TITLE
    EtherTalk 2.0: Networking Packet Formats and Protocols
Article ID:
Created:
Modified: 		5656
6/17/90
7/16/90
TOPIC



    1) With AppleTalk Phase 2, how does the SNAP (SubNetwork Address Protocol)
    work?

    If this works with the condition: DSAP=SSAP in LLC (Logical Link Control),
    the first 5 bytes of the LLC Information (LLC data) is the PID (Protocol
    Identifier). In this case, what are the DSAP and SSAP used?

    2) Also, if the above is correct, what is the Apple OUI (Organizational
    Unique Identifier) in the PID header?

    Notes
    -----
    IEEE 802.3 Frame

    Preamble
    Dest-Addr (6 bytes)
    Source-Addr (6 bytes)
    Length (2 bytes)
    LLC-DSAP (1 byte)
    LLC-SSAP (1 byte)
    LLC-control (1 byte)
    LLC-data (43-1497 bytes)

DISCUSSION

    1) With EtherTalk 2.0, Apple is adopting standard networking packet formats as
    defined by the Institute of Electrical and Electronics Engineers (IEEE) 802
    committee. The 802 committee defines standards for local area networking.

    The IEEE divides the OSI data link layer into the Logical Link Control (LLC)
    layer and the Medium Access Control (MAC) layer. The IEEE LLC layer
    provides error checking and reliable transfer of data. The LLC layer was
    defined to provide a uniform interface to the network layer, independent of
    the MAC and physical layers used. The LLC provides connectionless-oriented
    (Type 1) or connection-oriented (Type 2) services.

    The MAC layer provides some of the lower-level functions of the OSI
    data link layer and some functions that fit into the OSI physical
    layer. These functions include channel (media) access, channel contention
    arbitration, and data formatting. The MAC layer has three variants:

    - Carrier Sense Multiple Access/Collision Detect (CSMA/CD), which is similar
    to the Ethernet standard and is used in engineering and office automation
    applications.

    - Token Bus, used in factory automation applications.

    - Token Ring, which has been widely adopted by IBM and other companies for
    office automation applications.

    Ethernet and 802.3 CSMA/CD are often incorrectly considered identical.
    Ethernet is a de facto local area network standard developed by Digital
    Equipment Corporation, Xerox, and Intel. It forms the basis for 802.3, but
    the 802.3 packet format is slightly different. Unlike the Ethernet packet,
    the 802.3 packet has no protocol-type field and has a data length field.
    Workstations using Ethernet packets do not recognize workstations using
    802.3 packets, and vice versa.

    The Ethernet protocol-type field is used to distinguish higher-level
    protocols. The protocol-type field allows for 64 different protocol
    identifiers. However, with the maturation of local area network technology,
    64 different protocol identifiers are too few. To accommodate the large
    number of protocols, the IEEE developed the SubNetwork Address Protocol
    (SNAP) standard.

    SNAP allows multiple protocols to be used with one data link. It defines
    a 5-byte field to identify the protocol using the data link. It ensures
    that protocol identifiers from different vendors do not conflict. The IEEE
    has assigned Apple a SNAP identifier for AppleTalk and AARP (Apple
    Address Resolution Protocol).

    With EtherTalk 1.0, AppleTalk data was transmitted in the data field of
    an Ethernet packet. With EtherTalk 2.0, AppleTalk data is encapsulated in a
    SNAP packet, which is encapsulated in an LLC packet, and transmitted in a
    CSMA/CD 802.3 packet. The result is that EtherTalk 1.0 packets are not seen
    by EtherTalk 2.0 nodes, and EtherTalk 2.0 packets are not seen by EtherTalk
    1.0 nodes.

    Though EtherTalk 2.0 uses the SNAP interface, higher-level protocols can
    go directly to the LLC layer, and not use SNAP. SNAP allows connectionless
    communication services only. Software developers that need to use
    connection-oriented services must bypass SNAP and use the 802.2 LLC
    interface directly. Such developers would use Apple's Ethernet driver for
    our EtherTalk card, but would not use our higher-level EtherTalk 2.0
    software.

    EtherTalk 2.x (802.3) Frame
    ---------------------------
    802.3 Destination 6 bytes
    802.3 Source 6 bytes
    802.2 LLC Length 2 bytes
    802.2 LLC Header
    802.2 LLC DSAP 1 bytes (always $AA (SNAP SAP) for EtherTalk 2.x)
    802.2 LLC SSAP 1 bytes (always $AA (SNAP SAP) for EtherTalk 2.x)
    802.2 LLC Control 1 bytes (always $03)
    SNAP Header 5 bytes (always $080007809B for EtherTalk 2.x)
    DDP Header 13 bytes
    AppleTalk Data ?? bytes (586 bytes maximum)
    Padding (if needed) ?? bytes
    ---------------
    60 to 621 bytes

    DSAP - Destination Service Access Point
    SSAP - Source Service Access Point
    The SNAP SAP is defined as $AA.

    If the 802.3 packet is less than 60 bytes padding is added to make the
    packet 60 bytes long.

    In 802.2, DSAP and SSAP are almost always the same except in frames that
    are establishing an initial SNA connection.

    2) The SNAP (SubNetwork Address Protocol) unique identifier assigned to
    Apple by the IEEE is $0800070000. The first 3 bytes of the unique
    identifier is the vendor address. The last 2 bytes are the local
    administered (by Apple) identifiers. We (Apple) have defined $809B as
    EtherTalk. The unique identifier for EtherTalk 2.0 is $080007809B. This is
    considered to be a locally administered SNAP address.

    A global SNAP address has been defined by the IEEE for AARP packets. The
    SNAP number is $00000080F3.

    These values are the same for TokenTalk 2.0.



Document Information
Product Area: Communications-Networking
Category: Protocol and File Format Information
Sub Category: General Topics
Keywords:

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