1. Transport Layer Problems with network layer services
Transport layer services
Addressing
Connection establishment and release
Flow control and buffering
Multiplexing
TCP and UDP
Some performance guidelines
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2. Problems with Network Layer Services
Data corruption (while stored in node memory)
Packet loss (due to node failure)
Duplicate packets (if ACK is lost)
Virtual channel loss
Out of sequence
Congestion
Note: Some of the problems exist also at the link level. The problems here are end-to-end and more difficult to resolve due to large delay.
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3. Transport Layer Services
Error, loss, duplicate detection and recovery
(by software checksum and message ID)
Sequencing and flow control
(by sliding window protocol)
Connection establishment and release
(by 3-way handshake)
Multiplexing
(upward or downward)
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4. Primitives of Transport Services
socket primitives for TCP
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5. Addressing
TSAP (Transport Service Access Point) -- e.g. (IP addr, local port) in Internet
NSAP (Network Service Access Point) -- e.g. IP addr in Internet
Three alternative schemes to address TSAP:
1. stable well-known TSAP addresses for common services
(server processes attach themselves to well-known ports)
2. process server that acts as a proxy for less-heavily used servers (listens to a set of ports to wait for connection request, spawns off process to serve the client, goes back to listen for new requests)
3. name server that new services register to
(listens to a well-known TSAP, responds the TSAP of requested service)
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6. Connection Establishment and Release
Problems of delayed duplicates
Three-way handshake: both sides do not have to start with the same sequence number
host 1
host 2
host 1
host 2
host 1
host 2
host 1
host 2
CR (seq=x)
delayed duplicate DR DR
CR (seq=x)
ACK (seq=y, ACK=x) DR
lost
ACK DR
ACK (seq=y, ACK=x)
ACK
ACK
DATA (seq=x, ACK=y)
REJECT (ACK=y)
lost
timeout
release connection
normal operation
delayed duplicate
normal operation
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7. Flow Control and Buffering
Source buffering (for low-bandwidth bursty traffic) vs. destination buffering (for high bandwidth smooth traffic)
Two potential bottlenecks:
receive buffer space
subnet capacity
Dynamic sliding window flow control:
measure capacity c TPDUs/sec and cycle time r
compute window size as cr
adjust frequently to track changes in the carrying capacity and cycle time
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8. Multiplexing upward multiplexing downward multiplexing 4 3 2 1
layer
TSAP
TSAP 4
NSAP
NSAP 3 2 1
to router
upward multiplexing
downward multiplexing
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9. TCP (Transmission Control Protocol) A reliable end-to-end byte stream protocol over an unreliable internetwork
socket, connection, segment
TCP header
TCP connection management
TCP transmission management and silly window syndrome
TCP dynamic window congestion control
TCP dynamic timer management
UDP: encapsulating raw IP
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10. TCP Socket socket: IP+port (TSAP)
connection: (socket_source, socket_dest)
full-duplex, point-to-point, byte stream
segment: two limits: 65,535 bytes
network’s MTU (max transfer unit)
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11. TCP Header     11 Ying-Dar Lin@CIS.NCTU 32 Bits URG: urgent data
Urgent pointer: byte offset
PSH: PUSHed data (not to buffer it)
RST: reset a connection
SYN, ACK: (1,0)--connection request
(1,1)--connection reply
FIN: release a connection
Window size: receiver window size
Checksum: on header, data, psuedo header
Options: max TCP payload (default
536 bytes)
window scale factor (up to
216)
NAK for selective repeat
Source port
Destination port
Sequence number
Acknowledgement number
TCP
header
length U R G A C K P S H R S T S Y N F I N
Window size
Checksum
Urgent pointer 
Options (0 or more 32-bit words)  
Data (optional) 
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12. TCP Connection Management
(START)
CONNECT/SYN
CLOSED
host 1
host 2
CLOSE/-
LISTEN/-
CLOSE/-
SYN (SEQ=x)
SYN/SYN+ACK
LISTEN
RST/-
SEND/SYN
SYN (SEQ=y,ACK=x+1)
SYN
RCVD
SYN
SENT
SYN/SYN +ACK (simultaneous open)
(Data transfer stage)
ACK/-
SYN + ACK/ACK
(step 3 of the three-way handshake)
SYN (SEQ=x+1,ACK=y+1)
ESTABLISHED
CLOSE/FIN
CLOSE/FIN
FIN/ACK
Connection establishment
(Active close)
(Passive Close)
CLOSING
FIN/ACK
FIN
WAIT 1
CLOSED
WAIT
ACK/-
ACK/-
FIN+ACK/ACK
TIMED
WAIT
CLOSE/FIN
FIN
WAIT 1
LAST
ACK
FIN/ACK
(Timeout/)
CLOSED
ACK/-
(Go back to start)
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13. TCP Transmission Management and Silly Window Syndrome
Example: a TELNET TCP connection to an interactive editor
1. Sourcedest: 21-byte TCP data segment (41-byte IP datagram)
2. Sourcedest: acknowledgement segment (40-byte)
3. Sourcedest: window update segment (40-byte)(after editor reads the byte)
4. Sourcedest: echo segment (41-byte) (after editor processes the byted)
5. Repeat step 1, 162 bytes in 4 segments for each character types!!
Silly window syndrome: frequent but mall window updates
1. sending application to TCP one byte at a time
2. receiving application sucks the data up from TCP one byte at a time
Nagle’s algorithm to solve 1:
When data come into the sender one byte at a time, just send the first byte and buffer all the rest until the outstanding byte is acked.
Clark’s algorithm to solve 2:
The receiver should not send a window update until it can handle the max segment size it advitised when conn. was established, or its buffer is half empty, whichever is smaller.
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14. TCP Dynamic Window Congestion Control
Two limits: network capacity  congestion window
receiver capacity  receiver window
#bytes that may be sent: min(congestion window, receiver window)
Dynamic window control:
1. Initialization:
congestion windowone maximum segment
threshold64KB
2. Slow start: exponential up to threshold
congestion windowcongestion window + one max segment size for each acked segment
(Each successful acked burst doubles congestion window)
3. Congestion avoidance: linearly up to receiver window
congestion windowcongestion window + one max segment size for each acked burst
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15. TCP Congestion Window Transmission number
Timeout
Threshold
Congestion window (kilobytes)
Threshold
Transmission number
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16. TCP Dynamic Timer Management
Retransmission timer:
RTT=RTT + (1-)M
(M: measured round-trip delay, ~7/8)
D=D + (1-) |RTT-M|
Timeout=RTT+4*D
Persistent timer: prevent deadlock due to lost window update
Keepalive timer: for idle connections, controversial!
Close timer: for timed wait state while closing a connection double the max packet lifetime (2*120 sec)
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17. Design Guidelines at Transport Layer
CPU speed is more important than network speed
Reduce packet count to reduce software overhead
Minimize context switching
Minimize copying
You can buy more bandwidth but not lower delay
Avoiding congestion is better than recovering from it
Avoid timeouts
Speed up TPDU processing
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