Add new common chapter about QoS, IP DSCP and IEEE 802.1q PCP
Related: SYS#5427 Change-Id: Id344c29eda2a9b3e36376302b425e9db1f6c0f28
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@ -166,6 +166,9 @@
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system for telecommunication application providers
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https://www.etsi.org/deliver/etsi_ts/101200_101299/101220/12.00.00_60/ts_101220v120000p.pdf
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- [[[ieee-802.1q]]] IEEE 802.1Q: Bridges and Bridged Networks
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https://ieeexplore.ieee.org/document/6991462
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- [[[ietf-rfc768]]] IETF RFC 768: User Datagram Protocol
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https://tools.ietf.org/html/rfc768
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- [[[ietf-rfc791]]] IETF RFC 791: Internet Protocol
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@ -178,6 +181,9 @@
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https://tools.ietf.org/html/rfc1350
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- [[[ietf-rfc2131]]] IETF RFC 2131: Dynamic Host Configuration Protocol
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https://tools.ietf.org/html/rfc2131
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- [[[ietf-rfc2474]]] IETF RFC 2474: Definition of the Differentiated Services Field (DS Field) in the IPv44
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and IPv6 Headers
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https://tools.ietf.org/html/rfc2474
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- [[[ietf-rfc2719]]] IETF RFC 2719: Signal Transport over IP
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https://tools.ietf.org/html/rfc2719
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- [[[ietf-rfc3331]]] IETF RFC 3331: Message Transfer Part 2 User Adaptation Layer
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@ -81,6 +81,8 @@ DHCP::
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downlink::
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Direction of messages / signals from the network core towards the
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mobile phone
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DSCP::
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Diffrentiated Services Code Point (_IETF RFC 2474_ <<ietf-rfc2474>>
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DSP::
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Digital Signal Processor
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dvnixload::
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@ -264,6 +266,8 @@ PC::
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Point Code; an address in MTP
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PCH::
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Paging Channel on downlink Um interface; used by network to page an MS
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PCP::
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Priority Code Point (_IEEE 802.1Q_ <<ieee-802.11>>)
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PCU::
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Packet Control Unit; used to manage Layer 2 of the GPRS radio interface
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PDCH::
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@ -354,6 +358,8 @@ TCP::
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Transmission Control Protocol; (_IETF RFC 793_ <<ietf-rfc793>>)
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TFTP::
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Trivial File Transfer Protocol; (_IETF RFC 1350_ <<ietf-rfc1350>>)
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TOS::
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Type Of Service; bit-field in IPv4 header, now re-used as DSCP (_IETF RFC 791_ <<ietf-rfc791)
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TRX::
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Transceiver; element of a BTS serving a single carrier
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TS::
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@ -388,6 +394,8 @@ VCTCXO::
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Voltage Controlled, Temperature Compensated Crystal Oscillator; a
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precision oscillator, superior to a classic crystal oscillator, but
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inferior to an OCXO
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VLAN::
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Virtual LAN in the context of Ethernet (_IEEE 802.1Q_ <<ieee-802.1q>>)
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VLR::
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Visitor Location Register; volatile storage of attached subscribers in
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the MSC
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@ -0,0 +1,161 @@
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[[qos-dscp-pcp]]
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== QoS, DSCP/TOS, Priority and IEEE 802.1q PCP
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In many use cases operators want to apply different QoS classes for user plane
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vs. control plane traffic. IP Routers, Ethernet switches and other network gear
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can then perform intelligent queue management as required for the respective service.
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For example, voice user plane frames need a rather stable and short latency,
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while IP user plane and control plane traffic has less critical latency requirements.
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=== IP Level (DSCP)
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At IP level, different priorities / classes of traffic are expressed
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in accordance to <<ietf-rfc2474>> by the DSCP (Differentiated Services Code
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Point) field of the IP header. DSCP resembles the upper 6 bits of the
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field formerly known as the TOS bits as per <<ietf-rfc791>>.
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On Linux and other operating systems with BSD-style sockets API, the
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applications can request a specific DSCP value to be used for packets
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generated by those sockets.
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Osmocom CNI software such as osmo-bts and osmo-mgw support setting the
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DSCP value via VTY commands, see e.g. the `rtp ip-dscp` setting of the
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`bts` node in osmo-bts.
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=== Packet Priority
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In the Linux network stack, every packet is represented by `struct
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sk_buff`, which has an associated _priority_. Furthermore, every socket
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through which applications send data have an associated _socket
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priority_. Each time a packet is transmitted through a given socket,
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the packet inherits the packet priority from the socket priority.
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Furthermore, there is a mapping table that maps DSCP/TOS bits to
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priority. The sixteen different TOS bit values are mapped to priority
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values as follows:
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.Linux kernel default DSCP/TOS -> priority mapping
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[options="header",width="50%"]
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|===
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|TOS (binary)|DSCP (binary)|Priority (decimal)
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|xxx0000x|xxx000|0
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|xxx0001x|xxx000|0
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|xxx0010x|xxx001|0
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|xxx0011x|xxx001|0
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|xxx0100x|xxx010|2
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|xxx0101x|xxx010|2
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|xxx0110x|xxx011|2
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|xxx0111x|xxx011|2
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|xxx1000x|xxx100|6
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|xxx1001x|xxx100|6
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|xxx1010x|xxx101|6
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|xxx1011x|xxx101|6
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|xxx1100x|xxx110|4
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|xxx1101x|xxx110|4
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|xxx1110x|xxx111|4
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|xxx1111x|xxx111|4
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|===
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This table of default DSCP/TOS -> priority bit mappings cannot be
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modified.
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However, the per-packet _priority_ values can be set by various means
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of network policy, including
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* by packet filter rules (iptables, ip6tables, nftables)
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** if you use `iptables`, using `CLASSIFY --set-class` in the `mangle` table
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** if you use `nftables`, using `meta priority set` in the `mangle` table
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* by the application using the SO_PRIORITY socket option (currently not yet supported by Osmocom CNI)
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=== Ethernet Level (PCP)
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At Ethernet level, different priorities / QoS classes are expressed by
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the so-called PCP (Priority Code Point) field in the IEEE 802.1q (VLAN)
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header.
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NOTE:: This means that PCP functionality requires the use of IEEE 802.q
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VLAN. You cannot use PCP without VLAN
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The Linux kernel assigns IEEE 802.1q PCP bits based on a _mapping_
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between the _priority_ and the PCP value. Each VLAN network device
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maintains a separate map for both egress (transmit) and ingress
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(receive) path.
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The current priority mappings can be inspected via the `/proc`
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filesystem. For example, if you have a VLAN device `eth0.9` for
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VLAN ID 9 on the net-device `eth0`, you can use the following example:
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.Example: Inspecting the current egress QoS map
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----
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$ sudo cat /proc/net/vlan/eth0.9<1>
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eth0.9 VID: 9 REORDER_HDR: 1 dev->priv_flags: 1021
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total frames received 123340
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total bytes received 40668066
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Broadcast/Multicast Rcvd 1106
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total frames transmitted 10499
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total bytes transmitted 1570809
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Device: eth0
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INGRESS priority mappings: 0:0 1:0 2:0 3:0 4:0 5:0 6:0 7:0 <2>
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EGRESS priority mappings: <3>
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----
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<1> make sure to specify your specific VLAN interface name here instead of `eth0.9`
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<2> ingress priority mappings (all PCP values mapped to priority 0)
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<3> egress priority mappings (empty)
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As we can see in the above example, there are no egress priority
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mappings yet. Let's create three new mappings, mapping _priority_
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value 1 to PCP 1, _priority_ 2 to PCP 2, and _priority_ 3 to PCP 3:
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.Example: Creating three new egress QoS mappings
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----
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$ sudo ip link set dev eth0.9<1> type vlan egress-qos-map 1:1 2:2 3:3 <2>
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$ sudo cat /proc/net/vlan/eth0.9 <3>
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eth0.9 VID: 9 REORDER_HDR: 1 dev->priv_flags: 1021
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total frames received 123898
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total bytes received 40843611
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Broadcast/Multicast Rcvd 1106
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total frames transmitted 10517
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total bytes transmitted 1574357
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Device: eth0
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INGRESS priority mappings: 0:0 1:0 2:0 3:0 4:0 5:0 6:0 7:0
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EGRESS priority mappings: 1:1 2:2 3:3 <4>
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----
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<1> make sure to specify your specific VLAN interface name here instead of `eth0.9`
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<2> command to define three new egress QoS maps
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<3> command to re-display the current status
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<4> three new egress mappings are shown as given in `ip` command
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NOTE:: The settings of the `ip` command are volatile and only active until
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the next reboot (or the network device or VLAN is removed). Please refer to
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the documentation of your specific Linux distribution in order to find out how
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to make such settings persistent by means of an `ifup` hook whenever the interface
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comes up. For CentOS/RHEL 8 this can e.g. be achieved by means of an `/sbin/ifup-local
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script` (when using `network-scripts` and not NetworkManager). For Debian or Ubuntu,
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this typically involves adding `up` lines to `/etc/network/interfaces` or a `/etc/network/if-up.d`
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script.
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=== Putting things together
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Assuming one needs to set both the DSCP bits as well as the PCP for
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certain traffic, the above-mentioned mechanisms need to be combined as
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follows:
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. configure the osmocom program to set the DSCP value
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. use the default DSCP -> priority mapping, if possible
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. configure an egrees QoS map to map from priority to PCP
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If the desired combination of DSCP + PCP cannot be achieved that way,
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due to the rather static default kernel mapping table, one needs to go
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one step further:
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. configure the osmocom program to set the DSCP value
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. use packet filter rules to set the priority based on DSCP
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. configure an egrees QoS map to map from priority to PCP
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include::{srcdir}/chapters/qos-example.adoc[]
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@ -4,6 +4,7 @@ EXTRA_DIST = test-usermanual-docinfo.xml \
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vty \
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test2-vty-reference.xml \
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vty-test2 \
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chapters \
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$(NULL)
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# Do not install any of the test pdfs
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=== QoS example for example manual
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Quite exemplary.
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