Comcast download speed fix for Linksys eSeries

From DD-WRT Wiki

Revision as of 17:05, 4 January 2014 by JNavas (Talk | contribs)
Jump to: navigation, search

Contents

Introduction

[Culled from DD-WRT Forum thread Critical DSCP bug Affecting WiFi Download Speeds on Comcast]

Unlike many other service providers, IPv4 packets from the Comcast network are configured with DSCP 8, Priority higher than Default/Routine (0).

There is an edge case interaction between this Comcast DSCP value and improper condition handling in the WMM driver in certain Linksys/Cisco eSeries routers (and possibly others as well) which results in very low download speeds. This issue can appear in both stock and third-party (DD-WRT) firmware, although Cisco appears to have at some point released new WMM drivers that largely resolve the issue in current stock firmware.

Symptoms

Download speeds are normal with current stock firmware, and on wired connections with DD-WRT firmware, but are very low (less than 1 Mbps) on wireless connections with DD-WRT firmware.


Because DSCP pertains to QoS, it's possible this problem might only exist when QoS is enabled (and go away when QoS is disabled), but this has not been confirmed.

Testing

To see if you are getting the bad DSCP, take a look at your downloaded IPv4 packets with Wireshark, preferably directly connected to WAN, but you should see them even on Wi-Fi. (See Add DSCP Column in Wireshark)

If you see the problem, open a Telnet or SSH connection to DD-WRT, and run the following commands:

insmod xt_DSCP.ko
iptables -t mangle -A PREROUTING -i vlan2 -j DSCP --set-dscp 0

Assuming there are no errors, check download speed on wireless.

You may need to change vlan2 depending on your particular setup.

Configuration

To save the fix so it survives reboots, first navigate to Administration > Commands

In Command Shell, enter Commands:

insmod xt_DSCP.ko

and click Save Startup.

In Command Shell, enter Commands:

iptables -t mangle -A PREROUTING -i vlan2 -j DSCP --set-dscp 0

and click Save Firewall.

Methodology

This fix corrects the bad DSCP header information as soon as a packet hits the WAN interface on a router.

DSCP as received from Comcast's network is 0x08. This is the lowest priority possible, which WMM interprets as being unimportant to transmit quickly.

The fix changes DSCP on all packets to 0x00, which essentially means unclassified, which WMM handles properly. This is how virtually every other ISP uses DSCP and is why the issues is specific to Comcast.

References