Using netstat and dropwatch to observe packet loss on Linux servers

Anyone that is running a modern Operating System is most likely utilizing TCP/IP to send and receive data. Modern TCP/IP stacks are somewhat complex and have a slew of tunables to control their behavior. The choice of when and when not to tune is not always super clear cut, since documentation and the advice of various network “experts” doesn’t always jive.

When I’m looking into performance problems that are network related one of the first things I review is the netstat “-s” output:

$ netstat -s

Ip:
    25030820 total packets received
    269 with invalid addresses
    0 forwarded
    0 incoming packets discarded
    21629075 incoming packets delivered
    21110503 requests sent out
Icmp:
    12814 ICMP messages received
    0 input ICMP message failed.
    ICMP input histogram:
        destination unreachable: 2
        echo requests: 12809
        echo replies: 3
    12834 ICMP messages sent
    0 ICMP messages failed
    ICMP output histogram:
        destination unreachable: 22
        echo request: 3
        echo replies: 12809
IcmpMsg:
        InType0: 3
        InType3: 2
        InType8: 12809
        OutType0: 12809
        OutType3: 22
        OutType8: 3
Tcp:
    138062 active connections openings
    1440632 passive connection openings
    7 failed connection attempts
    2262 connection resets received
    8 connections established
    12225207 segments received
    10785279 segments send out
    10269 segments retransmited
    0 bad segments received.
    69115 resets sent
Udp:
    553643 packets received
    22 packets to unknown port received.
    0 packet receive errors
    6911684 packets sent
UdpLite:
TcpExt:
    33773 invalid SYN cookies received
    154132 TCP sockets finished time wait in fast timer
    6 time wait sockets recycled by time stamp
    72284 delayed acks sent
    3 delayed acks further delayed because of locked socket
    Quick ack mode was activated 269 times
    3359 packets directly queued to recvmsg prequeue.
    2592713 packets directly received from backlog
    4021 packets directly received from prequeue
    3557638 packets header predicted
    1732 packets header predicted and directly queued to user
    1939991 acknowledgments not containing data received
    3179859 predicted acknowledgments
    1631 times recovered from packet loss due to SACK data
    Detected reordering 1034 times using FACK
    Detected reordering 1007 times using SACK
    Detected reordering 622 times using time stamp
    1557 congestion windows fully recovered
    4236 congestion windows partially recovered using Hoe heuristic
    299 congestion windows recovered after partial ack
    2 TCP data loss events
    5 timeouts after SACK recovery
    5 timeouts in loss state
    2511 fast retransmits
    2025 forward retransmits
    88 retransmits in slow start
    5518 other TCP timeouts
    295 DSACKs sent for old packets
    35 DSACKs sent for out of order packets
    251 DSACKs received
    25247 connections reset due to unexpected data
    2248 connections reset due to early user close
    6 connections aborted due to timeout
    TCPSACKDiscard: 2707
    TCPDSACKIgnoredOld: 65
    TCPDSACKIgnoredNoUndo: 12
    TCPSackShifted: 4176
    TCPSackMerged: 2301
    TCPSackShiftFallback: 98834
IpExt:
    InMcastPkts: 2
    OutMcastPkts: 3390453
    InBcastPkts: 8837402
    InOctets: 5156017179
    OutOctets: 2509510134
    InMcastOctets: 80
    OutMcastOctets: 135618120
    InBcastOctets: 2127986990

The netstat output contains a slew of data you can be used to see how much data your host is processing, if it’s accepting and processing data efficiently and if the buffers that link the various layers (Ethernet -> IP -> TCP -> APP) are working optimally.

When I build new Linux machines via kickstart, I make sure my profile contains the ktune package. That is all the tuning I do to start, unless an application or database requires a specific setting (think large pages and SysV IPC settings for Oracle).

Once I’ve met with an application resource and a business analyst, I like to pound the application with a representative benchmark and compare the system performance before and after the stress test was run. By comparing the before and after results I can see where exactly the system is choking (this is very rare), or if the application needs to be modified to accommodate additional load. If the application is a standard TCP/IP based application that utilizes HTTP, I’ll typically turn to siege and iPerf to stress my applications and systems.

If during load-testing I notice that data is being dropped in one or more queues, I’ll fire up dropwatch to observe where in the TCP/IP stack data is being dropped:

$ dropwatch -l kas

Initalizing kallsyms db
dropwatch> start
Enabling monitoring...
Kernel monitoring activated.
Issue Ctrl-C to stop monitoring
1 drops at netlink_sendskb+14d (0xffffffff813df30e)
1 drops at ip_rcv_finish+32e (0xffffffff813f0c93)
4 drops at ip_local_deliver+291 (0xffffffff813f12d7)
64 drops at unix_stream_recvmsg+44a (0xffffffff81440fb9)
32 drops at ip_local_deliver+291 (0xffffffff813f12d7)
23 drops at unix_stream_recvmsg+44a (0xffffffff81440fb9)
1 drops at ip_rcv_finish+32e (0xffffffff813f0c93)
4 drops at .brk.dmi_alloc+1e60bd47 (0xffffffffa045fd47)
2 drops at skb_queue_purge+60 (0xffffffff813b6542)
64 drops at unix_stream_recvmsg+44a (0xffffffff81440fb9)

This allows you to see if data is being dropped at the link layer, the IP layer, the UDP/TCP layer or the application layer. If the drops are occurring somewhere in TCP/IP (i.e. inside the kernel) I will review the kernel documentation and source code to see what occurs at the specific areas of the kernel listed in the dropwatch output, and find the sysctl values that control the sizes of the buffers at that layer (some are dynamic, some are fixed).

Tuning applications to perform optimally has filled dozens and dozens of books, and it’s a fine art that you learn from seeing problems erupt in the field. It also helps to know how to intepret all the values in the netstat output, and I cannot recommend TCP/IP volume I, TCP/IP volume II and TCP/IP volume III enough! Everyone who runs an IP connected system should be required to read these before they are allowed access to the system. :)

5 Comments

Using netstat and dropwatch to observe packet loss on Linux servers | Linux Affinity  on July 11th, 2011

[...] more: Using netstat and dropwatch to observe packet loss on Linux servers Posted in: Server ADD [...]

Paul  on July 12th, 2011

Did you even notice that you linked to the second edition of TCP/IP Illustrated volume I, which will be released this October?

It will be interesting to read an updated version of this great book.

matty  on July 12th, 2011

@Paul — sure did. I have the first edition of TCP/IP illustrated I and you can bet I will be trading up to the second edition! I did this with UNIX network programming when it was updated and couldn’t be happier. Richard Stevens is a legend and is very much missed!

kayjay  on July 12th, 2011

Hi Matty,
Unrelated, but do you have a twitter I can follow ?

./kj

Rick  on July 22nd, 2011

Hi,

I’ve got quite a lot of dropped UDP packets with packet receive errors under the UDP section of netstat -su. I believe they are dropping at the kernel. I’ve tried increasing all the sysctl parameters that several websites have mentioneed. Using your tool, I see “4 drops at udp_queue_rcv_skb+229″

Do you know explicitly how to raise the receive queue size on the kernel?

Thanks,
Rick

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