Network Working Group R. Housley
Request for Comments: 3378 RSA Laboratories
Category: Informational S. Hollenbeck
VeriSign, Inc.
September 2002

EtherIP: Tunneling Ethernet Frames in IP Datagrams

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Abstract

This document describes the EtherIP, an early tunneling protocol, to
provide informational and historical context for the assignment of IP
protocol 97. EtherIP tunnels Ethernet and IEEE 802.3 media Access
control frames in IP datagrams so that non-IP traffic can traverse an
IP internet. The protocol is very lightweight, and it does not
provide protection against infinite loops.

1. IntrodUCtion

EtherIP was first designed and developed in 1991. This document was
written to document the protocol for informational purposes and to
provide historical context for the assignment of IP protocol 97 by
IANA.

The IETF Layer Two Tunneling Protocol Extensions (L2TPEXT) Working
Group and IETF Pseudo Wire Emulation Edge-to-Edge (PWE3) Working
Group are developing protocols that overcome the deficiencies of
EtherIP. In general, the standards track protocols produced by these
IETF working groups should be used instead of EtherIP.

The EtherIP protocol is used to tunnel Ethernet [DIX] and IEEE 802.3
[CSMA/CD] media access control (MAC) frames (including IEEE 802.1Q
[VLAN] datagrams) across an IP internet. Tunneling is usually
performed when the layer three protocol carried in the MAC frames is
not IP or when encryption obscures the layer three protocol control
information needed for routing. EtherIP may be implemented in an end
station to enable tunneling for that single station, or it may be
implemented in a bridge-like station to enable tunneling for multiple
stations connected to a particular local area network (LAN) segment.

EtherIP may be used to enable communications between stations that
implement Ethernet or IEEE 802.3 with a layer three protocol other
than IP. For example, two stations connected to different Ethernet
LANs using the Xerox Network Systems Internetwork Datagram Protocol
(IDP) [XNS] could employ EtherIP to enable communications across the
Internet.

EtherIP may be used to enable communications between stations that
encrypt the Ethernet or IEEE 802.3 payload. Regardless of the layer
three protocol used, encryption obscures the layer three protocol
control information, making routing impossible. For example, two
stations connected to different Ethernet LANs using IEEE 802.10b
[SDE] could employ EtherIP to enable encrypted communications across
the Internet.

EtherIP may be implemented in a single station to provide tunneling
of Ethernet or IEEE 802.3 frames for either of the reasons stated
above. Such implementations require processing rules to determine
which MAC frames to tunnel and which MAC frames to ignore. Most
often, these processing rules are based on the destination address or
the EtherType.

EtherIP may be implemented in a bridge-like station to provide
tunneling services for all stations connected to a particular LAN
segment. Such implementations promiscuously listen to all of the
traffic on the LAN segment, then apply processing rules to determine
which MAC frames to tunnel and which MAC frames to ignore. MAC
frames that require tunneling are encapsulated with EtherIP and IP,
then transmitted to the local IP router for delivery to the bridge-
like station serving the remote LAN. Most often, these processing
rules are based on the source address, the destination address, or
the EtherType. Care in establishing these rules must be exercised to
ensure that the same MAC frame does not get transmitted endlessly
between several bridge-like stations, especially when broadcast or
multicast destination MAC addresses are used as selection criteria.
Infinite loops can result if the topology is not restricted to a
tree, but the construction of the tree is left to the human that is
configuring the bridge-like stations.

1.1. Conventions Used In This Document

The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

2. Protocol Format

EtherIP segments are sent and received as internet datagrams. An
Internet Protocol (IP) header carries several information fields,
including an identifier for the next level protocol. An EtherIP
header follows the internet header, providing information specific to
the EtherIP protocol.

Internet Protocol version 4 (IPv4) [RFC791] defines an 8-bit field
called "Protocol" to identify the next level protocol. The value of
this field MUST be set to 97 decimal (141 octal, 61 hex) to identify
an EtherIP datagram.

EtherIP datagrams contain a 16-bit header and a variable-length
encapsulated Ethernet or IEEE 802.3 frame that immediately follows IP
fields.

+-----------------------+-----------------------------+

IP EtherIP Header Encapsulated Ethernet Frame

+-----------------------+-----------------------------+

Figure 1: EtherIP Datagram Description

The 16-bit EtherIP header field consists of two parts: a 4-bit
version field that identifies the EtherIP protocol version and a
12-bit field reserved for future use. The value of the version field
MUST be 3 (three, '0011' binary). The value of the reserved field
MUST be 0 (zero). Earlier versions of this protocol used an 8-bit
header field. The Xerox Ethernet Tunnel (XET) employed the 8-bit
header. The 16-bit header field provides memory alignment advantages
in some implementation environments.

In summary, the EtherIP Header has two fields:

Bits 0-3: Protocol version
Bits 4-15: Reserved for future use

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

VERSION RESERVED

+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

Figure 2: EtherIP Header Format (in bits)

The encapsulated Ethernet frame field contains a complete Ethernet or
IEEE 802.3 frame of any type less the frame check sequence (FCS)
value. The IP checksum does not provide integrity protection for the
Ethernet/IEEE 802.3 frame, so some higher-layer protocol encapsulated
by the Ethernet/IEEE 802.3 frame is eXPected to provide the integrity
protection.

3. Sending Procedures

This section describes the processing to encapsulate an Ethernet or
IEEE 802.3 MAC frame in an EtherIP datagram. First, the
implementation determines whether the MAC frame requires tunneling.
Then, if tunneling is required, the MAC frame is processed according
to the steps provided in this section. Stations processing VLAN
datagrams MAY need to examine the VLAN header to make appropriate
tunneling decisions.

An end station that implements EtherIP may tunnel some traffic, but
not all traffic. Thus, the first step in processing a MAC frame is
to determine if the frame needs to be tunneled or not. If the
recipient station is connected to the same LAN as the source station,
then tunneling is not needed. If the network connecting the stations
can route the layer three protocol, then tunneling is not needed.
Other environment specific criteria MAY also be applied. If
tunneling is not needed, skip all EtherIP processing and perform
normal data link layer processing to transmit the MAC frame.
Otherwise, follow the steps described below.

A bridge-like station promiscuously listens to all of the MAC frames
on the LAN. Each MAC frame read from the LAN is examined to
determine if it needs to be tunneled. If the recipient station is
connected to the same LAN as the source station, then tunneling is
not needed. If the destination MAC address is a router serving the
LAN, then tunneling is not needed. Other environment specific
criteria MAY also be applied. If tunneling is not needed, then
discard the MAC frame. Otherwise, follow the steps described below.

The EtherIP encapsulation process is as follows:

1. Prepend the 16-bit EtherIP header to the MAC frame. The EtherIP
Version field MUST be set to 3 (three), and the EtherIP Reserved
field MUST be set to 0 (zero). The MAC frame MUST NOT include the
FCS.

2. Determine the destination IP address of the remote EtherIP
station. This address is usually determined from the destination
MAC address.

3. Encapsulate the EtherIP datagram in an IP datagram with the
destination IP address determined in the previous step, and the
IPv4 Protocol field MUST be set to 97 (decimal).

4. Transmit the resulting IP datagram to the remote EtherIP station
via the IP router serving the LAN.

4. Receiving Procedures

This section describes the processing to decapsulate an Ethernet or
IEEE 802.3 MAC frame from an EtherIP datagram.

Since a bridge-like station promiscuously listens to all of the MAC
frames on the LAN, it may need to separate the MAC frames that
encapsulate IP datagrams addressed to it from MAC frames that are
candidates for decapsulation. The process for identifying MAC frames
that are candidates for decapsulation is as follows:

1. Perform normal data link layer processing to receive a suspected
EtherIP datagram.

2. If the recipient station is connected to the same LAN as the
source station, then ignore the frame. In most environments,
frames with a source MAC address other than the IP router serving
the LAN are ignored.

3. If the network connecting the stations can route the layer three
protocol, then decapsulation is not needed, and the frame is
ignored.

4. Ignore frames that do not contain an IP datagram.

5. Examine the IPv4 protocol field to confirm that the value of the
field is 97 (decimal). If not, ignore the frame.

Other environment specific criteria MAY also be applied.

Upon reception of an IPv4 datagram with the Protocol field set to 97
(decimal), the MAC frame is processed as follows:

1. Examine the 16-bit EtherIP header. Confirm that the value of the
version field is 3 (three), and that the value of the Reserved
field is 0 (zero). Frames with other values MUST be discarded.

2. Extract the encapsulated MAC frame from the EtherIP datagram.
Note that the extracted frame MUST NOT include a FCS value.

3. Perform normal data link layer processing to transmit the
extracted MAC frame to the destination station on the LAN. The
FCS MUST be calculated and appended to the frame as part of the
data link layer transmission processing.

5. IANA Considerations

IANA has assigned IP protocol value 97 (decimal) for EtherIP. No
further action or review is required.

6. Security Considerations

EtherIP can be used to enable the transfer of encrypted Ethernet or
IEEE 802.3 frame payloads. In this regard, EtherIP can improve
security. However, if a firewall permits EtherIP traffic to pass in
and out of a protected enclave, arbitrary communications are enabled.
Therefore, if a firewall is configured to permit communication using
EtherIP, then additional checking of each frame is probably necessary
to ensure that the security policy that the firewall is installed to
enforce is not violated.

Further, the addition of EtherIP can expose a particular environment
to additional security threats. Assumptions that might be
appropriate when all communicating nodes are attached to one Ethernet
segment or switch may no longer hold when nodes are attached to
different Ethernet segments or switches are bridged together with
EtherIP. It is outside the scope of this specification, which
documents an existing practice, to fully analyze and review the risks
of Ethernet tunneling. The IETF Pseudo-wire Emulation Working Group
is doing work in this area, and this group is expected to provide
information about general layering as well as specific Ethernet over
IP documents. An example should make the concern clear. A number of
IETF standards employ relatively weak security mechanisms when
communicating nodes are expected to be connected to the same local
area network. The Virtual Router Redundancy Protocol [RFC2338] is
one instance. The relatively weak security mechanisms represent a
greater vulnerability in an emulated Ethernet. One solution is to
protect the IP datagrams that carry EtherIP with IPsec [RFC2401].

The IETF Pseudo-wire Emulation Working Group may document other
security mechanisms as well.

7. Acknowledgements

This document describes a protocol that was originally designed and
implemented by Xerox Special Information Systems in 1991 and 1992.
An earlier version of the protocol was provided as part of the Xerox
Ethernet Tunnel (XET).

8. References

[CSMA/CD] Institute of Electrical and Electronics Engineers:
"Carrier Sense Multiple Access with Collision Detection
(CSMA/CD) Access Method and Physical Layer Specifications",
ANSI/IEEE Std 802.3-1985, 1985.

[DIX] Digital Equipment Corporation, Intel Corporation, and Xerox
Corporation: "The Ethernet -- A Local Area Network: Data
Link Layer and Physical Layer (Version 2.0)", November
1982.

[RFC791] Postel, J., "Internet Protocol", STD 5, RFC791, September
1981.

[RFC2119] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC2338] Knight, S., Weaver, D., Whipple, D., Hinden, R., Mitzel,
D., Hunt, P., Higginson, P., Shand, M. and A. Lindem,
"Virtual Router Redundancy Protocol", RFC2338, April 1998.

[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC2401, November 1998.

[SDE] Institute of Electrical and Electronics Engineers:
"Interoperable LAN/MAN Security (SILS) Secure Data Exchange
(SDE) (Clause 2)", IEEE Std 802.10b-1992, 1992.

[XNS] Xerox Corporation: "Internet Transport Protocols", XSIS
028112, December 1981.

[VLAN] Institute of Electrical and Electronics Engineers: "IEEE
Standard for Local and Metropolitan Area Networks: Virtual
Bridge Local Area Networks", ANSI/IEEE Std 802.1Q-1998,
1998.

9. Authors' Addresses

Russell Housley
RSA Laboratories
918 Spring Knoll Drive
Herndon, VA 20170
USA

EMail: rhousley@rsasecurity.com

Scott Hollenbeck
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA

EMail: shollenbeck@verisign.com

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