buglwIP - A Lightweight TCP/IP stack - Bugs: bug #63092, When the length of unack data is...

 
 

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bug #63092: When the length of unack data is less than pcb->cwnd, LWIP cannot send packets.

Submitter:  jiangheng <jinag12345>
Submitted:  Fri 23 Sep 2022 10:14:28 AM UTC
   
 
Category:  TCP Severity:  3 - Normal
Item Group:  None Status:  None
Privacy:  Public Assigned to:  None
Open/Closed:  Open Planned Release:  None
lwIP version:  2.1.3

Fri 23 Sep 2022 10:14:28 AM UTC, original submission:  

In tcp_output function:
if (lwip_ntohl(seg->tcphdr->seqno) - pcb->lastack + seg->len > wnd) {
    ....
    goto output_done;
}

In tcp_slowmgr function:
if (pcb->rtime >= pcb->rto) {
    pcb->cwnd = pcb->mss;
    tcp_rexmit_rto_commit(pcb);
}

tcp_rexmit_rto_commit ≈ tcp_output;

in tcp_output, The printf command output is as follows:
lwip_ntohl(seg->tcphdr->seqno) = 405161
pcb->lastack = 403701
seg->len = 1460
pcb->snd_wnd 65535
pcb->cwnd = 1460
so tcp_output will not send this tcp segment, It enters the slow start phase of congestion control and cannot exit.
Although unack data exists, the value of snd_wnd is 65535. lwip cant send data is correct this moment?

jiangheng <jinag12345>

 

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