Network Working Group D. Haskin
Request For Comments: 1863 Bay Networks, Inc.
Category: Experimental October 1995
A BGP/IDRP Route Server alternative to a full mesh routing
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Abstract
This document describes the use and detailed design of Route Servers
for dissemination of routing information among BGP/IDRP speaking
routers.
The intention of the proposed technique is to reduce overhead and
management complexity of maintaining numerous direct BGP/IDRP
sessions which otherwise might be required or desired among routers
within a single routing domain as well as among routers in different
domains that are connected to a common switched fabric (e.g. an ATM
cloud).
Current deployments of Exterior Routing protocols, such as the Border
Gateway Protocol [BGP4] and the adaptation of the ISO Inter-Domain
Routing Protocol [IDRP], require that all BGP/IDRP routers, which
participate in inter-domain routing (border routers) and belong to
the same routing domain, establish a full mesh connectivity with each
other for purpose of exchanging routing information acquired from
other routing domains. In large routing domains the number of intra-
domain connections that needs to be maintained by each border route
can be significant.
In addition, it may be desired for a border router to establish
routing sessions with all border routers in other domains which are
reachable via a shared communication media. We refer to routers that
are directly reachable via a shared media as adjacent routers. Such
direct peering allows a router to acquire "first hand" information
about destinations which are directly reachable through adjacent
routers and select the optimum direct paths to these destinations.
Establishment of BGP/IDRP sessions among all adjacent border routers
would result in a full mesh routing connectivity. Unfortunately for
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a switched media as ATM, SMDS or Frame Relay network which may
inter-connect a large number of routers, due to the number of
connections that would be needed to maintain a full mesh direct
peering between the routers, makes this approach impractical.
In order to alleviate the "full mesh" problem, this paper proposes to
use IDRP/BGP Route Servers which would relay external routes with all
of their attributes between client routers. The clients would
maintain IDRP/BGP sessions only with the assigned route servers
(sessions with more than one server would be needed if redundancy is
desired). All routes that are received from a client router would be
propagated to other clients by the Route Server. Since all external
routes and their attributes are relayed unmodified between the client
routers, the client routers would acquire the same routing
information as they would via direct peering. We refer to such
arrangement as virtual peering. Virtual peering allows client
routers independently apply selection criteria to the acquired
external routes according to their local policies as they would if a
direct peering were established.
The routing approach described in this paper assumes that border
routers possess a mechanism to resolve the media access address of
the next hop router for any route acquired from a virtual peer.
It is fair to note that the approach presented in this paper only
reduces the number of routing connection each border router needs to
maintain. It does not reduce the volume of routing information that
needs to maintained at each border router.
Besides addressing the "full mesh" problems, the proposal attempts
to achieve the following goals:
- to minimize BGP/IDRP changes that need to be implemented in client
routers in order to inter-operate with route servers;
- to provide for redundancy of distribution of routing information to
route server clients;
- to minimize the amount of routing updates that have to be sent to
route server clients;
- to provide load distribution between route servers;
- to avoid an excessive complexity of the interactions between Route
Servers themselves.
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The following terms and acronyms are used in this paper:
Routing Domain - a collection of routers with the same set of
routing policies. For IPv4 it can be identified
with an Autonomous System Number, for IPv6
it can be identified with a Routing Domain
Identifier.
Border Router (BR) - a router that acquires external routes, i.e.
routes to internet points outside its routing
domain.
Route Server (RS) - a process that collects routing information
from border routers and distributes this
information to 'client routers'.
RS Client (RC) - a router than peers with an RS in order to
acquire routing information. A server's client
can be a router or another route server.
RS Cluster (RSC) - two or more of route servers that share the same
subset of clients. A RS Cluster provides
redundancy of routing information to its
clients, i.e. routing information is provided
to all RS Cluster clients as long as there is
at least one functional route server in the RS
Cluster.
RCID - Cluster ID
In the proposed scheme a Route Server (RS) does not apply any
selection criteria to the routes received from border routers for the
purpose of distributing these routes to its clients. All routes
acquired from border routers or other Route Servers are relayed to
the client border routers.
There can be two classes of Route Servers: Route Servers that relay
external routes between routers in a single routing domain and Route
Servers that relay external routes between border routers in
different routing domains. The former are Intra-Domain Route Servers
and the latter are Inter-Domain Route Servers.
In the RS model proposed in this document there is no routing
exchange between Intra-Domain Route Servers and Inter-Domain Route
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Servers. Routes that cross a domain boundary must always pass
through a border router of such a domain which may apply
administrative filters to such routes.
Operations of Intra-Domain Route Servers and Inter-Domain Route
Servers are identical.
One or more Route Servers form an RS Cluster (RSC). For redundancy's
sake two or more RSs can be configured to operate in an RS Cluster.
All route servers in an RSC share the same clients, i.e. cluster
clients establish connections to all route servers in such an RSC for
the purpose of exchanging routing information. Each cluster is
assigned an unique RSC Identifier (RCID) represented by a 2-octet
unsigned integer.
Clusters which provide virtual connectivity between their clients
would be normally exchanging routing information among themselves so
that all external routes are propagated to all participating clients.
Though a Route Server Client (RC) can be associated with multiple
RSC, it seems that there is no real advantage of doing so except for
a short transition period to provide a graceful re-assignment from
one RSC to another or, if for some reason, there are multiple RS
groups that don't exchange routing information with each other.
The inter-cluster route exchange can be accomplished by forming a
full mesh routing adjacency between clusters. In this approach,
illustrated in the diagram below, each RS in each RSC would maintain
a routing connection with every RS in other RS clusters. Only routes
that are acquired from border routers are propagated to RSs in other
RS clusters.
BR11 BR12 BR1n BR21 BR22 BR2n
| | ... | | | ... |
----------------- ------------------
! RS11 RS12 ! --- ! RS21 RS22 !
----------------- ------------------
<RSC#1> \ / <RSC#2>
\ /
-----------------
! RS31 RS32 ! <RSC#3>
-----------------
| | ... |
BR31 BR32 BR3n
Another way to propagate routing information between clusters would
be to form a cluster hierarchy in which an RS in one cluster
maintains sessions only with RSs in designated clusters. In this
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approach an RS must advertise all acquired routes to an RS in another
cluster except the routes that are acquired from that cluster.
Nevertheless, it allows for minimizing the number of routing
sessions which can be highly desirable in some network. It is
important for the hierarchical scheme that the inter-cluster route
exchange links form a tree, i.e. there is only one route propagation
path between any two clusters, otherwise routing loops may result.
For detection and pruning of routing loops in a hierarchical cluster
topology, it is advisable to include the "RCID Path" attribute (see
4.3.4) in all routing updates sent between route servers. This
attribute lists IDs of all clusters in the route propagation path.
When a duplicate ID is detected in this attribute an offending route
needs to be discarded.
The diagram below which illustrates the hierarchical approach is
created from the diagram above by removing the route exchange link
between clusters 2 and 3.
BR11 BR12 BR1n BR21 BR22 BR2n
| | ... | | | ... |
----------------- ------------------
! RS11 RS12 ! --- ! RS21 RS22 !
----------------- ------------------
<RSC#1> \ <RSC#2>
\
-----------------
! RS31 RS32 ! <RSC#3>
-----------------
| | ... |
BR31 BR32 BR3n
It seems that the only disadvantage of the hierarchical model, is the
management headache of avoiding routing loops and redundant
information flow by insuring that inter-cluster links always form a
tree. But more study is needed to fully evaluate the comparative
merits of the full-mesh and hierarchical models.
Since RSs in the same cluster maintain routing sessions with the same
set of clients, it may seem that there is no need to exchange routing
information between RSs in the same cluster. Nevertheless, such a
route exchange may help to maintain identical routing databases in
the servers during client acquisition periods and when a partial
failure may affect some routing sessions.
Route servers in the same RS cluster exchange control messages in
attempt to subdivide the responsibilities of providing routing
information to their clients. In order to simplify the RS design,
the RS messaging is implemented on top of exterior protocol which is
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used by route servers for the routing information exchange.
Route servers act as concentrators for routes acquired by border
routers so that the border routers need to maintain routing
connections with only one or two designated route servers. Route
Servers distribute routing information that is provided to them by
the border routers to all their client.
If routing information were relayed to RS clients in UPDATE messages
with only those path attribute that are currently defined in the
BGP-4/IDRP specification, the RS clients would not be able to
associate external routes they receive with the border routers which
submitted that routes to route servers. Such an association is
necessary for making a correct route selection decision. Therefore,
the new path attribute, ADVERTISER, is defined.
The ADVERTISER is an optional non-transitive attribute that defines
the identifying address of the border router which originally
submitted the route to a router server in order for it to be relayed
to other RS clients. Type Code of the ADVERTISER attribute is 255.
This attribute must be included in every UPDATE message that is
relayed by route servers and must be recognized by RS clients.
An RS client establishes an BGP/IDRP connection to every route server
in the RS cluster to which the route client is assigned.
RS clients must be able to recognize the ADVERTISER path attribute
that is included in all UPDATE messages received from route servers.
Routes received in UPDATE messages from route servers are processed
as if they were received directly from the border routers specified
in the ADVERTISER attributes of the respective updates.
If an RS client receives a route from a Intra-Domain Route Server, is
assumed that the border router identified in the ADVERTISER attribute
is located in the receiving client's own routing domain.
If an RS client receives a route from a Inter-Domain Route Server,
the locality of the border router identified in the ADVERTISER
attribute can be determined from the BGP's AS_PATH attribute or
IDRP's RD_PATH attribute respectively.
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If no ADVERTISER attribute was included in an UPDATE message from a
route server it is assumed that the route server itself is the
advertiser of the corresponding route.
If the NEXT_HOP path attribute of an UPDATE message lists an address
of the receiving router itself, the route that is carried in such an
update message must be declared unreachable.
In addition, it is highly desirable, albeit not required, to
slightly modify the "standard" BGP/IDRP operation when acquiring
routes from RSs:
when a route is received from an RS and a route with the completely
identical attributes has been previously acquired from another RS
in the same cluster, the previously acquired route should be
replaced with the newly acquired route. Such a route replacement
should not trigger any route advertisement action on behalf of the
route.
RSs are designed to operate in such a way that eliminates the need to
keep multiple copies of the same route by RS clients and minimizes
the possibility of a route flap when the BGP/IDRP connection to one
of the redundant route servers is lost.
It is attempted to subdivide the route dissemination load between
route servers such that only one RS provides routing updates to a
given client. But since, for avoiding an excessive complexity, the
reconciliation algorithm does not eliminate completely the
possibility of races, it is still possible that a client may receive
updates from more than one route server. Therefore, the client's
ability to discard duplicate routes may reduce the need for a bigger
routing database.
A Route Server maintains BGP-4/IDRP sessions with its clients
according to the respective BGP-4/IDRP specification with exception
of protocol modifications outlined in this document.
UPDATE messages sent by route servers have the same format and
semantics as it respective BGP-4/IDRP counterparts but also carry the
ADVERTISER path attribute which specifies the BGP Identifier of the
border router that submitted the route advertised in the UPDATE
message. In addition, if the hierarchical model is deployed to
interconnect Route Server clusters, it is advisable to include the
"RCID Path" attribute in all routing updates sent between route
servers as described in 4.3.4.
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When route servers exchange OPEN messages they include the Route
Server protocol version (current version is 1) as well as Cluster IDs
of their respective clusters in an Optional Parameter of the OPEN
message. The value of Parameter Type for this parameter is 255. The
length of the parameter data is 3 octets. The format of parameter
data is shown below:
+-----------------------+------------------------------------+
| Version = 1 (1 octet) | Cluster ID (2 octets) |
+-----------------------+------------------------------------+
Also, route servers that belong to the same cluster send to each
other LIST messages with lists of clients to which they're providing
routing information. In the LIST message an RS specifies the Router
Identifier of each client to which that RS is providing routing
updates. Since LIST messages are relatively small there is no need to
add a processing complexity of generating incremental updates when a
list changes; instead the complete list is sent when RSs need to be
informed of the changes. The format of the LIST message is presented
in 4.3.1.
The LIST message contains the fixed BGP/IDRP header that is followed
with the fields shown below. The type code in the fixed header of
the LIST message is 255.
+----------+----------+----------+----------+
| Client Identifying Address | Repeated for each
+-------------------------------------------+ informed client
The number of Client Identifying Address" fields is not encoded
explicitly, but can be calculated as:
(<LIST message Length> - <Header Length>) / <Address Length>,
where <LIST message Length> is the value encoded in the fixed
BGP/IDRP header, <Header Length> is the length of that header, and
<Address Length> is 4 for IPv4 and 16 for IPv6.
A route server acquires external routes from RS clients that are also
border routers. A RS also may acquire external routes from other
RSs. Route servers relay all acquired routes unaltered to their
clients. No route selection is performed for purpose of route re-
advertisement to RS clients.
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While route servers receive and store routing data from all their
client, Routing Servers in the same cluster coordinate their route
advertisement in the attempt to ensure that only one RS provides
routing updates to a given client. If an RS fails, other Route
Servers in the cluster take over the responsibility of providing
routing updates to the clients that were previously served by the
failed RS. A route flap that can result from such switch-over can be
eliminated by the configuring client's "Hold Time" of their BGP-
4/IDRP sessions with the route servers to be larger than the switch-
over time. The switch-over time is determined by the Hold Time of
BGP-4/IDRP sessions between the route servers in the cluster and the
period that is needed for that route servers to reconcile their route
advertisement responsibilities. The reconciliation protocol is
described in 4.3.3.
The BGP-4/IDRP operations of route servers differs from the
"standard" operation in the following ways:
- when receiving routes from another RS, the RS Client mode of
operation is assumed, i.e., when a route with completely
identical attributes has been previously acquired from an RS
belonging to the same cluster as the RS that advertises the new
route, the previously acquired route should be discarded and
the newly acquired route should be accepted. Such a route
replacement should not trigger any route advertisement action
on behalf of the route.
- all acquired routes are advertised to a client router except
routes which were acquired from that client (no route echoing);
- if the hierarchical model of inter-cluster route exchange is
used, all acquired routes are advertised to an RS in another
RSC except routes that are acquired from that RSC. In the
full-mesh model, only routes which are acquired from border
routers are advertised to route servers in other clusters;
- if route servers in the same RS cluster are configured to
exchange routing information, only external routes that are
acquired from border routers are advertised to route servers in
the local cluster;
- the ADVERTISER path attribute is included in every UPDATE
messages that is generated by RS. This attribute must
specify the identifying address of the border router from which
information provided in UPDATE has been acquired. All other
routing attributes should be relayed to RS's peers unaltered.
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- when a route advertised by to an RS by a client becomes
unreachable such a route needs to be declared unreachable to
all other clients. In order to withdraw a route, the route
server sends an UPDATE for that route to each client (except
the client that this route was originally acquired) with the
NEXT_HOP path attribute set to the address of the client to
which this UPDATE is sent to. The the ADVERTISER path attribute
with the identifying address of the border router that
originally advertised the withdrawn route must be also included
in such an update message.
- if the hierarchical model is deployed to interconnect Route
Server clusters, it is advisable to include the RCID_PATH
attribute in all routing updates sent between route servers as
described in 4.3.4. The RCID_PATH attribute is never included
in UPDATE messages sent to border routers.
In order to coordinate route advertisement activities, route servers
which are members of the same RS cluster establish and maintain
BGP/IDRP connections between themselves forming a full-mesh
connectivity. Normally, there is no need for more than two-three
route servers in one cluster.
Route servers belonging to the same cluster send to each other LIST
messages with lists of clients to which they're providing routing
information; let's call such clients "informed clients".
Each RS maintains a separate "informed client" list for each RS in
the local cluster including itself. All such lists are linked in an
ascending order that is determined by the number of clients in each
list; the order among the lists with the same number of clients is
determined by comparing the identifying addresses of the
corresponding RSs -- an RS in such a "same number of clients" subset
is positioned after all RSs with the lower address.
An RS can be in one of two RS coordination states: 'Initiation' and
'Active'.
This is the initial state of route server that is entered upon RS
startup. When the Initiation state is entered the 'InitiationTimer'
is started. The Initiation state transits to the Active state upon
expiration of the 'InitiationTimer' or as soon as all configured
BGP/IDRP connections to other route servers in the local RS Cluster
are established and LIST messages from that route servers are
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received.
In the Initiation state an RS:
o tries to establish connections with other RSs in the local and
remote clusters.
o accepts BGP/IDRP connections from client routers.
o receives and process BGP/IDRP updates but doesn't send any
routing updates.
o stores "informed client" lists received from other RSs in the
local cluster - a newly received list replaces the existing list
for the same RS. If a LIST message is received from the route
server in another RS cluster, it should be silently ignored.
o initializes an empty "informed client" list for its own clients.
o as soon as a BGP/IDRP connection to an RS in the same RS Cluster
is established, transmits an empty LIST message to such an RS.
This state is entered upon expiration of the 'InitiationTimer' or as
soon as all configured BGP/IDRP connections to other route servers in
the local RS Cluster are established and LIST messages from that route
servers are received.
In the Active state an RS:
o continues attempts to establish connections with other route
servers in the local and remote clusters;
o accepts new BGP/IDRP connections;
o transmits a LIST message to an RS in the local cluster as soon
as an BGP/IDRP session with the RS is established and then
whenever the local "informed client" list changes;
o receives and process BGP/IDRP updates;
o receives and processes "informed client" lists as described
below:
a) If a LIST message is received from the route server in
another RS cluster, it should be silently ignored.
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b) If a LIST message is received from a route server that
belongs to the same RS Cluster, the differences between
the old and the new list are determined and the old "informed
client" list for that RS is replaced by the list from the new
message. For each client that was in the old list but not in
the new list it is checked whether the server has
an established BGP/IDRP connection to that client and
the client is not in any of the other "informed client"
lists. If both conditions are met, the processing described
for a new client takes place (see 4.3.3.3).
o for each new BGP/IDRP client (including connections established
in Initiation state), decides if that client should become an
"informed client", i.e. whether routing updates are to be sent
to the client or that client has been already taken care by
another RS in the local cluster. The decision process is
described in the next section.
Whenever an RS acquires a new BGP/IDRP peer it scans through all
"informed client" lists in order to determine if this peer has
already been receiving routing updates from another RS in the local
RS cluster. If the identifying address of the peer is found in one
of the list, no routing updates are sent to that peer.
If the peer's Router Id is not found, the route server initiates a
'DelayTimer' timer for that peer and the decision is postponed until
that timer expires. The delay value is calculated as followed:
the RS determines the relative position of its own "informed
client" list in the linked list of all "informed client" lists.
If such position is expressed with a number, say N, in the 1 to
"maximum number of lists" range, then the delay value is set to
(N-1)*<DelayGranularity>.
Upon expiration of the DelayTimer, the "informed client" lists are
scanned once again to see if the corresponding peer has already been
receiving routing updates from another RS in the local RS cluster.
If the Router Id of the peer is found in one of the lists as a result
of receiving a new LIST message, no routing updates are sent to that
peer. Otherwise, the peer's Router ID is entered in the "informed
client" list that belongs to the RS, the transmission of the updated
LIST message is immediately scheduled, and routing updates are sent
to the client.
The rational for the delay is to minimize races in the decision as
which RS among route servers in the same RSC is going to provide
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routing information to a given client. The RS with least number of
"informed clients" would have a shortest delay and is the most
probable to win the race. This helps to equalize the number of
"informed clients" between RSc in a cluster.
After an BGP/IDRP peer is placed in the "informed client" list, it is
only removed from the list when the BGP/IDRP connection to this peer
is lost. While an RS client is in the list it is accurately updated
with all routing changes.
If a route server loses a routing session with a route server in the
same cluster, it must consider taking the responsibilities of route
advertisement to the clients that are in the "informed client" list
of the remote route server of the failed session.
For each such client it is checked whether the server has an
established BGP/IDRP connection to that client and the client is not
in any of the "informed client" lists of active RS. If both
conditions are true, the processing described for a new client takes
place (see 4.3.3.3).
After advertisement responsibilities are reconciled the "informed
client" list associated with the failed session should be discarded.
The RCID_PATH is an optional non-transitive attribute that is
composed of a sequence of RS Cluster Identifiers (RCID) that
identifies the RS Cluster through which routing information carried
in the UPDATE message has passed. Type Code of the RCID_PATH
attribute is 254. The attribute value field contains one or more RS
Cluster Identifiers, each encoded as a 2-octets long field.
When a route server propagates a route which has been learned from
nother Route Server's UPDATE message, the following is performed with
respect to the the RCID_PATH attribute:
- if the destination of the route is not a route server, the
RCID_PATH Attribute is excluded from the UPDATE message sent to
that client.
- if the destination of the route is another route server that is
located in the advertising server's own RS cluster, the
RCID_PATH attribute is sent unmodified.
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- if the destination of the route is a route server in a different
RS cluster, the advertising route server shall verify that the
RCID of the destination speaker's cluster is not present in
the RCID_PATH attribute associated with route. If it does,
the route shall not be advertised and an event indicating
that a route loop was detected should be logged, otherwise
the advertising router shall prepend its own RCID to the RCID
sequence in the RCID_PATH attribute (put it in the leftmost
position).
When a route server propagates a route which has been learned from a
border router to another route server then:
- if the destination of the route is a route server that is
located in the advertising router's own RS cluster, an empty
RCID_PATH attribute shall be included in the UPDATE message
(an empty RCID_PATH attribute is one whose length field contains
the value zero).
- if the destination of the route is a route server in a different
RS cluster, the advertising route server shall include its own
RCID in the RCID_PATH attribute. In this case, the RCID of
advertising route server will be the only entry in the RCID_PATH
attribute.
In addition to the error codes defined in the BGP-4/IDRP
specification, the following error can be indicated in a NOTIFICATION
message that is sent by a route server:
255 LIST Message Error
The following error subcodes can be associated with the LIST Message
Error:
1 - Bad Address. This subcode indicates that a Client Identifying
Address in the received LIST message does not represent
a valid network layer address of a router interface.
The following additional UPDATE error subcodes are also defined:
255 - Invalid ADVERTISER Attribute. This subcode indicates that
a value of the ADVERTISER Attribute does not represent
a valid network layer address of a router interface.
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The InitiationTimer value of 5 minutes is suggested.
In order to avoid route flaps during an RS switch-over, a value of
DelayGranularity should be such so the maximum possible value of the
DelayTimer (see 4.3.3.3) combined with the Hold Time of inter-RS
connections would be shorter than two-third of the smallest Hold Time
interval of all BGP/IDRP connections between the route servers and
their clients (including RSs in other clusters). So in a cluster
with three RSs and the respective Hold Times of 30 and 90 seconds the
DelayGranularity of 15 seconds would be a recommended value.
For the same reason it is recommended that the Hold Time of BGP/IDRP
connections between route servers in the same cluster is set to one-
third of the smallest Hold Time of all BGP/IDRP connections between
the route servers and their clients (including RSs in other
clusters). So, if the smallest Hold Time of BGP/IDRP sessions with
clients is 90 seconds, the recommended value of the Hold Time of
BGP/IDRP connections between route servers in that cluster would be
30 seconds.
This document does not propose any mechanism for the dynamic RS
discovery by RS clients or/and by other route servers. It is assumed
that at minimum a manual configuration will be provided in
participating routers to achieve the needed connectivity.
Some design concepts presented in this paper benefited from
discussions with Tony Li (cisco Systems).
Author likes to thank John Krawczyk (Bay Networks) and Susan Harris
(Merit) for their review and valuable comments.
Also, author would like to thank Yakov Rekhter (IBM) for the review
of the earlier version of this document and constructive comments.
Special thanks to Ray Chang (Bay Networks) whose experience in
implementing the concepts presented in this document helped to refine
the route server design.
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[BGP4] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, T.J. Watson Research Center, IBM Corp.,
cisco Systems, March 1995.
[IDRP] Rekhter, Y., and P. Traina, "IDRP for IPv6", Work In Progress.