IOS TFTP vs FTP priority through switches

Unanswered Question
Aug 12th, 2008

I am benchmarking download times, using tftp, of switch IOS's (around 4-6MByte files) to a central server and find that over 2Mb WAN links they can vary between 28kB/s and 4kB/s with an average on most sites of around 12kB/s. If I tranfer a 4MBye file from a PC, using FTP, through the switches, back to the central server I find that the transfer rates are between 180KB/s and 1MB/s. My question is, do the switches prioritise user data through the switch. Does the user data take precedence over the IOS tftp file transfer.

I ask the question because I am investigating why identical switches on separate sites, connected to identical 2MB/s WAN links, take up to 3 times longer to transfer files to the central server.

I have this problem too.
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andrew.butterworth Tue, 08/12/2008 - 03:26

TFTP runs over UDP and doesn't have the advantage of TCP's error checking & windowing which FTP uses. Even on a local copy you will notice significant speed differences between the two protocols.

HTH

Andy

evansr255 Tue, 08/12/2008 - 03:39

Thanks Andy,

Yes, I appreciate the speed differences between TCP and UDP but the main isssue I have is the difference in transfer of times the IOS images on different sites that all have 2Mb/s wan links. The set up on each site is the same, in that the switches I am transferring the IOS from, are all directly connected to the WAN routers. I am trying to prove that the issue is with the service provider and not related to the indiviual sites. My reference to FTP transfers is because I was concerned that the switches would delay the IOS image download and give priority to user ftp traffic through the switch.

regards,

Roy

Richard Burts Tue, 08/12/2008 - 03:50

Royston

Andy is pointing in the right direction on this issue. While there is not much difference between TFTP and FTP in error checking and retransmission capability (it is built into the transport in FTP and coded in the application for TFTP, but both protocols will detect errors or missing packets and retransmit) there are significant differences in the protocols that produce a performance advantage for FTP.

TFTP sends a single packet and waits for an acknowledgment before transmitting the next packet while FTP can transmit multiple packets before it must wait for an acknowledgment (this is the windowing that Andy mentions). And TFTP uses a smaller frame size while FTP uses a larger frame size and so carries more data per packet. These are the reasons that TFTP does not perform quite as well as FTP.

[edit] having just seen your response to Andy, without knowing anything about how your switches are configured it is difficult to say what they will do - most especially whether they have any Quality of Service which might differentiate types of traffic. But in general we can certainly say that switches do not process any differently traffic that is TFTP from FTP.

HTH

Rick

Joseph W. Doherty Tue, 08/12/2008 - 04:13

"My question is, do the switches prioritise user data through the switch. Does the user data take precedence over the IOS tftp file transfer."

Depending upon the features of the network devices and their configurations, they might prioritize some traffic over other traffic, but I think it unlikely the cause for the performance differences you're seeing.

"I ask the question because I am investigating why identical switches on separate sites, connected to identical 2MB/s WAN links, take up to 3 times longer to transfer files to the central server."

Although you note the switches are identical, and the WAN links identical, is the drop rate, latency and link utilizations also identical? Of these three, drop rates and latency can hugely impact effective transfer rates for both TFTP and FTP.

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Posted August 12, 2008 at 3:11 AM
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