Variable sized packet - c++

Question

I am trying to define a packet whose length is determined during an ns-3 simulation (think of it as a packet sent on the downlink containing schedule information whose length depends on the number of nodes in the network which can join/leave the network during simulation). Does anyone have any idea how I could approach this?

Answer 1

The traditional solution is to send the length first, followed by the data:
+------------+---------------------+
| uint32_t n | n - 4 bytes of data |
+------------+---------------------+
To decode, read the first four bytes, and then use the value in those bytes to determine how much more data there is.

Answer 2

The simplest introduction on how to do this is http://www.nsnam.org/support/faq/miscellaneous/#table
In practice, if you want to extend this code to store a variable-sized data structure, you can do this:
class MyHeader : public Header
{
public:
// new methods
void AppendData (uint16_t data);
std::vector<uint16_t> GetData (void) const;
static TypeId GetTypeId (void);
// overridden from Header
virtual uint32_t GetSerializedSize (void) const;
virtual void Serialize (Buffer::Iterator start) const;
virtual uint32_t Deserialize (Buffer::Iterator start);
virtual void Print (std::ostream &os) const;
private:
std::vector<uint16_t> m_data;
};
I will skip the obvious GetData/AppendData methods. Instead, we can focus on the Serialize/Deserialize methods:
uint32_t
MyHeader::GetSerializedSize (void) const
{
// two bytes of data to store
return m_data.size() * 2;
}
void
MyHeader::Serialize (Buffer::Iterator start) const
{
start.WriteHtonU32(GetSerializedSize());
for (std::vector<uint16_t>::const_iterator i = m_data.begin(); i != m_data.end(); i++)
{
start.WriteHtonU16 (*i);
}
}
uint32_t
MyHeader::Deserialize (Buffer::Iterator start)
{
uint32_t len = start.ReadNtohU32 ();
for (uint32_t i = 0; i < len; i++) {
m_data.append(start.ReadNtohU16())
}
return 4+len*2;
}