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SCTP #1

This exercise is about Stream Control Transfer Protocol (SCTP).

Task for week 46

1. First, read the following three pages:

2. Second, think answers to following questions:

  1. What are the major differences when comparing SCTP to UDP? How about TCP?
    1. UDP:
      • SCTP provides reliable transmission and flow control
      • SCTP has a connection establishment (4-way handshake)
      • With UDP the address and port need to be forwarded to lower layers for each packet to be sent, SCTP uses this only for connection establishment
      • As UDP is packet oriented it has not got any streams, whereas SCTP supports multiple streams between connected devices
    2. TCP:
      • SCTP provides multiple streams to use whereas in TCP one stream is available per direction
      • SCTP is message oriented (preserves message boundaries) and TCP is stream oriented
      • SCTP provides multihoming with the association between connected devices, TCP is more point-to-point with strict connection tied to address and port
      • The handshakes are different; STCP has a 4-way which uses cookies, whereas TCP has a 3-way handshake
      • SCTP has no support for half-open connections as in TCP
        • shutdown() command with parameters does not work
  2. Why SCTP can be regarded as more secure as TCP?
    • The handshakes are different; TCP has a 3-way handshake whereas STCP has a 4-way which uses cookies → SYN flood attacks can be prevented
      • Cookies can be used for verification and authentication
      • Verification tag protects against blind masquerade attacks and stale packets from a previous association
      • On other hand, usually firewalls aren't configured to block SCTP
  3. How the SCTP benefits from Selective ACKs when compared to TCP?
    • The proceduce of SCTP follows the optional TCP SACK
      • Data is acknowledged by chunks, the range received is acknowledged but also are the gaps in additon to the amount of duplicate fields
      • Similarly to TCP delayed acks are used to send SACK
      • One SACK for every two received packets
      • With one SACK a large number of blocks can be acknowledged
        • vs. TCP 3/4
      • SACK packets can be piggybacked to save bandwidth (headers)
  4. What benefits does the multihoming feature of SCTP give?
    • Interfaces (and addresses) can be added and removed dynamically
    • If one route is disconnected, other available routes can be used
    • Multihoming enables more effective roaming
    • The fastest/least congested route can be utilized for communication
    • Better error tolerance
  5. 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.
    • When one TCP segment is lost and others arrive out of order, subsequent segment(s) are held until lost one is retransmitted → causes delay
    • with web servers, multiple images that are sent in pieces so they “appear” to the user at the same time
      • piece of first, second, third…, 2nd pieces, 3rd pieces, etc.
      • if the first packet is missing a segment → whole process of sending the pictures can be delayed!
    • 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

Some real life benefits of SCTP: SSH over SCTP, Using SCTP as a Transport Layer Protocol for HTTP

3. Third, only look at the SCTP example code and try to understand how it works.


Task for next week (week 47): SCTP #2


CT30A5002 - Games and Networking