210 lines
8.2 KiB
Text
210 lines
8.2 KiB
Text
/**
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\addtogroup uip
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@{
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*/
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/**
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* \defgroup sicslowpan 6LoWPAN implementation
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* @{
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6lowpan is a Working Group in IETF which defines the use of IPv6 on
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IEEE 802.15.4 links.
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Our implementation is based on RFC4944 <em>Transmission of IPv6
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Packets over IEEE 802.15.4 Networks</em>, draft-hui-6lowpan-interop-00
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<em>Interoperability Test for 6LoWPAN</em>, and draft-hui-6lowpan-hc-01
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<em>Compression format for IPv6 datagrams in 6lowpan Networks</em>.
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<HR>
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\section drafts Specifications implemented
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\note We currently only support 802.15.4 64-bit addresses.
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\subsection rfc4944 RFC 4944
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RFC4944 defines address configuration mechanisms based on 802.15.4
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16-bit and 64-bit addresses, fragmentation of IPv6 packets below IP
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layer, IPv6 and UDP header compression, a mesh header to enable link-layer
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forwarding in a mesh under topology, and a broadcast header to enable
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broadcast in a mesh under topology.
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We implement addressing, fragmentation, and header compression. We support
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the header compression scenarios defined in draft-hui-6lowpan-interop-00.
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This draft defines an interoperability scenario which was used between
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ArchRock and Sensinode implementations.
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We do not implement mesh under related features, as we target route over
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techniques.
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\subsection hc01 draft-hui-6lowpan-hc-01
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draft-hui-6lowpan-hc-01 defines a stateful header compression mechanism
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which should soon deprecate the stateless header compression mechanism
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defined in RFC4944. It is much more powerfull and flexible, in
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particular it allows compression of some multicast addresses and of all
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global unicast addresses.
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<HR>
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\section general Implementation overview
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6lowpan does not run as a separate process. It is called by the MAC %process
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when a 6lowpan packet is received, and by the tcpip %process when an
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IPv6 packet needs to be sent.
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It is initialized from the MAC %process, which calls sicslowpan_init
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(giving as argument a pointer to the mac_driver structure).
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The main 6lowpan functions are implemented in the sicslowpan.h and
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sicslowpan.c files. They are used to format packets between the
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802.15.4 and the IPv6 layers.
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6lowpan also creates a few IPv6 and link-layer dependencies which are
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detailed in the next section.
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<HR>
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\section implementation Implementation details
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\subsection Addressing
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<b>Link-layer addresses</b><br>
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The format of a 802.15.4 address is defined in uip.h.
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\code
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/** \brief 64 bit 802.15.4 address */
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struct uip_802154_shortaddr {
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u8_t addr[2];
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};
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/** \brief 16 bit 802.15.4 address */
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struct uip_802154_longaddr {
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u8_t addr[8];
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};
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/** \brief 802.15.4 address */
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typedef struct uip_802154_longaddr uip_lladdr_t;
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#define UIP_802154_SHORTADDR_LEN 2
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#define UIP_802154_LONGADDR_LEN 8
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#define UIP_LLADDR_LEN UIP_802154_LONGADDR_LEN
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\endcode
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<b>Neighbor Discovery Link Layer Address options </b><br>
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The format of ND link-layer address options depends on the length of
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the link-layer addresses.
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802.15.4 specificities regarding link-layer address options are implemented in uip-nd6.h.
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\code
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#define UIP_ND6_OPT_SHORT_LLAO_LEN 8
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#define UIP_ND6_OPT_LONG_LLAO_LEN 16
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#define UIP_ND6_OPT_LLAO_LEN UIP_ND6_OPT_LONG_LLAO_LEN
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\endcode
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<b>Address Autoconfiguration</b><br>
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The address autoconfiguration mechanism also depends on the format of
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the link-layer address. The dependency is reflected in the
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#uip_netif_addr_autoconf_set function in #uip-netif.c.
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\code
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#if (UIP_LLADDR_LEN == 8)
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memcpy(ipaddr->u8 + 8, lladdr, UIP_LLADDR_LEN);
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ipaddr->u8[8] ^= 0x02;
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\endcode
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\subsection io Packet Input/Output
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At initialization, the #input function in sicslowpan.c is set as the
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function to be called by the MAC upon packet reception. The #output
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function is set as the tcpip_output function.<br>
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At packet reception, the link-layer copies the 802.15.4 payload in the
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rime buffer, and sets its length. It also stores the source and
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destination link-layer addresses as two rime addresses.
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\code
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rimebuf_copyfrom(&rx_frame.payload, rx_frame.payload_length);
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rimebuf_set_datalen(rx_frame.payload_length);
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rimebuf_set_addr(RIMEBUF_ADDR_RECEIVER, (const rimeaddr_t *)&rx_frame.dest_addr);
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rimebuf_set_addr(RIMEBUF_ADDR_SENDER, (const rimeaddr_t *)&rx_frame.src_addr);
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\endcode
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It then calls the sicslowpan #input function. Similarly, when the IPv6 layer
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has a packet to send over the radio, it puts the packet in uip_buf,
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sets uip_len and calls the sicslowpan #output function.
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\subsection frag Fragmentation
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\li #output function: When an IP packet, after header compression, is
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too big to fit in a 802.15.4 frame, it is fragmented in several packets
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which are sent successively over the radio. The packets are formatted
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as defined in RFC 4944. Only the first fragment contains the IP/UDP
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compressed or uncompressed header fields.
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\li #input function: This function takes care of fragment
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reassembly. We do not assume that the fragments are received in order.
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When reassembly of a packet is ongoing, we discard any non fragmented
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packet or fragment from another packet. Reassembly times out after
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#SICSLOWPAN_REASS_MAXAGE = 20s.
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\note Fragmentation support is enabled by setting the #SICSLOWPAN_CONF_FRAG
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compilation option.
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\note As we do not support complex buffer allocation mechanism, for now
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we define a new 1280 bytes buffer (#sicslowpan_buf) to reassemble packets.
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At reception, once all the fragments are received, we copy the packet
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to #uip_buf, set #uip_len, and call #tcpip_input.
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\note #MAC_MAX_PAYLOAD defines the maximum payload
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length in a 802.15.4 frame. For now it is constant and equal to 102
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bytes (the 802.15.4 frame can be maximum 127 bytes long, and
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the header 25 bytes long).
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\subsection hc Header Compression
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<b>Compression schemes</b><br>
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The #SICSLOWPAN_CONF_COMPRESSION compilation option defines the
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compression scheme supported. We support HC1, HC01, and IPv6 compression.
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HC1 and IPv6 compression are defined in RFC4944, HC01 in
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draft-hui-6lowpan-hc. What we call IPv6 compression means sending packets
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with no compression, and adding the IPv6 dispatch before the IPv6 header.<br>
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If at compile time IPv6 "compression" is chosen, packets sent will never
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be compressed, and compressed packets will not be processed at reception.<br>
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If at compile time either HC1 or HC01 are chosen, we will try to compress
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all fields at sending, and will accept packets compressed with the
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chosen scheme, as well as uncompressed packets.<br>
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Note that HC1 and HC01 supports are mutually exclusive. HC01 should soon
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deprecate HC1.
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<b>Compression related functions</b><br>
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When a packet is received, the #input function is called. Fragmentation
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issues are handled, then we check the dispatch byte: if it is IPv6, we
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treat the packet inline. If it is HC1 or HC01, the corresponding
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decompression function (#uncompress_hdr_hc1 or #uncompress_hdr_hc01)
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is called.<br>
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When a packet needs to be sent, we try to compress it. If only the IPv6
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compression support is enabled, we just add the IPv6 dispatch before the
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802.15.4 payload. If HC1 or HC01 support is enabled, we call the
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corresponding compression function (#compress_hdr_hc1 or #compress_hdr_hc01)
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to compress the packet as much as possible.
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<b>HC1 comments</b><br>
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In HC1, if the IPv6 flow label is not compressed, we would need to copy
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the fields after the flow label starting in the middle of a byte (the
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flow label is 20 bits long). To avoid this, we compress the packets only
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if all fields can be compressed. If we cannot, we use the IPv6 dispatch
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and send all headers fields inline. This behavior is the one defined in
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draft-hui-6lowpan-interop-00.<br>
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In the same way, if the packet is an UDP packet, we compress the UDP
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header only if all fields can be compressed.<br>
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Note that HC1 can only compress unicast link local addresses. For this
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reason, we recommend using HC01.
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<b>HC01 comments</b><br>
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HC01 uses address contexts to enable compression of global unicast
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addresses. All nodes must share context (namely the global prefixes in
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use) to compress and uncompress such addresses successfully. The context
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number is defined by 2 bits. Context 00 is reserved for the link local
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context. Other contexts have to be distributed within the LoWPAN
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dynamically, by means of ND extensions yet to be defined.<br>
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Until then, if you want to test global address compression, you need
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to configure the global contexts manually.
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<HR>
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*/
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/** @} */
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/** @} */
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