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New Member

Donwloading an image


Is it possible to modify the speed of the TFTP or FTP client on Cisco's equipement?

My pb is that when i'm trying to download an image over a satellite link (10 Mbps data rate and 500 ms delay) it takes around 6 h !!

thks for ur help


Re: Donwloading an image

TFTP is a very simple protocol, and has no concept of transmission windows. Each packet has to be acknowledged individually, after it's received. This means you can only have one data packet "in flight" at once, which will obviously impair performance on a long fat pipe such as a satellite connection.

FTP should perform better than TFTP in this situation, subject to the negotiated TCP window sizes. IOS 12.2(8)T support the TCP window scaling option for "long fat pipes" such as satellite connections, which may help performance. You can set the 'ip tcp window-size [XXX]' option in the configuration to change the negotiated window size, but I can't think right now if this will influence the receive window size, which is what you need in order to transfer data into the router at higher speed. Start at a window size of 65,535. Try 131,072 if you have an IOS that supports window scaling. Again, I'm not 100% sure these will have an impact on your receive window. Don't set these options if your router is low on RAM.

New Member

Re: Donwloading an image

But i thought that TFTP was working over UDP that means no ack...

In fact, i've already tried why a FTP server and modified the windows size (ip tcp window-size 65535) and this is not working better !

thks again


Re: Donwloading an image

It is true that UDP, as the underlying transport layer, does not provide delivery and sequencing guarantees to the application layer as TCP does. However, most applications that run over UDP still require some guarantee of integrity, and TFTP is one of those applications. Your router wouldn't run well at all if part of IOS was missing or corrupted!

TFTP stands for "Trivial File Transfer Protocol". The "Trivial" part means it can be implemented with the bare minimum of code and CPU/memory resources. This is why it's the file transfer method of choice for routers in ROMMON mode as well as diskless workstations and even Cisco IP phones during the boot-up process. UDP instead of TCP. TCP needs a fair amount of code to implement properly: congestion control, acknowledgements, etc. Sending a UDP packet is as simple as writing a fairly static packet header and a checksum. This does, however, move the burden of error checking and retransmission into the application. TFTP therefore implements a simple, one-by-one transmit and acknowledge scheme.

More info about TFTP can be found in the standard document:

I'm not really sure if we can make FTP to the router go faster for you. Your best bet might be to download the image and a TFTP server to a PC at that location, and to TFTP the image from that PC. The PCs should be able to get more reasonable download performance over the satellite link.