Change in osmo-gsm-manuals[master]: Add new common chapter about QoS, IP DSCP and IEEE 802.1q PCP

laforge gerrit-no-reply at lists.osmocom.org
Wed Apr 28 09:28:10 UTC 2021


laforge 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...
URL: <http://lists.osmocom.org/pipermail/gerrit-log/attachments/20210428/25061a2f/attachment.htm>


More information about the gerrit-log mailing list