rfc1180.txt

RFC 的详细文档！

   For instance, the SNMP server, called an SNMP agent, always waits on
   port 161.  There can be only one SNMP agent per computer because
   there is only one UDP port number 161.  This port number is well
   known; it is a fixed number, an internet assigned number.  If an SNMP
   client wants service, it sends its request to port number 161 of UDP
   on the destination computer.

   When an application sends data out through UDP it arrives at the far
   end as a single unit.  For example, if an application does 5 writes
   to the UDP port, the application at the far end will do 5 reads from
   the UDP port.  Also, the size of each write matches the size of each
   read.

   UDP preserves the message boundary defined by the application.  It
   never joins two application messages together, or divides a single
   application message into parts.

6.2  Checksum

   An incoming IP packet with an IP header type field indicating "UDP"
   is passed up to the UDP module by IP.  When the UDP module receives
   the UDP datagram from IP it examines the UDP checksum.  If the
   checksum is zero, it means that checksum was not calculated by the
   sender and can be ignored.  Thus the sending computer's UDP module
   may or may not generate checksums.  If Ethernet is the only network
   between the 2 UDP modules communicating, then you may not need



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   checksumming.  However, it is recommended that checksum generation
   always be enabled because at some point in the future a route table
   change may send the data across less reliable media.

   If the checksum is valid (or zero), the destination port number is
   examined and if an application is bound to that port, an application
   message is queued for the application to read.  Otherwise the UDP
   datagram is discarded.  If the incoming UDP datagrams arrive faster
   than the application can read them and if the queue fills to a
   maximum value, UDP datagrams are discarded by UDP.  UDP will continue
   to discard UDP datagrams until there is space in the queue.

7.  Transmission Control Protocol

   TCP provides a different service than UDP.  TCP offers a connection-
   oriented byte stream, instead of a connectionless datagram delivery
   service.  TCP guarantees delivery, whereas UDP does not.

   TCP is used by network applications that require guaranteed delivery
   and cannot be bothered with doing time-outs and retransmissions.  The
   two most typical network applications that use TCP are File Transfer
   Protocol (FTP) and the TELNET.  Other popular TCP network
   applications include X-Window System, rcp (remote copy), and the r-
   series commands.  TCP's greater capability is not without cost: it
   requires more CPU and network bandwidth.  The internals of the TCP
   module are much more complicated than those in a UDP module.

   Similar to UDP, network applications connect to TCP ports.  Well-
   defined port numbers are dedicated to specific applications.  For
   instance, the TELNET server uses port number 23.  The TELNET client
   can find the server simply by connecting to port 23 of TCP on the
   specified computer.

   When the application first starts using TCP, the TCP module on the
   client's computer and the TCP module on the server's computer start
   communicating with each other.  These two end-point TCP modules
   contain state information that defines a virtual circuit.  This
   virtual circuit consumes resources in both TCP end-points.  The
   virtual circuit is full duplex; data can go in both directions
   simultaneously.  The application writes data to the TCP port, the
   data traverses the network and is read by the application at the far
   end.

   TCP packetizes the byte stream at will; it does not retain the
   boundaries between writes.  For example, if an application does 5
   writes to the TCP port, the application at the far end might do 10
   reads to get all the data.  Or it might get all the data with a
   single read.  There is no correlation between the number and size of



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   writes at one end to the number and size of reads at the other end.

   TCP is a sliding window protocol with time-out and retransmits.
   Outgoing data must be acknowledged by the far-end TCP.
   Acknowledgements can be piggybacked on data.  Both receiving ends can
   flow control the far end, thus preventing a buffer overrun.

   As with all sliding window protocols, the protocol has a window size.
   The window size determines the amount of data that can be transmitted
   before an acknowledgement is required.  For TCP, this amount is not a
   number of TCP segments but a number of bytes.

8.  Network Applications

   Why do both TCP and UDP exist, instead of just one or the other?

   They supply different services.  Most applications are implemented to
   use only one or the other.  You, the programmer, choose the protocol
   that best meets your needs.  If you need a reliable stream delivery
   service, TCP might be best.  If you need a datagram service, UDP
   might be best.  If you need efficiency over long-haul circuits, TCP
   might be best.  If you need efficiency over fast networks with short
   latency, UDP might be best.  If your needs do not fall nicely into
   these categories, then the "best" choice is unclear.  However,
   applications can make up for deficiencies in the choice.  For
   instance if you choose UDP and you need reliability, then the
   application must provide reliability.  If you choose TCP and you need
   a record oriented service, then the application must insert markers
   in the byte stream to delimit records.

   What network applications are available?

   There are far too many to list.  The number is growing continually.
   Some of the applications have existed since the beginning of internet
   technology: TELNET and FTP.  Others are relatively new: X-Windows and
   SNMP.  The following is a brief description of the applications
   mentioned in this tutorial.

8.1  TELNET

   TELNET provides a remote login capability on TCP.  The operation and
   appearance is similar to keyboard dialing through a telephone switch.
   On the command line the user types "telnet delta" and receives a
   login prompt from the computer called "delta".

   TELNET works well; it is an old application and has widespread
   interoperability.  Implementations of TELNET usually work between
   different operating systems.  For instance, a TELNET client may be on



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   VAX/VMS and the server on UNIX System V.

8.2  FTP

   File Transfer Protocol (FTP), as old as TELNET, also uses TCP and has
   widespread interoperability.  The operation and appearance is as if
   you TELNETed to the remote computer.  But instead of typing your
   usual commands, you have to make do with a short list of commands for
   directory listings and the like.  FTP commands allow you to copy
   files between computers.

8.3  rsh

   Remote shell (rsh or remsh) is one of an entire family of remote UNIX
   style commands.  The UNIX copy command, cp, becomes rcp.  The UNIX
   "who is logged in" command, who, becomes rwho.  The list continues
   and is referred to collectively to as the "r" series commands or the
   "r*" (r star) commands.

   The r* commands mainly work between UNIX systems and are designed for
   interaction between trusted hosts.  Little consideration is given to
   security, but they provide a convenient user environment.

   To execute the "cc file.c" command on a remote computer called delta,
   type "rsh delta cc file.c".  To copy the "file.c" file to delta, type
   "rcp file.c delta:".  To login to delta, type "rlogin delta", and if
   you administered the computers in a certain way, you will not be
   challenged with a password prompt.

8.4  NFS

   Network File System, first developed by Sun Microsystems Inc, uses
   UDP and is excellent for mounting UNIX file systems on multiple
   computers.  A diskless workstation can access its server's hard disk
   as if the disk were local to the workstation.  A single disk copy of
   a database on mainframe "alpha" can also be used by mainframe "beta"
   if the database's file system is NFS mounted on "beta".

   NFS adds significant load to a network and has poor utility across
   slow links, but the benefits are strong.  The NFS client is
   implemented in the kernel, allowing all applications and commands to
   use the NFS mounted disk as if it were local disk.

8.5  SNMP

   Simple Network Management Protocol (SNMP) uses UDP and is designed
   for use by central network management stations.  It is a well known
   fact that if given enough data, a network manager can detect and



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   diagnose network problems.  The central station uses SNMP to collect
   this data from other computers on the network.  SNMP defines the
   format for the data; it is left to the central station or network
   manager to interpret the data.

8.6  X-Window

   The X Window System uses the X Window protocol on TCP to draw windows
   on a workstation's bitmap display.  X Window is much more than a
   utility for drawing windows; it is entire philosophy for designing a
   user interface.

9.  Other Information

   Much information about internet technology was not included in this
   tutorial.  This section lists information that is considered the next
   level of detail for the reader who wishes to learn more.

     o administration commands: arp, route, and netstat
     o ARP: permanent entry, publish entry, time-out entry, spoofing
     o IP route table: host entry, default gateway, subnets
     o IP: time-to-live counter, fragmentation, ICMP
     o RIP, routing loops
     o Domain Name System

10.  References

   [1] Comer, D., "Internetworking with TCP/IP Principles, Protocols,
       and Architecture", Prentice Hall, Englewood Cliffs, New Jersey,
       U.S.A., 1988.

   [2] Feinler, E., et al, DDN Protocol Handbook, Volume 2 and 3, DDN
       Network Information Center, SRI International, 333 Ravenswood
       Avenue, Room EJ291, Menlow Park, California, U.S.A., 1985.

   [3] Spider Systems, Ltd., "Packets and Protocols", Spider Systems
       Ltd., Stanwell Street, Edinburgh, U.K. EH6 5NG, 1990.

11.  Relation to other RFCs

   This RFC is a tutorial and it does not UPDATE or OBSOLETE any other
   RFC.

12.  Security Considerations

   There are security considerations within the TCP/IP protocol suite.
   To some people these considerations are serious problems, to others
   they are not; it depends on the user requirements.



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   This tutorial does not discuss these issues, but if you want to learn
   more you should start with the topic of ARP-spoofing, then use the
   "Security Considerations" section of RFC 1122 to lead you to more
   information.

13.  Authors' Addresses

   Theodore John Socolofsky
   Spider Systems Limited
   Spider Park
   Stanwell Street
   Edinburgh EH6 5NG
   United Kingdom

   Phone:
     from UK        031-554-9424
     from USA 011-44-31-554-9424
   Fax:
     from UK        031-554-0649
     from USA 011-44-31-554-0649

   EMail: [e-mail]


   Claudia Jeanne Kale
   12 Gosford Place
   Edinburgh EH6 4BJ
   United Kingdom

   Phone:
     from UK        031-554-7432
     from USA 011-44-31-554-7432

   EMail: [e-mail]

















Socolofsky & Kale                                              [Page 28]