Explicit Rate Feedback

• PRCA was accepted by the ATM Forum Traffic Management Working Group.

• But many felt it wasn't fast enough because
  • Switch can only say congestion yes or no.
  • If AIR needs to be added 3 times, it will take 3 round trip delays.

• ATM-F-TMWG decided to include an Explicit Rate Feedback mechanism:
  • Source injects resource management (RM) cells for every group of Nrm cells.
  • For VCC, RM cells have payload type indicator (PTI) = 110.
  • For VPC, RM cells have PTI = 110 and VCI = 000...0110.
  • RM cells are marked by the source with a "desired" rate, ER.
  • Congested switches "mark down" the desired rate.
  • The RM cell is returned to the source by the destination.

Some fields of an RM cell

• DIR = 0 means the RM cell is traveling toward the destination (forward direction).
• DIR = 1 means the RM cell is traveling toward the source.
• CI = Congestion Indication bit - just like in a data cell, located in the payload field.
• NI = No Increase bit, set by a switch to indicate "keep allowed cell rate (ACR) constant".
• MCR = Minimum cell rate (part of negotiation phase).
• CCR = Current ACR cell rate.
• ER = Explicit Rate
  • Set by the source to the "desired" rate. In most cases set by the source to the peak cell rate (PCR).
  • Decreased by congested switches along the route.

EPRCA - Enhanced PRCA Algorithm

• Switches and sources must implement PRCA using EFCI (i.e. using the CI bit). This is called Binary Feedback (no RM cells).
• Switch or source may use NI and CI in RM cells. This is called Relative Rate Feedback.
• Switch or source may use the ER and CCR bits. This is called Explicit Rate Feedback.
• Big advantage of the ER mechanism is resetting the ACR in one round trip time.

What if one round trip time is anticipated to be too long?

Concept: VS/VD (Virtual Source/Virtual Destination)

• BECN - Backward Explicit Congestion Notification
  • If at a Non-VS/VD switch and an emergency arises, e.g. a bunch of connections come on all at one time, a switch can create an RM cell, set its "desired" rate, ER, low, and send it to the source directly.
  • This RM cell has DIR = 1 and BN = 1 (BN=BECN indicator).