WHAT YOU NEED TO KNOW ABOUT PATHALIAS
	   AND WHAT PATHALIAS NEEDS TO KNOW ABOUT YOU
			       or
		   HOW PATHALIAS MAKES DOMAINS

		        Christopher Seiwald

This note describes the host connectivity data and domain data
needed to effect UUCP domain-style address routing.  This
describes mostly the domain data, but also discusses how to
distribute connectivity data.  Look elsewhere for a discussion of
domains.

Briefly, the connectivity data (what's in mod.map) connects
all hosts in the UUCP network into one big directed graph, and
the domain data superimposes a domain tree onto that graph.
Pathalias converts these two sets of data into a routing database
which smail/rmail, a UUCP mail routing program, uses.

1.  Domains and Gateways for UUCP

For domains in the UUCP zone, the top of a subdomain is all
gateway hosts for that domain; the top of the UUCP zone will
probably be nearly a hundred hosts.  As a transition aid, we
also consider an individual host at the bottom of the domain tree a
subdomain "host.UUCP", with one gateway and no further subdomains.  
(We expect to phase this out eventually.)

A gateway host for a domain must do four things:
	I)	Pass mail bound for that domain to the
		appropriate host.
	II)	Pass mail bound for outside that domain to a
		gateway in the parent domain.
	III)	Pass mail bound for a subdomain to a gateway of
		that subdomain.
	IV)	Recognise the domain!user address syntax.

smail/rmail already provides (IV).  With the data described here,
pushed through pathalias, smail/rmail can then provide (I)-(III).

2.  The Zone Registry

For any sizeable zone, one gateway host supports the zone registry.
For other zones, such as BITNET, CSNET, DDN, etc., registries are
supported, using conventions appropriate to those zones.  Contact
electronic mail addresses are supported for queries, and updates to
configuration information may also be handled via mail.  In the UUCP
zone, the id's "uucpmap@cbosgd.ATT.COM" and "domains@cbosgd.ATT.COM"
serve to collect the connectivity and domain data, respectively, for
that zone.

The registry for a zone speaks for that zone, communicating
chiefly with its peers, the registry of the parent domain, and
the registries of the subdomains.

3.  Functions of Domain Data

Each gateway for a domain must map the domain-style names into
the UUCP host names for all hosts of the domain.  This host name
mapping provides (I) above.

Each gateway for a domain knows a) at least one gateway for each
immediate subdomain, and b) at least one gateway host of the
parent domain.  This provides (II) and (III) above.

For consistency across the gateways of a domain, each gateway for
the domain should know a) ALL gateways for each immediate
subdomain; and b) ALL gateways for the parent domain.  Pathalias
will pick the closest. In this way, one single database can
describe the domain structure for all gateways on a domain,
without variations for each gateway. 

In order to aid routing and avoid overloading the parent gateway,
gateways should also know most gateways for peer level domains.
This information is also provided by the map and used by pathalias.
When a new peer domain is created, traffic can be routed through the
parent (which must be updated immediately) until information about
the peer can be propagated.

Additionally, a gateway could know about domains more than one
level above or below it so that mail doesn't stop for address
resolution at every gateway along its path.

4.  Format of Domain Data

4.1  Host Name List

The host name list aliases the domain style address of a host to
the UUCP host name.  The pathalias input format is:

		uucp-name .domain-name[, ...]

The .UUCP suffix is implicit in the uucp-name (smail/rmail does
this), and is not needed.
Upper/lower case doesn't matter in a dotted domain name.
Examples:
	
		ihnp4 = .ATT.COM
		ucbvax = .Berkeley.EDU
		cbosgd = .osgd.cb.att.com, .cbosgd.att.com

Might produce from pathalias:

		ihnp4			mtxinu!ihnp4!%s
		.ihnp4.ATT.COM		mtxinu!ihnp4!%s
		ucbvax			ucbvax!%s
		.Berkeley.EDU		ucbvax!%s
		cbosgd			cbosgd!%s
		.osgd.cb.att.com	cbosgd!%s
		.cbosgd.att.com		cbosgd!%s

A single host may have more than one domain style address; in
fact, a host may be in several domains at once.  However, each
host must have a single primary location in the domain tree,
and other addresses should be viewed as transition aids.  For
example, cbosgd might be known as cbosgd, cbosgd.UUCP,
cbosgd.ATT.UUCP, cbosgd.btl.csnet, and cbosgd.ATT.COM, but
the primary name is the one in the organizational domain (COM)
which applies to all networks, and the others are temporary
names for upward compatibility only.

4.2  Domain Gateway List

The domain gateway list aliases the domain style address of a
domain to the UUCP host name of the closest gateway of that
domain.  This involves a trick in pathalias, and employs a
extra network name domain-gw.  The pathalias input format is:

		domain-gw .domain-name

Again, the .UUCP suffix is implicit in the uucp-name, and is
not needed.  
Examples:

		decwrl .DEC.COM
		decuac .DEC.COM

		cbosgd .ATT.COM
		clyde .ATT.COM

Might generate from pathalias:

		.DEC.COM	seismo!decuac!%s
		.ATT.COM	cbosgd!%s

Note that pathalias chooses the closest host from inside the {}'s.
The (DEAD)'s prevent pathalias from following along the mock network
called "domain-gw".

5.  Distribution of Domain Data

A zone registry maintains a Host Name List (in the format of 4.1
above) of all hosts within its domain and a Domain Gateway List
(in the format of 4.2 above) of all gateways of the domain.

Up: A registry collects the Domain Gateway List from the registry
of each of its subdomains, and transmits to the registry of its
parent domain its own Domain Gateway List and, if it chooses, the
Domain Gateway Lists of some or all of its subdomains.  Whether
it includes lists from its subdomains depends on how important it
considers them to the parent domain.

Down: Similarly, a registry collects the Domain Gateway List from
the registry of its parent domain, and transmits to the registry
of each of its subdomains its Domain Gateway List and the Domain
Gateway List of its parent domain.  Note that the Domain Gateway
List of the parent domain may include lists from the parent's
other subdomains.

A registry may decide not to use the parent domain's Domain
Gateway List or not to transmit it to its subdomains' registries.
(This should be done only with the consent of the subdomains.) In
this case, the registry must introduce a domain gateway alias for
all top level domains, to ensure that all the mail gets delivered.

Across: a registry transmits to each of the gateways of its
domain its Host Name List, its Domain Gateway List, and collected
Domain Gateway Lists.  The registry also transmits to each normal
host (one gateway, no subdomains) of its domain its Domain
Gateway List.

Together, "up," "down," and "across" insure that each gateway has
the Host Name List for its domain, and the Domain Gateway List of
its own domain and at least its parent domain and subdomains.
"Up" and "across" will probably take place on demand by mail.
"Down" will probably be broadcast via netnews on a regular
schedule.  In particular, the second level information for the UUCP
zone (one entry per organization) and the complete top level domain
information make up the postings to mod.map.

6.  Distribution of Connectivity Data

The distribution of connectivity data will probably follow the
path of domain data: registries passing connectivity data up,
down, and across the domain tree, with the exception that the
connectivity within a third (or lower) level domain will be
discouraged from leaving the domain, so the data the UUCP zone
registry distributes will include only the first and second
level gateways.  Local information about internal subdomains and
machines of organizations should not be included in globally
published information, but rather distributed locally as needed.

7.  Various Notes

The following are examples of data that should be joined together
as input to pathalias.

	Parent Domain Gateway List
	Parent Connectivity Data
	This Level Domain Gateway List
	This Level Host Name List
	This Level Connectivity Data
	Collected Subdomains' Domain Gateway Lists
	Collected Subdomains' Connectivity Data
	Private Additions
	Alias for "this host"

This note does not describe the inclusion of private additions to
the domain or connectivity data.

Because domain names intermix with host names (and the .UUCP
suffix is implicit), you can address hosts known at your gateway
as "uucp-host.UUCP".  We discourage this, because the address is 
then particular to the sender's location.


		/+\
5/1/85		+\	chris@cbosgd.att.com
		\+/

[Updated 5/9/86 by Mark Horton.]
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#@(#)Domains	2.5 (smail) 9/15/87
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