Hello Experts, Greetings!!!
Can someone explain or share a good article that details the journey of a data packet from the iPerf server to the UE in downlink, particularly when measuring DL-Throughput using the iPerf tool? I am specifically interested in the TCP and gNB stack portions.
This question arises from a statement in the PDCP spec (5G-NR Rel_15) that mentions:
“The maximum supported size of a PDCP SDU is 9000 bytes. The maximum supported size of a PDCP Control PDU is 9000 bytes.”
Does this imply that the TCP MTU size cannot exceed 9000 bytes? What happens if a data packet with TCP MTU set to > 9000 bytes (theoretically, TCP MTU can be 64K max) arrives at the PDCP layer?
I would like to understand the entire data path flow in both downlink and uplink to fully comprehend this specification statement. Any insights or pointers would be greatly appreciated.
TIA! 
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Does this help? Securing Backhaul with LTE.
This article is for LTE, but the principle remains the same.
Remember, if a higher layer packet size exceeds the lower layer’s limit, it will be fragmented accordingly.
Also, try searching for MTU in LTE or MTU in 5G—there are some good resources available online.
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Before even reaching the RAN backhaul, every network interface - such as UPF, SGW, or any IP Layer 3 node - is configured to handle a specific MTU size, typically less than 1500 bytes. This is usually reflected in an MSS (Maximum Segment Size) of around 1380 to 1440 bytes. If a packet larger than 1500 bytes passes through any of these interfaces, it will either be segmented at the transport layer (if the protocol supports segmentation, like TCP) or discarded (if it does not support segmentation, like UDP). As a result, such large packets would never reach the RAN in their original size.
Even if a large packet somehow made it through, it would ultimately be discarded by the PDCP layer, as SDUs exceeding the size limit cannot be passed to the lower layers. That being said, in real-world user scenarios, this is not a major concern, as modern applications - such as web browsing and video streaming - are designed to operate within the <1500 byte MTU limit.
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Thanks!
I’m specifically referring to a scenario where jumbo frames are enabled.
FYI, jumbo frames have a significantly higher MTU than a standard Ethernet frame.
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