Re: Sending huge file between machines on same subnet

Hello Åke,

Am 24.05.2022 um 17:00 schrieb Åke Olsson <konsult@xxxxxxxxxxxxxxx>:

I need to transfer a large save file between two IBM i machines. These are sitting on the same subnet but one of them is really old and with an older version of the OS as well.

How old is "older"? Which release are we talking about?

I have tried using regular FTP and this fails – apparently due to the file size. And yes: I have adjusted the time parameters in the FTP script to their maximum values.

Some FTP implementations had issues with files > 2GB (32 Bit signed integer overflow).

I tried the option in RDI to copy the file from machine a to machine b. It seems to be working but at a very pedestrian pace. With my math it seems as if it could possibly be completed in about a week or so – provided that the VPN does not falter or any other abrupt drop of connection happens.

You said, both machines are in the same subnet. So, the subnet is stretched over a L2 VPN? Does this VPN permit the full 1500 Bytes of IP packets to pass? Or only shorter ones? On the IP layer, TCP handshakes a MSS value (Maximum Segment Size), but this might fail if a L2 VPN (which is assumed my any equipment connected to the segment to be completely transparent, just like a cable) doesn't handle packet fragmentation — VPN usually means encryption, and encryption enlarges packets. But you rarely can send packets larger than 1500 Bytes over the Internet.

Many "smaller" things might have worked for ages, as long as the maximum packet size isn't exceeded.

I thought that it might be possible to download to my local PC and then as a next step upload from there to target machine. Alas the ftp client on my laptop simply refused to deal with files of this size.

See above. :-)

Is this a one-time copy job or a preparation for a later automatism?

So – how to do it?

Depends. Your question raises more questions to be answered before poking into the darkness. ;-)

Oh – just to be sure I tested the FTP with sending a much smaller save file
over (only a hundreg megabytes or so). Worked just fine.

So, it's most likely not a VPN/MTU issue. However, to give you some options, please elaborate on "old".

:wq! PoC