Streams may enter an unresponsive state due to "Fragment MTU exceeded"

[jianye-chen]

Describe the bug

Under some network conditions, streams may become unresponsive while connection remains intact.
After dropping the connection by an application level timeout, newly established connections may recover.

[0]8568.6468::2026/06/24-05:22:28.171623600 [Microsoft-Quic][C][TX][233] SH DestCid:70845077534919afd6 KP:0 SB:1 (Payload 1446 bytes)
[0]8568.6468::2026/06/24-05:22:28.171626500 [Microsoft-Quic][C][TX][233]   PING
[0]8568.6468::2026/06/24-05:22:28.171629300 [Microsoft-Quic][C][TX][233]   STREAM ID:292 Offset:0 Len:192 Fin
[0]8568.6468::2026/06/24-05:22:28.171633300 [Microsoft-Quic][C][TX][233]   PADDING Len:1244

Reason = 258(Fragment MTU exceeded). Direction = 0(Send). NBL = 0xFFFFC88FD3EDAA80.
[0]8568.6468::2026/06/24-05:22:28.171720000 [Microsoft-Windows-Winsock-AFD]sendmsg: 1: Process 0xFFFFC88FDB042200, Endpoint 0xFFFFC88FBE074010, Buffer Count 1, Buffer 0xFFFFC88F887D2190, Length 1472, Seq 3045, Status 0xC0000206(NT=The size of the buffer is invalid for the specified operation.)

[0]8568.6468::2026/06/24-05:22:28.171732400 [Microsoft-Quic][data][0x11F88429710] ERROR, 1784, WSASendMsg completion.

Workaround

Set QuicSettings.MinimumMtu and QuicSettings.MaximumMtu to a constant value 1248.

Affected OS

• Windows
• Linux
• macOS
• Other (specify below)

Additional OS information

No response

MsQuic version

v2.5.6

Steps taken to reproduce bug

1. establish a connection
2. create a stream
3. send some data

Expected behavior

Streams should eventually deliver all data enqueued.

Actual outcome

Streams enter an unresponsive state and stop delivering any data.

Additional details

No response

[guhetier]

The MTU of the network path was reduced (likely due to a route change), which MsQuic doesn't currently support.
MsQuic should listen to ICMP traffic and/or react to the proper send failure codes and re-evaluate the MTU.

Investigation notes:

The MTU discovery succeeds up to 1500.

173396 2026-06-24T09:21:51.388 8568.6468 e [Microsoft-Quic] [conn][0x0000011F88425F20] Path[0] Mtu Discovery Search Packet Sending with MTU 1500
173438 2026-06-24T09:21:51.388 8568.6468 e [Microsoft-Quic] [C][TX][15] SH DestCid:70845077534919afd6 KP:0 SB:1 (Payload 1446 bytes)
173445 2026-06-24T09:21:51.388 8568.6468 e [Microsoft-Quic] [data][0x0000011F88429710] Send 1472 bytes in 1 buffers (segment=0) Dst=[::ffff:172.206.140.111]:443 Src=192.168.15.6:63489
173446 2026-06-24T09:21:51.388 8568.6468 e [Microsoft-Windows-Winsock-AFD] sendmsg: 0: Process 0xFFFFC88FDB042200, Endpoint 0xFFFFC88FBE074010, Buffer Count 1, Buffer 0xFFFFC88FAA4B41B8, Length 1472, Addr 23.213.232.170:443, Seq 3100, Status STATUS_SUCCESS
173447 2026-06-24T09:21:51.388 8568.6468 e [Microsoft-Windows-TCPIP] UDP: Endpoint 0xFFFFC88FACF94D40 segment message. SegmentSize = 0 (0 == No Segmentation) MessageLength = 1472 HwDatagrams = 0 HwSegments = 0 SwSegments = 0 SubMssSegments = 0 Status = STATUS_SUCCESS.
173448 2026-06-24T09:21:51.388 8568.6468 e [Microsoft-Windows-TCPIP] UDP: endpoint 0xFFFFC88FACF94D40 (LocalAddress = [::ffff:0:0]:63489, RemoteAddress = [::ffff:172.206.140.111]:443) sending 1 messages and a total of 1472 bytes. PID = 34152 ProcessSeqNum 2814749767112670.
173449 2026-06-24T09:21:51.388 0004.A26C e [Microsoft-Windows-Winsock-AFD] sendmsg: 1: Process 0xFFFFC88FDB042200, Endpoint 0xFFFFC88FBE074010, Buffer Count 1, Buffer 0xFFFFC88F8DE18800, Length 35, Addr [::ffff:10.252.214.150]:53, Seq 3102, Status STATUS_SUCCESS

176422 2026-06-24T09:21:51.548 8568.6468 e [Microsoft-Quic] [C][TX][15] ACKed (159.813 ms)
176424 2026-06-24T09:21:51.548 8568.6468 e [Microsoft-Quic] [conn][0x0000011F88425F20] Path[0] MTU updated to 1500 bytes
176425 2026-06-24T09:21:51.548 8568.6468 e [Microsoft-Quic] [conn][0x0000011F88425F20] Path[0] Mtu Discovery Entering Search Complete at MTU 1500

Everything is fine until this send:

246846 2026-06-24T09:22:23.405 8568.6468 e [Microsoft-Quic] [data][0x0000011F88429710] Send 1472 bytes in 1 buffers (segment=1472) Dst=[::ffff:172.206.140.111]:443 Src=192.168.15.6:63489

Then things start failing:

247637 2026-06-24T09:22:24.588 8568.6468 e [Microsoft-Quic] [data][0x0000011F88429710] Send 1472 bytes in 1 buffers (segment=1472) Dst=[::ffff:172.206.140.111]:443 Src=192.168.15.6:63489
247638 2026-06-24T09:22:24.588 8568.6468 e [Microsoft-Windows-Winsock-AFD] sendmsg: 0: Process 0xFFFFC88FDB042200, Endpoint 0xFFFFC88FBE074010, Buffer Count 1, Buffer 0xFFFFC88FB133E1B8, Length 1472, Addr 239.255.255.250:1900, Seq 3100, Status STATUS_SUCCESS
247642 2026-06-24T09:22:24.588 8568.6468 e [Microsoft-Windows-TCPIP] TCPIP: Network layer (Protocol UDP, AddressFamily = IPV4) dropped 1 packet(s) on interface 6. SourceAddress = 192.168.15.6:0. DestAddress = 172.206.140.111:0. Reason = Fragment MTU exceeded. Direction = Send. NBL = 0xFFFFC88FABCDC2C0.
247643 2026-06-24T09:22:24.588 8568.6468 e [Microsoft-Windows-Winsock-AFD] sendmsg: 1: Process 0xFFFFC88FDB042200, Endpoint 0xFFFFC88FBE074010, Buffer Count 1, Buffer 0xFFFFC88FB133E1B8, Length 1472, Addr 239.255.255.250:1900, Seq 3200, Status The size of the buffer is invalid for the specified operation.

ICMP logs show this is likely due to a route change reducing the MTU

[13]0004.38EC::2026/06/24-05:22:23.406410900 [Microsoft-Windows-TCPIP]1(ICMP): 1(Receive)message. Type = 3(Destination Unreachable(v4)/Time Exceeded(v6)), Code = 4, CompartmentId = 1, SourceAddress = 192.168.15.1, DestAddress = 192.168.15.6
[27]0004.38EC::2026/06/24-05:33:21.823081900 [Microsoft-Windows-TCPIP]1(ICMP): 1(Receive)message. Type = 3(Destination Unreachable(v4)/Time Exceeded(v6)), Code = 4, CompartmentId = 1, SourceAddress = 192.168.15.1, DestAddress = 192.168.15.6
[18]0004.38EC::2026/06/24-05:43:30.102279100 [Microsoft-Windows-TCPIP]1(ICMP): 1(Receive)message. Type = 3(Destination Unreachable(v4)/Time Exceeded(v6)), Code = 4, CompartmentId = 1, SourceAddress = 192.168.15.1, DestAddress = 192.168.15.6
[30]0004.38EC::2026/06/24-05:59:50.051407300 [Microsoft-Windows-TCPIP]1(ICMP): 1(Receive)message. Type = 3(Destination Unreachable(v4)/Time Exceeded(v6)), Code = 4, CompartmentId = 1, SourceAddress = 192.168.15.1, DestAddress = 192.168.15.6
[21]0004.38EC::2026/06/24-06:18:42.720413300 [Microsoft-Windows-TCPIP]1(ICMP): 1(Receive)message. Type = 3(Destination Unreachable(v4)/Time Exceeded(v6)), Code = 4, CompartmentId = 1, SourceAddress = 192.168.15.1, DestAddress = 192.168.15.6