Which layer in TCP/IP model provides Quality of Service (QoS)? I am confused between Layer 2 and Layer 3?
According to Computer Networks, by Andrew S. Tanenbaum, Quality of Service is provided by the Network Layer (Layer 3).
A stream of packets from a source to a destination is called a flow (Clark, 1988). A flow might be all the packets of a connection in a connection-oriented network, or all the packets sent from one process to another process in a connectionless network. The needs of each flow can be characterized by four primary parameters: bandwidth, delay, jitter, and loss. Together, these determine the QoS (Quality of Service) the flow requires.
Computer Networks - Andrew S. Tanenbaum, 5th Edition, Chapter 5 (The Network Layer), Section 5.4 (Quality of Service), p. 404
Any layer can distinguish between different qualities of service. Layers 2 & 3 offer "best effort" delivery, where QoS can influence relative amount of effort. There are many layer-2 technologies, but ethernet is certainly a popular one. Ether standards committees have published various QoS specs, including IEEE P802.1p. There have been many others. Sometimes queuing discipline (packet drop policy) is of interest, and sometimes a reservation of committed bit-rate is the goal.
A QoS field has been baked into layer-3 IP headers since the beginning -- see Type of Service in https://www.rfc-editor.org/rfc/rfc791#page-12
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I have heard, Many companies like facebook are using UDP to fetch data from memcached. I have a doubt, How they make sure there is NO packet loss and order of received packet is per requirement.As we know tcp provide such facility but udp does not.
OSI Model has 7 layers which are:
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Splitting things to layers is very good approach to solve problems but it doesn't mean you have to do all the network operations in network layer.
As you have mentioned, TCP provides feedback to end systems when UDP doesn't but UDP has it's own advantages. First of UDP's datagram is simpler than TCP's one. And also most of huge systems like Facebook uses UDP because using TCP for these kind systems would not be very clever since all the data senders would have to keep track of sending rate, retransmissions rate for many many receivers. So if they've used TCP, their network layer would be under very very big presure.
So they make flow control in the application layer to reduce network traffic.
I'm trying to understand Network layers. There are five layers, application, transport, network, link and physical layer. I have studied from various sources but I couldn't understand clearly, especially transport, network and link layers. Can you explain those layers for a dummy person like me? Thanks.
So I think the hardest part people have with the OSI model is that they look at some of the layers and forget its talking about communication... application layer specifically people get confused as to what its talking about.
Application references the protocols applications use to communicate.
Examples being ftp, http, etc.
As stated above, the original OSI model (which is really just theory, nothing actually implements it exactly as it would be too inefficient) has 2 more layers in between application and transport (which are most likely wrapped into the application layer). These are Presentation and Session.
Presentation is responsible for things like HTTP's Accept-Encoding: gzip, deflate. encryption and character encoding are said to be Presentation layer, so SSL/TLS falls under this category (the s in https).
HTTP Sessions is literally would be a literal example of the Session layer (anything that keep your application state alive across connections). If a protocol is connection oriented, it might not have a session layer, which is why HTTP is probably the only example I can think of at the moment.
As you can see HTTP was the answer to all 3 of these top layers, which is why they have all been combined into Application layer in newer versions of the model. Cisco still uses the 7 layer model, but Microsoft uses the 5 layer you're using.
Transport is TCP... it contains information about reordering packets and can adjust the amount of packets per window, allows the computer to know if anything was lost and ask for re-transmission, etc.
UDP is also an example of the transport layer, however its a lot simpler of a protocol, no re-transmission of packets are done on UDP.
Network is the IP protocol (also IPX/SPX from the old netware days, and ICMP (pings) and IGMP (routers)) this allows for addressing computers that rely outside the a collision domain (things separated by switches or routers).
Link or DataLink layer is ethernet, (or ATM, or FDDI) which addresses computers physically connected to a hub or directly with a network cable to each other.
This layer adds the MAC addressing in ethernet and the frame part of a packet is the header it uses.
Physical layer (in original OSI model) is just your cables and network equipment.
Pretty much the only people who still talk about OSI is network techs.
They may still say Layer 2 or Layer 3 switches, etc which is in reference to OSI.. Layer 2 is an ethernet switch, a layer 3 switch adds routing.
The best way to see how this is used is to load up wireshark and snoop your own network traffic. It will actually show you the parts of a packet that are responsible for most of the layers.
Knowing the OSI model is not really overly useful, but it can help you organize the stages of network communication in your mind and help you troubleshoot.
Knowing the protocols and how they interact is extremely useful, learn how tcp negotiates connections, IP addressing and subnet masking, HTTP and Ethernet can help you whether you are a developer or a server admin, or network admin or even a DBA. There is nothing like a bad network card to ruin your weekend or even a whole month if no one thinks to check it, and the only way to tell is use tcpdump, and wireshark to see errors in the Ethernet frames.
The OSI reference model
The OSI model is used to connect to the open systems—these are the systems that are open and communicate with other systems. By using this model, we do not depend on an operating system anymore, so we are allowed to communicate with any operating system on any computer. This model contains seven layers, where each layer has a specific function and defines the way data is handled on certain different layers. The seven layers that are contained in this model are the Physical layer, Data Link layer, Network layer, Transport layer, Session layer, Presentation layer, and the Application layer.
THE PHYSICAL LAYER
This is the first layer in the OSI model and contains a definition of the network's physical specification, including the physical media (cables and connectors) and basic devices (repeaters and hubs). The layer is responsible for the input raw bits transmission data stream into zeros and for the ones that are on the communication channel. It then places the data onto the physical media. It is concerned with data transmission integrity and makes sure that the bits that are sent from one device are exactly the same as the data that is received by the other device
THE DATA LINK LAYER
The main role of the Data Link layer is to provide a link for raw data transmission. Before the data is transmitted, it is broken up into data frames, and the Data Link layer transmits them consecutively. The receiver will send back an acknowledge frame for each frame that has been sent if the service is reliable.
This layer consists of two sublayers: Logical Link Control (LLC) and Media Access Control (MAC). The LLC sublayer is responsible for transmission error checking and deals with frame transmission, while the MAC sublayer defines how to retrieve data from the physical media or store data in the physical media.
We can also find the MAC address, also called as the physical address, in this layer. The MAC address is used to identify every device that connects to the network because it is unique for each device.
The MAC address contains twelve hexadecimal characters, where two digits are paired with each other. The first six digits represent the organizationally unique identifier and the remaining digits represent the manufacturer serial number. If you are really curious to know what this number means, you can go to www.macvendorlookup.com and fill the text box with our MAC address to know more about it.
THE NETWORK LAYER
The Network layer is responsible for defining the best way to route the packets from a source to the destination device. It will generate routing tables using Internet Protocol (IP) as the routing protocol, and the IP address is used to make sure that the data gets its route to the required destination. There are two versions of IP nowadays: IPv4 and IPv6. In IPv4, we use 32-bit addresses to address the protocol and we use 128-bit addresses in IPv6. You are going to learn more about Internet Protocol, IPv4, and IPv6 in the next topic.
THE TRANSPORT LAYER
The Transport layer is responsible for transferring data from a source to destination. It will split up the data into smaller parts, or in this case segments, and then will join all the segments to restore the data to its initial form in the destination.
There are two main protocols that work in this layer: the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).
TCP supplies the delivery of data by establishing a session. The data will not be transmitted until a session is established. TCP is also known as the connection-oriented protocol, which means that the session has to be established before transmitting the data.
UDP is a method of delivering data with the best efforts, but does not give a guaranteed delivery because it does not establish a session. Therefore, UDP is also known as the connection-less protocol. In-depth explanation about TCP and UDP can be found in the next topic.
THE SESSION LAYER
The Session layer is responsible for the establishment, maintenance, and termination of the session. We can analogize the session like a connection between two devices on the network. For example, if we want to send a file from a computer to another, this layer will establish the connection first before the file can be sent. This layer will then make sure that the connection is still up until the file is sent completely. Finally, this layer will terminate the connection if it is no longer needed. The connection we talk about is the session.
This layer also makes sure that the data from a different application is not interchanged. For example, if we run the Internet browser, chat application, and download manager at the same time, this layer will be responsible for establishing the session for every single application and ensure that they remain separated from other applications.
There are three communication methods that are used by this layer: the simplex, half-duplex, or full-duplex method.
In the simplex method, data can only be transferred by one party, so the other cannot transfer any data. This method is no longer common in use, since we need applications that can interact with each other.
In the half-duplex method, any data can be transferred to all the involved devices, but only one device can transfer the data in the time, after it completes the sending process. Then, the others can also send and transfer data.
The full-duplex method can transfer data to all the devices at the same time. To send and receive data, this method uses different paths.
THE PRESENTATION LAYER
The Presentation layer role is used to determine the data that has been sent, to translate the data into the appropriate format, and then to present it. For example, we send an MP3 file over the network and the file is split up into several segments. Then, using the header information on the segment, this layer will construct the file by translating the segments.
Moreover, this layer is responsible for data compression and decompression because all the data transmitted over the Internet is compressed to save the bandwidth. This layer is also responsible for data encryption and decryption in order to secure communication between two devices.
THE APPLICATION LAYER
The Application layer deals with the computer application that is used by a user. Only the application that connects to a network will connect to this layer. This layer contains several protocols that are needed by a user, which are as follows:
The Domain Name System (DNS): This protocol is the one that finds the hostname of an IP address. With this system, we do not need to memorize every IP address any longer, just the hostname. We can easily remember a word in the hostname instead of a bunch of numbers in the IP address.
The Hypertext Transfer Protocol (HTTP): This protocol is the one that transmits data over the Internet on web pages. We also have the HTTPS format that is used to send encrypted data for security issues.
The File Transfer Protocol (FTP): This protocol is the one that is used to transfer files from or to an FTP server.
The Trivial FTP (TFTP): This protocol is similar to FTP, which is used to send smaller files.
The Dynamic Host Configuration Protocol (DHCP): This protocol is a method that is used to assign the TCP/IP configuration dynamically.
The Post Office Protocol (POP3): This protocol is an electronic mail protocol used to get back e-mails from POP3 servers. The server is usually hosted by an Internet Service Provider (ISP).
The Simple Mail Transfer Protocol (SMTP): This protocol is in contrast with POP3 and is used to send electronic mails.
The Internet Message Access Protocol (IMAP): This protocol is used to receive e-mail messages. With this protocol, users can save their e-mail messages on their folder on a local computer.
The Simple Network Management Protocol (SNMP): This protocol is used to manage network devices (routers and switches) and detect problems to report them before they become significant.
The Server Message Block (SMB): This protocol is an FTP that is used on Microsoft networks primarily for file and printer sharing.
This layer also decides whether enough network resources are available for network access. For instance, if you want to surf the Internet using an Internet browser, the Application layer decides whether access to the Internet is available using HTTP.
We can divide all the seven layers into two section layers: the Upper Layer and Lower Layer. The upper layer is responsible for interacting with the user and is less concerned about the low-level details, whereas the lower layer is responsible for transferring data over the network, such as formatting and encoding.
There are FIVE LAYERS in TCP/IP Model and SEVEN LAYERS in OSI Reference Model.
The primary difference between our five-layer model and the seven-layer OSI model is that the OSI model abstracts the application layer into three layers total.
The physical layer represents the physical devices that interconnect computers. This includes the specifications for the networking cables and the connectors that join devices together along with specifications describing how signals are sent over these connections. The physical layer is all about cabling, connectors and sending signals.
The second layer in our model is known as the data link layer. Also called the network interface or the network access layer. The data link layer is responsible for defining a common way of interpreting the signals, so network devices can communicate. Lots of protocols exist at the data link layer, but the most common is known as Ethernet.
The third layer, the network layer is also sometimes called the Internet layer. It's this layer that allows different networks to communicate with each other through devices known as routers.
The most common protocol used at this layer is known as IP or Internet Protocol. IP is the heart of the Internet and most small networks around the world.
While the data link layer is responsible for getting data across a
single link, the network layer is responsible for getting data
delivered across a collection of networks.
While the network layer delivers data between two individual nodes, the transport layer sorts out which client and server programs are supposed to get that data. The transport layer Protocol is known as TCP or Transmission Control Protocol and UDP or User Datagram Protocol.
The big difference between the two is that TCP provides mechanisms to
ensure that data is reliably delivered while UDP does not.
The network layer, in our case IP, is responsible for getting data
from one node to another. The transport layer, mostly TCP and UDP, is
responsible for ensuring that data gets to the right applications
running on those nodes.
The fifth layer is known as the application layer. There are lots of different protocols at this layer, and as you might have guessed from the name, they are application-specific. Protocols used to allow you to browse the web or send receive emails are some common ones. Application Layer Protocols are HTTP, SMTP, etc.
You can think of layers like different aspects of a package being
delivered. The physical layer is the delivery truck and the
roads. The data link layer is how the delivery trucks get from
one intersection to the next over and over. The network layer
identifies which roads need to be taken to get from address A to
address B. The transport layer ensures that the delivery
driver knows how to knock on your door to tell you your package has
arrived. And the application layer is the contents of the
package itself.
I was wondering if it is layer 7 for websocket as the application is actually the browser.
Websocket depends on TCP (OSI#4) and only the handshake phase is initialized by HTTP (OSI#7) 1. Although it uses TCP port 80 only.
According to the runtime behavior, I have to say WebSocket should be a special OSI#7 protocol. Then we can put SSL/TLS into OSI#6 (see wikipedia), and the implementation inside browser into OSI#5.
It is better to understand the layer using TCP/IP model rather than OSI model. WebSocket layers on top of TCP, considered as transport layer in TCP/IP model, and one can layer application layer protocol on top of WebSocket.
HTTP, SSL, HTTPS, WebSockets, etc. are all application layer protocols.
But the OSI protocol stack doesn't apply to TCP/IP, which has its own layer model: same names, different functions. It isn't helpful to keep using the obsolete OSI stack as though it actually reflected any reality. It doesn't.
Only the Handshake is interpreted by https server by upgrade request. Apart from that Websocket is independent TCP-based protocol. So i would say host layer #4 and #7.
https://www.rfc-editor.org/rfc/rfc6455#page-11
L1 does not have a map where a cable is digged in the soil (how deep, where), nor in wich cable certain wires delivering information is flowing, or where is is layed in cable self, nor it dictates how cable is marked. L1 is only physical layer, not where and how the wires are layed. So L0 is needed.
L1: "The physical layer is responsible for the transmission and reception of unstructured raw data between a device and a physical transmission medium. It converts the digital bits into electrical, radio, or optical signals. Layer specifications define characteristics such as voltage levels, the timing of voltage changes, physical data rates, maximum transmission distances, modulation scheme, channel access method and physical connectors. This includes the layout of pins, voltages, line impedance, cable specifications, signal timing and frequency for wireless devices. Bit rate control is done at the physical layer and may define transmission mode as simplex, half duplex, and full duplex. The components of a physical layer can be described in terms of a network topology. Physical layer specifications are included in the specifications for the ubiquitous Bluetooth, Ethernet, and USB standards. An example of a less well-known physical layer specification would be for the CAN standard."
Here is the part of end-to-end principle from Wikipedia
End-to-end connectivity is a property of the Internet that allows all
nodes of the network to send packets to all other nodes of the
network, without requiring intermediate network elements to further
interpret them.
But you should notice that routers (intermediate device) always among end nodes. So, as end-to-end principle above, not only NAT, but normally network with break end-to-end principle too.
Does it true ? If not, please explain for me why, normally network with router doesn't break end-to-end principle.
Thanks :)
A router has a special job in that it must facilitate the end-to-end principle.
A router's job, basically, is to interpret the destinatinaton IP address and determine the next router (or end node) to send it to. A router usually does two things:
Decrement the TTL
Find the next hop based on the destination IP address
It should not, in general, further interpret the packet.
Now, a NAT breaks end-to-end because it further interprets and transforms the packet. NAT might, for example, change the source address of the packet. In that case, you have broken end-to-end because another internet host will not be able to identify the specific host that sent the packet. They'll only be able to identify the source address of the NAT.
Edit: RFC 1812 makes this clear in section 2.2.1, where it describes the layering responsibilities:
o Transport Layer
The Transport Layer provides end-to-end communication services.
...
TCP is a reliable connection-oriented transport service that
provides end-to-end reliability, resequencing, and flow control.
...
o Internet Layer
All Internet transport protocols use the Internet Protocol (IP) to
carry data from source host to destination host. IP is a
connectionless or datagram internetwork service, providing no
end-to-end delivery guarantees.
In section 2.2.3, it goes on to state that:
Routers provide datagram transport only, and they seek to minimize
the state information necessary to sustain this service in the
interest of routing flexibility and robustness.
A "normal" router doesn't deal at all with the "end-to-end" principle. Its job is simply to deliver packets at the Internet layer, which is by definition "providing no end-to-end delivery guarantees". That's the Transport Layer's job. NAT "breaks end-to-end" by operating at the Transport layer rather than simply the Internet layer, and keeping too much state about each connection.
Hope this makes sense. I know it can be confusing.
I went through the article in wikipedia and found that there is a condition to it that it is only applicable to application-specific jobs not for other supportive tasks like address translations or redirecting, NAT etc. which are performed by intermediate routers... here is the part from that article
The end-to-end principle states that application-specific functions ought to reside in the end hosts of a network rather than in intermediary nodes – provided they can be implemented "completely and correctly" in the end hosts.
So, the principle is preserved as intermediate routers are not handling the application-specific functions , it is the end routers(nodes) which performs these functions.
In Internet Model there are four layers: Link -> Networking -> Transport -> Application.
I don't really know the difference between networking layer and transport layer. As I read:
Transport layer: include congestion control, flow control, reliability ...
Networking layer: route data from A to B
So, base on above properties, I see that there are some overlaps between those two layers.
1) Networking layer decides to move data from A to B. But, when data has known how to moved from A to B, what does it means for term "flow control" "congestion control" ... ? How and What does it control when the packet (and byte stream is in packet) has already known to moved across network.
2) Or other example, TCP protocol in Transport layer is ORDERED delivery of a stream. But, TCP doesn't decide how to move data, but Networking layer. So, how can TCP can do ?
So, I cannot get in those two terms. Please teach me.
Thanks :)
These are levels of abstraction.
Transport Layer is where the decision to use TCP/UDP is made. Among commonly used protocols in this layer, TCP is reliable, UDP isn't. Depending upon the choice made, the respective headers are attached to your packet. TCP for example just knows about SYN-ACK, Three-way handshake mechanisms, but does not know the address of the remote-endpoint, or the mechanism of getting the packet across the network.
Congestion control, Flow control help ensure that the network isn't flooded with packets, by regulating the number of packets being sent.
Now, after TCP/UDP header being appended, it moves on to the Network Layer. Till this step, the remote end-point's IP address wasn't a part of the packet at all. It is at this step that the Source & Destination IP addresses are added to the packet. This layer actually knows the remote-endpoint.
Sender Receiver
----------- ------------
| | virtual link | |
| Transport | -----------------> | Transport |
| | | |
----------- ------------
| |
| |
----------- -----------
| | virtual link | |
| Network | ------------------> | Network |
| | | |
----------- -----------
| |
| |
----------- -----------
| | | |
| Physical | | Physical |
| | | |
----------- -----------
↓ ↑
|____________real link____________|
The sender's Transport Layer data, is the exact data received by the receiver's transport layer.
As the packet travels down the sender, each layer is adding its own header information, but all of that is removed by the corresponding layer on the receiver.
The advantage is that a virtual link is established, like the one shown above, whereas the real link is only in the physical layer.
Transport layer:
Logical communication between processes.
Network layer:
Logical communication between hosts.
Transport layer:
Responsible for checking that data available in session layer are error free.
Network layer:
Responsible for logical addressing and translating logical addresses (ex. amazon.com) into physical addresses (ex. 180.215.206.136)
Transport layer: Protocols used at this layer are :
TCP(Transmission Control Protocol)
UDP(User Datagram Protocol )
SCTP(Stream Control Transmission Protocol)
Network layer: Protocols used at this layer are :
IP(Internet Protocol)
ICMP(Internet Control Message Protocol)
IGMP(Internet Group Message Protocol)
RARP(Reverse Address Resolution Protocol)
ARP(Address Resolution Protocol)
Transport layer:
This layer ensures that the protocols operated at this layer provide reliable end-to-end flow and error control.
Network layer:
This layer controls routing of data from source to destination plus the building and dismantling data packets.
Transport: Determines how data is to be sent: Reliably or unreliably. Defines well known services (ports.)
Network: Provides logical addressing, finds best path to a destination.
Transport Layer:
The fourth and “middle” layer of the OSI Reference Model protocol stack is the transport layer. I consider the transport layer in some ways to be part of both the lower and upper “groups” of layers in the OSI model. It is more often associated with the lower layers, because it concerns itself with the transport of data, but its functions are also somewhat high-level, resulting in the layer having a fair bit in common with layers 5 through 7 as well.
Recall that layers 1, 2 and 3 are concerned with the actual packaging, addressing, routing and delivery of data; the physical layer handles the bits; the data link layer deals with local networks and the network layer handles routing between networks. The transport layer, in contrast, is sufficiently conceptual that it no longer concerns itself with these “nuts and bolts” matters. It relies on the lower layers to handle the process of moving data between devices.
The transport layer really acts as a “liaison” of sorts between the abstract world of applications at the higher layers, and the concrete functions of layers one to three. Due to this role, the transport layer’s overall job is to provide the necessary functions to enable communication between software application processes on different computers. This encompasses a number of different but related duties
Modern computers are multitasking, and at any given time may have many different software applications all trying to send and receive data. The transport layer is charged with providing a means by which these applications can all send and receive data using the same lower-layer protocol implementation. Thus, the transport layer is sometimes said to be responsible for end-to-end or host-to-host transport (in fact, the equivalent layer in the TCP/IP model is called the “host-to-host transport layer”).
Network Layer:
The third-lowest layer of the OSI Reference Model is the network layer. If the data link layer is the one that basically defines the boundaries of what is considered a network, the network layer is the one that defines how internetworks (interconnected networks) function. The network layer is the lowest one in the OSI model that is concerned with actually getting data from one computer to another even if it is on a remote network; in contrast, the data link layer only deals with devices that are local to each other.
While all of layers 2 through 6 in the OSI Reference Model serve to act as “fences” between the layers below them and the layers above them, the network layer is particularly important in this regard. It is at this layer that the transition really begins from the more abstract functions of the higher layers—which don't concern themselves as much with data delivery—into the specific tasks required to get data to its destination. The transport layer, which is related to the network layer in a number of ways, continues this “abstraction transition” as you go up the OSI protocol stack.
Network Layer Functions
Some of the specific jobs normally performed by the network layer include:
Logical Addressing: Every device that communicates over a network has associated with it a logical address, sometimes called a layer three address. For example, on the Internet, the Internet Protocol (IP) is the network layer protocol and every machine has an IP address. Note that addressing is done at the data link layer as well, but those addresses refer to local physical devices. In contrast, logical addresses are independent of particular hardware and must be unique across an entire internetwork.
Routing: Moving data across a series of interconnected networks is probably the defining function of the network layer. It is the job of the devices and software routines that function at the network layer to handle incoming packets from various sources, determine their final destination, and then figure out where they need to be sent to get them where they are supposed to go. I discuss routing in the OSI model more completely in this topic on the topic on indirect device connection, and show how it works by way of an OSI model analogy.
Datagram Encapsulation: The network layer normally encapsulates messages received from higher layers by placing them into datagrams (also called packets) with a network layer header.
Fragmentation and Reassembly: The network layer must send messages down to the data link layer for transmission. Some data link layer technologies have limits on the length of any message that can be sent. If the packet that the network layer wants to send is too large, the network layer must split the packet up, send each piece to the data link layer, and then have pieces reassembled once they arrive at the network layer on the destination machine. A good example is how this is done by the Internet Protocol.
Error Handling and Diagnostics: Special protocols are used at the network layer to allow devices that are logically connected, or that are trying to route traffic, to exchange information about the status of hosts on the network or the devices themselves.
Cthulhu's explanation is ok but in order to understand a bit better, I recommend that you should read on the OSI Model
Transport layer handles port numbers, TCP, UDP, layer 4 PDU's and it's the first step in encapsulating and segmenting data in order to send it across the network
PDU = protocol data unit , it's a piece of information containing a header, the data segment and maybe a footer (see layer 2 encapsulation)
Network handles ip routing and delivery of data packets across the network
Each layer (regardless if it's OSI model or TCP/IP model with 4 layers) , each layer interacts with it's adjacent layer and provides an abstract framework for today's telecom purposes
Regarding your questions :
1). Flow control is a TCP mechanism for handling the size of the packet in order to prevent packet loss and retransmision, congestion control is another thing. Network layer doesn't decide anything, it simply tries to send your package over a network, if it fails, it will notify the upper layer about this problem and then the application or user should decide what to do).
2). TCP enables a 3way handshake mechanism to start a session, afterwards each packet is market with a counter and the receiver acknowledges receipt of senders package. In case he doesn't acknowledge, TCP resends that lost package. Networking layer will only forward packets and won't make any decisions regarding traffic control or packet ordering.
Further details are available in CCNA1 documentation or on the web.
Considering the ISO/OSI reference model transport layer is 4th layer. It mainly deals with the end to end delivery of the packets. End to end means it is responsible for delivering the packet to the appropriate port.
Network layer on the other hand is 3rd layer and is responsible for delivering the packet only to the host not to any specific [port/process in the system.
People do have the doubt that when you have transport layer which can deliver the packet from one end to other than why do we have network layer?
The simple answer to this question is that network layer is responsible for carrying the packet from sender to destination. But after reaching the destination it depends on the transport layer to deliver it to appropriate port number or process(in operating system terminology). Moreover at network layer we IP protocol which is heart of internet. You can read more about this here.