Re: [tcpm] SYN/ACK Payloads, draft 01

> |
> | int newsocket = accept(...)
> | getsockopt(newsocket, SOL_TCP, &tcpsa, ...) if (TCP_SADATA_REQ & 
> | tcpsa.tcpsa_flags) {
> |    // The first data from the socket is the client's 
> draft-agl handshake
> |    decode_tagged_data(newsocket);
> |    ...
> | } else {
> |    // No special case,
> |    continue_as_normal();
> | }
> 
> I'm wondering why an implementation in user space would 
> expect to find out anything about a TCP connection that 
> hadn't finished handshaking, i.e., the accept call above 

This has been done in some stacks today (like ours). The incoming SYN is
reported to the application and the application can suggest whether or
not to accept the connection. If the application gives a nod, proceed
with the 3 way handshake else send an RST. This is different from a 3
way handshake happening (which involves the TCB and the associated
baggage creation) and then informing the application and applications
can reject this connection during accept(). Applications wishing to use
this special property would set a socket option indicating the
notification of SYN when the listener is created.

The advantage is that you don't allocate the memory ahead of time if the
application isn't interested in accepting this connection. Other ways of
handling this is to have the application program the access control list
in TCP or below, so that an early discard can be made.

-Anantha

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