📄 rfc1483.txt
字号:
| 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).
Heinanen [Page 11]
RFC 1483 Multiprotocol over AAL5 July 1993
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
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. 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.
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
Heinanen [Page 12]
RFC 1483 Multiprotocol over AAL5 July 1993
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.
7. 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 RFCs.
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, "Draft Recommendation I.363". CCITT Study Group XVIII,
Geneva, 19 - 29 January, 1993.
[3] CCITT, "Draft Recommendation I.36x.1". CCITT Study Group XVIII,
Geneva, 19-29 January, 1993.
[4] Bradley, T., Brown, C., and Malis, A., "Multiprotocol
Interconnect over Frame Relay". RFC 1294, Wellfleet
Communications, Inc. and BBN Communications, January 1992.
Heinanen [Page 13]
RFC 1483 Multiprotocol over AAL5 July 1993
[5] CCITT, "Draft text for Q.93B". CCITT Study Group XI, 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] Postel, J. and Reynolds, J., "A Standard for the Transmission of
IP Datagrams over IEEE 802 Networks". RFC 1042, ISI, February,
1988.
Appendix A. Multiprotocol Encapsulation over FR-SSCS
I.36x.1 defines a Frame Relaying Specific Convergence Sublayer (FR-
SSCS) to be used on the top of the Common Part Convergence Sublayer
CPCS) of the AAL type 5 for Frame Relay/ATM interworking. The
service offered by FR-SSCS corresponds to the Core service for Frame
Relaying as described in I.233.
An FR-SSCS-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 FR-SSCS-PDU embedded in the Payload of an AAL5 CPCS-
PDU.
FR-SSCS-PDU in Payload of AAL5 CPCS-PDU
+-------------------------------+ -------
| Q.922 Address Field | FR-SSCS-PDU Header
| (2-4 octets) |
+-------------------------------+ -------
| . |
| . |
| Q.922 Information field | FR-SSCS-PDU Payload
| . |
| . |
+-------------------------------+ -------
| AAL5 CPCS-PDU Trailer |
+-------------------------------+
Routed and bridged PDUs are encapsulated inside the FR-SSCS-PDU as
defined in RFC 1294. The Q.922 Information field starts with a Q.922
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 FR-
SSCS-PDU has the following format:
Heinanen [Page 14]
RFC 1483 Multiprotocol over AAL5 July 1993
FR-SSCS-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 FR-
SSCS-PDU has the following format:
FR-SSCS-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
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.
Heinanen [Page 15]
RFC 1483 Multiprotocol over AAL5 July 1993
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
Heinanen [Page 16]
⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -