Seeing Dropped Packets / Tcp Retrans On Latest 4.4.1-10el6

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Hi All,

Was troubleshooting some odd VM network issues and discovered that we’re seeing dropped packets + retransmissions across multiple domU OS’s and dom0 hardware platforms.

xendev01 ~ # tshark -R “tcp.analysis.retransmission ” -i vif7.0

Running as user “root” and group “root”. This could be dangerous.

Capturing on vif7.0

3.054257 xxx.xxx.xxx.196 -> xxx.xxx.xxx.145 SSH 110 [TCP Fast Retransmission] Encrypted response packet len=44

3.061949 xxx.xxx.xxx.196 -> xxx.xxx.xxx.145 SSH 1434 [TCP Fast Retransmission] Encrypted response packet len=1368

3.383880 xxx.xxx.xxx.196 -> xxx.xxx.xxx.145 SSH 1434 [TCP Fast Retransmission] Encrypted response packet len=1368

3.630911 xxx.xxx.xxx.196 -> xxx.xxx.xxx.145 SSH 1434 [TCP Fast Retransmission] Encrypted response packet len=1368

3.635964 xxx.xxx.xxx.196 -> xxx.xxx.xxx.145 SSH 1434 [TCP Fast Retransmission] Encrypted response packet len=1368

I’ve confirmed this is happening with linux, windows and pfsense (bsd) domU’s. I’ve turned off every feature I can with ethtool on both the underlying bridge on the host, the vif’s, and the eth’s inside the domU’s. I also see it on traffic inbetween vms on the same host.

The domU sees packet errors on incoming traffic and outgoing looks fine, dumping on the dom0 indicates incoming packets are fine, but the reply from the domU is broken. This does not happen running the exact same VMs on some older xen 4.1.3 hosts. Reproduction is easy (for me at least), any burst of traffic will do it. I’ve just been running “ps auxf” over SSH to a vm to trigger.

Since I’m seeing it on the host when I sniff the vif, this feels like a bug?

– Nathan

4 thoughts on - Seeing Dropped Packets / Tcp Retrans On Latest 4.4.1-10el6

  • Hi All,

    Some more data on this, I’ve reproduced this on another host that’s a completely stock CentOS/xen deployment with a CentOS 6.6 domU.

    Since I’m seeing the retransmissions on the VIF, I don’t think it’s related to the network stack but just in case.. Each host is connected via LACP with vlan tagging to a pair of stacked cisco 3750’s. Host networking config is here:

    http://dpaste.com/1Q6NY3Y

    The vm is on br99 here.

    This is easily reproducable by just generating a 250mb random file and doing an scp, while watching with tshark:

    tshark -R “tcp.analysis.retransmission”

    There’s no visible impact to the connection the vast majority of the time, which is why I think this has gone unnoticed.

    Just to confirm this wasn’t related to hardware / nics, I’ve reproduced this on:

    – Dell PowerEdge M620 with broadcom nics
    – Dell C6220 with intel nics
    – Supermicro X8DTT with intel nics

    Any ideas? =)

    – Nathan

  • So I might have been misinterpreting things here and might be way off base. I think you can ignore this thread and I’ll follow up if I get anything concrete down the road =) The retranmissions I’m seeing and reproducing are probably within normal allowances and can’t reproduce the issue that originally lead me down this path.

    – Nathan

  • Hi All,

    I’ve tracked this down… We do rate limiting of our vms with a mix of ebtables/tc.

    Running these commands (replace vif1.0 with the correct vif for your VM) will reproduce this:

    ebtables -A FORWARD -i vif1.0 -j mark –set-mark 990 –mark-target CONTINUE

    tc qdisc add dev bond0 root handle 1: htb default 2
    tc class add dev bond0 parent 1: classid 1:0 htb rate 10000mbit

    tc class add dev bond0 parent 1: classid 1:990 htb rate 10000mbit tc filter add dev bond0 protocol ip parent 1:0 prio 990 handle 990 fw flowid 1:990

    Note that the speed limits being applied here are 10gb and I’m testing this on a 1gb network, so TC shouldn’t really be doing anything here except letting the packets through. These same commands worked fine on gentoo xen 4.1 / kernel 3.2.57, compared to this now not working on CentOS xen 4.4.1 / kernel 3.10.68.

    Easiest way to reproduce is simply generate a large file, scp it to a remote host and on the remote host run:
    tshark -Y “tcp.analysis.duplicate_ack_num”

    If you run the SSH in a loop + tshark in another window, you can see the Dup ACK’s begin immediately after adding the last filter rule:

    25790294 1752.756733 xxx.xxx.xxx.13 -> xxx.xxx.xxx.205 TCP 78 [TCP Dup ACK 25790286#4] SSH > 51515 [ACK] Seq994 AckP769840 Win44704 Len=0 TSvals8150929 TSecrB94944346 SLEP785768 SREP790596
    25790296 1752.756742 xxx.xxx.xxx.13 -> xxx.xxx.xxx.205 TCP 78 [TCP Dup ACK 25790286#5] SSH > 51515 [ACK] Seq994 AckP769840 Win44704 Len=0 TSvals8150929 TSecrB94944346 SLEP785768 SREP792044

    – Nathan

  • [cc’ing xen-users to see if anyone there has any familiarity with ebtables]

    So just to be clear, we have 3 variables to consider here?

    * gentoo -> CentOS
    * Xen 4.1 -> Xen 4.4
    * kernel 3.2.57 -> 3.10.68

    Unfortunately I’m not very familiar with ebtables, so I don’t have a clear idea what sort of thing might cause duplicate ACKs.

    Are you able to narrow down any of those?

    You can find CentOS packages for Xen 4.2 and kernel 3.4 here:

    http://vault.CentOS.org/6.4/xen4/x86_64/Packages/

    If you could build a more recent kernel and see if it’s been fixed, that might be helpful as well.

    -George

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