Ethernet Data-Link Protocols
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One of the most significant strengths of the Ethernet family of protocols is that these protocols
use the same data-link standard. In fact, the core parts of the data-link standard date
back to the original Ethernet standards.
The Ethernet data-link protocol defines the Ethernet frame: an Ethernet header at the front,
the encapsulated data in the middle, and an Ethernet trailer at the end. Ethernet actually
defines a few alternate formats for the header, with the frame format shown in Figure 2-14
being commonly used today.
Figure 2-14 Commonly Used Ethernet Frame Format
While all the fields in the frame matter, some matter more to the topics discussed in this
book. Table 2-4 lists the fields in the header and trailer and a brief description for reference,
with the upcoming pages including more detail about a few of these fields.
Table 2-4 IEEE 802.3 Ethernet Header and Trailer Fields
* The IEEE 802.3 specifi cation limits the data portion of the 802.3 frame to a minimum of 46 and a maximum
of 1500 bytes. The term maximum transmission unit (MTU) defi nes the maximum Layer 3 packet that can
be sent over a medium. Because the Layer 3 packet rests inside the data portion of an Ethernet frame, 1500
bytes is the largest IP MTU allowed over an Ethernet .
The source and destination Ethernet address fields play a huge role in how Ethernet LANs
work. The general idea for each is relatively simple: The sending node puts its own address
in the source address field and the intended Ethernet destination device’s address in the destination
address field. The sender transmits the frame, expecting that the Ethernet LAN, as a
whole, will deliver the frame to that correct destination.
Ethernet addresses, also called Media Access Control (MAC) addresses, are 6-byte-long (48-bitlong)
binary numbers. For convenience, most computers list MAC addresses as 12-digit hexadecimal
numbers. Cisco devices typically add some periods to the number for easier readability
as well; for example, a Cisco switch might list a MAC address as 0000.0C12.3456.
Most MAC addresses represent a single NIC or other Ethernet port, so these addresses are
often called a unicast Ethernet address. The term unicast is simply a formal way to refer to
the fact that the address represents one interface to the Ethernet LAN. (This term also contrasts
with two other types of Ethernet addresses, broadcast and multicast, which will be
defined later in this section.)
The entire idea of sending data to a destination unicast MAC address works well, but it
works only if all the unicast MAC addresses are unique. If two NICs tried to use the same
MAC address, there could be confusion. (The problem would be like the confusion caused to
the postal service if you and I both tried to use the same mailing address—would the postal
service deliver mail to your house or mine?) If two PCs on the same Ethernet tried to use the
same MAC address, to which PC should frames sent to that MAC address be delivered?
Ethernet solves this problem using an administrative process so that, at the time of
manufacture, all Ethernet devices are assigned a universally unique MAC address. Before
a manufacturer can build Ethernet products, it must ask the IEEE to assign the manufacturer
a universally unique 3-byte code, called the organizationally unique identifier
(OUI). The manufacturer agrees to give all NICs (and other Ethernet products) a MAC
address that begins with its assigned 3-byte OUI. The manufacturer also assigns a unique
value for the last 3 bytes, a number that manufacturer has never used with that OUI. As a
result, the MAC address of every device in the universe is unique.
NOTE The IEEE also calls these universal MAC addresses global MAC addresses.
Figure 2-15 shows the structure of the unicast MAC address, with the OUI.
Figure 2-15 Structure of Unicast Ethernet Addresses
Ethernet addresses go by many names: LAN address, Ethernet address, hardware address,
burned-in address, physical address, universal address, or MAC address. For example, the
term burned-in address (BIA) refers to the idea that a permanent MAC address has been
encoded (burned into) the ROM chip on the NIC. As another example, the IEEE uses the
term universal address to emphasize the fact that the address assigned to a NIC by a manufacturer
should be unique among all MAC addresses in the universe.
In addition to unicast addresses, Ethernet also uses group addresses. Group addresses identify
more than one LAN interface card. A frame sent to a group address might be delivered
to a small set of devices on the LAN, or even to all devices on the LAN. In fact, the IEEE
defines two general categories of group addresses for Ethernet:
Broadcast address: Frames sent to this address should be delivered to all devices on the
Ethernet LAN. It has a value of FFFF.FFFF.FFFF.
Multicast addresses: Frames sent to a multicast Ethernet address will be copied and forwarded
to a subset of the devices on the LAN that volunteers to receive frames sent to a
specific multicast address.
Table 2-5 summarizes most of the details about MAC addresses.
Table 2-5 LAN MAC Address Terminology and Features
Identifying Network Layer Protocols with the Ethernet Type Field
While the Ethernet header’s address fields play an important and more obvious role in
Ethernet LANs, the Ethernet Type field plays a much less obvious role. The Ethernet Type
field, or EtherType, sits in the Ethernet data link layer header, but its purpose is to directly
help the network processing on routers and hosts. Basically, the Type field identifies the
type of network layer (Layer 3) packet that sits inside the Ethernet frame.
First, think about what sits inside the data part of the Ethernet frame shown earlier in Figure
2-14. Typically, it holds the network layer packet created by the network layer protocol on
some device in the network. Over the years, those protocols have included IBM Systems
Network Architecture (SNA), Novell NetWare, Digital Equipment Corporation’s DECnet,
and Apple Computer’s AppleTalk. Today, the most common network layer protocols are
both from TCP/IP: IP version 4 (IPv4) and IP version 6 (IPv6).
The original host has a place to insert a value (a hexadecimal number) to identify the type
of packet encapsulated inside the Ethernet frame. However, what number should the sender
put in the header to identify an IPv4 packet as the type? Or an IPv6 packet? As it turns
out, the IEEE manages a list of EtherType values, so that every network layer protocol that
needs a unique EtherType value can have a number. The sender just has to know the list.
(Anyone can view the list; just go to www.ieee.org and search for EtherType.)
For example, a host can send one Ethernet frame with an IPv4 packet and the next Ethernet
frame with an IPv6 packet. Each frame would have a different Ethernet Type field value,
using the values reserved by the IEEE, as shown in Figure 2-16.
Figure 2-16 Use of Ethernet Type Field
Error Detection with FCS
Ethernet also defines a way for nodes to find out whether a frame’s bits changed while
crossing over an Ethernet link. (Usually, the bits could change because of some kind of electrical
interference, or a bad NIC.) Ethernet, like most data-link protocols, uses a field in the
data-link trailer for the purpose of error detection.
The Ethernet Frame Check Sequence (FCS) field in the Ethernet trailer—the only field in
the Ethernet trailer—gives the receiving node a way to compare results with the sender, to
discover whether errors occurred in the frame. The sender applies a complex math formula
to the frame before sending it, storing the result of the formula in the FCS field. The receiver
applies the same math formula to the received frame. The receiver then compares its own
results with the sender’s results. If the results are the same, the frame did not change; otherwise,
an error occurred and the receiver discards the frame.
Note that error detection does not also mean error recovery. Ethernet defines that the
errored frame should be discarded, but Ethernet does not attempt to recover the lost frame.
Other protocols, notably TCP, recover the lost data by noticing that it is lost and sending
the data again.