Why is my IPv4 packet size 588 most of the time?

[GBH]

Can anyone give me a clue on why majority of my IPv4 packets are limited to a packet size of 588?

Here are the details:

- 588 is the number returned from item->GetSize(), where item is a QueueDiscItem object

- the IPv4 packets are from BulkSendApplication to PacketSink (I made sure I set the SendSize attribute). See code below:

BulkSendHelper ftpSourceHelper("ns3::TcpSocketFactory",

InetSocketAddress(lanNodesInetInterfaces.GetAddress(i+1), port));

ftpSourceHelper.SetAttribute("MaxBytes", UintegerValue(maxBytes));

ftpSourceHelper.SetAttribute("SendSize", UintegerValue(1400)); // by default, 512

ApplicationContainer ftpSourceApps = ftpSourceHelper.Install(ftpNodes.Get(i));

- I made sure the MTUs from source node to destination node are all set to 1500. I printed them out.

Thanks!

[GBH]

I figured it out.

TypeId

TcpSocket::GetTypeId()

{

...

.AddAttribute(

"SegmentSize",

"TCP maximum segment size in bytes (may be adjusted based on MTU discovery)",

UintegerValue(536),

MakeUintegerAccessor(&TcpSocket::GetSegSize, &TcpSocket::SetSegSize),

MakeUintegerChecker<uint32_t>())

...

It is the TCP segment size setting that matters.  536 + 52 = 588.  I guess 52 is the various header sizes.