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Buffer and Queue Management

Hardware datapath configuration manages packet buffering, queueing and scheduling in hardware. To configure priority groups, and assign the scheduling alogorithm and weights, you edit the /etc/cumulus/datapath/traffic.conf.

The /usr/lib/python2.7/dist-packages/cumulus/__chip_config/[bcm|mlx]/datapath.conf assigns buffer space and egress queues. The default thresholds defined in the datapath.conf file are intended for data center environments, but certain workloads may require additional tuning. It is best to make small, incremental changes to validate the changes with your application performance. Be sure to back up the original file before making changes.

Each packet is assigned to an ASIC Class of Service (CoS) value based on the priority value of the packet stored in the 802.1p (Class of Service) or DSCP (Differentiated Services Code Point) header field. The choice to schedule packets based on COS or DSCP is a configurable option in the /etc/cumulus/datapath/traffic.conf file.

Priority groups include:

  • Control: Highest priority traffic
  • Service: Second-highest priority traffic
  • Bulk: All remaining traffic

The scheduler is configured to use a hybrid scheduling algorithm. It applies strict priority to control traffic queues and a weighted round robin selection from the remaining queues. Unicast packets and multicast packets with the same priority value are assigned to separate queues, which are assigned equal scheduling weights.

You can configure Quality of Service (QoS) for switches on the following platforms only:

  • Broadcom Tomahawk, Trident II, Trident II+ and Trident3
  • Mellanox Spectrum and Spectrum-2

Example Configuration File

The following example /etc/cumulus/datapath/traffic.conf datapath configuration file applies to 10G, 40G, and 100G switches on Broadcom Tomahawk, Trident II, Trident II+, or Trident3 and Mellanox Spectrum platforms only.

  • For the default source packet fields and mapping, each selected packet field must have a block of mapped values. Any packet field value that is not specified in the configuration is assigned to a default internal switch priority. The configuration applies to every forwarding port unless a custom remark configuration is defined for that port (see below).
  • For the default remark packet fields and mapping, each selected packet field should have a block of mapped values. Any internal switch priority value that is not specified in the configuration is assigned to a default packet field value. The configuration applies to every forwarding port unless a custom remark configuration is defined for that port (see below).
  • Per-port source packet fields and mapping apply to the designated set of ports.
  • Per-port remark packet fields and mapping apply to the designated set of ports.
click to see traffic.conf file

On switches with Spectrum ASICs, you must enable packet priority remark on the ingress port. A packet received on a remark-enabled port is remarked according to the priority mapping configured on the egress port. If you configure packet priority remark the same way on every port, the default configuration example above is correct. However, per-port customized configurations require two port groups: one for the ingress ports and one for the egress ports, as below:

remark.port_group_list = [ingress_remark_group, egress_remark_group]
remark.ingress_remark_group.packet_priority_remark_set = [dscp]
remark.remark_port_group.port_set = swp1-swp4,swp6
remark.egress_remark_group.port_set = swp10-swp20
remark.egress_remark_group.cos_0.priority_remark.dscp = [2]
remark.egress_remark_group.cos_1.priority_remark.dscp = [10]
remark.egress_remark_group.cos_2.priority_remark.dscp = [18]
remark.egress_remark_group.cos_3.priority_remark.dscp = [26]
remark.egress_remark_group.cos_4.priority_remark.dscp = [34]
remark.egress_remark_group.cos_5.priority_remark.dscp = [42]
remark.egress_remark_group.cos_6.priority_remark.dscp = [50]
remark.egress_remark_group.cos_7.priority_remark.dscp = [58]

On Broadcom switches, if you modify the configuration in the /etc/cumulus/datapath/traffic.conf file, you must restart switchd for the changes to take effect; run the cumulus@switch:~$ sudo systemctl restart switchd.service command.

On Mellanox switches with the Spectrum ASIC, the following options in the /etc/cumulus/datapath/traffic.conf file do not require you to restart switchd. However, you must run the echo 1 > /cumulus/switchd/config/traffic/reload command after you change the options.

  • DSCP/802.1p to COS remark assignments (traffic.*)
  • Explicit congestion notification (ECN) settings (ecn_red.*)
  • Priority flow control (PFC) settings (pfc.*)
  • Link Pause settings (link_pause.*)
  • Queue weight settings (priority_group.*.weight)

Configure Traffic Marking through ACL Rules

You can mark traffic for egress packets through iptables or ip6tables rule classifications. To enable these rules, you do one of the following:

  • Mark DSCP values in egress packets.
  • Mark 802.1p CoS values in egress packets.

To enable traffic marking, use cl-acltool. Add the -p option to specify the location of the policy file. By default, if you do not include the -p option, cl-acltool looks for the policy file in /etc/cumulus/acl/policy.d/.

The iptables-/ip6tables-based marking is supported with the following action extension:

-j SETQOS --set-dscp 10 --set-cos 5

For ebtables, the setqos keyword must be in lowercase, as in:

-A FORWARD -o swp5 -j setqos --set-cos 5

You can specify one of the following targets for SETQOS/setqos:

--set-cos INTSets the datapath resource/queuing class value. Values are defined in IEEE P802.1p.
--set-dscp valueSets the DSCP field in packet header to a value, which can be either a decimal or hex value.
--set-dscp-class classSets the DSCP field in the packet header to the value represented by the DiffServ class value. This class can be EF, BE or any of the CSxx or AFxx classes.

You can specify either --set-dscp or --set-dscp-class, but not both.

Here are two example rules:

-t mangle -A FORWARD --in-interface swp+ -p tcp --dport bgp -j SETQOS --set-dscp 10 --set-cos 5

-t mangle -A FORWARD --in-interface swp+ -j SETQOS --set-dscp 10

You can put the rule in either the mangle table or the default filter table; the mangle table and filter table are put into separate TCAM slices in the hardware.

To put the rule in the mangle table, include -t mangle; to put the rule in the filter table, omit -t mangle.

Priority Flow Control

Priority flow control, as defined in the IEEE 802.1Qbb standard, provides a link-level flow control mechanism that can be controlled independently for each Class of Service (CoS) with the intention to ensure no data frames are lost when congestion occurs in a bridged network.

PFC is not supported on switches with the Helix4 ASIC.

PFC is a layer 2 mechanism that prevents congestion by throttling packet transmission. When PFC is enabled for received packets on a set of switch ports, the switch detects congestion in the ingress buffer of the receiving port and signals the upstream switch to stop sending traffic. If the upstream switch has PFC enabled for packet transmission on the designated priorities, it responds to the downstream switch and stops sending those packets for a period of time.

PFC operates between two adjacent neighbor switches; it does not provide end-to-end flow control. However, when an upstream neighbor throttles packet transmission, it could build up packet congestion and propagate PFC frames further upstream: eventually the sending server could receive PFC frames and stop sending traffic for a time.

The PFC mechanism can be enabled for individual switch priorities on specific switch ports for RX and/or TX traffic. The switch port’s ingress buffer occupancy is used to measure congestion. If congestion is present, the switch transmits flow control frames to the upstream switch. Packets with priority values that do not have PFC configured are not counted during congestion detection; neither do they get throttled by the upstream switch when it receives flow control frames.

PFC congestion detection is implemented on the switch using xoff and xon threshold values for the specific ingress buffer which is used by the targeted switch priorities. When a packet enters the buffer and the buffer occupancy is above the xoff threshold, the switch transmits an Ethernet PFC frame to the upstream switch to signal packet transmission should stop. When the buffer occupancy drops below the xon threshold, the switch sends another PFC frame upstream to signal that packet transmission can resume. (PFC frames contain a quanta value to indicate a timeout value for the upstream switch: packet transmission can resume after the timer has expired, or when a PFC frame with quanta == 0 is received from the downstream switch.)

After the downstream switch has sent a PFC frame upstream, it continues to receive packets until the upstream switch receives and responds to the PFC frame. The downstream ingress buffer must be large enough to store those additional packets after the xoff threshold has been reached.

Priority flow control is fully supported on both Broadcom and Mellanox switches.

PFC is disabled by default in Cumulus Linux. To enable priority flow control (PFC), you must configure the following settings in the /etc/cumulus/datapath/traffic.conf file on the switch:

  • Specify the name of the port group in pfc.port_group_list in brackets; for example, pfc.port_group_list = [pfc_port_group].
  • Assign a CoS value to the port group in pfc.pfc_port_group.cos_list setting. pfc_port_group is the name of a port group you specified above and is used throughout the following settings.
  • Populate the port group with its member ports in pfc.pfc_port_group.port_set.
  • Set a PFC buffer size in pfc.pfc_port_group.port_buffer_bytes. This is the maximum number of bytes allocated for storing bursts of packets, guaranteed at the ingress port. The default is 25000 bytes.
  • Set the xoff byte limit in pfc.pfc_port_group.xoff_size. This is a threshold for the PFC buffer; when this limit is reached, an xoff transition is initiated, signaling the upstream port to stop sending traffic, during which time packets continue to arrive due to the latency of the communication. The default is 10000 bytes.
  • Set the xon delta limit in pfc.pfc_port_group.xon_delta. This is the number of bytes to subtract from the xoff limit, which results in a second threshold at which the egress port resumes sending traffic. After the xoff limit is reached and the upstream port stops sending traffic, the buffer begins to drain. When the buffer reaches 8000 bytes (assuming default xoff and xon settings), the egress port signals that it can start receiving traffic again. The default is 2000 bytes.
  • Enable the egress port to signal the upstream port to stop sending traffic (pfc.pfc_port_group.tx_enable). The default is true.
  • Enable the egress port to receive notifications and act on them (pfc.pfc_port_group.rx_enable). The default is true.
  • The switch priority value(s) are mapped to the specific ingress buffer for each targeted switch port. Cumulus Linux looks at either the 802.1p bits or the IP layer DSCP bits depending on which is configured in the traffic.conf file to map packets to internal switch priority values.

The following configuration example shows PFC configured for ports swp1 through swp4 and swp6:

# to configure priority flow control on a group of ports:
# -- assign cos value(s) to the cos list
# -- add or replace a port group names in the port group list
# -- for each port group in the list
#    -- populate the port set, e.g.
#       swp1-swp4,swp8,swp50s0-swp50s3
#    -- set a PFC buffer size in bytes for each port in the group
#    -- set the xoff byte limit (buffer limit that triggers PFC frame transmit to start)
#    -- set the xon byte delta (buffer limit that triggers PFC frame transmit to stop)
#    -- enable PFC frame transmit and/or PFC frame receive
# priority flow control
pfc.port_group_list = [pfc_port_group]
pfc.pfc_port_group.cos_list = []
pfc.pfc_port_group.port_set = swp1-swp4,swp6
pfc.pfc_port_group.port_buffer_bytes = 25000
pfc.pfc_port_group.xoff_size = 10000
pfc.pfc_port_group.xon_delta = 2000
pfc.pfc_port_group.tx_enable = true
pfc.pfc_port_group.rx_enable = true

Port Groups

A port group refers to one or more sequences of contiguous ports. You can define multiple port groups by adding:

  • A comma-separated list of port group names to the port_group_list.
  • The port_set, rx_enable, and tx_enable configuration lines for each port group.

You can specify the set of ports in a port group in comma-separate sequences of contiguous ports; you can see which ports are contiguous in the /var/lib/cumulus/porttab file. The syntax supports:

  • A single port (swp1s0 or swp5)
  • A sequence of regular swp ports (swp2-swp5)
  • A sequence within a breakout swp port (swp6s0-swp6s3)
  • A sequence of regular and breakout ports, provided they are all in a contiguous range. For example:

On a Broadcom switch, restart switchd with the sudo systemctl restart switchd.service command to allow the PFC configuration changes to take effect. On a Mellanox switch with the Spectrum ASIC, restarting switchd is not necessary.

The PAUSE frame is a flow control mechanism that halts the transmission of the transmitter for a specified period of time. A server or other network node within the data center may be receiving traffic faster than it can handle it, thus the PAUSE frame. In Cumulus Linux, you can configure individual ports to execute link pause by:

  • Transmitting pause frames when its ingress buffers become congested (TX pause enable)
  • Responding to received pause frames (RX pause enable).

Link pause is disabled by default. To enabling link pause, you must configure settings in the /etc/cumulus/datapath traffic.conf file.

What’s the difference between link pause and priority flow control?

  • Priority flow control is applied to an individual priority group for a specific ingress port.
  • Link pause (also known as port pause or global pause) is applied to all the traffic for a specific ingress port.

Here is an example configuration that enables both types of link pause for swp1 through swp4 and swp6:

# to configure pause on a group of ports:
# -- add or replace port group names in the port group list
# -- for each port group in the list
#    -- populate the port set, e.g.
#       swp1-swp4,swp8,swp50s0-swp50s3
#    -- set a pause buffer size in bytes for each port in the group
#    -- set the xoff byte limit (buffer limit that triggers pause frames transmit to start)
#    -- set the xon byte delta (buffer limit that triggers pause frames transmit to stop)

# link pause
link_pause.port_group_list = [pause_port_group]
link_pause.pause_port_group.port_set = swp1-swp4,swp6
link_pause.pause_port_group.port_buffer_bytes = 25000
link_pause.pause_port_group.xoff_size = 10000
link_pause.pause_port_group.xon_delta = 2000
link_pause.pause_port_group.rx_enable = true
link_pause.pause_port_group.tx_enable = true

On a Broadcom switch, restart switchd with the sudo systemctl restart switchd.service command to allow the PFC configuration changes to take effect. On a Mellanox switch with the Spectrum ASIC, restarting switchd is not necessary.

Cut-through Mode and Store and Forward Switching

Cut-through mode is disabled in Cumulus Linux by default on switches with Broadcom ASICs. With cut-though mode enabled and link pause is asserted, Cumulus Linux generates a TOVR and TUFL ERROR; certain error counters increment on a given physical port.

cumulus@switch:~$ sudo ethtool -S swp49 | grep Error
HwIfInDot3LengthErrors: 0
HwIfInErrors: 0
HwIfInDot3FrameErrors: 0
SoftInErrors: 0
SoftInFrameErrors: 0
HwIfOutErrors: 35495749
SoftOutErrors: 0

cumulus@switch:~$ sudo ethtool -S swp50 | grep Error
HwIfInDot3LengthErrors: 3038098
HwIfInErrors: 297595762
HwIfInDot3FrameErrors: 293710518

To work around this issue, disable link pause or disable cut-through mode in the /etc/cumulus/datapath/traffic.conf file.

To disable link pause, comment out the link_pause* section in the /etc/cumulus/datapath/traffic.conf file:

cumulus@switch:~$ sudo nano /etc/cumulus/datapath/traffic.conf 
#link_pause.port_group_list = [port_group_0]
#link_pause.port_group_0.port_set = swp45-swp54
#link_pause.port_group_0.rx_enable = true
#link_pause.port_group_0.tx_enable = true

To enable store and forward switching, set cut_through_enable to false in the /etc/cumulus/datapath/traffic.conf file:

cumulus@switch:~$ sudo nano /etc/cumulus/datapath/traffic.conf
cut_through_enable = false

On switches using Broadcom Tomahawk, Trident II, Trident II+, and Trident3 ASICs, Cumulus Linux supports store and forward switching but does not support cut-through mode.

On switches with the Mellanox Spectrum ASIC, Cumulus Linux supports cut-through mode but does not support store and forward switching.

Congestion Notification

Explicit Congestion Notification (ECN) is defined by RFC 3168. ECN enables the Cumulus Linux switch to mark a packet to signal impending congestion instead of dropping the packet, which is how TCP typically behaves when ECN is not enabled.

ECN is a layer 3 end-to-end congestion notification mechanism only. Packets can be marked as ECN-capable transport (ECT) by the sending server. If congestion is observed by any switch while the packet is getting forwarded, the ECT-enabled packet can be marked by the switch to indicate the congestion. The end receiver can respond to the ECN-marked packets by signaling the sending server to slow down transmission. The sending server marks a packet ECT by setting the least 2 significant bits in an IP header DiffServ (ToS) field to 01 or 10. A packet that has the least 2 significant bits set to 00 indicates a non-ECT-enabled packet.

The ECN mechanism on a switch only marks packets to notify the end receiver. It does not take any other action or change packet handling in any way, nor does it respond to packets that have already been marked ECN by an upstream switch.

On Trident II switches only, if ECN is enabled on a specific queue, the ASIC also enables RED on the same queue. If the packet is ECT marked (the ECN bits are 01 or 10), the ECN mechanism executes as described above. However, if it is entering an ECN-enabled queue but is not ECT marked (the ECN bits are 00), then the RED mechanism uses the same threshold and probability values to decide whether to drop the packet. Packets entering a non-ECN-enabled queue do not get marked or dropped due to ECN or RED in any case.

ECN is implemented on the switch using minimum and maximum threshold values for the egress queue length. When a packet enters the queue and the average queue length is between the minimum and maximum threshold values, a configurable probability value will determine whether the packet will be marked. If the average queue length is above the maximum threshold value, the packet is always marked.

The downstream switches with ECN enabled perform the same actions as the traffic is received. If the ECN bits are set, they remain set. The only way to overwrite ECN bits is to set the ECN bits to 11.

ECN is supported on Broadcom Tomahawk, Tomahawk2, Trident II, Trident II+ and Trident3, and Mellanox Spectrum ASICs.

Click to learn how to configure ECN

Check Interface Buffer Status

  • On switches with ASICs, you can collect a fine-grained history of queue lengths using histograms maintained by the ASIC; see the ASIC Monitoring for details.
  • On Broadcom switches, the buffer status is not visible currently.
iptables-extensions man page