4 ms·

Normally, an ethernet or wifi restricts an MTU of 1500. I use ping packets accordingly

dogma1138 11y ago
1500 is the MTU of Ethernet, it's often not sustainable on end to end connection (especially when you add frame overheads) if you are using that high of a payload size you'll get considerably worse performance than say limiting it to around 500 bytes, you are welcomed to try it. Also with how global traffic is managed smaller packets tend to get priority since they can be qued quicker backbone connectivity uses much bigger frames than Ethernet so more often than not generating more smaller packets would increase your overall throughput (to a limit) unless you are on a very controlled network.
- theptip 11y ago
  What do you mean by "it's often not sustainable"? Throughput on a server is higher at higher packet size, so if you're doing a download I'd expect the server to send 1500 byte packets. It's pretty easy to saturate a link with 1500 byte packets, and it's much harder to do so at lower packet sizes (from the server's perspective) since the per-packet processing costs start to dominate over the per-byte costs. Admittedly my knowledge of this sort of stuff is mostly intra-DC; is there some other factor that you're referring to that supersedes this on the web? I'm not aware of prioritizing smaller packets on the backbone, sounds like something that would be targeted at small flows (i.e. first N packets in a flow get a priority bump)? More info on that would be appreciated.
  - Ded7xSEoPKYNsDd 11y ago
    I don't know about cable, but DSL usually uses PPPoE. It has on overhead of 8 bytes, lowering the MTU of those connections to 1492 bytes.
    - dogma1138 11y ago
      It doesn't matter the MTU setting on your end for WAN and ISP / interlink grade networks is meaningless they don't use Ethernet, FDDI frame size is 4500 (ATM is about double that) bytes (minus what ever overhead, but usually 4200 and change) ISP/WAN routers don't care about how many mbit/s they transfer but how many packets they route at per given unit of time, as packets get packed into a single frame the smaller the packet the more packets they can transfer each frame. Also from a more high level point of view if you think about it the small packets are the most critical ones and they are at least as far as responsiveness goes DNS is limited to 512bytes over UDP, TCP 3-way handshake packets are tiny and those are the packets that need to get to and back from their destination as fast as possible, delays in data transfers means slower speeds, delays of handshakes mean that your application can fail or hang. Other important traffic such as VOIP[0] also uses very small packet sizes for this same reason most critical services need to transfer very little data (per given unit of time) but need to update data as frequently as possible to provide the illusion of real time and to mask the latency, same goes for other things like online/multi-player gaming and so on and on and on. Pretty much if you want your service to be as responsive as possible limit your packet size to the smallest size possible and increase your PPS this will ensure that your packets get quicker to their destination. [0]VOIP Packet Sizes http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/7934-bwidth-consume.html http://www.cisco.com/c/en/us/support/docs/voice/voice-qualit... The only time you would want to use large packets is pretty much when you can have a buffer, this means that you need to handle less packets per second which lowers CPU consumption (across the entire path) so video streaming and such can use pretty much as large of an MTU as they want unless they start getting fragments.
      - simoncion 11y ago
        > It doesn't matter the MTU setting on your end for WAN and ISP / interlink grade networks is meaningless [as] they don't use Ethernet... It absolutely does matter and is quite meaningful. :) If you set your edge router's Internet-facing MTU to 9k, and the upstream equipment's MTU is smaller than that, then either your packets will be dropped, or PTMU Discovery will try to figure out the MTU of the path. (Better hope everyone along the path is correctly handling ICMP! :) ) > The only time you would want to use large packets is pretty much when you can have a buffer... Or if you have high-volumes of data to move and want to dramatically increase the data:Ethernet_frame_boilerplate ratio. :) > Also ... if you think about it the small packets are the most critical ones... [because they need to be dispatched as quickly as possible.] Yes, but a larger MTU shouldn't affect this. Set whatever socket options are required to get those packets on their way as soon as they're created, and your system shouldn't wait to fill an Ethernet frame before sending that packet.
- vampire_dk 11y ago
  Thanks I'll look into this :)
- simoncion 11y ago
  > 1500 is the MTU of Ethernet... It's the de-facto MTU of much of The Internet. Baby Jumbo (MTU >1500 but <9k), Jumbo (MTU ~9k), and Super Jumbo (MTU substantially larger than 9k) frames exist, and are supported by many (but -sadly- not all) Ethernet devices. Edit: > Also with how global traffic is managed smaller packets tend to get priority... Do you have a reliable citation for this? I would expect that core and near-core devices would handle so much traffic, that they all would be using MTUs far higher than 1500 bytes per frame.
  - dogma1138 11y ago
    It's pretty standard QoS measure, network schedulers especially for multiplexed/aggregated networks will have a bias for small packets, you should be able to find performance statistics for various token bucket configurations that will demonstrate that.
    - simoncion 11y ago
      Do you have a cite for that? :) I know that CoDel doesn't bias for small packets; it treats all flows equally and tracks traffic on a bytes-transferred (rather than packets-transferred) basis.
- suprjami 11y ago
  Can you please clarify this comment? It sounds like you're saying Ethernet cannot maintain a line-rate transfer at the maximum MTU. But that can't be what you're saying, anyone could run an iperf/netperf or even large crafted packet transfer and prove this wrong.
suprjami 11y ago
If the layer above (i.e. IPv4) can create fragments, you can send up to the maximum payload of the L3 protocol. You can send up to a 64KiB IPv4 packet over 1500-byte Ethernet.

1500 is the MTU of Ethernet, it's often not sustainable on end to end connection (especially when you add frame overheads) if you are using that high of a payload size you'll get considerably worse performance than say limiting it to around 500 bytes, you are welcomed to try it. Also with how global traffic is managed smaller packets tend to get priority since they can be qued quicker backbone connectivity uses much bigger frames than Ethernet so more often than not generating more smaller packets would increase your overall throughput (to a limit) unless you are on a very controlled network.

theptip 11y ago
What do you mean by "it's often not sustainable"? Throughput on a server is higher at higher packet size, so if you're doing a download I'd expect the server to send 1500 byte packets. It's pretty easy to saturate a link with 1500 byte packets, and it's much harder to do so at lower packet sizes (from the server's perspective) since the per-packet processing costs start to dominate over the per-byte costs. Admittedly my knowledge of this sort of stuff is mostly intra-DC; is there some other factor that you're referring to that supersedes this on the web? I'm not aware of prioritizing smaller packets on the backbone, sounds like something that would be targeted at small flows (i.e. first N packets in a flow get a priority bump)? More info on that would be appreciated.
- Ded7xSEoPKYNsDd 11y ago
  I don't know about cable, but DSL usually uses PPPoE. It has on overhead of 8 bytes, lowering the MTU of those connections to 1492 bytes.
  - dogma1138 11y ago
    It doesn't matter the MTU setting on your end for WAN and ISP / interlink grade networks is meaningless they don't use Ethernet, FDDI frame size is 4500 (ATM is about double that) bytes (minus what ever overhead, but usually 4200 and change) ISP/WAN routers don't care about how many mbit/s they transfer but how many packets they route at per given unit of time, as packets get packed into a single frame the smaller the packet the more packets they can transfer each frame. Also from a more high level point of view if you think about it the small packets are the most critical ones and they are at least as far as responsiveness goes DNS is limited to 512bytes over UDP, TCP 3-way handshake packets are tiny and those are the packets that need to get to and back from their destination as fast as possible, delays in data transfers means slower speeds, delays of handshakes mean that your application can fail or hang. Other important traffic such as VOIP[0] also uses very small packet sizes for this same reason most critical services need to transfer very little data (per given unit of time) but need to update data as frequently as possible to provide the illusion of real time and to mask the latency, same goes for other things like online/multi-player gaming and so on and on and on. Pretty much if you want your service to be as responsive as possible limit your packet size to the smallest size possible and increase your PPS this will ensure that your packets get quicker to their destination. [0]VOIP Packet Sizes http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/7934-bwidth-consume.html http://www.cisco.com/c/en/us/support/docs/voice/voice-qualit... The only time you would want to use large packets is pretty much when you can have a buffer, this means that you need to handle less packets per second which lowers CPU consumption (across the entire path) so video streaming and such can use pretty much as large of an MTU as they want unless they start getting fragments.
    - simoncion 11y ago
      > It doesn't matter the MTU setting on your end for WAN and ISP / interlink grade networks is meaningless [as] they don't use Ethernet... It absolutely does matter and is quite meaningful. :) If you set your edge router's Internet-facing MTU to 9k, and the upstream equipment's MTU is smaller than that, then either your packets will be dropped, or PTMU Discovery will try to figure out the MTU of the path. (Better hope everyone along the path is correctly handling ICMP! :) ) > The only time you would want to use large packets is pretty much when you can have a buffer... Or if you have high-volumes of data to move and want to dramatically increase the data:Ethernet_frame_boilerplate ratio. :) > Also ... if you think about it the small packets are the most critical ones... [because they need to be dispatched as quickly as possible.] Yes, but a larger MTU shouldn't affect this. Set whatever socket options are required to get those packets on their way as soon as they're created, and your system shouldn't wait to fill an Ethernet frame before sending that packet.
vampire_dk 11y ago
Thanks I'll look into this :)
simoncion 11y ago
> 1500 is the MTU of Ethernet... It's the de-facto MTU of much of The Internet. Baby Jumbo (MTU >1500 but <9k), Jumbo (MTU ~9k), and Super Jumbo (MTU substantially larger than 9k) frames exist, and are supported by many (but -sadly- not all) Ethernet devices. Edit: > Also with how global traffic is managed smaller packets tend to get priority... Do you have a reliable citation for this? I would expect that core and near-core devices would handle so much traffic, that they all would be using MTUs far higher than 1500 bytes per frame.
- dogma1138 11y ago
  It's pretty standard QoS measure, network schedulers especially for multiplexed/aggregated networks will have a bias for small packets, you should be able to find performance statistics for various token bucket configurations that will demonstrate that.
  - simoncion 11y ago
    Do you have a cite for that? :) I know that CoDel doesn't bias for small packets; it treats all flows equally and tracks traffic on a bytes-transferred (rather than packets-transferred) basis.
suprjami 11y ago
Can you please clarify this comment? It sounds like you're saying Ethernet cannot maintain a line-rate transfer at the maximum MTU. But that can't be what you're saying, anyone could run an iperf/netperf or even large crafted packet transfer and prove this wrong.

suprjami 11y ago

If the layer above (i.e. IPv4) can create fragments, you can send up to the maximum payload of the L3 protocol. You can send up to a 64KiB IPv4 packet over 1500-byte Ethernet.