How to fix tcp dup ack


  • When is a Fast Retransmission Not a Fast Retransmission?
  • Detecting network errors and their impact on services
  • TCP series #3: network packet loss, retransmissions, and duplicate acknowledgements
  • The Visible Network
  • TCP SACK: In-Depth Review
  • When is a Fast Retransmission Not a Fast Retransmission?

    A word from my sponsor: Take your network monitoring to a new level with a Dynatrace! Share blog post Detecting errors like dropped packets or retransmissions on the network level is relatively easy. Figuring out if those errors affect the performance and connectivity of your services is however another matter.

    Some network errors are mitigated and compensated for by network protocols and active networking components, like network interfaces. Meanwhile, other network errors lead to performance problems that negatively affect your services. TCP, appropriate to the season, is the Santa Claus of protocols. Your services need not worry about retransmissions or network congestion.

    Nevertheless anybody running applications in production needs to understand TCP and its basics. Back in the days if two devices on the same Ethernet network e. With full duplex switches, where communication end-points can talk to each other at the same time, this potential error is obsolete. Even in wireless networks, which still work basically like hubs, network collisions can be neglected because there are procedures in place to avoid collisions in the first place e.

    With the help of checksums on the network level we are able to detect if a bit was toggled, missing, or duplicated by network data transmission. Checksums assure that received data is identical to the transmitted data.

    If the Ethernet checksum CRC is wrong the Ethernet frame is silently dropped by the network interface and is never seen by the operating system, not even with packet capturing tools.

    With the IP checksum and TCP checksum in the respective headers there are two additional supervisory bodies that can detect integrity errors.

    Full queues If the processing queue on a switch or router is overloaded, the incoming packets will be dropped. Also if the queue for incoming packets on the host you try to connect to is full, the packets will also be dropped. Every router that forwards an IP packet decreases the value of the field by one — it actually has nothing to do with time at all. Meanwhile, some network components drop packets with TTL equal to zero silently.

    This mechanism is useful for preventing packets from becoming caught up in an endless routing loop within your network. The observant reader and network veteran is familiar with this technique because traceroute uses it to identify all hops that a packet makes on its route to its destination. Packet retransmissions First off, retransmissions are essential for assuring reliable end-to-end communication in networks. Retransmissions are a sure sign that the self-healing powers of the TCP protocol are working — they are the symptom of a problem, not a problem in themselves.

    Common reasons for retransmissions include network congestion where packets are dropped either a TCP segment is lost on its way to the destination, or the associated ACK is lost on the way back to the sender , tight router QoS rules that give preferential treatment to certain protocols, and TCP segments that arrive out of order at their destination, usually because the order of segments became mixed up on the way from sender to destination.

    If the rate is higher, the user experience of your service may be affected. The good news is that most of the problems are findable using standard tools that are usually part of your operating system. The line that starts with RX contains information about received packets.

    The TX lines contain information about transmitted packets. RX information Details about received packets packets shows the number of successfully received packets. You can counter the first error by increasing the ring-buffer. TX information Details about transmitted packets packets shows the number of successfully transmitted packets. Again, increasing the TX buffer using ethtool may help. This parameter is relevant only for some queueing disciplines and can be overwritten using the tc command.

    For more information about queueing disciplines, take a look at this deep dive into Queueing in the Linux Network Stack and the tc-pfifo main page. This returns a lot of information and the output format is in a human-readable format, like the number of retransmitted and dropped packets sorted by protocol. If you want to focus on TCP retransmissions you can filter out the relevant information.

    Trying to connect to a closed port increases the TCP SYN retransmission counter ethtool This tool allows you to query and control the settings of the network interface and the network driver, as seen before.

    It shows you a detailed list of all errors that can occur on the network interface level, like CRC errors and carrier errors. If you have no retransmissions on the TCP layer but ifconfig still shows you a lot of erroneous packets, this is the place to look for the specifics. If a lot of errors show up in the ethtool output, it usually means that there is something wrong with the hardware NIC, cable, switchport. The next step would be to read the Linux Device Drivers book, digest it, and then start reading through the kernel source code e.

    Three helpful tools for gathering information about network errors tcpretrans tcpretrans is part of the perf-tools package. It offers you a live ticker of retransmitted TCP segments, including source and destination address and port, and TCP state information. If you suspect that more than one application or service is responsible for TCP retransmissions, tcpretrans allows you to debug your network connections if you call your services in isolation from each other and watch the output of tcpretrans.

    With a command line parameter you can write the output to a file for future analysis. Wireshark Wireshark , formerly ethereal, is the Swiss Army knife of network and protocol analyzer tools for Windows and Unix when it comes to analyzing TCP sessions, identifying failed connections, and seeing all network traffic that travels to and from your computer. You can configure it to listen on a specific network interface, specify filters to, for example, concentrate on a certain protocol, host, or port, and you can dump captured traffic to a file for an future analysis.

    Also, Wireshark can read tcpdump files, so you can capture traffic on one host on the command line and open the file for analysis in Wireshark on your computer for an analysis. Another feature of wireshark is that it knows a lot of common application protocols e. Network errors and analysis tools assigned to OSI layers Now you know how and where to find information about network errors. But what can you learn from this information?

    Though do you really need to investigate anything at all? After all, investigating each retransmitted or dropped packet is pointless—the network protocol stack has self-healing powers and some of the alleged errors are simply part of the game.

    What really counts Usually, more than one computer, switches, and routers are involved in networking. Ultimately, in more complex environments, you need tool support to stay on top of things. You need a monitoring tool that monitors all the hosts that are part of your infrastructure — a tool that notifies you when something out of the ordinary occurs.

    You need to be notified if any of these measurements fall in comparison to the baseline. Your main concern should be providing high-performance services that are easy to use and always available.

    There are a number of networking and service-related metrics you can measure and evaluate. The following three are a good starting point. Network traffic Measuring network traffic provides a good overview of the overall usage and performance of your service. Responsiveness Responsiveness measures the time from the last request packet that the service receives to the first response packet that the service sends. It measures the time a process needs to produce a response to a given request and should be watched in correlation with hardware resources.

    Connectivity Connectivity shows the percentage of properly established TCP connections compared to TCP connections that were refused or timed out.

    It shows when services were available to clients and when they were not, over time. Navigating from smartscape to process metrics Now the interesting part is how you can relate network errors to actual service response times. The really neat thing about Dynatrace is how well it integrates all this information to help me assess and fix this problem. Furthermore, you can see that the JavaScript error rate increased and that the root cause of this problem is a crashed couchDB process i.

    However monitoring connectivity and performance in more complex infrastructures with network overlays and encapsulation, virtual switches that run as applications, and intra-VM traffic that you never see on any physical network interface add additional layers of complexity. So stay tuned! There is a free usage tier so you can walk through all the functionality described here and see for yourself how well it works in your own environment.

    Detecting network errors and their impact on services

    In TCP acknowledgments are the source of proof to indicate that the recipient had received the packet. Acknowledgment makes the TCP a reliable protocol. Acknowledgments are the key component of TCP operation. Data sending host sends packets filled with data to the receiver, which receives it and sends an acknowledgment to sending host. It indicates to the sender that the receiver had received the data. Means if the sender has sent 10 data segments and received has only received few.

    As shown in above figure, Packet a bytes sent by server and client ACK at b. Next sent packet b bytes and client ACK at d. Now packet e, f lost and packet g ACKed in h. But this ACK g does not tell the server if the client has received all data from SEQ number however still indicates to the server that something had received by the client.

    The server will resend all the data back to the client from i to K and will eventually get ACK s in L. Above, the server resends the data in packet g again in packet K. Imagine in the real world how much data will be resent by the server which the client has already received causing load in the network.

    SACK takes 16 bits in the options field hex 04 02 seen in the figure. When the data packet lost and not received by the receiver. The sender sends subsequent packets following that lost packet again as discussed above. However, with SACK in action sender no need to resend those bytes again which are already received by received. It lets the sender to send only those bytes which not listed in the SACK option and hence reducing resends. Below figure is output from Wireshark taken at client side.

    This means Wireshark at the client-side has missed the sequence number of this TCP stream from the server. When this happens the client will send DUP acknowledgment in packet Next packet c is also ACKed with packet d.

    Packet e lost on the way to the client. Lastly ACK tells the server that now the client has received all bytes from 1 to

    TCP series #3: network packet loss, retransmissions, and duplicate acknowledgements

    Packets to probably arrived at the NetApp just 1. Halving packets per round trip is halving the data throughput. Just 25 packets to are transmitted in the next burst. We see 26, 27, 28, … packets per round trip eventually ramping all the way up to This is why we observe the linear throughput increase until the end. Only when it has packets in the correct order does it release them to us.

    From the retransmissions and their timing, we can infer that the first 21 packets of the burst of 79 arrived at the other side of our local ASA out-of-order and later than the others in the burst.

    The Visible Network

    H However, once the local ASA received the 24 late packets which were only about 1 ms late it released the full original burst of 79 packets to us in the correct order.

    It indicates to the sender that the receiver had received the data. Means if the sender has sent 10 data segments and received has only received few. As shown in above figure, Packet a bytes sent by server and client ACK at b.

    Next sent packet b bytes and client ACK at d. Now packet e, f lost and packet g ACKed in h. But this ACK g does not tell the server if the client has received all data from SEQ number however still indicates to the server that something had received by the client. The server will resend all the data back to the client from i to K and will eventually get ACK s in L. Above, the server resends the data in packet g again in packet K.

    Meanwhile, other network errors lead to performance problems that negatively affect your services. TCP, appropriate to the season, is the Santa Claus of protocols. Your services need not worry about retransmissions or network congestion. Nevertheless anybody running applications in production needs to understand TCP and its basics. Back in the days if two devices on the same Ethernet network e. With full duplex switches, where communication end-points can talk to each other at the same time, this potential error is obsolete.

    Even in wireless networks, which still work basically like hubs, network collisions can be neglected because there are procedures in place to avoid collisions in the first place e. With the help of checksums on the network level we are able to detect if a bit was toggled, missing, or duplicated by network data transmission. Checksums assure that received data is identical to the transmitted data.

    If the Ethernet checksum CRC is wrong the Ethernet frame is silently dropped by the network interface and is never seen by the operating system, not even with packet capturing tools. With the IP checksum and TCP checksum in the respective headers there are two additional supervisory bodies that can detect integrity errors. Full queues If the processing queue on a switch or router is overloaded, the incoming packets will be dropped.

    Also if the queue for incoming packets on the host you try to connect to is full, the packets will also be dropped. Every router that forwards an IP packet decreases the value of the field by one — it actually has nothing to do with time at all. Meanwhile, some network components drop packets with TTL equal to zero silently. This mechanism is useful for preventing packets from becoming caught up in an endless routing loop within your network.

    The observant reader and network veteran is familiar with this technique because traceroute uses it to identify all hops that a packet makes on its route to its destination. Packet retransmissions First off, retransmissions are essential for assuring reliable end-to-end communication in networks. Retransmissions are a sure sign that the self-healing powers of the TCP protocol are working — they are the symptom of a problem, not a problem in themselves.

    TCP SACK: In-Depth Review

    Common reasons for retransmissions include network congestion where packets are dropped either a TCP segment is lost on its way to the destination, or the associated ACK is lost on the way back to the sendertight router QoS rules that give preferential treatment to certain protocols, and TCP segments that arrive out of order at their destination, usually because the order of segments became mixed up on the way from sender to destination.

    If the rate is higher, the user experience of your service may be affected. The good news is that most of the problems are findable using standard tools that are usually part of your operating system. The line that starts with RX contains information about received packets.


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