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Is it possible to convert all outgoing udp traffic from a gateway router to tcp on the same port with iptables. Have looked at mangle but unsure how I can use that.
This is not possible in a generic way for at least the following reasons:
UDP is a datagram protocol, where each packet is independent from the other. TCP instead is a stream protocol. While it would be possible to concatenate the UDP packets to a TCP stream it is not clear at which boundaries the TCP stream should be split to generate the packets.
With UDP duplicates, packet loss and reordering of packets could happen. If you cannot be sure about the proper order and that you got all messages and exactly once you can not construct reliable a TCP stream from it.
TCP acknowledges when it receives packets and if an ACK is missing the packet gets retransmitted. Once you forward TCP to UDP these ACKs can not be done because UDP does not have such mechanism.
It might still be possible to do application specific translations, like UDP DNS to TCP DNS or similar. But this translation depends on the application protocol so you need to have an application specific gateway. iptables as a packet filter works on a lower layer and does not provide such functioniality.
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Before a browser can request for a webpage, A TCP connection is required to be established. What is the necessity of 3 way handshake in interacting with a server computer? why can't we simply send a web request and wait for the response?
Shouldn't the resolution of IP address be enough for this purpose?
Basically, I need to know the reason for establishing TCP connection.
thanks in advance
You are using a device named A and server is named B
Host A sends a TCP SYNchronize packet to Host B
Host B receives A's SYN
Host B sends a SYNchronize-ACKnowledgement
Host A receives B's SYN-ACK
Host A sends ACKnowledge
Host B receives ACK.
TCP socket connection is ESTABLISHED.
See more at: http://www.inetdaemon.com/tutorials/internet/tcp/3-way_handshake.shtml#sthash.F2f4b8Xn.dpuf
Because you need a TCP connection to send HTTP over, and TCP has a 3-way handshake.
Basically, I need to know the reason for establishing TCP connection.
Because HTTP runs over TCP. It doesn't exist in a vacuum.
TCP provides ordering, automatic retransmission, and congestion control. I'd say these are the obvious reasons why the design adopted TCP.
In contrast, e.g. UDP is fast. There is no handshaking. But UDP packets are not ordered, also packets can get lost (no automatic retransmission), and there is no congestion control.
You can try implementing your data transferring for things like HTML in UDP. It's not easy, you still need to reinvent ordering and retransmission for reliable lossless delivery.
If you don't care about lossy or a bit out-of-order transferring then you probably don't need TCP. (e.g. real time video)
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On the other hand, avoid TCP to get better performance is not necessarily a bad idea. Read about QUIC. (It also has features like loss recovery and congestion control, you should not expect it to be extremely lightweight.)
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Apparently there are two ways a client can access a port in the client machine. I've only found one so far.
The question:
When a client wishes to send a message to a server using TCP, it must establish a connection to a specific port and IP address. It must use a socket and a port on the client side to transport the data. Discuss the two ways that a client might get access to a port in the client machine.
I've read wikipedia and some other sites, and it looks like there is only one way TCP connects to a port? Am I not understanding this question?
Three 'ways a client might get access to a port in the client machine':
Specify a specific port and use the bind() system call.
Specify port zero and call bind(). The system will allocate a client port.
Don't call bind() at all. The system will again allocate a client port on connect().
Don't ask me which two your instructor wants, or whether he wants something else completely, but that's how I would answer the question. Unless there is more to it than this, it is very poorly posed indeed.
There are 2 types of network communication on TCP/IP protocol from a client to a server (or another client)
TCP protocol
UDP protocol
The main difference between those 2 are; TCP protocol works the way just like you described: The client should try to open a connection on a specific port to an IP address and the remote side should accept the connection properly in order to start sending and recieveing the data through the socket. But the UDP protocol does not need to open (establish) a connection from point to point. It allows you yo send any data at any time without any condition other than a valid IP address and port number.
Note that when talking about UDP protocol, unlike the TCP, you will have no guarantee that your data (actually called "packet") has been successfully sent or not to the remote address at all. You will never know that. And it's only recommended for small amount of data packages. The bigger the packet means the less change of proper transmission.
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I would like to create a new protocol, i.e having features of TCP and UDP. can you tell me what are tips and techniques are required if it is possible.
Thanks in advance
TCP gives you three features that UDP does not: (a) estimating sending rate, (b) retransmission, and (c) flow-control. In doing so, the tradefoff is that TCP becomes slower compared to UDP. So, if your application is delay sensitive, which is typically true for audio/video applications, then you need to start with UDP and keep whichever of the above three you want. Typically, UDP applications might add forward-error-correction or application layer packet-book-keeping to ensure retransmission.
There is yet another advantage that UDP offers which TCP does not: if you have an application that might use mulitcast. For such cases, UDP would be the right chioce since UDP can handle point to multipoint. Using TCP for multicast applications would be hard since now the sender would have to keep track of retransmissions/sending rate for multiple receivers.
So, in summary, UDP will offer you two features that TCP cannot: lower-delay and ability to do multicast. So, this way, we can actually reduce the scope of the question and ask what are the features of TCP that one would like to add to UDP since there is no way, one can add features of UDP to TCP.
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I know that when a HTTP request is made, packets are sent from a seemingly-random high-numbered port (e.g. 4575) on the client to port 80 on the server. Then the server sends the reply to the same high-numbered port, the router knows to route that to the client computer, and all is complete.
My question is: How is the return port (4575 in this example) determined? Is it random? If so, within what range? Are there any constraints on it? What happens, for example, if two computers in a LAN send HTTP requests with the same source port to the same website? How does the router know which one to route to which computer? Or maybe this situation is rare enough that no-one bothered to defend against it?
The NAT is going to decide/determine the outbound port for a NATed connection/session, via it's own internal means. Meaning, it will vary according to the implementation of the NAT. This means any responses back will come back to that same outbound port.
As for your question:
What happens, for example, if two computers in a LAN send HTTP
requests with the same source port to the same website?
It will assign different outbound ports for each. Thus, it can distinguish between the two in responses it receives. A NATs would create/maintain a mapping of translated ports, creating new outbound port numbers for new sessions. So even if if there were two different "internal" sessions, from two different machines, on the same port number, it would map to two different port numbers on the outgoing side. Thus, when packets came back in on the respective ports, it would know how to translate them back to the correct address/port on the inside LAN.
Diagram:
It depends on the NAT and on the protocol. For instance I'm writing this message behind a full cone NAT and this particular NAT is configured (potentially hard-wired) to always map an UDP private transport address UDP X:x to the public transport address UDP Y:x. It's quite easy to shed some light on this case with with a STUN server (google has some free stun servers), a cheap NAT, 2 laptops, wire shark and a really really light STUN client which uses a hard coded port like 777. Only the first call will get through and it will be mapped on the original port, the second one will be blocked.
NAT's are a hack, some of them are so bad that they actually override on return the public transport address not only in the header but even in the transported data which is kinda crazy.
ICE protocols has to xor the public address to bypass this issue.
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I am trying to understand TCP congestion avoidance mechanism, but I don't understand one thing:
TCP congestion avoidance is per flow or per link?
In other words: there are 2 routers A and B
A is sending B two TCP flows - when one TCP flow detects congestion, does it decrease window size in the other flow as well?
of course if this happens, the other flow will detect congestion in some time, but does the second flow "waits" until it detects congestion on its own? that would be quite uneffective...
thanks a lot
It decreases the window size for the current connection. Each connection's RTT and windows are maintained independently.
Routers operate on layer 3 (IP) and are not aware of layer 4 (TCP), because of this, routers do not take any part in TCP congestion avoidance mechanism. This mechanism is fully implemented by TCP endpoints. It is triggered by routers dropping IP packets, but (classic) routers are not aware what higher level protocol IP packets carry.
The fact that one flow does not affect the other is quite desirable from the security perspective. With NAT you can have many hosts sharing the same IP address. From the outside world all these hosts look as a single machine. So, if some server reduced throughput of all TCP connections coming from a single IP address in response to packets dropped within one of those connections that would open a door to quite nasty DoS attacks.
Another issue is that some routers may be configured to drop packets based on IP ToS field. For example, latency sensitive SSH traffic may set different ToS than bulk FTP download. If router is configured to take into account ToS field, it may drop packets belonging to FTP connection, which should trigger congestion avoidance, but is should not affect packets belonging to SSH connection, which may be handled with higher priority.