Computer Networking

Quality of Service (QoS) refers to a set of technologies that manage network traffic to reduce packet loss, latency, and jitter on the network. QoS controls and manages network resources by setting priorities for specific types of data.

The goal of QoS is to provide different priority levels to different applications, users, or data flows, guaranteeing a certain level of performance for critical traffic.

For example, in a corporate network, QoS can be used to prioritize:

• High Priority: Real-time traffic like VoIP phone calls and video conferencing, which are very sensitive to delay.
• Medium Priority: Important business applications.
• Low Priority: General web Browse or large file downloads, which are not as sensitive to delay.

SSL (Secure Sockets Layer) and its successor, TLS (Transport Layer Security), are cryptographic protocols designed to provide communications security over a computer network.

They are most famously used in HTTPS to secure web traffic. TLS provides three main services:

1. Encryption: It encrypts the data being transmitted, making it unreadable to anyone who might intercept it.
2. Authentication: It verifies the identity of the parties involved in the communication (typically, it authenticates the server to the client) using digital certificates.
3. Integrity: It ensures that the data has not been tampered with during transit using message authentication codes (MACs).

Although the term SSL is still widely used, the protocol itself is deprecated due to security vulnerabilities. TLS is the modern, secure standard.

A network topology refers to the arrangement of the elements (links, nodes, etc.) of a computer network.

• Bus Topology: All devices are connected to a single central cable, called the bus or backbone. Simple but outdated, as a single cable failure takes down the whole network.
• Ring Topology: All devices are connected in a circle. Data travels in one direction. A failure in one device or cable can break the loop.
• Star Topology: All devices are connected to a central hub or switch. This is the most common topology used in modern LANs. If one cable fails, only that one device is affected. However, if the central switch fails, the whole network goes down.
• Mesh Topology: Every device is connected to every other device. This provides high redundancy and reliability but is very expensive and complex to implement. Often used in WANs for core router connections.
• Hybrid Topology: A combination of two or more different topologies. For example, a star-bus network connects multiple star-configured networks together via a central bus.

Jitter is the variation in the latency (or delay) of packets as they travel across a network. In other words, it's the inconsistency in the arrival time of data packets.

For example, if four packets are sent at 10ms intervals and they arrive at 20ms, 22ms, 25ms, and 28ms, the delay is not constant. This variation is jitter.

Jitter is particularly problematic for real-time applications like:

• VoIP (Voice over IP): High jitter can cause words to be garbled or parts of the conversation to be missed.
• Video Streaming: It can lead to a choppy, stuttering video playback.
• Online Gaming: It can cause noticeable lag and an inconsistent gameplay experience.

Networking devices can use 'jitter buffers' to collect packets and send them out at a more regular interval to smooth out the effects of jitter.

Anycast is a network addressing and routing method in which a single destination IP address is shared by devices (usually servers) in multiple geographic locations.

When a client sends a request to an anycast address, routers direct the traffic to the server that is 'closest' or 'best' in terms of network topology. This is determined by routing protocols, typically meaning the server with the lowest latency or fewest network hops.

Key uses of anycast include:

• DNS Services: Many root DNS servers and major DNS providers use anycast to provide faster responses to users by directing them to a nearby server.
• Content Delivery Networks (CDNs): CDNs use anycast to route users to the nearest cache server, speeding up content delivery and improving reliability.
• DDoS Mitigation: By distributing traffic across many servers globally, anycast can help absorb and mitigate the impact of a DDoS attack.

A Content Delivery Network (CDN) is a geographically distributed group of servers that work together to provide fast delivery of Internet content.

The goal of a CDN is to improve website performance and availability by caching content (like images, videos, CSS, and JavaScript files) in servers located close to the end-users. These servers are called 'edge servers'.

When a user requests content from a website that uses a CDN, the request is redirected to the nearest edge server instead of the website's main 'origin' server. This provides two main benefits:

1. Reduced Latency: Since the content is delivered from a nearby server, the physical distance the data has to travel is much shorter, resulting in faster load times.
2. Reduced Load on Origin Server: By offloading the delivery of static content, the CDN reduces the traffic and processing load on the website's main server, improving its stability.

IMAP (Internet Message Access Protocol) and POP3 (Post Office Protocol version 3) are two protocols used to retrieve emails from a mail server.

SMTP (Simple Mail Transfer Protocol) is the standard protocol for sending electronic mail (email) from an email client to a mail server, and from one mail server to another.

While protocols like POP3 and IMAP are used to *pull* emails from a server, SMTP is used to *push* emails to a server and between servers.

It typically uses TCP Port 25 for server-to-server communication and Port 587 (or 465 for older, SSL-based connections) for client-to-server submissions.

A routing table is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics (distances) associated with those routes.

For every packet a router receives, it examines the destination IP address and consults its routing table to make a forwarding decision. Each entry in the table typically contains:

• Destination Network: The network address of the final destination.
• Subnet Mask: The mask for the destination network.
• Next Hop: The IP address of the next router to which the packet should be sent.
• Outgoing Interface: The physical network interface (e.g., GigabitEthernet0/1) the router should use to send the packet.
• Metric: A value indicating the 'cost' or 'desirability' of a route. Routers use this to select the best path when multiple routes to the same destination exist.

Routing tables can be configured manually by an administrator (static routing) or built dynamically using routing protocols (dynamic routing).

The term 'sticky bit' primarily comes from Unix/Linux file permissions and doesn't have a direct, standard definition in network security itself. However, the concept can be applied metaphorically or in specific contexts, such as session management.

A 'sticky session' or 'session affinity' in the context of load balancing is a relevant concept. When a load balancer is distributing traffic across multiple servers, session affinity ensures that all requests from a single user during a session are consistently sent to the same server.

This is critical for applications that maintain session state on the server (like a shopping cart on an e-commerce site). Without session affinity, a user's subsequent requests could be sent to different servers that don't have their session information, leading to a broken user experience.