Advanced Network Programming Chapter 1

1. Advanced Network ProgrammingChapter 1 Introduction to Transport Layer and TCP

2. Networking (OSI) Reference Model • OSI 7-layer Reference Model • Application (WEB, NFS, FTP, Telnet, etc.) • Presentation (Conversion, Compression, Cryptography) • Session (Synchronization) • Transport (End-to-end Messages) • Network (Packet Routing) • Data Link (Station-to-station Frames) • Physical (Bit Transmission) A. Özgit - Advanced Network Programming

3. Networking • Distinction between service and protocol is important! • This will be discussed later. • Some widely known transport protocols • UDP, TP0, TP4, SNA-APPN, DECnet-NSP, ATM, XTP, T/TCP, RTP, VMTP, NETBLT A. Özgit - Advanced Network Programming

4. Transport Layer (Layer-4) • Lowest layer that operates on an end-to-end basis. • Lies at the boundary between hosts and an internetwork of routers, bridges, and communication links. • A good transport layer service • Allows applications to use a standard set of primitives. • Run on variety of networks – w/o worrying about different network interfaces and reliabilities. • Isolates applications from the technology. A. Özgit - Advanced Network Programming

5. Transport Layer (Layer-4) • Layer-4 provides interprocess communication between two processes that most often are running on different hosts. • TCP and its companion UDP (User Datagram Protocol) are the most widely used protocols. • Other are from IBM’s SNA, and Digital’s (Compaq) DECnet. • Connection to proprietary protocol suites. • Ongoing research • tcp-impl WG of IETF (www.ietf.org) • end2end WG of IRTF (www.irtf.org) A. Özgit - Advanced Network Programming

6. Transport Layer (Layer-4) • Basic Issues • Addressing • Connection-oriented (CO) vs. Connectionless (CL) • Reliability • Loss • Duplicate • Ordering • Integrity • Blocking vs. Non-Blocking • Multicast, Unicast • Priority • Security • Status Reporting A. Özgit - Advanced Network Programming

7. Role of TCP • The Web • An example of client/server application • Web browser (client) • Runs on “local” machine • Communicates with a server on some “remote” machine • Uses an application layer protocol called the HTTP (Hypertext Transfer Protocol). • HTTP is a simple request/response protocol. • We will use HTTP 0.9 (the simplest) in examples. A. Özgit - Advanced Network Programming

8. Role of TCP • Web browser (client) • Access TCP’s service thru function calls that comprise that Transport Layer’s Application ++++-Programming Interface (API). • API provides • (at a minimum) functions to send and receive messages • e.g. Berkeley Sockets – read(), write() • Connection setup and close for CO protocols • e.g. connect(), close() A. Özgit - Advanced Network Programming

9. Terminology • Simplified Communication Model (OSI) – Figure-1 • User Sender / User Receiver at the top • Application Entities use the services of the transport layer • Peer Entities exchange Protocol Data Units (PDUs) • APDU • The request “get /index.html” • sent from client (application entity) to the server (its peer application entity). • Bi-directional Protocol • Both sides can send and receive data simultaneously. A. Özgit - Advanced Network Programming

10. Terminology • Transport Entity • Hardware and/or software within a given host that implements a particular transport service and protocol. • User Sender • submits a chunk of user data (Transport Service Data Unit – TSDU; informally a message) to the transport sender. • Transport Sender • transmits or sends this data to the transport receiver over a network which may provide different levels of reliability. A. Özgit - Advanced Network Programming

11. Terminology • Transport Receiver • receives the data that arrives from the network and delivers it to the user receiver. • TPDUs may flow in both directions even when user data flows only from sender to receiver • Control TPDUs • Separate and/or Piggybacked A. Özgit - Advanced Network Programming

12. Terminology • What happens to the request APDU? • APDU becoming a single TSDU, being encapsulated in a single TPDU, which in turn becomes a single NSDU, which is encapsulated in a single NPDU (Figure-2) • TCP’s TPDU  TCP segment • Packet  IP datagram (NPDU) or TCP segment (informally) • IP’s PDU  Datagram • Datagram  IP’s NPDU or UDP’s TPDU (informally) A. Özgit - Advanced Network Programming

13. Example TCP Connection (1) • Enter “http://ozgit.nom.tr/index.html” from web client. • http indicates application layer protocol to be used. • TCP port number 80 (implicitly) to be used. • “ozgit.nom.tr” is the host name (mapped to an IP number –144.122.71.91- by DNS) • Transport Service Access Point (TSAP) • TSAP  IP Address + TCP Port Number • One end point of a communication channel between a process on a local m/c and a process on a remote m/c. • “index.html” is the file being requested. • http request (APDU)  “GET /index.html” A. Özgit - Advanced Network Programming

14. Example TCP Connection (2) • Connection request to the transport entity at (144.122.71.91, 80). • By calling connect() • Local TCP initiates a 3-way handshake with the remote server. • TPDUs are exchanged between TCP entities to ensure reliable connection establishment and toestablish initial sequence numbers. • If 3-way handshake fails, TCP notifies the application. • Otherwise success code is returned -confirmation. • OSI Model: • Request – Indication • Response - Confirmation A. Özgit - Advanced Network Programming

15. Example TCP Connection (3) • Web client submits a request to send data (APDU – “GET /index.html”) • Local TCP sends this data most likely in a single TPDU. • TCP Segment  TSDU + Transport Layer Header A. Özgit - Advanced Network Programming

16. Example TCP Connection (4) • Remote TCP receives the TPDU, the data (APDU – “GET /index.html”) is buffered. • Delivered when Web server does a read() • This delivery is known as a data indication in OSI terminology. • Remote TCP also sends back an acknowledgement (ACK) -control TPDU- to the local TCP A. Özgit - Advanced Network Programming

17. Example TCP Connection (5) • The Web server responds with contents of “index.html”. • File may be too large to be efficiently submitted to TCP in one write() call –i.e., one TSDU. • Web Server divides APDU into multiple write() calls –i.e., multiple TSDUs. • Remote TCP then sends these TSDUs to local the TCP in multiple TPDUs. • TCP treats the data as a byte stream and segments it as necessary –i.e., does not care about TSDU boundaries. • Boundaries between APDUs, submitted TSDUs, TPDUs, and delivered TSDUs may all be different. A. Özgit - Advanced Network Programming

18. Example TCP Connection (6) • TCP must detect and recover from network errors. • As the remote TCP send the TPDUs, it includes a sequence number in each TPDU. • It also copies each TPDU into a buffer, and sets a timer. • Retransmits the TPDU if timer expires before getting an ACK. • Retransmission is done in a new TPDU. • Individual byte-stream sequence numbers are used. • TPDUs retransmitted may or may not correspond exactly to the original TPDUs. • Remote TCP also places a checksum is the TPDU header to detect bit errors. A. Özgit - Advanced Network Programming

19. Example TCP Connection (7) • As TPDUs are received by the local TCP • TPDUs with checksum errors are discarded. • It ensures that no pieces of the byte-stream are missing • Out-of-order arrivals are reordered. • It responds to the remote TCP with ACK TPDUs. • Duplicates are discarded (e.g., as a consequence of lost ACK TPDUs). • Pieces of byte-stream are buffered in local TCP • Web client requests them by doing read() calls. • Each read() results in delivery of a TSDU. A. Özgit - Advanced Network Programming

20. Example TCP Connection (8) • TCP connection is bi-directional. • Either side may initiate the closing of the connection • In first generation web systems the server initiates the close by calling close() function (Disconnect Request). • Disconnect is handled with a 4-way handshake procedure. A. Özgit - Advanced Network Programming

21. Transport Service • A transport service abstracts a set of functions that is provided to a higher layer. • A protocol, refers to the details of how a transport sender and a transport receiver cooperate to provide that service. • Distinction between service and protocol is important (Contribution of OSI Reference Model). A. Özgit - Advanced Network Programming

22. CO-message vs. CO-byte vs. CL • Two types of transport services • Connection-oriented (CO) • Provides for the establishment, maintenance, and termination of a logical connection between transport users (three distinct phases of operation). • Connection Establishment (T-Connect) • Data Transfer (T-Data) • Connection Termination (T-Disconnect) • CO service has two variations • Message-oriented (TP4) • Byte-stream • Connectionless (CL) • Provides only one phase of operation: data transfer. A. Özgit - Advanced Network Programming

23. Reliability • A service is reliable if and only if it satisfies all of the following: • No-loss • No-duplicates • Ordered • Data Integrity A. Özgit - Advanced Network Programming

24. No-loss vs. Uncontrolled-loss vs. Controlled-loss • No-loss (at-least-once delivery) service guarantees either of the two results: • The data is delivered to the user receiver, or • The user sender is notified that some data may not have been delivered. • Uncontrolled-loss (best-effort) • No assurance • Example: UDP • Controlled-loss • Loss may occur, but there is control over the degree of loss. • Example: k-XP A. Özgit - Advanced Network Programming

25. No-duplicates vs. Maybe-duplicates • No-duplicates • At-most-once delivery • e.g., TCP • Maybe-duplicates • Efforts by the protocol may or may not be made to avoid delivering duplicates. • e.g., UDP A. Özgit - Advanced Network Programming

26. Ordered vs. Unordered vs. Partially-ordered • Ordered service • Preserves user sender’s submission order of data. • e.g., TCP • Unordered service • Does not provide the above guarantee. • e.g., UDP • Partially-ordered service • Guarantees to deliver pieces of data in one of a set of permitted orders as predefined by a partial order relation agreed upon by the user sender and user receiver. • e.g., Multimedia comm., distributed databases. A. Özgit - Advanced Network Programming

27. Data-integrity vs. No-data-integrity vs. Partial-data-integrity • Data-integrity • Ensures with high probability that all data bits delivered to a user receiver are identical to those originally submitted. • Strength of the error detection method. • TCP uses 16-bit checksum. • No-data-integrity • Provide no guarantees regarding bit errors. • Partial-data-integrity • A controlled amount of bit errors (as a means of achieving higher throughput). • e.g., real-time multimedia application A. Özgit - Advanced Network Programming

28. Remarks on Reliability and CO vs. CL • All aspects of reliability (loss, duplicates, order, data-integrity) are orthogonal. • Data might get lost while the order is preserved. • Relationship between a service being CO or CL and whether or not it is reliable. • These two services are orthogonal. • CO service is assumed to be reliable. Why? A. Özgit - Advanced Network Programming

29. Remarks on Reliability and CO vs. CL • Whereas: TCP service is CO and TCP service is reliable, • Whereas: TP4 service is CO and TP4 service is reliable, • Whereas: X.25 service is CO and X.25 service is reliable • CO service  Reliable Service ? A. Özgit - Advanced Network Programming

30. Remarks on Reliability and CO vs. CL • Whereas: UDP service is CL and UDP service is unreliable, • CL service  Unreliable Service ? A. Özgit - Advanced Network Programming

31. Blocking vs. Non-blocking • Blocking service • Ensures that the transport layer is not overwhelmed with incoming data. • Provides flow control between user sender and transport sender. • Non-blocking service • Allows the user sender to submit data and continue processing w/o awaiting the transport sender’s OK. A. Özgit - Advanced Network Programming

32. Multicast vs. Unicast • Multicast service • Enables a user sender to submit data, a copy of which will be delivered to one or more user receiver(s). • Unicast service • Delivery of data to exactly one user receiver. A. Özgit - Advanced Network Programming

33. Priority vs. No-priority • Priority service • Enables a user sender to indicate the relative importance of various messages. • May be combined with uncontrolled-loss or controlled-loss service to drop lower priority data, thereby allowing the delivery of high-priority data with smaller delay and/or higher probability. • No-priority service • No differentiation of the importance of the classes of data. A. Özgit - Advanced Network Programming

34. Security vs. No-security • Security service • A security service provides one or more security functions such as authentication, access control, confidentiality, and integrity [ISO-7498-2]. • Authentication is the verification of user sender’s and user receiver’s identity. • Access control checks a user’s permission status, allowing the use of different resources. • Confidentiality guarantees that only the intended user receiver(s) can decode and understand the user sender’s data. • Integrity detects any modification, insertion, deletion, or replay of transport sender’s data. • e.g., TP4 • No-security service • Does not provide any of the above security functions. A. Özgit - Advanced Network Programming

35. Status-reporting vs. Non-status-reporting • Status-reporting service • Allows a user sender to obtain specific information about the transport entity or its connections. • Non-status reporting service • Does not provide any information about the transport entity and its connections. A. Özgit - Advanced Network Programming

36. QoS vs. No-QoS • QoS service • Allows a user sender to specify the quality of transmission service desired. • No-QoS service • Delivery of data to exactly one user receiver. A. Özgit - Advanced Network Programming

37. QoS Parameters (ISO) • Connection Establishment Delay • Connection Establishment Failure Probability • Throughput • Transit Delay • Residual Error Rate • Transfer Failure Probability • Connection Release Delay • Connection Release Failure Probability A. Özgit - Advanced Network Programming

38. QoS Parameters (ISO) • Protection • Priority • Resilience • The ATM environment supports only two QoS parameters: • (sustained) target, acceptable, and minimum throughput • Transit delay A. Özgit - Advanced Network Programming

39. Transport Protocol Features • CO vs. CL • Establishment and maintenance of state information • A record of characteristics and events related to the communication between the transport sender and receiver. • CO: state information is maintained • Three phases: • Connection Establishment • Data Transfer • Connection Termination • CL: no state information is maintained A. Özgit - Advanced Network Programming

40. Transport Protocol Features • Transaction Oriented • A single APDU (request) is sent by user sender • User receiver responds with a single APDU (response) • Characteristics: • Asymmetrical model • Simplex data transfer • Short duration • Low delay • Few data TPDUs • Message orientation • Need for a no-duplicate service A. Özgit - Advanced Network Programming

41. Transport Protocol Features • CO Protocol Features • Signaling – exchange of control (state) information • In-band (more suitable for short-lived connections) • Out-of-band (desirable for high-speed communication systems) • Unidirectional vs. Bidirectional A. Özgit - Advanced Network Programming

42. Transport Protocol Features • Connection Establishment (See Figure-3) • Implicit connect • Connection is established as soon as the first TPDU is sent or received. • 2-way-handshake connect • CR-TPDU (Connection Request) • CC-TPDU (Connection Confirm) • 3-way-handshake connect • CR-TPDU (Connection Request) • CC-TPDU (Connection Confirm) • ACK-CC-TPDU (ACK for Connection Confirm) A. Özgit - Advanced Network Programming

43. Transport Protocol Features • Connection Termination (See Figure-4) • Implicit disconnect • Time-out • Abortive disconnect • Close connection abnormally due to an error condition • 2-way-handshake disconnect • DR-TPDU (Disconnect Request) • DC-TPDU (Disconnect Confirm) • 4(3)-way-handshake disconnect • Two 2-way-handshakes one for each direction of data flow • 3-way if the first DC-TPDU also functions as a DR-TPDU for the reverse direction A. Özgit - Advanced Network Programming

44. Error Control • Guard against loss or damage of user data and control information • For realistic high-speed networks with low error rates, transport layer error control is more efficient than link layer error control. • Two phases: • Error detection • Error reporting and recovery A. Özgit - Advanced Network Programming

45. Error Control • Error Detection • Identifies lost, misordered, duplicated and corrupted TPDUs • Sequence numbers handles the first three problems • Corrupted data is discovered by means of: • Length fields • Error Detecting Codes (EDC) • The header/trailer, the data, or the both • Separate EDCs are recommended for multimedia applications A. Özgit - Advanced Network Programming

46. Error Control • Error Reporting and Recovery • Error reporting is a mechanism where receiver explicitly informs the sender about errors that have been detected. • Error recovery is a mechanism used by both sender and receiver to recover from errors whether or not they are explicitly reported. • Timers, sequence numbers and acknowledgements are used. A. Özgit - Advanced Network Programming

47. Error Control • Error Reporting and Recovery • A positive ACK (PACK) • PAR (Positive ACK with Retransmission) or ARQ (Automatic Repeat Request) • Upon receipt of an ACK, the sender updates its state information, discards buffered TPDUs that are acknowledged, and retransmits any TPDUs that are not acknowledged. • In case of timeout, it may assume something has gone wrong and retransmits unacknowledged TPDU(s). • No error reporting mechanism A. Özgit - Advanced Network Programming

48. Error Control • Error Reporting and Recovery • A negative ACK (NACK) aka Selective Reject • Explicitly identifies TPDUs that have not been received A. Özgit - Advanced Network Programming

49. Error Control • Piggybacking • Artificially delay returning an ACK hoping the receiver will soon submit its next message to be sent as a part of the reverse direction data flow. • When this occurs, the ACK is piggyback-ed as header information on the reverse direction data TPDU. A. Özgit - Advanced Network Programming

50. Error Control • Cumulative vs. Selective Acknowledgement • Cumulative PACK • Carries a sequence number indicating that all TPDUs with lower sequence numbers have been received. • A recent cumulative PACK incorporates the information of the previously lost one. • Unnecessary retransmissions of correctly received TPDUs. • Selective PACK • Acknowledges exactly one TPDU • Block PACK • Variation of selective PACK where blocks of individual TPDUs are selectively acknowledged. A. Özgit - Advanced Network Programming