If TCP is connection oriented why do packets follo

问题:

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.

回答1:

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

回答2:

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.

回答3:

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.

回答4:

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