TCP fast retransmission

During analysis of my ESX server, vsphere client communication I have encountered the following packet capture. My vsphere client keeps asking for a lost segment with ACK number 1583183 for 9 times after which according to wireshark analysis TCP Fast Retransmission which is one of the ways of handling congestion is engaged.

When I check RFC2581, it says  :

After receiving 3 duplicate ACKs, TCP performs a retransmission of what appears to be 
the missing segment, without waiting for the retransmission timer to expire.

TCP doesn’t wait for the re-transmission timer but isn’t this quite long to engage fast re-transmit? It is 9 duplicate acks in total. Maybe some of my duplicate acks are also lost? It might be. Anyway, it was good to check RFC once again for this retransmit incident.

About: rtoodtoo

Worked for more than 10 years as a Network/Support Engineer and also interested in Python, Linux, Security and SD-WAN, currently living in the Netherlands and works as a Network Support Engineer. // JNCIE-SEC #223 / RHCE / PCNSE


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