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  1. #1
    Join Date
    Mar 2003
    London, UK

    Degraded HTTP performance on particular IPs

    I have a Xen VPS with running Ubuntu Linux 8.04 on a 2.6.18 x86_64 kernel. Apache 2.2.8 is serving a large test file. The VPS does not have load issues.

    The measured throughput of wget from different IPs is as follows:

    Peered ~70Mbps
    ADSL IP .17, Ubuntu 9.04: ~0.5Mbps (starts/stops a lot) Vista: ~0.5Mbps (starts/stops a lot)
    ADSL IP .18, Ubuntu 9.04: 4.7Mbps (uplink max)
    ADSL IP .19, Ubuntu 9.04: ~0.5Mbps (starts/stops a lot)
    ADSL IP .20, Voyage Linux: 4.7Mbps (uplink max)
    ADSL IP .21, Mac OS X 10.5: 4.7Mbps (uplink max)

    Each of the ADSL IPs is on the same subnet and routed through the same switch and modem, assigned to different client machines. The two clients experiencing poor performance have a Tigon3 and Intel PRO/1000 NIC respectively. I tried an on-board RTL NIC in one as well, to no avail. Both clients have no problem receiving 4.7Mbps from any other peered mirrors (or > 250Mbps from LAN network mounts) and I have confirmed that all IPs use the same inbound and outbound routes to the VPS.

    Here's the kicker: if I swap the IPs of any pair of good/bad throughput clients, by reconfiguring the LAN DHCP server, their throughputs swap as well. Neither the switch nor the modem runs a firewall and I temporarily disabled the VPS's firewall to no effect. Throughputs to all other tested sites remains high from any IP.

    What on earth could be going on?

    Edit: To be clear, all of the IPs are publicly routable. There is no NAT configuration in place.
    Last edited by Jay Cornwall; 08-28-2009 at 07:10 PM.

  2. #2
    Join Date
    Aug 2000
    Have you tried running a tcpdump to see what could be going on?

  3. #3
    Join Date
    Mar 2003
    London, UK
    A packet capture does reveal something of interest. Here are two captures from Wireshark on the same machine, each running wget from a different IP (but to the same server).

    packetcap1.txt (bad IP)
    packetcap2.txt (good IP)

    If I am reading it correctly there is substantial packet loss on the first trace, causing many retransmissions and degrading the connection. The second trace is flawless.

    I've provided these traces to my ISP to see if they can fathom a cause for this, but it seems bizarre to me.

  4. #4
    Join Date
    Aug 2000
    Have you also asked John @ Clustered to see if he can shed some light on it? maybe get ask him to perform some tests going the other way from the node to the IP(s) having a problem?

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