1

Jaringan KomputerTransport Layer

2

The Transport LayerThe heart of the whole protocol hierarchyTask:

To provide reliable, cost-effective data transport from the source machine to the destination machine, independently of the physical network or networks currently in useThe service is provided to its users, normally processes in the application layerMakes use of the services provided by the network layer

Without the transport layer, the whole concept of layered protocols would make little sense

2

3

The Transport Layer

ServicesElementsA Simple Transport ProtocolInternet Transport Protocol:

UDPTCP

Performance Issues

4

The Transport Layer

ServicesElementsA Simple Transport ProtocolInternet Transport Protocol:

UDPTCP

Performance Issues

3

5

ServicesSimilar to Network Layer:

Connection Oriented & ConnectionlessBut:

Transport code runs entirely on the users' machines, the network layer mostly runs on the routers, which are operated by the carrier (at least for a wide area network)What happens if the network layer offers inadequate service? Suppose that it frequently loses packets? What happens if routers crash from time to time?

The existence of the transport layer makes it possible for the transport service to be more reliable than the underlying network service

6

ServicesMany people have traditionally made a distinction between layers 1 through 4 on the one hand and layer(s) above 4 on the other

The bottom four layers can be seen as the transport service providerThe upper layer(s) are the transport service user

This distinction of provider versus user has a considerable impact on the design of the layers and puts the transport layer in a key position, since it forms the major boundary between the provider and user of the reliable data transmission service

4

7

Service Primitives

8

TPDU

5

9

Connection ManagementA state diagram for

a simple connection

management scheme

Transitions labeled in italics are caused by packet arrivals. The solid lines show the client's state sequence. The dashed lines show the server's state sequence

5/2/2006 10

Berkeley Sockets (Internet)Another set of transport primitives, the socket primitives used in Berkeley UNIX for TCPWidely used for Internet programmingOffer more features and flexibility

6

11

The Transport Layer

ServicesElementsA Simple Transport ProtocolInternet Transport Protocol:

UDPTCP

Performance Issues

12

Elements of Transport Protocols

In some ways, transport protocols resemble the data link protocols Both have to deal with error control, sequencing, and flow controlSignificant differences due to major dissimilarities between the environments in which the two protocols operate

At the data link layer, two routers communicate directly via a physical channelAt the transport layer, this physical channel is replaced by the entire subnet

7

13

Environment

(a) Environment of the data link layer(b) Environment of the transport layer.

14

DifferencesRoute: In transport layer, explicit addressing of destinations is requiredEstablishing a connection:

Over the wire is simpleIn transport layer, initial connection establishment is more complicated

Potential existence of storage capacity in the subnetAmount rather than of kind

BufferingFlow control

8

15

Addressing

TSAP (Transport Service Access Point): to specify which one to connect to

16

AddressingTSAP addresses

Stable: every conceivable server listening at a well-known TSAPworks for services that never change (e.g. the Web server)

Not stable:works for user process that often want to talk to other user processes that only exist for a short time and do not have a TSAP address that is known in advanceSchemes:

initial connection protocol: each machine wishes to offer services to remote users has a special process server that acts as a proxy name server/directory server: when a new service is created, it must register itself with the name server, giving its service name and its TSAP

9

17

Connection EstablishmentIs it as simple as just send a CONNECTION REQUEST TPDU to the destination and wait for a CONNECTION ACCEPTED reply? NO!Problems occur:

Packet lostTraffic jam

Packet (delayed) duplication

Solution (?):Use throw-away transport address (create when needed)Connection identifier

Flaw: huge history information & lost if crashed

18

Other SolutionsRestrict packet lifetime:

Restricted subnetHop counterTimestamp

If machine losing all memory:Clock-based (Tomlinson):

equipping each host with a time-of-day clock low-order k bits of the clock are used as the initial sequence numberconnection must first be established

10

19

Other Solutions

Problem: Incorrect connection establishment

three-way handshake (Tomlinson)

(a) Normal operation (b) Duplicate CR(c) Duplicate CR and duplicate

ACK

20

Connection Release

Two styles of terminating a connection:asymmetric release:

when one party hangs up, the connection is brokenabrupt and may result in data loss

symmetric release: treats the connection as two separate unidirectional connectionsand requires each one to be released separatelya host can continue to receive data even after it has sent a DISCONNECT TPDUonly if it s obvious to terminate (fixed data and time is known)

11

21

Connection Release

Other situationProtocol that say: I am done. Are you done too? If responds: I am done too. Goodbye, the connection can be safely released. Problem:

two-army problem:2,3,4 (or more) way handshake protocol?

substitute ''disconnect'' for ''attack.'' If neither side is prepared to disconnect until it is convinced that the other side is prepared to disconnect too, the disconnection will never happen.

22

Protocols for connection release – 3 way is adequate

Four protocol scenarios for releasing a connection

(a) Normal case of three-way handshake

(b) Final ACK lost(c) Response lost(d) Response lost and

subsequent DRs lost

12

23

Flow Control and BufferingTL vs. DL:

Similarity: in both layers a sliding window or other scheme is needed on each connection to keep a fast transmitter from overrunning a slow receiverDifference: a router usually has relatively few lines, whereas a host may have numerous connections. This difference makes it impractical to implement the data link buffering strategy in the transport layer

If the network service is unreliable, the sender must buffer all TPDUs sent, just as in the data link layer

24

Buffer Size

(a) Chained fixed-size buffers (b) Chained variable-sized

buffers(c) One large circular buffer per

connection

13

25

Multiplexing

(a) Upward multiplexing. (b) Downward multiplexing

26

Crash RecoveryIf hosts and routers are subject to crashes, recovery from these crashes becomes an issue

If the transport entity is entirely within the hosts, recovery from network and router crashes is straightforwardIf the network layer provides datagram service, the transport entities expect lost TPDUs all the time and know how to cope with themIf the network layer provides connection-oriented service, then loss of a virtual circuit is handled by establishing a new one and then probing the remote transport entity to ask it which TPDUs it has received and which ones it has not received. The latter ones can be retransmitted

14

27

Strategy

Different combinations of client and server strategyA: ACK, W: write, C: crash

28

The Transport Layer

ServicesElementsA Simple Transport ProtocolInternet Transport Protocol:

UDPTCP

Performance Issues

15

29

The Example Service Primitives

5 primitives: CONNECT, LISTEN, DISCONNECT, SEND, and RECEIVEEach primitive corresponds exactly to a library procedure that executes the primitiveThe parameters for the service primitives and library procedures are as follows:

connum = LISTEN(local)connum = CONNECT(local, remote)status = SEND(connum, buffer, bytes)status = RECEIVE(connum, buffer, bytes)status = DISCONNECT(connum)

30

The Example Transport Entity

Use connection-oriented, reliable network serviceFocus on transport issues that do not occur in the lower layers

connection establishmentconnection releasecredit management

Transport entity may be part of the host's operating system, or it may be a package of library routines running within the user's address spaceAlgorithm

16

31

Graphical Form

The example protocol in graphical form. Transitions that leave the connection state unchanged have been omitted for simplicity

32

Packets & States

States:IDLE— Connection not established yet.WAITING— CONNECT has been executed and CALL REQUEST sent.QUEUED— A CALL REQUEST has arrived; no LISTEN yet.ESTABLISHED— The connection has been established.SENDING— The user is waiting for permission to send a packet.RECEIVING— A RECEIVE has been done.DISCONNECTING— A DISCONNECT has been done locally

17

33

The Transport Layer

ServicesElementsA Simple Transport ProtocolInternet Transport Protocol:

UDPTCP

Performance Issues

34

UDP (User Datagram Protocol)

Internet connectionless protocol in the transport layerUDP is basically just IP with a short header addedUDP provides a way for applications to send encapsulated IP datagrams and send them without having to establish a connectionUDP is described in RFC 768

18

35

UDP Segment & HeaderUDP transmits segments consisting of an 8-byte header followed by the payloadHeader:

36

Remote Procedure CallAllowing programs to call procedures located on remote hosts When a process on machine 1 calls a procedure on machine 2, the calling process on 1 is suspended and execution of the called procedure takes place on 2.

Information can be transported from the caller to the callee in the parameters and can come back in the procedure result.No message passing is visible to the programmer

Idea: to make a remote procedure call look as much as possible like a local one.

In the simplest form, to call a remote procedure, the client program must be bound with a small library procedure, called the client stub, that represents the server procedure in the client's address space.Similarly, the server is bound with a procedure called the server stub. These procedures hide the fact that the procedure call from the client to the server is not local

19

37

Remote Procedure Call

Steps in making a remote procedure call. The stubs are shaded

38

Remote Procedure CallProblems:

Passing pointers is impossible because the client and server are in different address spacesUnspecified array size (no way of determining how large they are)not always possible to deduce the types of the parameters, not even from a formal specification or the code itselfthe use of global variables

Restrictions are needed

20

39

Real-Time Transport ProtocolUsed in real-time multimedia applications

Internet radioInternet telephonymusic-on-demandVideoconferencingvideo-on-demand

Described in RFC 1889The basic function of RTP is to multiplex several real-time data streams onto a single stream of UDP packetsThe UDP stream can be sent to a single destination (unicasting) or to multiple destinations (multicasting)

40

Real-Time Transport Protocol

(a) The position of RTP in the protocol stack(b) Packet nesting

21

41

The Transport Layer

ServicesElementsA Simple Transport ProtocolInternet Transport Protocol:

UDPTCP

Performance Issues

42

TCP (Transmission Control Protocol)

For most Internet applications, reliable, sequenced delivery is needed – TCPSpecifically designed to provide a reliable end-to-end byte stream over an unreliable internetworkDesigned to dynamically adapt to properties of the internetwork and to be robust in the face of many kinds of failuresFormally defined in RFC 793Clarifications of changes and some bug fixes are detailed in RFC 1122Extensions are given in RFC 1323

22

43

TCP EntityEach machine supporting TCP has a TCP transport entity as:

library procedureuser process part of the kernel

It manages TCP streams and interfaces to the IP layerIt accepts user data streams from local processes, breaks them up into pieces not exceeding 64 KB (in practice, often 1460 data bytes in order to fit in a single Ethernet frame with the IP and TCP headers), and sends each piece as a separate IP datagramWhen datagrams containing TCP data arrive at a machine, they are given to the TCP entity, which reconstructs the original byte streams''TCP'' mean

the TCP transport entity (a piece of software) or the TCP protocol (a set of rules)

44

TCP Service Model TCP service is obtained by both the sender and receiver creating end points, called socketsEach socket has a socket number (address) consisting of the IP address of the host and a 16-bit number local to that host, called a portA port is the TCP name for a TSAPFor TCP service to be obtained, a connection must be explicitly established between a socket on the sending machine and a socket on the receiving machineA socket may be used for multiple connections at the same time (2 / more connections may terminate at same socket) Connections are identified by the socket identifiers at both ends. No virtual circuit numbers or other identifiers are used.

23

45

PortPort numbers below 1024 are called well-known ports and are reserved for standard services

For example, any process wishing to establish a connection to a host to transfer a file using FTP can connect to the destination host's port 21 to contact its FTP daemonThe list of well-known ports is given at www.iana.org. Over 300 have been assigned

46

TCP Service ModelAll TCP connections are full duplex and point-to-point. TCP does not support multicasting or broadcasting.A TCP connection is a byte stream, not a message stream. Message boundaries are not preserved end to end.

For example, if the sending process does four 512-byte writes to a TCP stream, these data may be delivered to the receiving process as four 512-byte chunks, two 1024-byte chunks, one 2048-byte chunk or some other way. There is no way for the receiver to detect the unit(s) in which the data were written.

24

47

TCP ProtocolEvery byte on a TCP connection has its own 32-bit sequence numberThe sending and receiving TCP entities exchange data in the form of segments. A TCP segment consists of a fixed 20-byte header (plus an optional part) followed by zero or more data bytesTwo limits restrict the segment size:

Each segment, including the TCP header, must fit in the 65,515-byte IP payloadEach network has a maximum transfer unit (MTU) and each segment must fit in the MTU. In practice, the MTU is generally 1500 bytes (the Ethernet payload size)

48

TCP ProtocolThe basic protocol used by TCP entities is the sliding window protocol

When a sender transmits a segment, it also starts a timerWhen the segment arrives at the destination, the receiving TCP entity sends back a segment (with data if any exist, otherwise without data) bearing an acknowledgement number equal to the next sequence number it expects to receiveIf the sender's timer goes off before the acknowledgement is received, the sender transmits the segment again

25

49

TCP Segment Header

50

TCP Connection EstablishmentBy means of the three-way handshake To establish a connection, one side passively waits for an incoming connection by executing the LISTEN and ACCEPT primitives, either specifying a specific source or nobody in particular.The other side, say, the client, executes a CONNECT primitive, specifying the IP address and port to which it wants to connect, the maximum TCP segment size it is willing to accept, and optionally some user data (e.g., a password)

26

51

TCP Connection EstablishmentThe CONNECT primitive sends a TCP segment with the SYN: on and ACK: off and waits for a responseWhen this segment arrives at the destination, the TCP entity there checks to see if there is a process that has done a LISTEN on the port given in the Destination port field. If not, it sends a reply with the RST bit on to reject the connectionIf some process is listening to the port, that process is given the incoming TCP segment. It can then accept or reject the connection. If it accepts, an acknowledgement segment is sent back

52

TCP Connection Establishment

a) TCP connection establishment in the normal case (b) Call collision

27

53

TCP Connection ReleaseEach simplex connection is released independently of its siblingTo release a connection, either party can send a TCP segment with the FIN bit set, which means that it has no more data to transmitWhen the FIN is acknowledged, that direction is shut down for new data. Data may continue to flow indefinitely in the other direction, however. When both directions have been shut down, the connection is releasedNormally, four TCP segments are needed to release a connection, one FIN and one ACK for each directionTo avoid the two-army problem, timers are used

54

TCP Connection Management

28

55

TCP Congestion ControlRealize that two potential problems exist

network capacity receiver capacity

Each sender maintains two windowsthe window the receiver has grantedthe congestion window

Each reflects the number of bytes the sender may transmit. The number of bytes that may be sent is the minimum of the two windowsIt uses threshold. When a timeout occurs, the threshold is set to half of the current congestion window, and the congestion window is reset to one maximum segment

56

TCP Congestion Control

29

57

TCP Timer Management

TCP uses multiple timers to do its workretransmission timer: wait for ACKpersistence timer: prevent deadlockkeepalive timer: idle before disconnect

58

Wireless TCP

In theoryTransport protocols should be independent of the technology of the underlying network layerTCP should not care whether IP is running over fiber or over radio

In practicemost TCP implementations have been carefully optimized based on assumptions that are true for wired networks but that fail for wireless networksPacket lost is the main cause for timeout

30

59

Wireless TCPThe principal problem is the congestion control algorithm

Nearly all TCP implementations nowadays assume that timeouts arecaused by congestion, not by lost packets. Consequently, when a timer goes off, TCP slows down and sends less vigorously The idea behind this approach is to reduce the network load and thus alleviate the congestion

Wireless transmission links are highly unreliableThey lose packets all the timeThe proper approach to dealing with lost packets is to send themagain, and as quickly as possibleSlowing down just makes matters worse

60

Transactional TCP A way to combine the efficiency of RPC using UDP (just two messages) with the reliability of TCPAn experimental TCP variant which is described in RFCs 1379 and 1644Idea: modify the standard connection setup sequence slightly to allow the transfer of data during setupAnother proposal is SCTP (Stream Control Transmission Protocol)

31

61

Transactional TCP

(a) RPC using normal TCP. (b) RPC using T/TCP

62

Performance

Rule #1: CPU Speed Is More Important Than Network SpeedRule #2: Reduce Packet Count to Reduce Software Overhead Rule #3: Minimize Context Switches Rule #4: Minimize Copying Rule #5: You Can Buy More Bandwidth but Not Lower Delay Rule #6: Avoiding Congestion Is Better Than Recovering from ItRule #7: Avoid Timeouts