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laforge gerrit-no-reply at lists.osmocom.orglaforge has uploaded this change for review. ( https://gerrit.osmocom.org/c/osmo-gsm-manuals/+/23926 ) Change subject: Add new common chapter about QoS, IP DSCP and IEEE 802.1q PCP ...................................................................... Add new common chapter about QoS, IP DSCP and IEEE 802.1q PCP Related: SYS#5427 Change-Id: Id344c29eda2a9b3e36376302b425e9db1f6c0f28 --- M common/chapters/bibliography.adoc A common/chapters/qos-dscp-pcp.adoc 2 files changed, 164 insertions(+), 0 deletions(-) git pull ssh://gerrit.osmocom.org:29418/osmo-gsm-manuals refs/changes/26/23926/1 diff --git a/common/chapters/bibliography.adoc b/common/chapters/bibliography.adoc index df69c44..8e08ba3 100644 --- a/common/chapters/bibliography.adoc +++ b/common/chapters/bibliography.adoc @@ -176,6 +176,9 @@ https://tools.ietf.org/html/rfc1350 - [[[ietf-rfc2131]]] IETF RFC 2131: Dynamic Host Configuration Protocol https://tools.ietf.org/html/rfc2131 +- [[[ietf-rfc2474]]] IETF RFC 2474: Definition of the Differentiated Services Field (DS Field) in the IPv44 + and IPv6 Headers + https://tools.ietf.org/html/rfc2474 - [[[ietf-rfc2719]]] IETF RFC 2719: Signal Transport over IP https://tools.ietf.org/html/rfc2719 - [[[ietf-rfc3331]]] IETF RFC 3331: Message Transfer Part 2 User Adaptation Layer diff --git a/common/chapters/qos-dscp-pcp.adoc b/common/chapters/qos-dscp-pcp.adoc new file mode 100644 index 0000000..a9233e3 --- /dev/null +++ b/common/chapters/qos-dscp-pcp.adoc @@ -0,0 +1,161 @@ +[[qos-dscp-pcp]] +== QoS, DCSP/TOS, Prioirty and IEEE 802.1q PCP + +In many use cases operators want to apply different QoS classes for user plane +vs. control plane traffic. IP Routers, Ethernet switches and other network gear +can then perform intelligent queue management as required for the respective service. + +For example, voice user plane frames need a rather stable and short latency, +while IP user plane and control plane traffic has less critical latency requirements. + +=== IP Level (DSCP) + +At IP level, different priorities / classes of traffic are expressed +in accordance to <<ietf-rfc2474>> by the DSCP (Differentiated Services Code +Point) field of the IP header. DSCP resembles the upper 6 bits of the +field formerly known as the TOS bits as per <<ietf-rfc791>>. + +On Linux and other operating systems with BSD-style sockets API, the +applications can request a specific DSCP value to be used for packets +generated by those sockets. + +Osmocom CNI software such as osmo-bts and osmo-mgw support setting the +DSCP value via VTY commands, see e.g. the `rtp ip-dscp` setting of the +`bts` node in osmo-bts. + + +=== Packet Priority + +In the Linux network stack, every packet is represented by `struct +sk_buff`, which has an associated _priority_. Furthermore, every socket +through which applications send data have an associated _socket +priority_. Each time a packet is transmitted through a given socket, +the packet inherits the packet priority from the socket priority. + +Furthermore, there is a mapping table that maps DSCP/TOS bits to +priority. The sixteen different TOS bit values are mapped to priority +values as follows: + +.Linux kernel default DSCP/TOS -> priority mapping +[options="header",width="50%"] +|=== +|TOS (binary)|DSCP (binary)|Priority (decimal) +|xxx0000x|xxx000|0 +|xxx0001x|xxx000|0 +|xxx0010x|xxx001|0 +|xxx0011x|xxx001|0 +|xxx0100x|xxx010|2 +|xxx0101x|xxx010|2 +|xxx0110x|xxx011|2 +|xxx0111x|xxx011|2 +|xxx1000x|xxx100|6 +|xxx1001x|xxx100|6 +|xxx1010x|xxx101|6 +|xxx1011x|xxx101|6 +|xxx1100x|xxx110|4 +|xxx1101x|xxx110|4 +|xxx1110x|xxx111|4 +|xxx1111x|xxx111|4 +|=== + +This table of default DSCP/TOS -> priority bit mappings cannot be +modified. + +However, the per-packet _priority_ values can be set by various means +of network policy, including + +* by packet filter rules (iptables, ip6tables, nftables) +** if you use `iptables`, using `CLASSIFY --set-class` in the `mangle` table +** if you use `nftables`, using `meta priority set` in the `mangle` table +* by the application using the SO_PRIORITY socket option (currently not yet supported by Osmocom CNI) + + +=== Ethernet Level (PCP) + +At Ethernet level, different priorities / QoS classes are expressed by +the so-called PCP (Priority Code Point) field in the IEEE 802.1q (VLAN) +header. + +NOTE:: This means that PCP functionality requires the use of IEEE 802.q +VLAN. You cannot use PCP without VLAN + +The Linux kernel assigns IEEE 802.1q PCP bits based on a _mapping_ +between the _priority_ and the PCP value. Each VLAN network device +maintains a separate map for both egress (transmit) and ingress +(receive) path. + +The current priority mappings can be inspected via the `/proc` +filesystem. For example, if you have a VLAN device `eth0.9` for +VLAN ID 9 on the net-device `eth0`, you can use the following example: + +.Example: Inspecting the current egress QoS map +---- +$ sudo cat /proc/net/vlan/eth0.9<1> +eth0.9 VID: 9 REORDER_HDR: 1 dev->priv_flags: 1021 + total frames received 123340 + total bytes received 40668066 + Broadcast/Multicast Rcvd 1106 + + total frames transmitted 10499 + total bytes transmitted 1570809 +Device: eth0 +INGRESS priority mappings: 0:0 1:0 2:0 3:0 4:0 5:0 6:0 7:0 <2> + EGRESS priority mappings: <3> +---- +<1> make sure to specify your specific VLAN interface name here instead of `eth0.9` +<2> ingress priority mappings (all PCP values mapped to priority 0) +<3> egress priority mappings (empty) + +As we can see in the above example, there are no egress priority +mappings yet. Let's create three new mappings, mapping _priority_ +value 1 to PCP 1, _priority_ 2 to PCP 2, and _priority_ 3 to PCP 3: + +.Example: Creating three new egress QoS mappings +---- +$ sudo ip link set dev eth0.9<1> type vlan egress-qos-map 1:1 2:2 3:3 <2> +$ sudo cat /proc/net/vlan/eth0.9 <3> +eth0.9 VID: 9 REORDER_HDR: 1 dev->priv_flags: 1021 + total frames received 123898 + total bytes received 40843611 + Broadcast/Multicast Rcvd 1106 + + total frames transmitted 10517 + total bytes transmitted 1574357 +Device: eth0 +INGRESS priority mappings: 0:0 1:0 2:0 3:0 4:0 5:0 6:0 7:0 + EGRESS priority mappings: 1:1 2:2 3:3 <4> +---- +<1> make sure to specify your specific VLAN interface name here instead of `eth0.9` +<2> command to define three new egress QoS maps +<3> command to re-display the current status +<4> three new egress mappings are shown as given in `ip` command + +NOTE:: The settings of the `ip` command are volatile and only active until +the next reboot (or the network device or VLAN is removed). Please refer to +the documentation of your specific Linux distribution in order to find out how +to make such settings persistent by means of an `ifup` hook whenever the interface +comes up. For CentOS/RHEL 8 this can e.g. be achieved by means of an `/sbin/ifup-local +script` (when using `network-scripts` and not NetworkManager). For Debian or Ubuntu, +this typically involves adding `up` lines to `/etc/network/interfaces` or a `/etc/network/if-up.d` +script. + + +=== Putting things together + +Assuming one needs to set both the DSCP bits as well as the PCP for +certain traffic, the above-mentioned mechanisms need to be combined as +follows: + +. configure the osmocom program to set the DSCP value +. use the default DSCP -> priority mapping, if possible +. configure an egrees QoS map to map from priority to PCP + +If the desired combination of DSCP + PCP cannot be achieved that way, +due to the rather static default kernel mapping table, one needs to go +one step further: + +. configure the osmocom program to set the DSCP value +. use packet filter rules to set the priority based on DSCP +. configure an egrees QoS map to map from priority to PCP + +include::{srcdir}/chapters/qos-example.adoc[] -- To view, visit https://gerrit.osmocom.org/c/osmo-gsm-manuals/+/23926 To unsubscribe, or for help writing mail filters, visit https://gerrit.osmocom.org/settings Gerrit-Project: osmo-gsm-manuals Gerrit-Branch: master Gerrit-Change-Id: Id344c29eda2a9b3e36376302b425e9db1f6c0f28 Gerrit-Change-Number: 23926 Gerrit-PatchSet: 1 Gerrit-Owner: laforge <laforge at osmocom.org> Gerrit-MessageType: newchange -------------- next part -------------- An HTML attachment was scrubbed... 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