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 packet will 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.

The User Datagram Protocol (UDP) is a very basic and simple protocol on top of the IP protocol. It was developed to allow for very simple data transmission without any error detection of any kind, and it is stateless. However, it is very well fit for query/response kind of applications, such as for example DNS, et cetera, since we know that unless we get a reply from the DNS server, the query was lost somewhere. Sometimes it may also be worth using the UDP protocol instead of TCP, such as when we want only error/loss detection but don't care about sequencing of the packets. This removes some overhead that comes from the TCP protocol. We may also do the other thing around, make our own protocol on top of UDP that only contains sequencing, but no error or loss detection.

The UDP protocol is specified in RFC 768 - User Datagram Protocol. It is a very short and brief RFC, which fits a simple protocol like this very well.

The UDP header can be said to contain a very basic and simplified TCP header. It contains destination-, source-ports, header length and a checksum as seen in the image below.

Source port - bit 0-15. This is the source port of the packet, describing where a reply packet should be sent. This can actually be set to zero if it doesn't apply. For example, sometimes we don't require a reply packet, and the packet can then be set to source port zero. In most implementations, it is set to some port number.

Destination port - bit 16-31. The destination port of the packet. This is required for all packets, as opposed to the source port of a packet.

Length - bit 32-47. The length field specifies the length of the whole packet in octets, including header and data portions. The shortest possible packet can be 8 octets long.

Checksum - bit 48-63. The checksum is the same kind of checksum as used in the TCP header, except that it contains a different set of data. In other words, it is a one's complement of the one's complement sum of parts of the IP header, the whole UDP header, theUDP data and padded with zeroes at the end when necessary.