Networking

What happens when you see a connection error saying: "connection reset by peer"?

the web server closed the connection when the web browser was expecting it. the server can also refuse the message or the browser can wait a really long time because the server isn't even responding with a refusal.

How does network congestion cause packet loss?

Since the amount of buffer space is finite, an arriving packet may find that the buffer is completely full with other packets wait- ing for transmission. In this case, packet loss will occur—either the arriving packet or one of the already-queued packets will be dropped.

Give an example of congestion as a result of output queue?

Suppose Hosts A and B are sending packets to Host E. Hosts A and B first send their packets along 10 Mbps Ethernet links to the first router. The router then directs these packets to the 1.5 Mbps link. If, during a short interval of time, the arrival rate of packets to the router (when converted to bits per second) exceeds 1.5 Mbps, congestion will occur at the router as packets queue in the link's output buffer before being transmitted onto the link.

What is transmission delay?

The transmission delay is L/R. This is the amount of time required to push (that is, transmit) all of the packet's bits into the link. it can be hundreds of milliseconds for large Internet packets sent over low-speed dial-up modem links.

How does the Application layer communicate with the other layers in the Internet Model?

While each application is different, it can reuse the well-defined API from the Application layer to the TCP or UDP service beneath. Applications typically want a bi-directional reliable byte stream.

What are the two predominant types of packet switches?

routers and link-layer switches

What is a segment?

transport-layer packet

Which layer is HTTP and SMTP implemented?

Application-layer protocols—such as HTTP and SMTP—are almost always implemented in software in the end systems; so are transport-layer protocols.

What is UDP (User Datagram Protocol)?

If an application doesn't need reliable delivery like video conferencing apps, it can use the much simpler UDP. UDP bundles up app data and hands it to the network layer for the delivery to the other end.

How does the 4 layer Internet Model compare with the 7-layer OSI Model?

Network engineers sometimes use the numbering system on the OSI model to refer to a specific layer.

Which layer is responsible for delivering data across a single hop?

The link layer.

What is a network node?

host or router.

What four properties characterize the IP Service Model?

1. It sends datagrams from end host to end host. 2. It is unreliable, 3. but makes a best-effort to deliver datagrams. 4. The network maintains no per-flow state associated with the datagrams

What is a consequence of layering when it comes to networking?

A consequence of layering is that a layer at the source host communicates with its peer layer at the destination, without concerning itself with the implementation details of the layers above and below.

What is a message in the application layer?

An application-layer protocol is distributed over multiple end systems, with the application in one end system using the protocol to exchange packets of information with the application in another end system. We'll refer to this packet of information at the application layer as a message.

What is the 4 layer Internet Model?

Application, Transport, Network, and Link Layer. All 4 layers are there to enable applications in the end-hosts to communicate reliably. Each layer has a different responsibility, with each layer building a service on top of the one below. All the way to the top, where we have the bidirectional byte stream communication between applications.

What is Queuing Delay?

At the queue, the packet experiences a queuing delay as it waits to be transmitted onto the link. The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission onto the link. If the queue is empty and no other packet is currently being transmitted, then our packet's queuing delay will be zero. On the other hand, if the traffic is heavy and many other packets are also waiting to be transmitted, the queuing delay will be long.

Explain the physical path that data takes down a sending end system's protocol stack, up and down the protocol stacks of an intervening link-layer switch and router, and then up the protocol stack at the receiving end system (encapsulation)?

At the sending host, an application-layer message (M in Figure 1.24) is passed to the transport layer. In the simplest case, the transport layer takes the message and appends additional information (so-called transport-layer header information, Ht in Figure 1.24) that will be used by the receiver-side transport layer. The application layer message and the transport-layer header information together constitute the transport-layer segment. The transport-layer segment thus encapsulates the application-layer message. The added information might include information allowing the receiver-side transport layer to deliver the message up to the appropriate application, and error-detection bits that allow the receiver to determine whether bits in the message have been changed in route. The transport layer then passes the segment to the network layer, which adds network-layer header information (Hn in Figure 1.24) such as source and destination end system addresses, creating a network-layer datagram. The datagram is then passed to the link layer, which will add its own link-layer header information and create a link-layer frame. Thus, we see that at each layer, a packet has two types of fields: header fields and a payload field. The payload is typically a packet from the layer above.

Where are the physical layers and data link layers typically implemented?

Because the physical layer and data link layers are responsible for handling communication over a specific link, they are typically implemented in a network interface card (for example, Ethernet or WiFi interface cards) associated with a given link.

How does Bit Torrent work from a networking perspective?

Bit Torrent is a program that allows people to share and exchange large files. In Bit Torrent, a client requests documents from other clients. So that a single client can request from many others in parallel, Bit Torrent breaks into chunks of data called pieces. When a client downloads a complete piece from another client, it then tells other clients that it has that piece so they could download it too. These collections of collaborating clients are called swarms. The tracker is a node that keeps track of members of the swarm.

How do networked computers/applications work?

Connectivity is the idea that two computers in different parts of the world can connect to one another and exchange data. If you connect your computer to internet, you can talk to all the other computers on the internet.

How does the router determine which link it should forward the packet onto?

Each router has a forwarding table that maps destination addresses (or portions of the destination addresses) to that router's outbound links. When a packet arrives at a router, the router examines the address and searches its forwarding table, using this destination address, to find the appropriate outbound link. The router then directs the packet to this outbound link.

What is an output buffer (output queue) on a router?

For each attached link, the packet switch has an output buffer, which stores packets that the router is about to send into that link. The output buffers play a key role in packet switching. If an arriving packet needs to be transmitted onto a link but finds the link busy with the transmission of another packet, the arriving packet must wait in the output buffer. Thus, in addition to the store-and-forward delays, packets suffer output buffer queuing delays. These delays are variable and depend on the level of congestion in the network.

How does the Byte Stream model work?

For instance, Computer B (web server) is waiting for other computers to connect to it. Computer A on the left wants to communicate with Computer B. A and B both set up a connection. When A writes data to the connection, it travels over the network and B can read it. Similarly, if B writes data, A can read it. Either side can close the connection. If the web browser is done requesting data from web server, it can close the connection.

What are link-layer packed called?

Frames

What is HTTP?

Hyper Text Transport Protocol. HTTP was designed to be a document-centric way for programs to communicate. HTTP is all in ASCI 11 text: it's human readable. Basic model is: client sends a request by writing to the connection, the server reads the request, processes it, and writes a response to the connection, which the client then reads.

How does the service provided by Internet Protocol (IP) work?

IP datagrams consist of a header and some data. When the transport layer has data to send, it hands a Transport Segment to the Network layer below. The Network layer puts the transport segment inside a new IP datagram. IP's job is to deliver the datagram to the other end. But first, the IP datagram has to make it over the first link to the first router. IP sends the datagram to the Link layer that puts it inside a Link frame, such as an Ethernet packet and ships it off to the first router.

What is the average throughput?

If the file consists of F bits and the transfer takes T seconds for Host B to receive all F bits, then the average throughput of the file transfer is F/T bits/sec.

Why is IP referred to as the "thin waist"?

If we want to use the internet, we need to use the Internet Protocol (IP), but we have lots of choices for link layers.

What is packet switching?

In a network app, end systems exchange information with each other. To send a message from a source end system to a destination end system, the source breaks long messages into smaller chunks of data known as packets. Between source and destination, each packet travels through communication links and packet switches. Packets are transmitted over each communication link at a rate equal to the full transmission rate of the link. So, if a source end system or a packet switch is sending a packet of L bits over a link with transmission rate R bits/sec, then the time to transmit the packet is L/R seconds.

How does Skype from a networking perspective?

In it's most simple mode, when you want to call someone, Skype is a client-client (not server) exchange. Sort of like HTTP, you the caller open a connection with the recipient, if recipient accepts, you both start exchanging data, voice, video. What happens when both clients are behind NAT's? Skype introduces a relay server to facilitate communication between both clients. They both open connections to relay and when client A open connection to relay and sends data, the relay forwards it client B.

When is the queuing delay large and when is it insignificant?

It depends on the rate at which traffic arrives at the queue, the transmission rate of the link, and the nature of the arriving traffic, that is, whether the traffic arrives periodically or arrives in bursts. If packets arrive periodically—that is, one packet arrives every L/R seconds—then every packet will arrive at an empty queue and there will be no queuing delay. On the other hand, if packets arrive in bursts but periodically, there can be a significant average queuing delay. When the traffic intensity is close to 1, there will be intervals of time when the arrival rate exceeds the transmission capacity (due to variations in packet arrival rate).

What is the link layer?

Link Layer- the internet is made up of end-hosts, links, routers. Data is delivered hop-by-hop over each link turn. Data is delivered in packets. A packet is consists of data we want to be delivered along with the header that tells the network where the packet is to be delivered. The link layers job is to carry the packet one link at a time. You've probably heard of wifi and ethernet as two type of link layers.

What is the network layer in the 4 layer Internet Model?

Must use the Internet Protocol (IP). IP makes a best-effort attempt to deliver our datagrams to the other end. But it makes no promises. If an application wants a guarantee that it's data will be transmitted when necessary and will be delivered to the application in order and without corruption, then it needs another protocol running on top of IP. This is the transport layer.

What is a propagation delay?

Once a bit is pushed into the link, it needs to propagate to router B. The time required to propagate from the beginning of the link to router B is the propagation delay. The bit propagates at the propagation speed of the link. The propagation speed depends on the physical medium of the link. The propagation delay is the distance between two routers divided by the propagation speed.

Store-and-Forward transmission

Store-and-forward transmission means that the packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link.

TCP

TCP provides a connection-oriented service to its applications. This service includes guaranteed delivery of application-layer messages to the destination and flow control (that is, sender/receiver speed matching). TCP also breaks long messages into shorter segments and provides a congestion-control mechanism, so that a source throttles its transmission rate when the network is congested.

Network Layer

The Internet's network layer is responsible for moving network-layer packets known as datagrams from one host to another. The Internet transport-layer protocol (TCP or UDP) in a source host passes a transport-layer segment and a destination address to the network layer. The network layer then provides the service of delivering the segment to the transport layer in the destination host. The Internet's network layer also contains rout- ing protocols that determine the routes that datagrams take between sources and destinations.

Transport Layer

The Internet's transport layer transports application-layer messages between application endpoints.

UDP

The UDP protocol provides a connectionless service to its applications. This is a no-frills service that provides no reliability, no flow control, and no congestion control.

What is network throughput?

The amount of data per second that can be transferred. computer networks necessarily constrain throughput between end systems, introduce delays between end systems, and can actually lose packets. On one hand, it is unfortunate that the physical laws of reality introduce delay and loss as well as constrain throughput.

As a packet travels from one node (host or router) to the subsequent node (host or router) along this path, what types of delays could the packet suffer from?

The most important of these delays are the nodal processing delay, queuing delay, transmission delay, and propagation delay; together, these delays accumulate to give a total nodal delay. The performance of many Internet applications—such as search, Web browsing, email, maps, instant messaging, and voice-over-IP are greatly affected by network delays.

How does the Network Layer work with the Link Layer?

The network layer hands the datagram to the link layer below, telling it to send the data over the first link. In other words, the link layer is providing a service to the network layer. At the other end of the link is a router, the link layer of the router accepts the datagram from the link and hands it up to the network layer in the router. The network layer of the router examines the destination address

Presentation Layer

The role of the presentation layer is to provide services that allow communicating applications to interpret the meaning of data exchanged. These services include data compression and data encryption as well as data description.

Session Layer

The session layer provides for delimiting and synchronization of data exchange, including the means to build a checkpointing and recovery scheme.

What does the socket API do?

The socket API provides computer applications with a reusable means to communicate with remote applications.

What is the processing delay in the context of networking?

The time required to examine the packet's header and determine where to direct the packet is part of the processing delay. The processing delay can also include other factors, such as the time needed to check for bit-level errors in the packet that occurred in transmitting the packet's bits from the upstream node to router A. It strongly influences a router's maximum throughput, which is the maximum rate at which a router can forward packets.

What is the difference between propagation and transmission delays?

The transmission delay is the amount of time required for the router to push out the packet; it is a function of the packet's length and the transmission rate of the link, but has nothing to do with the distance between the two routers. The propagation delay, on the other hand, is the time it takes a bit to propagate from one router to the next; it is a function of the distance between the two routers, but has nothing to do with the packet's length or the transmission rate of the link.

Application Communication - Bidirectional Byte Stream

This allows two programs running on different computers to exchange data. It abstracts the entire network to a simple read/write relationship.

Link Layer

To move a packet from one node (host or router) to the next node in the route, the network layer relies on the services of the link layer. at each node, the network layer passes the datagram down to the link layer, which delivers the datagram to the next node along the route. At this next node, the link layer passes the datagram up to the network layer. Examples of link- layer protocols include Ethernet, WiFi, and the cable access, etc. As datagrams typically need to traverse several links to travel from source to destination, a datagram may be handled by different link-layer protocols at different links along its route.

Protocol Layering?

To provide structure to the design of network protocols, network designers organize protocols—and the network hardware and software that implement the protocols— in layers. We are interested in the services that a layer offers to the layer above—the so-called service model of a layer. Each layer provides its service by (1) performing certain actions within that layer and by (2) using the services of the layer directly below it.

What is a traceroute program?

Traceroute is a simple program that can run in any Internet host. When the user specifies a destination hostname, the program in the source host sends multiple, special packets toward that destination. As these packets work their way toward the destination, they pass through a series of routers. When a router receives one of these special packets, it sends back to the source a short message that contains the name and address of the router. The source records the time that elapses between when it sends a packet and when it receives the corresponding return message. RFC 1393.

What is the transport layer?

Transmission Control Protocol (TCP) makes sure that data sent by an app at one end of the internet is correctly delivered - in the right order to the other end of the internet. If the network layer drops some datagrams, TCP will retransmit them, multiple times if need-be.

What two choices in the transport layer do applications have in transporting data?

UDP (User Datagram Protocol) and TCP (Transmission Control Protocol).

What is the GET request?

When a web client requests a page from a web server, the web client sends a GET request. This is one of the commands of HTTP. HTTP dictates that the GET command is sent as an ASC11 string. Along with a URL of the page being requested. As far as App layer is concerned, the GET request is sent directly to it's peer at the other end- web server. The App layer hands the GET request to the TCP layer, which provides the service of making sure it is reliably delivered.

How does the datagram service for the IP protocol work?

When we ask IP to send some data for us, it creates a datagram and puts our data inside. The datagram is a packet that is routed individually through the network based on the information in its header. In other words, the datagram is self-contained. The header contains the IP address of the destination, which we abbreviate here as "IP DA" for IP destination address. The forwarding decision at each router is based on the IP DA. The datagram header also contains an IP source address, or IP SA, saying where the packet came from, so the receiver knows where to send any response. Datagrams are routed hop-by-hop through the network from one router.

Application Layer

Where network applications and their application-layer protocols reside. The Internet's application layer includes many protocols, such as the HTTP protocol (which provides for Web document request and transfer), SMTP (which provides for the transfer of e-mail messages), and FTP (which provides for the transfer of files between two end systems).

What happens when a packet arrives at a full queue at a router?

With no place to store such a packet, a router will drop that packet; that is, the packet will be lost. The fraction of lost packets increases as the traffic intensity increases.

How does the World Wide Web (www) work?

World Wide Web uses HTTP (Hyper Text Transfer Protocol). When you see http:// in browser, that means it's communicating using http. In HTTP, a client opens a connection to a server and sends commands (GET, PUT, Delete) to it. Most common command is GET, which requests a page. For example, if I type http://www.stanford.edu/ into my browser, the browser opens a connection to server and sends a GET request for the root page of the site. The server receives the request, checks if it's valid and the user can access that page, and sends a response. The response has a numeric code associated with it like 200 OK response.

How do end systems and apps affect network delays?

an end system wanting to transmit a packet into a shared medium (e.g., as in a WiFi or cable modem scenario) may purposefully delay its transmission as part of its protocol for sharing the medium with other end systems.

What is an example of Store-and-Forward transmission?

consider a network consisting of two hosts connected by a single router. A router will have many incident links, since its job is to switch an incoming packet onto an outgoing link; In this example, the source has three packets, each consisting of L bits, to send to the destination. the source has transmitted some of packet 1, and the front of packet 1 has already arrived at the router. Because the router employs store-and-forwarding, at this instant of time, the router cannot transmit the bits it has received; instead it must first buffer (i.e., "store") the packet's bits. Only after the router has received all of the packet's bits can it begin to transmit (i.e., "forward") the packet onto the outbound link.

IP Protocol

defines the fields in the datagram as well as how the end systems and routers act on these fields.

How do end-systems connect to the internet?

end systems (PCs, smartphones, Web servers, mail servers, and so on) connect into the Internet via an access ISP. The access ISP can provide either wired or wireless connectivity, using an array of access technologies including DSL, cable, FTTH, Wi-Fi, and cellular. the access ISPs themselves must be interconnected. This is done by creating a network of networks understanding this phrase is the key to understanding the Internet.

What is the instantaneous throughput?

instantaneous throughput at any instant of time is the rate (in bits/sec) at which Host B is receiving the file from Host A.

How do you characterize/measure queuing delays?

one typically uses statistical measures, such as average queuing delay, variance of queuing delay, and the probability that the queuing delay exceeds some specified value.

What is the difference between routers and link-layer switches?

routers and link-layer switches organize their networking hardware and software into layers. But routers and link-layer switches do not implement all of the layers in the protocol stack; they typically implement only the bottom layers. link-layer switches implement layers 1 and 2; routers implement layers 1 through 3. This means, for example, that Internet routers are capable of implementing the IP protocol (a layer 3 protocol), while link-layer switches are not.

What do routers need to do before forwarding a packet?

routers need to receive, store, and process the entire packet before forwarding.

Give an example of how do forwarding tables get set?

the Internet has a number of special routing protocols that are used to automatically set the forwarding tables. A routing protocol may, for example, determine the shortest path from each router to each destination and use the shortest path results to configure the forwarding tables in the routers.

Physical Layer

the job of the physical layer is to move the individual bits within the frame from one node to the next. The protocols in this layer are again link dependent and further depend on the actual transmission medium of the link (for example, twisted-pair copper wire, single-mode fiber optics).

What is the role of the Network layer and what are packets?

the network layers job is to deliver packets end-to-end across the internet from source to destination. A packet is an important building block of network layer. A packet is a name we give to a self-contained collection of data, plus a header that describes what the data is, where it is going, and where it came from. Network layer packets are called datagrams. They consist of some data, and a head containing the "To" and "From" addresses.

What is the server- to-client throughput?

the server cannot pump bits through its link at a rate (Rs) faster than the transmission rate of the link between the server and the router; the router cannot forward bits at a rate (Rc) faster than the rate of the link between the router and the client. If Rs < Rc, then the bits pumped by the server will "flow" right through the router and arrive at the client at a rate of Rs bps, giving a throughput of Rs bps. If, on the other hand, Rc < Rs, then the router will not be able to forward bits as quickly as it receives them. In this case, bits will only leave the router at rate Rc, giving an end-to-end throughput of Rc.

What does throughput ultimately depend on?

throughput depends on the transmission rates of the links over which the data flows. We saw that when there is no other intervening traffic, the throughput can simply be approximated as the minimum transmission rate along the path between source and destination. The example in Figure 1.20(b) shows that more generally the throughput depends not only on the transmission rates of the links along the path, but also on the intervening traffic. In particular, a link with a high transmission rate may nonetheless be the bottleneck link for a file transfer if many other data flows are also passing through that link.