Chapter 12: Networks
Now the client must wait for server acknowledgment of the SYN packet and for a SYN packet acknowl-
edging the connection request, which is received bymeans of the normal TCP mechanisms (lower part of
Figure 12-29). This leads to thetcp_rcv_state_processdispatcher, which, in this case, directs the flow of
control totcp_rcv_synsent_state_process. The socket state is set to ESTABLISHED, andtcp_send_ack
returns another ACK packet to the server to conclude connection setup.Transmissionof Data Packets
Data are transferred between computers once a connection has been set up as described above. This
process is sometimes quite tricky because TCP has several features that call for comprehensive control
and security procedures between the communicating hosts:❑ Byte streams are transmitted in a guaranteed order.
❑ Lost packets are retransmitted by automated mechanisms.
❑ Data flow is controlled separately in each direction and is matched to the speeds of the hosts.Even though initially these requirements may not appear to be very complex, a relatively large number
of procedures and tricks are needed to satisfy them. Because most connections are TCP-based, the speed
and efficiency of the implementation are crucial. The Linux kernel therefore resorts to tricks and opti-
mizations, and unfortunately these don’t necessarilymake the implementation any easier to understand.Before turning our attention to how data transmission is implemented over an established connection,
it is necessary to discuss some of the underlying principles. We are particularly interested in the mecha-
nisms that come into play when packets are lost.The concept of packet acknowledgment based on sequence numbers is also adopted for normal data
packets. However, sequence numbers reveal more about data transmission than mentioned above.
According to which scheme are sequence numbers assigned? When a connection is set up, a random
number is generated (by the kernel usingsecure_tcp_sequence_numberfromdrivers/char/random.c).
Thereafter a system supporting the strict acknowledgment of all incoming data packets is used.A unique sequence number that builds on the number initially sent is assigned to each byte of a TCP
transmission. Let us assume, for example, that the initial random number of the TCP system is 100. The
first 16 bytes sent therefore have the sequence numbers 100, 101,..., 115.TCP uses acumulative acknowledgmentscheme. This means that an acknowledgment covers a contiguous
range of bytes. The number sent in theackfield acknowledges all bytes between the last and the current
ACK number of a data stream. (The initial sequence number is used as the starting point if an acknowl-
edgment has not yet been sent and there is therefore no last number.) The ACK number confirms receipt
of all data up to and including the byte that is 1 less than the number and therefore indicates which byte
is expected next. For instance, ACK number 166 acknowledges all bytes up to and including 165 and
expects bytes from 166 upward in the next packet.This mechanism is used to trace lost packets. Note that TCP does not feature an explicit re-request mech-
anism; in other words, the receiver cannot request the sender to retransmit lost packets. The onus is on
the sender to retransmit the missing segment automatically if it does not receive an acknowledgment
within a certain time-out period.How are these procedures implemented in the kernel? We assume that the connection was established as
described above so that the two sockets (on the different systems) both have the ESTABLISHED state.