Wired Goodput reless Goodput LAN: Absolute□ Percentage of max□ 100 WAN: Absolute Percentage of max Throughput 三 LL-TCP-AWARE LL-SMART LL-SMART-TCP-AWARE Figure 3. Performance of link-layer protocols: bit-error rate=1.9x10(I error/65536 bytes), socket buffer size= 32 KB. For each case there are two bars: the thick one corresponds to the scale on the left and denotes the throughput in Mbps: the thin one corresponds to the scale on the right and shows the throughput as a percentage of the maximum, i.e. in the absence of wireless errors(1.5 Mbps in the Lan environment and 1.35 Mbps in the WAN environment). Internet hops with minimal congestion" in order to study the the link and transport layers often lead to significant perfor impact of large delay-bandwidth products mance degradation. However, this is not the dominating effect when link layer schemes, such as LL, are used with Each run in the experiment consists of an 8 MByte transfer TCP Reno and its variants. These TCP implementations from the source to receiver across the wired net and the have coarse retransmission timeout granularities that WaveLAN link. We chose this rather long transfer size in order to limit the impact of transient behavior at the start of typically multiples of 500 ms, while link-layer protocols a TCP connection. During each run, we measure the typically have much finer timeout granularities. The real problem is that when packets are lost, link-layer protocol throughput at the receiver in Mbps, and the wired and wire- that do not attempt in-order delivery across the link(e.g less goodputs as percentages. In addition, all packet trans- LL) cause packets to reach the TCP receiver out-of-order missions on the ethernet and waveLan are recorded for This leads to the generation of duplicate acknowledgments nalysis using tcpdump [201, and the sender's TCP code by the TCP receiver, which causes the sender to invoke fast instrumented to record events such as coarse timeouts retransmission and recovery. This can potentially cause retransmission times, duplicate acknowledgment arrivals, degraded throughput and goodput, especially when the congestion window size changes, etc. The rest of this sec tion presents and discusses the results of these experiments delay-bandwidth product is large Our results substantiate this claim, as can be seen by com- 4.2 Link-Laver Protocols paring the LL and LL-TCP-AWARE results(Figure 3 and Traditional link-layer protocols operate independently of Table 2). For a packet size of 1400 bytes, a bit error rate of 1.9x10-0(1/65536 bytes)translates to a packet error rate of the higher-layer protocol, and consequently, do not neces- about 2.2 to 2.3%. Therefore, an optimal link-layer protocol sarily shield the sender from the lossy link. In spite of local that recovers from errors locally and does not compete with retransmissions, TCP performance could be poor for two TCP retransmissions should have a wireless goodput of tting of timers wo lay ers. an sary invocations of the TCP fast retransmission mechanism gestion. In the LAN experiments, the throughput difference between LL and LL-TCP-AWARE is about 10%. however due to out-of-order delivery of data In[10], the effects or the LL wireless goodput is only 95.5%, significantly less the first situation are simulated and analyzed for a TCP-like than LL- TCP-AWARE's wireless goodput of97.6%,which transport protocol (that closely tracks the round-trip time to set its retransmission timeout) and a reliable link-layer pro- is close to the maximum achievable goodput. When a loss tocol. The conclusion was that unless the packet loss rate is high(more than about 10%), competing retransmissions by atively quickly. However, enough packets are typically in transit to create more than 3 duplicate acknowledgments These duplicates eventually propagate to the sender and 2. WAN experiments across the US were performed between 10 trigger a fast retransmission and the associated congestion pm and 4 am, PST and we verified that no congestion losses control mechanisms. These fast retransmissions result in occurred in the runs reportedInternet hops with minimal congestion2 in order to study the impact of large delay-bandwidth products. Each run in the experiment consists of an 8 MByte transfer from the source to receiver across the wired net and the WaveLAN link. We chose this rather long transfer size in order to limit the impact of transient behavior at the start of a TCP connection. During each run, we measure the throughput at the receiver in Mbps, and the wired and wire￾less goodputs as percentages. In addition, all packet trans￾missions on the Ethernet and WaveLan are recorded for analysis using tcpdump [20], and the sender’s TCP code instrumented to record events such as coarse timeouts, retransmission times, duplicate acknowledgment arrivals, congestion window size changes, etc. The rest of this sec￾tion presents and discusses the results of these experiments. 4.2 Link-Layer Protocols Traditional link-layer protocols operate independently of the higher-layer protocol, and consequently, do not neces￾sarily shield the sender from the lossy link. In spite of local retransmissions, TCP performance could be poor for two reasons: (i) competing retransmissions caused by an incom￾patible setting of timers at the two layers, and (ii) unneces￾sary invocations of the TCP fast retransmission mechanism due to out-of-order delivery of data. In [10], the effects of the first situation are simulated and analyzed for a TCP-like transport protocol (that closely tracks the round-trip time to set its retransmission timeout) and a reliable link-layer pro￾tocol. The conclusion was that unless the packet loss rate is high (more than about 10%), competing retransmissions by the link and transport layers often lead to significant perfor￾mance degradation. However, this is not the dominating effect when link layer schemes, such as LL, are used with TCP Reno and its variants. These TCP implementations have coarse retransmission timeout granularities that are typically multiples of 500 ms, while link-layer protocols typically have much finer timeout granularities. The real problem is that when packets are lost, link-layer protocols that do not attempt in-order delivery across the link (e.g., LL) cause packets to reach the TCP receiver out-of-order. This leads to the generation of duplicate acknowledgments by the TCP receiver, which causes the sender to invoke fast retransmission and recovery. This can potentially cause degraded throughput and goodput, especially when the delay-bandwidth product is large. Our results substantiate this claim, as can be seen by com￾paring the LL and LL-TCP-AWARE results (Figure 3 and Table 2). For a packet size of 1400 bytes, a bit error rate of 1.9x10-6 (1/65536 bytes) translates to a packet error rate of about 2.2 to 2.3%. Therefore, an optimal link-layer protocol that recovers from errors locally and does not compete with TCP retransmissions should have a wireless goodput of 97.7% and a wired goodput of 100% in the absence of con￾gestion. In the LAN experiments, the throughput difference between LL and LL-TCP-AWARE is about 10%. However, the LL wireless goodput is only 95.5%, significantly less than LL-TCP-AWARE’s wireless goodput of 97.6%, which is close to the maximum achievable goodput. When a loss occurs, the LL protocol performs a local retransmission rel￾atively quickly. However, enough packets are typically in transit to create more than 3 duplicate acknowledgments. These duplicates eventually propagate to the sender and trigger a fast retransmission and the associated congestion control mechanisms. These fast retransmissions result in 2. WAN experiments across the US were performed between 10 pm and 4 am, PST and we verified that no congestion losses occurred in the runs reported. LL LL-TCP-AWARE LL-SMART LL-SMART-TCP-AWARE Throughput (Mbps) Wireless Goodput LAN: Absolute Wired Goodput Figure 3. Performance of link-layer protocols: bit-error rate = 1.9x10-6 (1 error/65536 bytes), socket buffer size = 32 KB. For each case there are two bars: the thick one corresponds to the scale on the left and denotes the throughput in Mbps; the thin one corresponds to the scale on the right and shows the throughput as a percentage of the maximum, i.e. in the absence of wireless errors (1.5 Mbps in the LAN environment and 1.35 Mbps in the WAN environment). Throughput 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Percentage of max. WAN: Absolute Percentage of max. Throughput (% of maximum) 0 10 20 30 40 50 60 70 80 90 100 95.5 97.9 1.20 95.5 98.4 0.82 97.6 100.0 1.36 97.6 100.0 1.19 95.5 98.3 1.29 95.3 99.4 0.93 97.7 100.0 1.39 97.6 100.0 1.22