network protocols handbook.pdf

252Protocols GuideISO Protocols - Application LayerProtocol NameX.400: Message Handling ServiceProtocolProtocol DescriptionX.400 is the Message Handling Service protocol for e-mailtransmission specified by the ITU-T and ISO. X.400 is commonin Europe and Canada and is an alternative to the more populare-mail protocol, Simple Mail Transfer Protocol (SMTP), whichis defined by IETF. X.400 uses a binary format so it is easy toinclude binary contents without encoding it for transfer. Also itis harder for people to fake email addresses and contents, thanwith STMP where text messages are used.X.400 and STMP have similar features but also unique featuresin themselves. Generally speaking, X.400 is a more complexprotocol with the following features that are not in the SMTP:• Delivery notifications - Delivery notifications are usedboth about delivery notifications (yes, the message gothere) and about non-delivery notifications (no, the messagedid not get there).• Receipt notifications - A receipt notification is passedback to the originating user indicating what happenedto the message after it was delivered (for instance that itwas read by the recipient). In X.400, receipt and non-receiptnotifications may include notifications of somethingbeing automatically forwarded, messages deleted, etc.• Security functions – X.400 defines a framework for mailtransmission securities. It defines the concept of a “securitylabel” and allows using an OID for identifying yoursecurity labeling scheme, but no labeling scheme is actuallyspecified in the protocol.• Priority markers (3 levels) – This feature is used for orderingthe queue of mails to send, so that “important”mails get sent before “less important” mails.• Deferred delivery – Schedule delivery time for messages.This feature has not been widely deployed.• Conversion in the network –such as converting Teletexto plain text, or fax images to text saying “there was apicture here, but you are not allowed to see it”. Conversionnever improves a message, and it is impossible tosupport security functions like signatures or encryptionwhile doing conversion in the network.• Reliable Transfer Service – This X.400 feature givesthe ability to continue transferring a document after thetransfer is interrupted.SMTP has some functions that X.400 does not, including thefollowing:• Standard functionality to check each recipient for validitybefore transferring the message; X.400 requires transferringthe complete message before checking recipients.• Optional functionality for checking whether a messageis too large to transfer before sending it.• Ability to insert any data into the header of a messagewith a fair probability of it being presented to the user• Ability (MIME Multipart/Alternative) to send several representationsof the same content in the same message,guaranteeing both interoperability with the lowest commondenominator and no loss of information betweencompatible UAs.In addition, an X.400 address is different from that of an STMP.X.400 consists of a set of bindings for country (c), administrativedomain (a), primary management domain (p), surname (s),and given name (g). An SMTP e-mail address that looks like thishypothetical address:Jeff.warson@javvin.subdomain.uslooks like this in an X.400 e-mail message:G=Jeff; S=warson; O=subdomain; OU=javvin; PRMD=attmail;ADMD=attmail; C=USProtocol StructureX.400 was designed with attributed addresses. The completeset of attributes is rather large:Attribute Type Abbreviation LabelGiven Name Given Name GInitial Initials ISurname Surname SGeneration Qualifier Generation QCommon Name Common Name CNOrganization Organization OOrganizational Unit 1 Org.Unit.1 OU1Organizational Unit 2 Org.Unit.2 OU2Organizational Unit 3 Org.Unit.3 OU3Organizational Unit 4 Org.Unit.4 OU4Private Management DomainNamePRMDPAdministration ManagementDomain NameADMDACountry Country CPhysical Delivery PersonalNamePD-personPD-PNExtension of Postal O/R AddressComponentsPD-ext. address PD-EAExtension of Physical DeliveryAddress ComponentsPD-ext. delivery PD-EDPhysical Delivery Office Number PD-office number PD-OFN