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First of all, thanks for taking the time to read.

I have a doubt.

Does QOS marking in the IP header occur on every packet or does it need TCP segmentation established (TCP handshake)?

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  • Packets are routed individually, without regard to any other packets. QoS markings are set in various ways, but they do nothing unless you have configured the network devices to do something with them, and the conditions are right (congestion).
    – Ron Maupin
    Jun 3 at 18:55
  • Of course, I need to be configured. My doubt is more study. Theoretical and not practical. Example, I have a rule configured applying priority 1 to destination port 443, in the initial syn it already marks or would wait for the connection established. I understand that no, QOS is in the IP header is protocol and session independent. It checks everything that is assigned to it, limiting it only to src ip, dst ip, src port and dst port.
    – Joao
    Jun 3 at 19:43

3 Answers 3

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Example, I have a rule configured applying priority 1 to destination port 443, in the initial syn it already marks or would wait for the connection established. I understand that no, QOS is in the IP header is protocol and session independent. It checks everything that is assigned to it, limiting it only to src ip, dst ip, src port and dst port.

Routing is only by IP address. Each packet is routed independently of any other packets, so each packet needs to be marked. Routers do not know anything about TCP ports.

A router has no idea if the packet payload is TCP, UDP, or any other transport protocol, and it certainly has no idea if the packet contains a TCP SYN. In fact, different packets containing segments of the same TCP connection could take different paths for various reasons, arriving out of order due to the varying latency of the different paths, which is why TCP is designed to reorder data to be able to present it to the application in order.

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Does QOS marking in the IP header occur on every packet or does it need TCP segmentation established (TCP handshake)?

Depending on your devices and their configurations, you need to mark each packet's differentiated services field.

Whether a packet is used for TCP's initial handshake or for application data doesn't matter. The mechanisms work on the packet level, the payload isn't relevant (unless you use layer-4 information to change the DSCP field on the fly).

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QoS markings are typically set at the client machine or at the access switchport that the client plugs in to and every single IP packet that meets the criteria for a QoS marking get's the mark. TCP is independent of QoS.

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