+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Cell Payload (48-bytes) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
"
"
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPI | VCI |PTI |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ATM Cell Payload (48-bytes) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: ATM Cell Mode Encapsulation
In the simplest case, this encapsulation can be used to transmit a
single ATM cell per Pseudowire PDU. However, in order to provide
better Pseudowire bandwidth efficiency, several ATM cells may be
optionally encapsulated into a single Pseudowire PDU.
The maximum number of concatenated cells in a packet is limited by
the MTU size of the session and also by the ability of the egress
LCCE to process them. For more details about ATM Maximum
Concatenated Cells, please refer to Section 6.
5.2.1. ATM VCC Cell Relay Service
A VCC cell relay service may be provided by mapping an ATM Virtual
Channel Connection to a single Pseudowire using cell mode
encapsulation as defined in Section 5.2.
An LCCE may map one or more VCCs to a single PW. However, a service
provider may wish to provision a single VCC to a PW in order to
satisfy QOS or restoration requirements.
The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931],
Attribute Type 68, MUST be present in the ICRQ messages and MUST
include the ATM cell transport VCC Mode PW Type of 0x0009.
5.2.2. ATM VPC Cell Relay Service
A Virtual Path Connection cell relay service may be provided by
mapping an ATM Virtual Path Connection to a single Pseudowire using
cell mode encapsulation as defined in Section 5.2.
An LCCE may map one or more VPCs to a single Pseudowire.
The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931],
Attribute Type 68, MUST be present in the ICRQ messages and MUST
include the ATM cell transport VPC Mode PW Type of 0x000A.
5.2.3. ATM Port Cell Relay Service
ATM port cell relay service allows an ATM port to be connected to
another ATM port. All ATM cells that are received at the ingress ATM
port on the LCCE are encapsulated as per Section 5.2, into Pseudowire
PDU and sent to peer LCCE.
Each LCCE MUST discard any idle/unassigned cells received on an ATM
port associated with ATMPWs.
The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931],
Attribute Type 68, MUST be present in the ICRQ messages and MUST
include the ATM Cell transport Port Mode PW Type of 0x0003.
5.3. OAM Cell Support
The OAM cells are defined in [I610-1], [I610-2], [I610-3] and
[ATMSEC] can be categorized as follows:
a. Fault Management
b. Performance monitoring and reporting
c. Activation/deactivation
d. System Management (e.g., security OAM cells)
OAM Cells are always encapsulated using cell mode encapsulation,
regardless of the encapsulation format used for user data.
5.3.1. VCC Switching
The LCCEs SHOULD be able to pass the F5 segment and end-to-end Fault
Management, Resource Management (RM cells), Performance Management,
Activation/deactivation, and System Management OAM cells.
F4 OAM cells are inserted or extracted at the VP link termination.
These OAM cells are not seen at the VC link termination and are
therefore not sent across the PW.
5.3.2. VPC Switching
The LCCEs MUST be able to pass the F4 segment and end-to-end Fault
Management, Resource Management (RM cells), Performance Management,
Activation/deactivation, and System Management OAM cells
transparently according to [I610-1].
F5 OAM cells are not inserted or extracted at the VP cross-connect.
The LCCEs MUST be able to pass the F5 OAM cells transparently across
the PW.
6. ATM Maximum Concatenated Cells AVP
The "ATM Maximum Concatenated Cells AVP", Attribute Type 86,
indicates that the egress LCCE node can process a single PDU with
concatenated cells up to a specified number of cells. An LCCE node
transmitting concatenated cells on this PW MUST NOT exceed the
maximum number of cells as specified in this AVP. This AVP is
applicable only to ATM Cell Relay PW Types (VCC, VPC, Port Cell
Relay). This Attribute value may not be same in both directions of
the specific PW.
The Attribute Value field for this AVP has the following format:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ATM Maximum Concatenated Cells|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this
AVP MAY be set to 0, but MAY vary (see Section 5.2 of [RFC3931]).
The length (before hiding) of this AVP is 8.
This AVP is sent in an ICRQ, ICRP during session negotiation or via
SLI control messages when LCCE changes the maximum number of
concatenated cells configuration for a given ATM cell relay circuit.
This AVP is OPTIONAL. If the egress LCCE is configured with a
maximum number of cells to be concatenated by the ingress LCCE, it
SHOULD signal this value to the ingress LCCE.
7. OAM Emulation Required AVP
An "OAM Emulation Required AVP", Attribute Type 87, MAY be needed to
signal OAM emulation in AAL5 SDU Mode, if an LCCE cannot support the
transport of OAM cells across L2TP sessions. If OAM cell emulation
is configured or detected via some other means on one side, the other
LCCE MUST support OAM cell emulation as well.
This AVP is exchanged during session negotiation (in ICRQ and ICRP)
or during the life of the session via SLI control messages. If the
other LCCE cannot support the OAM cell emulation, the associated L2TP
session MUST be torn down via CDN message with result code 22.
OAM Emulation AVP is a boolean AVP, having no Attribute Value. Its
absence is FALSE and its presence is TRUE. This AVP MAY be hidden
(the H bit MAY be 0 or 1). The M bit for this AVP SHOULD be set to
0, but MAY vary (see Section 5.2 of [RFC3931]). The Length (before
hiding) of this AVP is 6.
8. ATM Defects Mapping and Status Notification
ATM OAM alarms or circuit status is indicated via the Circuit Status
AVP as defined in Section 5.4.5 of [RFC3931]. For reference, usage
of this AVP is shown below.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |N|A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Value is a 16-bit mask with the two least significant bits
defined, and the remaining bits are reserved for future use.
Reserved bits MUST be set to 0 when sending and ignored upon receipt.
The A (Active) bit indicates whether the ATM circuit is ACTIVE (1) or
INACTIVE (0).
The N (New) bit indicates whether the ATM circuit status indication
is for a new ATM circuit (1) or an existing ATM circuit (0).
8.1. ATM Alarm Status AVP
An "ATM Alarm Status AVP", Attribute Type 88, indicates the reason
for the ATM circuit status and specific alarm type, if any, to its
peer LCCE node. This OPTIONAL AVP MAY be present in the SLI message
with the Circuit Status AVP.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Circuit Status Reason | Alarm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Circuit Status Reason is a 2-octet unsigned integer, and the
Alarm Type is also a 2-octet unsigned integer.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this
AVP SHOULD be set to 0, but MAY vary (see Section 5.2 of [RFC3931]).
The Length (before hiding) of this AVP is 10 octets.
This AVP is sent in the SLI message to indicate additional
information about the ATM circuit status.
Circuit Status Reason values for the SLI message are as follows:
0 - Reserved
1 - No alarm or alarm cleared (default for Active Status)
2 - Unspecified or unknown Alarm Received (default for
Inactive Status)
3 - ATM Circuit received F1 Alarm on ingress LCCE
4 - ATM Circuit received F2 Alarm on ingress LCCE
5 - ATM Circuit received F3 Alarm on ingress LCCE
6 - ATM Circuit received F4 Alarm on ingress LCCE
7 - ATM Circuit received F5 Alarm on ingress LCCE
8 - ATM Circuit down due to ATM Port shutdown on Peer LCCE
9 - ATM Circuit down due to loop-back timeout on ingress LCCE
The general ATM Alarm failures are encoded as below:
0 - Reserved
1 - No Alarm type specified (default)
2 - Alarm Indication Signal (AIS)
3 - Remote Defect Indicator (RDI)
4 - Loss of Signal (LOS)
5 - Loss of Pointer (LOP)
6 - Loss of Framer (LOF)
7 - Loopback cells (LB)
8 - Continuity Check (CC)
9. Applicability Statement
The ATM Pseudowire emulation described in this document allows for
carrying various ATM services across an IP packet switched network
(PSN). These ATM services can be PVC-based, PVP-based, or port-
based. In all cases, ATMPWs operate in a point-to-point deployment
model.
ATMPWs support two modes of encapsulation: ATM AAL5-SDU Mode and ATM
Cell Relay Mode. The following sections list their respective
characteristics in relationship to the native service.
9.1. ATM AAL5-SDU Mode
ATMPWs operating in AAL5-SDU Mode only support the transport of PVC-
based services. In this mode, the AAL5 CPCS-PDU from a single VCC is
reassembled at the ingress LCCE, and the AAL5 CPCS-SDU (i.e., the
AAL5 CPCS-PDU without CPCS-PDU Trailer or PAD octets, also referred
to as AAL5 CPCS-PDU Payload) is transported over the Pseudowire.
Therefore, Segmentation and Reassembly (SAR) functions are required
at the LCCEs. There is a one-to-one mapping between an ATM PVC and
an ATMPW operating in AAL5-SDU Mode, supporting bidirectional
transport of variable length frames. With the exception of
optionally transporting OAM cells, only ATM Adaptation Layer (AAL)
Type 5 frames are carried in this mode, including multiprotocol over
AAL5 packets [RFC2684].
The following considerations stem from ATM AAL5-SDU Mode Pseudowires
not transporting the ATM cell headers and AAL5 CPCS-PDU Trailer (see
Section 5.1):
o An ATMPW operating in AAL5-SDU Mode conveys EFCI and CLP
information using the G and C bits in the ATM-Specific Sublayer.
In consequence, the EFCI and CLP values of individual ATM cells
that constitute the AAL5 frame may be lost across the ATMPW, and
CLP and EFCI transparency may not be maintained. The AAL5-SDU
Mode does not preserve EFCI and CLP values for every ATM cell
within the AAL5 PDU. The processing of these bits on ingress
and egress is defined in Section 4.1.
o Only the least significant bit (LSB) from the CPCS-UU (User-to-
User indication) field in the CPCS-PDU Trailer is transported
using the ATM-Specific Sublayer (see Section 4.1). This bit
contains the Frame Relay C/R bit when FRF.8.1 Frame Relay / ATM
PVC Service Interworking [FRF8.1] is used. The CPCS-UU field is
not used in multiprotocol over AAL5 [RFC2684]. However,
applications that transfer user to user information using the
CPCS-UU octet would fail to operate.
o The CPI (Common Part Indicator) field in the CPCS-PDU Trailer is
also not transported across the ATMPW. This does not affect
multiprotocol over AAL5 applications since the field is used for
alignment and MUST be coded as 0x00 [RFC2684].
o The trailing CRC field in the CPCS-PDU is stripped at the
ingress LCCE and not transported over the ATMPW operating in
AAL5-SDU Mode. It is in turn regenerated at the egress LCCE.
Since the CRC has end-to-end significance, this means that
errors introduced in the ATMPW payload during encapsulation or
transit across the packet switched network may not be detected.
To allow for payload integrity checking transparency on ATMPWs
operating in AAL5-SDU Mode using L2TP over IP or L2TP over
UDP/IP, the L2TPv3 session can utilize IPsec as specified in
Section 4.1.3 of [RFC3931].
Some additional characteristics of the AAL5-SDU Mode are the
following:
o The status of the ATM PVC is signaled between LCCEs using the
Circuit Status AVP. More granular cause values for the ATM
circuit status and specific ATM alarm types are signaled using
the ATM Alarm Status AVP (see Section 8.1). Additionally, loss
of connectivity between LCCEs can be detected by the L2TPv3
keepalive mechanism (see Section 4.4 in [RFC3931]).
o F5 OAM cells’ relative order with respect to user data cells may
not be maintained. F5 OAM cells that arrive during the
reassembly of an AAL5 SDU are sent immediately over the PW and
before the AAL5 SDU payload. At egress, these OAM cells are
sent before the cells that comprise the AAL5-SDU. Therefore,
applications that rely on cell sequence integrity between OAM
and user data cells may not work. This includes Performance
Monitoring and Security OAM cells (see Section 5.1). In
addition, the AAL5-SDU service allows for OAM emulation in which
OAM cells are not transported over the ATMPW (see Section 7).
This is advantageous for AAL5-SDU Mode ATMPW implementations
that do not support cell transport using the T-bit.
o Fragmentation and Reassembly procedures MAY be used for managing
mismatched MTUs, as specified in Section 5 of [L2TPFRAG] or in
the underlying PSN (IP, etc.) between tunnel endpoints as
discussed in Section 4.1.4 of [RFC3931]. Only one of these
methods SHOULD be used for a given AAL5-SDU Mode ATMPW. The
procedures described in [L2TPFRAG] can be used to support the
maximum size of an AAL5 SDU, 2 ^ 16 - 1 (65535) octets.
However, relying on fragmentation on the L2TP/IPv4 packet
between tunnel endpoints limits the maximum size of the AAL5 SDU
that can be transported, because the maximum total length of an
IPv4 datagram is already 65535 octets. In this case, the
maximum AAL5 SDU that can be transported is limited to 65535
minus the encapsulating headers, 24-36 octets for L2TP-over-IPv4
or 36-48 octets for L2TP-over-UDP/IPv4. When the AAL5 payload
is IPv4, an additional option is to fragment IP packets before
tunnel encapsulation with L2TP/IP (see Section 4.1.4 of
[RFC3931]).
o Sequencing may be enabled on the ATMPW using the ATM-Specific
Sublayer Sequence Number field, to detect lost, duplicate, or
out-of-order frames on a per-session basis (see Section 4.2).
o Quality of Service characteristics such as throughput (cell
rates), burst sizes and delay variation can be provided by
leveraging Quality of Service features of the LCCEs and the
underlying PSN, increasing the faithfulness of ATMPWs. This
includes mapping ATM service categories to a compatible PSN
class of service.
9.2. ATM Cell Relay Mode
In this mode, no reassembly takes place at the ingress LCCE. There
are no SAR requirements for LCCEs. Instead, ATM-layer cells are
transported over the ATMPW. Consequently, all AAL types can be
transported over ATMPWs operating in Cell Relay Mode. ATM Cell Relay
Pseudowires can operate in three different modes (see Section 5.2):
ATM VCC, ATM VPC, and ATM Port Cell Relay Services. The following
are some of their characteristics:
o The ATM cells transported over Cell Relay Mode ATMPWs consist of
a 4-byte ATM cell header and a 48-byte ATM cell-payload (see
Section 5.2). The ATM Service Payload of a Cell Relay Mode
ATMPW is a multiple of 52 bytes. The Header Error Checksum
(HEC) in the ATM cell header containing a Cyclic Redundancy
Check (CRC) calculated over the first 4 bytes of the ATM cell
header is not transported. Accordingly, the HEC field may not
accurately reflect errors on an end-to-end basis; errors or
corruption in the 4-byte ATM cell header introduced in the ATMPW
payload during encapsulation or transit across the PSN may not
be detected. To allow for payload integrity checking
transparency on ATMPWs operating in Cell Relay Mode using L2TP
over IP or L2TP over UDP/IP, the L2TPv3 session can utilize
IPsec as specified in Section 4.1.3 of [RFC3931].
o ATM PWs operating in Cell Relay Mode can transport a single ATM
cell or multiple concatenated cells (see Section 6). Cell
concatenation improves the bandwidth efficiency of the ATMPW (by
decreasing the overhead) but introduces latency and delay
variation.
o The status of the ATM PVC is signaled between LCCEs using the
Circuit Status AVP. More granular cause values for the ATM
circuit status and specific ATM alarm types are signaled using
the ATM Alarm Status AVP (see Section 8.1). Additionally, loss
of connectivity between LCCEs can be detected by the L2TPv3
keepalive mechanism (see Section 4.4 in [RFC3931]).
o ATM OAM cells are transported in the same fashion as user cells,
and in the same order as they are received. Therefore,
applications that rely on cell sequence integrity between OAM
and user data cells are not adversely affected. This includes
performance management and security applications that utilize
OAM cells (see Section 5.3).
o The maximum number of concatenated cells is limited by the MTU
size of the session (see Section 5.2 and Section 6). Therefore,
Fragmentation and Reassembly procedures are not used for Cell
Relay ATMPWs. Concatenating cells to then fragment the
resulting packet defeats the purpose of cell concatenation.
Concatenation of cells and fragmentation act as inverse
functions, with additional processing but null net effect, and
should not be used together.
o Sequencing may be enabled on the ATMPW to detect lost,
duplicate, or out-of-order packets on a per-session basis (see
Section 4.2).
o Quality of Service characteristics such as throughput (cell
rates), burst sizes, and delay variation can be provided by
leveraging Quality of Service features of the LCCEs and the
underlying PSN, increasing the faithfulness of ATMPWs. This
includes mapping ATM service categories to a compatible PSN
class of service, and mapping CLP and EFCI bits to PSN classes
of service. For example, mapping a Constant Bit Rate (CBR) PVC
to a class of service with tight loss and delay characteristics,
such as an Expedited Forwarding (EF) Per-Hop Behavior (PHB) if
the PSN is an IP DiffServ-enabled domain. The following
characteristics of ATMPWs operating in Cell Relay Mode include
additional QoS considerations:
- ATM Cell transport VCC Pseudowires allow for mapping
multiple ATM VCCs to a single ATMPW. However, a user may
wish to map a single ATM VCC per ATMPW to satisfy QoS
requirements (see Section 5.2.1).
- Cell Relay ATMPWs allow for concatenating multiple cells in
a single Pseudowire PDU to improve bandwidth efficiency,
but may introduce latency and delay variation.
10. Congestion Control
As explained in [RFC3985], the PSN carrying the PW may be subject to
congestion, with congestion characteristics depending on PSN type,
network architecture, configuration, and loading. During congestion
the PSN may exhibit packet loss and packet delay variation (PDV) that
will impact the timing and data integrity of the ATMPW. During
intervals of acute congestion, some Cell Relay ATMPWs may not be able
to maintain service. The inelastic nature of some ATM services
reduces the risk of congestion because the rates will not expand to
consume all available bandwidth, but on the other hand, those ATM
services cannot arbitrarily reduce their load on the network to
eliminate congestion when it occurs.
Whenever possible, Cell Relay ATMPWs should be run over traffic-
engineered PSNs providing bandwidth allocation and admission control
mechanisms. IntServ-enabled domains providing the Guaranteed Service
(GS) or DiffServ-enabled domains using Expedited Forwarding (EF) are
examples of traffic-engineered PSNs. Such PSNs will minimize loss
and delay while providing some degree of isolation of the Cell Relay
ATMPW’s effects from neighboring streams.
If the PSN is providing a best-effort service, then the following
best-effort service congestion avoidance considerations apply: Those
ATMPWs that carry constant bit rate (CBR) and variable bit rate-real
time (VBR-rt) services across the PSN will most probably not behave