Kernel Korner - Analysis of the HTB Queuing Discipline

When defining the HTB configuration, the following options of the tc class commands were used in order to achieve the required results:

Most of the traffic in the Internet world is generated by TCP, so packet sizes representative of datagram sizes, such as 64 (TCP Ack), 512 (FTP) and 1,500, were included for all test cases.

Figure 6. Test 1: One Stream In, One Stream Out

From the results in Table 4, the following statements can be made:

Figure 7. Graphic Analysis of Packet Size vs. Output Bandwidth

Figure 7 illustrates the relationship between packet size and output bandwidth after testing various packet sizes. From the results in Table 4 and Figure 7, we can conclude two things: throughput accuracy can be controlled by changing the cburst/burst values and the accuracy bandwidth range size is 2Mbit/s when using packets of sizes between 64 and 1,500 bytes. To verify that a linear relationship exists between the burst/cbursts values and the rate bandwidth, multiple burst and cburst values were tested. Table 5 shows the significant portion of the sampled data from the test case.

18k/18k values were used as a starting point. The burst/cburst values were obtained by using the formula cburst/burst (Kbytes) = 18/(30M/Assign rate). From the results in Table 5, cburst/burst values can be defined dynamically for the rate value, such as when assuming a linear relationship.

Figure 8. Test 2: Three Streams In, One Stream Out

Table 6 shows an example of one level of inheritance. Class 2 and class 3 inherit the rate limit specification from class 1 (30Mbit/sec). In this test, the rate ceiling of the child classes is equal to the parent's rate limit, so class 2 and class 3 can borrow up to 30Mbit/sec. The linear relation assumption was used to calculate the cburst/burst values of all classes, based on their desired bandwidth.

Table 6 describes how the linear relationship works in the case of one level of inheritance. In this test, the input stream transmits continuous traffic of 39Mbit/s, and the accumulated output bandwidth is 31.8Mbit/s.

Figure 9. Test 3: Four Streams in, One Stream out

Table 7 shows the case of two levels of inheritance. Class 2 and class 3 inherit the rate limit specification from class 1 (30Mbit/s). Classes 4, 5 and 6 inherit the rate limit of class 3 (28Mbit/s) and share it based on their own rate limit specifications. In this test, the rate ceiling of the child classes is equal to the parent's rate limit, so classes 4, 5 and 6 can borrow up to 28Mbit/s. The linear relation assumption was used for calculating the cburst/burst values of all classes, based on their desired bandwidth.

From the results of Table 7, it can be observed that the linear relationship works in the case of two levels of inheritance. In this test the input port transmits continuous traffic of 50Mbit/s, and the accumulated output bandwidth is 32.05Mbit/s.