6LPWA Static Context Header Compression (SCHC) for IPV6 and UDP

Headers compression is mandatory to bring the internet protocols to the node within a LPWA network . Nevertheless, LPWA networks offer good properties for an efficient header compression: Topology is star oriented, therefore all the packets follows the same path. For the needs of this draft, the architecture can be summarized to End-Systems (ES) exchanging information with a single LPWA Compressor (LC). In most of the cases, End Systems and LC form a star topology. ESs and LC maintain a context for compression. Traffic flows are mostly deterministic, since End-Systems embed built-in applications. Contrary to computers or smartphones, new applications cannot be easily installed. First mechanisms such as RoHC use a context to store header field values and send smaller incremental differences on the link. The first version of RoHC targeted IP/UDP/RTP stack. RoHCv2 extends the principle to any protocol and introduces a formal notation describing the header and associating compression functions to each field. To be efficient the sender and the receiver must check that the context remains synchronized (i.e. contains the same values). Context synchronization imposes to periodically send a full header or at least dynamic fields. If fully compressed, the header can be compatible with LPWA constraints. However, the first exchanges or context resynchronisations impose to send uncompressed headers, which may be bigger than the original one. This will force the use of inefficient fragmentation mechanisms. For some LPWA technologies, duty cycle limits can also delay the resynchronization. illustrates this behavior.

| | t| | | +--+ | | +--+ | +------------+ +------------+ ]]> On the other hand, 6LoWPAN is context-free based on the fact that IPv6, its extensions or UDP headers do not contain incremental fields. The compression mechanism described in is based on sending a 2-byte bitmap, which describes how the header should be decompressed, either using some standard values or sending information after this bitmap. also allows for UDP compression. In the best case, when Hop limit is a standard value, flow label, DiffServ fields are set to 0 and Link Local addresses are used over a single hop network, the 6LoWPAN compressed header is reduced to 4 bytes. This compression ratio is possible because the IID are derived from the MAC addresses and the link local prefix is known from both sides. In that case, the IPv6 compression is 4 bytes and UDP compression is 2 bytes, which fills half of the payload of a SIGFOX frame, or more than 10% of a LoRaWAN payload (with spreading factor 12). The Static Context Header Compression (SCHC) combines the advantages of RoHC context, which offers a great level of flexibility in the processing of fields, and 6LoWPAN behavior to elide fields that are known from the other side. Static context means that values in the context field do not change during the transmission, avoiding complex resynchronization mechanisms, incompatible with LPWA characteristics. In most of the cases, IPv6/UDP headers are reduced to a small context identifier.

Static Context Header Compression (SCHC) avoids context synchronization, which is the most bandwidth-consuming operation in RoHC. Based on the fact that the nature of data flows is highly predictable in LPWA networks, a static context may be stored on the End-System (ES). The other end, the LPWA Compressor (LC) can learn the context through a provisionning protocol during the identification phase (for instance, as it learns the encryption key). The context contains an ordered list of rules. Each rule is a vector of entries. Each entry is composed of a field descriptor, a prescribed matching value, a matching rule for the compression side, a matching rule for the decompression side and a compression/decompression action. Contexts in the compressor and decompressor are the same. A rule is identified by a rule identifier. If the layer 2 allows it, the rule id can be carried in the layer 2 header. Otherwise the rule id is located in the first byte of the L2 payload. Being at the boundary between Layer 2 and Layer 3, the rule id will also be called a shim id. Different ES will use the same shim id to identify their own context. An LC may also use the ES device id to identify the appropriate rule.

The compression/decompression process follows several steps: compression rule selection: the goal is to identify which rule will be used to compress the headers. To each field is associated a matching rule for compression. Each header field's value is compared to the corresponding value stored in the rule for that field using the matching operator. If all the fields match, the packet is processed using this rule action functions and the rule list exploration is aborted. Otherwise the next rule is tested. If no rule is found, then the packet is dropped. compression: the action function indicates is the field is send on the link or not. A field can also be partially sent regarding the matching operator. The resulting compressed header must be aligned on byte boundaries. decompression rule selection, as for compression, a rule has to be selected to uncompress incoming packets. A matching operator is defined on the compress header and works as for compression. decompression: the same action function indicates how the field value can be rebuilt, either from bits received on the link, a value stored in the rule or by using a specific algorithm.

Matching a field with a value and header compression are related operations; If a field matches a rule containing the value, it is not necessary to send it on the link. Since context are synchronized, reading the rule's value is enough to reconstruct the field's value at the other end. On some other cases, the value need to be sent on the link to inform the other end. The field value may vary from one packet to another, therefore the field cannot be used to select the rule id. It may exist some intermediary cases, where part of the value may be used to select a field and a variable part has to be sent on the link. This is true for Least Significant Bits (LSB) where the most significant bit can be used to select a rule id and the least significant bits has to be sent on the link. Several matching operators are defined: = : a field value in a packet matches with a field value in a rule if they are equal. no : no check is done between a field value in a packet matches with a field value in the rule lbs(L) : a field value of length T in a packet matches with a field value in a rule if the most significant T-L bits are equal.

The action functions describe the action taken by the compression and inversely the action taken by the decompressor to restore the original value.

lists all the functions defined to compress and decompress a field. The first column gives the function's name. The second and third columns outlines the compression/decompression process. As with 6LoWPAN, the compression process may produce some data, where fields that were not compressed (or were partially compressed) will be sent in the order of the original packet. Information added by the compression phase must be aligned on byte boundaries, but each individual compression function may generate any size.

gives an example of function assignment to IPv6/UDP fields.

The compressor do not sent the field value on the link. The decompressor restore the field value with the one stored in the matched rule.

The compressor send the field value on the link, if the matching operator is "=". Otherwise the matching operator indicates the information that will be sent on the link. For a LSB operator only the Least Significant Bits are sent.

These functions are used to process respectively the End System and the LC Device Identifier (DID). The IID value is computed from device ID present in the Layer 2 header. The computation depends of the technology and the device ID size.

This section gives some scenarios of the compression mechanism for IPv6/UDP. The goal is to illustrate the SCHC behaviour.

The most common case will be a LPWA end-system embeds some applications running over CoAP. In this example, the first flow is for instance for the device management based on CoAP using Link Local addresses and UDP ports 123 and 124. The second flow will be a CoAP server for measurements done by the end-system (using ports 5683) and Global Addresses alpha::IID/64 to beta::1/64. The last flow is for legacy applications using different ports numbers, the destination is gamma::1/64. presents the protocol stack for this end-system. IPv6 and UDP are represented with dotted lines since these protocols are compressed on the radio link. The rule ID is represented by a shim id (respectively 0, 1 and 2).

Note that in some LPWA technologies, only End Systems have a device ID . Therefore it is necessary to define statically an IID for the Link Local address for the LPWA Compressor.

All the fields described in the three rules are present in the IPv6 and UDP headers. Two fields have been added at the begin, they are used to identify the rule id for decompression when the other end receives the compressed header. The shim id is read either from the L2 header or from the first bit in the payload depending on the technology. The ESDevice-ID value is found in the L2 header. The second and third rules use global addresses. The way the ES learn the prefix is not in the scope of the document. One possible way is to use a management protocol to set up in both end rules the prefix used on the LPWA network. The third rule compresses port numbers on 4 bits. This value is selected to maintain alignment on byte boundaries for the compressed header.

Thanks to Dominique Barthel, Alexander Pelov, Juan Carlos Zuniga for useful design consideration.