From help-file :
CBQ packet scheduler CONFIG_NET_SCH_CBQ Say Y here if you want to use the Class-Based Queueing (CBQ) packet scheduling algorithm for some of your network devices. This algorithm classifies the waiting packets into a tree-like hierarchy of classes; the leaves of this tree are in turn scheduled by separate algorithms (called "disciplines" in this context). See the top of net/sched/sch_cbq.c for references about the CBQ algorithm. CBQ is a commonly used scheduler, so if you're unsure, you should say Y here. Then say Y to all the queueing algorithms below that you want to use as CBQ disciplines. Then say Y to "Packet classifier API" and say Y to all the classifiers you want to use; a classifier is a routine that allows you to sort your outgoing traffic into classes based on a certain criterion. This code is also available as a module called sch_cbq.o ( = code which can be inserted in and removed from the running kernel whenever you want). If you want to compile it as a module, say M here and read Documentation/modules.txt.
From source-files :
Class-Based Queueing (CBQ) algorithm. ======================================= Sources:  Sally Floyd and Van Jacobson, "Link-sharing and Resource Management Models for Packet Networks", IEEE/ACM Transactions on Networking, Vol.3, No.4, 1995  Sally Floyd, "Notes on CBQ and Guaranted Service", 1995  Sally Floyd, "Notes on Class-Based Queueing: Setting Parameters", 1996  Sally Floyd and Michael Speer, "Experimental Results for Class-Based Queueing", 1998, not published. ----------------------------------------------------------------------- Algorithm skeleton was taken from NS simulator cbq.cc. If someone wants to check this code against the LBL version, he should take into account that ONLY the skeleton was borrowed, the implementation is different. Particularly: --- The WRR algorithm is different. Our version looks more reasonable (I hope) and works when quanta are allowed to be less than MTU, which is always the case when real time classes have small rates. Note, that the statement of  is incomplete, delay may actually be estimated even if class per-round allotment is less than MTU. Namely, if per-round allotment is W*r_i, and r_1+...+r_k = r < 1 delay_i <= ([MTU/(W*r_i)]*W*r + W*r + k*MTU)/B In the worst case we have IntServ estimate with D = W*r+k*MTU and C = MTU*r. The proof (if correct at all) is trivial. --- It seems that cbq-2.0 is not very accurate. At least, I cannot interpret some places, which look like wrong translations from NS. Anyone is advised to find these differences and explain to me, why I am wrong 8). --- Linux has no EOI event, so that we cannot estimate true class idle time. Workaround is to consider the next dequeue event as sign that previous packet is finished. This is wrong because of internal device queueing, but on a permanently loaded link it is true. Moreover, combined with clock integrator, this scheme looks very close to an ideal solution.