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RFC1483 - Multiprotocol Encapsulation over ATM Adaptation Layer 5

王朝other·作者佚名  2008-05-31
窄屏简体版  字體: |||超大  

Network Working Group Juha Heinanen

Reguest for Comments: 1483 Telecom Finland

July 1993

Multiprotocol Encapsulation over ATM Adaptation Layer 5

Status of this Memo

This RFCspecifies an IAB standards track protocol for the Internet

community, and requests discussion and suggestions for improvements.

Please refer to the current edition of the "IAB Official Protocol

Standards" for the standardization state and status of this protocol.

Distribution of this memo is unlimited.

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 Protocol Data Units (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".

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 ATM Adaptation Layer type 5 (AAL5) [2].

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 Service Specific Convergence Sublayer (FR-SSCS), as defined in

I.36x.1 [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 FR-SSCS-PDU as well as how IP and CLNP PDUs are

encapsulated over FR-SSCS according to RFC1294.

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

private ATM networks. 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:

AAL5 CPCS-PDU Format

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

.

.

CPCS-PDU Payload

up to 2^16 - 1 octets)

.

.

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

PAD ( 0 - 47 octets)

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

CPCS-UU (1 octet )

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

CPI (1 octet )

+-------------------------------+CPCS-PDU Trailer

Length (2 octets)

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

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 CPCS-UU (User-to-User indication) field is used to transparently

transfer CPCS user to user information. The field has no function

under the multiprotocol ATM encapsulation described in this memo and

can be set to any value.

The CPI (Common Part Indicator) field alings the CPCS-PDU trailer to

64 bits. Possible additional functions are for further study in

CCITT. When only the 64 bit alignment function is used, this field

shall be codes as 0x00.

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 0x00 is used for the abort function.

The CRC field protects the entire CPCS-PDU except the CRC field

itself.

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

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 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)

.

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

This is compatible with RFC1042 [7]. Any changes in the header

format specified in RFC1042 should be followed by this memo.

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)

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

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).

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.

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.

An ingress 802.6 bridge can abort an AAL5 CPCS-PDU by setting its

Length field to zero. If the egress bridge has already begun

transmitting segments of the PDU to an 802.6 subnetwork and then

notices that the AAL5 CPCS-PDU has been aborted, it may immediately

generate an EOM cell that causes the 802.6 PDU to be rejected at the

receiving bridge. Such an EOM cell could, for example, contain an

invalid value in the Length field of the Common PDU Trailer.

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

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.

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)

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

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

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

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". RFC1209, 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". RFC1294, Wellfleet

Communications, Inc. and BBN Communications, January 1992.

[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". RFC1042, 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 RFC1294. 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:

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 RFC1294 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 RFC1294 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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