IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Dispatch Type for SCHC
draft-gomez-6lo-schc-dispatch-00

Abstract

A new framework called Static Context Header Compression (SCHC) has been designed to support IPv6 over Low Power Wide Area Network (LPWAN) technologies [RFC8724]. One of the SCHC components is a header compression mechanism. If used properly, SCHC header compression allows a greater compression ratio than that achievable with traditional 6LoWPAN header compression [RFC6282]. For this reason, it may make sense to use SCHC header compression in some 6LoWPAN environments. In its current form, this document proposes a number of 6LoWPAN Dispatch type approaches to signal when a packet header has been compressed by using SCHC header compression.

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Table of Contents

• 1. Introduction
• 2. Conventions used in this document
• 3. Frame Format
• 4. SCHC Dispatch Type Approaches
• 4.1. Approach 1
• 4.2. Approach 2
• 4.3. Approach 3
• 5. IANA Considerations
• 6. Security Considerations
• 7. Acknowledgments
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Author's Address

1. Introduction

RFC 6282 is the main specification for IPv6 over Low power Wireless Personal Area Network (6LoWPAN) IPv6 header compression [RFC6282]. This RFC was designed assuming IEEE 802.15.4 as the layer below the 6LoWPAN adaptation layer, and it has also been reused (with proper adaptations) for IPv6 header compression over many other technologies relatively similar to IEEE 802.15.4 in terms of characteristics such as physical layer bit rate, layer 2 maximum payload size, etc. Examples of such technologies comprise BLE, DECT-ULE, ITU G.9959, MS/TP, NFC, and PLC. RFC 6282 provides additional functionality, such as a mechanism for UDP header compression.

In the best case, RFC 6282 allows to compress a 40-byte IPv6 header down to a 2-byte compressed header (in link-local interactions) or a 3-byte compressed header (when global IPv6 addresses are used). On the other hand, an RFC 6282 compressed UDP header has a typical size of 4 bytes. Therefore, in advantageous conditions, a 48-byte uncompressed IPv6/UDP header may be compressed down to a 7-byte format by using RFC 6282.

Recently, a new framework called Static Context Header Compression (SCHC) has been designed to support IPv6 over Low Power Wide Area Network (LPWAN) technologies [RFC8724]. SCHC comprises header compression and fragmentation functionality tailored to the extraordinary constraints of LPWAN technologies, which are more severe than those exhibited by IEEE 802.15.4 or other relatively similar technologies.

SCHC header compression allows a greater compression ratio than that of RFC 6282. If used properly, SCHC allows to compress an IPv6/UDP header down to e.g. a single byte. Therefore, it may make sense to use SCHC header compression in some 6LoWPAN environments [I-D.toutain-6lo-6lo-and-schc], considering its greater efficiency.

If SCHC header compression is added to the panoply of header compression mechanisms used in 6LoWPAN environments, then there is a need to signal when a packet header has been compressed by using SCHC. To this end, in its current form, the present document proposes a number of 6LoWPAN Dispatch type approaches for SCHC header compression, based on exploiting RFC 4944 and/or RFC 8025 Dispatch type space [RFC4944][RFC8025].

2. Conventions used in this document

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 [RFC2119].

3. Frame Format

Figure 1 illustrates the content of an encapsulated, SCHC compressed, IPv6 datagram:

    

         +---------------+-------------+---------+
         | SCHC Dispatch | SCHC Header | Payload |
         +---------------+-------------+---------+

        

Figure 1: Encapsulated, SCHC compressed IPv6 datagram

The SCHC Header corresponds to a packet header that has been compressed by using SCHC. As defined in [RFC8724], the SCHC Header comprises a Rule ID, and a compression residue. (Note: more details, including a discussion on padding, to be added.)

4. SCHC Dispatch Type Approaches

This section presents 3 different approaches for the SCHC Dispatch type to be discussed.

4.1. Approach 1

A first approach for the SCHC Dispatch Pattern is using Not a LoWPAN (NALP) Dispatch type space [RFC4944]. The first two bits in a NALP Dispatch type are 00. Approach 1 defines that a Dispatch starting by "001" indicates that a SCHC-compressed packet comes next.

SCHC Dispatch Pattern: 001XXXXX

The last 5 bits of the Dispatch (indicated as 'X' above) may be used to define 32 different Rule IDs.

This approach has pros and cons. A single byte is used for the Dispatch plus the Rule ID. However, is 32 a relatively low number of possible Rule ID values? On the other hand, there may be backwards compatibility issues with existing implementations, where SCHC-compressed packets might be misunderstood as other types of packets.

4.2. Approach 2

A second approach, that also uses NALP Dispatch type space, is:

SCHC Dispatch Pattern: 001YYYYY YYYYYYYY

The last 13 bits of the Dispatch (indicated as 'Y' above) may be used to define 8192 different Rule IDs.

With this approach, two bytes are used for the SCHC Dispatch plus the Rule ID, but 8192 possible Rule IDs can be used. The same backwards compatibility issues in Approach 1 may exist for Approach 2 as well.

4.3. Approach 3

A third approach, which is not based on using NALP space, is using the RFC 8025 concept of "pages", which would allocate one page for SCHC-compressed headers:

SCHC Dispatch Pattern: 1111ZZZZ (ZZZZ to be determined)

With this approach, and with the aim to minimize header overhead, a whole page is allocated for the SCHC Dispatch type. A 1-byte Rule ID follows the SCHC Dispatch Pattern.

In this case, two bytes are used for the SCHC Dispatch plus the Rule ID. 256 possible Rule IDs can be used. There are no backwards compatibility issues.

5. IANA Considerations

TBD

6. Security Considerations

TBD

7. Acknowledgments

Ana Minaburo and Laurent Toutain suggested for the first time the use of SCHC in environments where 6LoWPAN has traditionally been used.

Carles Gomez has been funded in part by the Spanish Government through project PID2019-106808RA-I00, and by Secretaria d'Universitats i Recerca del Departament d'Empresa i Coneixement de la Generalitat de Catalunya 2017 through grant SGR 376.

8. References

8.1. Normative References

8.2. Informative References

Author's Address