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courses:ct30a5000:assignment5notes [2012/11/22 17:25] (current)
julaakko created
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 +===== Answers to questions (without the examples) =====
 +  - What are the differences between SCTP one-to-one and one-to-many styles? Why there are 2 "​different"​ styles?
 +    * Differences:​
 +      * One-to-one is strict between connected devices as TCP connection.
 +      * One socket is peeled of from the listening socket (as with accept() in TCP) for each new connection using one-to-one.
 +      * The closing of the peeled sockets are done in different way as in one-to-many.
 +      * One-to-many provides full benefits of SCTP: multihoming,​ graceful shutdown, piggybacking of data in handshake messages, automatic initiation of handshake with first send and association events from SCTP kernel.
 +    * The purpose of the one-to-one style is to provide backwards compatibility for existing applications using TCP. However, with one-to-one style packet boundaries are preserved.
 +  - Explain the difference between a connection (TCP) and an association (SCTP).
 +    * Connection is strict between address + port pairs
 +    * Connection is between two addresses of peers, with association multiple addresses on peers can be utilized
 +    * Different handshakes (3-way vs. 4-way)
 +    * TCP connection is stream oriented and SCTP is message oriented
 +    * Association makes it possible to use multiple streams
 +  - Why SCTP can be regarded as more secure as TCP?
 +    * The handshakes are different; STCP has 4-way which uses cookies, whereas TCP has 3-way handshake -> SYN flood attacks can be prevented
 +    * Cookies can be used for verification and authentication
 +    * On other hand, usually firewalls aren't configured to block SCTP! 
 +  - What benefits does the multihoming feature of SCTP give? Give at least 3 examples.
 +      * Interfaces (and addresses) can be added and removed dynamically
 +      * Multihoming provides that if one path is disconnected other available one can be used
 +      * Multihoming enables more effective roaming
 +      * The fastest/​least congested route can be utilized for communication
 +      * Better error tolerance
 +  - Explain Head-of-line blocking. Why it is a problem with TCP? How SCTP handles this or how this affects SCTP? Give an example application/​application layer protocol that could benefit from SCTP where head-of-line blocking can occur.
 +    * A note: each stream is not stream of bytes - it is a sequence of messages
 +    * If the server sends e.g. four separate pieces of data independent of each other and fragments of first one are lost that will have to be retransmitted -> whole process is delayed. The server will send the data with retransmissions,​ client has to hold all the data and errors with one data packet would delay the whole process. ​
 +    * TCP: one connection (solution: multiple connections)
 +    * SCTP: multiple streams, reserve certain amount of streams per association and send each piece of data using separate stream / block of streams