MTU and # of packets

binaryhat

This question has been in my head:

Since the MTU is 1500 bytes...how many packets will be sent if the file size is say 10MB? Just an example...

bbarrick

Here's my guess, approximately 7,000.

Did a quick check and 10 Megabytes divided by 1500 bytes = 6,990.50667. But I'm sure there will be more packets sent, if you include the entire process.

NetworkVeteran

binaryhat wrote: »

This question has been in my head:

Since the MTU is 1500 bytes...how many packets will be sent if the file size is say 10MB? Just an example...

Well, IP adds 20 bytes and TCP adds 20 bytes, leaving a 1460-byte payload. 10,485,760 / 1460 = ~7,183.

xXErebuS

NetworkVeteran wrote: »

Well, IP adds 20 bytes and TCP adds 20 bytes, leaving a 1460-byte payload. 10,485,760 / 1460 = ~7,183.

I am assuming we are talking about the system mtu 1500 you see in CLI?

It's still a 1500 Ethernet payload (IP Payload + headers), the IP Payload is 1460. IMO it's 6,990.506. If you were talking about the IP MTU under and interface it's still 6,990.506 since that MTU # reflects payload + headers.

EDIT: Don't think I made clear what I was trying to say:

Ethernet payload (system mtu 1500) = payload (doesn't include headers)
IP payload (ip mtu 1500) = payload + headers

If I am wrong someone please point me in the right direction.

networker050184

IP MTU and the system MTU are calculated basically the same way with ethernet, IP headers and payload.

xXErebuS

networker050184 wrote: »

IP MTU and the system MTU are calculated basically the same way with ethernet, IP headers and payload.

When you specify the IP MTU # you are specifying the size for the payload + L3 headers; when you specify the system MTU you are specifying the size for just the payload . There is an additional 18/22 bytes added to the system MTU so they are not calculated the same way.

MTU manipulation - Packet Life <-- this is IMO a good explanation.

For instance say you need to transmit a 2000 byte payload TCP/IP; your IP MTU # would need to be 2040, and your ethernet mtu # would only have to be 2040 b/c the headers / CRC are not counted in this number but assumed.


EDIT: More -


http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_note09186a00801350c8.shtml
4507(config)#system mtu ? <1500-1552> MTU size in bytes 4507(config)#system mtu 1552 Global Ethernet MTU is set to 1552 bytes. Note: this is the Ethernet payload size, not the total Ethernet frame size, which includes the Ethernet header/trailer

networker050184

Right, but the "payload" with the MTU command is accounting for those L3 headers as well. It doesn't count ethernet headers, but IP headers are counted in this equation.

In other words the payload of an ethernet frame contains the entire L3 packet including headers.

xXErebuS

networker050184 wrote: »

Right, but the "payload" with the MTU command is accounting for those L3 headers as well. It doesn't count ethernet headers, but IP headers are counted in this equation.

In other words the payload of an ethernet frame contains the entire L3 packet including headers.

Correct, I see where we disagreed - I read the OP's question wrong . NetworkVeteran is correct; per packet your only getting 1460 bytes of data.