Page 296 - DCAP403_Operating System

Unit 13: Case Study: Linux

maintain hash tables which are used to lookup the addresses within incoming IP messages and Notes
direct them to the correct socket/sock pair. TCP is a connection oriented protocol and so there is
more information involved in processing TCP packets than there is in processing UDP packets.
UDP maintains a hash table of allocated UDP ports, the udp_hash table. This consists of pointers
to sock data structures indexed by a hash function based on the port number. As the UDP hash
table is much smaller than the number of permissible port numbers (udp_hash is only 128 or
UDP_HTABLE_SIZE entries long) some entries in the table point to a chain of sock data structures
linked together using each sock’s next pointer.
TCP is much more complex as it maintains several hash tables. However, TCP does not actually
add the binding sock data stucture into its hash tables during the bind operation, it merely checks
that the port number requested is not currently being used. The sock data structure is added to
TCP’s hash tables during the listen operation.

Making a Connection on an INET BSD Socket

Once a socket has been created and, provided it has not been used to listen for inbound connection
requests, it can be used to make outbound connection requests. For connectionless protocols like
UDP this socket operation does not do a whole lot but for connection orientated protocols like
TCP it involves building a virtual circuit between two applications.

An outbound connection can only be made on an INET BSD socket that is in the right state; that
is to say one that does not already have a connection established and one that is not being used
for listening for inbound connections. This means that the BSD socket data structure must be in
state SS_UNCONNECTED. The UDP protocol does not establish virtual connections between
applications, any messages sent are datagrams, one off messages that may or may not reach
their destinations. It does, however, support the connect BSD socket operation. A connection
operation on a UDP INET BSD socket simply sets up the addresses of the remote application; its
IP address and its IP port number. Additionally it sets up a cache of the routing table entry so that
UDP packets sent on this BSD socket do not need to check the routing database again (unless this
route becomes invalid). The cached routing information is pointed at from the ip_route_cache
pointer in the INET sock data structure. If no addressing information is given, this cached routing
and IP addressing information will be automatically be used for messages sent using this BSD
socket. UDP moves the sock’s state to TCP_ESTABLISHED.
For a connect operation on a TCP BSD socket, TCP must build a TCP message containing
the connection information and send it to IP destination given. The TCP message contains
information about the connection, a unique starting message sequence number, the maximum
sized message that can be managed by the initiating host, the transmit and receive window
size and so on. Within TCP all messages are numbered and the initial sequence number is used

as the first message number. Linux chooses a reasonably random value to avoid malicious
protocol attacks. Every message transmitted by one end of the TCP connection and successfully
received by the other is acknowledged to say that it arrived successfully and uncorrupted.
Unacknowledges messages will be retransmitted. The transmit and receive window size is the
number of outstanding messages that there can be without an acknowledgement being sent. The
maximum message size is based on the network device that is being used at the initiating end
of the request. If the receiving end’s network device supports smaller maximum message sizes
then the connection will use the minimum of the two. The application making the outbound TCP
connection request must now wait for a response from the target application to accept or reject
the connection request. As the TCP sock is now expecting incoming messages, it is added to the
tcp_listening_hash so that incoming TCP messages can be directed to this sock data structure.
TCP also starts timers so that the outbound connection request can be timed out if the target
application does not respond to the request.

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