Network Packet Transmission Issues

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Questions and Answers

What is the main issue highlighted regarding packet transmission in a network?

Transmission capacity is uniform across all links.

Packets are guaranteed to reach their destination without delay.

Packets can only be sent if all links are idle.

Some links may become bottlenecks due to traffic from multiple sources. (correct)

What does the term 'connectionless' refer to in this context?

Traffic must follow a predetermined path through the network.

Data packets are not dependent on one another in the transmission process. (correct)

Connections must be confirmed by both the sender and receiver before use.

All connections must be established before data transmission.

Which statement best describes the characteristics of connection-oriented services?

They transmit data without any error checking mechanisms.

They require minimal overhead and are always faster.

They ensure that all packets travel along the same path.

They rely on the network for managing states throughout the transmission. (correct)

Why is the classification of networks as either connectionless or connection-oriented considered restrictive?

Some networks exhibit characteristics of both types. Signup and view all the answers

In a packet-switched network, what is a potential issue with sending packets independently?

It may lead to inefficient use of bandwidth resources. Signup and view all the answers

What is the primary method used by a router to manage multiple incoming flow packets?

Round-robin Signup and view all the answers

In a round-robin service, all flows are treated equally regardless of what factor?

Packet size Signup and view all the answers

If a router handles two flows with different packet sizes, what could be an outcome of simple round-robin servicing?

Unequal bandwidth distribution favoring larger packets Signup and view all the answers

Which of the following is a challenge associated with fair queuing in routers?

Underutilization of available bandwidth Signup and view all the answers

How is the bandwidth allocation affected in a scenario where one flow has larger packets than another?

It favors the flow with larger packets Signup and view all the answers

What is a potential drawback of using round-robin queuing without considering packet size?

Inefficient use of link capacity Signup and view all the answers

In a scenario with a router that processes flows of different packet lengths, which flow would typically receive more bandwidth under simple round-robin servicing?

The flow with larger packets Signup and view all the answers

What should be taken into consideration to allocate bandwidth fairly when servicing different flows?

Packet size Signup and view all the answers

What is the primary function of the tail drop policy in networking?

To determine which packets get dropped Signup and view all the answers

What problem can occur in a priority queue system where users can set their own packet priorities?

The high-priority queue can starve out others Signup and view all the answers

In fair queuing, how is bandwidth ideally allocated among users?

Equally among all queues Signup and view all the answers

What is a characteristic feature of packets within each priority in a priority queue?

They are managed in a FIFO manner Signup and view all the answers

Which of the following best describes the convoy effect in networking?

A scenario where small packets get delayed by large packets Signup and view all the answers

What is one practical limitation of fair queuing in network management?

It is impossible to perfectly divide the bandwidth equally Signup and view all the answers

What does the term 'elephant and mice problem' refer to in networking?

The difficulty in managing priority for large and small packets Signup and view all the answers

Which of the following does NOT describe a characteristic of FIFO queuing?

Prioritization among packets can occur Signup and view all the answers

What happens when multiple variable bit applications exceed their average rate at a switch?

The excess data will be queued for processing. Signup and view all the answers

What two parameters describe the bandwidth characteristics of sources in the token bucket filter?

Token rate and bucket depth Signup and view all the answers

Why is knowing the long-term average bandwidth insufficient for video applications?

Scenes can change rapidly, leading to variable data rates. Signup and view all the answers

What occurs if enough sources send data exceeding the total allowed rate to a switch?

Data will overflow causing some packets to be lost. Signup and view all the answers

What is required to send a packet of a certain length in a token bucket filter mechanism?

An initial set of tokens must be available. Signup and view all the answers

What is likely to happen if the count of newly arriving packets increases?

The probability of packet drops increases. Signup and view all the answers

What action does Method #1 suggest if the current RTT exceeds the average RTT?

Decrease the congestion window by one-eighth. Signup and view all the answers

How is the change in the congestion window calculated according to Method #2?

By using (CurrentWindow - OldWindow) × (CurrentRTT - OldRTT). Signup and view all the answers

What happens if the value resulting from Method #2 is positive?

The congestion window is decreased by one. Signup and view all the answers

What is one of the challenges to supporting multimedia applications in packet-switched networks?

The need for higher-bandwidth links. Signup and view all the answers

In terms of Quality of Service (QoS), what becomes critical for multimedia applications?

Stable transmission rates over time. Signup and view all the answers

What represents a method of source-based congestion avoidance?

Taking into account both RTT and congestion window size. Signup and view all the answers

Why has the promise of packet-switched networks for multimedia not been fully realized?

Due to the need for higher-bandwidth links. Signup and view all the answers

What does Guaranteed Service in QoS ensure regarding packet delay?

There is a maximum delay specified for any packet. Signup and view all the answers

What is the main goal of Controlled Load Service in a heavily loaded network?

To emulate a lightly loaded network for certain applications. Signup and view all the answers

What role does flowspec play in RSVP?

It provides qualitative and quantitative information to the network. Signup and view all the answers

What does admission control process in RSVP determine?

If the network can provide the requested service. Signup and view all the answers

What is the purpose of resource reservation in RSVP?

To exchange information about service requests and decisions between users and the network. Signup and view all the answers

Which statement best describes packet scheduling in network switches and routers?

It manages how packets are queued and scheduled for transmission. Signup and view all the answers

What is the Tspec in the context of flowspec?

The flow’s traffic characteristics, including bandwidth. Signup and view all the answers

Which of the following is NOT a component of the RSVP protocol?

Routing Table Signup and view all the answers

Study Notes

Congestion Control and Resource Allocation

Networks need to effectively and fairly allocate resources to competing users.
Resources include bandwidth and buffers at routers and switches.
Packets contend for link use and are placed in queues.
Congestion occurs when too many packets contend for the same link, causing queue overflows and packet loss.
Congestion control mechanisms are needed to manage network resources effectively.

Issues in Resource Allocation

Resource allocation is complicated due to distributed resources within a network.
A network can take an active role in allocating resources or allow sources to send data as much as they want, and recover from congestion later.
The first approach avoids congestion, while the second approach can be disruptive if many packets are dropped before congestion is controlled.

Network Model

We examine resource allocation in packet-switched networks, consisting of multiple links and switches or routers.
In this environment, a source might have sufficient capacity on its outgoing link but encounter congested links later in the network.
Connectionless flows, while datagrams are switched independently, often transmit datagrams through a specific set of routers to support a flow.

Flow Abstractions

Abstractions like host-to-host or process-to-process flows are identical to channels, but visible to routers.
State information—like soft state (indirect flow state estimation) and hard state (explicit router/host info)—is used to facilitate resource allocation decisions.

Service Model

Best-effort service treats all packets equally without guarantees.
Quality of service supports prioritized service or guarantees (e.g., bandwidth, delay, variance) for applications like video streaming.

Taxonomy (1)

Router-centric: routers decide on packet forwarding and dropping.
Host-centric: hosts monitor network conditions and adjust sending rates.
Reservation-based: entities request capacity allocation from the network to prevent congestion.
Feedback-based: hosts send data without reservations and adjust transmission rates based on network feedback (e.g., explicit messages or implicit packet loss).

Taxonomy (2)

Window-based: network uses window advertisements to reserve buffer space.
Rate-based: sender behavior is controlled by the rate the receiver or network can absorb.

Evaluation Criteria

Effective resource allocation is measured by throughput and delay.
Greedy approach attempts to maximize throughput and minimize delay. However, higher throughput can lead to longer queues and packet delays potentially leading to network congestion.
Negotiation between throughput and delay evaluates resource allocation effectiveness.

Load vs. Power

The ratio of throughput to delay is maximized at an optimal load.

Fair Resource Allocation

Fairness is difficult to define explicitly, but reservation-based systems can be explicitly unfair.
Ideally, flows receive an equal share of bandwidth.
Jain's fairness index quantifies fairness among flows.

Queuing Disciplines

FIFO (First-In, First-Out) serves packets in the order they arrive at the router.
Tail drop discards packets when the queue is full, regardless of which flow the packet belongs to.
Priority queuing gives different priorities to packets, often using the IP header marks.
Fair queuing prioritizes flows evenly dividing bandwidth among them. Implementing perfect fair queuing is practically impossible due to diverse packet sizes.

TCP Congestion Control

TCP congestion control, developed by Van Jacobson, addresses congestion collapse by having hosts send packets at a rate the network can handle and making adjustments based on network congestion, in particular dropped packets, to restore network stability.
TCP's distributive approach involves each source estimating network capacity to avoid sending more packets increasing network congestion.
Self-clocking uses ACKs as signals for how many packets have been successfully sent and processed by a network to allow for sending additional packets, without pushing the network's capacity.

Vanilla TCP Congestion Control

The congestion window (cwnd) limits data in transit at once.
Advertised window (awnd) is the maximum data the receiver can receive.
TCP takes the minimum of both, and increments cwnd depending on acknowledgments received.

TCP Congestion Control, Challenge

TCP learning appropriate cwnd (congestion window) values relies on the receiving side's advertised window (awnd) information.

TCP Congestion Control, AIMD

The method of Additive Increase and Multiplicative Decrease is used to reduce the sending rate (by halving the congestion window) during periods of packet loss but increase the sending rate in proportion to each acknowledgment.

TCP Congestion Control, MSS

Maximum Segment Size (MSS) is the largest data size a network connection permits.
MSS is typically in the range of 128 bytes to MTU (Maximum Transmission Unit).
The congestion window is always larger than or equal to one MSS.
Incrementing the congestion window by an entire MSS for each acknowledgment.

TCP Congestion Control, Slow Start

Slow start rapidly increases the congestion window.
The window is increased exponentially for each acknowledgment.
The initial window starts from 1 packet.

TCP Congestion Control, Congestion Threshold

The "target" congestion window is half the cwnd value before the last packet loss.
Slow start is involved to rapidly increase the sending rate to the target value, and then additive increase is used beyond this point.

TCP Congestion Control, Implementation

Implementing TCP congestion control involves managing the congestion window (cwnd) which increments based on the receipt of acknowledgments (ACKs). The maxwin value defines an upper limit for the cwnd variable.

Improvement # 1: Fast Retransmit

A heuristic where a dropped packet results in a faster retransmission response by the sender.
This happens sooner than using a timeout mechanism.

Improvement # 2: Fast Recovery

When the fast retransmit mechanism notices congestion, the sender halves the congestion window value and resumes additive increases, not multiplicative increases.

Congestion Avoidance Mechanism

Reactive methods control congestion after it occurs.
TCP backs off or reduces its load after noticing congestion.
Proactive methods aim to avoid congestion in the first place, by anticipating potential congestion and adjusting the sending rate. Some protocols use congestion prediction methods, notably in ATM networks. This is not widely adopted in IP networks.

DEC Bit

A congestion bit is set in packets by routers when queues are long.
The destination copies this bit to the acknowledgment and the source uses this to reduce or increase its sending rate.

Random Early Detection (RED)

RED is a proactive queuing method that drops packets before queues get excessively full.
It uses a probability to drop packets in a random way when the queue length is between two thresholds.
The probability of dropping a packet increases when the queue length is closer to the maximum threshold.

RED, Drop Probability

The drop probability is a calculation of how full the queue is and how long ago the last drop occurred.
The probability formula accounts for factors of current average queue length, previous maximum queue depth, and time elapsed since the last dropped packet.

Source-Based Congestion Avoidance

Sources can detect congestion from changes in round trip time (RTT).
Methods use changes in RTT to reduce the congestion window, by making reductions proportional to the change in RTT.

Quality of Service

Multimedia applications demand quality of service (QoS) to guarantee data delivery and responsiveness.
Fine-grained approaches provide QoS to individual flows, such as with integrated services (RSVP).
Coarse-grained approaches provide QoS to large traffic classes, such as differentiated services.

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Description

Explore key concepts related to packet transmission in networks with this quiz. Delve into the characteristics of connectionless and connection-oriented services and the implications of sending packets independently in packet-switched networks. Test your understanding of the complexities of network communication.