1. Introduction
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].