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INFORMATIONAL
Internet Engineering Task Force (IETF)                             X. XuRequest for Comments: 7814                           Huawei TechnologiesCategory: Informational                                     C. JacquenetISSN: 2070-1721                                                   Orange                                                               R. Raszuk                                                                T. Boyes                                                            Bloomberg LP                                                                  B. Fee                                                        Extreme Networks                                                              March 2016Virtual Subnet: A BGP/MPLS IP VPN-Based Subnet Extension SolutionAbstract   This document describes a BGP/MPLS IP VPN-based subnet extension   solution referred to as "Virtual Subnet", which can be used for   building Layer 3 network virtualization overlays within and/or   between data centers.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7814.Xu, et al.                    Informational                     [Page 1]

RFC 7814                     Virtual Subnet                   March 2016Copyright Notice   Copyright (c) 2016 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .32.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .43.  Solution Description  . . . . . . . . . . . . . . . . . . . .53.1.  Unicast . . . . . . . . . . . . . . . . . . . . . . . . .53.1.1.  Intra-subnet Unicast  . . . . . . . . . . . . . . . .53.1.2.  Inter-subnet Unicast  . . . . . . . . . . . . . . . .63.2.  Multicast . . . . . . . . . . . . . . . . . . . . . . . .83.3.  Host Discovery  . . . . . . . . . . . . . . . . . . . . .93.4.  ARP/ND Proxy  . . . . . . . . . . . . . . . . . . . . . .93.5.  Host Mobility . . . . . . . . . . . . . . . . . . . . . .93.6.  Forwarding Table Scalability on Data-Center Switches  . .103.7.  ARP/ND Cache Table Scalability on Default Gateways  . . .103.8.  ARP/ND and Unknown Unicast Flood Avoidance  . . . . . . .103.9.  Path Optimization . . . . . . . . . . . . . . . . . . . .104.  Limitations . . . . . . . . . . . . . . . . . . . . . . . . .114.1.  Non-support of Non-IP Traffic . . . . . . . . . . . . . .114.2.  Non-support of IP Broadcast and Link-Local Multicast  . .114.3.  TTL and Traceroute  . . . . . . . . . . . . . . . . . . .115.  Security Considerations . . . . . . . . . . . . . . . . . . .126.  References  . . . . . . . . . . . . . . . . . . . . . . . . .126.1.  Normative References  . . . . . . . . . . . . . . . . . .126.2.  Informative References  . . . . . . . . . . . . . . . . .13   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .14   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .15Xu, et al.                    Informational                     [Page 2]

RFC 7814                     Virtual Subnet                   March 20161.  Introduction   For business continuity purposes, Virtual Machine (VM) migration   across data centers is commonly used in situations such as data-   center maintenance, migration, consolidation, expansion, or disaster   avoidance.  The IETF community has recognized that IP renumbering of   servers (i.e., VMs) after the migration is usually complex and   costly.  To allow the migration of a VM from one data center to   another without IP renumbering, the subnet on which the VM resides   needs to be extended across these data centers.   To achieve subnet extension across multiple cloud data centers in a   scalable way, the following requirements and challenges must be   considered:   a.  VPN Instance Space Scalability: In a modern cloud data-center       environment, thousands or even tens of thousands of tenants could       be hosted over a shared network infrastructure.  For security and       performance isolation purposes, these tenants need to be isolated       from one another.   b.  Forwarding Table Scalability: With the development of server       virtualization technologies, it's not uncommon for a single cloud       data center to contain millions of VMs.  This number already       implies a big challenge to the forwarding table scalability of       data-center switches.  Provided multiple data centers of such       scale were interconnected at Layer 2, this challenge would become       even worse.   c.  ARP/ND Cache Table Scalability: [RFC6820] notes that the Address       Resolution Protocol (ARP) / Neighbor Discovery (ND) cache tables       maintained by default gateways within cloud data centers can       raise scalability issues.  Therefore, mastering the size of the       ARP/ND cache tables is critical as the number of data centers to       be connected increases.   d.  ARP/ND and Unknown Unicast Flooding: It's well-known that the       flooding of ARP/ND broadcast/multicast messages as well as       unknown unicast traffic within large Layer 2 networks is likely       to affect network and host performance.  When multiple data       centers that each host millions of VMs are interconnected at       Layer 2, the impact of such flooding would become even worse.  As       such, it becomes increasingly important to avoid the flooding of       ARP/ND broadcast/multicast as well as unknown unicast traffic       across data centers.Xu, et al.                    Informational                     [Page 3]

RFC 7814                     Virtual Subnet                   March 2016   e.  Path Optimization: A subnet usually indicates a location in the       network.  However, when a subnet has been extended across       multiple geographically dispersed data-center locations, the       location semantics of such a subnet is not retained any longer.       As a result, traffic exchanged between a specific user and a       server that would be located in different data centers may first       be forwarded through a third data center.  This suboptimal       routing would obviously result in unnecessary consumption of the       bandwidth resources between data centers.  Furthermore, in the       case where traditional Virtual Private LAN Service (VPLS)       technology [RFC4761] [RFC4762] is used for data-center       interconnect, return traffic from a server may be forwarded to a       default gateway located in a different data center due to the       configuration of a virtual router redundancy group.  This       suboptimal routing would also unnecessarily consume the bandwidth       resources between data centers.   This document describes a BGP/MPLS IP VPN-based subnet extension   solution referred to as "Virtual Subnet", which can be used for data-   center interconnection while addressing all of the aforementioned   requirements and challenges.  Here, the BGP/MPLS IP VPN means both   BGP/MPLS IPv4 VPN [RFC4364] and BGP/MPLS IPv6 VPN [RFC4659].  In   addition, since Virtual Subnet is built mainly on proven technologies   such as BGP/MPLS IP VPN and ARP/ND proxy [RFC925] [RFC1027]   [RFC4389], those service providers that provide Infrastructure as a   Service (IaaS) cloud services can rely upon their existing BGP/MPLS   IP VPN infrastructure and take advantage of their BGP/MPLS VPN   operational experience to interconnect data centers.   Although Virtual Subnet is described in this document as an approach   for data-center interconnection, it can be used within data centers   as well.   Note that the approach described in this document is not intended to   achieve an exact emulation of Layer 2 connectivity, and therefore it   can only support a restricted Layer 2 connectivity service model with   limitations that are discussed inSection 4.  The discussion about   where this service model can apply is outside the scope of this   document.2.  Terminology   This memo makes use of the terms defined in [RFC4364].Xu, et al.                    Informational                     [Page 4]

RFC 7814                     Virtual Subnet                   March 20163.  Solution Description3.1.  Unicast3.1.1.  Intra-subnet Unicast                           +--------------------+    +------------------+   |                    |   +------------------+    |VPN_A:192.0.2.1/24|   |                    |   |VPN_A:192.0.2.1/24|    |              \   |   |                    |   |  /               |    |    +------+   \ ++---+-+                +-+---++/   +------+     |    |    |Host A+-----+ PE-1 |                | PE-2 +----+Host B|     |    |    +------+\    ++-+-+-+                +-+-+-++   /+------+     |    |     192.0.2.2/24 | | |                    | | |  192.0.2.3/24    |    |                  | | |                    | | |                  |    |     DC West      | | |  IP/MPLS Backbone  | | |     DC East      |    +------------------+ | |                    | | +------------------+                         | +--------------------+ |                         |                        |VRF_A :                  V                VRF_A : V+------------+---------+--------+      +------------+---------+--------+|   Prefix   |Next hop |Protocol|      |   Prefix   |Next hop |Protocol|+------------+---------+--------+      +------------+---------+--------+|192.0.2.1/32|127.0.0.1| Direct |      |192.0.2.1/32|127.0.0.1| Direct |+------------+---------+--------+      +------------+---------+--------+|192.0.2.2/32|192.0.2.2| Direct |      |192.0.2.2/32|   PE-1  |  IBGP  |+------------+---------+--------+      +------------+---------+--------+|192.0.2.3/32|   PE-2  |  IBGP  |      |192.0.2.3/32|192.0.2.3| Direct |+------------+---------+--------+      +------------+---------+--------+|192.0.2.0/24|192.0.2.1| Direct |      |192.0.2.0/24|192.0.2.1| Direct |+------------+---------+--------+      +------------+---------+--------+                  Figure 1: Intra-subnet Unicast Example   As shown in Figure 1, two hosts (i.e., Hosts A and B) belonging to   the same subnet (i.e., 192.0.2.0/24) are located in different data   centers (i.e., DC West and DC East), respectively.  PE routers (i.e.,   PE-1 and PE-2) that are used for interconnecting these two data   centers create host routes for their own local hosts respectively and   then advertise these routes by means of the BGP/MPLS IP VPN   signaling.  Meanwhile, an ARP proxy is enabled on Virtual Routing and   Forwarding (VRF) attachment circuits of these PE routers.   Let's now assume that Host A sends an ARP request for Host B before   communicating with Host B.  Upon receiving the ARP request, PE-1   acting as an ARP proxy returns its own MAC address as a response.   Host A then sends IP packets for Host B to PE-1.  PE-1 tunnels such   packets towards PE-2, which in turn forwards them to Host B.  Thus,Xu, et al.                    Informational                     [Page 5]

RFC 7814                     Virtual Subnet                   March 2016   Hosts A and B can communicate with each other as if they were located   within the same subnet.3.1.2.  Inter-subnet Unicast                           +--------------------+    +------------------+   |                    |   +------------------+    |VPN_A:192.0.2.1/24|   |                    |   |VPN_A:192.0.2.1/24|    |              \   |   |                    |   |  /               |    |  +------+     \ ++---+-+                +-+---++/     +------+   |    |  |Host A+-------+ PE-1 |                | PE-2 +-+----+Host B|   |    |  +------+\      ++-+-+-+                +-+-+-++ |   /+------+   |    |   192.0.2.2/24   | | |                    | | |  | 192.0.2.3/24  |    |   GW=192.0.2.4   | | |                    | | |  | GW=192.0.2.4  |    |                  | | |                    | | |  |    +------+   |    |                  | | |                    | | |  +----+  GW  +-- |    |                  | | |                    | | |      /+------+   |    |                  | | |                    | | |    192.0.2.4/24  |    |                  | | |                    | | |                  |    |     DC West      | | |  IP/MPLS Backbone  | | |      DC East     |    +------------------+ | |                    | | +------------------+                        | +--------------------+ |                        |                        |VRF_A :                 V                VRF_A : V+------------+---------+--------+      +------------+---------+--------+|   Prefix   |Next hop |Protocol|      |   Prefix   |Next hop |Protocol|+------------+---------+--------+      +------------+---------+--------+|192.0.2.1/32|127.0.0.1| Direct |      |192.0.2.1/32|127.0.0.1| Direct |+------------+---------+--------+      +------------+---------+--------+|192.0.2.2/32|192.0.2.2| Direct |      |192.0.2.2/32|  PE-1   |  IBGP  |+------------+---------+--------+      +------------+---------+--------+|192.0.2.3/32|   PE-2  |  IBGP  |      |192.0.2.3/32|192.0.2.3| Direct |+------------+---------+--------+      +------------+---------+--------+|192.0.2.4/32|   PE-2  |  IBGP  |      |192.0.2.4/32|192.0.2.4| Direct |+------------+---------+--------+      +------------+---------+--------+|192.0.2.0/24|192.0.2.1| Direct |      |192.0.2.0/24|192.0.2.1| Direct |+------------+---------+--------+      +------------+---------+--------+| 0.0.0.0/0  |   PE-2  |  IBGP  |      | 0.0.0.0/0  |192.0.2.4| Static |+------------+---------+--------+      +------------+---------+--------+                Figure 2: Inter-subnet Unicast Example (1)   As shown in Figure 2, only one data center (i.e., DC East) is   deployed with a default gateway (i.e., GW).  PE-2, which is connected   to GW, would either be configured with or have learned a default   route from GW with the next hop being pointed at GW.  Meanwhile, this   route is distributed to other PE routers (i.e., PE-1) as per normal   operation as described in [RFC4364].  Assume Host A sends an ARPXu, et al.                    Informational                     [Page 6]

RFC 7814                     Virtual Subnet                   March 2016   request for its default gateway (i.e., 192.0.2.4) prior to   communicating with a destination host outside of its subnet.  Upon   receiving this ARP request, PE-1 acting as an ARP proxy returns its   own MAC address as a response.  Host A then sends a packet for Host B   to PE-1.  PE-1 tunnels such a packet towards PE-2 according to the   default route learned from PE-2, which in turn forwards that packet   to GW.                           +--------------------+    +------------------+   |                    |   +------------------+    |VPN_A:192.0.2.1/24|   |                    |   |VPN_A:192.0.2.1/24|    |              \   |   |                    |   |  /               |    |  +------+     \ ++---+-+                +-+---++/     +------+   |    |  |Host A+----+--+ PE-1 |                | PE-2 +-+----+Host B|   |    |  +------+\   |  ++-+-+-+                +-+-+-++ |   /+------+   |    |  192.0.2.2/24 |  | | |                    | | |  | 192.0.2.3/24  |    |  GW=192.0.2.4 |  | | |                    | | |  | GW=192.0.2.4  |    |  +------+    |   | | |                    | | |  |    +------+   |    |--+ GW-1 +----+   | | |                    | | |  +----+ GW-2 +-- |    |  +------+\       | | |                    | | |      /+------+   |    |  192.0.2.4/24    | | |                    | | |    192.0.2.4/24  |    |                  | | |                    | | |                  |    |     DC West      | | |  IP/MPLS Backbone  | | |      DC East     |    +------------------+ | |                    | | +------------------+                        | +--------------------+ |                        |                        |VRF_A :                 V                VRF_A : V+------------+---------+--------+      +------------+---------+--------+|   Prefix   |Next hop |Protocol|      |   Prefix   |Next hop |Protocol|+------------+---------+--------+      +------------+---------+--------+|192.0.2.1/32|127.0.0.1| Direct |      |192.0.2.1/32|127.0.0.1| Direct |+------------+---------+--------+      +------------+---------+--------+|192.0.2.2/32|192.0.2.2| Direct |      |192.0.2.2/32|  PE-1   |  IBGP  |+------------+---------+--------+      +------------+---------+--------+|192.0.2.3/32|   PE-2  |  IBGP  |      |192.0.2.3/32|192.0.2.3| Direct |+------------+---------+--------+      +------------+---------+--------+|192.0.2.4/32|192.0.2.4| Direct |      |192.0.2.4/32|192.0.2.4| Direct |+------------+---------+--------+      +------------+---------+--------+|192.0.2.0/24|192.0.2.1| Direct |      |192.0.2.0/24|192.0.2.1| Direct |+------------+---------+--------+      +------------+---------+--------+| 0.0.0.0/0  |192.0.2.4| Static |      | 0.0.0.0/0  |192.0.2.4| Static |+------------+---------+--------+      +------------+---------+--------+                Figure 3: Inter-subnet Unicast Example (2)   As shown in Figure 3, in the case where each data center is deployed   with a default gateway, hosts will get ARP responses directly from   their local default gateways, rather than from their local PE routers   when sending ARP requests for their default gateways.Xu, et al.                    Informational                     [Page 7]

RFC 7814                     Virtual Subnet                   March 2016                                  +------+                           +------+ PE-3 +------+    +------------------+   |      +------+      |   +------------------+    |VPN_A:192.0.2.1/24|   |                    |   |VPN_A:192.0.2.1/24|    |              \   |   |                    |   |  /               |    |  +------+     \ ++---+-+                +-+---++/     +------+   |    |  |Host A+-------+ PE-1 |                | PE-2 +------+Host B|   |    |  +------+\      ++-+-+-+                +-+-+-++     /+------+   |    |  192.0.2.2/24    | | |                    | | |    192.0.2.3/24  |    |  GW=192.0.2.1    | | |                    | | |    GW=192.0.2.1  |    |                  | | |                    | | |                  |    |     DC West      | | |  IP/MPLS Backbone  | | |      DC East     |    +------------------+ | |                    | | +------------------+                         | +--------------------+ |                         |                        |VRF_A :                  V                VRF_A : V+------------+---------+--------+      +------------+---------+--------+|   Prefix   |Next hop |Protocol|      |   Prefix   |Next hop |Protocol|+------------+---------+--------+      +------------+---------+--------+|192.0.2.1/32|127.0.0.1| Direct |      |192.0.2.1/32|127.0.0.1| Direct |+------------+---------+--------+      +------------+---------+--------+|192.0.2.2/32|192.0.2.2| Direct |      |192.0.2.2/32|  PE-1   |  IBGP  |+------------+---------+--------+      +------------+---------+--------+|192.0.2.3/32|   PE-2  |  IBGP  |      |192.0.2.3/32|192.0.2.3| Direct |+------------+---------+--------+      +------------+---------+--------+|192.0.2.0/24|192.0.2.1| Direct |      |192.0.2.0/24|192.0.2.1| Direct |+------------+---------+--------+      +------------+---------+--------+| 0.0.0.0/0  |   PE-3  |  IBGP  |      | 0.0.0.0/0  |   PE-3  |  IBGP  |+------------+---------+--------+      +------------+---------+--------+                Figure 4: Inter-subnet Unicast Example (3)   Alternatively, as shown in Figure 4, PE routers themselves could be   configured as default gateways for their locally connected hosts as   long as these PE routers have routes to reach outside networks.3.2.  Multicast   To support IP multicast between hosts of the same Virtual Subnet,   Multicast VPN (MVPN) technologies [RFC6513] could be used without any   change.  For example, PE routers attached to a given VPN join a   default provider multicast distribution tree that is dedicated to   that VPN.  Ingress PE routers, upon receiving multicast packets from   their local hosts, forward them towards remote PE routers through the   corresponding default provider multicast distribution tree.  Within   this context, the IP multicast doesn't include link-local multicast.Xu, et al.                    Informational                     [Page 8]

RFC 7814                     Virtual Subnet                   March 20163.3.  Host Discovery   PE routers should be able to dynamically discover their local hosts   and keep the list of these hosts up-to-date in a timely manner to   ensure the availability and accuracy of the corresponding host routes   originated from them.  PE routers could accomplish local host   discovery by some traditional host-discovery mechanisms using ARP or   ND protocols.3.4.  ARP/ND Proxy   Acting as an ARP or ND proxy, a PE router should only respond to an   ARP request or Neighbor Solicitation (NS) message for a target host   when it has a best route for that target host in the associated VRF   and the outgoing interface of that best route is different from the   one over which the ARP request or NS message is received.  In the   scenario where a given VPN site (i.e., a data center) is multihomed   to more than one PE router via an Ethernet switch or an Ethernet   network, the Virtual Router Redundancy Protocol (VRRP) [RFC5798] is   usually enabled on these PE routers.  In this case, only the PE   router being elected as the VRRP Master is allowed to perform the   ARP/ND proxy function.3.5.  Host Mobility   During the VM migration process, the PE router to which the moving VM   is now attached would create a host route for that host upon   receiving a notification message of VM attachment (e.g., a gratuitous   ARP or unsolicited NA message).  The PE router to which the moving VM   was previously attached would withdraw the corresponding host route   when noticing the detachment of that VM.  Meanwhile, the latter PE   router could optionally broadcast a gratuitous ARP or send an   unsolicited NA message on behalf of that host with the source MAC   address being one of its own.  In this way, the ARP/ND entry of this   host that moved and that has been cached on any local host would be   updated accordingly.  In the case where there is no explicit VM   detachment notification mechanism, the PE router could also use the   following trick to detect the VM detachment: upon learning a route   update for a local host from a remote PE router for the first time,   the PE router could immediately check whether that local host is   still attached to it by some means (e.g., ARP/ND PING and/or ICMP   PING).  It is important to ensure that the same MAC and IP are   associated to the default gateway active in each data center, as the   VM would most likely continue to send packets to the same default   gateway address after having migrated from one data center to   another.  One possible way to achieve this goal is to configure theXu, et al.                    Informational                     [Page 9]

RFC 7814                     Virtual Subnet                   March 2016   same VRRP group on each location to ensure that the default gateway   active in each data center shares the same virtual MAC and virtual IP   addresses.3.6.  Forwarding Table Scalability on Data-Center Switches   In a Virtual Subnet environment, the MAC learning domain associated   with a given Virtual Subnet that has been extended across multiple   data centers is partitioned into segments, and each segment is   confined within a single data center.  Therefore, data-center   switches only need to learn local MAC addresses, rather than learning   both local and remote MAC addresses.3.7.  ARP/ND Cache Table Scalability on Default Gateways   When default gateway functions are implemented on PE routers as shown   in Figure 4, the ARP/ND cache table on each PE router only needs to   contain ARP/ND entries of local hosts.  As a result, the ARP/ND cache   table size would not grow as the number of data centers to be   connected increases.3.8.  ARP/ND and Unknown Unicast Flood Avoidance   In a Virtual Subnet environment, the flooding domain associated with   a given Virtual Subnet that was extended across multiple data   centers, is partitioned into segments and each segment is confined   within a single data center.  Therefore, the performance impact on   networks and servers imposed by the flooding of ARP/ND broadcast/   multicast and unknown unicast traffic is minimized.3.9.  Path Optimization   As shown in Figure 4, to optimize the forwarding path for the traffic   between cloud users and cloud data centers, PE routers located in   cloud data centers (i.e., PE-1 and PE-2), which are also acting as   default gateways, propagate host routes for their own local hosts to   remote PE routers that are attached to cloud user sites (i.e., PE-3).   As such, traffic from cloud user sites to a given server on the   Virtual Subnet that has been extended across data centers would be   forwarded directly to the data-center location where that server   resides, since traffic is now forwarded according to the host route   for that server, rather than the subnet route.  Furthermore, for   traffic coming from cloud data centers and forwarded to cloud user   sites, each PE router acting as a default gateway would forward   traffic according to the longest-match route in the corresponding   VRF.  As a result, traffic from data centers to cloud user sites is   forwarded along an optimal path as well.Xu, et al.                    Informational                    [Page 10]

RFC 7814                     Virtual Subnet                   March 20164.  Limitations4.1.  Non-support of Non-IP Traffic   Although most traffic within and across data centers is IP traffic,   there may still be a few legacy clustering applications that rely on   non-IP communications (e.g., heartbeat messages between cluster   nodes).  Since Virtual Subnet is strictly based on L3 forwarding,   those non-IP communications cannot be supported in the Virtual Subnet   solution.  In order to support those few non-IP traffic (if present)   in the environment where the Virtual Subnet solution has been   deployed, the approach following the idea of "route all IP traffic,   bridge non-IP traffic" could be considered.  In other words, all IP   traffic including both intra- and inter-subnet, would be processed   according to the Virtual Subnet design, while non-IP traffic would be   forwarded according to a particular Layer 2 VPN approach.  Such a   unified L2/L3 VPN approach requires ingress PE routers to classify   packets received from hosts before distributing them to the   corresponding L2 or L3 VPN forwarding processes.  Note that more and   more cluster vendors are offering clustering applications based on   Layer 3 interconnection.4.2.  Non-support of IP Broadcast and Link-Local Multicast   As illustrated before, intra-subnet traffic across PE routers is   forwarded at Layer 3 in the Virtual Subnet solution.  Therefore, IP   broadcast and link-local multicast traffic cannot be forwarded across   PE routers in the Virtual Subnet solution.  In order to support the   IP broadcast and link-local multicast traffic in the environment   where the Virtual Subnet solution has been deployed, the unified L2/   L3 overlay approach as described inSection 4.1 could be considered   as well.  That is, IP broadcast and link-local multicast messages   would be forwarded at Layer 2 while routable IP traffic would be   processed according to the Virtual Subnet design.4.3.  TTL and Traceroute   As mentioned before, intra-subnet traffic is forwarded at Layer 3 in   the Virtual Subnet context.  Since it doesn't require any change to   the Time-To-Live (TTL) handling mechanism of the BGP/MPLS IP VPN,   when doing a traceroute operation on one host for another host   (assuming that these two hosts are within the same subnet but are   attached to different sites), the traceroute output would reflect the   fact that these two hosts within the same subnet are actually   connected via a Virtual Subnet, rather than a Layer 2 connection   since the PE routers to which those two hosts are connected would be   displayed in the traceroute output.  In addition, for any other   applications that generate intra-subnet traffic with TTL set to 1,Xu, et al.                    Informational                    [Page 11]

RFC 7814                     Virtual Subnet                   March 2016   these applications may not work properly in the Virtual Subnet   context, unless special TTL processing and loop-prevention mechanisms   for such context have been implemented.  Details about such special   TTL processing and loop-prevention mechanisms are outside the scope   of this document.5.  Security Considerations   Since the BGP/MPLS IP VPN signaling is reused without any change,   those security considerations as described in [RFC4364] are   applicable to this document.  Meanwhile, since security issues   associated with the NDP are inherited due to the use of NDP proxy,   those security considerations and recommendations as described in   [RFC6583] are applicable to this document as well.   Inter-data-center traffic often carries highly sensitive information   at higher layers that is not directly understood (parsed) within an   egress or ingress PE.  For example, migrating a VM will often mean   moving private keys and other sensitive configuration information.   For this reason, inter-data-center traffic should always be protected   for both confidentiality and integrity using a strong security   mechanism such as IPsec [RFC4301].  In the future, it may be feasible   to protect that traffic within the MPLS layer [MPLS-SEC] though at   the time of writing, the mechanism for that is not sufficiently   mature to recommend.  Exactly how such security mechanisms are   deployed will vary from case to case, so securing the inter-data-   center traffic may or may not involve deploying security mechanisms   on the ingress/egress PEs or further "inside" the data centers   concerned.  Note though that if security is not deployed on the   egress/ingress PEs, there is a substantial risk that some sensitive   traffic may be sent in the clear and will therefore be vulnerable to   pervasive monitoring [RFC7258] or other attacks.6.  References6.1.  Normative References   [RFC925]   Postel, J., "Multi-LAN address resolution",RFC 925,              DOI 10.17487/RFC0925, October 1984,              <http://www.rfc-editor.org/info/rfc925>.   [RFC1027]  Carl-Mitchell, S. and J. Quarterman, "Using ARP to              implement transparent subnet gateways",RFC 1027,              DOI 10.17487/RFC1027, October 1987,              <http://www.rfc-editor.org/info/rfc1027>.Xu, et al.                    Informational                    [Page 12]

RFC 7814                     Virtual Subnet                   March 2016   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private              Networks (VPNs)",RFC 4364, DOI 10.17487/RFC4364, February              2006, <http://www.rfc-editor.org/info/rfc4364>.   [RFC4389]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery              Proxies (ND Proxy)",RFC 4389, DOI 10.17487/RFC4389, April              2006, <http://www.rfc-editor.org/info/rfc4389>.6.2.  Informative References   [MPLS-SEC] Farrel, A. and S. Farrell, "Opportunistic Security in MPLS              Networks", Work in Progress,draft-ietf-mpls-opportunistic-encrypt-01, March 2016.   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the              Internet Protocol",RFC 4301, DOI 10.17487/RFC4301,              December 2005, <http://www.rfc-editor.org/info/rfc4301>.   [RFC4659]  De Clercq, J., Ooms, D., Carugi, M., and F. Le Faucheur,              "BGP-MPLS IP Virtual Private Network (VPN) Extension for              IPv6 VPN",RFC 4659, DOI 10.17487/RFC4659, September 2006,              <http://www.rfc-editor.org/info/rfc4659>.   [RFC4761]  Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private              LAN Service (VPLS) Using BGP for Auto-Discovery and              Signaling",RFC 4761, DOI 10.17487/RFC4761, January 2007,              <http://www.rfc-editor.org/info/rfc4761>.   [RFC4762]  Lasserre, M., Ed. and V. Kompella, Ed., "Virtual Private              LAN Service (VPLS) Using Label Distribution Protocol (LDP)              Signaling",RFC 4762, DOI 10.17487/RFC4762, January 2007,              <http://www.rfc-editor.org/info/rfc4762>.   [RFC5798]  Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP)              Version 3 for IPv4 and IPv6",RFC 5798,              DOI 10.17487/RFC5798, March 2010,              <http://www.rfc-editor.org/info/rfc5798>.   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/              BGP IP VPNs",RFC 6513, DOI 10.17487/RFC6513, February              2012, <http://www.rfc-editor.org/info/rfc6513>.   [RFC6583]  Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational              Neighbor Discovery Problems",RFC 6583,              DOI 10.17487/RFC6583, March 2012,              <http://www.rfc-editor.org/info/rfc6583>.Xu, et al.                    Informational                    [Page 13]

RFC 7814                     Virtual Subnet                   March 2016   [RFC6820]  Narten, T., Karir, M., and I. Foo, "Address Resolution              Problems in Large Data Center Networks",RFC 6820,              DOI 10.17487/RFC6820, January 2013,              <http://www.rfc-editor.org/info/rfc6820>.   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an              Attack",BCP 188,RFC 7258, DOI 10.17487/RFC7258, May              2014, <http://www.rfc-editor.org/info/rfc7258>.Acknowledgements   Thanks to Susan Hares, Yongbing Fan, Dino Farinacci, Himanshu Shah,   Nabil Bitar, Giles Heron, Ronald Bonica, Monique Morrow, Rajiv Asati,   Eric Osborne, Thomas Morin, Martin Vigoureux, Pedro Roque Marque, Joe   Touch, Wim Henderickx, Alia Atlas, and Stephen Farrell for their   valuable comments and suggestions on this document.  Thanks to Loa   Andersson for his WG LC review on this document.  Thanks to Alvaro   Retana for his AD review on this document.  Thanks to Ronald Bonica   for his RtgDir review.  Thanks to Donald Eastlake 3rd for his Sec-DIR   review of this document.  Thanks to Jouni Korhonen for the OPS-Dir   review of this document.  Thanks to Roni Even for the Gen-ART review   of this document.  Thanks to Sabrina Tanamal for the IANA review of   this document.Xu, et al.                    Informational                    [Page 14]

RFC 7814                     Virtual Subnet                   March 2016Authors' Addresses   Xiaohu Xu   Huawei Technologies   No.156 Beiqing Rd   Beijing  100095   China   Email: xuxiaohu@huawei.com   Christian Jacquenet   Orange   4 rue du Clos Courtel   Cesson-Sevigne,  35512   France   Email: christian.jacquenet@orange.com   Robert Raszuk   Bloomberg LP   731 Lexington Avenue   New York City, NY  10022   United States   Email: robert@raszuk.net   Truman Boyes   Bloomberg LP   Email: tboyes@bloomberg.net   Brendan Fee   Extreme Networks   Email: bfee@extremenetworks.comXu, et al.                    Informational                    [Page 15]

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