The entire TCP connection transfer mechanism is synchronized
through control messages. To this purpose, we have implemented
a new communication protocol (THOP) in the standard TCP/IP stack.
The THOP protocol defines a small number of messages, that are
encapsulated into IP datagrams. Each message triggers
an appropriate action or notifies an event to the destination.
Let us describe its key role in the TCP handoff mechanism.
To minimize the processing overhead associated to communications, we chose to make the THOP protocol as light as possible.
THOP has a (albeit very
short) header, made up of two 16 bit fields:
the opcode identifies the message type,
the checksum is used to verify the message integrity.
The message body depends on the message type.
The current implementation supports the following messages.
The THOP_CREATE message encapsulates all
information necessary for transferring a socket to a Web server.
It contains the parameters of the tcp_opt structure
(which stores the state of a TCP connection) and information about
the client, such as the IP source address, the TCP source port and
the content of the HTTP request.
The THOP_NOTIFYCLS message is sent from a Web server
when the corresponding TCP connection is closed. The Web switch
does have to know when a TCP connection is closed, because it must
delete the corresponding entry from the table of transferred connections.
The only way a Web switch knows that a TCP connection is closed without
receiving an explicit message is through a TCP reset from
a client. The reset is forwarded to the server which
takes care of releasing the associated system resources.
The THOP_NOTIFYDROP message is sent from the Web
server to the Web switch when the duplication of a TCP connection
has not been possible for whatever reason (typically, the
maximum number of outstanding connection requests has been
We have previously stated that the server does not require
any kind of configuration for communicating with a switch node.
This is possible because, at connection transfer time,
the source IP address in the IP header of the Handoff request
is stored into a field of the newly created socket, namely swaddr.
The swaddr field indicates whether a socket has
been established through an Handoff or through a
three-way handshake. In the former case, it stores the
switch IP address; in the latter case, it stores an
It is worth observing that the THOP protocol provides a very
flexible communication mechanism among the nodes of the Web cluster.
For example, it is not only
suitable to synchronize the connection transferring process, but
it may be also extended to implement a distributed load
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