Protocol Overhead

Ethernet

Ethernet frame format:

6 byte dest addr

6 byte src addr

[4 byte optional 802.1q VLAN Tag]

2 byte length/type

46-1500 byte data (payload)

4 byte CRC

Ethernet overhead bytes:
  12 gap + 8 preamble + 14 header + 4 trailer = 38 bytes/packet w/o 802.1q
  12 gap + 8 preamble + 18 header + 4 trailer = 42 bytes/packet with 802.1q

Ethernet Payload data rates are thus:
  1500/(38+1500) = 97.5293 %   w/o 802.1q tags
  1500/(42+1500) = 97.2763 %   with 802.1q tags

TCP over Ethernet:
 Assuming no header compression (e.g. not PPP)
 Add 20 IPv4 header or 40 IPv6 header (no options)
 Add 20 TCP header
 Add 12 bytes optional TCP timestamps
 Max TCP Payload data rates over ethernet are thus:
  (1500-40)/(38+1500) = 94.9285 %  IPv4, minimal headers
  (1500-52)/(38+1500) = 94.1482 %  IPv4, TCP timestamps
  (1500-52)/(42+1500) = 93.9040 %  802.1q, IPv4, TCP timestamps
  (1500-60)/(38+1500) = 93.6281 %  IPv6, minimal headers
  (1500-72)/(38+1500) = 92.8479 %  IPv6, TCP timestamps
  (1500-72)/(42+1500) = 92.6070 %  802.1q, IPv6, ICP timestamps

UDP over Ethernet:
 Add 20 IPv4 header or 40 IPv6 header (no options)
 Add 8 UDP header
 Max UDP Payload data rates over ethernet are thus:
  (1500-28)/(38+1500) = 95.7087 %  IPv4
  (1500-28)/(42+1500) = 95.4604 %  802.1q, IPv4
  (1500-48)/(38+1500) = 94.4083 %  IPv6
  (1500-48)/(42+1500) = 94.1634 %  802.1q, IPv6

An excellent source of ethernet information is Charles Spurgeon’s Ethernet Web Site.

Notes:

1. 48-bit (6 byte) ethernet address have a 24-bit “Organizationally Unique Identifier” (OUI) assigned by IEEE + a 24-bit number assigned by the vendor.
2. The minimum ethernet payload (data field) is 46 bytes which makes a 64 byte ethernet packet including header and CRC.




3. The maximum ethernet payload (data field) is 1500 bytes which makes a 1518 byte ethernet packet including header and CRC. When 802.1q added an optional 4-byte VLAN Tag Header, they extended the allowed maximum frame size to 1522 bytes (22 byte header+CRC).




4. The bit speed of 100 Mbps ethernet on the wire/fiber is actually 125 Mbps due to 4B/5B encoding. Every four data bits gets mapped to one of 16 5-bit symbols. This leaves 16 non-data symbols. This encoding came from FDDI.




5. The original Ethernet II spec had a two byte type field which 802.3 changed to a length field, and later a length/type field depending on use: values 1536 and over are types, under 1536 lengths.

Gigabit Ethernet with Jumbo Frames

Gigabit ethernet is exactly 10 times faster than 100 Mbps ethernet, so for standard 1500 byte frames, the numbers above all apply, multiplied by 10. Many GigE devices however allow “jumbo frames” larger than 1500 bytes. The most common figure being 9000 bytes. For 9000 byte jumbo frames, potential GigE throughput becomes (from Bill Fink, the author of nuttcp):

Theoretical maximum TCP throughput on GigE using jumbo frames:

 (9000-20-20-12)/(9000+14+4+7+1+12)*1000000000/1000000 = 990.042 Mbps
                                      
  MTU          MTU             GigE      Mbps
                        
      IP      Ethernet           InterFrame Gap (IFG), aka
   Header       Header           InterPacket Gap (IPG), is
                          a minimum of 96 bit times
        TCP            FCS         from the last bit of the
     Header                        FCS to the first bit of
             Preamble        the preamble
    TCP                
       Options            Start
   (Timestamp)            Frame
        Delimiter
     (SFD)
          
       Inter
       Frame
         Gap
       (IFG)

Theoretical maximum UDP throughput on GigE using jumbo frames:

 (9000-20-8)/(9000+14+4+7+1+12)*1000000000/1000000 = 992.697 Mbps

Theoretical maximum TCP throughput on GigE without using jumbo frames:

 (1500-20-20-12)/(1500+14+4+7+1+12)*1000000000/1000000 = 941.482 Mbps

Theoretical maximum UDP throughput on GigE without using jumbo frames:

 (1500-20-8)/(1500+14+4+7+1+12)*1000000000/1000000 = 957.087 Mbps

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