ISDS 409 Ch 5 Network and transport layers
Quality of service
- A special type of connection-oriented messaging in which different connections are assigned different priorities. -Different classes of service are defined, each with different priorities When the transport layer software attempts to establish a connection, it specifies the class of service that connection requires. -When connection established the network ensures that no connections are established that exceed the maximum number of that class on a given circuit. -Resource reservation protocol and real-time streaming protocol both permit application layer software to request connections that have certain minimum data transfer capabilities. -RTSP-> geared toward audio/video streaming -RSVP-> more general purpose. -Both are used to create a connection and request a certain minimum guaranteed data rate. -Once the connection has been established they use Real-Time Transport protocol to send packets across the connection. -RTP contains info about the sending application, packet sequence number, and a time stamp so that the data in the RTP packet can be synchronized with other RTP packets by the application layer software. -RTP combined with UDO . Each real-time packet is first created using RTP and then surrounded by a UDP diagram, before being handed to the IP software at the network layer.
Segmenting
- Data link layer can transmit only messages of certain lengths. It is up the senders transport layer to break the data into several smaller segments that can be sent by the data link layer across the circuit. -At the other end the receivers transport layer must receive all these separate segments and recombine them into one large message. - Segmenting: To take one outgoing message from the application layer and break it into a set of small segments for transmission through the network. -Also means to take the incoming smaller segments from the network layer and reassemble them into one message for the application layer -Incoming packets can either be delivered one at a time or held until all packets have arrived and the message is complete. -TCP responsible for ensuring that the receiver has actually received all segments that have been sent. TCP therefore uses continuous ARQ.] -One challenge at the transport layer is deciding how big to make the segments -Transport layer at the sender negotiates with the transport layer at the receiver to settle on the best segment sizes to use. -The negotiation is done by establishing a TCP connection between the sender and receiver.
Connectionless messaging
- Each packet is treated separately and makes its own way through the network. -No connection is established. -Sends the packets as separate, unrelated, entities, possible that different packets will take different routes through the network, depending on the type of routing used and the amount of traffic. May arrive out of sequence at their destination, since traveling at different speeds. -Senders network layer puts a sequence number on each packet, in addition to information about the message stream to which the packet belongs. -Network layer must reassemble them in the correct order before passing the message to the application layer. TCP?IP can operate as either connection oriented or connectionless -When connection oriented messaging is desired TCP is used. -When connectionless is desired the TCP segment is replaced with a user datagram protocol (UDP) packet. UDP header is much smaller that the TCP header (only 8-bytes) -Connectionless most common when the application data or message can fit into one single message. -HTTP always uses TCP . All application layer uses TCP - UDP is most commonly used for control messages such as addressing, routing control messages and network management.
Transmission Control Protocol/ Internet Protocol (TCP/IP)
- Each transport and network layer protocol performs essentially the same functions, but each is incompatible with the other unless there is a special device to translate between them. - TCP/IP was developed for the US department of Defenses Advanced Research Project Agency network by Vinton Cerf and Bob Kahn in 1974 -It is the transport/network layer protocol used on the internet. -Worlds most popular protocol set, used by almost all BNs and WANs. -Allows reasonable efficient and error-free transmission. -Because it performs error checking, it can send large files across sometimes unreliable networks with great assurance that the data will arrive uncorrupted. -TCP/IP compatible with a variety of data link layers which is why its hecka popular. -Has two parts: -1.) TCP: is the transport layer protocol that links the application layer to the network layer. -It performs segmenting: Breaking the data into smaller PDUs called segments, numbering them, ensuring each segment is reliably delivered, and putting them in the proper order at the destination. 2.) IP: is the network layer protocol and perform addressing and routing. -Used at each of the intervening computers through which the message passes; it is IP that route the message of the final destination. The TCP software needs to be active only at the sender and the receiver because TCP is involved only when data comes from or goes to the application layer.
Transport Layer
- Links the application software in the application layer with the network and is responsible for the end-to-end delivery of the message. -Accepts outgoing messages from the application layer and segments them fro transmission. -The application layer software produces the SMTP packet which is then split into two smaller TCP segments by the transport layer.
Connection-orientated messaging
- Sets up a TCP connection ( also called a session) between the sender and receiver. -To establish a connection the transport layer on both the sender and the receiver must send a SYN ( Synchronize) and receive ACK segment. -This process starts with the sender (usually a client) sending a SYN to the receiver (usually a server). -The server responds with an ACK for the senders/clients SYN and thens ends it own SYN. SYN is usually a randomly generated number that identifies a packet. -The last step is when the client send an ACK for the serves SYN. This is called the three-way handshake, and this process also contains the segment size negotiation. -Once the connection is established the segments flow between the sender and receiver. TCP uses the continuous ARQ (sliding window) to make sure that all segments arrive and to provide flow control. -When the transmission is complete the session is transmitted using a four-way handshake. -Because TCP/IP connection is a full-duplex connection each side of the session has to terminate the connection independently -The sender will start by sending with a FIN to inform the receiver that is finished sending data. -The server acknowledges the FIN sending an ACK. The the serve sends a FIN to the client. -The connection is successfully terminated when the server receives the ACK for its FIN>
Internet Protocol (IP)
- The network layer protocol. Network Layer PDUs are called packets. Two forms of IP are currently in use. The oder from is IP version 4 (IPv4) which has a 192-bit header ( 24 byes). this header contains source and destination addresses, packet length, and packet number. -IP version 4 is being replaced by IP version 6 which as a 320-bit header ( 40 byes) - Reason for the increase in the packet size is an increase in the address size from 32 bits to 128 bits. IPv6 simpler packet structure make it easier to perform routing and support a variety of new approaches. -IPv6 developed because IP addresses were being depleted on the internet. -IPv4 has max of 4.2 billion address, 500 million of these addresses are reserved and cannot be used and the way addresses were assigned in the early days of the internet means that a small number of companies received several million addresses even when they didn't need them -Uses decimals to express addresses -IPv6 has a max of 3.4 X 10^38 addresses. -Uses hexadecimals (base 16) like Etherent to express addresses -Addresses are 8 sets of 2-byte number ( 2001:0809:0600:00d1) -There is a compressed notation that eliminates the leading zeros within each block and blocks that are all zeros (2001:890:600:d1) - Ethernet has a maximum packet size of 1,492 buys so the maximum size of a TCP message filed is IPv4 is used is 1,492-24 = 1,444
Linking the application layer
- When the transport layer receives an incoming message, the transport layer must decide which application program it must be delivered to. -With TCP/IP each application layer software package has a unique port address. -Any message sent to a computer must tell TCP the application layer port address that is to receive the message. -When an application layer generates an outgoing message, it tells the TCP software its own port address ( source port address) and the port address at the destination computer ( the destination port address) -These two port addresses are placed in the first two fields in the TCP segments -Port address can be any 16-bit (2-byte) number. -On the Internet all port addresses have been standardized. - Anyone using web server should set up the web server with a port address of 80, which is called the well known port. Web browser therefore automatically generate a port address of 80 for any web page you click on -Network managers are free to use whatever port addresses they want, but if they use a nonstandard port number then the application layer software on the client must specify the correct port number. ** Figure 5-5 121
Session
-Conversation of two computers. -When the sending computer wants to send a message to the receiver it usually starts by establishing a session with that computer. -The sender transmits the segments in sequence until the conversation is done and then the sender ends the session. This approach to session management is called connection-oriented messaging. - Sometimes sender only wants to send one short information message or a request, the sender may choose not to start a session but just send the one quick message and move on. This approach is called connectionless messaging.
Addressing
-One address is used by the data link layer another by the network layer and still another by the application layer. -Application layer address ( service name) : when a user work with application software -
Most commonly used set of transport and network layer protocols
-TCP/IP
Segment
-The protocol data unit PD at the transport later. -The network layer takes the messages from the transport layer and routes them through the network by selecting the best path from computer to computer through the network ( and adds an IP packet). -The data link layer adds an Ethernet frame and instructs the physical layer hardware when to transmit -Each layer in the network has its won set of protocol that are used to hold the data generated by higher layers, much like a set of matryoshka ( Russian dolls) -Network and transport layers also accept incoming messages from the data link layer and organize them into coherent messages that are passed to the application layer. -Transport layer at the sender would break the message into several smaller segments and give them to the network layer to route, which in turn gives them to the data link layer to transmit. -The network layer at the receiver would receive the individual packets from the data link layer, process them, and pass them to the transport layer, which would reassemble them into the one email message before giving it to he application layer.
Transmission Control Protocol
-Typical TCP segment has a 192-bit header ( 24 bytes) of control information -It contains the source and destination port identifier. The destination port tells the TCP software at the destination to which application layer program the application layer packet should be sent, whereas the source port tells the receiver which application layer program the packet is from. _TCP segment provides a sequence number so that the TCP software at the destination can assemble the segments into the correct order and make sure that no segments have been lost. -Option field; optional rarely used. Resulting in a 20- byte long TCP header. -Header length field is used to tell the receiver how long the TCP packet is- whether the options field is included. -TCP/IP has a second type of transport layer protocol called User Datagram Protocol (UDP) -UDP PDUs are called diagrams. UDP is used when the sender needs to send a single small packet to the receiver. -UDP Diagram, has only four fields ( 8-bytes of overhead) plus the application layer packet: source port, destination port, length, and a CRC-16. -UDP does not check for lost messages so occasionally a UDP datagram is lost and the message must be resent.