CPSC 3600 - Quantifying Network Performances

What are the three main performance metrics for networks?

Latency, Throughput, and Errors.

What does latency measure?

The time it takes for data to travel from the sender to the receiver.

What does throughput measure?

The amount of data successfully transmitted per unit of time.

What do errors measure in networking?

Whether the transmitted data arrives intact and usable.

What are the four main sources of latency in a network?

Transmission delay, Propagation delay, Processing delay, and Queuing delay.

What is the formula for total latency?

d_total = d_trans + d_prop + d_process + d_queue.

What is transmission (bandwidth) delay?

The time required to place all the bits of a packet onto the communication link.

How is transmission delay calculated?

d_trans = packet length (L) / transmission rate (R).

What is propagation delay?

The time it takes for the signal to travel from sender to receiver across the medium.

How is propagation delay calculated?

d_prop = distance / propagation speed (s_prop).

What are typical propagation speeds in different media?

~3 × 10^8 m/s for radio waves; ~2 × 10^8 m/s for electricity/light in copper or fiber.

What is processing delay?

The time a router or device takes to examine a packet's header and decide how to forward it.

What is queuing delay?

The time a packet spends waiting in a buffer or queue before being transmitted.

What is RFC 1149 and why is it notable?

It defines "IP over Avian Carriers" (IPoAC), a joke standard using carrier pigeons to carry IP packets.

Has RFC 1149 ever been implemented?

Yes, in 2001 the Bergen Linux group sent 9 packets over 5 km using pigeons, with an average latency of ~5.2 million ms and high packet loss.

What is processing delay in networking?

The time a device (like a router) takes to examine a packet, check for errors, and determine how to forward it.

How long is processing delay usually?

Very small, typically measured in microseconds.

Why can't we calculate processing delay with a simple formula?

Because it depends on hardware/software efficiency and varies per device.

What is the store-and-forward principle?

Routers must receive the entire packet before forwarding it, since forwarding information is inside the packet.

Why can't routers forward bits as they arrive?

The destination information is in the packet header, which may not be fully received until the end.

What do routers need because of store-and-forward?

Buffers to temporarily hold packets before forwarding.

How does store-and-forward add extra latency?

Each router must fully receive and then retransmit the packet, adding delay at every hop.

What is queuing delay?

The time a packet spends waiting in a buffer before being transmitted because the outgoing link is busy.

What determines queuing delay?

The ratio of incoming traffic rate (R_in) to outgoing link capacity (R_out).

What happens to queuing delay when R_in / R_out ≈ 0?

Average queuing delay is very small.

What happens when R_in / R_out ≈ 1?

Queuing delay becomes very large.

What happens when R_in / R_out > 1?

The buffer grows faster than it can empty, leading to infinite queuing delay in theory and dropped packets in practice.

Why are packets dropped when R_in / R_out > 1?

If the router's buffer is full, new packets arriving must be discarded.

Which source of delay is the most variable and problematic in real networks?

Queuing delay.

Review: What are the four sources of delay in the Internet?

Transmission delay, Propagation delay, Processing delay, and Queuing delay.

Mbps vs MBps

Mbps uses bits (lowercase b), MBps uses bytes (uppercase B). 8 Mbps = 1 MBps.

Course assumption for units

1 Kb = 1000 bits, 1 Mb = 1000 Kb, 1 Gb = 1000 Mb (not the binary 1024 system).

Traceroute purpose

Measures real Internet delays by timing packets sent to each router along a path.

Traceroute mechanism

Sends 3 packets to each router i, router replies, sender measures round-trip delay.

Traceroute commands

Windows uses tracert; macOS/Linux use traceroute.

Ladder diagram meaning

Visual representation of how packets travel across multiple links over time.

Ladder diagram axes

Horizontal = packet transmission, Height = transmission time, Slope = propagation time.

Example first packet

Took 200 ms to transmit + 100 ms to propagate, arrived at 600 ms.

Example second packet

Started transmission at 200 ms, arrived at 800 ms.

Simplified ladder diagrams

Sometimes ignore transmission time to highlight propagation delays.

Bandwidth vs Throughput

Bandwidth is the maximum possible data rate per second, Throughput is the actual achieved data rate per second.

Goodput

The amount of useful data transmitted per second (ignores errors and retransmissions).

Throughput when Rs < Rc

Average end-to-end throughput is Rs (the slower sender link is the bottleneck).

Throughput when Rs > Rc

Average end-to-end throughput is Rc (the slower receiver link is the bottleneck).

Bottleneck link definition

The slowest link along the end-to-end path that limits overall throughput.

Fair sharing example

If 10 connections share a link of capacity R, each connection gets R/10 throughput.

Per-connection throughput formula

min(Rc, Rs, R/10). In practice Rc or Rs is usually the bottleneck.

Throughput vs Latency

High propagation delay (latency) does not necessarily mean low throughput.

IPoAC example

Pigeons carrying dense storage may have high throughput despite very high latency.

Review of throughput concepts

Throughput = actual data sent per second, Bandwidth = maximum possible, Goodput = useful data only. Max throughput limited by slowest link. Throughput and latency are not always related.

Sources of network errors

Corrupted packets, Dropped packets, Misordered packets, Collisions

Corrupted packets

Packets may get altered in flight (e.g. bit flips). Since it's usually impossible to know which bit flipped, the entire packet must be discarded.

Dropped packets

Occur when router queues are full or due to corruption. Lost packets may be retransmitted from the previous node, the origin node, or not at all.

Misordered packets

Data is split into packets for transmission. Packets must arrive in order to be reassembled correctly. Out-of-order packets may be dropped if the system can't handle reordering.

Collisions

Occur when multiple hosts transmit on the same shared medium simultaneously (e.g. cable internet). Signals interfere, both packets are dropped, and retransmission is triggered. Collisions do not occur on non-shared media like dedicated Ethernet cables.

Error review

Network errors include corrupted packets, dropped packets, misordered packets, and collisions (on shared media).