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Lecture 9 SCTP Sockets

Lecture 9 SCTP Sockets. CPE 401 / 601 Computer Network Systems. slides are modified from Janardhan Iyengar , John Rumsey , Nimish Vartak. Transport. Transport. user-level. kernel. IP. IP. IP. IP. IP. IP. Wifi. Wifi. Eth. Eth. IP. IP. Where is SCTP in the stack?. Application.

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Lecture 9 SCTP Sockets

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  1. Lecture 9SCTP Sockets CPE 401 / 601Computer Network Systems slides are modified from Janardhan Iyengar,John Rumsey, Nimish Vartak

  2. Transport Transport user-level kernel IP IP IP IP IP IP Wifi Wifi Eth Eth IP IP Where is SCTP in the stack? Application Application Socket API Socket API UDP TCP SCTP SCTP UDP TCP

  3. SCTP – Motivation • New applications • Migration from PSTN to Packet based Internet • Telephony signaling messages • Shortcomings of existing protocols • TCP • “head-of-line blocking” • Byte-oriented, not message-oriented • Multi-homing support not built in • DoS attack prone • UDP • No Reliability • Absence of congestion control • Absence of flow control SCTP

  4. SCTP – Overview • “SCTP is a reliable transport protocol operating on top of a connectionless packet network such as IP. …” • RFC 2960 • Has built-in support for multi-homed hosts • Is message-based – conserves the message boundaries. • Classifies messages as: • sequenced delivery of user messages within multiple streams • with an option for un-ordered delivery of individual user messages • Additional security mechanisms SCTP

  5. SCTP Feature Summary Start with TCP: reliable (retransmissions) congestion controlled connection oriented Add: 4-way handshake to reduce vulnerability to DOS attacks framing preserve message boundaries multistreaming instead of one ordered stream, up to 64K independent ordered streams multihoming instead of one IP address per endpoint a set of IP addresses per endpoint SCTP

  6. TCP Connection Setup A B t=0 closed listen SYN SYN sent SYN-ACK 1RTT SYN recd (TCB created) ACK data established estab’d

  7. SYN Flooding Attack attackers Flooded!! victim 130.2.4.15 128.3.4.5 TCB SYN TCB 228.3.14.5 192.10.2.8 SYN TCB TCB 190.13.4.1 SYN TCB 221.3.5.10 Unavailable, reserved resources • There is no ACK in response to the SYN-ACK, hence connection • remains half-open • Other genuine clients cannot open connections to the victim • The victim is unable to provide service

  8. SCTP – Features (contd …) Connection setup (SYN) INIT (SYN-ACK) INIT-ACK COOKIE-ECHO COOKIE-ACK End-Point A End-Point Z SCTP

  9. What’s in a cookie? • Information from original INIT • Information from current INIT-ACK • Timestamp • Life span of cookie (Time to live) • Signature for authentication (SHA-1, MD5, etc.) SCTP

  10. SCTP Association Setup V: Verification tag I: Initiate tag closed A B t=0 INIT (V=0) (I=TagA) cookie wait INIT–ACK (V=TagA) (I=TagB) (StateCookie) closed 1RTT COOKIE–ECHO (V=TagB) (StateCookie) cookie echoed COOKIE–ACK (V=TagA) 2RTT estab’d data (V=TagB) established

  11. SCTP – Features (contd …) Connection close SHUTDOWN SHUTDOWN-ACK SHUTDOWN-CMPL End-Point A End-Point Z No Half Closed State SCTP

  12. Graceful Shutdown A B App signals shutdown (pending data) Shutdown pending SHUTDOWN Shutdown received Shutdown sent (pending data) SHUTDOWN-ACK Shutdown-Ack sent SHUTDOWN-COMPLETE Closed Closed

  13. CLOSED ESTABLISHED COOKIE_WAIT SHUTDOWN- SENT SHUTDOWN- ACK-SENT SHUTDOWN- PENDING SHUTDOWN- PENDING COOKIE_ECHOED CLOSED SCTP state diagram SCTP

  14. SCTP Feature Summary Start with TCP: reliable (retransmissions) congestion controlled connection oriented Add: 4-way handshake to reduce vulnerability to DOS attacks framing preserve message boundaries multistreaming instead of one ordered stream, up to 64K independent ordered streams multihoming instead of one IP address per endpoint a set of IP addresses per endpoint SCTP

  15. Message Boundaries • UDP honors message boundaries • Each app message becomes a datagram • TCP does not honor message boundaries • App messages become part of a byte stream • SCTP maintains message boundaries • Each app message is maintained as one or more data chunks SCTP

  16. An SCTP packet forms the payload of an IP packet Chunk N Chunk 1 SCTP Header Source Port Destination Port Verification Tag Flags Flags Type Type Value Value Checksum Length Length N 2 1 1 N 2 1 1 4 4 2 2 • An SCTP packet consists of a 12 byte common header and one or more “Chunks” • Control chunks bundled before Chunks in SCTP SCTP

  17. SCTP Header • Source Port & Destination Port • Uses same port concept as TCP and UDP • Verification Tag • Exchanged between endpoints at startup • To Validate the sender • Checksum • Protected by 32 bit checksum (CRC32 algorithm) SCTP Header Source Port Destination Port Verification Tag Checksum 4 4 2 2 SCTP

  18. SCTP Chunks • Type • Used to distinguish data chunks and different types of control chunks • Flags • Usage depends on Chunk type • Length • Required because chunks have a variable length • Value • Payload field Chunk Flags Type Value Length N 2 1 1 SCTP

  19. Type = 1 Chunk Flags Chunk Length Initiate Tag Advertised Receiver Window Credit (a_rwnd) Number of Outbound Streams Number of Inbound Streams Initial Transmission Sequence Number (TSN) Optional/Variable-Length Parameters INIT Chunk SCTP

  20. Type = 0 Reserv. Length U B E Transmission Sequence Number (TSN) Stream Identifier S Stream Sequence Number N Payload Protocol Identifier User Data (seq. n of Stream S) Data Chunk SCTP

  21. DATA chunk TSN=109 DATA chunk TSN=110 DATA chunk TSN=111 DATA chunk TSN=112 DATA chunk TSN=113 DATA chunk TSN=114 X DATA chunk TSN=115 X DATA chunk TSN=116 DATA chunk TSN=117 DATA chunk TSN=118 DATA chunk TSN=119 DATA chunk TSN=120 DATA chunk TSN=121 DATA chunk TSN=122 DATA chunk TSN=123 X DATA chunk TSN=124 DATA chunk TSN=125 DATA chunk TSN=126 SACK chunk Cumulative TSN=113 Gap Ack Block #1 Start = +3 End = +9 Gap Ack Block #2 Start = +11 End = +13 Selective Acknowledgement

  22. Type = 3 Chunk Flags Chunk Length Cumulative Transmission Sequence Number (TSN) Ack Advertised Receiver Window Credit (a_rwnd) Number of Gap Ack Blocks = N Number of Duplicate TSNs = X Gap Ack Block #1 Start Gap Ack Block #1 End Gap Ack Block #N Start Gap Ack Block #N End Duplicate Transmission Sequence Number (TSN) 1 Duplicate Transmission Sequence Number (TSN) N SACK Chunk SCTP

  23. SCTP Feature Summary Start with TCP: reliable (retransmissions) congestion controlled connection oriented Add: 4-way handshake to reduce vulnerability to DOS attacks framing preserve message boundaries multistreaming instead of one ordered stream, up to 64K independent ordered streams multihoming instead of one IP address per endpoint a set of IP addresses per endpoint SCTP

  24. Multi-streaming • A.k.a. partial ordering. Eliminates Head of Line (HOL) blocking • In TCP, all data must be sent in order; loss at head of line delays delivery of subsequent data • In SCTP, you can send over up to 64K independent streams, each ordered independently • A loss on one stream does not delay the delivery on other streams i.e. multi-streaming eliminates HOL blocking SCTP

  25. Head-of-Line Blocking in TCP S R’s App R 1 2 ACK 2 3 1 4 ACK 3 2 5 ACK 3 6 ACK 3 ACK 3 PDU 3 is blocking the head of the line.

  26. Head-of-line Blocking • TCP provides a single data stream • When a segment is lost, subsequent segments must wait to be processed. • Problem for some applications (telephony) • SCTP provides multiple independent streams per association SCTP

  27. SCTP Multistreaming • Logical separation of data within an assoc • Designed to prevent head-of-line blocking • Can be used to deliver multiple objects belonging to the same assoc • Eg: objects on a webpage, multimedia streams (audio/video/text), files in an FTP mget

  28. SCTP Feature Summary Start with TCP: reliable (retransmissions) congestion controlled connection oriented Add: 4-way handshake to reduce vulnerability to DOS attacks framing preserve message boundaries multistreaming instead of one ordered stream, up to 64K independent ordered streams multihoming instead of one IP address per endpoint a set of IP addresses per endpoint SCTP

  29. Multi-homing Internet End-Point A End-Point Z • In TCP, connections made between <IP addr,port> and <IP addr, port> • If a host is multi-homed, you have to choose ONE IP Addr only, at each end • If that interface goes down, so does the connection • With SCTP, you can list as many IP addresses per endpoint as you like • If host is still reachable through ANY of those addresses, connection stays up. SCTP

  30. IP B2 IP B1 IP A1 IP B3 IP A2 IP network SCTP Multi-Homing • Multiple src/dest ip addresses • Use of different physical paths not guaranteed • Peer reachability and path status are monitored (heartbeat) • One selectable default destination • Parameters per path (cwnd, ssthresh, RTT) SCTP

  31. A1 B1 ISP ISP Internet A2 B2 ISP ISP Host A Host B What is SCTP Multihoming? • Hosts pick 1 of 4 possible TCP connections: • {(A1, B1), (A1, B2), (A2, B1), (A2, B2)} • Hosts use 1 SCTP association: • ({A1,A2}, {B1,B2}) • Selectable “primary” dest: Host A → B1 ; Host B → A1 • New data sent only to primary destination SCTP

  32. 1 IP address B1 IP address A1 DATA SCTP Endpoint A SCTP Endpoint B 2 IP address A2 IP address B2 DATA Multihoming Operation SACK SCTP

  33. SCTP – Summary Well suited for Multimedia • Like TCP • Provides connection establishment • Ensures Reliability • Provisions for ordered and un-ordered data • Provides Congestion Control • In addition to TCP features • Provides multi-homing • Provides multi-streaming • Has security features SCTP

  34. SCTP Socket Types • SCTP socket API comes in two forms: one-to-one and one-to-many. • The one-to-many at one time was known by the “UDP style” socket. The one-to-one used to be called the a “TCP style” socket. • So what is the purpose of each socket style and how can it be used? SCTP

  35. One-to-One style • The purpose of the one-to-one style socket is to provide a smooth transition mechanism for those applications running on TCP and wishing to move to SCTP. • The same semantics used in TCP are used with this style. • A server will typically open the socket, make a call to listen (to accept associations), and call accept, blocking upon the arrival of a new association. • The only notable difference between a TCP socket and a SCTP socket is the socket call uses IPPROTO_SCTP instead of IPPROTO_TCP (or 0). SCTP

  36. One-to-One Example Server int sd, newfd, sosz; struct sockaddr_in6 sin6; sosz = sizeof(sin6); sd = socket(AF_INET6, SOCK_STREAM, IPPROTO_SCTP); listen(sd, 1); while (1) { newfd = accept(sd, (struct sockaddr *)&sin6, &sosz) do_child_stuff(newfd, &sin6, sosz); } SCTP

  37. One-to-Many style • A typical server using a one-to-many style socket will do a socket() call, followed by a listen() and recvfrom(). • A typical client will just sendto() the server of his choice. • Note that the connect() and accept() call are not needed. • The connect() call can be done by either side (server or client) but it is not needed. • Note that this style is more like what a UDP client/server would look like thus the previous name. SCTP

  38. One-to-many Example Server int sd, newfd, sosz, msg_flags; struct sockaddr_in6 sin6; struct sndrcvinfo snd_rcv; char buf[8000]; sosz = sizeof(sin6); sd = socket(AF_INET6, SOCK_SEQPKT, IPPROTO_SCTP); listen(sd, 1); while (1) { len = sctp_recvmsg(sd, buf, sizeof(buf), (sockaddr *)&sin6, &sosz, &snd_rcv, &msg_flags); do_child_stuff(newfd, buf, len, &sin6, &snd_rcv, msg_flags); } SCTP

  39. SCTP Notifications • The SCTP stack, at times, has information it may wish to share with its application (or Upper Layer Protocol ... ULP). • The ULP can turn off and on specific notifications via a socket options call. • By default ALL notifications are off. • We can get a notification • By reading data and looking at the msg_flags, if the message read is a notification, then "MSG_NOTIFICATION” is contained within the msg_flags argument upon return. SCTP

  40. Deciphering Notifications • Every Notification uses a TLV format as illustrated below: • Type of notifications • SCTP_ASSOC_CHANGE • SCTP_PEER_ADDR_CHANGE • SCTP_REMOTE_ERROR • SCTP_SEND_FAILED • SCTP_SHUTDOWN_EVENT • .... struct sctp_tlv { u_int16_t sn_type; u_int16_t sn_flags; u_int32_t sn_length; }; SCTP

  41. Socket Options • SCTP provides a host of socket options to perform a mirad of operations. • Some have unique structures others just turn things on and off with boolean's or integers. • SCTP_NODELAY • SCTP_MAXSEG • SCTP_ASSOCINFO • SCTP_AUTOCLOSE • SCTP_ADAPTION_LAYER • SCTP_DEFAULT_SEND_PARAM • SCTP_DISABLE_FRAGMENTS • ... SCTP

  42. Extended “system calls”. • sctp_connectx • Allows a user to specify multiple address to attempt to connect too. • sctp_bindx • Allows an application to bind a set of addresses instead of one or all addresses. • sctp_opt_info • Some implementations do not support a getsockopt() call that allows data to be passed both ways. This call is compatible with all implementations. • sctp_peeloff • this call is used to convert a single association that is part of a one-to-many socket into an individual new socket descriptor that is a one-to-one socket.

  43. Extended “system calls” • sctp_getpaddrs • This call will return a block of memory holding the peers addresses currently part of the association. • sctp_freepaddrs • This call is used to release the memory back that the sctp_getpaddrs call allocated. • sctp_getladdrs • This call will return a block of memory holding the local addresses bound to an association. • sctp_freeladdrs • This call should be used to release the memory allocated by sctp-getladdrs back to the system. SCTP

  44. Extended “system calls” • sctp_sendmsg • This call will allow the caller to specify on the command line things like the stream number and other SCTPish information to be sent with a message. • sctp_send • This call has a similar purpose to sctp_sendmsg but instead of a large number of command line options, a sctp_sendrcvinfo structure is used to pass the relevant information. • sctp_recvmsg • This call (as we saw previously) is used to receive a message but also a sctp_sendrcvinfo structure with details on the message (e.g. The stream number and stream sequence number). SCTP

  45. Summary • SCTP is a new transport protocol • available now in bleeding edge Linux and BSD kernels,and will make its way into the mainstream • It has some cool new features SCTP

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