[Cake] DSCP ramblings

Kevin Darbyshire-Bryant kevin at darbyshire-bryant.me.uk
Wed Apr 22 11:58:48 EDT 2020

During these strange times of lockdown I’ve been trying to keep myself occupied/entertained/sane(???) by ‘fiddling with stuff’ and improving my coding.  This started with an idea of learning Python which was great until the on-line bit of it ran out and someone posted an idea on the Openwrt forum about graphing Cake stats.

That had nothing to do with Python and involved (new to me) technologies such as ‘collectd’, ‘JSON’, a bit of javascript and my usual level of cobbling something together in ‘ash’…. So that course was well spent :-)

Anyway, data was collected and graphs produced in a very small household.  What’s immediately apparent from those graphs and cake in ‘diffserv4’ mode is that very, very few applications are using DSCP at all.  Most things are to port 443.

I was also a little surprised to see that my DNS over foo proxies such as stubby & https-dns-proxy don’t use DSCP coding.  It surprised me even more to see RFC recommendations that DNS be treated as ‘Best Effort’.  Now in the days of udp only and no dnssec (with fallback to tcp) this may be good enough, but I wonder if this is realistic these days?

So putting aside the discussion of what codepoint should be used, I then wondered how hard it would be to actually set a dscp in these applications.  And this is where I had another surprise.  For example https-dns-proxy uses libcurl.  libcurl has no standard ‘in-library’ method for setting a socket’s dscp.  I cobbled a workaround in the application https://github.com/aarond10/https_dns_proxy/pull/83 - it works.

Next I attacked stubby, which uses getdns.  getdns doesn’t even have a callback or parameters passing so you can set a dscp on the socket from a client application, pure ‘hack the library’ stuff.

To be blunt and on a small sample of 2 libraries/applications, it seems that DSCP is completely ignored.  Applications signalling ’this is/isnt latency sensitive/bulk’ isn’t going to happen if it isn’t easy to do.

Apple should be marking facetime calls as being ‘video conference’ or whatever.  BBC iplayer Radio apps should be marking ‘audio streaming’. But every f*ing thing is CS0 port 443.  And I’m wondering how much of this is because library support is simply missing.  Maybe gaming apps are better? (I don’t game)

Right, I’m off for a lie down.  Sorry for the rant.

Hack for getdns/stubby

diff --git a/src/stub.c b/src/stub.c
index 2547d10f..7e47aba5 100644
--- a/src/stub.c
+++ b/src/stub.c
@@ -52,6 +52,7 @@
 #include "platform.h"
 #include "general.h"
 #include "pubkey-pinning.h"
+#include <netinet/ip.h>

  * STUB_TCP_RETRY added to deal with edge triggered event loops (versus
@@ -381,6 +382,9 @@ tcp_connect(getdns_upstream *upstream, getdns_transport_list_t transport)
 # else
        static const int  enable = 1;
 # endif
+#if defined(IP_TOS)
+       int dscp = IPTOS_CLASS_CS4;
        int fd = -1;

@@ -390,6 +394,12 @@ tcp_connect(getdns_upstream *upstream, getdns_transport_list_t transport)
                   __FUNC__, (void*)upstream);
        if ((fd = socket(upstream->addr.ss_family, SOCK_STREAM, IPPROTO_TCP)) == -1)
                return -1;
+#if defined(IP_TOS)
+       if (upstream->addr.ss_family == AF_INET6)
+               (void)setsockopt(fd, IPPROTO_IPV6, IP_TOS, &dscp, sizeof(dscp));
+       else if (upstream->addr.ss_family == AF_INET)
+               (void)setsockopt(fd, IPPROTO_IP, IP_TOS, &dscp, sizeof(dscp));


Kevin D-B

gpg: 012C ACB2 28C6 C53E 9775  9123 B3A2 389B 9DE2 334A

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: Message signed with OpenPGP
URL: <https://lists.bufferbloat.net/pipermail/cake/attachments/20200422/db3ef7fc/attachment-0001.sig>

More information about the Cake mailing list