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In uIP, TCP packets are sent and ACK'ed one at a time. That is, after one TCP packet is sent, the next packet cannot be sent until the previous packet has been ACKed by the receiving side. The TCP protocol, of course, supports sending multiple packets which can be ACKed be the receiving time asynchronously. This one-packet-at-a-time logic is a simplification in the uIP design; because of this, uIP needs only a single packet buffer any you can use uIP in even the tiniest environments. This is a good thing for the objectives of uIP. Improvements in packet buffering is the essential improvement that you get if upgrade from Adam Dunkel's uIP to his stack.
The price that you pay is in memory usage. This one-at-a-time packet transfer does create a performance problem for uIP: RFC 1122 states that a host may delay ACKing a packet for up to 500ms but must respond with an ACK to every second segment. In the baseline uIP, the effectively adds a one half second delay between the transfer of every packet to a recipient that employs this delayed ACK policy! UIP has an option to work around this: It has logic that can be enable to split each packet into half, sending half as much data in each packet. Sending more, smaller packets does not sound like a performance improvement. This tricks the recipient that follows RFC 1122 into receiving the two, smaller back-to-back packets and ACKing the second immediately.
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References: and. The NuttX, low-level TCP/IP stack does not have the limitations of the uIP TCP/IP stack. It can send numerous TCP/IP packets regardless of whether they have been ACKed or not. That is because in NuttX, the accounting for which packets have been ACKed and which have not has been moved to a higher level in the architecture. NuttX includes a standard, BSD socket interface on top of the low-level TCP/IP stack. It is in this higer-level, socket layer where the ACK accounting is done, specifically in the function.
If you send a large, multi-packet buffer via send(), it will be broken up into individual packets and each packet will be sent as quickly as possible, with no concern for whether the previous packet has been ACKed or not. However, the NuttX send() function will not return to the caller until the final packet has been ACKed. It does this to assure that the callers data was sent succeffully (or not). This behavior means that if an odd number of packets were sent, there could still be a delay after the final packet before send() receives the ACK and returns.
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So the NuttX approach is similar to the uIP way of doing things, but does add one more buffer, the user provided buffer to send(), that can be used to improve TCP/IP performance (of course, this user provided buffer is also required by in order to be compliant with send()””. But what happens if all of the user buffer is smaller than the MSS of one TCP packet? Suppose the MTU is 1500 and the user I/O buffer is only 512 bytes?
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In this case, send() performance degenerates to the same behavior as uIP: An ACK is required for each packet before send() can return and before send() can be called again to send the next packet. A fix has recently been contributed by Yan T that works in a similar way to uIP split packet logic: In send(), the logic normally tries to send a full packet of data each time it has the opportunity to do so.