The network consists of three links of km max each and therefore, has a max one-way delay of 15 ms or kB at Mbps. The maximum receiver window is, thus, greater than twice the one-way delay.
We use a TCP timer granularity of ms. The timer is exercised only when there is packet loss. First, a set of headers and trailers are added to every TCP segment. This payload with padding requires 12 ATM cells of 48 data bytes each. Hence, the maximum receiver window of kB corresponds to cells over ATM. The algorithm measures the load and number of active sources over successive averaging intervals and tries to achieve a link utilization equal to the target.
This version includes the averaging feature for the number of sources and a large averaging interval of 5 ms, cells. The features and the reason for their choice is discussed in Section 5. All links run at Mbps. The links traversed by the connections are symmetric i. In our simulations, N is 15 and the link lengths may assume values , , and 1 km.
Feedback delay is the sum of the delay for feedback from the switch to reach the source and the delay for the new load from the sources to reach the switch. It is at least twice the one-way propagation delay from the source to the switch.
The feedback delay determines how quickly the feedback is conveyed to the sources and how quickly the new load is felt at the switch.
The maximum amplitude of the VBR source is VBR is given priority at the link, i. The link lengths yield a round trip time propagation of 30 ms and a feedback delay of 10 ms. The maximum source queue values third column are tabulated for every VC, while the maximum switch queue values fourth column are for all the VCs together. When there is no over ow the maximum source queue third column measured in units of cells is also presented as a fraction of the maximum receiver window.
The last column tabulates the aggregate TCP throughput. TCP then times out and retransmits the lost data. The remaining cells 0. The switch queues are zero because the sources are rate-limited by the ABR mechanism [9]. The TCP throughput The source can directly ow control the TCP source.
These edge routers may not be able to ow control the TCP sources except by dropping cells. All link lengths are km. The round trip time is 30 ms and the feedback delay is 10 ms. It also tries to achieve a target queueing delay. However, since the ABR capacity is scaled as a function of queue length, the queue maximum can be controlled. We vary the two VBR model parameters: the duty cycle d and the period p. Each parameter assumes three values. The duty cycle assumes values 0.
Observe that the maximum queues are small fractions of the round trip time. We observe that rows 5 and 6 have divergent unbounded queues. The duration of the OFF time determines how long such high rate feedback is given to sources. Possible solutions tes, a different artifact of TCP becomes apparent, and that is domination of one flow over the other.
At such high window The congestion-induced degradation in performance due sizes, the burst of data arriving from either of the flows is to synchronization or unfair bandwidth utilization due to much larger, and in most cases, the flow ahead of the queue domination can be rectified in a number of ways.
This manages to get through while the other one loses a lot of section examines two possible solutions—source rate con- cells. Once that occurs, the leading flow dominates the other trol and increased switch buffering—based on experimental which has backed off and continues to back off until the results.
These solutions are by no means the only methods dominating one finishes its session. Only then can the that exist to overcome these problems. For example, there backed-off flow proceed faster; however, due to the diffi- are ATM switches available in the market that implement culty in raising its congestion window much higher, it dis- packetdiscard algorithms [Early Packet Discard EPD or plays dismal throughput, much smaller than half the Partial Packet Discard PPD ] discussed in the literature throughput of the other flow.
This unfair behavior can [20,21]. There are ongoing experimental studies of these work against either flow8; it is completely random, as methods within Bellcore which are not available for pub- lication at the present time.
ABR switches will be able to 8 Domination effects are not clear on black-and-white paper. Authors can address these problems as well; however, they are just provide color graphs, if requested. Rate control the width of the high throughput section of the graph in Fig. The the amount of data into the ATM network; thus, into the glitch in Fig.
This can be provided by exercising rate timeout and retransmission events as was the case for the control at the ingress to the ATM network, either at the data glitch in Fig. As Fig. The difference they should be effective. These experiments are carried out between Figs. The average bandwidth assigned to each Fig. The bandwidth assigned to each flow is limited to longer used, maximum throughput is reached at very Mbps in this test, and an additional delay of 20 ms is small window sizes as in Figs.
The maximum throughput reaches 17— Note that the buffering they both sustain that level until the buffers on their source within the routers is the same kbyte. Note that As Figs. Sufficient buffering with the use of small burst sizes. Unfortunately, large burst sizes are found to yield significant throughput degradation Although rate shaping the streams at the entry to the ATM when multiple flows congest the small-buffer ATM switch, network is an effective solution, it requires provisioning by sending large bursts of data simultaneously to the DS3 parts of the total bandwidth for specific connections ahead ATM bottleneck.
Since all 40 Mbps each. Unfortunately, the smallest peak rate to aver- the connections, for each of which a portion of the band- age rate ratio that can be set is 2 a high oversubscription width is reserved, may not be active at all times, the active rate due to the limitations of the commercial equipment connections cannot utilize the unused bandwidth reserved available to this study at the time. A capability to choose for the inactive ones with CBR provisioning.
Furthermore, peak rates that yield smaller peak rate to average ratios VBR provisioning implemented in the IP routers is ineffec- should bring forth the advantages of leaky buckets for tive for small-buffer ATM switches.
In addition, provision- small-buffer ATM switches, and can then provide statistical ing connections requires manual intervention. Switched multiplexing and improved performance with the use of Virtual Circuits SVCs will address these problems; how- larger burst sizes. Unfortunately, current inflexible imple- ever, they are not available for the tests presented here. Sufficient buffering is a relative Figs. When the ratio of the band- the best performance under all circumstances.
This section tion to the ratio of the bandwidth assigned. Both of these tests are per flow, as shown in Fig. The performance in Fig. The major difference is that the performance curve in of either domination unfair bandwidth utilization between Fig.
The ratio of the buffer sizes is about The undesirable performance effects immediately after reaching small buffer switch has only a few hundreds of cell buffers its maximum throughput due to the small per port buffers per port, whereas the large buffer switch has a few tens of being exhausted rapidly.
If there was an additional delay of thousands of cells per port. There was also no added net- 20 ms, the high throughput section of the graph in Fig. Clearly, a switch buffer of a few port buffering in the ATM switch. For example, the sufficient cell buffers provide good throughput performance throughput in Fig.
A switch buffer of window sizes immediately after the window size at which tens of thousands of cells per port can comfortably accom- it reaches its maximum of Thus, other hand, a switch buffer of tens of thousands of cells per network delays that can possibly change during a TCP port performs well for a wide range of TCP window sizes; it session will not likely degrade the performance character- is evident from the maximum throughput around istics of the session.
However, with limited cell buffers, a Such problems do not exist for larger buffer [1] M. Larger buffers [3] J. Jacobson, R. The router requires buffer equal to the sum of the receiver window sizes of the participating TCP connections. Second, we introduce various patterns of VBR background traffic. View PDF on arXiv. Save to Library Save. Create Alert Alert. Share This Paper.
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