Chapter 6. What happens at the Transport Layer? Services provided Transport protocols UDP TCP Flow control Congestion control

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1 Chapter 6 What happens at the Transport Layer? Services provided Transport protocols UDP TCP Flow control Congestion control

2 OSI Model Hybrid Model Software outside the operating system Software inside the operating system Only Internet addresses used Physical addresses used

3 provide efficient, reliable, and cost-effective data transmission service to users (application processes); the Transport Layer makes use of the services provided by the Network Layer; the software and/or hardware within the Transport Layer that does the work is called the transport entity; the transport entity can be located in the operating system kernel, in a library package bound into network applications, in a separate user process, or even on the network interface card; connection-oriented and connectionless services

4 to allow users to access the transport service, the Transport Layer provides some operations to application programs - a transport service interface; provides a reliable service on top of an unreliable network; the Transport Layer can also provide an unreliable (datagram) service;

5 Transport Control Protocol (TCP) bidirectional connection-oriented protocol; provides reliable unicast end-to-end data transfer over an unreliable internetwork; User Datagram Protocol (UDP) simple, unreliable, best-effort datagram transfer; Stream Control Transmission Protocol (SCTP) provides a reliable transport service supporting sequenced delivery of messages within multiple streams, maintaining application protocol message boundaries; Datagram Congestion Control Protocol (DCCP) provides a congestion controlled, unreliable flow of datagrams suitable for use by applications such as streaming media;

6 UDP does not guarantee that messages arrive, does not guarantee messages arrive in the same order they are sent, and does not provide any mechanisms to control the rate at which information flows between a pair of communicating hosts. UDP messages can be lost, duplicated, or arrive out of order. Furthermore, packets can arrive faster than the recipient can process them.

7 the length of the UDP datagram including the UDP header counted in bytes the port number used by the sending application layer process the port number used by the receiving application layer process

8 IP address + port number IP address Transport Layer addresses identify communicating application processes (end-toend significance). 16-bit port numbers enable the multiplexing and demultiplexing of packets at the Transport Layer Port number allocation can be statically assigned or dynamic.

9 UDP messages contain protocol port numbers that are used to distinguish among multiple applications executing on a single computer. In addition to the data sent, each UDP message contains both a destination port number and a source port number, making it possible for the UDP software at the destination to deliver an incoming message to the correct recipient and for the recipient to send a reply.

10 The primitives for a simple transport service - they allow application programs to establish, use, and then release connections;

11 hostname 1 FTP hostname IP address resolver FTP 2 TCP TCP IP address + port number IP ARP ARP IP IP address Ethernet Driver Ethernet Driver 1. FTP client calls gethostbyname(host) 2. FTP client asks TCP to establish a connection with the IP address (CONNECT primitive) 3. TCP sends a CONNECTION REQUEST packet to the host IP address. 44. If the server is in LISTEN state, it responds with CONNECTION ACCEPTED. 55. Connection is now established. Data can now be sent and received using the SEND and RECEIVE primitives. When the connection is not required anymore, a DISCONNECT REQUEST will be sent.

12 the socket primitives as they are used for TCP; sockets were first released as part of the Berkeley UNIX distribution in 1983; now widely used for Internet programming on many operating systems; more features and flexibility;

13 TCP views the data stream as a sequence of octets that it divides into segments for transmission. Usually, each segment travels across the underlying internet in a single IP datagram. TCP uses a specialized sliding window mechanism that optimizes throughput and handles flow control. The TCP window mechanism makes it possible to send multiple segments before an acknowledgement arrives. The TCP form of a sliding window protocol also solves the end-to-end flow control problem by allowing the receiver to restrict transmission until it has sufficient buffer space to accommodate more data.

14 Each TCP connection is specified by a pair of endpoints that correspond to the pair of communicating applications. A connection consists of a virtual circuit between two application programs. TCP defines an endpoint to be a pair of integers (host, port), where host is the IP address for a host and port is a TCP port on that host. ( , 1069) and ( , 25) ( , 1184) and ( , 53) ( , 1184) and ( , 53) Because TCP identifies a connection by a pair of endpoints, a given TCP port number can be shared by multiple connections on the same machine.

15 The nesting of TPDUs, packets, and frames.

16 Wireshark used by the sending application layer process used by the receiving application layer process sequence number of the first data byte in the segment contains the next sequence number which the sender of the acknowledgement expects length of the TCP header including any options, counted in 32-bit words.

17 indicates the number of data bytes which the sender of the segment is willing to receive Internet checksum computed over the pseudo header, the TCP header and the data contained in the TCP segment series of flags points, relative to the actual segment number, to important data if the URG flag is set

18 Each message carries two protocol port numbers: a destination port number specifies a port on the destination computer to which the message has been sent, and a source port number specifies a port on the sending machine from which the message has been sent.

19 Application programs at both ends of the connection must agree that the connection is desired. The application program on one end performs a passive open by contacting the local operating system and indicating that it will accept an incoming connection for a specific port number. The protocol software prepares to accept a connection at the port. The application program on the other end can then perform an active open by requesting that a TCP connection be established.

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21 The states used in the TCP connection management finite state machine.

22 The finite state machine of TCP. Labels on state transitions show the input that created the transition followed by the output, if any.

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25 The performance of sliding window protocols depends on the window size and the speed at which the network accepts packets. Because a well-tuned sliding window protocol keeps the network completely saturated with packets, it can obtain substantially higher throughput than a simple stop-andwait protocol.

26 The TCP sliding window mechanism makes it possible to send multiple segments before an acknowledgement arrives. The TCP form of a sliding window protocol also solves the end-to-end flow control problem by allowing the receiver to restrict transmission until it has sufficient buffer space to accommodate more data. Window management in TCP.

27 Because TCP connections are full duplex, two transfers proceed simultaneously over each connection, one in each direction. Thus, TCP software on a computer maintains two windows per connection: one window slides along as the data stream is sent, while the other slides along as data is received. Window management in TCP.

28 TCP allows the window size to vary over time. Each acknowledgement, which specifies how many octets have been received, contains a receiver flow control window advertisement that specifies how many additional octets of data the receiver is prepared to accept beyond the data being acknowledged. Window management in TCP.

29 Because TCP sends data in variable length, an acknowledgement cannot easily refer to a datagram or a segment. Instead, an acknowledgement refers to a position in the stream using the stream sequence numbers. The receiver collects data octets from arriving segments and reconstructs an exact copy of the stream being sent. Because segments travel in IP datagrams, they can be lost or delivered out of order; the receiver uses the sequence number in each segment to know where the data in the segment fits into the stream. A TCP acknowledgement specifies the sequence number of the next octet that the receiver expects to receive.

30 Every time it sends a segment, TCP starts a timer and waits for an acknowledgement. If the timer expires before data in the segment has been acknowledged, TCP assumes that the segment was lost or corrupted and retransmits it. RTT is impossible to predict over the Internet. TCP uses an adaptive retransmission algorithm. That is, TCP monitors the round trip time on each connection and computes reasonable values for timeouts. As the performance of a connection changes, TCP revises its timeout value.

31 TCP s performance is also affected by congestion. Whereas flow control referred to the amount of data a receiver was able to process, congestion control refers to the amount of data that can be in transit over the network (defined by the congestion window size).

32 Let s check this with Wireshark! When too much traffic is offered, congestion sets in and performance degrades sharply.

33 The congestion window (cwnd) is maintained by a TCP sender in addition to the flow control receiver window (rwnd) which is advertised by the receiver. The sender uses these two windows to limit the data that is sent to the network and not yet received (flight size) to the minimum of the receiver and the congestion window: flightsize = min(cwnd; rwnd) The key problem to be solved is the dynamic estimation of the congestion window.

34 The initial window (IW) is usually initialized using the following formula: IW = min(4 SMSS; max(2smss; 4380bytes)) SMSS is the sender maximum segment size, the size of the largest segment that the sender can transmit. sstresh sstresh During slow start, the congestion window (cwnd) increases by at most SMSS bytes for every received acknowledgement that acknowledges data. Slow start ends when cwnd exceeds ssthresh or when congestion is observed.

35 This algorithm leads to an exponential increase if there are multiple segments acknowledged in the cwnd. During congestion avoidance, cwnd is incremented by one full-sized segment per round-trip time (RTT). Congestion avoidance continues until congestion is detected. sstresh sstresh One formula commonly used to update cwnd during congestion avoidance is given by the following equation: cwnd = cwnd + (SMSS * SMSS/cwnd)

36 When congestion is noticed (the retransmission timer expires), then cwnd is reset to 1 full-sized segment and the slow start threshold ssthresh is updated as follows: ssthresh = max(flightsize/2; 2*SMSS) sstresh sstresh

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