Re: [lwip-devel] MSG_MORE flag for send

[Jonathan Larmour]

Simon Goldschmidt wrote:
> Interesting patch (this functionality is required by the TCP RFCs by  
> the way),

Are you sure about that? We already only set PSH on the last segment of 
enqueued data from a single 'write', so isn't that all that is required? 
It seems more to me that this is an extension to give the user more 
control over when to send a PSH, rather than it being at the end of every 
'write'.

> but I'd rather solve this without adding an extra parameter  
> to a load of functions throughout the whole stack (I can already hear  
> people saying' why add an extra parameter (which might be put on the  
> stack when calling the funciton) for something I don't need?').

I can see you glancing at me ;-). But yes, I think this patch would need 
more discussion and development. I am a bit concerned with the 
consequences of running out of memory (segs, pbufs, netbufs, ...) - much 
more likely for us than other stacks of course. My concern, which may be 
unjustified, is that PSH can affect the remote end's behaviour for ACKs. 
And if so, could result in unnecessary delays before remote ACKs are 
transmitted, leading to a stalled local unacked queue. I'd have to look 
more closely at the RFCs and existing practice to find out for sure 
admittedly.

Certainly, adding an extra param is not a requirement; this could be done 
with a separate API message to set a flag in the pcb, e.g. in tcp.h:
#define TF_DELAY_PSH (u8_t)0x80U

And then in tcp_enqueue:
   if (seg != NULL && seglen > 0 && seg->tcphdr != NULL &&
       0 == (pcb->flags & TF_DELAY_PSH) ) {
     TCPH_SET_FLAG(seg->tcphdr, TCP_PSH);
   }

New equivalent functions can be provided in the higher APIs. The advantage 
there being that they are left out if unused (if using a linker with dead 
function garbage collection).

Rather than there being a specific API message to set this flag (which 
would take space that couldn't be removed without an lwipopts.h define), 
it may be better to have one to get/set any PCB flag, which has more 
generic purpose.

Jifl
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