Re: [asio-users] UDP - maximum receive size

[Green, C. <cli...@bo...>]

Don't confuse underlying networking packet fragmentation with IP datagrams. Read up on layered protocols if that causes any confusion.

Datagrams can't be arbitrarily large - for IPv4 it's 65,535 bytes. I found this by searching: "IPv4 datagram maximum size" and Google found an answer for me in under a second. For IPv6 it is the same, unless it is a jumbogram. Again, Google is my friend. The Wikipedia entries even have a nice paragraph on packet fragmentation.

BTW, I assume you know that UDP is an unreliable protocol. Datagrams can be, and will be, dropped at any time for any reason by the sending or receiving OS, or any equipment / software in between.

Cliff

From: Gheorghe Marinca [mailto:ghe...@gm...]
Sent: Wednesday, August 29, 2012 11:43 AM
To: asi...@li...
Subject: Re: [asio-users] UDP - maximum receive size

Thanks for replay Cliff. Looked over other frameworks/languages and it seems that receive generally gets the content of all UDP datagram and throws an exception if the client buffer is too small.
I find this quite strange considering that a UDP datagram can be arbitrary large - even if it will be fragmented to about 1500 bytes (or so) on most networks

On Wed, Aug 29, 2012 at 8:28 PM, Green, Cliff <cli...@bo...<mailto:cli...@bo...>> wrote:
This is not directly an Asio question, but a general TCP / UDP question. There are many references (books, online) that will help with these questions.

Datagrams are always received in one read, and Asio methods follow that same model. If the buffer / requested size is too small, the data will be truncated. This is different than TCP, which is a stream protocol.

Check out various non-Asio discussions about receive buffer size - I don't think Asio adds anything to the behavior.

Cliff


From: Gheorghe Marinca [mailto:ghe...@gm...<mailto:ghe...@gm...>]
Sent: Wednesday, August 29, 2012 9:33 AM
To: asi...@li...<mailto:asi...@li...>
Subject: [asio-users] UDP - maximum receive size

Hi,

I send requests to a name server for name resolution. It's not clear for me how can I tell when the receiving of data on the client socket has ended.
When I call "async_receive " on the socket I begin reading data from server. Will asio call my receive completed handler (along with data on my buffer) only when it has
detected that all the datagram has been received   (that is for *one call* to async_receive function) or is it possible to get that handler called multiple times as it happens with tcp (do I have to call async receive multiple times in this case?)

Also what is the effect on setting option receive buffer size on asio ? Any thoughts are appreciated.

Regards
-Ghita

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