OCS-Office Communications Server Exposed - New Ideas "Inside" – TCP, no UDP

November 14, 2007
If you want to know more about OCS, this and more than 100 other concepts are explained in a new course called SIP Essentials – OCS Special Edition available online or in a classroom. There is also a separate version for channel partners wanting to explore offering OCS to their customers. For more go to: http://www.techtionary.com/nacse/istudies-ocs-tc.htm
 
One of the interesting facets of OCS is that it only supports TCP-Transmission Control Protocol (connection-oriented transmission), not UDP-User Datagram Protocol (connection-less streaming transmission). I have been using a SIP service using UDP from SimpleSignal (www.simplesignal.com) successfully for more than a year. So, it comes as a surprise that Microsoft would only support TCP. The sequencing requirements of TCP are vital to transmission of account numbers and word sentences but not really important to voice transmission as the elegance of the human ear easily fills in for lost syllables and tolerates a vast amount of noise. UDP has always been the choice of streaming media and as the QoS of the internet has vastly increased the need for TCP for voice seems irrelevant. However, so that you can make up your own mind, here is an animation detailing both TCP and UDP.
 
Primary Features of TCP - Transport Layer 4 protocol:
- Data SEQuencing also known as segmenting - size (amount of data in window sequence), order (number) packets for orderly transmission and ACKnowledgement confirmation of delivery
- Connection-oriented - connection established, maintained via ACKnowledgements, data order or SEQuence (contains the data content) and retransmissions and properly disconnected or FINished
- Point-to-Point - used between two locations
- Reliability - TCP guarantees data delivery without loss, duplication or errors
- Full Duplex - simultaneously two-way (bi-directional) communications
- Flow control - to speed up or slow down - congestion management/avoidance
- Windowing - requires that both the receiver and sender determine the size (in bytes) of the data package called a window before data transmission begins.
- Buffering - cache data - memory/disk storage to save and hold data until bandwidth is available or other priority such as video goes first, then business data and then email). If the buffer is full, data may be discarded/lost.
- Source quenching - TCP receiver waits on sending SEQ. SEQuence (with the data content) is needed before sender begins again and also tells sender place or data sequence to begin again.
 
For some of the Cisco certifications, the following three statements are important for connection-oriented sessions:
- Segments delivered (transmitted) are ACKnowledged back to the sender upon their reception.
- Segments are sequenced back in order upon arrival a their destination
- A manageable data flow is maintained in order to avoid congestion, overloading (overflow) and data loss.
This question relates to the fact that TCP-Transmission Control Protocol creates a connection is a connection-less environment. TCP provides guaranteed delivery by establishing a "virtual circuit" between sender and receiver, the name of this virtual circuit is called a "socket." TCP establishes a random port on the sender and requests a specific port on the receiver - PORT + IP = Socket.
 
TCP verifies each SEQuence of data bytes before the next packet is sent; UDP sends or broadcast packets without verification. TCP is connection-oriented while UDP is connection-less. TCP creates a 3-way "hand shake" to create a virtual connection between the two parties.   (1) SYNchronize opens then connection to the receiver. If the receiver recognizes the sender (or rejects as a potential hacker), the receiver sends an (2) SYNchronize-ACKnowledgement. Then the sender transmits (sends) data in IP-Internet Protocol packets (divided into "segments") with (3) ACKnowledgement and begins sending segments of data called SEQuences with numbers and data. This is the so-called sliding window concept. When finished, the sender sends a FINished notification. Remember SYN, SYN-ACK, ACK for the CCNA/DA exams.


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