[lwip-users] Delay during transmission

[JM]

lwIP v.1.3.0

I'm sending data on a TCP connection that was created from a listen pcb with the code below:

 tcp_write(pcb, buf_ptr, strlen(buf_ptr), 0);
 tcp_output(pcb);

I'm sending about 4000 bytes.  The first two packets come out of lwIP very quickly, the other end ACKs immediately, then lwIP waits for exactly 2 seconds and does a TCP Window Update before sending the last packet immediately afterwards.  From there on everything is fast.  What could cause this delay?  Some more info below....



13    6.253169    192.168.0.112    192.168.0.100    HTTP    GET / HTTP/1.1
14    6.269166    192.168.0.100    192.168.0.112    HTTP    Continuation or non-HTTP traffic
18    8.271579    192.168.0.100    192.168.0.112    HTTP    Continuation or non-HTTP traffic
17    8.271129    192.168.0.100    192.168.0.112    TCP    [TCP Window Update] http > netrix-sftm [ACK] Seq=2921 Ack=368 Win=5840 Len=0
18    8.271579    192.168.0.100    192.168.0.112    HTTP    Continuation or non-HTTP traffic
19    8.271591    192.168.0.100    192.168.0.112    TCP    http > netrix-sftm [FIN, ACK] Seq=3934 Ack=368 Win=5840 Len=0
20    8.271614    192.168.0.112    192.168.0.100    TCP    netrix-sftm > http [ACK] Seq=368 Ack=3935 Win=64522 Len=0







Here's some relevant settings in lwipopts.h:
#define TCP_MSS                1460
#define PBUF_POOL_BUFSIZE    512
#define PBUF_POOL_SIZE        12
#define TCP_WND             (TCP_MSS*4)
#define TCP_SND_BUF         (TCP_MSS*4)
#define MEM_SIZE            1024
#define MEMP_NUM_PBUF        10
#define MEMP_NUM_TCP_SEG     (2 * TCP_SND_QUEUELEN)       
#define TCP_SND_QUEUELEN    (4 * (TCP_SND_BUF/TCP_MSS))