Packets received from Ethernet Interface getting discarded due to incorrect packet_type (and other issues)

Question

Packets received from Ethernet Interface getting discarded due to incorrect packet_type (and other issues)

Sulav Lal Shrestha 361

I am getting started with Azure RTOS. I managed to get the threadx working and for netx the netx driver also seems to be *working (I have not been able to do anything meaningful with netx yet).

I am using the netxdriver ATSAME54-XPRO and ported it to SAME70Q21B microcontroller (SAME70-Xplained board). However, even though the board receives the ethernet messages, it discards the messages.

Steps:

Configure the microcontroller board as DHCP client.
DHCP client sends DHCP discover message.
DHCP server replies back with offer.
The microcontroller board receives the DHCP offer but discards it in the function _nx_driver_transfer_to_netx.

I found some issues like:

In the function _nx_driver_transfer_to_netx, the nx_packet_prepend_ptr member in NX_PACKET *packet_ptr has address 2 greater than it should be.
The nx_packet_length is 4 more than it should be.
The nx_packet_append_ptr is 6 more than it should be.

This is causing the packet type to be calculated wrongly and hence getting discarded.

I manually fixed the above 3 parameters for this particular case. With this, the packet is getting identified as IP packet, but then it goes to ip packet handler and finds that the checksum is wrong and discards the packet.

Any ideas on why this is happening and how I can fix it?

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Answer 1

Sulav Lal Shrestha 361

The 4 bytes at the end seem to be ethernet padding. By copying the entire buffer two bytes to the right, so that the pointers align, the issue seems to be fixed. However, I am not sure yet why this was needed in the first place.

Tiejun Zhou 1,131 Reputation points Microsoft Employee

2021-10-18T01:04:16.163+00:00

Good to know the problem is solved. For performance consideration, NetX assumes TCP/IP headers are aligned to four bytes so we can read long integers from headers directly. Without that, we need to read data byte by byte which is not efficient.
António Sérgio Azevedo 7,671 Reputation points Microsoft Employee Moderator

2021-10-19T10:16:03.653+00:00
@Sulav Lal Shrestha Could you go ahead and mark the above as answer? Thank you so much.

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Please accept an answer if correct. Original posters help the community find answers faster by identifying the correct answer. Here is how.

Answer 2

Scott Larson 451

Does the ethernet controller apply hardware padding to the packet? This is a common feature in ethernet controllers to help with alignment.

Tiejun Zhou 1,131 Reputation points Microsoft Employee

2021-10-15T02:00:20.75+00:00
As Scott mentioned, the driver needs to pad 2 bytes for alignment in NetX TCP/IP stack.

Assuming you are comparing the actual packet length with the one reported in IP header. It is a normal behavior for Ethernet controller. In IP header processing, the packet will be trimmed to actual size.

I guess 6 is 2 padding at beginning and 4 padding at the end.

For packet type calculation, I think it can be solved with padding.
Sulav Lal Shrestha 361 Reputation points

2021-10-15T13:55:15.663+00:00

Added a figure in the question to provide more information on the issue.

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Packets received from Ethernet Interface getting discarded due to incorrect packet_type (and other issues)

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