SCTP: next generation signalling protocol and FreeBSD 7

1. SCTP: next generation signalling protocoland FreeBSD 7 Murat Balaban Director, R & Dendersys ltd.http://www.enderunix.org/murat/

2. SCTP Intro • new IP transport protocol, existing at an equivalent level as UDP and TCP • provide transport layer functions to all of the main Internet applications • IETF standart • RFC 2960 – RFC 3758

3. Like TCP • provides a reliable transport service • In-order delivery • connection-oriented • Congestion avoidance • Slow start

4. Unlike TCP • provides a number of functions that are considered critical for signaling transport • at the same time can provide transport benefits to other applications requiring additional performance and reliability. 

5. SCTP Core Features • Multi-streaming • Multi-homing

6. Multi-streaming • Name is derived from the multi-streaming function • In contrast, TCP provides a single stream of data and ensures that delivery of that stream takes place with perfect sequence preservation • For a number of applications, this characteristic of strict sequence preservation is not truly necessary

7. Multi-streaming contd. • Signalling Multiple streams can carry different signalls of varying importance Multiple calls can be placed on different streams whithin the same “association”

8. Multi-streaming contd. • Web page objects Objects can be placed on different independent streams, which results in a better perceived performance.

9. Multi-streaming: How? By creating independence between data transmission and data delivery.  In particular, each DATA “chunk” (or PDU) in the protocol uses two sets of sequence numbers, a Transmission Sequence Number that governs the transmission of messages and the detection of message loss, and the Stream ID/Stream Sequence Number pair, which is used to determine the sequence of delivery of received data.

10. Multi-homing • the ability for a single SCTP endpoint to support multiple IP addresses • Benefit: potentially greater survivability of the session in the presence of network failures • redundant LANs can be used to reinforce the local access, while various options are possible in the core network to reduce the dependency of failures for different addresses

11. App-3 App-2 App-1 OS NI-1 NI-2 NI-3 160.15.82.20 10.1.61.11 161.10.8.221 Multi-homing

12. Multi-homing contd. • Use of addresses with different prefixes can force routing to go through different carriers, for example, while route-pinning techniques or even redundant core networks can also be used if there is control over the network architecture and protocols.

13. Multi-homing: load sharing? • Nope. • Only for redundancy

14. Multi-homing: load sharing? • Nope. • Only for redundancy

15. Multi-homing: How? • SCTP endpoints can exchange lists of addresses during initiation of the association • A single port number is used across the entire address list at an endpoint for a specific session. • to reduce the potential for security problems, it is required that some response messages be sent specifically to the source address in the message that caused the response

16. Other features • a unicast protocol, and supports data exchange between exactly 2 endpoints, although these may be represented by multiple IP addresses. • provides reliable transmission • Full-duplex

17. Other features contd. • message oriented and supports framing of individual message boundaries.  In comparison, TCP is stream oriented and does not preserve any implicit structure within a transmitted byte stream. • Rate-adaptive • Unordered data delivery option

18. Other features contd. • Security cookie against connection flood attack (SYN flood) • Built-in heartbeat (reachability check) • Extensibility (TLV chunks)

19. Association Establishment • To establish this state, both sides go through a specific set of exchanges • TCP uses a 3-way handshake (SYN, SYN/ACK, ACK) • SCTP uses a 4-way handshake • In TCP, the communication relationship between two endpoints is called a “connection” • In SCTP, this is called an “association” this is because it is a broader concept than a single connection (i.e. multi-homing)

20. Endpoint A Endpoint Z INIT INIT-ACK Association Is Up * Association Is Up COOKIE-ECHO * COOKIE-ACK * -- User data can be attached Setting Up an Association

21. [Rcv INIT] Gen Cookie Send INIT-ACK CLOSED COOKIE_WAIT [ASSOCIATE] Create TCB Send INIT Start init timer [rcv valid COOKIE-ECHO] Create TCB Send COOKIE-ACK [rcv INIT-ACK] Send COOKIE-ECHO Stop init timer Start cookie timer COOKIE_ECHOED ESTABLISHED [rcv COOKIE-ACK] Stop cookie timer SCTP States I

22. ESTABLISHED [SHUTDOWN] Check outstanding data chunks [rcv SHUTDOWN] Check outstanding data chunks SHUTDOWN- PENDING SHUTDOWN- PENDING [No More Outstanding data chunks] Send SHUTDOWN Start shutdown timer [No More Outstanding data chunks] Send SHUTDOWN-ACK Start shutdown- timer NEXT-SLIDE NEXT-SLIDE SCTP States II

23. FROM STATES-2 FROM STATES-2 SHUTDOWN- SENT SHUTDOWN- ACK-SENT [rcv SHUTDOWN-ACK] send SHUTDOWN_COMPETE Stop shutdown timer Delete TCB [rcv SHUTDOWN-COMPLETE] Stop shutdown timer delete TCB CLOSED SCTP States III

24. IP Header SCTP Common Header Chunk 1 ... Chunk N SCTP Header with IP

25. Source Port Destination Port Verification Tag CRC-32c Checksum Common Header

26. Chunk Type Chunk Length Chunk Flags Chunk Data SCTP Chunks • Chunk Type: 8-bit value indicating the type of chunk • Chunk Flags: 8-bit flags, defined on per chunk type basis • Chunk Length: 16-bit length in bytes, including the chunk type, chunk flags, and chunk length fields. • Note that chunks are padded to 32-bit boundaries within an SCTP packet. Any padding bytes (0x00) used are NOT included in the chunk length

27. SCTP Chunk Types • There are 20 chunk types currently defined in SCTP (including non-RFC/Internet Draft extensions): • (1) DATA (0x00) • (2) INITIATION [INIT] (0x01) • (3) INITIATION-ACKNOWLEDGMENT [INIT-ACK] (0x02) • (4) SELECTIVE-ACKNOWLEDGMENT [SACK] (0x03) • (5) HEARTBEAT (0x04)

28. Flags=0 Type=a Length=variable State Cookie from INIT-ACK Cookie Echo Chunk

29. Type=0x00 Flags=UBE Length=variable TSN Value Stream Sequence Num Stream Identifier Payload Protocol Identifier Variable Length User Data DATA Chunk Flag Bits ‘UBE’ are used to indicate: • U – Unordered Data • B – Beginning of Fragmented Message • E – End of Fragmented Message • A user message that fits in one chunk would have both the B and E bits set

30. FreeBSD ready? • Not yet. • -CURRENT • Planned for 7.0-RELEASE • Thanks to Randal Steward

31. -- Thanks -- Q & A Murat Balaban EnderUNIX Yazılım Geliştirme Takımımurat@enderunix.orghttp://www.enderunix.org/murat/