However, it adds not 17 bytes but 21 bytes of padding, 4 bytes more than is necessary. When 192.168.1.2 receives the packet, it replies and also needs to add padding. When that 43-byte packet hits the wire, it will be padded by 17 bytes to meet the minimum 60 byte (data) size. Whenever you see packets less than the minimum frame size, this is a tell-tale sign of the capture point. ?įirst, the capture was taken at the host 192.168.1.1, so Wireshark gets the packet before the transmitter adds the padding, thus the reason it's only 43 bytes. So it seems that a frame check should only exist if the total length of the frame is 64 bytes, and in such case the last 4 bytes are to be read as the frame check sequence. (or the maybe the h/w itself does this?).įurthermore, neither host adds a frame check to frames 3 and 4. It seems like the Ethernet driver at host A was not implemented to add padding/FCS to the end of the Ethernet frame, but host B's driver was. The question I have is, why doesn't host A apply this same padding and FCS (in frame 1)? Frame 1, sent by host A, does not add such padding: This results in a frame that meets the minimum size of 64 bytes. The key difference that I noticed between these two frames is that in frame 2, sent by host B, there is a 17 byte padding, plus 4 byte frame check (CRC) appended to the end of the frame. In frame 1 and frame 2, I see that they both contain 43 bytes of "valid data," yet the value shown in the length column shows 43 bytes for frame 1, but 64 bytes for frame 2. But I'm not sure why this padding and FRC are not always added. I was studying the capture you posted to cloudshark above in trying to understand why this padding and FCS are present.įrom what I can tell, it seems that the padding is added to bring the data size up to 60 bytes, with the FCS being an additional 4 bytes (after the padding), which results in a total size of 64 bytes.
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