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.
Jennifer Hsu
Juniper Networks, Inc.
Extended Ethernet Frame Size Support for IP and CLNS
draft-kaplan-isis-ext-eth-ip-clns-1-00.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 the current Ethernet Frame
standards to support payloads greater than 1500 Bytes for IP and
ISO CLNS. 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 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 frames with IP or CLNS 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 CLNS Frames in the presence of IP Frames
Some ISO protocols commonly used in the Internet, such as ESIS
and ISIS are carried as CLNS packets. There is no reserved
Ethertype for CLNS. CLNS packets can only use the
IEEE 802.3/802.2 encoding, and so are limited in length to
1500 bytes.
IP packets are encapsulated within Ethernet II frames, which do
not have length fields, and so IP packets are not limited in
length to 1500 bytes by framing.
6. Proposed Ethernet Frame Extension
Large CLNS frames may be supported by the following:
+ Define an Ethertype, 0x8872, for CLNS and encoding these frames as
802.2 within Ethernet II:
+----+----+------+------+------+------+------+-----+
| DA | SA | Type | DSAP | SSAP | Ctrl | Data | FCS |
+----+----+------+------+------+------+------+-----+
=== 802.2 Header ===
DA Destination MAC Address (6 bytes)
SA Source MAC Address (6 bytes)
Type Ethertype for CLNS (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 CLNS packets encapsulated
as 802.3/802.2 because 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 packets, including IP and CLNS,
can be larger 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
Jennifer Hsu
Juniper Networks, Inc.
385 Ravendale Drive
Mountain View, CA 94043
301-924-7237
email: jhsu@juniper.net