One document matched: draft-ietf-atm-multipro-03.txt
Differences from draft-ietf-atm-multipro-02.txt
Network Working Group Juha Heinanen
Reguest for Comments: DRAFT Telecom Finland
Expires March 16, 1993 October 16, 1992
Multiprotocol Interconnect over ATM Adaptation Layer 5
Status of this Memo
This document is an Internet Draft. 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. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a
``working draft'' or ``work in progress.'' Please check the 1id-
abstracts.txt listing contained in the internet-drafts Shadow
Directories on nic.ddn.mil, nnsc.nsf.net, nic.nordu.net,
ftp.nisc.sri.com, or munnari.oz.au to learn the current status of any
Internet Draft.
Abstract
The purpose of this memo is to define approaches for multi-protocol
operation over ATM AAL5. After field experience is obtained, some of
the approaches may or may not be progressed to Internet Standards.
Distribution of this memo is unlimited.
1. Introduction
Asynchronous Transfer Mode (ATM) based networks are of increasing
interest for both local and wide area applications. This memo
describes three different methods for carrying connectionless network
interconnect traffic (routed and bridged PDUs) over an ATM network.
The first method approaches ATM from the LAN perspective and does
higher-layer protocol multiplexing by prefixing the carried PDU by an
IEEE 802.2 Logical Link Control (LLC) header. It is in the following
called "LLC Encapsulation". The second method is functionally
equivalent to the first, but approaches ATM from the WAN perspective
by prefixing the carried PDU by an ISO/CCITT Network Layer Protocol
ID (NLPID). It is in the following called "NLPID Encapsulation".
The third method does higher-layer protocol multiplexing implicitly
by ATM Virtual Circuits (VCs) and 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, but proposes
that they are carried in the Payload field of Common Part Convergence
Sublayer (CPCS) PDU of AAL5 [1]. AAL5 is a new simple and efficient
ATM Adaptation Layer currently being standardized both in ANSI and
CCITT.
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. Encapsulation based approach to protocol multiplexing (LLC
Encapsulation or NLPID 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.
As already mentioned in the introduction, the two proposed
encapsulation based multiplexing methods are functionally equivalent.
A subset of LLC Encapsulation has been earlier defined for SMDS [2],
whereas NLPID Encapsulation is the same as that used for Frame Relay
[3]. The later also allows interworking of ATM and Frame Relay
stations according to I.555 [4]. Selection between the two
encapsulation based multiplexing methods for ATM may thus depend on
compatibility requirements with other network technologies. This
memo doesn't prefer one or the other, but leaves the choice to the
market. Later, when practical experience has been obtained and the
market response is known, a new memo may be issued that states the
Internet policy regarding the two encapsulation methods.
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.
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The format of the AAL5 CPCS-PDU is given below:
AAL5 CPCS-PDU Format
+-------------------------------+
| CPCS-PDU Header |
+-------------------------------+
| . |
| . |
| Payload |
| (up to 2^16 - 1 octets) |
| . |
| . |
+-------------------------------+
| PAD ( 0 - 47 octets) |
+-------------------------------+\
| Reserved (2 octets) | |
+-------------------------------+ |
| Length (2 octets) | > CPCS-PDU Trailer
+-------------------------------| |
| CRC (4 octets) | |
+-------------------------------+/
The need and the functions of the CPCS-PDU Header are currently for
further study in CCITT. It is assumed in this memo that if the
Header is included, its length will be a multiple of four 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
Encapsulation Based Multiplexing is needed when several protocols are
carried over the same VC. In order to allow the receiver to properly
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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 covers protocols that operate over LLC Type 1
(unacknowledged connectionless mode) service, the same encapsulation
principle applies also to protocols operating over LLC 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 based 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 the appendix) 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 the appendix.
An NLPID value of 0x00 is defined in ISO/IEC TR 9577 as the Null Network
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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 shall, however, not be used. Instead, IP is encapsulated
like all other routed non-ISO protocols by identifying it in the SNAP
header that 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:
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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) |
| . |
+-------------------------------+
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 the
appendix.
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 of the bridged PDU
at a four octet boundary.
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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) |
+-------------------------------+
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) |
+-------------------------------+
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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).
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) |
+-------------------------------+
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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.
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.
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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) |
| |
+-------------------------------+
| LAN FCS |
+-------------------------------+
5. NLPID Encapsulation
In NLPID Encapsulation, the protocol of the routed or bridged PDU is
identified by prefixing the PDU by a one octet NLPID, which is possibly
followed by a SNAP header. For compatibility with RFC 1294 and I.555,
the AAL5 CPCS-PDU Payload field always starts with three extra octets
corresponding to the Address and Control fields of Q.922. In some
cases, additional padding is needed before the NLPID octet in order to
align the routed PDU or the payload of the bridged PDU at a four octet
boundary.
Although this memo only describes how NLPID encapsulation is used to
carry connectionless PDUs over an ATM network, the same encapsulation
principle also applies to connection-oriented protocols, such as X.25
and Q.931, for which appropriate NLPID values have been allocated.
5.1. NLPID Encapsulation for Routed Protocols
In NLPID encapsulation the protocol of a routed PDU is identified by
Network Level Protocol ID, which is a one octet field administered by
ISO and CCITT. Values for this field are defined in ISO/IEC TR 9577
[6]. A NLPID value of 0x00 is defined within 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. Some of the currently defined
NLPID values are listed in the appendix.
As it can be seen from the appendix, a unique NLPID value has been
assigned for some routed protocols, such as IP and CLNP. In such cases
the format of the AAL5 CPCS-PDU Payload field shall be as follows:
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RFC DRAFT Multiprotocol Interconnect over ATM AAL5 October 1992
Payload Format for Routed NLPID PDUs
+-------------------------------+
| XXXXXXX0 (Q.922 Addr MSO) |
+-------------------------------+
| XXXXXXX1 (Q.922 Addr LSO) |
+-------------------------------+
| 0x03 (Q.922 Control) |
+-------------------------------+
| NLPID (1 octet) |
+-------------------------------+
| . |
| Carried PDU |
| (up to 2^16 - 5 octets) |
| . |
+-------------------------------+
The value of the first two octets is constrained so that the they
form a valid two octet Q.922 Address field. This means that the last
bit of the first octet must be set to 0 and the last bit of the
second octet must be set to 1. All other bits (marked with letter X)
can be set to any value. The third octet is the Q.922 Control field
and shall according to RFC 1294 have the value 0x03. Note that these
three octets together with the NLPID octet align the carried PDU at a
four octet boundary.
In the particular case of an Internet IP PDU, the NLPID is 0xCC:
Payload Format for Routed IP PDUs
+-------------------------------+
| XXXXXXX0 (Q.922 Addr MSO) |
+-------------------------------+
| XXXXXXX1 (Q.922 Addr LSO) |
+-------------------------------+
| 0x03 (Q.922 Control) |
+-------------------------------+
| NLPID 0xCC |
+-------------------------------+
| . |
| IP PDU |
| (up to 2^16 - 5 octets) |
| . |
+-------------------------------+
In the particular case of a CLNP PDU, the NLPID is 0x81:
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Payload Format for Routed CLNP PDUs
+-------------------------------+
| XXXXXXX0 (Q.922 Addr MSO) |
+-------------------------------+
| XXXXXXX1 (Q.922 Addr LSO) |
+-------------------------------+
| 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 a SNAP header follows.
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 two-octet
Protocol Identifier (PID) which follows. Together they identify a
distinct protocol. Note that OUI 0x00-00-00 specifies that the
following PID is an EtherType.
The format of the AAL5 CPCS-PDU Payload field for routed non-NLPID
PDUs shall thus be as follows:
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Payload Format for Routed non-NLPID PDUs
+-------------------------------+
| XXXXXXX0 (Q.922 Addr MSO) |
+-------------------------------+
| XXXXXXX1 (Q.922 Addr LSO) |
+-------------------------------+
| 0x03 (Q.922 Control) |
+-------------------------------+
| PAD 0x00-00-00 |
+-------------------------------+
| NLPID 0x80 |
+-------------------------------+
| OUI 0x00-00-00 |
+-------------------------------+
| EtherType (2 octets) |
+-------------------------------+
| . |
| Carried PDU |
| (up to 2^16 - 9 octets) |
| . |
+-------------------------------+
The three octet PAD field is used to align the carried PDU at a four
octet boundary.
According to the above described encapsulation scheme, PDUs of those
protocols, such as IP and CLNP, that have a unique NLPID assigned to
them, could be encapsulated in two different ways. In order to
eliminate the conflict, the SNAP variation shall only be used if no
NLPID value is defined for the given protocol. This results in a
smaller frame and less processing overhead at the receiver.
5.2. NLPID Encapsulation for Bridged Protocols
In NLPID Encapsulation bridged PDUs are encapsulated by identifying the
type of the bridged media in the SNAP header. As with routed non-NLPID
protocols, the presence of the SNAP header is indicated by the NLPID
value 0x80. 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 FCS is preserved within the bridged PDU.
The media type (PID) values that can be used in ATM encapsulation are
listed in the Appendix.
The AAL5 CPCS-PDU Payload field carrying a bridged PDU shall, therefore,
have one of the following formats. Padding is added before the NLPID
field if necessary in order to align the user information of the bridged
PDU at a four octet boundary.
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Payload Format for Bridged Ethernet/802.3 PDUs
+-------------------------------+
| XXXXXXX0 (Q.922 Addr MSO) |
+-------------------------------+
| XXXXXXX1 (Q.922 Addr LSO) |
+-------------------------------+
| 0x03 (Q.922 Control) |
+-------------------------------+
| PAD 0x00 |
+-------------------------------+
| NLPID 0x80 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-01 or 0x00-07 |
+-------------------------------+
| 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
+-------------------------------+
| XXXXXXX0 (Q.922 Addr MSO) |
+-------------------------------+
| XXXXXXX1 (Q.922 Addr LSO) |
+-------------------------------+
| 0x03 (Q.922 Control) |
+-------------------------------+
| PAD 0x00 |
+-------------------------------+
| NLPID 0x80 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-02 or 0x00-08 |
+-------------------------------+
| PAD 0x00 |
+-------------------------------+
| Frame Control (1 octet) |
+-------------------------------+
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| LAN FCS (if PID is 0x00-02) |
+-------------------------------+
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Payload Format for Bridged 802.5 PDUs
+-------------------------------+
| XXXXXXX0 (Q.922 Addr MSO) |
+-------------------------------+
| XXXXXXX1 (Q.922 Addr LSO) |
+-------------------------------+
| 0x03 (Q.922 Control) |
+-------------------------------+
| PAD 0x00 |
+-------------------------------+
| NLPID 0x80 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-03 or 0x00-09 |
+-------------------------------+
| PAD 0xXX |
+-------------------------------+
| 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 a
PAD field, which can be set to any value (XX).
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Payload Format for Bridged FDDI PDUs
+-------------------------------+
| XXXXXXX0 (Q.922 Addr MSO) |
+-------------------------------+
| XXXXXXX1 (Q.922 Addr LSO) |
+-------------------------------+
| 0x03 (Q.922 Control) |
+-------------------------------+
| PAD 0x00 |
+-------------------------------+
| NLPID 0x80 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-04 or 0x00-0A |
+-------------------------------+
| PAD 0x00 |
+-------------------------------+
| Frame Control (1 octet) |
+-------------------------------+
| MAC destination address |
+-------------------------------+
| |
| (remainder of MAC frame) |
| |
+-------------------------------+
| LAN FCS (if PID is 0x00-04) |
+-------------------------------+
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Payload Format for Bridged 802.6 PDUs
+-------------------------------+
| XXXXXXX0 (Q.922 Addr MSO) |
+-------------------------------+
| XXXXXXX1 (Q.922 Addr LSO) |
+-------------------------------+
| 0x03 (Q.922 Control) |
+-------------------------------+
| PAD 0x00-00-00 |
+-------------------------------+
| NLPID 0x80 |
+-------------------------------+
| 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.
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.
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One should 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
+-------------------------------+
| XXXXXXX0 (Q.922 Addr MSO) |
+-------------------------------+
| XXXXXXX1 (Q.922 Addr LSO) |
+-------------------------------+
| 0x03 (Q.922 Control) |
+-------------------------------+
| NLPID 0x80 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-0E |
+-------------------------------+
| |
| BPDU as defined by |
| 802.1(d) or 802.1(g) |
| |
+-------------------------------+
| LAN FCS |
+-------------------------------+
5.3. Fragmentation Issues
In the event of I.555 interworking with Frame Relay, a fragmented frame
may (rarely) be encountered. Frame fragments have an OUI of 0x00-80-C2
and a PID of 0x00-0D. For optional fragmentation procedures, see
section 9 of RFC 1294. In general, fragmentation is not recommended.
6. 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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6.1. 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) |
| . |
| . |
+-------------------------------+
6.2. 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) |
| |
+-------------------------------+
| LAN FCS |
+-------------------------------+
In case of Ethernet, 802.3, 802.4, 802.5, and FDDI PDUs the presense
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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.
7. 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.
8. 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
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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
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 3. 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.
9. 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. Also, this memo didn't try to specify which one (if
any) of the two encapsulation based multiplexing methods should be
preferred. Practical experience with real ATM networks is needed
before these issues can be fully resolved.
Acknowledgements
This document has evolved from RFCs [2] and [3] 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] CCITT, "Report of WPXVIII". CCITT Study Group XVIII, Working
Party 8, TD 62, Annex 5, Section C, Geneva 8-19 June, 1992.
[2] Piscitello, D. and Lawrence, C., "The Transmission of IP
Datagrams over the SMDS Service". RFC 1209, Bell Communications
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Research, March 1991.
[3] Bradley, T., Brown, C., and Malis, A., "Multiprotocol
Interconnect over Frame Relay". RFC 1294, Wellfleet
Communications, Inc. and BBN Communications, January 1992.
[4] CCITT, "Draft Recommendation I.555". CCITT Study Group XVIII,
Working Party 2, TD 36, Annex 4, Geneva 8-19 June, 1992.
[5] CCITT, "Draft text for Q.93B". CCITT Study Group XI, Working
Party XI/6-37, March 9-20, 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
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
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
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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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