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27 May 2026

Reading time ~11 minutes

Deep-Dive: How AWS Nitro v6 (c8i) Changes TCP Connection Tracking and How to Fix Silent Drops

Upgrading your AWS EC2 workloads to the latest generation—such as the Intel-based c8i or Graviton-based m8g/r8g families—is a standard task for modernizing infrastructure. However, many engineering teams encounter a baffling post-migration symptom: sudden application timeouts, microservice connection drops, and hanging RPC calls.

If your architecture utilizes long-lived, idle TCP connections (such as persistent database pools, gRPC streams, or internal RPCs) routed through Network Load Balancers (NLB) or Gateway Load Balancers (GWLB), you are likely running directly into a critical architectural change introduced in the AWS Nitro v6 platform.

This post dives deep into the root cause of these silent drops, shares real-world tcpdump packet traces from a replication lab, and details how to implement permanent fixes at both the network and OS levels.

###The Culprit: Nitro v6 Connection Tracking Thresholds When security groups or load balancers are associated with an Elastic Network Interface (ENI), AWS uses stateful Connection Tracking (conntrack) on the physical Nitro card to monitor TCP session states.

To optimize hardware resources for high-throughput modern instances, the Nitro v6 platform introduces a much more aggressive default configuration for the TCP established state timeout (TcpEstablishedTimeout):

Nitro Platform GenerationInstance FamiliesDefault TcpEstablishedTimeoutNitro v5 and earlierc6a, m6i, r6g, etc.432,000 seconds (5 days)Nitro v6 and newerc8i, m8g, r8i, etc.350 seconds (~5.8 minutes)

###The Silent Drop Mechanism If a TCP connection remains entirely idle (no packets transmitted) for more than 350 seconds on a Nitro v6 instance, the ENI state engine silently evicts the session from its local tracking table.

When your client or server eventually attempts to transmit data again:

  1. The packet hits the Nitro card’s network processor.
  2. The card checks its connection tracking table and finds no matching record.
  3. Because the state is missing, the card treats this as an invalid/untracked out-of-order packet and silently drops it.
  4. The sender never receives an ACK, gets stuck in a retransmission loop, and eventually throws a connection timeout or connection reset error. ###Lab Replication & Packet-Level Analysis To fully understand this behavior on the wire, we constructed a minimal replication environment using Ubuntu 26.04 LTS instances:
    • RPC Server: c6a.large (Nitro v5) listening on port 13888
    • RPC Client: c8i.large (Nitro v6) connecting over TCP

We captured three distinct packet traces representing:

  1. The Default Behavior (Failure after 350s idle).
  2. The Corrected Behavior (Setting TcpEstablishedTimeout to 5 days).
  3. The Keepalive Mitigation (Resetting the countdown timer dynamically).

Let’s analyze the exact packet sequences for each scenario.

###Case 1: Default Behavior - The Silent Eviction (traffic_13888_client_c8i_default.txt) In this test, we established a connection, successfully sent initial payloads, paused for over 350 seconds, and then tried to send the string “789”. Here is the exact packet stream captured from the client’s perspective:

# 1. TCP Three-Way Handshake
15:46:23.860999 gif0  Out IP 172.31.41.192.51972 > 172.31.12.10.13888: Flags [S], seq 2310427052, win 62727, options [mss 8961,sackOK,TS val 3631753175 ecr 0,nop,wscale 9], length 0
15:46:23.861521 gif0  In  IP 172.31.12.10.13888 > 172.31.41.192.51972: Flags [S.], seq 4066315758, ack 2310427053, win 62643, options [mss 8961,sackOK,TS val 2100133385 ecr 3631753175,nop,wscale 9], length 0
15:46:23.861534 gif0  Out IP 172.31.41.192.51972 > 172.31.12.10.13888: Flags [.], ack 1, win 123, options [nop,nop,TS val 3631753175 ecr 2100133385], length 0

# 2. Active Session: Client sends "123" and "456", Server ACKs immediately
15:46:25.602768 gif0  Out IP 172.31.41.192.51972 > 172.31.12.10.13888: Flags [P.], seq 1:5, ack 1, win 123, options [nop,nop,TS val 3631754916 ecr 2100133385], length 4
15:46:25.603357 gif0  In  IP 172.31.12.10.13888 > 172.31.41.192.51972: Flags [.], ack 5, win 123, options [nop,nop,TS val 2100135127 ecr 3631754916], length 0

15:46:27.619204 gif0  Out IP 172.31.41.192.51972 > 172.31.12.10.13888: Flags [P.], seq 5:9, ack 1, win 123, options [nop,nop,TS val 3631756933 ecr 2100135127], length 4
15:46:27.619675 gif0  In  IP 172.31.12.10.13888 > 172.31.41.192.51972: Flags [.], ack 9, win 123, options [nop,nop,TS val 2100137143 ecr 3631756933], length 0

# 3. Idle Gap (Client sits silent from 15:46:27 to 15:55:03 -> 516 seconds)
# This gap of 516s is significantly larger than the 350s Nitro v6 tracking limit.

# 4. Post-Idle Transmission Attempt: Client tries to send "789" (seq 9:13)
15:55:03.940940 gif0  Out IP 172.31.41.192.51972 > 172.31.12.10.13888: Flags [P.], seq 9:13, ack 1, win 123, options [nop,nop,TS val 3632273255 ecr 2100137143], length 4

# 5. Exponential Backoff Retransmissions (No ACKs ever returned)
15:55:04.147832 gif0  Out IP 172.31.41.192.51972 > 172.31.12.10.13888: Flags [P.], seq 9:13, ack 1, win 123, options [nop,nop,TS val 3632273462 ecr 2100137143], length 4  # Retransmit 1 (+206ms)
15:55:04.355832 gif0  Out IP 172.31.41.192.51972 > 172.31.12.10.13888: Flags [P.], seq 9:13, ack 1, win 123, options [nop,nop,TS val 3632273670 ecr 2100137143], length 4  # Retransmit 2 (+208ms)
15:55:04.763827 gif0  Out IP 172.31.41.192.51972 > 172.31.12.10.13888: Flags [P.], seq 9:13, ack 1, win 123, options [nop,nop,TS val 3632274078 ecr 2100137143], length 4  # Retransmit 3 (+408ms)
15:55:05.587820 gif0  Out IP 172.31.41.192.51972 > 172.31.12.10.13888: Flags [P.], seq 9:13, ack 1, win 123, options [nop,nop,TS val 3632274902 ecr 2100137143], length 4  # Retransmit 4 (+824ms)
15:55:07.251838 gif0  Out IP 172.31.41.192.51972 > 172.31.12.10.13888: Flags [P.], seq 9:13, ack 1, win 123, options [nop,nop,TS val 3632276566 ecr 2100137143], length 4  # Retransmit 5 (+1.66s)
15:55:10.515843 gif0  Out IP 172.31.41.192.51972 > 172.31.12.10.13888: Flags [P.], seq 9:13, ack 1, win 123, options [nop,nop,TS val 3632279830 ecr 2100137143], length 4  # Retransmit 6 (+3.26s)

#####Packet Analysis:

  • The Handshake & Data: At 15:46:25 and 15:46:27, data payload chunks of 4 bytes (seq 1:5 and seq 5:9) are cleanly received and matched with an incoming acknowledgement (In IP … Flags [.]).
  • The Eviction: After the last data exchange at 15:46:27.619675, we wait until 15:55:03.940940 before typing again. This represents an idle gap of 516.32 seconds. Since this exceeds 350 seconds, the Nitro card drops the connection’s state from memory.
  • The Retransmission Loop: When the client sends the next 4 bytes (seq 9:13), the Nitro engine drops the packet. It never exits the local node onto the network, or the incoming return packet is discarded.
  • Notice the classic TCP exponential backoff retry pattern in the timestamps: the retransmissions trigger at intervals of ~200ms, 200ms, 400ms, 800ms, 1.6s, and 3.2s. No ACK (In) packets are ever received from the server. The connection is completely dead. ###Case 2: Adjusted Timeout Configuration - Success (traffic_13888_client_c8i_timeout432000.txt) For this run, we applied the ENI-level attribute fix, changing the connection tracking established timeout to 432000 seconds (5 days) via the AWS EC2 API. We then repeated the experiment, waiting 400 seconds (exceeding the default 350s limit) before transmitting again. ```

    1. TCP Three-Way Handshake

    17:47:03.440943 gif0 Out IP 172.31.41.192.36120 > 172.31.12.10.13888: Flags [S], seq 1741915011, win 62727, options [mss 8961,sackOK,TS val 1224121321 ecr 0,nop,wscale 9], length 0 17:47:03.441475 gif0 In IP 172.31.12.10.13888 > 172.31.41.192.36120: Flags [S.], seq 3620295978, ack 1741915012, win 62643, options [mss 8961,sackOK,TS val 1387741948 ecr 1224121321,nop,wscale 9], length 0 17:47:03.441488 gif0 Out IP 172.31.41.192.36120 > 172.31.12.10.13888: Flags [.], ack 1, win 123, options [nop,nop,TS val 1224121321 ecr 1387741948], length 0

2. Active Session: Initial Payloads Sent & Acknowledged

17:47:04.554064 gif0 Out IP 172.31.41.192.36120 > 172.31.12.10.13888: Flags [P.], seq 1:5, ack 1, win 123, options [nop,nop,TS val 1224122434 ecr 1387741948], length 4 17:47:04.554515 gif0 In IP 172.31.12.10.13888 > 172.31.41.192.36120: Flags [.], ack 5, win 123, options [nop,nop,TS val 1387743061 ecr 1224122434], length 0

17:47:06.380127 gif0 Out IP 172.31.41.192.36120 > 172.31.12.10.13888: Flags [P.], seq 5:9, ack 1, win 123, options [nop,nop,TS val 1224124260 ecr 1387743061], length 4 17:47:06.380572 gif0 In IP 172.31.12.10.13888 > 172.31.41.192.36120: Flags [.], ack 9, win 123, options [nop,nop,TS val 1387744887 ecr 1224124260], length 0

17:47:09.010113 gif0 Out IP 172.31.41.192.36120 > 172.31.12.10.13888: Flags [P.], seq 9:13, ack 1, win 123, options [nop,nop,TS val 1224126890 ecr 1387744887], length 4 17:47:09.010726 gif0 In IP 172.31.12.10.13888 > 172.31.41.192.36120: Flags [.], ack 13, win 123, options [nop,nop,TS val 1387747517 ecr 1224126890], length 0

3. Idle Gap (Client sits silent from 17:47:09 to 17:53:49 -> 400 seconds)

Note: 400s is greater than the default 350s timeout.

4. Post-Idle Transmission: Client sends data (seq 13:17) and Server ACKs immediately!

17:53:49.388919 gif0 Out IP 172.31.41.192.36120 > 172.31.12.10.13888: Flags [P.], seq 13:17, ack 1, win 123, options [nop,nop,TS val 1224527269 ecr 1387747517], length 4 17:53:49.389511 gif0 In IP 172.31.12.10.13888 > 172.31.41.192.36120: Flags [.], ack 17, win 123, options [nop,nop,TS val 1388147896 ecr 1224527269], length 0

Subsequent exchanges continue normally

17:53:51.054075 gif0 Out IP 172.31.41.192.36120 > 172.31.12.10.13888: Flags [P.], seq 17:21, ack 1, win 123, options [nop,nop,TS val 1224528934 ecr 1388147896], length 4

#####Packet Analysis:
- The Idle Window: The connection is kept completely quiet between 17:47:09.010726 and 17:53:49.388919 (exactly 400.37 seconds).
- Immediate Response: When the client sends the next segment (seq 13:17), instead of dropping it, the Nitro card preserves the tracking state. It permits the packet out to the network, and the server receives and acknowledges it instantly: 17:53:49.389511 In IP ... Flags [.], ack 17 (latency: 0.59ms).
- The Session Stays Warm: Data continues to exchange normally because the conntrack engine has a lifetime threshold of 5 days.
###Case 3: TCP Keepalive Mitigation - Success (traffic_13888_client_c8i_keepalive300.txt)
If you cannot modify the AWS ENI connection tracking infrastructure parameters directly, an excellent alternative is utilizing OS/application-level TCP Keep-Alives. In this trace, the client's operating system is configured to send keepalive probes every 300 seconds (5 minutes).
Let’s inspect how keepalives maintain the Nitro conntrack table:

1. Active Session: Payloads exchange up to 21:12:53

21:12:53.480752 gif0 Out IP 172.31.36.171.42162 > 172.31.12.131.13888: Flags [P.], seq 9:13, ack 1, win 123, options [nop,nop,TS val 1463280441 ecr 300321629], length 4 21:12:53.481325 gif0 In IP 172.31.12.131.13888 > 172.31.36.171.42162: Flags [.], ack 13, win 123, options [nop,nop,TS val 300324105 ecr 1463280441], length 0

2. Idle Phase begins. At exactly 307.32 seconds of silence, a Keep-Alive is sent:

21:18:00.802859 gif0 Out IP 172.31.36.171.42162 > 172.31.12.131.13888: Flags [.], ack 1, win 123, options [nop,nop,TS val 1463587764 ecr 300324105], length 0 21:18:00.803299 gif0 In IP 172.31.12.131.13888 > 172.31.36.171.42162: Flags [.], ack 13, win 123, options [nop,nop,TS val 300631427 ecr 1463280441], length 0

3. User types new data later at 21:20:33 (153 seconds after keep-alive probe)

21:20:33.726634 gif0 Out IP 172.31.36.171.42162 > 172.31.12.131.13888: Flags [P.], seq 13:17, ack 1, win 123, options [nop,nop,TS val 1463740687 ecr 300631427], length 4 21:20:33.727081 gif0 In IP 172.31.12.131.13888 > 172.31.36.171.42162: Flags [.], ack 17, win 123, options [nop,nop,TS val 300784351 ecr 1463740687], length 0

#####Packet Analysis:
- The Probe: At 21:18:00.802859, exactly 307.32 seconds after the last ACK, the client's TCP stack automatically fires a keep-alive probe: Flags [.] with length 0.
- The ACK: The server responds immediately at 21:18:00.803299 acknowledging the probe (ack 13).
- The Core Effect: Although this exchange contains 0 bytes of application data, it resets the Nitro card's 350-second idle countdown timer back to zero.
- When the user finally sends payload "123" at 21:20:33 (which is overall 460 seconds since the last data transaction), the tracking path is still wide open. The client and server exchange data effortlessly.

###Solutions & Action Items
To prevent packet drops when upgrading to Nitro v6 instances (c8i, m8g, etc.), implement one of the following two solutions:
**Solution 1:** Modify the ENI Connection Tracking Timeout (Infrastructure Fix)The most robust fix is to configure the target ENI connection tracking rules to match previous-generation settings. You can do this at launch or inline using the EC2 API or AWS CLI.
Run the following command to update TcpEstablishedTimeout to 5 days (432000 seconds):

aws ec2 modify-network-interface-attribute
–network-interface-id \ --connection-tracking-specification TcpEstablishedTimeout=432000

#####Automation Tip: 
If you run Auto Scaling Groups or EKS Node Groups, incorporate this command into your Launch Templates or node bootstrap/User Data script. Alternatively, you can query dynamic interfaces on boot:

TOKEN=$(curl -s -X PUT “http://169.254.169.254/latest/api/token”) MAC=$(curl -s -H “X-aws-ec2-metadata-token: $TOKEN” http://169.254.169.254/latest/meta-data/mac) ENI_ID=$(curl -s -H “X-aws-ec2-metadata-token: $TOKEN” http://169.254.169.254/latest/meta-data/network/interfaces/macs/${MAC}/interface-id) aws ec2 modify-network-interface-attribute –network-interface-id $ENI_ID –connection-tracking-specification TcpEstablishedTimeout=432000 –region your-region

**Solution 2:** Tune OS/Application TCP Keepalives (System/Code Fix)
If you do not want to alter AWS infrastructure properties, you must force your applications to keep the socket alive by sending lightweight packet probes before the 350-second tracking timer expires.

#####Linux System Configuration (sysctl)
By default, the Linux kernel TCP keep-alive time is set to a conservative 7200 seconds (2 hours). Optimize this globally on your EC2 instances by applying these settings to /etc/sysctl.conf:

Send keepalive probes after 5 minutes of inactivity (300 seconds)

net.ipv4.tcp_keepalive_time = 300

Send up to 5 probes if no response is received

net.ipv4.tcp_keepalive_probes = 5

Wait 15 seconds between individual probes

net.ipv4.tcp_keepalive_intvl = 15

Apply the changes instantly via:

sudo sysctl -p

#####Application-Level Sockets
Ensure your application clients (e.g., Database Connectors, HTTP clients, or gRPC frameworks) explicitly request keepalives. For example, in Go:

dialer := &net.Dialer{ KeepAlive: 300 * time.Second, // Fires a probe if the connection is idle for 5 mins } ``` ###Conclusion Migrating to AWS Nitro v6 instances like c8i delivers phenomenal performance optimizations, but the transition demands strict attention to network-level defaults. When migrating:

  1. Audit your idle timeouts: If your target load balancers have idle timeouts configured above 350 seconds, ensure you adjust your instances’ TcpEstablishedTimeout.
  2. Apply global OS-level keepalives to prevent silent session blackholing.
  3. Use packet captures (tcpdump / Wireshark) during migrations to track retransmissions post-idle windows and catch silent discards proactively.

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