HomeDocsIETF standardsRFCRFC 6200 — 6299
Text version
Internet Engineering Task Force (IETF)
Request for Comments: 6282
Updates: 4944
Category: Standards Track
ISSN: 2070-1721
J. Hui, Ed.
Arch Rock Corporation
P. Thubert
Cisco
September 2011

Compression Format for IPv6 Datagrams

over IEEE 802.15.4-Based Networks

Abstract

This document updates RFC 4944, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks". This document specifies an IPv6 header compression format for IPv6 packet delivery in Low Power Wireless Personal Area Networks (6LoWPANs). The compression format relies on shared context to allow compression of arbitrary prefixes. How the information is maintained in that shared context is out of scope. This document specifies compression of multicast addresses and a framework for compressing next headers. UDP header compression is specified within this framework.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6282.

Copyright Notice

Copyright © 2011 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 ....................................................3
      1.1. Requirements Language ......................................4
   2. Specific Updates to RFC 4944 ....................................4
   3. IPv6 Header Compression .........................................5
      3.1. LOWPAN_IPHC Encoding Format ................................6
           3.1.1. Base Format .........................................6
           3.1.2. Context Identifier Extension .......................10
      3.2. IPv6 Header Encoding ......................................11
           3.2.1. Traffic Class and Flow Label Compression ...........11
           3.2.2. Deriving IIDs from the Encapsulating Header ........12
           3.2.3. Stateless Multicast Address Compression ............13
           3.2.4. Stateful Multicast Address Compression .............14
   4. IPv6 Next Header Compression ...................................15
      4.1. LOWPAN_NHC Format .........................................15
      4.2. IPv6 Extension Header Compression .........................15
      4.3. UDP Header Compression ....................................17
           4.3.1. Compressing UDP Ports ..............................17
           4.3.2. Compressing UDP Checksum ...........................18
           4.3.3. UDP LOWPAN_NHC Format ..............................20
   5. IANA Considerations ............................................20
   6. Security Considerations ........................................21
   7. Acknowledgements ...............................................22
   8. References .....................................................22
      8.1. Normative References ......................................22
      8.2. Informative References ....................................23

1. Introduction

The [IEEE802.15.4] standard specifies an MTU of 127 bytes, yielding about 80 octets of actual Media Access Control (MAC) payload with security enabled, on a wireless link with a link throughput of 250 kbps or less. The 6LoWPAN adaptation format [RFC4944] was specified to carry IPv6 datagrams over such constrained links, taking into account limited bandwidth, memory, or energy resources that are expected in applications such as wireless sensor networks. [RFC4944] defines a Mesh Addressing header to support sub-IP forwarding, a Fragmentation header to support the IPv6 minimum MTU requirement [RFC2460], and stateless header compression for IPv6 datagrams (LOWPAN_HC1 and LOWPAN_HC2) to reduce the relatively large IPv6 and UDP headers down to (in the best case) several bytes.

LOWPAN_HC1 and LOWPAN_HC2 are insufficient for most practical uses of IPv6 in 6LoWPANs. LOWPAN_HC1 is most effective for link-local unicast communication, where IPv6 addresses carry the link-local prefix and an Interface Identifier (IID) directly derived from IEEE 802.15.4 addresses. In this case, both addresses may be completely elided. However, though link-local addresses are commonly used for local protocol interactions such as IPv6 Neighbor Discovery [RFC4861], DHCPv6 [RFC3315], or routing protocols, they are usually not used for application-layer data traffic, so the actual value of this compression mechanism is limited.

Routable addresses must be used when communicating with devices external to the 6LoWPAN or in a route-over configuration where IP forwarding occurs within the 6LoWPAN. For routable addresses, LOWPAN_HC1 requires both IPv6 source and destination addresses to carry the prefix in-line. In cases where the Mesh Addressing header is not used, the IID of a routable address must be carried in-line. However, LOWPAN_HC1 requires 64 bits for the IID when carried in-line and cannot be shortened even when it is derived from the IEEE 802.15.4 16-bit short address. When the destination is an IPv6 multicast address, LOWPAN_HC1 requires the full 128-bit address to be carried in-line.

As a result, this document defines an encoding format, LOWPAN_IPHC, for effective compression of Unique Local, Global, and multicast IPv6 Addresses based on shared state within contexts. In addition, this document also introduces a number of additional improvements over the header compression format defined in [RFC4944].

LOWPAN_IPHC allows for compression of some commonly used IPv6 Hop Limit values. If the 6LoWPAN is a mesh-under stub, a Hop Limit of 1 for inbound and a default value such as 64 for outbound are usually enough for application-layer data traffic. Additionally, a Hop Limit value of 255 is often used to verify that a communication occurs over a single-hop. This specification enables compression of the IPv6 Hop Limit field in those common cases, whereas LOWPAN_HC1 does not.

This document also defines LOWPAN_NHC, an encoding format for arbitrary next headers. LOWPAN_IPHC indicates whether the following header is encoded using LOWPAN_NHC. If so, the bits immediately following the compressed IPv6 header start the LOWPAN_NHC encoding. In contrast, LOWPAN_HC1 could be extended to support compression of next headers using LOWPAN_HC2, but only for UDP, TCP, and ICMPv6. Furthermore, the LOWPAN_HC2 octet sits between the LOWPAN_HC1 octet and uncompressed IPv6 header fields. This specification moves the next header encoding bits to follow all IPv6-related bits, allowing for a properly layered structure and direct support for IPv6 extension headers.

Using LOWPAN_NHC, this document defines a compression mechanism for UDP. While [RFC4944] defines a compression mechanism for UDP, that mechanism does not enable checksum compression when rendered possible by additional upper-layer mechanisms such as upper-layer Message Integrity Check (MIC). This specification adds the capability to elide the UDP checksum over the 6LoWPAN, which enables saving of a further two octets.

Also, using LOWPAN_NHC, this document defines encoding formats for IPv6-in-IPv6 encapsulation as well as IPv6 Extension Headers. With LOWPAN_HC1 and LOWPAN_HC2, chains of next headers cannot be encoded efficiently.

1.1. 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].

2. Specific Updates to RFC 4944

This document specifies a header compression format that is intended to replace that defined in Section 10 of [RFC4944]. Implementation of Section 10 of [RFC4944] is now NOT RECOMMENDED. New implementations MAY implement decompression according to Section 10 of [RFC4944] but SHOULD NOT send packets compressed according to Section 10 of [RFC4944].

A compliant implementation of [RFC4944] as updated by this document MUST be able to properly process a packet received that makes use of the provisions of this document. A compliant implementation MAY implement additional LOWPAN_NHC types (Section 4) that may be registered (Section 5) in the future. It is out of scope of this document how a compressor learns that a decompressor has additional capabilities.

Section 5.3 of [RFC4944] also defines how to fragment compressed IPv6 datagrams that do not fit within a single link frame. Section 5.3 of [RFC4944] defines the fragment header's datagram_size and datagram_offset values as the size and offset of the IPv6 datagram before compression. As a result, all fragment payload outside the first fragment must carry their respective portions of the IPv6 datagram before compression. This document does not change that requirement. When using the fragmentation mechanism described in Section 5.3 of [RFC4944], any header that cannot fit within the first fragment MUST NOT be compressed.

The header compression format defined in this document preempts the ESC dispatch value defined in Section 5.1 of [RFC4944]. Instead, the value of 01 000000 is reserved as a replacement value for ESC, to be finally assigned with the first assignment of extension bytes.

3. IPv6 Header Compression

In this section, we define the LOWPAN_IPHC encoding format for compressing the IPv6 header. To enable effective compression, LOWPAN_IPHC relies on information pertaining to the entire 6LoWPAN. LOWPAN_IPHC assumes the following will be the common case for 6LoWPAN communication: Version is 6; Traffic Class and Flow Label are both zero; Payload Length can be inferred from lower layers from either the 6LoWPAN Fragmentation header or the IEEE 802.15.4 header; Hop Limit will be set to a well-known value by the source; addresses assigned to 6LoWPAN interfaces will be formed using the link-local prefix or a small set of routable prefixes assigned to the entire 6LoWPAN; addresses assigned to 6LoWPAN interfaces are formed with an IID derived directly from either the 64-bit extended or the 16-bit short IEEE 802.15.4 addresses. 

    +-------------------------------------+----------------------------
    | Dispatch + LOWPAN_IPHC (2-3 octets) | In-line IPv6 Header Fields
    +-------------------------------------+----------------------------

Figure 1: LOWPAN_IPHC Header

The LOWPAN_IPHC encoding utilizes 13 bits, 5 of which are taken from the rightmost bits of the dispatch type. The encoding may be extended by another octet to support additional contexts. Any information from the uncompressed IPv6 header fields carried in-line follow the LOWPAN_IPHC encoding, as shown in Figure 1. In the best case, the LOWPAN_IPHC can compress the IPv6 header down to two octets (the dispatch octet and the LOWPAN_IPHC encoding) with link-local communication.

When routing over multiple IP hops, LOWPAN_IPHC can compress the IPv6 header down to 7 octets (1-octet dispatch, 1-octet LOWPAN_IPHC, 1-octet Hop Limit, 2-octet Source Address, and 2-octet Destination Address). The Hop Limit may not be compressed because it needs to decremented at each hop and may take any value. Stateful address compression must be applied to the source and destination IPv6 addresses because they do not statelessly match the source and destination link-layer addresses on intermediate hops.

3.1. LOWPAN_IPHC Encoding Format

This section specifies the format of the LOWPAN_IPHC encoding that describes how an IPv6 header is compressed. The encoding can be 2 octets long for the base encoding or 3 octets long when an additional context encoding is present. The IPv6 header fields that are not fully elided are placed immediately after the LOWPAN_IPHC, either in a compressed form if the field is partially elided or literally.

3.1.1. Base Format

       0                                       1
       0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
     | 0 | 1 | 1 |  TF   |NH | HLIM  |CID|SAC|  SAM  | M |DAC|  DAM  |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

Figure 2: LOWPAN_IPHC base Encoding

   TF: Traffic Class, Flow Label:  As specified in [RFC3168], the 8-bit
      IPv6 Traffic Class field is split into two fields: 2-bit Explicit
      Congestion Notification (ECN) and 6-bit Differentiated Services
      Code Point (DSCP).
   
      00:  ECN + DSCP + 4-bit Pad + Flow Label (4 bytes)
   
      01:  ECN + 2-bit Pad + Flow Label (3 bytes), DSCP is elided.
   
      10:  ECN + DSCP (1 byte), Flow Label is elided.
   
      11:  Traffic Class and Flow Label are elided.

NH:

Next Header:

0:

Full 8 bits for Next Header are carried in-line.

1:

The Next Header field is compressed and the next header is encoded using LOWPAN_NHC, which is discussed in Section 4.1.

HLIM:

Hop Limit: 

      00:  The Hop Limit field is carried in-line.
      
      01:  The Hop Limit field is compressed and the hop limit is 1.
      
      10:  The Hop Limit field is compressed and the hop limit is 64.
      
      11:  The Hop Limit field is compressed and the hop limit is 255.

CID:

Context Identifier Extension:

0:

No additional 8-bit Context Identifier Extension is used. If context-based compression is specified in either Source Address Compression (SAC) or Destination Address Compression (DAC), context 0 is used.

1:

An additional 8-bit Context Identifier Extension field immediately follows the Destination Address Mode (DAM) field.

SAC: 

        Source Address Compression

0:

Source address compression uses stateless compression.

      1: Source address compression uses stateful, context-based
         compression.

SAM:

Source Address Mode:

If SAC=0:

00: 128 bits. The full address is carried in-line.

         01:  64 bits.  The first 64-bits of the address are elided.
            The value of those bits is the link-local prefix padded with
            zeros.  The remaining 64 bits are carried in-line.
         
         10:  16 bits.  The first 112 bits of the address are elided.
            The value of the first 64 bits is the link-local prefix
            padded with zeros.  The following 64 bits are 0000:00ff:
            fe00:XXXX, where XXXX are the 16 bits carried in-line.
         
         11:  0 bits.  The address is fully elided.  The first 64 bits
            of the address are the link-local prefix padded with zeros.
            The remaining 64 bits are computed from the encapsulating
            header (e.g., 802.15.4 or IPv6 source address) as specified
            in Section 3.2.2.

If SAC=1: 

         00:  The UNSPECIFIED address, ::
         
         01:  64 bits.  The address is derived using context information
            and the 64 bits carried in-line.  Bits covered by context
            information are always used.  Any IID bits not covered by
            context information are taken directly from the
            corresponding bits carried in-line.  Any remaining bits are
            zero.
         
         10:  16 bits.  The address is derived using context information
            and the 16 bits carried in-line.  Bits covered by context
            information are always used.  Any IID bits not covered by
            context information are taken directly from their
            corresponding bits in the 16-bit to IID mapping given by
            0000:00ff:fe00:XXXX, where XXXX are the 16 bits carried in-
            line.  Any remaining bits are zero.
         
         11:  0 bits.  The address is fully elided and is derived using
            context information and the encapsulating header (e.g.,
            802.15.4 or IPv6 source address).  Bits covered by context
            information are always used.  Any IID bits not covered by
            context information are computed from the encapsulating
            header as specified in Section 3.2.2.  Any remaining bits
            are zero.

M: 

      Multicast Compression

0:

Destination address is not a multicast address.

1:

Destination address is a multicast address.

DAC: 

        Destination Address Compression

0:

Destination address compression uses stateless compression. 

      1: Destination address compression uses stateful, context-based
      
         compression.

DAM:

Destination Address Mode: 

      If M=0 and DAC=0  This case matches SAC=0 but for the destination
         address:

00: 128 bits. The full address is carried in-line.

         01:  64 bits.  The first 64-bits of the address are elided.
            The value of those bits is the link-local prefix padded with
            zeros.  The remaining 64 bits are carried in-line.
         
         10:  16 bits.  The first 112 bits of the address are elided.
            The value of the first 64 bits is the link-local prefix
            padded with zeros.  The following 64 bits are 0000:00ff:
            fe00:XXXX, where XXXX are the 16 bits carried in-line.
         
         11:  0 bits.  The address is fully elided.  The first 64 bits
            of the address are the link-local prefix padded with zeros.
            The remaining 64 bits are computed from the encapsulating
            header (e.g., 802.15.4 or IPv6 destination address) as
            specified in Section 3.2.2.

If M=0 and DAC=1: 

         00:  Reserved.
         
         01:  64 bits.  The address is derived using context information
            and the 64 bits carried in-line.  Bits covered by context
            information are always used.  Any IID bits not covered by
            context information are taken directly from the
            corresponding bits carried in-line.  Any remaining bits are
            zero.
         
         10:  16 bits.  The address is derived using context information
            and the 16 bits carried in-line.  Bits covered by context
            information are always used.  Any IID bits not covered by
            context information are taken directly from their
            corresponding bits in the 16-bit to IID mapping given by
            0000:00ff:fe00:XXXX, where XXXX are the 16 bits carried in-
            line.  Any remaining bits are zero.
         
         11:  0 bits.  The address is fully elided and is derived using
            context information and the encapsulating header (e.g.
            802.15.4 or IPv6 destination address).  Bits covered by
            context information are always used.  Any IID bits not
            covered by context information are computed from the
            encapsulating header as specified in Section 3.2.2.  Any
            remaining bits are zero.

If M=1 and DAC=0:

00: 128 bits. The full address is carried in-line.

         01:  48 bits.  The address takes the form ffXX::00XX:XXXX:XXXX.

10: 32 bits. The address takes the form ffXX::00XX:XXXX.

11: 8 bits. The address takes the form ff02::00XX.

If M=1 and DAC=1: 

         00:  48 bits.  This format is designed to match Unicast-Prefix-
            based IPv6 Multicast Addresses as defined in [RFC3306] and
            [RFC3956].  The multicast address takes the form ffXX:XXLL:
            PPPP:PPPP:PPPP:PPPP:XXXX:XXXX. where the X are the nibbles
            that are carried in-line, in the order in which they appear
            in this format.  P denotes nibbles used to encode the prefix
            itself.  L denotes nibbles used to encode the prefix length.
            The prefix information P and L is taken from the specified
            context.
         
         01:  reserved
         
         10:  reserved
         
         11:  reserved

3.1.2. Context Identifier Extension

This specification expects that a conceptual context is shared between the node that compresses a packet and the node(s) that needs to expand it. How the contexts are shared and maintained is out of scope. What information is contained within a context information is out of scope. Actions in response to unknown and/or invalid contexts are out of scope. The specification enables a node to use up to 16 contexts. The context used to encode the source address does not have to be the same as the context used to encode the destination address.

If the CID field is set to '1' in the LOWPAN_IPHC encoding, then an additional octet extends the LOWPAN_IPHC encoding following the DAM bits but before the IPv6 header fields that are carried in-line. The additional octet identifies the pair of contexts to be used when the IPv6 source and/or destination address is compressed. The context identifier is 4 bits for each address, supporting up to 16 contexts. Context 0 is the default context. The encoding is shown in Figure 3. 

                       0   1   2   3   4   5   6   7
                     +---+---+---+---+---+---+---+---+
                     |      SCI      |      DCI      |
                     +---+---+---+---+---+---+---+---+

Figure 3: LOWPAN_IPHC Encoding

SCI: Source Context Identifier. Identifies the prefix that is used

when the IPv6 source address is statefully compressed.

DCI: Destination Context Identifier. Identifies the prefix that is

used when the IPv6 destination address is statefully compressed.

3.2. IPv6 Header Encoding

Fields carried in-line (in part or in whole) appear in the same order as they do in the IPv6 header format [RFC2460]. The Version field is always elided. Unicast IPv6 addresses may be compressed to 64 or 16 bits or completely elided. Multicast IPv6 addresses may be compressed to 8, 32, or 48 bits. The IPv6 Payload Length field MUST always be elided and inferred from lower layers using the 6LoWPAN Fragmentation header or the IEEE 802.15.4 header.

3.2.1. Traffic Class and Flow Label Compression

The Traffic Class field in the IPv6 header comprises 6 bits of Diffserv extension [RFC2474] and 2 bits of Explicit Congestion Notification (ECN) [RFC3168]. The TF field in the LOWPAN_IPHC encoding indicates whether the Traffic Class and Flow Label are carried in-line in the compressed IPv6 header. When Flow Label is included while the Traffic Class is compressed, an additional 4 bits are included to maintain byte alignment. Two of the 4 bits contain the ECN bits from the Traffic Class field.

To ensure that the ECN bits appear in the same location for all encodings that include them, the Traffic Class field is rotated right by 2 bits in the compressed IPv6 header. The encodings are shown below: 

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |ECN|   DSCP    |  rsv  |             Flow Label                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: TF = 00: Traffic Class and Flow Label carried in-line 

                          1                   2
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |ECN|rsv|             Flow Label                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: TF = 01: Flow Label carried in-line

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |ECN|   DSCP    |
     +-+-+-+-+-+-+-+-+

Figure 6: TF = 10: Traffic Class carried in-line

3.2.2. Deriving IIDs from the Encapsulating Header

LOWPAN_IPHC elides the IIDs of source or destination addresses when SAM = 3 or DAM = 3, respectively. In this mode, the IID is derived from the encapsulating header. When the encapsulating header carries IPv6 addresses, bits for the source and destination addresses are copied from the source and destination addresses of the encapsulating IPv6 header.

The remainder of this section defines the mapping from IEEE 802.15.4 [IEEE802.15.4] link-layer addresses to IIDs for both short and extended IEEE 802.15.4 addresses. IID bits not covered by the context information MAY be elided if they match the link-layer address mapping and MUST NOT be elided if they do not.

An extended IEEE 802.15.4 address takes the form of an IEEE EUI-64 address. Generating an IID from an extended address is identical to that defined in Appendix A of [RFC4291]. The only change needed to transform an IEEE EUI-64 identifier to an interface identifier is to invert the universal/local bit.

A short IEEE 802.15.4 address is 16 bits in length. Short addresses are mapped into the restricted space of IEEE EUI-64 addresses by setting the middle 16 bits to 0xfffe, the bottom 16 bits to the short address, and all other bits to zero. As a result, an IID generated from a short address has the form: 

      0000:00ff:fe00:XXXX

where XXXX carries the short address. The universal/local bit is zero to indicate local scope.

This mapping for non-EUI-64 identifiers differs from that presented in Appendix A of [RFC4291]. Using the restricted space ensures no overlap with IIDs generated from unrestricted IEEE EUI-64 addresses. Also, including 0xfffe in the middle of the IID helps avoid overlap with other locally managed IIDs.

This mapping from a short IEEE 802.15.4 address to 64-bit IIDs is also used to reconstruct any part of an IID not covered by context information.

3.2.3. Stateless Multicast Address Compression

LOWPAN_IPHC supports stateless compression of multicast addresses when M = 1 and DAC = 0. An IPv6 multicast address may be compressed down to 48, 32, or 8 bits using stateless compression. The format supports compression of the Solicited-Node Multicast Address (ff02:: 1:ffXX:XXXX) as well as any IPv6 multicast address where the upper bits of the multicast group identifier are zero. The 8-bit compressed form only carries the least-significant bits of the multicast group identifier. The 48- and 32-bit compressed forms carry the multicast scope and flags in-line, in addition to the least-significant bits of the multicast group identifier. 

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Flags | Scope |              Group Identifier                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Group Identifier       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7: DAM = 01. 48-bit Compressed Multicast Address

(ffFS::00GG:GGGG:GGGG)

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Flags | Scope |              Group Identifier                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 8: DAM = 10. 32-bit Compressed Multicast Address

(ffFS::00GG:GGGG)

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |   Group ID    |
     +-+-+-+-+-+-+-+-+
     
     Figure 9: DAM = 11. 8-bit Compressed Multicast Address (ff02::GG)

3.2.4. Stateful Multicast Address Compression

LOWPAN_IPHC supports stateful compression of multicast addresses when M = 1 and DAC = 1. This document currently defines DAM = 00: context-based compression of Unicast-Prefix-based IPv6 Multicast Addresses [RFC3306][RFC3956]. In particular, the Prefix Length and Network Prefix can be taken from a context. As a result, LOWPAN_IPHC can compress a Unicast-Prefix-based IPv6 Multicast Address down to 6 octets by only carrying the 4-bit Flags, 4-bit Scope, 8-bit Rendezvous Point Interface ID (RIID), and 32-bit Group Identifier in- line. 

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Flags | Scope | Rsvd / RIID   |       Group Identifier        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Group Identifier       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 10: DAM = 00. Unicast-Prefix-based IPv6 Multicast

Address Compression

Note that the Reserved field MUST carry the reserved bits from the multicast address format as described in [RFC3306]. When a Rendezvous Point is encoded in the multicast address as described in [RFC3956], the Reserved field carries the RIID bits in-line.

4. IPv6 Next Header Compression

LOWPAN_IPHC elides the IPv6 Next Header field when the NH bit is set to 1. This also indicates the use of 6LoWPAN next header compression, LOWPAN_NHC. The value of IPv6 Next Header is recovered from the first bits in the LOWPAN_NHC encoding. The following bits are specific to the IPv6 Next Header value. Figure 11 shows the structure of an IPv6 datagram compressed using LOWPAN_IPHC and LOWPAN_NHC. 

   +-------------+-------------+-------------+-----------------+--------
   | LOWPAN_IPHC | In-line     | LOWPAN_NHC  | In-line Next    | Payload
   |   Encoding  |   IP Fields |   Encoding  |   Header Fields |
   +-------------+-------------+-------------+-----------------+--------

Figure 11: Typical LOWPAN_IPHC/LOWPAN_NHC Header Configuration

4.1. LOWPAN_NHC Format

Compression formats for different next headers are identified by a variable-length bit-pattern immediately following the LOWPAN_IPHC compressed header. When defining a next header compression format, the number of bits used SHOULD be determined by the perceived frequency of using that format. However, the number of bits and any remaining encoding bits SHOULD respect octet alignment. The following bits are specific to the next header compression format. This document defines a compression format for IPv6 Extension and UDP headers.

+----------------+---------------------------

| var-len NHC ID | compressed next header...

+----------------+---------------------------

Figure 12: LOWPAN_NHC Encoding

4.2. IPv6 Extension Header Compression

A necessary property of encoding headers using LOWPAN_NHC is that the immediately preceding header must be encoded using either LOWPAN_IPHC or LOWPAN_NHC. In other words, all headers encoded using the 6LoWPAN encoding format defined in this document must be contiguous. As a result, this document defines a set of LOWPAN_NHC encodings for selected IPv6 Extension Headers such that the UDP Header Compression defined in Section 4.3 may be used in the presence of those extension headers.

The LOWPAN_NHC encodings for IPv6 Extension Headers are composed of a single LOWPAN_NHC octet followed by the IPv6 Extension Header. The format of the LOWPAN_NHC octet is shown in Figure 13. The first 7 bits serve as an identifier for the IPv6 Extension Header immediately following the LOWPAN_NHC octet. The remaining bit indicates whether or not the following header utilizes LOWPAN_NHC encoding. 

                       0   1   2   3   4   5   6   7
                     +---+---+---+---+---+---+---+---+
                     | 1 | 1 | 1 | 0 |    EID    |NH |
                     +---+---+---+---+---+---+---+---+

Figure 13: IPv6 Extension Header Encoding

EID:

IPv6 Extension Header ID: 

      0: IPv6 Hop-by-Hop Options Header [RFC2460]
      
      1: IPv6 Routing Header [RFC2460]
      
      2: IPv6 Fragment Header [RFC2460]
      
      3: IPv6 Destination Options Header [RFC2460]
      
      4: IPv6 Mobility Header [RFC6275]

5: 

         Reserved

6: 

         Reserved

7: 

         IPv6 Header

NH:

Next Header:

0:

Full 8 bits for Next Header are carried in-line.

1:

The Next Header field is elided and the next header is encoded using LOWPAN_NHC, which is discussed in Section 4.1.

For the most part, the IPv6 Extension Header is carried unmodified in the bytes immediately following the LOWPAN_NHC octet, with two important exceptions: Length field and Next Header field.

The Next Header field contained in IPv6 Extension Headers is elided when the NH bit is set in the LOWPAN_NHC encoding octet. Note that doing so allows LOWPAN_NHC to utilize no more overhead than the non- encoded IPv6 Extension Header.

The Length field contained in a compressed IPv6 Extension Header indicates the number of octets that pertain to the (compressed) extension header following the Length field. Note that this changes the Length field definition in [RFC2460] from indicating the header size in 8-octet units, not including the first 8 octets. Changing the Length field to be in units of octets removes wasteful internal fragmentation.

IPv6 Hop-by-Hop and Destination Options Headers may use a trailing Pad1 or PadN to achieve 8-octet alignment. When there is a single trailing Pad1 or PadN option of 7 octets or less and the containing header is a multiple of 8 octets, the trailing Pad1 or PadN option MAY be elided by the compressor. A decompressor MUST ensure that the containing header is padded out to a multiple of 8 octets in length, using a Pad1 or PadN option if necessary. Note that Pad1 and PadN options that appear in locations other than the end MUST be carried in-line as they are used to align subsequent options.

Note that specifying units in octets means that LOWPAN_NHC MUST NOT be used to encode IPv6 Extension Headers that have more than 255 octets following the Length field after compression.

When the identified next header is an IPv6 Header (EID=7), the NH bit of the LOWPAN_NHC encoding is unused and MUST be set to zero. The following bytes MUST be encoded using LOWPAN_IPHC as defined in Section 3.

4.3. UDP Header Compression

This document defines a compression format for UDP headers using LOWPAN_NHC. The UDP compression format is shown in Figure 14. Bits 0 through 4 represent the NHC ID and '11110' indicates the specific UDP header compression encoding defined in this section.

4.3.1. Compressing UDP Ports

This specification allows a particular range of ports number (0xf0b0 to 0xf0bf) to be compressed down to 4 bits. This is a stateless compression that is inherited from [RFC4944], as opposed to a new stateful compression.

The range of ports compressible down to 4 bits is not in a reserved range. A network stack implementation that is designed to communicate over a 6LoWPAN should avoid using those ports as dynamic ports whenever possible.

Considering that this represents only 16 contiguous ports, it can be expected that many incompatible applications will use the same value of port numbers for their own end-to-end needs. Thus, a port number in the (0xf0b0 to 0xf0bf) range provides very little information about the application at the remote end.

The overloading of the 0xf0bX ports increases the risk of getting the wrong type of payload and misinterpreting the content compared to ports that are reserved at IANA. As a result, it is recommended that the use of those ports be associated with a mechanism such as a Transport Layer Security (TLS) [RFC5246] Message Integrity Check (MIC) that makes sure that the content is what is expected and is checked.

4.3.2. Compressing UDP Checksum

The UDP checksum operation is mandatory with IPv6 [RFC2460] for all packets. For that reason, [RFC4944] disallows the compression of the UDP checksum.

With this specification, a compressor in the source transport endpoint MAY elide the UDP Checksum if it is authorized by the upper layer. The compressor MUST NOT set the C bit unless it has received such authorization. Requiring upper-layer authorization ensures that the intended transport peer will have sufficient means to deal with any data corruption that occurs before reaching the destination. The upper layer MUST NOT provide the authorization unless one of the following cases is satisfied: 

   Tunneling:  In this case, 6LoWPAN is deployed as a wireless pseudo-
      fieldbus by tunneling existing field protocols over UDP.  If the
      tunneled Protocol Data Unit (PDU) possesses its own addressing,
      security and integrity check (e.g., IPsec Encapsulating Security
      Payload tunnel mode [RFC4303] or IP over UDP encapsulation), the
      tunneling mechanism MAY authorize eliding the UDP checksum in
      order to save on the encapsulation overhead.
   
   Message Integrity Check:  In this case, either IPsec Authentication
      Header [RFC4302] or some other form of integrity check in the UDP
      payload that covers at least the same information as the UDP
      checksum (pseudo-header, data) and has at least the same strength.

To help ensure that the UDP Checksum will be properly restored when expanding a 6LoWPAN packet, an additional integrity check (e.g., a Layer 2 (L2) Message Integrity Check) MUST be used whenever transmitting link frames that carry a compressed UDP datagram that elides the checksum. Without this additional integrity check, a UDP packet may be delivered to an unintended destination since corruption in data covered by the pseudo-header can go undetected.

A compressor MUST verify the UDP Checksum before it is elided and MUST ensure that the additional integrity check is in place before verifying and eliding the checksum. If verification of the UDP Checksum fails, the compressor MUST drop the packet.

A decompressor that expands a 6LoWPAN packet with the C bit set MUST compute the UDP Checksum on behalf of the source node and place that value in the restored UDP header as specified in the incumbent standards [RFC0768], [RFC2460]. The decompressor MUST unambiguously determine that an additional integrity check was put in place by the compressor and verify the integrity check and SHOULD do so after restoring the UDP Checksum. If the decompressor cannot unambiguously determine the presence of an integrity check or verification fails, the decompressor MUST drop the packet.

The recommended ordering of computing and verifying the UDP Checksum and additional integrity check ensures that data is never stored unprotected in memory. In practice, functionality separation between layers may preclude the recommended ordering. However, implementors should take special note and understand the risks when dealing with unprotected data covered by the pseudo-header.

To allow intermediate nodes to compress the UDP Checksum, a forwarding node MAY infer upper-layer authorization for an incoming packet if it has the C bit set and it can unambiguously determine that an integrity check covering the same data as the UDP Checksum was in place while the UDP Checksum was elided. A forwarding node MUST NOT infer authorization if it cannot unambiguously determine the presence of and verify an integrity check while the UDP Checksum was elided.

4.3.3. UDP LOWPAN_NHC Format

                       0   1   2   3   4   5   6   7
                     +---+---+---+---+---+---+---+---+
                     | 1 | 1 | 1 | 1 | 0 | C |   P   |
                     +---+---+---+---+---+---+---+---+

Figure 14: UDP Header Encoding

C:

Checksum:

0:

All 16 bits of Checksum are carried in-line.

1:

All 16 bits of Checksum are elided. The Checksum is recovered by recomputing it on the 6LoWPAN termination point.

P:

Ports: 

      00:  All 16 bits for both Source Port and Destination Port are
         carried in-line.
      
      01:  All 16 bits for Source Port are carried in-line.  First 8
         bits of Destination Port is 0xf0 and elided.  The remaining 8
         bits of Destination Port are carried in-line.
      
      10:  First 8 bits of Source Port are 0xf0 and elided.  The
         remaining 8 bits of Source Port are carried in-line.  All 16
         bits for Destination Port are carried in-line.
      
      11:  First 12 bits of both Source Port and Destination Port are
         0xf0b and elided.  The remaining 4 bits for each are carried
         in-line.

Fields carried in-line (in part or in whole) appear in the same order as they do in the UDP header format [RFC0768]. The UDP Length field MUST always be elided and is inferred from lower layers using the 6LoWPAN Fragmentation header or the IEEE 802.15.4 header.

5. IANA Considerations

This document defines a new IPv6 header compression format for 6LoWPAN. The document allocates the following 32 Dispatch type field values for LOWPAN_IPHC: 

     01 100000
      through
     01 111111

This assignment preempts the assignment of 01 111111 for ESC [RFC4944]; this preemption is possible because extension bytes that would enable the use of ESC have not been allocated yet. Instead, the value:

  1. 000000

is reserved as a replacement value for ESC, to be finally assigned with the first assignment of extension bytes.

This document also creates a new IANA registry for the LOWPAN_NHC header type, with the following initial content: 

     00000000 to 11011111: (unassigned)
     1110000N: IPv6 Hop-by-Hop Options Header       [RFC6282]
     1110001N: IPv6 Routing Header                  [RFC6282]
     1110010N: IPv6 Fragment Header                 [RFC6282]
     1110011N: IPv6 Destination Options Header      [RFC6282]
     1110100N: IPv6 Mobility Header                 [RFC6282]
     1110111N: IPv6 Header                          [RFC6282]
     11110CPP: UDP Header                           [RFC6282]
     11111000 to 11111110: (unassigned)

Capital letters in bit positions represent class-specific bit assignments. N indicates whether or not additional LOWPAN_NHC encodings follow, as defined in Section 4.2. CPP represents variables specific to UDP header compression defined in Section 4.3.

The policy for this registry [RFC5226] is IETF Review. In this process, new values SHOULD be assigned in a way that preserves the NHC ID abstraction of Section 4 (i.e., k one-bits followed by one zero-bit identify the general class of NHC, followed by class- specific bit assignments).

6. Security Considerations

The definition of LOWPAN_IPHC permits the compression of header information on communication that could take place in its absence, albeit in a less efficient form. It recognizes that a IEEE 802.15.4 PAN may have associated with it a number of prefixes through shared context. How the shared context is assigned and managed is beyond the scope of this document.

The overloading of the 0xf0bX ports increases the risk of getting the wrong type of payload and misinterpreting the content compared to ports that reserved at IANA. It is thus recommended that the use of those ports be associated with a mechanism such as a Transport Layer Security (TLS) [RFC5246] Message Integrity Check (MIC) that validates that the content is expected and checked for integrity.

7. Acknowledgements

Thanks to Julien Abeille, Robert Assimiti, Dominique Barthel, Carsten Bormann, Robert Cragie, Stephen Dawson-Haggerty, Mathilde Durvy, Erik Nordmark, Christos Polyzois, Joseph Reddy, Shoichi Sakane, Zach Shelby, Dario Tedeschi, Tony Viscardi, and Jay Werb for useful design consideration and implementation feedback. Special thanks to David Black, Lars Eggert, and Carsten Bormann for their contribution in closing the security issues around UDP compression.

8. References

8.1. Normative References

   [RFC0768]       Postel, J., "User Datagram Protocol", STD 6, RFC 768,
                   August 1980.
   
   [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.
   
   [RFC3168]       Ramakrishnan, K., Floyd, S., and D. Black, "The
                   Addition of Explicit Congestion Notification (ECN) to
                   IP", RFC 3168, September 2001.
   
   [RFC4291]       Hinden, R. and S. Deering, "IP Version 6 Addressing
                   Architecture", RFC 4291, February 2006.
   
   [RFC4944]       Montenegro, G., Kushalnagar, N., Hui, J., and D.
                   Culler, "Transmission of IPv6 Packets over IEEE
                   802.15.4 Networks", RFC 4944, September 2007.
   
   [RFC5226]       Narten, T. and H. Alvestrand, "Guidelines for Writing
                   an IANA Considerations Section in RFCs", BCP 26,
                   RFC 5226, May 2008.
   
   [RFC6275]       Perkins, C., Ed., Johnson, D., and J. Arkko,
                   "Mobility Support in IPv6", RFC 6275, July 2011.

8.2. Informative References

   [IEEE802.15.4]  IEEE Computer Society, "IEEE Std. 802.15.4-2006",
                   October 2006.
   
   [RFC3306]       Haberman, B. and D. Thaler, "Unicast-Prefix-based
                   IPv6 Multicast Addresses", RFC 3306, August 2002.
   
   [RFC3315]       Droms, R., Bound, J., Volz, B., Lemon, T., Perkins,
                   C., and M. Carney, "Dynamic Host Configuration
                   Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
   
   [RFC3956]       Savola, P. and B. Haberman, "Embedding the Rendezvous
                   Point (RP) Address in an IPv6 Multicast Address",
                   RFC 3956, November 2004.
   
   [RFC4302]       Kent, S., "IP Authentication Header", RFC 4302,
                   December 2005.
   
   [RFC4303]       Kent, S., "IP Encapsulating Security Payload (ESP)",
                   RFC 4303, December 2005.
   
   [RFC4861]       Narten, T., Nordmark, E., Simpson, W., and H.
                   Soliman, "Neighbor Discovery for IP version 6
                   (IPv6)", RFC 4861, September 2007.
   
   [RFC5246]       Dierks, T. and E. Rescorla, "The Transport Layer
                   Security (TLS) Protocol Version 1.2", RFC 5246,
                   August 2008.

Authors' Addresses

   Jonathan W. Hui (editor)
   Arch Rock Corporation
   501 2nd St. Ste. 410
   San Francisco, California  94107
   USA
   
   Phone: +415 692 0828
   EMail: jhui@archrock.com
   
   Pascal Thubert
   Cisco Systems
   Village d'Entreprises Green Side
   400, Avenue de Roumanille
   Batiment T3
   Biot - Sophia Antipolis  06410
   FRANCE
   
   Phone: +33 4 97 23 26 34
   EMail: pthubert@cisco.com