Compression Format for IPv6 Datagrams in 6LoWPAN Networks

1
Compression Format for IPv6 Datagrams in 6LoWPAN
Networks

• Jonathan Hui
• 6LoWPAN WG Meeting
• 71st IETF Meeting
• Philadelphia, PA

2
RFC 4944 IPv6 Header Compression
0
1
2
3
4
5
6
7
SA
DA
NH
TF
uncompressed fields
HC2

• Most effective when communicating with link-local
  addresses
• Prefix must be carried in-line when not
  link-local
• Route-over, ROLL
• Communicating with devices outside PAN
• Suffix must be 64 bits when carried in-line
• No provision to shorten it even when IID is
  derived from short 802.15.4 address.
• Multicast must carry all 128 bits in-line
• Even for commonly used multicast addresses (e.g.
  link-local all nodes, IPv6 ND, etc.)
• Hop-limit always carried in-line

3
RFC 4944 Next Header Compression

• Defined for UDP header
• No way to elide Checksum
• End-to-end integrity checks may be provided by
  other end-to-end mechanisms (e.g. security).
• No support for future compression of arbitrary
  next headers
• UDP, TCP, or ICMPv6 only

4
Proposed 6LoWPAN HC

• Generalize LOWPAN_HC1/HC2
• Broader range of communication paradigms
• Mesh-under, route-over, communication with
  external devices, multicast
• Framework for compression of arbitrary next
  headers
• UDP compression initially defined within this
  framework
• IPv6 Hop Limit and UDP Checksum compression
• Carry forward design concepts
• Minimize state
• Rely on shared context

5
LOWPAN_IPHCIPv6 Header Compression
0
1
2
3
4
5
6
7
VTF
NH
HLIM
SA
DA
uncompressed fields

• VTF Version, Traffic Class, Flow Label
• NH Next Hop
• HLIM Hop Limit
• SA Source Address
• DA Destination Address
• rsv reserved
• Payload Length always elided

6
LOWPAN_IPHCAddress Compression
Full 128-bit Address In-Line
00 128 bits
64-bit Suffix In-Line
CP Implicit
01 64 bits
SA
CP Implicit
0s
10 16 bits
CP Implicit
From Lower Layers
11 0 bits

• Common Prefix (CP)
• Implicit when prefix is elided
• Link-local (LL) or Common Routable Prefix (CRP)
• Identified by different 6LoWPAN Dispatch values
• SA derived from IEEE 802.15.4 Short Address
• Elided suffix derived from lower-layers

7
LOWPAN_IPHCObtaining the Common Routable Prefix

• Assumption
• 6LoWPAN network operate under a single
  administrative domain
• Single-homed
• CRP is trivial (the only prefix assigned to the
  PAN).
• Renumbering inconsistencies caught with
  pseudo-header checksum
• Multi-homed
• Need to specify a protocol and think through the
  operational details
• Can we go without for now?

8
LOWPAN_IPHCIID Derived from 802.15.4 Short
Addresses

• RFC 4944
• Includes PAN ID and 0xFFEE
• Is there a need to assign the same prefix to gt1
  PAN?
• Instead, prefix Short Address with zeros
• u/l-bit is zero, indicating local scope
• Could also be some fixed bit-pattern, other than
  0s.

SA
0s
9
LOWPAN_IPHCHop Limit Compression

• 1 bit to indicate compression
• 1 bit to indicate 63 (egress) or 1 (ingress)
• Most useful for mesh-under
• All nodes connected via a single IP hop
• Not as useful for route-over
• Forwarding nodes have to expand anyway

10
LOWPAN_IPHCMulticast Address Compression

• For commonly-used, well-known multicast addresses
• Divide 16-bit compressed address into ranges
• Unicast 0xxxxxxxxxxxxxxx
• Multicast 101xxxxxxxxxxxxx
• Prefix (8-bits) Compressed to 3-bit range
• Flags (4-bits) Assumed to be zero
• permanent, not derived from prefix, doesnt embed
  RP
• Scope (4-bits) Carried in-line
• Group ID (112-bits) Mapped to 9-bits
• Currently defined All Nodes (1) and All Routers
  (2)

128 bits
FF
Flags
Scope
Group ID
1
2
3
5
6
7
9
0
1
3
4
5
0
4
8
2
1
0
0
Scope
Group ID
11
LOWPAN_NHCNext Header Compression
0
1
2
3
4
5
6
7
ID
SP
DP
C
rsv
uncompressed fieds

• IPHC NH indicates next header compression
• IPv6 Next Header elided, derived from first bits
  in NHC
• Encoding gives shorter bit-patterns to frequently
  used next headers
• ID 0 for UDP, 1 for other
• SP Source Port
• DP Destination Port
• C Checksum
• Length always elided
• Checksum MUST NOT be elided when no other
  end-to-end integrity cover the pseudo-header, UDP
  header, and UDP payload

12
Unicast Examples

• Link-Local, Mesh-Under (9 bytes)
• Link-Local, Route-Over (4 bytes)
• Routable, Mesh-Under (9 bytes)
• Routable Addresses, Route-Over (9 bytes)

5
1
1
1
1
Disp.
IPHC
NHC
Ports
6LoWPAN Mesh Header
15.4
1
1
1
1
Disp.
IPHC
NHC
Ports
15.4
5
1
1
1
1
Disp.
IPHC
NHC
Ports
6LoWPAN Mesh Header
15.4
1
1
1
1
1
2
2
Disp.
IPHC
HLIM
Src Addr
Dst Addr
NHC
Ports
15.4
13
Multicast Examples

• Link-Local, Mesh-Under (11 bytes)
• Link-Local, Route-Over (6 bytes)
• Routable, Mesh-Under (11 bytes)
• Routable Addresses, Route-Over (9 bytes)

1
1
1
1
1
1
5
Disp.
IPHC
NHC
Ports
6LoWPAN Mesh Header
15.4
Disp.
Bcast
Disp.
IPHC
NHC
Ports
15.4
Dst Addr
1
1
1
1
1
1
5
Disp.
IPHC
NHC
Ports
6LoWPAN Mesh Header
15.4
Disp.
Bcast
1
1
1
1
1
2
2
Disp.
IPHC
HLIM
Src Addr
Dst Addr
NHC
Ports
15.4
14
6LoWPAN HC Summary

• Generalize LOWPAN_HC1/HC2
• Broader range of communication paradigms
• Mesh-under, route-over, communication with
  external devices, multicast
• Framework for compression of arbitrary next
  headers
• UDP compression initially defined within this
  framework
• IPv6 Hop Limit and UDP Checksum compression
• Carry forward design concepts
• Minimize state
• Rely on shared context

15
Discussion

• Should 6lowpan-hc become a WG doc?

16
Combining IPHC and NHC(From Discussion with
Pascal Thubert)

• Another dispatch for combining LOWPAN_IPHC/NHC?
• Fully elided Source and Destination addresses and
  Hop Limit
• Next header compression
• Pro Save an additional octet
• Con Additional code overhead

0
1
2
3
4
5
6
7
5
VTF
rsv
uncompressed fields
SP
DP
C
HLIM
0