[lwip-users] LWIP not ACKing data in retransmissions

[Fabian Koch]

Hey all,

 

we have a weird behavior of one of our devices and I wonder if anyone could provide any pointers as to what is going on.

(pcap attached)

 

Scenario:

• Our device is 172.22.66.200 in this capture
• Other device is 172.22.67.200
• Pcap contains only one TCP stream between the two
• We are using the socket API on top of LWIP 1.4.1

 

Good behavior:

• In #9, the peer sends a retransmission because our device didn’t answer for 240ms
• In #10, we ACK that retransmission with no payload and everything looks fine

 

Weird behavior:

• In #92 there is another retransmission by the peer after 527ms and we even take another 478ms to answer
• #93 ACKs the retransmission but does not increase the ACK number to include the payload of the retransmission
• So that retransmission is sent again and again (exponential back-off) but the payload is never ACKed
• Meanwhile our device happily sends payloads and the gateway ACKs them

 

What is happening here?

 

Kind regards

Fabian

LWIP_retrans_ACK.pcapng
Description: LWIP_retrans_ACK.pcapng