Transcript Slides - University of Washington

Introduction to Computer Networks
Transport Layer Overview
(§6.1.2-6.1.4)
David Wetherall ([email protected])
Professor of Computer Science & Engineering
Where we are in the Course
• Starting the Transport Layer!
– Builds on the network layer to deliver
data across networks for applications
with the desired reliability or quality
Application
Transport
Network
Link
Physical
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Recall
• Transport layer provides end-to-end connectivity
across the network
app
app
TCP
TCP
IP
IP
IP
IP
802.11
802.11
Ethernet
Ethernet
Host
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Router
Host
3
Recall (2)
• Segments carry application data
across the network
• Segments are carried within
packets within frames
Segment
802.11
IP
TCP
App, e.g., HTTP
Packet
Frame
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Transport Layer Services
• Provide different kinds of data
delivery across the network to
applications
Unreliable
Reliable
Messages Datagrams (UDP)
Bytestream
Streams (TCP)
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Comparison of Internet Transports
• TCP is full-featured, UDP is a glorified packet
TCP (Streams)
Connections
Bytes are delivered once,
reliably, and in order
Arbitrary length content
Flow control matches
sender to receiver
Congestion control matches
sender to network
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UDP (Datagrams)
Datagrams
Messages may be lost,
reordered, duplicated
Limited message size
Can send regardless
of receiver state
Can send regardless
of network state
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User Datagram Protocol (UDP)
• Used by apps that don’t want
reliability or bytestreams
– Voice-over-IP (unreliable)
– DNS, RPC (message-oriented)
– DHCP (bootstrapping)
(If application wants reliability and
messages then it has work to do!)
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Datagram Sockets
Client (host 1) Time Server (host 2)
request
reply
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Datagram Sockets (2)
Client (host 1)
Time
Server (host 2)
4: sendto
request
1: socket
2: bind
3: recvfrom*
5: recvfrom*
reply
6: sendto
1: socket
7: close
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7: close
*= call blocks
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UDP Buffering
Application
App
App
App
Ports
Transport
(TCP)
Message queues
Network (IP)
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Port Mux/Demux
packet
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UDP Header
• Uses ports to identify sending and
receiving application processes
• Datagram length up to 64K
• Checksum (16 bits) for reliability
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Topic
• How to set up connections
– We’ll see how TCP does it
SYNACK!
SYN! ACK!
Network
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Connection Establishment
• Both sender and receiver must be ready
before we start the transfer of data
– Need to agree on a set of parameters
– e.g., the Maximum Segment Size (MSS)
• This is signaling
– It sets up state at the endpoints
– Like “dialing” for a telephone call
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Three-Way Handshake
• Used in TCP; opens connection for
data in both directions
Active party
(client)
Passive party
(server)
• Each side probes the other with a
fresh Initial Sequence Number (ISN)
– Sends on a SYNchronize segment
– Echo on an ACKnowledge segment
• Chosen to be robust even against
delayed duplicates
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Three-Way Handshake (2)
• Three steps:
–
–
–
–
Active party
(client)
Client sends SYN(x)
Server replies with SYN(y)ACK(x+1)
Client replies with ACK(y+1)
SYNs are retransmitted if lost
• Sequence and ack numbers
carried on further segments
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Passive party
(server)
1
2
3
Time
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Three-Way Handshake (3)
• Suppose delayed, duplicate
copies of the SYN and ACK
arrive at the server!
Active party
(client)
Passive party
(server)
– Improbable, but anyhow …
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Three-Way Handshake (4)
• Suppose delayed, duplicate
copies of the SYN and ACK
arrive at the server!
Active party
(client)
Passive party
(server)
– Improbable, but anyhow …
• Connection will be cleanly
rejected on both sides 
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X
REJECT
X
REJECT
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Topic
• How to release connections
– We’ll see how TCP does it
FIN!
FIN!
Network
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Connection Release
• Orderly release by both parties when
done
– Delivers all pending data and “hangs up”
– Cleans up state in sender and receiver
• Key problem is to provide reliability
while releasing
– TCP uses a “symmetric” close in which
both sides shutdown independently
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TCP Connection Release
• Two steps:
Active party
Passive party
– Active sends FIN(x), passive ACKs
– Passive sends FIN(y), active ACKs
– FINs are retransmitted if lost
• Each FIN/ACK closes one
direction of data transfer
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TCP Connection Release (2)
• Two steps:
– Active sends FIN(x), passive ACKs
– Passive sends FIN(y), active ACKs
– FINs are retransmitted if lost
• Each FIN/ACK closes one
direction of data transfer
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Active party
Passive party
1
2
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