Network Working Group Srihari Ramachandra
Internet Draft Yakov Rekhter
Expiration Date: May 2001 Rex Fernando
John G. Scudder
Cisco Systems
Enke Chen
Redback Networks
Graceful Restart Mechanism for BGP
draft-ramachandra-bgp-restart-04.txt
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 except that the right to
produce derivative works is not granted.
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2. Abstract
This document proposes a mechanism for BGP that would help minimize
the negative effects on routing caused by BGP restart. An End-of-RIB
marker is specified and can be used to convey routing convergence
information. A new BGP capability, termed "Graceful Restart
Capability", is defined which would allow a BGP speaker to express
its ability to preserve forwarding state during BGP restart. Finally,
procedures are outlined for temporarily retaining routing information
across a TCP transport reset.
3. Introduction
Usually when BGP on a router restarts, all the BGP peers detect that
the session went down, and then came up. This "down/up" transition
results in a "routing flap" and causes BGP route re-computation,
generation of BGP routing updates and flap the forwarding tables. It
could spread across multiple routing domains. Such routing flaps may
create transient forwarding blackholes and/or transient forwarding
loops. They also consume resources on the control plane of the
routers affected by the flap. As such they are detrimental to the
overall network performance.
This document proposes a mechanism for BGP that would help minimize
the negative effects on routing caused by BGP restart. An End-of-RIB
marker is specified and can be used to convey routing convergence
information. A new BGP capability, termed "Graceful Restart
Capability", is defined which would allow a BGP speaker to express
its ability to preserve forwarding state during BGP restart. Finally,
procedures are outlined for temporarily retaining routing information
across a TCP transport reset.
4. Marker for End-of-RIB
An UPDATE message with empty withdrawn NLRI is specified as the End-
Of-RIB Marker that can be used by a BGP speaker to indicate to its
peer the completion of the initial routing update after the session
is established. For IPv4 unicast address family, the End-Of-RIB
Marker is an UPDATE message with the minimum length [BGP-4]. For any
other address family, it is an UPDATE message that contains only
MP_UNREACH_NLRI [BGP-MP] with no withdrawn routes for that <AFI, Sub-
AFI>.
Although the End-of-RIB Marker is specified for the purpose of BGP
graceful restart, it is noted that the generation of such a marker
upon completion of the initial update would be useful for routing
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convergence in general, and thus the practice is recommended.
In addition, it would be beneficial for routing convergence if a BGP
speaker can indicate to its peer up-front that it will generate the
End-Of-RIB marker, regardless of its ability to preserve its
forwarding state during BGP restart. This can be accomplished using
the Graceful Restart Capability described in the next section.
5. Graceful Restart Capability
The Graceful Restart Capability is a new BGP capability [BGP-CAP]
that can be used by a BGP speaker to indicate its ability to preserve
its forwarding state during BGP restart. It can also be used to
convey to its peer its intention of generating the End-Of-RIB marker
upon the completion of its initial routing updates.
This capability is defined as follows:
Capability code: 64
Capability length: variable
Capability value: Consists of the "Restart Flags" field,
"Restart Time" field, and zero or more of the tuples <AFI,
Sub-AFI, Flags for address family> as follows.
+--------------------------------------------------+
| Restart Flags (4 bits) |
+--------------------------------------------------+
| Restart Time in seconds (12 bits) |
+--------------------------------------------------+
| Address Family Identifier (16 bits) |
+--------------------------------------------------+
| Subsequent Address Family Identifier (8 bits) |
+--------------------------------------------------+
| Flags for Address Family (8 bits) |
+--------------------------------------------------+
| ... |
+--------------------------------------------------+
| Address Family Identifier (16 bits) |
+--------------------------------------------------+
| Subsequent Address Family Identifier (8 bits) |
+--------------------------------------------------+
| Flags for Address Family (8 bits) |
+--------------------------------------------------+
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The use and meaning of the fields are as follows:
Restart Flags:
This field contains bit flags related to restart.
The most significant bit is defined as the Restart State bit
which can be used to avoid possible deadlock caused by waiting
for the End-of-RIB marker when multiple BGP speakers peering
with each other restart. When set (value 1), this bit indicates
that the BGP speaker has restarted, and its peer should not wait
for the End-of-RIB marker from the speaker before advertising
routing information to the speaker.
The remaining bits are reserved.
Restart Time:
This is the estimated time (in seconds) it will take for the BGP
session to be re-established after a restart. This can be used to
speed up routing convergence by its peer in case that the BGP
speaker does not come back after a restart.
Address Family Identifier (AFI):
This field carries the identity of the Network Layer protocol
for which the Graceful Restart support is advertised. Presently
defined values for this field are specified in RFC1700 (see
the Address Family Numbers section).
Subsequent Address Family Identifier (Sub-AFI):
This field provides additional information about the type of
the Network Layer Reachability Information carried in the
attribute.
Flags for Address Family:
This field contains bit flags for the <AFI, Sub-AFI>.
The most significant bit is defined as the Forwarding State
bit which can be used to indicate if the forwarding state for
the <AFI, Sub-AFI> has indeed been preserved during the previous
BGP restart. When set (value 1), the bit indicates that the
forwarding state has been preserved.
The remaining bits are reserved.
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The advertisement of this capability by a BGP speaker also implies
that it will generate the End-of-RIB marker upon completion of its
initial routing update to its peer. The value of the "Restart Time"
field is irrelevant in the case that the capability does not carry
any <AFI, Sub-AFI>.
6. Operation
A BGP speaker may advertise the Graceful Restart Capability for an
address family to its peer only if it has the ability to preserve its
forwarding state for the address family when BGP restarts.
Even if the speaker does not have the ability to preserve its
forwarding state for any address family during BGP restart, it is
still recommended that the speaker advertise the Graceful Restart
Capability to its peer to indicate its intention of generating the
End-of-RIB marker upon the completion of its initial routing updates.
The End-of-RIB marker should be sent by a BGP speaker to its peer
once it completes the initial routing update (including the case when
there is no update to send) for an address family after the BGP
session is established.
It is noted that the normal BGP procedures MUST be followed when the
TCP session terminates due to the sending or receiving of a BGP
NOTIFICATION message.
In general the Restart Time SHOULD NOT be greater than the HOLDTIME
carried in the OPEN.
In the following sections, "Restarting Speaker" refers to a router
whose BGP has restarted, and "Receiving Speaker" refers to a router
that peers with the restarting speaker.
Consider that the Graceful Restart Capability for an address family
is advertised by the Restarting Speaker, and is understood by the
Receiving Speaker, and a BGP session between them is established.
The following sections detail the procedures that shall be followed
by the Restarting Speaker as well as the Receiving Speaker once the
Restarting Speaker restarts.
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6.1. Procedures for the Restarting Speaker
When the Restarting Speaker restarts, if possible it shall retain the
forwarding state for the BGP routes in the Loc-RIB, and shall mark
them as stale. It should not differentiate between stale and other
information during forwarding.
To re-establish the session with its peer, the Restarting Speaker
must set the "Restart State" bit in the Graceful Restart Capability
of the OPEN message. Unless allowed via configuration, the
"Forwarding State" bit for an address family in the capability can be
set only if the forwarding state has indeed been preserved for that
address family during the restart.
Once the session between the Restarting Speaker and the Receiving
Speaker is re-established, the Restarting Speaker will receive and
process BGP messages from its peers. However, it shall defer route
selection for an address family until it receives the End-of-RIB
marker from all its peers (excluding the ones with the "Restart
State" bit set in the received capability). It is noted that prior to
route selection, the speaker has no routes to advertise to its peers
and no routes to update the forwarding state.
In situations where both IGP and BGP have restarted, it might be
advantageous to wait for IGP to converge before the BGP speaker
performs route selection.
After the BGP speaker performs route selection, the forwarding state
of the speaker shall be updated and any previously marked stale
information shall be removed. The Adj-RIB-Out can then be advertised
to its peers. Once the initial update is complete for an address
family (including the case that there is no routing update to send),
the End-of-RIB marker shall be sent.
To put an upper bound on the amount of time a router defers its route
selection, an implementation must support a (configurable) timer that
imposes this upper bound.
6.2. Procedures for the Receiving Speaker
When the Restarting Speaker restarts, the Receiving Speaker may or
may not detect the termination of the TCP session with the Restarting
Speaker, depending on the underlying TCP implementation, whether or
not [BGP-AUTH] is in use, and the specific circumstances of the
restart. In case it does not detect the TCP reset and still
considers the BGP session as being established, it shall treat the
subsequent open connection from the Restarting Speaker as an
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indication of TCP reset and act accordingly.
When the TCP reset is detected by the Receiving Speaker, it shall
retain the routes received from the Restarting Speaker for all the
address families that were previously received in the Graceful
Restart Capability, and shall mark them as stale routing information.
To deal with possible consecutive restarts, a route (from the
Restarting Speaker) previously marked as stale shall be deleted. The
router should not differentiate between stale and other routing
information during forwarding.
In re-establishing the session, the "Restart State" bit in the
Graceful Restart Capability of the OPEN message sent by the Receiving
Speaker shall not be set unless the Receiving Speaker has also
restarted. The presence and the setting of the "Forwarding State" bit
for an address family depends upon the actual forwarding state and
configuration.
If the session does not get re-established within the "Restart Time"
that the Restarting Speaker advertised previously, the Receiving
Speaker shall delete all the stale routes from the Restarting Speaker
that it is retaining.
Once the session is re-established, if the "Forwarding State" bit for
an address family is not set in the received Graceful Restart
Capability, or if the capability is not received for an address
family, the Receiving Speaker shall immediately remove all the stale
routes from the Restarting Speaker that it is retaining for that
address family.
The Receiving Speaker shall send the End-of-RIB marker once it
completes the initial update for an address family (including the
case that it has no routes to send) to the Restarting Speaker.
The Receiving Speaker shall replace the stale routes by the routing
updates received from the Restarting Speaker. Once the End-of-RIB
marker for an address family is received from the Restarting Speaker,
it shall immediately remove any routes from the Restarting Speaker
that are still marked as stale for that address family.
To put an upper bound on the amount of time a router retains the
stale routes, an implementation may support a (configurable) timer
that imposes this upper bound.
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7. Deployment Considerations
While the procedures described in this document would help minimize
the effect of routing flaps, it is noted, however, that when a BGP
Graceful-Restart capable router restarts, there is a potential for
transient routing loops or blackholes in the network if routing
information changes before the involved routers complete routing
updates and convergence. Also, depending on the network topology, if
not all IBGP speakers are Graceful-Restart capable, there could be an
increased exposure to transient routing loops or blackholes when the
Graceful-Restart procedures are exercised.
The Restart Time, the upper bound for retaining routes and the upper
bound for deferring route selection may need to be tuned as more
deployment experience is gained.
Finally, it is noted that there is little benefit deploying BGP
Graceful-Restart in an AS whose IGPs and BGP are tightly coupled
(i.e., BGP and IGPs would both restart), and IGPs have no similar
Graceful-Restart capability.
8. Security Considerations
Since with this proposal a new connection can cause an old one to be
terminated, it might seem to open the door to denial of service
attacks. However, it is noted that unauthenticated BGP is already
known to be vulnerable to denials of service through attacks on the
TCP transport. The TCP transport is commonly protected through use
of [BGP-AUTH]. Such authentication will equally protect against
denials of service through spurious new connections.
It is thus concluded that this proposal does not change the
underlying security model (and issues) of BGP-4.
9. Acknowledgments
The authors would like to thank Alvaro Retana, Satinder Singh, David
Ward, Naiming Shen and Bruce Cole for their review and comments.
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10. References
[BGP-4] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-
4)", RFC 1771, March 1995.
[BGP-MP] Bates, T., Chandra, R., Katz, D., and Rekhter, Y.,
"Multiprotocol Extensions for BGP-4", RFC 2283, March 1998.
[BGP-CAP] Chandra, R., Scudder, J., "Capabilities Advertisement with
BGP-4", RFC 2842, May 2000.
[BGP-AUTH] Heffernan A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
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11. Author Information
Srihari Ramachandra
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
e-mail: rsrihari@cisco.com
Yakov Rekhter
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
e-mail: yakov@cisco.com
Rex Fernando
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
e-mail: rex@cisco.com
John G. Scudder
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
e-mail: jgs@cisco.com
Enke Chen
Redback Networks, Inc.
350 Holger Way
San Jose, CA 95134
e-mail: enke@redback.com
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