Transport Level Protocols and UDP

Tom Kelliher, CS 325

Feb. 28, 2011

Administrivia

Outline

Transport level protocols introduction.
Multiplexing and demultiplexing.
UDP.

Transport Level Protocols Introduction

Transport level protocols provide the illusion of a direct connection between hosts:

$\begin{figure}\centering\includegraphics[width=4in]{Figures/fig03_01.eps}\end{figure}$

Terminology:

Segment: TCP.
Datagram: UDP and IP.
IP: network-level protocol.

Things to note:

Network-level protocols provide host-to-host communications.
USPS analogy -- to your house.
Only needs IP address.
Transport-level protocols provide process-to-process communications.
USPS -- To you.
IP is a best-effort delivery service. No guarantees: dropped, duplicated, mangled datagrams.
UDP adds process delivery and data error detection.
TCP adds reliability and congestion control.

Multiplexing and Demultiplexing

Multiplexing: Taking application data and adding necessary header information to ensure delivery to appropriate process on destination host. Multiple application streams (possibly to same destination host) multiplexed into network-level protocol.

Demultiplexing: Using header information to pass application data to correct process.

Illustration:

$\begin{figure}\centering\includegraphics[width=5in]{Figures/fig03_02.eps}\end{figure}$

Use of port numbers to identify sockets:

$\begin{figure}\centering\includegraphics[]{Figures/fig03_03.eps}\end{figure}$

Source port is also part of the ``return'' address.
Servers listen on ``well known'' port numbers. Clients use ``any old'' port number.
A process creates a socket; the OS then associates I/O on that socket to that process.
Only one process can open a socket on a given port, but...
multiple TCP sockets (processes) might be using the same socket -- the ``main'' process and forked children.
A UDP socket is identified by an ordered pair: (destination IP address, destination port).
A single process receives all datagrams sent to a given port.
A TCP socket is identified by a four-tuple: (source IP address, source port, destination IP address, destination port).
Multiple processes can receive segments sent to a given port.
Example: Two SSH clients (one for Christine, one for Joe) on kingfisher (10.67.1.43) connecting to the SSH server (port 22) on phoenix (10.32.3.39).

Linux example of connection status (produced with netstat -t -p -e -a):

Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address               Foreign Address             
State       User       Inode      PID/Program name   

tcp        0      0 *:printer                   *:*                         
LISTEN      root       328119     16106/xinetd        

tcp        0      0 *:5252                      *:*                         
LISTEN      smmsp      328461     16305/milter-greyli 

tcp        0      0 *:netbios-ssn               *:*                         
LISTEN      root       328886     16491/smbd          

tcp        0      0 localhost.localdomain:783   *:*                         
LISTEN      root       328561     16348/spamd.pid     

tcp        0      0 *:sunrpc                    *:*                         
LISTEN      root       327576     15887/portmap       

tcp        0      0 *:10000                     *:*                         
LISTEN      root       328048     16085/beremote      

tcp        0      0 *:auth                      *:*                         
LISTEN      root       328118     16106/xinetd        

tcp        0      0 phoenix.goucher.edu:domain  *:*                         
LISTEN      named      328000     16055/named         

tcp        0      0 localhost.localdomai:domain *:*                         
LISTEN      named      327998     16055/named         

tcp        0      0 *:ipp                       *:*                         
LISTEN      root       644033     25518/cupsd         

tcp        0      0 *:postgres                  *:*                         
LISTEN      postgres   589356     19320/postmaster    

tcp        0      0 *:824                       *:*                         
LISTEN      root       327612     15906/rpc.statd     

tcp        0      0 *:smtp                      *:*                         
LISTEN      root       328522     16328/sendmail: acc 

tcp        0      0 localhost.localdomain:rndc  *:*                         
LISTEN      named      328003     16055/named         

tcp        0      0 localhost.lo:x11-ssh-offset *:*                         
LISTEN      kelliher   433569     32014/0             

tcp        0      0 *:xfs                       *:*                         
LISTEN      xfs        328807     16482/xfs           

tcp        0      0 *:microsoft-ds              *:*                         
LISTEN      root       328885     16491/smbd          

tcp        0      0 phoenix.goucher.edu:5252    bluebird.goucher.edu:54880  
ESTABLISHED smmsp      419692     16305/milter-greyli 

tcp        0      0 phoenix.goucher.edu:39161   bluebird.goucher.edu:5252   
ESTABLISHED smmsp      433791     16305/milter-greyli 

tcp        0      0 *:5252                      *:*                         
LISTEN      smmsp      328460     16305/milter-greyli 

tcp        0      0 *:http                      *:*                         
LISTEN      root       328689     16417/httpd         

tcp        0      0 *:ssh                       *:*                         
LISTEN      root       328027     16077/sshd          

tcp        0      0 *:postgres                  *:*                         
LISTEN      postgres   589355     19320/postmaster    

tcp        0      0 localhost:x11-ssh-offset    *:*                         
LISTEN      kelliher   433570     32014/0             

tcp        0      0 *:https                     *:*                         
LISTEN      root       328691     16417/httpd         

tcp        0      0 *:xfs                       *:*                         
LISTEN      xfs        328806     16482/xfs           

tcp        0      0 phoenix.goucher.edu:http    lj511775.crawl.yahoo.:36548 
TIME_WAIT   root       0          -                   

tcp        0      0 phoenix.goucher.edu:ssh     bluebird.goucher.edu:46079  
ESTABLISHED root       433527     32012/sshd: kellihe 

tcp        0      0 phoenix.goucher.edu:http    crawl-8.cuill.com:38700     
TIME_WAIT   root       0          -                   

tcp        0      0 phoenix.goucher.edu:http    crawl-8.cuill.com:49221     
TIME_WAIT   root       0          -                   

tcp        0      0 phoenix.goucher.edu:http    ppp-69-218-215-238.dsl:1278 
FIN_WAIT2   apache     660717     25594/httpd

Connection-oriented demultiplexing:

$\begin{figure}\centering\includegraphics[]{Figures/fig03_05.eps}\end{figure}$

UDP

Little overhead.
No guarantees -- application protocols must build in reliability.
Reliability not always needed -- telephony.
Sometimes, getting through is more important: SNMP, DNS, RIP.
Sometimes, low overhead in a local networking environment matters most: NFS.

$\begin{figure}\centering\includegraphics[width=5in]{Figures/fig03_06.eps}\end{figure}$

UDP datagram format:

$\begin{figure}\centering\includegraphics[]{Figures/fig03_07.eps}\end{figure}$

Header is eight bytes -- low overhead.
Length is for entire datagram.
Checksum is computed over entire datagram and pseudo-header.
Pseudo-header fields:
1. Source and Destination IP address.
2. IP protocol field.
Double-check that IP didn't route datagram to wrong host or transport-level protocol.
Importance of end-to-end checks: even if all links perform error checking, errors can and have been introduced in routers. Some link-level protocols (SLIP) do not perform error checking.

Open UDP sockets on phoenix:

Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address               Foreign Address             
State       User       Inode      PID/Program name   

udp        0      0 *:who                       *:*                                     
root       328156     16132/rwhod         

udp        0      0 *:syslog                    *:*                                     
root       327536     15859/syslogd       

udp        0      0 phoenix.gouc:netbios-ns     *:*                                     
root       328833     16495/nmbd          

udp        0      0 *:netbios-ns                *:*                                     
root       328828     16495/nmbd          

udp        0      0 phoenix.gou:netbios-dgm     *:*                                     
root       328834     16495/nmbd          

udp        0      0 *:netbios-dgm               *:*                                     
root       328829     16495/nmbd          

udp        0      0 *:xdmcp                     *:*                                     
root       329172     16578/gdm-binary    

udp        0      0 *:818                       *:*                                     
root       327602     15906/rpc.statd     

udp        0      0 phoenix.goucher.edu:domain  *:*                                     
named      327999     16055/named         

udp        0      0 localhost.locald:domain     *:*                                     
named      327997     16055/named         

udp        0      0 *:821                       *:*                                     
root       327609     15906/rpc.statd     

udp        0      0 *:38749                     *:*                                     
named      328001     16055/named         

udp        0      0 *:sunrpc                    *:*                                     
root       327575     15887/portmap       

udp        0      0 localhost.localdomain:44528 localhost.localdomain:44528 
ESTABLISHED postgres   589363     19320/postmaster    

udp        0      0 *:ipp                       *:*                                     
root       644034     25518/cupsd         

udp        0      0 phoenix.goucher.edu:ntp     *:*                                     
root       328166     16123/ntpd          

udp        0      0 localhost.localdomain:ntp   *:*                                     
root       328165     16123/ntpd          

udp        0      0 *:ntp                       *:*                                     
root       328163     16123/ntpd          

udp        0      0 *:38750                     *:*                                     
named      328002     16055/named         

udp        0      0 *:ntp                       *:*                                     
root       328164     16123/ntpd

Thomas P. Kelliher 2011-02-28

Tom Kelliher

Transport Level Protocols and UDP

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