Janitorial Tasks to clean up the bufferbloat

There are a ton of tiny code improvements that can be made across the tens of thousands of applications “out there” to reduce their bufferbloat, far more than any one person can do. But it is extremely simple for someone to pick up a given codebase, make a small, basically mechanical change, test it, and submit back to mainline. It had generally been my hope that this would happen on everything - or that many, many folk, would “scratch their itch” to fix something right in front of them. A worldwide bufferbloat-reducing hackathon, with 1000 people participating, would knock out a goodly chunk of the problem in a day!

A daydream was to see an university class (or dozens) tackle a dozen applications with bufferbloat-fighting measures like the below, do them, and measure before and after.

BQL has generally been shown to be a win on every driver it has been implemented on. It is a very few lines of code to add, however that code requires having the device in front of you and rigorous testing, thus BQL support entering the kernel has been fairly slow, with only a few dozen BQL enabled drivers out of the hundreds of devices “out there”. Scratching this itch is a good introduction to kernel programming, and a net win for fighting bufferbloat, and thus folk doing a teeny bit of work on this here and there will gradually make a difference (especially by fixing the driver(s) you are actually using in front of you).

TCP_NOTSENT_LOWAT on interactive tcp applications

The TCP_NOTSENT_LOWAT option has been shown to improve interactivity on quite a few application types, notably screen sharing and interactive web traffic

It is a matter of a few minutes coding to add to various apps and services that could use it, but more than a few minutes to see if it is useful or not.

It is not a standard socket option as yet in Linux and only recently exported in OSX. A good setting is in the range 16-128k.

It can also be set globally to a good figure for all applications. This is useful on desktops and servers that have few context switch related issues:

And: See #450

TCP Congestion control selection

Most applications do not allow for setting what congestion control algorithm is used. Being able to select a lower priority, delay based CC, is of benefit to some apps - for example, file uploads to flickr or facebook. You can enable a different congestion control algorithm via:

    char  cong_have[16];
    int my_len = 15;
    const char cong_control[] = "cdg"; // others might be reno, vegas,westwood, dctcp, cubic, etc
    setsockopt(socket, protocol, TCP_CONGESTION, cong_control, strlen(cong_control));
    if(getsockopt(socket, protocol, TCP_CONGESTION, cong_have, &my_len) ==
           SOCKET_ERROR) { perror("can't get cong control"); } 
    if (strncmp(cong_control,cong_have,strlen(cong_control)) != 0) {
           perror("can't set desired %s congestion control algorithm",cong_control);

Most OSes do not have a wide variety of TCP congestion control algorithms available, so you should also check for success or failure as per the above. Under linux you can see and set the available and enabled congestion control algorithms via:

cat /proc/sys/net/ipv4/tcp_available_congestion_control 
cat /proc/sys/net/ipv4/tcp_allowed_congestion_control 

Some may need to be modprobed before being able to use. Usually that is modprobe tcp_X where X= the congestion control algorithm desired.

Correct classification

The internet is rife with applications that actually do try to apply the diffserv field, but either select something out of the old style TOS bits, or do not work correctly with IPv6 (using IP_TOS rather than IPV6_TCLASS).

Any place where you see IP_TOS being set, and not IPV6_TCLASS you should put in a:

#ifdef IPV6_TCLASS
    const int dscp = 0xc0 // CS6 network control in this example, CS1 is background...
    rc = setsockopt(s, IPPROTO_IPV6, IPV6_TCLASS, &dscp, sizeof(dscp));

It is OK if this call is applied to a IPv4 socket (and fails) - and dscp should be an int, rather than a char, as in IP_TOS.

Some OSes allow you to use ECN (2) on udp packets, which should be used very carefully, as in mosh. Extracting the received IP headers appropriately to deal with ECT is beyond the current scope of this document.


TCP fast open is a socket option that allows for sending data in the initial syn portion of a TCP transaction. It requires that both servers and clients support it properly - notably that the connection’s intent be idempotent.

ECN generation and awareness

Over 54% of the alexa top 1m (and all modern linux distros) will enable ECN if asked for.

Apple is enabling ECN support universally across iOS and OSX in the hope that this will also drive demand and deployment of network queueing algorithms that will mark, rather than drop, packets.

ECN can be easily enabled for many OSes.

But without a qdisc on the bottleneck links that respect it, turning it on on the tcps has little effect.

fq_codel enables ECN by default, but this is presently turned off in openwrt’s qos-scripts and in some circumstances in the sqm case, and off by default in pie and red. Particularly on higher bandwidth links, we are reasonably confident that ecn marking behaviors are sane in fq_codel, pie, and red, (but not codel by itself as presently implemented). Cake does ecn marking by default with good overflow protection and is also on by default. As more ecn rolls out, we expect to have to improve ECN behaviors across all queue algorithms.

ECN has great applicability in DCTCP environments, for which a new CE_THRESHOLD option just landed for codel and fq_codel.

TCP Pacing where appropriate

TCP pacing has become a very attractive option, especially in conditions where the new sch_fq qdisc can be applied on an outward facing server.

When the application has a fixed maximum rate, applications can override the (pretty reasonable) defaults to get better behavior.

u32 val = 1000000;
setsockopt(sockfd, SOL_SOCKET, SO_MAX_PACING_RATE, &val, sizeof(val));

Moving Quic along

A great deal of google generated traffic has moved away from tcp to the quic protocol. Standardization efforts are being started in the IETF. If you are running chrome, Quic is enabled by default in many circumstances, and can be enabled easily if not already enabled

A quic server handles thousands (millions ?) of flows. Having one kernel socket per flow would be way too expensive, so having sch_fq here does not help as much as you might want it to, but it and sch_fq or fq_codel remain a good combination. Also the rx path in UDP is not optimized for 4-tuple hashing.

Quic uses an initial burst of 10 packets (IW10) followed by 22 paced packets in it’s initial configuration, all managed from userspace, no sch_fq needed. While this may seem large, quic also tends to use less distinct flows overall than http users use.

Internally to the server application there are 2 hash tables, one lookup on destination_IP:destination_port, and one on *:destination_port. For a quic server, all sockets would share same keys.

Public work on quic is in the chrome web browser codebase and public work on a library, client and server is taking place on github

Fq_codel or cake on edge routers

We’ve spent tons of time on trying to get smart queue management right - most recently defeated by GRO offloads in new routers, which is only fixed in sch_tbf and cake, as yet.

To edit this page, submit a pull request to the Github repository.
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Bufferbloat Related Projects

OpenWrt Project
Congestion Control Blog
Flent Network Test Suite
The Cake shaper
CeroWrt (where it all started)

Network Performance Related Resources

Jim Gettys' Blog - The chairman of the Fjord
Toke's Blog - Karlstad University's work on bloat
Voip Users Conference - Weekly Videoconference mostly about voip
Candelatech - A wifi testing company that "gets it".