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"ACKed segment that wasn't captured", but also "This is an ACK to the segment in frame: nnn".

asked 2022-11-10 15:03:53 +0000

IanW gravatar image

updated 2022-11-10 15:13:43 +0000

On Wireshark Version 4.0.1 (v4.0.1-0-ge9f3970b1527).

I have a lot of packets in a capture marked "ACKed segment that wasn't captured (common at capture start)", but also "This is an ACK to the segment in frame: nnn". Does this make sense?

I'd prepared a screen capture showing this but I'm forbidden to upload it due to lack of points. Sorry. Here's a packet export instead:

Frame 116064: 54 bytes on wire (432 bits), 54 bytes captured (432 bits)
Ethernet II, Src: 00:00:00_00:00:00 (00:00:00:00:00:00), Dst: 00:00:00_00:00:00 (00:00:00:00:00:00)
Internet Protocol Version 4, Src: 10.27.187.1, Dst: 10.22.61.4
Transmission Control Protocol, Src Port: 62326, Dst Port: 5018, Seq: 3345995, Ack: 3916774, Len: 0
    Source Port: 62326
    Destination Port: 5018
    [Stream index: 5]
    [Conversation completeness: Incomplete (12)]
    [TCP Segment Len: 0]
    Sequence Number: 3345995    (relative sequence number)
    Sequence Number (raw): 4121653282
    [Next Sequence Number: 3345995    (relative sequence number)]
    Acknowledgment Number: 3916774    (relative ack number)
    Acknowledgment number (raw): 80821650
    0101 .... = Header Length: 20 bytes (5)
    Flags: 0x010 (ACK)
    Window: 8192
    [Calculated window size: 8192]
    [Window size scaling factor: -1 (unknown)]
    Checksum: 0xd45b [unverified]
    [Checksum Status: Unverified]
    Urgent Pointer: 0
    [Timestamps]
    [SEQ/ACK analysis]
        [This is an ACK to the segment in frame: 116013]
        [The RTT to ACK the segment was: 0.010588000 seconds]
        [TCP Analysis Flags]
            [Expert Info (Warning/Sequence): ACKed segment that wasn't captured (common at capture start)]
                [ACKed segment that wasn't captured (common at capture start)]
                [Severity level: Warning]
                [Group: Sequence]

Frame 116013: 108 bytes on wire (864 bits), 62 bytes captured (496 bits)
Ethernet II, Src: 00:00:00_00:00:00 (00:00:00:00:00:00), Dst: 00:00:00_00:00:00 (00:00:00:00:00:00)
Internet Protocol Version 4, Src: 10.22.61.4, Dst: 10.27.187.1
Transmission Control Protocol, Src Port: 5018, Dst Port: 62326, Seq: 3916720, Ack: 3345995, Len: 54
    Source Port: 5018
    Destination Port: 62326
    [Stream index: 5]
    [Conversation completeness: Incomplete (12)]
    [TCP Segment Len: 54]
    Sequence Number: 3916720    (relative sequence number)
    Sequence Number (raw): 80821596
    [Next Sequence Number: 3916774    (relative sequence number)]
    Acknowledgment Number: 3345995    (relative ack number)
    Acknowledgment number (raw): 4121653282
    0101 .... = Header Length: 20 bytes (5)
    Flags: 0x018 (PSH, ACK)
    Window: 32736
    [Calculated window size: 32736]
    [Window size scaling factor: -1 (unknown)]
    Checksum: 0x0488 [unverified]
    [Checksum Status: Unverified]
    Urgent Pointer: 0
    [Timestamps]
    [SEQ/ACK analysis]
        [This is an ACK to the segment in frame: 116010]
        [The RTT to ACK the segment was: 0.000249000 seconds]
        [Bytes in flight: 54]
        [Bytes sent since last PSH flag: 54]
        [TCP Analysis Flags]
            [Expert Info (Warning/Sequence): ACKed segment that wasn't captured (common at capture start)]
                [ACKed segment that wasn't captured (common at capture start)]
                [Severity level: Warning]
                [Group: Sequence]
    TCP payload (54 bytes)
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answered 2022-11-10 20:08:43 +0000

Guy Harris gravatar image

updated 2022-11-10 20:09:29 +0000

Does this make sense?

No, it doesn't; it makes no sense whatsoever, as it's self-contradictory.

Please file a bug on this on the Wireshark issue list. Attach the raw capture file (rather than a screenshot or the export text above) if you can, as that may make it easier to determine what the problem is and will make it easier to

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Or it could mean that the frame in question acks not only data in the frame in question but also asks data in some other segment that's not a frame in the capture. But even in that case it should be phrased in a way to make that more obvious.

Guy Harris gravatar imageGuy Harris ( 2022-11-10 21:15:58 +0000 )edit

Many thanks, Guy.

The problem appears to start after a break in a capture file. All packets after the break appear to be flagged.

I've tested with 3.6.9 and the problem doesn't appear in that.

I shall report to the issue list as you request.

Thanks

Ian

IanW gravatar imageIanW ( 2022-11-14 08:22:35 +0000 )edit

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Asked: 2022-11-10 15:03:53 +0000

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Last updated: Nov 10 '22