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Versions: (draft-touch-intarea-tunnels) 00 01 02 03 04 05 06 07 08 09

Internet Area WG                                               J. Touch
Internet Draft                                                  USC/ISI
Intended status: Informational                              M. Townsley
Updates: 4459                                                     Cisco
Expires: January 2017                                      July 6, 2016




                  IP Tunnels in the Internet Architecture
                     draft-ietf-intarea-tunnels-03.txt


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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   described in the Simplified BSD License.

Abstract

   This document discusses the role of IP tunnels in the Internet
   architecture, in which IP datagrams are carried as payloads in non-
   link layer protocols. It explains their relationship to existing
   protocol layers and the challenges in supporting IP tunneling based
   on the equivalence of tunnels to links.

Table of Contents


   1. Introduction...................................................3
   2. Conventions used in this document..............................6
      2.1. Key Words.................................................6
      2.2. Terminology...............................................6
   3. The Tunnel Model...............................................9
      3.1. What is a tunnel?........................................10
      3.2. View from the Outside....................................11
      3.3. View from the Inside.....................................12
      3.4. Location of the Ingress and Egress.......................12
      3.5. Implications of This Model...............................13
      3.6. Fragmentation............................................14
         3.6.1. Outer Fragmentation.................................14
         3.6.2. Inner Fragmentation.................................15
         3.6.3. The necessity of Outer Fragmentation................16
   4. IP Tunnel Requirements........................................16
      4.1. Minimum MTU Considerations...............................17
      4.2. Fragmentation............................................18
      4.3. MTU discovery............................................21
      4.4. IP ID exhaustion.........................................22
      4.5. Hop Count................................................23
      4.6. Signaling................................................24


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      4.7. Relationship of Header Fields............................26
      4.8. Congestion...............................................27
      4.9. Checksums................................................27
      4.10. Numbering...............................................27
      4.11. Multicast...............................................28
      4.12. Multipoint..............................................28
      4.13. NAT / Load Balancing....................................29
      4.14. Recursive tunnels.......................................29
   5. Observations (implications)...................................29
      5.1. Tunnel protocol designers................................29
      5.2. Tunnel implementers......................................30
      5.3. Tunnel operators.........................................30
      5.4. Diagnostics..............................................30
      5.5. For existing standards...................................31
         5.5.1. Generic UDP Encapsulation (GUE - IP in UDP in IP)...31
         5.5.2. Generic Packet Tunneling in IPv6....................31
         5.5.3. Geneve (NVO3).......................................32
         5.5.4. GRE (IP in GRE in IP)...............................33
         5.5.5. IP in IP / mobile IP................................33
         5.5.6. IPsec tunnel mode (IP in IPsec in IP)...............35
         5.5.7. L2TP................................................36
         5.5.8. L2VPN...............................................36
         5.5.9. L3VPN...............................................36
         5.5.10. LISP...............................................36
         5.5.11. MPLS...............................................37
         5.5.12. PWE................................................37
         5.5.13. SEAL/AERO..........................................37
         5.5.14. TRILL..............................................37
         5.5.15. RTG DT encapsulations..............................38
      5.6. For future standards.....................................38
   6. Security Considerations.......................................39
   7. IANA Considerations...........................................40
   8. References....................................................40
      8.1. Normative References.....................................40
      8.2. Informative References...................................40
   9. Acknowledgments...............................................44
   APPENDIX A: Fragmentation efficiency.............................45
      A.1. Selecting fragment sizes.................................45
      A.2. Packing..................................................46

1. Introduction

   The Internet is loosely based on the ISO seven layer stack, in which
   data units traverse the stack by being wrapped inside data units one
   layer down. A tunnel is a mechanism for transmitting data units
   between endpoints by wrapping them as data units of the same or
   higher layers, e.g., IP in IP (Figure 1) or IP in UDP (Figure 2).


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                        +----+----+--------------+
                        | IP'| IP |     Data     |
                        +----+----+--------------+

                           Figure 1 IP inside IP

                     +----+-----+----+--------------+
                     | IP'| UDP | IP |     Data     |
                     +----+-----+----+--------------+

                   Figure 2 IP in UDP in IP in Ethernet

   This document focuses on tunnels that transit IP packets, i.e., in
   which an IP packet is the payload of another protocol. Tunnels
   provide a virtual link that can help decouple the network topology
   seen by transiting packets from the underlying physical network
   [To98][RFC2473]. Tunnels were critical in the development of
   multicast because not all routers were capable of processing
   multicast packets [Er94]. Tunnels allowed multicast packets to
   transit between multicast-capable routers over paths that did not
   support multicast. Similar techniques have been used to support other
   protocols, such as IPv6 [RFC2460].

   Use of tunnels is common in the Internet. The word "tunnel" occurs in
   over 100 RFCs, and is supported within numerous protocols, including:

   o  IP in IP / mobile IP - IPv4 in IPv4 tunnels
      [RFC2003][RFC2473][RFC5944]

   o  IP in IPv6 - IPv6 or IPv4 in IPv6 [RFC2473]

   o  IPsec - includes a tunnel mode to enable encryption or
      authentication of the an entire IP datagram [RFC4301]

   o  Generic Router Encapsulation (GRE) - a shim layer for tunneling
      any network layer in any other network layer, IP in GRE in IP
      [RFC2784][RFC7588][RFC7676]

   o  Generic UDP Encapsulation (GUE) - IP in UDP (in IP)[He15]

   o  Automatic Multicast Tunneling (AMT) - IP in UDP for multicast
      [RFC7450]

   o  L2TP - PPP over IP, to extend a subscriber's DSL/FTTH connection
      from an access line provider to an ISP [RFC3931]




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   o  L2VPNs - provides a link topology different from that provided by
      physical links [RFC4664]

   o  L3VPNs - provides a network topology different from that provided
      by ISPs [RFC4176]

   o  LISP - reduces routing table load within an enclave of routers at
      the expense of more complex ingress encapsulation tables [RFC6830]

   o  MPLS - IP over a circuit-like path in which identifiers are
      rewritten on each hop, often used for traffic provisioning
      [RFC3031]

   o  NVO3 - data center network sharing (which includes use of GUE,
      above) [RFC7364]

   o  PWE3 - emulates wire-like services over packet-switched services
      [RFC3985]

   o  SEAL/AERO -IP in IP tunneling with an additional shim header
      designed to overcome the limitations of RFC2003 [RFC5320][Te16]

   o  TRILL - enables L3 routing (typically IS-IS) in an enclave of
      Ethernet bridges [RFC5556][RFC6325]

   The variety of tunnel mechanisms raises the question of the role of
   tunnels in the Internet architecture and the potential need for these
   mechanisms to have similar and predictable behavior. In particular,
   the ways in which packet sizes (i.e., Maximum Transmission Unit or
   MTU) mismatch and error signals (e.g., ICMP) are handled may benefit
   from a coordinated approach.

   Regardless of the layer in which encapsulation occurs, tunnels
   emulate a link. The only difference is that a link operates over a
   physical communication channel, whereas a tunnel operates over
   software protocol layers. Because tunnels are links, they are subject
   to the same issues as any link, e.g., MTU discovery, signaling, and
   the potential utility of native support for broadcast and multicast
   [RFC2460][RFC3819]. They have advantages over native links, being
   potentially easier to reconfigure and control.

   The first attempt to use large-scale tunnels transit multicast across
   the Internet in 1988 lead to tunnel collapse. At the time, tunnels
   were not implemented as encapsulation-based virtual links, but rather
   as loose source routes on un-encapsulated IP datagrams [RFC1075].
   Using encapsulation tunnels instead avoided that collapse [Er94] and
   eventually to AMT [RFC7450].


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   The remainder of this document describes the general principles of IP
   tunneling and discusses the key considerations in the design of a
   protocol that tunnels IP datagrams. It derives its conclusions from
   the equivalence of tunnels and links. Note that all considerations
   are in the context of existing standards and requirements.

2. Conventions used in this document

2.1. Key Words

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

2.2. Terminology

   This document uses the following terminology. These definitions are
   given in the most general terms, but will be used primarily to
   discuss IP tunnels in this document. They are presented in order from
   most fundamental to those derived on earlier definitions:

   o  Messages: variable length data labeled with globally-unique
      endpoint IDs, also known as a datagram for IP messages [RFC791].

   o  Network node (node): a device that can act as an endpoint or
      forwarder. For datagrams (IP messages), these are hosts or
      gateways/routers, respectively.

   o  Endpoint or host: a node that sources or sinks messages labeled
      from/to its IDs, typically known as a host for both IP and higher-
      layer protocol messages [RFC1122].

   o  Forwarder: a node that relays messages using destination IDs and
      local context, also known as a gateway or router for IP messages
      [RFC1812]. Note that most forwarders also act as endpoints when
      they source or sink messages.

   o  Source (sender): the node that generates a message.

   o  Destination (receiver): the node that consumes a message.

   o  Link: a device (or medium) that transfers messages between nodes,
      i.e., by which a message can traverse between nodes without being
      processed by a forwarder. Note that the notion of forwarder is
      relative to the layer at which message processing is considered
      [To16].



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   o  Link interface (sometimes known as a network interface): a
      location on a link co-located with a node where messages depart
      onto that link or arrive from that link.

   o  Path: a sequence of one or more links or tunnels over which a
      message can traverse between nodes (hosts or forwarders), which
      may or may not involve being processed by a forwarder.

   o  Tunnel: a protocol mechanism that transits messages using
      encapsulation to allow a path to appear as a single link. Note
      that a protocol can be used to tunnel itself (IP over IP) and that
      this includes the conventional layering of the ISO stack (i.e., by
      this definition, Ethernet is a tunnel for IP). A tunnel can be
      considered a virtual link.

   o  Ingress: the virtual link interface of a tunnel which receives
      messages within a node, encapsulates them according to the tunnel
      protocol, and transmits them into the tunnel. This is the tunnel
      equivalent of the outgoing (departing) network interface of a
      link. Note that the ingress virtual link interface and traffic
      source node can be co-located.

   o  Egress: a virtual link interface that receives messages that have
      finished transiting a tunnel and presents them to a node. This is
      the tunnel equivalent of the incoming (arriving) network interface
      of a link. The egress decapsulates messages for further transit to
      the destination. Note that the egress virtual link interface and
      traffic destination node can be co-located.

   o  Tunnel transit packet (TTP): the packet arriving at a node
      connected to a tunnel that enters the ingress and exits the
      egress, i.e., the packet carried over the tunnel. This is
      sometimes known as the "tunneled packet", i.e., the packet carried
      over the tunnel. This is the tunnel equivalent of a network layer
      packet as it would traverse a link.

   o  Tunnel link packet (TLP): packets that traverse from ingress to
      egress, in which resides all or part of a tunnel transit packet.
      This is sometimes known as the "tunnel packet", i.e., the packet
      of the tunnel itself. This is the tunnel equivalent of a link
      layer packet as it would traverse a link.








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   o  Link MTU (LMTU): the largest message that can transit a link. It
      typically does not include link-layer information, e.g., link
      layer headers or trailers, i.e., it refers to the message that the
      link can carry rather than the message as it appears on the link.
      This is thus the largest network layer packet (including network
      layer headers, e.g., IP datagram) that can transit a link. Note
      that this need not be the native size of messages on the link,
      i.e., the link may internally fragment and reassemble messages.
      For IPv4, the smallest LMTU is 68 bytes [RFC791], and for IPv6 the
      smallest LMTU is 1280 bytes [RFC2460].

   o  Path MTU (PMTU): the largest message that can transit a path.
      Typically, this is the minimum of the link MTUs of the links of
      the path, and represents the largest network layer message
      (including network layer headers) that can transit a path. Note
      that this is not the largest network packet that can be sent
      between a source and destination; this is the largest network
      network packet that can be sent without requiring reassembly at
      the network layer of the destination.

   o  Reassembly MTU (RMTU): the largest message that can be reassembled
      by a destination, which is not directly related to the link or
      path MTU. Sometimes also referred to as "receiver MTU". For IPv4,
      this is 576 bytes [RFC793] and for IPv6 it is 1500 bytes
      [RFC2460]; note that in both cases, the size refers to the message
      transferred at the network layer, which includes the network layer
      headers.

   o  Tunnel MTU (TMTU): the largest message that can transit a tunnel,
      i.e., this is the tunnel equivalent of a link MTU. Typically, this
      is limited by the egress reassembly MTU. Note that this value may
      have no relation to the path MTU between the tunnel ingress and
      egress.

   o  Tunnel internal MTU (TIMTU): the largest message that a tunnel
      egress can emit into a tunnel without requiring further
      fragmentation to reach the tunnel egress. This the path MTU
      between the ingress and egress.

   o  Egress reassembly MTU (ERMTU): the largest message that can be
      reassembled by an egress. This is the size of the RMTU of a tunnel
      minus the encapsulation overhead of that tunnel. Sometimes also
      referred to as the "egress MTU".






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3. The Tunnel Model

   A network architecture is an abstract description of a distributed
   communications system, its components and their relationships, the
   requisite properties of those components and the emergent properties
   of the system that result [To03]. Such descriptions can help explain
   behavior, as when the OSI seven-layer model is used as a teaching
   example [Zi80]. Architectures describe capabilities - and, just as
   importantly, constraints.

   A network can be defined as a system of endpoints and relays
   interconnected by communication paths, abstracting away issues of
   naming in order to focus on message forwarding. To the extent that
   the Internet has a single, coherent interpretation, its architecture
   is defined by its core protocols (IP [RFC791], TCP [RFC793], UDP
   [RFC768]) and messages, hosts, routers, and links [Cl88][To03], as
   shown in Figure 3:

               +------+    ------      ------    +------+
               |      |   /      \    /      \   |      |
               | HOST |--+ ROUTER +--+ ROUTER +--| HOST |
               |      |   \      /    \      /   |      |
               +------+    ------      ------    +------+

                   Figure 3 Basic Internet architecture

   As a network architecture, the Internet is a system of hosts and
   routers interconnected by links that exchange messages when possible.
   "When possible" defines the Internet's "best effort" principle. The
   limited role of routers and links represents the End-to-End Principle
   [Sa84] and longest-prefix match enables hierarchical forwarding.

   Although the definitions of host, router, and link seem absolute,
   they are often relative as viewed within the context of one OSI
   layer, each of which can be considered a distinct network
   architecture. An Internet gateway is a Layer 3 router when it
   transits IP datagrams but it acts as a Layer 2 host as it sources or
   sinks Layer 2 messages on attached links to accomplish this transit
   capability. In this way, a single node (Internet gateway) behaves as
   different components (router, host) at different layers.

   Even though a single node may have multiple roles - even concurrently
   - at a given layer, each role is typically static and determined by
   context. An Internet gateway always acts as a Layer 2 host and that
   behavior does not depend on where the gateway is viewed from within
   Layer 2. In the context of a single layer, a node's behavior is
   modeled as a single component from all viewpoints in that layer.


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3.1. What is a tunnel?

   A tunnel can be modeled as a link in another network
   [To98][To01][To03]. In Figure 4, a source host (Hsrc) and destination
   host (Hdst) communicating over a network M in which two routers (Ra
   and Rd) are connected by a tunnel. Keep in mind that it is possible
   that both network N and network M can both be components of the
   Internet, i.e., there may be regular traffic as well as tunneled
   traffic over any of the routers shown.

                     --_                         --
         +------+   /  \                        /  \   +------+
         | Hsrc |--+ Ra +      --      --      + Rd +--| Hdst |
         +------+   \  //\    /  \    /  \    /\\  /   +------+
                     --/I \--+ Rb +--+ Rc +--/E \--
                       \  /   \  /    \  /   \  /
                        \/     --      --     \/
                       <------ Network N ------->
         <-------------------- Network M --------------------->

                         Figure 4 The big picture

   The tunnel consists of two elements (ingress I, egress E), that lie
   along a path connected by a (possibly different) network N.
   Regardless of how the ingress and egress are connected, the tunnel
   serves as a link to the nodes it connects (here, Ra and Rd).

   IP packets arriving at the ingress are encapsulated to traverse
   network N. We call these packets "tunnel transit packets" (TTPs)
   because they will now transit the tunnel inside one or more "tunnel
   link packets" (TLPs). TLPs use the source address of the ingress and
   the destination address of the egress - using whatever address is
   appropriate to the Layer at which the ingress and egress operate
   (Layer 2, Layer 3, Layer 4, etc.). The egress decapsulates those
   messages, which then continue on network M as if emerging from a
   link. To tunnel transit packets, and to the routers the tunnel
   connects (Ra and Rd), the tunnel acts as a link and the ingress and
   egress act as network interfaces to that link.

   The model of each component (ingress, egress) and the entire system
   (tunnel) depends on the layer from which you view the tunnel. From
   the perspective of the outermost hosts (Hsrc and Hdst), the tunnel
   appears as a link between two routers (Ra and Rd). For routers along
   the tunnel (e.g., Rb and Rc), the ingress and egress appear as the
   endpoint hosts and Hsrc and Hdst are invisible.




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   When the tunnel network (N) is implemented using the same protocol as
   the endpoint network (M), the picture looks flatter (Figure 5), as if
   it were running over a single network. However, note that this
   appearance is incorrect - nothing has changed. From the perspective
   of the endpoints, Rb and Rc and network N don't exist and aren't
   visible, and from the perspective of the tunnel, network M doesn't
   exist. The fact that network N and M use the same protocol, and may
   traverse the same links is irrelevant.

                   --_         --      --          --
       +------+   /  \  /\    /  \    /  \    /\  /  \   +------+
       | Hsrc |--+ Ra +/I \--+ Rb +--+ Rc +--/E \+ Rd +--| Hdst |
       +------+   \  / \  /   \  /    \  /   \  / \  /   +------+
                   --   \/     --      --     \/   --
                       <------ Network N ------->
       <---------------------- Network M ----------------------->

                     Figure 5 IP in IP network picture

3.2. View from the Outside

   From outside the tunnel, to network M, the entire tunnel acts as a
   link (Figure 6). It may be numbered or unnumbered and the addresses
   associated with the ingress and egress are irrelevant from outside.

                   --_                             --
       +------+   /  \                            /  \   +------+
       | Hsrc |--+ Ra +--------------------------+ Rd +--| Hdst |
       +------+   \  /                            \  /   +------+
                   --                              --

                Figure 6 Tunnels as viewed from the outside

   A tunnel is effectively invisible to the network in which it resides,
   except that it behaves exactly as a link. Consequently [RFC3819]
   requirements for links supporting IP also apply to tunnels.

   E.g., the IP datagram hop count (IPv4 Time-to-Live [RFC791] and IPv6
   Hop Limit [RFC2460]) are decremented when traversing a router, not by
   traversing a link - or thus a tunnel. Tunnels have a tunnel MTU - the
   largest datagram that can transit, just as links have a corresponding
   link MTU. A link MTU may not reflect the native link message sizes
   (ATM AAL5 48 byte messages support a 9KB MTU) and the same is true
   for a tunnel.





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3.3. View from the Inside

   Within network N, i.e., from inside the tunnel itself, the ingress is
   a source of tunnel link packets and the egress is a sink - both are
   hosts on network N (Figure 7). Consequently [RFC1122] Internet host
   requirements apply to ingress and egress nodes when Network N uses IP
   (and thus the ingress/egress use IP encapsulation).

                   _           --      --
                        /\    /  \    /  \    /\
                       /I \--+ Rb +--+ Rc +--/E \
                       \  /   \  /    \  /   \  /
                        \/     --      --     \/
                       <------ Network N ------->

            Figure 7 Tunnels, as viewed from within the tunnel

   Viewed from within the tunnel, the outer network (M) doesn't exist.
   Tunnel link packets can be fragmented by the source (ingress) and
   reassembled at the destination (egress), just as at any endpoint. The
   path between ingress and egress may have a path MTU but the endpoints
   can exchange messages as large as can be reassembled at the
   destination (egress), i.e., an egress MTU. Information about the
   network - i.e., regarding MTU sizes, network reachability, etc. - are
   relayed from the destination (egress) and intermediate routers back
   to the source (ingress), without regard for the external network (M).

3.4. Location of the Ingress and Egress

   The ingress and egress are endpoints of the tunnel and the tunnel is
   a link. The ingress and egress are thus link endpoints at the network
   nodes the tunnel interconnects. Such link endpoints are typically
   described as "network interfaces".

   Tunnel interfaces may be physical or virtual. The interface may be
   implemented inside the node where the tunnel attaches, e.g., inside a
   host or router. The interface may also be implemented as a "bump in
   the wire" (BITW), somewhere along a link between the two nodes the
   link interconnects. IP in IP tunnels are often implemented as
   interfaces, where IPsec tunnels are sometimes implemented as BITW.
   These implementation variations determine only whether information
   available at the link endpoints (ingress/egress) can be easily shared
   with the connected network nodes.






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3.5. Implications of This Model

   This approach highlights a few key features of a tunnel as a network
   architecture construct:

   o  To the tunnel transit packets (TTPs), tunnels turn a network
      (Layer 3) path into a (Layer 2) link

   o  To nodes the tunnel traverses, the tunnel ingress and egress act
      as hosts that source and sink tunnel link packets (TLPs)

   The consequences of these features are as follow:

   o  Like a link, a tunnel has an MTU defined by the reassembly MTU of
      the receiving interface (egress).

   o  Like any other link, the MTU inside a tunnel are not relevant to
      the transited traffic. There is no mechanism or protocol by which
      they are measured or confirmed.

   o  Path MTU discovery in the network layer (i.e., outer network M)
      has no direct relation to the MTU of the hops within the link
      layer of the links (or thus tunnels) that connect its components.

   o  Hops remain defined as the number of routers encountered on a path
      or the time spent at a router [RFC1812]. Hops are not decremented
      solely by the transit of a link, e.g., a packet with a hop count
      of zero should successfully transit a link (and thus a tunnel)
      that connects two hosts. Routers, not links, alter hopcounts.

   o  The addresses of a tunnel ingress and egress correspond to link
      layer addresses to the tunnel transit packet and outer network M.
      Like point-to-point links, point-to-point tunnels can be
      unnumbered in the network in which they reside (even though they
      must have addresses in the network they transit).

   o  Like network interfaces, the ingress and egress are never a direct
      source of ICMP messages but may provide information to their
      attached host or router to generate those ICMP messages.

   o  Like network interfaces and links, two nodes may be connected by
      any combination of tunnels and links, including multiple tunnels.
      As with multiple links, existing routing determines which traffic
      uses each link or tunnel.

   These observations make it much easier to determine what a tunnel
   must do to transit IP packets, notably it must satisfy all


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   requirements expected of a link [RFC1122][RFC3819]. The consequence
   of these observations are that tunnels are no different from links,
   except only that a link has a physical instantiation.

3.6. Fragmentation

   There are two places where fragmentation can occur in a tunnel,
   called Outer Fragmentation and Inner Fragmentation. This document
   assumes that only Outer Fragmentation is viable because it is the
   only approach that works for IPv4 datagrams with DF=1 and for IPv6.

3.6.1. Outer Fragmentation

   The simplest case is Outer Fragmentation, as shown in Figure 8. The
   bottom of the figure shows the network topology, where packets start
   at the source, enter the tunnel at the encapsulator, exit the tunnel
   at the decapsulator, and arrive finally at the destination. The
   packet traffic is shown above the topology, where the end-to-end
   packets are shown at the top. The packets are composed of an inner
   header (iH) and inner data (iD); the term "inner") is relative to the
   tunnel, as will become apparent. When the packet (iH,iD) arrives at
   the encapsulator, it is placed inside the tunnel packet structure,
   here shown as adding just an outer header, oH, in step (a).

    +----+----+                                              +----+----+
    | iH | iD |------+ -  -  -  -  -  -  -  -  -  -  +------>| iH | iD |
    +----+----+      |                               |       +----+----+
                     v                               |
              +----+----+----+               +----+----+----+
          (a) | oH | iH | iD |               | oH | iH | iD | (c)
              +----+----+----+               +----+----+----+
                     |                               ^
                     |       +----+----+-----+       |
                (b1) +----- >| oH'| iH | iD1 |-------+
                     |       +----+----+-----+       |
                     |                               |
                     |       +----+-----+            |
                (b2) +----- >| oH"| iD2 |------------+
                             +----+-----+
   +-----+         +---+                           +---+         +-----+
   |     |        /     \ ======================= /     \        |     |
   | Src |=======|  Enc  |=======================|  Dec  |=======| Dst |
   |     |        \     / ======================= \     /        |     |
   +-----+         +---+                           +---+         +-----+

                Figure 8 Fragmentation of the outer packet



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   When the encapsulated packet exceeds the tunnel MTU, the packet needs
   to be fragmented. In this case we fragment the packet at the outer
   header, with the fragments shown as (b1) and (b2). Note that the
   outer header indicates fragmentation (as ' and "),the inner header
   occurs only in the first fragment, and the inner data is broken
   across the two packets. These fragments are reassembled at the
   encapsulator in step (c), and the resulting packet is decapsulated
   and sent on to the destination.

   Outer fragmentation isolates Source and Destination from tunnel
   encapsulation duties. This can be considered a benefit in clean,
   layered network design, but also may result in complex decapsulator
   design, especially where tunnels aggregate large amounts of traffic,
   such as IP ID overload (see Sec. 4.4). Outer fragmentation is valid
   for any tunnel encapsulation protocol that supports fragmentation
   (e.g., IPv4 or IPv6), where the tunnel endpoints act as the host
   endpoints of that protocol.

   Along the tunnel, the inner header is contained only in the first
   fragment, which can interfere with mechanisms that 'peek' into lower
   layer headers, e.g., as for ICMP, as discussed in Sec. 4.6.

3.6.2. Inner Fragmentation

   Inner Fragmentation distributes the impact of tunneling across both
   the decapsulator and destination, and is shown in Figure 9; this can
   be especially important when the tunnel aggregates large amounts of
   traffic. However, this mechanism is thus valid only when the original
   source packets can be fragmented on-path, e.g., as in IPv4 datagrams
   with DF=0.

   Again, the network topology is shown at the bottom of the figure, and
   the original packets show at the top. Packets arrive at the
   encapsulator, and are fragmented there based on the inner header into
   (a1) and (a2). The fragments arrive at the decapsulator, which
   removes the outer header and forwards the resulting fragments on to
   the destination. The destination is then responsible for reassembling
   the fragments into the original packet.

   Along the tunnel, the inner headers are copied into each fragment,
   and so are available to mechanisms that 'peek' into headers (e.g.,
   ICMP, as discussed in Sec. 4.6). Because fragmentation happens on the
   inner header, the impact of IP ID is reduced.






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   +----+----+                                               +----+----+
   | iH | iD |-------+-  -  -  -  -  -  -  -  -  -  -  -  - >| iH | iD |
   +----+----+       |                                       +----+----+
                     v                                            ^
                +----+-----+                    +----+-----+      |
           (a1) | iH'| iD1 |                    | iH'| iD1 |------+
                +----+-----+                    +----+-----+      |
                                                                  |
                +----+---                       +----+-----+      |
           (a2) | iH"| iD2 |                    | iH"| iD2 |------+
                +----+-----+                    +----+-----+
                     |                               ^
                     |       +----+----+-----        |
                (b1) +----- >| oH | iH'| iD1 |-------+
                     |       +----+----+-----+       |
                     |                               |
                     |       +----+----+-----+       |
                (b2) +----- >| oH | iH"| iD2 |-------+
                             +----+----+-----+
   +-----+         +---+                           +---+         +-----+
   |     |        /     \ ======================= /     \        |     |
   | Src |=======|  Enc  |=======================|  Dec  |=======| Dst |
   |     |        \     / ======================= \     /        |     |
   +-----+         +---+                           +---+         +-----+

                Figure 9 Fragmentation of the inner packet

3.6.3. The necessity of Outer Fragmentation

   Fragmentation is critical tunnels that support TTP packets for
   protocols with minimum MTU requirements, while operating over tunnel
   paths using protocols with minimum MTU requirements. Depending on the
   amount of space used by encapsulation, these two minimums will
   ultimately interfere, and the TTP will need to be fragmented to both
   support a TTP minimum MTU while traversing tunnels with their own TLP
   minimum MTUs.

   Outer Fragmentation is the only solution that supports all IPv4 and
   IPv6 traffic, because inner fragmentation is allowed only for IPv4
   datagrams with DF=0. As a result, the remainder of this document
   assumes Outer Fragmentation.

4. IP Tunnel Requirements

   The requirements of an IP tunnel are defined by the requirements of
   an IP link because both transit IP packets. A tunnel thus must
   transit the IP minimum MTU, i.e., 68 bytes for IPv4 [RFC793] and 1280


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   bytes for IPv6 [RFC2460] and a tunnel must support address resolution
   when there is more than one egress.

   The requirements of the tunnel ingress and egress are defined by the
   network over which they exchange messages (tunnel link packets). For
   IP-over-IP, this means that the ingress MUST NOT exceed the IPv4
   Identification (fragment) field uniqueness requirements [RFC6864].

   These requirements remain even though tunnels have some unique
   issues, including the need for additional space for encapsulation
   headers and the potential for tunnel MTU variation.

4.1. Minimum MTU Considerations

   There are a variety of values of minimum MTU to consider, both in a
   conventional network and in a tunnel as a link in that network. These
   are indicated in Figure 10, an annotated variant of Figure 4.

     (a) LMTU    <->
     (b) PMTU    <------------------------------------>
     (c) <-RMTU----------------------------------------------->
     (d) TMTU          <------------------------>
     (e) TIMTU             <---------------->
     (f) ERMTU         <------------------------>
                     --_                         --
         +------+   /  \                        /  \   +------+
         | Hsrc |--+ Ra +      --      --      + Rd +--| Hdst |
         +------+   \  //\    /  \    /  \    /\\  /   +------+
                     --/I \--+ Rb +--+ Rc +--/E \--
                       \  /   \  /    \  /   \  /
                        \/     --      --     \/
                       <------ Network N ------->
         <-------------------- Network M --------------------->

                    Figure 10 The variety of MTU values

   Consider the following example values. For IPv6, the minimum LMTU (a)
   is 1280 bytes, which is also the minimum PMTU (b). The minimum RMTU
   (c) is 1500 bytes, which is also the minimum MTU for endpoint-to-
   endpoint communication. This means that IPv6 already assumes that
   endpoint-to-endpoint communication may require source fragmentation
   to transit IPv6-compatible links, even without considering tunnels.

   The TMTU (d) is the tunnel equivalent of a LMTU, and thus also needs
   to be 1280 bytes for IPv6. Assuming the links of a tunnel traverse
   IPv6 hops (e.g., I to Rb, Rb to Rc, and Rc to E), the TIMTU (e) is
   equivalent to the PMTU between I and E, which is 1280 - encaps (where


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   "encaps" is the tunnel encapsulation overhead). This value is
   insufficient to satisfy the requirement of an IPv6 link (which must
   transit at least 1280 bytes unfragmented), but this is not a problem.
   The TMTU (d) is not limited by TIMTU (e), but by ERMTU (f), the
   tunnel equivalent of RMTU (c). For a tunnel using IPv6 over IPv6, the
   ERMTU is the RMTU of tne underlying network N minus space for
   encapsulation, i.e., 1500 - encaps bytes, and the tunnel is viable as
   long as ERMTU >= 1280. Even though the tunnel will ultimately transit
   ERMTU - encaps byte messages between the ingress and egress, each hop
   within the tunnel transits only TIMTU - encaps byte messages. The
   difference between TIMTU and ERMTU is the reason why the tunnel
   ingresses need to support fragmentation and tunnel egresses need to
   support reassembly. The high cost of fragmentation and reassembly is
   why it is useful for applications to avoid sending messages too close
   to the PMTU, even the PMTU at their own layer.

4.2. Fragmentation

   A tunnel interacts with fragmentation in two different ways. As a
   link in network M, it messages might be fragmented before they reach
   the tunnel - i.e., at the TTP layer either during source
   fragmentation (if generated at the same node as the ingress
   interface) or forwarding fragmentation (for IPv4 DF=0 datagrams). In
   addition, messages traversing the tunnel may require fragmentation by
   the ingress - i.e., source fragmentation at the TLP layer by the
   ingress. These two fragmentation operations are no more related than
   are conventional IP fragmentation and ATM segmentation and
   reassembly; one occurs at the network layer, the other at the
   (virtual) link layer.

   As with any link layer, a tunnel MTU (TMTU) is defined as the largest
   message that can transit the tunnel. For a tunnel, this is the egress
   reassembly MTU (ERMTU), which is the reassembly MTU (RMTU) of the
   egress interface minus the space needed for the tunnel encapsulation
   headers. This value must also satisfy the requirements of the IP
   packets that the tunnel transits.

   Note that many of the issues with tunnel fragmentation and MTU
   handling were discussed in [RFC4459], but that document described a
   variety of alternatives as if they were independent. This document
   explains the combined approach that is necessary.

   Like any other link, an IPv4 tunnel must transit 68 byte packets
   without requiring source fragmentation [RFC791][RFC1122] and an IPv6
   tunnel must transit 1280 byte packets without requiring source
   fragmentation [RFC2460]. The tunnel MTU interacts with routers or
   hosts it connects the same way as would a link MTU. In the following


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   pseudocode, TTPsize is the size of the tunnel transit packet (TTP),
   and ERMTU is the reassembly MTU of the egress. As with any link, the
   link MTU (LMTU) is defined not by the native path of the link (or,
   for a tunnel, the path MTU of encapsulated packets inside the tunnel)
   but by the egress reassembly capability. This is because the ICMP
   "packet too big" message indicates failure of a link to transit a
   packet, not a preference for a size that matches that inside the
   mechanism of the link. There is no ICMP message for "larger than I'd
   like, but I can still transit it".

   These rules apply at the host/router where the tunnel is attached,
   i.e., at the network layer of the TTP (we assume that all tunnels,
   including multipoint tunnels, have a single, uniform TMTU). These are
   basic source fragmentation rules (or transit refragmentation for IPv4
   DF=0 datagrams), and have no relation to the tunnel itself other than
   to consider the TMTU as the effective LMTU of the next hop:

      if (TTP > TMTU) then
         if (TTP can be fragmented, e.g., IPv4 DF=0) then
            split TTP into fragments of TMTU size
            and send each fragment to the tunnel ingress
         else
            drop TTP and send ICMP "too big" to TTP source
         endif
      else
         send TTP to the tunnel ingress
      endif


   These rules apply at the tunnel ingress, in its role as host on the
   tunnel path, i.e., as source fragmentation of TLP messages (we assume
   that all tunnels, even multipoint tunnels, have a single, uniform
   TIMTU), where "encaps" is the encapsulation overhead:

      if (TTP <= (TIMTU + encaps)) then
         encapsulate the TTP and process as if arriving at the node
      else
         if ((TIMTU + encaps) < TTP <= (ERMTU - encaps)) then
            fragment TTP into TIMTU chunks
            encapslate each chunk and process as if arriving at the node
         else
            {never happens; host/router already dropped by now}
         endif
      endif





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   There is one path above that never occurs - i.e., a network interface
   should never receive a message larger than its MTU, and a tunnel
   should thus never receive a message larger than its (ERMTU - encaps)
   limit. A router attempting to process such a message would generate
   an ICMP error (packet too big, fragmentation needed) and the packet
   would already have been dropped before entering into this algorithm.

   As an example, consider IPv4 over IPv6 or IPv6 over IPv6 tunneling,
   where IPv6 encapsulation adds a 40 byte fixed header plus IPv6
   options (i.e., IPv6 header extensions) of total size TOptSz. From
   [RFC2460] it follows that the TMTU must be at least 1280 bytes and
   the ERMTU must be at least 1500 - (40 + TOptSz) bytes. The TIMTU must
   be a minimum of 1280 - (40 + TOptSz) bytes. Considering these minimum
   values, the previous algorithm becomes:

      if (TTP <= (1240 - TOptSz)) then
         encapsulate the TTP and and process as if arriving at the node
      else
         if ((1240 - TOptSz) < TTP <= (1460 - TOptSz))   then
            fragment TTP into (1240 - TOptSz) chunks
            encapslate each chunk and process as if arriving at the node
         else
            {never happens; host/router already dropped by now}
         endif
      endif


   This tunnel supports IPv6 transit only if TOptSize is smaller than
   180 bytes, and supports IPv4 transit if TOptSize is smaller than 884
   bytes. IPv6 TTPs of 1280 bytes may be guaranteed transit the outer
   network (M) without needing fragmentation there but they may require
   ongoing fragmentation and reassembly if the TMTU is not at least 1320
   bytes.

   When using IP directly over IP, the minimum ERMTU for IPv4 is 576
   bytes and for IPv6 is 1500 bytes. This means that tunnels of IPv4-
   over-IPv4, IPv4-over-IPv6, and IPv6-over-IPv6 are possible without
   additional requirements, but this may involve ingress fragmentation
   and egress reassembly. IPv6 cannot be tunneled directly over IPv4
   without additional requirements, notably that the ERMTU is at least
   1280 bytes. Fragmentation and reassembly cannot be avoided for IPv6-
   over-IPv6 without similar requirements.

   When ongoing ingress fragmentation and egress reassembly would be
   prohibitive or costly, larger MTUs can be supported by design and
   confirmed either out-of-band (by design) or in-band (e.g., using
   PLPMTUD [RFC4821], as done in SEAL [RFC5320] and AERO [Te16]).


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   Alternately, an ingress can encapsulate packets that fit and shut
   down once fragmentation is needed, but it must not continue to
   forward smaller packets while dropping larger packets that are still
   within required limits.

4.3. MTU discovery

   MTU discovery enables a network path to support a larger PMTU than it
   can assume from the minimum requirements of protocol over which it
   operates. A tunnel has two different LMTU-like values: TMTU and the
   TIMTU.

   There is temptation to optimize tunnel traversal so that packets are
   not fragmented between ingress and egress, i.e., to attempt tune the
   network PMTU to the TIMTU rather than the TMTU, to avoid ingress
   fragmentation. This is hazardous for many reasons:

   o  The tunnel is capable of transiting packets as large as the ERMTU,
      which is always at least as large as the TIMTU and typically is
      larger.

   o  ICMP has only one type of error message regarding large packets -
      "too big", i.e., too large to transit. There is no optimization
      message of "bigger than I'd like, but I can deal with if needed".

   o  IP tunnels often involve some level of recursion, i.e.,
      encapsulation over itself [RFC4459].

   Recursive tunneling occurs whenever a protocol ends up encapsulated
   in itself. This happens directly, as when IPv4 is encapsulated in
   IPv4, or indirectly, as when IP is encapsulated in UDP which then is
   a payload inside IP. It can involve many layers of encapsulation
   because a tunnel provider isn't always aware of whether the packets
   it transits are already tunneled.

   Recursion is impossible when the tunnel transit packets are limited
   to that of the native size of the TIMTU. Arriving tunnel transit
   packets have a minimum supported size (1280 for IPv6) and the tunnel
   PMTU has the same requirement; there would be no room for the
   additional encapsulation headers. The result would be an IPv6 tunnel
   that cannot satisfy IPv6 transit requirements.

   It is more appropriate to require the tunnel to satisfy IP transit
   requirements and enforce that requirement at design time or during
   operation (the latter using PLPMTUD [RFC4821]). Conventional path MTU
   discovery (PMTUD) relies on existing endpoint ICMP processing of
   explicit negative feedback from routers along the path via "message


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   to big" ICMP packets in the reverse direction of the tunnel
   [RFC1191]. This technique is susceptible to the "black hole"
   phenomenon, in which the ICMP messages never return to the source due
   to policy-based filtering [RFC2923]. PLPMTUD requires a separate,
   direct control channel from the egress to the ingress that provides
   positive feedback; the direct channel is not blocked by policy
   filters and the positive feedback ensures fail-safe operation if
   feedback messages are lost [RFC4821].

4.4. IP ID exhaustion

   In IPv4, the IP Identification (ID) field is a 16-bit value that is
   unique for every packet for a given source address, destination
   address, and protocol, such that it does not repeat within the
   Maximum Segment Lifetime (MSL) [RFC791][RFC1122]. Although the ID
   field was originally intended for fragmentation and reassembly, it
   can also be used to detect and discard duplicate packets, e.g., at
   congested routers (see Sec. 3.2.1.5 of [RFC1122]). For this reason,
   and because IPv4 packets can be fragmented anywhere along a path, all
   packets between a source and destination of a given protocol must
   have unique ID values over a period of an MSL, which is typically
   interpreted as two minutes (120 seconds). These requirements have
   recently been somewhat relaxed in recognition of the primary use of
   this field for reassembly and the need to handle only fragment
   misordering at the receiver [RFC6864].

   The uniqueness of the IP ID is a known problem for high speed nodes,
   because it limits the speed of a single protocol between two
   endpoints [RFC4963]. Although this suggests that the uniqueness of
   the IP ID is moot, tunnels exacerbate this condition. A tunnel often
   aggregates traffic from a number of different source and destination
   addresses, of different protocols, and encapsulates them in a header
   with the same ingress and egress addresses, all using a single
   encapsulation protocol. The result is one of the following:

   1. The IP ID rules are enforced, and the tunnel throughput is
      severely limited.

   2. The IP ID rules are enforced, and the tunnel consumes large
      numbers of ingress/egress IP addresses solely to ensure ID
      uniqueness.

   3. The IP ID rules are ignored.

   The last case is the most obvious solution, because it corresponds to
   how endpoints currently behave. Fortunately, fragmentation is
   somewhat rare in the current Internet at large, but it can be common


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   along a tunnel. Fragments that repeat the IP ID risk being
   reassembled incorrectly, especially when fragments are reordered or
   lost. Reassembly errors are not always detected by other protocol
   layers (see Sec. 4.9), and even when detected they can result in
   excessive overall packet loss and can waste bandwidth between the
   egress and ultimate packet destination.

4.5. Hop Count

   This section considers the selection of the value of the hop count of
   the tunnel link header, as well as the potential impact on the tunnel
   transit header. The former is affected by the number of hops within
   the tunnel. The latter determines whether the tunnel has visible
   effect on the transit packet.

   In general, the Internet hop count field is used to detect and avoid
   forwarding loops that cannot be corrected without a synchronized
   reboot. The IPv4 Time-to-Live (TTL) and IPv6 Hop Limit field each
   serve this purpose [RFC791][RFC2460].

   The IPv4 TTL field was originally intended to indicate packet
   expiration time, measured in seconds. A router is required to
   decrement the TTL by at least one or the number of seconds the packet
   is delayed, whichever is larger [RFC1812]. Packets are rarely held
   that long, and so the field has come to represent the count of the
   number of routers traversed. IPv6 makes this meaning more explicit.

   These hop count fields represent the number of network forwarding
   elements traversed by an IP datagram. An IP datagram with a hop count
   of zero can traverse a link between two hosts because it never visits
   a router (where it would need to be decremented and would have been
   dropped).

   An IP datagram traversing a tunnel thus need not have its hopcount
   modified, i.e., the tunnel transit header need not be affected. A
   zero hop count datagram should be able to traverse a tunnel as easily
   as it traverses a link. A router MAY be configured to decrement
   packets traversing a particular link (and thus a tunnel), which may
   be useful in emulating a path as if it had traversed one or more
   routers, but this is strictly optional. The ability of the outer
   network and tunnel network to avoid indefinitely looping packets does
   not rely on the hop counts of the tunnel traversal packet and tunnel
   link packet being related in any way at all.

   The hop count field is also used by several protocols to determine
   whether endpoints are "local", i.e., connected to the same subnet
   (link-local discovery and related protocols [RFC4861]). A tunnel is a


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   way to make a remote address appear directly-connected, so it makes
   sense that the other ends of the tunnel appear local and that such
   link-local protocols operate over tunnels unless configured
   explicitly otherwise. When the interfaces of a tunnel are numbered,
   these can be interpreted the same way as if they were on the same
   link subnet.

4.6. Signaling

   In the current Internet architecture, signaling goes upstream, either
   from routers along a path or from the destination, back toward the
   source. Such signals are typically contained in ICMP messages, but
   can involve other protocols such as RSVP, transport protocol signals
   (e.g., TCP RSTs), or multicast control or transport protocols.

   A tunnel behaves like a link and acts like a link interface at the
   nodes where it is attached. As such, it can provide information that
   enhances IP signaling (e.g., ICMP), but itself does not directly
   generate ICMP messages.

   For tunnels, this means that there are two separate signaling paths.
   The outer network M nodes can each signal the source of the tunnel
   transit packets, Hsrc (Figure 11). Inside the tunnel, the inner
   network N nodes can signal the source of the tunnel link packets, the
   ingress I (Figure 12).

           +--------+---------------------------+--------+
           |        |                           |        |
           v        --_                         --       v
        +------+   /  \                        /  \   +------+
        | Hsrc |--+ Ra +      --      --      + Rd +--| Hdst |
        +------+   \  //\    /  \    /  \    /\\  /   +------+
                    --/I \--+ Rb +--+ Rc +--/E \--
                      \  /   \  /    \  /   \  /
                       \/     --      --     \/
                        <---- Network N ----->
        <-------------------- Network M --------------------->

                   Figure 11 Signals outside the tunnel










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                        +-----+-------+------+
                    --_ |     |       |      |  --
        +------+   /  \ v     |       |      | /  \   +------+
        | Hsrc |--+ Ra +      --      --      + Rd +--| Hdst |
        +------+   \  //\    /  \    /  \    /\\  /   +------+
                    --/I \--+ Rb +--+ Rc +--/E \--
                      \  /   \  /    \  /   \  /
                       \/     --      --     \/
                        <----- Network N ---->
        <--------------------- Network M -------------------->

                    Figure 12 Signals inside the tunnel

   These two signal paths are inherently distinct except where
   information is exchanged between the network interface of the tunnel
   (the ingress) and its attached node (Ra, in both figures).

   It is always possible for a network interface to provide hints to its
   attached node (host or router), which can be used for optimization.
   In this case, when signals inside the tunnel indicate a change to the
   tunnel, the ingress (i.e., the tunnel network interface) can provide
   information to the router (Ra, in both figures), so that Ra can
   generate the appropriate signal in return to Hsrc. This relaying may
   be difficult, because signals inside the tunnel may not return enough
   information to the ingress to support direct relaying to Hsrc.

   In all cases, the tunnel ingress needs to determine how to relay the
   signals from inside the tunnel into signals back to the source. For
   some protocols this is either simple or impossible (such as for
   ICMP), for others, it can even be undefined (e.g., multicast). In
   some cases, the individual signals relayed from inside the tunnel may
   result in corresponding signals in the outside network, and in other
   cases they may just change state of the tunnel interface. In the
   latter case, the result may cause the router Ra to generate new ICMP
   errors when later messages arrive from Hsrc or other sources in the
   outer network.

   The meaning of the relayed information must be carefully translated.
   In the case of soft or hard ICMP errors, the translation may be
   obvious. ICMP "packet too big" messages from inside the tunnel might
   update TIMTU at the ingress, but may have no effect on the tunnel as
   visible to the router where it is attached (Ra).

   In addition to ICMP, messages typically considered for translation
   include Explicit Congestion Notification (ECN [RFC6040]) and
   multicast (IGMP, e.g.).



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4.7. Relationship of Header Fields

   Some tunnel specifications attempt to relate the fields of the tunnel
   transit packet and tunnel link packet, i.e., the packet arriving at
   the ingress and the encapsulation header. These two headers are
   effectively independent and there is no utility in requiring their
   contents to be related.

   In specific, the encapsulation header source and destination
   addresses are network endpoints in the tunnel network N, but have no
   meaning in the outer network M, even when the tunneled packet
   traverses the same network. The addresses are effectively
   independent, and the tunnel endpoint addresses are link addresses to
   the tunnel transit packet.

   Because the tunneled packet uses source and destination addresses
   with a separate meaning, it is inappropriate to copy or reuse the
   IPv4 Identification or IPv6 Fragment ID fields of the tunnel transit
   packet. These fields need to be generated based on the context of the
   encapsulation header, not the tunnel transit header.

   Similarly, the DF field need not be copied from the tunnel transit
   packet to the encapsulation header of the tunnel link packet
   (presuming both are IPv4). Path MTU discovery inside the tunnel does
   not directly correspond to path MTU discovery outside the tunnel,
   i.e., inside the tunnel it would update the TIMTU used for outer
   fragmentation at the ingress, but has no effect on the TMTU reported
   to the device where the ingress is attached as a network interface.

   The same is true for most other fields. When a field value is
   generated in the encapsulation header, its meaning should be derived
   from what is desired in the context of the tunnel as a link. When
   feedback is received from these fields, they should be presented to
   the tunnel ingress and egress as if they were network interfaces. The
   behavior of the node where these interfaces attach should be
   identical to that of a conventional link.

   There are exceptions to this rule that are explicitly intended to
   relay signals from inside the tunnel to outside the tunnel. The
   primary example is ECN [RFC6040], which copies the ECN bits from the
   tunnel transit header to the tunnel link header during encapsulation
   at the ingress and modifies the tunnel transit header at egress based
   on a combination of the bits of the two headers. This is intended to
   allow congestion notification within the tunnel to be interpreted as
   if it were on the direct path. Other examples may involve the DSCP
   flags. In both cases, it is assumed that the intent of copying values
   on encapsulation and merging values on decapsulation has the effect


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   of allowing the tunnel to act as if it participates in the same type
   of network as outside the tunnel (network M).

4.8. Congestion

   In general, tunnels carrying IP traffic need not react directly to
   congestion any more than would any other link layer [RFC5405]. IP
   traffic is not generally expected to be congestion reactive.

   [text from David Black on ECN relaying?]

4.9. Checksums

   IP traffic transiting a tunnel needs to expect a similar level of
   error detection and correction as it would expect from any other
   link. In the case of IPv4, there are no such expectations, which is
   partly why it includes a header checksum [RFC791].

   IPv6 omitted the header checksum because it already expects most link
   errors to be detected and dropped by the link layer and because it
   also assumes transport protection [RFC2460]. When transiting IPv6
   over IPv6, the tunnel fails to provide the expected error detection.
   This is why IPv6 is often tunneled over layers that include separate
   protection, such as GRE [RFC2784].

   The fragmentation created by the tunnel ingress can increase the need
   for stronger error detection and correction, especially at the tunnel
   egress to avoid reassembly errors. The Internet checksum is known to
   be susceptible to reassembly errors that could be common [RFC4963],
   and should not be relied upon for this purpose. This is why SEAL and
   AERO include a separate checksum [RFC5320][Te16]. This requirement
   can be undermined when using UDP as a tunnel with no UDP checksum (as
   per [RFC6935][RFC6936]) when fragmentation occurs because the egress
   has no checksum with which to validate reassembly. For this reason,
   it is safe to use UDP with a zero checksum for atomic (non-
   fragmented, non-fragmentable) tunnel link packets only; when used on
   fragments, whether generated at the ingress or en-route inside the
   tunnel, omission of such a checksum can result in reassembly errors
   that can cause additional work (capacity, forwarding processing,
   receiver processing) downstream of the egress.

4.10. Numbering

   Tunnel ingresses and egresses have addresses associated with the
   encapsulation protocol. These addresses are the source and
   destination (respectively) of the encapsulated packet while
   traversing the tunnel network.


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   Tunnels may or may not have addresses in the network whose traffic
   they transit (e.g., network M in Figure 4). In some cases, the tunnel
   is an unnumbered interface to a point-to-point virtual link. When the
   tunnel has multiple egresses, tunnel interfaces require separate
   addresses in network M.

   To see the effect of tunnel interface addresses, consider traffic
   sourced at router Ra in Figure 4. Even before being encapsulated by
   the ingress, that traffic needs a source IP network address that
   belongs to the router. One option is to use an address associated
   with one of the other interfaces of the router [RFC1122]. Another
   option is to assign a number to the tunnel interface itself.
   Regardless of which address is used, the resulting IP packet is then
   encapsulated by the tunnel ingress using the ingress address as a
   separate operation.

4.11. Multicast

   [To be addressed]

   Note that PMTU for multicast is difficult. PIM carries an option that
   may help in the Population Count Extensions to PIM [RFC6807].

   IMO, again, this is no different than any other multicast link.

4.12. Multipoint

   Multipoint tunnels are tunnels with more than two ingress/egress
   endpoints. Just as tunnels emulate links, multipoint tunnels emulate
   multipoint links.

   Multipoint links require a support for egress determination, just as
   multipoint links do. This function is typically supported by ARP
   [RFC826] or ARP emulation (e.g., LAN Emulation, known as LANE
   [RFC2225]) for multipoint links. For multipoint tunnels, a similar
   mechanism is required for the same purpose - to determine the egress
   address for proper ingress encapsulation.

   All multipoint systems - tunnels and links - might support different
   MTUs between each ingress/egress (or link entrance/exit) pair. In
   most cases, it is simpler to assume a uniform MTU throughout the
   multipoint system, e.g., the minimum MTU supported across all
   ingress/egress pairs. This applies to both the ERMTU and TIMETU (the
   latter as used only by the ingress).

   A multipoint tunnel MUST have support for broadcast and multicast, in
   exactly the same way as this is already required for multipoint links


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   [RFC3819]. Both modes can be supported either by a native mechanism
   inside the tunnel or by emulation using serial replication at the
   tunnel ingress, in the same way that links may provide the same
   support either natively (e.g., via promiscuous or automatic
   replication in the link itself) or network interface emulation (e.g.,
   as for non-broadcast multiaccess networks, i.e., NBMAs).

4.13. NAT / Load Balancing

   [To be addressed]

   Talk about ECMP / LAG here

4.14. Recursive tunnels

   [IS THIS REDUNDANT?]

   The rules described in this document already support tunnels over
   tunnels, sometimes known as "recursive" tunnels, in which IP is
   transited over IP either directly or via intermediate encapsulation
   (IP-UDP-IP).

   There are known hazards to recursive tunneling, notably that the
   independence of the tunnel transit header and tunnel link header hop
   counts can result in a tunneling loop. Such looping can be avoided
   when using direct encapsulation (IP in IP) by use of a header option
   to track the encapsulation count and to limit that count [RFC2473].
   This looping cannot be avoided when other protocols are used for
   tunneling, e.g., IP in UDP in IP, because the encapsulation count may
   not be visible where the recursion occurs.

5. Observations (implications)

   [Leave this as a shopping list for now]

5.1. Tunnel protocol designers

   Recursive tunneling + minimum MTU = frag/reassembly is inevitable, at
   least to be able to split/join two fragments

   Account for egress MTU/path MTU differences.

   Include a stronger checksum.

   Ensure the egress MTU is always larger than the path MTU.




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   Ensure that the egress reassembly can keep up with line rate OR
   design PLPMTUD into the tunneling protocol.

5.2. Tunnel implementers

   Detect when the egress MTU is exceeded.

   Detect when the egress MTU drops below the required minimum and shut
   down the tunnel if that happens - configuring the tunnel down and
   issuing a hard error may be the only way to detect this anomaly, and
   it's sufficiently important that the tunnel SHOULD be disabled. This
   is always better than blindly assuming the tunnel has been deployed
   correctly, i.e., that the solution has been engineered.

   Do NOT decrement the TTL as part of being a tunnel. It's always
   already OK for a router to decrement the TTL based on different next-
   hop routers, but TTL is a property of a router not a link.

5.3. Tunnel operators

   Keep the difference between "enforced by operators" vs. "enforced by
   active protocol mechanism" in mind. It's fine to assume something the
   tunnel cannot or does not test, as long as you KNOW you can assume
   it. When the assumption is wrong, it will NOT be signaled by the
   tunnel. Do NOT decrement the TTL as part of being a tunnel. It's
   always already OK for a router to decrement the TTL based on
   different next-hop routers, but TTL is a property of a router not a
   link.

   Do NOT decrement the TTL as part of being a tunnel. It's always
   already OK for a router to decrement the TTL based on different next-
   hop routers, but TTL is a property of a router not a link.

   >>>> PLPMTUD can give incorrect information during ECMP or LAG

5.4. Diagnostics

   Some current implementations include diagnostics to support
   monitoring the impact of tunneling, especially the impact on
   fragmentation and reassembly resources, the status of path MTU
   discovery, etc.

   >> Because a tunnel ingress/egress is a network interface, it SHOULD
   have similar resources as any other network interface. This includes
   resources for packet processing as well as monitoring.




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5.5. For existing standards

5.5.1. Generic UDP Encapsulation (GUE - IP in UDP in IP)

   [He15]

   Consistent with this doc:

   Inconsistent with this doc:

      Imports RFC4459

      Appears to allow both pre and post-encapsulation fragmentation

   Recommendations:

      Should not encourage pre-encaps fragmentation

      See recommendations for RFC4459

5.5.2. Generic Packet Tunneling in IPv6

   [RFC2473]

   Consistent with this doc:

      Considers the endpoints of the tunnel as virtual interfaces.

      Considers the tunnel a virtual link.

      Requires source fragmentation at the ingress and reassembly at the
   egress.

      Includes a recursion limit to prevent unlimited re-encapsulation.

      Sets tunnel transit header hop limit independently.

      Sends ICMPs back at the ingress based on the arriving tunnel
   transit packet and its relation to the tunnel MTU (though it uses the
   incorrect value of the tunnel MTU; see below).

      Allows for ingress relaying of internal tunnel errors (but see
   below; it does not discuss retaining state about these).

   Inconsistent with this doc:




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      Decrements the tunnel transit header by 1, i.e., incorrectly
   assuming that tunnel endpoints occur at routers only and that the
   tunnel, rather than the router, is responsible for this decrement.

      This doc goes to pains to describe the decapsulation process as if
   it were distinct from conventional protocol processing by the
   receiver (when it should not be).

      Copies traffic class from tunnel link to tunnel transit header (as
   one variant).

      Treats the tunnel MTU as the tunnel path MTU, rather than the
   tunnel egress MTU.

      Incorrectly fragments IPv4 DF=0 tunnel transit packets that arrive
   larger than the tunnel MTU at the IPv6 layer; the relationship
   between IPv4 and the tunnel is more complex (as noted in this doc).

      Fails to retain state from the tunnel based on ingress receiving
   ICMP messages from inside the tunnel, e.g., such as might cause
   future tunnel transit packets arriving at the ingress to be discarded
   with an ICMP error response rather than allowing them to proceed into
   the tunnel.

   Recommendation:

      This doc should update 2473 for TTL decrement, tunnel MTU, and
   fragmentation. Other issues are less critical.

5.5.3. Geneve (NVO3)

   [RFC7364] info, [Gr16] stds - ISSUE US AS BCP; Gr16 should follow

   Consistent with this doc:

      Generation of the link header fields is not discussed and presumed
   independent of transit packet.

      Reportedly treats an ingress/egress as applying to multiple
   tunnels, rather than considering them logically independent for each
   tunnel. This appears to confuse implementation aggregation with
   architecture.

      Reportedly treats tunnels as supporting traffic for multiple
   virtual networks, rather than considering them logically independent.
   This appears to confuse implementation aggregation with architecture.



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   Inconsistent with this doc:

      Tries to match transit to tunnel path MTU rather than egress MTU.

   Recommendation:

      Gr16 should be updated to follow us

5.5.4. GRE (IP in GRE in IP)

   IPv4 [RFC2784] stds, [RFC7588] info, [RFC7676] stds - NO CHANGES

   Consistent with this doc:

      Does not address link header generation.

      Non-default behavior allows fragmentation of link packet to match
   tunnel path MTU up to the limit of the egress MTU.

      Default behavior sets link DF independently.

      Shuts the tunnel down if the tunnel path MTU isn't >= 1280.

   Inconsistent with this doc:

      Based on tunnel path MTU, not egress MTU.

      Claims that the tunnel (GRE) mechanism is responsible for
   generating ICMP error messages.

      Default behavior fragments transit packet (where possible) based
   on tunnel path MTU (it should fragment based on egress MTU).

      Default behavior does not support the minimum MTU of IPv6 when run
   over IPv6.

      Non-default behavior allows copying DF for IPv4 in IPv4.

   Recommendations:

      No changes - existing docs largely describe legacy deployment.

5.5.5. IP in IP / mobile IP

   IPv4 [RFC2003] stds, [RFC4459] info:

   Consistent with this doc:


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      Generate link ID independently

      Generate link DF independently when transit DF=0

      Generate ECN/update ECN based on sharing info [RFC6040]

      Set link TTL to transit to egress only (independently)

      Do not decrement TTL on entry except when part of forwarding

      Do not decrement TTL on exit except when part of forwarding

      Options not copied, but used as a hint to desired services.

      Generally treat tunnel as a link, e.g., for link-local.

   Inconsistent with this doc

      Set link DF when transit DF=1 (won't work unless I-E runs PLPMTUD)

      Drop at egress if transit TTL=0 (wrong TTL for host-host tunnels)

      Drop when transit source is router's IP (prevents tun from router)

      Drop when transit source matches egress (prevents tun to router)

      Use tunnel ICMPs to generate upper ICMPs, copying context (ICMPs
   are now coming from inside a link!); these should be handled by
   setting errors as a "network interface" and letting the attached
   host/router figure out what to send.

      Using tunnel MTU discovery to tune the transit packet to the
   tunnel path MTU rather than egress MTU.

   Recommendations:

      IMO, ought to update 2003! (no "update" to informational), esp.
   regarding TTL issues, transit source drop issues, and tunnel MTU.

   IPv6 [RFC2473] std:

   Consistent with this doc:

      Doesn't discuss lots of header fields, but implies they're set
   independently.

      Sets link TTL independently.


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   Inconsistent with this doc:

      Tunnel issues ICMP PTBs.

      ICMP PTB issued if larger then 1280 - header, rather than egress
   reassembly MTU.

      Fragments IPv6 over IPv6 fragments only if transit is <= 1280
   (i.e., forces all tunnels to have a max MTU of 1280).

      Fragments IPv4 over IPv6 fragments only if IPv4 DF=0
   (misinterpreting the "can fragment the IPv4 packet" as permission to
   fragment at the IPv6 link header)

      Considers encapsulation a forwarding operation and decrements the
   transit TTL.

   Recommendation:

      Should UPDATE 2473; tunnel should not issue PTBs (router should),
   issue them correctly, fragment correctly, and not TTL decrement.

5.5.6. IPsec tunnel mode (IP in IPsec in IP)

   [RFC4301] std

   Consistent with this doc:

      Most of the rules, except as noted below.

   Inconsistent with this doc:

      Writes its own header copying rules (Sec 5.1.2), rather than
   referring to existing standards, but that makes sense for security
   reasons.

      Uses policy to set, clear, or copy DF (policy isn't the issue)

      Intertwines tunneling with forwarding rather than presenting the
   tunnel as a network interface; this can be corrected by using IPsec
   transport mode with an IP-in-IP tunnel [RFC3884].

   Recommendations:

      None.




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5.5.7. L2TP

   [RFC3931] std

   Consistent with this doc:

      Does not address most link headers, which are thus independent.

   Inconsistent with this doc:

      Manages tunnel access based on tunnel path MTU, instead of egress
   MTU.

      Refers to RFC2473 (IPv6 in IPv6), which is inconsistent with this
   doc as noted above.

   Recommendations:

      Should update to use correct tunnel MTU.

5.5.8. L2VPN

   [RFC4664]

   Consistent with this doc:

   Inconsistent with this doc:

   Recommendations:

5.5.9. L3VPN

   [RFC4176]

   Consistent with this doc:

   Inconsistent with this doc:

   Recommendations:

5.5.10. LISP

   [RFC6830]

   Consistent with this doc:

   Inconsistent with this doc:


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   Recommendations:

5.5.11. MPLS

   [RFC3031]

   Consistent with this doc:

   Inconsistent with this doc:

   Recommendations:

5.5.12. PWE

   [RFC3985]

   Consistent with this doc:

   Inconsistent with this doc:

   Recommendations:

5.5.13. SEAL/AERO

   [RFC5320][Te16]

   Consistent with this doc:

   Inconsistent with this doc:

   Recommendations:

5.5.14. TRILL

   [RFC5556][RFC6325]

   Consistent with this doc:

      Puts IP in Ethernet, so most of the issues don't come up.

      Ethernet doesn't have TTL or fragment.

      Rbridge (trill) TTL header is independent of transit packet.

   Inconsistent with this doc:

      None.


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   Recommendations:

      None.

5.5.15. RTG DT encapsulations

   [No16], refers to NVO3 and other encapsulations

   Includes info on tables for multipoint tunnels, additional info for
   headers, etc.

   Consistent with this doc:

   Inconsistent with this doc:

      Assumes MTU can be managed to avoid fragmentation. This is
   impossible as long as any one layer is used recursively and that
   layer includes a mandatory minimum MTU. A "trust but verify" policy
   is better than assuming engineered MTU deployment is sufficient.

      Relies on ICMP PTB to correct for tunnel path MTU issues.

      Allows encaps protocols to not support fragmentation.

   Recommendations:

      That doc should refer to this regarding general tunneling issues,
   including fragmentation, tunnel MTU, and TTL, including the "trust
   but verify" issue for engineered MTU deployment.

      All encaps protocols for IP over IP (eventually) MUST support
   fragm.

5.6. For future standards

   Larger IPv4 MTU (2K? or just 2x path MTU?) for reassembly

   Always include frag support for at least two frags; do NOT try to
   deprecate fragmentation.

   Limit encapsulation option use/space.

   Augment ICMP to have two separate messages: PTB vs P-bigger-than-
   optimal

   Include MTU as part of BGP as a hint - SB



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   Hazards of multi-MTU draft-van-beijnum-multi-mtu-04

6. Security Considerations

   Tunnels may introduce vulnerabilities or add to the potential for
   receiver overload and thus DOS attacks. These issues are primarily
   related to the fact that a tunnel is a link that traverses a network
   path and to fragmentation and reassembly. ICMP signal translation
   introduces a new security issue and must be done with care. ICMP
   generation at the router or host attached to a tunnel is already
   covered by existing requirements (e.g., should be throttled).

   Tunnels traverse multiple hops of a network path from ingress to
   egress. Traffic along such tunnels may be susceptible to on-path and
   off-path attacks, including fragment injection, reassembly buffer
   overload, and ICMP attacks. Some of these attacks may not be as
   visible to the endpoints of the architecture into which tunnels are
   deployed and these attacks may thus be more difficult to detect.

   Fragmentation at routers or hosts attached to tunnels may place an
   undue burden on receivers where traffic is not sufficiently diffuse,
   because tunnels may induce source fragmentation at hosts and path
   fragmentation (for IPv4 DF=0) more for tunnels than for other links.
   Care should be taken to avoid this situation, notably by ensuring
   that tunnel MTUs are not significantly different from other link
   MTUs.

   Tunnel ingresses emitting IP datagrams MUST obey all existing IP
   requirements, such as the uniqueness of the IP ID field. Failure to
   either limit encapsulation traffic, or use additional ingress/egress
   IP addresses, can result in high speed traffic fragments being
   incorrectly reassembled.

   Tunnels are susceptible to attacks at both the inner and outer
   network layers. The tunnel ingress/egress endpoints appear as network
   interfaces in the outer network, and are as susceptible as any other
   network interface. This includes vulnerability to fragmentation
   reassembly overload, traffic overload, and spoofed ICMPs that
   misreport the state of those interfaces. Similarly, the
   ingress/egress appear as hosts to the path traversed by the tunnel,
   and thus are as susceptible as any other host to attacks as well.

   [management?]

   [Access control?]




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   describe relationship to [RFC6169] - JT (as per INTAREA meeting
   notes, don't cover Teredo-specific issues in RFC6169, but include
   generic issues here)

7. IANA Considerations

   This document has no IANA considerations.

   The RFC Editor should remove this section prior to publication.

8. References

8.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2. Informative References

   [Cl88]    Clark, D., "The design philosophy of the DARPA internet
             protocols," Proc. Sigcomm 1988, p.106-114, 1988.

   [Er94]    Eriksson, H., "MBone: The Multicast Backbone,"
             Communications of the ACM, Aug. 1994, pp.54-60.

   [Gr16]    Gross, J., et al., "Geneve: Generic Network Virtualization
             Encapsulation," draft-ietf-nvo3-geneve-01, Jan. 2016.

   [He15]    Herbert, T., L. Yong, O. Zia, "Generic UDP Encapsulation,"
             draft-ietf-nvo3-gue-04, Jul. 2016.

   [No16]    Nordmark, E. (Ed.), A. Tian, J. Gross, J. Hudson, L.
             Kreeger, P. Garg, P. Thaler, T. Herbert, "Encapsulation
             Considerations," draft-ietf-rtgwg-dt-encap-01, Mar. 2016.

   [RFC768]  Postel, J, "User Datagram Protocol," RFC 768, Aug. 1980

   [RFC791]  Postel, J., "Internet Protocol," RFC 791 / STD 5, September
             1981.

   [RFC793]  Postel, J, "Transmission Control Protocol," RFC 793, Sept.
             1981.

   [RFC826]  Plummer, D., "An Ethernet Address Resolution Protocol -- or
             -- Converting Network Protocol Addresses to 48.bit Ethernet
             Address for Transmission on Ethernet Hardware," RFC 826,
             Nov. 1982.


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   [RFC1075] Waitzman, D., C. Partridge, S. Deering, "Distance Vector
             Multicast Routing Protocol," RFC 1075, Nov. 1988.

   [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
             Communication Layers," RFC 1122 / STD 3, October 1989.

   [RFC1191] Mogul, J., S. Deering, "Path MTU discovery," RFC 1191,
             November 1990.

   [RFC1812] Baker, F., "Requirements for IP Version 4 Routers," RFC
             1812, June 1995.

   [RFC2003] Perkins, C., "IP Encapsulation within IP," RFC 2003,
             October 1996.

   [RFC2225] Laubach, M., J. Halpern, "Classical IP and ARP over ATM,"
             RFC 2225, Apr. 1998.

   [RFC2460] Deering, S., R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification," RFC 2460, Dec. 1998.

   [RFC2473] Conta, A., "Generic Packet Tunneling in IPv6
             Specification," RFC 2473, Dec. 1998.

   [RFC2784] Farinacci, D., T. Li, S. Hanks, D. Meyer, P. Traina,
             "Generic Routing Encapsulation (GRE)", RFC 2784, March
             2000.

   [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery," RFC
             2923, September 2000.

   [RFC2473] Conta, A., S. Deering, "Generic Packet Tunneling in IPv6
             Specification," RFC 2473, Dec. 1998.

   [RFC3031] Rosen, E., A. Viswanathan, R. Callon, "Multiprotocol Label
             Switching Architecture", RFC 3031, January 2001.

   [RFC3819] Karn, P., Ed., C. Bormann, G. Fairhurst, D. Grossman, R.
             Ludwig, J. Mahdavi, G. Montenegro, J. Touch, L. Wood,
             "Advice for Internet Subnetwork Designers," RFC 3819 / BCP
             89, July 2004.

   [RFC3884] Touch, J., L. Eggert, Y. Wang, "Use of IPsec Transport Mode
             for Dynamic Routing," RFC 3884, September 2004.





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   [RFC3931] Lau, J., Ed., M. Townsley, Ed., I. Goyret, Ed., "Layer Two
             Tunneling Protocol - Version 3 (L2TPv3)," RFC 3931, March
             2005.

   [RFC3985] Bryant, S., P. Pate (Eds.), "Pseudo Wire Emulation Edge-to-
             Edge (PWE3) Architecture", RFC 3985, March 2005.

   [RFC4176] El Mghazli, Y., Ed., T. Nadeau, M. Boucadair, K. Chan, A.
             Gonguet, "Framework for Layer 3 Virtual Private Networks
             (L3VPN) Operations and Management," RFC 4176, October 2005.

   [RFC4301] Kent, S., and K. Seo, "Security Architecture for the
             Internet Protocol," RFC 4301, December 2005.

   [RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
             Network Tunneling," RFC 4459, April 2006.

   [RFC4664] Andersson, L., Ed., E. Rosen, Ed., "Framework for Layer 2
             Virtual Private Networks (L2VPNs)," RFC 4664, September
             2006.

   [RFC4821] Mathis, M., J. Heffner, "Packetization Layer Path MTU
             Discovery," RFC 4821, March 2007.

   [RFC4861] Narten, T., E. Nordmark, W. Simpson, H. Soliman, "Neighbor
             Discovery for IP version 6 (IPv6)," RFC 4861, Sept. 2007.

   [RFC4963] Heffner, J., M. Mathis, B. Chandler, "IPv4 Reassembly
             Errors at High Data Rates," RFC 4963, July 2007.

   [RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and
             Adaptation Layer (SEAL)," RFC 5320, Feb. 2010.

   [RFC5405] Eggert, L., G. Fairhurst, "Unicast UDP Usage Guidelines for
             Application Designers," RFC 5405, Nov. 2008.

   [RFC5556] Touch, J., R. Perlman, "Transparently Interconnecting Lots
             of Links (TRILL): Problem and Applicability Statement," RFC
             5556, May 2009.

   [RFC5944] Perkins, C., Ed., "IP Mobility Support for IPv4, Revised"
             RFC 5944, Nov. 2010.

   [RFC6040] Briscoe, B., "Tunneling of Explicit Congestion
             Notification," RFC 6040, Nov. 2010.




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   [RFC6169] Krishnan, S., D. Thaler, J. Hoagland, "Security Concerns
             With IP Tunneling," RFC 6169, Apr. 2011.

   [RFC6325] Perlman, R., D. Eastlake, D. Dutt, S. Gai, A. Ghanwani,
             "Routing Bridges (RBridges): Base Protocol Specification,"
             RFC 6325, July 2011.

   [RFC6807] Farinacci, D., G. Shepherd, S. Venaas, Y. Cai, "Population
             Count Extensions to Protocol Independent Multicast (PIM),"
             RFC 6807, Dec. 2012.

   [RFC6830] Farinacci, D., V. Fuller, D. Meyer, D. Lewis, "The
             Locator/ID Separation Protocol," RFC 6830, Jan. 2013.

   [RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field,"
             Proposed Standard, RFC 6864, Feb. 2013.

   [RFC6935] Eubanks, M., P. Chimento, M. Westerlund, "IPv6 and UDP
             Checksums for Tunneled Packets," RFC 6935, Apr. 2013.

   [RFC6936] Fairhurst, G., M. Westerlund, "Applicability Statement for
             the Use of IPv6 UDP Datagrams with Zero Checksums," RFC
             6936, Apr. 2013.

   [RFC7364] Narten, T., Gray, E., Black, D., Fang, L., Kreeger, L., M.
             Napierala, "Problem Statement: Overlays for Network
             Virtualization", RFC 7364, Oct. 2014.

   [RFC7450] Bumgardner, G., "Automatic Multicast Tunneling," RFC 7450,
             Feb. 2015.

   [RFC7588] Bonica, R., C. Pignataro, J. Touch, "A Widely-Deployed
             Solution to the Generic Routing Encapsulation Fragmentation
             Problem," RFC 7588, July 2015.

   [RFC7676] Pignataro, C., R. Bonica, S. Krishnan, "IPv6 Support for
             Generic Routing Encapsulation (GRE)," RFC 7676, Oct 2015.

   [Sa84]    Saltzer, J., D. Reed, D. Clark, "End-to-end arguments in
             system design," ACM Trans. on Computing Systems, Nov. 1984.

   [Te16]    Templin, F., "Asymmetric Extended Route Optimization,"
             draft-templin-aerolink-67, Jun. 2016.

   [To01]    Touch, J., "Dynamic Internet Overlay Deployment and
             Management Using the X-Bone," Computer Networks, July 2001,
             pp. 117-135.


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   [To03]    Touch, J., Y. Wang, L. Eggert, G. Finn, "Virtual Internet
             Architecture," USC/ISI Tech. Report 570, Aug. 2003.

   [To16]    Touch, J., "Middleboxes Models Compatible with the
             Internet," USC/ISI Tech. Report <TBD>, July 2016.

   [To98]    Touch, J., S. Hotz, "The X-Bone," Proc. Globecom Third
             Global Internet Mini-Conference, Nov. 1998.

   [Zi80]    Zimmermann, H., "OSI Reference Model - The ISO Model of
             Architecture for Open Systems Interconnection," IEEE Trans.
             on Comm., Apr. 1980.

9. Acknowledgments

   This document originated as the result of numerous discussions among
   the authors, Jari Arkko, Stuart Bryant, Lars Eggert, Ted Faber, Gorry
   Fairhurst, Dino Farinacci, Matt Mathis, and Fred Templin. It
   benefitted substantially from detailed feedback from Toerless Eckert,
   Vincent Roca, and Lucy Yong, as well as other members of the Internet
   Area Working Group.

   This document was prepared using 2-Word-v2.0.template.dot.

Authors' Addresses

   Joe Touch
   USC/ISI
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695
   U.S.A.

   Phone: +1 (310) 448-9151
   Email: touch@isi.edu


   W. Mark Townsley
   Cisco
   L'Atlantis, 11, Rue Camille Desmoulins
   Issy Les Moulineaux, ILE DE FRANCE 92782

   Email: townsley@cisco.com







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APPENDIX A: Fragmentation efficiency

A.1. Selecting fragment sizes

   There are different ways to fragment a packet. Consider a network
   with an MTU as shown in Figure 13, where packets are encapsulated
   over the same network layer as they arrive on (e.g., IP in IP). If a
   packet as large as the MTU arrives, it must be fragmented to
   accommodate the additional header.

                 X===========================X (MTU)
                 +----+----------------------+
                 | iH | DDDDDDDDDDDDDDDDDDDD |
                 +----+----------------------+
                   |
                   |  X===========================X (MTU)
                   |  +---+----+------------------+
               (a) +->| H'| iH | DDDDDDDDDDDDDDDD |
                   |  +---+----+------------------+
                   |      |
                   |      |  X===========================X (MTU)
                   |      |  +----+---+----+-------------+
                   | (a1) +->| nH'| H | iH | DDDDDDDDDDD |
                   |      |  +----+---+----+-------------+
                   |      |
                   |      |  +----+-------+
                   | (a2) +->| nH"| DDDDD |
                   |         +----+-------+
                   |
                   |  +---+------+
               (b) +->| H"| DDDD |
                      +---+------+
                          |
                          |  +----+---+------+
                     (b1) +->| nH'| H"| DDDD |
                             +----+---+------+

                   Figure 13Fragmenting via maximum fit

   Figure 13 shows this process, using Outer Fragmentation as an example
   (the situation is the same for Inner Fragmentation, but the headers
   that are affected differ). The arriving packet is first split into
   (a) and (b), where (a) is of the MTU of the network. However, this
   tunnel then traverses over another tunnel, whose impact the first
   tunnel ingress has not accommodated. The packet (a) arrives at the
   second tunnel ingress, and needs to be encapsulated again, but
   because it is already at the MTU, it needs to be fragmented as well,


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   into (a1) and (a2). In this case, packet (b) arrives at the second
   tunnel ingress and is encapsulated into (b1) without fragmentation,
   because it is already below the MTU size.

   In Figure 14, the fragmentation is done evenly, i.e., by splitting
   the original packet into two roughly equal-sized components, (c) and
   (d). Note that (d) contains more packet data, because (c) includes
   the original packet header because this is an example of Outer
   Fragmentation. The packets (c) and (d) arrive at the second tunnel
   encapsulator, and are encapsulated again; this time, neither packet
   exceeds the MTU, and neither requires further fragmentation.


                 X===========================X (MTU)
                 +----+----------------------+
                 | iH | DDDDDDDDDDDDDDDDDDDD |
                 +----+----------------------+
                   |
                   |  X===========================X (MTU)
                   |  +---+----+----------+
               (c) +->| H'| iH | DDDDDDDD |
                   |  +---+----+----------+
                   |      |
                   |      |  X===========================X (MTU)
                   |      |  +----+---+----+----------+
                   | (c1) +->| nH | H'| iH | DDDDDDDD |
                   |         +----+---+----+----------+
                   |
                   |  +---+--------------+
               (d) +->| H"| DDDDDDDDDDDD |
                      +---+--------------+
                          |
                          |  +----+---+--------------+
                     (d1) +->| nH | H"| DDDDDDDDDDDD |
                             +----+---+--------------+

                       Figure 14 Fragmenting evenly

A.2. Packing

   Encapsulating individual packets to traverse a tunnel can be
   inefficient, especially where headers are large relative to the
   packets being carried. In that case, it can be more efficient to
   encapsulate many small packets in a single, larger tunnel payload.
   This technique, similar to the effect of packet bursting in Gigabit
   Ethernet (regardless of whether they're encoded using L2 symbols as
   delineators), reduces the overhead of the encapsulation headers


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   (Figure 15). It reduces the work of header addition and removal at
   the tunnel endpoints, but increases other work involving the packing
   and unpacking of the component packets carried.

                     +-----+-----+
                     | iHa | iDa |
                     +-----+-----+
                           |
                           |     +-----+-----+
                           |     | iHb | iDb |
                           |     +-----+-----+
                           |           |
                           |           |     +-----+-----+
                           |           |     | iHc | iDc |
                           |           |     +-----+-----+
                           |           |           |
                           v           v           v
                +----+-----+-----+-----+-----+-----+-----+
                | oH | iHa | iHa | iHb | iDb | iHc | iDc |
                +----+-----+-----+-----+-----+-----+-----+

                  Figure 15 Packing packets into a tunnel

   [NOTE: PPP chopping and coalescing?]

























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