CODEWITHSUNDEEP
network-programmingtcpip
Stiffo
Stiffo

Reputation: 818

What is the actual bit transfer rate over a network transferring 100kb/s?

When transferring, say, 1GB worth of data over the internet this data is split into packets, each packet containing a small piece of data and aach of these packets are part of a frame.

Eg. Windows reports that you are transferring the file in 100kb/s over a TCP connection, but this appears to be the amount of data from the file being transferred per second, and does not seem to include the ip or tcp header, or ethernet frame.

What is the actual amount of traffic on the network needed to transfer at this speed? Or is that data actually already included in the transfer speed, but just small enough that it makes no significant difference?

Also, IP supports up to 1500 byte / packet (I think?), but what is the common size of data packets when loading, say, an HD image on reddit?

Sorry for the rather basic questions I probably should have figured out myself by now...

Upvotes: 4

Views: 1094

Answers (3)

Danny_ds
Danny_ds

Reputation: 11406

It depends on where you look at the transfer rate:

  • Task Manager will report all the transferred bytes (i.e. the sum of all the packets including their headers).
  • A file transfer program will report the transmitted payload.

Task Manager

If you look at Task Manger / Network, you can see the transmitted bytes together with the number of transmitted packets (unicast or non-unicast).

That data comes from the network driver (or at least something close to it), so it makes sense to report the total amount of data here (otherwise each packet would need to be inspected to calculate the payload).

There is also a graph showing the transfer rate. Those numbers could easily be compared with the reported numbers in file transfer software.

File transfer program

A file transfer program on the other hand, does not know the details about the packets being created in the lower layers (those could be any size). So the only option here is to report the amount of transmitted payload data / part of the file, which also makes more sense to the user.

Network packets

On normal networks (there could also be jumbo frames), a TCP-packet (full ethernet frame) is around 1500 bytes when fully loaded (on my system (IPv4) the packets are 1514 bytes with a total header size of 54 bytes -- 14 for the Ethernet header, 20 for the IP header and 20 for the TCP header). Those could be split in smaller packets along the way in the network, but in most cases they won't.

Transfer rate

When transferring a file (or other large datastream), on average 2 full packets (1514 bytes) will be sent each time, and 1 small packet (54 bytes) is received (the [ACK] packet). In this optimal case we have 2 x 1460 payload, and 2 x 54 bytes of overhead on the sending side + 54 bytes on the receiving side. When comparing to the maximum transfer rate of the internet connection, we also have to consider some latency.

Not all transmissions are optimal:

  • There could be packets that never arrived or where the checksum was wrong, so a retransmit would be needed.

  • In some cases data could be sent in smaller parts, causing a higher overhead/payload ratio (but with small chunks Nagle's algorithm could take care of that).

  • Certain software could be reading the file contents into small buffers (say 4096 bytes). Those could then be split in 2 x 1460 and 1 x 1176, introducing some extra overhead.

Conclusion

It's hard to tell or calculate the exact ratio transferred_bytes/payload. It depends on the quality of the internet connection (lost packets, retransmits), the software or API calls used to transfer the data, and even the underlying network (small frames vs jumbo frames for example).

Upvotes: 2

gen-y-s
gen-y-s

Reputation: 881

I'm pretty sure that the reported transer rate doen't include all the headers and overhead of the different layers in the protocal stack, since the reported thruput usually comes from some user-space application which would only get the actual data from the network stream object. It would need to do additional work to find out about all the headers and frames and other overhead that occurred in the different layers and affected the actual physical transmission.

Upvotes: -1

Erobrere
Erobrere

Reputation: 388

A typical full-size TCP/IPv4 packet on the Internet is of size 1500B (maximum transmission unit (MTU)), of which (minimum) 20B are of TCP header and (minimum) 20B are of IPv4. This MTU was chosen to be compatible with Ethernet. Furthermore, there are application headers (e.g., HTTP for web, SIP/RTP/RTCP for voice call, etc.) included in this packet. The minimum MTU is 576B for IPv4 and 1280B for IPv6. One can see MTU on Linux with ifconfig command.

The best way to figure these values is by using a pcap tool/network analyzer such as Wireshark. Also refer to wiki pages or a good networking book for headers and fields of the protocols.

Upvotes: 1

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