Quoting Sally Floyd: | * Open issue: Add possible mechanisms for limited the maximum | burst size? Using a token bucket size based on the | current rate? Or not? Email from Eddie Kohler and Gerrit | Renker. Ouch. Didn't read that one. The token bucket question is actually a phantom, the only possible logical conclusion here is that the bucket size is zero. Below is the copy from the patch which has the details if anyone is interested; the short conclusion is that the bucket size must be zero. And this agrees with Eddie's answer to the token bucket question. [CCID 3]: Avoid accumulating of large send credit Problem: -------- Large backlogs of packets which can be sent immediately currently accumulate when (i) the application idles, or (ii) the application emits at a rate slower than the allowed rate X/s, or (iii) due to scheduling inaccuracy (resolution only up to HZ). The consequence is that a huge burst of packets can be sent immediately, which violates the allowed sending rate and can (worst case) choke the network. Fix: ---- Avoid any backlog of sending time which is greater than one whole t_ipi. This permits the coarse-granularity bursts mentioned in [RFC 3448, 4.6], but disallows the disproportionally large bursts. D e t a i l e d J u s t i f i c a t i o n [not commit message] ------------------------------------------------------------------ Let t_nom < t_now be such that t_now = t_nom + n*t_ipi + t_r, where n is a natural number and t_r < t_ipi. Then t_nom - t_now = - (n*t_ipi + t_r) First consider n=0: the current packet is sent immediately, and for the next one the send time is t_nom' = t_nom + t_ipi = t_now + (t_ipi - t_r) Thus the next packet is sent t_r time units earlier. The result is burstier traffic, as the inter-packet spacing is reduced; this burstiness is mentioned by [RFC 3448, 4.6]. Now consider n=1. This case is illustrated below |<----- t_ipi -------->|<-- t_r -->| |----------------------|-----------| t_nom t_now Not only can the next packet be sent t_r time units earlier, a third packet can additionally be sent at the same time. This case can be generalised in that the packet scheduling mechanism now acts as a Token Bucket Filter whose bucket size equals n: when n=0, a packet can only be sent when the next token arrives. When n>0, a burst of n packets can be sent immediately in addition to the tokens which arrive with rate rho = 1/t_ipi. The aim of CCID 3 is an on average smooth traffic with allowed sending rate X. The following determines the required bucket size n for the purpose of achieving, over the period of one RTT R, an average allowed sending rate X. The number of bytes sent during this period is X*R. Tokens arrive with rate rho at the bucket, whose size n shall be determined now. Over the period of R, the TBF allows s * (n + R * rho) bytes to be sent, since each token represents a packet of size s. Hence we have the equation s * (n + R * rho) = X * R <=> n + R/t_ipi = X/s * R = R / t_ipi which shows that n must be 0. Hence we can not allow a `credit' of t_nom - t_now > t_ipi time units to accrue in the packet scheduling.
