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Versions: (draft-briscoe-tsvwg-rfc6040update-shim) 00 01 02 03 04 05

Transport Area Working Group                                  B. Briscoe
Internet-Draft                                                 CableLabs
Updates: 6040, 2661, 2784, 3931, 4380,                 November 12, 2017
         7450 (if approved)
Intended status: Standards Track
Expires: May 16, 2018


 Propagating Explicit Congestion Notification Across IP Tunnel Headers
                          Separated by a Shim
                 draft-ietf-tsvwg-rfc6040update-shim-05

Abstract

   RFC 6040 on "Tunnelling of Explicit Congestion Notification" made the
   rules for propagation of ECN consistent for all forms of IP in IP
   tunnel.  This specification updates RFC 6040 to clarify that its
   scope includes tunnels where two IP headers are separated by at least
   one shim header that is not sufficient on its own for wide area
   packet forwarding.  It surveys widely deployed IP tunnelling
   protocols separated by such shim header(s) and updates the
   specifications of those that do not mention ECN propagation (L2TPv2,
   L2TPv3, GRE, Teredo and AMT).  This specification also updates RFC
   6040 with configuration requirements needed to make any legacy tunnel
   ingress safe.

Status of This Memo

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

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

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

   This Internet-Draft will expire on May 16, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Scope of RFC 6040 . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Feasibility of ECN Propagation between Tunnel Headers . .   4
     3.2.  Desirability of ECN Propagation between Tunnel Headers  .   5
   4.  Making a non-ECN Tunnel Ingress Safe by Configuration . . . .   5
   5.  IP-in-IP Tunnels with Tightly Coupled Shim Headers  . . . . .   6
     5.1.  Specific Updates to Protocols under IETF Change Control .   9
       5.1.1.  L2TP (v2 and v3) ECN Extension  . . . . . . . . . . .   9
       5.1.2.  GRE . . . . . . . . . . . . . . . . . . . . . . . . .  12
       5.1.3.  Teredo  . . . . . . . . . . . . . . . . . . . . . . .  13
       5.1.4.  AMT . . . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   8.  Comments Solicited  . . . . . . . . . . . . . . . . . . . . .  16
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   RFC 6040 on "Tunnelling of Explicit Congestion Notification"
   [RFC6040] made the rules for propagation of Explicit Congestion
   Notification (ECN [RFC3168]) consistent for all forms of IP in IP
   tunnel.

   A common pattern for many tunnelling protocols is to encapsulate an
   inner IP header (v4 or v6) with shim header(s) then an outer IP
   header (v4 or v6).  Some of these shim headers are designed as
   generic encapsulations, so they do not necessarily directly
   encapsulate an inner IP header.  Instead they can encapsulate headers
   such as link-layer (L2) protocols that in turn often encapsulate IP.





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   To clear up confusion, this specification clarifies that the scope of
   RFC 6040 includes any IP-in-IP tunnel, including those with shim
   header(s) and other encapsulations between the IP headers.  Where
   necessary, it updates the specifications of the relevant
   encapsulation protocols with the specific text necessary to comply
   with RFC 6040.

   This specification also updates RFC 6040 to state how operators ought
   to configure a legacy tunnel ingress to avoid unsafe system
   configurations.

2.  Terminology

   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]
   when, and only when, they appear in all capitals, as shown here.

   This specification uses the terminology defined in RFC 6040
   [RFC6040].

3.  Scope of RFC 6040

   In section 1.1 of RFC 6040, its scope is defined as:

      "...ECN field processing at encapsulation and decapsulation for
      any IP-in-IP tunnelling, whether IPsec or non-IPsec tunnels.  It
      applies irrespective of whether IPv4 or IPv6 is used for either
      the inner or outer headers. ..."

   This was intended to include cases where shim header(s) sit between
   the IP headers.  Many tunnelling implementers have interpreted the
   scope of RFC 6040 as it was intended, but it is ambiguous.
   Therefore, this specification updates RFC 6040 by adding the
   following scoping text after the sentences quoted above:

      It applies in cases where an outer IP header encapsulates an inner
      IP header either directly or indirectly by encapsulating other
      headers that in turn encapsulate (or might encapsulate) an inner
      IP header.

   There is another problem with the scope of RFC 6040.  Like many IETF
   specifications, RFC 6040 is written as a specification that
   implementations can choose to claim compliance with.  This means it
   does not cover two important cases:






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   1.  those cases where it is infeasible for an implementation to
       access an inner IP header when adding or removing an outer IP
       header;

   2.  those implementations that choose not to propagate ECN between IP
       headers.

   However, the ECN field is a non-optional part of the IP header (v4
   and v6).  So any implementation that creates an outer IP header has
   to give the ECN field some value.  There is only one safe value a
   tunnel ingress can use if it does not know whether the egress
   supports propagation of the ECN field; it has to clear the ECN field
   in any outer IP header to 0b00.

   However, an RFC has no jurisdiction over implementations that choose
   not to comply with it or cannot comply with it, including all those
   implementations that pre-dated the RFC.  Therefore it would have been
   unreasonable to add such a requirement to RFC 6040.  Nonetheless, to
   ensure safe propagation of the ECN field over tunnels, it is
   reasonable to add requirements on operators, to ensure they configure
   their tunnels safely (where possible).  Before stating these
   configuration requirements in Section 4, the factors that determine
   whether propagating ECN is feasible or desirable will be briefly
   introduced.

3.1.  Feasibility of ECN Propagation between Tunnel Headers

   In many cases shim header(s) and an outer IP header are always added
   to (or removed from) an inner IP packet as part of the same
   procedure.  We call this a tightly coupled shim header.  Processing
   the shim and outer together is often necessary because the shim(s)
   are not sufficient for packet forwarding in their own right; not
   unless complemented by an outer header.  In these cases it will often
   be feasible for an implementation to propagate the ECN field between
   the IP headers.

   In some cases a tunnel adds an outer IP header and a tightly coupled
   shim header to an inner header that is not an IP header, but that in
   turn encapsulates an IP header (or might encapsulate an IP header).
   For instance an inner Ethernet (or other link layer) header might
   encapsulate an inner IP header as its payload.  We call this a
   tightly coupled shim over an encapsulating header.

   Digging to arbitrary depths to find an inner IP header within an
   encapsulation is strictly a layering violation so it cannot be a
   required behaviour.  Nonetheless, some tunnel endpoints already look
   within a L2 header for an IP header, for instance to map the Diffserv
   codepoint between an encapsulated IP header and an outer IP header



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   [RFC2983].  In such cases at least, it should be feasible to also
   (independently) propagate the ECN field between the same IP headers.
   Thus, access to the ECN field within an encapsulating header can be a
   useful and benign optimization.  The guidelines in section 6 of
   [I-D.ietf-tsvwg-ecn-encap-guidelines] give the conditions for this
   layering violation to be benign.

3.2.  Desirability of ECN Propagation between Tunnel Headers

   Developers and network operators are encouraged to implement and
   deploy tunnel endpoints compliant with RFC 6040 (as updated by the
   present specification) in order to provide the benefits of wider ECN
   deployment [RFC8087].  Nonetheless, propagation of ECN between IP
   headers, whether separated by shim headers or not, has to be optional
   to implement and to use, because:

   o  Legacy implementations of tunnels without any ECN support already
      exist

   o  A network might be designed so that there is usually no bottleneck
      within the tunnel

   o  If the tunnel endpoints would have to search within an L2 header
      to find an encapsulated IP header, it might not be worth the
      potential performance hit

4.  Making a non-ECN Tunnel Ingress Safe by Configuration

   Even when no specific attempt has been made to implement propagation
   of the ECN field at a tunnel ingress, it ought to be possible for the
   operator to render a tunnel ingress safe by configuration.  The main
   safety concern is to disable (clear to zero) the ECN capability in
   the outer IP header at the ingress if the egress of the tunnel does
   not implement ECN logic to propagate any ECN markings into the packet
   forwarded beyond the tunnel.  Otherwise the non-ECN egress could
   discard any ECN marking introduced within the tunnel, which would
   break all the ECN-based control loops that regulate the traffic load
   over the tunnel.

   Therefore this specification updates RFC 6040 by inserting the
   following text just before the last paragraph of section 4.3:

      When the outer tunnel header is IP (v4 or v6), if possible, the
      operator MUST configure the ingress to zero the outer ECN field in
      any of the following cases:






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      *  if it is known that the tunnel egress does not support
         propagation of the ECN field (RFC 6040, RFC 4301 or the full
         functionality mode of RFC 3168)

      *  or if the behaviour of the egress is not known or an egress
         with unknown behaviour might be dynamically paired with the
         ingress.

      *  or if an IP header might be encapsulated within a non-IP header
         that the tunnel ingress is encapsulating, but the ingress does
         not inspect within the encapsulation.

   In order that the network operator can comply with the above safety
   rules, even if a tunnel ingress does not claim to support RFC 6040,
   RFC 4301 or the full functionality mode of RFC 3168, the
   implementation of the tunnel ingress:

   o  MUST make propagation of the ECN field between inner and outer IP
      headers independent of any configuration of Diffserv codepoint
      propagation;

   o  SHOULD zero the outer ECN field in its default configuration.

   There might be concern that the above "MUST" makes compliant
   equipment non-compliant at a stroke.  However, any equipment that is
   still treating the ToS octet (IPv4) or the Traffic Class octet (IPv6)
   as a single 8-bit field is already non-compliant, and has been since
   1998 when the upper 6 bits were separated off for the Diffserv
   codepoint (DSCP) [RFC2474].  For instance, copying the ECN field as a
   side-effect of copying the DSCP is a seriously unsafe bug that risks
   breaking the feedback loops that regulate load on a tunnel.

   Permanently zeroing the outer ECN field is safe, but it is not
   sufficient to claim compliance with RFC 6040 because it does not meet
   the aim of introducing ECN support to tunnels (see Section 4.3 of
   [RFC6040]).

5.  IP-in-IP Tunnels with Tightly Coupled Shim Headers

   There follows a list of specifications of encapsulations with tightly
   coupled shim header(s), in rough chronological order.  The list is
   confined to standards track or widely deployed protocols.  The list
   is not necessarily exhaustive so, for the avoidance of doubt, the
   scope of RFC 6040 is defined in Section 3 and is not limited to this
   list.

   o  PPTP (Point-to-Point Tunneling Protocol) [RFC2637];




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   o  L2TP (Layer 2 Tunnelling Protocol), specifically L2TPv2 [RFC2661]
      and L2TPv3 [RFC3931], which not only includes all the L2-specific
      specializations of L2TP, but also derivatives such as the Keyed
      IPv6 Tunnel [RFC8159];

   o  GRE (Generic Routing Encapsulation) [RFC2784] and NVGRE (Network
      Virtualization using GRE) [RFC7637];

   o  GTP (GPRS Tunnelling Protocol), specifically GTPv1 [GTPv1], GTP v1
      User Plane [GTPv1-U], GTP v2 Control Plane [GTPv2-C];

   o  Teredo [RFC4380];

   o  CAPWAP (Control And Provisioning of Wireless Access Points)
      [RFC5415];

   o  LISP (Locator/Identifier Separation Protocol) [RFC6830];

   o  AMT (Automatic Multicast Tunneling) [RFC7450];

   o  VXLAN (Virtual eXtensible Local Area Network) [RFC7348] and VXLAN-
      GPE [I-D.ietf-nvo3-vxlan-gpe];

   o  SFC (Service Function Chaining) [RFC7665];

   o  Geneve [I-D.ietf-nvo3-geneve];

   o  GUE (Generic UDP Encapsulation) [I-D.ietf-intarea-gue];

   o  Direct tunnelling of an IP packet within a UDP/IP datagram (see
      Section 3.1.11 of [RFC8085]);

   o  TCP Encapsulation of IKE and IPsec Packets (see Section 12.5 of
      [RFC8229]).

   Some of the listed protocols enable encapsulation of a variety of
   network layer protocols as inner and/or outer.  This specification
   applies in the cases where there is an inner and outer IP header as
   described in Section 3.  Otherwise
   [I-D.ietf-tsvwg-ecn-encap-guidelines] gives guidance on how to design
   propagation of ECN into other protocols that might encapsulate IP.

   Where protocols in the above list need to be updated to specify ECN
   propagation and they are under IETF change control, update text is
   given in the following subsections.  For those not under IETF
   control, it is RECOMMENDED that implementations of encapsulation and
   decapsulation comply with RFC 6040.  It is also RECOMMENDED that




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   their specifications are updated to add a requirement to comply with
   RFC 6040 (as updated by the present document).

   PPTP is not under the change control of the IETF, but it has been
   documented in an informational RFC [RFC2637].  However, there is no
   need for the present specification to update PPTP because L2TP has
   been developed as a standardized replacement.

   NVGRE is not under the change control of the IETF, but it has been
   documented in an informational RFC [RFC7637].  NVGRE is a specific
   use-case of GRE (it re-purposes the key field from the initial
   specification of GRE [RFC1701] as a Virtual Subnet ID).  Therefore
   the text that updates GRE in Section 5.1.2 below is also intended to
   update NVGRE.

   Although the definition of the various GTP shim headers is under the
   control of the 3GPP, it is hard to determine whether the 3GPP or the
   IETF controls standardization of the _process_ of adding both a GTP
   and an IP header to an inner IP header.  Nonetheless, the present
   specification is provided so that the 3GPP can refer to it from any
   of its own specifications of GTP and IP header processing.

   The specification of CAPWAP already specifies RFC 3168 ECN
   propagation and ECN capability negotiation.  Without modification the
   CAPWAP specification already interworks with the backward compatible
   updates to RFC 3168 in RFC 6040.

   LISP made the ECN propagation procedures in RFC 3168 mandatory from
   the start.  RFC 3168 has since been updated by RFC 6040, but the
   changes are backwards compatible so there is still no need for LISP
   tunnel endpoints to negotiate their ECN capabilities.

   VXLAN is not under the change control of the IETF but it has been
   documented in an informational RFC.  In contrast, VXLAN-GPE (Generic
   Protocol Extension) is being documented under IETF change control.
   It is RECOMMENDED that VXLAN and VXLAN-GPE implementations comply
   with RFC 6040 when the VXLAN header is inserted between (or removed
   from between) IP headers.  The authors of any future update to these
   specifications are encouraged to add a requirement to comply with RFC
   6040 as updated by the present specification.

   Although the Service Function Chaining (SFC) architecture [RFC7665]
   depends on a shim-based encapsulation to identify the service
   function path (SFP), it does not specify the processing of ECN when
   handling transport encapsulation.

   The specifications of Geneve and GUE already refer to RFC 6040 for
   ECN encapsulation.



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   Section 3.1.11 of the UDP usage guidelines [RFC8085] already explains
   that a tunnel that encapsulates an IP header directly within a UDP/IP
   datagram needs to follow RFC 6040 when propagating the ECN field
   between inner and outer IP headers.  The requirements in Section 4
   update RFC 6040 so, by reference, they automatically update RFC 8085
   to add the important but previously unstated requirement that, if the
   UDP tunnel egress does not, or might not, support ECN propagation, a
   legacy UDP tunnel ingress has to clear the outer IP ECN field to
   0b00, e.g. by configuration.

   Section 12.5 of TCP Encapsulation of IKE and IPsec Packets [RFC8229]
   already recommends the compatibility mode of RFC 6040 in this case,
   because there is not a one-to-one mapping between inner and outer
   packets.

5.1.  Specific Updates to Protocols under IETF Change Control

5.1.1.  L2TP (v2 and v3) ECN Extension

   The L2TP terminology used here is defined in [RFC2661] and [RFC3931].

   L2TPv3 [RFC3931] is used as a shim header between any packet-switched
   network (PSN) header (e.g.  IPv4, IPv6, MPLS) and many types of layer
   2 (L2) header.  The L2TPv3 shim header encapsulates an L2-specific
   sub-layer then an L2 header that is likely to contain an inner IP
   header (v4 or v6).  Then this whole stack of headers can be
   encapsulated optionally within an outer UDP header then an outer PSN
   header that is typically IP (v4 or v6).

   L2TPv2 is used as a shim header between any PSN header and a PPP
   header, which is in turn likely to encapsulate an IP header.

   Even though these shims are rather fat (particularly in the case of
   L2TPv3), they still fit the definition of a tightly coupled shim
   header over an encapsulating header (Section 3.1), because all the
   headers encapsulating the L2 header are added (or removed) together.
   L2TPv2 and L2TPv3 are therefore within the scope of RFC 6040, as
   updated by Section 3 above.

   L2TP maintainers are RECOMMENDED to implement the ECN extension to
   L2TPv2 and L2TPv3 defined in Section 5.1.1.2 below, in order to
   provide the benefits of ECN [RFC8087], whenever a node within an L2TP
   tunnel becomes the bottleneck for an end-to-end traffic flow.








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5.1.1.1.  Safe Configuration of a 'Non-ECN' Ingress LCCE

   The following text is appended to both Section 5.3 of [RFC2661] and
   Section 4.5 of [RFC3931] as an update to the base L2TPv2 and L2TPv3
   specifications:

      The operator of an LCCE that does not support the ECN Extension in
      Section 5.1.1.2 of RFCXXXX MUST follow the configuration
      requirements in Section 4 of RFCXXXX to ensure it clears the outer
      IP ECN field to 0b00 when the outer PSN header is IP (v4 or v6).
      {RFCXXXX refers to the present document so it will need to be
      inserted by the RFC Editor}

   In particular, for an LCCE implementation that does not support the
   ECN Extension, this means that configuration of how it propagates the
   ECN field between inner and outer IP headers MUST be independent of
   any configuration of the Diffserv extension of L2TP [RFC3308].

5.1.1.2.  ECN Extension for L2TP (v2 or v3)

   When the outer PSN header and the payload inside the L2 header are
   both IP (v4 or v6), to comply with RFC 6040, an LCCE will follow the
   rules for propagation of the ECN field at ingress and egress in
   Section 4 of RFC 6040 [RFC6040].

   Before encapsulating any data packets, RFC 6040 requires an ingress
   LCCE to check that the egress LCCE supports ECN propagation as
   defined in RFC 6040 or one of its compatible predecessors ([RFC4301]
   or the full functionality mode of [RFC3168]).  If the egress supports
   ECN propagation, the ingress LCCE can use the normal mode of
   encapsulation (copying the ECN field from inner to outer).
   Otherwise, the ingress LCCE has to use compatibility mode [RFC6040]
   (clearing the outer IP ECN field to 0b00).

   An LCCE can determine the remote LCCE's support for ECN either
   statically (by configuration) or by dynamic discovery during setup of
   each control connection between the LCCEs, using the Capability AVP
   defined in Section 5.1.1.2.1 below.

   Where the outer PSN header is some protocol other than IP that
   supports ECN, the appropriate ECN propagation specification will need
   to be followed, e.g.  "Explicit Congestion Marking in MPLS"
   [RFC5129].  Where no specification exists for ECN propagation by a
   particular PSN, [I-D.ietf-tsvwg-ecn-encap-guidelines] gives general
   guidance on how to design ECN propagation into a protocol that
   encapsulates IP.





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5.1.1.2.1.  LCCE Capability AVP for ECN Capability Negotiation

   The LCCE Capability Attribute-Value Pair (AVP) defined here has
   Attribute Type ZZ.  The Attribute Value field for this AVP is a bit-
   mask with the following 16-bit format:

          0                   1
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |X X X X X X X X X X X X X X X E|
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 1: Value Field for the LCCE Capability Attribute

   This AVP MAY be present in the following message types: SCCRQ and
   SCCRP (Start-Control-Connection-Request and Start-Control-Connection-
   Reply).  This AVP MAY be hidden (the H-bit set to 0 or 1) and is
   optional (M-bit not set).  The length (before hiding) of this AVP
   MUST be 8 octets.  The Vendor ID is the IETF Vendor ID of 0.

   Bit 15 of the Value field of the LCCE Capability AVP is defined as
   the ECN Capability flag (E).  When the ECN Capability flag is set to
   1, it indicates that the sender supports ECN propagation.  When the
   ECN Capability flag is cleared to zero, or when no LCCE Capabiliy AVP
   is present, it indicates that the sender does not support ECN
   propagation.  All the other bits are reserved.  They MUST be cleared
   to zero when sent and ignored when received or forwarded.

   An LCCE initiating a control connection will send a Start-Control-
   Connection-Request (SCCRQ) containing an LCCE Capability AVP with the
   ECN Capability flag set to 1.  If the tunnel terminator supports ECN,
   it will return a Start-Control-Connection-Reply (SCCRP) that also
   includes an LCCE Capability AVP with the ECN Capability flag set to
   1.  Then, for any sessions created by that control connection, both
   ends of the tunnel can use the normal mode of RFC 6040, i.e. it can
   copy the IP ECN field from inner to outer when encapsulating data
   packets.

   If, on the other hand, the tunnel terminator does not support ECN it
   will ignore the ECN flag in the LCCE Capability AVP and send an SCCRP
   to the tunnel initiator without a Capability AVP (or with a
   Capability AVP but with the ECN Capability flag cleared to zero).
   The tunnel initiator interprets the absence of the ECN Capability
   flag in the SCCRP as an indication that the tunnel terminator is
   incapable of supporting ECN.  When encapsulating data packets for any
   sessions created by that control connection, the tunnel initiator
   will then use the compatibility mode of RFC 6040 to clear the ECN
   field of the outer IP header to 0b00.



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   If the tunnel terminator does not support this ECN extension, the
   network operator is still expected to configure it to comply with the
   safety provisions set out in Section 5.1.1.1 above, when it acts as
   an ingress LCCE.

5.1.2.  GRE

   The GRE terminology used here is defined in [RFC2784].  GRE is often
   used as a tightly coupled shim header between IP headers.  Sometimes
   the GRE shim header encapsulates an L2 header, which might in turn
   encapsulate an IP header.  Therefore GRE is within the scope of RFC
   6040 as updated by Section 3 above.

   GRE tunnel endpoint maintainers are RECOMMENDED to support [RFC6040]
   as updated by the present specification, in order to provide the
   benefits of ECN [RFC8087] whenever a node within a GRE tunnel becomes
   the bottleneck for an end-to-end IP traffic flow tunnelled over GRE
   using IP as the delivery protocol (outer header).

   GRE itself does not support dynamic set-up and configuration of
   tunnels.  However, control plane protocols such as Mobile IPv4 (MIP4)
   [RFC5944], Mobile IPv6 (MIP6) [RFC6275], Proxy Mobile IP (PMIP)
   [RFC5845] and IKEv2 [RFC5996] are sometimes used to set up GRE
   tunnels dynamically.

   When these control protocols set up IP-in-IP or IPSec tunnels, it is
   likely that they propagate the ECN field as defined in RFC 6040 or
   one of its compatible predecessors (RFC 4301 or the full
   functionality mode of RFC 3168).  However, if they use a GRE
   encapsulation, this presumption is less sound.

   Therefore, If the outer delivery protocol is IP (v4 or v6) the
   operator is obliged to follow the safe configuration requirements in
   Section 4 above.  Section 5.1.2.1 below updates the base GRE
   specification with this requirement, to emphasize its importance.

   Where the delivery protocol is some protocol other than IP that
   supports ECN, the appropriate ECN propagation specification will need
   to be followed, e.g Explicit Congestion Marking in MPLS [RFC5129].
   Where no specification exists for ECN propagation by a particular
   PSN, [I-D.ietf-tsvwg-ecn-encap-guidelines] gives more general
   guidance on how to propagate ECN to and from protocols that
   encapsulate IP.








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5.1.2.1.  Safe Configuration of a 'Non-ECN' GRE Ingress

   The following text is appended to Section 3 of [RFC2784] as an update
   to the base GRE specification:

      The operator of a GRE tunnel ingress MUST follow the configuration
      requirements in Section 4 of RFCXXXX when the outer delivery
      protocol is IP (v4 or v6).  {RFCXXXX refers to the present
      document so it will need to be inserted by the RFC Editor}

5.1.3.  Teredo

   Teredo [RFC4380] provides a way to tunnel IPv6 over an IPv4 network,
   with a UDP-based shim header between the two.

   For Teredo tunnel endpoints to provide the benefits of ECN, the
   Teredo specification would have to be updated to include negotiation
   of the ECN capability between Teredo tunnel endpoints.  Otherwise it
   would be unsafe for a Teredo tunnel ingress to copy the ECN field to
   the IPv6 outer.

   It is believed that current implementations do not support
   propagation of ECN, but that they do safely zero the ECN field in the
   outer IPv6 header.  However the specification does not mention
   anything about this.

   To make existing Teredo deployments safe, it would be possible to add
   ECN capability negotiation to those that are subject to remote OS
   update.  However, for those implementations not subject to remote OS
   update, it will not be feasible to require them to be configured
   correctly, because Teredo tunnel endpoints are generally deployed on
   hosts.

   Therefore, until ECN support is added to the specification of Teredo,
   the only feasible further safety precaution available here is to
   update the specification of Teredo implementations with the following
   text, as a new section 5.1.3:

      "5.1.3 Safe 'Non-ECN' Teredo Encapsulation

      A Teredo tunnel ingress implementation that does not support ECN
      propagation as defined in RFC 6040 or one of its compatible
      predecessors (RFC 4301 or the full functionality mode of RFC 3168)
      MUST zero the ECN field in the outer IPv6 header."







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5.1.4.  AMT

   Automatic Multicast Tunneling (AMT [RFC7450]) is a tightly coupled
   shim header that encapsulates an IP packet and is itself encapsulated
   within a UDP/IP datagram.  Therefore AMT is within the scope of RFC
   6040 as updated by Section 3 above.

   AMT tunnel endpoint maintainers are RECOMMENDED to support [RFC6040]
   as updated by the present specification, in order to provide the
   benefits of ECN [RFC8087] whenever a node within an AMT tunnel
   becomes the bottleneck for an IP traffic flow tunnelled over AMT.

   To comply with RFC 6040, an AMT relay and gateway will follow the
   rules for propagation of the ECN field at ingress and egress
   respectively, as described in Section 4 of RFC 6040 [RFC6040].

   Before encapsulating any data packets, RFC 6040 requires an ingress
   AMT relay to check that the egress AMT gateway supports ECN
   propagation as defined in RFC 6040 or one of its compatible
   predecessors (RFC 4301 or the full functionality mode of RFC 3168).
   If the egress gateway supports ECN, the ingress relay can use the
   normal mode of encapsulation (copying the IP ECN field from inner to
   outer).  Otherwise, the ingress relay has to use compatibility mode,
   which means it has to clear the outer ECN field to zero [RFC6040].

   An AMT tunnel is created dynamically (not manually), so the relay
   will need to determine the remote gateway's support for ECN using the
   ECN capability declaration defined in Section 5.1.4.2 below.

5.1.4.1.  Safe Configuration of a 'Non-ECN' Ingress AMT Relay

   The following text is appended to Section 4.2.2 of [RFC7450] as an
   update to the AMT specification:

      The operator of an AMT relay that does not support RFC 6040 or one
      of its compatible predecessors (RFC 4301 or the full functionality
      mode of RFC 3168) MUST follow the configuration requirements in
      Section 4 of RFCXXXX to ensure it clears the outer IP ECN field to
      zero. {RFCXXXX refers to the present document so it will need to
      be inserted by the RFC Editor}

5.1.4.2.  ECN Capability Declaration of an AMT Gateway









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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  V=0  |Type=3 |  Reserved |E|P|            Reserved           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Request Nonce                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


               Figure 2: Updated AMT Request Message Format

   Bit 14 of the AMT Request Message counting from 0 (or bit 7 of the
   Reserved field counting from 1) is defined here as the AMT Gateway
   ECN Capability flag (E), as shown in Figure 2.  The definitions of
   all other fields in the AMT Request Message are unchanged from RFC
   7450.

   When the E flag is set to 1, it indicates that the sender of the
   message supports RFC 6040 ECN propagation.  When it is cleared to
   zero, it indicates the sender of the message does not support RFC
   6040 ECN propagation.  An AMT gateway "that supports RFC 6040 ECN
   propagation" means one that propagates the ECN field to the forwarded
   data packet based on the combination of arriving inner and outer ECN
   fields, as defined in Section 4 of RFC 6040.

   The other bits of the Reserved field remain reserved.  They will
   continue to be cleared to zero when sent and ignored when either
   received or forwarded, as specified in Section 5.1.3.3. of RFC 7450.

   An AMT gateway that does not support RFC 6040 MUST NOT set the E flag
   of its Request Message to 1.

   An AMT gateway that supports RFC 6040 ECN propagation MUST set the E
   flag of its Relay Discovery Message to 1.

   The action of the corresponding AMT relay that receives a Request
   message with the E flag set to 1 depends on whether the relay itself
   supports RFC 6040 ECN propagation:

   o  If the relay supports RFC 6040 ECN propagation, it will store the
      ECN capability of the gateway along with its address.  Then
      whenever it tunnels datagrams towards this gateway, it MUST use
      the normal mode of RFC 6040 to propagate the ECN field when
      encapsulating datagrams (i.e. it copies the IP ECN field from
      inner to outer).






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   o  If the discovered AMT relay does not support RFC 6040 ECN
      propagation, it will ignore the E flag in the Reserved field, as
      per section 5.1.3.3. of RFC 7450.

      If the AMT relay does not support RFC 6040 ECN propagation, the
      network operator is still expected to configure it to comply with
      the safety provisions set out in Section 5.1.4.1 above.

6.  IANA Considerations

   IANA is requested to assign the following L2TP Control Message
   Attribute Value Pair:

              +----------------+----------------+-----------+
              | Attribute Type | Description    | Reference |
              +----------------+----------------+-----------+
              | ZZ             | ECN Capability | RFCXXXX   |
              +----------------+----------------+-----------+

   [TO BE REMOVED: This registration should take place at the following
   location: https://www.iana.org/assignments/l2tp-parameters/l2tp-
   parameters.xhtml ]

7.  Security Considerations

   The Security Considerations in [RFC6040] and
   [I-D.ietf-tsvwg-ecn-encap-guidelines] apply equally to the scope
   defined for the present specification.

8.  Comments Solicited

   Comments and questions are encouraged and very welcome.  They can be
   addressed to the IETF Transport Area working group mailing list
   <tsvwg@ietf.org>, and/or to the authors.

9.  Acknowledgements

   Thanks to Ing-jyh (Inton) Tsang for initial discussions on the need
   for ECN propagation in L2TP and its applicability.  Thanks also to
   Carlos Pignataro, Tom Herbert, Ignacio Goyret, Alia Atlas, Praveen
   Balasubramanian, Joe Touch, Mohamed Boucadair, David Black, Jake
   Holland and Sri Gundavelli for helpful advice and comments.  "A
   Comparison of IPv6-over-IPv4 Tunnel Mechanisms" [RFC7059] helped to
   identify a number of tunnelling protocols to include within the scope
   of this document.






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   Bob Briscoe was part-funded by the Research Council of Norway through
   the TimeIn project.  The views expressed here are solely those of the
   authors.

10.  References

10.1.  Normative References

   [I-D.ietf-tsvwg-ecn-encap-guidelines]
              Briscoe, B., Kaippallimalil, J., and P. Thaler,
              "Guidelines for Adding Congestion Notification to
              Protocols that Encapsulate IP", draft-ietf-tsvwg-ecn-
              encap-guidelines-09 (work in progress), July 2017.

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

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
              G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
              RFC 2661, DOI 10.17487/RFC2661, August 1999,
              <https://www.rfc-editor.org/info/rfc2661>.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              DOI 10.17487/RFC2784, March 2000,
              <https://www.rfc-editor.org/info/rfc2784>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3931]  Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
              "Layer Two Tunneling Protocol - Version 3 (L2TPv3)",
              RFC 3931, DOI 10.17487/RFC3931, March 2005,
              <https://www.rfc-editor.org/info/rfc3931>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.



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   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              DOI 10.17487/RFC4380, February 2006,
              <https://www.rfc-editor.org/info/rfc4380>.

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January
              2008, <https://www.rfc-editor.org/info/rfc5129>.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <https://www.rfc-editor.org/info/rfc6040>.

10.2.  Informative References

   [GTPv1]    3GPP, "GPRS Tunnelling Protocol (GTP) across the Gn and Gp
              interface", Technical Specification TS 29.060.

   [GTPv1-U]  3GPP, "General Packet Radio System (GPRS) Tunnelling
              Protocol User Plane (GTPv1-U)", Technical Specification TS
              29.281.

   [GTPv2-C]  3GPP, "Evolved General Packet Radio Service (GPRS)
              Tunnelling Protocol for Control plane (GTPv2-C)",
              Technical Specification TS 29.274.

   [I-D.ietf-intarea-gue]
              Herbert, T., Yong, L., and O. Zia, "Generic UDP
              Encapsulation", draft-ietf-intarea-gue-04 (work in
              progress), May 2017.

   [I-D.ietf-nvo3-geneve]
              Gross, J., Ganga, I., and T. Sridhar, "Geneve: Generic
              Network Virtualization Encapsulation", draft-ietf-
              nvo3-geneve-05 (work in progress), September 2017.

   [I-D.ietf-nvo3-vxlan-gpe]
              Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol
              Extension for VXLAN", draft-ietf-nvo3-vxlan-gpe-05 (work
              in progress), October 2017.

   [RFC1701]  Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
              Routing Encapsulation (GRE)", RFC 1701,
              DOI 10.17487/RFC1701, October 1994,
              <https://www.rfc-editor.org/info/rfc1701>.






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   [RFC2637]  Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little,
              W., and G. Zorn, "Point-to-Point Tunneling Protocol
              (PPTP)", RFC 2637, DOI 10.17487/RFC2637, July 1999,
              <https://www.rfc-editor.org/info/rfc2637>.

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

   [RFC3308]  Calhoun, P., Luo, W., McPherson, D., and K. Peirce, "Layer
              Two Tunneling Protocol (L2TP) Differentiated Services
              Extension", RFC 3308, DOI 10.17487/RFC3308, November 2002,
              <https://www.rfc-editor.org/info/rfc3308>.

   [RFC5415]  Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
              Ed., "Control And Provisioning of Wireless Access Points
              (CAPWAP) Protocol Specification", RFC 5415,
              DOI 10.17487/RFC5415, March 2009,
              <https://www.rfc-editor.org/info/rfc5415>.

   [RFC5845]  Muhanna, A., Khalil, M., Gundavelli, S., and K. Leung,
              "Generic Routing Encapsulation (GRE) Key Option for Proxy
              Mobile IPv6", RFC 5845, DOI 10.17487/RFC5845, June 2010,
              <https://www.rfc-editor.org/info/rfc5845>.

   [RFC5944]  Perkins, C., Ed., "IP Mobility Support for IPv4, Revised",
              RFC 5944, DOI 10.17487/RFC5944, November 2010,
              <https://www.rfc-editor.org/info/rfc5944>.

   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5996, DOI 10.17487/RFC5996, September 2010,
              <https://www.rfc-editor.org/info/rfc5996>.

   [RFC6275]  Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
              Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
              2011, <https://www.rfc-editor.org/info/rfc6275>.

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,
              <https://www.rfc-editor.org/info/rfc6830>.

   [RFC7059]  Steffann, S., van Beijnum, I., and R. van Rein, "A
              Comparison of IPv6-over-IPv4 Tunnel Mechanisms", RFC 7059,
              DOI 10.17487/RFC7059, November 2013,
              <https://www.rfc-editor.org/info/rfc7059>.




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   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
              <https://www.rfc-editor.org/info/rfc7348>.

   [RFC7450]  Bumgardner, G., "Automatic Multicast Tunneling", RFC 7450,
              DOI 10.17487/RFC7450, February 2015,
              <https://www.rfc-editor.org/info/rfc7450>.

   [RFC7637]  Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network
              Virtualization Using Generic Routing Encapsulation",
              RFC 7637, DOI 10.17487/RFC7637, September 2015,
              <https://www.rfc-editor.org/info/rfc7637>.

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,
              <https://www.rfc-editor.org/info/rfc7665>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8087]  Fairhurst, G. and M. Welzl, "The Benefits of Using
              Explicit Congestion Notification (ECN)", RFC 8087,
              DOI 10.17487/RFC8087, March 2017,
              <https://www.rfc-editor.org/info/rfc8087>.

   [RFC8159]  Konstantynowicz, M., Ed., Heron, G., Ed., Schatzmayr, R.,
              and W. Henderickx, "Keyed IPv6 Tunnel", RFC 8159,
              DOI 10.17487/RFC8159, May 2017,
              <https://www.rfc-editor.org/info/rfc8159>.

   [RFC8229]  Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
              of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229,
              August 2017, <https://www.rfc-editor.org/info/rfc8229>.

Author's Address

   Bob Briscoe
   CableLabs
   UK

   EMail: ietf@bobbriscoe.net
   URI:   http://bobbriscoe.net/




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