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Versions: 00 01 02 draft-ietf-isis-ext-eth

Network Working Group                                   Mike O'Dell
Internet Draft                                          Jed Kaplan
Expiration Date: November 1999                          UUNET Technologies, Inc.

                                                        John Hayes
                                                        Ted Schroeder
                                                        Alteon WebSystems, Inc.

                                                        P.J. Singh
                                                        Packet Engines, Inc.

                                                        Daemon Morrell
                                                        Juniper Networks, Inc.

                                                        Jennifer Hsu



                  Extended Ethernet Frame Size Support

                    draft-kaplan-isis-ext-eth-02.txt


1. Status of this Memo

        This document is an Internet-Draft and is in full conformance with
        all provisions of Section 10 of RFC2026.

        Internet-Drafts are working documents of the Internet Engineering
        Task Force (IETF), its areas, and its working groups. Note that
        other groups may also distribute working documents as Internet-
        Drafts.

        Internet-Drafts are draft documents valid for a maximum of six months
        and may be updated, replaced, or obsoleted by other documents at any
        time. It is inappropriate to use Internet-Drafts as reference
        material or to cite them other than as "work in progress."

        The list of current Internet-Drafts can be accessed at
        http://www.ietf.org/ietf/lid-abstracts.txt

        The list of Internet-Draft Shadow Directories can be accessed at
        http://www.ietf.org/shadow.html


2. Abstract

        This document presents an extension to current Ethernet Frame
        standards to support payloads greater than 1500 Bytes for Ethernet_II
        and 802.3 frames. This is useful for Gigabit Ethernet technology,
        providing a means to carry large MTU packets without fragmentation
        over a high-speed broadcast network.

3. Overview

        There are two fundamental frame types defined for Ethernet:
        Ethernet II [ETH] [RFC894] and 802.3 [IEEE802.3]. 802.3 headers
        may be followed by a Logical Link Control header,
        802.2 [IEEE802.2]. Both types of encapsulations can co-exist on
        the same media at the same time. Encodings for Ethernet II and 802.3
        frames evolved such that, as long as payloads were less than 1500
        bytes, Ethernet II frames could always be distinguished from
        IEEE 802.3 frames.

        However, when the payload is greater than 1500 bytes frames may
        not be uniquely distinguishable as conforming to Ethernet II or
        802.3 formats. This document extends the Ethernet frame format
        to allow Ethernet_II or 802.3 frame payloads larger than 1500 bytes
        to be uniquely distinguished.

4. Ethernet Frame Formats

        A. Ethernet II

                +----+----+------+------+-----+
                | DA | SA | Type | Data | FCS |
                +----+----+------+------+-----+

                DA      Destination MAC Address (6 bytes)
                SA      Source MAC Address      (6 bytes)
                Type    Protocol Type           (2 bytes)
                Data    Protocol Data           (46 - 1500 bytes)
                FCS     Frame Checksum          (4 bytes)

        B. IEEE 802.3 and derivatives

                +----+----+------+------+-----+
                | DA | SA | Len  | Data | FCS |
                +----+----+------+------+-----+

                DA      Destination MAC Address (6 bytes)
                SA      Source MAC Address      (6 bytes)
                Len     Length of Data field    (2 bytes)
                Data    Protocol Data           (46 - 1500 bytes)
                FCS     Frame Checksum          (4 bytes)

        The derivatives include LLC (802.2) and SNAP which prefix the
        data field with an LLC header.  In these instances the Len field
        then corresponds to the combined size of both the data portion
        of the frame and the LLC header.

        On reception, the two formats are differentiated based on the
        magnitude of the Type/Length field, as follows:

        > 1500 bytes:   value corresponds to a type field.  The frame is an
                        Ethernet II frame, with type values starting
                        at 1536 (600 hex).

        <= 1500 bytes:  value corresponds to a length field.  The frame is
                        an IEEE 802.3 format (or derivative) with a maximum
                        data length of 1500 bytes.

5. Problem with Large 802.3 Frames in the presence of Ethernet_II Frames

        Some protocols commonly used in the Internet have no reserved Ethertype.
        An example is the set of ISO Network layer protocols, of which
        ISIS is a member. Such protocols are only defined to use the IEEE
        802.3/802.2 encoding, and so their packets are limited in length to
        1500 bytes.

        Ethernet_II frames have no length field. Protocols encapsulated in
        Ethernet II frames, such as IP, are not limited in length to 1500
        bytes by framing.

6. Proposed Ethernet Frame Extension

        Large 802.3 and Ethernet_II frames can be supported by the following:

        +  Define an Ethertype for 802.3, 0x8870, and encode large frames
           (where the data field is greater than 1500 bytes),
           exclusive of the Destination MAC address, Source MAC address,
           and Data length fields, within Ethernet II.

           Large 802.3/802.2 frames would have the following fields:

                +----+----+------+------+------+------+------+-----+
                | DA | SA | Type | DSAP | SSAP | Ctrl | Data | FCS |
                +----+----+------+------+------+------+------+-----+
                                  === 802.2 Header ===

                DA      Destination MAC Address                 (6 bytes)
                SA      Source MAC Address                      (6 bytes)
                Type    0x8870 (Ethertype)                      (2 bytes)
                DSAP    802.2 Destination Service Access Point  (1 byte)
                SSAP    802.2 Source Service Access Point       (1 byte)
                Ctrl    802.2 Control Field                     (1 byte)
                Data    Protocol Data                           ( > 46 bytes)
                FCS     Frame Checksum                          (4 bytes)

        +  Allow Ethernet II frames to have payloads greater than 1500 bytes.

        There is no loss of information from 802.3/802.2 frames. Although
        the 802.3 length field is missing, the frame length is known by
        virtue of the frame being accepted by the network interface.

        In this manner, all Ethernet II frames, including large 802.3
        packets, can be longer than 1500 bytes, yet are uniquely identified.


7. References

[ETH] "The Ethernet - A Local Area Network", version 1.0, Digital
Equipment Corporation, September 1980, and "The Ethernet, A Local
Area Network" Data Link Layer and Physical Layer Specifications",
Digital, Intel, and Xerox, November, 1982.

[RFC894] IETF RFC 894

[IEEE802.3] IEEE Std 802.3

[IEEE802] IEEE Std 802

[IEEE802.3Z] IEEE Std 802.3z

[EXT.FRAME] "Use of Extended Frame Sizes in Ethernet Networks", draft
2.1, Alteon Networks, Inc.


8. Author's Addresses

Mike O'Dell
UUNET an MCI WorldCom Company
3060 WIllaims Drive
Fairfax, Va. 22031-4648
703-206-5890
email: mo@uu.net

Jed Kaplan
UUNET an MCI WorldCom Company
3060 WIllaims Drive
Fairfax, Va. 22031-4648
914-701-5309
email: jkaplan@uu.net

John Hayes
Alteon WebSystems, Inc.
50 Great Oaks Blvd.
San Jose, CA 95119
408-360-5507
email: hayes@alteon.com

Ted Schroeder
Alteon WebSystems, Inc.
50 Great Oaks Blvd.
San Jose, CA 95119
408-360-5500
email: ted@alteon.com

P.J. Singh
Packet Engines, Inc.
11707 East Sprague #101
Spokane WA  99206
509-777-7000
email: pjsingh@packetengines.com

Daemon Morrell
Juniper Networks, Inc.
12343-D Sunrise Valley Drive
Reston, VA 20191
email: dmorrell@juniper.net

Jennifer Hsu
jhsu@mur.com


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