Ecn (Explicit Congestion Notification); Data Center And Virtualization - Fujitsu PSWITCH 2048T User Manual

User's Guide
by priority control. The IEEE 802.1p protocol defines a total of eight priority levels
and class of service queues.

3.1.7.1. ECN (Explicit Congestion Notification)

Explicit Congestion Notification (ECN) is defined in RFC 3168. Conventional TCP
networks signal congestion by dropping packets. A Random Early Discard scheme
provides earlier notification than a tail drop scheme by dropping packets already
queued for transmission. ECN marks congested packets that would otherwise have
been dropped and expects an ECN-capable receiver to signal congestion back to
the transmitter without the need to retransmit the packet that would have been
dropped.
ECN uses the two least significant bits of Diffserv field (TOS octet in IPv4/Traffic
Class octet in IPv6) and codes them as follows:
00: Non ECN-Capable Transport – Non-ECT
10: ECN Capable Transport – ECT(0)
01: ECN Capable Transport – ECT(1)
11: Congestion Encountered – CE
This software WRED considers packets for early discard only when the number of
packets queued for transmission on a port exceeds the relevant minimum WRED
threshold. Four thresholds are available for configuration. The green, yellow, and
red thresholds operate on TCP packets. The fourth threshold operates on non-TCP
packets.
When ECN is enabled and congestion is experienced, packets that are marked
ECN-capable, are queued for transmission, and are randomly selected for discard
by WRED are instead marked CE and are transmitted rather than dropped. This
includes packets that exceed the WRED upper threshold. If the switch experiences
severe congestion (no buffers available), then packets are discarded.
This software implements ECN capability as part of the WRED configuration process.
Eligible packets are marked by hardware based on the WRED configuration. The
network operator can configure any CoS queue to operate in ECN marking mode
and can configure different discard thresholds for each color.

3.1.8. Data Center and Virtualization

The FUJITSU PSWITCH supports the following Data Center and virtualization
oriented features.
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