TCP/UDP Traffic indication in LTE


In LTE, how does RLC/MAC knows DL/UL data is TCP or UDP?

RLC/ MAC don’t need to know if the data is TCP or UDP type. Although VOIP is generally UDP type and it uses RLC UM. But this is by design, there is no interworking.


Hi Sanjay,
This should be pertaining to QCI, RRC connection Reconfiguration carries AM (TCP type) and UM (UDP type) against the different configured QCI during bearer activation, which is configurable under ENB/GNB.

QCI1 (VoLTE) only UM which uses RTP over UDP,
Rest all QCIs use TCP.

Thank you.

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Its true that each QCI has certain RLC Mode i.e. QCI1 always uses RLC UM, whereas QCI8 would be using RLC AM. That doesn’t mean you will always have TCP type traffic on QCI8. For example if you are using an application like Skype for video conferencing, Skype would use UDP over the Internet but on the LTE side you wouldn’t be configured with QCI1 while using Skype. Instead you would be using QCI8 , QCI9 etc.

RLC/MAC do not need to know traffic is TCP or UDP. PDCP layer will configure RLC entity as TM, UM or AM based on PDU data that PDCP received from higher layer.
For AM RLC, there are retransmission, ack report.

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Voice over IP may be partial exception to this… however, it can’t be complete UDP following implementation complexity.
If you have any PCAP having it tested, please share if you can.
Thank you.

Hi @Hainm, Thanks it is food for thought, have you ever seen any applications or session flow where PDCP configuring data (QCI) contrary to it’s RLC configuration under ENB/GNB layer for each QCI to ensure QoS.

Would be happy to see it flowing in L2 traces?