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Information request/reply
The information request and reply types are obsolete since there are protocols on top of the IP protocol that can now take care of this when necessary (DHCP, etc). The information request generates a reply from any answering host on the network that we are attached to.
The host that wishes to receive information creates a packet with the source address set to the network we are attached to (for example, 192.168.0.0), and the destination network set to 0. The reply will contain information about our numbers (netmask and ip address).
The information request is run through ICMP type 15 while the reply is sent via type 16.
SCTP Characteristics
Stream Control Transmission Protocol (SCTP) is a relatively new protocol in the game, but since it is growing in usage and complements the TCP and UDP protocols, I have chosen to add this section about it. It has an even higher reliability than TCP, and at the same time a lower overhead from protocol headers.
SCTP has a couple of very interesting features that can be interesting. For those who wish to learn more about this, read the RFC 3286 - An Introduction to the Stream Control Transmission Protocol and RFC 2960 - Stream Control Transmission Protocol document. The first document is an introduction to SCTP and should be very interesting to people who are still in need of more information. The second document is the actual specification for the protocol, which might be less interesting unless you are developing for the protocol or are really interested.
The protocol was originally developed for Telephony over IP, or Voice over IP (VoIP), and has some very interesting attributes due to this. Industry grade VoIP requires very high reliability for one, and this means that a lot of resilience has to be built into the system to handle different kind of problems. The following is a list of the basic features of SCTP.
• Unicast with Multicast properties. This means it is a point-to-point protocol but with the ability to use several addresses at the same end host. It can in other words use different paths to reach the end host. TCP in comparison breaks if the transport path breaks, unless the IP protocol corrects it.
• Reliable transmission. It uses checksums and SACK to detect corrupted, damaged, discarded, duplicated and reordered data. It can then retransmit data as necessary. This is pretty much the same as TCP, but SCTP is more resilient when it comes to reordered data and allows for faster pickups.
• Message oriented. Each message can be framed and hence you can keep tabs on the structure and order of the datastream. TCP is byte oriented and all you get is a stream of bytes without any order between different data inside. You need an extra layer of abstraction in TCP in other words.
• Rate adaptive. It is developed to cooperate and co-exist with TCP for bandwidth. It scales up and down based on network load conditions just the same as TCP. It also has the same algorithms for slow starting when packets where lost. ECN is also supported.
• Multi-homing. As previously mentioned, it is able to set up different end nodes directly in the protocol, and hence doesn't have to rely on the IP layer for resilience.
• Multi-streaming. This allows for multiple simultaneous streams inside the same stream. Hence the name Stream Control Transmission Protocol. A single stream can for example be opened to download a single webpage, and all the images and html documents can then be downloaded within the same stream simultaneously. Or why not a database protocol which can create a separate control stream and then use several streams to receive the output from the different queries simultaneously.
• Initiation. 4 packet initiation of co
This list could be made even longer, but I will not. Most of this information is gathered from the RFC 3286 - An Introduction to the Stream Control Transmission Protocol document, so read on there for more information
Note In SCTP we talk about chunks, not packets or windows anymore. An SCTP frame can contain several different chunks since the protocol is message oriented. A chunk can either be a control or a data chunk. Control chunks is used to control the session, and data chunks are used to send actual data.
Initialization and association
Each co
The initialization is done through 4 packets. First an INIT chunk is sent, which is replied to with an INIT ACK containing a cookie, after this the co
Data sending and control session
SCTP can at this point send data. In SCTP there are control chunks and data chunks, as previously stated. Data chunks are sent using DATA chunks, and DATA chunks are acknowledged by sending a SACK chunk. This works practically the same as a TCP SACK. SACK chunks are control chunks.
On top of this, there are some other control chunks that can be seen. HEARTBEAT and HEARTBEAT ACK chunks for one, and ERROR chunks for another. HEARTBEATs are used to keep the co
Shutdown and abort
The SCTP co
When the user/application wants to close the SCTP socket gracefully, it tells the protocol to SHUTDOWN. SCTP then sends all the data still in its buffers, and then sends a SHUTDOWN chunk. When the other end receives the SHUTDOWN, it will stop accepting data from the application and finish sending all the data. Once it has gotten all the SACK's for the data, it will send a SHUTDOWN ACK chunk and once the closing side has received this chunk, it will finally reply with a SHUTDOWN COMPLETE chunk. The whole session is now closed.
Another way of closing a session is to ABORT it. This is an ungraceful way of removing an SCTP association. When a co