UDP datagram counting - c++

Question

Hi I have a server communicating with clients over UDP. Basically clients streams to the server UDP packets. Each packet consist from header and payload. in header, there is only one short int - i call it seqnum, running from 0 to SHORT_MAX. When client reaches SHORT_MAX on sending, it starts again from 0.
On the server I need to reconstruct stream in this way:
a) If packet arrives with expected seqnum append to the stream.
b) If packet arrives with lower than expected seqnum - drop it - it is packet which arrived to late
c) If packet arrives with higher seqnum than expected - consider packets between expected and actual seqnum as lost and try to reconstruct them and after that append actual packet
I am dealing now with two problems connected to overflow of counter:
1) how to detect situation c) on SHORT_MAX boundary (e.g. expected is SHORT_MAX-2, actual seqnum in packet is 2) - in my scenario it would be wrongly detected as situation b)
2) same problem with situation b) wrongly detected as c)
Thx a lot

Answer 1

Assuming that SHORT_MAX actually means SHRT_MAX, then you have some 30000 or so packets that are missing if you find scenario 2. Which probably means you can't reconstruct anyway, and the link has been going wrong for quite some time. You may solve that by having a "timeout" (e.g. if no correct packet has been received in X seconds, give up and start over at some suitable point - or whatever you can do if LOTS of packets have gone missing - you can of course test this by unplugging the cable or something like that).
And you can detect "wraparound" by doing some modulo math.
#include <iostream>
#include <algorithm>
using namespace std;
#define MAX_SEQ_NO 16
#define THRESHOLD 6 // Max number of "missing packets" that is acceptable
void check_seq_no(int seq_no)
{
static int expected = 0;
cout << "Got seq_no=" << seq_no << " expected=" << expected << endl;
if (seq_no == expected)
{
expected = (expected + 1) % MAX_SEQ_NO;
}
else
{
if ((seq_no + THRESHOLD) % MAX_SEQ_NO > (expected + THRESHOLD) % MAX_SEQ_NO)
{
int missing;
if (seq_no > expected)
{
missing = seq_no - expected;
}
else
{
missing = MAX_SEQ_NO + seq_no - expected;
}
cout << "Packets missing ..." << missing << endl;
expected = (seq_no+1) % MAX_SEQ_NO;
}
else
{
cout << "Old packet received ... " << endl;
}
}
}
int main()
{
int seq_no = 0;
bool in_sim = false;
int old_seq_no = 0;
for(;;)
{
int r = rand() % 50;
if (!in_sim)
{
old_seq_no = seq_no;
switch(r)
{
// Low number: Resend an older packet
case 4:
seq_no --;
case 3:
seq_no --;
case 2:
seq_no --;
case 1:
seq_no --;
in_sim = true;
break;
// High number: "lose" a packet or four.
case 46:
seq_no++;
case 47:
seq_no++;
case 48:
seq_no++;
case 49:
seq_no++;
in_sim = true;
break;
default:
break;
}
if (old_seq_no > seq_no)
{
cout << "Simulating resend of old packets: " << old_seq_no - seq_no << endl;
}
else if (old_seq_no < seq_no)
{
cout << "Simulating missing packets: " << seq_no - old_seq_no << endl;
old_seq_no = seq_no;
}
}
if (old_seq_no == seq_no)
{
in_sim = false;
}
check_seq_no(seq_no % MAX_SEQ_NO);
seq_no++;
}
}

Answer 2

I would suggest that anytime you get a packet within some near threshold of SHORT_MAX, and it is within a similar threshold of 0, you consider it as a candidate for reconstruction. You can also compensate somewhat for the loss of packets by building a sane acknowledgment system between the client and server. Think of this less as a "reconstruction" problem as it is a prioritization problem wherein you must discard "old" or possibly re-transmitted data.
Another strategy I've used in the past is to define channels with (potentially) different configurations in terms of thresholds, ACKing, and overall reliability. In the ideal world you'll have packets that are 100% reliable (TCP-style, guaranteed in-order delivery) and packets that are 100% unreliable, and then possibly streams that are somewhere in between -- a good UDP-based protocol will support these. This gives your application code more direct control over the protocol's algorithms, which is the real point of UDP and where it shines for applications like games and video.
You will likely find that often you are re-implementing bits of TCP, and you may even consider using TCP for your 100% reliable-channel -- it's worth noting that TCP is often given preference by backbones, because they know they will eventually have to re-transmit those packets, if they don't make it through on this trip.

Answer 3

Why don't use an unsigned short, you get twice as much. For those situations that you comment, you need to specify a threshold. It's a dilemma you need to face, I mean, if you're expecting packet 30 and receive packet 60, it's a correct packet or an old lost one. That's why you need to put a threshold
For example;
If (NumberReceived < NumerberExpected)
{
threshold = (USHRT_MAX - NumerberExpected) + NumberReceived ;
// in here you have to decided how many is the threshold 10, 20, 50 …
if (threshold < 10) It is a correct packet and has started over
else It is a lost packet
}
else
If (NumberReceived > NumerberExpected)
{
threshold = NumerberReceived - NumberExpected ;
// in here you have to decided how many is the threshold 10, 20, 50 …
if (threshold < 10) It is a correct packet and I've lost some packets
else It is a lost packet;
}
else It is a correct packet