The TCP headers must be able to perform all of the tasks above. We have already
explained when and where some of the headers are used, but there are still other
areas that we haven't touched very deeply at. Below you see an image of the
complete set of TCP headers. It is formatted in 32 bit words per row, as you can
see.
Source port - bit 0 - 15. This is the source port of the packet. The source port
was originally bound directly to a process on the sending system. Today, we use
a hash between the IP addresses, and both the destination and source ports to
achieve this uniqueness that we can bind to a single application or program.
Destination port - bit 16 - 31. This is the destination port of the TCP packet.
Just as with the source port, this was originally bound directly to a process
on the receiving system. Today, a hash is used instead, which allows us to have
more open connections at the same time. When a packet is received, the
destination and source ports are reversed in the reply back to the originally
sending host, so that destination port is now source port, and source port is
destination port.
Sequence Number - bit 32 - 63. The sequence number field is used to set a
number on each TCP packet so that the TCP stream can be properly sequenced
(e.g., the packets winds up in the correct order). The Sequence number is then
returned in the ACK field to ackonowledge that the packet was properly
received.
Acknowledgment Number - bit 64 - 95. This field is used when we acknowledge a
specific packet a host has received. For example, we receive a packet with
one Sequence number set, and if everything is okey with the packet, we reply
with an ACK packet with the Acknowledgment number set to the same as the
original Sequence number.
Data Offset - bit 96 - 99. This field indicates how long the TCP header is, and
where the Data part of the packet actually starts. It is set with 4 bits, and
measures the TCP header in 32 bit words. The header should always end at an
even 32 bit boundary, even with different options set. This is possible
thanks to the Padding field at the very end of the TCP header.
Reserved - bit 100 - 103. These bits are reserved for future usage. In RFC 793
this also included the CWR and ECE bits. According to RFC 793 bit 100-105
(i.e., this and the CWR and ECE fields) must be set to zero to be fully
compliant. Later on, when we started introducing ECN, this caused a lot of
troubles because a lot of Internet appliances such as firewalls and routers
dropped packets with them set. This is still true as of writing this.
CWR - bit 104. This bit was added in RFC 3268 and is used by ECN. CWR stands
for Congestion Window Reduced, and is used by the data sending part to inform
the receiving part that the congestion window has been reduced. When the
congestion window is reduced, we send less data per timeunit, to be able to
cope with the total network load.
ECE - bit 105. This bit was also added with RFC 3268 and is used by ECN. ECE
stands for ECN Echo. It is used by the TCP/IP stack on the receiver host to
let the sending host know that it has received an CE packet. The same thing
applies here, as for the CWR bit, it was originally a part of the reserved
field and because of this, some networking appliances will simply drop the
packet if these fields contain anything else than zeroes. This is actually
still true for a lot of appliances unfortunately.
URG - bit 106. This field tells us if we should use the Urgent Pointer
field or not. If set to 0, do not use Urgent Pointer, if set to 1, do use
Urgent pointer.
ACK - bit 107. This bit is set to a packet to indicate that this is in reply
to another packet that we received, and that contained data. An Acknowledgment
packet is always sent to indicate that we have actually received a packet, and
that it contained no errors. If this bit is set, the original data sender will
check the Acknowledgment Number to see which packet is actually acknowledged,
and then dump it from the buffers.
PSH - bit 108. The PUSH flag is used to tell the TCP protocol on any
intermediate hosts to send the data on to the actual user, including the TCP
implementation on the receiving host. This will push all data through,
unregardless of where or how much of the TCP Window that has been pushed
through yet.
RST - bit 109. The RESET flag is set to tell the other end to tear down the
TCP connection. This is done in a couple of different scenarios, the main
reasons being that the connection has crashed for some reason, if the
connection does not exist, or if the packet is wrong in some way.
SYN - bit 110. The SYN (or Synchronize sequence numbers) is used during the
initial establishment of a connection. It is set in two instances of the
connection, the initial packet that opens the connection, and the reply SYN/ACK
packet. It should never be used outside of those instances.
FIN - bit 111. The FIN bit indicates that the host that sent the FIN bit has no
more data to send. When the other end sees the FIN bit, it will reply with a
FIN/ACK. Once this is done, the host that originally sent the FIN bit can no
longer send any data. However, the other end can continue to send data until it
is finished, and will then send a FIN packet back, and wait for the final
FIN/ACK, after which the connection is sent to a CLOSED state.
Window - bit 112 - 127. The Window field is used by the receiving host to tell
the sender how much data the receiver permits at the moment. This is done by
sending an ACK back, which contains the Sequence number that we want to
acknowledge, and the Window field then contains the maximum accepted sequence
numbers that the sending host can use before he receives the next ACK packet.
The next ACK packetwill update accepted Window which the sender may use.
Checksum - bit 128 - 143. This field contains the checksum of the whole TCP
header. It is a one's complement of the one's complement sum of each 16 bit word
in the header. If the header does not end on a 16 bit boundary, the additional
bits are set to zero. While the checksum is calculated, the checksum field is
set to zero. The checksum also covers a 96 bit pseudoheader containing the
Destination-, Source-address, protocol, and TCP length. This is for extra
security.
Urgent Pointer - bit 144 - 159. This is a pointer that points to the end of the
data which is considered urgent. If the connection has important data that
should be processed as soon as possible by the receiving end, the sender can set
the URG flag and set the Urgent pointer to indicate where the urgent data ends.
Options - bit 160 - **. The Options field is a variable length field and
contains optional headers that we may want to use. Basically, this field
contains 3 subfields at all times. An initial field tells us the length of the
Options field, a second field tells us which options are used, and then we have
the actual options. A complete listing of all the TCP Options can be found in
TCP options.
Padding - bit **. The padding field pads the TCP header until the whole header
ends at a 32-bit boundary. This ensures that the data part of the packet begins
on a 32-bit boundary, and no data is lost in the packet. The padding always
consists of only zeros.
