Published on Jan 03, 2023
When designing systems for load-balancing, process migration, or fail-over, there is eventually the point where one would like to be able to "move" a socket from one machine to another one, without losing the connection on that socket, similar to file descriptor passing on a single host. Such a move operation usually involves at least three elements:
1. Moving any application space state related to the connection to the new owner. E.g. in the case of a Web server serving large static files, the application state could simply be the file name and the current position in the file.
2. Making sure that packets belonging to the connection are sent to the new owner of the socket. Normally this also means that the previous owner should no longer receive them.
3. Last but not least, creating compatible network state in the kernel of the new connection such that it can resume the communication where the previous owner left off.
We shall call the host from which ownership of the connection endpoint is taken the origin, the host to which it is transferred the destination, and the host on the other end of the connection (which does not change) the peer. Details of moving the application state are beyond the scope of this paper, and we will only sketch relatively simple examples. Similarly, we will mention a few ways for how the redirection in the network can be accomplished, but without going into too much detail. The complexity of the kernel state of a network connection, and the difficulty of moving this state from one host to another, varies greatly with the transport protocol being used.
Among the two major transport protocols of the Internet, UDP  and TCP , the latter clearly presents more of a challenge in this regard. Nevertheless, some issues also apply to UDP. tcpcp (TCP Connection Passing) is a proof of concept implementation of a mechanism that allows applications to transport the kernel state of a TCP endpoint from one host to another, while the connection is established, and without requiring the peer to cooperate in any way.
tcpcp is not a complete process migration or load-balancing solution, but rather a building block that can be integrated into such systems. tcpcp consists of a kernel patch (at the time of writing for version 2.6.4 of the Linux kernel) that implements the operations for dumping and restoring the TCP connection endpoint, a library with wrapper functions (see Section 3), and a few applications for debugging and demonstration.
The remainder of this paper is organized as follows: this section continues with a description of the context in which connection passing exists. Section 2 explains the connection passing operation in detail. Section 3 introduces the APIs tcpcp provides. The information that defines a TCP connection and its state is described in Section 4. Sections 5 and 6 discuss congestion control and the limitations TCP imposes on check pointing. Security implications of the availability and use of tcpcp are examined in Section 7. We conclude with an outlook on future direction the work on tcpcp will take in Section 8, and the conclusions in Section 9. The excellent "TCP/IP Illustrated"  is recommended for readers who wish to refresh their memory of TCP/IP concepts and terminology.
The key feature of tcpcp is that the peer can be left completely unaware that the connection is passed from one host to another. In detail, this means:
" The peer's networking stack can be used "as is," without modification and without requiring non-standard functionality
" The connection is not interrupted
" The peer does not have to stop sending
" No contradictory information is sent to the peer
" These properties apply to all protocol layers visible to the peer
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