Computer Communications
Lecture 17-18
The Transport Layer
Required Reading: Tanenbaum (chapter 6 and section 5.3)
Transport Service
• Two types of service: connectionless and connection-oriented
– Implemented by a transport protocol between the two transport entities
• Interface to upper layers (user processes in the application layer) via transport
service primitives (e.g., socket API in the Internet)
The network, transport,
and application layers.
The nesting of TPDUs,
packets, and frames.
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Elements of Transport Protocols
• Addressing
• Multiplexing
• Connection Establishment
• Connection Release
• Flow Control and Buffering
• Congestion Control
• Sequencing
• Error Control
• Crash Recovery
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Transport vs. Data Link Protocols
• Transport layer similarities with data link layer
– Need to deal with error control, sequencing, flow control, etc.
• Transport layer differences from data link layer
– Needs explicit addressing
– Complicated initial connection establishment
– Deal with existence of storage capacity in the subnet
– Greater flow control and buffering requirements due to large and
dynamically varying number of connections
(a) Environment of the data link layer. (b) Environment of the transport layer.4
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Addressing
• Transport layer address:
Transport Service Access Point
(TSAP)
– “port” (16-bit) in the Internet
• Network layer address:
Network Service Access Point
(NSAP)
– “IP address” (32-bit) in the
Internet
• Mechanisms for knowing
TSAPs
– Well-known TSAPs
– Initial connection protocol and
a special process server (e.g.,
Internet daemon – inetd)
TSAPs, NSAPs and transport connections.
– Name (directory) server
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Multiplexing
• Upward multiplexing: De-multiplex arriving packets (TPDUs) to
corresponding processes using transport addresses
• Downward multiplexing: distribute traffic from a process among multiple
network links, e.g., ISDN line
(a) Upward multiplexing.
(b) Downward multiplexing.
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The Internet Transport Protocols: UDP
• User Datagram Protocol (UDP) – RFC 768
– Connectionless protocol for sending/receiving encapsulated IP datagrams
(IP packets wrapped with a short header)
» In Internet, transport layer messages are called “segments”
– Supports (de-)multiplexing of multiple processes over same IP address
– No flow control, error control, sequencing
• UDP Applications
– Client-server (request-reply) interactions
» E.g., Domain Name System (DNS), Remote Procedure Call (RPC)
– Real-time multimedia applications
» E.g., Real-time Transport Protocol (RTP)
The UDP header.
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The Internet Transport Protocols: TCP
• Transmission Control Protocol (TCP) – RFC 793, RFC 1122, RFC 1323
– Connection-oriented
– Provides a end-to-end reliable “byte” stream
» Every byte has a (32-bit) sequence number
» Uses a sliding window protocol
» Separate 32-bit sequence numbers for acknowledgements and window
mechanism
– Full-duplex and point-to-point
– Communicates in segments
» Segment size bound by maximum IP payload limit and network maximum
transfer unit (MTU)
• TCP applications
– FTP (21), Telnet (23), HTTP (80), NNTP (119), SMTP (25), POP-3 (110)
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The TCP Segment Header
TCP Header.
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TCP Checksum
• Checksums header, data and conceptual pseudoheader (below)
• Checksum computation:
– Set checksum part of the header to zero, pad data field with an additional
zero if odd length
– Add up all the 16-bit words in 1’s complement take 1’s complement of
the sum as the checksum
• Same procedure also used for UDP
The pseudoheader included in the TCP checksum.
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Connection Establishment
• Problems arise when network can lose, store and duplicate packets
• Main problem: existence of delayed duplicates
• Common solution due to Tomlinson (1975):
– Use of connection identifiers (sequence numbers) with clock-based setting
of initial sequence numbers
– Bounded packet lifetimes
– Three-way handshake
• Similar solution also used for TCP connection establishment
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TCP Connection Establishment
• Three-way handshake
(a) TCP connection establishment in the normal case.
(b) Call collision.
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Connection Release
• Asymmetric release: connection broken if one side releases
– May result in data loss
• Symmetric release: each side releases separately
Abrupt disconnection with loss of data.
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Connection Release (2)
• Illustrating the impossibility of devising a correct connection release
protocol
The two-army problem.
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Connection Release (3)
• Practical approach: a three-way handshake with timeouts
– Can lead to half-open connections
Four protocol scenarios for releasing a connection. (a) Normal case of a
three-way handshake. (b) final ACK lost.
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Connection Release (4)
• Practical approach: a three-way handshake with timeouts
– Can lead to half-open connections
(c) Response lost. (d) Response lost and subsequent DRs lost.
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TCP Connection Release
• View a connection as a pair of
simplex connections and release
each simplex connection
independently
• Timers used to avoid two-army
problem
– If no response to FIN within two
packet lifetimes, sender of the FIN
releases connection
– Other side eventually notices that
no one is listening to it any more and
will time out as well
Figure from Kurose & Ross
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Flow Control and Buffering
• Flow control problem in transport layer similar to data link layer, except
that the former has to deal with large number of connections
– Allocating dedicated buffers per connection at sender and receiver may not
be practical
– Different approaches for buffer organization
» Chained fixed-size buffers
» Chained variable-sized buffers
» One large circular buffer per connection
– Type of traffic carried by a connection also influences buffering strategy
– As connections come and go, and traffic types changes, need to able to
adjust buffer allocations dynamically
» Using variable-sized sliding windows
» Decouple window management from acknowledgements
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Dynamic Buffer Allocation
Dynamic buffer allocation. The arrows show the direction of transmission. An
ellipsis (…) indicates a lost TPDU.
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TCP Window Management
• Additional optimizations
– Cumulative ACKs
– Delayed acknowledgements to
reduce acknowledgement and window
update traffic
– Nagle’s algorithm to avoid sending
many short packets with few bytes of
data (interactive traffic) and make
efficient use of network bandwidth
» Send the first byte and buffer the
rest until outstanding byte is
acknowledged
» After receiving acknowledgement,
send all buffered characters in one
TCP segment and start buffering until
all sent data is acknowledged or
enough data waiting to be sent (half
the window or max segment)
– Clark’s solution to address “silly
window syndrome”
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TCP Window Management (2)
• Silly window syndrome: when data
consumed one byte at a time at
receiver
• Clark’s solution: receiver delays
sending window update segment until
it has buffer to hold max sized
segment or is half empty
• Additionally need to deal with out-oforder segments
Silly window syndrome.
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TCP Timer Management
• Retransmission timer setting difficult with TCP compared to data link
layer because of high RTT variance and network dynamics
• Retransmission timeout calculation algorithm due to Jacobson (1988)
Timeout = RTTest + 4 x RTT_Devest
RTTest =
α
RTTest + (1 – ) RTT_Sample, where
RTT_Devest =
α
α
α
= 7/8 (typically)
RTT_Devest + (1 – ) |RTTest – RTT_Sample|
α
– Karn’s algorithm to deal with retransmitted segments in timeout calculation
» Do not update RTT on any retransmitted segments
» Double timeout so long as retransmission attempt fails
• Persistence timer to prevent deadlocks by triggering periodically probes
from sender to receiver
• Keepalive timer used when a connection has been idle for a long time
• Timer during connection release set to twice the max packet lifetime
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Congestion
• Happens when offered load close to
network capacity
• Several factors can cause
congestion
– Sudden traffic bursts
» Role of memory (buffering
capacity)
– Slow processors
– Low bandwidth links
• Congestion control vs. flow control
– Congestion control involves the
whole network
– Only a pair of nodes (sender and
receiver of a flow) are involved in flow
control
– But both may require same response
from senders, i.e., reducing sending
rate
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General Principles of Congestion Control
• Open loop solutions: focus on congestion prevention, oblivious of
current network state
– Source-oriented vs. destination-oriented
• Closed loop solutions based on feedback loop
1. Monitor the system to detect when and where congestion occurs
– Metrics: packet drop percentage, mean queue lengths, mean and standard
deviation of packet delays, #retransmitted packets
2. Pass this information to places where action can be taken
– Using special packets or marking existing packets to inform traffic sources
(explicit feedback)
– Implicit feedback (e.g., packet loss in wired Internet)
– Proactive probing from hosts
3. Adjust system operation to correct the problem
– Choosing appropriate timescale for adaptation critical
– Increase resources or decrease load
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Congestion Prevention Policies (Jain, 1990)
Policies that affect congestion.
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Congestion Control
• Virtual circuit subnets
• Datagram subnets
– Admission control
– Rerouting new virtual circuits around
congested areas
– Resource reservation during virtual
circuit setup
• Jitter (delay variation) control for
audio/video applications
– Can be bounded by computing
expected transit time
– Prioritizing packets that are behind
their schedule the most can help
control jitter
– Buffering at receiver can help for
video on demand or stored audio/video
streaming apps, but not for live apps
like Internet telephony or video
conferencing
– Monitor resource (e.g., output line)
utilization and enter a “warning” state
when it exceeds a threshold
– On packet arrival for the resource in
warning state, inform traffic source
using:
» Warning bit
» Choke packets
» Hop-by-hop choke packets
» Load shedding (random vs.
application-aware)
- E.g., Random Early Detection
(RED) with TCP
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TCP Congestion Control
• Congestion can be dealt with by
employing a principle borrowed from
physics: law of conservation of packets,
i.e., refrain from injecting a new packet
until an old one leaves the network
• TCP achieves this goal by adjusting
transmission rate via dynamic
manipulation of sender window = min
(receiver window, congestion window)
a) A fast network feeding a low capacity
receiver flow control problem (receiver
window)
b) A slow network feeding a highcapacity receiver congestion control
problem (congestion window)
• Congestion detection via monitoring
retransmission timeouts because losses
mainly due to congestion in wired
Internet and losses lead to timeouts
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TCP Congestion Control (2)
• When connection
established, congestion
window = max segment size
and threshold = 64KB
• Congestion window is
increased exponentially in
response to
acknowledgements until either
a timeout occurs or threshold
is reached or receiver window
is reached (slow start phase)
• When timeout, threshold is
set to half the current
congestion window and
congestion window reset to
max segment size
Illustration of TCP congestion control algorithm.
• After crossing threshold,
congestion window increases
linearly till a timeout occurs or
receiver window is reached
(congestion avoidance phase)
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