Network Working Group Juha Heinanen
Reguest for Comments: DRAFT Telecom Finland
Expires June 17, 1993 December 17, 1992
Multiprotocol Interconnect over ATM Adaptation Layer 5
Status of this Memo
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Abstract
This memo describes two encapsulations methods for carrying network
interconnect traffic over ATM AAL5. The first method allows
multiplexing of multiple protocols over a single ATM virtual circuit
whereas the second method assumes that each protocol is carried over
a separate ATM virtual circuit.
1. Introduction
Asynchronous Transfer Mode (ATM) based networks are of increasing
interest for both local and wide area applications. This memo
describes two different methods for carrying connectionless network
interconnect traffic (routed and bridged PDUs) over an ATM network.
The first method allows multiplexing of multiple protocols over a
single ATM virtual circuit. The protocol of a carried PDU is
identified by prefixing the PDU by an IEEE 802.2 Logical Link Control
(LLC) header. This method is in the following called "LLC
Encapsulation" and a subset of it has been earlier defined for SMDS
[1]. The second method does higher-layer protocol multiplexing
implicitly by ATM Virtual Circuits (VCs). It is in the following
called "VC Based Multiplexing".
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ATM is a cell based transfer mode that requires variable length user
information to be segmented and reassembled to/from short, fixed
length cells. This memo doesn't specify a new Segmentation And
Reassembly (SAR) method for bridged and routed PDUs. Instead, the
PDUs are carried in the Payload field of Common Part Convergence
Sublayer (CPCS) PDU of AAL5 [2]. AAL5 is a new simple and efficient
ATM Adaptation Layer currently being standardized both in ANSI and
CCITT.
Note that this memo only describes how routed and bridged PDUs are
carried directly over the CPCS of AAL5, i.e., when the Service
Specific Convergence Sublayer (SSCS) of AAL5 is empty. If Frame
Relay Specific Convergence Sublayer (FRCS), as defined in I.555 [3],
is used over the CPCS of AAL5, then routed and bridged PDUs are
carried using the NLPID multiplexing method described in RFC 1294
[4]. Appendix A (which is for information only) shows the format of
the FRCS-PDU as well as how IP and CLNP PDUs are encapsulated over
FRCS according to RFC 1294.
2. Selection of the Multiplexing Method
It is envisioned that VC Based Multiplexing will be dominant in
environments where dynamic creation of large numbers of ATM VCs is
fast and economical. These conditions are likely to first prevail in
ATM LANs. LLC Encapsulation, on the other hand, may be desirable
when it is not practical for one reason or another to have a separate
VC for each carried protocol. This is the case, for example, if the
ATM network only supports (semi) Permanent Virtual Circuits (PVCs) or
if charging depends heavily on the number of simultaneous VCs.
When two ATM stations wish to exchange connectionless network
interconnect traffic, selection of the multiplexing method is done
either by manual configuration (in case of PVCs) or by B-ISDN
signalling procedures (in case of Switched VCs). The details of B-
ISDN signalling are still under study in CCITT [5]. It can, however,
be assumed that B-ISDN signalling messages include a "Low layer
compatibility" information element, which will allow negotiation of
AAL5 and the carried (encapsulation) protocol.
3. AAL5 Frame Format
No matter which multiplexing method is selected, routed and bridged
PDUs shall be encapsulated within the Payload field of AAL5 CPCS-PDU.
The format of the AAL5 CPCS-PDU is given below:
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AAL5 CPCS-PDU Format
+-------------------------------+
| . |
| . |
| CPCS-PDU Payload |
| (up to 2^16 - 1 octets) |
| . |
| . |
+-------------------------------+
| PAD ( 0 - 47 octets) |
+-------------------------------+ -------
| Reserved (2 octets) |
+-------------------------------+
| Length (2 octets) | CPCS-PDU Trailer
+-------------------------------|
| CRC (4 octets) |
+-------------------------------+ -------
The Payload field contains user information up to 2^16 - 1 octets.
The PAD field pads the CPCS-PDU to fit exactly into the ATM cells
such that the last 48 octet cell payload created by the SAR sublayer
will have the CPCS-PDU Trailer right justified in the cell.
The Reserved field is coded 0x00-00 and is used to achieve 32 bit
alignment in the CPCS-PDU trailer. Additional functions besides the
32 bit alignment are for further study in CCITT.
The Length field indicates the length, in octets, of the Payload
field. The maximum value for the Length field is 65535 octets. A
Length field coded as zero is used for the abort function.
The CRC field protects the CPCS-PDU Header (if included) + the
Payload field + the PAD field + the Reserved field + the Length
field.
4. LLC Encapsulation
LLC Encapsulation is needed when several protocols are carried over
the same VC. In order to allow the receiver to properly process the
incoming AAL5 CPCS-PDU, the Payload Field must contain information
necessary to identify the protocol of the routed or bridged PDU. In
LLC Encapsulation this information is encoded in an LLC header placed
in front of the carried PDU.
Although this memo only deals with protocols that operate over LLC
Type 1 (unacknowledged connectionless mode) service, the same
encapsulation principle applies also to protocols operating over LLC
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Type 2 (connection-mode) service. In the latter case the format
and/or contents of the LLC header would differ from what is shown
below.
4.1. LLC Encapsulation for Routed Protocols
In LLC Encapsulation the protocol of the routed PDU is identified by
prefixing the PDU by an IEEE 802.2 LLC header, which is possibly
followed by an IEEE 802.1a SubNetwork Attachment Point (SNAP) header.
In LLC Type 1 operation, the LLC header consists of three one octet
fields:
+------+------+------+
| DSAP | SSAP | Ctrl |
+------+------+------+
In LLC Encapsulation for routed protocols, the Control field has
always value 0x03 specifying Unnumbered Information Command PDU.
The LLC header value 0xFE-FE-03 identifies that a routed ISO PDU (see
[6] and Appendix B) follows. The Control field value 0x03 specifies
Unnumbered Information Command PDU. For routed ISO PDUs the format
of the AAL5 CPCS-PDU Payload field shall thus be as follows:
Payload Format for Routed ISO PDUs
+-------------------------------+
| LLC 0xFE-FE-03 |
+-------------------------------+
| . |
| ISO PDU |
| (up to 2^16 - 4 octets) |
| . |
+-------------------------------+
The routed ISO protocol is identified by a one octet NLPID field that is
part of Protocol Data. NLPID values are administered by ISO and CCITT.
They are defined in ISO/IEC TR 9577 [6] and some of the currently
defined ones are listed in Appendix C.
An NLPID value of 0x00 is defined in ISO/IEC TR 9577 as the Null Network
Layer or Inactive Set. Since it has no significance within the context
of this encapsulation scheme, a NLPID value of 0x00 is invalid under the
ATM encapsulation.
It would also be possible to use the above encapsulation for IP, since,
although not an ISO protocol, IP has an NLPID value 0xCC defined for it.
This format must not be used. Instead, IP is encapsulated like all
other routed non-ISO protocols by identifying it in the SNAP header that
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immediately follows the LLC header.
The presence of a SNAP header is indicated by the LLC header value
0xAA-AA-03. A SNAP header is of the form
+------+------+------+------+------+
| OUI | PID |
+------+------+------+------+------+
The three-octet Organizationally Unique Identifier (OUI) identifies an
organization which administers the meaning of the following two octet
Protocol Identifier (PID). Together they identify a distinct routed or
bridged protocol. The OUI value 0x00-00-00 specifies that the following
PID is an EtherType.
The format of the AAL5 CPCS-PDU Payload field for routed non-ISO PDUs
shall thus be as follows:
Payload Format for Routed non-ISO PDUs
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-00-00 |
+-------------------------------+
| EtherType (2 octets) |
+-------------------------------+
| . |
| Non-ISO PDU |
| (up to 2^16 - 9 octets) |
| . |
+-------------------------------+
In the particular case of an Internet IP PDU, the Ethertype value is
0x08-00:
Payload Format for Routed IP PDUs
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-00-00 |
+-------------------------------+
| EtherType 0x08-00 |
+-------------------------------+
| . |
| IP PDU |
| (up to 2^16 - 9 octets) |
| . |
+-------------------------------+
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4.2. LLC Encapsulation for Bridged Protocols
In LLC Encapsulation bridged PDUs are encapsulated by identifying the
type of the bridged media in the SNAP header. As with routed non-ISO
protocols, the presence of the SNAP header is indicated by the LLC
header value 0xAA-AA-03. With bridged protocols the OUI value in the
SNAP header is the 802.1 organization code 0x00-80-C2 and the actual
type of the bridged media is specified by the two octet PID.
Additionally, the PID indicates whether the original Frame Check
Sequence (FCS) is preserved within the bridged PDU. The media type
(PID) values that can be used in ATM encapsulation are listed in
Appendix B.
The AAL5 CPCS-PDU Payload field carrying a bridged PDU shall, therefore,
have one of the following formats. Padding is added after the PID field
if necessary in order to align the user information field of the bridged
PDU at a four octet boundary.
Payload Format for Bridged Ethernet/802.3 PDUs
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-01 or 0x00-07 |
+-------------------------------+
| PAD 0x00-00 |
+-------------------------------+
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| LAN FCS (if PID is 0x00-01) |
+-------------------------------+
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Payload Format for Bridged 802.4 PDUs
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-02 or 0x00-08 |
+-------------------------------+
| PAD 0x00-00-00 |
+-------------------------------+
| Frame Control (1 octet) |
+-------------------------------+
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| LAN FCS (if PID is 0x00-02) |
+-------------------------------+
Payload Format for Bridged 802.5 PDUs
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-03 or 0x00-09 |
+-------------------------------+
| PAD 0x00-00-XX |
+-------------------------------+
| Frame Control (1 octet) |
+-------------------------------+
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| LAN FCS (if PID is 0x00-03) |
+-------------------------------+
Note that the 802.5 Access Control (AC) field has no significance
outside the local 802.5 subnetwork. It can thus be regarded as
the last octet of the three octet PAD field, which can be set to
any value (XX).
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Payload Format for Bridged FDDI PDUs
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-04 or 0x00-0A |
+-------------------------------+
| PAD 0x00-00-00 |
+-------------------------------+
| Frame Control (1 octet) |
+-------------------------------+
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| LAN FCS (if PID is 0x00-04) |
+-------------------------------+
Payload Format for Bridged 802.6 PDUs
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-0B |
+---------------+---------------+ ------
| Reserved | BEtag | Common
+---------------+---------------+ PDU
| BAsize | Header
+-------------------------------+ -------
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| |
| Common PDU Trailer |
| |
+-------------------------------+
Note that in bridged 802.6 PDUs, there is only one choice for the
PID value, since the presence of a CRC-32 is indicated by the CIB
bit in the header of the MAC frame.
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The Common Protocol Data Unit (PDU) Header and Trailer are
conveyed to allow pipelining at the egress bridge to an 802.6
subnetwork. Specifically, the Common PDU Header contains the
BAsize field, which contains the length of the PDU. If this field
is not available to the egress 802.6 bridge, then that bridge
cannot begin to transmit the segmented PDU until it has received
the entire PDU, calculated the length, and inserted the length
into the BAsize field. If the field is available, the egress
802.6 bridge can extract the length from the BAsize field of the
Common PDU Header, insert it into the corresponding field of the
first segment, and immediately transmit the segment onto the 802.6
subnetwork. Thus, the bridge can begin transmitting the 802.6 PDU
before it has received the complete PDU.
Note that the Common PDU Header and Trailer of the encapsulated
frame should not be simply copied to the outgoing 802.6 subnetwork
because the encapsulated BEtag value may conflict with the
previous BEtag value transmitted by that bridge.
Payload Format for BPDUs
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-0E |
+-------------------------------+
| |
| BPDU as defined by |
| 802.1(d) or 802.1(g) |
| |
+-------------------------------+
5. VC Based Multiplexing
In VC Based Multiplexing, the carried network interconnect protocol is
identified implicitly by the VC connecting the two ATM stations, i.e.
each protocol must be carried over a separate VC. There is therefore no
need to include explicit multiplexing information in the Payload of the
AAL5 CPCS-PDU. This results in minimal bandwidth and processing overhead.
As indicated above, the carried protocol can be either manually
configured or negotiated dynamically during call establishment using
signalling procedures. The signalling details will be defined later in
other RFCs when the relevant standards have become available.
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5.1. VC Based Multiplexing of Routed Protocols
PDUs of routed protocols shall be carried as such in the Payload of the
AAL5 CPCS-PDU. The format of the AAL5 CPCS-PDU Payload field thus becomes:
Payload Format for Routed PDUs
+-------------------------------+
| . |
| Carried PDU |
| (up to 2^16 - 1 octets) |
| . |
| . |
+-------------------------------+
5.2. VC Based Multiplexing of Bridged Protocols
PDUs of bridged protocols shall be carried in the Payload of the AAL5
CPCS-PDU exactly as described in section 4.2 except that only the
fields after the PID field are included. The AAL5 CPCS-PDU Payload
field carrying a bridged PDU shall, therefore, have one of the
following formats.
Payload Format for Bridged Ethernet/802.3 PDUs
+-------------------------------+
| PAD 0x00-00 |
+-------------------------------+
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| LAN FCS (VC dependent option) |
+-------------------------------+
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Payload Format for Bridged 802.4/802.5/FDDI PDUs
+-------------------------------+
| PAD 0x00-00-00 or 0x00-00-XX |
+-------------------------------+
| Frame Control (1 octet) |
+-------------------------------+
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| LAN FCS (VC dependent option) |
+-------------------------------+
Note that the 802.5 Access Control (AC) field has no significance
outside the local 802.5 subnetwork. It can thus be regarded as
the last octet of the three octet PAD field, which in case of
802.5 can be set to any value (XX).
Payload Format for Bridged 802.6 PDUs
+---------------+---------------+ -------
| Reserved | BEtag | Common
+---------------+---------------+ PDU
| BAsize | Header
+-------------------------------+ -------
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| |
| Common PDU Trailer |
| |
+-------------------------------+
Payload Format for BPDUs
+-------------------------------+
| |
| BPDU as defined by |
| 802.1(d) or 802.1(g) |
| |
+-------------------------------+
In case of Ethernet, 802.3, 802.4, 802.5, and FDDI PDUs the presense
or absence of the trailing LAN FCS shall be identified implicitly by
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the VC, since the PID field is not included. PDUs with the LAN FCS
and PDUs without the LAN FCS are thus considered to belong to
different protocols even if the bridged media type would be the same.
6. Address Resolution
An ATM network provides VCs that form the basis for connections
between stations attached to it. A VC may also span over several ATM
networks in an "ATM internet" consisting of an arbitrary
concatenation of private ATM and public ATM networks. ATM VCs can be
establish either (semi)permanently by the operator of the ATM network
or dynamically by an ATM signalling protocol being defined by CCITT.
In either case, each VC is identified by a Virtual Path Identifier
(VPI) and a Virtual Channel Identifier (VCI). These identifiers have
only local significance at each ATM interface.
The support of multicasting in ATM networks is also presently under
study in CCITT. If an ATM network supports multicasting, a special
VPI/VCI pair can be used to indicate the sending of ATM cells to all
stations in a particular multicast group. An ATM station may use the
multicasting capability to dynamically resolve a protocol address to
a hardware address using the standard Address Resolution Protocol
(ARP) [7]. ARP packets are encapsulated within an LLC encoded CPCS-
PDU Payload field as described in section 4. The details of
multicast based address resolution will be described in a future RFC
when more information is available on the ATM multicast mechanism.
Multicast based address resolution will not be practical over large
public or private ATM networks. In such cases it might be possible
to apply a technique similar to "shortcut routing" [8] to augment the
address resolution process. Address resolution could also work using
a "well known" VC that connects to one or more address resolution
servers. Another possibility might be to use DNS to store both the
internet address and the physical ATM address of the destination.
Finally, as proposed in [9], an ATM network could support signalling
based on internet addresses in which case no address resolution would
be needed. Further elaboration of address resolution mechanisms is
outside the scope of this memo.
7. Bridging in an ATM Network
An ATM interface acting as a bridge must be able to flood, forward,
and filter bridged PDUs.
Flooding is performed by sending the PDU to all possible appropriate
destinations. In the ATM environment this means sending the PDU
through each relevant VC. This may be accomplished by explicitly
copying it to each VC or by using a multicast VC.
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To forward a PDU, a bridge must be able to associate a destination
MAC address with a VC. It is unreasonable and perhaps impossible to
require bridges to statically configure an association of every
possible destination MAC address with a VC. Therefore, ATM bridges
must provide enough information to allow an ATM interface to
dynamically learn about foreign destinations beyond the set of ATM
stations.
To accomplish dynamic learning, a bridged PDU shall conform to the
encapsulation described within section 4. In this way, the receiving
ATM interface will know to look into the bridged PDU and learn the
association between foreign destination and an ATM station.
8. For Further Study
Due to incomplete standardization of ATM multicasting, addressing,
and signalling mechanisms, details related to the negotiation of the
multiplexing method as well as address resolution had to be left for
further study.
Acknowledgements
This document has evolved from RFCs [1] and [4] from which much of
the material has been adopted. Thanks to their authors T. Bradley,
C. Brown, A. Malis, D. Piscitello, and C. Lawrence. In addition,
the expertise of the ATM working group of the IETF has been
invaluable in completing the document. Special thanks Brian
Carpenter of CERN, Rao Cherukuri of IBM, Dan Grossman of Motorola,
Joel Halpern of Network Systems, Bob Hinden of Sun Mircosystems, and
Gary Kessler of MAN Technology Corporation for their detailed
contributions.
Security Considerations
Security issues are not addressed in this memo.
References
[1] Piscitello, D. and Lawrence, C., "The Transmission of IP
Datagrams over the SMDS Service". RFC 1209, Bell Communications
Research, March 1991.
[2] CCITT, "AAL Type 5, Draft Recommendation text for section 6 of
I.363". CCITT Study Group XVIII/8-5, Report of Rapporteur's
Meeting on AAL type 5, Annex 2, Copenhagen, 19-21 October, 1992.
[3] CCITT, "Draft Recommendation I.555". CCITT Study Group XVIII,
Working Party 2, TD 36, Annex 4, Geneva 8-19 June, 1992.
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[4] Bradley, T., Brown, C., and Malis, A., "Multiprotocol
Interconnect over Frame Relay". RFC 1294, Wellfleet
Communications, Inc. and BBN Communications, January 1992.
[5] CCITT, "Draft text for Q.93B". CCITT Study Group XI, Working
Party XI/6, 23 September - 2 October, 1992.
[6] Information technology - Telecommunications and Information
Exchange Between Systems, "Protocol Identification in the
Network Layer". ISO/IEC TR 9577, October 1990.
[7] Plummer, David C., "An Ethernet Address Resolution Protocol".
RFC 826, Symbolics, Inc., November 1982.
[8] Tsuchiya, Paul, "Discovery and Routing over Large Public Data
Networks". Internet Draft, Bellcore, July 1992.
[9] Lyon, T., Liaw, F., and Romanow, A., "Network Layer Architecture
for ATM Networks". Internet Draft, Sun Microsystems, July 1992.
Appendix A. Multiprotocol Encapsulation over FRCS
I.555 defines a Frame Relaying Specific Convergence Sublayer (FRCS)
to be used on the top of the Common Part of the AAL for Frame
Relay/ATM interworking. The service offered by FRCS corresponds to
the Core service for Frame Relaying as described in I.233.
An FRCS-PDU consists of Q.922 Address field followed by Q.922
Information field. The Q.922 flags and the FCS are omitted, since
the corresponding functions are provided by the AAL. The figure
below shows an FRCS-PDU embedded in the Payload of an AAL5 CPCS-PDU.
FRCS-PDU in Payload of AAL5 CPCS-PDU
+-------------------------------+ -------
| Q.922 Address Field | FRCS-PDU Header
| (2-4 octets) |
+-------------------------------+ -------
| . |
| . |
| Q.922 Information field | FRCS-PDU Payload
| . |
| . |
+-------------------------------+ -------
| AAL5 CPCS-PDU Trailer |
+-------------------------------+
Routed and bridged PDUs are encapsulated inside the FRCS-PDU as
defined in RFC 1294. The Q.922 Information field starts with a Q.922
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Control field followed by an optional Pad octet that is used to align
the remainder of the frame to a convenient boundary for the sender.
The protocol of the carried PDU is then identified by prefixing the
PDU by an ISO/CCITT Network Layer Protocol ID (NLPID).
In the particular case of an IP PDU, the NLPID is 0xCC and the FRCS-
PDU has the following format:
FRCS-PDU Format for Routed IP PDUs
+-------------------------------+
| Q.922 Addr Field |
| (2 or 4 octets) |
+-------------------------------+
| 0x03 (Q.922 Control) |
+-------------------------------+
| NLPID 0xCC |
+-------------------------------+
| . |
| IP PDU |
| (up to 2^16 - 5 octets) |
| . |
+-------------------------------+
Note that according to RFC 1294 the Q.922 Address field shall be
either 2 or 4 octets, i.e., a 3 octet Address field is not supported.
In the particular case of a CLNP PDU, the NLPID is 0x81 and the
FRCS-PDU has the following format:
FRCS-PDU Format for Routed CLNP PDUs
+-------------------------------+
| Q.922 Addr Field |
| (2 or 4 octets) |
+-------------------------------+
| 0x03 (Q.922 Control) |
+-------------------------------+
| NLPID 0x81 |
+-------------------------------+
| . |
| Rest of CLNP PDU |
| (up to 2^16 - 5 octets) |
| . |
+-------------------------------+
Note that in case of ISO protocols the NLPID field forms the first
octet of the PDU itself and shall thus not be repeated.
The above encapsulation applies only to those routed protocols that
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have a unique NLPID assigned. For other routed protocols (and for
bridged protocols), it is necessary to provide another mechanism for
easy protocol identification. This can be achieved by using an NLPID
value 0x80 to indicate that an IEEE 802.1a SubNetwork Attachment
Point (SNAP) header follows.
See RFC 1294 for more details related to multiprotocol encapsulation
over FRCS.
Appendix B. List of Locally Assigned values of OUI 00-80-C2
with preserved FCS w/o preserved FCS Media
------------------ ----------------- --------------
0x00-01 0x00-07 802.3/Ethernet
0x00-02 0x00-08 802.4
0x00-03 0x00-09 802.5
0x00-04 0x00-0A FDDI
0x00-05 0x00-0B 802.6
0x00-0D Fragments
0x00-0E BPDUs
Appendix C. Partial List of NLPIDs
0x00 Null Network Layer or Inactive Set (not used with ATM)
0x80 SNAP
0x81 ISO CLNP
0x82 ISO ESIS
0x83 ISO ISIS
0xCC Internet IP
Author's Address
Juha Heinanen Telecom Finland, PO Box 228, SF-33101 Tampere, Finland
Phone: +358 49 500 958
Email: Juha.Heinanen@datanet.tele.fi
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