-----Original Message----- From: owner-postfix-us...@postfix.org [mailto:owner-postfix-us...@postfix.org] On Behalf Of Glenn English Sent: Saturday, 31 August 2013 12:52 AM To: postfix-users@postfix.org Subject: Re: newbie check Was [Re: port 25 submission settings sanity check]
On Aug 30, 2013, at 7:07 AM, Terry Gilsenan wrote: >> As attachments get larger, and end users use email rather than ftp for file >> transfer for convenience sake, a UDP implementation, perhaps using UDP as a >> data >streaming channel could become a very useful configuration, and the >> transfer speed over high latency links (think satellite etc) could improve >> immensely. > >I don't think so. As Weitse said yesterday, to run SMTP successfully on UDP, >he'd have to duplicate TCP's back-and-forth to make sure those bits got there, >they >were correct, in the right order, etc. If that were done, you'd be back >at TCP overhead and speed (and reliability). I am not talking about implementing SMTP on UDP, I am taking about the possibility of adding a side-channel for bulk data that would use UDP. If EC is handled within the application then it will work ok. UDP and TCP differ in that with TCP there is an overhead within the protocol that guarantees delivery. Data transfer over UDP can be just as reliable as TCP if the delivery guarantee mechanism is handled at the application layer. On reliable links with large RTT (latency) UDP with EC in the application is a whole world better than TCP. Try it some time with IPTables and netem, and setup a local router that will introduce a 1000ms delay into your network, then try using a vpn like say, OpenVPN in UDP mode (UDP 1194) and then in TCP (TCP 443) mode and you will soon see what I am talking about. The UDP connection will far outshine the TCP connection where RTT is 400ms+ Use netem to drop the RTT down to 200ms and the advantage reduces, drop the RTT to 30ms and the improvement reduces again. On satellite links (they are millions of these) the latency significantly impacts the size of the TCP window, and drastically impacts the maximum available bandwidth. This effect is almost completely moot when UDP is used. T ===[Disclaimer]=== This electronic transmission, including any attachments, is confidential, may contain privileged information and should be read or retained only by the intended recipient. If you received this message in error, please delete it from your system and notify the sender immediately. Any review, dissemination or other use of this information by persons or entities other than the intended recipient is strictly prohibited. ===[End]===
