Asynchronous Transfer Mode (ATM) over Layer 2 Tunneling Protocol Version 3 (L2TPv3)
updated by
- rfc-5641 — Layer 2 Tunneling Protocol Version 3 (L2TPv3) Extended Circuit Status Values
Extracted elements (30)
An LCCE MAY defer session establishment until an ATM circuit becomes ACTIVE rather than initiating immediately upon provisioning, to delay resource allocation until it is actually needed. The document explicitly notes this as a preferred approach.
Cell concatenation in ATM Cell Relay Mode improves pseudowire bandwidth efficiency by reducing per-cell overhead, but introduces latency and delay variation. The maximum concatenated cells value is negotiated per-direction via the ATM Maximum Concatenated Cells AVP to allow asymmetric limits.
The ATM-Specific Sublayer is REQUIRED for AAL5 SDU Mode (to carry T, G, C, U bits needed for EFCI/CLP/C-R transparency and OAM cell signaling) but is OPTIONAL for ATM Cell Mode (where sequencing support is the only reason to include it).
Fragmentation and reassembly (per [L2TPFRAG]) MUST NOT be used for Cell Relay ATMPWs—concatenating cells then fragmenting the resulting packet is counterproductive. For AAL5-SDU Mode, only one fragmentation method (L2TP-layer or IP-layer) SHOULD be used for a given session.
In AAL5-SDU Mode, the trailing CRC of the CPCS-PDU is stripped at the ingress LCCE and regenerated at egress; errors introduced in the ATMPW payload during transit may not be detected. Similarly, EFCI and CLP values of individual ATM cells within an AAL5 frame may be lost across the ATMPW.
In Cell Relay Mode, the ATM Header Error Checksum (HEC) is not transported across the ATMPW; errors or corruption in the 4-byte ATM cell header introduced during encapsulation or transit may not be detected on an end-to-end basis.
An L2TP Control Connection MUST be established per RFC 3931 Section 3.3 to emulate ATM Pseudowires. The SCCRQ and SCCRP MUST include supported ATM Pseudowire types in the Pseudowire Capabilities List AVP.
An LCCE transmitting concatenated cells on a PW MUST NOT exceed the maximum number of cells specified in the ATM Maximum Concatenated Cells AVP (Attribute Type 86).
ATMPWs carrying CBR and VBR-rt traffic SHOULD be halted when acute congestion is detected, to allow other traffic or network infrastructure to continue. LCCEs SHOULD monitor for congestion and SHOULD terminate the ATM service by tearing down the L2TP session via CDN when severe congestion is detected.
Each LCCE MUST be able to uniquely identify itself in SCCRQ and SCCRP messages via a globally unique value, by default advertised via the structured Router ID AVP; the unstructured Hostname AVP MAY also be used.
If an LCCE signals OAM emulation via the OAM Emulation Required AVP and the other LCCE cannot support it, the associated L2TP session MUST be torn down via CDN with result code 22.
In ATM Port Cell Relay Service, each LCCE MUST discard any idle/unassigned cells received on an ATM port associated with ATMPWs.
In VPC Switching, LCCEs MUST be able to pass F4 segment and end-to-end OAM cells (Fault Management, RM, Performance Management, etc.) transparently, and MUST pass F5 OAM cells transparently across the PW.
The Circuit Status AVP MUST be present in the ICRQ and ICRP messages for ATMPWs, and MAY be present in the SLI message.
The Remote End ID AVP MUST be present in the ICRQ in order for the remote LCCE to associate the session to the ATM circuit. If LCCE B cannot find the circuit, a CDN is returned and LCCE A SHOULD exhibit a periodic retry mechanism with configurable period and maximum retries.
The SLI message MUST be sent whenever there is a status change reportable by any value in the Circuit Status AVP, except for the initial ICRQ, ICRP, and CDN messages.
When an ATM circuit is unprovisioned at either LCCE, the associated L2TP session MUST be torn down via the CDN message defined in RFC 3931 Section 3.4.3.
Four ATM Pseudowire types are defined: 0x0002 (ATM AAL5 SDU VCC transport), 0x0003 (ATM Cell transparent Port Mode), 0x0009 (ATM Cell transport VCC Mode), and 0x000A (ATM Cell transport VPC Mode), signaled in the Pseudowire Type AVP.
In AAL5 SDU Mode, the ingress LCCE reassembles the AAL5 CPCS-SDU (excluding the AAL5 trailer and padding bytes) and transports it over the PW; SAR functions are required at both LCCEs.
OAM cells are always encapsulated using cell mode encapsulation regardless of the encapsulation format used for user data. In AAL5-SDU Mode, F5 OAM cells arriving during AAL5 reassembly are sent immediately over the PW, ahead of the AAL5 SDU payload.
The G (EFCI) and C (CLP) bits in the ATM-Specific Sublayer convey the EFCI and CLP quality bits across the ATMPW. The ingress LCCE SHOULD set each to 1 if the corresponding ATM cell bit is set; the egress LCCE SHOULD propagate them to all outgoing cells.
The T bit in the ATM-Specific Sublayer distinguishes between ATM admin cells (T=1, encapsulated as VCC cell relay) and AAL5 payload (T=0), enabling OAM cell transport within an AAL5 SDU Mode session.
Four ATM Pseudowire Types are registered (0x0002, 0x0003, 0x0009, 0x000A); L2-Specific Sublayer Type 2 (ATM) is registered; AVP Attribute Types 86, 87, 88 are assigned; Result Code 22 added to CDN message; new ATM Alarm Status AVP Values registry and ATM-Specific Sublayer Bits registry created.
ATM over L2TPv3 is subject to the security considerations defined in RFC 3931 with no additional ATM-specific considerations. To address the loss of end-to-end AAL5 CRC integrity and HEC integrity across ATMPWs, the L2TPv3 session can utilize IPsec as specified in RFC 3931 Section 4.1.3.
ATM circuit creation triggers L2TP session establishment (ICRQ → ICRP → ICCN three-way handshake). An LCCE MAY initiate immediately on circuit creation or wait until the circuit is ACTIVE. Circuit unprovisioning triggers session teardown via CDN; status changes trigger SLI messages.
ATM Cell Mode encapsulation consists of an optional ATM-Specific Sublayer, followed by a 4-byte ATM cell header (VPI, VCI, PTI, CLP) and a 48-byte ATM cell payload; HEC is not included. Multiple cells may be concatenated in a single PDU.
The ATM Alarm Status AVP (Attribute Type 88) contains a 2-octet Circuit Status Reason and a 2-octet Alarm Type, for a total length of 10 octets before hiding.
The ATM Maximum Concatenated Cells AVP (Attribute Type 86) has a 16-bit unsigned integer Attribute Value field; total AVP length before hiding is 8 bytes.
The ATM-Specific Sublayer is a 32-bit field with bits: x (reserved), S (sequence), B (fragmentation begin), E (fragmentation end), T (transport type: 1=admin cell, 0=AAL5), G (EFCI), C (CLP), U (command/response), followed by a 24-bit Sequence Number.
The OAM Emulation Required AVP (Attribute Type 87) is a boolean AVP with no Attribute Value field; its presence is TRUE, its absence is FALSE; total length before hiding is 6 bytes.