Contents

Architecture

The Two Bit DiffServ Architecture at the core routers allows for the coexistance of AF and EF. The figure shows the architecture.

 Figure 2: Two Bit DiffServ Architecture

 

The first level queue in the architecture is the ds_mark queuing discipline, which will be directly associated with the output device. This queuing discipline will extract the TOS byte from the incoming packet and pass it on to the tc_index classifier. The first level tc_index classifier extracts the DSCP from the TOS byte by masking with 0xfc and then right shifting by 2. For example, if the TOS is 0xb8,

TOS          0xb8        10111000
mask         0xfc        10111000
shift        2           00101110 (0x2e, the codepoint for EF)

It then stores the DSCP in skb->tc_index and feeds it onto the next level of tc_index filters. These filters are used to identify the AF class and drop precedence to which the packet belongs, if at all it belongs to the AF class. If it does, then it creates a new class based on the AF codepoint and stores the result of the classification in skb->tc_index. The value stored in the AF codepoint will be based on the AF class and the drop precedence within the AF class to which the packet belongs. The TOS byte, the corresponding DSCP and the class ids for the different AF classes are summarised below.

TOS(decimal)	hex	binary	DSCP(decimal)	hex	binary	Classid	AFClass
40	0x28	00101000	10	0x0a	00001010	1:111	AF 11
48	0x30	00110000	12	0x0c	00001100	1:112	AF 12
56	0x38	00111000	14	0x0e	00001110	1:113	AF 13
72	0x48	01001000	18	0x12	00010010	1:121	AF 21
80	0x50	01010000	20	0x14	00010100	1:122	AF 22
88	0x58	01011000	22	0x16	00010110	1:123	AF 23
104	0x68	01101000	26	0x1a	00011010	1:131	AF 31
112	0x70	01110000	28	0x1c	00011100	1:132	AF 32
120	0x78	01111000	30	0x1e	00011110	1:133	AF 33
136	0x88	10001000	34	0x22	00100010	1:141	AF 41
144	0x90	10010000	36	0x24	00100100	1:142	AF 42
152	0x98	10011000	38	0x26	00100110	1:143	AF 43

Once this classification is done, the packet is then handed over to a Class Based Queue (CBQ) 2:0.This CBQ will be used to differentiate between AF, EF and best effort. When the packets enter this CBQ, the EF packets are directed to class 2:1 by the 0x2e filter and all other packets belonging to the different AF classes and BE are sent to class 2:2 by the handle 0 filter. For EF, a pfifo is associated with the class 2:1. The EF class is configured to be bounded and isolated to ensure that it receives the bandwidth allocated for it. Now another CBQ 3:0 is derived from class 2:2 for AF and BE. In turn for each of the AF classes and BE traffic, classes 3:1 .. 3:5 are present. Each of the AF classes is associated with a GRED to allow for multiple drop precedences (there is a virtual queue for every drop precedence within an AF class) and the BE class has a RED queuing discipline. In this 3:0 CBQ, there is the 0xf0 filter which extracts the class number from the skb->tc_index. Then there are tc_index filters for each of the classes with handles 1,2,3,4 and 5 respectively. The AF packets are then queued in the appropriate Generalised Random Early Detection (GRED) queuing discipline. The GRED is more generalised form of the RED. In times of congestion, when it has to drop packets, it will drop them with the drop probability that is associated with the queue. The GRED then queues up the packet in the CBQ, which in turn sends the packet to the ds_mark queuing discipline for the packet to be transmitted.

1999-12-19