Optimizing AIX 5L performance: Tuning network performance, Part 1

Network overviewMonitoring the hardware

This three-part series on AIX® networking focuses on the challenges of optimizing network performance. In the first installment, you’ll review some best practices for network configuration, and utilize those practices for efficient monitoring of your hardware by taking advantage of network tools such as netstat, netpmon, entstat, and nmon. You'll also examine certain tools used to look at your interfaces to see how to configure them.


Ken Milberg, Future Tech UNIX Consultant, Technology Writer, and Site Expert, Future Tech

Ken Milberg is a Technology Writer and Site Expert for techtarget.com and provides Linux technical information and support at searchopensource.com. He is also a writer and technical editor for IBM Systems Magazine, Open Edition. Ken holds a bachelor's degree in computer and information science and a master's degree in technology management from the University of Maryland. He is the founder and group leader of the NY Metro POWER-AIX/Linux Users Group. Through the years, he has worked for both large and small organizations and has held diverse positions from CIO to Senior AIX Engineer. Today, he works for Future Tech, a Long Island-based IBM business partner. Ken is a PMI certified Project Management Professional (PMP), an IBM Certified Advanced Technical Expert (CATE, IBM System p5 2006), and a Solaris Certified Network Administrator (SCNA). You can contact him at kmilberg@gmail.com.

20 November 2007

Also available in Chinese Russian

About this series

Part 1 of this three-part series (see Resources) on AIX® networking provides a networking overview and discusses the tools that help you monitor your hardware. Part 2 covers tuning the Network File System (NFS) with monitoring utilities, such as nfsstat and nmon, and it also goes over how to tune with nfso. Part 3 shows you how to monitor network packets and how to use netstat for this purpose. You'll learn how to tune your network subsystem using the no utility. This series also expounds on various best practices of network I/O performance tuning.


The first thing that usually comes to mind when a system administrator hears that there might be some network contention issues is to run netstat. netstat, the equivalent of using vmstat or iostat for your memory reports, is a quick and dirty way of getting an overview of how your network is configured. Unlike vmstat or iostat, the defaults usually do not give you as much information as you probably would like. You need to understand the correct usage of netstat and how best to utilize it when monitoring your system.

netstat is really not a monitoring tool in the sense of vmstat and iostat. You can use other tools more suitable (discussed later in the article) to help monitor your network subsystem. At the same time, you can't really start to monitor unless you have a thorough understanding of the various components related to network performance. These components include your network adapters, your switches and routers, and how you are using virtualization on your host logical partitions. If you determine you are indeed having a network bottleneck, fixing the problem might actually lay outside of your immediate host machine. There is little you can do if the network switch is improperly configured on the other end. Of course, you might be able to point the network team in the right direction. You should also spend time gathering overall information about your network. How are you going to be able to understand how to troubleshoot your network devices unless you really understand your network? In this article, you'll look at specific AIX network tracing tools, such as netpmon, and how they can help you isolate your bottlenecks.

Finally, no matter which subsystem you are looking to tune, you must think of systems tuning must as an ongoing process. As stated before, the best time to start monitoring your systems is at the beginning, before you have any problems and users aren't screaming. You must have a baseline of network performance so that you know what the system looks like when it is behaving normally. Finally, when making changes, be careful to make changes only one at a time so that you can really assess the impact of your change.

Network I/O overview

This section provides an overview of the network as it relates to AIX and covers the physical aspects of the network (device drives and adapters), the AIX networking stack, and how to make some changes to your adapter.

Understanding the network subsystem, as it relates to AIX, is not an easy undertaking. When examining the CPU and memory bottlenecks, there are far fewer areas that you need to examine from a hardware and software aspect. Disk I/O tuning is more complex, as there are many more issues that impact performance, particularly during the architectural and build-out of your systems. In this respect, tuning your network is probably most like tuning your disk I/O, which is actually not too surprising, as they both relate to I/O. Let's start. Figure 1 illustrates the AIX Transmission Control Protocol/Internet Protocol (TCP/IP) layers.

Figure 1. The AIX TCP/IP layers
The AIX TCP/IP layers

You can clearly see there is more to network monitoring than running netstat and looking for collisions. From the application layer through the media layer, there are areas that need to be configured, monitored, and tuned. At this point, you should notice some similarities between this illustration and the Open Systems Interconnection Basic Reference Model (OSI Model). The OSI Model has seven layers (bottom to top):

  • Physical
  • Data-link
  • Network
  • Transport
  • Session
  • Presentation
  • Application

Perhaps the most important concept to understand is that on the host machine each layer communicates with its corresponding layer on the remote machine. The actual application programs transmit data using either User Datagram Protocol (UDP) or Transmission Control Protocol (TCP) transport layer protocols. They receive the data from whatever application you are using and divide them into packets. The packets themselves differ, depending on whether it is a UDP or TCP packet. Generally speaking, UDP is faster, while TCP is more secure. There are many tunable parameters to look at—you'll get to these parameters during subsequent phases of the series. You might want to start to familiarize yourself with the no command, which is the utility designed to make the majority of your network changes. From a hardware perspective, it is critical that you understand the components that need to be configured appropriately to optimize performance. Though you might work together with the network teams that manage your switches and routers, it is unlikely that you will be configuring them, unless you are a small shop or a one-person IT department. The most important component you will be working with is your network adapter. In 2007, most of your adapters will probably be some version that supports Gigabit Ethernet, such as a 10/100/1000 Mbps Ethernet card. There are several important concepts you will need to work with here.

Maximum Transfer Unit

Maximum Transfer Unit (MTU) is defined as the largest packet that can be sent over a network. The size depends on the type of network. For example, 16-bit token ring has a default MTU size of 17914, while Fiber Distributed Data Interface (FDDI) has a default size of 4352. Ethernet has a default size of 1500 (9000 with jumbo frames enabled). Larger packets require less packet transfers, which result in higher bandwidth utilization on your system. An exception to this is if your application prefers smaller packets. If you are using a Gigabit Ethernet, you can use a jumbo frames option. To support the use of jumbo frames, it's important to note that your switch must also be configured, accordingly.

To change to jumbo frames, use this fastpath: # smit devices.

Then go to Communication>Ethernet>Adapter>Change/show characteristics of an Ethernet adapter. Try to change the transmit jumbo frames option from "No" to "Yes" (see Listing 1).

Listing 1. Characteristics of an Ethernet adapter screen
              Change / show characteristics of an Ethernet adapter

Type or select values in entry fields.
Press Enter AFTER making all desired changes.

[TOP]                                                   [Entry Fields]
  Ethernet Adapter                                    ent1
  Description                                         Virtual I/O Ethernet >
  Status                                              Available
  Enable ALTERNATE ETHERNET address                   no                     +
  ALTERNATE ETHERNET address                         [0x000000000000]        +
  Minimum Tiny Buffers                               [512]                   +#
  Maximum Tiny Buffers                               [2048]                  +#
  Minimum Small Buffers                              [512]                   +#
  Maximum Small Buffers                              [2048]                  +#
  Maximum Medium Buffers                             [128]                   +#
  Maximum Medium Buffers                             [256]                   +#
  Minimum Large Buffers                              [24]                    +#

F1=Help             F2=Refresh          F3=Cancel           F4=List
F5=Reset            F6=Command          F7=Edit             F8=Image
F9=Shell            F10=Exit            Enter=Do

Where is the jumbo frames option? In this case, you cannot make the change. The reason for this is because you are only using the Virtual I/O Ethernet on this system—this topic is discussed in more detail later. Remember, you must understand the network on the host you are administering!

Let's check this system (see Listing 2).

Listing 2. Checking the system
              Change / show characteristics of an Ethernet adapter

Type or select values in entry fields.
Press Enter AFTER making all desired changes.

                                                        [Entry Fields]
  Ethernet Adapter                                    ent1
  Description                                         10/100/1000 Base-TX P>
  Status                                              Available
  Location                                            1j-08
  RX descriptor queue size                           [1024]                  +#
  TX descriptor queue size                           [1024]                  +#
  Software transmit queue size                       [8192]                  +#
  Transmit jumbo frames                               yes                    +
  Enable hardware TX TCP resegmentation               yes                    +
  Enable hardware transmit and receive checksum       yes                    +
  Media speed                                         Auto_Negotiation       +
  Enable ALTERNATE ETHERNET address                   no                     +
  ALTERNATE ETHERNET address                         [0x000000000000]        +
  Apply change to DATABASE only                       no                     +

F1=Help             F2=Refresh          F3=Cancel           F4=List
F5=Reset            F6=Command          F7=Edit             F8=Image
F9=Shell            F10=Exit            Enter=Do

You have now changed the field to support jumbo frames.

Media speed

Adapters communicate with other devices based on how your media speed is configured. Though there are other choices, you need to configure your card for either 100_full_duplex or auto-negotiation. With auto-negotiation, both adapters attempt to communicate using the highest possible speed. Though you might find in the documentation that it should be configured this way (IBM® even defaults it this way on the system), most senior AIX administrators I know prefer to set it to full duplex, to ensure that you are receiving the fastest possible adapter speed. If it doesn't work properly, you should then work with the appropriate network teams to resolve the problems prior to deployment. I would prefer taking more time initially versus setting the adapter to a setting that might cause slower speeds as a result of poorly configured switches. The lsattr command gives you the information that you need. The en prefix displays your driver parameters, while the ent prefix displays your hardware parameters. Let's display your hardware parameters (see Listing 3).

Listing 3. Displaying the hardware parameters
testsys:/home/test>lsattr -El ent0
alt_addr        0x000000000000   Alternate Ethernet Address        True
busintr         166              Bus interrupt level               False
busmem          0xc8030000       Bus memory address                False
chksum_offload  yes              Enable RX Checksum Offload        True
intr_priority   3                Interrupt priority                False
ipsec_offload   no               IPsec Offload                     True
large_send      no               Enable TCP Large Send Offload     True
media_speed     Auto_Negotiation Media Speed                       True
poll_link       no               Enable Link Polling               True
poll_link_timer 500              Time interval for Link Polling    True
rom_mem         0xc8000000       ROM memory address                False
rx_hog          1000             RX Descriptors per RX Interrupt   True
rxbuf_pool_sz   1024             Receive Buffer Pool Size          True
rxdesc_que_sz   1024             RX Descriptor Queue Size          True
slih_hog        10               Interrupt Events per Interrupt    True
tx_preload      1520             TX Preload Value                  True
tx_que_sz       8192             Software TX Queue Size            True
txdesc_que_sz   512              TX Descriptor Queue Size          True
use_alt_addr    no               Enable Alternate Ethernet Address True

In this case, your interface is set as auto-negotiate.

You should also check your firmware levels to make sure they are up-to-date. I've seen many network problems fixed when updating to the latest levels of firmware. The lscfg command gives you the firmware information (see Listing 4).

Listing 4. Using the lscfg command for firmware information
testsys:/home/test >lscfg -vp | grep -p ROM
      10/100 Mbps Ethernet PCI Adapter II:
        Part Number.................09P5023
        FRU Number..................09P5023
        EC Level....................H10971A
        Manufacture ID..............YL1021
        Network Address.............0002556FC98B
        ROM Level.(alterable).......SCU015
        Product Specific.(Z0).......A5204207
        Device Specific.(YL)........U0.1-P1-I1/E1

      10/100/1000 Base-TX PCI-X Adapter:
        Part Number.................00P3056
        FRU Number..................00P3056
        EC Level....................H11635A
        Manufacture ID..............YL1021
        Network Address.............00096B2E31BD
        ROM Level.(alterable).......GOL002
        Device Specific.(YL)........U0.1-P1/E2

See the Resources section at the end of the article for a link to the most current release information for your adapter. In this case, you're going to find the history for the 10/100/1000 Base-TX PCI-X adapter:

  • OLxxx—This is a table place holder for future firmware revisions.
  • GOL021—This level of firmware corrects the vendor device ID on EMC Class B adapters so that the adapter is recognized during AIX Network Installation Management (NIM). Checksum from the AIX 60;sum61; command is 38603.
  • GOL012—In OF, there is a very small possibility that the adapter can hang the system when the adapter (hardware) does not function properly during transmit. The change prevents the adapter from trying to send packets forever. After trying for a predetermined time, a timer is added to timeout.
  • GOL002—If the user selected 10/auto or 100/auto and did not ping the switch first, the updated firmware resolves the problem; the open firmware would not change the settings to auto/auto before passing it to AIX. If AIX sees a parameter as 10/auto or 100/auto, it does not understand how to deal with that, and the system stops during ioconfig with a code 607. The firmware now changes any combination of 10/auto, 100/auto, auto/full, or auto/half, which are invalid selections, to auto/auto.
  • GOL001—Original (GA) Open Firmware level.

A quick glance at the history shows that you are two levels down from where you should be. You need to look for some downtime to upgrade the firmware, particularly if you've been having some intermittent network problems.

Though the series focuses on tuning in subsequent parts, you might want to start to familiarize yourself with the memory management facility of network subsystems. What you need to know at this point is that it relates to data structures called mbufs. These are used to store kernel data for incoming and outbound traffic. The buffer sizes themselves can range from 32 to 16384 bytes. They are created by making allocation requests to the Virtual Memory Manager (VMM). In an SMP box, each memory pool is split evenly for every processor . The monitoring section below shows you how to view mbufs. An important concept to note is that processors cannot borrow from the memory pool outside of its own processor.

Two other concepts you should be familiar with are virtual Ethernet and shared Ethernet.

  • Virtual Ethernet: Virtual Ethernet, supported on AIX 5.3 on POWER5™, allows for inter-partition- and IP-based communications between logical partitions on the same frame. This is done by the use of a virtual I/O switch. The Ethernet adapters themselves are created and configured using the HMC. If you recall, you tried to change an adapter earlier that was configured with virtual Ethernet.
  • Shared Ethernet: Shared Ethernet is one of the features of Advanced POWER Virtualization. It allows for the use of Virtual I/O Servers (VIOs), where several host machines can actually share one physical network adapter. Typically, this is used in environments that do not require substantial network bandwidth.

While the scope of this series is not on virtualization, you should understand that if you are using virtualization, there might be other reasons for your bottleneck outside of what you are doing on your host machine. While virtualization is a wonderful thing, be careful not to share too many adapters from your VIO Server, or you might pay a large network I/O penalty. Using appropriate monitoring tools should inform you if you have a problem. Further, you might also want to familiarize yourself with concepts such as Address Resolution Protocol (ARP) and Domain Name Server (DNS), which can also impact network performance and reliability in different ways.


This section provides an overview of general network monitoring commands and specific AIX tools available to you. Some of the tools allow you to quickly troubleshoot a performance problem and others capture data for historical trending and analysis.

Let's get back to the old standby, netstat, which displays overall network statistics. Probably one of the most common commands you type in is netstat -in (see Listing 5).

Listing 5. Using netstat with the -in option
@lpar7ml162f_pub[/home/u0004773] > netstat -in
Name  Mtu   Network     Address            Ipkts Ierrs    Opkts Oerrs  Coll
en1   1500  link#2      2a.   21005666     0   175389     0     0
en1   1500  10.153        21005666     0   175389     0     0
en0   1500  link#3      2a.   328241182    0     1189     0     0
en0   1500  172.29.128    328241182    0     1189     0     0
lo0   16896 link#1                           62223     0    62234     0     0
lo0   16896 127            62223     0    62234     0     0
lo0   16896 ::1                              62223     0    62234     0     0
root@lpar7ml162f_pub[/home/u0004773] >

Here is what it means:

  • Name: Interface name.
  • MTU: Interface Maximum Transfer Unit size.
  • Network: The actual network address that the interface connects to.
  • Address: Mac and IP address.
  • Ipkts: The total amount of packets received by the interface.
  • Ierrs: The amount of errors reported back from the interface.
  • Opkts: The amount of packets transmitted from the interface.
  • Oerrs: The amount of error packets transmitted from the interface.
  • Coll: The amount of collisions on the adapter. If you are using Ethernet, you won't see anything here.

Another handy netstat flag is the -m option. This flag allows you to view the Kernel malloc statistics; the mbuf memory requests, including the size of the buffers, the amount in use and the failures by CPU (see Listing 6).

Listing 6. netstat with -m option
root@lpar7ml162f_pub[/home/u0004773] > netstat -m

Kernel malloc statistics:

******* CPU 0 *******
By size           inuse     calls failed   delayed    free   hiwat   freed
32                  194      5203      0         2      62    2620       0
64                  484      3926      0         7      28    2620       0
128                 309     14913      0         8     875    1310       0
256                 392     14494      0        22     136    2620       0
512                2060 261283179      0       261      60    3275       0
1024                 31      2714      0         8      25    1310       0
2048                587      1237      0       292       5    1965       0
4096                  9      8367      0         2       2     655       0
8192                  2        12      0         2       1     327       0
16384               224       354      0        29       2     163       0
32768                48       183      0        13       3      81       0
65536                84       142      0        42       0      81       0
131072                3         4      0         0      51     102       0

******* CPU 1 *******
By size           inuse     calls failed   delayed    free   hiwat   freed
32                   17        96      0         0     111    2620       0
64                  295      1214      0         5      25    2620       0
128                 151     93806      0         5     713    1310       0
256                  83       273      0         5      29    2620       0
512                1577  86936634      0       199      23    3275       0
1024                  4        18      0         2       4    1310       0
2048                515       516      0       257       1    1965       0
4096                  1       707      0         0       1     655       0
8192                  1         1      0         1       4     327       0
16384                32        32      0         4       0     163       0
65536                34        34      0        17       0      81       0
131072                0         0      0         0      44      88       0

If you are using an Ethernet, you can also use the entstat command to display device-driver statistics. This provides a potpourri of information (see Listing 7).

Listing 7. Using the enstat command to display device driver statistics
testsys:/home/test>entstat -d en1
Device Type: 10/100 Mbps Ethernet PCI Adapter II (1410ff01)
Hardware Address: 00:02:55:6f:c9:9b
Elapsed Time: 5 days 12 hours 14 minutes 46 seconds

Transmit Statistics:                          Receive Statistics:
--------------------                          -------------------
Packets: 803536                               Packets: 2095253
Bytes: 511099654                              Bytes: 1099945394
Interrupts: 520                               Interrupts: 2074913
Transmit Errors: 0                            Receive Errors: 0
Packets Dropped: 0                            Packets Dropped: 0
                                              Bad Packets: 0
Max Packets on S/W Transmit Queue: 38
S/W Transmit Queue Overflow: 0
Current S/W+H/W Transmit Queue Length: 1

Broadcast Packets: 535                        Broadcast Packets: 997476
Multicast Packets: 2                          Multicast Packets: 5477
No Carrier Sense: 0                           CRC Errors: 0
DMA Underrun: 0                               DMA Overrun: 0
Lost CTS Errors: 0                            Alignment Errors: 0
Max Collision Errors: 0                       No Resource Errors: 0
Late Collision Errors: 0                      Receive Collision Errors: 0
Deferred: 0                                   Packet Too Short Errors: 0
SQE Test: 0                                   Packet Too Long Errors: 0
Timeout Errors: 0                             Packets Discarded by Adapter: 0
Single Collision Count: 0                     Receiver Start Count: 0
Multiple Collision Count: 0
Current HW Transmit Queue Length: 1

General Statistics:
No mbuf Errors: 0
Adapter Reset Count: 0
Adapter Data Rate: 200
Driver Flags: Up Broadcast Running
        Simplex AlternateAddress 64BitSupport
        ChecksumOffload PrivateSegment DataRateSet

10/100 Mbps Ethernet PCI Adapter II (1410ff01) Specific Statistics:
Link Status : up
Media Speed Selected: Auto negotiation
Media Speed Running: 100 Mbps Full Duplex
Receive Pool Buffer Size: 1024
No Receive Pool Buffer Errors: 0
Receive Buffer Too Small Errors: 0
Entries to transmit timeout routine: 0
Transmit IPsec packets: 0
Transmit IPsec packets dropped: 0
Receive IPsec packets: 0
Receive IPsec packets dropped: 0
Inbound IPsec SA offload count: 0
Transmit Large Send packets: 0
Transmit Large Send packets dropped: 0
Packets with Transmit collisions:
 1 collisions: 0           6 collisions: 0          11 collisions: 0
 2 collisions: 0           7 collisions: 0          12 collisions: 0
 3 collisions: 0           8 collisions: 0          13 collisions: 0
 4 collisions: 0           9 collisions: 0          14 collisions: 0
 5 collisions: 0          10 collisions: 0          15 collisions: 0


You won't see many collisions, as you'll probably be working in a switched environment. Look for transmit errors and make sure they are not increasing too fast. You need to learn to troubleshoot collision and error problems before you even begin to think about tuning. Alternatively, you can use netstat -v, which provides similar information.

Let's look at netpmon. netpmon provides information on CPU usage as it relates to the network, and it also includes data about network device driver I/O, Internet socket calls, and other various statistics. Similar to its other trace brethren, tprof and filemon, it starts a trace and runs in the background until you stop it with the trcstop command. I like netpmon, because it really gives you a detailed overview of network activity and also captures data for trending and analysis (though it is not as useful as nmon for this purpose). Here you'll use a trace buffer size of two million bytes (see Listing 8).

Listing 8. netpmom with -T option
root@lpar7ml162f_pub[/etc] > netpmon -T 2000000 -o /tmp/net.out
Wed Sep  5 05:30:27 2007
System: AIX 5.3 Node: lpar7ml162f_pub Machine: 00C22F2F4C00

Run trcstop command to signal end of trace.

Now you'll stop it (see Listing 9).

Listing 9. Stopping netpmom
root@lpar7ml162f_pub[/etc] >
root@lpar7ml162f_pub[/etc] > trcstop
root@lpar7ml162f_pub[/etc] > [netpmon: Reporting started]

[netpmon: Reporting completed]
[               4 traced cpus               ]
[         245.464 secs total preempt time   ]

[netpmon: 164.813 secs in measured interval]

root@lpar7ml162f_pub[/etc] >

Let's look at the data. Here is just a small sampling of the output (see Listing 10).

Listing 10. Sample output
# more net.out

Process CPU Usage Statistics:
Process (top 20)             PID  CPU Time   CPU %   CPU %
UNKNOWN                    15920  151.2735  36.558   0.000
UNKNOWN                     7794  104.8801  25.346   0.000
UNKNOWN                     6876   73.8785  17.854   0.000
UNKNOWN                     5402   50.6225  12.234   0.000
xmwlm                      13934   15.0469   3.636   0.000
-ksh                        5040    0.0371   0.009   0.000
getty                      18688    0.0280   0.007   0.000
sshd:                      28514    0.0224   0.005   0.000
syncd                      10068    0.0212   0.005   0.000
gil                         3870    0.0163   0.004   0.004
swapper                        0    0.0135   0.003   0.000
spray                       5400    0.0085   0.002   0.000
send-mail                  18654    0.0084   0.002   0.000
rmcd                       15026    0.0081   0.002   0.000
ping                        5036    0.0068   0.002   0.000
ksh                        26642    0.0062   0.002   0.000
trcstop                     5404    0.0057   0.001   0.000
rpc.lockd                  22032    0.0052   0.001   0.000
mail                        6872    0.0039   0.001   0.000
IBM.ServiceRMd             28126    0.0032   0.001   0.000
Total (all processes)             395.9176  95.681   0.004
Idle time                          70.3216  16.995


First Level Interrupt Handler CPU Usage Statistics:
FLIH                              CPU Time   CPU %   CPU %
PPC decrementer                    18.4640   4.462   0.000
queued interrupt                    6.2882   1.520   0.000
external device                     0.6343   0.153   0.000
data page fault                     0.0220   0.005   0.000
Total (all FLIHs)                  25.4085   6.140   0.000

TCP Socket Call Statistics (by Process):
                                   ------ Read -----   ----- Write -----
Process (top 20)             PID   Calls/s   Bytes/s   Calls/s   Bytes/s
sshd:                      28514      0.47      7754      0.65        40
sshd:                      29596      0.04       596      0.05         3
Total (all processes)                 0.51      8350      0.70        43


NFSv3 Client RPC Statistics (by Server):

Server                     Calls/s
p650                          0.03
Total (all servers)           0.03


PROCESS: ping   PID: 5036
reads:                  12
  read sizes (bytes):   avg 192.0   min 192     max 192     sdev 0.0
  read times (msec):    avg 7.927   min 7.136   max 12.806  sdev 1.496
writes:                 12
  write sizes (bytes):  avg 64.0    min 64      max 64      sdev 0.0
  write times (msec):   avg 0.052   min 0.039   max 0.063   sdev 0.007

As you can see, there is little overall network I/O activity going on during this time. The top section is most important, as it really helps you get an understanding of what processes are eating up network I/O time.

lsattr (used earlier to view the hardware parameters) is another command you will be using frequently to display statistics on your interfaces. The attributes that you see here are configured using either the chdev or no commands. Let's display your driver parameters (see Listing 11).

Listing 11. Displaying the driver parameters using lsattr
testsys:/home/testsys >lsattr -El en0
alias4               IPv4 Alias including Subnet Mask           True
alias6               IPv6 Alias including Prefix Length         True
arp           on     Address Resolution Protocol (ARP)          True
authority            Authorized Users                           True
broadcast            Broadcast Address                          True
mtu           1500   Maximum IP Packet Size for This Device     True
netaddr              Internet Address                           True
netaddr6             IPv6 Internet Address                      True
netmask              Subnet Mask                                True
prefixlen            Prefix Length for IPv6 Internet Address    True
remmtu        576    Maximum IP Packet Size for REMOTE Networks True
rfc1323              Enable/Disable TCP RFC 1323 Window Scaling True
security      none   Security Level                             True
state         detach Current Interface Status                   True
tcp_mssdflt          Set TCP Maximum Segment Size               True
tcp_nodelay          Enable/Disable TCP_NODELAY Option          True
tcp_recvspace        Set Socket Buffer Space for Receiving      True
tcp_sendspace        Set Socket Buffer Space for Sending        True

I also like to use the spray command to troubleshoot possible problems. The spray command sends a one-way stream of packets to the remote host machines from your host. It shows you the amount of packets as well as the packet transfer rate (see Listing 12).

Listing 12. spray command
root@lpar7ml162f_pub[/etc] > /usr/etc/spray lpar8test  -c 2000 -l 1400 -d 1
sending 2000 packets of length 1402 to
        lpar8test ...

        34 packets (1.700%) dropped by lpar8test
        23667 packets/second, 33181234 bytes/second
root@lpar7ml162f_pub[/etc] >

In this example, 2000 packets were sent to the lpar8test host, with a delay of one micro-second. Each packet was sent 1400 bytes. Before using spray, make sure that sprayd is not commented out of inetd (defaulted in AIX), and don't forget to refresh inetd. If you are seeing a substantial amount of dropped packets, that is obviously not good.

Finally, let's look at nmon in Listing 13.

Listing 13. nmon
?nmon????????p=Partitions???????Host=lpar7ml162f_pubRefresh=2 secs???05:43.15?????????
? Network ????????????????????????????????????????????????????????????????????????????
?I/F Name Recv=KB/s Trans=KB/s packin packout insize outsize Peak->Recv Trans     ?
?    en1     2.1     0.0      46.3      0.0    46.0    0.0        2.1     0.0        ?
?    en0    43.8     0.3     575.2      0.5    77.9  674.0       43.8     0.6        ?
?    lo0     0.0     0.0       0.0      0.0     0.0    0.0        0.0     0.0        ?
?  Total     0.0     0.0 in Mbytes/second                                            ?
?I/F Name  MTU  ierror oerror collision Mbits/s Description                          ?
?    en1   1500      0      0      0   2047 Standard Ethernet Network Interface      ?
?    en0   1500      0      0      0   2047 Standard Ethernet Network Interface      ?
?    lo0  16896      0      0      0      0 Loopback Network Interface

If you've been following the other series on AIX (see Resources), you know I love nmon and you should also, once you start using it. With nmon (type in n after startup), you have a quick snapshot of everything going on in your network, including adapter details, MTU, error counters and collisions, and megabit rating.

Further, you also have the ability to capture data with nmon. Using the nmon analyzer, you can print out pretty graphical reports directly from Microsoft® Excel spreadsheets. See Resources for a link to an IBM Wiki for the nmon manual or for downloads.


This article covered the relative importance of the network I/O subsystem, and defined the AIX network I/O layers and how it relates to the OSI Model. You learned some best practices for network configuration, changed your Ethernet settings to support jumbo frame, and viewed interface hardware and driver data. You even examined the monitoring tools available to you and captured data using netpmon and nmon. In the next part of the series, you'll tune NFS, find out more about monitoring utilities, such as nfsstat and nmon, and discover how to tune with nfso.



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