Comtech EF Data CDM-IP 300L - Manual (Page 47)

CDM-IP 300L IP-Centric Satellite Modem Rev. 1
CD/CDMIP300L.IOM
19
Weighted Random Early Detection (WRED) – Without Weighted Random Early Detection
(WRED), output buffers fill during periods of congestion. When the buffers are full, tail drop
occurs; all additional packets are dropped. Since the packets are dropped all at once, global
synchronization of TCP hosts can occur as multiple TCP hosts reduce their transmission rates.
As the congestion clears, the TCP hosts increase their transmissions rates, resulting in waves of
congestion followed by periods where the transmission link is not fully used.
WRED allows for more graceful dropping of packets, as QoS queues get full. In the previous
CDM-IP release, a simple tail drop algorithm was applied to packets as they were being added to
the QoS queues. This can result in large number of contiguous packets being dropped which
causes many protocols such as RTP and TCP to ungracefully degrade performance in a over-
consumed or bursty scenario. WRED applies a randomization which means that the percentage
change to drop packets increases as the queue becomes full, and minimizes the chances of global
synchronization. Thus, WRED allows the transmission line to be used fully at all times.
Filtering – Any specific flow can be designated as filtered (see Maximum
Bandwidth/Priority QoS).
QoS Rule Hierarchy – The QoS Rule Hierarchy is the same as Maximum
Bandwidth/Priority QoS.
QoS Statistics - QoS Statistics are displayed as Maximum Bandwidth/Priority QoS.
1.4.4.3 D
IFF
S
ERV
Q
O
S M
ODE
The CDM-IP QoS can also be set to DiffServ Mode to make it fully compliant to the Differential
Services QoS standards.
Class Selector DiffServ Code Points (DSCP) – Some implementations of DiffServ will
prioritize traffic by Class Selector assignment. This is defined in the DiffServ Code Points
(DSCP) within the IP header. The first 3 bits of the DSCP define the Class Selector Precedence
(or Priority):
Class Selector DSCP CDM-IP Priority
Precedence 1 001 000 1
Precedence 2 010 000 2
Precedence 3 011 000 3
Precedence 4 100 000 4
Precedence 5 101 000 5
Precedence 6 110 000 6
Precedence 7 111 000 7
Default 000 000 9
The CDM-IP will prioritize the traffic based upon the DSCP Class Selector Precedence.
NOTE: All traffic that does not have the DSCP Class Selector Precedence defined (000
000) will be placed in the Default Queue and have a Precedence of 9.