA with a 512-bit
   signature (256-bit key) would suffice for equivalent cryptographic
   strength.  The larger signature size of RSA used with a SAS
   undermines the advantages of the SAS, because the average hop count,
   i.e., the number of ASes, for a route is about 3.8.  In the end, it
   may turn out that the SAS has more complexity and does not provide
   sufficient savings in PDU size or RIB size to merit its use.  Further
   exploration of this is needed to better understand SAS properties and
   applicability for BGPsec.  There is also a concern that the SAS is
   not a time-tested cryptographic technique, and thus its adoption is
   potentially risky.






Sriram                        Informational                    [Page 18]

RFC 8374                  BGPsec Design Choices               April 2018


4.4.  Protocol Extensibility

   There is clearly a need to specify a transition path from a current
   protocol specification to a new version.  When changes to the
   processing of the BGPsec path signatures are required, a new version
   of BGPsec will be required.  Examples of this include changes to the
   data that is protected by the BGPsec signatures or adoption of a
   signature algorithm in which the number of signatures in the
   signature block may not correspond to one signature per AS in the
   AS path (e.g., aggregate signatures).

4.4.1.  Decision

   This protocol-version transition mechanism is analogous to the
   algorithm transition discussed in Section 4.2.  During the transition
   period from one protocol version (i.e., the current version) to the
   next (new) version, updates will carry two sets of signatures (i.e.,
   two Signature_Blocks), one corresponding to each version.  A
   protocol-version identifier is associated with each Signature_Block.
   Hence, each Signature_Block will be preceded by its type-length field
   and a protocol-version identifier.  A BGPsec speaker that has been
   upgraded to handle the new version should validate both
   Signature_Blocks and then add its corresponding signature to each
   Signature_Block for forwarding the update to the next AS.  A BGPsec
   speaker that has not been upgraded to handle the new protocol version
   will strip off the Signature_Block of the new version and then will
   forward the update with an attachment of its own signature to the
   Signature_Block of the current version.

   Note: The details of protocol extensibility (i.e., transition to a
   new version of BGPsec) in the published BGPsec protocol specification
   (see Section 6.3 in [RFC8205]) differ somewhat from the above.  In
   particular, the protocol-version identifier is not part of the BGPsec
   update.  Instead, it is negotiated during the BGPsec capability
   exchange portion of BGPsec session negotiation.

4.4.2.  Discussion

   In the case that a change to BGPsec is 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") that is designed to accommodate the
   desired changes to BGPsec.  At this point, a transition would begin
   that is analogous to the algorithm transition discussed in
   Section 4.2.  During the transition period, all BGPsec speakers will
   simultaneously include both the BGPsec_PATH (current) attribute (see
   Section 3 of RFC 8205) and the new BGPsec_PATH_TWO attribute.  Once
   the transition is complete, the use of BGPsec_PATH could then be



Sriram                        Informational                    [Page 19]

RFC 8374                  BGPsec Design Choices               April 2018


   deprecated, at which point BGPsec speakers will include only the new
   BGPsec_PATH_TWO attribute.  Such a process could facilitate a
   transition to new BGPsec semantics in a backwards-compatible fashion.

4.5.  Key per Router (Rogue Router Problem)

4.5.1.  Decision

   Within each AS, each individual BGPsec router can have a unique pair
   of private and public keys [RFC8207].

4.5.2.  Discussion

   Given a unique key pair per router, if a router is compromised, its
   key pair can be revoked independently, without disrupting the other
   routers in the AS.  Each per-router key pair will be represented in
   an end-entity certificate issued under the certification authority
   (CA) certificate of the AS.  The Subject Key Identifier (SKI) in the
   signature points to the router certificate (and thus the unique
   public key) of the router that affixed its signature, so that a
   validating router can reliably identify the public key to use for
   signature verification.

4.6.  Router ID

4.6.1.  Decision

   The router certificate subject name will be the string "ROUTER"
   followed by a decimal representation of a 4-byte ASN followed by the
   router ID.  (Note: The details are specified in Section 3.1 in
   [RFC8209].)

4.6.2.  Discussion

   Every X.509 certificate requires a subject name [RFC6487].  The
   stylized subject name adopted here is intended to facilitate
   debugging by including the ASN and router ID.














Sriram                        Informational                    [Page 20]

RFC 8374                  BGPsec Design Choices               April 2018


5.  Optimizations and Resource Sizing

5.1.  Update Packing and Repacking

   With traditional BGP [RFC4271], an originating BGP router normally
   packs multiple prefix announcements into one update if the prefixes
   all share the same BGP attributes.  When an upstream BGP router
   forwards eBGP updates to its peers, it can also pack multiple
   prefixes (based on the shared AS path and attributes) into one
   update.  The update propagated by the upstream BGP router may include
   only a subset of the prefixes that were packed in a received update.

5.1.1.  Decision

   Each update contains exactly one prefix.  This avoids a level of
   complexity that would otherwise be inevitable if the origin had
   packed and signed multiple prefixes in an update and an upstream AS
   decided to propagate an update containing only a subset of the
   prefixes in that update.  BGPsec recommendations regarding packing
   and repacking may be revisited when optimizations are considered in
   the future.

5.1.2.  Discussion

   Currently, with traditional BGP, there are, on average, approximately
   four prefixes announced per update [RIB_size].  So, the number of BGP
   updates (carrying announcements) is about four times fewer, on
   average, as compared to the number of prefixes announced.

   The current decision is to include only one prefix per secured update
   (see Section 2.2.2).  When optimizations are considered in the
   future, the possibility of packing multiple prefixes into an update
   can also be considered.  (Please see Section 5.2 for a discussion of
   signature per prefix vs. signature per update.)  Repacking could be
   performed if signatures were generated on a per-prefix basis.
   However, one problem regarding this approach -- multiple prefixes in
   a BGP update but with a separate signature for each prefix -- is that
   the resulting BGP update violates the basic definition of a BGP
   update: the different prefixes will have different signatures and
   Expire Time attributes, while a BGP update (by definition) must have
   the same set of shared attributes for all prefixes it carries.










Sriram                        Informational                    [Page 21]

RFC 8374                  BGPsec Design Choices               April 2018


5.2.  Signature per Prefix vs. Signature per Update

5.2.1.  Decision

   The initial design calls for including exactly one prefix per update;
   hence, there is only one signature in each secured update (modulo
   algorithm transition conditions).

5.2.2.  Discussion

   Some notes to assist in future optimization discussions follow:

   In the general case of one signature per update, multiple prefixes
   may be signed with one signature together with their shared AS path,
   next ASN, and Expire Time.  If the "signature per update" technique
   is used, then there are potential savings in update PDU size as well
   as RIB memory size.  But if there are any changes made to the
   announced prefix set along the AS path, then the AS where the change
   occurs would need to insert an Explicit Path Attribute (EPA)
   [Secure-BGP].  The EPA conveys information regarding what the prefix
   set contained prior to the change.  There would be one EPA for each
   AS that made such a modification, and there would be a way to
   associate each EPA with its corresponding AS.  This enables an
   upstream AS to know and verify what was announced and signed by prior
   ASes in the AS path (in spite of changes made to the announced prefix
   set along the way).  The EPA adds complexity to processing (signature
   generation and validation); further increases the size of updates
   and, thus, of the RIB; and exposes data to downstream ASes that would
   not otherwise be exposed.  Not all of the pros and cons of packing
   and repacking in the context of signature per prefix vs. signature
   per update (with packing) have been evaluated.  But the current
   recommendation is for having only one prefix per update (no packing),
   so there is no need for the EPA.

5.3.  Maximum BGPsec Update PDU Size

   The current BGP update message PDU size is limited to 4096 bytes
   [RFC4271].  The question was raised as to whether or not BGPsec would
   require a larger update PDU size.

5.3.1.  Decision

   The current thinking is that the maximum PDU size should be increased
   to 64 KB [BGP-Ext-Msg] so that there is sufficient room to
   accommodate two Signature_Blocks (i.e., one block with a current
   algorithm and another block with a new signature algorithm during a
   future transition period) for long AS paths.




Sriram                        Informational                    [Page 22]

RFC 8374                  BGPsec Design Choices               April 2018


   Note: RFC 8205 states the following: "All BGPsec UPDATE messages MUST
   conform to BGP's maximum message size.  If the resulting message
   exceeds the maximum message size, then the guidelines in Section 9.2
   of RFC 4271 [RFC4271] MUST be followed."

5.3.2.  Discussion

   The current maximum message size for BGP updates is 4096 octets.  An
   effort is underway in the IETF to extend it to a larger size
   [BGP-Ext-Msg].  BGPsec will conform to whatever maximum message size
   is available for BGP while adhering to the guidelines in Section 9.2
   of RFC 4271 [RFC4271].

   Note: Estimates for the average and maximum sizes anticipated for
   BGPsec update messages are provided in [MsgSize].

5.4.  Temporary Suspension of Attestations and Validations

5.4.1.  Decision

   If a BGPsec-capable router needs to temporarily suspend/defer signing
   and/or validation of BGPsec updates during periods of route processor
   overload, the router may do so even though such suspension/deferment
   is not desirable; the specification does not forbid it.  Following
   any temporary suspension, the router should subsequently send signed
   updates corresponding to the updates for which validation and signing
   were skipped.  The router also may choose to skip only validation but
   still sign and forward updates during periods of congestion.

5.4.2.  Discussion

   In some situations, a BGPsec router may be unable to keep up with the
   workload of performing signing and/or validation.  This can happen,
   for example, during BGP session recovery when a router has to send
   the entire routing table to a recovering router in a neighboring AS
   (see [CPUworkload]).  So, it is possible that a BGPsec router
   temporarily pauses performing the validation or signing of updates.
   When the workload eases, the BGPsec router should clear the
   validation or signing backlog and send signed updates corresponding
   to the updates for which validation and signing were skipped.  During
   periods of overload, the router may simply send unsigned updates
   (with signatures dropped) or may sign and forward the updates with
   signatures (even though the router itself has not yet verified the
   signatures it received).







Sriram                        Informational                    [Page 23]

RFC 8374                  BGPsec Design Choices               April 2018


   A BGPsec-capable AS may request (out of band) that a BGPsec-capable
   peer AS never downgrade a signed update to an unsigned update.
   However, in partial-deployment scenarios, it is not possible for a
   BGPsec router to require a BGPsec-capable eBGP peer to send only
   signed updates, except for prefixes originated by the peer's AS.

   Note: If BGPsec has not been negotiated with a peer, then a BGPsec
   router forwards only unsigned updates to that peer; the sending
   router does so by following the reconstruction procedure in
   Section 4.4 of [RFC8205] to generate an AS_PATH attribute
   corresponding to the BGPsec_PATH attribute in a received signed
   update.  If the above-mentioned temporary suspension is ever applied,
   then the same AS_PATH reconstruction procedure should be utilized.

6.  Incremental Deployment and Negotiation of BGPsec

6.1.  Downgrade Attacks

6.1.1.  Decision

   No attempt will be made in the BGPsec design to prevent downgrade
   attacks, i.e., a BGPsec-capable router sending unsigned updates when
   it is capable of sending signed updates.

6.1.2.  Discussion

   BGPsec allows routers to temporarily suspend signing updates (see
   Section 5.4).  Therefore, it would be contradictory if we were to try
   to incorporate in the BGPsec protocol a way to detect and reject
   downgrade attacks.  One proposed way to detect downgrade attacks was
   considered, based on signed peering registrations (see Section 9.5).

6.2.  Inclusion of Address Family in Capability Advertisement

6.2.1.  Decision

   It was decided that during capability negotiation, the address family
   for which the BGPsec speaker is advertising support for BGPsec will
   be shared using the Address Family Identifier (AFI).  Initially, two
   address families would be included, namely, IPv4 and IPv6.  BGPsec
   for use with other address families may be specified in the future.
   Simultaneous use of the two (i.e., IPv4 and IPv6) address families
   for the same BGPsec session will require that the BGPsec speaker
   include two instances of this capability (one for each address
   family) during BGPsec capability negotiation.






Sriram                        Informational                    [Page 24]

RFC 8374                  BGPsec Design Choices               April 2018


6.2.2.  Discussion

   If new address families are supported in the future, they will be
   added in future versions of the specification.  A comment was made
   that too many version numbers are bad for interoperability.
   Renegotiation on the fly to add a new address family (i.e., without
   changeover to a new version number) is desirable.

6.3.  Incremental Deployment: Capability Negotiation

6.3.1.  Decision

   BGPsec will be incrementally deployable.  BGPsec routers will use
   capability negotiation to agree to run BGPsec between them.  If a
   BGPsec router's peer does not agree to run BGPsec, then the BGPsec
   router will run only traditional BGP with that peer, i.e., it will
   not send BGPsec (i.e., signed) updates to the peer.

   Note: See Section 7.9 of [RFC8205] for a discussion of incremental /
   partial-deployment considerations.  Also, Section 6 of [RFC8207]
   describes how edge sites (stub ASes) can sign updates that they
   originate but can receive only unsigned updates.  This facilitates a
   less expensive upgrade to BGPsec in resource-limited stub ASes and
   expedites incremental deployment.

6.3.2.  Discussion

   The partial-deployment approach to incremental deployment will result
   in "BGPsec islands".  Updates that originate within a BGPsec island
   will generally propagate with signed AS paths to the edges of that
   island.  As BGPsec adoption grows, the BGPsec islands will expand
   outward (subsuming non-BGPsec portions of the Internet) and/or pairs
   of islands may join to form larger BGPsec islands.

6.4.  Partial Path Signing

   "Partial path signing" means that a BGPsec AS can be permitted to
   sign an update that was received unsigned from a downstream neighbor.
   That is, the AS would add its ASN to the AS path and sign the
   (previously unsigned) update to other neighboring (upstream)
   BGPsec ASes.

6.4.1.  Decision

   It was decided that partial path signing in BGPsec will not be
   allowed.  A BGPsec update must be fully signed, i.e., each AS in the
   AS path must sign the update.  So, in a signed update, there must be
   a signature corresponding to each AS in the AS path.



Sriram                        Informational                    [Page 25]

RFC 8374                  BGPsec Design Choices               April 2018


6.4.2.  Discussion

   Partial path signing (as described above) implies that the AS path is
   not rigorously protected.  Rigorous AS path protection is a key
   requirement of BGPsec [RFC7353].  Partial path signing clearly
   reintroduces the following attack vulnerability: if a BGPsec speaker
   is allowed to sign an unsigned update and if signed (i.e., partially
   or fully signed) updates would be preferred over unsigned updates,
   then a faulty, misconfigured, or subverted BGPsec speaker can
   manufacture any unsigned update it wants (by inserting a valid origin
   AS) and add a signature to it to increase the chance that its update
   will be preferred.

6.5.  Consideration of Stub ASes with Resource Constraints: Encouraging
      Early Adoption

6.5.1.  Decision

   The protocol permits each pair of BGPsec-capable ASes to
   asymmetrically negotiate the use of BGPsec.  Thus, a stub AS (or
   downstream customer AS) can agree to perform BGPsec only in the
   transmit direction and speak traditional BGP in the receive
   direction.  In this arrangement, the ISP's (upstream) AS will not
   send signed updates to this stub or customer AS.  Thus, the stub AS
   can avoid the need to hardware-upgrade its route processor and RIB
   memory to support BGPsec update validation.

6.5.2.  Discussion

   Various other options were also considered for accommodating a
   resource-constrained stub AS, as discussed below:

   1.  An arrangement that can be effected outside of the BGPsec
       specification is as follows.  Through a private arrangement
       (invisible to other ASes), an ISP's AS (upstream AS) can truncate
       the stub AS (or downstream AS) from the path and sign the update
       as if the prefix is originating from the ISP's AS (even though
       the update originated unsigned from the customer AS).  This way,
       the path will appear fully signed to the rest of the network.
       This alternative will require the owner of the prefix at the stub
       AS to issue a ROA for the upstream AS, so that the upstream AS is
       authorized to originate routes for the prefix.

   2.  Another type of arrangement that can also be effected outside of
       the BGPsec specification is as follows.  The stub AS does not
       sign updates, but it obtains an RPKI (CA) certificate and issues
       a router certificate under that CA certificate.  It passes on the
       private key for the router certificate to its upstream provider.



Sriram                        Informational                    [Page 26]

RFC 8374                  BGPsec Design Choices               April 2018


       That ISP (i.e., the second-hop AS) would insert a signature on
       behalf of the stub AS using the private key obtained from the
       stub AS.  This arrangement is called "proxy signing" (see
       Section 6.6).

   3.  An extended ROA is created that includes the stub AS as the
       originator of the prefix and the upstream provider as the
       second-hop AS, and partial signatures would be allowed (i.e., the
       stub AS need not sign the updates).  It is recognized that this
       approach is also authoritative and not trust based.  It was
       observed that the extended ROA is not much different from what is
       done with the ROA (in its current form) when a Provider-
       Independent (PI) address is originated from a provider's AS.
       This approach was rejected due to possible complications with the
       creation and use of a new RPKI object, namely, the extended ROA.
       Also, the validating BGPsec router has to perform a level of
       indirection with this approach, i.e., it must detect that an
       update is not fully signed and then look for the extended ROA to
       validate.

   4.  Another method, based on a different form of indirection, would
       be as follows.  The customer (stub) AS registers something like a
       Proxy Signer Authorization, which authorizes the second-hop
       (i.e., provider) AS to sign on behalf of the customer AS using
       the provider's own key [Dynamics].  This method allows for fully
       signed updates (unlike the approach based on the extended ROA).
       But this approach also requires the creation of a new RPKI
       object, namely, the Proxy Signer Authorization.  In this
       approach, the second-hop AS and validating ASes have to perform a
       level of indirection.  This approach was also rejected.

   The various inputs regarding ISP preferences were taken into
   consideration, and eventually the decision in favor of asymmetric
   BGPsec was reached (Section 6.5.1).  An advantage for a stub AS that
   does asymmetric BGPsec is that it only needs to minimally upgrade to
   BGPsec so it can sign updates to its upstream AS while it receives
   only unsigned updates.  Thus, it can avoid the cost of increased
   processing and memory needed to perform update validations and to
   store signed updates in the RIBs, respectively.

6.6.  Proxy Signing

6.6.1.  Decision

   An ISP's AS (or upstream AS) can proxy-sign BGP announcements for a
   customer (downstream) AS, provided that the customer AS obtains an
   RPKI (CA) certificate, issues a router certificate under that CA
   certificate, and passes on the private key for that certificate to



Sriram                        Informational                    [Page 27]

RFC 8374                  BGPsec Design Choices               April 2018


   its upstream provider.  That ISP (i.e., the second-hop AS) would
   insert a signature on behalf of the customer AS using the private key
   provided by the customer AS.  This is a private arrangement between
   the two ASes and is invisible to other ASes.  Thus, this arrangement
   is not part of the BGPsec protocol specification.

   BGPsec will not make any special provisions for an ISP to use its own
   private key to proxy-sign updates for a customer's AS.  This type of
   proxy signing is considered a bad idea.

6.6.2.  Discussion

   Consider a scenario when a customer's AS (say, AS8) is multihomed to
   two ISPs, i.e., AS8 peers with AS1 and AS2 of ISP-1 and ISP-2,
   respectively.  In this case, AS8 would have an RPKI (CA) certificate;
   it issues two separate router certificates (corresponding to AS1 and
   AS2) under that CA certificate, and it passes on the respective
   private keys for those two certificates to its upstream providers AS1
   and AS2.  Thus, AS8 has a proxy-signing service from both of its
   upstream ASes.  In the future, if AS8 were to disconnect from ISP-2,
   then it would revoke the router certificate corresponding to AS2.

6.7.  Multiple Peering Sessions between ASes

6.7.1.  Decision

   No problems are anticipated when BGPsec-capable ASes have multiple
   peering sessions between them (between distinct routers).

6.7.2.  Discussion

   In traditional BGP, multiple peering sessions between different pairs
   of routers (between two neighboring ASes) may be simultaneously used
   for load sharing.  Similarly, BGPsec-capable ASes can also have
   multiple peering sessions between them.  Because routers in an AS can
   have distinct private keys, the same update, when propagated over
   these multiple peering sessions, will result in multiple updates that
   may differ in their signatures.  The peer (upstream) AS will apply
   its normal procedures for selecting a best path from those multiple
   updates (and updates from other peers).

   This decision regarding load balancing (vs. using one peering session
   as the primary for carrying data and another as the backup) is
   entirely local and is up to the two neighboring ASes.







Sriram                        Informational                    [Page 28]

RFC 8374                  BGPsec Design Choices               April 2018


7.  Interaction of BGPsec with Common BGP Features

7.1.  Peer Groups

   In traditional BGP, the idea of peer groups is used in BGP routers to
   save on processing when generating and sending updates.  Multiple
   peers for whom the same policies apply can be organized into peer
   groups.  A peer group can typically have tens of ASes (and maybe as
   many as 300) in it.

7.1.1.  Decision

   It was decided that BGPsec updates are generated to target unique AS
   peers, so there is no support for peer groups in BGPsec.

7.1.2.  Discussion

   BGPsec router processing can make use of peer groups preceding the
   signing of updates to peers.  Some of the update processing prior to
   forwarding to members of a peer group can be done only once per
   update, as is done in traditional BGP.  Prior to forwarding the
   update, a BGPsec speaker adds the peer's ASN to the data that needs
   to be signed and signs the update for each peer AS in the group
   individually.

   If updates were to be signed per peer group, information about the
   forward AS set that constitutes a peer group would have to be
   divulged (since the ASN of each peer would have to be included in the
   update).  Some ISPs do not like to share this kind of information
   globally.

7.2.  Communities

   The need to provide protection in BGPsec for the community attribute
   was discussed.

7.2.1.  Decision

   Community attribute(s) will not be included in any message that is
   signed in BGPsec.

7.2.2.  Discussion

   From a security standpoint, the community attribute, as currently
   defined, may be inherently defective.  A substantial amount of work
   on the semantics of the community attribute is needed, and additional
   work on its security aspects also needs to be done.  The community
   attribute is not necessarily transitive; it is often used only



Sriram                        Informational                    [Page 29]

RFC 8374                  BGPsec Design Choices               April 2018


   between neighbors.  In those contexts, transport-security mechanisms
   suffice to provide integrity and authentication.  (There is no need
   to sign data when it is passed only between peers.)  It was suggested
   that one could include only the transitive community attributes in
   any message that is signed and propagated (across the AS path).  It
   was noted that there is a flag available (i.e., unused) in the
   community attribute, and it might be used by BGPsec (in some
   fashion).  However, little information is available at this point
   about the use and function of this flag.  It was speculated that this
   flag could potentially be used to indicate to BGPsec whether or not
   the community attribute needs protection.  For now, community
   attributes will not be secured by BGPsec path signatures.

7.3.  Consideration of iBGP Speakers and Confederations

7.3.1.  Decision

   An iBGP speaker that is also an eBGP speaker and that executes BGPsec
   will by necessity carry BGPsec data and perform eBGPsec functions.
   Confederations are eBGP clouds for administrative purposes and
   contain multiple Member-ASes.  A Member-AS is not required to sign
   updates sent to another Member-AS within the same confederation.
   However, if BGPsec signing is applied in eBGP within a confederation,
   i.e., each Member-AS signs to the next Member-AS in the path within
   the confederation, then upon egress from the confederation, the
   Member-AS at the boundary must remove any and all signatures applied
   within the confederation.  The Member-AS at the boundary of the
   confederation will sign the update to an eBGPsec peer using the
   public ASN of the confederation and its private key.  The BGPsec
   specification will not specify how to perform this process.

   Note: In RFC 8205, signing a BGPsec update between Member-ASes within
   a confederation is required if the update were to propagate with
   signatures within the confederation.  A Confed_Segment flag exists in
   each Secure_Path segment, and when set, it indicates that the
   corresponding signature belongs to a Member-AS.  At the confederation
   boundary, all signatures with Confed_Segment flags set are removed
   from the update.  RFC 8205 specifies in detail how all of this is
   done.  Please see Figure 5 in Section 3.1 of [RFC8205], as well as
   Section 4.3 of [RFC8205], for details.

7.3.2.  Discussion

   This topic may need to be revisited to flesh out the details
   carefully.






Sriram                        Informational                    [Page 30]

RFC 8374                  BGPsec Design Choices               April 2018


7.4.  Consideration of Route Servers in IXPs

7.4.1.  Decision

   [BGPsec-Initial] made no special provisions to accommodate route
   servers in Internet Exchange Points (IXPs).

   Note: The above decision subsequently changed: RFC 8205 allows the
   accommodation of IXPs, especially for transparent route servers.  The
   pCount (AS prepend count) field is set to zero for transparent route
   servers (see Section 4.2 of [RFC8205]).  The operational guidance for
   preventing the misuse of pCount=0 is given in Section 7.2 of
   RFC 8205.  Also, see Section 8.4 of RFC 8205 for a discussion of
   security considerations concerning pCount=0.

7.4.2.  Discussion

   There are basically three methods that an IXP may use to propagate
   routes: (A) direct bilateral peering through the IXP, (B) BGP peering
   between clients via peering with a route server at the IXP (without
   the IXP inserting its ASN in the path), and (C) BGP peering with an
   IXP route server, where the IXP inserts its ASN in the path.
   (Note: The IXP's route server does not change the NEXT_HOP attribute
   even if it inserts its ASN in the path.)  It is very rare for an IXP
   to use Method C because it is less attractive for the clients if
   their AS path length increases by one due to the IXP.  A measure of
   the extent of the use of Method A vs. Method B is given in terms of
   the corresponding IP traffic load percentages.  As an example, at a
   major European IXP, these percentages are about 80% and 20% for
   Methods A and B, respectively (this data is based on private
   communication with IXPs circa 2011).  However, as the IXP grows (in
   terms of number of clients), it tends to migrate more towards
   Method B because of the difficulties of managing up to n x (n-1)/2
   direct interconnections between n peers in Method A.

   To the extent that an IXP is providing direct bilateral peering
   between clients (Method A), that model works naturally with BGPsec.
   Also, if the route server in the IXP plays the role of a regular
   BGPsec speaker (minus the routing part for payload) and inserts its
   own ASN in the path (Method C), then that model would also work well
   in the BGPsec Internet and this case is trivially supported in
   BGPsec.









Sriram                        Informational                    [Page 31]

RFC 8374                  BGPsec Design Choices               April 2018


7.5.  Proxy Aggregation (a.k.a. AS_SETs)

7.5.1.  Decision

   Proxy aggregation (i.e., the use of AS_SETs in the AS path) will not
   be supported in BGPsec.  There is no provision in BGPsec to sign an
   update when an AS_SET is part of an AS path.  If a BGPsec-capable
   router receives an update that contains an AS_SET and also finds that
   the update is signed, then the router will consider the update
   malformed (i.e., a protocol error).

   Note: Section 5.2 of RFC 8205 specifies that a receiving BGPsec
   router "MUST handle any syntactical or protocol errors in the
   BGPsec_PATH attribute by using the 'treat-as-withdraw' approach as
   defined in RFC 7606 [RFC7606]."

7.5.2.  Discussion

   Proxy aggregation does occur in the Internet today, but it is very
   rare.  Only a very small fraction (about 0.1%) of observed updates
   contain AS_SETs in the AS path [ASset].  Since traditional BGP
   currently allows for proxy aggregation with the inclusion of AS_SETs
   in the AS path, it is necessary that BGPsec specify what action a
   receiving router must take if such an update is received with
   attestation.  BCP 172 [RFC6472] recommends against the use of AS_SETs
   in updates, so it is anticipated that the use of AS_SETs will
   diminish over time.

7.6.  4-Byte AS Numbers

   Not all (currently deployed) BGP speakers are capable of dealing with
   4-byte ASNs [RFC6793].  The standard mechanism used to accommodate
   such speakers requires a peer AS to translate each 4-byte ASN in the
   AS path to a reserved 2-byte ASN (23456) before forwarding the
   update.  This mechanism is incompatible with the use of BGPsec, since
   the ASN translation is equivalent to a route modification attack and
   will cause signatures corresponding to the translated 4-byte ASNs to
   fail validation.

7.6.1.  Decision

   BGP speakers that are BGPsec capable are required to process
   4-byte ASNs.

7.6.2.  Discussion

   It is reasonable to assume that upgrades for 4-byte ASN support will
   be in place prior to the deployment of BGPsec.



Sriram                        Informational                    [Page 32]

RFC 8374                  BGPsec Design Choices               April 2018


8.  BGPsec Validation

8.1.  Sequence of BGPsec Validation Processing in a Receiver

   It is natural to ask in what sequence a receiver must perform BGPsec
   update validation so that if a failure were to occur (i.e., the
   update was determined to be invalid) the processor would have spent
   the least amount of processing or other resources.

8.1.1.  Decision

   There was agreement that the following sequence of receiver
   operations is quite meaningful; the following steps are included in
   [BGPsec-Initial].  However, the ordering of these validation-
   processing steps is not a normative part of the BGPsec specification.

   1.  Verify that the signed update is syntactically correct.  For
       example, check to see if the number of signatures matches the
       number of ASes in the AS path (after duly accounting for AS
       prepending).

   2.  Verify that the origin AS is authorized to advertise the prefix
       in question.  This verification is based on data from ROAs and
       does not require any cryptographic operations.

   3.  Verify that the advertisement has not yet expired.

   4.  Verify that the target ASN in the signature data matches the ASN
       of the router that is processing the advertisement.  Note that
       the target-ASN check is also a non-cryptographic operation and
       is fast.

   5.  Validate the signature data starting from the most recent AS to
       the origin.

   6.  Locate the public key for the router from which the advertisement
       was received, using the SKI from the signature data.

   7.  Hash the data covered by the signature algorithm.  Invoke the
       signature validation algorithm on the following three inputs: the
       locally computed hash, the received signature, and the public
       key.  There will be one output: valid or invalid.

   8.  Repeat steps 5 and 6 for each preceding signature in the
       Signature_Block until (a) the signature data for the origin AS is
       encountered and processed or (b) either of these steps fails.





Sriram                        Informational                    [Page 33]

RFC 8374                  BGPsec Design Choices               April 2018


   Note: Significant refinements to the above list occurred in the
   progress towards RFC 8205.  The detailed syntactic-error checklist is
   presented and explained in Section 5.2 of [RFC8205].  Also, a logical
   sequence of steps to be followed in the validation of
   Signature_Blocks is described in Section 5.2 of [RFC8205].

8.1.2.  Discussion

   If the goal is to minimize computational costs associated with
   cryptographic operations, the sequence of receiver operations that is
   suggested above is viewed as appropriate.  One additional interesting
   suggestion was that when there are two Signature_Blocks in an update,
   the validating router can first verify which of the two algorithms is
   cheaper, to save on processing.  If that Signature_Block verifies,
   then the router can skip validating the other Signature_Block.

8.2.  Signing and Forwarding Updates when Signatures Failed Validation

8.2.1.  Decision

   A BGPsec router should sign and forward a signed update to upstream
   peers if it selected the update as the best path, regardless of
   whether the update passed or failed validation (at this router).

8.2.2.  Discussion

   The availability of RPKI data at different routers (in the same AS or
   different ASes) may differ, depending on the sources used to acquire
   RPKI data.  Hence, an update may fail validation in one AS, and the
   same update may pass validation in another AS.  Also, an update may
   fail validation at one router in an AS, and the same update may pass
   validation at another router in the same AS.

   A BCP may be published later that will identify some update-failure
   conditions that may present unambiguous cases for rejecting the
   update (in which case the router would not select the AS path in the
   update).  These cases are "TBD" (to be determined).














Sriram                        Informational                    [Page 34]

RFC 8374                  BGPsec Design Choices               April 2018


8.3.  Enumeration of Error Conditions

   Enumeration of error conditions and the recommendations for how to
   react to them are still under discussion.

8.3.1.  Decision

   TBD.  Also, please see Section 8.5 for the decision and discussion
   specifically related to syntactic errors in signatures.

   Note: Section 5.2 of RFC 8205 describes the detection of syntactic
   and protocol errors in BGPsec updates as well as how the updates with
   such errors are to be handled.

8.3.2.  Discussion

   The following list is a first attempt to provide some possible error
   conditions and recommended receiver reactions in response to the
   detection of those errors.  Refinements will follow after further
   discussions.

   E1  Abnormalities where a peer (i.e., the preceding AS) should
       definitely not have propagated to a receiving eBGPsec router.
       For example, (A) the number of signatures does not match the
       number of ASes in the AS path (after accounting for AS
       prepending), (B) there is an AS_SET in the received update and
       the update has signatures, or (C) other syntactic errors with
       signatures have occurred.

       Reaction: See Section 8.5.

   E2  Situations where a receiving eBGPsec router cannot find the
       certificate for an AS in the AS path.

       Reaction: Mark the update as "Invalid".  It is acceptable to
       consider the update in the best-path selection.  If it is chosen,
       then the router should sign and propagate the update.

   E3  Situations where a receiving eBGPsec router cannot find a ROA for
       the {prefix, origin} pair in the update.

       Reaction: Same as in (E2) above.

   E4  Situations where the receiving eBGPsec router verifies signatures
       and finds that the update is "Invalid" (even though its peer
       might not have known, e.g., due to RPKI skew).

       Reaction: Same as in (E2) above.



Sriram                        Informational                    [Page 35]

RFC 8374                  BGPsec Design Choices               April 2018


       In some networks, the best-path-selection policy may specify
       choosing an unsigned update over one with invalid signature(s).
       Hence, the signatures must not be stripped even if the update is
       "Invalid".  No evil bit is set in the update (when it is
       "Invalid") because an upstream peer may not get that same answer
       when it tries to validate.

8.4.  Procedure for Processing Unsigned Updates

   An update may come in unsigned from an eBGP peer or internally (e.g.,
   as an iBGP update).  In the latter case, the route is being
   originated from within the AS in question.

8.4.1.  Decision

   If an unsigned route is received from an eBGP peer and if it is
   selected, then the route will be forwarded unsigned to other eBGP
   peers -- even BGPsec-capable peers.  If the route originated in this
   AS (IGP or iBGP) and is unsigned, then it should be signed and
   announced to external BGPsec-capable peers.

8.4.2.  Discussion

   It is also possible that an update received in IGP (or iBGP) may have
   private ASNs in the AS path.  These private ASNs would normally
   appear in the rightmost portion of the AS path.  It was noted that in
   this case the private ASNs to the right would be removed (as done in
   traditional BGP), and then the update will be signed by the
   originating AS and announced to BGPsec-capable eBGP peers.

   Note: See Section 7.5 of [RFC8205] for operational considerations for
   BGPsec in the context of private ASNs.

8.5.  Response to Syntactic Errors in Signatures and Recommendations for
      How to React to Them

   Note: The contents of this subsection (i.e., Section 8.5) differ
   substantially from the recommendations in RFC 8205 regarding the
   handling of syntactic errors and protocol errors.  Hence, the reader
   may skip this subsection and instead read Section 5.2 of [RFC8205].
   This subsection (Section 8.5) is kept here for the sake of archival
   value concerning design discussions.

   Different types of error conditions were discussed in Section 8.3.
   Here, the focus is only on syntactic-error conditions in signatures.






Sriram                        Informational                    [Page 36]

RFC 8374                  BGPsec Design Choices               April 2018


8.5.1.  Decision

   If there are syntactic-error conditions such as (A) AS_SET and
   BGPsec_PATH both appearing in an update, (B) the number of signatures
   not matching the number of ASes (after accounting for any AS
   prepending), or (C) a parsing issue occurring with the BGPsec_PATH
   attribute, then the update (with the signatures stripped) will still
   be considered in the best-path-selection algorithm.  (**Note: This is
   not true in RFC 8205**.)  If the update is selected as the best path,
   then the update will be propagated unsigned.  The error condition
   will be logged locally.

   A BGPsec router will follow whatever the current IETF (IDR WG)
   recommendations are for notifying a peer that it is sending malformed
   messages.

   In the case when there are two Signature_Blocks in an update, and one
   or more syntactic errors are found to occur within one of them but
   the other one is free of any syntactic errors, then the update will
   still be considered in the best-path-selection algorithm after the
   syntactically bad Signature_Block has been removed.  (**Note: This is
   not true in RFC 8205**.)  If the update is selected as the best path,
   then the update will be propagated with only one (i.e., the
   error-free) Signature_Block.  The error condition will be logged
   locally.

8.5.2.  Discussion

   As stated above, a BGPsec router will follow whatever the current
   IETF (IDR WG) recommendations are for notifying a peer that it is
   sending malformed messages.  Question: If the error is persistent and
   a full BGP table dump occurs, then would there be 500K such errors
   resulting in 500K "notify" messages sent to the peer that is
   generating the errors?  Answer: Rate limiting would be applied to the
   notify messages and should prevent any overload due to these
   messages.

8.6.  Enumeration of Validation States

   Various validation conditions are possible that can be mapped to
   validation states for possible input to the BGPsec decision process.
   These conditions can be related to whether an update is signed,
   Expire Time is checked, route origin validation is checked against a
   ROA, signature verification passed, etc.







Sriram                        Informational                    [Page 37]

RFC 8374                  BGPsec Design Choices               April 2018


8.6.1.  Decision

   It was decided that BGPsec validation outcomes will be mapped to one
   of only two validation states: (1) Valid -- passed all validation
   checks (i.e., Expire Time check, route origin and Signature_Block
   validation) and (2) Invalid -- all other possibilities.  "Invalid"
   would include situations such as the following:

   1.  Due to a lack of RPKI data or insufficient RPKI data, validation
       was not performed.

   2.  The signature Expire Time check failed.

   3.  Route origin validation failed.

   4.  Signature checks were performed, and one or more of them failed.

   Note: Expire Time is obsolete (see the notes in Sections 2.2.1 and
   2.2.2).  RFC 8205 uses the states "Valid" and "Not Valid", but only
   with respect to AS path validation (i.e., not including the result of
   origin validation); see Section 5.1 of [RFC8205].  "Not Valid"
   includes all conditions in which path validation was attempted but a
   "Valid" result could not be reached.  (Note: Path validation is not
   attempted in the case of syntactic or protocol errors in a BGPsec
   update; see Section 5.2 of [RFC8205].)  Each Relying Party (RP) is
   expected to devise its own policy to suitably factor the results of
   origin validation [RFC6811] and path validation [RFC8205] into its
   path-selection decision.

8.6.2.  Discussion

   It may be noted that the result of update validation is just an
   additional input for the BGP decision process.  The router's local
   policy ultimately has control over what action (regarding BGP path
   selection) is taken.

   Initially, four validation states were considered:

   1.  The update is not signed.

   2.  The update is signed, but the router does not have corresponding
       RPKI data to perform a validation check.

   3.  The validation check was performed, and the check failed
       (Invalid).

   4.  The validation check was performed, and the check passed (Valid).




Sriram                        Informational                    [Page 38]

RFC 8374                  BGPsec Design Choices               April 2018


   As stated above, it was later decided that BGPsec validation outcomes
   will be mapped to one of only two validation states.  It was observed
   that an update can be invalid for many different reasons.  To begin
   to differentiate these numerous reasons and to try to enumerate
   different flavors of the Invalid state will not likely be
   constructive in route-selection decisions and may even introduce new
   vulnerabilities in the system.  However, some questions remain, such
   as the following:

   Question: Is there a need to define a separate validation state for
   the case when an update is not signed but the {prefix, origin} pair
   matches the ROA information?  After some discussion, a tentative
   conclusion was reached: this is in principle similar to validation
   based on partial path signing (which was ruled out; see Section 6.4).
   So, there is no need to add another validation state for this case;
   treat it as "Invalid", considering that it is unsigned.

   Another remaining question: Would the RP want to give the update a
   higher preference over another unsigned update that failed origin
   validation or over a signed update that failed both signature and ROA
   validation?

8.7.  Mechanism for Transporting Validation State through iBGP

8.7.1.  Decision

   BGPsec validation need be performed only at eBGP edges.  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 will determine how the validation status is conveyed
   internally, using various preexisting mechanisms, e.g., setting a BGP
   community, or modifying a metric value such as Local_Pref or MED.  A
   signed update that cannot be validated (except those with syntax
   errors) should be forwarded with signatures from the ingress router
   to the egress router, where it is signed when propagated towards
   other eBGPsec speakers in neighboring ASes.  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.
   The latter approach may be used at an operator's discretion, under
   circumstances when RPKI skew is known to happen at different routers
   within an AS.

   Note: An extended community for carrying the origin validation state
   in iBGP has been specified in RFC 8097 [RFC8097].







Sriram                        Informational                    [Page 39]

RFC 8374                  BGPsec Design Choices               April 2018


8.7.2.  Discussion

   The attribute used to represent the validation state can be carried
   between ASes, if desired.  ISPs may like to carry it over their eBGP
   links between their own ASes (e.g., sibling ASes).  A peer (or
   customer) may receive it over an eBGP link from a provider and may
   want to use it to shortcut their own validation check.  However, the
   peer (or customer) should be aware that this validation-state
   attribute is just a preview of a neighbor's validation and must
   perform their own validation check to be sure of the actual state of
   the update's validation.  Question: Should validation-state
   propagation be protected by attestation in cases where it is useful
   for diagnostics purposes?  The decision was made to not protect the
   validation-state information using signatures.

   The following validation states may be needed for propagation via
   iBGP between edge routers in an AS:

   o  Validation states communicated in iBGP for an unsigned update
      (route origin validation result): (1) Valid, (2) Invalid,
      (3) NotFound (see [RFC6811]), (4) Validation Deferred.

      *  An update could be unsigned for either of the following two
         reasons, but they need not be distinguished: (a) it had no
         signatures (i.e., came in unsigned from an eBGP peer) or
         (b) signatures were present but stripped.

   o  Validation states communicated in iBGP for a signed update:
      (1) Valid, (2) Invalid, (3) Validation Deferred.

   The reason for conveying the additional "Validation Deferred" state
   may be illustrated as follows.  An ingress edge Router A receiving an
   update from an eBGPsec peer may not attempt to validate signatures
   (e.g., in a processor overload situation), and in that case Router A
   should convey "Validation Deferred" state for that signed update (if
   selected for best path) in iBGP to other edge routers.  An egress
   edge Router B, upon receiving the update from ingress Router A, would
   then be able to perform its own validation (origin validation for an
   unsigned update or origin/signature validation for a signed update).
   As stated before, the egress router (Router B in this example) may
   always choose to perform its own validation when it receives an
   update from iBGP (independently of the update's validation status
   conveyed in iBGP) to account for the possibility of RPKI data skew at
   different routers.  These various choices are local and entirely at
   the operator's discretion.






Sriram                        Informational                    [Page 40]

RFC 8374                  BGPsec Design Choices               April 2018


9.  Operational Considerations

   Note: Significant thought has been devoted to operations and
   management considerations subsequent to the writing of
   [BGPsec-Initial].  The reader is referred to [RFC8207] and Section 7
   of [RFC8205] for details.

9.1.  Interworking with BGP Graceful Restart

   BGP Graceful Restart (BGP-GR) [RFC4724] is a mechanism currently used
   to facilitate nonstop packet forwarding when the control plane is
   recovering from a fault (i.e., the BGP session is restarted) but the
   data plane is functioning.  Two questions were raised: Are there any
   special concerns about how BGP-GR works while BGPsec is operational?
   Also, what happens if the BGP router operation transitions from
   traditional BGP operation to BGP-GR to BGPsec, in that order?

9.1.1.  Decision

   No decision was made relative to this issue (at the time that
   [BGPsec-Initial] was written).

   Note: See Section 7.7 of [RFC8205] for comments concerning the
   operation of BGP-GR with BGPsec.  They are consistent with the
   discussion below.

9.1.2.  Discussion

   BGP-GR can be implemented with BGPsec, just as it is currently
   implemented with traditional BGP.  The Restart State bit, Forwarding
   State bit, End-of-RIB marker, staleness marker (in the Adj-RIB-In),
   and Selection_Deferral_Timer are key parameters associated with
   BGP-GR [RFC4724].  These parameters would apply to BGPsec, just as
   they apply to traditional BGP.

   Regarding what happens if the BGP router transitions from traditional
   BGP to BGP-GR to BGPsec, the answer would simply be as follows.  If
   there is a software upgrade to BGPsec during BGP-GR (assuming that
   the upgrade is being done on a live BGP speaker), then the BGP-GR
   session should be terminated before a BGPsec session is initiated.
   Once the eBGPsec peering session is established, the receiving
   eBGPsec speaker will see signed updates from the sending (newly
   upgraded) eBGPsec speaker.  There is no apparent harm (it may, in
   fact, be desirable) if the receiving speaker continues to use
   previously learned unsigned BGP routes from the sending speaker until
   they are replaced by new BGPsec routes.  However, if the Forwarding
   State bit is set to zero by the sending speaker (i.e., the newly
   upgraded speaker) during BGPsec session negotiation, then the



Sriram                        Informational                    [Page 41]

RFC 8374                  BGPsec Design Choices               April 2018


   receiving speaker would mark all previously learned unsigned BGP
   routes from that sending speaker as "stale" in its Adj-RIB-In.  Then,
   as BGPsec updates are received (possibly interspersed with unsigned
   BGP updates), the "stale" routes will be replaced or refreshed.

9.2.  BCP Recommendations for Minimizing Churn: Certificate Expiry/
      Revocation and Signature Expire Time

9.2.1.  Decision

   Work related to this topic is still in progress.

9.2.2.  Discussion

   BCP recommendations for minimizing churn in BGPsec have been
   discussed.  There are various potential strategies on how routers
   should react to such events as certificate expiry/revocation and
   signature Expire Time exhaustion [Dynamics].  The details will be
   documented in the near future after additional work is completed.

9.3.  Outsourcing Update Validation

9.3.1.  Decision

   Update signature validation and signing can be outsourced to an
   off-board server or processor.

9.3.2.  Discussion

   Possibly, an off-router box (one or more per AS) can be used that
   performs path validation.  For example, these capabilities might be
   incorporated into a route reflector.  At an ingress router, one needs
   the Adj-RIB-In entries validated but not the RIB-out entries.  So,
   the off-router box is probably unlike the traditional route
   reflector; it sits at the network edge and validates all incoming
   BGPsec updates.  Thus, it appears that each router passes each BGPsec
   update it receives to the off-router box and receives a validation
   result before it stores the route in the Adj-RIB-In.  Question: What
   about failure modes here?  The failure modes would be dependent on
   the following:

   1.  How much of the control plane is outsourced.

   2.  How reliable the off-router box is (or, equivalently,
       communication to and from it).

   3.  How centralized vs. distributed this arrangement is.




Sriram                        Informational                    [Page 42]

RFC 8374                  BGPsec Design Choices               April 2018


   When any kind of outsourcing is done, the user needs to be watchful
   and ensure that the outsourcing does not cross trust/security
   boundaries.

9.4.  New Hardware Capability

9.4.1.  Decision

   It is assumed that BGPsec routers (Provider Edge (PE) routers and
   route reflectors) will require significantly upgraded hardware --
   much more memory for RIBs and hardware cryptographic assistance.
   However, stub ASes would not need to make such upgrades because they
   can negotiate asymmetric BGPsec capability with their upstream ASes,
   i.e., they sign updates to the upstream AS but receive only unsigned
   BGP updates (see Section 6.5).

9.4.2.  Discussion

   It is accepted that it might take several years to go beyond test
   deployment of BGPsec because of the need for additional route
   processor CPU and memory.  However, because BGPsec deployment will be
   incremental and because signed updates are not sent outside of a set
   of contiguous BGPsec-enabled ASes, it is not clear how much
   additional (RIB) memory will be required during initial deployment.
   See [RIB_size] for preliminary results on modeling and estimation of
   BGPsec RIB size and its projected growth.  Hardware cryptographic
   support reduces the computation burden on the route processor and
   offers good security for router private keys.  However, given the
   incremental-deployment model, it also is not clear how substantial a
   cryptographic processing load will be incurred in the early phases of
   deployment.

   Note: There are recent detailed studies that considered software
   optimizations for BGPsec.  In [Mehmet1] and [Mehmet2], computational
   optimizations for cryptographic processing (i.e., ECDSA speedup) are
   considered for BGPsec implementations on general-purpose CPUs.  In
   [V_Sriram], software optimizations at the level of update processing
   and path selection are proposed and quantified for BGPsec
   implementations.












Sriram                        Informational                    [Page 43]

RFC 8374                  BGPsec Design Choices               April 2018


9.5.  Signed Peering Registrations

9.5.1.  Decision

   The idea of signed BGP peering registrations (for the purpose of path
   validation) was rejected.

9.5.2.  Discussion

   The idea of using a secure map of AS relationships to "validate"
   updates was discussed and rejected: such solutions were not pursued
   because they cannot provide strong guarantees regarding the validity
   of updates.  Using these techniques, one can say only that an update
   is "plausible"; one cannot say that it is "definitely" valid (based
   on signed peering relations alone).

10.  Security Considerations

   This document requires no security considerations.  See [RFC8205] for
   security considerations for the BGPsec protocol.

11.  IANA Considerations

   This document has no IANA actions.

12.  Informative References

   [ASset]    Sriram, K. and D. Montgomery, "Measurement Data on AS_SET
              and AGGREGATOR: Implications for {Prefix, Origin}
              Validation Algorithms", IETF SIDR WG presentation,
              IETF 78, July 2010, <http://www.nist.gov/itl/antd/upload/
              AS_SET_Aggregator_Stats.pdf>.

   [BGP-Ext-Msg]
              Bush, R., Patel, K., and D. Ward, "Extended Message
              support for BGP", Work in Progress, draft-ietf-idr-bgp-
              extended-messages-24, November 2017.

   [BGPsec-Initial]
              Lepinski, M., "BGPSEC Protocol Specification", Work in
              Progress, draft-lepinski-bgpsec-protocol-00, March 2011.

   [BGPsec-Rollover]
              Weis, B., Gagliano, R., and K. Patel, "BGPsec Router
              Certificate Rollover", Work in Progress, draft-ietf-
              sidrops-bgpsec-rollover-04, December 2017.





Sriram                        Informational                    [Page 44]

RFC 8374                  BGPsec Design Choices               April 2018


   [Borchert]
              Borchert, O. and M. Baer, "Subject: Modifiation [sic]
              request: draft-ietf-sidr-bgpsec-protocol-14", message to
              the IETF SIDR WG Mailing List, 10 February 2016,
              <https://www.ietf.org/mail-archive/web/sidr/current/
              msg07509.html>.

   [CiscoIOS]
              "Cisco IOS: Configuring Route Dampening", February 2014,
              <https://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/
              configuration/guide/fipr_c/1cfbgp.html>.

   [CPUworkload]
              Sriram, K. and R. Bush, "Estimating CPU Cost of BGPSEC on
              a Router", Presented at RIPE-63; also at IETF 83 SIDR WG
              Meeting, March 2012, <https://www.ietf.org/proceedings/
              83/slides/slides-83-sidr-7.pdf>.

   [Dynamics]
              Sriram, K., Montgomery, D., Borchert, O., Kim, O., and P.
              Gleichmann, "Potential Impact of BGPSEC Mechanisms on
              Global BGP Dynamics", Presentation to the BGPsec
              authors/designers team, October 2009,
              <https://www.nist.gov/file/448631>.

   [Gueron]   Gueron, S. and V. Krasnov, "Fast and side channel
              protected implementation of the NIST P-256 Elliptic Curve
              for x86-64 platforms", OpenSSL patch ID 3149,
              October 2013, <https://rt.openssl.org/Ticket/
              Display.html?id=3149&user=guest&pass=guest>.

   [JunOS]    "Juniper JunOS: Using Routing Policies to Damp BGP Route
              Flapping", November 2010, <http://www.juniper.net/
              techpubs/en_US/junos10.4/topics/usage-guidelines/
              policy-using-routing-policies-to-damp-bgp-route-
              flapping.html>.

   [Mandelberg1]
              Mandelberg, D., "Subject: wglc for draft-ietf-sidr-bgpsec-
              protocol-11 (Specific topic: Include Address Family
              Identifier in the data protected under signature -- to
              alleviate a security concern)", message to the IETF SIDR
              WG Mailing List, 10 February 2015, <https://www.ietf.org/
              mail-archive/web/sidr/current/msg06930.html>.







Sriram                        Informational                    [Page 45]

RFC 8374                  BGPsec Design Choices               April 2018


   [Mandelberg2]
              Mandelberg, D., "Subject: draft-ietf-sidr-bgpsec-
              protocol-13's security guarantees (Specific topic: Sign
              all of the preceding signed data (rather than just the
              immediate, previous signature) -- to alleviate a security
              concern)", message to the IETF SIDR WG Mailing List,
              26 August 2015, <https://www.ietf.org/mail-archive/
              web/sidr/current/msg07241.html>.

   [Mao02]    Mao, Z., et al., "Route Flap Damping Exacerbates Internet
              Routing Convergence", August 2002,
              <http://www.eecs.umich.edu/~zmao/Papers/sig02.pdf>.

   [Mehmet1]  Adalier, M., "Efficient and Secure Elliptic Curve
              Cryptography Implementation of Curve P-256", NIST Workshop
              on ECC Standards, June 2015,
              <http://csrc.nist.gov/groups/ST/ecc-workshop-2015/papers/
              session6-adalier-Mehmet.pdf>.

   [Mehmet2]  Adalier, M., Sriram, K., Borchert, O., Lee, K., and D.
              Montgomery, "High Performance BGP Security: Algorithms and
              Architectures", North American Network Operators Group
              Meeting NANOG69, February 2017,
              <https://www.nanog.org/meetings/abstract?id=3043>.

   [MsgSize]  Sriram, K., "Decoupling BGPsec Documents and Extended
              Messages draft", Presented at the IETF SIDROPS WG
              Meeting, IETF 98, March 2017,
              <https://www.ietf.org/proceedings/98/slides/
              slides-98-sidrops-decoupling-bgpsec-documents-and-
              extended-messages-draft-00.pdf>.

   [Replay-Protection]
              Sriram, K. and D. Montgomery, "Design Discussion and
              Comparison of Protection Mechanisms for Replay Attack and
              Withdrawal Suppression in BGPsec", Work in Progress,
              draft-sriram-replay-protection-design-discussion-10,
              April 2018.

   [RFC2439]  Villamizar, C., Chandra, R., and R. Govindan, "BGP Route
              Flap Damping", RFC 2439, DOI 10.17487/RFC2439,
              November 1998, <https://www.rfc-editor.org/info/rfc2439>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.




Sriram                        Informational                    [Page 46]

RFC 8374                  BGPsec Design Choices               April 2018


   [RFC4724]  Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
              Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
              DOI 10.17487/RFC4724, January 2007,
              <https://www.rfc-editor.org/info/rfc4724>.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/info/rfc4760>.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090,
              DOI 10.17487/RFC6090, February 2011,
              <https://www.rfc-editor.org/info/rfc6090>.

   [RFC6472]  Kumari, W. and K. Sriram, "Recommendation for Not Using
              AS_SET and AS_CONFED_SET in BGP", BCP 172, RFC 6472,
              DOI 10.17487/RFC6472, December 2011,
              <https://www.rfc-editor.org/info/rfc6472>.

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
              February 2012, <https://www.rfc-editor.org/info/rfc6480>.

   [RFC6482]  Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
              Origin Authorizations (ROAs)", RFC 6482,
              DOI 10.17487/RFC6482, February 2012,
              <https://www.rfc-editor.org/info/rfc6482>.

   [RFC6483]  Huston, G. and G. Michaelson, "Validation of Route
              Origination Using the Resource Certificate Public Key
              Infrastructure (PKI) and Route Origin Authorizations
              (ROAs)", RFC 6483, DOI 10.17487/RFC6483, February 2012,
              <https://www.rfc-editor.org/info/rfc6483>.

   [RFC6487]  Huston, G., Michaelson, G., and R. Loomans, "A Profile for
              X.509 PKIX Resource Certificates", RFC 6487,
              DOI 10.17487/RFC6487, February 2012,
              <https://www.rfc-editor.org/info/rfc6487>.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,
              <https://www.rfc-editor.org/info/rfc6793>.







Sriram                        Informational                    [Page 47]

RFC 8374                  BGPsec Design Choices               April 2018


   [RFC6811]  Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
              Austein, "BGP Prefix Origin Validation", RFC 6811,
              DOI 10.17487/RFC6811, January 2013,
              <https://www.rfc-editor.org/info/rfc6811>.

   [RFC7132]  Kent, S. and A. Chi, "Threat Model for BGP Path Security",
              RFC 7132, DOI 10.17487/RFC7132, February 2014,
              <https://www.rfc-editor.org/info/rfc7132>.

   [RFC7353]  Bellovin, S., Bush, R., and D. Ward, "Security
              Requirements for BGP Path Validation", RFC 7353,
              DOI 10.17487/RFC7353, August 2014,
              <https://www.rfc-editor.org/info/rfc7353>.

   [RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
              Patel, "Revised Error Handling for BGP UPDATE Messages",
              RFC 7606, DOI 10.17487/RFC7606, August 2015,
              <https://www.rfc-editor.org/info/rfc7606>.

   [RFC8097]  Mohapatra, P., Patel, K., Scudder, J., Ward, D., and R.
              Bush, "BGP Prefix Origin Validation State Extended
              Community", RFC 8097, DOI 10.17487/RFC8097, March 2017,
              <https://www.rfc-editor.org/info/rfc8097>.

   [RFC8205]  Lepinski, M., Ed., and K. Sriram, Ed., "BGPsec Protocol
              Specification", RFC 8205, DOI 10.17487/RFC8205,
              September 2017, <https://www.rfc-editor.org/info/rfc8205>.

   [RFC8207]  Bush, R., "BGPsec Operational Considerations", BCP 211,
              RFC 8207, DOI 10.17487/RFC8207, September 2017,
              <https://www.rfc-editor.org/info/rfc8207>.

   [RFC8208]  Turner, S. and O. Borchert, "BGPsec Algorithms, Key
              Formats, and Signature Formats", RFC 8208,
              DOI 10.17487/RFC8208, September 2017,
              <https://www.rfc-editor.org/info/rfc8208>.

   [RFC8209]  Reynolds, M., Turner, S., and S. Kent, "A Profile for
              BGPsec Router Certificates, Certificate Revocation Lists,
              and Certification Requests", RFC 8209,
              DOI 10.17487/RFC8209, September 2017,
              <https://www.rfc-editor.org/info/rfc8209>.

   [RIB_size]
              Sriram, K., et al., "RIB Size Estimation for BGPSEC",
              May 2011, <http://www.nist.gov/itl/antd/upload/
              BGPSEC_RIB_Estimation.pdf>.




Sriram                        Informational                    [Page 48]

RFC 8374                  BGPsec Design Choices               April 2018


   [RIPE580]  Bush, R., et al., "RIPE-580: RIPE Routing Working Group
              Recommendations on Route Flap Damping", January 2013,
              <http://www.ripe.net/ripe/docs/ripe-580>.

   [Secure-BGP]
              Lynn, C., Mikkelson, J., and K. Seo, "Secure BGP (S-BGP)",
              Work in Progress, draft-clynn-s-bgp-protocol-01,
              June 2003.

   [V_Sriram]
              Sriram, V. and D. Montgomery, "Design and analysis of
              optimization algorithms to minimize cryptographic
              processing in BGP security protocols", Computer
              Communications, Vol. 106, pp. 75-85,
              DOI 10.1016/j.comcom.2017.03.007, July 2017,
              <https://www.sciencedirect.com/science/article/pii/
              S0140366417303365>.

Acknowledgements

   The author would like to thank Jeff Haas and Wes George for serving
   as reviewers for this document for the Independent Submissions
   stream.  The author is grateful to Nevil Brownlee for shepherding
   this document through the Independent Submissions review process.
   Many thanks are also due to Michael Baer, Oliver Borchert, David
   Mandelberg, Sean Turner, Alvaro Retana, Matthias Waehlisch, Tim Polk,
   Russ Mundy, Wes Hardaker, Sharon Goldberg, Ed Kern, Chris Hall, Shane
   Amante, Luke Berndt, Doug Maughan, Pradosh Mohapatra, Mark Reynolds,
   Heather Schiller, Jason Schiller, Ruediger Volk, and David Ward for
   their review, comments, and suggestions during the course of
   this work.

Contributors

   The following people made significant contributions to this document
   and should be considered co-authors:

   Rob Austein
   Dragon Research Labs
   Email: sra@hactrn.net

   Steven Bellovin
   Columbia University
   Email: smb@cs.columbia.edu

   Russ Housley
   Vigil Security, LLC
   Email: housley@vigilsec.com



Sriram                        Informational                    [Page 49]

RFC 8374                  BGPsec Design Choices               April 2018


   Stephen Kent
   Independent
   Email: kent@alum.mit.edu

   Warren Kumari
   Google
   Email: warren@kumari.net

   Matt Lepinski
   New College of Florida
   Email: mlepinski@ncf.edu

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

   Chris Morrow
   Google, Inc.
   Email: morrowc@google.com

   Sandy Murphy
   Parsons, Inc.
   Email: sandy@tislabs.com

   Keyur Patel
   Arrcus
   Email: keyur@arrcus.com

   John Scudder
   Juniper Networks
   Email: jgs@juniper.net

   Samuel Weiler
   W3C/MIT
   Email: weiler@csail.mit.edu

Author's Address

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

   Email: ksriram@nist.gov






Sriram                        Informational                    [Page 50]

