While reading the book on TCP/IP I came across the words which are as "Although it looks as though the use of the flow label may make the source and destination addresses useless, the parts of the Internet that use connection-less service at the network layer still keep these addresses for several reasons.One reason is that part of the packet path may still be using the connection-less service. Another reason is that the protocol at the network layer is designed with these addresses and it may take a while before they can be changed". Now my question to you is if a connection has been formed between hosts in a connection-oriented manner then how come a path of a packet may still be using the connection-less services. Because as per my knowledge prevails the virtual path always be formed at while 3-way handshake is taking place which is the TCP/IP connection (which uses a connection-oriented service) ? And my second question for the second reason is that which protocol they are talking about since these words are stated below the Heading of "Connection-Oriented Services" therefore, it's making me pissed off to understand the literal meaning behind the words(The core conceptual understanding). And correct if anyone thinks I am having a wrong concept at any place. I'll be obliged. Thanks.
TCP as a connection-oriented protocol runs on top of IP which is connection-less. The routers used in transport only look at the IP packet, the TCP segment is simply payload and transported along. TCP provides several algorithms to form a virtual connection over a connection-less network.
The IP packet goes from hop to hop. On each hop, a router makes a forwarding decision solely based on the destination IP address. (More sophisticated devices may inspect more packet elements including source address and payload, but they aren't simple routers.)
The "path" is made up of all these individual hops. Because each hop is based on an independent routing decision the path can change at any time and for any packet. The path is not laid out by the TCP handshake.
Basically, you have to look at each protocol layer individually. Each one serves its own function.
I hope this also answers the second question.
Related
As I understand it TCP is required for congestion control and error recovery or reliable delivery of information from one node to another and its not the fastest of protocols for delivering information.
Some routing protocols such as EIGRP and OSPF ride directly on top of IP. Even ICMP rides directly over IP.
Why is UDP even required at all? Is it only required so that developers/programmers can identify what application the inbound packet should be sent to based on the destination port number contained within the packet?
If that is the case then how is information gathered from protocols that ride directly on top of IP sent to the appropriate process when there is no port number information present?
Why are voice and video sent over UDP? Why not directly over IP?
(Note that I do understand thoroughly the use case for TCP. I am not asking why use UDP over TCP or vice versa. I am asking why use UDP at all and how can some protocols use directly the IP layer. Whats the added advantage or purpose of UDP over IP?)
Your question makes more sense in terms of why is UDP useful (than why is UDP required).
UDP is a recognized protocol by the Internet Assigned Numbers Authority. UDP can be useful if you want to write a network protocol that's datagram based and you want to play more nicely with Internet devices.
Routers can have rules to do things like drop any packet that doesn't make sense. So if you try and send packets using say an unassigned IP protocol number between hosts separated by one or more routers, the packets may well never get delivered as you've intended. The same could happen with packets from an unrecognized UDP protocol but that's at least one less door to worry about whether your packet can make it through.
Internet endpoints (like hosts) may do similar filtering too. If you want to write your own datagram based protocol and use a typical host operating system, you're more likely to need to write your software as a privileged process if not as a kernel extension if you're trying to ride it as its own IP protocol (than if you'll be using UDP).
Hope this answer is useful!
First of all, IP and UDP are protocols on the different layers, IP by definition is Internet layer when UDP is transport layer. Layers were introduced to simplify network protocols architecture and to separate concerns. Application layer protocols are supposed to be based on transport layer (with some exceptions).
Most popular transport protocols (in IP network) are UDP and TCP. While TCP is feature rich but with many tradeoffs UDP is very simple but gives a lot of freedom and so typically is a base for other protocols.
The main feature of UDP is multiplexing: ports that allow multiple protocol instances (aka sockets) to coexist on the same node. This means that implementing your own protocol over IP instead of UDP either you won't be able to have multiple instances of your protocol on the same machine or you'll have to implement multiplexing yourself.
There're other features like segmentation and checksum. These features are not mandatory.
And as was mentioned in another answer there're lots of middleware like routers, NATs and firewalls that can ruin the idea of a custom "right over IP" protocol, but it's more like a collateral damage than a feature of UDP.
I know, how to write a C# application that works through a local network.
I mean I know, how to make my client-side application access my server-side application in a single local network.
But I wonder: How do such apps, as Skype, TeamViewer, and many other connect via global network?
I apologise, if this question is simple or obvious, but I couldn't find any information about this stuff.
Please, help me, I'll be very grateful. Any information is accepted - articles, plain info, books,and so on...
Question is very wide and I try to do short overview.
Following major difference between LAN (Local Area Network) and WAN (Wide Area Network):
Network quality:
LAN is more or less stable, WAN can be with network issues like:
Packet loss (you need use loss-tolerant transport like TCP or UDP with retransmits or packet loss concealment)
Packet jitter (interpacket intervals may differ a lot from sending part). Most common thing is packets bursts.
Packet reordering
Packet duplication
Network connectivity
WAN is less stable than LAN. So you need properly handle all things like:
Connection stale
Connection loss
Errors in the middle of the connection (if you use UDP for example)
Addresses:
In WAN you deal with different network equipment between client and server (or peers in case of peer-to-peer communication). You need to take in account:
NATs - most of the clients are behind NAT and you need to pass them through. According technics are called "NAT traversal"
Firewalls - may ISP has own rules what client can do or can't. So if you do something specific like custom transport protocol you may bump into ISP firewalls.
Routing - especially multicast and broadcast communication. In common case multicast is not possible to route. Broadcasts are never routed. So you need to avail this type of communication if you want to use WAN.
May be I forgot something. But these points are major. You can read many articles about any of them.
I've seen while searching that it is possible to use two different programs on the same computer communicating over the network using the same port and same network interface provided one use UDP and the other TCP. However I didn't get a good explanation, how does it actually work and why this is possible?
Is it also possible for multiple programs to use the same UDP port since UDP does not establish a real connection between the peers, but just sends the packets to an address? I understand it's not possible with TCP as it creates a synchronized connection between the server and the client, but what about UDP?
Please explain in details if possible, or link a good article on the topic.
The other answers are correct but somewhat incomplete.
An IP (aka "INET") socket "connection" (i.e. communication between two processes, possibly on different machines) is defined by a 5-tuple: protocol, source address, source port, destination address, destination port. You can see that this is not limited to a stateful connection such as TCP.
This means that you can bind different processes to any unique instance of that 5-tuple. Because the "protocol" (e.g. TCP and UDP) is part of the differentiating factor, each can have a different process.
Theoretically, you could bind different services to the same TCP port if they bind to different interfaces (network cards, loopback, etc.) though I've never tried it.
It is standard practice, however, to always use the same service on the same port number. If both UDP and TCP are supported, they're just different ways of communicating with that same service. DNS, for example, uses UDP on port 53 for lookup because they are small requests and it's faster than creating a TCP connection but DNS also uses TCP on port 53 for "transfers" which are infrequent and can have large amounts of data.
Lastly, in complete accuracy, it isn't necessarily a 5-tuple. IP uses the "protocol" to pass to the next layer such as TCP and UDP though there are others. TCP and UDP each seperately differentiate connections based on the remaining 4 items. It's possible to create other protocols on top of IP that use completely different (perhaps port-less) differentiation mechanisms.
And then there are different socket "domains", such as the "unix" socket domain, which is completely distinct from "inet" and uses the filesystem for addressing.
The destination isn't identified by IP Addr:Port alone. There is another thing - IP header has a field called Protocol which differentiates the TCP and UDP endpoint. As such it becomes possible for two process to bind to same IP:Port as long as communication protocol is different.
The endpoint of a connection is for UDP and TCP defined by IP, protocol (TCP or UDP) and port. This means as long as you use a different protocol the endpoint of the communication is different too.
Because they are not the only component of the means of address. It's the same as why you can have two houses with the same number on different streets, or why you know John Whorfin is not the same Red Lectroid as John Bigbooté.
Each IP packet contains a field that says which transport-layer protocol is to be used, and within the domain of that protocol is a set of ports that can be the same as in any other protocol because they are actually a completely separate set.
As for the second question, there are answers elsewhere.
According to my knowledge if an internet application has to be designed, we should use either a connection-oriented service or connection-less service, but not both.
Internet's connection oriented service is TCP and connection-less service is UDP, and both resides in the transport layer of Internet Protocol stack.
Internet's only network layer is IP, which is a connection-less service. So it means whatever application we design it eventually uses IP to transmit the packets.
Connection-oriented services use the same path to transmit all the packets, and connection-less does not.
Therefore my problem is
if a connection oriented application has been designed, it should transmit the packets using the same path. But IP breaks that rule by using different routes.So how do both TCP and IP work together in this sense? It totally confuses me.
You, my friend, are confusing the functionality of two different layers.
TCP is connection oriented in the sense that there's a connection establishment, between the two ends where they may negotiate different things like congestion-control mechanism among other things.
The transport layer protocols' general purpose is to provide process-to-process delivery meaning that it doesn't know anything about routes; how your packets reach the end system is beyond their scope, they're only concerned with how packets are being transmitted between the two end PROCESSES.
IP, on the other hand, the Network layer protocol for the Internet, is concerned with data-delivery between end-systems yet it's connection-less, it maintains no connection so each packet is handled independently of the other packets.
Leaving your system, each router will choose the path that it sees fit for EACH packet, and this path may change depending on availability/congestion.
How does that answer your question?
TCP will make sure packets reach the other process, it won't care HOW they got there.
IP, on the other hand, will not care if they reach the other end at all, it'll simply forward each different packet according to what it sees most fit for a particular packet.
Note:
Let's assume that IP was connection-oriented, would that mean packets would follow the same-path?
Not necessarily, it depends on what the word 'connection' at this layer means, if it means negotiating certain options related to security, for instance, you may still have all your packets being forwarded through different routes over the Internet.
EDIT:
Not to confuse you though, most connection-oriented services at the network-layer and below mean that the connection, when established, also establishes a virtual-path that all 'packets' must follow, for further information read about:
Virtual circuit and frame-relay networks
This link answers your question pretty well http://www.tcpipguide.com/free/t_ConnectionOrientedandConnectionlessProtocols-3.htm
Some people consider this (TCP) to be like a “simulation” of circuit-switching at higher network layers; this is perhaps a bit of a dubious analogy. Even though a TCP connection can be used to send data back and forth between devices, all that data is indeed still being sent as packets; there is no real circuit between the devices. This means that TCP must deal with all the potential pitfalls of packet-switched communication, such as the potential for data loss or receipt of data pieces in the incorrect order.
The TCP protocol deals with the problem of IP packets arriving out of order or being lost, to give you the feeling they arrive through a single FIFO channel. Yes, TCP is smart enough to do that, there's no need for a dedicated underlying channel.
The TCP protocal is implemented by the sending/receiving machines, once the packets leave the sending machine, the routers they travel along know nothing about TCP, they just use IP to get the packets from the source the to destination. Then, it is the destination machines job to, using TCP, make sure that all the packets arrive and that they arrive in the correct order. The internet itself doesn't know anything about TCP, it's just a layer (often software) that gives connection to a connectionless medium (the internet).
So onces a packet leaves a destination, it can go along any path (mostly) as long as it gets to the desintation, regardless of the higher level protocol (such as TCP or UDP).
I mean, it's a bit more complicated then that, but as far as I can remember that's the general Idea.
Refer my short points properly,
1) Connection oriented means ==> reserving resources(buffer,cpu,bandwidth etc.)..but "Where??".(where resources are reserved?? This where is reason of your confusion, so following is ans.).
2) Connection oriented at Transport Layer means ==> Reserving the resources at Both End processes/Ports.(Since TCP is a transport layer,then its responsibility is to reserve the resources at both end processes only,irrespective of whats happening in the intermediate path.)
3) Connection oriented at Network Layer means ==> Reserving the resources at Network Layers.(Now In the whole journey of a packet from source to destination, Network layer is found at all intermediate routers too(but not transport layer). Hence if any protocol at Network layer is connection oriented then,its responsibility is to reserve resources at all intermediate routeres too i.e. all packets will have to follow same intermediate path, But IP is connection less hence,intermediate resources will not be reserved. i.e journey of a packets may follow different paths etc.)
#CONCLUSION:==> Intermediate path is decided by Network Layer, hence if IP then paths may be different.(IP may contain TCP),But TCP is responsible for Resource reservation at both End processes ,irrespective of Intermediate path of packet.
router works on three layers only (physical , data link and Network layers) , so routers will take decision depending only on the info. of network layer (IP protocol ) hence there is no information available about its TCP or UDP at the router
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.