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Can't find a clear answer in Google. Both drop packets when the queue is filled. So why two terms are used? If the shaper includes the scheduler, then what else do the shaper besides the dropping?

Or if the shaper just drops packets and the scheduler just decides what to do with such dropped packets, then why https://en.wikipedia.org/wiki/Network_scheduler says that "the scheduler must choose which packets to drop" ?

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The authoritative source would be RFC7640 in which you will find the following definition:

  • Traffic scheduling: provides traffic classification within the network device by directing packets to various types of queues and applies a dispatching algorithm to assign the forwarding sequence of packets.

  • Traffic shaping: controls traffic by actively buffering and smoothing the output rate in an attempt to adapt bursty traffic to the configured limits.

So shapping is about controlling the bandwidth used while scheduling is about giving priority to some kind of traffic against others.

Note however that I never saw a device claiming to be "RFC7640 compliant" and you may well find implementation that doesn't strictly adhere to those definitions. There's often some mix and match in this area.

Actually this RFC status is:

This document is not an Internet Standards Track specification; it is published for informational purposes.

and therefore nobody has any obligation to adhere to it.

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Traffic shaping is a general process, a scheduler may be a specific part in it. Apart from "just" dropping excess traffic (the fine art is selecting which traffic to drop), shaping can also reorder packets to lower the latency for sensitive protocols or packets (e.g. prioritize VoIP, or TCP ACKs on asymmetric links).

Often, multiple queues are used for packets depending on their priority level. The scheduler runs those queues with a defined weighting (e.g. three queues with 4:2:1 scheduling) and either forwards or drops the packets.

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  • 1
    The scheduler reorders things, shaping is a simple hammer that doesn't care what the traffic is.
    – Ricky
    Jul 28 at 11:18

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