[Rtg-dt-encap-considerations] Fwd: New Version Notification for draft-rtg-dt-encap-02.txt

Erik Nordmark <nordmark@acm.org> Fri, 22 May 2015 16:21 UTC

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Date: Fri, 22 May 2015 09:21:15 -0700
From: Erik Nordmark <nordmark@acm.org>
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The design team has updated this document based on comments we received 
in the meetings and hallways in the Dallas.

I think the chairs are planning to ask for WG adoption, so it would be 
good for folks to take a look at the draft.

The change log section summarizes the changes:

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RTGWG                                                   E. Nordmark (ed)
Internet-Draft                                           Arista Networks
Intended status: Informational                                   A. Tian
Expires: November 22, 2015                                 Ericsson Inc.
                                                                 J. Gross
                                                                   VMware
                                                                J. Hudson
                                          Brocade Communications Systems,
                                                                     Inc.
                                                               L. Kreeger
                                                      Cisco Systems, Inc.
                                                                  P. Garg
                                                                Microsoft
                                                                P. Thaler
                                                     Broadcom Corporation
                                                               T. Herbert
                                                                   Google
                                                             May 21, 2015

                       Encapsulation Considerations
                          draft-rtg-dt-encap-02

Abstract

    The IETF Routing Area director has chartered a design team to look at
    common issues for the different data plane encapsulations being
    discussed in the NVO3 and SFC working groups and also in the BIER
    BoF, and also to look at the relationship between such encapsulations
    in the case that they might be used at the same time.  The purpose of
    this design team is to discover, discuss and document considerations
    across the different encapsulations in the different WGs/BoFs so that
    we can reduce the number of wheels that need to be reinvented in the
    future.

Status of this Memo

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

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

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

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    material or to cite them other than as "work in progress."

    This Internet-Draft will expire on November 22, 2015.

Copyright Notice

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

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

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Table of Contents

    1.  Design Team Charter  . . . . . . . . . . . . . . . . . . . . .  4
    2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
    3.  Common Issues  . . . . . . . . . . . . . . . . . . . . . . . .  6
    4.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
    5.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . . .  7
    6.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  8
    7.  Entropy  . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
    8.  Next-protocol indication . . . . . . . . . . . . . . . . . . .  9
    9.  MTU and Fragmentation  . . . . . . . . . . . . . . . . . . . . 11
    10. OAM  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
    11. Security Considerations  . . . . . . . . . . . . . . . . . . . 14
      11.1.  Encapsulation-specific considerations . . . . . . . . . . 14
      11.2.  Virtual network isolation . . . . . . . . . . . . . . . . 16
      11.3.  Packet level security . . . . . . . . . . . . . . . . . . 17
      11.4.  In summary: . . . . . . . . . . . . . . . . . . . . . . . 17
    12. QoS  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
    13. Congestion Considerations  . . . . . . . . . . . . . . . . . . 18
    14. Header Protection  . . . . . . . . . . . . . . . . . . . . . . 20
    15. Extensibility Considerations . . . . . . . . . . . . . . . . . 22
    16. Layering Considerations  . . . . . . . . . . . . . . . . . . . 25
    17. Service model  . . . . . . . . . . . . . . . . . . . . . . . . 26
    18. Hardware Friendly  . . . . . . . . . . . . . . . . . . . . . . 27
      18.1.  Considerations for NIC offload  . . . . . . . . . . . . . 28
    19. Middlebox Considerations . . . . . . . . . . . . . . . . . . . 32
    20. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 33
    21. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
    22. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 34
    23. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 35
    24. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
      24.1.  Normative References  . . . . . . . . . . . . . . . . . . 35
      24.2.  Informative References  . . . . . . . . . . . . . . . . . 37
    Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40

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1.  Design Team Charter

    There have been multiple efforts over the years that have resulted in
    new or modified data plane behaviors involving encapsulations.  That
    includes IETF efforts like MPLS, LISP, and TRILL but also industry
    efforts like VXLAN and NVGRE.  These collectively can be seen as a
    source of insight into the properties that data planes need to meet.
    The IETF is currently working on potentially new encapsulations in
    NVO3 and SFC and considering working on BIER.  In addition there is
    work on tunneling in the INT area.

    This is a short term design team chartered to collect and construct
    useful advice to parties working on new or modified data plane
    behaviors that include additional encapsulations.  The goal is for
    the group to document useful advice gathered from interacting with
    ongoing efforts.  An Internet Draft will be produced for IETF92 to
    capture that advice, which will be discussed in RTGWG.

    Data plane encapsulations face a set of common issues such as:
    o  How to provide entropy for ECMP
    o  Issues around packet size and fragmentation/reassembly
    o  OAM - what support is needed in an encapsulation format?
    o  Security and privacy.
    o  QoS
    o  Congestion Considerations
    o  IPv6 header protection (zero UDP checksum over IPv6 issue)
    o  Extensibility - e.g., for evolving OAM, security, and/or
       congestion control
    o  Layering of multiple encapsulations e.g., SFC over NVO3 over BIER
    The design team will provide advice on those issues.  The intention
    is that even where we have different encapsulations for different
    purposes carrying different information, each such encapsulation
    doesn't have to reinvent the wheel for the above common issues.

    The design team will look across the routing area in particular at
    SFC, NVO3 and BIER.  It will not be involved in comparing or
    analyzing any particular encapsulation formats proposed in those WGs
    and BoFs but instead focus on common advice.

2.  Overview

    The references provide background information on NVO3, SFC, and BIER.
    In particular, NVO3 is introduced in [RFC7364], [RFC7365], and
    [I-D.ietf-nvo3-arch].  SFC is introduced in
    [I-D.ietf-sfc-architecture] and [I-D.ietf-sfc-problem-statement].
    Finally, the information on BIER is in
    [I-D.shepherd-bier-problem-statement],

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    [I-D.wijnands-bier-architecture], and
    [I-D.wijnands-mpls-bier-encapsulation].  We assume the reader has
    some basic familiarity with those proposed encapsulations.  The
    Related Work section points at some prior work that relates to the
    encapsulation considerations in this document.

    Encapsulation protocols typically have some unique information that
    they need to carry.  In some cases that information might be modified
    along the path and in other cases it is constant.  The in-flight
    modifications has impacts on what it means to provide security for
    the encapsulation headers.
    o  NVO3 carries a VNI Identifier edge to edge which is not modified.
       There has been OAM discussions in the WG and it isn't clear
       whether some of the OAM information might be modified in flight.
    o  SFC carries service meta-data which might be modified or
       unmodified as the packets follow the service path.  SFC talks of
       some loop avoidance mechanism which is likely to result in
       modifications for for each hop in the service chain even if the
       meta-data is unmodified.
    o  BIER carries a bitmap of egress ports to which a packet should be
       delivered, and as the packet is forwarded down different paths
       different bits are cleared in that bitmap.

    Even if information isn't modified in flight there might be devices
    that wish to inspect that information.  For instance, one can
    envision future NVO3 security devices which filter based on the
    virtual network identifier.

    The need for extensibility is different across the protocols
    o  NVO3 might need some extensions for OAM and security.
    o  SFC is all about carrying service meta-data along a path, and
       different services might need different types and amount of meta-
       data.
    o  BIER might need variable number of bits in their bitmaps, or other
       future schemes to scale up to larger network.
    The extensibility needs and constraints might be different when
    considering hardware vs. software implementations of the
    encapsulation headers.  NIC hardware might have different constraints
    than switch hardware.

    As the IETF designs these encapsulations the different WGs solve the
    issues for their own encapsulation.  But there are likely to be
    future cases when the different encapsulations are combined in the
    same header.  For instance, NVO3 might be a "transport" used to carry
    SFC between the different hops in the service chain.

    Most of the issues discussed in this document are not new.  The IETF
    and industry as specified and deployed many different encapsulation

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    or tunneling protocols over time, ranging from simple IP-in-IP and
    GRE encapsulation, IPsec, pseudo-wires, session-based approached like
    L2TP, and the use of MPLS control and data planes.  IEEE 802 has also
    defined layered encapsulation for Provider Backbone Bridges (PBB) and
    IEEE 802.1Qbp (ECMP).  This document tries to leverage what we
    collectively have learned from that experience and summarize what
    would be relevant for new encapsulations like NVO3, SFC, and BIER.

3.  Common Issues

    [This section is mostly a repeat of the charter but with a few
    modifications and additions.]

    Any new encapsulation protocol would need to address a large set of
    issues that are not central to the new information that this protocol
    intends to carry.  The common issues explored in this document are:
    o  How to provide entropy for Equal Cost MultiPath (ECMP) routing
    o  Issues around packet size and fragmentation/reassembly
    o  Next header indication - each encapsulation might be able to carry
       different payloads
    o  OAM - what support is needed in an encapsulation format?
    o  Security and privacy
    o  QoS
    o  Congestion Considerations
    o  Header protection
    o  Extensibility - e.g., for evolving OAM, security, and/or
       congestion control
    o  Layering of multiple encapsulations e.g., SFC over NVO3 over BIER
    o  Importance of being friendly to hardware and software
       implementations

    The degree to which these common issues apply to a particular
    encapsulation can differ based on the intended purpose of the
    encapsulation.  But it is useful to understand all of them before
    determining which ones apply.

4.  Scope

    It is important to keep in mind what we are trying to cover and not
    cover in this document and effort.  This is
    o  A look across the three new encapsulations, while taking lots of
       previous work into account
    o  Focus on the class of encapsulations that would run over IP/UDP.
       That was done to avoid being distracted by the data-plane and
       control-plane interaction, which is more significant for protocols
       that are designed to run over "transports" that maintain session

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       or path state.
    o  We later expanded the scope somewhat to consider how the
       encapsulations would play with MPLS "transport", which is
       important because SFC and BIER seem to target being independent of
       the underlying "transport"

    However, this document and effort is NOT intended to:
    o  Design some new encapsulation header to rule them all
    o  Design yet another new NVO3 encapsulation header
    o  Try to select the best encapsulation header
    o  Evaluate any existing and proposed encapsulations

    While the origin and focus of this document is the routing area and
    in particular NVO3, SFC, and BIER, the considerations apply to other
    encapsulations that are being defined in the IETF and elsewhere.
    There seems to be an increase in the number of encapsulations being
    defined to run over UDP, where there might already exist an
    encapsulation over IP or Ethernet.  Feedback on how these
    considerations apply in those contexts is welcome.

5.  Assumptions

    The design center for the new encapsulations is a well-managed
    network.  That network can be a datacenter network (plus datacenter
    interconnect) or a service provider network.  Based on the existing
    and proposed encapsulations in those environment it is reasonable to
    make these assumptions:
    o  The MTU is carefully managed and configured.  Hence an
       encapsulation protocol can make the packets bigger without
       resulting in a requirement for fragmentation and reassembly
       between ingress and egress.  (However, it might be useful to
       detecting MTU misconfigurations.)
    o  In general an encapsulation needs some approach for congestion
       management.  But the assumptions are different than for arbitrary
       Internet paths in that the underlay might be well-provisioned and
       better policed at the edge, and due to multi-tenancy, the
       congestion control in the endpoints might be even less trusted
       than on the Internet at large.

    The goal is to implement these encapsulations in hardware and
    software hence we can't assume that the needs of either
    implementation approach can trump the needs of the other.  In
    particular, around extensibility the needs and constraints might be
    quite different.

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6.  Terminology

    The capitalized keyword MUST is used as defined in
    http://en.wikipedia.org/wiki/Julmust

    TBD: Refer to existing documents for at least NVO3 and SFC
    terminology.  We use at least the VNI ID in this document.

7.  Entropy

    In many cases the encapsulation format needs to enable ECMP in
    unmodified routers.  Those routers might use different fields in TCP/
    UDP packets to do ECMP without a risk of reordering a flow.

    The common way to do ECMP-enabled encapsulation over IP today is to
    add a UDP header and to use UDP with the UDP source port carrying
    entropy from the inner/original packet headers as in LISP [RFC6830].
    The total entropy consists of 14 bits in the UDP source port (using
    the ephemeral port range) plus the outer IP addresses which seems to
    be sufficient for entropy; using outer IPv6 headers would give the
    option for more entropy should it be needed in the future.

    In some environments it might be fine to use all 16 bits of the port
    range.  However, middleboxes might make assumptions about the system
    ports or user ports.  But they should not make any assumptions about
    the ports in the Dynamic and/or Private Port range, which have the
    two MSBs set to 11b.

    The UDP source port might change over the lifetime of an encapsulated
    flow, for instance for DoS mitigation or re-balancing load across
    ECMP.

    There is some interaction between entropy and OAM and extensibility
    mechanism.  It is desirable to be able to send OAM packets to follow
    the same path as network packets.  Hence OAM packets should use the
    same entropy mechanism as data packets.  While routers might use
    information in addition the entropy field and outer IP header, they
    can not use arbitrary parts of the encapsulation header since that
    might result in OAM frames taking a different path.  Likewise if
    routers look past the encapsulation header they need to be aware of
    the extensibility mechanism(s) in the encapsulation format to be able
    to find the inner headers in the presence of extensions; OAM frames
    might use some extensions e.g. for timestamps.

    Architecturally the entropy and the next header field are really part
    of enclosing delivery header.  UDP with entropy goes hand-in-hand
    with the outer IP header.  Thus the UDP entropy is present for the

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    underlay IP routers the same way that an MPLS entropy label is
    present for LSRs.  The entropy above is all about providing entropy
    for the outer delivery of the encapsulated packets.

    It has been suggested that when IPv6 is used it would not be
    necessary to add a UDP header for entropy, since the IPv6 flow label
    can be used for entropy.  (This assumes that there is an IP protocol
    number for the encapsulation in addition to a UDP destination port
    number since UDP would be used with IPv4 underlay.  And any use of
    UDP checksums would need to be replaced by an encaps-specific
    checksum or secure hash.)  While such an approach would save 8 bytes
    of headers when the underlay is IPv6, it does assume that the
    underlay routers use the flow label for ECMP, and it also would make
    the IPv6 approach different than the IPv4 approach.  Currently the
    leaning is towards recommending using the UDP encapsulation for both
    IPv4 and IPv6 underlay.  The IPv6 flow label can be used for
    additional entropy if need be.

    Note that in the proposed BIER encapsulation
    [I-D.wijnands-mpls-bier-encapsulation], there is an an 8-bit field
    which specifies an entropy value that can be used for load balancing
    purposes.  This entropy is for the BIER forwarding decisions, which
    is independent of any outer delivery ECMP between BIER routers.  Thus
    it is not part of the delivery ECMP discussed in this section.
       [Note: For any given bit in BIER (that identifies an exit from the
       BIER domain) there might be multiple immediate next hops.  The
       BIER entropy field is used to select that next hop as part of BIER
       processing.  The BIER forwarding process may do equal cost load
       balancing, but the load balancing procedure MUST choose the same
       path for any two packets have the same entropy value.]

    In summary:
    o  The entropy is associated with the transport, that is an outer IP
       header or MPLS.
    o  In the case of IP transport use >=14 bits of UDP source port, plus
       outer IPv6 flowid for entropy.

8.  Next-protocol indication

    Next-protocol indications appear in three different context for
    encapsulations.

    Firstly, the transport delivery mechanism for the encapsulations we
    discuss in this document need some way to indicate which
    encapsulation header (or other payload) comes next in the packet.
    Some encapsulations might be identified by a UDP port; others might
    be identified by an Ethernet type or IP protocol number.  Which

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    approach is used is a function of the preceding header the same way
    as IPv4 is identified by both an Ethernet type and an IP protocol
    number (for IP-in-IP).  In some cases the header type is implicit in
    some session (L2TP) or path (MPLS) setup.  But this is largely beyond
    the control of the encapsulation protocol.  For instance, if there is
    a requirement to carry the encapsulation after an Ethernet header,
    then an Ethernet type is needed.  If required to be carried after an
    IP/UDP header, then a UDP port number is needed.  For UDP port
    numbers there are considerations for port number conservation
    described in [I-D.ietf-tsvwg-port-use].

    It is worth mentioning that in the MPLS case of no implicit protocol
    type many forwarding devices peek at the first nibble of the payload
    to determine whether to apply IPv4 or IPv6 L3/L4 hashes for load
    balancing [RFC7325].  That behavior places some constraints on other
    payloads carried over MPLS and some protocol define an initial
    control word in the payload with a value of zero in its first nibble
    [RFC4385] to avoid confusion with IPv4 and IPv6 payload headers.

    Secondly, the encapsulation needs to indicate the type of its
    payload, which is in scope for the design of the encapsulation.  We
    have existing protocols which use Ethernet types (such as GRE).  Here
    each encapsulation header can potentially makes its own choices
    between:
    o  Reuse Ethernet types - makes it easy to carry existing L2 and L3
       protocols including IPv6, IPv6, and Ethernet.  Disadvantages are
       that it is a 16 bit number and we probably need far less than 100
       values, and the number space is controlled by the IEEE 802 RAC
       with its own allocation policies.
    o  Reuse IP protocol numbers - makes it easy to carry e.g., ESP in
       addition to IP and Etnernet but brings in all existing protocol
       numbers many of which would never be used directly on top of the
       encapsulation protocol.  IANA managed eight bit values, presumably
       more difficult to get an assigned number than to get a transport
       port assignment.
    o  Define their own next-protocol number space, which can use fewer
       bits than an Ethernet type and give more flexibility, but at the
       cost of administering that numbering space (presumably by the
       IANA).

    Thirdly, if the IETF ends up defining multiple encapsulations at
    about the same time, and there is some chance that multiple such
    encapsulations can be combined in the same packet, there is a
    question whether it makes sense to use a common approach and
    numbering space for the encapsulation across the different protocols.
    A common approach might not be beneficial as long as there is only
    one way to indicate e.g., SFC inside NVO3.

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    Many Internet protocols use fixed values (typically managed by the
    IANA function) for their next-protocol field.  That facilitates
    interpretation of packets by middleboxes and e.g., for debugging
    purposes, but might make the protocol evolution inflexible.  Our
    collective experience with MPLS shows an alternative where the label
    can be viewed as an index to a table containing processing
    instructions and the table content can be managed in different ways.
    Encapsulations might want to consider the tradeoffs between such more
    flexible versus more fixed approaches.

    In summary:
    o  Would it be useful for the IETF come up with a common scheme for
       encapsulation protocols?  If not each encapsulation can define its
       own scheme.

9.  MTU and Fragmentation

    A common approach today is to assume that the underlay have
    sufficient MTU to carry the encapsulated packets without any
    fragmentation and reassembly at the tunnel endpoints.  That is
    sufficient when the operator of the ingress and egress have full
    control of the paths between those endpoints.  And it makes for
    simpler (hardware) implementations if fragmentation and reassembly
    can be avoided.

    However, even under that assumption it would be beneficial to be able
    to detect when there is some misconfiguration causing packets to be
    dropped due to MTU issues.  One way to do this is to have the
    encapsulator set the don't-fragment (DF) flag in the outer IPv4
    header and receive and log any received ICMP "packet too big" (PTB)
    errors.  Note that no flag needs to be set in an outer IPv6 header
    [RFC2460].

    Encapsulations could also define an optional tunnel fragmentation and
    reassembly mechanism which would be useful in the case when the
    operator doesn't have full control of the path, or when the protocol
    gets deployed outside of its original intended context.  Such a
    mechanism would be required if the underlay might have a path MTU
    which makes it impossible to carry at least 1518 bytes (if offering
    Ethernet service), or at least 1280 (if offering IPv6 service).  The
    use of such a protocol mechanism could be triggered by receiving a
    PTB.  But such a mechanism might not be implemented by all
    encapsulators and decapsulators.  [Aerolink is one example of such a
    protocol.]

    Depending on the payload carried by the encapsulation there are some
    additional possibilities:

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    o  If payload is IPv4/6 then the underlay path MTU could be used to
       report end-to-end path MTU.
    o  If the payload service is Ethernet/L2, then there is no such per
       destination reporting mechanism.  However, there is a LLDP TLV for
       reporting max frame size; might be useful to report minimum to end
       stations, but unmodified end stations would do nothing with that
       TLV since they assume that the MTU is at least 1518.

    In summary:
    o  In some deployments an encapsulation can assume well-managed MTU
       hence no need for fragmentation and reassembly related to the
       encapsulation.
    o  Even so, it makes sense for ingress to track any ICMP packet too
       big addressed to ingress to be able to log any MTU
       misconfigurations.
    o  Should an encapsulation protocol be depoyed outside of the
       original context it might very well need support for fragmentation
       and reassembly.

10.  OAM

    The OAM area is seeing active development in the IETF with
    discussions (at least) in NVO3 and SFC working groups, plus the new
    LIME WG looking at architecture and YANG models.

    The design team has take a narrow view of OAM to explore the
    potential OAM implications on the encapsulation format.

    In terms of what we have heard from the various working groups there
    seem to be needs to:
    o  Be able to send out-of-band OAM messages - that potentially should
       follow the same path through the network as some flow of data
       packets.
       *  Such OAM messages should not accidentally be decapsulated and
          forwarded to the end stations.
       *  Be able to add OAM information to data packets that are
          encapsulated.  Discussions have been around
       *  Using a bit in the OAM to synchronize sampling of counters
          between the encapsulator and decapsulator.
       *  Optional timestamps, sequence numbers, etc for more detailed
          measurements between encapsulator and decapsulator.
    o  Usable for both proactive monitoring (akin to BFD) and reactive
       checks (akin to traceroute to pin-point a failure)

    To ensure that the OAM messages can follow the same path the OAM
    messages need to get the same ECMP (and LAG hashing) results as a
    given data flow.  An encapsulator can choose between one of:

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    o  Limit ECMP hashing to not look past the UDP header i.e. the
       entropy needs to be in the source/destination IP and UDP ports
    o  Make OAM packets look the same as data packets i.e. the initial
       part of the OAM payload has the inner Ethernet, IP, TCP/UDP
       headers as a payload.  (This approach was taken in TRILL out of
       necessity since there is no UDP header.)  Any OAM bit in the
       encapsulation header must in any case be excluded from the
       entropy.

    There can be several ways to prevent OAM packets from accidentally
    being forwarded to the end station using:
    o  A bit in the frame (as in TRILL) indicating OAM
    o  A next-protocol indication with a designated value for "none" or
       "oam".
    This assumes that the bit or next protocol, respectively, would not
    affect entropy/ECMP in the underlay.  However, the next-protocol
    field might be used to provide differentiated treatement of packets
    based on their payload; for instance a TCP vs. IPsec ESP payload
    might be handled differently.  Based on that observation it might be
    undesirable to overload the next protocol with the OAM drop behavior,
    resulting in a preference for having a bit to indicate that the
    packet should be forwarded to the end station after decapsulation.

    There has been suggestions that one (or more) marker bits in the
    encaps header would be useful in order to delineate measurement
    epochs on the encapsulator and decapsulator and use that to compare
    counters to determine packet loss.

    A result of the above is that OAM is likely to evolve and needs some
    degree of extensibility from the encapsulation format; a bit or two
    plus the ability to define additional larger extensions.

    An open question is how to handle error messages or other reports
    relating to OAM.  One can think if such reporting as being associated
    with the encapsulation the same way ICMP is associated with IP.
    Would it make sense for the IETF to develop a common Encapsulation
    Error Reporting Protocol as part of OAM, which can be used for
    different encapsulations?  And if so, what are the technical
    challenges.  For instance, how to avoid it being filtered as ICMP
    often is?

    A potential additional consideration for OAM is the possible future
    existence of gateways that "stitch" together different dataplane
    encapsulations and might want to carry OAM end-to-end across the
    different encapsulations.

    In summary:

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    o  It makes sense to reserve a bit for "drop after decapsulation" for
       OAM out-of-band.
    o  An encapsulation needs sufficient extensibility for OAM (such as
       bits, timestamps, sequence numbers).  That might be motivated by
       in-band OAM but it would make sense to leverage the same
       extensions for out-of band OAM.
    o  OAM places some constraints on use of entropy in forwarding
       devices.
    o  Should IETF look into error reporting that is independent of the
       specific encapsulation?

11.  Security Considerations

    Different encapsulation use cases will have different requirements
    around security.  For instance, when encapsulation is used to build
    overlay networks for network virtualization, isolation between
    virtual networks may be paramount.  BIER support of multicast may
    entail different security requirements than encapsulation for
    unicast.

    In real deployment, the security of the underlying network may be
    considered for determining the level of security needed in the
    encapsulation layer.  However for the purposes of this discussion, we
    assume that network security is out of scope and that the underlying
    network does not itself provide adequate or as least uniform security
    mechanisms for encapsulation.

    There are at least three considerations for security:
    o  Anti-spoofing/virtual network isolation
    o  Interaction with packet level security such as IPsec or DTLS
    o  Privacy (e.g., VNI ID confidentially for NVO3)

    This section uses a VNI ID in NVO3 as an example.  A SFC or BIER
    encapsulation is likely to have fields with similar security and
    privacy requirements.

11.1.  Encapsulation-specific considerations

    Some of these considerations appear for a new encapsulation, and
    others are more specific to network virtualization in datacenters.
    o  New attack vectors:
       *  DDOS on specific queued/paths by attempting to reproduce the
          5-tuple hash for targeted connections.
       *  Entropy in outer 5-tuple may be too little or predictable.
       *  Leakage of identifying information in the encapsulation header
          for an encrypted payload.

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       *  Vulnerabilities of using global values in fields like VNI ID.
    o  Trusted versus untrusted tenants in network virtualization:
       *  The criticality of virtual network isolation depends on whether
          tenants are trusted or untrusted.  In the most extreme cases,
          tenants might not only be untrusted but may be considered
          hostile.
       *  For a trusted set of users (e.g. a private cloud) it may be
          sufficient to have just a virtual network identifier to provide
          isolation.  Packets inadvertently crossing virtual networks
          should be dropped similar to a TCP packet with a corrupted port
          being received on the wrong connection.
       *  In the presence of untrusted users (e.g. a public cloud) the
          virtual network identifier must be adequately protected against
          corruption and verified for integrity.  This case may warrant
          keyed integrity.
    o  Different forms of isolation:
       *  Isolation could be blocking all traffic between tenants (or
          except as allowed by some firewall)
       *  Could also be about performance isolation i.e. one tenant can
          overload the network in a way that affects other tenants
       *  Physical isolation of traffic for different tenants in network
          may be required, as well as required restrictions that tenants
          may have on where their packets may be routed.
    o  New attack vectors from untrusted tenants:
       *  Third party VMs with untrusted tenants allows internally borne
          attacks within data centers
       *  Hostile VMs inside the system may exist (e.g. public cloud)
       *  Internally launched DDOS
       *  Passive snooping for mis-delivered packets
       *  Mitigate damage and detection in event that a VM is able to
          circumvent isolation mechanisms
    o  Tenant-provider relationship:
       *  Tenant might not trust provider, hypervisors, network
       *  Provider likely will need to provide SLA or a least a statement
          on security
       *  Tenant may implement their own additional layers of security
       *  Regulation and certification consuderations
    o  Trend towards tighter security:
       *  Tenants' data in network increases in volume and value, attacks
          become more sophisticated
       *  Large DCs already encrypt everything on disk
       *  DCs likely to encrypt inter-DC traffic at this point, use TLS
          to Internet.
       *  Encryption within DC is becoming more commonplace, becomes
          ubiquitous when cost is low enough.
       *  Cost/performance considerations.  Cost of support for strong
          security has made strong network security in DCs prohibitive.

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       *  Are there lessons from MacSec?

11.2.  Virtual network isolation

    The first requirement is isolation between virtual networks.  Packets
    sent in one virtual network should never be illegitimately received
    by a node in another virtual network.  Isolation should be protected
    in the presence of malicious attacks or inadvertent packet
    corruption.

    The second requirement is sender authentication.  Sender identity is
    authenticated to prevent anti-spoofing.  Even if an attacker has
    access to the packets in the network, they cannot send packets into a
    virtual network.  This may have two possibilities:
    o  Pairwise sender authentication.  Any two communicating hosts
       negotiate a shared key.
    o  Group authentication.  A group of hosts share a key (this may be
       more appropriate for multicast of encapsulation).

    Possible security solutions:
    o  Security cookie: This is similar to L2TP cookie mechanism
       [RFC3931].  A shared plain text cookie is shared between
       encapsulator and decapsulator.  A receiver validates a packet by
       evaluating if the cookie is correct for the virtual network and
       address of a sender.  Validation function is F(cookie, VNI ID,
       source addr).  If cookie matches, accept packet, else drop.  Since
       cookie is plain text this method does not protect against an
       eavesdropping.  Cookies are set and may be rotated out of band.
    o  Secure hash: This is a stronger mechanism than simple cookies that
       borrows from IPsec and PPP authentication methods.  In this model
       security field contains a secure hash of some fields in the packet
       using a shared key.  Hash function may be something like H(key,
       VNI ID, addrs, salt).  The salt ensures the hash is not the same
       for every packet, and if it includes a sequence number may also
       protect against replay attacks.

    In any use of a shared key, periodic re-keying should be allowed.
    This could include use of techniques like generation numbers, key
    windows, etc.  See [I-D.farrelll-mpls-opportunistic-encrypt] for an
    example application.

    We might see firewalls that are aware of the encapsulation and can
    provide some defense in depth combined with the above example anti-
    spoofing approaches.  An example would be an NVO3-aware firewall
    being able to check the VNI ID.

    Separately and in addition to such filtering, there might be a desire
    to completely block an encapsulation protocol at certain places in

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    the network, e.g., at the edge of a datacenter.  Using a fixed
    standard UDP destination port number for each encapsulation protocol
    would facilitate such blocking.

11.3.  Packet level security

    An encapsulated packet may itself be encapsulated in IPsec (e.g.
    ESP).  This should be straightforward and in fact is what would
    happen today in security gateways.  In this case, there is no special
    consideration for the fact that packet is encapsulated, however since
    the encapsulation layer headers are included (part of encrypted data
    for instance) we lose visibility in the network of the encapsulation.

    The more interesting case is when security is applied to the
    encapsulation payload.  This will keep the encapsulation headers in
    the outer header visible to the network (for instance in nvo3 we may
    way to firewall based on VNI ID even if the payload is encrypted).
    One possibility is to apply DTLS to the encapsulation payload.  In
    this model the protocol stack may be something like IP|UDP|Encap|
    DTLS|encrypted_payload.  The encapsulation and security should be
    done together at an encapsulator and resolved at the decapsulator.
    Since the encapsulation header is outside of the security coverage,
    this may itself require security (like described above).

    In both of the above the security associations (SAs) may be between
    physical hosts, so for instance in nvo3 we can have packets of
    different virtual networks using the same SA-- this should not be an
    issue since it is the VNI ID that ensures isolation (which needs to
    be secured also).

11.4.  In summary:

    o  Encapsulations need extensibility mechanisms to be able to add
       security features like cookies and secure hashes protecting the
       encapsulation header.
    o  NVO3 proably has specific higher requirements relating to
       isolation for network virtualization, which is in scope for the
       NVO3 WG/
    o  Our collective IETF experience is that succesful protocols get
       deployed outside of the original intended context, hence the
       initial assumptions about the threat model might become invalid.
       That needs to be considered in the standardization of new
       encapsulations.

12.  QoS

    In the Internet architecture we support QoS using the Differentiated

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    Services Code Points (DSCP) in the formerly named Type-of-Service
    field in the IPv4 header, and in the Traffic-Class field in the IPv6
    header.  The ToS and TC fields also contain the two ECN bits.

    We have existing specifications how to process those bits.  See
    [RFC2983] for diffserv handling, which specifies how the received
    DSCP value is used to set the DSCP value in an outer IP header when
    encapsulating.  (There are also existing specifications how DSCP can
    be mapped to layer2 priorities.)

    Those specifications apply whether or not there is some intervening
    headers (e.g., for NVO3 or SFC) between the inner and outer IP
    headers.  Thus the encapsulation considerations in this area are
    mainly about applying the framework in [RFC2983].

    Note that the DSCP and ECN bits are not the only part of an inner
    packet that might potentially affect the outer packet.  For example,
    [RFC2473] specifies handling of inner IPv6 hop-by-hop options that
    effectively result in copying some options to the outer header.  It
    is simpler to not have future encapsulations depend on such copying
    behavior.

    There are some other considerations specific to doing OAM for
    encapsulations.  If OAM messages are used to measure latency, it
    would make sense to treat them the same as data payloads.  Thus they
    need to have the same outer DSCP value as the data packets which they
    wish to measure.

    Due to OAM there are constraints on middleboxes in general.  If
    middleboxes inspect the packet past the outer IP+UDP and
    encapsulation header and look for inner IP and TCP/UDP headers, that
    might violate the assumption that OAM packets will be handled the
    same as regular data packets.  That issue is broader than just QoS -
    applies to firewall filters etc.

    In summary:
    o  Leverage the existing approach in [RFC2983] for DSCP handling.

13.  Congestion Considerations

    Additional encapsulation headers does not introduce anything new for
    Explicit Congestion Notification.  It is just like IP-in-IP and IPsec
    tunnels which is specified in [RFC6040] in terms of how the ECN bits
    in the inner and outer header are handled when encapsulating and
    decapsulating packets.  Thus new encapsulations can more or less
    include that by reference.

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    There are additional considerations around carrying non-congestion
    controlled traffic.  These details have been worked out in
    [I-D.ietf-mpls-in-udp].  As specified in [RFC5405]: "IP-based traffic
    is generally assumed to be congestion-controlled, i.e., it is assumed
    that the transport protocols generating IP-based traffic at the
    sender already employ mechanisms that are sufficient to address
    congestion on the path Consequently, a tunnel carrying IP-based
    traffic should already interact appropriately with other traffic
    sharing the path, and specific congestion control mechanisms for the
    tunnel are not necessary".  Those considerations are being captured
    in [I-D.ietf-tsvwg-rfc5405bis].

    For this reason, where an encapsulation method is used to carry IP
    traffic that is known to be congestion controlled, the UDP tunnels
    does not create an additional need for congestion control.  Internet
    IP traffic is generally assumed to be congestion-controlled.
    Similarly, in general Layer 3 VPNs are carrying IP traffic that is
    similarly assumed to be congestion controlled.

    However, some of the encapsulations (at least NVO3) will be able to
    carry arbitrary Layer 2 packets to provide an L2 service, in which
    case one can not assume that the traffic is congestion controlled.

    One could handle this by adding some congestion control support to
    the encapsulation header (one instance of which would end up looking
    like DCCP).  However, if the underlay is well-provisioned and managed
    as opposed to being arbitrary Internet path, it might be sufficient
    to have a slower reaction to congestion induced by that traffic.
    There is work underway on a notion of "circuit breakers" for this
    purpose.  See See [I-D.ietf-tsvwg-circuit-breaker].  Encapsulations
    which carry arbitrary Layer 2 packets want to consider that ongoing
    work.

    If the underlay is provisioned in such a way that it can guarantee
    sufficient capacity for non-congestion controlled Layer 2 traffic,
    then such circuit breakers might not be needed.

    Two other considerations appear in the context of these
    encapsulations as applied to overlay networks:
    o  Protect against malicious end stations
    o  Ensure fairness and/or measure resource usage across multiple
       tenants
    Those issues are really orthogonal to the encapsulation, in that they
    are present even when no new encapsulation header is in use.
    However, the application of the new encapsulations are likely to be
    in environments where those issues are becoming more important.
    Hence it makes sense to consider them.

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    One could make the encapsulation header be extensible to that it can
    carry sufficient information to be able to measure resource usage,
    delays, and congestion.  The suggestions in the OAM section about a
    single bit for counter synchronization, and optional timestamps
    and/or sequence numbers, could be part of such an approach.  There
    might also be additional congestion-control extensions to be carried
    in the encapsulation.  Overall this results in a consideration to be
    able to have sufficient extensibility in the encapsulation to be
    handle to handle potential future developments in this space.

    Coarse measurements are likely to suffice, at least for circuit-
    breaker-like purposes, see [I-D.wei-tsvwg-tunnel-congestion-feedback]
    and [I-D.briscoe-conex-data-centre] for examples on active work in
    this area via use of ECN.  [RFC6040] Appendix C is also relevant.
    The outer ECN bits seem sufficient (at least when everything uses
    ECN) to do this course measurements.  Needs some more study for the
    case when there are also drops; might need to exchange counters
    between ingress and egress to handle drops.

    Circuit breakers are not sufficient to make a network with different
    congestion control when the goal is to provide a predictable service
    to different tenants.  The fallback would be to rate limit different
    traffic.

    In summary:
    o  Leverage the existing approach in [RFC6040] for ECN handling.
    o  If the encapsulation can carry non-IP, hence non-congestion
       controlled traffic, then leverage the approach in
       [I-D.ietf-mpls-in-udp].
    o  "Watch this space" for circuit breakers.

14.  Header Protection

    Many UDP based encapsulations such as VXLAN [RFC7348] either
    discourage or explicitly disallow the use of UDP checksums.  The
    reason is that the UDP checksum covers the entire payload of the
    packet and switching ASICs are typically optimized to look at only a
    small set of headers as the packet passes through the switch.  In
    these case, computing a checksum over the packet is very expensive.
    (Software endpoints and the NICs used with them generally do not have
    the same issue as they need to look at the entire packet anyways.)

    The lack a header checksum creates the possibility that bit errors
    can be introduced into any information carried by the new headers.
    Specifically, in the case of IPv6, the assumption is that a transport
    layer checksum - UDP in this case - will protect the IP addresses
    through the inclusion of a pseudoheader in the calculation.  This is

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    different from IPv4 on which many of these encapsulation protocols
    are initially deployed which contains its own header checksum.  In
    addition to IP addresses, the encapsulation header often contains its
    own information which is used for addressing packets or other high
    value network functions.  Without a checksum, this information is
    potentially vulnerable - an issue regardless of whether the packet is
    carried over IPv4 or IPv6.

    Several protocols cite [RFC6935] and [RFC6936] as an exemption to the
    IPv6 checksum requirements.  However, these are intended to be
    tailored to a fairly narrow set of circumstances - primarily relying
    on sparseness of the address space to detect invalid values and well
    managed networks - and are not a one size fits all solution.  In
    these cases, an analysis should be performed of the intended
    environment, including the probability of errors being introduced and
    the use of ECC memory in routing equipment.

    Conceptually, the ideal solution to this problem is a checksum that
    covers only the newly added headers of interest.  There is little
    value in the portion of the UDP checksum that covers the encapsulated
    packet because that would generally be protected by other checksums
    and this is the expensive portion to compute.  In fact, this solution
    already exists in the form of UDP-Lite and UDP based encapsulations
    could be easily ported to run on top of it.  Unfortunately, the main
    value in using UDP as part of the encapsulation header is that it is
    recognized by already deployed equipment for the purposes of ECMP,
    RSS, and middlebox operations.  As UDP-Lite uses a different protocol
    number than UDP and it is not widely implemented in middleboxes, this
    value is lost.  A possible solution is to incorporate the same
    partial-checksum concept as UDP-Lite or other header checksum
    protection into the encapsulation header and continue using UDP as
    the outer protocol.  One potential challenge with this approach is
    the use of NAT or other form of translation on the outer header will
    result in an invalid checksum as the translator will not know to
    update the encapsulation header.

    The method chosen to protect headers is often related to the security
    needs of the encapsulation mechanism.  On one hand, the impact of a
    poorly protected header is not limited to only data corruption but
    can also introduce a security vulnerability in the form of
    misdirected packets to an unauthorized recipient.  Conversely, high
    security protocols that already include a secure hash over the
    valuable portion of the header (such as by encrypting the entire IP
    packet using IPsec, or some secure hash of the encap header) do not
    require additional checksum protection as the hash provides stronger
    assurance than a simple checksum.

    If the sender has included a checksum, then the receiver should

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    verify that checksum or, if incapable, drop the packet.  The
    assumption is that configuration and/or control-plane capability
    exchanges can be used when different receiver have different checksum
    validation capabilities.

    In summary:
    o  Encapsulations need extensibility to be able to add checksum/CRC
       for the encapsulation header itself.
    o  When the encapsulation has a checksum/CRC, include the IPv6
       pseudo-header in it.
    o  The checksum/CRC can potentially be avoided when cryptographic
       protection is applied to to the encapsulation.

15.  Extensibility Considerations

    Protocol extensibility is the concept that a networking protocol may
    be extended to include new use cases or functionality that were not
    part of the original protocol specification.  Extensibility may be
    used to add security, control, management, or performance features to
    a protocol.  A solution may allow private extensions for
    customization or experimentation.

    Extending a protocol often implies that a protocol header must carry
    new information.  There are two usual methods to accomplish this:
    1.  Define or redefine the meaning of existing fields in a protocol
        header.
    2.  Add new (optional) fields to the protocol header.
    It is also possible to create a new protocol version, but this is
    more associated with defining a protocol than extending it (IPv6
    being a successor to IPv4 is an example of protocol versioning).

    In some cases it might be more appropriate to define a new inner
    protocol which can carry the new functionality instead of extending
    the outer protocol.  Examples where this works well is in the IP/
    transport split, where the earlier architecture had a single NCP
    protocol which carried both the hop-by-hop semantics which are now in
    IP, and the end-to-end semantics which are now in TCP.  Such a split
    is effective when different nodes need to act upon the different
    information.  Applying this for general protocol extensibility
    through nesting is not well understood, and does result in longer
    header chains.  Furthermore, our experience with IPv6 extension
    headers [RFC2460] in middleboxes indicates that the approach does not
    help with middlebox traversal.

    Many protocol definitions include some number of reserved fields or
    bits which can be used for future extension.  VXLAN is an example of
    a protocol that includes reserved bits which are subsequently being

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    allocated for new purposes.  Another technique employed is to
    repurpose existing header fields with new meanings.  A classic
    example of this is the definition of DSCP code point which redefines
    the ToS field originally specified in IPv4.  When a field is
    redefined, some mechanism may be needed to ensure that all interested
    parties agree on the meaning of the field.  The techniques of
    defining meaning for reserved bits or redefining existing fields have
    the advantage that a protocol header can be kept a fixed length.  The
    disadvantage is that the extensibility is limited.  For instance, the
    number reserved bits in a fixed protocol header is limited.  For
    standard protocols the decision to commit to a definition for a field
    can be wrenching since it is difficult to retract later.  Also, it is
    difficult to predict a priori how many reserved fields or bits to put
    into a protocol header to satisfy the extensions create over the
    lifetime of the protocol.

    Extending a protocol header with new fields can be done in several
    ways.
    o  TLVs are a very popular method used in such protocols as IP and
       TCP.  Depending on the type field size and structure, TLVs can
       offer a virtually unlimited range of extensions.  A disadvantage
       of TLVs is that processing them can be verbose, quite complicated,
       several validations must often be done for each TLV, and there is
       no deterministic ordering for a list of TLVs.  TCP serves as an
       example of a protocol where TLVs have been successfully used (i.e.
       required for protocol operation).  IP is an example of a protocol
       that allows TLVs but are rarely used in practice (router fast
       paths usually that assume no IP options).  Note that TCP TLVs are
       implemented in software as well as (NIC) hardware handling various
       forms of TCP offload.
    o  Extension headers are closely related to TLVs.  These also carry
       type/value information, but instead of being a list of TLVs within
       a single protocol header, each one is in its own protocol header.
       IPv6 extension headers and SFC NSH are examples of this technique.
       Similar to TLVs these offer a wide range of extensibility, but
       have similarly complex processing.  Another difference with TLVs
       is that each extension header is idempotent.  This is beneficial
       in cases where a protocol implements a push/pop model for header
       elements like service chaining, but makes it more difficult group
       correlated information within one protocol header.
    o  A particular form of extension headers are the tags used by IEEE
       802 protocols.  Those are similar to e.g., IPv6 extension headers
       but with the key difference that each tag is a fixed length header
       where the length is implicit in the tag value.  Thus as long as a
       receiver can be programmed with a tag value to length map, it can
       skip those new tags.

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    o  Flag-fields are a non-TLV like method of extending a protocol
       header.  The basic idea is that the header contains a set of
       flags, where each set flags corresponds to optional field that is
       present in the header.  GRE is an example of a protocol that
       employs this mechanism.  The fields are present in the header in
       the order of the flags, and the length of each field is fixed.
       Flag-fields are simpler to process compared to TLVs, having fewer
       validations and the order of the optional fields is deterministic.
       A disadvantage is that range of possible extensions with flag-
       fields is smaller than TLVs.

    The requirements for receiving unknown or unimplemented extensible
    elements in an encapsulation protocol (flags, TLVs, optional fields)
    need to be specified.  There are two parties to consider, middle
    boxes and terminal endpoints of encapsulation (at the decapsulator).

    A protocol may allow or expect nodes in a path to modify fields in an
    encapsulation (example use of this is BIER).  In this case, the
    middleboxes should follow the same requirements as nodes terminating
    the encapsulation.  In the case that middle boxes do not modify the
    encapsulation, we can assume that they may still inspect any fields
    of the encapsulation.  Missing or unknown fields should be accepted
    per protocol specification, however it is permissible for a site to
    implement a local policy otherwise (e.g. a firewall may drop packets
    with unknown options).

    For handling unknown options at terminal nodes, there are two
    possibilities: drop packet or accept while ignoring the unknown
    options.  Many Internet protocols specify that reserved flags must be
    set to zero on transmission and ignored on reception.  L2TP is
    example data protocol that has such flags.  GRE is a notable
    exception to this rule, reserved flag bits 1-5 cannot be ignored
    [RFC2890].  For TCP and IPv4, implementations must ignore optional
    TLVs with unknown type; however in IPv6 if a packet contains an
    unknown extension header (unrecognized next header type) the packet
    must be dropped with an ICMP error message returned.  The IPv6
    options themselves (encoded inside the destinations options or hop-
    by-hop options extension header) have more flexibility.  There bits
    in the option code are used to instruct the receiver whether to
    ignore, silently drop, or drop and send error if the option is
    unknown.  Some protocols define a "mandatory bit" that can is set
    with TLVs to indicate that an option must not be ignored.
    Conceptually, optional data elements can only be ignored if they are
    idempotent and do not alter how the rest of the packet is parsed or
    processed.

    Depending on what type of protocol evolution one can predict, it
    might make sense to have an way for a sender to express that the

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    packet should be dropped by a terminal node which does not understand
    the new information.  In other cases it would make sense to have the
    receiver silently ignore the new info.  The former can be expressed
    by having a version field in the encapsulation, or a notion of
    "mandatory bit" as discussed above.

    A security mechanism which use some form secure hash over the
    encapsulation header would need to be able to know which extensions
    can be changed in flight.

    In summary:
    o  Encapsulations need the ability to be extended to handle e.g., the
       OAM or security aspects discussed in this document.
    o  Practical experience seems to tell us that extensibility
       mechanisms which are not in use on day one might result in
       immediate ossification by lack of implementation support.  In some
       cases that has occurred in routers and in other cases in
       middleboxes.  Hence devising ways where the extensibility
       mechanisms are in use seems important.

16.  Layering Considerations

    One can envision that SFC might use NVO3 as a delivery/transport
    mechanism.  With more imagination that in turn might be delivered
    using BIER.  Thus it is useful to think about what things look like
    when we have BIER+NVO3+SFC+payload.  Also, if NVO3 is widely deployed
    there might be cases of NVO3 nesting where a customer uses NVO3 to
    provide network virtualization e.g., across departments.  That
    customer uses a service provider which happens to use NVO3 to provide
    transport for their customers.Thus NVO3 in NVO3 might happen.

    A key question we set out to answer is what the packets might look
    like in such a case, and in particular whether we would end up with
    multiple UDP headers for entropy.

    Based on the discussion in the Entropy section, the entropy is
    associated with the outer delivery IP header.  Thus if there are
    multiple IP headers there would be a UDP header for each one of the
    IP headers.  But SFC does not require its own IP header.  So a case
    of NVO3+SFC would be IP+UDP+NVO3+SFC.  A nested NVO3 encapsulation
    would have independent IP+UDP headers.

    The layering also has some implications for middleboxes.
    o  A device on the path between the ingress and egress is allowed to
       transparently inspect all layers of the protocol stack and drop or
       forward, but not transparently modify anything but the layer in
       which they operate.  What this means is that an IP router is

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       allowed modify the outer IP ttl and ECN bits, but not the
       encapsulation header or inner headers and payload.  And a BIER
       router is allowed to modify the BIER header.
    o  Alternatively such a device can become visible at a higher layer.
       E.g., a middlebox could become an decapsulate + function +
       encapsulate which means it will generate a new encapsulation
       header.

    The design team asked itself some additional questions:
    o  Would it make sense to have a common encapsulation base header
       (for OAM, security?, etc) and then followed by the specific
       information for NVO3, SFC, BIER?  Given that there are separate
       proposals and the set of information needing to be carried
       differs, and the extensibility needs might be different, it would
       be difficult and not that useful to have a common base header.
    o  With a base header in place, one could view the different
       functions (NVO3, SFC, and BIER) as different extensions to that
       base header resulting in encodings which are more space optimal by
       not repeating the same base header.  The base header would only be
       repeated when there is an additional IP (and hence UDP) header.
       That could mean a single length field (to skip to get to the
       payload after all the encapsulation headers).  That might be
       technically feasible, but it would create a lot of dependencies
       between different WGs making it harder to make progress.  Compare
       with the potential savings in packet size.

17.  Service model

    The IP service is lossy and subject to reordering.  In order to avoid
    a performance impact on transports like TCP the handling of packets
    is designed to avoid reordering packets that are in the same
    transport flow (which is typically identified by the 5-tuple).  But
    across such flows the receiver can see different ordering for a given
    sender.  That is the case for a unicast vs. a multicast flow from the
    same sender.

    There is a general tussle between the desire for high capacity
    utilization across a multipath network and the import on packet
    ordering within the same flow (which results in lower transport
    protocol performance).  That isn't affected by the introduction of an
    encapsulation.  However, the encapsulation comes with some entropy,
    and there might be cases where folks want to change that in response
    to overload or failures.  For instance, might want to change UDP
    source port to try different ECMP route.  Such changes can result in
    packet reordering within a flow, hence would need to be done
    infrequently and with care e.g., by identifying packet trains.

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    There might be some applications/services which are not able to
    handle reordering across flows.  The IETF has defined pseudo-wires
    [RFC3985] which provides the ability to ensure ordering (implemented
    using sequence numbers and/or timestamps).

    Architectural such services would make sense, but as a separate layer
    on top of an encapsulation protocol.  They could be deployed between
    ingress and egress of a tunnel which uses some encaps.  Potentially
    the tunnel control points at the ingress and egress could become a
    platform for fixing suboptimal behavior elsewhere in the network.
    That would clearly be undesirable in the general case.  However,
    handling encapsulation of non-IP traffic hence non-congestion-
    controlled traffic is likely to be required, which implies some
    fairness and/or QoS policing on the ingress and egress devices.

    But the tunnels could potentially do more like increase reliability
    (retransmissions, FEC) or load spreading using e.g.  MP-TCP between
    ingress and egress.

18.  Hardware Friendly

    Hosts, switches and routers often leverage capabilities in the
    hardware to accelerate packet encapsulation, decapsulation and
    forwarding.

    Some design considerations in encapsulation that leverage these
    hardware capabilities may result in more efficiently packet
    processing and higher overall protocol throughput.

    While "hardware friendliness" can be viewed as unnecessary
    considerations for a design, part of the motivation for considering
    this is ease of deployment; being able to leverage existing NIC and
    switch chips for at least a useful subset of the functionality that
    the new encapsulation provides.  The other part is the ease of
    implementing new NICs and switch/router chips that support the
    encapsulation at ever increasing line rates.

    [disclaimer] There are many different types of hardware in any given
    network, each maybe better at some tasks while worse at others.  We
    would still recommend protocol designers to examine the specific
    hardware that are likely to be used in their networks and make
    decisions on a case by case basis.

    Some considerations are:
    o  Keep the encap header small.  Switches and routers usually only
       read the first small number of bytes into the fast memory for
       quick processing and easy manipulation.  The bulk of the packets

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       are usually stored in slow memory.  A big encap header may not fit
       and additional read from the slow memory will hurt the overall
       performance and throughput.
    o  Put important information at the beginning of the encapsulation
       header.  The reasoning is similar as explained in the previous
       point.  If important information are located at the beginning of
       the encapsulation header, the packet may be processed with smaller
       number of bytes to be read into the fast memory and improve
       performance.
    o  Avoid full packet checksums in the encapsulation if possible.
       Encapsulations should instead consider adding their own checksum
       which covers the encapsulation header and any IPv6 pseudo-header.
       The motivation is that most of the switch/router hardware make
       switching/forwarding decisions by reading and examining only the
       first certain number of bytes in the packet.  Most of the body of
       the packet do not need to be processed normally.  If we are
       concerned of preventing packet to be misdelivered due to memory
       errors, consider only perform header checksums.  Note that NIC
       chips can typically already do full packet checksums for TCP/UDP,
       while adding a header checksum might require adding some hardware
       support.
    o  Place important information at fixed offset in the encapsulation
       header.  Packet processing hardware may be capable of parallel
       processing.  If important information can be found at fixed
       offset, different part of the encapsulation header may be
       processed by different hardware units in parallel (for example
       multiple table lookups may be launched in parallel).  It is easier
       for hardware to handle optional information when the information,
       if present, can be found in ideally one place, but in general, in
       as few places as possible.  That facilitates parallel processing.
       TLV encoding with unconstrained order typically does not have that
       property.
    o  Limit the number of header combinations.  In many cases the
       hardware can explore different combinations of headers in
       parallel, however there is some added cost for this.

18.1.  Considerations for NIC offload

    This section provides guidelines to provide support of common
    offloads for encapsulation in Network Interface Cards (NICs).
    Offload mechanisms are techniques that are implemented separately
    from the normal protocol implementation of a host networking stack
    and are intended to optimize or speed up protocol processing.
    Hardware offload is performed within a NIC device on behalf of a
    host.

    There are three basic offload techniques of interest:

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    o  Receive multi queue
    o  Checksum offload
    o  Segmentation offload

18.1.1.  Receive multi-queue

    Contemporary NICs support multiple receive descriptor queues (multi-
    queue).  Multi-queue enables load balancing of network processing for
    a NIC across multiple CPUs.  On packet reception, a NIC must select
    the appropriate queue for host processing.  Receive Side Scaling
    (RSS) is a common method which uses the flow hash for a packet to
    index an indirection table where each entry stores a queue number.

    UDP encapsulation, where the source port is used for entropy, should
    be compatible with multi-queue NICs that support five-tuple hash
    calculation for UDP/IP packets as input to RSS.  The source port
    ensures classification of the encapsulated flow even in the case that
    the outer source and destination addresses are the same for all flows
    (e.g. all flows are going over a single tunnel).

18.1.2.  Checksum offload

    Many NICs provide capabilities to calculate standard ones complement
    payload checksum for packets in transmit or receive.  When using
    encapsulation over UDP there are at least two checksums that may be
    of interest: the encapsulated packet's transport checksum, and the
    UDP checksum in the outer header.

18.1.2.1.  Transmit checksum offload

    NICs may provide a protocol agnostic method to offload transmit
    checksum (NETIF_F_HW_CSUM in Linux parlance) that can be used with
    UDP encapsulation.  In this method the host provides checksum related
    parameters in a transmit descriptor for a packet.  These parameters
    include the starting offset of data to checksum, the length of data
    to checksum, and the offset in the packet where the computed checksum
    is to be written.  The host initializes the checksum field to pseudo
    header checksum.  In the case of encapsulated packet, the checksum
    for an encapsulated transport layer packet, a TCP packet for
    instance, can be offloaded by setting the appropriate checksum
    parameters.

    NICs typically can offload only one transmit checksum per packet, so
    simultaneously offloading both an inner transport packet's checksum
    and the outer UDP checksum is likely not possible.  In this case
    setting UDP checksum to zero (per above discussion) and offloading
    the inner transport packet checksum might be acceptable.

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    There is a proposal in [I-D.herbert-remotecsumoffload] to leverage
    NIC checksum offload when an encapsulator is co-resident with a host.

18.1.2.2.  Receive checksum offload

    Protocol encapsulation is compatible with NICs that perform a
    protocol agnostic receive checksum (CHECKSUM_COMPLETE in Linux
    parlance).  In this technique, a NIC computes a ones complement
    checksum over all (or some predefined portion) of a packet.  The
    computed value is provided to the host stack in the packet's receive
    descriptor.  The host driver can use this checksum to "patch up" and
    validate any inner packet transport checksum, as well as the outer
    UDP checksum if it is non-zero.

    Many legacy NICs don't provide checksum-complete but instead provide
    an indication that a checksum has been verified (CHECKSUM_UNNECESSARY
    in Linux).  Usually, such validation is only done for simple TCP/IP
    or UDP/IP packets.  If a NIC indicates that a UDP checksum is valid,
    the checksum-complete value for the UDP packet is the "not" of the
    pseudo header checksum.  In this way, checksum-unnecessary can be
    converted to checksum-complete.  So if the NIC provides checksum-
    unnecessary for the outer UDP header in an encapsulation, checksum
    conversion can be done so that the checksum-complete value is derived
    and can be used by the stack to validate an checksums in the
    encapsulated packet.

18.1.3.  Segmentation offload

    Segmentation offload refers to techniques that attempt to reduce CPU
    utilization on hosts by having the transport layers of the stack
    operate on large packets.  In transmit segmentation offload, a
    transport layer creates large packets greater than MTU size (Maximum
    Transmission Unit).  It is only at much lower point in the stack, or
    possibly the NIC, that these large packets are broken up into MTU
    sized packet for transmission on the wire.  Similarly, in receive
    segmentation offload, small packets are coalesced into large, greater
    than MTU size packets at a point low in the stack receive path or
    possibly in a device.  The effect of segmentation offload is that the
    number of packets that need to be processed in various layers of the
    stack is reduced, and hence CPU utilization is reduced.

18.1.3.1.  Transmit Segmentation Offload

    Transmit Segmentation Offload (TSO) is a NIC feature where a host
    provides a large (larger than MTU size) TCP packet to the NIC, which
    in turn splits the packet into separate segments and transmits each
    one.  This is useful to reduce CPU load on the host.

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    The process of TSO can be generalized as:
    o  Split the TCP payload into segments which allow packets with size
       less than or equal to MTU.
    o  For each created segment:
       1.  Replicate the TCP header and all preceding headers of the
           original packet.
       2.  Set payload length fields in any headers to reflect the length
           of the segment.
       3.  Set TCP sequence number to correctly reflect the offset of the
           TCP data in the stream.
       4.  Recompute and set any checksums that either cover the payload
           of the packet or cover header which was changed by setting a
           payload length.

    Following this general process, TSO can be extended to support TCP
    encapsulation UDP.  For each segment the Ethernet, outer IP, UDP
    header, encapsulation header, inner IP header if tunneling, and TCP
    headers are replicated.  Any packet length header fields need to be
    set properly (including the length in the outer UDP header), and
    checksums need to be set correctly (including the outer UDP checksum
    if being used).

    To facilitate TSO with encapsulation it is recommended that optional
    fields should not contain values that must be updated on a per
    segment basis-- for example an encapsulation header should not
    include checksums, lengths, or sequence numbers that refer to the
    payload.  If the encapsulation header does not contain such fields
    then the TSO engine only needs to copy the bits in the encapsulation
    header when creating each segment and does not need to parse the
    encapsulation header.

18.1.3.2.  Large Receive Offload

    Large Receive Offload (LRO) is a NIC feature where packets of a TCP
    connection are reassembled, or coalesced, in the NIC and delivered to
    the host as one large packet.  This feature can reduce CPU
    utilization in the host.

    LRO requires significant protocol awareness to be implemented
    correctly and is difficult to generalize.  Packets in the same flow
    need to be unambiguously identified.  In the presence of tunnels or
    network virtualization, this may require more than a five-tuple match
    (for instance packets for flows in two different virtual networks may
    have identical five-tuples).  Additionally, a NIC needs to perform
    validation over packets that are being coalesced, and needs to
    fabricate a single meaningful header from all the coalesced packets.

    The conservative approach to supporting LRO for encapsulation would

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    be to assign packets to the same flow only if they have identical
    five-tuple and were encapsulated the same way.  That is the outer IP
    addresses, the outer UDP ports, encapsulated protocol, encapsulation
    headers, and inner five tuple are all identical.

18.1.3.3.  In summary:

    In summary, for NIC offload:
    o  The considerations for using full UDP checksums are different for
       NIC offload than for implementations in forwarding devices like
       routers and switches.
    o  Be judicious about encapsulations that change fields on a per-
       packet basis, since such behavior might make it hard to use TSO.

19.  Middlebox Considerations

    This document has touched upon middleboxes in different section.  The
    reason for this is as encapsulations get widely deployed one would
    expect different forms of middleboxes might become aware of the
    encapsulation protocol just as middleboxes have been made aware of
    other protocols where there are business and deployment
    opportunities.  Such middleboxes are likely to do more than just drop
    packets based on the UDP port number used by an encapsulation
    protocol.

    We note that various forms of encapsulation gateways that stitch one
    encapsulation protocol together with another form of protocol could
    have similar effects.

    An example of a middlebox that could see some use would be an NVO3-
    aware firewall that would filter on the VNI IDs to provide some
    defense in depth inside or across NVO3 datacenters.

    A question for the IETF is whether we should document what to do or
    what not to do in such middleboxes.  This document touches on areas
    of OAM and ECMP as it relates to middleboxes and it might make sense
    to document how encapsulation-aware middleboxes should avoid
    unintended consequences in those (and perhaps other) areas.

    In summary:
    o  We are likely to see middleboxes that at least parse the headers
       for succesful new encapsulations.
    o  Should the IETF document considerations for what not to do in such
       middleboxes?

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20.  Related Work

    The IETF and industry has defined encapsulations for a long time,
    with examples like GRE [RFC2890], VXLAN [RFC7348], and NVGRE
    [I-D.sridharan-virtualization-nvgre] being able to carry arbitrary
    Ethernet payloads, and various forms of IP-in-IP and IPsec
    encapsulations that can carry IP packets.  As part of NVO3 there has
    been additional proposals like Geneve [I-D.gross-geneve] and GUE
    [I-D.herbert-gue] which look at more extensibility.  NSH
    [I-D.quinn-sfc-nsh] is an example of an encapsulation that tries to
    provide extensibility mechanisms which target both hardware and
    software implementations.

    There is also a large body of work around MPLS encapsulations
    [RFC3032].  The MPLS-in-UDP work [I-D.ietf-mpls-in-udp] and GRE over
    UDP [I-D.ietf-tsvwg-gre-in-udp-encap] have worked on some of the
    common issues around checksum and congestion control.  MPLS also
    introduced a entropy label [RFC6790].  There is also a proposal for
    MPLS encryption [I-D.farrelll-mpls-opportunistic-encrypt].

    The idea to use a UDP encapsulation with a UDP source port for
    entropy for the underlay routers' ECMP dates back to LISP [RFC6830].

    The pseudo-wire work [RFC3985] is interesting in the notion of
    layering additional services/characteristics such as ordered delivery
    or timely deliver on top of an encapsulation.  That layering approach
    might be useful for the new encapsulations as well.  For instance,
    the control word [RFC4385].  There is also material on congestion
    control for pseudo-wires in [I-D.ietf-pwe3-congcons].

    Both MPLS and L2TP [RFC3931] rely on some control or signaling to
    establish state (for the path/labels in the case of MPLS, and for the
    session in the case of L2TP).  The NVO3, SFC, and BIER encapsulations
    will also have some separation between the data plane and control
    plane, but the type of separation appears to be different.

    IEEE 802.1 has defined encapsulations for L2 over L2, in the form of
    Provider backbone Bridging (PBB) [IEEE802.1Q-2014] and Equal Cost
    Multipath (ECMP) [IEEE802.1Q-2014].  The latter includes something
    very similar to the way the UDP source port is used as entropy: "The
    flow hash, carried in an F-TAG, serves to distinguish frames
    belonging to different flows and can be used in the forwarding
    process to distribute frames over equal cost paths"

    TRILL, which is also a L2 over L2 encapsulation, took a different
    approach to entropy but preserved the ability for OAM frames
    [RFC7174] to use the same entropy hence ECMP path as data frames.  In
    [I-D.ietf-trill-oam-fm] there 96 bytes of headers for entropy in the

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    OAM frames, followed by the actual OAM content.  This ensures that
    any headers, which fit in those 96 bytes except the OAM bit in the
    TRILL header, can be used for ECMP hashing.

    As encapsulations evolve there might be a desire to fit multiple
    inner packets into one outer packet.  The work in
    [I-D.saldana-tsvwg-simplemux] might be interesting for that purpose.

21.  Acknowledgements

    The authors acknowledge the comments from Alia Atlas, Fred Baker,
    David Black, Bob Briscoe, Stewart Bryant, Mike Cox, Andy Malis, Radia
    Perlman, Michael Smith, and Lucy Yong.

22.  Open Issues

    o  Middleboxes:
       *  Due to OAM there are constraints on middleboxes in general.  If
          middleboxes inspect the packet past the outer IP+UDP and
          encapsulation header and look for inner IP and TCP/UDP headers,
          that might violate the assumption that OAM packets will be
          handled the same as regular data packets.  That issue is
          broader than just QoS - applies to firewall filters etc.
       *  Firewalls looking at inner payload?  How does that work for OAM
          frames?  Even if it only drops ...  TRILL approach might be an
          option?  Would that encourage more middleboxes making the
          network more fragile?
       *  Editorially perhaps we should pull the above two into a
          separate section about middlebox considerations?
    o  Next-protocol indication - should it be common across different
       encapsulation headers?  We will have different ways to indicate
       the presence of the first encapsulation header in a packet (could
       be a UDP destination port, an Ethernet type, etc depending on the
       outer delivery header).  But for the next protocol past an
       encapsulation header one could envision creating or adoption a
       common scheme.  Such a would also need to be able to identify
       following headers like Ethernet, IPv4/IPv6, ESP, etc.
    o  Common OAM error reporting protocol?
    o  There is discussion about timestamps, sequence numbers, etc in
       three different parts of the document.  OAM, Congestion
       Considerations, and Service Model, where the latter argues that a
       pseudo-wire service should really be layered on top of the
       encapsulation using its own header.  Those recommendations seem to
       be at odds with each other.  Do we envision sequence numbers,
       timestamps, etc as potential extensions for OAM and CC?  If so,
       those extensions could be used to provide a service which doesn't

Nordmark (ed), et al.   Expires November 22, 2015              [Page 34]
Internet-Draft        Encapsulation Considerations              May 2015

       reorder packets.

23.  Change Log

    The changes from draft-rtg-dt-encap-01 based on feedback at the
    Dallas IETF meeting:
    o  Setting the context that not all common issues might apply to all
       encapsulations, but that they should all be understood before
       being dismissed.
    o  Clarified that IPv6 flow label is useful for entropy in
       combination with a UDP source port.
    o  Editorially added a "summary" set of bullets to most sections.
    o  Editorial clarifications in the next protocol section to more
       clearly state the three areas.
    o  Folded the two next protocol sections into one.
    o  Mention the MPLS first nibble issue in the next protocol section.
    o  Mention that viewing the next protocol as an index to a table with
       processing instructions can provide additional flexibility in the
       protocol evolution.
    o  For the OAM "don't forward to end stations" added that defining a
       bit seems better than using a special next-protocol value.
    o  Added mention of DTLS in addition to IPsec for security.
    o  Added some mention of IPv6 hob-by-hop options of other headers
       than potentially can be copied from inner to outer header.
    o  Added text on architectural considerations when it might make
       sense to define an additional header/protocol as opposed to using
       the extensibility mechanism in the existing encapsulation
       protocol.
    o  Clarified the "unconstrained TLVs" in the hardware friendly
       section.
    o  Clarified the text around checksum verification and full vs.
       header checksums.
    o  Added wording that the considerations might apply for encaps
       outside of the routing area.
    o  Added references to draft-ietf-pwe3-congcons,
       draft-ietf-tsvwg-rfc5405bis, RFC2473, and RFC7325
    o  Removed reference to RFC3948.
    o  Updated the acknowledgements section.
    o  Added this change log section.

    Erik (for the design team)



-------- Forwarded Message --------
Subject: 	New Version Notification for draft-rtg-dt-encap-02.txt
Date: 	Thu, 21 May 2015 19:03:07 -0700
From: 	internet-drafts@ietf.org
To: 	Jon Hudson <jon.hudson@gmail.com>om>, Tom Herbert 
<therbert@google.com>om>, Lawrence Kreeger <kreeger@cisco.com>om>, Patricia 
Thaler <pthaler@broadcom.com>om>, Patricia Thaler <pthaler@broadcom.com>om>, 
Jon Hudson <jon.hudson@gmail.com>om>, Albert Tian 
<albert.tian@ericsson.com>om>, Jesse Gross <jgross@vmware.com>om>, Albert Tian 
<albert.tian@ericsson.com>om>, Pankaj Garg <pankajg@microsoft.com>om>, Pankaj 
Garg <pankajg@microsoft.com>om>, Tom Herbert <therbert@google.com>om>, Jesse 
Gross <jgross@vmware.com>om>, Erik Nordmark <nordmark@arista.com>om>, Lawrence 
Kreeger <kreeger@cisco.com>om>, Erik Nordmark <nordmark@arista.com>



A new version of I-D, draft-rtg-dt-encap-02.txt
has been successfully submitted by Erik Nordmark and posted to the
IETF repository.

Name:		draft-rtg-dt-encap
Revision:	02
Title:		Encapsulation Considerations
Document date:	2015-05-21
Group:		Individual Submission
Pages:		41
URL:            https://www.ietf.org/internet-drafts/draft-rtg-dt-encap-02.txt
Status:         https://datatracker.ietf.org/doc/draft-rtg-dt-encap/
Htmlized:       https://tools.ietf.org/html/draft-rtg-dt-encap-02
Diff:           https://www.ietf.org/rfcdiff?url2=draft-rtg-dt-encap-02

Abstract:
    The IETF Routing Area director has chartered a design team to look at
    common issues for the different data plane encapsulations being
    discussed in the NVO3 and SFC working groups and also in the BIER
    BoF, and also to look at the relationship between such encapsulations
    in the case that they might be used at the same time.  The purpose of
    this design team is to discover, discuss and document considerations
    across the different encapsulations in the different WGs/BoFs so that
    we can reduce the number of wheels that need to be reinvented in the
    future.

                                                                                   


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