IPv6 Header Compression for Global Addresses

07/24/2007 69th IETF Meeting - 6LoWPAN WG 1

IPv6 Header Compression for Global Addresses

Jonathan Hui

David Culler

draft-hui-6lowpan-hc1g-00 – “Stateless IPv6 Header Compression for Globally Routable Packets in 6LoWPAN Subnetworks”


“IPv6 Communication over IEEE 802.15.4” …

• 6LoWPAN supports IP communication using global (i.e., routable) IPv6 addresses– But it never compresses them!

• HC1 compression only applies to the special link local prefix.– Shared by all nodes in the PAN

• So if you use IP over 802.15.4 for what IP is typically used for (internetworking) you lose much of the virtue of 6LoWPAN.

=> Complement HC1 with HC1g stateless compression for a shared global prefix


Motivation• Why global addresses?

– End-to-end communication in networks with heterogeneous links• without a stateful (possibly application specific) translation gateway• data collection point may be multiple IP hops away

– End-to-end communication across different PANs– IP connects different subnetworks

• ZigBee does not

• LOWPAN_HC1 supports arbitrary addresses, but…– Only allows compression of the link-local (fe80::/64) prefix– Uncompressed addresses consume 32 bytes

• 1/4th the 802.15.4 MTU before link and 6LoWPAN headers!

• The Issue is compressing the origin / final IP addresses– Not the number of PAN hops between them


Why Global Addresses?• Monitoring and Control Applications

– Collector OR controller generally not a 802.15.4 device– Conventional device on conventional link

Collector/Controller

802.15.4 802.3

hartcomm.org

Transit Network

Sensor Patch

Patch Network

Data Service

Intranet/Internet

Client Data Browsingand Processing

Sensor Node

Gateway

Other information sources

Sensor Node

Transit Network

Sensor Patch

Patch Network

Data Service

Intranet/InternetIntranet/Internet

Client Data Browsingand ProcessingClient Data Browsingand Processing

Sensor Node

Gateway

Other information sources

Sensor Node


Why Global Addresses?• Monitoring and Control Applications

– Collector/controller often connected using other links

• Link-local requires a stateful translation gateway• What about multiple 802.15.4 egress points?

– Good for redundancy– But requires a translation gateway at each– How is state managed across them?


802.15.4

Translation Gateway

802.3


• Route directly to the data collector• Possibly through different egress points

Why Global Addresses?


802.15.4 802.3


• Multiple PANs– May be different in PAN ID, channel, MAC, network-

wide keys, etc.


802.15.4802.15.4

802.15.4


• Assigned IP addresses within a PAN– IP addresses are useful for naming– If assigned IP address is used to name source or

destination, no HC1 compression• Assigned prefix or assigned interface identifier


802.15.4


HC1g - In a Nutshell

• Shared context of the PAN includes its prefix• Define compression for prefix other than link-local

– PAN has a compressible global prefix (CGP)

• Compression of Interface IDs derived from short addrs• Similar to LOWPAN_HC1 encoding

– Compresses to 2 octets in best case– Allows arbitrary IPv6 addresses if needed– Same encoding octet– Uses lower-layer information when possible

• Support for compressed well-known multicast addresses– Neighbor discovery, DHCPv6– Uses 6LoWPAN short address encoding


HC1g Compliments HC1

• HC1 and HC1g identified by different dispatch values• HC1 is efficient for link-local communication• HC1g is efficient for off-link communication

– Or on-link using non-default prefix

802.15.4

6LoWPAN Mesh/Frag

LOWPAN_HC1 LOWPAN_HC1g

Upper Layer Protocols

Global CommunicationLink-Local Communication


LOWPAN_HC1g Addresses

• PAN has a compressible global prefix (CGP) associated with it– Prefix is elided whenever it matches the CGP

• Four HC1g address forms:– Full 128 bits for arbitrary IPv6 addresses

– 64 bits: prefix derived from CGP, IID carried inline

– 16 bits: prefix derived from CGP, 6LoWPAN encoded short inline

– 0 bits: prefix derived from CGP, IID derived from lower layers (e.g. 802.15.4 src/dest or 6LoWPAN orig/final)

Arbitrary Prefix Arbitrary IID

Elided Arbitrary IID

Elided SA

Elided


HC1g Encoding

• Source Compression

• Destination Compression

• Version, Traffic, Flow

• Next Header– in-line, UCP, ICMP, TCP

• L4 Compression– HC2 followsSC

0 1 2 3 4 5 6 7

DC V NH L4


LoWPAN HC1g 16-bit Addr

• 16-bit short address follows 6LoWPAN encoding

• Compression of IIDs derived from 802.15.4 short address– 802.15.4 short address when first bit is 0

– Upper 48-bits of IID is assumed to be zero

– U/L bit is also zero, indicating local scope

• Well-known multicast addresses– 16 bits starts with ’101’

– Allocates another range in 6LoWPAN short address encoding

• Neither are supported in LOWPAN_HC1


Multicast Address Compression• For commonly-used, well-known multicast addresses• Use 6LoWPAN short address encoding

– Prefix (8-bits): Compressed to 3-bit range

– Flags (4-bits): Assumed to be zero • permanent, not derived from prefix, doesn’t embed RP

– Scope (4-bits): Carried in-line

– Group ID (112-bits): Mapped to 9-bits• Only all-nodes and all-routers currently defined

• Can be used in LOWPAN_HC1g or Mesh Addressing

1 0 1 Scope Group ID0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

1

FF Flags Scope Group ID

128 bits


Other Differences

• Changes made based on lessons learned from LOWPAN_HC1 implementations

• Possible lack of octet alignment– Can consume several hundred bytes of code

– Octet alignment broken in uncommon cases

– Saves at most one octet in best case (when used with HC_UDP)

– HC1g include 4-bit version field with Traffic Class and Flow Label

• HC_UDP not possible when using IP extension headers– HC2 bit changed to specify layer 4 compression

• Only UDP is currently valid, ICMP/TCP not yet defined


HC1g summary

• Compression for prefixes other than link-local– PAN has a compressible global prefix (CGP)

• Compression of Interface IDs derived from short addrs• Similar to LOWPAN_HC1 encoding, but for CGP

– Compresses to 2 octets in best case– Allows arbitrary IPv6 addresses if needed– Same encoding octet, same HC2– Uses lower-layer information when possible

• Support for compressed well-known multicast addresses– Neighbor discovery, DHCPv6– Uses 6LoWPAN short address encoding

Documents

IPv6 Header Compression for Global Addresses