1. Transport layer issues Johan Lukkien
Computer Networks 2002/2003
Transport layer issues
Johan Lukkien
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

2. Provided service
Communication between applications on disparate machines
add multiplexing to the network layer
need additional addressing to distinguish applications: ports, ....
Connection oriented / Connectionless
Quality (optional)
independent of required service
reliability. confidentiality
dependent of required service
quality of service: timeliness, bandwidth
in cooperation with required service
congestion control
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

3. Transport service Must deal with
many concurrent transport connections
with many different machines
Provided interface available to application programmers
Balance
hide network level details
admit high performance
either network abstraction is with low cost
or network services are accessible
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

4. Packing TPDUs TPDUs, also called segments
Minimal segment size: MSS – configured
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

5. Required service Packets between machines
just send and receive to an addressed machine
general fault model: loss, duplication, out-of-order, delay
Possibly, entry points for quality control
specific functions, e.g., bandwidth reservation, connection setup
though the entire network layer then must support it!
over-dimensioning
idem
Note: usually, only the end-stations run the transport protocol
so, the TPDU’s are not interpreted in the network
Question: is this always the case? If not, what are the consequences?
all connection information is in the transport layer
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

6. Overview Services Connection control Communication control types
addressing
packet format
Connection control
setup and destroy
client and server roles
Communication control
feedback mechanisms
policies and mechanisms for acknowledgements, timeouts, flow control, buffering
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

7. Services User datagram Reliable connection oriented service
datagram service between applications
just add multiplexing to basic packet network service
internet: UDP (user datagram protocol)
address: (machine, port)
includes multicasting, many-to-one
Reliable connection oriented service
between applications
add multiplexing and connections to basic network service, hiding the faults
internet: TCP (transmission control protocol)
point-to-point channel, identified by
(source mach., source port) – (dest. mach., dest. port)
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

8. UDP Header Uses port: 0-65535 (0..1023: reserved, e.g. ftp, DNS)
length: includes header
checksum: (why?) – may be omitted
Uses
if connection orientation not needed
e.g. simple request/reply
if reliable streams are not needed
hence do not waste performance
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

9. The Real-Time Transport Protocol
Transport protocol on top UDP
Typical uses: multi-media
can use for multicasting (e.g. Internet radio)
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

10. Setting up a connection
Usually, an ‘active’ and a ‘passive’ partner
active: seeking contact – client
passive: awaiting contact – server
The client needs to know (where to find) the server
At least one partner must be active
in principle, two active partners would work
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

11. Provided interface Operations to access a service: primitives
Minimal set:
must support client and server roles
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

12. State diagram Transitions labeled in italics: through packet arrivals
Solid lines: client
Dashed lines: server
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

13. Berkeley Sockets Standard interface for TCP 18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

14. TCP addresses A server waits at a fixed meeting point
e.g. an FTP server
If the connection is identified by just (machine, port) no machine can serve two similar tasks
e.g. two FTP sessions
For the client this problem does not exist
it just needs an unused port
A TCP connection is determined on both endpoints by
(src, src port, dst, dst port)
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

15. Congestion and flow control
Congestion control
the subnet must be able to carry the flow
effective and fair sharing
Flow control
sender and receiver issue
don’t flood the receiver
Note: flow control techniques (e.g., ‘shut up’) can be used to avoid or solve congestion
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

16. Congestion
What happens if traffic goes from a ‘wide’ into a ‘small’ link? (a source is ‘wide’)
delay per packet increases
hence retransmission occurs, possibly unneeded
packets are dropped
thus wasting all work along the path, also for others
chain of events into complete congestion
irrespective of buffer capacity or
knowledge of the exact packets
that got lost
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

17. Congestion control Feedback cycle Measurements Actions taken
measure, interpret, adjust
no single location to take action
Measurements
discarded packets, locally
lack of buffer space, length of queues
# timed-out packets
routers mark outgoing packets with local condition
Actions taken
inform source
....increases load, slowed down by congestion
drop packets
policies: retain old (wine), or new (milk)
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

18. Relation to network uses
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

19. Congestion control at the endpoints
Measure congestion at sender
timeout
or retransmission request
Action
adjust transmission rate
maximal unacknowledged window: C (at least MSS)
Threshold T
round trip time: rtt
C/rtt is effective transmission rate
algorithm – slow start AIMD
reduce T until C/2 upon failure; set C to 1 MSS
upon ack, increase by 1 (= slow start: exponential growth)
upon reaching C reaching T: only increase linearly
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

20. Behavior Roughly 75% of available bandwidth used
Convergence to fairness, if everybody cooperates
however, multiple TCPs increases share
“TCP-friendly”
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

21. TCP: some assigned ports
Protocol
Use 21
FTP
File transfer 23
Telnet
Remote login 25
SMTP 69
TFTP
Trivial File Transfer Protocol 79
Finger
Lookup info about a user 80
HTTP
World Wide Web
110
POP-3
Remote access
119
NNTP
USENET news
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

22. The TCP Segment Header 18-Sep-18 Johan J. Lukkien, j.j.lukkien@tue.nl
TU/e Computer Science, System Architecture and Networking

23. Pseudoheader Included in the TCP checksum Mix of layers 18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

24. Sequence numbers Needed when message order is important
sliding window protocols
(soft) state updates, e.g. router tables
Finite range: wrap-around
Dangers
long-lived messages
delayed duplicate messages
delayed acknowledgements
(replay attacks)
restart after failure
Approach: use aging
hop count
timestamp
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

25. Timestamping Use synchronized clocks to put the time on a message
e.g. through an external reference clock
Define minimal packet life-time
Minimal time after packet traces have gone: T
Use a crash-resilient counter to generate sequence numbers
so a machine knows ‘where it was’ after a crash
wrap-around must not be possible within T
counter increment faster than send rate
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

26. Forbidden region
Forbidden region represents sequence numbers that might be used after a restart
can be entered from the bottom
fast transmission
or from the left
slow transmission
applications must check and resynchronize
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

27. Getting started: three-way handshake
CR denotes CONNECTION REQUEST.
Normal operation
Old CONNECTION REQUEST appearing out of nowhere
Duplicate CONNECTION REQUEST and duplicate ACK.
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

28. TCP Connection Establishment
6-31
(a) TCP connection establishment in the normal case.
(b) Call collision: single connection remains
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

29. TCP Transmission Policy
Flow control through specifying available window
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

30. Silly windows... 18-Sep-18 Johan J. Lukkien, j.j.lukkien@tue.nl
TU/e Computer Science, System Architecture and Networking

31. TCP Timer Management
(a) Probability density of ACK arrival times in the data link layer.
(b) Probability density of ACK arrival times for TCP
hence, dynamically determine timer value (e.g. double upon failure, linear decrease)
18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

32. Transactional TCP E.g. using HTTP 18-Sep-18
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking