1.1 --- a/thesis/tex/4-MasqmailsFuture.tex	Sat Jan 31 20:07:58 2009 +0100
     1.2 +++ b/thesis/tex/4-MasqmailsFuture.tex	Sat Jan 31 21:39:53 2009 +0100
     1.3 @@ -40,7 +40,7 @@
     1.4  
     1.5  \paragraph{\RF1: Incoming and outgoing channels}
     1.6  \label{rf1}
     1.7 -\sendmail-compatible \mta{}s must support at least two incoming channels: mail submitted using the \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}.
     1.8 +\sendmail-compatible \MTA{}s must support at least two incoming channels: mail submitted using the \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}.
     1.9  
    1.10  \SMTP\ is the primary mail transport protocol today, but with the increasing need for new protocols (see section \ref{sec:what-will-be-important}) in mind, support for more than just \SMTP\ is good to have. New protocols will show up, maybe multiple protocols need to be supported then. This leads to multiple remote channels, one for each supported protocol as it was done in other \MTA{}s. Best would be interfaces to add further protocols as modules.
    1.11  
    1.12 @@ -69,7 +69,7 @@
    1.13  
    1.14  \paragraph{\RF2: Mail queuing}
    1.15  \label{rf2}
    1.16 -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 used 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.
    1.17 +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 used 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.
    1.18  
    1.19  The mail queue and the module(s) to manage it are the central part of the whole system. This demands especially for robustness and reliability, as a failure here can lead to loosing mail. An \MTA\ takes over responsibility for mail in accepting it, hence loosing mail messages is absolutely to avoid. This covers any kind of crash situation too. The worst thing acceptable to happen is an already sent mail to be sent again.
    1.20  
    1.21 @@ -151,7 +151,7 @@
    1.22  \label{rf8}
    1.23  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}).
    1.24  
    1.25 -As spam is, by increasing the amount of mail messages, not just a nuisance for end users, but also for the infrastructure---the \mta{}s---they need to protect themselves.
    1.26 +As spam is, by increasing the amount of mail messages, not just a nuisance for end users, but also for the infrastructure---the \MTA{}s---they need to protect themselves.
    1.27  
    1.28  Filtering spam can be done by either refusing spam during the \SMTP\ dialog or by checking for spam after the mail was accepted and queued. Both ways have advantages and disadvantages, so modern \MTA{}s use them in combination.
    1.29  
    1.30 @@ -169,7 +169,7 @@
    1.31  \label{rf9}
    1.32  Related to spam is malicious content (short: \name{malware}) like viruses, worms, trojan horses. They, in contrast to spam, do not affect the \MTA\ itself, as they are in the mail's body. \MTA{}s searching for malware is equal to real world's post offices opening 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, so it is often done there.
    1.33  
    1.34 -In any way should malware checking be performed by external programs that may be invoked by the \mta. But \NAME{MDA}s are better points to invoke content scanners.
    1.35 +In any way should malware checking be performed by external programs that may be invoked by the \MTA. But \NAME{MDA}s are better points to invoke content scanners.
    1.36  
    1.37  A popular email filter framework is \name{amavis} which integrates various spam and malware scanners. The common setup includes a receiving \MTA\ which sends it to \name{amavis} using \SMTP, \name{amavis} processes the mail and sends it then to a second \MTA\ that does the outgoing transfer. Having interfaces to such scanners is nice to have, though. (This setup with two \MTA\ instances is discussed in more detail in section \ref{sec:current-code-security}).
    1.38  
    1.39 @@ -201,7 +201,7 @@
    1.40  
    1.41  
    1.42  \paragraph{\RG2: Reliability}
    1.43 -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 are distributed systems, 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.
    1.44 +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 are distributed systems, 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.
    1.45  
    1.46  The goal is to transfer exactly one copy of the message. \person{Tanenbaum} evaluates the situation and comes to the conclusion that ``in general, there is no way to arrange this.'' \cite[pages~377--379]{tanenbaum02}. Only strategies where now mail gets lost are acceptable; he identifies three of them, but one generates more duplicates than the others, so two strategies remain. (1) The client always reissues the transfer; the server first sends an acknowledgment, then handles the transfer. (2) The client reissues the transfer only if no acknowledgment was received; the server first handles the transfer and sends the acknowledgment afterwards. The first strategy does not need acknowledgments at all, however, it will lose mail if the second transfer fails too.
    1.47  
    1.48 @@ -241,7 +241,7 @@
    1.49  
    1.50  
    1.51  \paragraph{\RG10: Usability}
    1.52 -Usability, not mentioned by \person{Hafiz} (he focuses on architecture) but by \person{Spinellis} and \person{Kan}, is a property 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; and 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 focused must be the most common form of configuration: choosing one of several common setups.
    1.53 +Usability, not mentioned by \person{Hafiz} (he focuses on architecture) but by \person{Spinellis} and \person{Kan}, is a property 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; and 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 focused must be the most common form of configuration: choosing one of several common setups.
    1.54  
    1.55  %fixme: << masqmail as portable app? >>
    1.56  
    1.57 @@ -269,7 +269,7 @@
    1.58  	\label{fig:masqmail-arch}
    1.59  \end{figure}
    1.60  
    1.61 -\sendmail\ improved its old architecture by adding the milter interface, to include further functionality by invoking external programs. \exim\ was designed, and is carefully maintained, with a modular-like code structure in mind. \qmail\ started from scratch with a ``security-first'' approach, \postfix\ improved on it, and \name{sendmail X}/\name{MeTA1} tries to adopt the best of \qmail\ and \postfix\ to completely replace the old \sendmail\ architecture. \person{Hafiz} describes this evolution of \mta\ architecture very well \cite{hafiz05}.
    1.62 +\sendmail\ improved its old architecture by adding the milter interface, to include further functionality by invoking external programs. \exim\ was designed, and is carefully maintained, with a modular-like code structure in mind. \qmail\ started from scratch with a ``security-first'' approach, \postfix\ improved on it, and \name{sendmail X}/\name{MeTA1} tries to adopt the best of \qmail\ and \postfix\ to completely replace the old \sendmail\ architecture. \person{Hafiz} describes this evolution of \MTA\ architecture very well \cite{hafiz05}.
    1.63  
    1.64  Every one of these programs is more modular, or became more modular over time, than \masqmail\ is. Modern requirements like spam protection and future requirements like---probably---the use of new mail transport protocols demand for modular designs in order to keep the software simple. Simplicity is a key property for security. ``the essence of security engineering is to build systems that are as simple as possible.'' \cite[page 45]{graff03}.
    1.65  
    1.66 @@ -389,7 +389,7 @@
    1.67  
    1.68  \section{Work to do}
    1.69  
    1.70 -After the requirements for modern \mta{}s were identified in section \ref{sec:mta-requirements} and \masqmail's features were set against them in section \ref{sec:fulfilled-requirements}, here the work that is left to do is identified. Table \ref{tab:requirements} lists all requirements with importance and the work needed to achieve them. The column ``Focus'' shows the attention a work task should get. The focus depends on the task's importance and the amount of work it includes.
    1.71 +After the requirements for modern \MTA{}s were identified in section \ref{sec:mta-requirements} and \masqmail's features were set against them in section \ref{sec:fulfilled-requirements}, here the work that is left to do is identified. Table \ref{tab:requirements} lists all requirements with importance and the work needed to achieve them. The column ``Focus'' shows the attention a work task should get. The focus depends on the task's importance and the amount of work it includes.
    1.72  
    1.73  \begin{table}
    1.74  	\begin{center}