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I have captured a pure TCP ack using Microsoft Network Monitor. It shows the captured frame length as 54 bytes. IP header (20 bytes) + TCP Header (20 bytes) + Src MAC (6 bytes) + Dst MAC (6 bytes) + Frame Type (2 bytes). I don't see a CRC (4 byte) field. I know the minimum frame length for Ethernet is 64 bytes (46 + 18) and the maximum is 1518. Why don't I see this in Network Monitor? The value of the data field for Ethernet frame is 46-1500. Is the IP dataframe padded with zeros in 6 octets to make the total frame length 46?
Found Answer: The minimum length of the data field of a packet sent over an Ethernet is 46 octets. If necessary, the data field should be padded (with octets of zero) to meet the Ethernet minimum frame size. This padding is not part of the IP packet and is not included in the total length field of the IP header. http://www.ietf.org/rfc/rfc0894.txt
What is minimum overhead for a frame in IEEE 802.11. What is the maximum and minimum frame size there? What will be the frame size of a pure TCP Ack over WiFi?
I will also accept partial answers
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One of the smallest packets commonly seen on networks is a TCP ACK. This has a 20 byte IP header and a 20 byte TCP header, adding up to 40 bytes. Because this is smaller than ethernet's minimum payload size of 46 bytes, it is automatically padded prior to transmission to bring it up to size.
RFC 5681 mandates that an acknowledgement be sent for at least every other full-size segment, and that no more than 500ms expire before any segment is acknowledged. The resulting behavior is that, for longer transfers, acknowledgements are only sent for every two segments received ("ack every other").
The standard size of a TCP packet has a minimum size of 20 bytes, and a maximum of 60 bytes. The UDP packet consists of only 8 bytes for each packet.
What is an ACK packet? ACK is short for "acknowledgement." An ACK packet is any TCP packet that acknowledges receiving a message or series of packets. The technical definition of an ACK packet is a TCP packet with the "ACK" flag set in the header.
Wireshark answers your question in FAQ 7.10.
Most OSes do not support capturing the FCS of a frame on Ethernet, and probably do not support it on most other link-layer types. Some drivers on some OSes do support it, such as some (all?) Ethernet drivers on NetBSD and possibly the driver for Apple's gigabit Ethernet interface in Mac OS X; in those OSes, you might always get the FCS, or you might only get the FCS if you capture in promiscuous mode (you'd have to determine which is the case).
IEEE Std 802.11 Page 60:
The Frame Body field is of variable size. The maximum frame body size is determined by the maximum MSDU size (2304 octets) plus any overhead from security encapsulation.
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