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Versions: 00 01 02 draft-ietf-v6ops-ipv6rtr-reqs

Network Working Group                                            Z. Kahn
Internet-Draft                                                  LinkedIn
Intended status: Informational                             J. Brzozowski
Expires: May 2, 2017                                             Comcast
                                                                R. White
                                                                S. Zandi
                                                               P. Lumbis
                                                        Cumulus Networks
                                                                F. Baker
                                                        October 29, 2016

                     Requirements for IPv6 Routers


   An example.

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

   This Internet-Draft will expire on May 2, 2017.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Review of the Internet Architecture . . . . . . . . . . . . .   3
     2.1.  Robustness Principle  . . . . . . . . . . . . . . . . . .   3
     2.2.  Complexity and Elegance . . . . . . . . . . . . . . . . .   4
     2.3.  Layered Structure . . . . . . . . . . . . . . . . . . . .   6
     2.4.  Routers . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Requirements Related to Device Management . . . . . . . . . .   9
     3.1.  Configuration . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Device Access . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Zero Touch Provisioning . . . . . . . . . . . . . . . . .  10
     3.4.  Device Protection against Denial of Service Attacks . . .  11
   4.  Requirements Related to Telemetry . . . . . . . . . . . . . .  11
     4.1.  Device Status and Error Logging . . . . . . . . . . . . .  12
     4.2.  Network Topology and Traceability . . . . . . . . . . . .  12
     4.3.  Traffic Flow and Traceability . . . . . . . . . . . . . .  13
   5.  Requirements Related to IPv6 Forwarding and Addressing  . . .  13
     5.1.  The IPv6 Address is not a Host Identifier . . . . . . . .  13
     5.2.  Router Handling of IPv6 Addresses . . . . . . . . . . . .  13
     5.3.  Maximum Transmission Unit and Jumbo Frames  . . . . . . .  14
     5.4.  ICMP Considerations . . . . . . . . . . . . . . . . . . .  16
     5.5.  Machine Access to the Forwarding Table  . . . . . . . . .  16
     5.6.  Processing IPv6 Extension Headers . . . . . . . . . . . .  17
     5.7.  IPv6 Only Forwarding  . . . . . . . . . . . . . . . . . .  17
   6.  Future Considerations . . . . . . . . . . . . . . . . . . . .  17
     6.1.  Segment Routing . . . . . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   This memo defines and discusses requirements for devices that perform
   forwarding for Internet Protocol version 6 (IPv6).  This can include
   (but is not limited to) the devices described below.

   o  Devices which are primarily designed to forward traffic between
      multiple interfaces.  These are normally referred to by the

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      Internet community as routers or, in some cases, intermediate

   o  Devices which are designed to modify packets rather than "just"
      forwarding them.  These are often referred to by the Internet
      community as middle boxes.

   Readers should recognize that while this memo applies to IPv6,
   devices processing IPv6 packets will often also process IPv4 packets,
   forward based on MPLS labels, and potentially process many other
   protocols.  This memo will only interact with IPv4, MPLS, and other
   protocols as they impact the behavior of an IPv6 forwarding device;
   no attempt is made to specify requirements for protocols other than

   The reader should, therefore, not count on this document as a "sole
   source of truth," but rather use this document as a guide.

   This document is broken into the following sections: a review of
   Internet architecture and principles, requirements relating to device
   management, requirements related to telemetry, requirements related
   to IPv6 forwarding and addressing, and future considerations.
   Following these sections, a short conclusion is provided for review.

2.  Review of the Internet Architecture

   The Internet relies or interacts with a number of basic concepts and
   considerations.  These concepts are not explicitly called out in any
   specification, nor do they necessarily impact protocol design or
   packet forwarding directly.  This section provides an overview of
   these concepts and considerations to help the reader understand the
   larger context of this document.

2.1.  Robustness Principle

   Every point where multiple protocols, or multiple implementations of
   the same protocol, interact, is an interaction surface that can
   threaten the robustness of the overall system.  While it may seem the
   global Internet has achieved a level of stability that makes it
   immune to such considerations, the reality is every network is a
   complex system, and is therefore subject to massive non repeatable
   unanticipated failures.  Postel's Robustness Principle countered this
   problem with a simple statement: "Be conservative in what you do, and
   liberal in what you accept from others."  [RFC7922]

   However, since this time, it has been noted that following this law
   allows errors in protocols to accumulate over time, with overall
   negative effects on the system as a whole.  RFC1918 [RFC1918]

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   describes several points in conjunction with this principle that bear
   updating based on further experience with large scale protocol and
   network deployments within the Internet community, including:

   o  Software should be written to deal with error states gracefully.
      Software should not degrade in a way that will cause the failure
      of adjacent systems when possible.  For instance, when a routing
      protocol implementation fails, it should not do so in a way that
      will cause the spreading of or continued existence of false
      reachability information, nor should it fail in a way that
      overloads adjacent routers or interacting protocols and causing a
      cascading failure.

   o  It is best to assume the network is filled with poor
      implementations and malevolent actors, both of which will find
      every possible failure mode over time.

   o  It is best to assume every technology will be used to the limits
      of its technical capabilities, rather than assuming a particular
      protocol's scope of use will align (in any way) with the intent of
      the original designer(s).  Successful implementations attract more
      functionality, much like a few nodes in a scale free graph
      eventually become connectivity hubs.

   o  Protocols and implementations change over time.  A corollary of
      the assumption that protocols will be used until they reach their
      technical limits rather than staying within a tightly scoped
      purpose, is that protocols will change over time as they gain new
      functionality.  Protocol and implementation design should take
      into account use cases that have not yet been thought of by
      building flexibility into protocols.

   o  Obscure, but legal, protocol features are often ignored or left
      unimplemented.  It is important to work within the bounds of what
      is actually implemented in any given protocol, and to leave corner
      cases for another day.  It is never helpful to boil the ocean
      whether in a design, an implementation, or a protocol.

2.2.  Complexity and Elegance

   Complexity, as articulated by Mike O'Dell (see RFC3439 [RFC3439]), is
   "the primary mechanism which impese efficient scaling, and as a
   result is the primary driver of increases in both capital
   expenditures (CAPEX) and operational expenditures (OPEX)."  At the
   same time, complexity cannot be "solved," but rather must be
   "managed."  The simplest and most obvious solution to any problem is
   often easy to design, deploy, and manage.  It's also often wrong and/
   or broken.  As much as developers, designers, and operators might

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   like to make things as simple as possible, hard problems require
   complex solutions.  The following observations apply to the
   management of complexity in both protocol and network design.

   Elegance is the ultimate goal.  Rather than seeking out simple
   solutions because they are simple, seek out solutions that will solve
   the problem in the simplest way possible.  Often this will require
   seeing the problem from different angles, trying to break the problem
   up in multiple ways, and trying, abandoning, and rebuilding ideas and
   implementations until a better way is found.

   There are always tradeoffs.  For any protocol, network, or
   operational design decision, there will always be a tradeoff between
   at least two competing goals.  If some problem appears to have a
   single solution without tradeoffs, this doesn't mean the tradeoffs
   don't exist.  Rather, it means the tradeoffs haven't been discovered
   yet.  In the area of protocol and network design, these tradeoffs
   often take the form of common "choose two or three" situations, such
   as "quick, cheap, high quality."  In networks, however, the tradeoffs
   are often:

   o  The amount of state carried in the system, or simply the state.
      The amount of state required to operate a system as it scales
      tends to be nonlinear.  Some instances of this are described in
      section 2.2.1, the Amplification Principle, in RFC3439.  [RFC3439]

   o  The number of interaction surfaces between the components that
      make up the complete system, and the depth of those interaction
      surfaces.  Some examples of surfaces are described in section
      2.2.2, the Coupling Principle, in RFC3439.  [RFC3439] Layering is
      essentially a form of abstraction; all abstractions are subject to
      the law of leaky abstractions, which states: "all nontrivial
      absractions leak."

   o  The desired optimization, or the resulting optimization of
      resources and support for applications running on top of the

   These three make up a "triangle problem."  For instance, to increase
   the optimization of traffic flow through a network generally requires
   adding more state to the control plane, leading to problems in
   complexity due to amplification.  To reduce amplification, the
   control plane (or perhaps the various functions the control plane
   serves) can be broken up into subsystems, or modules.  Breaking the
   control plane up into subsystems, however, introduces interaction
   surfaces between the components, which is another form of complexity.

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2.3.  Layered Structure

   The Internet data plane is organized around broad top and bottom
   layers, and much thinner middle layer.  This is illustrated in the
   figure below.

   \                         /
    \ HTTP, FTP, SNMP, ETC. /
     \                     /
       \     TCP, UDP    /
        \               /
          \    IPv6   /
          /   (MPLS)  \
        /               \
       /                 \
     /  Ethernet, Wireless \
    /    Physical Media     \
   /                         \

                                 Figure 1

   This layering emulates or mirrors many naturally occurring systems,
   and is a common strategy for managing complexity.  The single
   protocol in the center, IPv6, serves to separate the complexity of
   the lower layers from the complexity of the upper layers.  This
   center layer of the Internet ecosystem has traditionally been called
   the Network Layer, in reference to the Department of Defense (DoD)
   [DoD] and OSI models.  [OSI] The Internet ecosystem includes three
   different protocols in this central location.

   o  IPv4, an older network protocol that, it is anticipated, will be
      replaced over time as the Internet ecosystem standardizes on IPv6

   o  IPv6, a newer network protocol that is being adopted

   o  MPLS, or MultiProtocol Label Switching, which is often used as a
      data plane within an autonomous system

   These protocols are often treated as if they exist in strict
   hierarchical layers with a well defined and followed Application
   Programming Interface (API), data models, Remote Procedure Calls
   (RPCs), sockets, etc.  The reality, however, is there are often solid
   reasons for violating these layers, creating interaction surfaces
   that are often deeper than intended or understood without some
   experience.  Beyond this, such layering mechanisms act as information
   abstractions.  It is well known that all such abstractions leak.
   Because of these intentional and unintentional leakages of
   information, the interactions between protocols is often subtle.

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2.4.  Routers

   A router connects to two or more logical interfaces and at least one
   physical interface.  A router processes packets by:

   o  Receiving a packet through an interface

   o  Stripping the data link, physical header, or tunnel encapsulation
      off the packet

   o  Examining the packet for errors, information that must be handled
      locally, etc.

   o  Looking up the destination in a local forwarding table

   o  Rewriting the data link and/or physical layer header

   o  Transmitting the packet out an interface

   When consulting the forwarding table, the router searches for the
   longest prefix containing the destination address, and uses the
   information in the table to determine the next hop, or rather the
   next logically connected device to forward the packet to.  The next
   hop will either be another router, which will presumably carry the
   packet closer to the final destination, or it will be the destination
   host itself.  The following figure provides a conceptual model of a
   router; not all routers actually have this set of tables and
   interactions, and some have many more moving parts.  This model is
   simply used as a common reference to promote understanding.

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   +-------------+            +-------------+
   | Candidate   |            | Startup     |
   | Config      |<--+    +-->| Config      |
   +--+----------+   |    |   +-------+-----+
      |              |    |           |
      v              |    |           v
   | Running Configuration                  +------>----------+
   +---+----------+----------+----------+---+                 |
       |          |          |          |                     |
       v          |          |          |                     |
   +-------+      |          |          |                     |
   | IS-IS |<-----------------------------------> Adjacent    |
   +---+---+      v          |          |         Routers     |
       |      +-------+      |          |                     |
       |      |  BGP  |<------------------------> Peers       |
       |      +---+---+      v          |                     |
       |          |      +-------+      |                     |
       |          |      | OSPF  |<-------------> Adjacent    |
       |          |      +---+---+      v         Routers     |
       |          |          |      +-------+                 |
       |          |          |      | Other |                 |
       |          |          |      +---+---+                 |
       |          |          |          |                     |
   +---+----------+----------+----------+---+                 |
   | RIB Manager                            |                 |
   +---+------------------------------------+                 |
       |                                                      |
   +---+------------------------------------+                 |
   | Routing Information Base (RIB)         |                 |
   +---+------------------------------------+                 |
       |                                                      |
   +---+------------------------------------+                 |
   | Forwarding Information Base (FIB)      |                 |
   +---+----------+---------------------+---+                 |
       |          |                     |                     |
   +---+---+  +---+---+             +---+---+                 |
   | Int 1 |  | Int 2 |     ...     | Int X | <---------------+
   +-------+  +-------+             +-------+
       ^                                |
       |                                v
   Packets In                       Packets Out

                                 Figure 2

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3.  Requirements Related to Device Management

   Network engineering began in the era of Command Line Interfaces
   (CLIs), and has generally stayed with these CLIs even as the
   Graphical User Interface (GUI) has become the standard way of
   interacting with almost every other computing device.  Direct human
   interaction with networking devices in large scale and complex
   environments, however, tends to result in an unacceptably low Mean
   Time Between Mistakes (MTBM), directly impacting the overall
   availability of the network.  In reaction to this, operators have
   increased their reliance on automation, specifically targetting
   machine to machine, socket, and Application Programming Interface
   (API) solutions in deploying and configuring devices.  This section
   considers the various components of device management.

3.1.  Configuration

   Configuration primarily relates to the startup, candidate, and
   running configurations in the router model shown above.  In order to
   deploy networks at any scale, operators rely on automated management
   of network device configuration.  This effort has traditionally
   focused on Simple Network Management Protocol (SNMP) Management
   Information Base (MIBs).  In the future, operators expect to move
   towards open source/open standards YANG models.

   Network devices should place a priority on supporting machine
   readable APIs, rather than human interaction, particularly interfaces
   that understand and accept configuration and other information
   carried in YANG models.

   To support automated network device configuration, IPv6 routers and
   network devices SHOULD support YANG and SNMP configuration, including
   (but not limited to):

   o  Openconfig models [OPENCONF] related to the protocols configured
      on the device, interface state, and device state

   o  [RFC7223]: A YANG Data Model for Interface Management

   o  [RFC7224]: IANA Interface Type YANG Module

   o  [RFC7277]: A YANG Data Model for IP Management

   o  [RFC7317]: A YANG Data Model for System Management

   o  Simple Network Management Protocol (SNMP) MIBs as appropriate

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3.2.  Device Access

   To operate a network at scale, operators rely on the ability to
   access a device to troubleshoot and gather state manually and
   programmatically through a number of different interfaces.  These
   interfaces should provide current device configuration, current
   device state (such as interface state, packets drops, etc.), and
   current control plane contents (such as the RIB in the figure above).
   In other words, manual and programmable interfaces should provide
   information about the network device (the whole device stack).

   To support automated state gathering and troubleshooting, routers
   supporting IPv6 SHOULD support:

   o  TELNET ([RFC0854]): TELNET SHOULD be disabled by default, but
      should be available for operational purposes as required or as
      configured by the operator

   o  SSH ([RFC4253]): SSH SHOULD be the default access for IPv6 capable
      network devices

   NETCONF [RFC6241] and RESTCONF [I-D.ietf-netconf-restconf] are
   currently standard mechanisms designed to carry YANG models for
   device configuration and telemetry.  However, gRPC [GRPC] is becoming
   a common alternative for telemtry.  Devices SHOULD support one or
   both of these access methods to gather telemetry information.

3.3.  Zero Touch Provisioning

   To operate a network at scale, operators rely on protocols and
   mechanisms that reduce provisioning time to a minimum.  The preferred
   state is zero touch provisioning; plug a new network device in and it
   just works without any manual configuration.  The closer an operator
   can come to this ideal, the more MTBM and Operational Expenses (OPEX)
   can be reduced -- an important goals in the real world.

   To reach this goal, IPv6 network devices should support several
   standards, including, but not limited to:

   o  [I-D.ietf-dhc-rfc3315bis]: Dynamic Configuration Protocol for IPv6

   o  SLAAC ([RFC7217] and [RFC7527]): SLAAC SHOULD be enabled by
      default on all network device interfaces

   SLAAC SHOULD be able to be disabled by operators who prefer to use
   some other mechanism for address management and assignment.

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3.4.  Device Protection against Denial of Service Attacks

   Denial of Service (DoS) and Distributed Denial of Service (DDoS)
   attacks are unfortunately common in the Internet globally; these
   types of attacks cost network operators a great deal in opportunity
   and operational costs in prevention and responses.  The network
   control plane, as well as protocols used to access the device.  To
   provide for effective counters to DoS and DDoS attacks directly on
   network devices:

   o  Manufacturers and system integrators should test and clearly
      report the packet/traffic load handling capabilities of devices
      with and without various encryption methods enabled

   o  Network devices should be able to police traffic destined to the
      control plane based on the rate of traffic received, including the
      ability ot police individual flows, targeted services, etc., at
      individual rates

   o  Ideally, network devices should be able to statefully filter
      traffic destined to the control plane

   There are other useful techniques for dealing with DDoS attacks,
   including: transferring sessions to a new address and abandoning the
   address under attack, using BGP communities to spread the attack over
   multiple ingress ports and "eat" it, and requiring mutual
   authenication before allocating larger resource pools to a
   connection, these techniques are not "device level," and hence are
   not considered further here.

4.  Requirements Related to Telemetry

   Telemetry relates to information devices push to systems used to
   monitor and track the state of the network.  This applies to
   individual devices as well as the network as a system.  Two major
   challenges face operators in the area of telemetry:

   o  Information that is laid out for human, rather than machine,

   o  Software systems that require information to be queried (or polled
      or even pushed) on a per-item basis.  This form of organization
      can produce a lot of information very quickly, overwhelming
      monitoring systems and consuming a large amount of available
      network resources.  Instead, telemetry should be focused on bulk

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   There are three broad categories of telemetry: device state, topology
   state, and flow state.  These three roughly correspond to the
   management plane, the control plane, and the forwarding plane of the
   network.  Each of the sections below considers one of these three
   telemetry types.

4.1.  Device Status and Error Logging

   IPv6 network devices should be able to report errors and status
   changes in a number of different formats.  The supported formats
   should focus on machine readability, rather than human readability.
   Specifically, IPv6 network devices SHOULD support:

   o  NETCONF/RESTCONF transporting telemetry formatted according to
      YANG (see above)

   o  Syslog (RFC5424) [RFC5424]

   o  SNMP as appropriate

   NETCONF/RESTCONF transporting telemetry formatted according to YANG
   should be the current focus of development for vendor provided and
   open source software; ideally, the entire network could be monitored
   using a single modeling language to ease implementation of telemetry
   systems and increase the pace at which new software can be deployed
   in production environments.

4.2.  Network Topology and Traceability

   IPv6 network devices are part of a system of devices that, combined,
   make up the entire network.  Viewing the network as a system is often
   crucial for operational purposes, such as understanding flows,
   tracing changes in the topology, utilization, and other factors over
   time.  To support systemic monitoring of the network topology, IPv6
   devices SHOULD support at least:

   o  [RFC5424]: North-Bound Distribution of Link-State and Traffic
      Engineer (TE) Information using BGP

   o  [I-D.ietf-i2rs-yang-l2-network-topology]: An I2RS model for layer
      2 topologies

   o  [I-D.ietf-i2rs-yang-l3-topology]: An I2RS model for layer 3

   o  [I-D.ietf-i2rs-yang-network-topo]: A Data Model for Network

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   (More to be added)

4.3.  Traffic Flow and Traceability

   (To be added)

5.  Requirements Related to IPv6 Forwarding and Addressing

   There are a number of capabilities that a device should have to be
   deployed into an IPv6 network, and several forwarding plane
   considerations operators and vendors need to bear in mind.  The
   sections below explain these considerations.

5.1.  The IPv6 Address is not a Host Identifier

   The IPv6 address is commonly treated as a host identifier; it is not.
   Rather, it is an interface identifier that describes the topological
   point where a particular host connects to the Internet.  It is
   generally harmful to embed IPv6 addresses inside upper layer headers
   to identify a particular host.

5.2.  Router Handling of IPv6 Addresses

   Internet Routing Registries may allocate a network operator a wide
   range of prefix lengths (see RFC6177 [RFC6177] for further
   information).  Within this allocation, network operators will often
   suballocate address space along nibble boundaries (/48, /52, /56,
   /60, and /64) for ease of configuration and management.  Several
   common practices are:

   o  Each multiaccess interface is allocated a /64

   o  Point-to-point links are allocated a /64, but may be addressed
      with a longer prefix length to prevent certain kinds of denial of
      service attacks

   o  Although aggregation may only performed to the nibble boundaries
      noted above, variances are possible

   o  Loopback addresses are assigned a /128

   Given these common practices, routers designed to run IPv6 SHOULD
   support the following addressing conventions:

   o  The default prefix length on any interface other than a loopback
      SHOULD be a /64

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   o  Configuring a prefix length longer than a /64 on any interface not
      configured as a point-to-point should require additional
      configuration steps to prevent manual configuration errors

   o  Network devices SHOULD NOT assume IPv6 prefix lengths only on
      nibble boundaries, but rather should support any prefix length
      shorter than the /64, /128, and longer prefixes used for point-to-
      point interfaces

   o  Loopback interfaces SHOULD default to a /128 prefix length unless
      some additional configuration is undertaken to override this
      default setting

   o  Link Local addressing SHOULD be configurable and useable as the
      primary address on all interfaces on a device.

5.3.  Maximum Transmission Unit and Jumbo Frames

   The long history of the Maximum Transmission Unit (MTU) in networks
   is not a happy one.  Specific problems with MTU sizing include:

   o  Many different default sizes on different media types, from very
      small (576 bytes on X.25) to very large (17914 bytes on 16Mbps
      Token Ring)

   o  Many different ways to calcualte the MTU on any given link; for
      instance a 9000 byte MTU can be calculated as 8184 bytes on one
      operating system, 8972 on another, and 9000 on a third

   o  The increasing use of tunnel encapsulations in the network; for
      instance MPLS over GRE over IP over...

   o  The wide variety of default MTUs across many different end hosts
      and operating systems

   o  The general ineffectiveness of path MTU discovery to operate
      correctly in the face of packet filters and rate limiters

   o  Lower speed links at the network edge which require a lot of time
      to serialize a packet with a large MTU

   o  Increased jitter caused by increasing the disparity between large
      and small packet size across a lower bandwidth links

   The final point requires some further elucidation.  The time required
   to serialize various packets at various speeds are:

   o  64 byte packet onto a 10Mb/s link: .5ms

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   o  1500 byte packet onto a 10Mb/s link: 1.2ms

   o  9000 byte packet onto a 10Mb/s link: 7.2ms

   o  64 byte packet onto a 100Mb/s link: .05ms

   o  1500 byte packet onto a 100Mb/s link: .12ms

   o  9000 byte packet onto a 100Mb/s link: .72ms

   A 64 byte packet trapped behind a single 1500 byte packet on a 10Mb/s
   link suffers 1.7ms of serialization delay.  Each additional 1500 byte
   packet added to the queue in front of the 64 byte packet adds and
   addtional 1.2ms of delay.  In contrast, a 64 byte packet trapped
   behind a single 9000 byte packet on a 10Mb/s link suffers 7.7ms of
   serialization delay.  Each additional 9000 byte packet added to the
   queue adds an additional 7.2ms of serialization delay.  The practical
   result is that larger MTU sizes on lower speed links can add a
   significant amount of delay and jitter into a flow.  On the other
   hand, increasing the MTU on higher speed links appears to add
   megligable additional delay and jitter.

   The result is that it costs less in terms of overall systemic
   performance to use higher MTUs on higher speed links than on lower
   speed links.  Based on this, increasing the MTU across any particular
   link may not increase overall end-to-end performance, but can greatly
   enhance the performance of local applications (such as a local BGP
   peering session, or a large/long standing elephant flow used to
   transfer data across a local fabric), while also providing room for
   tunnel encapsulations to be added with less impact on lower MTU end

   The general rule of thumb is to assume the largest size MTU should be
   used on higher speed transit only links in order to support a wide
   array of available link sizes, default MTUs, and tunnel
   encapsulations.  Routers designed for a network or data center core
   SHOULD support at least 9000 byte MTUs on all interfaces.  MTU
   detection mechanisms, such as IS-IS hello padding, [RFC7922] SHOULD
   be enabled to ensure correct point-to-point MTU configuration.
   Devices SHOULD also support:

   o  [RFC1191]: Path MTU Discovery

   o  [RFC1981]: Path MTU Discovery for IP version 6

   o  [RFC4821]: Packetization Layer Path MTU Discovery

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5.4.  ICMP Considerations

   Internet Control Message Protocool (ICMP) is described in [RFC0792]
   and [RFC4443].  ICMP is often used to perform a traceroute through a
   network (normally by using a TTL expired ICMP message), for Path MTU
   discovery, and, in IPv6, for autoconfiguration and neighbor
   discovery.  ICMP is often blocked by middle boxes of various kinds
   and/or ICMP filters configured on the ingress edge of a provider
   network.  Routers implementing IPv6 SHOULD:

   o  NOT filter ICMP by default, as this has negative impacts on many
      aspects of IPv6 operation, particularly path MTU

   o  Rate limit the generation of ICMP messages relative to the ability
      of the device to generate packets and to block the use of ICMP
      packets being used as part of a distributed denial of service

   o  Implement the filtering suggestions in

5.5.  Machine Access to the Forwarding Table

   In order to support treating the "network as a whole" as a single
   programmable system, it is important for each network device have the
   ability to directly program forwarding information.  This
   programmatic interface allows controllers, which are programmed to
   support specific business logic and applications, to modify and
   filter traffic flows without interfering with the distributed control
   plane.  While there are several programmatic interfaces available,
   this document suggests that the I2RS interface to the RIB be
   supported in all IPv6 network devices.  Specifically, these drafts
   should be supported to enable network programmability:

   o  [I-D.ietf-i2rs-fb-rib-data-model]: Filter-Based RIB Data Model

   o  [I-D.ietf-i2rs-fb-rib-info-model]: Filter-Based RIB Information

   o  [I-D.ietf-i2rs-rib-data-model]: A YANG Data Model for Routing
      Information Base (RIB)

   o  [RFC7922]: I2RS Traceability

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5.6.  Processing IPv6 Extension Headers

   (To be added)

5.7.  IPv6 Only Forwarding

   (To be added)

6.  Future Considerations

   (To be added)

6.1.  Segment Routing

   (To be added)

7.  Security Considerations

   This document addresses several ways in which devices designed to
   support IPv6 forwarding can and should be designed and/or configured
   to support increased security.  These recommendations do not,
   however, modify protocol behavior, and therefore do not create new
   security vulnerabilities that need to be considered.

8.  Conclusion

   (To be added)

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

9.2.  Informative References

   [DoD]      Wikipedia, "The Internet Protocol Suite", 2016,

   [GRPC]     gRPC, "gRPC", 2016, <http://www.grpc.io>.

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              Gont, F., Hunter, R., Massar, J., and S. LIU, "Defeating
              Attacks which employ Forged ICMP/ICMPv6 Error Messages",
              draft-gont-opsec-icmp-ingress-filtering-02 (work in
              progress), March 2016.

              Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
              bis", draft-ietf-dhc-rfc3315bis-05 (work in progress),
              June 2016.

              Hares, S., Kini, S., Dunbar, L., Krishnan, R., Bogdanovic,
              D., and R. White, "Filter-Based RIB Data Model", draft-
              ietf-i2rs-fb-rib-data-model-00 (work in progress), June

              Kini, S., Hares, S., Dunbar, L., Ghanwani, A., Krishnan,
              R., Bogdanovic, D., and R. White, "Filter-Based RIB
              Information Model", draft-ietf-i2rs-fb-rib-info-model-00
              (work in progress), June 2016.

              Wang, L., Ananthakrishnan, H., Chen, M.,
              amit.dass@ericsson.com, a., Kini, S., and N. Bahadur, "A
              YANG Data Model for Routing Information Base (RIB)",
              draft-ietf-i2rs-rib-data-model-06 (work in progress), July

              Dong, J. and X. Wei, "A YANG Data Model for Layer-2
              Network Topologies", draft-ietf-i2rs-yang-l2-network-
              topology-03 (work in progress), July 2016.

              Clemm, A., Medved, J., Varga, R., Tkacik, T., Liu, X.,
              Bryskin, I., Guo, A., Ananthakrishnan, H., Bahadur, N.,
              and V. Beeram, "A YANG Data Model for Layer 3 Topologies",
              draft-ietf-i2rs-yang-l3-topology-04 (work in progress),
              September 2016.

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              Clemm, A., Medved, J., Varga, R., Tkacik, T., Bahadur, N.,
              Ananthakrishnan, H., and X. Liu, "A Data Model for Network
              Topologies", draft-ietf-i2rs-yang-network-topo-06 (work in
              progress), September 2016.

              Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", draft-ietf-netconf-restconf-18 (work in
              progress), October 2016.

              OpenConfig, "Openconfig release YANG models", 2016,

   [OSI]      Wikipedia, "OSI Model", 2016,

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,

   [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, RFC 854, DOI 10.17487/RFC0854, May
              1983, <http://www.rfc-editor.org/info/rfc854>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
              1996, <http://www.rfc-editor.org/info/rfc1981>.

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   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              DOI 10.17487/RFC2629, June 1999,

   [RFC3439]  Bush, R. and D. Meyer, "Some Internet Architectural
              Guidelines and Philosophy", RFC 3439,
              DOI 10.17487/RFC3439, December 2002,

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <http://www.rfc-editor.org/info/rfc4253>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", RFC 4443,
              DOI 10.17487/RFC4443, March 2006,

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,

   [RFC5424]  Gerhards, R., "The Syslog Protocol", RFC 5424,
              DOI 10.17487/RFC5424, March 2009,

   [RFC6177]  Narten, T., Huston, G., and L. Roberts, "IPv6 Address
              Assignment to End Sites", BCP 157, RFC 6177,
              DOI 10.17487/RFC6177, March 2011,

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,

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   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,

   [RFC7224]  Bjorklund, M., "IANA Interface Type YANG Module",
              RFC 7224, DOI 10.17487/RFC7224, May 2014,

   [RFC7277]  Bjorklund, M., "A YANG Data Model for IP Management",
              RFC 7277, DOI 10.17487/RFC7277, June 2014,

   [RFC7317]  Bierman, A. and M. Bjorklund, "A YANG Data Model for
              System Management", RFC 7317, DOI 10.17487/RFC7317, August
              2014, <http://www.rfc-editor.org/info/rfc7317>.

   [RFC7527]  Asati, R., Singh, H., Beebee, W., Pignataro, C., Dart, E.,
              and W. George, "Enhanced Duplicate Address Detection",
              RFC 7527, DOI 10.17487/RFC7527, April 2015,

   [RFC7922]  Clarke, J., Salgueiro, G., and C. Pignataro, "Interface to
              the Routing System (I2RS) Traceability: Framework and
              Information Model", RFC 7922, DOI 10.17487/RFC7922, June
              2016, <http://www.rfc-editor.org/info/rfc7922>.

Authors' Addresses

   Zaid Ali Kahn
   xxx, CA  xxx

   Email: zaid@linkedin.com

   John Brzozowski
   xxx, xxx  xxx

   Email: John_Brzozowski@comcast.com

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   Russ White
   144 Warm Wood Lane
   Apex, NC  27539

   Email: russ@riw.us

   Shawn Zandi
   xxx, CA  xxx

   Email: szandi@linkedin.com

   Pete Lumbis
   Cumulus Networks
   xxx, xxx  xxx

   Email: plumbis@cumulusnetworks.com

   Fred Baker
   xxx, xxx  xxx

   Email: fredbaker.ietf@gmail.com

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