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
draft-ietf-isis-ext-eth-01.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
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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 4.2.7.1.
[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
email: jkaplan@allegronetworks.com
P.J. Singh
Allegro Networks, Inc.
6399 San Ignacio Blvd.
San Jose, Ca. 95119
408-281-5500
email: pjsingh@allegronetworks.com
Mike O'Dell
email: mo@ccr.net
John Hayes
Archduke Design, Inc.
24700 Skyland Road
Los Gatos, CA 95033
(408) 691-6891
email: hayes@archdukedesign.com
Ted Schroeder
email: ted@alteon.com
Daemon Morrell
Juniper Networks, Inc.
12343-D Sunrise Valley Drive
Reston, VA 20191
email: dmorrell@juniper.net
Jennifer Hsu
jhsu@mur.com
10. Appendix 1. Following is the response from the IEEE to
draft-kaplan-isis-ext-eth-02.txt.
From: Geoff Thompson, Chair, IEEE 802.3
To: Scott O. Bradner, IETF
Re: 802.3 Position on Extended Ethernet Frame Size Support
Scott-
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 -
draft-kaplan-isis-ext-eth-02.txt
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
frames.
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
incomplete.
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 4.2.7.1), then the Length/Type
field indicates the number of MAC client data octets contained
in the subsequent data field of the frame (Length
interpretation).
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
design.
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. http://www.cis.ohio-state.edu/~jain/papers/xie1.htm
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.
Datatracker
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