Re: [lwip-users] Getting the server to use an MSS larger than 536

[Mason]

Simon Goldschmidt wrote:

> Mason wrote:
> 
>> Note that Google sends only 590-byte frames. How do I get lwip
>> to advertize larger MSS?
>>
>> NB: I have the following TCP-related definitions in my lwipopts.h
>>
>> #define TCP_MSS 1460
>> #define TCP_WND (40*TCP_MSS)
>> #define TCP_SND_BUF (8*TCP_MSS)
>> #define TCP_SND_QUEUELEN 16
>>
>> There's relevant code in src/core/tcp.c
>>
>>   pcb->snd_wnd = TCP_WND;
>>   /* As initial send MSS, we use TCP_MSS but limit it to 536.
>>      The send MSS is updated when an MSS option is received. */
>>   pcb->mss = (TCP_MSS > 536) ? 536 : TCP_MSS;
>> #if TCP_CALCULATE_EFF_SEND_MSS
>>   pcb->mss = tcp_eff_send_mss(pcb->mss, ipaddr);
>> #endif /* TCP_CALCULATE_EFF_SEND_MSS */
> 
> In that case (if you have set netif->mtu to 1500), tcp_eff_send_mss()
> should return 1460 (1500 - IP_HLEN - TCP_HLEN). If it doesn't,
> possible problems could be that ip_route() doesn't return your netif
> or that netif->mtu isn't set to 1500.
> 
>> When I receive Google's SYN+ACK packet with MSS=1430 in
>> tcp_parseopt, the pcb mss is correctly set to 1430. Perhaps
>> this is only the /write/ MSS?
> 
> Yes, this is lwIP's send MSS, while the initial pcb->mss is used to
> construct an MSS option in the initial SYN packet sent on connect.

Applying the following patch makes the distant system send larger
frames (1484 bytes in Google's case, 1430-byte segment).

--- tcp.c.orig  2012-02-23 10:57:12.843750000 +0100
+++ tcp.c       2012-03-21 17:29:25.515625000 +0100
@@ -749,7 +749,7 @@
   pcb->snd_wnd = TCP_WND;
   /* As initial send MSS, we use TCP_MSS but limit it to 536.
      The send MSS is updated when an MSS option is received. */
-  pcb->mss = (TCP_MSS > 536) ? 536 : TCP_MSS;
+  pcb->mss = TCP_MSS;
 #if TCP_CALCULATE_EFF_SEND_MSS
   pcb->mss = tcp_eff_send_mss(pcb->mss, ipaddr);
 #endif /* TCP_CALCULATE_EFF_SEND_MSS */

I don't understand the comment explaining why the MSS is clipped.
Does this come from an RFC?

Relevant commits are:
http://git.savannah.gnu.org/cgit/lwip.git/commit/?id=2d5908f4dedeac4ae2f568bd6f06ea218de5a125
http://git.savannah.gnu.org/cgit/lwip.git/commit/?id=aee9c4c8e6c9ec54783f3657c40be96753c72dd9

-- 
Regards.