4.1.2.7  LIST and NLST Commands .......................   31
            4.1.2.8  SITE Command .................................   32
            4.1.2.9  STOU Command .................................   32
            4.1.2.10  Telnet End-of-line Code .....................   32
            4.1.2.11  FTP Replies .................................   33
            4.1.2.12  Connections .................................   34
            4.1.2.13  Minimum Implementation; RFC-959 Section .....   34
         4.1.3  SPECIFIC ISSUES ...................................   35
            4.1.3.1  Non-standard Command Verbs ...................   35
            4.1.3.2  Idle Timeout .................................   36
            4.1.3.3  Concurrency of Data and Control ..............   36
            4.1.3.4  FTP Restart Mechanism ........................   36
         4.1.4  FTP/USER INTERFACE ................................   39




 



RFC1123                       INTRODUCTION                  October 1989


            4.1.4.1  Pathname Specification .......................   39
            4.1.4.2  "QUOTE" Command ..............................   40
            4.1.4.3  Displaying Replies to User ...................   40
            4.1.4.4  Maintaining Synchronization ..................   40
         4.1.5   FTP REQUIREMENTS SUMMARY .........................   41
      4.2  TRIVIAL FILE TRANSFER PROTOCOL -- TFTP .................   44
         4.2.1  INTRODUCTION ......................................   44
         4.2.2  PROTOCOL WALK-THROUGH .............................   44
            4.2.2.1  Transfer Modes ...............................   44
            4.2.2.2  UDP Header ...................................   44
         4.2.3  SPECIFIC ISSUES ...................................   44
            4.2.3.1  Sorcerer's Apprentice Syndrome ...............   44
            4.2.3.2  Timeout Algorithms ...........................   46
            4.2.3.3  Extensions ...................................   46
            4.2.3.4  Access Control ...............................   46
            4.2.3.5  Broadcast Request ............................   46
         4.2.4  TFTP REQUIREMENTS SUMMARY .........................   47

   5.  ELECTRONIC MAIL -- SMTP and RFC-822 ........................   48
      5.1  INTRODUCTION ...........................................   48
      5.2  PROTOCOL WALK-THROUGH ..................................   48
         5.2.1  The SMTP Model ....................................   48
         5.2.2  Canonicalization ..................................   49
         5.2.3  VRFY and EXPN Commands ............................   50
         5.2.4  SEND, SOML, and SAML Commands .....................   50
         5.2.5  HELO Command ......................................   50
         5.2.6  Mail Relay ........................................   51
         5.2.7  RCPT Command ......................................   52
         5.2.8  DATA Command ......................................   53
         5.2.9  Command Syntax ....................................   54
         5.2.10  SMTP Replies .....................................   54
         5.2.11  Transparency .....................................   55
         5.2.12  WKS Use in MX Processing .........................   55
         5.2.13  RFC-822 Message Specification ....................   55
         5.2.14  RFC-822 Date and Time Specification ..............   55
         5.2.15  RFC-822 Syntax Change ............................   56
         5.2.16  RFC-822  Local-part ..............................   56
         5.2.17  Domain Literals ..................................   57
         5.2.18  Common Address Formatting Errors .................   58
         5.2.19  Explicit Source Routes ...........................   58
      5.3  SPECIFIC ISSUES ........................................   59
         5.3.1  SMTP Queueing Strategies ..........................   59
            5.3.1.1 Sending Strategy ..............................   59
            5.3.1.2  Receiving strategy ...........................   61
         5.3.2  Timeouts in SMTP ..................................   61
         5.3.3  Reliable Mail Receipt .............................   63
         5.3.4  Reliable Mail Transmission ........................   63
         5.3.5  Domain Name Support ...............................   65




 



RFC1123                       INTRODUCTION                  October 1989


         5.3.6  Mailing Lists and Aliases .........................   65
         5.3.7  Mail Gatewaying ...................................   66
         5.3.8  Maximum Message Size ..............................   68
      5.4  SMTP REQUIREMENTS SUMMARY ..............................   69

   6. SUPPORT SERVICES ............................................   72
      6.1 DOMAIN NAME TRANSLATION .................................   72
         6.1.1 INTRODUCTION .......................................   72
         6.1.2  PROTOCOL WALK-THROUGH .............................   72
            6.1.2.1  Resource Records with Zero TTL ...............   73
            6.1.2.2  QCLASS Values ................................   73
            6.1.2.3  Unused Fields ................................   73
            6.1.2.4  Compression ..................................   73
            6.1.2.5  Misusing Configuration Info ..................   73
         6.1.3  SPECIFIC ISSUES ...................................   74
            6.1.3.1  Resolver Implementation ......................   74
            6.1.3.2  Transport Protocols ..........................   75