Network Working Group Srihari R. Sangli (Procket Networks)
Internet Draft Yakov Rekhter (Juniper Networks)
Expiration Date: March 2004 Rex Fernando (Procket Networks)
John G. Scudder (Cisco Systems)
Enke Chen (Redback Networks)
Graceful Restart Mechanism for BGP
draft-ietf-idr-restart-08.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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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.
The mechanisms described in this document are applicable to all
routers, both those with the ability to preserve forwarding state
during BGP restart and those without (although the latter need to
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implement only a subset of the mechanisms described in this
document).
3. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
4. 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.
5. Marker for End-of-RIB
An UPDATE message with no reachable NLRI and 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 the MP_UNREACH_NLRI attribute [BGP-MP] with no
withdrawn routes for that <AFI, SAFI>.
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
convergence in general, and thus the practice is recommended.
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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.
6. 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, SAFI, 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) |
+--------------------------------------------------+
The use and meaning of the fields are as follows:
Restart Flags:
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This field contains bit flags related to restart.
0 1 2 3
+-+-+-+-+
|R|Resv.|
+-+-+-+-+
The most significant bit is defined as the Restart State (R)
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, and SHOULD be set to zero by
the sender and ignored by the receiver.
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 [IANA-AFI].
Subsequent Address Family Identifier (SAFI):
This field provides additional information about the type of
the Network Layer Reachability Information carried in the
attribute. Presently defined values for this field are
specified in [IANA-SAFI].
Flags for Address Family:
This field contains bit flags for the <AFI, SAFI>.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|F| Reserved |
+-+-+-+-+-+-+-+-+
The most significant bit is defined as the Forwarding State (F)
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bit which can be used to indicate if the forwarding state for
the <AFI, SAFI> 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, and SHOULD be set to zero by
the sender and ignored by the receiver.
When a sender of this capability doesn't include any <AFI, SAFI> in
the capability, it means that the sender is not capable of preserving
its forwarding state during BGP restart, but supports procedures for
the Receiving Speaker (as defined in Section 6.2 of this document).
In that case the value of the "Restart Time" field advertised by the
sender is irrelevant.
A BGP speaker SHOULD NOT include more than one instance of the
Graceful Restart Capability in the capability advertisement [BGP-
CAP]. If more than one instance of the Graceful Restart Capability
is carried in the capability advertisement, the receiver of the
advertisement SHOULD ignore all but the last instance of the Graceful
Restart Capability.
Including <AFI=IPv4, SAFI=unicast> into the Graceful Restart
Capability doesn't imply that the IPv4 unicast routing information
should be carried by using the BGP Multiprotocol extensions [BGP-MP]
- it could be carried in the NLRI field of the BGP UPDATE message.
7. Operation
A BGP speaker MAY advertise the Graceful Restart Capability for an
address family to its peer if it has the ability to preserve its
forwarding state for the address family when BGP restarts. In
addition, 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 (as mentioned before this is done by not
including any <AFI, SAFI> in the advertised capability). There are
two reasons for doing this. First, to indicate its intention of
generating the End-of-RIB marker upon the completion of its initial
routing updates, as doing this would be useful for routing
convergence in general. Second, to indicate its support for a peer
which wishes to perform a graceful restart.
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.
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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.
7.1. Procedures for the Restarting Speaker
When the Restarting Speaker restarts, possible it SHOULD retain, if
possible, 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 and excluding the ones
which do not advertise the graceful restart 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
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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.
If one wants to apply graceful restart only when the restart is
planned (as opposed to both planned and unplanned restart), then one
way to accomplish this would be to set the Forwarding State bit to 1
after a planned restart, and to 0 in all other cases. Other
approaches to accomplish this are outside the scope of this document.
7.2. 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 peer as an indication of TCP
reset and act accordingly (when the Graceful Restart Capability has
been received from the peer).
"Acting accordingly" in this context means that the previous TCP
session SHOULD be closed, and the new one retained. Note that this
behavior differs from the default behavior, as specified in [BGP-4]
section 6.8. Since the previous connection is considered to be
reset, no NOTIFICATION message should be sent -- the previous TCP
session is simply closed.
When the Receiving Speaker detects TCP reset for a BGP session with a
peer that has advertised the Graceful Restart Capability, it SHALL
retain the routes received from the peer 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 peer) 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 restarted.
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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 peer advertised previously, the Receiving Speaker SHALL
delete all the stale routes from the peer that it is retaining.
Once the session is re-established, if the "Forwarding State" bit for
a specific address family is not set in the newly received Graceful
Restart Capability, or if a specific address family is not included
in the newly received Graceful Restart Capability, or if the Graceful
Restart Capability isn't received in the re-established session at
all, then Receiving Speaker SHALL immediately remove all the stale
routes from the peer 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 peer.
The Receiving Speaker SHALL replace the stale routes by the routing
updates received from the peer. Once the End-of-RIB marker for an
address family is received from the peer, it SHALL immediately remove
any routes from the peer 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.
8. 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 the benefits of deploying BGP Graceful
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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 are reduced relative to the scenario where IGPs do
have similar Graceful Restart capability.
9. 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.
10. Acknowledgments
The authors would like to thank Bruce Cole, Bill Fenner, Eric Gray
Jeffrey Haas, Alvaro Retana, Naiming Shen, Satinder Singh, David
Ward, Shane Wright and Alex Zinin for their review and comments.
11. Normative 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", RFC2858, June 2000.
[BGP-CAP] Chandra, R., Scudder, J., "Capabilities Advertisement with
BGP-4", draft-ietf-idr-rfc2842bis-02.txt, April 2002.
[BGP-AUTH] Heffernan A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[IANA-AFI] http://www.iana.org/assignments/address-family-numbers.
[IANA-SAFI] http://www.iana.org/assignments/safi-namespace.
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12. Author Information
Srihari R. Sangli
Procket Networks, Inc.
1100 Cadillac Court
Milpitas, CA 95035
e-mail: srihari@procket.com
Yakov Rekhter
Juniper Networks, Inc.
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
e-mail: yakov@juniper.net
Rex Fernando
Procket Networks, Inc.
1100 Cadillac Court
Milpitas, CA 95035
e-mail: rex@procket.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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