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NVO3 WG                                                      E. Nordmark
Internet-Draft                                                C. Appanna
Intended status: Standards Track                                   A. Lo
Expires: April 3, 2016                                   Arista Networks
                                                                Oct 2015


     Layer-Transcending Traceroute for Overlay Networks like VXLAN
             draft-nordmark-nvo3-transcending-traceroute-01

Abstract

   Tools like traceroute have been very valuable for the operation of
   the Internet.  Part of that value comes from being able to display
   information about routers and paths over which the user of the tool
   has no control, but the traceroute output can be passed along to
   someone else that can further investigate or fix the problem.

   In overlay networks such as VXLAN and NVGRE the prevailing view is
   that since the overlay network has no control of the underlay there
   needs to be special tools and agreements to enable extracting traces
   from the underlay.  We argue that enabling visibility into the
   underlay and using existing tools like traceroute has been overlooked
   and would add value in many deployments of overlay networks.

   This document specifies an approach that can be used to make
   traceroute transcend layers of encapsulation including details for
   how to apply this to VXLAN.  The technique can be applied to other
   encapsulations used for overlay networks.  It can also be implemented
   using current commercial silicon.

Status of this Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 3, 2016.




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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   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.  Solution Overview  . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Goals and Requirements . . . . . . . . . . . . . . . . . . . .  5
   4.  Definition Of Terms  . . . . . . . . . . . . . . . . . . . . .  6
   5.  Example Topologies . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Controlling and selecting ttl behavior . . . . . . . . . . . . 10
   7.  Introducing a ttl copyin flag in the encapsulation header  . . 10
   8.  Encapsulation Behavior . . . . . . . . . . . . . . . . . . . . 11
   9.  Decapsulating Behavior . . . . . . . . . . . . . . . . . . . . 14
   10. Other ICMP errors  . . . . . . . . . . . . . . . . . . . . . . 15
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     14.1.  Normative References  . . . . . . . . . . . . . . . . . . 16
     14.2.  Informative References  . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
















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1.  Introduction

   Tools like traceroute have been very valuable for the operation of
   the Internet.  Part of that value comes from being able to display
   information about routers and paths over which the user of the tool
   has no control, but the traceroute output can be passed along to
   someone else that can further investigate or fix the problem.  The
   output of traceroute can be included in an email or a trouble ticket
   to report the problem.  This provide a lot more information than the
   mere indication that A can't communicate with B, in particular when
   the failures are transient.  The ping tool provides some of the same
   benefits in being able to return ICMP errors such as host unreachable
   messages.

   This document shows how those tools can be used to gather information
   for both the overlay and underlay parts of an end-to-end path by
   providing the option to have some packets use a uniform time-to-live
   (ttl) model for the tunnels, and associated ICMP error handling.
   These changes are limited to the tunnel ingress and egress points.

   The desire to make traceroute provide useful information for overlay
   network is not an argument against also using a layered approach for
   OAM as specified in e.g., [I-D.tissa-lime-yang-oam-model].  Such
   approaches are quite appropriate for continuos monitoring at
   different layers and across different domains.  A layer transcending
   traceroute complements the ability to do layered and/or continuos
   monitoring.

   The traceroute tool relies on receiving ICMP errors [RFC0792] in
   combination with using different IP time-to-live values.  That
   results in the packet making it further and further towards the
   destination with ICMP ttl exceeded errors being received from each
   hop.  That provides the user the working path even if the packets are
   black holed eventually, and also provides any errors like ICMP host
   unreachable.  The fundamental assumption is that the ttl is
   decremented for each hop and that the resulting ICMP ttl exceeded
   errors are delivered back to the host.

   When some encapsulation is used to tunnel packets there is an
   architectural question how those tunnels should be viewed from the
   rest of the network.  Different models were described first for
   diffserv in [RFC2983] and then applied to MPLS in [RFC3270] and
   expanded to MPLS ttl handling in [RFC3443] and those models apply to
   other forms of direct or indirect IP in IP tunnels.  Those RFCs
   define two models for ttl that are of interest to us:

   o  A pipe model, where the tunnel is invisible to the rest of the
      network in that it looks like a direct connection between the



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      tunnel ingress and egress.

   o  A uniform model, where the ttl decrements uniformly for hops
      outside and inside the tunnel.

   The tunneling mechanisms discussed in NVO3 (such as VXLAN [RFC7348],
   NVGRE [I-D.sridharan-virtualization-nvgre], GENEVE
   [I-D.gross-geneve], and GUE [I-D.herbert-gue]), have either been
   specified to provide the pipe model of a tunnel or are silent on the
   setting of the outer ttl.  Those protocols can be extended to have an
   optional uniform tunnel model when the payload is IP, following the
   same model as in [RFC3443].  Note that these encapsulations carry
   Ethernet frames hence are not even aware that the payload is IP.
   However, IP is the bulk of what is carried over such tunnels and the
   ingress NVE can inspect the IP part of the Ethernet frame.

   However, for general application traffic the pipe model is fine and
   might even be expected by some applications.  In general, when the
   source and destination IP are in the same IP subnet the ttl should
   not be decremented.  Thus it makes sense to have a way to selectively
   enable the uniform model perhaps based on some method to identify
   packets associated with traceroute or some marker in the packet
   itself that the traceroute tool can set.


2.  Solution Overview

   The pieces needed to accomplish this are:

   o  One or more ways to select the uniform model packets at the tunnel
      ingress.

   o  Tunnel ingress copying out the original ttl from a selected packet
      to the outer IP header, and then doing a check and decrement of
      that ttl.

   o  If that ttl check results in ttl expiry at the tunnel ingress,
      then deliver an ICMP ttl exceeded packet back to the host.

   o  A mechanism by which the tunnel egress knows which packets should
      have uniform model, for instance a bit in the encapsulation
      header.

   o  The tunnel egress copying in the ttl (for identified packets) from
      the outer header to the inner IP header, then doing a check and
      decrement of that ttl.





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   o  If ttl check results in ttl expiry at the tunnel egress, then
      deliver an ICMP error back to the original host (or, perhaps
      better, to tunnel ingress the same way as underlay routers do).

   o  IP routers in the underlay will deliver any ICMP errors to the
      source IP address of the packet.  For tunneled packets that will
      be the tunnel ingress.  Hence the tunnel ingress needs to be able
      to take such ICMP errors and form corresponding ICMP errors that
      are sent back to the host.  The requirement in [RFC1812] ensures
      that the ICMP errors will contain enough headers to form such an
      ICMP error.

   The idea to reflect (some) ICMP errors from inside a tunnel back to
   the original source goes back to IPv6 in IPv4 encapsulation as
   specified in [RFC1933] and [RFC2473].  However, those drafts did not
   advocate using a uniform ttl model for the tunnels but did handle
   ICMP packet too big and other unreachable messages.  Those drafts
   specify how to reflect ICMP errors received from underlay routers to
   ICMP errors sent to the original host.  The addition of handling ICMP
   ttl exceeded errors for uniform tunnel model is straight forward.

   The information carried in the ICMP errors are quite limited - the
   original packet plus an ICMP type and code.  However, there are
   extension mechanisms specified in [RFC4884] and used for MPLS in
   [RFC4950] which include TLVs with additional information.  If there
   are additional information to include for overlay networks that
   information could be added by defining new ICMP Extensions Objects
   based on [RFC4884].  Such extensions are for further study.


3.  Goals and Requirements

   The following goals and requirements apply:

   o  No changes needed in the underlay.

   o  Optional changes on the decapsulating end.

   o  ECMP friendly.  If the underlay employs equal cost multipath
      routing then one should be able to use this mechanism to trace the
      same path as a given TCP or UDP flow is using.  In addition, one
      should be able to explore different ECMP paths by varying the IP
      addresses and port numbers in the packets originated by traceroute
      on the host.

   o  Provide output which makes it possible to compare a regular
      overlay traceroute with the layer-transcending output.




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4.  Definition Of Terms

   The keywords "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].

   The terminology such as NVE, and TS are used as specified in
   [RFC7365]:

   o  Network Virtualization Edge (NVE): An NVE is the network entity
      that sits at the edge of an underlay network and implements L2
      and/or L3 network virtualization functions.

   o  Tenant System (TS): A physical or virtual system that can play the
      role of a host or a forwarding element such as a router, switch,
      firewall, etc.

   o  Virtual Access Points (VAPs): A logical connection point on the
      NVE for connecting a Tenant System to a virtual network.

   o  Virtual Network (VN): A VN is a logical abstraction of a physical
      network that provides L2 or L3 network services to a set of Tenant
      Systems.

   o  Virtual Network Context (VN Context) Identifier: Field in an
      overlay encapsulation header that identifies the specific VN the
      packet belongs to.

   We use the VTEP term in [RFC7348] as synonymous with NVE, and VNI as
   synonymous to VN Context Identifier.


5.  Example Topologies

   The following example topologies illustrate different cases where we
   want a tracing capability.  The examples are for overlay technologies
   such as VXLAN which provide a layer 2 overlay on IP.  The cases for
   layer 3 overlay on top of IP are simpler and not shown in this
   document.

   The VXLAN term VTEP is used as synonymous to NVO3's NVE term.










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   -----------                -----------
   |    H1   |                |    H2   |
   | 1.0.1.1 |                | 1.0.1.2 |
   |         |                |         |
   -----------                -----------
        |                          |
        |                          |
   -----------   -----------  -----------
   |  VtepA  |   |    R1   |  |  VtepB  |
   | 2.0.1.1 | --| 2.0.1.2 |  | 2.0.2.1 |
   |         |   | 2.0.2.2 |--|         |
   -----------   -----------  -----------

                             Simple L2 overlay

   The figure above shows two hosts connected using an underlay which
   provides a layer two service.  Thus H1 and H2 are in the same subnet
   and unaware of the existence of the underlay.  Thus a normal ping or
   traceroute would not be able to provide any information about the
   nature of a failure; either packets get through or they do not.  When
   the packets get through traceroute would output something like:

   traceroute to 1.0.1.2 (1.0.1.2), 30 hops max, 60 byte packets
    1  1.0.2.1 (1.0.2.1)  1.104 ms  1.235 ms  1.729 ms

   In this case it would be desirable to be able to traceroute from H1
   to H2 (and vice versa) and observe VtepA, R1, VtepB and H2.  Thus in
   the case of packets getting through traceroute would output:

   traceroute to 1.0.1.2 (1.0.1.2), 30 hops max, 60 byte packets
    1  2.0.1.1 (2.0.1.1)  1.104 ms  1.235 ms  1.729 ms
    2  2.0.1.2 (2.0.1.2)  2.106 ms  2.007 ms  2.156 ms
    3  2.0.2.1 (2.0.2.1)  35.034 ms  24.490 ms  21.626 ms
    4  1.0.1.2 (1.0.1.2)  40.830 ms  44.694 ms  75.620 ms

   Note that the underlay and overlay might exist in completely separate
   addressing domains.  Thus H1 might not be able to reach any of the
   underlay addresses.  And the underlay IP addresses might overlap the
   overlay IP addresses.  For example, it would be completely valid to
   see e.g.  VtepA having the same IP address as H1.  The user of this
   tool need to understand that the utility of the traceroute output is
   to get information to determine whether the issue is in the underlay
   or overlay, and be able to pass the underlay information to the
   operator of the underlay.

   In overlay networks without any ARP/ND optimizations ARP/ND packets
   would be flooded between the tunnel endpoints.  Thus if there is some
   communication failure between H1 and H2, then H1 above might not have



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   an ARP entry for H2.  This results in traceroute not being able to
   output any data.  This implies that in order to use traceroute to
   trouble shoot the issue one would need some workaround, such as
   installing some temporary ARP entries on the hosts.

   -----------                -----------  -----------  -----------
   |    H1   |                |    R2   |  |    R3   |  |    H4   |
   | 1.0.1.1 |                | 1.0.2.2 |--| 1.0.2.3 |  |         |
   |         |                | 1.0.1.2 |  | 1.0.3.3 |--| 1.0.3.4 |
   -----------                -----------  -----------  -----------
        |                          |
        |                          |
   -----------   -----------  -----------
   | VtepA  |   |    R1   |  |  VtepB  |
   | 2.0.1.1 | --| 2.0.1.2 |  | 2.0.2.1 |
   |         |   | 2.0.2.2 |--|         |
   -----------   -----------  -----------

                   L2 overlay as part of larger network

   The figure above has a overlay router the nexthop as seen by H1.  In
   this case a normal overlay traceroute would be able to display the
   overlay path i.e.

   traceroute to H4, 30 hops max, 60 byte packets
    1  R2
    2  R3
    3  H4

   The layer-transcending traceroute would show the combination of the
   underlay and overlay paths i.e.,

   traceroute to H4, 30 hops max, 60 byte packets
    1  VtepA
    2  R1
    3  VtepB
    4  R2
    5  R3
    6  H4












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   -----------             -------------------             -----------
   |    H1   |             |       R5        |             |    H6   |
   | 1.0.1.1 |             |                 |             |         |
   |         |             | 1.0.1.2 1.0.5.5 |             | 1.0.5.6 |
   -----------             |-----------------|             -----------
        |                  |    |       |    |                  |
        |                  |    |       |    |                  |
   ----------- ----------- |-----------------| ----------- -----------
   | VtepA   | |   R1    | |  VtepB    VtepC | |   R6    | |  VtepD  |
   | 2.0.1.1 |-| 2.0.1.2 | | 2.0.2.1 3.0.1.1 |-| 3.0.1.2 | |         |
   |         | | 2.0.2.2 |-|                 | | 3.0.2.2 |-| 3.0.3.1 |
   ----------- ----------- ------------------- ----------- -----------

                       Multiple L2 overlays in path

   The figure above has multiple overlay network segments, that are
   connected in one router which provides the tunnel endpoints for both
   overlay segments plus routing for the overlay.  A more general
   picture would be to have an overlay routed path between the two NVEs
   e.g., VtepB and VtepC connected to different routers in the overlay.
   However, such a drawing in ASCII art doesn't fit on the page.

   An normal overlay traceroute in the above topology would show the
   overlay router i.e.,

   traceroute to H6, 30 hops max, 60 byte packets
    1  R5
    2  H6

   The layer-transcending traceroute would show the combination of the
   underlay and overlay paths i.e.,

   traceroute to H6, 30 hops max, 60 byte packets
    1  VtepA
    2  R1
    3  VtepB
    4  R5
    5  VtepC
    6  R6
    7  VtepD
    8  H6

   Note that the R3 device, which include VtepB and VtepC, appears as
   three hops in the traceroute output.  That is needed to be able to
   correlate the output with the overlay output which has R3.  That
   correlation would be hard if the R3 device only appeared as VtepB in
   the LTTON output.  The three-hop representation also stays invariant
   whether or not the NVEs and overlay router are implemented by a



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   single device or multiple devices.


6.  Controlling and selecting ttl behavior

   The network admin needs to be able to control who can use the layer
   transcending traceroute, since the operator might not want to
   disclose the underlay topology to all its users all the time.  There
   are different approaches for this such as designating particular
   ports (Virtual Access Points in NVO3 terminology) on a NVE to have
   uniform ttl tunnel model.  We have found it useful to be able to
   enable this capability on a per port and/or virtual network basis, in
   addition to having a global setting per NVE.

   When enabled on the NVEs the user on the TS needs to be able to
   control which traffic is subject to which tunnel mode.  The normal
   traffic would use the pipe ttl tunnel model and only explicit trace
   applications are likely to want to use the uniform ttl tunnel model.
   Hence it makes sense to use some marker in the packets sent by the TS
   to select those packets for uniform model on the NVE.  Such a
   mechanism should usable so that the user can perform both a regular
   traceroute and a LTTON.

   Potentially different fields in the packets originated by traceroute
   on the TS can be used to mark the packets for uniform ttl tunnel
   model.  However, many of those fields such as source and destination
   port numbers and protocol might be used in hashing for ECMP.  The
   marking that can be used without impacting ECMP is the DSCP field in
   the packet.  That field can be set with an option (--tos) in at least
   some existing traceroute implementations.

   Note that when DSCP is used for such marking it is a configured
   choice subject to agreement between the operator of the TS and NVE.
   The matching on the NVE should ignore the ECN bits as to not
   interfere with ECN.

   However, the DSCP value used in the overlay might have an impact on
   the forwarding of the packets.  In such a case one can use an
   alternative selector such as the UDP source port number.  That has
   the downside of affecting the has values used for ECMP and link
   aggregation port selection.


7.  Introducing a ttl copyin flag in the encapsulation header

   When this approach is applied to VXLAN [RFC7348] the decapsulating
   NVE has to be able to identify packets that have to be processed in
   the uniform ttl tunnel model way.  For that purpose we define a new



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   flag which is sent by the encapsulating NVE on selected packets, and
   is used by the decapsulating NVE to perform the ttl copyin, decrement
   and check.

   In addition to the one I-flag defined in [RFC7348] we define a new
   T-flag to capture this the trace behavior at the decapsulating tunnel
   endpoint.

      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |R|R|R|R|I|R|R|T|            Reserved                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                VXLAN Network Identifier (VNI) |   Reserved    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   New fields:

   T-flag:        When set indicates that decapsulator should take the
                  outer ttl and copy it to the inner ttl, and then check
                  and decrement the resulting ttl.


8.  Encapsulation Behavior

   If the uniform ttl model is enabled for the input, and the received
   naked packet matches the selector, then the ingress NVE will perform
   these additional operations as part of encapsulating an IPv4 or IPv6
   packet:

   o  Examine the IPv4 TTL (or IPv6 hopcount, respectively) on receipt
      and if 1 or less, then drop the packet and send an ICMPv4 (or
      ICMPv6) ttl exceeded back to the original host.  Since the NVE is
      operating on a L2 packet, it might not have any layer 3 interfaces
      or routes for the originating host.  Thus it sends the packet back
      to the source L2 address of the packet back out the ingress port -
      without any IP address lookup.

   o  If ttl did not expire, then decrement the above ttl/hopcount and
      place it in the outer IP header.  Encapsulate and send the packet
      as normal.

   o  If some other errors prevent sending the packet (such as unknown
      VN Context Id, no flood list configured), then the NVE SHOULD send
      an ICMP host unreachable back to the host.

   The ingress NVE will receive ICMP errors from underlay routers and
   the egress NVE; whether due to ttl exceeded or underlay issues such



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   as host unreachable, or packet too big errors.  The NVE should take
   such errors, and in addition to any local syslog etc, generate an
   ICMP error sent back to the host.  The principle for this is
   specified in [RFC1933] and [RFC2473].  Just like in those
   specifications, for the inner and outer IP header could be off
   different version.  A common case of that might be an IPv6 overlay
   with an IPv4 underlay.  That case requires some changes in the ICMP
   type and code values in addition to recreating the packets.  The
   place where LTTON differs from those specifications is that there is
   an NVO3 header and (for L2 over L3) and L2 header in the packet.

   The figures below show an example of ICMP header re-generation at
   VtepA for the case of IPv6 overlay with IPv4 underlay.  The case of
   IPv4 over IPv4 is similar and simpler since the ICMP header is the
   same for both overlay and underlay.  The example uses VXLAN
   encapsulation to provide the concrete details, but the approach
   applies to other NVO3 proposals.


































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                +--------------+
                | IPv4 Header  |
                | src = R1     |
                | dst = VtepA  |
                +--------------+
                |    ICMPv4    |
                |    Header    |
                |   type = X   |
                |   code = Y   |
         - -    +--------------+
                | IPv4 Header  |
                | src = VtepA  |
        IPv4    | dst = VtepB  |
                +--------------+
       Packet   |     UDP      |
                | dst = VXLAN  |
         in     +--------------+
                |   Ethernet   |
       Error    | DA = H2 mac  |
                | SA = H1 mac  |
                +--------------+   - -
                |    IPv6      |
                | src = H1 ipv6|
                | dst = H2 ipv6|   Original IPv6
                +--------------+   Packet.
                |  Transport   |   Used to
                |    Header    |   generate an
                +--------------+   ICMPv6
                |              |   error message
                ~     Data     ~   back to the source.
                |              |
         - -    +--------------+   - -

            ICMPv4 Error Message Returned to Encapsulating Node

   The above underlay ICMPv4 is used to form an overlay ICMPv6 packet by
   extracting the Ethernet DA from the inner Ethernet SA, and forming an
   IPv6 header where the source address is based on the source address
   of the ICMPv4 error.  The ICMPv6 type and code values are set based
   on the ICMPv4 type and code values.











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                +--------------+
                |   Ethernet   |
                | DA = H1 mac  |   From ICMPv4 packet
                | SA = VtepA   |   in error
                +--------------+
                | IPv6 Header  |
                | src = ::R1   |   96 zeros + IPv4 address
                | dst = H1 ipv6|
                +--------------+
                |    ICMPv6    |
                |    Header    |
                |   type = X'  |   Type and code mapped
                |   code = Y'  |   from v4 to v6 values
         - -    +--------------+   - -
                |    IPv6      |
        IPv6    | src = H1 ipv6|
                | dst = H2 ipv6|   Unmodified from
       Packet   +--------------+   ICMPv4 error
                |  Transport   |
         in     |    Header    |
                +--------------+
       Error    |              |
                ~     Data     ~
                |              |
         - -    +--------------+   - -

             Generated ICMPv6 Error Message for Overlay Source

   In the case of IPv6 over IPv4 the above example setting of the IPv6
   source address results in this type of traceroute output:

   traceroute to 2000:0:0:40::2, 30 hops max, 80 byte packets
    1  ::2.0.1.1 (::2.0.1.1)  1.231 ms  1.004 ms  1.126 ms
    2  ::2.0.1.2 (::2.0.1.2)  1.994 ms  2.301 ms  2.016 ms
    3  ::2.0.2.1 (::2.0.2.1)  18.846 ms  30.582 ms  19.776 ms
    4  2000:0:0:40::2 (2000:0:0:40::2)  48.964 ms  60.131 ms  53.895 ms


9.  Decapsulating Behavior

   If this uniform ttl model is enabled on the decapsulating NVE, and
   the overlay header indicates that uniform ttl model applies (the
   T-bit in the case of VXLAN), then the NVE will perform these
   additional operations as part of decapsulating a packet where the
   inner packet is an IPv4 or IPv6 packet:

   o  Examine the outer IPv4 TTL (or outer IPv6 hopcount, respectively)
      on receipt and if 1 or less, then drop the packet and send an



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      outer ICMPv4 (or ICMPv6) ttl exceeded back to the source of the
      outer packet i.e., the ingress NVE.  This ICMP packet should look
      the same as an ICMP error generated by an underlay router, and the
      requirement in [RFC1812] on the size of the packet in error
      applies.

   o  If ttl did not expire, then decrement the above ttl/hopcount and
      place it in the inner IP header.  If the inner IP header is IPv4
      then update the IPv4 header checksum.  Then decapsulate and send
      the packet as for other decapsulated packets.

   o  If some other errors prevent sending the packet (such as unknown
      VN Context Id), then the NVE SHOULD send an ICMP host unreachable
      instead of a ttl exceeded error.


10.  Other ICMP errors

   The technique for selecting ttl behavior specified in this draft can
   also be used to trigger other ICMPv4 and ICMPv6 errors.  For example,
   [RFC1933] specifies how ICMP packet too big from underlay routers can
   be used to report over ICMP packet too big errors to the original
   source.  Other errors that are more specific to the overlay protocol
   might also be useful, such as not being able to find a VNI ID for the
   incoming port,vlan, or not being able to flood the packet if the
   packet is a Broadcast, Unknown unicast, or Multicast packet.


11.  Security Considerations

   The considerations in [I-D.ietf-nvo3-security-requirements] apply.

   In addition, the use of the uniform ttl tunnel model will result in
   ICMP errors being generated by underlay routers and consumed by NVEs.
   That presents an attack vector which does not exist in a pipe ttl
   tunnel model.  However, ICMP errors should be rate limited [RFC1812].
   Implementations should also take appropriate measures in rate
   limiting the input rate for ICMP errors that are processed by limited
   CPU resources.

   Some implementations might handle the trace packets (with uniform ttl
   model) in software while the pipe ttl model packets can be handled in
   hardware.  In such a case the implementation should have mechanisms
   to avoid starvation of limited CPU resources due to these packets.







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12.  IANA Considerations

   TBD


13.  Acknowledgements

   The authors acknowledge the helpful comments from David Black and
   Diego Garcia del Rio.


14.  References

14.1.  Normative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <http://www.rfc-editor.org/info/rfc792>.

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,
              <http://www.rfc-editor.org/info/rfc1812>.

   [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>.

   [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, DOI 10.17487/RFC7348, August 2014,
              <http://www.rfc-editor.org/info/rfc7348>.

   [RFC7365]  Lasserre, M., Balus, F., Morin, T., Bitar, N., and Y.
              Rekhter, "Framework for Data Center (DC) Network
              Virtualization", RFC 7365, DOI 10.17487/RFC7365,
              October 2014, <http://www.rfc-editor.org/info/rfc7365>.

14.2.  Informative References

   [I-D.gross-geneve]
              Gross, J., Sridhar, T., Garg, P., Wright, C., Ganga, I.,
              Agarwal, P., Duda, K., Dutt, D., and J. Hudson, "Geneve:
              Generic Network Virtualization Encapsulation",
              draft-gross-geneve-02 (work in progress), October 2014.




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   [I-D.herbert-gue]
              Herbert, T., Yong, L., and O. Zia, "Generic UDP
              Encapsulation", draft-herbert-gue-03 (work in progress),
              March 2015.

   [I-D.ietf-nvo3-security-requirements]
              Hartman, S., Zhang, D., Wasserman, M., Qiang, Z., and M.
              Zhang, "Security Requirements of NVO3",
              draft-ietf-nvo3-security-requirements-05 (work in
              progress), June 2015.

   [I-D.sridharan-virtualization-nvgre]
              Garg, P. and Y. Wang, "NVGRE: Network Virtualization using
              Generic Routing Encapsulation",
              draft-sridharan-virtualization-nvgre-08 (work in
              progress), April 2015.

   [I-D.tissa-lime-yang-oam-model]
              Senevirathne, T., Finn, N., Kumar, D., Salam, S., Wu, Q.,
              and Z. Wang, "Generic YANG Data Model for Operations,
              Administration, and Maintenance (OAM)",
              draft-tissa-lime-yang-oam-model-06 (work in progress),
              August 2015.

   [RFC1933]  Gilligan, R. and E. Nordmark, "Transition Mechanisms for
              IPv6 Hosts and Routers", RFC 1933, DOI 10.17487/RFC1933,
              April 1996, <http://www.rfc-editor.org/info/rfc1933>.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <http://www.rfc-editor.org/info/rfc2473>.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,
              <http://www.rfc-editor.org/info/rfc2983>.

   [RFC3270]  Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
              P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
              Protocol Label Switching (MPLS) Support of Differentiated
              Services", RFC 3270, DOI 10.17487/RFC3270, May 2002,
              <http://www.rfc-editor.org/info/rfc3270>.

   [RFC3443]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
              in Multi-Protocol Label Switching (MPLS) Networks",
              RFC 3443, DOI 10.17487/RFC3443, January 2003,
              <http://www.rfc-editor.org/info/rfc3443>.

   [RFC4884]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro,



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              "Extended ICMP to Support Multi-Part Messages", RFC 4884,
              DOI 10.17487/RFC4884, April 2007,
              <http://www.rfc-editor.org/info/rfc4884>.

   [RFC4950]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro, "ICMP
              Extensions for Multiprotocol Label Switching", RFC 4950,
              DOI 10.17487/RFC4950, August 2007,
              <http://www.rfc-editor.org/info/rfc4950>.


Authors' Addresses

   Erik Nordmark
   Arista Networks
   Santa Clara, CA
   USA

   Email: nordmark@arista.com


   Chandra Appanna
   Arista Networks
   Santa Clara, CA
   USA

   Email: achandra@arista.com


   Alton Lo
   Arista Networks
   Santa Clara, CA
   USA

   Email: altonlo@arista.com

















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