Step 2: Kernel Priority (sk_prio) or DSCP to UP Mapping

Intel® Ethernet 800 Series Linux Flow Control

Configuration Guide for RDMA Use Cases

ID	Date	Version	Classification
635330	11/13/2025		Public

Option A: PCP/VLAN PFC: ToS to Kernel Priority Mapping to UP Mapping

Linux automatically maps IP ToS (Type of Service) values to kernel priorities (sk_prio) using a hardcoded function (ip_tos2prio) in net/ipv4/route.c. These kernel priorities are then mapped to User Priorities (UP) used in VLAN tagging and Priority Flow Control (PFC).

Default mappings to set priority values 0, 2 ,4, and 6 are:

ToS 0 --> sk_prio 0 --> UP 0 ToS 8 --> sk_prio 2 --> UP 2 ToS 24 --> sk_prio 4 --> UP 4 ToS 16 --> sk_prio 6 --> UP 6

For more details, refer to:

http://linux-tc-notes.sourceforge.net/tc/doc/priority.txt

Also, refer to man tc-prio. Although tc-prio is part of Linux traffic control and not applicable to RDMA traffic on Intel 800 Series NICs, it includes a useful chart showing ToS-to-priority mappings.

Linux Source Notes:

PCP Mappings are implemented in the Linux kernel and drivers using rt_tos2priority. For example, prio = rt_tos2priority(tos) (from drivers/infiniband/core/cma.c).

This is how 0, 2, 4, and 6 priority values occur.

In order to use other priority values (i.e., 1, 3, 5, 7), a VLAN is required to be set up using the egress-qos-map option. For example to map all priority 0 as priority 3:

# ip link add link <ifname> name <vlan-ifname> type vlan id <vlan-id> egress-qos-map 0:3 1:0 2:0 3:0 4:0 5:0 6:0 7:0 Notes:

UP is the priority used in PFC.

Competing technologies might use mqprio qdisc (see man tc-mqprio) to adjust this mapping.

The 800 Series ADQ feature uses mqprio to direct traffic. ADQ and PFC cannot be used at the same time.

Option B: DSCP to UP Mapping

DSCP mapping is implemented in the Linux kernel and drivers using ToS to DSCP direct mapping. Note that ToS is deprecated in favor of DSCP. The two low-order bits are used for ECN, while the upper six bits are used for the DSCP value (the priority). DSCP uses the upper 6 bits of the 8-bit ToS field, allowing up to 64 code points. DSCP values are effectively 4x the PCP ToS values. DSCP to UP translation table has 64 entries and provides a translation from every one of 64 DSCP values to a 3-bit UP value.

The following table shows the commonly used DSCP Values and their priority values.

DSCP Value	Decimal Value	ToS (4 x DSCP)	UP
000 000	0	0	0
001 000	8	32	1
010 000	16	64	2
010 100	20	80	2
011 000	24	96	3
011 010	26	104	3
100 000	32	128	4
100 110	38	152	4
101 000	40	160	5
101 110	46	184	5
110 000	48	192	6
111 000	56	224	7

Note:The DSCP to UP mapping table contains 64 entries, one for each possible DSCP value (0-63). The table above shows only a subset of commonly used DSCP values for clarity.