Thursday, July 13, 2017

Linux 4.11 kernel extends packet sampling support

Linux 4.11 on Linux Kernel Newbies describes the features added in the April 30, 2017 release. Of particular interest is the new netlink sampling channel:
Introduce psample, a general way for kernel modules to sample packets, without being tied to any specific subsystem. This netlink channel can be used by tc, iptables, etc. and allow to standardize packet sampling in the kernel commit
The psample netlink channel delivers sampled packet headers along with associated metadata from the Linux kernel to user space. The psample fields map directly into sFlow Version 5 sampled_header export structures:

netlink psamplesFlowDescription
PSAMPLE_ATTR_IIFINDEXinputInterface packet was received on.
PSAMPLE_ATTR_OIFINDEXoutputInterface packet was sent on.
PSAMPLE_ATTR_SAMPLE_GROUPdata sourceThe location within network device that generated packet sample.
PSAMPLE_ATTR_GROUP_SEQdropsNumber of times that the sFlow agent detected that a packet marked to be sampled was dropped due to lack of resources. Agent calculates drops by tracking discontinuities in PSAMPLE_ATTR_GROUP_SEQ
PSAMPLE_ATTR_SAMPLE_RATEsampling_rateThe Sampling Rate specifies the ratio of packets observed at the Data Source to the samples generated. For example a sampling rate of 100 specifies that, on average, 1 sample will be generated for every 100 packets observed.
PSAMPLE_ATTR_ORIGSIZEframe_lengthOriginal length of packet before sampling
PSAMPLE_ATTR_DATAheader<>Header bytes

Linux is widely used for switch network operating systems, including: Arista EOS, Cumulus Linux, Dell OS10, OpenSwitch, SONiC, and Open Network Linux. The adoption of Linux by network vendors and cloud providers is driving increased support for switch hardware by the Linux kernel community.

Hardware support for sFlow packet sampling is widely implemented in switch ASICs, including: Broadcom, Mellanox, Intel, Marvell, Barefoot Networks, Cavium, and Innovium. A standard Linux interface to ASIC sampling simplifies the implementation of sFlow agents (e.g. Host sFlow) and ensures consistent behavior across hardware platforms to deliver real-time network-wide visibility using industry standard sFlow protocol.

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