1.1 --- a/thesis/tex/4-MasqmailsFuture.tex	Sat Feb 07 11:42:45 2009 +0100
     1.2 +++ b/thesis/tex/4-MasqmailsFuture.tex	Sat Feb 07 12:00:11 2009 +0100
     1.3 @@ -44,7 +44,7 @@
     1.4  The requirements are named ``\NAME{RF}'' for ``requirement, functional''.
     1.5  
     1.6  
     1.7 -\paragraph{\RF1: Incoming and outgoing channels}
     1.8 +\paragraph{\RF\,1: Incoming and outgoing channels}
     1.9  \label{rf1}
    1.10  \sendmail-compatible \MTA{}s must support at least two incoming channels: mail submitted using the \path{sendmail} command, and mail received on a \NAME{TCP} port. Thus it is common to split the incoming channels into local and remote. This is done by \qmail\ and \postfix. The same way is \person{Hafiz}'s view \cite{hafiz05}.
    1.11  \index{incoming channels}
    1.12 @@ -80,7 +80,7 @@
    1.13  
    1.14  
    1.15  
    1.16 -\paragraph{\RF2: Mail queuing}
    1.17 +\paragraph{\RF\,2: Mail queuing}
    1.18  \label{rf2}
    1.19  \index{mail queue}
    1.20  Mail queuing removes the need to deliver instantly as a message is received. The queue provides fail-safe storage of mails until they are delivered. Mail queues are probably used in all \MTA{}s, even in some simple forwarders. The mail queue is essential for \masqmail, as \masqmail\ is intended for non-permanent online connections. This means, mail must be queued until a online connection is available to send the message. This may be after a reboot. Hence the mail queue must provide persistence.
    1.21 @@ -93,7 +93,7 @@
    1.22  
    1.23  
    1.24  
    1.25 -\paragraph{\RF3: Header sanitizing}
    1.26 +\paragraph{\RF\,3: Header sanitizing}
    1.27  \label{rf3}
    1.28  \index{header sanitizing}
    1.29  Mail coming into the system often lacks important header lines. At least the required ones must be added by the \MTA. One example is the \texttt{Date:} header, another is the, not required but recommended, \texttt{Message-ID:} header. Apart from adding missing headers, rewriting headers is important, too. Changing the locally known domain part of email addresses to globally known ones is an example. \masqmail\ needs to be able to rewrite the domain part dependent on the route used to send the message, to prevent messages to get classified as spam.
    1.30 @@ -104,7 +104,7 @@
    1.31  
    1.32  
    1.33  
    1.34 -\paragraph{\RF4: Aliasing}
    1.35 +\paragraph{\RF\,4: Aliasing}
    1.36  \label{rf4}
    1.37  \index{aliases}
    1.38  Email addresses can have aliases, thus they need to be expanded. Aliases can be of different kind: another local user, a remote user, a list of local and remote users, or a command. Most important are the aliases in the \path{aliases} file, usually located at \path{/etc/aliases}. Addresses expanding to lists of users lead to more envelopes. Aliases changing the recipient's domain part may require a different route to be used.
    1.39 @@ -112,7 +112,7 @@
    1.40  
    1.41  
    1.42  
    1.43 -\paragraph{\RF5: Route management}
    1.44 +\paragraph{\RF\,5: Route management}
    1.45  \label{rf5}
    1.46  \index{online routes}
    1.47  One key feature of \masqmail\ is its ability to send mail out over different routes. The online state defines the active route to be used. A specific route may not be suited for all messages, thus these messages are hold back until a suiting route is active. For more information on this concept see section~\ref{sec:masqmail-routes}.
    1.48 @@ -120,7 +120,7 @@
    1.49  
    1.50  
    1.51  
    1.52 -\paragraph{\RF6: Authentication}
    1.53 +\paragraph{\RF\,6: Authentication}
    1.54  \label{rf6}
    1.55  \label{requirement-authentication}
    1.56  \index{auth}
    1.57 @@ -153,7 +153,7 @@
    1.58  
    1.59  
    1.60  
    1.61 -\paragraph{\RF7: Encryption}
    1.62 +\paragraph{\RF\,7: Encryption}
    1.63  \label{rf7}
    1.64  \label{requirement-encryption}
    1.65  \index{enc}
    1.66 @@ -185,7 +185,7 @@
    1.67  
    1.68  
    1.69  
    1.70 -\paragraph{\RF8: Spam handling}
    1.71 +\paragraph{\RF\,8: Spam handling}
    1.72  \label{rf8}
    1.73  \index{spam}
    1.74  Spam is a major threat nowadays, but it is a war that is hard to win. The goal is to provide state-of-the-art spam protection, but not more. (See section~\ref{sec:swot-analysis}.)
    1.75 @@ -211,7 +211,7 @@
    1.76  
    1.77  
    1.78  
    1.79 -\paragraph{\RF9: Malware handling}
    1.80 +\paragraph{\RF\,9: Malware handling}
    1.81  \label{rf9}
    1.82  \index{malware}
    1.83  Related to spam is malicious content (short: \name{malware}) like viruses, worms, and trojan horses. They, in contrast to spam, do not affect the \MTA\ itself, as they are in the mail's body. \MTA{}s that search for malware are equal to post offices that open letters to check if they contain something that could harm the recipient. This is not a mail transport job. But by many people the \MTA\ which is responsible for the recipient is seen to be at a good position to do this work, thus it is often done there. Though, it is nice to have interfaces to such scanners within the \MTA.
    1.84 @@ -223,7 +223,7 @@
    1.85  
    1.86  
    1.87  
    1.88 -\paragraph{\RF10: Archiving}
    1.89 +\paragraph{\RF\,10: Archiving}
    1.90  \label{rf10}
    1.91  \index{archiving}
    1.92  Mail archiving and auditability become more important as email establishes as technology for serious business communication. Archiving is a must for companies in many countries. In the United States, the \name{Sarbanes-Oxley Act} \cite{sox} covers this topic.
    1.93 @@ -246,7 +246,7 @@
    1.94  These non-functional requirements are named ``\NAME{RG}'' for ``requirement, general''.
    1.95  
    1.96  
    1.97 -\paragraph{\RG1: Security}
    1.98 +\paragraph{\RG\,1: Security}
    1.99  \index{security}
   1.100  \MTA{}s are critical points for computer security as they are accessible from external networks. They must be secured with high effort. Properties like the need for high privilege level, from outside influenced work load, work on unsafe data, and demand for reliability, increase the need for security. This is best done by modularization, also called \name{compartmentalization}, as described in section~\ref{sec:discussion-mta-arch}.
   1.101  \index{compartmentalization}
   1.102 @@ -255,7 +255,7 @@
   1.103  \index{masqmail!security}
   1.104  
   1.105  
   1.106 -\paragraph{\RG2: Reliability}
   1.107 +\paragraph{\RG\,2: Reliability}
   1.108  \index{reliability}
   1.109  Reliability is the second essential quality property for an \MTA. Mail for which the \MTA\ took responsibility must never get lost while it is within the \MTA's responsibility. The \MTA\ must not be \emph{the cause} of any mail loss, no matter what happens. Unreliable \MTA{}s are of no value. However, as the mail transport infrastructure is a distributed system, one of the communication partners or the transport medium may crash at any time during mail transfer. Thus reliability is needed for mail transfer communication, too.
   1.110  \index{mail loss}
   1.111 @@ -266,27 +266,27 @@
   1.112  \index{duplicates}
   1.113  
   1.114  
   1.115 -\paragraph{\RG3: Robustness}
   1.116 +\paragraph{\RG\,3: Robustness}
   1.117  \index{robustness}
   1.118  Being robust means handling errors properly. Small errors may get corrected, large errors may kill a process. Killed processes should get restarted automatically and lead to a clean state again. Log messages should be written in every case. Robust software does not need a special environment, it creates a friendly environment itself. \person{Raymond}'s \name{Rule of Robustness} and his \name{Rule of Repair} are good descriptions \cite[pages~18--21]{raymond03}.
   1.119  
   1.120  
   1.121 -\paragraph{\RG4: Extendability}
   1.122 +\paragraph{\RG\,4: Extendability}
   1.123  \index{extendability}
   1.124  \masqmail's architecture needs to be extendable to allow new features to be added afterwards. The reasons for this need are the changing requirements. New requirements will appear, like more efficient mail transfer of large messages or a final solution to the spam problem. Extendability is the ability of software to include new function with little work.
   1.125  
   1.126  
   1.127 -\paragraph{\RG5: Maintainability}
   1.128 +\paragraph{\RG\,5: Maintainability}
   1.129  \index{maintainability}
   1.130  Maintaining software takes much time and effort. \person{Spinellis} guesses ``40\,\% to 70\,\% of the effort that goes into a software system is expended after the system is written first time.'' \cite[page~1]{spinellis03}. This work is called \emph{maintaining}. Hence making software good to maintain will ease all further work.
   1.131  
   1.132  
   1.133 -\paragraph{\RG6: Testability}
   1.134 +\paragraph{\RG\,6: Testability}
   1.135  \index{testability}
   1.136  Good testability make maintenance easier too, because functionality is directly verifiable when changes are done, thus removing the uncertainty. Modularized software makes testing easier, because parts can be tested without external influences. \person{Spinellis} sees testability as a sub-quality of maintainability.
   1.137  
   1.138  
   1.139 -\paragraph{\RG7: Performance}
   1.140 +\paragraph{\RG\,7: Performance}
   1.141  \index{performance}
   1.142  Also called ``efficiency''. Efficient software requires few time and few resources. The merge of communication hardware and its move from service providers to homes and to mobile devices demand smaller and more resource-friendly software. The amount of mail will be lower even if much more mail will be sent, thus time performance is less important. \masqmail\ is not a program to be used on large servers, but on small devices. Thus more important for \masqmail\ will be energy and heat saving, maybe also system resources.
   1.143  
   1.144 @@ -294,18 +294,18 @@
   1.145  
   1.146  
   1.147  
   1.148 -\paragraph{\RG8: Availability}
   1.149 +\paragraph{\RG\,8: Availability}
   1.150  \index{availability}
   1.151  Availability is important for server programs. They must stay operational by blocking \name{denial of service} attacks and the like. Automated restarts into a clean state after fatal errors are also required.
   1.152  
   1.153  
   1.154 -\paragraph{\RG9: Portability}
   1.155 +\paragraph{\RG\,9: Portability}
   1.156  \index{portability}
   1.157 -Source code that compiles and runs on various operation systems is called portable. Portability can be achieved by using standard features of the programming language and common libraries. Basic rules to achieve portable code are defined by \person{Kernighan} and \person{Pike} \cite{kernighan99}. Portable code lets software spread faster. Portability among the various flavors of Unix systems is a goal for \masqmail, because these systems are the ones \MTA{}s usually run on. No special care needs to be taken for non-\unix\ platforms.
   1.158 +Source code that compiles and runs on various operation systems is called portable. Portability can be achieved by using standard features of the programming language and common libraries. Basic rules to achieve portable code are defined by \person{Kernighan} and \person{Pike} \cite{kernighan99}. Portable code lets software spread faster. Portability among the various flavors of Unix systems is a goal for \masqmail, because these systems are the ones \MTA{}s usually run on. No special care needs to be taken for non-Unix platforms.
   1.159  
   1.160  
   1.161  
   1.162 -\paragraph{\RG10: Usability}
   1.163 +\paragraph{\RG\,10: Usability}
   1.164  \index{usability}
   1.165  Usability, not mentioned by \person{Hafiz} (he focuses on architecture) but by \person{Spinellis} and \person{Kan}, is a property which is very important from the user's point of view. Software with bad usability is rarely used, no matter how good it is. If substitutes with better usability exist, the user will switch to one of them. Here, usability includes setting up and configuring; the term ``users'' includes administrators. Having \MTA{}s on home servers and workstations requires easy and standardized configuration. The common setups should be configurable with little action by the user. Complex configuration should be possible, but the focus should be on the most common form of configuration: choosing one of several common setups.
   1.166  
   1.167 @@ -380,55 +380,55 @@
   1.168  Here follows a description of how far the requirements are already fulfilled by \masqmail.
   1.169  
   1.170  
   1.171 -\paragraph{\RF1: In/out channels}
   1.172 +\paragraph{\RF\,1: In/out channels}
   1.173  \index{incoming channels}
   1.174  \index{outgoing channels}
   1.175 -The incoming and outgoing channels that \masqmail\ already has (depicted in figure~\ref{fig:masqmail-channels} on page \pageref{fig:masqmail-channels}) are the ones required for an \MTA{}s at the moment. Currently, support for other protocols seems not to be necessary, although new protocols and mailing concepts are likely to appear (see section~\ref{sec:email-trends}). As other protocols are not required today, \masqmail\ is regarded to fulfill \RF1. Without any support in \masqmail\ for adding further protocols, the best strategy is to delaying such work until the functionality is essential, anyway.
   1.176 +The incoming and outgoing channels that \masqmail\ already has (depicted in figure~\ref{fig:masqmail-channels} on page \pageref{fig:masqmail-channels}) are the ones required for an \MTA{}s at the moment. Currently, support for other protocols seems not to be necessary, although new protocols and mailing concepts are likely to appear (see section~\ref{sec:email-trends}). As other protocols are not required today, \masqmail\ is regarded to fulfill \RF\,1. Without any support in \masqmail\ for adding further protocols, the best strategy is to delaying such work until the functionality is essential, anyway.
   1.177  
   1.178  %fixme: << smtp submission >> %fixme
   1.179  
   1.180 -\paragraph{\RF2: Queuing}
   1.181 +\paragraph{\RF\,2: Queuing}
   1.182  \index{mail queue}
   1.183  One single mail queue is used in \masqmail. It satisfies all current requirements.
   1.184  
   1.185 -\paragraph{\RF3: Header sanitizing}
   1.186 +\paragraph{\RF\,3: Header sanitizing}
   1.187  \index{header sanitizing}
   1.188  The envelope and mail headers are generated when the mail is put into the queue. The requirements are fulfilled.
   1.189  
   1.190 -\paragraph{\RF4: Aliasing}
   1.191 +\paragraph{\RF\,4: Aliasing}
   1.192  \index{aliases}
   1.193  Aliasing is done on delivery. All common kinds of aliases in the global aliases file are supported. So called \name{.forward} aliasing is not supported, but this is less common and seldom used.
   1.194  
   1.195 -\paragraph{\RF5: Route management}
   1.196 +\paragraph{\RF\,5: Route management}
   1.197  \index{online routes}
   1.198  Querying the name of the active route is done on delivery. Headers can get rewritten a second time then. This part does provide all the functionality required.
   1.199  
   1.200 -\paragraph{\RF6: Authentication}
   1.201 +\paragraph{\RF\,6: Authentication}
   1.202  \index{auth}
   1.203  Static authentication, based on \NAME{IP} addresses, can be achieved with \person{Venema}'s \NAME{TCP} \name{Wrapper} \cite{venema92}, by editing the \path{hosts.allow} and \path{hosts.deny} files. This is only relevant to authenticate hosts that try to submit mail into the system. Dynamic (secret-based) \SMTP\ authentication is already supported in form of \NAME{SMTP-AUTH} and \SMTP-after-\NAME{POP}, but only for outgoing connections. For incoming connections only address-based authentication is supported.
   1.204  \index{auth!smtp-after-pop}
   1.205  \index{auth!smtp-auth}
   1.206  
   1.207 -\paragraph{\RF7: Encryption}
   1.208 +\paragraph{\RF\,7: Encryption}
   1.209  \index{enc}
   1.210  Similar is the situation for encryption which is also only available for outgoing channels; here a tunnel application, like \name{openssl}, is needed. A secure tunnel can be created to send mail trough. State-of-the-art, however, is using \NAME{STARTTLS}, but this is not supported. For incoming channels, no encryption is available. The only possible setup to provide encryption of incoming channels is using an application like \name{stunnel} to crypt between the secure connection to the remote host and the plain connection to the \MTA. Unfortunately, this suffers from the problem explained on page \pageref{fig:stunnel} in figure~\ref{fig:stunnel}. Anyway, it would still be no \NAME{STARTTLS} support.
   1.211  \index{secure tunnel}
   1.212  
   1.213 -\paragraph{\RF8: Spam handling}
   1.214 +\paragraph{\RF\,8: Spam handling}
   1.215  \index{spam!handling}
   1.216  \masqmail\ does not provide special support for spam filtering. Spam prevention by not accepting spam during the \SMTP\ dialog is not possible at all. Spam filtering is only possible by using two \masqmail\ instances with an external spam filter in between. The mail flow is from the receiving \MTA\ instance, which accepts mail, to the filter application that processes and possible modifies it, to the second \MTA\ which is responsible for further delivery of the mail. This is a concept that works in general, and it is good to separate different work with clear interfaces. But the need of two instances of the same \MTA, with doubled setup, makes it rather a work-around. Better is to have this data flow respected in the \MTA\ design, like it was done in \postfix. Anyway, the more important part of spam handling, for sure, is done during the \SMTP\ dialog by completely refusing unwanted mail.
   1.217  
   1.218 -\paragraph{\RF9: Malware handling}
   1.219 +\paragraph{\RF\,9: Malware handling}
   1.220  \index{malware!handling}
   1.221  For malware handling applies nearly the same as for spam handling, except that all checks are done after mail is accepted. The possible setup is the same with the two \MTA\ instances and the filter in between. \masqmail\ does support such a setup, but not in a nice way.
   1.222  
   1.223 -\paragraph{\RF10: Archiving}
   1.224 +\paragraph{\RF\,10: Archiving}
   1.225  \index{archiving}
   1.226  There is currently no way for archiving every message that does through \masqmail.
   1.227  
   1.228  
   1.229  
   1.230 -\paragraph{\RG1: Security}
   1.231 +\paragraph{\RG\,1: Security}
   1.232  \index{security}
   1.233  \masqmail's current security is bad. However, it seems acceptable for using \masqmail\ on workstations and private networks, if the environment is trustable and \masqmail\ is protected against remote attacks. In environments where untrusted components or persons have access to \masqmail, its security is too low. Its author states that \masqmail\ ``is not designed to'' such usage \citeweb{masqmail:homepage2}. This is a clear indicator for being careful. Issues like high memory consumption, low performance, and denial-of-service attacks---things not regarded by design---may cause serious problems. In any way, a security report that confirms \masqmail's security level is missing.
   1.234  \index{masqmail!security}
   1.235 @@ -436,7 +436,7 @@
   1.236  \masqmail\ uses conditional compilation to exclude unneeded functionality from the executable at compile time. Excluding code means excluding all bugs and weaknesses within this code, too. Excluding unused code is a good concept to improve security.
   1.237  \index{conditional compilation}
   1.238  
   1.239 -\paragraph{\RG2: Reliability}
   1.240 +\paragraph{\RG\,2: Reliability}
   1.241  \index{reliability}
   1.242  Its reliability is also not good enough. Situations where only one part of a sent message was removed from the queue and the other part remained as garbage, showed off \citeweb{debian:bug245882}. Problems with large mail messages in conjunction with small bandwidth were also reported \citeweb{debian:bug216226}. Fortunately, lost email was no big problem yet, but \person{Kurth} warns:
   1.243  \index{masqmail!bugs}
   1.244 @@ -449,22 +449,22 @@
   1.245  In summary: Current reliability needs to be improved.
   1.246  %fixme: state machine
   1.247  
   1.248 -\paragraph{\RG3: Robustness}
   1.249 +\paragraph{\RG\,3: Robustness}
   1.250  \index{robustness}
   1.251  The logging behavior of \masqmail\ is good, although it does not cover the whole code. For example, if the queue directory is world writeable by accident (or as action of an intruder), any user can remove messages from the queue or replace them with own ones. \masqmail\ does not even write a debug message in this case. The origin of this problem, however, is \masqmail's trust in its environment.
   1.252  %fixme: rule of robustness, rule of repair
   1.253  
   1.254 -\paragraph{\RG4: Extendability}
   1.255 +\paragraph{\RG\,4: Extendability}
   1.256  \index{extendability}
   1.257  \masqmail's extendability is very poor. This is a general problem of monolithic software, but can though be provided with high effort. \exim\ is an example for good extendability in a monolithic program.
   1.258  
   1.259 -\paragraph{\RG5: Maintainability}
   1.260 +\paragraph{\RG\,5: Maintainability}
   1.261  \index{maintainability}
   1.262  The maintainability of \masqmail\ is equivalent to other software of similar kind. Missing modularity and therefore more complexity makes the maintainer's work harder. Conditional compilation might be good for security, but \name{ifdef}s scattered throughout the source code is a pain for maintenance. In summary is \masqmail's maintainability bearable, like in average Free Software projects.
   1.263  
   1.264  
   1.265  
   1.266 -\paragraph{\RG6: Testability}
   1.267 +\paragraph{\RG\,6: Testability}
   1.268  \index{testability}
   1.269  The testability suffers from missing modularity, too. Testing program parts is hard to do. Nevertheless, it is done by compiling parts of the source to two special test programs: One tests reading input from a socket, the other tests constructing messages and sending it directly. Neither is designed for automated testing of source parts, they are rather to help the programmer during development.
   1.270  
   1.271 @@ -473,21 +473,21 @@
   1.272  
   1.273  %fixme: think about clean-room testing
   1.274  
   1.275 -\paragraph{\RG7: Performance}
   1.276 +\paragraph{\RG\,7: Performance}
   1.277  \index{performance}
   1.278  The performance---efficiency---of \masqmail\ is good enough for its target field of operation, where this is a minor goal.
   1.279  
   1.280 -\paragraph{\RG8: Availability}
   1.281 +\paragraph{\RG\,8: Availability}
   1.282  \index{availability}
   1.283  This applies equal to availability. Hence no further work needs to be done her.
   1.284  
   1.285 -\paragraph{\RG9: Portability}
   1.286 +\paragraph{\RG\,9: Portability}
   1.287  \index{portability}
   1.288  The code's portability is good with view on Unix-like operation systems. At least \name{Debian}, \name{Red Hat}, \NAME{SUSE}, \name{Slackware}, \name{Free}\NAME{BSD}, \name{Open}\NAME{BSD}, and \name{Net}\NAME{BSD} are reported to be able to compile and run \masqmail\ \citeweb{masqmail:homepage2}. Special requirements for the underlying file system are not known. Thus, the portability is already good.
   1.289  \index{masqmail!supported systems}
   1.290  
   1.291  
   1.292 -\paragraph{\RG10: Usability}
   1.293 +\paragraph{\RG\,10: Usability}
   1.294  \index{usability}
   1.295  The usability is very good, from the administrator's point of view. \masqmail\ was developed to suite a specific, limited job---its configuration does perfect match. The user's view does not reach to the \MTA, as it is hidden behind the \MUA. Configuration could be eased even more by providing configuration generators that enable \masqmail\ to be used right ``out of the box'' after running one of several configuration scripts for common setups. This would improve \masqmail's usability for not technical educated people.
   1.296  \index{out-of-the-box usage}
   1.297 @@ -517,34 +517,34 @@
   1.298  These tasks are presented in more detail in a todo list, now. The list is sorted by focus and then by importance.
   1.299  
   1.300  
   1.301 -\subsubsection*{\TODO1: Encryption (\RF7)}
   1.302 +\subsubsection*{\TODO\,1: Encryption (\RF\,7)}
   1.303  \index{enc}
   1.304  Encryption is chosen for number one as it is essential to provide privacy. Using \NAME{STARTTLS} for encryption is definitely needed and should be added first; encrypted data transfer is hardly possible without support for it.
   1.305  
   1.306  
   1.307 -\subsubsection*{\TODO2: Authentication (\RF6)}
   1.308 +\subsubsection*{\TODO\,2: Authentication (\RF\,6)}
   1.309  \index{auth}
   1.310  Authentication of incoming \SMTP\ connections is also highly needed and should be added second. It is important to restrict access and to prevent relaying. For workstations and local networks, this has only medium importance and address-based authentication is sufficient in most times. But secret-based authentication is mandatory to receive mail from the Internet. Additionally it is a guard against spam.
   1.311  
   1.312  
   1.313 -\subsubsection*{\TODO3: Security (\RG1)}
   1.314 +\subsubsection*{\TODO\,3: Security (\RG\,1)}
   1.315  \index{security}
   1.316  \masqmail's security is bad, thus the program is forced into a limited field of operation. This field of operation even shrinks as security becomes more important and networking and interaction increases. Secure and trusted environment become rare, thus improving security is an important thing to do. The focus should be on adding compartments to split \masqmail\ into separate modules. (See section~\ref{sec:discussion-mta-arch}.) Furthermore, \masqmail's security should be tested throughout to get a definitive view how good it really is and where the weak spots are.
   1.317  \index{modularity}
   1.318  
   1.319  
   1.320 -\subsubsection*{\TODO4: Reliability (\RG2)}
   1.321 +\subsubsection*{\TODO\,4: Reliability (\RG\,2)}
   1.322  \index{reliability}
   1.323  Reliability is also to improve. It is a key quality property for an \MTA, and not good enough in \masqmail. Reliability is strong related to the queue, thus improvements there are favorable. Applying ideas of \name{crash-only software} \cite{candea03} will be a good step. \person{Candea} and \person{Fox} see in killing the process the best way to stop a running program. Doing so inevitably demands for good reliability of the queue, and the start up process inevitably demands for good recovery. Those critical situations for reliability are nothing special anymore, they are common. Hence they are regularly tested and will definitely work.
   1.324  \index{crash-only software}
   1.325  
   1.326  
   1.327 -\subsubsection*{\TODO5: Spam handling (\RF8)}
   1.328 +\subsubsection*{\TODO\,5: Spam handling (\RF\,8)}
   1.329  \index{spam!handling}
   1.330  As authentication can be a guard against spam, filter facilities have lower priority. But basic spam filtering and interfaces for external tools should be implemented in future. Configuration guides for a setup of two \masqmail\ instances with a spam scanner in between should be written. And at least a basic kind of spam prevention during the \SMTP\ dialog should be implemented.
   1.331  
   1.332  
   1.333 -\subsubsection*{\TODO6: Extendability (\RG4)}
   1.334 +\subsubsection*{\TODO\,6: Extendability (\RG\,4)}
   1.335  \index{extendability}
   1.336  \masqmail\ lacks an interface to plug in modules with additional functionality. There exists no add-on or module system. The code is only separated by function into various source files. Some functional parts can be included or excluded by conditional compilation. But the \name{ifdef}s are scattered through all the code. This situation needs to be improved by collecting related function into single places that interact through clear interfaces with other parts. Also should these interfaces allow efficient adding of further functionality.
   1.337  \index{conditional compilation}
   1.338 @@ -578,7 +578,7 @@
   1.339  \index{interposition filter}
   1.340  
   1.341  
   1.342 -The requirements are now regarded, each on its own, and are linked to the development strategy that is preferred to reach each specific requirement. If some requirement is well achievable by using different strategies then it is linked to all of them. Implementing encryption (\TODO1) and authentication (\TODO2), for example, are limited to a narrow region in the code. Such features are addable to the current code base without much problem. In contrast can quality properties like reliability (\TODO4), extendability (\TODO6), and maintainability hardly be added to code afterwards---if at all. Security (\TODO3) is improvable in a new design, of course, but also with wrappers or interposition filters.
   1.343 +The requirements are now regarded, each on its own, and are linked to the development strategy that is preferred to reach each specific requirement. If some requirement is well achievable by using different strategies then it is linked to all of them. Implementing encryption (\TODO\,1) and authentication (\TODO\,2), for example, are limited to a narrow region in the code. Such features are addable to the current code base without much problem. In contrast can quality properties like reliability (\TODO\,4), extendability (\TODO\,6), and maintainability hardly be added to code afterwards---if at all. Security (\TODO\,3) is improvable in a new design, of course, but also with wrappers or interposition filters.
   1.344  
   1.345  This linking of requirements to the strategies is shown in table~\ref{tab:strategies}. The requirements are ordered by their focus.
   1.346  
   1.347 @@ -720,7 +720,7 @@
   1.348  Redesigning a software as requirements change helps keeping it alive. % fixme: add quote: ``one thing surely remains: change'' (something like that)
   1.349  \index{redesign}
   1.350  
   1.351 -Another danger is the dead end of complexity which is likely to appear by constant work on the same code base. It is even more likely if the code base has a monolithic architecture. A good example for simplicity is \qmail\ which consists of small independent modules, each with only about one thousand lines of code. Such simple code makes it obvious to understand what it does. The \name{suckless} project \citeweb{suckless.org} for example advertises such a philosophy of small and simple software by following the thoughts of the \unix\ inventors \cite{kernighan84} \cite{kernighan99}. Simple, small, and clear code avoids complexity and is thus also a strong prerequisite for security.
   1.352 +Another danger is the dead end of complexity which is likely to appear by constant work on the same code base. It is even more likely if the code base has a monolithic architecture. A good example for simplicity is \qmail\ which consists of small independent modules, each with only about one thousand lines of code. Such simple code makes it obvious to understand what it does. The \name{suckless} project \citeweb{suckless.org} for example advertises such a philosophy of small and simple software by following the thoughts of the Unix inventors \cite{kernighan84} \cite{kernighan99}. Simple, small, and clear code avoids complexity and is thus also a strong prerequisite for security.
   1.353  \index{suckless}
   1.354  
   1.355