Network Working Group                                           S. Jiang
Internet-Draft                              Huawei Technologies Co., Ltd
Intended status: Informational                                     G. Li
Expires: May 1, 2020                                 Huawei Technologies
                                                            B. Carpenter
                                                       Univ. of Auckland
                                                        October 29, 2019

                    Asymmetric IPv6 for IoT Networks


   This document describes a new approach to IPv6 header compression for
   use in scenarios where minimizing packet size is crucial but routing
   performance must be maximised.

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
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   This Internet-Draft will expire on May 1, 2020.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Proposed Solution . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Address Transformation at the Gateway . . . . . . . . . . . .   5
   4.  Routing without Decompression . . . . . . . . . . . . . . . .   6
   5.  Address Configuration . . . . . . . . . . . . . . . . . . . .   6
   6.  Compatibility with Existing Protocols . . . . . . . . . . . .   7
   7.  Relationship to Static Context Header Compression . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  Change log [RFC Editor: Please remove] . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The large address space of IPv6 is essential for the massive
   expansion of the network edge that will be caused by "Internet of
   Things" (IoT) technology over low-power or 5G links.  However, the
   size of a raw IPv6 packet header causes difficulty due to the small
   maximum transmission units (MTU) allowed by typical low-power, low-
   cost link layers.  For 5G, this aspect is discussed in
   [I-D.ietf-dmm-5g-uplane-analysis].  Thus header compression,
   including address compression, is an important issue.  This decreases
   the size of raw packets, but compressed IP addresses are not
   routeable except by decompressing them completely in every forwarding
   node.  There are two issues here.  The first is the extra computation
   resource needed for compressing or decompressing in constrained IoT
   nodes.  The second is that full-length IPv6 routing will consume more
   memory to store routing tables and packet queues.  Such resource
   consumption is very undesirable in constrained nodes with limited
   storage, CPU power, and battery capacity.

   To mitigate these issues, here we propose a solution enabling the
   shortening of IPv6 addresses inside packets, and the routing of
   packets according to short addresses, without needing the overhead of
   a decompression step prior to route lookup.  Considering that the
   scale and size of edge networks may vary widely, different lengths of
   short address can be used in different domains.

   As an illustrative example, consider an edge network which is known
   to never require more than a few hundred nodes, which in most cases
   will communicate either with each other, or with application layer

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   gateways to the rest of the Internet.  Rather than needing 128-bit
   addresses, such a network could very well operate with 16-bit
   addresses.  Also, it could very likely operate without needing
   enhancements such as differentiated services, ECN or flow labels.  If
   only IPv6 is supported, the version number field is pointless.  There
   is no reason for IPv6 packets within such a network to contain
   40-byte headers as specified in [RFC8200].  Therefore, the useful
   information could be carried in 8 bytes (see Figure 1).  Furthermore,
   routers within the edge network can route packets directly on 16-bit
   addresses, reducing RIB and FIB sizes and the lookup time.

      |         Payload Length        |  Next Header  |   Hop Limit   |
      |         Source Address        |  Destination Address          |

                                 Figure 1

   This work is distinct from previous work on address compression
   [RFC6282] [RFC7400].  Although those solutions tackle the problem of
   small MTU size, they do not address the problem of decompression

   This work is also distinct from ongoing work on static context header
   compression [I-D.ietf-lpwan-ipv6-static-context-hc], as discussed in
   more detail below.

   Finally, this work is distinct from the 6LoWPAN Routing Header
   [RFC8138], which can support truncated addresses in a different way.

2.  Proposed Solution

   The use of IPv6 naturally implies 128-bit addresses for both source
   and destination.  However, this address size is huge by the standards
   of IoT edge networks.  We propose the use of a context parameter to
   indicate the effective length of the IP address for every node in a
   local domain.  If the effective length is N bits, then all addresses
   in the domain are assumed to be preceded by a common prefix of 128-N
   bits, when a full size IPv6 address is needed.  Any node in the
   domain that needs the full address, such as a gateway node to the
   Internet, can therefore easily synthesize it.

   The address length parameter may be needed by every node in the
   domain.  It can be spread by various techniques:

   o  Configure the address length in every node.

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   o  Obtain the address length from a gateway (next hop router) node.

   o  Negotiate the address length between neighbors.

   The solution operates by shortening IP address fields to save
   overhead.  To enhance this, we propose a new field named Flexible
   Header Encoding (FHE).  It consists of 8 bits, each indicating
   whether the corresponding IPv6 header field [RFC8200] exists.

      Bit 0 indicates the Modified Version field

      Bit 1 indicates the Traffic Class field

      Bit 2 indicates the Flow Label field.

      Bit 3 indicates the Payload Length field.

      Bit 4 indicates the Next Header field.  (Zero implies "No Next
      Header", value 59)

      Bit 5 indicates the Hop Limit field.

      Bit 6 indicates the Source Address field.

      Bit 7 indicates the Destination Address field.

   The "Version" field is a special case.  In the context of FHE, all
   packets are presumed to be IPv6 so the normal version field has no
   purpose.  The Modified Version field, if present, has the following
   encoded meanings:

      0b0000: The source address (if exist) has pre-determined length
      inside the domain and the destination address (if exist) uses
      standard 128-bit IPv6 address.  (Outward traffic)

      0b0001: The source address (if exist) uses standard 128-bit IPv6
      address and the destination address (if exist) has pre-determined
      length inside the domain.  (Inward traffic)

      0b0010: The source address and destination address have the same
      length inside the domain.  The address length will be pre-

      0b0110: Reserved for IPv6 compatible case.

      0b0100: Reserved for IPv4 compatible case.

      0b0011~0b1111(except 0b0110, 0b0100): Reserved.

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   All fields, including the Modified Version field, follow the FHE in
   the same order as in [RFC8200], with no padding.  There are no
   alignment requirements, but when a packet is decompressed to a normal
   IPv6 format, padding options as defined in RFC8200 must be inserted.

   Compared to the illustrative example in Figure 1, the actual packet
   size would therefore be 10 bytes, a considerable improvement on the
   standard 40 bytes.

   One implication of the above is that the source and destination
   addresses may be elided completely if they are implicit.  Sourceless
   packets were originally suggested in [crowcroft].

   Figure 2 illustrates an example of the FHE format.  In this example
   the traffic class, flow label and source address are elided, and the
   destination address is truncated to 16 bits.  The modified version
   field could be 0b0001 or 0b0010.

                                                         FHE octet
       Modified                                       +-+-+-+-+-+-+-+-+
       Version                                        |1 0 0 1 1 1 0 1|
      |0 0 0 1|       Payload Length          |  Next Header  |  Hop  |
      | Limit | Truncated Destination Address |                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                       |
      |                                                               |
      +               Transport payload                               |
      |                                                               |

                                 Figure 2

   Note that Asymmetric IPv6 does not contain any special handling for
   IPv6 fragmentation, which will operate exactly as described in
   [RFC8200], with Asymmetric IPv6 applied to each fragment packet.
   However, we assume that in IoT deployment scenarios, packets whose
   length exceeds the IPv6 minimum link MTU before applying Asymmetric
   IPv6 will be rare.  If the underlying link layer cannot carry
   complete packets even after applying Asymmetric IPv6 compression, an
   adaptation layer will be necessary exactly as for normal IPv6.

3.  Address Transformation at the Gateway

   Truncated intra-domain addresses will be used to identify nodes
   inside the domain.  When a packet is sent from an IoT node to an
   external IPv6 host , the node's intra-domain address, which is unique
   in the domain, will be carried in the source address field.  When the

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   packet is forwarded outside the domain by a gateway, the intra-domain
   address will be transformed to a complete IPv6 address.  To achieve
   this, the gateway should will maintain a globally routeable prefix
   for all the nodes in the domain.  When a packet with an intra-domain
   source address is received, the gateway extracts this address and
   concatenates it to the prefix to form a standard, globally unique
   IPv6 address.  Vice versa, when IPv6 packets are received from the
   Internet, the prefix will be removed to recover the intra-domain
   short address.

   There are two options for handling the addresses of external hosts
   within the domain.  One is to use their full IPv6 addresses via
   Modified Version codes 0b0000 and 0b0001.  The other is effectively a
   specialized form of Network Address Translation.  Here, the gateway
   will maintain a dynamic mapping table between synthetic intra-domain
   addresses and IPv6 addresses.  As packets are received, the gateway
   performs the appropriate mapping.  The transformation must be
   checksum-neutral for the transport layer, so the methods designed for
   NAT46 should be adapted.

   NOTE IN DRAFT: Details and references TBD.

   It is an engineering choice whether this method is preferable to
   carrying full 128-bit addresses on the IOT side.

4.  Routing without Decompression

   Routing mechanisms may readily be adapted to truncated address sizes.
   If there is routing with an HFE domain, we assume that the truncated
   address size will be split into a prefix and an interface identifier,
   but this will not be at the traditional /64 boundary.  If routing
   between different length addresses is required, a suitably modified
   Forwarding Information Base (FIB) structure is needed, as for any
   variable length addressing scheme.  A truncated address needs to be
   virtually expanded to 128 bits at the router's inbound interface,
   although this may not be the physical implementation.

   A possible routing choice for IOT edge networks is RPL [RFC6550],
   although a more complete survey can be found in [talwar].

5.  Address Configuration

   The simplest approach to address configuration is simply to run
   normal IPv6 procedures (SLAAC or DHCPv6), on the argument that this
   is a rare process and the overhead does not matter.  If the truncated
   address size is less than 64 bits, it will be necessary to use
   shorter interface identifiers than normal, but this is not a major
   change.  Once a node has acquired an IPv6 address and has learned the

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   local address length parameter as outlined in Section 2, it can
   continue in FHE mode.

6.  Compatibility with Existing Protocols

   Although HFE nodes can only talk directly to each other, they are
   essentially a special form of IPv6 node and they can communicate with
   the whole IPv6 Internet via gateways.  The complexity is not greater
   than 6LoWPAN.  If appropriate, the 6LoWPAN adaptation layer [RFC4944]
   could be used, with a specific dispatch type.

7.  Relationship to Static Context Header Compression

   Static Context Header Compression (SCHC)
   [I-D.ietf-lpwan-ipv6-static-context-hc] is a powerful mechanism for
   reducing IPv6 packet size in an IoT application environment.  In
   particular it includes a profile for UDP over IPv6, and a somewhat
   modified version of this profile could achieve much of what
   Asymmetric IPv6 proposes.  In addition, SCHC provides support for
   fragmentation in the case of very small link MTUs.  However, SCHC is
   by design static, and once a context is established the fields to be
   compressed do not change.  Asymmetric IPv6 transmits the FHE and
   Modified Version bytes with every packet, so it provides dynamic
   choice as to which header elements are compressed or elided.

   In a context where the desirable compression is fixed, e.g. every
   address is the same length, the flow label is never used, etc., SCHC
   can used to the same effect as Asymmetric IPv6.  However, if the
   behavior needs to be dynamic, the signaling power of the FHE and
   Modified Version bytes in Asymmetric IPv6 is needed.

   Further study is needed whether the advantages of the two mechanisms
   can be combined.

8.  Security Considerations

   HFE is essentially only a non-cryptographic compression technique so
   it neither adds to nor reduces the intrinsic security of an IPv6
   packet.  The address length parameter is not a secret, since all
   nodes in the domain must know it.  The mechanism for distributing
   this parameter must be no less secure than any other configuration
   mechanism in us.

   Address-based privacy issues must be considered in deciding on the
   address length.  If the number of bits available for the interface
   identifier is significantly less than the 64 currently in use,
   address traceability and guessability will be affected.  However, if
   the traffic with short addresses is confined to within the edge

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   network, the privacy issue will be minimized.  [RFC7721] and
   [RFC7217] should be consulted prior to deciding the address length.

9.  IANA Considerations

   This document makes no request of the IANA.

   NOTE IN DRAFT: If the solution of a 6LoWPAN dispatch type is adopted,
   a suitable assignment request will be added.

10.  Acknowledgements

   Useful comments were received from Cheng Li, Pascal Thubert, Laurent
   Toutain and others.

11.  References

              Crowcroft, J. and M. Bagnulo, "SNA: Sourceless Network
              Architecture", University of Cambridge Computer Laboratory
              Technical Report UCAM-CL-TR-849, 2014.

              Homma, S., Miyasaka, T., Matsushima, S., and D. Voyer,
              "User Plane Protocol and Architectural Analysis on 3GPP 5G
              System", draft-ietf-dmm-5g-uplane-analysis-02 (work in
              progress), July 2019.

              Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J.
              Zuniga, "Static Context Header Compression (SCHC) and
              fragmentation for LPWAN, application to UDP/IPv6", draft-
              ietf-lpwan-ipv6-static-context-hc-21 (work in progress),
              July 2019.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,

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   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,

   [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,

   [RFC7400]  Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
              IPv6 over Low-Power Wireless Personal Area Networks
              (6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November
              2014, <>.

   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,

   [RFC8138]  Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
              "IPv6 over Low-Power Wireless Personal Area Network
              (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
              April 2017, <>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

              OF THINGS: A SURVEY", Indian J.Sci.Res. 12(1):417-423,

Appendix A.  Change log [RFC Editor: Please remove]

   draft-jiang-asymmetric-ipv6-00, 2019-06-03:

   Initial version

   draft-jiang-asymmetric-ipv6-01, 2019-06-21:

   Fixed reference error

   draft-jiang-asymmetric-ipv6-02, 2019-10-29:

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   Added illustrative example

   Discussed fragmentation

   Discussed relationship to SCHC

   Fixed bit pattern errors

Authors' Addresses

   Sheng Jiang
   Huawei Technologies Co., Ltd
   Q14, Huawei Campus, No.156 Beiqing Road
   Hai-Dian District, Beijing, 100095
   P.R. China


   Guangpeng Li
   Huawei Technologies
   Q14, Huawei Campus
   No.156 Beiqing Road
   Hai-Dian District, Beijing  100095
   P.R. China


   Brian Carpenter
   The University of Auckland
   School of Computer Science
   University of Auckland
   PB 92019
   Auckland  1142
   New Zealand


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