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Critical DSCP bug Affecting WiFi Download Speeds on Comcast Cisco E-Series and Others.

Discussion in 'DD-WRT Firmware' started by lightsword, Sep 10, 2013.

  1. lightsword

    lightsword Serious Server Member

    There is a critical bug that is likely crippling thousands of routers and pretty much only comcast has this issue. It is an edge case interaction caused by bad DSCP information coming from Comcast and poor condition handling by the WMM driver on certain linksys/cisco eseries routers and possibly others as well. This issue can appear on both stock and modded routers. Comcasts network is misconfigured and has been for years and has likely caused many people to needlessly replace fully functional routers, IPv6 speeds always worked for me because DSCP was configured correctly for that but it is incorrect for IPv4. Since the only packets with this bad information are the ones that are being downloaded upload is not affected nearly as badly.

    This is the fix I made and used on an e2000 and e3000 running shibby 112 although I suspect it would work on dd-wrt with minor modification:

    put this in init scripts section:
    insmod xt_DSCP.ko

    and this in wan up scripts:
    iptables -t mangle -A PREROUTING -i vlan2 -j DSCP —set-dscp 0

    these may need to be modified slightly depending on configuration, replace vlan2 with your wan interface and xt_DSCP.ko with the version for your router if it is different.

    This 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 interpretes as being unimportant to transmit quickly.

    The scripts listed changes the 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.

    I have confirmed this issue on connections in both Chicago and Boulder, CO and it is likely to be present on Comcast's entire network.

    Cisco appears to have at some point released new WMM drivers for stock firmware that largely resolve the issue on current stock firmware, otherwise implementing my temp fix as a GUI option would probably be a good idea for the affected routers. The ability to assign DSCP to all traffic on chosen interfaces also might be a good thing to add anyways.

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