domain name and its actual representative mail server. Under normal circumstances the algorithm for mail message routing is the following: ⢠Mail message ...
Mail Message Routing in Case of Internet Link Failures Oleg Gusak, Tugrul Dayar Department of Computer Engineering and Information Science Bilkent University, 06533, Ankara, Turkey {gusak, tugrul}@cs.bilkent.edu.tr
Abstract. Today, the Internet mailing system plays an important role as a means of communication. The stability and reliability of this system depends on the reliability of the basic communication network hardware. In case of an organization Internet link failure, it becomes impossible to send e-mail messages outside the organization. In this paper, we propose redirecting (that is, manually routing) mail messages through a secondary regional link. Keywords. Mail message routing, Internet, link failure, network reliability.
1 Introduction. Today, Internet plays an important role as a communication medium in everyday life of people. One of the assets of this medium is its e-mail system. It has entered our life and has become ubiquitous among other telecommunication services and has even replaced some of them. It is obvious that the reliability of this system is very important. The quality of service of such a system depends on the communication network hardware and its topology. Let us consider the typical structure of the interconnections of organizations as in Figure 1.
Internet
Link A Regional domain Organization 1
Link B
Organization 2
Figure 1. Domain interconnections.
The main link, link A, connects organization 1’s local area network to the Internet backbone. All messages addressed to external recipients that reside on the Global Area Network are routed through link A. Another channel, link B, maintains connection inside regional domain (for example, country). The purpose of having this channel is to reduce the main link workload and to provide fast connection to regional sites. On the other hand, this secondary channel does not require the essential expenses involved in having a direct link to the Internet since it connects geographically close organizations (in the same country). According to the discussed interconnection structure, each regional organization maintains at least two outgoing channels: one to the Global Area Network (i.e., Internet), another to the regional domain. Such a communication scheme requires certain rules and agreements for the interconnection and interoperation of these channels. The routing tables of corporate networks are set in such a way that: (i) mail messages addressed to external recipients residing on the Global Area Network are sent through channels of type A under all conditions, (ii) mail messages addressed to regional domain recipients are sent through channels of type B. In section 2, we discuss the details of mail message routing and in section 3, we introduce a solution to the failure of a type A link.
2 Mail message routing. Before going into the details of mail message routing let us briefly describe conventions about Internet e-mail addresses and principles of mail exchange. It is well known that an e-mail address consists of two parts separated by the symbol “@” (at). The first part is the name of the user and the second part is the name of the host or the domain to which the user belongs. In the beginning of the Internet, it was assumed that the second part of an e-mail address is always the name of the host. Under this condition, mail exchange is a simple procedure: the mail transfer agent at the sending host opens a direct connection to a host with the name defined in the second part of the e-mail address. This type of communication required each host that can be a potential mail receiver to be in “stand-by” mode. It is obvious that this scheme is not applicable to personal computers and workstations, which are not stable and work for a limited time on the Internet. For this purpose, special type of servers, mail servers were introduced. A mail server is responsible for receiving and delivering mail for a given group of hosts. Now, the second part of an e-mail address can be an arbitrary name that describes this group which is usually called domain. In this case, the mail message delivery procedure required an additional step to find out the correspondence between the domain name and its actual representative mail server. Under normal circumstances the algorithm for mail message routing is the following: • Mail message, after being prepared by a mail user agent (Pine, Netscape, etc.), is sent to a predefined Simple Mail Transfer Protocol (SMTP) server, the server responsible for exchanging mail for a given domain. Usually, this is the name of the nearest local SMTP server. • After this step, the SMTP server has to decide where to send the designated message using the following procedure (RFC 974): ◊ First, the SMTP server sends a request to the default Domain Server (DS) to resolve the address of the next hop on a delivering path for a given message. The Domain Server stores information as a series of resource records, each of which contains a particular piece of
information about a given domain name as described in Figure 2. There are two types of records that are points of interest for mail delivery. The first one is the Mail eXchange (MX) record that contains a domain name in the first field, and the name of a domain representative or a domain in the second field. When a domain name appears in the second field, the item designated in the first field is a subdomain, which is an alias for the domain specified in the second field. The third field indicates the type of the record. The fouth field of the MX record contains the priority of the given MX record. It is used in case there is more than one MX record for a given domain name. The smallest value corresponds to the highest priority. domain.org server.domain.org
Server.domain.org 0.0.0.1
MX WKS
10 TCP
SMTP
Figure 2. Domain server record structure. Another type of record, named Well Known Service (WKS), sets up the correspondence between a particular host and its Internet Protocol (IP) address. It also describes type of service (for our example it is the SMTP server) and type of transport protocol used for communication with a given host. The entry point for the Domain Server Database is a domain of interest and the result on a given query is the representative host names of a given domain. Possible answers on a request to Domain Server are the following: (1) Empty string indicates that addressed domain has representative with the same name, i.e., mailer should attempt to deliver mail directly to designated name in a mail message. (2) String that contains addresses of possibly more than one representative of a given domain. ◊ Having chosen the proper domain representative name, in the last step, the SMTP server issues a WKS query to the Domain Server to obtain the actual IP address of the given domain representative. • When the local SMTP server has resolved the IP address of the remote host, routing involves standard procedure for mail delivery (i.e., mail transfer protocol). Let us consider the procedure for mail message forwarding on a given network structure as in Figure 3 for two cases: sending mail to remote host in Global Area Network and sending mail to remote host in regional domain. The following conventions are adopted for the given example: (i) the local domain has name Reg and its subdomains of organizations 1 and 2 are named Org1 and Org2, respectively, (ii) the first column of the Domain Server Database contains domain names and the second column describes corresponding domain representatives, (iii) for simplicity, in the Domain Server Database only records of MX type are listed, and consequently, the router tables of routers contain conventional host names instead of actual IP numbers, (iv) the first column of a routing table contains the name of the host of interest and the second column contains the name of the next hop on the route to a given host. Let the prepared message be destined from local host LHost.Org1.Reg to remote host RHost.Domain.Inet. The mail message is first sent to the local SMTP server SMTP.Org1.Reg. This host issues a request to default Domain Server DS.Org1.Reg, and as a
result gets the address of a representative of the addressed domain (i.e., SMTP.Domain.Inet). Finally, the given message is delivered to host SMTP.Doman.Inet through link A. It should be emphasized that the message can be delivered to the given remote destination only through link A. According to the Routing Information Protocol (RIP) /RFC 1058/ that handles IP packet routing in the Internet, each router advertises, that is, broadcasts a list of networks and hosts which are reachable from a given router. All other routers upon receiving this list update their routing tables accordingly. In our case, router Router.Org2.Reg of the regional subdomain Org2 does not advertise a route to router Router.Org1.Reg through itself to the Internet. That is, Router.Org1.Reg knows nothing about link A’ of domain Org2.Reg (see Figure 3, routing table of router Router.Org1.Reg) to the Internet. In the second case, when a local user sends a message to the regional recipient Remote.Org2.Reg from Org1, as a response on a query to local Domain Server DS.Org1.Reg, the SMTP server SMTP.Org1.Reg gets SMTP.Org2.Reg as the address of the remote SMTP server, and establishes connection with that server through link B.
RHost.Domain.Inet
DS.Domain.Inet 1. Org1.Reg 2. Org2.Reg …
SMTP.Org1.Reg SMTP.Org2.Reg
SMTP.Domain.Inet
Router.Domain.Inet 1. SMTP.Org1.Reg 2. SMTP.Org2.Reg …
Router.Org1.Reg Router.Org2.Reg
Internet
Link A
Link A’
Router.Org1.Reg 1. SMTP.Domain.Inet 2. SMTP.Org2.Reg …
Router.Domain.Inet Router.Org2.Reg
DS.Org1.Reg 1.Domain.Inet 2. Org2.Reg
Router.Org2.Reg
Link B
SMTP.Org1.Reg
SMTP.Domain.Inet SMTP.Org2.Reg
…
1. SMTP.Domain.Inet 2. SMTP.Org1.Reg …
SMTP.Org2.Reg
DS.Org2.Reg 1.Domain.Inet 2. Org1.Reg
LHost.Org1.Reg
Router.Domain.Inet Router.Org1.Reg
SMTP.Domain.Inet SMTP.Org1.Reg
…
Organization 1
LHost.Org2.Reg
Organization 2
Figure 3. Mail Message Routing.
3 Internet link failure. Let us consider the failure of Link A. This condition does not affect mail message routing that have been addressed to regional domain Reg. Such regional messages still can reach recipients through internal link B. However, what happens if host Lhost.Org1.Reg sends a message to an address on the Global Area Network? After resolving the destination address to be SMTP.Domain.Inet, the SMTP server SMTP.Org1.Reg tries to establish connection with the remote host. All its attempts will be unsuccessful because router Router.Org1.Reg has no alternative route to the Global Area Network. It knows only about link A, which is out of order. The obvious solution for this problem is to use the capabilities of Link B and regional subdomain Org2. That is, we have to transfer our message to a mailer system in subdomain Org2 which has an alternative route to the Global Area Network through link A’. As we have mentioned before, a mail user agent program by default uses the nearest local SMTP server as the first hop on a mail message route. Now we have to redefine the name of this server and point our mail program to server SMTP.Org2.Reg. For instance, in PINE this option is located at SETUP->Config->SMTP-server, in Netscape Communicator it is located at Edit->Preference->Mail Server->Outgoing mail server. After this is done, the mail user agent, using the mail transfer protocol on top of Transmission Control Protocol (TCP) will connect to the mail server SMTP.Org2.Reg through link B (using information from the routing table of Router.Org1.Reg), which in turn will send it to the designated destination. The same procedure under link A failure can be applied to messages sent from the Global Area Network to subdomain Org1. If host RHost.Domain.Inet tries to reach domain Org1 by the default route (i.e., through mail server SMTP.Domain.Inet), it will be unsuccessful because of the failure of link A. To overcome this situation, it has to establish connection and send the mail message directly to server SMTP.Org2.Reg, which has a route to domain Org1.
References RFC 974, Mail Routing and the Domain System RFC 1058, Routing Information Protocol
