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Host-to-host (transport) layer protocols 127

Figure 7.3


TCP connection establishment

As mentioned before, TCP generates pseudo-random sequence numbers by means of a
32-bit software counter that resets at boot-up and then increments every 4 microseconds.
The host establishing the connection reads a value ‘x’ from the counter where x can vary
between 0 and 2
32
–1) and inserts it in the sequence number field. It then sets the SYN
flag = 1 and transmits the header (no data yet) to the appropriate IP address and port
number. Assuming that the chosen sequence number was 132, this action would then be
abbreviated as SYN 132.
The receiving host (e.g. the server) acknowledges this by incrementing the received
sequence number by one, and sending it back to the originator as an acknowledgment
number. It also sets the ACK flag = 1 to indicate that this is an acknowledgment. This
results in an ACK 133. The first byte expected would therefore be numbered 133. At the
same time the server obtains its own sequence number (y), inserts it in the header, and
also sets the SYN flag in order to establish a connection in the opposite direction. The
header is then sent off to the originator (the client), conveying the message e.g. SYN 567.
The composite ‘message’ contained within the header would thus be ACK 133, SYN 567.
The originator receives this, notes that its own request for a connection has been
complied with, and also acknowledges the other node’s request with an ACK 568. Two-
way communication is now established.
7.1.8 Closing a connection
An existing connection can be terminated in several ways.
Firstly, one of the hosts can request to close the connection by setting the FIN flag. The
other host can acknowledge this with an ACK, but does not have to close immediately as

128 Practical TCP/IP and Ethernet Networking
it may need to transmit more data. This is known as a half-close. When the second host is
also ready to close, it will send a FIN that is acknowledged with an ACK. The resulting
situation is known as a full close.
Secondly, either of the nodes can terminate its connection with the issue of RST,
resulting in the other node also relinquishing its connection and (although not necessarily)
responding with an ACK.
Both situations are depicted in Figure 7.4.


Figure 7.4
Closing a connection
7.1.9 The push operation
TCP normally breaks the data stream into what it regards are appropriately sized
segments, based on some definition of efficiency. However, this may not be swift enough
for an interactive keyboard application. Hence the push instruction (PSH bit in the code
field) used by the application program forces delivery of bytes currently in the stream and
the data will be immediately delivered to the process at the receiving end.
7.1.10 Maximum segment size
Both the transmitting and receiving nodes need to agree on the maximum size segments
they will transfer. This is specified in the options field.
On the one hand TCP ‘prefers’ IP not to perform any fragmentation as this leads to a
reduction in transmission speed due to the fragmentation process, and a higher probability
of loss of a packet and the resultant retransmission of the entire packet.
On the other hand, there is an improvement in overall efficiency if the data packets are
not too small and a maximum segment size is selected that fills the physical packets that
are transmitted across the network. The current specification recommends a maximum
segment size of 536 (this is the 576 byte default size of an X.25 frame minus 20 bytes
each for the IP and TCP headers). If the size is not correctly specified, for example too
Host-to-host (transport) layer protocols 129

small, the framing bytes (headers etc) consume most of the packet size resulting in
considerable overhead. Refer to RFC 879 for a detailed discussion on this issue.
7.1.11 The TCP frame
The TCP Frame consists of a header plus data and is structured as follows:

Figure 7.5
TCP frame format
The various fields within the header are as follows:
Source port: 16 bits
The source port number.
Destination port: 16 bits
The destination port number.
Sequence number: 32 bits
The sequence number of the first data byte in the current segment, except when the
SYN flag is set. If the SYN flag is set, a connection is still being established and the
sequence number in the header is the initial sequence number (ISN). The first subsequent
data byte is ISN+1.
Refer to the discussion on sequence numbers.
Acknowledgment number: 32 bits
If the ACK flag is set, this field contains the value of the next sequence number the
sender of this message is expecting to receive. Once a connection is established this is
always sent.
130 Practical TCP/IP and Ethernet Networking
Refer to the discussion on acknowledgment numbers.
Data offset: 4 bits
The number of 32 bit words in the TCP header. (Similar to IHL in the IP header.) This
indicates where the data begins. The TCP header (even one including options) is always
an integral number of 32 bits long.
Reserved: 6 bits
Reserved for future use. Must be zero.

Control bits (flags): 6 bits
(From left to right)
URG: Urgent pointer field significant
ACK: Acknowledgment field significant
PSH: Push function
RST: Reset the connection
SYN: Synchronize sequence numbers
FIN: No more data from sender
Checksum: 16 bits
The checksum field is the 16-bit one’s complement of the one’s complement sum of all
16-bit words in the header and text. If a segment contains an odd number of header and
text octets to be check-summed, the last octet is padded on the right with zeros to form a
16-bit word for checksum purposes. The pad is not transmitted as part of the segment.
While computing the checksum, the checksum field itself is replaced with zeros.
This is known as the standard Internet checksum, and is the same as the one used for
the IP header.
The checksum also covers a 96-bit ‘pseudo header’ conceptually appended to the TCP
header. This pseudo header contains the source IP address, the destination IP address, the
protocol number (06), and TCP length. It must be emphasized that this pseudo header is
only used for computation purposes and is NOT transmitted. This gives TCP protection
against misrouted segments.


Figure 7.6
Pseudo TCP header format
Window: 16 bits
The number of data octets beginning with the one indicated in the acknowledgement
field, which the sender of this segment is willing or able to accept.
Refer to the discussion on sliding windows.
Urgent pointer: Urgent data is placed in the beginning of a frame, and the urgent

pointer points at the last byte of urgent data (relative to the sequence number i.e. the
number of the first byte in the frame). This field is only being interpreted in segments
with the URG control bit set.
Options: Options may occupy space at the end of the TCP header and are a multiple of
8 bits in length. All options are included in the checksum.

Host-to-host (transport) layer protocols 131
7.2 UDP (user datagram protocol)
7.2.1 Basic functions
The second protocol that occupies the host-to-host layer is UDP. As in the case of TCP, it
makes use of the underlying IP protocol to deliver its datagrams.
UDP is a ‘connectionless’ or non-connection-oriented protocol and does not require a
connection to be established between two machines prior to data transmission. It is
therefore said to be an ‘unreliable’ protocol – the word ‘unreliable’ used here as opposed
to ‘reliable’ in the case of TCP.
As in the case of TCP, packets are still delivered to sockets or ports. However, no
connection is established beforehand and therefore UDP cannot guarantee that packets are
retransmitted if faulty, received in the correct sequence, or even received at all. In view
of this, one might doubt the desirability of such an unreliable protocol. There are,
however, some good reasons for its existence.
Sending a UDP datagram involves very little overhead in that there are no
synchronization parameters, no priority options, no sequence numbers, no retransmit
timers, no delayed acknowledgement timers, and no retransmission of packets. The
header is small; the protocol is quick, and streamlined functionally. The only major
drawback is that delivery is not guaranteed. UDP is therefore used for communications
that involve broadcasts, for general network announcements, or for real-time data. A
particularly good application is with streaming video and streaming audio where low
transmission overheads are a prerequisite, and where retransmission of lost packets is not
only unnecessary but also definitely undesirable.
7.2.2 The UDP frame

The format of the UDP frame and the interpretation of its fields are described RFC 768.
The frame consists of a header plus data and contains the following fields:

Figure 7.7
UDP frame format
Source port: 16 bits
This is an optional field. When meaningful, it indicates the port of the sending process,
and may be assumed to be the port to which a reply should be addressed in the absence of
any other information. If not used, a value of zero is inserted.
Destination port: 16 bits
As for source port
Message length: 16 bits
This is the length in bytes of this datagram including the header and the data. (This
means the minimum value of the length is eight.)