3 ms·

Well exposing it per application kinda defeats a bit of the purpose behind it. The point is that you get rid of the buffer so you get packet loss when the link

Well exposing it per application kinda defeats a bit of the purpose behind it. The point is that you get rid of the buffer so you get packet loss when the link to the other peer(s) is(are) saturated and allow the TCP congestion algorithms to work properly. Even in a high latency high bandwidth situation having huge buffers is self-defeating. Anybody that has used Bittorrent and assumed their ISP was doing some sort of throttling were you have weird latency spikes and reduced performance... this is a direct result of buffer bloat in the majority of cases. Large buffers only help in the case of a single TCP connection using up as much as the bandwidth as possible, which helps network routers and such things look good in benchmarks. What you are talking about is some sort of Quality of Service type thing. Probably most usefully expressed at the edge of networks for prioritizing traffic and at the ISP level so they can route traffic through different internet links based on requirements. ISPs have to deal with choosing different links to other networks and what it costs. They can do choose stuff like use main backbones versus secondary links and you get different trade-offs based cost versus latency and things of that nature. So ideally there should be some sort of flag you can set in the TCP packet that would indicate latency importance or some such thing. Or just making internet routing equipment application-protocol-aware.

Peaker 14y ago
By "expose it to the application", I mean in the flow-control sense. That send() (or select/epoll/etc) will block until there is room in the TCP window, rather than in the pre-determined buffer which will always be too small or too big. The way it works now, the kernel is basically forcing applications to have buffer-bloat (fill a 0.5MB socket buffer), or auto-detect RTT or manually select a proper buffer size. All are bad options. Also, in high-latency-high-bandwidth situations, the default 0.5MB buffer will simply fail to make use of the available bandwidth, so increasing the buffer size does not defeat the purpose. Latency spikes are a different situation, there are cases of constant high latency (e.g: inter-continental 1Gbps links).
- natermer 14y ago
  > Also, in high-latency-high-bandwidth situations, the default 0.5MB buffer will simply fail to make use of the available bandwidth, so increasing the buffer size does not defeat the purpose. To find out how much buffer is required you multiply the latency it takes to process the packet by the maximum rate. Say it takes 0.2 msecs to send a packet out on the network then a 1Gbps connection would need a buffer of 0.03 MB or so. So that 0.5MB should be sufficient to saturate a 10Gbps link. It seems that if you are trying to avoid blocking in a application when writing to a socket you should take a different strategy. > Latency spikes are a different situation, there are cases of constant high latency (e.g: inter-continental 1Gbps links). Latency spikes in terms of this discussion can be caused by situations were you have a low speed network connected to a high speed network with a large buffer between them. In this situation the server (or whatever) will write to the buffer on the bridge/router at a maximum amount rate allowed by the network. What should happen is that the router should block causing packet loss which will cause the TCP congestion algorithms to kick in and slow down the rate at which the packets are sent. What actually happens is that the massive buffers fill up and when you have multiple connections competing for buffer space you will all of a sudden hit a brick wall. So what comes up with high latency spikes, weird packet loss patterns, and such things. Small buffers means high performance...
halayli 14y ago
> So ideally there should be some sort of flag you can set in the TCP packet that would indicate latency importance or some such thing. Such field exist in the IP header and is called TOS (Type of Service).
noselasd 14y ago
Regarding bittorrent, I'm wondering if that's not just as much the fault of crappy NAT modems. Around here everyone gets a Zyxel, and bittorrent totally chokes it periodically. The hash table used for NAT grows, the CPU can't handle it, to the point you can't even log in to the modem, and new TCP sessions get dropped until the NAT clears out the old TCP connections. (With a proper network device, this shouldn't be an issue, but it's one I see all the time with the cheap modems we get from the ISPs)

Peaker 14y ago

By "expose it to the application", I mean in the flow-control sense. That send() (or select/epoll/etc) will block until there is room in the TCP window, rather than in the pre-determined buffer which will always be too small or too big. The way it works now, the kernel is basically forcing applications to have buffer-bloat (fill a 0.5MB socket buffer), or auto-detect RTT or manually select a proper buffer size. All are bad options. Also, in high-latency-high-bandwidth situations, the default 0.5MB buffer will simply fail to make use of the available bandwidth, so increasing the buffer size does not defeat the purpose. Latency spikes are a different situation, there are cases of constant high latency (e.g: inter-continental 1Gbps links).

natermer 14y ago
> Also, in high-latency-high-bandwidth situations, the default 0.5MB buffer will simply fail to make use of the available bandwidth, so increasing the buffer size does not defeat the purpose. To find out how much buffer is required you multiply the latency it takes to process the packet by the maximum rate. Say it takes 0.2 msecs to send a packet out on the network then a 1Gbps connection would need a buffer of 0.03 MB or so. So that 0.5MB should be sufficient to saturate a 10Gbps link. It seems that if you are trying to avoid blocking in a application when writing to a socket you should take a different strategy. > Latency spikes are a different situation, there are cases of constant high latency (e.g: inter-continental 1Gbps links). Latency spikes in terms of this discussion can be caused by situations were you have a low speed network connected to a high speed network with a large buffer between them. In this situation the server (or whatever) will write to the buffer on the bridge/router at a maximum amount rate allowed by the network. What should happen is that the router should block causing packet loss which will cause the TCP congestion algorithms to kick in and slow down the rate at which the packets are sent. What actually happens is that the massive buffers fill up and when you have multiple connections competing for buffer space you will all of a sudden hit a brick wall. So what comes up with high latency spikes, weird packet loss patterns, and such things. Small buffers means high performance...

halayli 14y ago

> So ideally there should be some sort of flag you can set in the TCP packet that would indicate latency importance or some such thing. Such field exist in the IP header and is called TOS (Type of Service).

noselasd 14y ago

Regarding bittorrent, I'm wondering if that's not just as much the fault of crappy NAT modems. Around here everyone gets a Zyxel, and bittorrent totally chokes it periodically. The hash table used for NAT grows, the CPU can't handle it, to the point you can't even log in to the modem, and new TCP sessions get dropped until the NAT clears out the old TCP connections. (With a proper network device, this shouldn't be an issue, but it's one I see all the time with the cheap modems we get from the ISPs)