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:

//  File:   ~/contiki-2.7/core/net/packetbuf.h
#define PACKETBUF_ATTR_PACKET_TYPE_DATA      0
#define PACKETBUF_ATTR_PACKET_TYPE_ACK       1
#define PACKETBUF_ATTR_PACKET_TYPE_STREAM    2
#define PACKETBUF_ATTR_PACKET_TYPE_STREAM_END 3
#define PACKETBUF_ATTR_PACKET_TYPE_TIMESTAMP 4

Now, to set a attribute we have to call the follwing function:

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;
}

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

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;