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FTP Single-Thread Upload Is Slow Due to a Long Ping Delay on Users Connected to the NE40E

Publication Date:  2013-09-03 Views:  98 Downloads:  0
Issue Description
1. For the networking, see the attachment.
2. Symptom: On a DCN in a city, when users connected to the NE40E find that the FTP single-thread upload is slow. The upload speed is only 1 Mbit/s in the environment where 100 Mbit/s bandwidth is available.
Handling Process

Possible causes:
1. The FTP server is faulty.
2. The NE40E discards some data during data forwarding.
3. The link quality is poor.
Analysis procedure:
1. Replace the FTP server. The fault persists.
2. Collect and send diagnostic statistics on the NE40E to the R&D personnel. No board discards packets.
3. Capture packets for analysis. The response time for a TCP packet is long.
In the environment with the 30 ms ping delay, capture and analyze packets as follows:

The number of exchanged packets returns to zero every around 20 ms. The network is idle for 10 ms. Click the idle part in the diagram to locate the packet details. After verification using the timestamp, the local end waits for around 10 ms for ACK packets from the peer end after sending FTP data packets.
Change the cost value of the NE40E and redirects traffic to another link to decrease the ping delay to 3 ms. Capture and analyze packets are follows:

The network is seldom idle.
Root Cause
The delay in the intermediate link is long, wasting link resources.
1. Change the cost value to redirect traffic to another link with a smaller delay.
2. Troubleshoot the intermediate link to verify the cause of the delay.
1. Use the multi-thread upload/download tool.
For the single-thread upload tool, a delay of dozens of milliseconds may cause low network bandwidth usage. This is because TCP is a connection-based reliable transport layer protocol. After sending data, the sender must wait for ACK packets from the peer (TCP sliding window). For the single-thread tool (there is only one TCP connection), the network is idle during the waiting time of ACK plackets, so the network bandwidth usage is low.
For the multi-thread upload tool, multiple TCP connections are set up. When one TCP connection is in waiting state, other connections can make full use of network resources.

2. Change the cost of the link with a large delay.
The delay for ping packets on the directly connected device is similar to the device forwarding delay. When the network interface bandwidth is the same, a larger delay of ping packets indicates a higher cost. You need to adjust the cost of the link so that IS-IS does not select the link.