---
title: "Short note: Feedback suppression in multicast"
subject: "Distributed System"
chapter: 9
key: "semester_7/distributed_system:feedback-suppression-note"
source: https://www.bctengineer.com/q/semester_7/distributed_system/feedback-suppression-note
site: BCT Engineering
---

# Short note: Feedback suppression in multicast

Distributed System · Chapter 9: Fault Tolerance

- Asked in: 2075 Asoj, 3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/distributed_system/feedback-suppression-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/distributed_system/chapter-9

## Question

Write a short note on feedback suppression mechanism in M-cast communication.

## Short answer

Feedback suppression reduces NACK implosion in reliable multicast. A receiver that misses a message waits a random time and multicasts its NACK; other receivers missing the same message suppress their own NACKs. The sender then multicasts the retransmission to the group.

## Answer

**Feedback suppression** is a technique in scalable reliable multicast (e.g. SRM, Scalable Reliable Multicast) for reducing the number of feedback messages (NACKs) from receivers to the sender.

If receivers each send a NACK for every lost message, a single loss at a router can cause hundreds of NACKs: a **feedback implosion**. In feedback suppression:
1. A receiver that misses a message **waits for a random time** before sending its NACK, and **multicasts** the NACK to the whole group (including other receivers).
2. Any other receiver that was going to send a NACK for the same message and now sees someone else's NACK **suppresses its own**.
3. The sender gets one (or very few) NACK and **multicasts** the retransmission to everybody (or the nearest member who has the message can retransmit).

Advantages: fewer messages, scalable. Disadvantages: receivers which did not lose the message get the retransmission too; the random delay has to be tuned, and it needs a multicast service and may cause delay.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What problem does feedback suppression solve?
   A. Message reordering
   B. Feedback implosion of NACKs at the sender
   C. Sender crash
   D. Corrupted messages
   Answer: B. One loss can trigger hundreds of NACKs; suppression cuts this to one or few. (Key point: Purpose)

2. What does a receiver that misses a message do first?
   A. Immediately sends a NACK to the sender only
   B. Waits a random time, then multicasts its NACK to the whole group
   C. Discards the missed sequence number
   D. Requests an ACK from everyone
   Answer: B. The random delay gives some receiver the chance to send first, and multicasting lets others see it. (Key point: Random wait)

3. When does a receiver suppress its own NACK?
   A. When it sees another receiver's NACK for the same message
   B. When its buffer is full
   C. When it receives a marker
   D. When the sender is idle
   Answer: A. Someone else's NACK already asks for the retransmission, so duplicates are unnecessary. (Key point: Suppression rule)

4. Which is a disadvantage of feedback suppression?
   A. Receivers that had the message also get the retransmission
   B. More NACKs reach the sender
   C. It cannot scale
   D. It needs no tuning
   Answer: A. Retransmission is multicast, so non-loss receivers get it too; the random delay also needs tuning and adds delay. (Key point: Disadvantage)

## Related in Distributed System

- [Need of multicast and consensus](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Amulticast-consensus-need) — Distributed System, chapter 6

## Related in other subjects

- [Feedback vs maximum ratio combining diversity](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afeedback-vs-mrc-diversity) — Wireless Communication, chapter 5
- [Working mechanism of WDM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Awdm-working) — Data Communication, chapter 6