SMU CSE 4344

transport layer

SMU CSE 4344

transport layer

• end-to-end protocols– transport code runs only on endpoint hosts

• encapsulates network communications– app layer sees only simple transport primitives

• connectionless service• connection-oriented service

– establishment, data transfer, release

SMU CSE 4344

transport layer in situ

• transport entities/protocol handlers

SMU CSE 4344

challenges of “best effort” network layer

• dropped packets

• reordered packets

• duplicated packets

• finite sized packets

• packets delivered after long delay

SMU CSE 4344

services provided by transport layer

• guaranteed message delivery

• in-order message delivery

• no more than one copy of each message

• messages of arbitrary size

• sender/receiver synchronization

• receiver-applied flow control

• multiplexed end-to-end flows

SMU CSE 4344

transport protocol “suite”

a transport protocol need not do all of these things

we merely ask that, if required, a transport protocol be available to do at least some of these things

and, that a suite of transport protocols cover all of these things in aggregate

SMU CSE 4344

protocol unit nesting for transmission

• TPDUs sometimes called “segments”

SMU CSE 4344

transport connection

• basis for multiplexing end-to-end messages• TSAP: transport service access point

SMU CSE 4344

connection establishment

• clients requesting available service• process server on “well-known” TSAP calls desired

service process, and hands off connection

SMU CSE 4344

SMU CSE 4344

intro to Internet transport layer

• Internet Protocol (IP, network layer) provides unreliable datagram service between hosts

• transport protocols

– end-to-end delivery between connection endpoints

• e.g., processes or programs

• User Datagram Protocol (UDP)

– datagram service (best effort)

• Transmission Control Protocol (TCP)

– reliable data delivery

SMU CSE 4344

UDP

• does not improve on best effort delivery• checksum• multiplexes messages

– multiple process endpoints per host– keeps concurrent end-to-end messages separate

• host process ID + endpoint process ID– port (TSAP or “mailbox”)– IP port field: 16 bits, 64K available ports

• transport message ID:– ((source IP, port ID), (sink ID, port ID))

SMU CSE 4344

TCP

• TCP is most widely used transport protocol

• provides reliable data delivery by using IP unreliable datagram delivery

• compensates for loss, delay, duplication, and similar problems in Internet components

• reliable delivery is high-level, familiar model for construction of applications

SMU CSE 4344

features of TCP

• connection oriented

– application requests connection to destination and then uses connection to deliver data to transfer data

• point-to-point

– a TCP connection has two endpoints

• reliability

– TCP guarantees data will be delivered without loss, duplication, or transmission errors

SMU CSE 4344

• full duplex– the endpoints of a TCP connection can exchange

data in both directions simultaneously

• stream interface– application delivers data to TCP as a continuous

stream, with no record boundaries; TCP makes no guarantees that data will be received in same blocks as transmitted

• reliable connection startup– three-way handshake guarantees reliable,

synchronized startup between endpoints

SMU CSE 4344

• graceful connection shutdown– TCP guarantees delivery of all data after endpoint

shutdown by application

Application process

Writebytes

TCPSend buffer

Segment Segment Segment

Transmit segments

Application process

Readbytes

TCPReceive buffer

…

… …

SMU CSE 4344

connection establishment

• TCP uses three-way handshake for reliable connection establishment and termination– host 1 sends segment with SYN bit set and

random sequence number– host 2 responds with segment with SYN bit

set, acknowledgement to Host 1 and random sequence number

– host 1 responds with acknowledgement

SMU CSE 4344

connection establishment and termination

Active participant(client)

Passive participant(server)

SYN, SequenceNum = x

SYN + ACK, SequenceNum = y,

ACK, Acknowledgment = y + 1

Acknowledgment = x + 1

SMU CSE 4344

IP for data delivery

• TCP uses IP for data delivery (like UDP)• endpoints are identified by ports• allows multiple connections on each host• IP treats TCP like data and does not interpret

any contents• Internet routers only look at IP header to

forward datagrams• TCP at destination interprets TCP messages

SMU CSE 4344

link layer vs. transport layer

• both offer “end-to-end” service between endpoints– error control, flow control, sequencing

• transport layer complications– addressing, connection mgt, packets wandering subnet

SMU CSE 4344

transport layer (vs. data link layer)

• potentially connects many different hosts– need explicit connection establishment and termination

• potentially different RTT– need adaptive timeout mechanism

• potentially long delay in network– need to be prepared for arrival of very old packets

• potentially different capacity at destination – need to accommodate different node capacity

• potentially different network capacity– need to be prepared for network congestion

SMU CSE 4344

reliable delivery using TCP

• TCP can recover from

– lost packets

– duplicate packets

– delayed packets

– corrupted data

– transmission speed mismatches

– congestion

– system reboots

SMU CSE 4344

lost packets

• TCP uses positive acknowledgement with retransmission to achieve reliable data delivery

• recipient sends acknowledgment control messages (ACK) to sender to verify successful receipt of data

• sender sets timer when data transmitted; if timer expires before ACK arrives, sender retransmits (with new timer)

SMU CSE 4344

• setting of delay critical, done dynamically

– timeout should be based on round trip time (RTT)

– sender picks retransmission timeout (RTO) based on previous RTTs

– specific method is called adaptive retransmission algorithm

SMU CSE 4344

adaptive retransmission (simplified)

• measure SampleRTT for each segment/ ACK pair

• compute weighted average of RTT– EstRTT = ( x EstRTT ) + ( x SampleRTT )

– where + = 1

– 0.8 < < 0.9

– 0.1 < < 0.2

• set timeout based on EstRTT– TimeOut = 2 x EstRTT

current implementations account for RTT variance

SMU CSE 4344

segments and sequence numbers

• application delivers arbitrarily large chunks of data to TCP as a “byte stream”

• TCP breaks data into segments, each of which fits into an IP datagram

• original stream is numbered by bytes

• segment contains sequence number of data bytes

• ACK does not acknowledge segments per se

• receiver ACKs segment with sequence number of acknowledged data

• so, one ACK can acknowledge many segments

SMU CSE 4344

segment format

Options (variable)

Data

Checksum

SrcPort DstPort

HdrLen 0 Flags

UrgPtr

AdvertisedWindow

SequenceNum

Acknowledgment

0 4 10 16 31

SMU CSE 4344

TCP flow control

• TCP uses sliding window for flow control

• receiver specifies window

– called window advertisement

– specifies which bytes in data stream can be sent

– carried in the segment along with ACK

• sender can transmit any bytes, in any size segment, between last acknowledged byte and within window size

SMU CSE 4344

issues related to advertisement

• sliding window can result in transmission of many small segments, as follows

• if receiver window is full, and receiving application consumes a few data bytes, receiver will advertise small window

• sender will immediately send small segment to fill window

SMU CSE 4344

• wastes processing cycles, network bandwidth

• solutions:

– receiver delays advertising new window

– sender delays sending data when window is small

SMU CSE 4344

transport wrinkle: RPC

• remote procedure calls (RPCs)– client call (with parameters), server return values

– needs reliable delivery, process-to-process

– TCP connection machinery overhead is too much

• must support: – large messages

– synchronization of request/reply

– delivery to/from correct host processes

• SunRPC, DCE-RPC (Open Softward Fdn.)