What is the total length of pure TCP Ack sent over ethernet?

5
votes

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

2
tcpwifiethernet

2 Answers

6
votes

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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5
votes

First, comparing normal ethernet and wifi headers is almost an apples and oranges comparison... the wifi frame has a variety of options that could expand the size of the 802.11 frame, and in some rare circumstances the wifi framing overhead alone could be 74 bytes. Also, the payload of a wifi frame is allowed to be 0 bytes, unlike the Ethernet II encapsulation which requires at least 46 bytes of payload.

So, to give you solid numbers for the expected wifi size with a TCP ACK...

30-bytes (wifi ethernet hdr)
20-bytes (IP Header)
20-bytes (TCP Header)
 4-bytes (MIC, if WPA)
 4-bytes (CRC)
------------------------
 > 74 bytes

The maximum size of an 802.11 frame (including headers) is 2346 bytes.