net: IPv6: malformed packet in fragmented echo reply

[rmstoi]

Prerequisites: enable CONFIG_NET_IPV6_FRAGMENT=y

There seems to be an issue with IPv6 fragmentation. For example, testing native posix echo server:

ping6 -I zeth 2001:db8::1 -s 1300 -c 1

To echo request Zephyr sends echo reply with the malformed second fragment. Wireshark marks that packet as malformed and ping6 does not succeed.

[finikorg]

I have this issue for my netusb samples, interesting that if I disable IPv6 fragmentation I can send 1500 bytes - headers, while when fragmentation is enabled I can not send even 1300 bytes.

[rmstoi]

The malformed fragment seems to occur after second echo request. The first echo reply has wrong checksum and its second fragment is missing.

fragv6malform.pcapng.gz

Also, I've changed the amount of buffers:

-CONFIG_NET_PKT_RX_COUNT=16
-CONFIG_NET_PKT_TX_COUNT=16
-CONFIG_NET_BUF_RX_COUNT=64
-CONFIG_NET_BUF_TX_COUNT=64
+CONFIG_NET_PKT_RX_COUNT=160
+CONFIG_NET_PKT_TX_COUNT=160
+CONFIG_NET_BUF_RX_COUNT=640
+CONFIG_NET_BUF_TX_COUNT=640

[jukkar]

I have this issue for my netusb samples, interesting that if I disable IPv6 fragmentation I can send 1500 bytes - headers, while when fragmentation is enabled I can not send even 1300 bytes.

The IPv6 MTU is 1280 bytes, although in this case we could and should send 1500 bytes just fine. If you feel like it, please create issue for that.

[finikorg]

I have this issue for my netusb samples, interesting that if I disable IPv6 fragmentation I can send 1500 bytes - headers, while when fragmentation is enabled I can not send even 1300 bytes.

The IPv6 MTU is 1280 bytes, although in this case we could and should send 1500 bytes just fine. If you feel like it, please create issue for that.

Added Issue #14659 (feature request)