📄 rfc1932.txt
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Cole, Shur & Villamizar Informational [Page 16]
RFC 1932 IP over ATM: A Framework Document April 1996
.----------.
---------< Non-ATM :
.-------. / /-< Subnet >-\
:Sub-ES >--/ : ---------- :
------- : :
: :
.--^---. .--^---.
:Router: :Router:
-v-v-- -v-v--
: : : :
.--------. : : .--------. : : .--------.
.-------. : >-/ \-< >-/ \-< : .-------.
:Sub-ES :---: Subnet :-----: Subnet :-----: Subnet :---:Sub-ES :
------- : : : : : : -------
-------- ---v---- --------
:
.--^----.
:Sub-ES :
-------
Figure 3: A configuration with both ATM-based and non-ATM based
subnets.
For example, figure 3 shows an end-to-end configuration consisting of
four components, three of which are ATM technology based, while the
fourth is a standard IP subnet based on non-ATM technology. End-
systems (either hosts or routers) attached to the ATM-based networks
may communicate either using the Classical IP model or directly via
ATM (subject to policy constraints). Such nodes may communicate
directly at the IP level without necessarily needing an intermediate
router, even if end-systems do not share a common IP-level network
prefix. Communication with end-systems on the non-ATM-based
Classical IP subnet takes place via a router, following the Classical
IP model (see Section 8.1 below).
Many of the problems and issues associated with creating such direct
connections across subnet boundaries were originally being addressed
in the IETF's IPLPDN working group and the IP over ATM working group.
This area is now being addressed in the Routing over Large Clouds
working group. Examples of work performed in the IPLPDN working
group include short-cut routing (proposed by P. Tsuchiya) and
directed ARP RFC-1433 [5] over SMDS networks. The ROLC working group
has produced the distributed ARP server architectures and the NBMA
Address Resolution Protocol (NARP) [7]. The Next Hop Resolution
Protocol (NHRP) is still work in progress, though the ROLC WG is
considering advancing the current document. Questions/issues
specifically related to defining a capability to cross IP subnet
boundaries include:
Cole, Shur & Villamizar Informational [Page 17]
RFC 1932 IP over ATM: A Framework Document April 1996
o How can routing be optimized across multiple logical IP subnets
over both a common ATM based and a non-ATM based infrastructure.
For example, in Figure 3, there are two gateways/routers between
the non-ATM subnet and the ATM subnets. The optimal path
from end-systems on any ATM-based subnet to the non ATM-based
subnet is a function of the routing state information of the two
routers.
o How to incorporate policy routing constraints.
o What is the proper coupling between routing and address
resolution particularly with respect to off-subnet communication.
o What are the local procedures to be followed by hosts and
routers.
o Routing between hosts not sharing a common IP-level (or L3)
network prefix, but able to be directly connected at the NBMA
media level.
o Defining the details for an efficient address resolution
architecture including defining the procedures to be followed by
clients and servers (see RFC-1433 [5], RFC-1735 [7] and NHRP).
o How to identify the need for and accommodate special purpose SVCs
for control or routing and high bandwidth data transfers.
For ATM (unlike other NBMA media), an additional complexity in
supporting IP routing over these ATM internets lies in the
multiplicity of address formats in UNI 3.0 [4]. NSAP modeled address
formats only are supported on "private ATM" networks, while either 1)
E.164 only, 2) NSAP modeled formats only, or 3) both are supported on
"public ATM" networks. Further, while both the E.164 and NSAP
modeled address formats are to be considered as network points of
attachment, it seems that E.164 only networks are to be considered as
subordinate to "private networks", in some sense. This leads to some
confusion in defining an ARP mechanism in supporting all combinations
of end-to-end scenarios (refer to the discussion in Appendix A on the
possible scenarios to be supported by ARP).
7. Extensions to IP Routing
RFC-1620 [3] describes the problems and issues associated with direct
connections across IP subnet boundaries in greater detail, as well as
possible solution approaches. The ROLC WG has identified persistent
routing loop problems that can occur if protocols which lose
information critical to path vector routing protocol loop suppression
are used to accomplish direct connections across IP subnet
Cole, Shur & Villamizar Informational [Page 18]
RFC 1932 IP over ATM: A Framework Document April 1996
boundaries.
The problems may arise when a destination network which is not on the
NBMA network is reachable via different routers attached to the NBMA
network. This problem occurs with proposals that attempt to carry
reachability information, but do not carry full path attributes (for
path vector routing) needed for inter-AS path suppression, or full
metrics (for distance vector or link state routing even if path
vector routing is not used) for intra-AS routing.
For example, the NHRP protocol may be used to support the
establishment of direct connections across subnetwork boundaries.
NHRP assumes that routers do run routing protocols (intra and/or
inter domain) and/or static routing. NHRP further assumes that
forwarding tables constructed by these protocols result in a steady
state loop-free forwarding. Note that these two assumptions do not
impose any additional requirements on routers, beyond what is
required in the absence of NHRP.
NHRP runs in addition to routing protocols, and provides the
information that allows the elimination of multiple IP hops (the
multiple IP hops result from the forwarding tables constructed by the
routing protocols) when traversing an NBMA network. The IPATM and
ROLC WGs have both expended considerable effort in discussing and
coming to understand these limitations.
It is well-known that truncating path information in Path Vector
protocols (e.g., BGP) or losing metric information in Distance Vector
protocols (e.g., RIP) could result in persistent forwarding loops.
These loops could occur without ATM and without NHRP.
The combination of NHRP and static routing alone cannot be used in
some topologies where some of the destinations are served by multiple
routers on the NBMA. The combination of NHRP and an intra-AS routing
protocol that does not carry inter-AS routing path attributes alone
cannot be used in some topologies in which the NBMA will provide
inter-AS transit connectivity to destinations from other AS served by
multiple routers on the NBMA.
Figure 4 provides an example of the routing loops that may be formed
in these circumstances. The example illustrates how the use of NHRP
in the environment where forwarding loops could exist even without
NHRP (due to either truncated path information or loss of metric
information) would still produce forwarding loops.
There are many potential scenarios for routing loops. An example is
given in Figure 4. It is possible to produce a simpler example where
a loop can form. The example in Figure 4 illustrates a loop which
Cole, Shur & Villamizar Informational [Page 19]
RFC 1932 IP over ATM: A Framework Document April 1996
will persist even if the protocol on the NBMA supports redirects or
can invalidate any route which changes in any way, but does not
support the communication of full metrics or path attributes.
.----. .----.
: H1 >----< S1 : Notes:
---- vvvv H#n == host #n
/ : \ R#n == router #n
/ : \ S#n == subnet #n
/------/ : \
: : \ S2 to R3 breaks
.--^---. .----. .-^--.
: : : R4 : : R6 :
: NBMA : --v- --v- See the text for
: : : : details of the
-v--v- = = looping conditions
: \ = SLOW = and mechanisms
: .-^--. = LINK =
: : R2 : = =
: --v- : :
: : .--^-. .--^-.
.-^--. : : R5 : : R7 :
: R8 : : --v- --v-
--v- \ : :
: \ / :
\ .-^^-. .--^-.
\ : S2 : : S4 :
\ --v- --v-
\ \ /
\ \ /
\ .^--^.
\ : R3 : path before the break is
\ -v-- H1->S1->R1->NBMA->R2->S2->R3->H2
\ /
.----. .-^^-. path after the break is
: H2 >---< S3 : H1->S1->R1->NBMA->R2->S2->R5->R4->S1
---- ---- \------<--the-loop--<-------/
Figure 4: A Routing Loop Due to Lost PV Routing Attributes.
In the example in Figure 4, Host 1 is sending traffic toward Host 2.
In practice, host routes would not be used, so the destination for
the purpose of routing would be Subnet 3. The traffic travels by way
of Router 1 which establishes a "cut-through" SVC to the NBMA next-
hop, shown here as Router 2. Router 2 forwards traffic destined for
Subnet 3 through Subnet 2 to Router 3. Traffic from Host 1 would
then reach Host 2.
Cole, Shur & Villamizar Informational [Page 20]
RFC 1932 IP over ATM: A Framework Document April 1996
Router 1's cut-through routing implementation caches an association
between Host 2's IP address (or more likely all of Subnet 3) and
Router 2's NBMA address. While the cut-through SVC is still up, Link
1 fails. Router 5 loses it's preferred route through Router 3 and
must direct traffic in the other direction. Router 2 loses a route
through Router 3, but picks up an alternate route through Router 5.
Router 1 is still directing traffic toward Router 2 and advertising a
means of reaching Subnet 3 to Subnet 1. Router 5 and Router 2 will
see a route, creating a loop.
This loop would not form if path information normally carried by
interdomain routing protocols such as BGP and IDRP were retained
across the NBMA. Router 2 would reject the initial route from Router
5 due to the path information. When Router 2 declares the route to
Subnet 3 unreachable, Router 1 withdraws the route from routing at
Subnet 1, leaving the route through Router 4, which would then reach
Router 5, and would reach Router 2 through both Router 1 and Router
5. Similarly, a link state protocol would not form such a loop.
Two proposals for breaking this form of routing loop have been
discussed. Redirect in this example would have no effect, since
Router 2 still has a route, just has different path attributes. A
second proposal is that is that when a route changes in any way, the
advertising NBMA cut-through router invalidates the advertisement for
some time period. This is similar to the notion of Poison Reverse in
distance vector routing protocols. In this example, Router 2 would
eventually readvertise a route since a route through Router 6 exists.
When Router 1 discovers this route, it will advertise it to Subnet 1
and form the loop. Without path information, Router 1 cannot
distinguish between a loop and restoration of normal service through
the link L1.
The loop in Figure 4 can be prevented by configuring Router 4 or
Router 5 to refuse to use the reverse path. This would break backup
connectivity through Router 8 if L1 and L3 failed. The loop can also
be broken by configuring Router 2 to refuse to use the path through
Router 5 unless it could not reach the NBMA. Special configuration of
Router 2 would work as long as Router 2 was not distanced from Router
3 and Router 5 by additional subnets such that it could not determine
which path was in use. If Subnet 1 is in a different AS or RD than
Subnet 2 or Subnet 4, then the decision at Router 2 could be based on
path information.
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