RFC: 814
NAME, ADDRESSES, PORTS, AND ROUTES
David D. Clark
MIT Laboratory for Computer Science
Computer Systems and Communications Group
July, 1982
1. IntrodUCtion
It has been said that the principal function of an operating system
is to define a number of different names for the same object, so that it
can busy itself keeping track of the relationship between all of the
different names. Network protocols seem to have somewhat the same
characteristic. In TCP/IP, there are several ways of referring to
things. At the human visible interface, there are character string
"names" to identify networks, hosts, and services. Host names are
translated into network "addresses", 32-bit values that identify the
network to which a host is attached, and the location of the host on
that net. Service names are translated into a "port identifier", which
in TCP is a 16-bit value. Finally, addresses are translated into
"routes", which are the sequence of steps a packet must take to reach
the specified addresses. Routes show up eXPlicitly in the form of the
internet routing options, and also implicitly in the address to route
translation tables which all hosts and gateways maintain.
This RFCgives suggestions and guidance for the design of the
tables and algorithms necessary to keep track of these various sorts of
identifiers inside a host implementation of TCP/IP.
2
2. The Scope of the Problem
One of the first questions one can ask about a naming mechanism is
how many names one can expect to encounter. In order to answer this, it
is necessary to know something about the expected maximum size of the
internet. Currently, the internet is fairly small. It contains no more
than 25 active networks, and no more than a few hundred hosts. This
makes it possible to install tables which exhaustively list all of these
elements. However, any implementation undertaken now should be based on
an assumption of a much larger internet. The guidelines currently
recommended are an upper limit of about 1,000 networks. If we imagine
an average number of 25 hosts per net, this would suggest a maximum
number of 25,000 hosts. It is quite unclear whether this host estimate
is high or low, but even if it is off by several factors of two, the
resulting number is still large enough to suggest that current table
management strategies are unacceptable. Some fresh techniques will be
required to deal with the internet of the future.
3. Names
As the previous section suggests, the internet will eventually have
a sufficient number of names that a host cannot have a static table
which provides a translation from every name to its associated address.
There are several reasons other than sheer size why a host would not
wish to have such a table. First, with that many names, we can expect
names to be added and deleted at such a rate that an installer might
spend all his time just revising the table. Second, most of the names
will refer to addresses of machines with which nothing will ever be
3
exchanged. In fact, there may be whole networks with which a particular
host will never have any traffic.
To cope with this large and somewhat dynamic environment, the
internet is moving from its current position in which a single name
table is maintained by the NIC and distributed to all hosts, to a
distributed approach in which each network (or group of networks) is
responsible for maintaining its own names and providing a "name server"
to translate between the names and the addresses in that network. Each
host is assumed to store not a complete set of name-address
translations, but only a cache of recently used names. When a name is
provided by a user for translation to an address, the host will first
examine its local cache, and if the name is not found there, will
communicate with an appropriate name server to oBTain the information,
which it may then insert into its cache for future reference.
Unfortunately, the name server mechanism is not totally in place in
the internet yet, so for the moment, it is necessary to continue to use
the old strategy of maintaining a complete table of all names in every
host. Implementors, however, should structure this table in such a way
that it is easy to convert later to a name server approach. In
particular, a reasonable programming strategy would be to make the name
table Accessible only through a subroutine interface, rather than by
scattering direct references to the table all through the code. In this
way, it will be possible, at a later date, to replace the subroutine
with one capable of making calls on remote name servers.
A problem which occasionally arises in the ARPANET today is that
4
the information in a local host table is out of date, because a host has
moved, and a revision of the host table has not yet been installed from
the NIC. In this case, one attempts to connect to a particular host and
discovers an unexpected machine at the address obtained from the local
table. If a human is directly observing the connection attempt, the
error is usually detected immediately. However, for unattended
operations such as the sending of queued mail, this sort of problem can
lead to a great deal of confusion.
The nameserver scheme will only make this problem worse, if hosts
cache locally the address associated with names that have been looked
up, because the host has no way of knowing when the address has changed
and the cache entry should be removed. To solve this problem, plans are
currently under way to define a simple facility by which a host can
query a foreign address to determine what name is actually associated
with it. SMTP already defines a verification technique based on this
approach.
4. Addresses
The IP layer must know something about addresses. In particular,
when a datagram is being sent out from a host, the IP layer must decide
where to send it on the immediately connected network, based on the
internet address. Mechanically, the IP first tests the internet address
to see whether the network number of the recipient is the same as the
network number of the sender. If so, the packet can be sent directly to
the final recipient. If not, the datagram must be sent to a gateway for
further forwarding. In this latter case, a second decision must be
5
made, as there may be more than one gateway available on the immediately
attached network.
When the internet address format was first specified, 8 bits were
reserved to identify the network. Early implementations thus
implemented the above algorithm by means of a table with 256 entries,
one for each possible net, that specified the gateway of choice for that
net, with a special case entry for those nets to which the host was
immediately connected. Such tables were sometimes statically filled in,
which caused confusion and malfunctions when gateways and networks moved
(or crashed).
The current definition of the internet address provides three
different options for network numbering, with the goal of allowing a
very large number of networks to be part of the internet. Thus, it is
no longer possible to imagine having an exhaustive table to select a
gateway for any foreign net. Again, current implementations must use a
strategy based on a local cache of routing information for addresses
currently being used.
The recommended strategy for address to route translation is as
follows. When the IP layer receives an outbound datagram for
transmission, it extracts the network number from the destination
address, and queries its local table to determine whether it knows a
suitable gateway to which to send the datagram. If it does, the job is
done. (But see RFC816 on Fault Isolation and Recovery, for
recommendations on how to deal with the possible failure of the
gateway.) If there is no such entry in the local table, then select any
6
accessible gateway at random, insert that as an entry in the table, and
use it to send the packet. Either the guess will be right or wrong. If
it is wrong, the gateway to which the packet was sent will return an
ICMP redirect message to report that there is a better gateway to reach
the net in question. The arrival of this redirect should cause an
update of the local table.
The number of entries in the local table should be determined by
the maximum number of active connections which this particular host can
support at any one time. For a large time sharing system, one might
imagine a table with 100 or more entries. For a personal computer being
used to support a single user telnet connection, only one address to
gateway association need be maintained at once.
The above strategy actually does not completely solve the problem,
but only pushes it down one level, where the problem then arises of how
a new host, freshly arriving on the internet, finds all of its
accessible gateways. Intentionally, this problem is not solved within
the internetwork architecture. The reason is that different networks
have drastically different strategies for allowing a host to find out
about other hosts on its immediate network. Some nets permit a
broadcast mechanism. In this case, a host can send out a message and
expect an answer back from all of the attached gateways. In other
cases, where a particular network is richly provided with tools to
support the internet, there may be a special network mechanism which a
host can invoke to determine where the gateways are. In other cases, it
may be necessary for an installer to manually provide the name of at
7
least one accessible gateway. Once a host has discovered the name of
one gateway, it can build up a table of all other available gateways, by
keeping track of every gateway that has been reported back to it in an
ICMP message.
5. Advanced Topics in Addressing and Routing
The preceding discussion describes the mechanism required in a
minimal implementation, an implementation intended only to provide
operational service access today to the various networks that make up
the internet. For any host which will participate in future research,
as contrasted with service, some additional features are required.
These features will also be helpful for service hosts if they wish to
obtain access to some of the more exotic networks which will become part
of the internet over the next few years. All implementors are urged to
at least provide a structure into which these features could be later
integrated.
There are several features, either already a part of the
architecture or now under development, which are used to modify or
expand the relationships between addresses and routes. The IP source
route options allow a host to explicitly direct a datagram through a
series of gateways to its foreign host. An alternative form of the ICMP
redirect packet has been proposed, which would return information
specific to a particular destination host, not a destination net.
Finally, additional IP options have been proposed to identify particular
routes within the internet that are unacceptable. The difficulty with
implementing these new features is that the mechanisms do not lie
8
entirely within the bounds of IP. All the mechanisms above are designed
to apply to a particular connection, so that their use must be specified
at the TCP level. Thus, the interface between IP and the layers above
it must include mechanisms to allow passing this information back and
forth, and TCP (or any other protocol at this level, such as UDP), must
be prepared to store this information. The passing of information
between IP and TCP is made more complicated by the fact that some of the
information, in particular ICMP packets, may arrive at any time. The
normal interface envisioned between TCP and IP is one across which
packets can be sent or received. The existence of asynchronous ICMP
messages implies that there must be an additional channel between the
two, unrelated to the actual sending and receiving of data. (In fact,
there are many other ICMP messages which arrive asynchronously and which
must be passed from IP up to higher layers. See RFC816, Fault
Isolation and Recovery.)
Source routes are already in use in the internet, and many
implementations will wish to be able to take advantage of them. The
following sorts of usages should be permitted. First, a user, when
initiating a TCP connection, should be able to hand a source route into
TCP, which in turn must hand the source route to IP with every outgoing
datagram. The user might initially obtain the source route by querying
a different sort of name server, which would return a source route
instead of an address, or the user may have fabricated the source route
manually. A TCP which is listening for a connection, rather than
attempting to open one, must be prepared to receive a datagram which
contains a IP return route, in which case it must remember this return
route, and use it as a source route on all returning datagrams.
9
6. Ports and Service Identifiers
The IP layer of the architecture contains the address information
which specifies the destination host to which the datagram is being
sent. In fact, datagrams are not intended just for particular hosts,
but for particular agents within a host, processes or other entities
that are the actual source and sink of the data. IP performs only a
very simple dispatching once the datagram has arrived at the target
host, it dispatches it to a particular protocol. It is the
responsibility of that protocol handler, for example TCP, to finish
dispatching the datagram to the particular connection for which it is
destined. This next layer of dispatching is done using "port
identifiers", which are a part of the header of the higher level
protocol, and not the IP layer.
This two-layer dispatching architecture has caused a problem for
certain implementations. In particular, some implementations have
wished to put the IP layer within the kernel of the operating system,
and the TCP layer as a user domain application program. Strict
adherence to this partitioning can lead to grave performance problems,
for the datagram must first be dispatched from the kernel to a TCP
process, which then dispatches the datagram to its final destination
process. The overhead of scheduling this dispatch process can severely
limit the achievable throughput of the implementation.
As is discussed in RFC817, Modularity and Efficiency in Protocol
Implementations, this particular separation between kernel and user
leads to other performance problems, even ignoring the issue of port
10
level dispatching. However, there is an acceptable shortcut which can
be taken to move the higher level dispatching function into the IP
layer, if this makes the implementation substantially easier.
In principle, every higher level protocol could have a different
dispatching algorithm. The reason for this is discussed below.
However, for the protocols involved in the service offering being
implemented today, TCP and UDP, the dispatching algorithm is exactly the
same, and the port field is located in precisely the same place in the
header. Therefore, unless one is interested in participating in further
protocol research, there is only one higher level dispatch algorithm.
This algorithm takes into account the internet level foreign address,
the protocol number, and the local port and foreign port from the higher
level protocol header. This algorithm can be implemented as a sort of
adjunct to the IP layer implementation, as long as no other higher level
protocols are to be implemented. (Actually, the above statement is only
partially true, in that the UDP dispatch function is subset of the TCP
dispatch function. UDP dispatch depends only protocol number and local
port. However, there is an occasion within TCP when this exact same
subset comes into play, when a process wishes to listen for a connection
from any foreign host. Thus, the range of mechanisms necessary to
support TCP dispatch are also sufficient to support precisely the UDP
requirement.)
The decision to remove port level dispatching from IP to the higher
level protocol has been questioned by some implementors. It has been
argued that if all of the address structure were part of the IP layer,
11
then IP could do all of the packet dispatching function within the host,
which would lead to a simpler modularity. Three problems were
identified with this. First, not all protocol implementors could agree
on the size of the port identifier. TCP selected a fairly short port
identifier, 16 bits, to reduce header size. Other protocols being
designed, however, wanted a larger port identifier, perhaps 32 bits, so
that the port identifier, if properly selected, could be considered
probabilistically unique. Thus, constraining the port id to one
particular IP level mechanism would prevent certain fruitful lines of
research. Second, ports serve a special function in addition to
datagram delivery: certain port numbers are reserved to identify
particular services. Thus, TCP port 23 is the remote login service. If
ports were implemented at the IP level, then the assignment of well
known ports could not be done on a protocol basis, but would have to be
done in a centralized manner for all of the IP architecture. Third, IP
was designed with a very simple layering role: IP contained exactly
those functions that the gateways must understand. If the port idea had
been made a part of the IP layer, it would have suggested that gateways
needed to know about ports, which is not the case.
There are, of course, other ways to avoid these problems. In
particular, the "well-known port" problem can be solved by devising a
second mechanism, distinct from port dispatching, to name well-known
ports. Several protocols have settled on the idea of including, in the
packet which sets up a connection to a particular service, a more
general service descriptor, such as a character string field. These
special packets, which are requesting connection to a particular
12
service, are routed on arrival to a special server, sometimes called a
"rendezvous server", which examines the service request, selects a
random port which is to be used for this instance of the service, and
then passes the packet along to the service itself to commence the
interaction.
For the internet architecture, this strategy had the serious flaw
that it presumed all protocols would fit into the same service paradigm:
an initial setup phase, which might contain a certain overhead such as
indirect routing through a rendezvous server, followed by the packets of
the interaction itself, which would flow directly to the process
providing the service. Unfortunately, not all high level protocols in
internet were expected to fit this model. The best example of this is
isolated datagram exchange using UDP. The simplest exchange in UDP is
one process sending a single datagram to another. Especially on a local
net, where the net related overhead is very low, this kind of simple
single datagram interchange can be extremely efficient, with very low
overhead in the hosts. However, since these individual packets would
not be part of an established connection, if IP supported a strategy
based on a rendezvous server and service descriptors, every isolated
datagram would have to be routed indirectly in the receiving host
through the rendezvous server, which would substantially increase the
overhead of processing, and every datagram would have to carry the full
service request field, which would increase the size of the packet
header.
In general, if a network is intended for "virtual circuit service",
13
or things similar to that, then using a special high overhead mechanism
for circuit setup makes sense. However, current directions in research
are leading away from this class of protocol, so once again the
architecture was designed not to preclude alternative protocol
structures. The only rational position was that the particular
dispatching strategy used should be part of the higher level protocol
design, not the IP layer.
This same argument about circuit setup mechanisms also applies to
the design of the IP address structure. Many protocols do not transmit
a full address field as part of every packet, but rather transmit a
short identifier which is created as part of a circuit setup from source
to destination. If the full address needs to be carried in only the
first packet of a long exchange, then the overhead of carrying a very
long address field can easily be justified. Under these circumstances,
one can create truly extravagant address fields, which are capable of
extending to address almost any conceivable entity. However, this
strategy is useable only in a virtual circuit net, where the packets
being transmitted are part of a established sequence, otherwise this
large extravagant address must be transported on every packet. Since
Internet explicitly rejected this restriction on the architecture, it
was necessary to come up with an address field that was compact enough
to be sent in every datagram, but general enough to correctly route the
datagram through the catanet without a previous setup phase. The IP
address of 32 bits is the compromise that results. Clearly it requires
a substantial amount of shoehorning to address all of the interesting
places in the universe with only 32 bits. On the other hand, had the
14
address field become much bigger, IP would have been susceptible to
another criticism, which is that the header had grown unworkably large.
Again, the fundamental design decision was that the protocol be designed
in such a way that it supported research in new and different sorts of
protocol architectures.
There are some limited restrictions imposed by the IP design on the
port mechanism selected by the higher level process. In particular,
when a packet goes awry somewhere on the internet, the offending packet
is returned, along with an error indication, as part of an ICMP packet.
An ICMP packet returns only the IP layer, and the next 64 bits of the
original datagram. Thus, any higher level protocol which wishes to sort
out from which port a particular offending datagram came must make sure
that the port information is contained within the first 64 bits of the
next level header. This also means, in most cases, that it is possible
to imagine, as part of the IP layer, a port dispatch mechanism which
works by maSKINg and matching on the first 64 bits of the incoming
higher level header.
1楼  RFC814 - Name, addresses, ports, and routes | |
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NAME, ADDRESSES, PORTS, AND ROUTES
David D. Clark
MIT Laboratory for Computer Science
Computer Systems and Communications Group
July, 1982
1. IntrodUCtion
It has been said that the principal function of an operating system
is to define a number of different names for the same object, so that it
can busy itself keeping track of the relationship between all of the
different names. Network protocols seem to have somewhat the same
characteristic. In TCP/IP, there are several ways of referring to
things. At the human visible interface, there are character string
"names" to identify networks, hosts, and services. Host names are
translated into network "addresses", 32-bit values that identify the
network to which a host is attached, and the location of the host on
that net. Service names are translated into a "port identifier", which
in TCP is a 16-bit value. Finally, addresses are translated into
"routes", which are the sequence of steps a packet must take to reach
the specified addresses. Routes show up eXPlicitly in the form of the
internet routing options, and also implicitly in the address to route
translation tables which all hosts and gateways maintain.
This RFCgives suggestions and guidance for the design of the
tables and algorithms necessary to keep track of these various sorts of
identifiers inside a host implementation of TCP/IP.
2
2. The Scope of the Problem
One of the first questions one can ask about a naming mechanism is
how many names one can expect to encounter. In order to answer this, it
is necessary to know something about the expected maximum size of the
internet. Currently, the internet is fairly small. It contains no more
than 25 active networks, and no more than a few hundred hosts. This
makes it possible to install tables which exhaustively list all of these
elements. However, any implementation undertaken now should be based on
an assumption of a much larger internet. The guidelines currently
recommended are an upper limit of about 1,000 networks. If we imagine
an average number of 25 hosts per net, this would suggest a maximum
number of 25,000 hosts. It is quite unclear whether this host estimate
is high or low, but even if it is off by several factors of two, the
resulting number is still large enough to suggest that current table
management strategies are unacceptable. Some fresh techniques will be
required to deal with the internet of the future.
3. Names
As the previous section suggests, the internet will eventually have
a sufficient number of names that a host cannot have a static table
which provides a translation from every name to its associated address.
There are several reasons other than sheer size why a host would not
wish to have such a table. First, with that many names, we can expect
names to be added and deleted at such a rate that an installer might
spend all his time just revising the table. Second, most of the names
will refer to addresses of machines with which nothing will ever be
3
exchanged. In fact, there may be whole networks with which a particular
host will never have any traffic.
To cope with this large and somewhat dynamic environment, the
internet is moving from its current position in which a single name
table is maintained by the NIC and distributed to all hosts, to a
distributed approach in which each network (or group of networks) is
responsible for maintaining its own names and providing a "name server"
to translate between the names and the addresses in that network. Each
host is assumed to store not a complete set of name-address
translations, but only a cache of recently used names. When a name is
provided by a user for translation to an address, the host will first
examine its local cache, and if the name is not found there, will
communicate with an appropriate name server to oBTain the information,
which it may then insert into its cache for future reference.
Unfortunately, the name server mechanism is not totally in place in
the internet yet, so for the moment, it is necessary to continue to use
the old strategy of maintaining a complete table of all names in every
host. Implementors, however, should structure this table in such a way
that it is easy to convert later to a name server approach. In
particular, a reasonable programming strategy would be to make the name
table Accessible only through a subroutine interface, rather than by
scattering direct references to the table all through the code. In this
way, it will be possible, at a later date, to replace the subroutine
with one capable of making calls on remote name servers.
A problem which occasionally arises in the ARPANET today is that
4
the information in a local host table is out of date, because a host has
moved, and a revision of the host table has not yet been installed from
the NIC. In this case, one attempts to connect to a particular host and
discovers an unexpected machine at the address obtained from the local
table. If a human is directly observing the connection attempt, the
error is usually detected immediately. However, for unattended
operations such as the sending of queued mail, this sort of problem can
lead to a great deal of confusion.
The nameserver scheme will only make this problem worse, if hosts
cache locally the address associated with names that have been looked
up, because the host has no way of knowing when the address has changed
and the cache entry should be removed. To solve this problem, plans are
currently under way to define a simple facility by which a host can
query a foreign address to determine what name is actually associated
with it. SMTP already defines a verification technique based on this
approach.
4. Addresses
The IP layer must know something about addresses. In particular,
when a datagram is being sent out from a host, the IP layer must decide
where to send it on the immediately connected network, based on the
internet address. Mechanically, the IP first tests the internet address
to see whether the network number of the recipient is the same as the
network number of the sender. If so, the packet can be sent directly to
the final recipient. If not, the datagram must be sent to a gateway for
further forwarding. In this latter case, a second decision must be
5
made, as there may be more than one gateway available on the immediately
attached network.
When the internet address format was first specified, 8 bits were
reserved to identify the network. Early implementations thus
implemented the above algorithm by means of a table with 256 entries,
one for each possible net, that specified the gateway of choice for that
net, with a special case entry for those nets to which the host was
immediately connected. Such tables were sometimes statically filled in,
which caused confusion and malfunctions when gateways and networks moved
(or crashed).
The current definition of the internet address provides three
different options for network numbering, with the goal of allowing a
very large number of networks to be part of the internet. Thus, it is
no longer possible to imagine having an exhaustive table to select a
gateway for any foreign net. Again, current implementations must use a
strategy based on a local cache of routing information for addresses
currently being used.
The recommended strategy for address to route translation is as
follows. When the IP layer receives an outbound datagram for
transmission, it extracts the network number from the destination
address, and queries its local table to determine whether it knows a
suitable gateway to which to send the datagram. If it does, the job is
done. (But see RFC816 on Fault Isolation and Recovery, for
recommendations on how to deal with the possible failure of the
gateway.) If there is no such entry in the local table, then select any
6
accessible gateway at random, insert that as an entry in the table, and
use it to send the packet. Either the guess will be right or wrong. If
it is wrong, the gateway to which the packet was sent will return an
ICMP redirect message to report that there is a better gateway to reach
the net in question. The arrival of this redirect should cause an
update of the local table.
The number of entries in the local table should be determined by
the maximum number of active connections which this particular host can
support at any one time. For a large time sharing system, one might
imagine a table with 100 or more entries. For a personal computer being
used to support a single user telnet connection, only one address to
gateway association need be maintained at once.
The above strategy actually does not completely solve the problem,
but only pushes it down one level, where the problem then arises of how
a new host, freshly arriving on the internet, finds all of its
accessible gateways. Intentionally, this problem is not solved within
the internetwork architecture. The reason is that different networks
have drastically different strategies for allowing a host to find out
about other hosts on its immediate network. Some nets permit a
broadcast mechanism. In this case, a host can send out a message and
expect an answer back from all of the attached gateways. In other
cases, where a particular network is richly provided with tools to
support the internet, there may be a special network mechanism which a
host can invoke to determine where the gateways are. In other cases, it
may be necessary for an installer to manually provide the name of at
7
least one accessible gateway. Once a host has discovered the name of
one gateway, it can build up a table of all other available gateways, by
keeping track of every gateway that has been reported back to it in an
ICMP message.
5. Advanced Topics in Addressing and Routing
The preceding discussion describes the mechanism required in a
minimal implementation, an implementation intended only to provide
operational service access today to the various networks that make up
the internet. For any host which will participate in future research,
as contrasted with service, some additional features are required.
These features will also be helpful for service hosts if they wish to
obtain access to some of the more exotic networks which will become part
of the internet over the next few years. All implementors are urged to
at least provide a structure into which these features could be later
integrated.
There are several features, either already a part of the
architecture or now under development, which are used to modify or
expand the relationships between addresses and routes. The IP source
route options allow a host to explicitly direct a datagram through a
series of gateways to its foreign host. An alternative form of the ICMP
redirect packet has been proposed, which would return information
specific to a particular destination host, not a destination net.
Finally, additional IP options have been proposed to identify particular
routes within the internet that are unacceptable. The difficulty with
implementing these new features is that the mechanisms do not lie
8
entirely within the bounds of IP. All the mechanisms above are designed
to apply to a particular connection, so that their use must be specified
at the TCP level. Thus, the interface between IP and the layers above
it must include mechanisms to allow passing this information back and
forth, and TCP (or any other protocol at this level, such as UDP), must
be prepared to store this information. The passing of information
between IP and TCP is made more complicated by the fact that some of the
information, in particular ICMP packets, may arrive at any time. The
normal interface envisioned between TCP and IP is one across which
packets can be sent or received. The existence of asynchronous ICMP
messages implies that there must be an additional channel between the
two, unrelated to the actual sending and receiving of data. (In fact,
there are many other ICMP messages which arrive asynchronously and which
must be passed from IP up to higher layers. See RFC816, Fault
Isolation and Recovery.)
Source routes are already in use in the internet, and many
implementations will wish to be able to take advantage of them. The
following sorts of usages should be permitted. First, a user, when
initiating a TCP connection, should be able to hand a source route into
TCP, which in turn must hand the source route to IP with every outgoing
datagram. The user might initially obtain the source route by querying
a different sort of name server, which would return a source route
instead of an address, or the user may have fabricated the source route
manually. A TCP which is listening for a connection, rather than
attempting to open one, must be prepared to receive a datagram which
contains a IP return route, in which case it must remember this return
route, and use it as a source route on all returning datagrams.
9
6. Ports and Service Identifiers
The IP layer of the architecture contains the address information
which specifies the destination host to which the datagram is being
sent. In fact, datagrams are not intended just for particular hosts,
but for particular agents within a host, processes or other entities
that are the actual source and sink of the data. IP performs only a
very simple dispatching once the datagram has arrived at the target
host, it dispatches it to a particular protocol. It is the
responsibility of that protocol handler, for example TCP, to finish
dispatching the datagram to the particular connection for which it is
destined. This next layer of dispatching is done using "port
identifiers", which are a part of the header of the higher level
protocol, and not the IP layer.
This two-layer dispatching architecture has caused a problem for
certain implementations. In particular, some implementations have
wished to put the IP layer within the kernel of the operating system,
and the TCP layer as a user domain application program. Strict
adherence to this partitioning can lead to grave performance problems,
for the datagram must first be dispatched from the kernel to a TCP
process, which then dispatches the datagram to its final destination
process. The overhead of scheduling this dispatch process can severely
limit the achievable throughput of the implementation.
As is discussed in RFC817, Modularity and Efficiency in Protocol
Implementations, this particular separation between kernel and user
leads to other performance problems, even ignoring the issue of port
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level dispatching. However, there is an acceptable shortcut which can
be taken to move the higher level dispatching function into the IP
layer, if this makes the implementation substantially easier.
In principle, every higher level protocol could have a different
dispatching algorithm. The reason for this is discussed below.
However, for the protocols involved in the service offering being
implemented today, TCP and UDP, the dispatching algorithm is exactly the
same, and the port field is located in precisely the same place in the
header. Therefore, unless one is interested in participating in further
protocol research, there is only one higher level dispatch algorithm.
This algorithm takes into account the internet level foreign address,
the protocol number, and the local port and foreign port from the higher
level protocol header. This algorithm can be implemented as a sort of
adjunct to the IP layer implementation, as long as no other higher level
protocols are to be implemented. (Actually, the above statement is only
partially true, in that the UDP dispatch function is subset of the TCP
dispatch function. UDP dispatch depends only protocol number and local
port. However, there is an occasion within TCP when this exact same
subset comes into play, when a process wishes to listen for a connection
from any foreign host. Thus, the range of mechanisms necessary to
support TCP dispatch are also sufficient to support precisely the UDP
requirement.)
The decision to remove port level dispatching from IP to the higher
level protocol has been questioned by some implementors. It has been
argued that if all of the address structure were part of the IP layer,
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then IP could do all of the packet dispatching function within the host,
which would lead to a simpler modularity. Three problems were
identified with this. First, not all protocol implementors could agree
on the size of the port identifier. TCP selected a fairly short port
identifier, 16 bits, to reduce header size. Other protocols being
designed, however, wanted a larger port identifier, perhaps 32 bits, so
that the port identifier, if properly selected, could be considered
probabilistically unique. Thus, constraining the port id to one
particular IP level mechanism would prevent certain fruitful lines of
research. Second, ports serve a special function in addition to
datagram delivery: certain port numbers are reserved to identify
particular services. Thus, TCP port 23 is the remote login service. If
ports were implemented at the IP level, then the assignment of well
known ports could not be done on a protocol basis, but would have to be
done in a centralized manner for all of the IP architecture. Third, IP
was designed with a very simple layering role: IP contained exactly
those functions that the gateways must understand. If the port idea had
been made a part of the IP layer, it would have suggested that gateways
needed to know about ports, which is not the case.
There are, of course, other ways to avoid these problems. In
particular, the "well-known port" problem can be solved by devising a
second mechanism, distinct from port dispatching, to name well-known
ports. Several protocols have settled on the idea of including, in the
packet which sets up a connection to a particular service, a more
general service descriptor, such as a character string field. These
special packets, which are requesting connection to a particular
12
service, are routed on arrival to a special server, sometimes called a
"rendezvous server", which examines the service request, selects a
random port which is to be used for this instance of the service, and
then passes the packet along to the service itself to commence the
interaction.
For the internet architecture, this strategy had the serious flaw
that it presumed all protocols would fit into the same service paradigm:
an initial setup phase, which might contain a certain overhead such as
indirect routing through a rendezvous server, followed by the packets of
the interaction itself, which would flow directly to the process
providing the service. Unfortunately, not all high level protocols in
internet were expected to fit this model. The best example of this is
isolated datagram exchange using UDP. The simplest exchange in UDP is
one process sending a single datagram to another. Especially on a local
net, where the net related overhead is very low, this kind of simple
single datagram interchange can be extremely efficient, with very low
overhead in the hosts. However, since these individual packets would
not be part of an established connection, if IP supported a strategy
based on a rendezvous server and service descriptors, every isolated
datagram would have to be routed indirectly in the receiving host
through the rendezvous server, which would substantially increase the
overhead of processing, and every datagram would have to carry the full
service request field, which would increase the size of the packet
header.
In general, if a network is intended for "virtual circuit service",
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or things similar to that, then using a special high overhead mechanism
for circuit setup makes sense. However, current directions in research
are leading away from this class of protocol, so once again the
architecture was designed not to preclude alternative protocol
structures. The only rational position was that the particular
dispatching strategy used should be part of the higher level protocol
design, not the IP layer.
This same argument about circuit setup mechanisms also applies to
the design of the IP address structure. Many protocols do not transmit
a full address field as part of every packet, but rather transmit a
short identifier which is created as part of a circuit setup from source
to destination. If the full address needs to be carried in only the
first packet of a long exchange, then the overhead of carrying a very
long address field can easily be justified. Under these circumstances,
one can create truly extravagant address fields, which are capable of
extending to address almost any conceivable entity. However, this
strategy is useable only in a virtual circuit net, where the packets
being transmitted are part of a established sequence, otherwise this
large extravagant address must be transported on every packet. Since
Internet explicitly rejected this restriction on the architecture, it
was necessary to come up with an address field that was compact enough
to be sent in every datagram, but general enough to correctly route the
datagram through the catanet without a previous setup phase. The IP
address of 32 bits is the compromise that results. Clearly it requires
a substantial amount of shoehorning to address all of the interesting
places in the universe with only 32 bits. On the other hand, had the
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address field become much bigger, IP would have been susceptible to
another criticism, which is that the header had grown unworkably large.
Again, the fundamental design decision was that the protocol be designed
in such a way that it supported research in new and different sorts of
protocol architectures.
There are some limited restrictions imposed by the IP design on the
port mechanism selected by the higher level process. In particular,
when a packet goes awry somewhere on the internet, the offending packet
is returned, along with an error indication, as part of an ICMP packet.
An ICMP packet returns only the IP layer, and the next 64 bits of the
original datagram. Thus, any higher level protocol which wishes to sort
out from which port a particular offending datagram came must make sure
that the port information is contained within the first 64 bits of the
next level header. This also means, in most cases, that it is possible
to imagine, as part of the IP layer, a port dispatch mechanism which
works by maSKINg and matching on the first 64 bits of the incoming
higher level header.
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