Network Working Group                                   Jed Kaplan
Internet Draft                                          P.J. Singh
Expiration Date: November 2001                          Allegro Networks, Inc.

                                                        Mike O'Dell

                                                        John Hayes
                                                        Archduke Design, Inc.

                                                        Ted Schroeder
                                                        Alteon WebSystems, Inc.

                                                        Daemon Morrell
                                                        Juniper Networks, Inc.

                                                        Jennifer Hsu

                  Extended Ethernet Frame Size Support


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-

        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

        The list of current Internet-Drafts can be accessed at

        The list of Internet-Draft Shadow Directories can be accessed at

2. Abstract

        This document presents an extension to current Ethernet Frame
        specifications for hardware and frame format to support payloads
        greater than 1500 Bytes for Type interpretation and Length
        interpretation 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:
        Type interpretation [IEEE-ISO] [RFC894] and Length interpretation
        [IEEE-ISO]. Length interpretation 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 Type interpretation and Length interpretation
        frames evolved such that, as long as payloads were less than 1500
        bytes, Type interpretation frames could always be distinguished from
        Length interpretation frames.

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

4. Ethernet Frame Formats

        A. Type interpretation

                | 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. Length interpretation 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:

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

                        (600 hex).

        <= maxValidFrame 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 Length interpretation Frames in the presence of Type
   Interpretation 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.

        Type Interpretation frames have no length field. Protocols
        encapsulated in Type interpretation frames, such as IP, are not
        limited in length to 1500 bytes by framing.

6. Proposed Ethernet Frame Extension

        Large Length interpretation and Type interpretation 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 Type interpretation frames.

           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 Type interpretation 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 Type interpretation frames, including large
        Length interpretation frames, can be longer than 1500 bytes, yet
        are uniquely identified.

7. Default MTU

        The motivation for this draft was support of FDDI MTU, 4470,
        without fragmention.

        Supporting MPLS over Ethernet without fragmentation requires 4 or 8
        additional bytes beyond Ethernet MTU.

        Some servers support MTU of 9180. Additionally RFC 1626 prescribes
        an MTU of 9180 for ATM AAL5 supporting IP over ATM. Some equipment
        providers claim significant cost to support this MTU.

        Assuming that there isn't a great need for an MTU of 9180, an MTU
        of 4470 is recommended, to support the expected predominate
        applications of extended Ethernet frames, without causing undue
        burden on equipment providers.

8. References

[IEEE-ISO] IEEE Std. 802.3 [2000 Edition], ISO/IEC 8802-3:2000.

[RFC894] IETF RFC 894

[IEEE802] IEEE Std 802

[IEEE802.3Z] IEEE Std 802.3z

[802.3X] IEEE Std. 802.3X [March 20, 1997], sec

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

9. Author's Addresses

Jed Kaplan
Allegro Networks, Inc.
12700 Failr Lakes Circle
Suite 100
Fairfax, Va 22033

P.J. Singh
Allegro Networks, Inc.
6399 San Ignacio Blvd.
San Jose, Ca. 95119

Mike O'Dell

John Hayes
Archduke Design, Inc.
24700 Skyland Road
Los Gatos, CA 95033
(408) 691-6891

Ted Schroeder

Daemon Morrell
Juniper Networks, Inc.
12343-D Sunrise Valley Drive
Reston, VA 20191

Jennifer Hsu

10. Appendix 1. Following is the response from the IEEE to

        From:   Geoff Thompson, Chair, IEEE 802.3
        To:     Scott O. Bradner, IETF
        Re:     802.3 Position on Extended Ethernet Frame Size Support


        This is in response to your query for a position regarding the
        publication of Extended Ethernet Frame Size Support -
        draft-kaplan-isis-ext-eth-02.txt - as an informational RFC. This
        response was approved in concept and draft by 802.3 during its
        closing plenary at Hilton Head on March 15. The final form was
        drafted by myself and reviewed by an ad hoc that was formed during
        our closing plenary. It should be considered the position of the
        802.3 Working Group.

        The response is composed of two parts, specific comments on the
        draft and general comments on the use of jumbo frames in Ethernet
        networks, however, virtually all traffic uses the type/length field
        as a type field. It seems unlikely that the implementations using
        the length format would take advantage of longer
        packets. Therefore, the draft conveys a very limited value.

        Specific comments on: Extended Ethernet Frame Size Support -

        The draft makes no mention that extended frames are not likely to be
        successfully handled by Ethernet equipment unless the network is
        composed entirely of equipment that is specifically designed, beyond
        the specifications of the Ethernet Standard, to relay extended size

        In section 2, Abstract, the document asserts that it presents an
        extension to the "current Ethernet Frame Standards to support
        payloads greater than 1500 bytes..." Neither the original Ethernet
        specification (it was not a "Standard") nor IEEE Std. 802.3 is a
        "frame standard". They are, rather, complete specifications for
        hardware and frame format with the expectation that parameters from
        one portion of the standard can be taken as a given in other
        portions of the Standard. Moreover, this draft is not an
        "extension" to those documents but rather a proposal to violate
        specific provisions of those documents.

        In section 3, the draft refers to "Ethernet II [ETH] and points to
        the reference [ETH] The reference, as cited, is incorrect or

        Ethernet II would seem to point to Ethernet Version 2.0. That would
        specifically not be "version 1.0...September 1980". The citation in
        fact points to 2 different documents and fails to note that the
        November 1982 edition is in fact Version 2.0. Further, both of these

        are obsolete references and have been superceded by IEEE Std. 802.3
        and ISO/IEC 8802-3. The current version of these Standards is IEEE
        Std. 802.3 [2000 Edition] and ISO/IEC 8802-3 : 2000.

        The details of section 4 are badly out of date. IEEE Std.  802.3
        has included both Type and Length encoded packets ever since the
        adoption of IEEE Std. 802.3x on March 20, 1997. The current text of
        the 802.3 text covering this reads:
        3.2.6 Length/Type Field
        This two-octet field takes one of two meanings,depending on  its
        numeric value. For numerical evaluation, the first octet is the most
        significant octet of this field.
          a)If the value of this field is less than or equal to the value of
            maxValidFrame (as specified in, then the Length/Type
            field indicates the number of MAC client data octets contained
            in the subsequent data field of the frame (Length
          b)If the value of this field is greater than or equal to 1536
            decimal (equal to 0600 hexadecimal),then the Length/Type field
          indicates the nature of the MAC client protocol (Type
          interpretation). The Length and Type interpretations of this
          field are mutually exclusive.
        Please note that any value over "the value of maxValidFrame" is NOT
        a valid value for encoding length. Additionally, the values between
        maxValidFrame and "1536 decimal" are undefined in the Ethernet
        standard.  The behavior of equipment at these values is not
        specified and can not be depended on. The draft implies that these
        values are valid type fields. This is not true. These values are
        not valid for either Type or Length.

        Section 4 Re: "...are not limited in length to 1500 bytes by
        framing." While this seems to be true, it is not necessarily true
        for a number of sometimes subtle reasons, some of which are noted
        in the "General" section below.

        Section 5: Regarding the statement "Although the 802.3 length field
        is missing, the frame length is missing, the frame length is known
        by virtue of the frame being accepted by the network interface."
        This statement is not correct. Many Ethernet interfaces,
        particularly those of relay equipment, accept frames without regard
        for packet type or content. There is no reasonable expectation that
        standards based Ethernet/802.3 equipment will reject the proposed
        frames. They may very well accept the frame and corrupt it before
        passing it on. This corruption may consist of truncation or
        alteration of the data within the packet.

        General comments on the use of jumbo frames in Ethernet networks:

        Consideration #1: The expectation of no more than 15-1600 bytes
        between frames and an interpacket gap before the next frame is
        deeply ingrained throughout the design and implementation of
        standardized Ethernet/802.3 hardware. This shows up in buffer
        allocation schemes, clock skew and tolerance compensation and fifo

        Consideration #2: For some Ethernet/802.3 hardware (repeaters are
        one specific example) it is not possible to design compliant
        equipment which meets all of the requirements and will still pass
        extra long frames.  Further, since clock frequency may vary with
        time and temperature, equipment may successfully pass long frames
        at times and corrupt them at other times.  Therefore, attempts to
        verify the ability to send long frames over a path may produce
        inaccurate results.

        Consideration #3: The error checking mechanism embodied in the 4
        byte checksum has not been well characterized at greater frame
        lengths, but is known to degrade. Therefore the data reliability of
        transfers in long frame transfers will have a greater rate of
        undetected frame errors.

        Consideration #4: The length of frames proposed by this draft can
        not be assured to pass through standards conformant hardware. The
        huge value of Ethernet/802.3 systems in the data networking
        universe is their standardization and the resulting assurance that
        systems will all interoperate. No such assurance can be provided
        for oversize frames with both the current broadly accepted standard
        and the large installed base ofstandards based equipment.

        In summary with regard to greatly longer frames for Ethernet, much
        of the gear produced today would be intolerant of greatly longer
        frames.  There is no way proposed to distinguish between frame
        types in the network as they arrive from the media. Bridges might
        and repeaters would drop or truncate frames (and cause errors doing
        so) right and left for uncharacterized reasons. It would be a
        mess. What might seem okay for small carefully characterized
        networks would be enormously difficult or impossible to do across
        the Standard.

        The choice of frame size for Ethernet packets is really the domain
        of 802.3 (CSMA/CD) and 802.1 (Bridging, VLANs). The only time the
        frame size has been modified over the history of the Standard was
        in order to increase maximum length by four bytes in order to
        accommodate VLANs, 802.1 initiated this work and 802.3 also
        modified the Ethernet standard to include these few extra
        bytes. The people with the experience dealing with this sort of
        thing attend IEEE 802.  It's easy to define a new ethertype, but
        it's not too easy to figure out what happens when these
        non-standard frames are given to standardized transmission
        equipment e.g. bridges.  We would expect discussions of this type
        to take place in both 802.3 & 802.1.

        The giant frame issue has been mentioned several times over the
        years in 802.3, discussed in the back halls and considered each
        time we move to a higher speed. It has never had consensus support
        in that context.  It has never been brought forward as a separate
        proposal. Backward compatibility has always been more important
        than ease of performance improvement. The problem is that the
        change is very easy to do in the standard and hard to do in the
        world. It is just like changing the gauge on railroad tracks. All
        you have to do is change one line in the standard, never mind all
        of the rails you have to move.

        The Kaplan draft is just meant for carrying IS-IS routing protocol
        frames (the IS-IS working group is the intended sponsor of this
        draft).  We expect those vendors supporting the larger frame will
        support this will show up and support this proposal. Those vendors
        not supporting the larger frame as well as those protecting the
        installed base will not support this activity nor having this sort
        of item standardized outside IEEE 802.3.

        With best regards,

        Geoff Thompson, Chair, IEEE 802.3

11. Appendix 2. Comments from the draft's authors.

        With respect to Consideration #2, paragraph 1:

        With the advent of full duplex ethernet and higher speed ethernet
        phys, transcievers have gone from transmitting when they have data
        ready send to transmitting constantly, sending IDLEs when they have
        nothing to send. Any clock drift due to time and temperature will
        affect the system without regard to the frame lengths being used.

        With respect to Consideration #3, paragraph 1:

        The error checking mechanism of ethernet (CRC-32) was characterized
        by R. Jain, "Error Characteristics of Fiber Distributed Data
        Interface (FDDI)," IEEE Transactions on Communications, Vol. 38,
        No. 8, August 1990,
        pp. 1244-1252.

        FDDI and Ethernet use the same error checking mechanism; CRC-32.
        The probability of undetected errors remains constant for frame
        sizes between 3007 and 91639 bits (approximately 376 to 11455
        bytes).  Setting the maximum size of jumbo frames to 9018 bytes
        falls well within this range.  There is no increase in undetected
        errors when using jumbo frames and the existing CRC-32 as the error
        detection mechanism.

        With respect to Consideration #4, paragraph 1:

        This proposal provides a workable mechanism to identify and
        handle jumbo frames through systems that do support jumbo frames.