Ethernet frame

A data packet on an Ethernet link is called an Ethernet frame. A frame begins with Preamble and Start Frame Delimiter. Following which, each Ethernet frame continues with an Ethernet header featuring destination and source MAC addresses. The middle section of the frame is payload data including any headers for other protocols (e.g. Internet Protocol) carried in the frame. The frame ends with a 32-bit cyclic redundancy check which is used to detect any corruption of data in transit.

Contents

Structure

A data packet on the wire is called a frame and consists of binary data. A frame viewed on the physical wire would show Preamble and Start Frame Delimiter, in addition to the other data. These are required by all physical hardware.[note 1]

The table below shows the complete Ethernet frame, as transmitted, for the MTU of 1500 octets (some implementations of gigabit Ethernet and higher speeds support larger jumbo frames).[note 2] One octet is eight bits of data (i.e., a byte on most modern computers).

802.3 Ethernet frame structure
Preamble Start of frame delimiter MAC destination MAC source 802.1Q tag (optional) Ethertype or length Payload Frame check sequence (32-bit CRC) Interframe gap
7 octets of 10101010 1 octet of 10101011 6 octets 6 octets (4 octets) 2 octets 46–1500 octets 4 octets 12 octets
64–1522 octets
72–1530 octets
84–1542 octets

Note: 2nd row of above table is misleading. It should be refactored. It suggests that if optional 802.1Q tag is used then min valid frame length is 68 octets. Its actually still 64 octets. This is because the additional 4 octets used due to the 802.1Q tag is considered to be part of the payload (or in the parlance of the IEEE 802.3-2008 standard, part of MAC CLIENT DATA).

Preamble and Start Frame Delimiter

See also: Syncword

The Start Frame Delimiter (SFD) is the 8-bit (1-byte) value marking the end of the preamble of an Ethernet frame. The SFD has the value 10101011 and is immediately followed by the destination MAC address. The preamble of an Ethernet frame consists of a 56-bit (7-byte) pattern of alternating 1 and 0 bits, which allows devices on the network to easily detect a new incoming frame. The SFD is designed to break this pattern, and signal the start of the actual frame.

10/100M transceiver chips (MII PHY) work with 4-bits (one nibble) at a time. Therefore the preamble will consist of 7 instances of 0101 + 0101, and the Start Frame Delimiter 0101 + 1101. 8-bit values are sent low 4-bit first and then high 4-bit. 1000M transceiver chips (GMII) work with 8-bits at a time, and 10 Gbit/s (XGMII) PHY works with 32-bits at a time. Note that when using octets, first 7 octets of 01010101 are sent, and then one octet of 11010101. But because the low 4-bit nibble 0101 is sent first, and later the high 4-bit nibble 1101, the Start-of-frame sequence 1101 will be sent after the preamble not before the last 4-bits of the preamble as one might otherwise be led to believe. There are also differences between DIX V2.0 (Ethernet 2) & IEEE 802.3 in the frame format.

Header

The header features source and destination MAC addresses which have 48 bits (6 bytes) each, the EtherType protocol identifier field and optional IEEE 802.1Q VLAN tag indicating VLAN membership and traffic priority.

EtherType or Length

This is a two-octet (2 bytes) field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the Payload of an Ethernet Frame.

Payload or data of an Ethernet frame

The minimum frame payload is 368 bit (46 bytes) (dictated by the slot time of the Ethernet LAN architecture) and the maximum frame payload is 12000 bits (1500 bytes).

Frame check sequence

The frame check sequence (CRC) is a 32-bit (4-byte) cyclic redundancy check which enables detection of corrupted data within the entire frame.

Interframe gap

Main article: Interframe gap

After a frame has been sent, transmitters are required to transmit a minimum of 96 bits (12 octets) of idle line state before transmitting the next frame.

Ethernet frame types

There are several types of Ethernet frames. The different frame types have different formats and MTU values, but can coexist on the same physical medium.

In addition, all four Ethernet frames types may optionally contain a IEEE 802.1Q tag to identify what VLAN it belongs to and its IEEE 802.1p priority (quality of service). This encapsulation is defined in the IEEE 802.3ac specification and increases the maximum frame by 4 bytes.

The IEEE 802.1Q tag, if present, is placed between the Source Address and the EtherType or Length fields. The first two bytes of the tag are the Tag Protocol Identifier (TPID) value of 0x8100. This is located in the same place as the EtherType/Length field in untagged frames, so an EtherType value of 0x8100 means the frame is tagged, and the true EtherType/Length is located after the Q-tag. The TPID is followed by two bytes containing the Tag Control Information (TCI) (the IEEE 802.1p priority (quality of service) and VLAN id). The Q-tag is followed by the rest of the frame, using one of the types described above.

Ethernet II

Ethernet II framing (also known as DIX Ethernet, named after DEC, Intel and Xerox, the major participants in its design[1]), defines the two-octet EtherType field in an Ethernet frame, preceded by destination and source MAC addresses, that identifies an upper layer protocol encapsulating the frame data. For example, an EtherType value of 0x0800 signals that the frame contains an IPv4 datagram. Likewise, an EtherType of 0x0806 indicates an ARP frame, 0x8100 indicates an IEEE 802.1Q frame and 0x86DD indicates an IPv6 frame.

As this industry-developed standard went through a formal IEEE standardization process, the EtherType field was changed to a (data) length field in the new 802.3 standard.[note 4] Since the packet recipient still needs to know how to interpret the packet, the standard required an IEEE 802.2 header to follow the length and specify the packet type. Many years later, the 802.3x-1997 standard, and later versions of the 802.3 standard, formally approved of both types of framing. In practice, both formats are in wide use, with original Ethernet framing the most common in Ethernet local area networks, due to its simplicity and lower overhead.

In order to allow some packets using Ethernet v2 framing and some packets using the original version of 802.3 framing to be used on the same Ethernet segment, EtherType values must be greater than or equal to 1536 (0x0600). That value was chosen because the maximum length of the data field of an Ethernet 802.3 frame is 1500 bytes (0x05DC). Thus if the field's value is greater than or equal to 1536, the frame must be an Ethernet v2 frame, with that field being a type field.[2] If it's less than or equal to 1500, it must be an IEEE 802.3 frame, with that field being a length field. Values between 1500 and 1536, exclusive, are undefined.[3] This convention allows software to determine whether a frame is an Ethernet II frame or an IEEE 802.3 frame, allowing the coexistence of both standards on the same physical medium.

802.2 LLC

Some protocols, particularly those designed for the OSI stack, operate directly on top of IEEE 802.2 LLC encapsulation, which provides both connection-oriented and connectionless network services.

IEEE 802.2 LLC encapsulation is not in widespread use on common networks currently, with the exception of large corporate NetWare installations that have not yet migrated to NetWare over IP. In the past, many corporate networks used IEEE 802.2 to support transparent translating bridges between Ethernet and Token Ring or FDDI networks.

There exists an Internet standard for encapsulating IPv4 traffic in IEEE 802.2 LLC SAP/SNAP frames.[4] It is almost never implemented on Ethernet, although it is used on FDDI, Token Ring, IEEE 802.11, and other IEEE 802 LANs. IP traffic cannot be encapsulated in IEEE 802.2 LLC frames without SNAP because, although there is a LLC SAP protocol type for IP, there is no such type for ARP, which is required for operation of any medium to large network. IPv6 can also be transmitted over Ethernet using IEEE 802.2 LLC SAP/SNAP, but, again, that's almost never used.

SNAP

By examining the 802.2 LLC header, it is possible to determine whether it is followed by a SNAP header. The LLC header includes two additional eight-bit address fields, called service access points (SAPs) in OSI terminology; when both source and destination SAP are set to the value 0xAA, the SNAP service is requested. The SNAP header allows EtherType values to be used with all IEEE 802 protocols, as well as supporting private protocol ID spaces. In IEEE 802.3x-1997, the IEEE Ethernet standard was changed to explicitly allow the use of the 16-bit field after the MAC addresses to be used as a length field or a type field.

Mac OS uses IEEE 802.2 LLC SAP/SNAP encapsulation for the AppleTalk v2 protocol suite on Ethernet (“EtherTalk”).

Novell raw 802.3

Novell's "raw" 802.3 frame format was based on early IEEE 802.3 work. Novell used this as a starting point to create the first implementation of its own IPX Network Protocol over Ethernet. They did not use any LLC header but started the IPX packet directly after the length field. This does not conform to the IEEE 802.3 standard, but since IPX has always FF at the first two bytes (while in IEEE 802.2 LLC that pattern is theoretically possible but extremely unlikely), in practice this mostly coexists on the wire with other Ethernet implementations, with the notable exception of some early forms of DECnet which got confused by this.

Novell NetWare used this frame type by default until the mid-nineties, and since NetWare was very widespread back then, while IP was not, at some point in time most of the world's Ethernet traffic ran over "raw" 802.3 carrying IPX. Since NetWare 4.10, NetWare now defaults to IEEE 802.2 with LLC (NetWare Frame Type Ethernet_802.2) when using IPX.[5]

Maximum throughput

We may calculate the protocol efficiency for Ethernet:

\text{Protocol efficiency} = \frac{\text{Payload size}}{\text{Frame size}}

Maximum efficiency is achieved with largest allowed payload size and is

\frac{1500}{1538} = 97.53%

for untagged Ethernet packets, since the frame size is maximum 1500 byte payload + 8 byte preamble + 14 byte header + 4 byte trailer + minimum interframe gap corresponding to 12 bytes = 1538 bytes. The maximum efficiency is

\frac{1500}{1542} = 97.28%

when 802.1Q VLAN tagging is used.

The throughput may be calculated from the efficiency:

\text{Throughput} = \text{Efficiency} \times \text{Net bit rate}\,\!,

where the physical layer net bit rate (the wire bit rate) depends on the Ethernet physical layer standard, and may be 10 Mbit/s, 100 Mbit/s, 1 Gbit/s or 10 Gbit/s. Maximum throughput for 100BASE-TX Ethernet is consequently 97.53 Mbit/s without 802.1Q, and 97.28 Mbit/s with 802.1Q.

Channel utilization is concept often confused with protocol efficiency. It considers only the use of the channel disregarding the nature of the data transmitted – either payload or overhead. At the physical layer, the link channel and equipment do not know the difference between data and control frames. We may calculate the link utilization or channel utilization:

\text{Channel utilization} = \frac{\text{Time spent transmitting data}}{\text{Total time}}

The total time considers the round trip time along the channel, the processing time in the hosts and the time transmitting data and acknowledge packets. The time spent transmitting data includes data and acknowledge packets.

Runt frames

A runt frame is an Ethernet frame that is less than the IEEE 802.3 minimum length of 64 bytes. Possible causes are collision, underruns, bad network card or software.[6]

Notes

  1. ^ Preamble and Start Frame Delimiter are not displayed by packet sniffing software because these bits are stripped away at OSI Layer 1 by the Ethernet adapter before being passed on to the OSI Layer 2 which is where packet sniffers collect their data from. There are OSI Physical Layer sniffers which can capture and display the Preamble and Start Frame but they are expensive and mainly used to detect physical related problems.
  2. ^ The bit patterns in the preamble and start of frame delimiter are written as bit strings, with the first bit transmitted on the left (not as byte values, which in Ethernet are transmitted least significant bit(s) first). This notation matches the one used in the IEEE 802.3 standard.
  3. ^ A version 1 Ethernet frame was used for early Ethernet prototypes and featured 8-bit MAC addresses and was never commercially deployed.
  4. ^ Original Ethernet packets define their length with the framing that surrounds it, rather than with an explicit length count.

References

  1. ^ Drew Heywood; Zubair Ahmad (2001). Drew Heywood's Windows 2000 Network Services. Sams. p. 53. ISBN 0672317419. 
  2. ^ LAN MAN Standards Committee of the IEEE Computer Society (20 March 1997). IEEE Std 802.3x-1997 and IEEE Std 802.3y-1997. The Institute of Electrical and Electronics Engineers, Inc.. pp. 28–31. 
  3. ^ IEEE Std 802.3-2005, 3.2.6
  4. ^ "RFC1042: A Standard for the Transmission of IP Datagrams over IEEE 802 Networks". Network Working Group of the IETF. February 1988. http://tools.ietf.org/html/rfc1042. 
  5. ^ Don Provan (1993-09-17). "Ethernet Framing". comp.sys.novell. (Web link).  — a classic series of Usenet postings by Novell's Don Provan that have found their way into numerous FAQs and are widely considered the definitive answer to the Novell Frame Type usage.
  6. ^ "Troubleshooting Ethernet". Cisco Systems. http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1904.html. 
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