ATM Signaling Support for IP over ATM
Extracted elements (26)
Best Effort capability is identified as the most convenient model for IP over ATM because IP itself is a best-effort service with no throughput guarantees. When Best Effort is unavailable, requesting a fraction (e.g., 1/10th) of link bandwidth is RECOMMENDED rather than full link rate, to reduce the probability of call rejection.
Even when a B-LLI specifying IP as layer-3 owner (layer_3_id=3, IPI=0xCC) is used, LLC/SNAP encapsulation is still required on all datagrams. This is because while called parties MUST accept this call form, they may choose whether to multiplex through LLC—the decision is local to the receiver's protocol architecture.
The B-LLI IE is the only element in UNI 3.1 for designating protocol endpoints. The framework principle is that all protocols in SETUP IEs MUST be used to encapsulate the call, but the highest-layer protocol specified SHOULD manage the call, creating a hierarchy that supports future higher-level protocol and application bindings.
B-LLI negotiation allows the calling endsystem to include up to three B-LLI IE instances in descending preference order in the SETUP message. The called endsystem MUST select the first supported encapsulation; if none are supported it SHALL clear with cause #88 (incompatible destination); if no B-LLI IE is present in SETUP, the call SHALL be cleared with cause #21.
FR-SSCS (used for Frame Relay interworking) cannot be negotiated as an alternative to LLC encapsulation or VC-multiplexing; RFC 1490 over FR-SSCS is mutually exclusive with null-SSCS encapsulations. This creates interoperability restrictions between FR-SSCS and non-FR-SSCS ATM endsystems even when both carry IP.
UNI 3.1 and UNI 3.0 are incompatible. Key differences: UNI 3.0 lacks the 'ATM Endsystem Address' type (NSAP type=001 must be used in subaddress IEs); cause codes differ (#93 vs #78 for AAL parameter not supported; #51 vs #37 for user cell rate unavailable); and the QoS coding standard field should be '11' in UNI 3.0 rather than '00'.
When a call fails with causes such as #1 (unallocated number), #3 (no route), #17 (user busy), #27 (destination out of order), or #38 (network out of order), the situation SHOULD be treated as Network or Host Unreachable per RFC 1122. Retryable failures such as #37 (user cell rate unavailable), #49 (QoS unavailable), or #73 (unsupported traffic parameter combination) MAY prompt a retry with modified IEs.
A SETUP message establishing an ATM connection for IP and multiprotocol interconnection MUST contain AAL Parameters, ATM Traffic Descriptor, Broadband Bearer Capability, Broadband Low Layer Information, QoS Parameter, Called Party Number, and Calling Party Number IEs. Although AAL Parameters and B-LLI are optional in UNI 3.1, they MUST be included for IP over ATM.
All IP over ATM endsystems MUST support a required subset of traffic management parameter combinations in the SETUP message, comprising: (1) token-bucket SCR/MBS style with BCOB-X/C and PCR+SCR+MBS(CLP=0+1); (2) CBR 'pipe' with BCOB-X, PCR(CLP=0+1), timing required; and (3) Best Effort non-guaranteed bitrate with PCR(CLP=0+1) and best_effort_indication. QoS Class 0 is required for all three.
If a receiver accepts more than one call from a single source, it MUST accept incoming PDUs on all additional connections, and MAY transmit on them. Receivers SHOULD NOT accept an incoming call only to close it or ignore PDUs from that connection.
If an IP router associates IP addresses of multiple LISs with distinct ATM addresses differentiated by the SEL field, its signaling entity MUST carry the ATM addresses corresponding to the particular IP entity in the Calling and Called Party Number IEs of the SETUP message.
LLC/SNAP encapsulation MUST be implemented by all endstations as the default encapsulation per RFC 1577, and MUST be signaled in the B-LLI IE. Only LLC is indicated in the B-LLI; it is the LLC layer's responsibility to inspect the encapsulation header of subsequent packets.
RFC 1483 specifies IP encapsulation over AAL 5; therefore AAL 5 MUST be indicated in the 'AAL type' field of the AAL Parameters IE. The SSCS type field SHOULD be absent or coded to Null SSCS for multiprotocol interconnect over AAL5.
Systems implementing a Public ATM UNI interface MUST support a configurable inactivity timer to clear idle connections; the timer's range SHOULD span from a few minutes to 'infinite', with a default of 20 minutes RECOMMENDED. The inactivity timer MUST monitor traffic in both directions of the connection.
Systems MUST support multiple simultaneous connections between peer systems. Algorithms to prevent call collision such as those in ISO/IEC 8473 section 8.4.3.5 MUST NOT be implemented, as opening multiple connections is specifically permitted.
When a VCC is shared among multiple higher-layer protocol entities, the ATM endsystem MUST monitor traffic at the lowest multiplexing layer to properly detect idle connections. If a shared VCC is not to be disconnected per RFC 1577, the ATMARP client or server SHALL NOT disconnect the call.
When configured for single-connection-only mode and a colliding incoming call arrives while a call request is pending, the incoming call MUST be rejected. Each system MUST then wait at least a configurable collision retry time in the range 1–10 seconds before retrying, and MUST add a random increment with exponential backoff.
UNI point-to-point signaling messages are: SETUP, CALL PROCEEDING, CONNECT, CONNECT ACKNOWLEDGE (call setup) and RELEASE, RELEASE COMPLETE (call release). The network indicates the newly allocated VPCI/VCI in its first response to SETUP (either CALL PROCEEDING or CONNECT).
Not all security issues relating to IP over ATM are clearly understood at this time, due to the fluid state of ATM specifications and the newness of the technology. Future revisions of this specification will address security capabilities that future ATM signaling standards may offer to IP over ATM signaling.
Call setup: calling endpoint sends SETUP → network responds with CALL PROCEEDING or CONNECT (allocating VPCI/VCI) → destination sends CONNECT to accept or RELEASE COMPLETE to reject. Call release: either endpoint sends RELEASE → network responds RELEASE COMPLETE, frees resources, and sends RELEASE toward the other endpoint which replies RELEASE COMPLETE.
AAL Parameters IE for IP over ATM: aal_type=5 (AAL 5), fwd_max_sdu_size_identifier=140, fwd_max_sdu_size=65535, bkw_max_sdu_size_identifier=129, bkw_max_sdu_size=65535, sscs_type_identifier=132, sscs_type=0 (null SSCS). Forward and Backward Maximum CPCS-SDU Size values may range from 0 to 65535; endstations are encouraged to support up to 64k.
ATM Traffic Descriptor IE for Best Effort service: fwd_peak_cell_rate_0+1_identifier=132, fwd_peak_cell_rate_0+1=(link rate), bkw_peak_cell_rate_0+1_identifier=133, bkw_peak_cell_rate_0+1=(link rate), best_effort_indication=190. No SCR, MBS, or tagging fields are present for Best Effort.
B-LLI IE for LLC/SNAP encapsulation: layer_2_id=2, user_information_layer=12 (lan_llc - ISO 8802/2). An optional IP-ownership variant adds layer_3_id=3 and ISO/IEC TR 9577 IPI=204 (0xCC), but LLC/SNAP encapsulation is still required on all datagrams even when this variant is used.
Broadband Bearer Capability IE with Best Effort: bearer_class=16 (BCOB-X), traffic_type=0 (no indication), timing_reqs=0 (no indication), susceptibility_to_clipping=0, user_plane_configuration=0 (point-to-point). IP over ATM signaling MUST permit both BCOB-C and BCOB-X.
Called Party Number IE fields: type_of_number (international number / unknown), addr_plan_ident (ISDN / ATM Endsystem Address), addr_number (E.164 or ATM Endsystem Address). Calling Party Number additionally carries presentation_indic (presentation allowed) and screening_indic (user provided verified and passed).
QoS Parameter IE: qos_class_fwd=0 (Class 0 - Unspecified), qos_class_bkw=0. Class 0 is the only QoS class all networks must support and the only class allowed with Best Effort service. The coding standard field SHOULD be set to '00' per ITU-T standardization.