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Winston [Page 0]
Network Working Group Ira Winston
Request for Comments: 948 University of Pennsylvania
June 1985
TWO METHODS FOR THE TRANSMISSION OF IP DATAGRAMS OVER
IEEE 802.3 NETWORKS
Status of this Memo
This memo describes two methods of encapsulating Internet
Protocol (IP) [1] datagrams on an IEEE 802.3 network [2]. This RFC
suggests a proposed protocol for the ARPA-Internet community, and
requests discussion and suggestions for improvements. Distribution
of this memo is unlimited.
Introduction
The IEEE 802 project has defined a family of standards for Local Area
Networks (LANs) that deals with the Physical and Data Link Layers as
defined by the ISO Open System Interconnection Reference Model
(ISO/OSI). Several Physical Layer standards (802.3, 802.4, and
802.5) [2, 3, 4] and one Data Link Layer Standard (802.2) [5] have
been defined. The IEEE Physical Layer standards specify the ISO/OSI
Physical Layer and the Media Access Control Sublayer of the ISO/OSI
Data Link Layer. The 802.2 Data Link Layer standard specifies the
Logical Link Control Sublayer of the ISO/OSI Data Link Layer.
The 802.3 standard is based on the Ethernet Version 2.0 standard [6].
The Ethernet Physical Layer and the 802.3 Physical Layer are
compatible for all practical purposes however, the Ethernet Data Link
Layer and the 802.3/802.2 Data Link Layer are incompatible.
There are many existing Ethernet network installations that transmit
IP datagrams using the Ethernet compatible standard described in [7].
IEEE 802.3 Physical Layer compatible connections can be added to
these networks using an an Ethernet Data Link Layer compatible method
for transmitting IP datagrams without violating the 802.3 standard.
Alternatively, an 802.2/802.3 Data Link Layer compatible method for
transmitting IP datagrams can be used.
Ethernet Compatible Method
IEEE 802.3 networks must use 48-bit physical addresses and 10
megabit/second bandwidth in order to be Ethernet compatible.
The IEEE 802.3 packet header is identical to Ethernet packet header
except for the meaning assigned to one of the fields in the header.
In an Ethernet packet header this field is used as a protocol type
field and in an 802.3 packet header the field is used as a length
field. The maximum allowed length field value on a 10 megabit/second
Winston [Page 1]RFC 948 June 1985
Transmission of IP Datagrams Over IEEE 802.3 Networks
802.3 network is 1500. The 802.3 standard states that packets with a
length field greater than the maximum allowed length field may be
ignored, discarded, or used in a private manner. Therefore, the
length field can be used in a private manner as a protocol type field
as long as the protocol types being used are greater than 1500. The
protocol type for IP, ARP and trailer encapsulation are all greater
than 1500. Using this technique, the method for transmitting IP
datagrams on Ethernet networks described in [7] can be used to
transmit IP datagrams on IEEE 802.3 networks in an Ethernet
compatible manner.
IEEE 802.2/802.3 Compatible Method
Frame Format
IP datagrams are transmitted in standard 802.2/802.3 LLC Type 1
Unnumbered Information format with the DSAP and SSAP fields of the
802.2 header set to 96, the IEEE assigned global SAP value for
IP [8]. The data field contains the IP header followed
immediately by the IP data.
IEEE 802.3 packets have minimum size restrictions based on network
bandwidth. When necessary, the data field should be padded (with
octets of zero) to meet the 802.3 minimum frame size requirements.
This padding is not part of the IP packet and is not included in
the total length field of the IP header.
IEEE 802.3 packets have maximum size restrictions based on the
network bandwidth. Implementations are encouraged to support
full-length packets.
Gateway implementations MUST be prepared to accept full-length
packets and fragment them when necessary.
Host implementations should be prepared to accept full-length
packets, however hosts MUST NOT send datagrams longer than 576
octets unless they have explicit knowledge that the destination
is prepared to accept them. A host may communicate its size
preference in TCP based applications via the TCP Maximum
Segment Size option [9].
Note: Datagrams on 802.3 networks may be longer than the general
Internet default maximum packet size of 576 octets. Hosts
connected to an 802.3 network should keep this in mind when
sending datagrams to hosts not on the same 802.3 network. It may
Winston [Page 2]RFC 948 June 1985
Transmission of IP Datagrams Over IEEE 802.3 Networks
be appropriate to send smaller datagrams to avoid unnecessary
fragmentation at intermediate gateways. Please see [9] for
further information on this point.
Address Mappings
The mapping of 32-bit Internet addresses to 16-bit or 48-bit 802.3
addresses can be done in several ways. A static table could be
used, or a dynamic discovery procedure could be used.
Static Table
Each host could be provided with a table of all other hosts on
the local network with both their 802.3 and Internet addresses.
Dynamic Discovery
Mappings between 32-bit Internet addresses and 802.3 addresses
could be accomplished through a protocol similar to the
Ethernet Address Resolution Protocol (ARP) [10]. Internet
addresses are assigned arbitrarily on some Internet networks.
Each host's implementation must know its own Internet address
and respond to 802.3 Address Resolution packets appropriately.
It should also use ARP to translate Internet addresses to 802.3
addresses when needed.
Broadcast Address
The broadcast Internet address (the address on that network
with a host part of all binary ones) should be mapped to the
broadcast 802.3 address (of all binary ones).
The use of the ARP dynamic discovery procedure is strongly
recommended.
Trailer Formats
Some versions of Unix 4.2bsd use a different encapsulation method
in order to get better network performance with the VAX virtual
memory architecture. Consenting systems on the same 802.3 network
may use this format between themselves. Details of the trailer
encapsulation method may be found in [11].
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Transmission of IP Datagrams Over IEEE 802.3 Networks
Byte Order
As described in Appendix B of the Internet Protocol specification
[1], the IP datagram is transmitted over 802.2/802.3 networks as a
series of 8-bit bytes.
Conclusion
The two encapsulation methods presented can be mixed on the same
local area network; however, this would partition the network into
two incompatible subnetworks. One host on a network could support
both methods and act as a gateway between the two subnetworks;
however, this would introduce a significant performance penalty and
should be avoided.
The IEEE 802.2/802.3 compatible encapsulation method is preferable to
the Ethernet compatible method because the IEEE 802.2 and IEEE 802.3
standards have been accepted both nationally and internationally and
because the same encapsulation method could be used on other IEEE 802
Physical Layer implementations. However, there are many existing
installations that are using IP on Ethernet and a controlled
transition from Ethernet to IEEE 802.2/802.3 is necessary.
To this end, all new implementations should allow for a static choice
of encapsulation methods and all existing implementations should be
modified to provide this static choice as well. During the
transition, all hosts on the same network would use the Ethernet
compatible method. After 802.2/802.3 support has been added to all
existing implementations, the IEEE 802.2/802.3 method would be used
and the transition would be complete.
References
[1] Postel, J. "Internet Protocol". RFC-791, USC/Information
Sciences Institute, September 1981.
[2] The Institute of Electronics and Electronics Engineers, Inc.
"IEEE Standards for Local Area Networks: Carrier Sense Multiple
Access with Collision Detection (CSMA/CD) Access Method and
Physical Layer Specifications". The Institute of Electronics
and Electronics Engineers, Inc., New York, New York, 1985.
[3] The Institute of Electronics and Electronics Engineers, Inc.
"IEEE Standards for Local Area Networks: Token-Passing Bus
Access Method and Physical Layer Specifications". The Institute
of Electronics and Electronics Engineers, Inc., New York, New
York, 1985.
Winston [Page 4]RFC 948 June 1985
Transmission of IP Datagrams Over IEEE 802.3 Networks
[4] The Institute of Electronics and Electronics Engineers, Inc.
"IEEE Standards for Local Area Networks: Token Ring Access
Method and Physical Layer Specifications". The Institute of
Electronics and Electronics Engineers, Inc., New York, New York,
1985.
[5] The Institute of Electronics and Electronics Engineers, Inc.
"IEEE Standards for Local Area Networks: Logical Link Control".
The Institute of Electronics and Electronics Engineers, Inc.,
New York, New York, 1985.
[6] "The Ethernet, Physical and Data Link Layer Specifications,
Version 2.0". Digital Equipment Corporation, Intel Corporation,
and Xerox Corporation, 1982.
[7] Hornig, C. "A Standard for the Transmission of IP Datagrams
over Ethernet Networks". RFC-894, Symbolics Cambridge Research
Center, April 1984.
[8] Reynolds, J., and Postel, J. "Assigned Numbers". RFC-943,
USC/Information Sciences Institute, April 1985.
[9] Postel, J. "The TCP Maximum Segment Size Option and Related
Topics". RFC-879, USC/Information Sciences Institute,
November 1983.
[10] Plummer, D. "An Ethernet Address Resolution Protocol".
RFC-826, Symbolics Cambridge Research Center, November 1982.
[11] Leffler, S., and Karels, M. "Trailer Encapsulations". RFC-893,
University of California at Berkeley, April 1984.
Winston [Page 5]
