I am a big fan of tools that lets you observe the behavior of low level systems, as it is great for learning. For example, hardware performance counters are great for observing processor behavior that is otherwise transparent for the developer. Similarly, the herdtools7 suite is great for observing the behavior of your processor's memory model.

I've been running a networking book club at my workplace, and I'm happy that the the ip -s -s link show command can give me similar low-level visibility into the networking stack of my machine (although in much less detail than the previous two tools). The kernel documentation describes the output of this tool in a lot of detail. The only thing I was confused by was what the transns entry in the TX errors row meant, as I couldn't find it in the documentation:

$ ip -s -s link show dev wlo1
3: wlo1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DORMANT group default qlen 1000
    link/ether 50:c2:e8:ee:f3:57 brd ff:ff:ff:ff:ff:ff
    RX:  bytes packets errors dropped  missed   mcast
     478520149  452566      0       0       0       0
    RX errors:  length    crc   frame    fifo overrun
                     0      0       0       0       0
    TX:  bytes packets errors dropped carrier collsns
      44078438  164580      0       3       0       0
    TX errors: aborted   fifo  window heartbt transns
                     0      0       0       0       4
    altname wlp41s0
    altname wlx50c2e8eef357

Thankfully, we can go spelunking in the iproute2 repository to see what exactly it represents. Searching the for the string transns finds us the code we want relatively easily, but it's still not clear to me what a carrier_change is, or what a carrier even is in the first place. The thorough git commit message helps us a lot though:

commit 30b557929f2aaeeee59e1bbaad7c804bcae40e7b
Author: david decotigny <decot@googlers.com>
Date:   Mon May 5 20:38:18 2014 -0700

    iproute2: show counter of carrier on<->off transitions

    This patch allows to display the current counter of carrier on<->off
    transitions (IFLA_CARRIER_CHANGES, see kernel commit "expose number of
    carrier on/off changes"):

      ip -s -s link show dev eth0
      32: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 ...
        link/ether ................. brd ff:ff:ff:ff:ff:ff
        RX: bytes  packets  errors  dropped overrun mcast
        125552461  258881   0       0       0       10150
        RX errors: length  crc     frame   fifo    missed
                   0        0       0       0       0
        TX: bytes  packets  errors  dropped carrier collsns
        40426119   224444   0       0       0       0
        TX errors: aborted fifo    window  heartbeat transns
                   0        0       0       0        3

    Tested:
      - kernel with patch "net-sysfs: expose number of carrier on/off
        changes": see "transns" column above
      - kernel wthout the patch: "transns" not displayed (as expected)

    Signed-off-by: David Decotigny <decot@googlers.com>

Sadly, I wasn't able to immediately figure out what a carrier was from the mentioned kernel commit either. Some resources online seem to imply that carrier_changes count the number of times a specific interface went up or down, which seems plausible as the transns number seems to go up whenever I manually toggle my computer's wlo1 interface on and off.