Interoperation of Controlled-Load Service and Guaranteed Service with ATM
Extracted elements (29)
The ATM QoS Class follows the ATM service category rather than the IP service to avoid combinations unlikely to be supported. For example, if only nrtVBR is available for GS, choosing QoS Class 1 (real-time) would likely cause connection failure.
The IWF should anticipate which cells the ATM UPC will tag and preemptively tag all cells of an affected IP packet uniformly. If tagging is left to ATM-layer UPC, the network will drop some cells of a packet while carrying others, causing the receiver to discard the entire partial packet.
The recommended service mappings are GS→CBR or rtVBR, CLS→nrtVBR or ABR (with MCR), and BE→UBR or ABR. All possible combinations are documented because ATM deployment may not make all preferred service categories universally available.
The UNI 3.x PCR CLP=0-only parameter is not recommended for interoperability reasons with TM/UNI 4.0 equipment. TM/UNI 4.0 does not support tagging for traffic in excess of PCR, which restricts CBR VCs to solutions other than tagging for handling excess traffic.
UNI 3.0 and UNI 3.1 are incompatible in actual codepoints but semantically equivalent; rtVBR and ABR service categories are new in TM/UNI 4.0 and unavailable in UNI 3.x. GS with rtVBR is not supported under UNI 3.x because BCOB-C cannot represent real-time behavior and UNI 3.1 Appendix F precludes VBR with timing required.
AAL type 5 encoding SHOULD be used as specified in RFC 1483 and RFC 1755. For AAL-5, specification of the maximum SDU size in both forward and reverse directions is REQUIRED; the TSpec M parameter SHOULD be used as the maximum SDU.
All rates and token bucket depth parameters mapped from IP-level to ATM parameters MUST be corrected for added headers and cell segmentation overhead. IP parameters are in bytes/bytes-per-sec; ATM parameters are in cells/cells-per-sec.
Excess traffic SHOULD be supported on a best-effort basis, but MUST NOT interfere with the QoS (delay and loss) of conforming CLS and GS traffic, nor with normal service of non-reserved best effort traffic.
For CBR or BCOB-A connections supporting GS, specification of a Peak Cell Rate (PCR) is REQUIRED by ATM standards. When rtVBR is specified, both PCR and SCR are REQUIRED.
For CLS over ABR, the ATM Minimum Cell Rate (MCR) MUST be directly converted from the token bucket rate of the receiver TSpec. ABR with a positive MCR aligns with the CLS concept of 'best effort with a floor.'
For GS, the ATM network MUST meet the delay performance advertised through Adspec parameters MPL, C, and D. Both service break bits MUST be set if reservations are honored but sufficient resources to avoid congestion loss are not allocated.
For IP over ATM, Frame Discard MUST always be indicated if available. This TM/UNI 4.0 feature enables partial packet discard (dropped cell causes remaining cells of the same AAL-5 PDU to be dropped), improving end-to-end TCP/IP efficiency.
For multipoint VCs where branch characteristics differ, egress routers MUST correct Adspec parameters (APB, MPL, Path MTU, C_ATM, D_ATM) in PATH messages as they exit the ATM network. This correction is REQUIRED for APB, MPL, Path MTU, and ATM-specific GS Adspec parameters.
Real-time support is REQUIRED for Guaranteed Service. In UNI 3.x, Bearer Class BCOB-A (or equivalent BCOB-X) MUST be used; in TM/UNI 4.0, CBR or rtVBR MUST be chosen. Non-real-time categories (nrtVBR, UBR, ABR) cannot guarantee consistently low delay.
The ingress edge device MUST choose a CLR value that provides the appropriate IP-level packet loss rate. The CLR determination MUST account for packet size distribution and ATM Frame Discard mode, which can change effective packet loss rate by orders of magnitude.
The IS break bit <1,2> MUST be left unmodified by IS-aware implementations. The GS and CLS service-specific break bits <2,2> and <5,2> MUST be set if the corresponding ATM service category does not adequately support the service; corresponding hop counters MUST be incremented if not set.
The QoS Class MUST NOT be interpreted as a way to add real-time behavior to an inherently non-real-time service. The QoS Class follows the ATM service category, not the IP service, to avoid unsupported combinations.
VBR conformance definition VBR.3 SHOULD always be used with VBR service supporting IP integrated services, as it is the only VBR conformance definition that supports tagging and applies dual GCRAs (one for PCR on aggregate CLP=0+1, another for SCR on CLP=0 only).
When tagging is in use and congestion occurs, an ATM switch MUST attempt to discard tagged (CLP=1) cells in preference to CLP=0 cells. Since GS and CLS REQUIRE excess traffic to be treated as best effort, the tagging option SHOULD always be chosen in VC setup if supported.
ATM Cell Loss Priority (CLP) bit marks cells as low priority (CLP=1, 'tagged'). Tagging may be done by the source to indicate relative priority, or by a switch for policing violations. Options for tagging are decided at call setup time.
ATM subnet-specific GS Adspec components are C_ATM (set to zero for a 'pure delay' element unless ATM service rate equals R) and D_ATM (captures CDV/variable queueing delay). When an RESV triggers VC setup, CDV and CTD are set as CDV=D_ATM+S_ATM and CTD=D_ATM+MPL+S_ATM using available slack.
ATM traffic descriptors consist of Peak Cell Rate (PCR), and for VBR services also Sustainable Cell Rate (SCR) and Maximum Burst Size (MBS). SCR and MBS form a leaky bucket pair. CDVT (bucket depth for PCR) is not signalled but is determined by the network operator.
Canonical mapping of GS onto rtVBR sets ATM traffic descriptor parameters as PCR=p_r (receiver peak rate), SCR=R (RSpec allocated rate), MBS=b_r (receiver bucket depth). Variations are permitted based on edge device buffering and economic tradeoffs.
In TM/UNI 4.0 three ATM QoS parameters may be individually signalled: Cell Loss Ratio (CLR), Cell Transfer Delay (CTD), and Cell Delay Variation (CDV). QoS Class is set to 0 for UBR/ABR (REQUIRED), 1 for CBR/rtVBR, and 3 for nrtVBR.
Simple mapping of CLS onto nrtVBR sets PCR=p_r, SCR=r_r, MBS=b_r from the receiver TSpec. For ABR VCs, TSpec rate r_r sets the MCR; since no signalled bucket depth exists, the edge device SHOULD buffer at least b_r bytes plus jitter margin.
The edge device InterWorking Function (IWF) translates IP-level RSVP reservations into ATM VC UNI semantics. It handles service mapping, VC management, AAL segmentation/reassembly, and cell scheduling/shaping on both the IP and ATM sides.
The GS worst-case delay formula is: delay <= b_r/R + C_TOT/R + D_TOT + MPL. C and D are Adspec parameters accumulated by addition along the path; MPL includes propagation delay; b_r/R accounts for source bursts; R is the receiver-requested service rate.
The document contains a Security Considerations section (Section 8) but provides no specific security discussion, stating only that this document raises no security issues not addressed in the referenced specifications (RSVP, IntServ, ATM UNI).
IP packets are encoded into ATM cells as follows: each packet is wrapped in an AAL-5 PDU (H bytes of LLC/SNAP headers + B bytes IP payload + 8-byte AAL-5 trailer), then segmented into 53-byte ATM cells (5-byte header + 48-byte payload). Cell count is C = floor[(H + B + 8 + 47)/48].