ATM Signalling Support for IP over ATM - UNI Signalling 4.0 Update
Extracted elements (28)
For IP router interconnection with ABR, a non-zero Minimum Cell Rate provides a bandwidth commitment analogous to a provisioned leased-line capacity, which is distinct from per-flow IIS reservations and helps prevent starvation of the aggregate best-effort traffic.
Frame discard is based on 'Partial and Early Packet Discard' strategy [ROM94] and applies to all ATM services except loss-less CBR; it is recommended for all IP SVCs because network entities that do not support it must still forward the signaling, enabling graceful degradation.
LIJ (Leaf Initiated Join), ATM Anycast, and Switched Virtual Path capabilities from SIG 4.0 are intentionally not discussed in this memo because models for their use in IP over ATM environments had not yet been defined at time of publication.
Traffic parameters cannot be renegotiated after a call is active in SIG 4.0; negotiation is only possible during call establishment using the Minimum Acceptable or Alternative ATM Traffic Descriptor IEs, which is why IP entities should proactively negotiate PCR to link rate with a minimum of zero.
UBR is identified as the most natural ATM service category for best-effort IP because non-IIS IP has no notion of the performance requirements of the higher layers it supports; it therefore cannot provide a flexible, application-specific traffic signaling interface.
In SIG 4.0, a SETUP or CONNECT message may include ONLY one of the Minimum Acceptable ATM Traffic Descriptor IE or the Alternative ATM Traffic Descriptor IE; including both in the same message is not permitted.
Several traffic parameter combination entries in Appendix C are included for backward compatibility with UNI 3.1 and ITU-T Recommendations (Notes 4, 5, 10); with these conformance definitions the CLR commitment applies only to the CLP=0 traffic stream.
The QoS Parameter IE may be omitted from a SETUP message if and only if the Extended QoS Parameters IE is included; certain ATM service categories also permit use of the Extended QoS Parameters IE and End-to-End Transit Delay IE in place of the generic QoS Parameter IE.
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.
A system implementing a Public ATM UNI interface serving as the called party of a VCC MUST NOT use an inactivity timer on such a VCC by default, to support layer 3 signalling protocols like RSVP where VCs correspond to reserved flows.
Although AAL Parameters and Broadband Low Layer Information IEs are optional in SIG 4.0 SETUP messages generally, in support of IP over ATM these two IEs MUST be included.
For VBR-nrt IP over ATM connections, an MBS value of N*(Maximum MTU) is RECOMMENDED where N>=1 with a default of 2 and Maximum MTU equals 192 cells (consistent with an IP MTU of 9180 bytes per RFC 1626).
Frame discard signaling MUST be supported by all IP over ATM entities, and it is RECOMMENDED that frame discard be signaled for all IP SVCs because it has been proven to increase throughput under network congestion.
If a site's configuration allows use of UBR, users SHOULD signal for it using the IEs and parameters pertaining to the UBR ATM Transfer Capability, as UBR is the most natural ATM service category for best-effort IP.
IP over ATM entities SHOULD take advantage of traffic parameter negotiation whenever possible by specifying PCR equal to the link rate in the ATM Traffic Descriptor IE of the SETUP message and a minimum of zero PCR in the Minimum Acceptable ATM Traffic Descriptor IE.
The Unspecified QoS Class (QoS Class 0) MUST be requested for ABR connections; QoS Class 0 corresponds to a network-specific objective for the cell loss ratio.
When the network does not enforce PCR and traffic shaping is not a concern, the user SHOULD specify 'no tagging' (UBR.1) in the SETUP message so that cells are dropped during congestion rather than randomly marked as low priority.
When the network enforces PCR via the UPC function and traffic shaping is not enabled at the source, the source SHOULD signal for tagging (UBR.2) so that non-conforming cells are tagged and forwarded rather than dropped.
The AAL Parameters IE carries ATM adaptation layer information for the connection; for IP over ATM it specifies AAL type 5, forward and backward maximum SDU size of 65,535 bytes (desired IP MTU), and a null SSCS type.
The ATM Traffic Descriptor IE defines the traffic characteristics of the VC including PCR, SCR/MBS, Minimum Cell Rate (new in SIG 4.0 for ABR), tagging bits (forward and backward), frame discard bits, and best-effort indication.
The Broadband Bearer Capability IE in SIG 4.0 includes the new 'ATM Transfer Capability' (ATC) field that explicitly specifies the desired ATM service category: CBR, rt-VBR, nrt-VBR, or ABR. Bearer class may be BCOB-A, BCOB-X, BCOB-C, or VP.
The Broadband Low Layer Information (B-LLI) IE is used together with the AAL Parameters IE to select an encapsulation for IP over ATM; it specifies LLC/SNAP encapsulation via layer_2_id=2 and user_information_layer=12 (LAN LLC, ISO 8802/2). B-LLI negotiation procedures follow Appendix D of [PER95].
Two new IEs defined for ABR signaling in SIG 4.0 are: ABR Setup Parameters IE (containing forward/backward ABR Initial Cell Rate, Transient Buffer Exposure, Cumulative RM Fixed Round Trip Time, Rate Increment/Decrement Factors) and ABR Additional Parameters IE (containing a compressed encoding of ABR parameters).
Two new SIG 4.0 IEs enable traffic parameter negotiation during call establishment: the Minimum Acceptable ATM Traffic Descriptor IE (for PCR negotiation) and the Alternative ATM Traffic Descriptor IE (for other parameters). A SETUP or CONNECT message MUST include at most one of these two IEs.
ATM Forum security mechanisms (authentication, data integrity, confidentiality/encryption) were not yet standardized at time of publication, preventing authentication of the originator of signalling messages such as connection setup or teardown requests. IP Security (RFC 1825) can be applied to IP datagrams over ATM VCs to compensate for the lack of ATM-layer security.
The AAL Parameters IE encodes aal_type=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) for IP over ATM connections.
The B-LLI IE for LLC/SNAP encapsulation encodes layer_2_id=2 and user_information_layer=12 (LAN LLC, ISO 8802/2).
The Called Party Number IE encodes type_of_number (international/unknown), addr_plan_ident (ISDN E.164 or ATM Endsystem Address), and addr_number. The calling party learns its own ATM address via the ATM Forum ILMI Specification 4.0.