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Documentation: Packet buffers

Simon Duquennoy edited this page Nov 9, 2018 · 6 revisions

This page, intended for protocol developers, describes the different types of buffers used in Contiki-NG. The focus is on the 6LoWPAN stack, but all information about Packetbuf and Queuebuf also applies to NullNet.

Uip buffer

At the network layer and above, packet payloads are stored in uip_buf.

To send data, upper-layer protocols or applications are expected to write to this buffer and then trigger a transmission. The uIP stack first adds the required IPv6 header and possible extension headers. 6LoWPAN will then compress the headers and fragment the datagram if needed. Each fragment will be, in turn, passed to the MAC layer for transmission.

At reception time, the reverse procedure takes place. The MAC layer will call 6LoWPAN, which will decompress headers and reassemble the datagram into uip_buf, before calling the uIP stack. The datagram is then accessible for processing by uIP and upper layers.

Access rules for uip_buf:

  • only from 6LoWPAN, uIP, or above, but not from any layer below
  • only outside of interrupt context

Packetbuf

6LoWPAN will build the link-layer packets directly into the global packetbuf. In addition to the payload, packetbuf carries a number of attributes / meta-data (see packetbuf.h). Once the packet is ready, 6LoWPAN passes it to the MAC layer for transmission. Likewise, when receiving packets, the MAC layer passes packets to 6LoWPAN via packetbuf. The packetbuf API is detailed at doxygen:packetbuf.

Access rules for packetbuf:

  • only from 6LoWPAN or below, but not from any layer above
  • only outside of interrupt context

Queuebuf

The queuebuf module provides a way to manage multiple packets at a time. The content of every queuebuf instance is basically the same as in the global packetbuf. Modules that need queuebuf are responsible for maintaining pointers to them -- there is no global pointer to queuebuf like there is for packetbuf.

For instance, 6LoWPAN uses queuebuf when fragmenting IPv6 datagrams into multiple packets. The MAC layers CSMA and TSCH also use queuebuf for their transmit queues.

Access rules for queuebuf:

  • only from 6LoWPAN or below, but not from any layer above
  • outside of interrupt context, or from interrupt context if the queuebuf instance is protected with a lock (like in TSCH)

When enabled, the queuebuf module will be initialised automatically by Contiki-NG. Depending on your platform's configuration, it may be possible to disable the queuebuf module by setting:

#define QUEUEBUF_CONF_ENABLED 0

Keep in mind that some Contiki-NG features strictly need the queuebuf module (e.g. CSMA, TSCH, 6LoWPAN fragmentation support), so you should only try to disable it if you do need any of this functionality.

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