Ubigate iBG2016 Configuration Guide/Ed.00© SAMSUNG Electronics Co., Ltd. 199Buffer Reservation for InterfacesWhen a wan interface is created, a certain amount of queue buffers will bereserved for it’s transmitting queue from the global buffer pool. Bufferreservation is only done for wan interfaces and not GE interfaces because weexpect congestion and hence queue build-ups only on wan interfaces.The number of buffers reserved for a wan interface’s queue is such that itprovides 15 msec of buffering for 100 byte packets at a rate equal to theinterface bandwidth. This amount of buffering is always guaranteed for a waninterface. A wan interface can use more than its reserved amount of buffers byborrowing queue buffers from the global pool if available. However, only theportion allocated for transmit queues can be borrowed. Therefore it is notpossible for traffic, flooding a WAN interface, to consume all buffers andstarve other interfaces(including Ethernet) of receive buffers. The calculationbelow shows how the number of reserved buffers is calculated for a WANinterface.MAX((NxBCR_BUFS_RED * Bc)/AVG_PKTSIZE_FOR_BUF_CALC, 20) +(Bc/MIN_PKTSIZE_FOR_BUF_CALC)Where,The value of NxBCR_BUFS_RED is 3,The value of _PKTSIZE_FOR_BUF_CALC is 100 bytes,Bc is interface bandwidth proportional to scheduling interval(5 msec),The value of MIN_PKTSIZE_FOR_BUF_CALC 64 bytes.The first part of expression gives 15 msec worth of packet buffering, andsecond part of expression gives extra buffers accounting for buffers which arenot freed right away after completing transmission of packet from the driverqueue.
