lecture 7 tcp/ip transport layer (1)
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Lecture 7 TCP/IP Transport Layer (1). Outline (Transport Layer). Principles behind transport layer services: multiplexing/demultiplexing principles of reliable data transfer learn about transport layer protocols in the Internet: UDP: connectionless transport - PowerPoint PPT PresentationTRANSCRIPT
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Lecture 7
TCP/IP Transport Layer (1)
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Outline (Transport Layer)
• Principles behind transport layer services:– multiplexing/demultiplexing– principles of reliable data transfer
• learn about transport layer protocols in the Internet:– UDP: connectionless transport– TCP: connection-oriented transport
• segment structure
• reliable data transfer
• flow control
• connection management
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Transport Services and Protocols• provide logical communication between application processes running
on different hosts• transport protocols run in end systems
– sender side: breaks application messages into segments, passes to network layer
– receiver side: reassembles segments into messages, passes to application layer
• more than one transport protocol available to applications, e.g.– Internet: TCP and UDP
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Transport vs. Network Layer
• network layer: logical communication between hosts
• transport layer: logical communication between processes – relies on, enhances,
network layer services
Household analogy:• 5 kids from house A sending letters to 5
kids in house B• In house A, Ali collects letters from
each kid and wrap them in single envelope. Ali also distributes received mails in house A. Babu does same task in house B.
• Postal service is used to send mails– processes = kids
– app messages = letters from each kid
– hosts = houses
– transport protocol = Ali and Babu
– network-layer protocol = postal service
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Internet Transport Layer protocols
• User Datagram Protocol (UDP)– unreliable unordered data transfer between sending
and receiving process– does not provide: connection setup, reliability, flow
control, congestion control, timing, or bandwidth guarantee
• Transmission Control Protocol (TCP)– reliable, in-order delivery– reliable transport: acknowledgement, retransmissions– flow control: sender won’t overwhelm receiver– congestion control: for benefit of the Internet– Does not provide: timing, minimum bandwidth
guarantees
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Outline (Transport Layer)
• Principles behind transport layer services:– multiplexing/demultiplexing– principles of reliable data transfer
• learn about transport layer protocols in the Internet:– UDP: connectionless transport– TCP: connection-oriented transport
• segment structure
• reliable data transfer
• flow control
• connection management
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Multiplexing/demultiplexing
application
transport
network
link
physical
P1 application
transport
network
link
physical
application
transport
network
link
physical
P2P3 P4P1
host 1 host 2 host 3
= process= socket
delivering received segmentsto correct application layers processes.
Demultiplexing at rcv host:gathering data from multipleapplication layer process, enveloping data with header (later used for demultiplexing)
Multiplexing at send host:
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
How Demultiplexing Works• host receives IP datagrams
– each datagram has source IP address, destination IP address
– each datagram carries 1 transport-layer segment
– each segment has source, destination port number
– The port number is a 16-bit number, ranging from 0 to 65535. Port numbers ranging from 0 - 1023 are called well-known port numbers and are used by well-known application protocols such as HTTP, FTP, Telnet etc.
• host uses IP addresses & port numbers to direct segment to appropriate socket
source port # dest port #
32 bits
applicationdata
(message)
other header fields
TCP/UDP segment format
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Demultiplexing
ClientIP:B
P2
client IP: A
P1P1P3
serverIP: C
SP: 6428
DP: 9157
D-IP:A
SP: 9157
DP: 6428
D-IP:C
SP: 6428
DP: 5775
D-IP:B
SP: 5775
DP: 6428
D-IP:C
SP provides “return address”
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Outline (Transport Layer)
• Principles behind transport layer services:– multiplexing/demultiplexing– principles of reliable data transfer
• learn about transport layer protocols in the Internet:– UDP: connectionless transport– TCP: connection-oriented
transport• segment structure
• reliable data transfer
• flow control
• connection management
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
UDP: User Datagram Protocol• “best effort” service, UDP segments
may be:– lost– delivered out of order to app
• connectionless:– no handshaking between UDP
sender, receiver– each UDP segment handled
independently of others• Why is there a UDP?
– no connection establishment (which can add delay)
– simple: no connection state at sender, receiver
– small segment header– no congestion control: UDP can
blast away as fast as desired
• often used for streaming multimedia applications– loss tolerant– rate sensitive
• other UDP uses– DNS– SNMP
• reliable transfer over UDP: add reliability at application layer– application-specific error
recovery!
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
UDP Segment Format
source port # dest port #
32 bits
Applicationdata
(message)
UDP segment format
length checksumLength, in
bytes of UDPsegment,including
header
identifiesreceiving process(for multiplexingdemultiplexing)
For UDP headerand UDP data(optional)
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
UDP Checksum
Sender:• treat segment contents
as sequence of 16-bit integers
• checksum: addition (1’s complement sum) of segment contents
• sender puts checksum value into UDP checksum field
Receiver:• compute checksum of
received segment• check if computed checksum
equals checksum field value:– NO - error detected– YES - no error detected.
Does not guarantee error free.
Goal: detect “errors” (e.g., flipped bits) in transmitted segment
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Outline (Transport Layer)
• Principles behind transport layer services:– multiplexing/demultiplexing– principles of reliable data transfer
• learn about transport layer protocols in the Internet:– UDP: connectionless transport– TCP: connection-oriented transport
• segment structure
• reliable data transfer
• flow control
• connection management
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Principles of Reliable Data Transfer
• important in application, transport, link layers
• top-10 list of important networking topics!
• characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Reliable Data Transfer
sendside
receiveside
rdt_send(): called from above, (e.g., by app.). Passed data to deliver to receiver upper layer
udt_send(): called by rdt,to transfer packet over unreliable channel to
receiver
rdt_rcv(): called when packet arrives on rcv-side of channel
deliver_data(): called by rdt to deliver data to
upper
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Reliable Data TransferWe will:• incrementally develop sender, receiver sides of
reliable data transfer protocol (rdt)• consider only unidirectional data transfer
– but control info will flow on both directions!
• use finite state machines (FSM) to specify sender, receiver
state1
state2
event causing state transitionactions taken on state transition
state: when in this “state” next state
uniquely determined by next event
eventactions
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt1.0: Reliable Transfer Over a Reliable Channel
• underlying channel perfectly reliable– no bit errors– no loss of packets
• separate FSMs for sender, receiver:– sender sends data into underlying channel– receiver reads data from underlying channel
Wait for call from above
packet = make_pkt(data)udt_send(packet)
rdt_send(data)
extract (packet,data)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt2.0: Channel with Bit Errors• all transmitted packets are received in the order in which
they were sent.• underlying channel may flip bits in packet
– checksum to detect bit errors
• the question: how to recover from errors:– acknowledgements (ACKs): receiver explicitly tells sender that
packet received OK– negative acknowledgements (NAKs): receiver explicitly tells sender
that packet had errors– sender retransmits packet on receipt of NAK
• new mechanisms in rdt2.0 (beyond rdt1.0):– error detection– receiver feedback: control msgs (ACK,NAK) receiver->sender
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt2.0: FSM Specification
Wait for call from above
snkpkt = make_pkt(data, checksum)udt_send(sndpkt)
extract(rcvpkt,data)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) && isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) && isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) && corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Sender sends one packet, then waits for receiver response
stop and wait
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt2.0 Flaws
What happens if ACK/NAK corrupted?
• sender doesn’t know what happened at receiver!– add enough checksum bits
to allow the sender to not only detect, but recover from, bit errors.
– resend the current data packet when it receives a garbled ACK or NAK packet. possible duplicate
Handling duplicates: • sender adds sequence
number to each packet
• sender retransmits current packet if ACK/NAK garbled
• receiver discards (doesn’t deliver up) duplicate packet
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt2.1: Mechanism
Sender:• sequence number added
to packet• two sequence numbers
(0,1) will suffice.• must check if received
ACK/NAK corrupted • twice as many states
– state must “remember” whether “current” packet has 0 or 1 sequence number.
Receiver:• must check if received
packet is duplicate– state indicates whether 0
or 1 is expected packet sequence number
• note: receiver can not know if its last ACK/NAK received OK at sender
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt2.1: Sender Handles Garbled ACK/NAKs
Wait for call 0 from
above
sndpkt = make_pkt(0, data, checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1, data, checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt2.1: Receiver Handles Garbled ACK/NAKs
Wait for 0 from below
sndpkt = make_pkt(NAK, chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq0(rcvpkt)
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt)
extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(ACK, chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq0(rcvpkt)
extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(ACK, chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) && (corrupt(rcvpkt)
sndpkt = make_pkt(ACK, chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq1(rcvpkt)
rdt_rcv(rcvpkt) && (corrupt(rcvpkt)
sndpkt = make_pkt(ACK, chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK, chksum)udt_send(sndpkt)
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt2.2: a NAK-free Protocol
• same functionality as rdt2.1, using ACKs only• instead of NAK, receiver sends ACK for last packet
received OK– receiver must explicitly include sequence number of packet
being ACKed
• duplicate ACK at sender results in same action as NAK: retransmit current packet
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt2.2: Sender Receiver FSM
Wait for call 0 from
above
sndpkt = make_pkt(0, data, checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,1) )
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt)
extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(ACK1, chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) && (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt3.0: Channels with Errors and LossNew assumption: • underlying channel can also lose packets (data or ACKs)• how to detect packet loss and what to do when this occurs.
– use of checksum, sequence numbers, ACK packets, and retransmissions
Approach: • sender waits “reasonable” amount of time for ACK • retransmits if no ACK received in this time• if packet (or ACK) just delayed (not lost):
– retransmission will be duplicate, but use of sequence numbers already handles this
– receiver must specify sequence number of packet being ACKed
• requires countdown timer
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt3.0 sender
sndpkt = make_pkt(0, data, checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isACK(rcvpkt,1) )
Wait for call 1 from
above
sndpkt = make_pkt(1, data, checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0)
rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isACK(rcvpkt,0) )
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt3.0 in Action
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt3.0 in Action
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Performance of Rdt3.0
• rdt3.0 works, but performance stinks• example: 1 Gbps link, 15 ms e-e prop. delay, 1KB
packet:Ttransmi
t
= 8kb/pkt10**9 b/sec
= 8 microsec
– U sender: utilization – fraction of time sender busy sending
– 1KB packet every 30 msec -> 33kB/sec throughput over 1 Gbps link– network protocol limits use of physical resources!
L (packet length in bits)R (transmission rate, bps)
=
U sender
= .008
30.008 = 0.00027
microseconds
L / R
RTT + L / R =
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Rdt3.0: stop-and-wait operation
first packet bit transmitted, t = 0
sender receiver
RTT
last packet bit transmitted, t = L / R
first packet bit arriveslast packet bit arrives, send ACK
ACK arrives, send next packet, t = RTT + L / R
U sender
= .008
30.008 = 0.00027
microseconds
L / R
RTT + L / R =
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Pipelined Protocols
Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged packets– range of sequence numbers must be increased– buffering at sender and/or receiver
• Two generic forms of pipelined protocols: go-Back-N, selective repeat
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Pipelining: Increased Utilization
first packet bit transmitted, t = 0
sender receiver
RTT
last bit transmitted, t = L / R
first packet bit arriveslast packet bit arrives, send ACK
ACK arrives, send next packet, t = RTT + L / R
last bit of 2nd packet arrives, send ACKlast bit of 3rd packet arrives, send ACK
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Go-Back-NSender:• k-bit sequence number in packet header• “window” of up to N, consecutive unack’ed packets allowed
• ACK(n): ACKs all packets up to, including sequence number n - “cumulative ACK”– may deceive duplicate ACKs (see receiver)
• timer for each in-flight packet• timeout(n): retransmit packet n and all higher sequence number packets in window
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
GBN: Sender Extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])…udt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum < base+N) { sndpkt[nextseqnum] = make_pkt(nextseqnum,data,chksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ }else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) && corrupt(rcvpkt)
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
GBN: Receiver Extended FSM
• ACK-only: always send ACK for correctly-received packet with highest in-order sequence number– may generate duplicate ACKs– need only remember expectedseqnum
• out-of-order packet: – discard (don’t buffer) -> no receiver buffering!– Re-ACK packet with highest in-order sequence
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) && notcurrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum)
extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(expectedseqnum,ACK,chksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnum,ACK,chksum)
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
GBN in Action
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Selective Repeat
• receiver individually acknowledges all correctly received packets– buffers packets, as needed, for eventual in-order
delivery to upper layer
• sender only resends packets for which ACK not received– sender timer for each unACKed packet
• sender window– N consecutive sequence numbers– again limits sequence numbers of sent, unACKed
packets
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Selective Repeat: Sender, Receiver Windows
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Selective Repeat
data from above :• if next available sequence
number in window, send packet
timeout(n):• resend packet n, restart
timerACK(n) in [sendbase,sendbase+N]:
• mark packet n as received• if n smallest unACKed
packet, advance window base to next unACKed sequence number
senderpacket n in [rcvbase,
rcvbase+N-1]• send ACK(n)• out-of-order: buffer• in-order: deliver (also deliver
buffered, in-order packets), advance window to next not-yet-received packet
packet n in [rcvbase-N,rcvbase-1]
• ACK(n)
otherwise: • ignore
receiver
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Selective Repeat in Action
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Selective Repeat: Dilemma
Example: • seq #’s: 0, 1, 2, 3• window size=3
• receiver sees no difference in two scenarios!
• incorrectly passes duplicate data as new in (a)
• a window size that is one smaller than the size of the sequence number space won't work.
• the window size must be less than or equal to half the size of the sequence number space.
Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking
Reading Material• Chapter 3 – text3 (Kurose)
• Chapter 6 – text2 (Tanenbaum)