It seems udp packet fragmented.

[yutayu]

Please provide the following information

libtorrent version (or branch):
libtorrent: 1.2.9.0
platform/architecture:
linux arm (raspi)
compiler and compiler version:
Deluge-gtk
Client: 2.0.3
Server: 2.0.3

please describe what symptom you see, what you would expect to see instead and
how to reproduce it.
11:28:11.293770 IP 180.42.xxx.73.56881 > 106.154.xxx.198.19251: UDP, length 1472
11:28:11.294334 IP 180.42.xxx.73 > 106.154.xxx.198: ip-proto-17
11:28:11.294359 IP 180.42.xxx.73.56881 > 106.154.xxx.198.19251: UDP, length 1472
11:28:11.294945 IP 180.42.xxx.73 > 106.154.xxx.198: ip-proto-17

This is ethernet-bridge router.
Behind Modem , MTU is 1500 but , Beyond modem MTU is 1454.
So I set MTU is 1454 at eth0.

[Seeker2]

Arvidn explained how libtorrent tries to adjust to MTU of the interface here:
Tribler/tribler#2620 (comment)

[yutayu]

@PriitUring
I use delgue with raspberry os, This deluge version is little bit old and I cannot upgrade it manually.
Then I close this issue.
I like deluge :)