Packetbuffer Basics

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Introduction

This tutorial is a introduction to different operations on the packetbuf on ContikiOS 2.7. We will also cover brief details about each of the functions and how they work.

You will learn

Through this tutorial you will learn about different funcions and usage components of packetbuf. This will help you in any projects since packetpuf is necessary for any networking project.

Source Codes

~/contiki-2.7/core/net/packetbuf.c

~/contiki-2.7/core/net/packetbuf.h

Description

struct packetbuf_attr packetbuf_attrs[PACKETBUF_NUM_ATTRS];
struct packetbuf_addr packetbuf_addrs[PACKETBUF_NUM_ADDRS];

These two structures are used to store the attributes of the packet and the addresses related to the packet i.e. source and destination address. The respective structures are defined as follows.

typedef uint16_t packetbuf_attr_t;

struct packetbuf_attr {
/*   uint8_t type; */
  packetbuf_attr_t val;
};
struct packetbuf_addr {
/*   uint8_t type; */
  rimeaddr_t addr;
};

Therefore packetbuf_attrs is an array of packets attributes. The array size is PACKETBUF_NUM_ATTRS. Similarly packetbuf_addrs is an array of packet addresses. The array size is PACKETBUF_NUM_ADDRS.

The sizes are defined as follows where PACKETBUF_ATTR_MAX=28:

//  File:   ~/contiki-2.7/core/net/packetbuf.h
#define PACKETBUF_NUM_ADDRS 4
#define PACKETBUF_NUM_ATTRS (PACKETBUF_ATTR_MAX - PACKETBUF_NUM_ADDRS)

The reason behind such definition is that the PACKETBUF_ATTR_MAX also counts the packets's sender and receiver address as the packet's attribute.

The val field in the packetbuf_attr structure can be assigned any of the follwing values: Now, to set a attribute we have to call the following function. We have to supply with the type and the value:

static inline int
packetbuf_set_attr(uint8_t type, const packetbuf_attr_t val)
{
/*   packetbuf_attrs[type].type = type; */
  packetbuf_attrs[type].val = val;
  return 1;
}


The types are defined as follows
<source lang="c">
enum {
  PACKETBUF_ATTR_NONE,

  /* Scope 0 attributes: used only on the local node. */
  PACKETBUF_ATTR_CHANNEL,
  PACKETBUF_ATTR_NETWORK_ID,
  PACKETBUF_ATTR_LINK_QUALITY,
  PACKETBUF_ATTR_RSSI,
  PACKETBUF_ATTR_TIMESTAMP,
  PACKETBUF_ATTR_RADIO_TXPOWER,
  PACKETBUF_ATTR_LISTEN_TIME,
  PACKETBUF_ATTR_TRANSMIT_TIME,
  PACKETBUF_ATTR_MAX_MAC_TRANSMISSIONS,
  PACKETBUF_ATTR_MAC_SEQNO,
  PACKETBUF_ATTR_MAC_ACK,

  /* Scope 1 attributes: used between two neighbors only. */
  PACKETBUF_ATTR_RELIABLE,
  PACKETBUF_ATTR_PACKET_ID,
  PACKETBUF_ATTR_PACKET_TYPE,
  PACKETBUF_ATTR_REXMIT,
  PACKETBUF_ATTR_MAX_REXMIT,
  PACKETBUF_ATTR_NUM_REXMIT,
  PACKETBUF_ATTR_PENDING,
  
  /* Scope 2 attributes: used between end-to-end nodes. */
  PACKETBUF_ATTR_HOPS,
  PACKETBUF_ATTR_TTL,
  PACKETBUF_ATTR_EPACKET_ID,
  PACKETBUF_ATTR_EPACKET_TYPE,
  PACKETBUF_ATTR_ERELIABLE,

  /* These must be last */
  PACKETBUF_ADDR_SENDER,
  PACKETBUF_ADDR_RECEIVER,
  PACKETBUF_ADDR_ESENDER,
  PACKETBUF_ADDR_ERECEIVER,
  
  PACKETBUF_ATTR_MAX
};

Therefore to choose an attribute you have to choose from the first 24 entries.e.g., packetbuf_set_attr(PACKETBUF_ATTR_RSSI,8) will set the RSSI attribute packet of the packet to be 8.

Similarly to get an attribute value we have to use following function

static inline packetbuf_attr_t
packetbuf_attr(uint8_t type)
{
  return packetbuf_attrs[type].val;
}

E.g. packetbuf_attr(PACKETBUF_ATTR_RSSI) will return the RSSI value associated with the packet.

static uint16_t buflen, bufptr; static uint8_t hdrptr;

/* The declarations below ensure that the packet buffer is aligned on

  an even 16-bit boundary. On some platforms (most notably the
  msp430), having apotentially misaligned packet buffer may lead to
  problems when accessing 16-bit values. */

static uint16_t packetbuf_aligned[(PACKETBUF_SIZE + PACKETBUF_HDR_SIZE) / 2 + 1]; static uint8_t *packetbuf = (uint8_t *)packetbuf_aligned;

static uint8_t *packetbufptr;