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Network Working Group                                           K. Patel
Internet-Draft                                               Arrcus, Inc
Intended status: Standards Track                          A. Vyavaharkar
Expires: September 2, 2018                                 Cisco Systems
                                                           N. Fazlollahi
                                                            Unaffiliated
                                                           A. Przygienda
                                                        Juniper Networks
                                                             Mar 1, 2018


                Extension to BGP's Route Refresh Message
                 draft-idr-bgp-route-refresh-options-04

Abstract

   [RFC2918] defines a route refresh capability to be exchanged between
   BGP speakers.  BGP speakers that support this capability are
   advertising that they can resend the entire BGP Adj-RIB-Out on
   receipt of a refresh request.  By supporting this capability, BGP
   speakers are more flexible in applying any inbound routing policy
   changes as they no longer have to store received routes in their
   unchanged form or reset the session when an inbound routing policy
   change occurs.  The route refresh capability is advertised per AFI,
   SAFI combination.

   There are newer AFI, SAFI types that have been introduced to BGP that
   support a variety of route types (e.g.  IPv4/MVPN, L2VPN/EVPN).
   Currently, there is no way to request a subset of routes in a Route
   Refresh message for a given AFI, SAFI.  This draft defines route
   refresh capability extensions that help BGP speakers to request a
   subset of routes for a given address family.  This is expected to
   reduce the amount of update traffic being generated by route refresh
   requests as well as lessen the burden on the router servicing such
   requests.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 2, 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
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   Without obtaining an adequate license from the person(s) controlling
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   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Use Case Examples . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Route Refresh Options Capability  . . . . . . . . . . . . . .   4
   4.  Route Refresh Sub-Types . . . . . . . . . . . . . . . . . . .   5
   5.  Route Refresh Option format . . . . . . . . . . . . . . . . .   5
   6.  Route Refresh Option Length . . . . . . . . . . . . . . . . .   6
   7.  Route Refresh ID  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Route Refresh Option Flags  . . . . . . . . . . . . . . . . .   7
   9.  Route Refresh Options . . . . . . . . . . . . . . . . . . . .   8
   10. Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   11. Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  11
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  12



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   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     15.2.  Information References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Sequence Number Binary Arithmetic  . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   [RFC2918] defines a route refresh capability to be exchanged between
   BGP speakers.  BGP speakers that support this capability are
   advertising that they can resend the entire BGP Adj-RIB-Out on
   receipt of a refresh request.  By supporting this capability, BGP
   speakers are more flexible in applying inbound routing policy changes
   as they no longer have to store copies of received routes in their
   unchanged form or reset the session when an inbound routing policy
   change occurs.  The route refresh capability is advertised per AFI,
   SAFI combination.

   Route refresh allows routers to dynamically request a full Adj-RIB-
   Out update from their peers when there's an inbound routing policy
   change.  This is useful because routers that mutually support this
   capability no longer have to flap the peering session or store an
   extra copy of received routes in their original form.  This helps by
   reducing memory requirements as well as eliminating the unnecessary
   churn caused by session flaps.  [RFC2918] does not define a way for
   routers to request a subset of the Adj-RIB-Out for a given AFI, SAFI.

   This draft defines new extensions to route refresh that will allow
   requesting routers to ask for a subset of the Adj-RIB-Out for a given
   AFI, SAFI combination.  For example, routers could ask for specific
   route types from those address families that support multiple route
   types or, they could ask for a specific prefix.

   As part of the new extensions, this draft combines elements of
   [RFC7313] and [RFC5291] and adds a new set of options to the route
   refresh message that will specify filters that can be applied to
   limit the scope of the refresh being requested.  The new option
   format will apply to all new option types that may be defined moving
   forward.

1.1.  Use Case Examples

   The authors acknowledge that while the extensions being proposed in
   this draft could potentially be addressed by Route Target Constrain
   described in [RFC4684] by using route targets to identify desired
   subset of routes, this proposal includes address families where RT
   Constrain extension is not supported and avoids the necessity to



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   assign and manage the route targets per desired set of routes.  The
   approach in this draft is intended to be a single-hop refresh only,
   i.e., propagation of the refreshes in a way similar to RT Constrain
   routes is NOT intended.

   Several possible use cases are discernible today:

   o  The capacity to refresh routes of a certain type within an address
      family is needed, e.g., auto discovery routes within the EVPN AF
      [RFC7432].

   o  In VPN scenarios where RT Constrain is not supported or
      configured, RDs can be used.

   o  In BGP LS [RFC7752] cases a speaker may choose to hold only a
      subset of routes and depending on configuration request a subset
      of routes.  This document could provide further filters to support
      those use cases.

   o  On changes in inbound policy, when previously configured filters
      have been removed, only the according subset of routes may be
      requested.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Route Refresh Options Capability

   A BGP speaker will use the BGP Capabilities Advertisement [RFC5492]
   to advertise the Route Refresh Options Capability to its peers.  This
   new capability will be advertised using the Capability code [TBD]
   with a capability length of 0.

   By advertising the Route Refresh Options Capability to a peer, a BGP
   speaker indicates that it is capable of receiving and processing the
   route refresh options described below.  This new capability can be
   advertised along with the Enhanced Route Refresh Capability described
   in [RFC7313].  However, if the Route Refresh Options Capability has
   been negotiated by both sides of the BGP session, then it will
   override the Enhanced Route Refresh Capability.








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4.  Route Refresh Sub-Types

   [RFC7313] defines route refresh BGP message sub-types that utilize
   the "Reserved" field of the Route Refresh message originally defined
   in [RFC2918].  Currently, there are three sub-types defined and this
   draft proposes three additional sub-types which will be used to
   indicate a Route Refresh message that includes options before any ORF
   field of the Route Refresh message as well as BoRR and EoRR Route
   Refresh messages with options.

               0 - Normal route refresh request [RFC2918]
                   with/without Outbound Route Filtering (ORF) [RFC5291]
               1 - Demarcation of the beginning of a route refresh
                   (BoRR) operation
               2 - Demarcation of the ending of a route refresh
                   (EoRR) operation
             + 3 - Route Refresh request with options and optional
                   ORF [RFC5291]
             + 4 - BoRR with options
             + 5 - EoRR with options
             255 - Reserved

   When the Route Refresh Options Capability has been negotiated by both
   sides of a BGP session, both peers MUST use message types 3, 4 and 5.
   The requesting speaker MUST use the refresh ID for all refresh
   requests including those without any options, i.e., requests for the
   full BGP Adj-RIB-Out.

   The Route Refresh Request Message with options will now be formatted
   as shown below

        0               1               2               3
        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           A F I               |     Res.      |   S A F I     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Total Option Length        |      Refresh ID#      | Flags |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 One or more Route Refresh Options             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


5.  Route Refresh Option format

   [RFC2918] defines the route refresh BGP message that includes only
   the AFI, SAFI of the routes being requested.  This draft proposes
   extending the basic message by including options that will indicate
   to the remote BGP speaker that a subset of the entire Adj-RIB-Out is



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   being requested.  The remote BGP speaker will select routes that
   match the specified options and the flag settings.

   As described in the previous section, the options will be added to
   the Route Refresh message before the ORF field of the message.
   Outbound Route Filtering is described in [RFC5291].  The options will
   assume the following format

        0               1               2               3
        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Length Of Options Field  |  Refresh ID#          | Flags |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           One or more Route Refresh Options                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


6.  Route Refresh Option Length

   The Option Length field will occupy the two octets immediately
   following the Route Refresh message containing the AFI, SAFI and sub-
   type.  The purpose of this field is to allow the BGP speaker to
   calculate the length of any attached ORF fields by subtracting the
   Option Length from the Route Refresh message length.

7.  Route Refresh ID

   The Refresh ID field will occupy twelve bits following the Route
   Refresh Options Length.  It is infeasible to use a wide number like a
   64-bit unsigned integer since this number must be stored per route
   entry associated with any peer supporting this feature, at least
   during the time any refresh is pending.  It is a value assigned by
   the requesting BGP speaker.  It MUST be a strictly monotonically
   increasing number per peer AFI and SAFI using sequence number
   arithmetic based on two-complements given in Appendix A.  It is
   comparable to the calculations standardized in [RFC1982] but fixes
   several of its anomalies.  The purpose of this field is to allow the
   requesting BGP speaker to correlate concurrent, overlapping refresh
   requests and ultimately delete correct stale routes.  The Refresh ID
   MUST be reflected in the BoRR and EoRR messages sent by the BGP
   speaker servicing the refresh request.

   A Refresh ID value MUST NOT be reused until an EoRR with this ID has
   been received by the requesting speaker or the last resort time has
   expired.  The behavior is unspecified otherwise.  More specifically,
   defining the interval [ LID, HID ] by the values





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   LID = MAX(lowest requested Refresh ID# without BoRR,
             lowest received BoRR without EoRR)

   and

   HID = highest requested Refresh ID#

   the requesting speaker MUST only use values V where V >: LID and V >:
   HID as defined by the relation given in Appendix A.  Beside that, HID
   =>: LID MUST hold by the same algebra.

   If no such number V exists, LID must catch up to HID, i.e. no further
   requests can be issued.  To use a 3 bit example in Appendix A, if LID
   was 1 and HID was 4, we cannot progress to unsigned 5 since 1 ? 5.
   When LID progresses to unsigned 2 however, we have 5 >: 2 and 5 >: 4
   and we can choose a V.

   Value of 0 MUST NEVER be used as Refresh ID and is considered an
   "invalid" ID.

   The sending speaker MUST NOT reorder the BoRR messages on sending in
   case it received multiple requests, i.e., the BoRRs MUST follow in
   the same sequence as the requested Route Refresh IDs.

8.  Route Refresh Option Flags

   This draft defines several route refresh option flags:

   o  'O'-bit specifies whether the receiving BGP speaker MUST logically
      OR the attached options or logically AND them (in case of the bit
      being clear).  When the flag is clear, the router on the receiving
      end SHOULD logically AND the options and only refresh routes that
      match all received options.  If the option flag is set, the router
      SHOULD select routes that match using a logical OR of the options.
      In any case the set of routes sent between the according BoRR and
      EoRR MUST contain at least the logically requested set.

   o  'C' bit indicates that the receiving BGP speaker MUST clear
      immediately all the received Route Refresh Requests with Options,
      either pending or being processed.  EoRRs MUST NOT be sent.  The
      Refresh ID# on the request MUST be set as the (in unsigned terms)
      next possible number L for which LID >: L and HID >: L per
      Appendix A or in other words we "wrap around the sequence number
      space" on reset.  The C flag MUST NOT be set on BoRR or EoRR
      messages and CAN be used only with refresh requests.

   o  by 'S' bit indicate a refresh is being spontaneously originated by
      the BGP speaker which received requests and has them pending.  The



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      receiving BGP speaker MUST immediately clear all their pending
      Route Refresh requests with the sending peer.  The Refresh ID# on
      the request MUST be set as the the largest unsigned number L for
      which LID >: L and HID >: L.  When this flag is set, the receiving
      BGP speaker MUST use this sequence number for its next request.
      To use example from Appendix A, if the peer received LID 4 and HID
      5 (i.e. it didn't send BoRR for 4 yet but received request for 5
      already) it will reset the sequence number to 1 by those rules.
      Now, if there is a request with 6 in flight, it will be seen as 1
      >: 6 when arriving.

   The precise format is indicated below


                0 1 2 3 4 5 6 7
               +-+-+-+-+-+-+-+-+
               |  .... |C|O|S|R|
               +-+-+-+-+-+-+-+-+

               C    Clear pending requests and reset Refresh ID# space.

               O    Use logical OR of attached options

               S    Synchronize sequence numbers

               R    Reserved bit


9.  Route Refresh Options

   This draft introduces new options carried within the Route Refresh
   message as shown in the following figure

        0               1               2               3
        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Type        |            Length             |     Value     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Value (cont'd).                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The option Type is a 1 octet field that uniquely identifies
   individual options.  The Length is a 2 octet field that contains the
   length of the option Value field in octets.  The option Value is a
   variable length field that is interpreted according to the value of
   the option Type field.




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   The following types are being defined in this draft and additional
   types can be defined subsequently as needed

   + 1 - Route Type
   + 2 - NLRI Prefix
   + 3 - Route Distinguisher Prefix

   The Route Type option would specify a particular route type that is
   being requested.  This option applies specifically to those AFI/SAFI
   combinations that support multiple route types, e.g.  L2VPN/EVPN and
   MUST be otherwise ignored.  The value field would be the route type
   specifying which route type was being requested.  The length of the
   option depends on the AFI/SAFI.

   The NLRI Prefix option would specify a request for all matching
   address prefixes with their lengths equal to or greater than the
   specified prefix per AFI/SAFI definitions.  The value field would
   contain the address prefix according to the NLRI specification of the
   AFI/SAFI contained in the Route Refresh message.  For those AFI/SAFI
   combinations that specify NLRIs containing a type and/or RD, the
   value field MUST exclude the type and RD and SHOULD only include any
   remaining NLRI fields.  If the requesting speaker expects its peer to
   also match the type and/or RD, the speaker CAN include the type and
   RD prefix options accordingly.  The length field would contain the
   length of the value field in bits.

   The Route Distinguisher prefix option would specify an RD prefix that
   is being requested for AFs that support it.  The receiving BGP
   speaker would then refresh all routes in the specified AFI/SAFI that
   matched the requested RDs.  The Value field would contain the RD, its
   length and the mask length of the RD prefix.  This option applies
   specifically to those AFI/SAFI combinations that support route
   distinguishers and MUST be otherwise ignored.

10.  Operation

   A BGP speaker that understands and supports Route Refresh Options
   SHOULD advertise the Route Refresh Options Capability in its Open
   message.  The following procedures for route refresh are only
   applicable if the BGP speaker originating the route refresh has
   received the route refresh options capability and supports it.

   When originating a Route Refresh message, a BGP speaker SHOULD use
   and set these options if it wants to restrict the scope of updates
   being refreshed.  The specific options being sent will be set
   according to the operator's command.





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   When a BGP speaker receives a route refresh message that includes any
   options, it MUST parse the options and strongly SHOULD use them to
   filter outgoing NLRIs when refreshing the Adj-RIB-Out to the
   requesting BGP speaker.

   If a BGP speaker receives the route refresh message with the message
   subtype set to BoRR with options as described above, then it needs to
   process all the included options and MUST mark all matching routes as
   stale as described in [RFC7313].

   If a BGP speaker receives the route refresh message with the message
   subtype set to EoRR with options as described above, then it needs to
   process all the included options and delete any remaining stale
   routes that match the options received with the EoRR as described in
   [RFC7313].

   A BGP speaker responding to a route refresh request MUST set the
   message subtypes of the BoRR and EoRR messages so that each BoRR
   message has a matching EoRR message.  This means a BoRR message
   without options SHOULD only be followed eventually by an EoRR message
   without options.  Similarly, a BoRR message with options MUST
   eventually be followed by an EoRR message with the same options.  If
   BoRR and EoRR message options do not match, the outcome is
   unpredictable as remaining staled routes pending a refresh may get
   inadvertently deleted.  BGP speakers MUST NOT summarize EoRR messages
   by combining options in order to allow the requesting BGP speaker to
   uniquely identify the included sets of routes when concurrent
   refreshes are originated with overlapping sets of routes.

   Observe that overlapping refreshes with different options are
   possible and in such case the according BoRR and EoRR messages are
   associated by using their Refresh ID#.  The BGP speaker responding to
   the route refresh requests MAY perform the refreshes in parallel.  In
   case of concurrent refreshes overlapping same routes, the responding
   speaker MUST ensure that the sent advertisements will result in
   deletion of the omitted routes at the time all EoRRs have been
   received by the remote speaker or it MUST explicitly advertise
   withdrawals to correct any anomalies.

   The BGP speaker requesting a refresh from its peers SHOULD maintain a
   locally configurable upper bound on how long it will keep matching
   stale routes once a BoRR has been received.  Each subsequent BoRR
   SHOULD reset this period so that any remaining stale routes are only
   flushed after the last BoRR has been received in case there are
   multiple back-to-back refreshes being sent out and the last matching
   EoRR is never received or arrives too late.  This is an
   implementation specific detail.




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   A BGP speaker may spontaneously originate a refresh to one or more of
   its peers depending on operator intervention, or due to a policy or
   configuration change, etc.  In such a case, the speaker MUST refresh
   the entire Adj-RIB-Out. The speaker MUST also send BoRR/EoRR with the
   options field with the 'S' flag set and a sequence number which lies
   outside the range of the sequence numbers that are currently in use
   with the receiving BGP speaker.

11.  Error Handling

   The handling of malformed options MUST follow the procedures
   mentioned in [RFC7606].  This draft obsoletes some of the error
   handling procedures in [RFC7313] if the Route Refresh Options
   Capability is sent.  In addition, this draft mandates the following
   behavior at the receiver of the route refresh request upon detection
   of:

   Length errors - If the message length minus the fixed-size message
   header is less than 4, the procedure in [RFC7313] MUST be followed.
   Also, if the overall length of all the options or any individual
   option length exceeds the total number of remaining bytes, the same
   procedure MUST be followed.

   Option type errors - Any unknown option type CAN be ignored for
   AND'ed options.  In case of OR'ed options the receiving speaker MUST
   ignore all the options and de-facto treat it as a full AFI/SAFI Adj-
   RIB-Out refresh.  Such event SHOULD be logged in either case to
   notify the operator.

   Option value errors - Length errors which cannot be distinguished
   from value field errors at the receiver are treated the same as value
   errors.  The receiver MUST send a NOTIFICATION message with the Error
   Code "ROUTE-REFRESH Message Error" and the subcode of Invalid Message
   Length to the peer.  The Data field of the NOTIFICATION message MUST
   contain the complete ROUTE-REFRESH message.

   BoRR with "unknown" or "invalid" Refresh ID# - The receiver MUST
   discard all pending requests and issue a Route Refresh Request with
   Options.  The options MUST be empty and the clear flag MUST be set to
   resynchronize the RIBs.  "Unknown" means here a BoRR which is not in
   the interval

    [ MAX(lowest requested Refresh ID# without BoRR,
          highest received BoRR+1 respecting sequence number arithmetic),
      highest requested Refresh ID# ]

   EoRR with unknown Refresh ID# - Those SHOULD be ignored and a warning
   or error MUST be logged.



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   BoRR or EoRR with incorrect options - analogous to BoRR with unknown
   Refresh ID#.

   EoRR with known Refresh ID# but without preceding BoRR - analogous to
   EoRR with unknown Refresh ID#. Observe that this can be caused by the
   peer expiring last resort timer and reusing the ID# for another
   request before the EoRR is received.  This should be extremely
   unlikely given the size of the refresh ID space.

12.  IANA Considerations

   This draft defines a new route refresh options format for BGP Route
   Refresh messages.

   This draft defines a new route refresh capability for BGP Route
   Refresh messages.  We request IANA to record this capability to
   create a new registry under BGP Capability Codes as follows:

                   +74     Route Refresh Options Capability

   This draft defines 3 new route refresh message subtypes for BGP Route
   Refresh messages.  We request IANA to record these subtypes to create
   a new registry under BGP Route Refresh Subcodes as follows:

              + 3 - Route Refresh with options
              + 4 - BoRR with options
              + 5 - EoRR with options

13.  Security Considerations

   This extension to BGP does not change the underlying security issues
   inherent in the existing [RFC7313] and [RFC4271].

14.  Acknowledgements

   The authors would like to thank Anant Utgikar for initial discussions
   resulting in this work.  John Scudder and Jeff Hass provided further
   comments.

15.  References

15.1.  Normative References

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              DOI 10.17487/RFC1982, August 1996,
              <https://www.rfc-editor.org/info/rfc1982>.





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   [RFC2918]  Chen, E., "Route Refresh Capability for BGP-4", RFC 2918,
              DOI 10.17487/RFC2918, September 2000,
              <https://www.rfc-editor.org/info/rfc2918>.

   [RFC4684]  Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
              R., Patel, K., and J. Guichard, "Constrained Route
              Distribution for Border Gateway Protocol/MultiProtocol
              Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
              Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC4684,
              November 2006, <https://www.rfc-editor.org/info/rfc4684>.

   [RFC5291]  Chen, E. and Y. Rekhter, "Outbound Route Filtering
              Capability for BGP-4", RFC 5291, DOI 10.17487/RFC5291,
              August 2008, <https://www.rfc-editor.org/info/rfc5291>.

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
              2009, <https://www.rfc-editor.org/info/rfc5492>.

   [RFC7313]  Patel, K., Chen, E., and B. Venkatachalapathy, "Enhanced
              Route Refresh Capability for BGP-4", RFC 7313,
              DOI 10.17487/RFC7313, July 2014,
              <https://www.rfc-editor.org/info/rfc7313>.

   [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>.

15.2.  Information References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.






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   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [Wikipedia]
              Wikipedia,
              "https://en.wikipedia.org/wiki/Serial_number_arithmetic",
              2016.

Appendix A.  Sequence Number Binary Arithmetic

   The only reasonably reference to a cleaner than [RFC1982] sequence
   number solution is given in [Wikipedia].  It basically converts the
   problem into two complement's arithmetic.  Assuming a straight two
   complement's substractions on the bit-width of the sequence number
   the according >: and =: relations are defined as:

      U_1, U_2 are 12-bits aligned unsigned version number

      D_f is  ( U_1 - U_2 ) interpreted as two complement signed 12-bits
      D_b is  ( U_2 - U_1 ) interpreted as two complement signed 12-bits

      U_1 >: U_2 IIF D_f > 0 AND D_b < 0
      U_1 =: U_2 IIF D_f = 0

   The >: relationsship is symmetric but not transitive.  Observe that
   this leaves the case of the numbers having maximum two complement
   distance, e.g. ( 0 and 0x800 ) undefined in our 12-bits case since
   D_f and D_b are both -0x7ff.

   A simple example of the relationship in case of 3-bit arithmetic
   follows as table indicating D_f/D_b values and then the relationship
   of U_1 to U_2:

           U2 / U1   0    1    2    3    4    5    6    7
           0        +/+  +/-  +/-  +/-  -/-  -/+  -/+  -/+
           1        -/+  +/+  +/-  +/-  +/-  -/-  -/+  -/+
           2        -/+  -/+  +/+  +/-  +/-  +/-  -/-  -/+
           3        -/+  -/+  -/+  +/+  +/-  +/-  +/-  -/-
           4        -/-  -/+  -/+  -/+  +/+  +/-  +/-  +/-
           5        +/-  -/-  -/+  -/+  -/+  +/+  +/-  +/-
           6        +/-  +/-  -/-  -/+  -/+  -/+  +/+  +/-
           7        +/-  +/-  +/-  -/-  -/+  -/+  -/+  +/+






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          U2 / U1   0    1    2    3    4    5    6    7
          0         =    >    >    >    ?    <    <    <
          1         <    =    >    >    >    ?    <    <
          2         <    <    =    >    >    >    ?    <
          3         <    <    <    =    >    >    >    ?
          4         ?    <    <    <    =    >    >    >
          5         >    ?    <    <    <    =    >    >
          6         >    >    ?    <    <    <    =    >
          7         >    >    >    ?    <    <    <    =

Authors' Addresses

   Keyur Patel
   Arrcus, Inc
   USA

   Email: keyur@arrcus.com


   Aamod Vyavaharkar
   Cisco Systems
   821 Alder Drive
   Milpitas, CA  95035
   USA

   Email: avyavaha@cisco.com


   Niloofar Fazlollahi
   Unaffiliated
   USA

   Email: Niloofar_fazlollahi@yahoo.com


   Tony Przygienda
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA  94089
   USA

   Email: prz@juniper.net









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