Ethernet Frames

Ethernet Frame Types • There are three Ethernet frame types that TCP/IP can use: – Ethernet II – Ethernet 802.2 Logical Link Control (LLC) – Ethernet 802.2 Sub-Network Access Protocol (SNAP)

Ethernet II Frame Structure •

The Ethernet II frame type consists of the following values, fields, and structure: – Preamble – Destination Address field – Source Address field – Type field – Data field – Frame Check Sequence field

Ethernet II Frame Structure

Ethernet II Frame Structure Preamble • The preamble is eight bytes long and consists of alternating ones and zeroes • This special string of bits precedes the actual Ethernet frame itself, and is not counted as part of the overall frame length Destination Address Field • The Destination Address field is six bytes long and indicates the data link address (also referred to as the hardware address or MAC address) of the destination IP host • The destination address may be broadcast, multicast, or unicast

Ethernet II Frame Structure Source Address Field •

The Source Address field is six bytes long and indicates the sender’s hardware address

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This field can only contain a unicast address—it cannot contain a broadcast or multicast address

Type Field • The Type field is two bytes long and identifies the protocol that is using this frame type

Assigned Protocol Types (by Number)

Ethernet II Frame Structure Frame Check Sequence Field •

The Frame Check Sequence field is four bytes long and includes the result of the CRC calculation

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Upon receipt of an Ethernet II frame, an IP host checks the validity of the contents by performing a CRC check on its contents, and comparing the result to the value contained in the Frame Check Sequence field

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At the Data Link layer, the frame is examined to determine the actual destination address (broadcast, multicast, or unicast)

Ethernet 802.2 LLC Frame Structure • Figure 3-2 depicts the format of an Ethernet 802.2 Logical Link Control (LLC) frame • The Ethernet 802.2 LLC frame type consists of the following fields: – Preamble – Start Frame Delimiter (SFD) field – Destination Address field – Source Address field – Length field – Destination Service Access Point (DSAP) field – Source Service Access Point (SSAP) field – Control field – Data field – Frame Check Sequence (FCS) field

Ethernet 802.2 LLC Frame Structure Preamble •

This preamble does not end in consecutive ones Start Frame Delimiter (SFD) Field

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The 1-byte SFD field consists of the pattern 10101011 and indicates the start of the Destination Address field Length Field

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The 2-byte Length field indicates the number of bytes in the data portion of the frame This frame does not use a Type field in this location—it uses a Service Access Point (SAP) field to indicate the upcoming protocol

Ethernet 802.2 LLC Frame Structure Destination Service Access Point (DSAP) Field • •

This 1-byte field indicates the destination protocol Table 3-2 lists some of the assigned SAP numbers (defined by the IEEE) Source Service Access Point (SSAP) Field

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This 1-byte field indicates the source protocol (typically the same as the destination protocol)

Assigned SAP Numbers

Ethernet 802.2 LLC Frame Structure Control Field • This 1-byte field indicates whether this frame is unnumbered format (connections) or informational/supervisory format (for connection-oriented and management purposes) Ethernet SNAP Frame Structure • RFC 1042, “A Standard for the Transmission of IP Datagrams over IEEE 802 Networks,” specifies how IP traffic should be encapsulated to 802.2 LLC frames that include the Sub-Network Access Protocol (SNAP) portion

Ethernet 802.2 LLC Frame Structure Ethernet SNAP Frame Structure •

Although Windows 2000 defaults to transmitting IP and ARP communications over the Ethernet II frame type, you can edit the Registry to support transmission of IP and ARP over the Ethernet 802.2 SNAP frame structure by adding the ArpUseEtherSNAP Registry setting, as listed in Table 3-3.

ArpUseEtherSNAP Registry Setting

Ethernet 802.2 LLC Frame Structure Ethernet SNAP Frame Structure • The Registry entry ArpUseEtherSNAP must be set to 1 to enable use of the Ethernet 802.2 SNAP frame format for IP and ARP traffic over Ethernet • Figure 3-3 depicts the format of an Ethernet SNAP frame

Ethernet 802.2 LLC Frame Structure Organization Code Field •

This 3-byte field identifies the organization that assigned the Ethernet type number used in the Ether Type field that follows Ether Type Field

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The 2-byte Ether Type field indicates the network protocol that is using this Ethernet SNAP frame format