Having SACK permitted can help improve the performance as the server doesn't have the retransmit all the packets again, from the point when a packet went missing. Also add info of additional Wireshark features where appropriate, like special statistics of this protocol. If it has value SACK permitted then that device allows selective acknowledgment. SACK is the method of resending only the necessary packets that never reached the receiver and hence help eliminate the redundant traffic. Capturing Packets After downloading and installing Wireshark, you can launch it and double-click the name of a network interface under Capture to start capturing packets on that interface. This is prone to cause unnecessary retransmission of data, which can exacerbate a congestion condition. The default TCP behavior is to send all the packets again after the original packet was lost. If either one has a smaller value then the smaller MSS size will be used in the rest of the TCP session. What happens if the MSS size of the Client & Server is not the same?ĭuring a three-way handshake, if both sides have the same MSS value than that is what it will be used throughout the TCP session.Smaller MSS value impacts network throughput. If the TCP MSS value is missing then a default size will be used and that is 536 bytes.
What if the MSS size is missing in the header?.If the value is 255/128/64 (/60 in some cases), 99.9% chance it was captured at the client-side. TTL value in the SYN packet can give you a big hint. This link details some of the default TTL values across devises/Operating systems. So far I have not come across a TTL of 255.
Usually, you will see a TTL of 128 or 64/60. The TTL can have a maximum value of 255 ( 8 bit header). TTL value gets decrement as the packet moves through every intermediary device on its way to its destination. Time-to-live (TTL) refers to the amount of time or “hops” that a packet is set to exist inside a network before it is discarded by a router. There is a lot of interesting and useful information you can learn from just analyzing TCP three-way handshake. In this post, I will focus on the TCP three-way handshake and share some tips & tricks I use to understand what is happening at the TCP/IP layer using Wireshark. Having a basic knowledge to analyze TCP packets can be a useful skill to have as a performance engineer. There are times when I need to look at the TCP packets to help design a load test script or understand what is happening under the hood.
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