SCTP #1

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

1. First, read the following three pages:

• Better networking with SCTP by IBM/M. Tim Jones
• SCTP Primer by Asim Iqbal
• Why is SCTP needed given TCP and UDP are widely available? by Randall Stewart, Paul D. Amer

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