Transport Layer TCP and UDP. CT542. Content. Transport layer services provided to upper layers Transport service p rotocols Berkley Sockets An Example of Socket Programming Internet file server Elements of transport protocols Adressing , establishing and releasing a connection
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Transport LayerTCP and UDP
LISTEN – in a client server application, the server executes this primitive, blocking the server until a client turns up
CONNECT – when a client wants to talk to the server, it executes this primitive; the transport entity caries out this primitive (by sending a connection packet request to the server and waiting for a connection accepted response), blocking the caller until the connection is established.
SEND/RECEIVE primitives can be used to exchange data after the connection has been established; either party could do (blocking) RECEIVE to wait for the other party to do SEND; when the TPDU arrives, the receiver is unblocked, does the required processing and sends back a reply
DISCONNECT – when a connection is no longer needed, it must be released in order to free up tables in the transport entities; it can be asymmetric (either end sends a disconnection TPDU to the remote transport entity; upon arrival, the connection is released) or symmetric (each direction is closed separately)
File server example:
File server example:
How a user process in host 1 establishes a connection with a time-of-day server in host 2.
(a) Normal operation.
(b) Old duplicate CONNECTION REQUEST appearing out of nowhere.
(c) Duplicate CONNECTION REQUEST and duplicate ACK.
Four protocol scenarios for releasing a connection.
(a) Normal case of a three-way handshake
(b) final ACK lost – situation saved by the timer
(c) Response lost
(d) Response lost and subsequent DRs lost
Step 1: Client calling the client stub. This is a local procedure call, with the parameters pushed into the stack in the normal way
Step 2: the client stub is packing the parameters into a message and makes a system call to send the message; packing the parameters is called marshaling, and it is a very wide use technique.
Step 3: the kernel is sending the message from the client machine to the server machine, over a transport and network protocol.
Step 4: the kernel on the receiving machine is passing the message from the transport stack to the server stub.
Step 5: the server stub is calling the server procedure with the unmarchaled parameters.
The reply traces the same path in the other direction
(a) Four 512-byte segments sent as separate IP datagrams.
(b) The 2048 bytes of data delivered to the application in a single READ CALL.
Source and destination ports identify the local endpoints for the connection; each host may decide for itself how to allocate the its own ports starting at 1024; a port number plus its host IP form a 48 bits unique TSAP
Sequence Number is associated with every byte that is sent. It is used for a number of different purposes: it is used to re-arrange the data at the receiving end, before passing it to the application; it is used to detect duplicate data and Acknowledgement number fields specifies the next byte expected
TCP header length tells how many 32 bit words are contained in the TCP header; this is required because the Options field is of variable length; technically it indicates the start of data within the segment, measured in 32 bits words.
URG flag is set to 1 if urgent pointer is in use; the urgent pointer is used to indicate a byte offset from the current sequence number at which urgent data is to be found; this facilitates interrupt messages without getting the TCP itself involved in carrying such message types
ACK flag is set to 1 to indicate that the acknowledgement number is valid; if set to 0, then the packet doesn’t contain an acknowledgement, so the appropriate field is ignored (the Acknowledgement number field is ignored)
PSH flag indicates pushed data, so the receiver is requested to deliver the received data to the application upon arrival, without buffering it to form a full buffer has been received
RST flag is used to restart a connection that has become confused due to a host crash or any other reasons; it is also used to reject an invalid segment or refuse an attempt to open a connection
SYN flag is used to establish connections; the connection requests have SYN = 1 and ACK = 0 to indicate that the piggyback acknowledgement field is not in use; the connection response does bear an acknowledgment so it has SYN = 1 and ACK = 1;
FIN flag is used to release connections; it specifies that the sender has no more data to transmit; Both SYN and FYN segments have sequence number and thus guaranteed to be processed in correct order
Flow control in TCP is done using variable size sliding window; the Window size field tells how many bytes may be sent starting at the byte acknowledged; a window size field with value 0 is legal and means that bytes up to (Acknowledgement number -1) have been received, and no more accepted
Checksum is provided for extreme reliability. It checksums the header, the data and the conceptual pseudo-header shown on the next slide; when performing the computation, the data field is padded with an additional zero byte if its length is an odd number; the checksum is simply the sum in 1’s complement
Options field was designated to provide extra facilities not covered by the regular header; the most important one is the one that allows each host to specify the maximum TCP payload is willing to accept; all Internet hosts are required to accept at least 536 +20 TCP segments
It is use the three-way handshake protocol
a) Normal case
b) Call collision case, when two hosts are trying to establish a connection between same two sockets
 the server must be prepared to accept an incoming connection; this is normally done by calling socket, bind and listen and it is called passive open
 the client, after the creation of a new socket, issues an active open by calling connect. This causes the client TCP to send a SYN segment (which stands for synchronize) to tell the server the client’s initial sequence number for the data that the client will send on the connection; normally there is no data sent with SYN: it just contains an IP header, a TCP header and possible TCP options
 the server must acknowledge the client’s SYN and the server must also send its own SYN containing the initial sequence number for the data that the server will send on the connection. The server sends SYN and the ACK of the client’s SYN in a single segment
the client must acknowledge the server’s SYN
Note that SYN consumes one byte of sequence space so it can be acknowledged unambiguously
The initial sequence number on a connection is not 0 (to avoid confusion when a host crashes). A clock based scheme is used, with a clock tick every 4 us. For additional safety, when a host crashes, it may not reboot for the maximum packet life time (120s) to make sure that no packets from previous connections are still roaming around the Internet, somewhere.
 the other end receives the FIN and performs a passive close. The received SYN is acknowledged by the TCP; the receipt of the FIN is also passed onto the application as an end-of-file; upon the receipt of FIN, the application will not receive anymore any data on the socket
 sometime latter, the application that received the end-of-file will close the socket; this will cause its TCP to send a FIN packet
 the TCP on the system that receives the final FIN (the end that did the active close) acknowledges the FIN
 one application calls close first, and we say that this end performs the active close. This end’s TCP sends a FIN segment, which means it is finished sending data
Quality of service
RTTP, RTCP and Transactional TCP
The P bit indicates that the packet has been padded to a multiple of 4 bytes; the last padding byte tells how many bytes were added
The X bit tells that an extension header is present; the format and meaning of the extension header are not defined; the only thing that is defined is the first word of the extension that gives the length; this is an escape mechanism for unforeseen requirements
The CC field tells how many contributing sources are present
The M bit is an application specific marker bit; it can be used to mark the start of a video frame
The Payload type field tells which encoding algorithm has been used (MP3, PCM, etc); since every packet carries this field, the encoding can change during transmission
Sequence number is just a counter that increments on each RTP packet sent; it is used to detect lost packets
The timestamp is produced by the stream’s source to note when the first sample in the packet was made; this field can help reduce jitter by decoupling the playback from the packet arrival time
Synchronization source identifier tells which stream the packet belongs to; it is the method used to multiplex and demultiplex multiple data streams over one single stream of UDP packets.
Contributing source identifier, if any, is used when mixers are present in the studio; in that case, the mixer is the synchronization source and the streams being mixed are listed here.
(a) RPC using normal TPC (b) RPC using T/TCP.