network
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... host
makes appropriate use of the TOS facility, its network service should
be at least as good as (and hopefully better than) it would have been
...
... which did not meet this goal, no matter how good it might be in other
respects, would ever become widely deployed and used. A particular
consequence of this goal is that if a network cannot provide the TOS
requested in a packet, the network ...
... network cannot provide the TOS
requested in a packet, the network does not discard the packet but
instead delivers it the same way it would have been delivered had
none of the TOS ...
...
The second field, labeled "TOS" above, denotes how the network should
make tradeoffs between throughput ...
... guarantee that the path taken by the datagram will have a delay that
the user considers "low". The network will attempt to choose the
lowest delay path available, based on its (often imperfect)
information about path delay. The network ...
... network will attempt to choose the
lowest delay path available, based on its (often imperfect)
information about path delay. The network will not discard the
datagram simply because it believes that the delay of the available
...
... datagram simply because it believes that the delay of the available
paths is "too high" (actually, the network manager can override this
behavior through creative use of routing metrics, but this is
...
... datagrams which contain user
data. Although it might seem intuitively correct to always
request that the network minimize delay for segments containing
acknowledgements but no data ...
... [15] lists the TOS values to be used by a number of common network
applications. For other applications, it is the responsibility of
the application's designer or programmer to make a suitable
...
... application.
It is essential for many sorts of network diagnostic applications,
and desirable for other applications, that the user of the
...
... Destination Unreachable
when an unreachable destination (network or host) would have been
reachable had a different TOS ...
... 2 -- redirect datagrams for the type of service and network
3 -- redirect datagrams for the type of service ...
... datagram. Inside such a routing domain, the network manager may
choose to limit the size of the routing database ...
... router) wishes to send an IP
packet to a destination on another network or subnet, it needs to
choose an appropriate router ...
... host treats code 0 (redirect datagrams for the network) Redirects
as if they were code 1 (redirect datagrams for the host ...
... host treats code 2
(redirect datagrams for the network and type of service) Redirects
as if they were code 3 (redirect datagrams ...
... TOS value (0000). Static routes have
their TOS values assigned by the network manager.
When a router ...
... ICMP Destination Unreachable is
returned to the source. Normally, the Unreachable uses code 0
(Network unreachable) or 1 (Host unreachable). If, however, a
route ...
... destination exists which has a different TOS value
and a non-infinite metric then code 11 (Network unreachable for
type of service) or code 12 (Host ...
... TOS field in a datagram primarily affects the path chosen through
the network, but an implementor may choose to have TOS also affect
...
... host or
router might choose to give preferential queuing on network output
queues to datagrams ...
... routing
is a choice made by the network manager, a user requiring a free
path might not get one if the packet has to pass through a routing
...
... willing to pay. Thus, the TOS value defined in this memo merely
requests that the network "minimize monetary cost".
...
...
(1) To define a new type of service requesting that the network
"minimize monetary cost"
...
... pairs of bits, it is unclear that the range of network service
provided by various paths could usefully be subdivided in so fine
a manner. If some of these non-useful combinations of bits ...
... Very Weak TOS will make little practical difference, since (except
where the network manager has intentionally set things up
otherwise) there will be a route with the default TOS ...
... regard.
Under the first option, if the network manager neglects some
pieces of the configuration the likely consequence is that some
packets which would benefit from TOS ...
... routed as if they had requested the default TOS. Under the second
option, however, a network manager can easily (accidently)
configure things in such a way that packets which request a
certain TOS ...
... the first option would seem to have a slight edge with regard to
robustness in the face of errors by the network manager.
It has been also been suggested that the first option provides the
...
... routing loops) would result. The mechanisms
specified in this memo reflect the first option because that will
probably be more intuitive to most network managers. Internet
routing ...
... topology changes, routes with infinite
metrics occur only as the result of deliberate action (or serious
error) on the part of the network manager. Thus, packets are
unlikely to be discarded unless the network manager has taken
...
... error) on the part of the network manager. Thus, packets are
unlikely to be discarded unless the network manager has taken
deliberate action to cause them to be. Some people believe that
this is an important feature of the specification, allowing the
...
... deliberate action to cause them to be. Some people believe that
this is an important feature of the specification, allowing the
network to (for example) keep packets which have requested that
cost be minimized off of a link that is so expensive that the
...
... cost be minimized off of a link that is so expensive that the
network manager feels confident that the users would want their
packets to be dropped. Others (including the author of this memo)
believe that this "feature" will prove not to be useful, and that
...
... routing choices
particularly more intuitive. It is also worth noting that this is
another case that a network manager has to try rather hard to
create: since OSPF ...
... non-zero TOS, a
network manager would have to await the development of a new
routing protocol or create ...
... are two reasons why this is so:
(1) Not all networks will consider the value of the TOS field
when deciding how to handle and route ...
... route packets. Partly this
is a transition issue: there will be a (probably lengthy)
period when some networks will use equipment that predates
this specification. Even long term, however, many networks
...
... period when some networks will use equipment that predates
this specification. Even long term, however, many networks
will not be able to provide better service by considering the
...
... value of the TOS field. For example, the best path through a
network composed of a homogeneous collection of
interconnected LANs is probably the same for any possible TOS ...
... LANs is probably the same for any possible TOS
value. Inside such a network, it would make little sense to
require routers and routing protocols ...
... example, an application may use the TOS field to request that
the network choose a path which maximizes throughput, but
cannot use that mechanism to say that it needs or wants a
...
... particular number of kilobytes or megabytes per second.
Because the network cannot know what the application
requires, it would be inappropriate for the network to decide
...
... Because the network cannot know what the application
requires, it would be inappropriate for the network to decide
to discard a packet which requested maximal throughput
...
...
The inability to provide resource guarantees is a serious drawback
for certain kinds of network applications. For example, a system
using packetized voice simply creates ...
... available bandwidth is inadequate to deliver intelligible speech.
Likewise, the network oughtn't even bother to deliver a voice
packet that has suffered more delay in the network ...
... network oughtn't even bother to deliver a voice
packet that has suffered more delay in the network than the
application can tolerate. Unfortunately, resource guarantees are
problematic in connectionless networks ...
... network than the
application can tolerate. Unfortunately, resource guarantees are
problematic in connectionless networks. Internet researchers are
...
... semantics.
(2) This specification assumes that network managers will do "the
right thing". If a routing domain ...
... routing domain uses TOS, the network
manager must configure the routers in such a way that a
reasonable path is chosen for each TOS ...
... reasonable path is chosen for each TOS. While this ought not
to be terribly difficult, a network manager could accidently
or intentionally violate our rule that using the TOS facility
...