Updates:

• All metrics have now been explained upstream in https://www.kernel.org/doc/html/latest/networking/snmp_counter.html, so there is no need for that page anymore.
• 2018-09-20 - Initial version

TL;DR

Explanation of all the Linux network statistics exported by the kernel, usually available through the netstat –statistics or nstat commands.

For each metric I’ll try to provide:

• human readable description (eventually from RFC + link)
• pointer to Linux kernel source

Only the statistics provided by nstat are described here right now.

Metrics?

As already explained in a previous blog post, a lot of network statistics are exported by the kernel:

$ netstat --statistics|head -10
Ip:
    68728385 total packets received
    1 with invalid headers
    56871060 forwarded
    0 incoming packets discarded
    4093729 incoming packets delivered
    57898402 requests sent out
    65 outgoing packets dropped
    8 fragments dropped after timeout
    3240 reassemblies required

$ nstat | shuf |head -n 10
IpInReceives                    32                 0.0
TcpInSegs                       5                  0.0
TcpOutSegs                      3                  0.0
IpExtInBcastOctets              980                0.0
IpExtInOctets                   1946               0.0
IpExtOutOctets                  1400               0.0
IpOutRequests                   9                  0.0
IpExtInBcastPkts                20                 0.0
TcpExtTCPPureAcks               3                  0.0
IpInDelivers                    5                  0.0

I am not going to explain the netstat output, since net-tools is officially obsolete in favour of iproute2, quote from linuxfoundation.org

Please keep in mind that most net-tools programs are obsolete now

Why this page?

While many of these metrics are standardized through several RFCs (for ip/tcp/udp/icmp), some are not part of any standard SNMP Mib, and have been added when the need arose. (I guess)

Unfortunately, AFAIK, there is no effort to document all of them, sometimes a dive into the Linux kernel source code is mandatory to understand what is their purpose.

These are my notes I wrote while reading the related areas of the Linux kernel source code, with the hope to make them more understandable.

Metrics

Metrics collected on the following system with netstat –zero

• OS: Debian Stretch
• linux kernel: 4.18
• nstat: iproute2-ss180129

Categories and status

This document is not complete yet, here is the current status:

• Ip
• Icmp
• Tcp
• Udp
• Udp-Lite
• Ip6 in progress
• Icmp6 in progress
• Udp6 in progress
• Udp-Lite6 in progress
• TcpExt in progress
• IpExt in progress

Ip

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Icmp

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Tcp

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Udp

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Udp-Lite

UDP-Lite (Lightweight User Datagram Protocol) is a connectionless protocol that allows a potentially damaged data payload to be delivered to an application rather than being discarded by the receiving station. This is useful as it allows decisions about the integrity of the data to be made in the application layer (application or the codec), where the significance of the bits is understood. UDP-Lite is described in RFC 3828

rfc5097 provides a MIB for this protocol.

Surprisingly, a couple of rfc defined statistics are not yet available in the Linux kernel, e.g.

• udpliteInPartialCov
• udpliteInBadChecksum
• udpliteOutPartialCov
• udpliteEndpointViolCoverage

Udp and Udp Lite share lot of code in the Linux kernel, SNMP statistics are kept separated thanks to the UDP_INC_STATS and IS_UDPLITE macros:

 #define UDP_INC_STATS(net, field, is_udplite)		      do { \
 	if (is_udplite) SNMP_INC_STATS((net)->mib.udplite_statistics, field);       \
 	else		SNMP_INC_STATS((net)->mib.udp_statistics, field);  }  while(0)

#define IS_UDPLITE(__sk) (__sk->sk_protocol == IPPROTO_UDPLITE)

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Ip6

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Icmp6

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Udp6

Udp-Lite6

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TcpExt

All these statistics are Linux specific (i.e. not described in a MIB rfc).

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IpExt

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