Internet Draft 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