1. CS4470 Computer Networking Protocols
4/4/2019
CS Computer Networking Protocols
13. Transport layer
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline Overview of transport layer Network Address Translation (NAT)
4/4/2019
Outline
Overview of transport layer
Network Address Translation (NAT)
User Datagram Protocol (UDP)
Dynamic Host Configuration Protocol (DHCP)
13. Transport layer CS4470

3. Transport layer overview
4/4/2019
Transport layer overview
Application layer
Application layer
Data
Data
Data
Transport layer
Transport layer H
Data H
segment H
Data T
Network layer
Network layer H
Data T
datagram
13. Transport layer CS4470

4. Transport vs. network layer
Provides computer-to-computer communication
Source and destination addresses are computers
Called host-to-host
Transport layer
Provides application-to-application communication
Need extended addressing scheme to identify applications
Called end-to-end
13. Transport layer CS4470

5. Transport Protocol Functionality
Identify sending and receiving applications
Optionally provide
Reliability
Flow control
Congestion control
Note: not all transport protocols provide above facilities
13. Transport layer CS4470

6. Internet transport-layer protocols
UDP
Unreliable, unordered delivery:
Lightweight transport
Becoming more popular (IP telephony)
Best-effort delivery
TCP
Reliable, in-order delivery
connection setup
congestion control
flow control
13. Transport layer CS4470

7. Identifying an application
Cannot extend IP address
No unused bits
Cannot use OS-dependent quantity
Process ID
Task number
Job name
Must work on all computer systems
13. Transport layer CS4470

8. Identifying an application (cont.)
Invent new abstraction
Called protocol port number
Used to identify sending or receiving application
unambiguously
Independent of underlying operating system
Used only with TCP/IP protocols
13. Transport layer CS4470

9. Protocol Port Example
Domain name server application is assigned port 53
Application using DNS obtains port 28900
UDP datagram sent from application to DNS server has
Source port number 28900
Destination port number 53
When DNS server replies, UDP datagram has
Source port number 53
Destination port number 28900
13. Transport layer CS4470

10. Network Address Translation (NAT)
Extension of original addressing scheme
Motivated by exhaustion of IP address space
Allows multiple computers to share single address
Requires device to perform packet translation
Implementations available
Stand-alone hardware device
IP router with NAT functionality embedded
13. Transport layer CS4470

11. NAT (cont.) A subnet NAT Three blocks of IP addresses are reserved
Obtains single, valid IP address
Assigns a private address to each computer
Uses NAT box to connect to Internet
NAT
Translates addresses in IP datagrams
Three blocks of IP addresses are reserved
13. Transport layer CS4470

12. Illustration Of NAT Single valid IP address needed
Computers at site assigned private, non-routable addresses
13. Transport layer CS4470

13. NAT Example Site uses private network 10.0.0.0 / 8 internally
First computer assigned
Second computer assigned
And so on . . .
Site obtains valid IP address (e.g., ).
Assume computer sends to
NAT translates IP source address of outgoing datagram
NAT translates destination address of incoming datagram
13. Transport layer CS4470

14. Illustration Of NAT Translation
Transparent to each end
Computer at site sends and receives datagrams normally
Computer in Internet receives datagrams from NAT box
13. Transport layer CS4470

15. NAT Details Implementation: NAT router must:
outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)
. . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr.
remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair
incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table
13. Transport layer CS4470

16. NAT example All datagrams leaving local Datagrams with source or
rest of
Internet
local network
(e.g., home network)
10.0.0/24
All datagrams leaving local
network have same single source NAT IP address: ,
different source port numbers
Datagrams with source or
destination in this network
have /24 address for
source, destination (as usual)
13. Transport layer CS4470

17. WAN side addr LAN side addr
NAT example
NAT translation table
WAN side addr LAN side addr
1: host
sends datagram to
, 80
2: NAT router
changes datagram
source addr from
, 3345 to
, 5001,
updates table
, , 3345
…… ……
S: , 3345
D: , 80 1
S: , 80
D: , 3345 4
S: , 5001
D: , 80 2
S: , 80
D: , 5001 3
4: NAT router
changes datagram
dest addr from
, 5001 to , 3345
3: Reply arrives
dest. address:
, 5001
13. Transport layer CS4470

18. Dynamic Assignment of IP addresses
Dynamic assignment of IP addresses is desirable for several reasons:
IP addresses are assigned on-demand
Avoid manual IP configuration
Support mobility of laptops
Three Protocols
RARP (until 1985, no longer used)
BOOTP ( )
DHCP (since 1993)
Only DHCP is widely used today.
13. Transport layer CS4470

19. DHCP Client-server: host requests an IP address from a DHCP server
Since this is an application, DHCP server does not have to be on the same network as the host.
Must designate a relay agent who know the address of DHCP server
There can be a few DHCP servers per site, rather than one on each network
13. Transport layer CS4470

20. 4/4/2019
Relay agent
13. Transport layer CS4470

21. DHCP protocol: 4 steps DHCP server discovery.
Use DHCP discover message
DHCP client creates an IP datagram
Source IP address:
Destination IP address:
The IP datagram is passed down to the adapter which encapsulates the datagram in a frame
Source MAC address: send’s MAC address
Destination MAC address: FF-FF-FF-FF-FF-FF
13. Transport layer CS4470

22. DHCP protocol: 4 steps DHCP server offer.
A DHCP server receiving a DHCP discover message responds to the client with a DHCP offer message
Each server offer message contains
Transaction ID of the received discover message
The proposed IP address for the client
The network mask
IP address lease time – the amount of time for which the IP address will be valid
13. Transport layer CS4470

23. DHCP protocol: 4 steps DHCP Request
The newly arriving client will choose from among one or more server offers
The client responds to its selected offer with a DHCP request message, echoing back the configuration parameters
13. Transport layer CS4470

24. DHCP protocol: 4 steps DHCP ACK
The server responds to the DHCP request message with a DHCP ACK message, confirming the requested parameters
Once the client receives the DHCP ACK, the interaction is complete and the client can use the DHCP-allocated IP address for the lease duration
13. Transport layer CS4470

25. 4/4/2019
13. Transport layer CS4470

26. UDP: User Datagram Protocol [RFC 768]
Connectionless service
Best-effort semantics
Each message encapsulated in IP datagram
Uses protocol ports to identify applications
13. Transport layer CS4470

27. Why is there a UDP? no connection establishment (which can add delay)
4/4/2019
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
13. Transport layer CS4470

28. UDP Semantics
Same best-effort semantics as IP (i.e., unreliable transfer)
Message can be
Lost
Duplicated
Delayed
Delivered out of order
13. Transport layer CS4470

29. UDP: more other UDP uses often used for streaming multimedia apps
loss tolerant
rate sensitive
other UDP uses
DNS
SNMP
reliable transfer over UDP: add reliability at application layer
application-specific error recovery!
13. Transport layer CS4470

30. UDP segment format Application data (message) source port #
dest port #
32 bits
Application
data
(message)
length
checksum
Length, in
bytes of UDP
segment,
including
header
13. Transport layer CS4470

31. Destinantion IP address
UDP pseudo header
Source IP address
UDP
Pseudo
header
32 bits
Application
data
(message)
Destinantion IP address
UDP length
Protocol=17
UDP header
It’s not transmitted to the IP layer
It’s only used for computing the checksum
13. Transport layer CS4470

32. UDP checksum The checksum includes three sections: Goal
A pseudo header
The UDP header
The application data
Goal
detect “errors” (e.g., flipped bits) in transmitted segment
13. Transport layer CS4470

33. UDP Encapsulation Two levels of encapsulation
UDP datagram size cannot exceed maximum IP payload
13. Transport layer CS4470