Extracting packet header info in LTE mac for uplink transmission

[Sab NT]

Hi everyone,

I want to intercept packets at the mac layer during LTE uplink transmission. I went through the ns3 LTE design document and understood the flow diagram for uplink transmission to some extent as follows: After the scheduler decides the MCS and hence the data size that can be transmitted through the uplink, the mac notifies the TxOpportunity with the specific size to the RLC . Then RLC compares this transmittable size with its buffer status size and does segmentation/concatenation as required to transmit PDU to MAC.  Now, I want to intercept the application packets associated with the MAC pdu before it transmits in DoTransmitPdu(). But since RLC does segmentation/concatenation, I don't know how to get the packets from the pdu.

I have the packet sequence number added in the application header of the packets. Can anyone please suggest how can I get the application packets from the mac pdu? 

Is there any option to disable the segmentation/concatenation at the RLC?  Any help will be greatly appreciated.

Thanks in advance.

[Sab NT]

Hi,

After some more reading my thinking is that I have to remove upper layer headers from the mac PDU in order to get the application level header info.  But  I am not sure how many headers I need to decapsulate from the PDU received in enb-mac in order to reach the application layer hear info. In my setup, I have an end-to-end UDP communication between UE and remote host through LTE-EPC. I tried removing the following headers one by one from mac pdu : rlcHeader, pdcpHeader, rrcHeader, ipHeader and udpHeader to reach to application level info but getting some assert-error e.g. " assert failed. cond="(verIhl >> 4) == 4 " in ipv4-header.

Am I following right way of doing this? Is there any other header needs to be removed in the LTE protocol stack?  Can someone please advise.

Thank you very much in advance.

[Tommaso Pecorella]

Hi,

enable packet printing and print the packet, you'll see all its headers.

About the approach, remember that not all the packets will have all the headers.

T.

[Sab NT]

Hi Tommaso,

Thank you very much for your reply. Yes the assert was failing because some headers are missing in some of the packets. 

Now, I am using alternate approach using packet tag at the MAC layer. But I am facing some problem. I added a uint32_t field in the lte-radio-bearer-tag, I added/modified get, set, serialize, Deserialize methods; also changed the GetSerialized Size() to accommodate 4 more bytes (for 32 bit field). Till this point it is fine and I can add the field in the tag in function call. However, when I do a RemovePacketTag and read the field, it round offs the field as 8 bit size and I am getting back (field_value%255). I checked my GetField method and member variables, everything is set as uint32_t. Could you please advise if I am missing anything. 

Thanks in advance.

[Sab NT]

Sorry, found my mistake while double checking. I had uint8_t variable in the Deserialize() method. After I changed, it is working fine.

Thanks for your help.