![]() ![]() This is also sometimes referred to as TCP retransmission due to RTO (Retransmit Timeout). TCP Retransmissions - These are the normal (althought not so normal if it happens quite often) everyday retransmissions in which the Packets sent are not acknowledged by receiver within a certain amount time (derived from SRTT. Here is a good write-up RFC 6298 if you want to get more information on how RTO is calculated. This timer is dynamically adjusted according to RTT and other factors. Every TCP segment as it is sent to the IP layer has a timer associated with it and an ACK should be received before this timer expires. In Wireshark, TCP retransmissions are classified as one of three categories. After some investigation, I found out what these mysterious Spurious retransmissions really are. started to wonder what this really means. It reads " not being what it purports to be false or fake". It can be useful to check for incrementing retransmits here as well (and may be faster than creating a trace, installing Wireshark, and viewing).First time I saw on "TCP Spurious Retransmissions" on Wireshark, I had to look up the definition of Spurious on Google as I've never heard that word before :). In ONTAP 9.5 and above, it is called Rexmit In ONTAP 9.1 and below it is called Retransmits In ONTAP 9.1 and below (including Data ONTAP 8), or 9.5 and above, the netstat command will have a retransmit column. This is the amount of time needed for bytes to be sent by a sender, the receiver to acknowledge the bytes and the sender to receive the acknowledgement It is calculated by estimating how much congestion there is on the link. The congestion window is a means of stopping a link between the sender and the receiver from becoming overloaded with too much traffic Note that this is not to be confused with the sliding window size which is maintained by the receiver The congestion window is maintained by the sender In TCP, the congestion window is one of the factors that determines the number of bytes that can be sent out at any time If, for example, a sender receives an ack which acknowledges byte 4000 and specifies a receive window of 10000 (bytes), the sender will not send packets after byte 14000, even if the congestion window allows it The receiver may be overwhelmed by data if for example it is very busy (such as a Web server)Įach TCP segment contains the current value of the receive window The congestion window tries not to exceed the capacity of the network (congestion control) the receive window tries not to exceed the capacity of the receiver to process data (flow control) ![]() The throughput of a communication is limited by two windows: the congestion window and the receive window The round trip time includes not only the propagation delay of the wires, and the switch latency, but also any buffering within the switch, the host or the storage system while exchanging trafficĪ switch that switches between different link speeds should provide buffer memory in this range on the participating ports. The bandwidth-delay product can be estimated by multiplying the ports link speed (in Bits per second) divided by 10, with the round trip time under load across the switch - typically in the order of around 1 millisecond: 40 Gbps / 10 ~= 4 GB/sec * 0.001 sec = 4.2 MB buffer memory The result, an amount of data measured in bits (or bytes), is equivalent to the maximum amount of data on the network circuit at any given time, i.e., data that has been transmitted but not yet acknowledged The product of a data link's capacity (in bits per second) and its round-trip delay time (in seconds) ![]() ![]() How to determine packet loss and the possible reasons it could be occurring This Wireshark filter will let you see those packets: > 0.Some systems will flag a TCP flag called SACK (such as ONTAP), which can be used to identify how many packets are missing at a time.To see which packets Wireshark has flagged as a retransmission:.When a packet is lost, the receiver stops responding to new packets until it is retransmitted, causing delays that could last as long as half a second.Badwidth Delay Product (BDP) associates this congestion window, with the round trip time, to give us an average expected throughput.The limitation set by this algorithm is the congestion window(cwin) and in a lossy network an average congestion window is used to see how much data is sent before packet loss.Bad read performance for reads after upgrading to 9.6 and later - NetApp Knowledge Base.When packet loss is seen, congestion algorithms limit the amount of tcp data on the network to prevent further loss.The goal of this article is to describe how packet loss typically causes performance issues, not the reason(s) that loss happens.There are numerous reasons why packet loss can cause performance impact. ![]()
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