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6MAN                                                           W. Kumari
Internet-Draft                                                    Google
Intended status: Best Current Practice                        J. Jaeggli
Expires: April 24, 2014                                            Zynga
                                                               R. Bonica
                                                        Juniper Networks
                                                        October 21, 2013


       Operational Issues Associated With Long IPv6 Header Chains
                     draft-wkumari-long-headers-02

Abstract

   This memo specifies requirements for IPv6 forwarders as they process
   packets with long header chains.  It also provides guidance for
   application developers whose applications might rely on long headers
   chains.

   As background, this memo explains how many ASIC-based IPv6 forwarders
   process packets and why processing of packets with long header chains
   might be problematic.

Requirements Language

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

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

   This Internet-Draft will expire on April 24, 2014.

Copyright Notice




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   Copyright (c) 2013 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Forwarder Information Requirements  . . . . . . . . . . . . .   4
   3.  Requirements For IPv6 Forwarders  . . . . . . . . . . . . . .   5
   4.  Recommendations For Application Developers  . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Changes / Author Notes.  . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   IPv6 [RFC2460] forwarders can acquire information from the following
   sources:

   o  The IPv6 header

   o  One or more IPv6 extension headers

   o  An upper-layer header

   Section 2 of this document explains how IPv6 forwarders use
   information from the IPv6 header and IPv6 extension headers to
   provide traditional forwarding services.  It also explains how IPv6
   forwarders use information from the upper-layer header to provide
   enhanced forwarding services.

   When a software-based forwarder processes an IPv6 datagram, it parses
   the header chain, regardless of its length, acquires the required



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   information and makes a forwarding decision.  Typically, software-
   based forwarders process a relatively small number of packets per
   second.  Therefore, they can perform the above mentioned procedure
   within the constraints of their processing budget.

   By contrast, ASIC-based forwarders process many more packets per
   second.  In order to fulfill this requirement, ASIC-based forwarders
   copy a fixed number of bytes from the beginning of the packet to on-
   chip memory.  Forwarders do this because they can access on-chip
   memory much more quickly than they can access off-chip memory.  Once
   the beginning of the packet has been transferred to on-chip memory,
   subsequent processing can proceed very quickly.

   The act of copying bytes from the beginning of a packet to on-chip
   memory consumes:

   o  Processor cycles

   o  On-chip memory

   o  Wall-time

   Therefore, the number of bytes copied to on-chip memory must be
   chosen wisely.  If a forwarder copies more bytes than it needs, it
   wastes resources and adversely impacts performance.  If it copies too
   few bytes, it may not have sufficient information to make a correct
   forwarding decision.

   The IPv6 header chain is a variable-length data structure, whose size
   can exceed 64 kilobytes.  However, packets with header chains
   exceeding 256 bytes are rarely observed on the Internet.  Therefore,
   most ASIC-based forwarders copy a relatively small number of bytes
   from the beginning of a packet into on-chip memory.  While this small
   number varies from platform to platform, it is generally much closer
   to 256 bytes than it is to 64 kilobytes.

   IPv6 forwarders MUST behave in a predictable manner when they process
   a packet whose header chain length exceeds the number of bytes copied
   to on-chip memory.  Section 3 of this memo defines required
   behaviors.

   Application developers should be aware of how ASIC-based forwarders
   process packets with long extension header chains.  Therefore,
   Section 4 of this document provides guidance to application
   developers.

1.1.  Terminology




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   For the purposes of this document, the terms "header chain" and
   "upper-layer" header are used as defined in
   [I-D.ietf-6man-oversized-header-chain].

   This document also introduces the following terms:

   o  forwarding service - a service that accepts a packet from one
      interface and forwards it through another

   o  traditional forwarding service - a forwarding service in which all
      parameters to the forwarding algorithm are drawn from the IPv6
      header, the hop-by-hop extension header, and the routing extension
      header

   o  enhanced forwarding service - a forwarding service in which
      parameters to the forwarding algorithm can be drawn from any
      portion of the IPv6 header chain

2.  Forwarder Information Requirements

   When an IPv6 forwarder provides traditional forwarding services, it
   extracts all information required by the forwarding algorithm from
   the IPv6 header, the hop-by-hop extension header (if present), and
   the routing extension header (if present).  In the nominal case, the
   IPv6 header contains all information required by the forwarding
   algorithm.  However, the hop-by-hop and routing extension headers can
   also impact forwarding behavior.

   Section 4.2 of [RFC2460] explains how the hop-by-hop extension header
   impacts forwarding behavior.  When the forwarder processes a hop-by-
   hop extension header, it examines each option contained by the
   header.  If forwarder encounters an unrecognized hop-by-hop option,
   and the high-order bits of the option type are "00", the forwarder
   skips over the option and continues to process subsequent options.
   However, if an forwarder encounters an unrecognized option, and the
   high-order bits of the option type are "01", "10" or "11", the
   forwarder discards the packet.














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   Section 4.4 of [RFC2460] explains how the routing extension header
   impacts forwarding behavior.  When the forwarder processes a packet
   whose destination address is local to itself, it scans the header
   chain, searching for a routing extension header.  If the packet
   contains a routing extension header and the forwarder recognizes the
   routing header type, it processes the header.  If the forwarder does
   not recognize the routing header type, the required behavior depends
   upon the Segments Left field.  If the Segments Left field is equal to
   zero, the forwarder ignores the routing extension header.  Otherwise,
   the forwarder discards the packet.  [RFC6275] and [RFC6554] describe
   currently defined routing extension header types.

   Some IPv6 forwarders provide enhanced forwarding services, such as
   firewall filtering, rate limiting and load balancing.  In order to
   provide these services, the forwarder requires access to an upper
   layer header.  The following are examples of enhanced services that
   require the forwarder to examine the upper layer header:

   o  Discard all packets directed to TCP port 25

   o  Rate limit packets destined for a particular address whose payload
      is TCP and have the TCP SYN bit set

   o  Load balance packets across parallel links so that all packet
      belonging to particular TCP session traverse the same link

3.  Requirements For IPv6 Forwarders

   The following requirements apply to all IPv6 forwarders:

   o  REQ-1: An IPv6 forwarder SHOULD NOT discard a valid packet because
      of its header chain length.  However, the forwarder MAY support a
      configuration option that causes it to discard packets whose
      header chain length exceeds a specified value.

   o  REQ-2: When processing packet that contains a hop-by-hop extension
      header, an IPv6 forwarder MUST process the entire hop-by-hop
      extension header, regardless of its length.  The forwarder MUST
      process each option as specified in Section 4.2 of [RFC2460].

   o  REQ-3: When processing a packet whose destination address is local
      to itself, an IPv6 forwarder MUST scan the entire header chain,
      regardless of its length, in order to determine whether the packet
      contains a routing extension header.  If the packet contains a
      routing extension header, the forwarder MUST process routing
      extension header as specified in Section 4.4 of [RFC2460].





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   The length of the IPv6 header plus the length of the hop-by-hop
   extension header can exceed the number of bytes that an ASIC-based
   forwarder copies into on-chip memory.  Therefore, in order to support
   REQ-2, ASIC-based forwarders typically support a special processing
   mechanism for packets containing hop-by-hop extensions.

   Also, the combined length of all headers preceding the routing
   extension header may exceed the number of bytes that an ASIC-based
   forwarder copies into on-chip memory.  Therefore, in order to support
   REQ-3, ASIC-based forwarders typically support a special processing
   mechanism for packets whose IPv6 destination address is local to the
   forwarder.  This forwarding mechanism is capable of processing the
   routing extension header, even if it begins beyond of the portion of
   the packet that was copied to on-chip memory.

   The following requirements apply to IPv6 forwarders that provide
   enhanced forwarding services:

   o  REQ-4: If a forwarder's ability to deliver enhanced services is
      limited in any way by extension header length, that limitation
      MUST be reflected in user documentation.  For example, assume that
      a forwarder provides a load balancing service, and that it
      acquires information required by the service from the IPv6 header
      and the upper-layer header.  If the service behaves in one manner
      when all required information is contained by the first N bytes of
      the header chain and in another manner when all required
      information is not contained by the first N bytes of the header
      chain, user documentation MUST reflect both behaviors as well as
      the value of N.

   o  REQ-5: If a forwarder's ability to deliver an enhanced service is
      limited by extension header length, the policy specification
      language used to configure the enhanced service MUST be
      sufficiently robust to address the limitation.  For example,
      assume that the forwarder provides a firewall service.  The
      firewall service is capable of filtering packets directed to a
      particular TCP port, but only if the TCP header is contained by
      the first N bytes of the header chain.  In this case, it MUST be
      possible to configure one policy for packets directed to the
      specified port, another policy for packet not directed to the
      specified port, and a third policy for packets whose TCP
      destination port is unknown.









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4.  Recommendations For Application Developers

   Applications developers should be aware that many ISPs and
   enterprises filter or severely rate limit packets containing long
   header chains.  They do this because of limitations imposed by the
   ASIC-based forwarders deployed at their edges.  ISPs and enterprises
   accept these limitations as part of an engineering trade off, in
   which high-speed forwarding is achieved at the cost of limiting
   enhanced services for packets with long extension headers.

   For example, assume that an enterprise deploys the following firewall
   filtering policy at its edge:

   o  Permit all packets whose destination is TCP port 80

   o  Discard all packets whose destination is not TCP port 80

   o  Discard all packets whose header chain is so long that TCP port
      information is not accessible to the filtering function

   In this case, the enterprise discards all packets whose destination
   cannot be determined by the filtering function.

   Aside from the issue of header chain length, operators may filter
   packets containing extension headers that may either compromise the
   network's security posture or require inordinate processing
   resources.

   This memo does not specify a maximum header chain length.  However,
   this memo does note that at the time of its publication, the number
   of bytes that ASIC-based forwarders copy from the beginning of a
   packet to on-chip memory varies from platform to platform.  Typical
   platforms copy between 128 and 384 bytes.  Therefore, application
   developers should avoid sending packets who header chain length is in
   that range, unless they have some assurance that their packets will
   not be discarded.

5.  IANA Considerations

   This document makes no requests of the IANA

6.  Security Considerations

   TBD

7.  Acknowledgements





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   The authors wish to thank Paul Hoffman, KK and Fernando Gont.  The
   authors also express their gratitude to an anonymous donor, without
   whom this document would not have been written.

8.  References

8.1.  Normative References

   [I-D.ietf-6man-oversized-header-chain]
              Gont, F., Manral, V., and R. Bonica, "Implications of
              Oversized IPv6 Header Chains", draft-ietf-6man-oversized-
              header-chain-08 (work in progress), October 2013.

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

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

   [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, December
              1998.

8.2.  Informative References

   [RFC6275]  Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
              in IPv6", RFC 6275, July 2011.

   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554, March
              2012.

Appendix A.  Changes / Author Notes.

   [RFC Editor: Please remove this section before publication ]

   Template to -00

   o  Initial submission.

   -00 to -01

   o  Added maximum header chain recommendation.

   o  Rewrite the forwarding description.




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Authors' Addresses

   Warren Kumari
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Email: warren@kumari.net


   Joel Jaeggli
   Zynga
   675 East Middlefield
   Mountain View, CA
   USA

   Email: jjaeggli@zynga.com


   Ronald P Bonica
   Juniper Networks
   2251 Corporate Park Drive
   Herndon, VA
   USA

   Email: rbonica@juniper.net
























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