I am using lwip-1.4.0 in RAW mode on a NXP LPC1788 (ARM Cortex-M3, 32-but
arch). I have implemented the tcp_echo example and all works well when sending
simple packets. However, when the packets get a little larger, things start to
break down. I have traced it to the following:
In my lowest level driver, I receive an Ethernet packet with a payload of 594
bytes. This gets copied into an lwip buffer that is allocated using:
p = pbuf_alloc(PBUF_RAW, PBUF_POOL_BUFSIZE, PBUF_POOL);
The defaults in my lwip\opt.h file are (in reverse order):
#define PBUF_POOL_BUFSIZE LWIP_MEM_ALIGN_SIZE(TCP_MSS+40+PBUF_LINK_HLEN)
#define PBUF_LINK_HLEN (14 + ETH_PAD_SIZE)
#define ETH_PAD_SIZE 0
#define TCP_MSS 536
And my lwipopts.h file has
#define MEM_ALIGNMENT 4
And so the PBUF_POOL_BUFSIZE is only LWIP_MEM_ALIGN_SIZE(590) which results in
592 - less than the 594 bytes coming in .
Why am I getting 594 bytes coming in and I only have 592 bytes allocated to
hold it? How do I fix this? (Scaling the TCP_MSS scales the problem).
-Gary
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