Layer 3 Discovery and Liveness
draft-ietf-lsvr-l3dl-02

Document Type Active Internet-Draft (lsvr WG)
Last updated 2019-07-07
Replaces draft-ietf-lsvr-lsoe
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Network Working Group                                            R. Bush
Internet-Draft                        Arrcus & Internet Initiative Japan
Intended status: Standards Track                              R. Austein
Expires: January 8, 2020                                        K. Patel
                                                                  Arrcus
                                                            July 7, 2019

                     Layer 3 Discovery and Liveness
                        draft-ietf-lsvr-l3dl-02

Abstract

   In Massive Data Centers, BGP-SPF and similar routing protocols are
   used to build topology and reachability databases.  These protocols
   need to discover IP Layer 3 attributes of links, such as logical link
   IP encapsulation abilities, IP neighbor address discovery, and link
   liveness.  This Layer 3 Discovery and Liveness protocol collects
   these data, which may then be disseminated using BGP-SPF and similar
   protocols.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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

   This Internet-Draft will expire on January 8, 2020.

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Copyright Notice

   Copyright (c) 2019 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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Top Level Overview  . . . . . . . . . . . . . . . . . . . . .   5
   5.  Inter-Link Protocol Overview  . . . . . . . . . . . . . . . .   6
     5.1.  L3DL Ladder Diagram . . . . . . . . . . . . . . . . . . .   7
   6.  Transport Layer . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  The Checksum  . . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  TLV PDUs  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Logical Link Endpoint Identifier  . . . . . . . . . . . . . .  13
   10. HELLO . . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   11. OPEN  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   12. ACK . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     12.1.  Retransmission . . . . . . . . . . . . . . . . . . . . .  18
   13. The Encapsulations  . . . . . . . . . . . . . . . . . . . . .  18
     13.1.  The Encapsulation PDU Skeleton . . . . . . . . . . . . .  19
     13.2.  Encapsulaion Flags . . . . . . . . . . . . . . . . . . .  20
     13.3.  IPv4 Encapsulation . . . . . . . . . . . . . . . . . . .  21
     13.4.  IPv6 Encapsulation . . . . . . . . . . . . . . . . . . .  21
     13.5.  MPLS Label List  . . . . . . . . . . . . . . . . . . . .  22
     13.6.  MPLS IPv4 Encapsulation  . . . . . . . . . . . . . . . .  22
     13.7.  MPLS IPv6 Encapsulation  . . . . . . . . . . . . . . . .  23
   14. VENDOR - Vendor Extensions  . . . . . . . . . . . . . . . . .  24
   15. KEEPALIVE - Layer 2 Liveness  . . . . . . . . . . . . . . . .  25
   16. Layers 2.5 and 3 Liveness . . . . . . . . . . . . . . . . . .  26
   17. The North/South Protocol  . . . . . . . . . . . . . . . . . .  26
     17.1.  Use BGP-LS as Much as Possible . . . . . . . . . . . . .  26
     17.2.  Extensions to BGP-LS . . . . . . . . . . . . . . . . . .  26
   18. Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     18.1.  HELLO Discussion . . . . . . . . . . . . . . . . . . . .  27
     18.2.  HELLO versus KEEPALIVE . . . . . . . . . . . . . . . . .  27

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   19. VLANs/SVIs/Sub-interfaces . . . . . . . . . . . . . . . . . .  27
   20. Implementation Considerations . . . . . . . . . . . . . . . .  28
   21. Security Considerations . . . . . . . . . . . . . . . . . . .  28
   22. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
     22.1.  PDU Types  . . . . . . . . . . . . . . . . . . . . . . .  29
     22.2.  Signature Type . . . . . . . . . . . . . . . . . . . . .  29
     22.3.  Flag Bits  . . . . . . . . . . . . . . . . . . . . . . .  30
     22.4.  Error Codes  . . . . . . . . . . . . . . . . . . . . . .  30
   23. IEEE Considerations . . . . . . . . . . . . . . . . . . . . .  30
   24. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  30
   25. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     25.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     25.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   The Massive Data Center (MDC) environment presents unusual problems
   of scale, e.g.  O(10,000) forwarding devices, while its homogeneity
   presents opportunities for simple approaches.  Approaches such as
   Jupiter Rising [JUPITER] use a central controller to deal with
   scaling, while BGP-SPF [I-D.ietf-lsvr-bgp-spf] provides massive
   scale-out without centralization using a tried and tested scalable
   distributed control plane, offering a scalable routing solution in
   Clos [Clos0][Clos1] and similar environments.  But BGP-SPF and
   similar higher level device-spanning protocols, e.g.
   [I-D.malhotra-bess-evpn-lsoe], need logical link state and addressing
   data from the network to build the routing topology.  They also need
   prompt but prudent reaction to (logical) link failure.

   Layer 3 Discovery and Liveness (L3DL) provides brutally simple
   mechanisms for devices to

   o  Discover each other's unique endpoint identification,

   o  Discover mutually supported layer 3 encapsulations, e.g.  IP/MPLS,

   o  Discover Layer 3 IP and/or MPLS addressing of interfaces of the
      encapsulations,

   o  Present these data, using a very restricted profile of a BGP-LS
      [RFC7752] API, to BGP-SPF which computes the topology and builds
      routing and forwarding tables,

   o  Enable layer 3 link liveness such as BFD, and finally

   o  Provide Layer 2 keep-alive messages for session continuity.

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   This protocol may be more widely applicable to a range of routing and
   similar protocols which need layer 3 discovery and characterisation.

2.  Terminology

   Even though it concentrates on the inter-device layer, this document
   relies heavily on routing terminology.  The following attempts to
   clarify the use of some possibly confusing terms:

   ASN:       Autonomous System Number [RFC4271], a BGP identifier for
              an originator of Layer 3 routes, particularly BGP
              announcements.
   BGP-LS:    A mechanism by which link-state and TE information can be
              collected from networks and shared with external
              components using the BGP routing protocol.  See [RFC7752].
   BGP-SPF    A hybrid protocol using BGP transport but a Dijkstra
              Shortest Path First decision process.  See
              [I-D.ietf-lsvr-bgp-spf].
   Clos:      A hierarchic subset of a crossbar switch topology commonly
              used in data centers.
   Datagram:  The L3DL content of a single Layer 2 frame.  A full L3DL
              PDU may be packaged in multiple Datagrams.
   Encapsulation:  Address Family Indicator and Subsequent Address
              Family Indicator (AFI/SAFI).  I.e. classes of layer 2.5
              and 3 addresses such as IPv4, IPv6, MPLS, etc.
   Frame:     A Layer 2 packet.
   Link or Logical Link:  A logical connection between two logical ports
              on two devices.  E.g. two VLANs between the same two ports
              are two links.
   LLEI:      Logical Link Endpoint Identifier, the unique identifier of
              one end of a logical link, see Section 9.
   MAC Address:  48-bit Layer 2 addresses are assumed since they are
              used by all widely deployed Layer 2 network technologies
              of interest, especially Ethernet.  See [IEEE.802_2001].
   MDC:       Massive Data Center, commonly composed of thousands of Top
              of Rack Switches (TORs).
   MTU:       Maximum Transmission Unit, the size in octets of the
              largest packet that can be sent on a medium, see [RFC1122]
              1.3.3.
   PDU:       Protocol Data Unit, an L3DL application layer message.  A
              PDU may need to be broken into multiple Datagrams to make
              it through MTU or other restrictions.
   RouterID:  An 32-bit identifier unique in the current routing domain,
              see [RFC6286].
   Session:   An established, via OPEN PDUs, session between two L3DL
              capable link end-points,
   SPF:       Shortest Path First, an algorithm for finding the shortest
              paths between nodes in a graph; AKA Dijkstra's algorithm.

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   System Identifier:  An eight octet ISO System Identifier a la
              [RFC1629] System ID
   TOR:       Top Of Rack switch, aggregates the servers in a rack and
              connects to aggregation layers of the Clos tree, AKA the
              Clos spine.
   ZTP:       Zero Touch Provisioning gives devices initial addresses,
              credentials, etc. on boot/restart.

3.  Background

   L3DL is primarily designed for a Clos type datacenter scale and
   topology, but can accommodate richer topologies which contain
   potential cycles.

   While L3DL is designed for the MDC, there are no inherent reasons it
   could not run on a WAN.  The authentication and authorization needed
   to run safely on a WAN need to be considered, and the appropriate
   level of security options chosen.

   L3DL assumes a new IEEE assigned EtherType (TBD).

   The number of addresses of one Encapsulation type on an interface
   link may be quite large given a TOR with tens of servers, each server
   having a few hundred micro-services, resulting in an inordinate
   number of addresses.  And highly automated micro-service migration
   can cause serious address prefix disaggregation, resulting in
   interfaces with thousands of disaggregated prefixes.

   Therefore the L3DL protocol is session oriented and uses incremental
   announcement and widrawal with session restart, a la BGP ([RFC4271]).

4.  Top Level Overview

   o  Devices discover each other on logical links

   o  Logical Link Endpoint Identifiers are exchanged

   o  Layer 2 Liveness Checks may be started

   o  Encapsulation data are exchanged and IP-Level Liveness Checks
      enabled

   o  A BGP-like upper layer protocol is assumed to use these data to
      discover and build a topology database

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   +-------------------+   +-------------------+   +-------------------+
   |      Device       |   |      Device       |   |      Device       |
   |                   |   |                   |   |                   |
   |+-----------------+|   |+-----------------+|   |+-----------------+|
   ||                 ||   ||                 ||   ||                 ||
   ||     BGP-SPF     <+---+>     BGP-SPF     <+---+>     BGP-SPF     ||
   ||                 ||   ||                 ||   ||                 ||
   |+--------^--------+|   |+--------^--------+|   |+--------^--------+|
   |         |         |   |         |         |   |         |         |
   |         |         |   |         |         |   |         |         |
   |+--------+--------+|   |+--------+--------+|   |+--------+--------+|
   ||  Encapsulations ||   ||  Encapsulations ||   ||  Encapsulations ||
   ||    Addresses    ||   ||    Addresses    ||   ||    Addresses    ||
   ||   L2 Liveness   ||   ||   L2 Liveness   ||   ||   L2 Liveness   ||
   |+--------^--------+|   |+--------^--------+|   |+--------^--------+|
   |         |         |   |         |         |   |         |         |
   |         |         |   |         |         |   |         |         |
   |+--------v--------+|   |+--------v--------+|   |+--------v--------+|
   ||                 ||   ||                 ||   ||                 ||
   ||Inter-Device PDUs<+---+>Inter-Device PDUs<+---+>Inter-Device PDUs||
   ||                 ||   ||                 ||   ||                 ||
   |+-----------------+|   |+-----------------+|   |+-----------------+|
   +-------------------+   +-------------------+   +-------------------+

   There are two protocols, the inter-device per-link layer 3 discovery
   and the API to the upper level BGP-like routing prototol:

   o  Inter-device PDUs are used to exchange device and logical link
      identities and layer 2.5 and 3 identifiers (not payloads), e.g.
      device IDs, port identities, VLAN IDs, Encapsulations, and IP
      addresses.

   o  A Link Layer to BGP API presents these data up the stack to a BGP
      protocol or an other device-spanning upper layer protocol,
      presenting them using the BGP-LS BGP-like data format.

   The upper layer BGP family routing protocols cross all the devices,
   though they are not part of these L3DL protocols.

   To simplify this document, Layer 2 framing is not shown.  L3DL is
   about layer 3.

5.  Inter-Link Protocol Overview

   Two devices discover each other and their respective identities by
   sending multicast HELLO PDUs (Section 10).  To assure discovery of
   new devices coming up on a multi-link topology, devices on such a
   topology send periodic HELLOs forever, see Section 18.1.

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   Once a new device is recognized, both devices attempt to negotiate
   and establish a session by sending unicast OPEN PDUs (Section 11).
   In an established session, the Encapsulations (Section 13) configured
   on an end point may be announced and modified.  Note that these are
   only the encapsuation and addresses configured on the announcing
   interface; though a device's loopback and overlay interface(s) may
   also be announced.  When two devices on a link have compatible
   Encapsulations and addresses, i.e. the same AFI/SAFI and the same
   subnet, the link is announced via the BGP-LS API.

5.1.  L3DL Ladder Diagram

   The HELLO, Section 10, is a priming message.  It is a small L3DL PDU
   encapsulated in an Ethernet multicast frame with the simple goal of
   discovering the identities of logical link endpoint(s) reachable from
   a Logical Link Endpoint, Section 9.

   The HELLO and OPEN, Section 11, PDUs, which are used to discover and
   exchange detailed Logical Link Endpoint Identifiers, LLEIs, and the
   ACK/ERROR PDU, are mandatory; other PDUs are optional; though at
   least one encapsulation SHOULD be agreed at some point.

   The following is a ladder-style diagram of the L3DL protocol
   exchanges:

   |             HELLO           | Logical Link Peer discovery
   |---------------------------->|
   |             HELLO           | Mandatory
   |<----------------------------|
   |                             |
   |                             |
   |             OPEN            | MACs, IDs, etc.
   |---------------------------->|
   |            ACK              |
   |<----------------------------|
   |                             |
   |             OPEN            | Mandatory
   |<----------------------------|
   |            ACK              |
   |---------------------------->|
   |                             |
   |                             |
   |   Interface IPv4 Addresses  | Interface IPv4 Addresses
   |---------------------------->| Optional
   |            ACK              |
   |<----------------------------|
   |                             |
   |   Interface IPv4 Addresses  |

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   |<----------------------------|
   |            ACK              |
   |---------------------------->|
   |                             |
   |                             |
   |   Interface IPv6 Addresses  | Interface IPv6 Addresses
   |---------------------------->| Optional
   |            ACK              |
   |<----------------------------|
   |                             |
   |   Interface IPv6 Addresses  |
   |<----------------------------|
   |            ACK              |
   |---------------------------->|
   |                             |
   |                             |
   |   Interface MPLSv4 Labels   | Interface MPLSv4 Labels
   |---------------------------->| Optional
   |            ACK              |
   |<----------------------------|
   |                             |
   |   Interface MPLSv4 Labels   | Interface MPLSv4 Labels
   |<----------------------------| Optional
   |            ACK              |
   |---------------------------->|
   |                             |
   |                             |
   |   Interface MPLSv6 Labels   | Interface MPLSv6 Labels
   |---------------------------->| Optional
   |            ACK              |
   |<----------------------------|
   |                             |
   |   Interface MPLSv6 Labels   | Interface MPLSv6 Labels
   |<----------------------------| Optional
   |            ACK              |
   |---------------------------->|
   |                             |
   |                             |
   |        L3DL KEEPALIVE       | Layer 2 Liveness
   |---------------------------->| Optional
   |        L3DL KEEPALIVE       |
   |<----------------------------|

6.  Transport Layer

   L3DL PDUs are carried by a simple transport layer which allows PDUs
   to occupy many Ethernet frames.  An L3DL Ethernet frame is referred
   to as a Datagram.

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   The L3DL Transport Layer encapsulates each Datagram using a common
   transport header.

   If a PDU does not fit in a single datagram, it is broken into
   multiple datagrams and reassembled by the receiver a la [RFC0791]
   Section 2.3 Fragmentation.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Version   |L|                Datagram Number                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Datagram Length        |            Checksum           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                               |           Payload...          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of the L3DL Transport Header are as follows:

   Version:  Seven-bit Version number of the protocol, currently 0.
      Values other than 0 MUST BE treated as an error.  The protocol
      version nees to be in one and only one place, so it is in the
      datagram as opposed to, for example, the PDU header.

   L: A bit that set to one if this Datagram is the last Datagram of the
      PDU.  For a PDU which fits in only one Datagram, it is set to one.
      Note that this is the inverse of the marking technique used by
      [RFC0791].

   Datagram Number:  A monotonically increasing 24-bit value which
      starts at zero for each PDU.  This is used to reassemble frames
      into PDUs a la [RFC0791] Section 2.3.  Note that this limits an
      L3DL PDU to 2^24 frames.

   Datagram Length:  Total number of octets in the Datagram including
      all payloads and fields.  Note that this limits a datagram to 2^16
      octets.

   Checksum:  A 32 bit hash over the Datagram to detect bit flips, see
      Section 7.

   Payload:  The PDU being transported or a fragment thereof.

   To avoid the need for a receiver to reassemble two PDUs at the same
   time, a sender MUST NOT send a subsequent PDU when a PDU is already
   in flight and not yet acknowledged if it is an ACKed PDU Type.

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7.  The Checksum

   There is a reason conservative folk use a checksum in UDP.  And as
   many operators stretch to jumbo frames (over 1,500 octets) longer
   checksums are the prudent approach.

   For the purpose of computing a checksum, the checksum field itself is
   assumed to be zero.

   The following code describes the suggested algorithm.

   Sum up 32-bit unsigned ints in a 64-bit long, then take the high-
   order section, shift it right, rotate, add it in, repeat until zero.

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   <CODE BEGINS>
   #include <stddef.h>
   #include <stdint.h>

   /* The F table from Skipjack, and it would work for the S-Box. */
   static const uint8_t sbox[256] = {
   0xa3,0xd7,0x09,0x83,0xf8,0x48,0xf6,0xf4,0xb3,0x21,0x15,0x78,
   0x99,0xb1,0xaf,0xf9,0xe7,0x2d,0x4d,0x8a,0xce,0x4c,0xca,0x2e,
   0x52,0x95,0xd9,0x1e,0x4e,0x38,0x44,0x28,0x0a,0xdf,0x02,0xa0,
   0x17,0xf1,0x60,0x68,0x12,0xb7,0x7a,0xc3,0xe9,0xfa,0x3d,0x53,
   0x96,0x84,0x6b,0xba,0xf2,0x63,0x9a,0x19,0x7c,0xae,0xe5,0xf5,
   0xf7,0x16,0x6a,0xa2,0x39,0xb6,0x7b,0x0f,0xc1,0x93,0x81,0x1b,
   0xee,0xb4,0x1a,0xea,0xd0,0x91,0x2f,0xb8,0x55,0xb9,0xda,0x85,
   0x3f,0x41,0xbf,0xe0,0x5a,0x58,0x80,0x5f,0x66,0x0b,0xd8,0x90,
   0x35,0xd5,0xc0,0xa7,0x33,0x06,0x65,0x69,0x45,0x00,0x94,0x56,
   0x6d,0x98,0x9b,0x76,0x97,0xfc,0xb2,0xc2,0xb0,0xfe,0xdb,0x20,
   0xe1,0xeb,0xd6,0xe4,0xdd,0x47,0x4a,0x1d,0x42,0xed,0x9e,0x6e,
   0x49,0x3c,0xcd,0x43,0x27,0xd2,0x07,0xd4,0xde,0xc7,0x67,0x18,
   0x89,0xcb,0x30,0x1f,0x8d,0xc6,0x8f,0xaa,0xc8,0x74,0xdc,0xc9,
   0x5d,0x5c,0x31,0xa4,0x70,0x88,0x61,0x2c,0x9f,0x0d,0x2b,0x87,
   0x50,0x82,0x54,0x64,0x26,0x7d,0x03,0x40,0x34,0x4b,0x1c,0x73,
   0xd1,0xc4,0xfd,0x3b,0xcc,0xfb,0x7f,0xab,0xe6,0x3e,0x5b,0xa5,
   0xad,0x04,0x23,0x9c,0x14,0x51,0x22,0xf0,0x29,0x79,0x71,0x7e,
   0xff,0x8c,0x0e,0xe2,0x0c,0xef,0xbc,0x72,0x75,0x6f,0x37,0xa1,
   0xec,0xd3,0x8e,0x62,0x8b,0x86,0x10,0xe8,0x08,0x77,0x11,0xbe,
   0x92,0x4f,0x24,0xc5,0x32,0x36,0x9d,0xcf,0xf3,0xa6,0xbb,0xac,
   0x5e,0x6c,0xa9,0x13,0x57,0x25,0xb5,0xe3,0xbd,0xa8,0x3a,0x01,
   0x05,0x59,0x2a,0x46
   };

   /* non-normative example C code, constant time even */

   uint32_t sbox_checksum_32(const uint8_t *b, const size_t n)
   {
     uint32_t sum[4] = {0, 0, 0, 0};
     uint64_t result = 0;
     for (size_t i = 0; i < n; i++)
       sum[i & 3] += sbox[*b++];
     for (int i = 0; i < sizeof(sum)/sizeof(*sum); i++)
       result = (result << 8) + sum[i];
     result = (result >> 32) + (result & 0xFFFFFFFF);
     result = (result >> 32) + (result & 0xFFFFFFFF);
     return (uint32_t) result;
   }
   <CODE ENDS>

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8.  TLV PDUs

   The basic L3DL application layer PDU is a typical TLV (Type Length
   Value) PDU.  It includes a signature to provide optional integrity
   and authentication.  It may be broken into multiple Datagrams, see
   Section 6.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    PDU Type   |                 Payload Length                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |                  Payload ...                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Sig Type   |        Signature Length       |               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
   ~                           Signature                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of the basic L3DL header are as follows:

   PDU Type:  An integer differentiating PDU payload types.  See
      Section 22.1.

   Payload Length:  Total number of octets in the Payload field.

   Payload:  The application layer content of the L3DL PDU.

   Sig Type:  The type of the Signature, see Section 22.2.  Type 0, a
      null signature, is defined in this document.

      Sig Type 0 indicates a null Signature.  For a trivial PDU such as
      KEEPALIVE, the underlying Datagram checksum may be sufficient for
      integrity, though it lacks authentication.

      Other Sig Types may be defined in other documents.

   Signature Length:  The length of the Signature, possibly including
      padding, in octets.  If Sig Type is 0, Signature Length MUST BE 0.

   Signature:  The result of running the signature algorithm specified
      in Sig Type over all octets of the PDU except for the Signature
      itself.

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9.  Logical Link Endpoint Identifier

   L3DL discovers neighbors on logical links and establishes sessions
   between the two ends of all consenting discovered logical links.  A
   logical link is described by a pair of Logical Link Endpoint
   Identifiers, LLEIs.

   An LLEI is a variable length descriptor which could be an ASN, a
   classic RouterID, a catenation of the two, an eight octet ISO System
   Identifier [RFC1629], or any other identifier unique to a single
   logical link endpoint in the topology.

   An L3DL deployment will choose and define an LLEI which suits its
   needs, simple or complex.  Two extremes are as follows:

   A simplistic view of a link between two devices is two ports,
   identified by unique MAC addresses, carrying a layer 3 protocol
   conversation.  In this case, the MAC addresses might suffice for the
   LLEIs.

   Unfortunately, things can get more complex.  Multiple VLANs can run
   between those two MAC addresses.  In practice, many real devices use
   the same MAC address on multiple ports and/or sub-interfaces.

   Therefore, in the general circumstance, a fully described LLEI might
   be as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                       System Identifier                       +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            ifIndex                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   System Identifier, a la [RFC1629], is an eight octet identifier
   unique in the entire operational space.  Routers and switches usually
   have internal MAC Addresses which can be padded with high order zeros
   and used if no System ID exists on the device.  If no unique
   identifier is burned into a device, the local L3DL configuration
   SHOULD create and assign a unique one by configuration.

   ifIndex is the SNMP identifier of the (sub-)interface, see [RFC1213].
   This uniquely identifies the port.

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   For a layer 3 tagged sub-interface or a VLAN/SVI interface, Ifindex
   is that of the logical sub-interface, so no further disambiguation is
   needed.

   L3DL PDUs learned over VLAN-ports may be interpreted by upper layer-3
   routing protocols as being learned on the corresponding layer-3 SVI
   interface for the VLAN.

   LLEIs are big-endian.

10.  HELLO

   The HELLO PDU is unique in that it is encapsulated in a multicast
   Ethernet frame.  It solicits response(s) from other LLEI(s) on the
   link.  See Section 18.1 for why multicast is used.  The destination
   multicast MAC Addressees to be used MUST be one of the following, See
   Clause 9.2.2 of [IEEE802-2014]:

   01-80-C2-00-00-0E:  Nearest Bridge = Propagation constrained to a
      single physical link; stopped by all types of bridges (including
      MPRs (media converters)).  This SHOULD BE used when the link is
      known to be a simple point to point link.
   To Be Assigned:  When a switch receives a frame with a multicast
      destination MAC it does not recognize, it forwards to all ports.
      This destination MAC is to be sent when the interface is known to
      be connected to a switch.  See Section 23.  This SHOULD BE used
      when the link may be a multi-point link.

   All other L3DL PDUs are encapsulated in unicast frames, as the peer's
   destination MAC address is known after the HELLO exchange.

   When an interface is turned up on a device, it SHOULD issue a HELLO
   if it is to participate in L3DL sessions.

   If a constrained Nearest Bridge destination address is configured for
   a point-to-point interface, see above, then the HELLO SHOULD NOT be
   repeated once a session has been created by an exchange of OPENs.

   If the configured destination address is one that is propagated by
   switches, the HELLO SHOULD be repeated at a configured interval, with
   a default of 60 seconds.  This allows discovery by new devices which
   come up on the layer-2 mesh.

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PDU Type = 0 |               Payload Length = 0              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |  Sig Type = 0 |      Signature Length = 0     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If more than one device responds, one adjacency is formed for each
   unique source LLEI response.  L3DL treats each adjacency as a
   separate logical link.

   When a HELLO is received from a source MAC address with which there
   is no established L3DL session, the receiver SHOULD respond with an
   OPEN PDU.  The two devices establish an L3DL session by exchanging
   OPEN PDUs.

   The Payload Length is zero as there is no payload.

   HELLO PDUs can not be signed as keying material has yet to be
   exchanged.  Hence the signature MUST always be the null type.

11.  OPEN

   Each device has learned the other's MAC Address from the HELLO
   exchange, see Section 10.  Therefore the OPEN and subsequent PDUs
   MUST BE unicast, as opposed to the HELLO's multicast frame.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PDU Type = 1 |                 Payload Length                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |                     Nonce                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |  LLEI Length  |            My LLEI            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-~
   ~                               |   AttrCount   |               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~       Attribute List ...      |   Auth Type   |   Key Length  ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |                    Key ...                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Serial Number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Sig Type   |        Signature Length       | Signature ... |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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   The Payload Length is the number of octets in all fields of the PDU
   from the Nonce through the Serial Number, not including the three
   final signature fields.

   The Nonce enables detection of a duplicate OPEN PDU.  It SHOULD be
   either a random number or a high resolution timestamp.  It is needed
   to prevent session closure due to a repeated OPEN caused by a race or
   a dropped or delayed ACK.

   My LLEI is the sender's LLEI, see Section 9.

   AttrCount is the number of attributes in the Attribute List.
   Attributes are single octets the semantics of which are operator-
   defined.

   A node may have zero or more operator-defined attributes, e.g.:
   spine, leaf, backbone, route reflector, arabica, ...

   Attribute syntax and semantics are local to an operator or
   datacenter; hence there is no global registry.  Nodes exchange their
   attributes only in the OPEN PDU.

   Auth Type is the Signature algorithm suite, see Section 8.

   Key Length is a 16-bit field denoting the length in octets of the Key
   itself, not including the Auth Type or the Key Length.  If there is
   no Key, the Auth Type and key Length MUST both be zero.

   The Key is specific to the operational environment.  A failure to
   authenticate is a failure to start the L3DL session, an ERROR PDU
   MUST BE sent (Error Code 2), and HELLOs MUST be restarted.

   The Serial Number is that of the last received and processed PDU.
   This allows a receiver sending an OPEN to tell the sender that the
   receiver wants to resume a session and the sender only needs to send
   data more recent than the Serial Number.  If this OPEN is not trying
   to restart a lost session, the Serial Number MUST BE set to zero.

   The Signature fields are described in Section 8 and in an asymmetric
   key environment serve as a proof of possession of the signing auth
   data by the sender.

   Once two logical link endpoints know each other, and have ACKed each
   other's OPEN PDUs, Layer 2 KEEPALIVEs (see Section 15) MAY be started
   to ensure Layer 2 liveness and keep the session semantics alive.  The
   timing and acceptable drop of KEEPALIVE PDUs are discussed in
   Section 15.

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   If a sender of OPEN does not receive an ACK of the OPEN PDU, then
   they MUST resend the same OPEN PDU, with the same Nonce.  Resending
   an unacknowledged OPEN PDU, like other ACKed PDUs, SHOULD use
   exponential back-off, see [RFC1122].

   If a properly authenticated OPEN arrives with a new Nonce from an
   LLEI with which the receiving logical link endpoint believes it
   already has an L3DL session (OPENs have already been exchanged), and
   the Serial Number in the OPEN is non-zero, the receiver SHOULD
   establish a new session by sending an OPEN with the Serial Number of
   the last data it received.  Each party MUST resume sending
   encapsulations etc. subsequent to the other party's Sequence Number.
   And each MUST retain all previously discovered encapsulation and
   other data.

   If a properly authenticated OPEN arrives with a new Nonce from an
   LLEI with which the receiving logical link endpoint believes it
   already has an L3DL session (OPENs have already been exchanged), and
   the Serial Number in the OPEN is zero, then the receiver MUST assume
   that the sending LLEI or entire device has been reset.  All
   previously discovered encapsulation data MUST NOT be kept and MUST be
   withdrawn via the BGP-LS API and the recipient MUST respond with a
   new OPEN.

12.  ACK

   The ACK PDU acknowledges receipt of a PDU and reports any error
   condition which might have been raised.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PDU Type = 3 |               Payload Length = 5              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |   ACKed PDU   | EType |       Error Code      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Error Hint          |    Sig Type   |Signature Leng.~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |                 Signature ...                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The ACK acknowledges receipt of an OPEN, Encapsulation, VENDOR PDU,
   etc.

   The ACKed PDU is the PDU Type of the PDU being acknowledged, e.g.,
   OPEN, one of the Encapsulations, etc.

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   If there was an error processing the received PDU, then the EType is
   non-zero.  If the EType is zero, Error Code and Error Hint MUST also
   be zero.

   A non-zero EType is the receiver's way of telling the PDU's sender
   that the receiver had problems processing the PDU.  The Error Code
   and Error Hint will tell the sender more detail about the error.

   The decimal value of EType gives a strong hint how the receiver
   sending the ACK believes things should proceed:

      0 - No Error, Error Code and Error Hint MUST be zero
      1 - Warning, something not too serious happened, continue
      2 - Session should not be continued, try to restart
      3 - Restart is hopeless, call the operator
      4-15 - Reserved

   The Error Codes, noting protocol failures listed in thi document, are
   listed in Section 22.4.  Someone stuck in the 1990s might think the
   catenation of EType and Error Code as an echo of 0x1zzz, 0x2zzz, etc.
   They might be right; or not.

   The Error Hint is any additional data the sender of the error PDU
   thinks will help the recipient or the debugger with the particular
   error.

   The Signature fields are described in Section 8.

12.1.  Retransmission

   If a PDU sender expects an ACK, e.g. for an OPEN, an Encapsulation, a
   VENDOR PDU, etc., and does not receive the ACK for a configurable
   time (default one second), and the interface is live at layer 2, the
   sender resends the PDU using exponential back-off, see [RFC1122].
   This cycle MAY be repeated a configurable number of times (default
   three) before it is considered a failure.  The session MAY BE
   considered closed in case of this ACK failure.

   If the link is broken at layer 2, retransmission MAY BE retried when
   the link is restored.

13.  The Encapsulations

   Once the devices know each other's LLEIs, know each other's upper
   layer identities, have means to ensure link state, etc., the L3DL
   session is considered established, and the devices SHOULD exchange L3
   interface encapsulations, L3 addresses, and L2.5 labels.

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   The Encapsulation types the peers exchange may be IPv4
   (Section 13.3), IPv6 (Section 13.4), MPLS IPv4 (Section 13.6), MPLS
   IPv6 (Section 13.7), and/or possibly others not defined here.

   The sender of an Encapsulation PDU MUST NOT assume that the peer is
   capable of the same Encapsulation Type.  An ACK (Section 12) merely
   acknowledges receipt.  Only if both peers have sent the same
   Encapsulation Type is it safe to assume that they are compatible for
   that type.

   A receiver of an encapsulation might recognize an addressing
   conflict, such as both ends of the link trying to use the same
   address.  In this case, the receiver SHOULD respond with an error
   (Error Code 1) ACK.  As there may be other usable addresses or
   encapsulations, this error might log and continue, letting an upper
   layer topology builder deal with what works.

   Further, to consider a logical link of a type to formally be
   established so that it may be pushed up to upper layer protocols, the
   addressing for the type must be compatible, e.g. on the same IP
   subnet.

13.1.  The Encapsulation PDU Skeleton

   The header for all encapsulation PDUs is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    PDU Type   |                 Payload Length                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |                     Count                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Serial Number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Encapsulation List...             |    Sig Type   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Signature Length       |         Signature ...         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An Encapsulation PDU describes zero or more addresses of the
   encapsulation type.

   The 24-bit Count is the number of Encapsulations in the Encapsulation
   list.

   The Serial Number is a monotonically increasing 32-bit value
   representing the sender's state in time.  It may be an integer, a

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   timestamp, etc.  On session restart (new OPEN), a receiver MAY send
   the last received Session Number to tell the sender to only send
   newer data.

   If a sender has multiple links on the same interface, separate state:
   data, ACKs, etc. must be kept for each peer session.

   Over time, multiple Encapsulation PDUs may be sent for an interface
   as configuration changes.

   If the length of an Encapsulation PDU exceeds the Datagram size limit
   on media, the PDU is broken into multiple Datagrams.  See Section 8.

   The Signature fields are described in Section 8.

   The Receiver MUST acknowledge the Encapsulation PDU with a Type=3,
   ACK PDU (Section 12) with the Encapsulation Type being that of the
   encapsulation being announced, see Section 12.

   If the Sender does not receive an ACK in a configurable interval
   (default one second), and the interface is live at layer 2, they
   SHOULD retransmit.  After a user configurable number of failures, the
   L3DL session should be considered dead and the OPEN process SHOULD be
   restarted.

   If the link is broken at layer 2, retransmission MAY BE retried if
   data have not changed in the interim.

13.2.  Encapsulaion Flags

    0           1            2            3            4  ...       7
   +------------+------------+------------+------------+------------+
   |  Ann/With  |   Primary  | Under/Over |  Loopback  | Reserved ..|
   +------------+------------+------------+------------+------------+

   Each encapsulation in an Encapsulation PDU of Type T may announce new
   and/or withdraw old encapsulations of Type T.  It indicates this with
   the Ann/With Encapsulation Flag, Announce == 1, Withdraw == 0.

   Each Encapsulation interface address in an Encapsulation PDU is
   either a new encapsulation be announced (Ann/With == 1) (yes, a la
   BGP) or requests one be withdrawn (Ann/With == 0).  Adding an
   encapsulation which already exists SHOULD raise an Announce/Withdraw
   Error (see Section 22.4); the EType SHOULD be 2, suggesting a session
   restart (see Section 12 so all encapsulations will be resent.

   If an LLEI has multiple addresses for an encapsulation type, one and
   only one address SHOULD be configured to be marked as primary

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   (Primary Flag == 1).  Only one address on an interface MAY be marked
   as primary for a particular encapsulation type.

   An Encapsulation interface address in an Encapsulation PDU MAY be
   marked as a loopback, in which case the Loopback bit is set.
   Loopback addresses are generally not seen directly on an external
   interface.  One or more loopback addresses MAY be exposed by
   configuration on one or more L3DL speaking external interfaces, e.g.
   for iBGP peering.  They SHOULD be marked as such, Loopback Flag == 1.

   Each Encapsulation interface address in an Encapsulation PDU is that
   of the direct 'underlay interface (Under/Over == 1), or an 'overlay'
   address (Under/Over == 0), likely that of a VM or container guest
   bridged or configured on to the interface already having an underlay
   address.

13.3.  IPv4 Encapsulation

   The IPv4 Encapsulation describes a device's ability to exchange IPv4
   packets on one or more subnets.  It does so by stating the
   interface's addresses and the corresponding prefix lengths.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PDU Type = 4 |                 Payload Length                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |                     Count                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Serial Number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Encaps Flags  |                  IPv4 Address                 ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |   PrefixLen   |    more ...   |    Sig Type   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Signature Length       |         Signature ...         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The 24-bit Count is the number of IPv4 Encapsulations being announced
   and/or withdrawn.

13.4.  IPv6 Encapsulation

   The IPv6 Encapsulation describes a logical link's ability to exchange
   IPv6 packets on one or more subnets.  It does so by stating the
   interface's addresses and the corresponding prefix lengths.

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PDU Type = 5 |                 Payload Length                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |                     Count                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Serial Number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Encaps Flags  |                                               |
   +-+-+-+-+-+-+-+-+                                               +
   |                                                               |
   +                                                               +
   |                                                               |
   +                                                               +
   |                          IPv6 Address                         |
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |   PrefixLen   |    more ...   |    Sig Type   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Signature Length       |         Signature ...         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The 24-bit Count is the number of IPv6 Encapsulations being announced
   and/or withdrawn.

13.5.  MPLS Label List

   As an MPLS enabled interface may have a label stack, see [RFC3032], a
   variable length list of labels is needed.  These are the labels the
   sender will accept for the prefix to which the list is attached.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Label Count  |                 Label                 | Exp |S|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Label                 | Exp |S|    more ...   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A Label Count of zero is an implicit withdraw of all labels for that
   prefix on that interface.

13.6.  MPLS IPv4 Encapsulation

   The MPLS IPv4 Encapsulation describes a logical link's ability to
   exchange labeled IPv4 packets on one or more subnets.  It does so by
   stating the interface's addresses the corresponding prefix lengths,
   and the corresponding labels which will be accepted fpr each address.

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PDU Type = 6 |                 Payload Length                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |                     Count                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Serial Number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Encaps Flags  |      MPLS Label List ...      |               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                  IPv4 Address                 |   PrefixLen   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    more ...   |    Sig Type   |        Signature Length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Signature                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The 24-bit Count is the number of MPLSv4 Encapsulation being
   announced and/or withdrawns.

13.7.  MPLS IPv6 Encapsulation

   The MPLS IPv4 Encapsulation describes a logical link's ability to
   exchange labeled IPv4 packets on one or more subnets.  It does so by
   stating the interface's addresses, the corresponding prefix lengths,
   and the corresponding labels which will be accepted for each address.

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PDU Type = 7 |                 Payload Length                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |                     Count                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Serial Number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Encaps Flags  |      MPLS Label List ...      |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
   |                                                               |
   +                                                               +
   |                                                               |
   +                                                               +
   |                          IPv6 Address                         |
   +                                               +-+-+-+-+-+-+-+-+
   |                                               |   Prefix Len  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    more ...   |    Sig Type   |        Signature Length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Signature ...                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The 24-bit Count is the number of MPLSv6 Encapsulations being
   announced and/or withdrawn.

14.  VENDOR - Vendor Extensions

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | PDU Type = 255|                 Payload Length                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |                 Serial Number                 ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |               Enterprise Number               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Ent Type   |              Enterprise Data ...              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |    Sig Type   |        Signature Length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Signature ...                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Vendors or enterprises may define TLVs beyond the scope of L3DL
   standards.  This is done using a Private Enterprise Number [IANA-PEN]

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   followed by Enterprise Data in a format defined for that Enterprise
   Number and Ent Type.

   Ent Type allows a VENDOR PDU to be sub-typed in the event that the
   vendor/enterprise needs multiple PDU types.

   As with Encapsulation PDUs, a receiver of a VENDOR PDU MUST respond
   with an ACK or an ERROR PDU.  Similarly, a VENDOR PDU MUST only be
   sent over an open session.

15.  KEEPALIVE - Layer 2 Liveness

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PDU Type = 2 |               Payload Length = 0              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |  Sig Type = 0 |      Signature Length = 0     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   L3DL devices SHOULD beacon frequent Layer 2 KEEPALIVE PDUs to ensure
   session continuity.  A receiver may choose to ignore KEEPALIVE PDUs.

   An operational deployment MUST BE configured whether to use
   KEEPALIVEs or not, either globally, or down to per-link granularity.
   Disagreement MAY result in repeated session break and
   reestablishment.

   KEEPALIVEs SHOULD be beaconed at a configured frequency.  One per
   second is the default.  Layer 3 liveness, such as BFD, may be more
   (or less) aggressive.

   When a sender transmits a PDU which is not a KEEPALIVE, the sender
   SHOULD reset the KEEPALIVE timer.  I.e. sending any PDU acts as a
   keepalive.  Once the last fragment has been sent, the KEEPALIVE timer
   SHOULD BE restarted.  Do not wait for the ACK.

   If a KEEPALIVE or other PDUs have not been received from a peer with
   which a receiver has an open session for a configurable time (default
   30 seconds), the link SHOULD BE presumed down.  The devices MAY keep
   configuration state and restore it without retransmission if no data
   have changed.  Otherwise, a new session SHOULD BE established and new
   Encapsulation PDUs exchanged.

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16.  Layers 2.5 and 3 Liveness

   Layer 2 liveness may be continuously tested by KEEPALIVE PDUs, see
   Section 15.  As layer 2.5 or layer 3 connectivity could still break,
   liveness above layer 2 MAY be frequently tested using BFD ([RFC5880])
   or a similar technique.

   This protocol assumes that one or more Encapsulation addresses may be
   used to ping, run BFD, or whatever the operator configures.

17.  The North/South Protocol

   Thus far, a one-hop point-to-point logical link discovery protocol
   has been defined.

   The devices know their unique LLEIs and know the unique peer LLEIs
   and Encapsulations on each logical link interface.

   Full topology discovery is not appropriate at the L3DL layer, so
   Dijkstra a la IS-IS etc. is assumed to be done by higher level
   protocols such as BGP-SPF.

   Therefore the LLEIs, link Encapsulations, and state changes are
   pushed North via a small subset of the BGP-LS API.  The upper layer
   routing protocol(s), e.g.  BGP-SPF, learn and maintain the topology,
   run Dijkstra, and build the routing database(s).

   For example, if a neighbor's IPv4 Encapsulation address changes, the
   devices seeing the change push that change Northbound.

17.1.  Use BGP-LS as Much as Possible

   BGP-LS [RFC7752] defines BGP-like Datagrams describing logical link
   state (links, nodes, link prefixes, and many other things), and a new
   BGP path attribute providing Northbound transport, all of which can
   be ingested by upper layer protocols such as BGP-SPF; see Section 4
   of [I-D.ietf-lsvr-bgp-spf].

   For IPv4 links, TLVs 259 and 260 are used.  For IPv6 links, TLVs 261
   and 262.  If there are multiple addresses on a link, multiple TLV
   pairs are pushed North, having the same ID pairs.

17.2.  Extensions to BGP-LS

   The Northbound protocol needs a few minor extensions to BGP-LS.
   Luckily, others have needed the same extensions.

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   Similarly to BGP-SPF, the BGP protocol is used in the Protocol-ID
   field specified in table 1 of
   [I-D.ietf-idr-bgpls-segment-routing-epe].  The local and remote node
   descriptors for all NLRI are the IDs described in Section 11.  This
   is equivalent to an adjacency SID or a node SID if the address is a
   loopback address.

   Label Sub-TLVs from [I-D.ietf-idr-bgp-ls-segment-routing-ext]
   Section 2.1.1, are used to associate one or more MPLS Labels with a
   link.

18.  Discussion

   This section explores some trade-offs taken and some considerations.

18.1.  HELLO Discussion

   A device with multiple Layer 2 interfaces, traditionally called a
   switch, may be used to forward frames and therefore packets from
   multiple devices to one logical interface (LLEI), I, on an L3DL
   speaking device.  Interface I could discover a peer J across the
   switch.  Later, a prospective peer K could come up across the switch.
   If I was not still sending and listening for HELLOs, the potential
   peering with K could not be discovered.  Therefore, on multi-link
   interfaces MUST continue to send HELLOs as long as they are turned
   up.

18.2.  HELLO versus KEEPALIVE

   Both HELLO and KEEPALIVE are periodic.  KEEPALIVE might be eliminated
   in favor of keeping only HELLOs.  But KEEPALIVEs are unicast, and
   thus less noisy on the network, especially if HELLO is configured to
   transit layer-2-only switches, see Section 18.1.

19.  VLANs/SVIs/Sub-interfaces

   One can think of the protocol as an instance (i.e. state machine)
   which runs on each logical link of a device.

   As the upper routing layer must view VLAN topologies as separate
   graphs, L3DL treats VLAN ports as separate links.

   L3DL PDUs learned over VLAN-ports may be interpreted by upper layer-3
   routing protocols as being learned on the corresponding layer-3 SVI
   interface for the VLAN.

   As Sub-Interfaces each have their own LLIEs, they act as separate
   interfaces, forming their own links.

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20.  Implementation Considerations

   An implementation SHOULD provide the ability to configure a logical
   interface as L3DL speaking or not.

   An implementation SHOULD provide the ability to configure whether
   HELLOs on an L3DL enabled interface send Nearest Bridge or the MAC
   which is propagated by switches from that interface; see Section 10.

   An implementation SHOULD provide the ability to distribute one or
   more loopback addresses or interfaces into L3DL on an external L3DL
   speaking interface.

   An implementation SHOULD provide the ability to distribute one or
   more overlay and/or underlay addresses or interfaces into L3DL on an
   external L3DL speaking interface.

   An implementation SHOULD provide the ability to configure one of the
   addresses of an encapsulation as primary on an L3DL speaking
   interface.  If there is only one address for a particular
   encapsulation, the implementation MAY mark it as primary by default.

   An implementation MAY allow optional configuration which updates the
   local forwarding table with overlay and underlay data both learned
   from L3DL peers and configured locally.

21.  Security Considerations

   The protocol as is MUST NOT be used outside a datacenter or similarly
   closed environment due to lack of formal definition of the
   authentication and authorization mechanism.  Sufficient mechanisms
   may be described in separate documents.

   Many MDC operators have a strange belief that physical walls and
   firewalls provide sufficient security.  This is not credible.  All
   MDC protocols need to be examined for exposure and attack surface.
   In the case of L3DL, Authentication and Integrity as provided in
   [draft-ymbk-l3dl-signing] is strongly recommended.

   It is generally unwise to assume that on the wire Layer 2 is secure.
   Strange/unauthorized devices may plug into a port.  Mis-wiring is
   very common in datacenter installations.  A poisoned laptop might be
   plugged into a device's port, form malicious sessions, etc. to
   divert, intercept, or drop traffic.

   Similarly, malicious nodes/devices could mis-announce addressing.

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   If OPENs are not being authenticated, an attacker could forge an OPEN
   for an existing session and cause the session to be reset.

   For these reasons, the OPEN PDU's authentication data exchange SHOULD
   be used.

   If the KEEPALIVE PDU is not signed (as suggested in Section 8) to
   save computation, then a MITM could fake a session being alive.

22.  IANA Considerations

22.1.  PDU Types

   This document requests the IANA create a registry for L3DL PDU Type,
   which may range from 0 to 255.  The name of the registry should be
   L3DL-PDU-Type.  The policy for adding to the registry is RFC Required
   per [RFC5226], either standards track or experimental.  The initial
   entries should be the following:

             PDU
             Code      PDU Name
             ----      -------------------
               0       HELLO
               1       OPEN
               2       KEEPALIVE
               3       ACK
               4       IPv4 Announcement
               5       IPv6 Announcement
               6       MPLS IPv4 Announcement
               7       MPLS IPv6 Announcement
               8-254   Reserved
               255     VENDOR

22.2.  Signature Type

   This document requests the IANA create a registry for L3DL Signature
   Type, AKA Sig Type, which may range from 0 to 255.  The name of the
   registry should be L3DL-Signature-Type.  The policy for adding to the
   registry is RFC Required per [RFC5226], either standards track or
   experimental.  The initial entries should be the following:

             Number      Name
             ------      -------------------
                 0       Null
                 1-255   Reserved

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22.3.  Flag Bits

   This document requests the IANA create a registry for L3DL PL Flag
   Bits, which may range from 0 to 7.  The name of the registry should
   be L3DL-PL-Flag-Bits.  The policy for adding to the registry is RFC
   Required per [RFC5226], either standards track or experimental.  The
   initial entries should be the following:

             Bit     Bit Name
             ----    -------------------
              0      Announce/Withdraw (ann == 0)
              1      Primary
              2      Underlay/Overlay (under == 0)
              3      Loopback
              4-7    Reserved

22.4.  Error Codes

   This document requests the IANA create a registry for L3DL Error
   Codes, a 16 bit integer.  The name of the registry should be L3DL-
   Error-Codes.  The policy for adding to the registry is RFC Required
   per [RFC5226], either standards track or experimental.  The initial
   entries should be the following:

             Error
             Code    Error Name
             ----    -------------------
               0     No Error
               1     Logical Link Addressing Conflict
               2     Authorization Failure in OPEN
               3     Signature Failure in PDU
               4     Announce/Withdraw Error

23.  IEEE Considerations

   This document requires a new EtherType.

   This document requires a new multicast MAC address that will be
   broadcast through a switch.

24.  Acknowledgments

   The authors thank Cristel Pelsser for multiple reviews, Harsha Kovuru
   for comments during implementation, Jeff Haas for review and
   comments, Joe Clarke for a useful review, John Scudder for deeply
   serious review and comments, Larry Kreeger for a lot of layer 2 clue,
   Martijn Schmidt for his contribution, Neeraj Malhotra for review,

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   Russ Housley for checksum discussion and sBox, and Steve Bellovin for
   checksum advice.

25.  References

25.1.  Normative References

   [I-D.ietf-idr-bgp-ls-segment-routing-ext]
              Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
              and M. Chen, "BGP Link-State extensions for Segment
              Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-16
              (work in progress), June 2019.

   [I-D.ietf-idr-bgpls-segment-routing-epe]
              Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
              S., and J. Dong, "BGP-LS extensions for Segment Routing
              BGP Egress Peer Engineering", draft-ietf-idr-bgpls-
              segment-routing-epe-19 (work in progress), May 2019.

   [I-D.ietf-lsvr-bgp-spf]
              Patel, K., Lindem, A., Zandi, S., and W. Henderickx,
              "Shortest Path Routing Extensions for BGP Protocol",
              draft-ietf-lsvr-bgp-spf-04 (work in progress), December
              2018.

   [IANA-PEN]
              "IANA Private Enterprise Numbers",
              <https://www.iana.org/assignments/enterprise-numbers/
              enterprise-numbers>.

   [IEEE.802_2001]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks: Overview and Architecture", IEEE 802-2001,
              DOI 10.1109/ieeestd.2002.93395, July 2002,
              <http://ieeexplore.ieee.org/servlet/opac?punumber=7732>.

   [IEEE802-2014]
              Institute of Electrical and Electronics Engineers, "Local
              and Metropolitan Area Networks: Overview and
              Architecture", IEEE Std 802-2014, 2014.

   [RFC1213]  McCloghrie, K. and M. Rose, "Management Information Base
              for Network Management of TCP/IP-based internets: MIB-II",
              STD 17, RFC 1213, DOI 10.17487/RFC1213, March 1991,
              <http://www.rfc-editor.org/info/rfc1213>.

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   [RFC1629]  Colella, R., Callon, R., Gardner, E., and Y. Rekhter,
              "Guidelines for OSI NSAP Allocation in the Internet",
              RFC 1629, DOI 10.17487/RFC1629, May 1994,
              <http://www.rfc-editor.org/info/rfc1629>.

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

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <http://www.rfc-editor.org/info/rfc3032>.

   [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,
              <http://www.rfc-editor.org/info/rfc4271>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <http://www.rfc-editor.org/info/rfc5880>.

   [RFC6286]  Chen, E. and J. Yuan, "Autonomous-System-Wide Unique BGP
              Identifier for BGP-4", RFC 6286, DOI 10.17487/RFC6286,
              June 2011, <http://www.rfc-editor.org/info/rfc6286>.

   [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,
              <http://www.rfc-editor.org/info/rfc7752>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <http://www.rfc-editor.org/info/rfc8174>.

25.2.  Informative References

   [Clos0]    Clos, C., "A study of non-blocking switching networks
              [PAYWALLED]", Bell System Technical Journal 32 (2), pp
              406-424, March 1953.

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   [Clos1]    "Clos Network",
              <https://en.wikipedia.org/wiki/Clos_network/>.

   [I-D.malhotra-bess-evpn-lsoe]
              Malhotra, N., Patel, K., and J. Rabadan, "LSoE-based PE-CE
              Control Plane for EVPN", draft-malhotra-bess-evpn-lsoe-00
              (work in progress), March 2019.

   [JUPITER]  Singh, A., Germano, P., Kanagala, A., Liu, H., Provost,
              J., Simmons, J., Tanda, E., Wanderer, J., HAP.lzle, U.,
              Stuart, S., Vahdat, A., Ong, J., Agarwal, A., Anderson,
              G., Armistead, A., Bannon, R., Boving, S., Desai, G., and
              B. Felderman, "Jupiter rising", Communications of the
              ACM Vol. 59, pp. 88-97, DOI 10.1145/2975159, August 2016.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <http://www.rfc-editor.org/info/rfc791>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <http://www.rfc-editor.org/info/rfc1122>.

Authors' Addresses

   Randy Bush
   Arrcus & Internet Initiative Japan
   5147 Crystal Springs
   Bainbridge Island, WA  98110
   US

   Email: randy@psg.com

   Rob Austein
   Arrcus, Inc

   Email: sra@hactrn.net

   Keyur Patel
   Arrcus
   2077 Gateway Place, Suite #400
   San Jose, CA  95119
   US

   Email: keyur@arrcus.com

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