TCP window flow control issue

Question

Hello WIZNet Community: We are using the WIZ550io SPI <-> Ethernet module interfacing with the NetBurner™ MOD5541X module. Pretty much successfully implemented and working nicely - mostly… The WIZ550io is set up as a server to interface with a laptop as a diagostic tool for our system. A main function is to download, from the NetNurner to the laptop up to megabytes of data, in 2000 byte blocks. Wireshark shows the following interchange for transaction of one block: WIZ550io (Server): Laptop (Client) <------- Request for next packet [PSH, ACK] Len = 2 [ACK] --------> Len = 0 [TCP Window Update] --------> Len = 0 1st data segment [PSH, ACK] ------> Len = 1460 2nd data segment [PSH, ACK] ------> Len = 546 <-------- [ACK] Len = 0 <------- Request for next packet [PSH, ACK] Len = 2 .... So the problem is the [TCP Window Update], which supposedly serves to negotiate the “window size” field. Not at all sure why this is even there. So fIrst, when we do an analysis (via Wireshark) of the SAME FUNCTION, but via one of the NetBurner Ethernet ports, we do NOT see any [TCP Window Update] msgs. Interesting. Normally, however, this additional msg from WIZ550io does NOT seem to impeded the data downloads. HOWEVER, occasionally, what we see is a ONE SECOND time interval between the [ACK] (msg 3) ) from the WIZ550io and the transmission of the [TCP Window Update] (msg 4) )!! and this occurs at EVERY sequence, as shown above. So for 10 MB of data, at 2K/second… do the math, and we’re looking at more than an hour! So this is a problem. IS there a quick & easy way to tell WIZ550io to NOT try to do any window size negotiation - i.e., just eliminate the sending [TCP Window Update] Messages entirely? That would be the simplest solution. Thanks for any help or insight. Ron (in Pittsburgh Strong)

becky · Answer

Hello, [TCP Window Update] Packet is a packet sent because the PC’s Window Size is not enough. If the PC has sufficient Window Size, the W5500 will not send a [TCP Window Update] packet. So I think that the W5500 sends data without the [TCP Window Update] message, the PC will not receive it and the W5500 will retransmit the data Anyway, there is no way to disable the [TCP Window Update] packet from being sent. Thank you

Eugeny · Answer

RonPeL: If you wish, I can send WireShark captures of both interactions. That would be good. Sounds like you have no-delayed ACK for the socket turned off (disabled). What value socket register Sn_MR is being initialized to?

RonPeL · Answer

Привет, Евгений, и спасибо за ваш быстрый ответ. К сожалению, английский язык намного легче для меня, моя способность к русскому языку ограничена и очень плоха. L And so the rest will be in English… I am setting the no-delayed ACK bit in S0_MR (I am only using socket 0 for our application.) S0_MR is initialized to 0x21. I will send along the WireShark captures shortly. Best regards, Ron

Eugeny · Answer

RonPeL: I will send along the WireShark captures shortly. Attach them here changing their externaion or packing into ZIP/RAR.