Clone the repository of ns3 - git clone https://gitlab.com/nsnam/ns-3-dev
Description of the issue: https://gitlab.com/nsnam/ns-3-dev/-/issues/209
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Peek() and DoPeek() function in ns-3 is used to get a copy of next element to be dequed from the queue. However, the implementation of Peek() and DoPeek() is such that Peek() calls DoPeek() function and first an Item is dequeued and stored in a pointer variable called m_requeued. Even if the Item is dequeued it is still considered as a part of the queue. This behaviour of the Peek() and DoPeek() is motivated from the Linux Kernal implementation of the function qdisc_peek_dequeued. This is recommended for the queue discs in which the next packet to be extracted is not obvious.
Default DoPeek() function in NS-3:
https://gitlab.com/nsnam/ns-3-dev/-/blob/master/src/traffic-control/model/queue-disc.cc#L935
Complete view of router queue given below, when a peek is called-
Router queue = Output port queue + m_requeued
--------------------------------------------------------------------
| -------------------------------------------- ------------ |
| | | | | | |
| | | | | | |
| -------------------------------------------- ------------ |
| Output port queue m_requeued |
--------------------------------------------------------------------
CoDel
Codel is an AQM(Active Queue Management) algorithm, which works at the output port, in dequeue time. It calculates current_queue_delay during dequeue time and based on its value, decides whether to drop or mark this packet / Item.
To read more about CoDel: https://datatracker.ietf.org/doc/html/rfc8289
Existing Peek() and DoPeek() have some issues when it comes to the implementation of codel. As Peek() in codel, inherits the Peek() and DoPeek() of queue-disc.cc. So, on calling of peek function it first dequeue the packet and perfom all the marking needed at the time of dequeue and then store it in m_requeued variable. But, the packet is still inside the queue. Hence, when we call Dequeue(), then it will first check m_requeued variable and if it is not empty then we will dequeue that m_requeued varible first without doing any marking. This is a problem with the codel because codel works at dequeue time. We are doing all the computation on calling Peek() which needed to be done on calling Dequeue().
Class diagram:
-------------------------
| CodelQueueDisc |
|-----------------------|
| DoDequeue() |
| DoEnqueue() |
-------------------------
|
|
| Inherits
'--------------> -------------------------
| QueueDisc |
|-----------------------|
| Peek() |
| DoPeek() |
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CodelQueueDisc class don't implement it's own Peek() & DoPeek(). It uses the functions defined in its base class QueueDics.Hence, to overcome this problem, it is required to override the Peek() & "Dopeek() function in CodelQueueDisc class, which doesn't perform a real internal dequeue.
Fq-Codel: It is an extended version of CoDel, designed to manage multiple flows of different types.
To read more about Fq-CoDel: https://datatracker.ietf.org/doc/html/rfc8290