NVO3                                                          S. Dikshit
Internet-Draft                                                  V. Joshi
Intended status: Informational                                Aruba, HPE
Expires: 5 February 2022                                 A. Sujeet Nayak
                                                                   Cisco
                                                           4 August 2021


                   MTU propagation over EVPN Overlays
         draft-saum-nvo3-mtu-propagation-over-evpn-overlays-00

Abstract

   Path MTU Discovery between end-host-devices/Virtual-Machines/servers/
   workloads connected over an EVPN-Overlay Network in an
   Datacenter/Campus/enterprise deployment, is a problem, yet to be
   resolved in the standards forums.  It needs a converged solution to
   ensure optimal usage of network and computational resources of the
   underlay routers/switches forming the basis of the overlay network.
   This documents takes leads from the guidelines presented in
   [RFC4459].

   The overlay connectivity can pan across various sites (geographically
   seperated or collocated) for realizing a Datacenter Interconnect or
   intersite VPNs between campus sites (buildings, branch offices etc).

   This literature intends to solve problem of icmp error propagation
   from an underlay routing/switching device to an end-host (hooked to
   EVPN overlay), thus facilitating "accurate MTU" learnings.

   This document also leverages the icmp multipart message extension,
   mentioned in [RFC4884] to carry the original packet in the icmp PDU.

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."




Dikshit, et al.          Expires 5 February 2022                [Page 1]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   This Internet-Draft will expire on 5 February 2022.

Copyright Notice

   Copyright (c) 2021 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.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     2.2.  Solution Requirements . . . . . . . . . . . . . . . . . .   4
   3.  Problem Description . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Issues in MTU propagation in an underlay  . . . . . . . .   5
       3.1.1.  Inaccurate MTU relayed to end hosts . . . . . . . . .   5
       3.1.2.  Packet_Too_Big not-relayed to host  . . . . . . . . .   7
   4.  Solution(s) . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Discovery of end-to-end Path MTU  . . . . . . . . . . . .   7
       4.1.1.  ICMP extensions leveraged for MTU propagation . . . .   8
       4.1.2.  Packet Path Processing  . . . . . . . . . . . . . . .   8
       4.1.3.  ICMP(v6) Error Translation  . . . . . . . . . . . . .  17
   5.  Inter-site MTU Propagation  . . . . . . . . . . . . . . . . .  27
   6.  Same subnet Considerations  . . . . . . . . . . . . . . . . .  28
   7.  Ecmp Considerations . . . . . . . . . . . . . . . . . . . . .  28
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  29
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  29
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     11.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31










Dikshit, et al.          Expires 5 February 2022                [Page 2]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


1.  Introduction

   There is an operational disconnect between underlay network
   provisioned as the underlay network, and the overlay network which
   intends to connect islands of customer deployments.  The deployments
   can range from cloud based services to storage applications or
   web(over the top) servers hosted over virtual machines or any other
   end devices like blade servers.  Overlay network are provisioned as
   tunnels leveraging Vxlan (and associated ones like gpe, geneve, gue),
   NVGRE, MPLS and other overlay encapsulations.

   The end hosts (VMs, workloads, user-devices) in a datacenter/campus
   deployment are connected to gateway.  In case the core network is
   laid out with EVPN-overlays, the gateways are Vteps (Vxlan-fabric
   gateways).  These are the networking devices which encapsulate the
   packet in an Overlay construct and relays it over the underlay
   network.

   For campus deployments, various branch offices can be provisioned
   with EVPN-overlays and the vpn connectivity between them can be
   realized via EVPN-overlays (VXLAN, MPLS, NVGRE fabrics) itself.  Thus
   it involves interworking/stitching of same/different overlays at DCI/
   VPN transit routing/switching devices.  The transit devices can be
   on-premise WAN gateways (SDWAN or otherwize) or service provider
   network entry points.

   IPv6/IPv4 enabled hosts/end-points, triggering PMTUD, may not get the
   right/inconsistent (or none) information from (over) the underlay
   network in case MTU errors are encountered in the packet path
   (encapsulated in the overlay).  This document validates the detailed
   solution for Vxlan-fabric (though equally applicable to other EVPN-
   overlays like Geneve, GUE, GPE, NVGRE) faciliated by an underlay
   network (via any routing protocol like BGP, OSPF, ISIS, EIGRP etc).
   This solution is equally applicable to other tunnel/overlay
   specifications falling into EVPN-overlay category.

   The proposal in this document, formulates an integrated approach
   which falls inline with OAM modelling discussed in NVO3.

2.  Requirements

2.1.  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 [RFC2119].





Dikshit, et al.          Expires 5 February 2022                [Page 3]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   When used in lowercase, these words convey their typical use in
   common language, and they are not to be interpreted as described in
   [RFC2119].

2.2.  Solution Requirements

   This section describes the advantages of the proposed solution,
   considering deployment in a typical EVPN-overlay underlay network:

   (a)  Optimal use of bandwidth in underlay and end-host network.

   (b)  In case Vxlan Gateway nodes complies to this solution, it MAY
        avoid black holing of icmp errors generated by underlay network
        devices.

   (c)  All end host applications (like web servers) can tailor the MSS
        accordingly against their respective transports.

   (d)  Facilitates seamless integration of IPv6 or dual stack
        applications over IPv4 based overlays and vice versa.

   (e)  The proposed solution is applicable to all encapsulations
        [RFC7348], [I-D.draft-ietf-nvo3-vxlan-gpe],
        [I-D.draft-ietf-nvo3-gue], [I-D.draft-gross-geneve] and
        [RFC7637].  Although the problem and solution refers to VXLAN
        [RFC7348] as a use-case in this document.

3.  Problem Description

   In current vendor implementation(s) of Vxlan-Gateway or other network
   devices, which form part of the underlay network and is configured
   with an overlay(tunnel) mechanism to transport packets from one
   customer end point to another, are incapable of relaying the errors
   encountered in routing/switching path in their networks (underlay
   network) to the customer end points (hosts/vm/blade-servers).  This
   deems right, as the underlay network should be transparent and water-
   tight with respect to leaking any public (underlay) network
   information to customer devices (and vice versa), thus ensuring
   seclusion between different customers provisioning tunneled over the
   common underlay network.

   For example, the information carried in the IP header of a Vxlan
   encapsulated packet is transparent to the payload (end-point
   generated packet).  Hence, any network-specific information related
   to IPv6/IPv4 native functionality is carried to the end-point
   devices, as is the case with an end-to-end private network.  The
   information generated in the underlay network devices while
   processing packets destined-to/sourced-from end-point devices, need



Dikshit, et al.          Expires 5 February 2022                [Page 4]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   to be percolated from underlay encapsulation to end customer specific
   payload.  This is something which is NOT directed by any standards,
   and also NOT implemented by current deployment(s) of routers and
   switches.

   Thus end-host sending out packets may never know about a lingering
   problem, impacting it's traffic in the underlay network.

   Note that terms "icmpv6" or "icmpv4" are used in the document with an
   intention to refer to both icmp and icmpv6, in case same context
   applies to both.

3.1.  Issues in MTU propagation in an underlay

   As mentioned in the [RFC1981], IPV6 PMTUD is based on the "Packet too
   big" icmpv6 error code, generated by the networking device which is
   capable of generating such messages on encountering packet paths
   which go over link with MTU size smaller than packet size.

   There are problems getting this working when end-point device
   initiates a "Path MTU Discovery" to remote end-point device.  It may
   lead to black-holing as per the current implementations.

   The following bullets provides pointers to potential black holing of
   PMTUD packets,

   (1)  Vxlan Gateway MAY not set the DF bit in the outer IP header
        encapsulation.

   (2)  Vxlan Gateway is incapable of relaying icmp error "Fragmentation
        Needed and Don't Fragment was Set", generated by IPv4 enabled
        underlay network device, to IPv6 enabled end-point host/vm/
        server(source of the original packet).

   The problems are discussed in detail in the following sub-sections.

3.1.1.  Inaccurate MTU relayed to end hosts

   Figure 1 depicts the topology referenced in the document for
   explaining the problem statement and the solution.











Dikshit, et al.          Expires 5 February 2022                [Page 5]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   +----------+                    +----------+
   |    H1    |                    |    H2    |
   |          |                    |          |
   |(H1_IPv6) |                    |(H2_IPv6) |
   +----------+                    +----------+
        |                               |
        |                               |
   +------------+   +----------+   +------------+
   |(VtepA_IPv6)|   |          |   |(VtepB_IPv6)|
   |   VtepA    |   |    R1    |   |  VtepB     |
   |(VtepA_IPv4)|---| (R1_IPv4)|---|(VtepB_IPv4)|
   +------------+   +----------+   +------------+

                  Figure 1. L3 Overlay

          LEGEND:
          MAC address : <Node_name>_MAC
          IP address  : <Node_name>_IPv4
          IPv6 address: <Node_name>_IPv6
          <Node_name> : node names in the above topology are
                        H1, VtepA, R1, VtepB, H2.
          VtepA, VtepB: Vxlan gateways
                    R1: Intermediate router in underlay network
                 H1,H2: End-point devices communicating withe each other

   H1 and H2 are the end point hosts in different subnet connected over
   Vxlan Overlays in the underlay network.  The Vtep tunnel end points,
   christened as VtepA and VtepB, are reachabile over an underlay IPv4
   network.  In this example, VtepA and VtepB are dual stack enabled and
   act as Vxlan gateways to connected hosts.  Link mtu between VtepA, R1
   and VtepB is configured as 1300 bytes; where as for the links between
   H1 and VtepA, H2 and VtepB, it is configured as 1500 bytes.

   H1 sends out a packet obliging to 1500 bytes MTU packet size
   containment over the H1 and VtepA link.  VtepA encapsulates the
   packet with (Vxlan + UDP) header and outer IP header corresponding to
   underlay reachability to destination tunnel end-point, that is VtepB,
   to reach out to H2.

   If size of encapsulated packet to be send over the link VtepA-R1
   exceeds the MTU (1300 bytes).  IPv4 packet with (IP header + UDP
   header + Vxlan header + Original L2 Packet from H1 containing the
   IPv6 Payload) SHOULD be fragmented.  In case Vxlan gateway, VtepA,
   does not sets the DF-bit in the outer IP header, the packet gets
   fragmented, with the reassembly done at the egress gateway (VtepB).






Dikshit, et al.          Expires 5 February 2022                [Page 6]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   The re-assembled packet is routed by VtepB to H2.  This can
   potentially lead to inaccurate Path MTU calculation at H1.  H1
   assumes it to be 1500 bytes as no icmp error is received.  This opens
   the door for fragment/reassembly and more cpu cycles on networking
   devices in the underlay network.

3.1.2.  Packet_Too_Big not-relayed to host

   In figure 1, assume that link between VtepA and R1 is 1500 as the
   only change from the figure 1 topology.  Hence the packet send by H1,
   leads to VtepA setting the DF-bit in the outer IP header(as part of
   Vxlan Encapsulation).  When R1 receives the packet and the routing
   table lookup points to the outgoing link with mtu size R1_VtepB_MTU
   bytes, less than the packet size (1500 bytes).  As DF-bit is set, R1
   generates icmpv4 error directed towards the src-ip (VtepA_IPv4).  It
   encapsulates the inner PDU of the original packet.  However, VtepA
   drops the icmp error packet and fails to relay it to H1.  This leads
   to black-holing.

   The above two sub-sections lay down potential problems for IPv6 Path
   MTU Discovery mechanism in an Overlay network.  Although these
   problem are generic to any combination of underlay and overlay
   network types (IPv4 or IPv6), the use-case topology in this document
   is specific to IPv6 end-point devices connected over Vxlan network,
   wherein, the underlay is connected over IPv4 network, unless
   mentioned specifically.

4.  Solution(s)

4.1.  Discovery of end-to-end Path MTU

   Since Vxlan Gateway is the one, which encapsulates the Vxlan (or any
   other overlay) header onto the packet traversing through the overlay
   network and also decapsulates the overlay header for packets
   egressing out of same and heading towards the end devices, the
   solution becomes more apt to be installed on devices playing such
   role.

   Firstly, It is a MUST that Vxlan gateways (VtepA and VtepB) SHOULD
   set the DF-bit in Outer header encapsulation for client packets that
   are wrapped with vxlan, related encapsulation, for Path MTU
   Discovery.  Thus ensuring that icmp error packet is generated for
   packet size exceeding the link MTU in underlay network.

   Secondly, it is MUST that Vxlan gateway devices translates the icmp
   error "Destination Unreachable" with code 'Fragmentation Needed and
   Don't Fragment was Set', into a icmpv6 error 'Packet too big' packet.
   This mandates that original packet carried in the icmp error message



Dikshit, et al.          Expires 5 February 2022                [Page 7]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   MUST carry information about the inner payload(original packet), and
   it is an IPv6 Packet, originated from the end-point device (H1 for
   VtepA in figure 1), connected to the Vxlan gateway over L3/L2
   network.

   Thirdly, it is MUST that Vxlan gateway devices translates the icmpv6
   error 'Packet too big' into a icmp error 'Destination Unreachable'
   with code 'Fragmentation Needed and Don't Fragment was Set' packet.
   Successfully translation mandates that, original packet carried in
   the icmp error message gives information about the inner payload
   (original packet), and it is an IPv4 packet, which originated from
   the end-point device connected to gateway over L3/L2 network.

   Fourthly, incase both, the client side network connected to Vxlan
   Gateway and the underlay network are same, that is, either both are
   ipv4 or both are ipv6, then icmp error code error translation is NOT
   required.  Rest of the process to retrieve original packet is
   identical.

4.1.1.  ICMP extensions leveraged for MTU propagation

   This solution leverages extensions in icmp and icmpv6 standards,
   [RFC4884], for the maximum size of the original packet that can be
   encapsulated in icmp error message with code as "Fragmentation
   Required(icmp)" or "Packet too big(icmpv6)" respectively.  As the
   host info is encapsulated in the inner payload, this requires
   additional bytes of data in icmp packet: (Outer IP Header + UDP
   Header + Vxlan + Inner L2 Header + Inner IPv6 SRC/DST IPs).

   In case Vxlan underlay network is provisioned over IPv6 underlay,
   then similar extensions are applicable to icmpv6.

   The processing of icmpv6 packet is extended from the current
   standards of 'non-delivery of icmpv6 packets to upper-layers on Vxlan
   gateways' to 'relaying it to the end-point devices'.

4.1.2.  Packet Path Processing

   Packet Path handling and processing is explained in this section.
   The assumptions are made with respect to network topology mentioned
   in Section 3.1.1.  The packet format in each flow captures packet
   fields which are significant with respect to this solution.  To
   understand the solution, the packet flow is explained which leads to
   generation of icmp or icmpv6 error by intermediate node in underlay
   network.






Dikshit, et al.          Expires 5 February 2022                [Page 8]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   IPv6 packet is sent by host H1 destined to host H2, both are in
   different IPv6 subnets.This packet is referred to as P1 in the
   document.

       +----------------------------------------------------+
   H1--|L2_Hdr(14 bytes): src-mac:H1_MAC, dest-mac:VtepA_MAC|-->VtepA
       +----------------------------------------------------+
       |IPv6_Hdr(40 bytes): src-ip:H1_IPV6, dest-ip:H2_IPv6 |
       +----------------------------------------------------+
       |Host/App specific Payload                           |
       +----------------------------------------------------+
       Figure 2a. Packet P1 sent by host H1 to host H2

   VtepA re-writes the mac addresses in 'P1' as part of Vxlan
   encapsulation.  This encapsulation is referred as 'P2' in the
   document.

       +------------------------------------------------------+
   H1--|L2_Hdr(14 bytes):src-mac:VtepA_MAC, dest-mac:VtepB_MAC|-->VtepA
       +------------------------------------------------------+
       |IPv6_Hdr(40 bytes): src-ip:H1_IPV6, dest-ip:H2_IPv6   |
       +------------------------------------------------------+
       |Host/App specific Payload                             |
       +------------------------------------------------------+
       Figure 2b. Packet P1 re-written by VtepA

4.1.2.1.  Packet Processing at Vxlan Gateway

   Processing at VtepA, in packet path from H1 to H2.

   (1)  VtepA(Vxlan gateway) performs the Vxlan encapsulation over the
        packet received from H1, based on route lookup.  The detail for
        encap are mentioned in [RFC7348].

   (2)  VtepA MUST set the DF-bit in the Outer IP header.

   (3)  Since the MTU of outgoing link is more than the packet, packet
        is sent out towards the underlay next hop, R1.

   (4)  P3 packets encapsulation is shown in figure 3.  P3 may find a
        reference without outer header encapsulation [RFC7348] provides
        details of the vxlan encapsulation.









Dikshit, et al.          Expires 5 February 2022                [Page 9]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


       +----------------------------------------------------------+
 VtepA-|L2_Hdr(14bytes):src-mac:VtepA_Mac, dest-mac:R1_MAC        |-->R1
       +----------------------------------------------------------+
       |IPv4_Hdr(20 bytes):src-ip:VtepA_IPv4,dest-ip:VtepB_IPv4,DF|
       +----------------------------------------------------------+
       |UDP(8 bytes): src-port: ephemeral-port, dest-port: 4789   |
       +----------------------------------------------------------+
       |Vxlan(8 bytes): Vxlan network identifier                  |
       +----------------------------------------------------------+
       |P2 packet (refer to H1 to VtepA flow for details of P1)   |
       +----------------------------------------------------------+
       Figure 3. Vxlan Encap packet sent by Vxlan Gateway to underlay

4.1.2.2.  Underlay Generates ICMP error

   In case the underlay is ipv6 and not ipv4, icmpv6 error is generated.

   Processing at R1:

   (1)  Packet Size (1500 bytes) is more than the outgoing link's mtu
        (1300 bytes) and DF-bit is set in the Outer IPv4 header added as
        part of Vxlan encapsulation at VtepA.

   (2)  R1 MUST generate icmp error message (Destination Unreachable)
        with error code (Fragmentation Needed and Don't Fragment was
        Set).  For ease of solution description, mtu is assumed to be
        symmetric over the reverse path, hence reverse path mtu from R1
        to VtepA is 1500 bytes. icmpv6 error message MUST include MTU of
        link between R1 and VtepB.

   (3)  In a nut shell, the icmp PDU encapsulation SHOULD be performed
        as mentioned in [RFC4884] , [RFC4443].  These standards atleast
        ensure, that original packet carried in icmp error PDU captures
        enough bytes to include the inner packets IPv6 header atleast.
        The capture of application specific details depends on the size
        of the Optional header in the original packet (generated by H1
        as in Figure 2b) and subsequent transport header.  This helps
        Vxlan Gateway to trace(L3 reachability) the original packet
        generator (end-point device) atleast and translate icmp error
        generted by underlay into icmpv6 one and relay it to end-point
        device.  The length field in icmp PDU, include the maximum
        possible length permissible in reverse path MTU

   For simplicity, not including the original packet header in the flow
   diagram in figure 4.  icmp PDU details are depicted in the follow up
   figure 5.





Dikshit, et al.          Expires 5 February 2022               [Page 10]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   +-----------------------------------------------------------+
R1-|L2_Hdr(14 bytes): src-mac:R1_MAC, dest-mac:VtepA_MAC       |-->VtepA
   +-----------------------------------------------------------+
   |IPv4_Hdr(20 bytes): src-ip:R1_IPv4, dest-ip:VtepA_IPv4     |
   +-----------------------------------------------------------+
   |ICMP PDU,type:3,code:4,R1_VtepB_MTU, P3(No outer L2 Header)|
   +-----------------------------------------------------------+
            Figure 4. Flow diagram from R1 to VtepA

   The details of icmp PDU are in the following figure.  Type '3' is
   "Destination Unreachable".  Code '4' is "Fragmentation Needed and
   Don't Fragment bit is set".

 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 4s 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|     Type=3    |     Code=4    |          Checksum             | ICMP
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type=3
|     unused    |    Length     |   Next Hop Mtu = R1_VtepB_MTU | Code=4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|  Ver=4|IHL=5  |  TOS          |     Total length              |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|         Id                    |Flags| Fragment Offset         |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|  TTL          | Protocol=UDP  | Header Checksum               |(Outer)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Max 40
|            src-ip  : VtepA_IPv4                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|            dest-ip : VtepB_IPv4                               |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|   Source UDP Port (ephemeral) |  Dest UDP Port = 4789 (Vxlan) |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+8 bytes
|   Length                      |  Checksum                     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
| | | | | | | | |     Reserved                                  |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+8 bytes
|Vxlan Network identifier (VNI)                 | Reserved      |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -------
|                  Inner Packet Dest-Mac = VtepB_MAC            |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                               |    Inner Packet Src-Mac =     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Inner
|                       VtepA_MAC                               |14 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
| Inner Vlan if present         |Ethtype = 0X86dd (IPv6)        |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|Ver=6  |Traffic Class  |      Flow Label                       |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|payload length                 |Next Header    | Hop Limit     |  |



Dikshit, et al.          Expires 5 February 2022               [Page 11]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               src-ipv6   =     H1_IPv6                        |IPv6
|                                                               |Header
|                                                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               dest-ipv6   =     H2_IPv6                       |  |
|                                                               |  |
|                                                               |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|    ~       Optional Headers and  transport header/Payload ~   | Varies
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
             Figure 5. ICMP PDU Original Packet Capture in Detail

4.1.2.3.  Relay ICMP(v6) Error to End Devices

   This sub-section can also be generalized as: "handling of icmp
   errors, which are generated by underlay network in response to end-
   device packets, by Vxlan Gateway".

   Processing at VtepA: Processing of icmp error message with code
   (Fragmentation Needed and Don't Fragment was Set):

   (1)  The icmp error is processed by Vxlan gateways as per the
        standards defined in [RFC1981] , [RFC4884] and [RFC4443] .

   (2)  If error code is (Fragmentation Needed and Don't Fragment was
        Set), it SHOULD perform further inspection of the original
        packet, P3(ethernet payload without its header) carried as data
        in icmp PDU in extension to standards referred in previous
        bullet.  The extension processing MUST be done prior to taking a
        decision to either drop the packet or deliver to upper-layer
        protocols.

   (3)  In extension to above, Vxlan gateway device SHOULD perform the
        vxlan decap as defined in [RFC7348], to arrive at the inner
        packet (P2, original packet with VtepA rewrite).  The underlay
        encap is not carrying the layer-2 header in the icmp error
        packet.  Once this processing is done, P2 is the packet which
        needs attention now, as it carries the credentials of actual
        host which should receive the relayed icmp packet.









Dikshit, et al.          Expires 5 February 2022               [Page 12]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   (4)  Post encap, the VNI should be cached and a check should be made
        if its a Layer-2 VNI (L2VNI) or an Layer-3 VNI (L3VNI).  If it's
        an L2VNI, go to the next step to check on the payload (ethernet)
        type.  If and only if, it is ipv6 or ipv4, then only process it
        further else terminate the processing.  If it's an L3VNI, then
        the mapping VRF should found to perform the route lookup for
        inner packet source IP address.

   (5)  The layer-3 payload type SHOULD be verified using ethernet type
        field in ethernet header.  In case it point to IPv6, src-ipv6
        field should be picked up to check for reahability, as the icmp
        packet MUST be sent to original sender, that is, H1.  In case H1
        is reachable, icmp packet SHOULD be constructed as mentioned in
        the following bullet.

   (6)  Now that P2 is out in the open, it's L2 header is decapsulated,
        and the leftover, in the figure 6, is run through the icmpv6
        processing as mentioned in [RFC4443].

   (7)  It SHOULD generate icmpv6 error message with type (Packet too
        big) destined to H1_IPv6, that is inner ipv6 packet's source
        ipv6 address.  The mtu 'R1_VtepB_MTU' is copied from icmp error
        packet recieved from the underlay.

   (8)  The IPv6 header is constructed from original payload as shown in
        figure 5.  The source ipv6 address is picked as local ipv6
        address "VtepA_IPv6".  The destination ipv6 address is set as
        the "src-ipv6" in original payload, H1_IPv6.  The Next Header is
        set as "58" which denote icmpv6.  The derivation of ethernet
        header is based on next hop to mac address mapping as is
        performed in any L3 lookup.  The follow up figure 9, shows the
        icmpv6 error packet sent out to node H1.  H1 is the original
        IPv6 packet generator as mentioned in Figure 2b.

   (9)  The route lookup is performed for H1_IPv6 in the VNI mapped VRF,
        as also mentioned in one of the earlier bullets.  Thus {inner
        packet source IP, VNI} as a tuple is required to resolve the
        path back to the inner packet source.













Dikshit, et al.          Expires 5 February 2022               [Page 13]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|Ver=6  |Traffic Class  |      Flow Label                       |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|payload length                 |Next Header    | Hop Limit     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               src-ipv6   =     H1_IPv6                        | Inner
|                                                               | IPv6
|                                                               | 40 byt)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               dest-ipv6   =     H2_IPv6                       |  |
|                                                               |  |
|                                                               |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|    ~  Optional Headers and  Transport/Application Payload ~   | Varies
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
      Figure 6. Original IPv6 Packet sent from H1 directed to H2

   Figure 6 gives a typical IPv6 format sent by end-host, H1 towards H2
   and encapsulated by Vxlan gateway, to translate the icmp error
   generated by underlay hop, R1, to the one understood in right context
   by H1.



























Dikshit, et al.          Expires 5 February 2022               [Page 14]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


 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=2     | Code=0        |      CheckSum                 |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type=2
|                 Mtu = R1_VtepB_MTU                            |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|Ver=6  |Traffic Class  |      Flow Label                       |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|payload length                 |Next Header    | Hop Limit     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               src-ipv6   =     H1_IPv6                        | Orig
|                                                               | Packet
|                                                               |40 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               dest-ipv6   =     H2_IPv6                       |  |
|                                                               |  |
|                                                               |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -----
|    ~ Optional/Transport Headers and Application Payload ~     |varies
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
 Figure 7. ICMPv6 "Packet Too Big" PDU relayed
           to H1 by Vxlan Gateway (VtepA)



























Dikshit, et al.          Expires 5 February 2022               [Page 15]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|                  Dest-Mac = H1_MAC                            |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                               |    Inner Packet Src-Mac =     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+eth hdr
|                       VtepA_MAC                               |14 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
| Inner Vlan if present         |Ethtype = 0X86dd (IPv6)        |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|Ver=6  |Traffic Class  |      Flow Label                       |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|payload length                 |Next Hdr = 58  | Hop Limit     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               src-ipv6   =     VtepA_IPv6                     | IPv6
|                                                               |header
|                                                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               dest-ipv6   =     H1_IPv6                       |  |
|                                                               |  |
|                                                               |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
Figure 8. Ethernet and IPv6 encap for ICMPv6 PDU mentioned in
          figure 7

   The translated icmp packet encapsulation looks similar to, figure 7
   and figure 8 put together in reverse order.  The flow diagram in
   figure 9 gives a concise form of "packet too big" icmpv6 error
   relayed by VtepA (Vxlan Gateway) towards H1 (end point device).

         +--------------------------------------------------------+
  VtepA--|L2_Hdr(14): src-mac:VtepA_MAC and Dest_Mac: H1_MAC      |-->H1
         +--------------------------------------------------------+
         |IPv6_Hdr(40 bytes): src-ip:Vtep_IPv6, dest-ip:H1_IPv6   |
         +--------------------------------------------------------+
         |ICMPv6: Packet_Too_Big, mtu, data: first 128 bytes of P3|
         +--------------------------------------------------------+
  Figure 9. Flow diagram:  VtepA to H1

   There are few more potential flows worth mentioning in this section.
   These cases are related to, icmp error getting generated from,
   ingress Vxlan gateway (VtepA) and egress Vxlan gateway (VtepB) with
   respect to packet sent from H1 to H2.  For ingress Vxlan gateway
   (VtepA) case, the legacy IPv6 PMTUD rules from [RFC4443] SHOULD be
   applied as no Vxlan encap is involved.





Dikshit, et al.          Expires 5 February 2022               [Page 16]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   Where as, egress Vxlan gateway (VtepB) SHOULD send packet P3 (without
   L2 header) in the icmp data, even though mtu calculation MAY be done
   post vxlan decapsulation.  That is when the outgoing link is
   identified as the one from VtepB to H2.  It MAY buffer packet P3
   prior to lookup based on inner packet (P2) credentials, so that P3
   can be encapsulated in the icmp packet.  This also ensures the packet
   format consistency, when accessed at the VtepA for translation before
   relaying it to H1.

4.1.3.  ICMP(v6) Error Translation

   This section specifically mentions about icmp and icmpv6 packet
   translation, generated in an underlay network to the one which is,
   understood by the end point device, with encapsulation aligning with
   the network-type(IPv4 and IPv6), end-point device and underlay is
   provisioned with.  The last leg processing mentioned in previous sub-
   section is specific to the topology mentioned in Section 3.1.1.
   However, this subsection elaborates on all possible topology
   combination of underlay and end-device networks with respect to IPv4
   or IPv6.  The explanation provided in form of figures for error
   generated by underlay and the translated one relayed to the end-point
   device by Vxlan gateway.

   (a)  End-Point is IPv6 connected and Underlay is IPv4 provisioned.

   (b)  End-Point is IPv4 connected and Underlay is IPv6 provisioned.

   (c)  Both End-Point and Underlay are provisioned with IPv6.

   (d)  Both End-Point and Underlay are provisioned with IPv4.

4.1.3.1.  End-Point is IPv6 connected and Underlay is IPv4 provisioned

   This case is similar to the last leg processing described in
   Section 4.1.2 and does not needs any more description.

4.1.3.2.  End-Point is IPv4 connected and Underlay is IPv6 provisioned

   Topology drawn in figure 10, provides for the icmpv6 PDU encap
   generated by R1.  H1_IPv4 and H2_IPv4 are in distinct ipv4 subnets.
   R1_IPv6 represents IPv6 addresses falling in both subnets connecting
   to VtepA and VtepB.









Dikshit, et al.          Expires 5 February 2022               [Page 17]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   Another difference between an IPv4 and IPv6 underlay is that for IPv6
   underlay there is no concept of DF-bit.  The fragmentation can only
   be done at ingress.  At all other underlay nodes "Packet too big"
   icmpv6 error is generated.  Vxlan Gateway SHOULD ensure that
   fragmentation is avoided at Vxlan Gateway and icmp error is sent back
   to H1.  This procedure is applicable if and only if, original packet
   contains DF-bit set in it's IP header.

   +----------+                    +----------+
   |    H1    |                    |    H2    |
   |          |                    |          |
   |(H1_IPv4) |                    |(H2_IPv4) |
   +----------+                    +----------+
        |                               |
        |                               |
   +------------+   +----------+   +------------+
   |(VtepA_IPv4)|   |          |   |(VtepB_IPv4)|
   |   VtepA    |   |    R1    |   |  VtepB     |
   |(VtepA_IPv6)|---| (R1_IPv6)|---|(VtepB_IPv6)|
   +------------+   +----------+   +------------+

                  Figure 10. L3 Overlay

          LEGEND:
          MAC address : <Node_name>_MAC
          IPv4 address: <Node_name>_IPv4
          IPv6 address: <Node_name>_IPv6
          <Node_name> : node names in the above topology are
                        H1, VtepA, R1, VtepB, H2.
          VtepA, VtepB: Vxlan gateways to underlay network

 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=2    |     Code=0    |          Checksum             | ICMPv6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type=2
|        Next Hop Mtu = R1_VtepB_MTU                            | Code=0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|Ver=6  |Traffic Class  |      Flow Label                       |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|payload length                 |Next Hdr       | Hop Limit     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               src-ipv6   =     R1_IPv6                        | IPv6
|                                                               |40 byte)
|                                                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               dest-ipv6  =     VtepA_IPv6                     |  |



Dikshit, et al.          Expires 5 February 2022               [Page 18]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


|                                                               |  |
|                                                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|         ~      Extension Headers ~ (payload type is UDP)      |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|   Source UDP Port (ephemeral) |  Dest UDP Port = 4789 (Vxlan) |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 8 byte
|   Length                      |  Checksum                     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
| | | | | | | | |     Reserved                                  |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 8 byte
|Vxlan Network identifier (VNI)                 | Reserved      |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|                  Inner Packet Dest-Mac = VtepA_MAC            |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                               |    Inner Packet Src-Mac =     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+eth hdr
|                       VtepB_MAC                               |14 byte
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
| Inner Vlan if present         |Ethtype = 0X0800 (IPv4)        |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|  Ver=4|IHL=5  |  TOS          |     Total length              |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|         Id                    |Flags| Fragment Offset         |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|  TTL          | Protocol      | Header Checksum               | Orig
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Hdr
|            src-ip  : H1_IPv4                                  |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|            dest-ip : H2_IPv4                                  |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|    ~     transport-header and Application specific Payload ~  | varies
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
        Figure 11. ICMPV6 PDU Sent by R1 to VtepA

   R1 sends an icmpv6 error "Packet Too Big" directed towards VtepA.
   The icmpv6 PDU is shown in Figure 11.  VtepA receives the packet with
   this icmpv6 PDU and translates it to icmp PDU with type "Destination
   Unreachable" and code "Fragmentation Needed" before relaying it to H1
   over ipv4 network.  Figure 12, reflects the relayed packet sent by
   VtepA to H1.  All other references SHOULD be taken as it is from
   Section 4.1.2.









Dikshit, et al.          Expires 5 February 2022               [Page 19]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|                  Dest-Mac = H1_MAC                            |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                               |    Inner Packet Src-Mac =     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+eth hdr
|                       VtepA_MAC                               |14 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
| Inner Vlan if present         |Ethtype = 0X0800 (IPv4)        |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|  Ver=4|IHL=5  |  TOS          |     Total length              |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|         Id                    |Flags| Fragment Offset         |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|  TTL          | Protocol=1    | Header Checksum               | IPv4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|            src-ip  : VtepA_IPv4                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|            dest-ip : H1_IPv4                                  |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|            Optional Header                                    |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|     Type=3    |     Code=4    |          Checksum             | ICMP
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type=3
|     unused    |    Length     |   Next Hop Mtu = R1_VtepB_MTU | Code=4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|  Ver=4|IHL=5  |  TOS          |     Total length              |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|         Id                    |Flags| Fragment Offset         |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|  TTL          | Protocol      | Header Checksum               |Orig
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+iPv4
|            src-ip  : H1_IPv4                                  |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|            dest-ip : H2_IPv4                                  |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|       Optional and Transport Header and Application data      | varies
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
Figure 12. ICMPv4 error Packet relayed to end point Host, H1

4.1.3.3.  Both End-Point and Underlay are provisioned with IPv6

   Topology is mentioned in Figure 13 with minor changes along with the
   legend.  Figure 14, outlines the icmpv6 PDU, encapsulation generated
   by R1.  H1_IPv6 and H2_IPv6 in different ipv6 subnets.  R1_IPv6
   reflects both subnets connecting to VtepA and VtepB.





Dikshit, et al.          Expires 5 February 2022               [Page 20]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   +----------+                    +----------+
   |    H1    |                    |    H2    |
   |          |                    |          |
   |(H1_IPv6) |                    |(H2_IPv6) |
   +----------+                    +----------+
        |                               |
        |                               |
   +------------+   +----------+   +------------+
   |(VtepA_IPv6)|   |          |   |(VtepB_IPv6)|
   |   VtepA    |   |    R1    |   |  VtepB     |
   |(VtepA_IPv6)|---| (R1_IPv6)|---|(VtepB_IPv6)|
   +------------+   +----------+   +------------+

                  Figure 13. L3 Overlay

          LEGEND:
          MAC address : <Node_name>_MAC
          IPv6 address: <Node_name>_IPv6
          <Node_name> : node names in the above topology are
                        H1, VtepA, R1, VtepB, H2.
          VtepA, VtepB: Vxlan gateways to underlay network

 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=2    |     Code=0    |          Checksum             | ICMPv6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type=2
|        Next Hop Mtu = R1_VtepB_MTU                            | Code=0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|Ver=6  |Traffic Class  |      Flow Label                       |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|payload length                 |Next Hdr       | Hop Limit     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               src-ipv6   =     R1_IPv6                        |  IPv6
|                                                               | Header
|                                                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               dest-ipv6  =     VtepA_IPv6                     |  |
|                                                               |  |
|                                                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|         ~      Extension Headers ~ (payload type is UDP)      |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|   Source UDP Port (ephemeral) |  Dest UDP Port = 4789 (Vxlan) |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+8 bytes
|   Length                      |  Checksum                     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------



Dikshit, et al.          Expires 5 February 2022               [Page 21]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


| | | | | | | | |     Reserved                                  |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+8 bytes
|Vxlan Network identifier (VNI)                 | Reserved      |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|                  Inner Packet Dest-Mac = VtepB_MAC            |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                               |    Inner Packet Src-Mac =     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+eth hdr
|                       VtepA_MAC                               |14 byte
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
| Inner Vlan if present         |Ethtype = 0X0800 (IPv4)        |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|Ver=6  |Traffic Class  |      Flow Label                       |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|payload length                 |Next Hdr       | Hop Limit     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               src-ipv6   =     VtepA_IPv6                     |Inner
|                                                               | Ipv6
|                                                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               dest-ipv6  =     H1_IPv6                        |  |
|                                                               |  |
|                                                               |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|         ~ Extension and Transport Headers, Application Data ~ | varies
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
Figure 14. ICMPv6 PDU generated by Intermediate Hop, R1 in Vxlan Network

   R1 sends an icmpv6 error "Packet Too Big" directed towards VtepA.
   The icmpv6 PDU is shown in Figure 14.  VtepA receives the packet with
   this icmpv6 PDU and relays it to H1 without any translation as H1 is
   connected to VtepA over ipv6 network.  All other references about
   original packet to be include in the icmpv6 PDU can be taken as it is
   from Section 4.1.2.















Dikshit, et al.          Expires 5 February 2022               [Page 22]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|                  Dest-Mac = H1_MAC                            |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                               |    Inner Packet Src-Mac =     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+eth hdr
|                       VtepA_MAC                               |14 byte
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
| Inner Vlan if present         |Ethtype = 0X86dd (IPv6)        |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|Ver=6  |Traffic Class  |      Flow Label                       |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|payload length                 |Next Hdr       | Hop Limit     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               src-ipv6   =     VtepA_IPv6                     |IPv6
|                                                               | Header
|                                                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               dest-ipv6  =     H1_IPv6                        |  |
|                                                               |  |
|                                                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|         ~      Extension Headers ~ (payload type is ICMPV6)   |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|     Type=2    |     Code=0    |          Checksum             | ICMPv6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type=2
|        Next Hop Mtu = R1_VtepB_MTU                            | Code=0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|Ver=6  |Traffic Class  |      Flow Label                       |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|payload length                 |Next Hdr       | Hop Limit     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               src-ipv6   =     H1_IPv6                        |Orig
|                                                               |IPv6
|                                                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|               dest-ipv6  =     H2_IPv6                        |  |
|                                                               |  |
|                                                               |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
| ~ Extension and Transport Headers and Applcation data     ~   | varies
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
Figure 15. ICMPv6 error Complete Packet sent to H1 by VtepA




Dikshit, et al.          Expires 5 February 2022               [Page 23]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


4.1.3.4.  Both End-Point and Underlay are provisioned with IPv4

   Topology is mentioned in figure 16, with minor changes along with the
   legend, figure 17, provides the icmp PDU encap generated by R1.
   H1_IPv4 and H2_IPv4 are in different ipv4 subnets.

   +----------+                    +----------+
   |    H1    |                    |    H2    |
   |          |                    |          |
   |(H1_IPv4) |                    |(H2_IPv4) |
   +----------+                    +----------+
        |                               |
        |                               |
   +------------+   +----------+   +------------+
   |(VtepA_IPv4)|   |          |   |(VtepB_IPv4)|
   |   VtepA    |   |    R1    |   |  VtepB     |
   |(VtepA_IPv4)|---| (R1_IPv4)|---|(VtepB_IPv4)|
   +------------+   +----------+   +------------+

                  Figure 16. L3 Overlay

          LEGEND:
          MAC address : <Node_name>_MAC
          IPv4 address: <Node_name>_IPv4
          <Node_name> : node names in the above topology are
                        H1, VtepA, R1, VtepB, H2.
          VtepA, VtepB: Vxlan gateways to underlay network
























Dikshit, et al.          Expires 5 February 2022               [Page 24]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


 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=3    |     Code=4    |          Checksum             | ICMP
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type=3
|     unused    |    Length     |   Next Hop Mtu = R1_VtepB_MTU | Code=4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|  Ver=4|IHL=5  |  TOS          |     Total length              |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|         Id                    |Flags| Fragment Offset         |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|  TTL          | Protocol=UDP  | Header Checksum               | IPv4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Header
|            src-ip  : VtepA_IPv4                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|            dest-ip : H1_IPv4                                  |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|   Source UDP Port (ephemeral) |  Dest UDP Port = 4789 (Vxlan) |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+8 bytes
|   Length                      |  Checksum                     |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
| | | | | | | | |     Reserved                                  |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+8 bytes
|Vxlan Network identifier (VNI)                 | Reserved      |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|                  Inner Packet Dest-Mac = VtepB_MAC            |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                               |    Inner Packet Src-Mac =     |inner
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+packet
|                       VtepA_MAC                               |eth hdr
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
| Inner Vlan if present         |Ethtype = 0X0800 (IPv4)        |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|  Ver=4|IHL=5  |  TOS          |     Total length              |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|         Id                    |Flags| Fragment Offset         |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|  TTL          | Protocol      | Header Checksum               | IPv4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ hdr
|            src-ip  : H1_IPv4                                  |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|            dest-ip : H2_IPv4                                  |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
| ~ Optional and Transport Header and Application Payload ~     |varies
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------

 Figure 17. ICMP PDU generated by R1 towards VtepA





Dikshit, et al.          Expires 5 February 2022               [Page 25]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   R1 sends an icmp error directed towards VtepA.  The icmp PDU is shown
   in figure 17.  VtepA receives the packet with this icmp PDU and
   relays it to H1 over ipv4 network.  Figure 16, displays the packet
   sent by VtepA to H1.  All other references can be taken as it is from
   Section 4.1.2.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|                  Dest-Mac = H1_MAC                            |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                               |    Src-Mac =                  |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  eth
|                       VtepA_MAC                               |header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
| Inner Vlan if present         |Ethtype = 0X0800 (IPv4)        |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|  Ver=4|IHL=5  |  TOS          |     Total length              |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|         Id                    |Flags| Fragment Offset         |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|  TTL          | Protocol=1    | Header Checksum               |IPv4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Header
|            src-ip  : VtepA_IPv4                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|            dest-ip : H1_IPv4                                  |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|            Optional Header                                    |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|     Type=3    |     Code=4    |          Checksum             | ICMP
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type=3
|     unused    |    Length     |   Next Hop Mtu = R1_VtepB_MTU | Code=4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|  Ver=4|IHL=5  |  TOS          |     Total length              |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|         Id                    |Flags| Fragment Offset         |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|  TTL          | Protocol      | Header Checksum               |Orig
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+IPv4
|            src-ip  : H1_IPv4                                  |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|            dest-ip : H2_IPv4                                  |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|~ Optional and Transport Header and Application Payload     ~  | varies
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------

Figure 18. Complete ICMP error Packet sent to H1 by VtepA





Dikshit, et al.          Expires 5 February 2022               [Page 26]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


5.  Inter-site MTU Propagation

                             +--+
                             |CE|
                             +--+
                               |
                            +----+
                       +----| PE |----+
           +---------+ |    +----+    | +---------+
   +----+  |        +---+            +---+        |  +----+
   |NVE1|--|        |   |            |   |        |--|NVE3|
   +----+  |---U1---|GW1|-----P------|GW3|---U2---|  +----+
           |        +---+            +---+        |
           |  NVO-1   |       WAN      |    NVO-2 |
           |        +---+            +---+        |
           |        |   |            |   |        |
   +----+  |        |GW2|            |GW4|        |  +----+
   |NVE2|--|        +---+            +---+        |--|NVE4|
   +----+  +---------+ |              | +---------+  +----+
                       +--------------+
   Figure 18. Datacenter/Site Interconnect Between Remote EVPN fabrics

   This section specifically calls out the relay of icmp errors
   generated by underlay in an intersite/interfabric connectivity across
   EVPN-overlays.  The reference diagram shown above is picked up from
   [RFC9014].

   The topology in the above diagram describes two disparate NVO fabrics
   connected across WAN; leveraging an EVPN provisioned overlay.  Lets
   consider the interconnect as EVPN-Overlay over the WAN network
   (between GW1/2 and GW3/4).  Thus there is a multi-hop overlay
   (tunnel) reachability between hosts in fabric(s) behind the edges
   NVE1/2 and NVE3/4.  There is a EVPN-Overlay tunnel between NVEs and
   their respective gateways, i.e., between NVE1/2 and GW1/2 and another
   one between NVE3/4 and GW3/4.  There is an intersite connect
   leveraging EVPN-Overlay, thus ensuring end to end connectivity
   between NVEs across the WAN.  The fabric in dataplane can be Vxlan,
   MPLS, NVGRE, GENEVE, GUE, GPE etc.

   The packet traversing between networks behind NVE1 to NVE3 shall
   transit through three EVPN-Overlay tunnels.  First one, between NVE1
   and GW1; second one, between the WAN gateways GW1 and GW3 and the
   third one, between GW3 and NVE3.  There is an EVPN-Overlay handoff at
   all the EVPN-tunnel end-points in the packet path, GW1 and GW2
   respectively.






Dikshit, et al.          Expires 5 February 2022               [Page 27]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   There is a possibility that the overlay encapsulated packet hits the
   MTU blockage at one of the underlay routers, lets say, P3 in this
   case.  P3 generates icmp error targetted towards GW3 as the tunnel
   end-point.  GW3 should check the credentials of the original PDU,
   carried in the icmp error and perform the route lookup.  It's very
   likely that the path to reach packet source (behind NVE1) is also via
   the EVPN-Overlay tunnel from GW3 to GW1.  The icmp error is relayed
   back over the EVPN-Overlay construct towards GW3.  In the same flow
   GW1 should peek into the original PDU credentials to get the
   reachability to the inner packet source.  As luck may have it, the
   packet source is reachable over the EVPN-overlay tunnel from GW1 to
   NVE1.  It should go through the similar decap/re-encap as mentioned
   in earlier sections.  The EVIs at each stitching point may be
   different, although ensuring that routes are exported between the
   VNIs.  The first-hop vtep towards the source i.e. NVE1 should perform
   procedures mentioned in Section 4.1.2, to relay out the icmp packet
   to the original source of the packet.

6.  Same subnet Considerations

   This section proposes propagation of icmp or icmpv6 error (specific
   to MTU) at source Vtep to inner packet source, which is generated by
   an underlay device for a case, when, inner packet source and
   destination ipv4(or ipv6) addresses are in the same subnet.

   The steps in section Section 4.1.2.3, elaborate on the check to be
   performed, if the icmp error is carrying the original PDU
   encapsulated with an L2VNI or L3VNI.  In case it is an L2VNI, then
   the possibility of the inner packet traffic being a "same subnet" one
   is, very high.  Hence Section 4.1.2.3, also talks about doing the
   ethernet type check in the inner packet payload.  If and only if,
   it's ipv4 or ipv6, relay the icmp error back to the inner packet
   source ip (or ipv6) address.  Else, don't process the relay message
   further.  As the inner packet is a non layer-3 PDU, it does not makes
   sense to relay back the icmp error.

7.  Ecmp Considerations

   Ecmp considerations are driven by the packet sent by the end host
   application and the way it's leveraged.

   To ensure "MTU propagation" via "icmpv6 error", is agnostic to ecmp
   paths in a Vxlan network, there are few more consideration.  In Vxlan
   Gateway, the route look-up is done based on attributes carried in
   packet generated by end point host.  The packet generated can
   potentially be from a tcp based end host application (although should
   not be generalized).




Dikshit, et al.          Expires 5 February 2022               [Page 28]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   Where as, for an intermediate node, (lets say, Spine node in Clos
   topology) in underlay network the look ups are based on Outer Encap
   (Vtep ip addresses and and UDP Header).

   The packet traversing from site behind NE1 to NVE3 shall transit
   through three EVPN-Overlay tunnels.  First one, between NVE1 and GW1;
   second one, between the WAN gateways GW1 and GW3 and the third one,
   between GW3 and NVE3.  There is an EVPN-Overlay handoff at all the
   EVPN-tunnel end-points in the packet path, GW1 and GW2 respectively.
   On another note, for an L2 gateway case, wherein Vxlan gateway (Vtep
   Node) bridges (and not routes) host packets destined to same subnet
   destination, MTU calculation SHOULD come into play only in the Spine
   devices.

   As a potential solution,the MTU values recieved over ECMP underlay
   paths can be cached at the ingress Vteps.  The Vtep MAY propagate/
   relay the lowest of the all MTUs received across ECMP underlay paths,
   to the end-host.

8.  Security Considerations

   This document inherits all the security considerations discussed in
   [RFC1981] and [RFC1191].

9.  IANA Considerations

   TBD

10.  Acknowledgements

   Thanks to Vengada Prasad Govindan, Deepak Kumar, Matthew Bocci and
   Rohit Mendiratta for providing the inputs.

11.  References

11.1.  Normative References

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

11.2.  Informative References









Dikshit, et al.          Expires 5 February 2022               [Page 29]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   [I-D.draft-gross-geneve]
              Gross, J., Sridhar, T., Garg, P., Wright, C., Ganga, G.,
              Agarwal, P., Duda, C., Dutt, D., and J. Hudson, "Geneve:
              Generic Network Virtualization Encapsulation", Work in
              Progress, Internet-Draft, draft-gross-geneve-02, 25
              October 2015, <http://www.ietf.org/internet-drafts/draft-
              gross-geneve-02.txt>.

   [I-D.draft-ietf-nvo3-gue]
              Herbert, T., Yong, L., and O. Zia, "Generic Protocol
              Extension for VXLAN", Work in Progress, Internet-Draft,
              draft-ietf-nvo3-gue-03, 6 March 2015,
              <http://www.ietf.org/internet-drafts/draft-ietf-nvo3-gue-
              03.txt>.

   [I-D.draft-ietf-nvo3-vxlan-gpe]
              Quinn, P., Manur, R., Kreeger, L., Lewis, D., Maino, F.,
              Smith, M., Agarwal, P., Yong, L., Xu, X., Elzur, U., and
              D. Melman, "Generic Protocol Extension for VXLAN", Work in
              Progress, Internet-Draft, draft-ietf-nvo3-vxlan-gpe-02, 1
              May 2015, <http://www.ietf.org/internet-drafts/draft-ietf-
              nvo3-vxlan-gpe-02.txt>.

   [I-D.nordmark-nvo3-transcending-traceroute]
              Nordmark, E., Appanna, C., and A. Lo, "Layer-Transcending
              Traceroute for Overlay Networks like VXLAN", Work in
              Progress, Internet-Draft, draft-nordmark-nvo3-
              transcending-traceroute-02, 4 March 2015,
              <http://www.ietf.org/internet-drafts/draft-nordmark-nvo3-
              transcending-traceroute-02.txt>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              1 November 1990,
              <http://www.rfc-editor.org/rfc/rfc1191.txt>.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996,
              <http://www.rfc-editor.org/rfc/rfc1981.txt>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (icmpv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006,
              <http://www.rfc-editor.org/rfc/rfc4443.txt>.

   [RFC4459]  Savola, P., "MTU and Fragmentation Issues with In-the-
              Network Tunneling", RFC 9014, April 2006,
              <http://www.rfc-editor.org/rfc/rfc9014.txt>.




Dikshit, et al.          Expires 5 February 2022               [Page 30]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007,
              <http://www.rfc-editor.org/rfc/rfc4821.txt>.

   [RFC4884]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
              "Extended ICMP to Support Multi-Part Messages", RFC 4884,
              April 2007, <http://www.rfc-editor.org/rfc/rfc4884.txt>.

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, August 2014,
              <http://www.rfc-editor.org/rfc/rfc7348.txt>.

   [RFC7637]  Yang, S. and M. Garg, "Network Virtualization Using
              Generic Routing Encapsulation", RFC 7637, September 2015,
              <http://www.rfc-editor.org/rfc/rfc7637.txt>.

   [RFC9014]  Rabadan, J., Sathappan, S., Henderickx, W., Sajassi, A.,
              and W. Drake, "Interconnect Solution for Ethernet VPN
              (EVPN) Overlay Networks", RFC 9014, May 2021,
              <http://www.rfc-editor.org/rfc/rfc9014.txt>.

Authors' Addresses

   Saumya Dikshit
   Aruba Networks, HPE
   Mahadevpura
   Bangalore 560 048
   Karnataka
   India

   Email: saumya.dikshit@hpe.com


   Vinayak Joshi
   Aruba Networks, HPE
   Mahadevpura
   Bangalore 560 048
   Karnataka
   India

   Email: vinayak.joshi@hpe.com







Dikshit, et al.          Expires 5 February 2022               [Page 31]


Internet-Draft     MTU propagation over EVPN Overlays        August 2021


   A. Sujeet Nayak
   Cisco
   Cessna Business Park
   Bangalore 560 087
   Karnataka
   India

   Email: sua@cisco.com











































Dikshit, et al.          Expires 5 February 2022               [Page 32]