SCTP Init abort问题求助：发送端Verification tag始终为0

Let's start by clarifying the SCTP spec basics here—this mismatch is often a case of either a stack bug or a misimplementation on one side:

Per RFC 4960 §3.3.1: The Verification Tag in an INIT chunk MUST be set to 0. The receiver (your server) responds with an INIT ACK that includes a randomly generated Verification Tag (this is required to prevent spoofing attacks).

So your sending side is actually following the spec by using VT=0 on its INIT—but the abort suggests either your server is mishandling this valid VT, or the sender isn't correctly using the server's random VT for subsequent packets. Here's how to fix this:

1. Check Your Server's SCTP Stack Compliance #

• If you're using a custom SCTP implementation or a modified off-the-shelf stack, double-check that it adheres to RFC 4960. Some buggy implementations incorrectly flag an INIT with VT=0 as invalid, triggering an abort instead of sending an INIT ACK.
• For open-source stacks like Linux kernel SCTP or lksctp-tools, make sure you're running a recent stable version. Older releases had edge-case bugs around INIT chunk validation. You can check your kernel version with uname -r and update if needed.

2. Verify the Sender's INIT ACK Handling #

• The sender must extract the Initiate Tag field from the server's INIT ACK and use this value as the Verification Tag for all future packets sent to the server. If your sender keeps using VT=0 after receiving the INIT ACK, the server will reject those packets with an abort (since it expects its own random VT).
• Dig into your sender's code: Look for the section where it processes the INIT ACK response. Ensure it's correctly overwriting the outgoing VT value with the Initiate Tag from the server. For example, in C with lksctp-tools, this would involve updating the sctp_initmsg struct or using sctp_sendmsg with the correct association context.

3. Capture and Analyze Traffic with tcpdump/Wireshark #

• Use tcpdump to capture the full INIT/INIT ACK exchange:

  tcpdump -i <your-network-interface> sctp port <your-sctp-port> -w sctp_init_capture.pcap
  

• Open the capture in Wireshark and verify these key points:
  • The sender's INIT chunk shows Verification Tag: 0 (this is correct).
  • The server's INIT ACK has a non-zero random Verification Tag, and its Initiate Tag field matches that random value.
  • Any packets the sender sends after the INIT ACK are using the server's Initiate Tag as their Verification Tag.
• If the sender is still using VT=0 post-INIT ACK, that's your root cause. If the server aborts immediately after receiving the valid INIT, the server stack has a bug or misconfiguration.

4. Rule Out Firewall/Middlebox Interference #

• Some firewalls or network devices with deep packet inspection (DPI) may incorrectly modify SCTP chunks—including Verification Tags. If the INIT ACK's Initiate Tag gets altered in transit, the sender will use the wrong VT, leading to an abort.
• Test the connection directly between the sender and server (bypassing any intermediate fireboxes or proxies) to eliminate this possibility.

内容的提问来源于stack exchange，提问作者Sachin Aravind