Re: How to force a nod to drop packets?

[Soulimane Mammar]

Hi,

Just sent you a reply on https://groups.google.com/g/ns-3-users/c/iWS1aWRwEgo

On Friday, February 18, 2022 at 12:07:50 PM UTC+1 Nadia1899 wrote:

Hi,

I have the simulation attached in the file. A client 10.1.1.1 in LAN1 sending udpechoPackets to a server 10.1.10.1 in LAN10.

I added the trace source PhyRxDrop to visualize the packets dropped by the server. However I can't see any dropped packets as numdrop in my simulation is always 0 (even If I increase the number of packets sent). Can you please tell me how I can force the server to drop some packets?

Thank you.

[Boumkheld Nadia]

Hi,

Thank you for your answer. And what if I want to add a droptailQueue to drop the packets when there is no space for them.

Thanks again

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[Soulimane Mammar]

Hi,

You can always attach a Tx Queue to a netdevice. The queue can be a DropTailQueue a RedQueue or whatever you want.

This can be done as follows:

auto client_csma_interface = StaticCast<CsmaNetDevice>(LAN1devices.Get (0));

// The default mode of DropTailQueue is QUEUE_MODE_PACKETS  and MaxPackets == 100

Ptr<DropTailQueue> dtq = CreateObject<DropTailQueue>() ;

client_csma_interface.SetQueue(dtq)

// In case you want to change the Queue size, use the config system

// i refers to the node number in the scenario (not int the container)

Config::Set("/NodeList/[i]/DeviceList/[i]/$ns3::CsmaNetDevice/TxQueue/$ns3::DropTailQueue", UintegerValue(new_size))

Regards

[Soulimane Mammar]

Hi,

Sorry, That was just a sketch not meant to be in real code

For recent implementation of DropTailQueue, the class is no more specific to packets, it is rather a template that we can specialize to Packet type.

Furthermore, there is no attribute MaxPackets for this template, it has been replaced by a more general attribute MaxSize 

So a working code looks like:

auto client_csma_interface = StaticCast<CsmaNetDevice>(LAN1devices.Get (0));

Ptr<DropTailQueue<Packet>> dtq = CreateObject<DropTailQueue<Packet>> ();

dtq->SetMaxSize(QueueSize (QueueSizeUnit::PACKETS, 3)); // setting the size of the queue to 3 packets

client_csma_interface->SetQueue (dtq);

Regards

On Friday, February 18, 2022 at 7:21:31 PM UTC+1 Nadia1899 wrote:

When I add this code for DroptailQueue in test2 File you attached. It generates some errors.

./scratch/test2.cc: In function ‘int main(int, char**)’:
../scratch/test2.cc:145:18: error: type/value mismatch at argument 1 in template parameter list for ‘template<class T> class ns3::Ptr’
  145 | Ptr<DropTailQueue> dtq = CreateObject<DropTailQueue> () ;
      |                  ^
../scratch/test2.cc:145:18: note:   expected a type, got ‘DropTailQueue’
../scratch/test2.cc:145:55: error: no matching function for call to ‘CreateObject<template<class Item> class ns3::DropTailQueue>()’
  145 | Ptr<DropTailQueue> dtq = CreateObject<DropTailQueue> () ;
      |                                                       ^
In file included from ./ns3/scheduler.h:25,
                 from ./ns3/calendar-scheduler.h:25,
                 from ./ns3/core-module.h:23,
                 from ../scratch/test2.cc:1:
./ns3/object.h:576:8: note: candidate: ‘template<class T, class ... Args> ns3::Ptr<T> ns3::CreateObject(Args&& ...)’
  576 | Ptr<T> CreateObject (Args&&... args)
      |        ^~~~~~~~~~~~
./ns3/object.h:576:8: note:   template argument deduction/substitution failed:
../scratch/test2.cc:146:23: error: ‘class ns3::Ptr<ns3::CsmaNetDevice>’ has no member named ‘SetQueue’
  146 | client_csma_interface.SetQueue(dtq)

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[Boumkheld Nadia]

Thank you. For some reason, even with the DroptailQueue I still can't see any packets dropped even if the client sends 1000 packets. In this case I don't use the Error Model.

Best.

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[Soulimane Mammar]

Hi,

You have to understand how things work.

You have a ppp link with a data rate of 10 Mbps and a client application that sends 1024 Bytes each 1ms

Using simple math, you can calculate the application data rate: (1024*8)/10^-3 = 8 Mbps which way less than 10 Mbps. In this case there no congestion and hence no packet will be dropped (considering a perfect link) even with no queue.

You have to create  a congestion situation (for instance sending 1024 Bytes each 0.5 ms) if you want to seed drops.

Don't forget, queues are good for bursty traffic not cbr

Regards

[Boumkheld Nadia]

Ok thank you so much for your explanation.

All the best

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