I had a strange idea. I heard of software which from my understanding uses UDP to transfer files decreasing the overhead found in TCP packets.
If my app requires TCP and my LAN has software set up to communicate with another datacenter on the other side of the coast with software setup on their end. Would it be possible to send the actual data via UDP but than simulating TCP on both ends?
Does anyone have any ideas or information about such projects?
If you're asking if you can use UDP as a Layer 2, then the answer is yes, sort of. There are various protocols that allow you to create a tunnel to another network using a UDP transport, such as L2TP and even IPsec (with NAT traversal). You could also do it at the application layer.
If you're asking if TCP can be implemented in UDP, the answer is no. First, TCP packets and UDP packets have an incompatible format. Second, TCP and UDP have different protocol numbers (seen in the IP header) which means that TCP traffic destined for a UDP port would not be passed to the correct upper-layer protocol.
Both TCP and UDP are built on top of the IP, but the TCP uses different packet structure and at the layer-2 it is not possible to mimic the TCP using UDP packets.
Of course, if you have the control on both the source and destination, then it is possible to create a reliable UDP tunnel for the TCP packets. This would require some internal information (packet number, ack/nack flags) in the body of the UDP packet.
There is an interesting project http://udt.sourceforge.net/
It is a broadcast-capable reliable file transfer mechanism built on top the UDP.
PseudoTCP is a protocol which implements TCP algorithms on top of the UDP. It was introduced since the NAT traversal for TCP is much more complicated than UDP. But some P2P applications do need a reliable data transfer among nodes.
So far as I know, there are two PseudoTCP variations: Libjingle and Libnice.Libjingle is an open source library from google which was initially for gtalk. You could take a look at file sharing example from libjingle: https://developers.google.com/talk/libjingle/file_share. Recently, Chrome desktop also use PseudoTCP implementation from libjingle for reliable connections.
Yes, you can develop a protocol on UDP that simulates TCP. However, if you simulated TCP fully, it would technically have more overhead. Because TCP is implement as the packet and your simulated TCP is implemented in the body of the packet.
If you only need one or two features of TCP (such as basic ordering), then implementing it in UDP is useful.
Halo uses 2-3 (IIRC) UDP protocols that simulate different features of TCP, then full fledged TCP for initializing game-states. I Shot You First Networking, GDC publication
For example, in one case, they send 3 duplicate UDP packets to overcome packet loss.
If you control the software on both ends, and it is cost-effective to build your own protocol, then UDP can be versatile.
One way to do it now on Linux-3.18+ is to use Foo over UDP (FOU) which implements Generic UDP Encapsulation (GUE). Here's a good introduction to FOU, and the man page for ip-fou.
Or if you want an [open source] UDP based file transfer system there are things like UDT, UFTP, Tsunami-UDP, and even Google's QUIC (Now deprecated in favour of IETF QUIC).
Update: The QUIC protocol now has been standardised by the IETF which provides for secure reliable and unreliable transport over UDP as an alternative to TCP. There's a wide range of QUIC implementations available. There is also a growing set of protocol mappings on to QUIC such as HTTP/3, DNS over QUIC, etc
If my app requires TCP and my LAN has software setup to communicate
with another datacenter on the other side of the coast with software
setup on their end. Would it be possible to send the actual data via
UDP but than simulating TCP on both ends?
No. A UDP socket is in a different namespace from a TCP socket. You will be unable to write UDP at one end and send or receive TCP at the other end. TCP and UDP are peer protocols; both exist at the layer above IP. You can't use one to spoof the other.
Hmm, I believe so. You'd need to use a proxy at both ends, but it should be possible.
The biggest problem you are going to run into is that UDP is designed with the idea that you don't care if some of the packets don't ever make it to the other end.
Here's a link with some more info:
http://www.cyberciti.biz/faq/key-differences-between-tcp-and-udp-protocols/
IMHO, it's not a good idea to transmit files via UDP.
TCP's problems are in its algorithms, not its headers.
You certainly could implement the TCP algorithms on top of UDP. That would effectively be the same as tunneling TCP datagrams inside of UDP datagrams. But all this accomplishes is to add a few more bytes of overhead to each packet, and require another endpoint to unwrap the packets.
UDP itself is just thin shim on top of IP: its a convenient way to access IP packet switched networking without having to dive into kernels or receive special handling from routers. The main reason to implement reliable transport on top of UDP is to get away from TCP algorithms in favor of something more efficient. FileCatalyst was mentioned above as one company which does this, and my own company Data Expedition, Inc. does so as well.
So you could implement TCP algorithms on top of UDP, but you wouldn't want to.
You can simulate something like a connection over UDP, and you as well can add reliability checks and ordering and retransmission and so on. - but then, it still isn't TCP, it just acts the way.
Of course, one of the ends can be a kind of "hub" or "proxy" which does an adaption. Then you don't have a 2-end solution, but in fact a 4 end solution - one pair with "real" TCP and the other with the "self-knitted" "TCP" - which you put together with an appropriately crafted program.
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'm currently developping a highly responsive game in python using client-server model. The speed of the data exchanges between the client and the server needs to be very fast.
Is it possible to make both, udp and tcp connections ? Udp would be used only during the game. And Tcp will be used for more reliability-needed messages like connection, name changes, chat,...
Is it also a good way of thinking ? Or should i use only UDP ?
Yes, this can be a good idea. With UDP, a single lost packet does not stall the entire stream. On the other hand you need retry and congestion control.
I'd try to send messages using UDP and if no confirmation arrives within a short amount of time re-send them on a TCP connection that has been kept open. That should move 99% of the load to UDP and use TCP for congestion control and reliability.
The H2O database does it that way.
You can bind your UDP and TCP connections on different ports or even on the same port. As for which to use, it is up to you. Try both out and if TCP is too slow or UDP is too unreliable then you always have the option to switch.
You can also use a 3rd-party library that builds reliability layer over UDP and specify required reliability per packet. As an example you can check Raknet.
i have a question about the trasport layer of the iso/osi network stack.
It's common to think that TCP is reliable but slow and UDP is fast but not reliable.
In several network applications, it is mandatory to implement an application-level ACK system also with TCP transport protocol in order to manage link interruptions and so on.
Assuming that the application scenario is this: local (small) network, typical small packet size (max 512 bytes), application-level ack system implementation. Why not use the protocol UDP as transport protocol in order to exploit its features such as speed, low latency and broadcast communication?
Thanks to all.
If you don't care about the Delivery (failure or successful delivery) of your message than UDP will be always a best choice.. But if you want to be sure whether the message is delivered or failed than think about other protocols rather than UDP.
In a small LAN it shouldnt be a big problem to use UDP, when the transferd Data isnt too "important" (which means if a packet is lost its no problem). More informations about the scenario are nessesary, if you use UDP you have to implement functions that check if you received all the information of the sender manually.
if you need the speed and if some lost packets are no problem (Voice Chat for instance) then use UDP.
I was told that for a High Frequency Trading (HFT) system that requires low-latency, TCP is used over UDP. I was told that with TCP you can make point to point connections, whereas you cannot with UDP, however from my understanding you can send UDP packets to specific IP/port.
There are several arguments used in this article as to why UDP > TCP for gaming but I can see relevance for HFT.
Why would TCP be a better protocol to use for HFT?
(Admins: My previous post of this question was silently removed with no explanation. If I am violating terms of use please alert me of this instead of silently removing the question)
UDP is superior to TCP if you don't need some of the features TCP provides. Every feature has a cost, and so if you don't need features, you are paying that cost for no reason.
In an HFT application, you need pretty much every feature TCP requires. So if you picked UDP, you'd have to implement those features yourself. That means you'd have to implement connection establishment, connection teardown, retransmissions, transmit pacing, windows, and so on.
If there was a way to do all those things that was better than the way TCP was doing it, TCP would be doing it that way. You'd have one hand tied behind your back because TCP is heavily optimized by some of the best minds on the planet and implemented in/with the kernel.
There's no reasons to expect a stream of data over an already-established TCP connection would be slower than the same data over UDP, plus you get checksumming, retries, and all the other TCP goodness. UDP mainly wins in cases where you can afford to discard the reliability or where the overhead of many TCP handshakes would be too expensive, such as with common DNS queries.
TCP is faster for when using a few connections, the important difference is that modern NICs perform significant amounts of acceleration on TCP and not really that much for UDP. This means there is more overhead to process each UDP packet and as such they cannot compete unless you need to send to multiple recipients simultaneously.
However the UDP multicast route still suffers the same problems as unicast UDP per datagram overheads. Therefore many HFT systems use hardware accelerated systems that can multiplex the streams across many NICs via TCP, example Solace.
These days though you want to completely bypass the kernel with say a userspace IP stack such as by Solarflare or Mellanox, or even skip both the kernel and IP stack with RDMA.
Quite simply, if you need connection reliability (ensuring that every byte of data transmitted is received), you should be using TCP regardless.
As you mentioned, UDP is more suitable for games, where 100% accurate real-time tracking of every object would use quite a large amount of bandwidth and is unnecessary (this is where slow connections encounter lag).
There is no special difference between a TCP port and a UDP port, beyond the type of connection being used (send the packet and forget it, UDP style, or negotiate a connection and sustain it, TCP style) and the service listening on the server side. e.g. TCP/25 would usually reveal a SMTP server, whereas UDP/25 would not.
Basically, modern TCP implementations are going to be just as fast as UDP, if you're keeping the connection alive. If TCP is having to resend a packet, you'd need to resend it in UDP too. Plus for UDP you're going to end up implementing the same reliability code (retransmission of dropped packets) that TCP has already implemented.
Are there any networking protocols that are not strictly TCP or UDP but can be used with either one?
For example, HTTP, FTP, STMP, RTMP are always TCP.
DNS, SNMP, DHCP, RIP are always UDP.
Is there anything that can be either TCP or UDP? Or am I wrong in the above assertions?
RTSP is one weird one I know of that uses both, TCP for the control port but UDP for audio/video/quality, but it has strict requirements of what gets sent of each.
I'm asking about standard, published, or at least commonly used protocols, not custom ones.
DNS can use either UDP or TCP; TCP is required when the response data exceeds 512 bytes.
If you examine a Windows' services file you will see a number of protocols registered for both TCP and UDP. Path: C:\Windows\System32\drivers\etc In fact, most of the listings in the services file use both TCP and UDP protocols.
As far as well known apps that use both, I would think that most chat applications use both. sms-chat definitely does but probably most others.
Edit:
From that file, here's a few of the protocols that can be sent over either TCP or UDP (there are exactly 100 listed protocols that use both in the file, many internal MS protocols):
echo
discard
daytime
qotd (Quote of the day)
chargen (Character generator)
time
SIP can use UDP, TCP or SCTP. Using a reliable transport becomes important in SIP if your messages get to be at all large (i.e., significantly larger than the smallest MTU in between user agents). A good example is shared- or bridged-line appearances, which use a form of presence with XML bodies. The larger the number of SIP clients in the shared-line group, the larger the packets are likely to be, making fragmentation and retransmission an issue.
SIP can be either UDP or TCP. However, the reality is that UDP is mostly used for this protocol.
SNMP almost always runs over UDP, but it can and does run over TCP. Theory says that it's a bad idea to do SNMP over an error-correcting transport because because some of the very errors that SNMP intends to detect are masked.