router
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... routes separately for each type of service. These new routing
protocols make it practical for routers to consider the requested
type of service when making routing ...
...
This specification defines in detail how hosts and routers use the
TOS facility. Section 2 introduces the primary considerations that
...
... TOS field of that octet may contain. Section 5 describes
how a host (or router) chooses appropriate values to insert into the
TOS fields of the IP datagrams ...
... how TOS affects path choice by both hosts and routers. Section 8
describes some additional ways in which TOS may optionally affect
...
... host implementations
should not interact badly with hosts and routers which implement the
specifications of this memo, since TOS support is almost non-existent
...
... specifications of this memo, since TOS support is almost non-existent
in routers which predate this specification. However, this memo does
attempt to be compatible with the treatment of IP TOS ...
... TOS field
and to the decision that hosts and routers should be able to handle a
new type of service correctly without having to understand its
...
... Internet protocol experiment which
makes use of that bit). Routers and recipients of datagrams ignore
the value of this field. This field is copied on fragmentation ...
... TOS values is no
longer meaningful. For example, it would be a serious error for a
router to choose a low delay path for a packet whose requested TOS
was 1110 simply because the router ...
... router to choose a low delay path for a packet whose requested TOS
was 1110 simply because the router noted that the former "delay bit"
was set.
...
... TOS values, and
hosts and routers must not preclude their use in any way. As will
become clear after reading the remainder of this memo, only the
default TOS ...
... default TOS is in any way special. A host or router need not (and
except as described in Section 8 should not) make any distinction
between TOS ...
... Layer. This
section describes how a host or router chooses appropriate TOS
values for ICMP messages ...
... ICMP message types 8 (Echo), 10
(Router Solicitation), 13 (Timestamp), 15 (Information
Request -- now obsolete), and 17 (Address ...
... reply messages include ICMP message types 0 (Echo
Reply), 9 (Router Advertisement), 14 (Timestamp Reply), 16
(Information Reply -- also obsolete), and 18 (Address ...
... type of service
A router generates a code 11 or code 12 Destination Unreachable
when an unreachable destination ...
... host) would have been
reachable had a different TOS value been specified. A router
generates a code 0 or code 1 Destination Unreachable in other
...
... host
A router generates a code 3 Redirect when the Redirect applies
only to IP packets which request a particular TOS ...
... only to IP packets which request a particular TOS value. A router
generates a code 1 Redirect instead when the the optimal next hop
...
... value. In order to minimize the potential for host confusion,
routers should refrain from using codes 0 and 2 in Redirects
[3, 6 ...
... domains the paths are
sufficiently homogeneous in nature that there is no reason for
routers to choose different paths based up the TOS field in a
datagram ...
... TOS.
Neither hosts nor routers should need to have any explicit knowledge
of whether TOS affects routing ...
...
When a host (which is not also a router) wishes to send an IP
packet to a destination on another network ...
... network or subnet, it needs to
choose an appropriate router to send the packet to. According to
the IP Architecture ...
... Architecture, it does so by maintaining a route cache and a
list of default routers. Each entry in the route cache lists a
destination ...
... destination (IP address) and the appropriate router to use to
reach that destination. The host ...
... ICMP Redirect mechanism. The host
learns the list of default routers either from static
configuration information or by using the ICMP Router Discovery ...
... routers either from static
configuration information or by using the ICMP Router Discovery
mechanism [8]. When the host ...
... route matching the destination
address in the packet. If one is found it is used; if not, the
packet is sent to one of the default routers. All of this is
described in greater detail in section 3.3.1 of RFC-1122std3 [1 ...
...
type 2: { destination, *, router }
where type 1 entries result from the receipt of code 3 (or code 1)
...
... destination address of the packet. If
either of these searches finds a matching entry, the packet is
sent to the router listed in the matching entry. Otherwise, the
packet is sent to one of the routers on the list of default
...
... sent to the router listed in the matching entry. Otherwise, the
packet is sent to one of the routers on the list of default
routers.
...
... and requested TOS match the type 1 entry. Because the type 2
entry may well specify the correct router for some TOS values
other than the one specified in the type 1 entry, saving the type
...
... created a new type 2 entry (thereby causing the new type 1 entry
to be flushed). That can happen, for example, if only some of the
routers on the local net are part of a routing domain that
...
... destination
for which the host should use the same router regardless of the
requested TOS. There is not yet sufficient experience with the
...
... TOS field when choosing an appropriate path over which to
forward an IP packet. How a router does this is a part of the
more general issue of how a router picks appropriate paths. This
...
... IP packet. How a router does this is a part of the
more general issue of how a router picks appropriate paths. This
larger issue can be extremely complex [4], and is beyond the scope
...
... discussion should therefore be considered only
an overview. Implementors should consult the Router Requirements
specification [3 ...
... routing
protocols they implement for details.
A router associates a TOS value with each route in its forwarding
...
... network manager.
When a router wants to forward a packet, it first looks up the
destination address in its forwarding table. This yields a set of
...
... datagram is handled. For example, a host or
router might choose to give preferential queuing on network output
queues ...
... queues to datagrams which have requested that delay be minimized.
Similarly, a router forced by overload to discard packets might
attempt to avoid discarding packets that have requested that
...
... Additionally, some Link Layer protocols have their own quality of
service mechanisms. When a router or host transmits an IP packet, it
...
... Internet is becoming increasingly commercialized, a
number of participants in the IETF's Router Requirements Working
Group felt it would be important to have a TOS ...
... TOS values but would require special code in both
hosts and routers. Also, it would not be helpful to users who
want their packets to travel via the least-cost path but can
accept some level of cost when necessary. Finally, since whether
...
... IP option would be a more
appropriate mechanism, since there are precedents for having IP
options that all routers are required to honor, and an IP option
could include parameters such as the maximum amount the user was
...
... routing to work correctly when new types of service are
deployed. If routers have different ideas about the size of the
TOS field they make inconsistent decisions that may lead to
...
... bit TOS field. Fortunately, this should not be
much of a problem in practice because routers which route based on
a three bit ...
... Strong TOS was quickly rejected. Because it requires that each
router a packet traverses have a route with the requested TOS,
...
... TOS makes a route
better). Since a router would need to understand the semantics of
all of the TOS ...
... TOS in their
routing decisions and routers that do not. Whether that is true
in all cases is unknown. It is certainly the case, however, that
under the second option it would not work to mix routers ...
... routers that do not. Whether that is true
in all cases is unknown. It is certainly the case, however, that
under the second option it would not work to mix routers that
consider TOS and routers ...
... All in all, there were no truly compelling arguments for choosing
one way or the other, but it was nontheless necessary to make a
choice: if different routers were to make the choice differently,
chaos (in the form of routing loops) would result. The mechanisms
...
... TOS.
It is commonly accepted that a router which has a default route
should nonetheless discard a packet if the router ...
... router which has a default route
should nonetheless discard a packet if the router has a more
specific route to the destination ...
... routing (described in Appendix
B.4), but that idea was rejected because it would add complexity
to routers without necessarily making their routing choices
particularly more intuitive. It is also worth noting that this is
...
... facility defined in RFC-791std5 or accepting that things wouldn't work
right until all routers in the Internet supported the TOS facility.
...
... value. Inside such a network, it would make little sense to
require routers and routing protocols to do the extra work
needed to consider the value of the TOS ...
... domain uses TOS, the network
manager must configure the routers in such a way that a
reasonable path is chosen for each TOS. While this ought not
...
... Deering, S., "ICMP Router Discovery Messages", RFC 1256prop, Xerox PARC, September 1991. ...
... Braden, Ross Callon, Vint Cerf, Noel Chiappa, Deborah Estrin, Phill
Gross, Bob Hinden, Steve Huston, Jon Postel, Greg Vaudreuil, John
Wobus, and the Router Requirements Working Group.
...