Internet DRAFT - draft-jiang-asymmetric-ipv6
draft-jiang-asymmetric-ipv6
Network Working Group S. Jiang
Internet-Draft Huawei Technologies Co., Ltd
Intended status: Informational G. Li
Expires: 19 May 2021 Huawei Technologies
B. E. Carpenter
Univ. of Auckland
15 November 2020
Asymmetric IPv6 for Resource-constrained IoT Networks
draft-jiang-asymmetric-ipv6-04
Abstract
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
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This Internet-Draft will expire on 19 May 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 3
3. Address Transformation at the Gateway . . . . . . . . . . . . 6
4. Routing without Decompression . . . . . . . . . . . . . . . . 6
5. Address Configuration . . . . . . . . . . . . . . . . . . . . 7
6. Compatibility with Existing Protocols . . . . . . . . . . . . 7
7. Relationship to Static Context Header Compression . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Change log [RFC Editor: Please remove] . . . . . . . 10
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, the importance of header overhead in small
packets is discussed in [NGMN-5G]. 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 (assuming that
routing is not bypassed by tunnelling). 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
gateways to the rest of the Internet. Rather than needing 128-bit
addresses, such a network could very well operate with 16-bit
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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
overhead.
This work is also distinct from the work on static context header
compression [RFC8724], 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. If a client
communicates with a server that is in the local domain, short
addresses will be used end-to-end.
The address length parameter may be needed by every node in the
domain. It can be spread by various techniques:
* Configure the address length in every node.
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* Obtain the address length from a gateway (next hop router) node.
* 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-
determined.
* 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.
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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
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 [RFC6145].
It is an engineering choice whether this method is preferable to
carrying full 128-bit addresses on the IOT side. Which type of
resource is more expensive should be seriously considered to choose
the appropriate ways, e.g. computing, memory, or transmiting in
various resource-constrained IoT networks.
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].
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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
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) [RFC8724] 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.
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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
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 Uma Chunduri, Cheng Li, Pascal
Thubert, Laurent Toutain and others.
11. References
[Crowcroft]
Crowcroft, J. and M. Bagnulo, "SNA: Sourceless Network
Architecture", University of Cambridge Computer Laboratory
Technical Report UCAM-CL-TR-849, 2014.
[NGMN-5G] Thibault, I., "5G Extreme Requirements: Operators' views
on fundamental trade-offs", NGMN Alliance , 2017.
[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,
<https://www.rfc-editor.org/info/rfc4944>.
[RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
<https://www.rfc-editor.org/info/rfc6145>.
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[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,
<https://www.rfc-editor.org/info/rfc6282>.
[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,
<https://www.rfc-editor.org/info/rfc6550>.
[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,
<https://www.rfc-editor.org/info/rfc7217>.
[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, <https://www.rfc-editor.org/info/rfc7400>.
[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,
<https://www.rfc-editor.org/info/rfc7721>.
[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, <https://www.rfc-editor.org/info/rfc8138>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zúñiga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/info/rfc8724>.
[Talwar] Talwar, M., "Routing Techniques and Protocols for Internet
of Things: a Survey", Indian J.Sci.Res. 12(1):417-423,
2015.
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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:
- Added illustrative example
- Discussed fragmentation
- Discussed relationship to SCHC
- Fixed bit pattern errors
* draft-jiang-asymmetric-ipv6-03, 2020-05-15:
- Minor technical and editorial fixes
- Converted to xml2rfc v3
* draft-jiang-asymmetric-ipv6-04, 2020-11-15:
- Explicitly limit the scope to resource-constrained domain
- Add engineering choice considerations accordingly
Authors' Addresses
Sheng Jiang
Huawei Technologies Co., Ltd
Q14, Huawei Campus, No.156 Beiqing Road
Hai-Dian District, Beijing, 100095
P.R. China
Email: jiangsheng@huawei.com
Guangpeng Li
Huawei Technologies
Q14, Huawei Campus
No.156 Beiqing Road
Hai-Dian District, Beijing
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100095
P.R. China
Email: liguangpeng@huawei.com
Brian Carpenter
The University of Auckland
School of Computer Science
University of Auckland
PB 92019
Auckland 1142
New Zealand
Email: brian.e.carpenter@gmail.com
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