Chaladi's Blog

I have been haunted by this weird TCP spurious retransmissions and TCP DUP ACK issue since past 1 month – It almost started/I’ve noticed on November last week. Our production FTP server is a Red Lion device See here sitting in our manufacturing site, whereas our source servers are hosted on Hyper-V clusters. This setup has no Firewalls; only Cisco Nexus Switches 3064  & 3048 Models – that’s 3 3064’s and 2 3048 models connected in a HA model. Our Hyper-V clusters are connected to Cisco 3064 Switches in HA model; 2 Nic cables pulled from each VM Host to 2 3064 Switches for HA. Red Lion – FTP/HTTP device has been attached to 3048 model. These 3064’s are connected to the 3048 Switches directly – no firewalls.

STP is configured properly and running A-okay. Other than Red Lion device, I was able to route traffic as desired and can reach data transfer rates at 250 MB/s. But if this same Red Lion device is moved and connected to a different network that’s having Cisco Catalyst switches, this Device is working fine. No retransmissions issue.

There are a lot of packet retransmissions happening just before the FTP application failing with error – BTW, I am using Filezilla client to transfer data to the FTP Box. Same is the case when browsing the FTP/HTTP site hosted on the Red Lion box via IE from my machines.

Wireshark Analysis

I’ve analysed the network connection between these servers in question and noticed that there are a lot of packet retransmissions happening. TCP “RST” (RESET), “Spurious Retransmissions” (Source Retransmitted the packet even though the DEST ACK; assuming the DEST hasn’t ACK) are noticed in high numbers. This is not the case when I tried to capture traffic between the other sources.

TCP RST couldn’t be considered as the issue normally because this happens after every session closure. But in our case the packet retransmissions and failing communication are resetting the RPC port communication and thus these messages are seen. So obviously, in both success and failure cases we will see this kind of messages.

TCP Segment Length

I have noticed that the Maximum Segment Size; MSS of the destination server – Redlion box is “1280” and the source server is “1460”. Pinging with 1460 without fragmentation to the destination server which has 1280 MSS value is responding fine; data that remote server responds with has same data length size – “data.len>1460” filter applied shows that ICMP data of 1460 is transmittable both ways. Both the source and destination servers acknowledged to communicate using 1280 MSS value as they should be per application protocols standards; verified this as per “tcp.len>1200” filter applied and could see traffic generated has no TCP segment length that is using higher segment size than 1280 size in the application communications and thus eliminating the possible MSS size issue for packet retransmissions.

Port Query Results

ICMP packets are fine, they don’t have any issues. Only FTP/HTTP traffic is getting affected. This means no issues until Network layer, but with Application/session layer the traffic is getting worse. And at times the Portqry too failing with Filtered messages on port 21 from Source to destination FTP box.

Right now I am doubting the Speed/duplex settings on these switches and VM Hosts. Our VM Hosts are 10G capable NICs and Switches too. It is hard-coded in Nexus switches regarding speed at VM Hosts interface, so technically switches are controlling the speed, so I got nothing to do on VM Hosts speed/duplex settings; anything I want to modify is left with Nexus switch.  End device Red Lion FTP box is only 100 MB Capable. Cannot blame if source talking at full 10 Gig speed and end device is failing to respond with same speed. Because the normal SYNC, ACK communication too getting affected with the TCP retransmissions; at this same time, I cannot assume this couldn’t be the reason. It still needs analysis to rule out things.

Worked with Cisco and they say Nexus switches don’t support buffering, so 10 Gig source and 100 MB destination don’t work in the nexus environment. Buffering is not capable they say in Nexus switches. An alternative they propose to fix is to update the IOS on these Nexus switches; but that’s tentative solution.

—— Update on 23rd Jan 2016—–

<<We’ve updated the Nexus IOS version to the latest, yet we see the same issues. Still banging head to get this fixed.>>

I will keep on updating this thread as more progress is made… Comments are welcome.

Cheers!

Chaladi