wdiff draft-ietf-idr-bgp-nh-cost-01.txt draft-ietf-idr-bgp-nh-cost-02.txt

Internet Engineering Task Force I. Varlashkin
Internet-Draft Easynet Global Services Google
Intended status: Standards Track R. Raszuk
Expires: September 28, 2012 NTT MCL November 16, 2015 Mirantis Inc.
March 27, 2012
K. Patel
M. Bhardwaj
S. Bayraktar
Cisco Systems
May 15, 2015

Carrying next-hop cost information in BGP
draft-ietf-idr-bgp-nh-cost-01
draft-ietf-idr-bgp-nh-cost-02

Abstract

This document describes

BGPLS provides a mechanism by which Link state and traffic
engineering information can be collected from internal networks and
shared with external network routers using BGP. BGPLS defines a new
Address Family to exchange this information using BGP.

BGP SAFI Optimal Route Reflection (ORR) provides a mechanism for a
centralized BGP Route Reflector to acheive requirements of a Hot
Potato Routing as described in Section 11 of [RFC4456]. Optimal
Route Reflection requires BGP ORR to overwrite the default IGP
location placement of the route reflector; which is used for
determining cost to the nexthop contained in the path.

This draft augments BGPLS and defines a new extensions to exchange
cost information to next-hops for the purpose of calculating best
path from a peer perspective rather than local BGP speaker own
perspective.

Status of this This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

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."
This Internet-Draft will expire on September 28, 2012. November 16, 2015.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.

Table of Contents

1. Motivation . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2
2. NEXT-HOP INFORMATION BASE . . . . . . . . . . . . . . . . . . . 3
3. BGP BEST PATH SELECTION MODIFICATION Bestpath Selection Modification . . . . . . . . . . . . . 3 4
4. USING BGP TO POPULATE NHIB . BGPLS Extensions . . . . . . . . . . . . . . . . . 4
4.1. NEXT-HOP SAFI . . . . . 4
4.1. RIB Metrics Prefix Descriptor . . . . . . . . . . . . . . 4
4.2. RIB Protocol ID . . . . 4
4.2. CAPABILITY ADVERTISEMENT . . . . . . . . . . . . . . . . . 4
4.3. INFORMATION ENCODING . Information Exchange . . . . . . . . . . . . . . . . . . 4 5
4.4. SESSION ESTABLISHMENT . . . . . . . . . . . . . . . Termination of the session carrying next-hop cost . . . . 5
4.5. INFORMATION EXCHANGE . . . . . . . . Graceful Restart and Route-Refresh . . . . . . . . . . . 5
4.6. TERMINATION OF NH SAFI SESSION . . . . . . . . . . . . . . 6
4.7. GRACEFUL RESTART AND ROUTE REFRESH . . . . . . . . . . . . 6
5. Security considerations . . . . . . . . . . . . . . . . . . . . 6 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 5
7. Acknowledgment Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 7 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7 6
8.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. USAGE SCENARIOS . . . . . . . . . . . . . . . . . . . 7
A.1. Trivial case . . . . . . . . . . . . . . . . . . . . . . . 7
A.2. Non-IGP based cost . . . . . . . . . . . . . . . . . . . . 8 7
A.3. Multiple route-reflectors . . . . . . . . . . . . . . . . . 8
A.4. Inter-AS MPLS VPN . . . . . . . . . . . . . . . . . . . . . 9 8
A.5. Corner case . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9

1. Motivation Introduction

In a certain situation situation, route-reflector clients may not get optimum
path to certain destinations. ADDPATH solves this problem by letting
route-reflector to advertise multiple paths for a given prefix. If
number of advertised paths are sufficiently big, route-reflector
clients can choose same route as they would in case of full-mesh.
This approach however places an additional burden on the control
plane. Solutions proposed by [BGP-ORR] use different approach -
instead of calculating best path from the local speaker speaker's own
perspective the calculations are done using cost from the client to
the next-hops. Although they eliminate need for transmitting
redundant routing information between peers, there are scenarios
where cost to the next-hop cannot be obtained accurately using this these
methods. For example, if next-hop information itself has been
learned via BGP then simple SPF run on link-state database won't be
sufficient to obtain cost information. There are also scenarios
where while a Route Reflector can reach its clients, the client to
client connectivity MAY be down.

BGPLS [I-D.ietf-idr-bgp-orr] provides a mechanism by which Link state
and traffic engineering information can be collected from internal
networks and shared with external network routers using BGP. BGPLS
defines a new Address Family to exchange this information using BGP.

To address such scenarios scenarios, this document proposes a
solution where draft defines extensions to BGPLS to
carry cost information of the next-hops. In particular, this draft
defines a new Protocol ID to announce a Router's IGP routes, and a
Prefix Descriptor to carry the next-hops is carried within
BGP itself using dedicated SAFI. cost information of the IGP routes
used towards resolving next-hops.

2. NEXT-HOP INFORMATION BASE

To facilitate further description of the proposed solution we
introduce a new table for all known next hops next-hops and costs to it from
various routers on the network.

Next-Hop Information Base (NHIB) stores cost to reach next-hop from
an arbitrary router on the network. This information is essential
for choosing best path from a peer perspective rather than BGP-speaker BGP-
speaker own perspective. In canonical form NHIB entry is triplet
(router, next-hop, cost), however this specification does not impose
any restriction on how BGP implementations store that information
internally. The cost in NHIB is does not have to be an IGP cost, but
all costs in NHIB MUST be comparable with each other.

NHIB can be populated from various sources both including static routing
and dynamic.
This dynamic routing. However, this document focuses on populating
NHIB using BGP. However it is
possible that protocols other than

An implementation implementing the BGP extension described in this
draft MAY provide an operator-controlled configuration knob
significant to an individual BGP speaker that treats next-hop cost
information received from two or more clients as equivalent. For
example a route-reflector could be receive next-hop cost only from R1
but it will use it while calculating best-path also used for R2, R3, Rn
because it has been instructed to populate
NHIB. do so by locally-significant
configuration. Multiple sources can be used for redundancy purpose.

3. BGP BEST PATH SELECTION MODIFICATION Bestpath Selection Modification

This section applies regardless of method used to populate NHIB.

When BGP speaker conforming to this specification selects routes to
be advertised to a peer it SHOULD use cost information from NHIB
rather than its own IGP cost to the next-hop after step (d) of
9.1.2.2 in [RFC4271].

4. USING BGP TO POPULATE NHIB

This section describes extension to base BGP specification that
allows BGP to be used for exchanging next-hop information between BGP
speakers via new SAFI in order to populate NHIB. Although next-hops
costs are exchanged via dedicated SAFI, this information is vital to
best path selection process for other AFI/SAFI (e.g. IPv4 and IPv6
unicast). It's therefore recommended that next-hop cost information
is exchanged before other AFI/SAFI. BGPLS Extensions

4.1. NEXT-HOP SAFI RIB Metrics Prefix Descriptor

This document introduces Next-Hop SAFI (NH SAFI) draft defines a new Prefix Descriptor known as a Cost Prefix
Descriptor with a TLV code point value to be assigned by IANA and purpose of exchanging information about cost to
next-hops.

4.2. CAPABILITY ADVERTISEMENT

A BGP speaker willing to exchange next-hop information MUST advertise
this in the OPEN message using BGP Capability IANA. The
Cost descriptor looks like:

Cost Value is a 4 byte Metric value assigned computed by IANA for NH SAFI. Note
that if BGP speaker whishes to exchange cost information for both
IPv4 and IPv6, then it MUST advertise two capabilities: one NH SAFI
for IPv4 and one NH SAFI for IPv6.

4.3. INFORMATION ENCODING

Routers use standard BGP UPDATE messages to exchange NH SAFI
information. a Router's
local RIB.

The Cost to reachable next-hops value is communicated using
MP_REACH_NLRI (attribute 14) with NLRI part as described below.
Requests are also sent using MP_REACH_NLRI. Informing a neighbour
about unreachable next-hop is done using MP_UNREACH_NLRI. All NH
SAFI messages MUST contain BGP COMMUNITY attribute with value
NO_ADVERTISE (0xFFFFFF02) and their propagation MUST follow normal
BGP rules (i.e. they're not to be propagated).

To request cost to associated with a next-hop from peer or to inform peer about cost
to prefix by a next-hop BGP attribute 14 is used as follow:

1. AFI Router. The
cost is set to indicate typically computed by the routing procotols that owns a
route.

4.2. RIB Protocol ID

This draft defines a new protocol ID for IPv4 or and IPv6 (whichever is appropriate)

2. SAFI is set to NH SAFI
3. Network Address of Next-Hop field is zeroed out

4. Topology
Prefix NLRI field is encoded as shown in the next figure

Flags - 1 octet field. Least significant bit MUST be set to 1 for
Request and a RIB Protocol ID. The RIB Protocol ID has a
value to zero for Response

AFI/SAFI fields can be set either assigned by IANA. The Prefix NLRI with RIB Protocol ID
is used to one of announce all the registered values to
indicate local and IGP computated routes that next-hop cost info applies only to specified AFI/SAFI.
Alternatively when both fields are be set to zero,
installed in the cost
information applies to any compatible AFI/SAFI negotiated RIB along with given
peer.

Next-hop - IPv4 or IPv6 address for which cost is being communicated
or requested. Type is determined from context, and length is
inferred from total length of attribute. its Cost is 32-bit unsigned integer (value described below), and NEXT_HOP
is AFI-specific address of the next-hop cost to which is being
communicated or requested. Size of NEXT_HOP field is inferred from
total length of attribute 14.

To inform peer that particular next-hop is unreachable
MP_UNREACH_NLRI attribute is used with same NLRI format as described
above. In this case cost field SHOULD be set to 0xFFFFFFFF.

4.4. SESSION ESTABLISHMENT

BGP speakers willing to exchange next-hop information SHOULD NOT
establish more then one session for given AFI and NH SAFI, even using
different transport addresses. This can be ensured for example by
checking peer's Router Id.

4.5. INFORMATION EXCHANGE value.

4.3. Information Exchange

Typically NH SAFI BGPLS sessions will be established between route-
reflectors route-reflectors
and its internal peers (both clients and non-clients). As soon as
the NH SAFI BGPLS session is ESTABLISHED requests for ESTABLISHED, all the RIB routes used to resolve
next-hop cost and information information about next-hop costs MAY be sent
independently. That is, route-reflector MAY send multiple requests
without waiting for response, and
immediately by clients to its peers MAY send cost information
before or after receiving such request. On the other hand, Router
Reflectors SHOULD request cost information from their internal peers
as soon as possible (due route-reflector. Implementations are
advised to reasons stated in section "BGP best path
selection modification"). announce BGP speaker does not need updates for this SAFI before any other SAFIs
to track
outstanding requests facilitate faster convergence of other SAFIs on Route Reflectors.

Each internal neighbor of a route-reflector announces its IGP RIB
Prefix information and its RIB metrics to the peer.

When Route Reflector using a BGP speaker receives request for cost information it MUST
reply
BGPLS session and a new NLRI Protocol ID and RIB metric Prefix
Descriptor. Each neighbor updates Route Reflector with actual cost (not necessarily its IGP cost, but whatever has
been chosen to be carried in NH SAFI) to given next-hop or with
prefix cost everytime a cost
set to all-ones indicating that next-hop is unreachable. If next-hop
information is obtained from sender's routing table, then sender MUST
perform lookup exactly the same way as it would for resolving next-
hop in BGP UPDATE message. For example, for non-labelled
destinations (e.g. AFI/SAFI 1/1 or 2/1) lookup would be done using
longest match, whereas for labelled IPv4 (AFI/SAFI 1/4, 1/128 or 2/4)
exact-match would be used.

When an IGP route changes.

Upon a BGP speaker detects change receipt of a BGPLS route and its associated cost, a Route
Reflector stores the prefix, cost, and neighbor information in its
local NHRIB database. It then uses the received cost towards
calculation of bestpath from the respective clients perpective as
opposed to previously advertised its own IGP cost.

4.4. Termination of the session carrying next-hop with delta equal or exceeding configured advertisement
threshold, it SHOULD inform peer by sending MP_UNREACH_NLRI as
described earlier. cost

When a BGP speaker discovers new the BGPLS session carrying next-hop among candidate routes it
SHOULD request cost information from the peer.

4.6. TERMINATION OF NH SAFI SESSION

When BGP speaker terminates (for
whatever reason) NH SAFI session
with a peer, it reason), the BGP speaker SHOULD remove invalidate all the next-hop
cost information received from (i.e same treatment that
peer unless instructed by configuration applies to do otherwise.

4.7. GRACEFUL RESTART AND ROUTE REFRESH

NH SAFI the next-hop
cost as to any other BGP learned information).

4.5. Graceful Restart and Route-Refresh

BGPLS sessions carrying next-hop cost could use graceful restart Graceful Restart
[RFC4724] and route refresh Route Refresh [RFC7313] mechanisms in the same way as
it's used for IPv4 and IPv6 unicast -
preservation and purge of next-hop cost information follows normal GR
rules. unicast.

5. Security considerations

This document does not introduce new security issues are introduced to the BGP protocol by this
specification. considerations above
and beyond those already specified in [RFC4271], [I-D.ietf-idr-bgp-
orr] and [I-D.ietf-idr-bgp-ls].

6. IANA Considerations

This draft defines a new protocol id value for RIB Protocol ID. This
draft requests IANA is requested to allocate a value for Next-Hop Subsequent Address
Family Identifier. a RIB Protocol ID from
BGPLS Protocol ID Registry.

This draft defines a new RIB Metrics Prefix Descriptor value. This
draft request IANA to allocate a TLV code value for the new
descriptor from the Prefix Descriptor registry.

7. Acknowledgment

Authors Acknowledgements

The authors would like to thank Keyur Patel, acknowledge David Ward, Anton Elita,
Nagendra Kumar and Burjiz Pithawala for their critical reviews and
feedback.

8. References

8.1. Normative References

[I-D.ietf-idr-bgp-optimal-route-reflection]
Raszuk, R., Cassar, C., Aman, E., Decraene, B., and S.
Litkowski, "BGP Optimal Route Reflection (BGP-ORR)",
draft-ietf-idr-bgp-optimal-route-reflection-09 (work in
progress), April 2015.

[I-D.ietf-idr-ls-distribution]
Gredler, H., Medved, J., Previdi, S., Farrel, A., and S.
Ray, "North-Bound Distribution of Link-State and TE
Information using BGP", draft-ietf-idr-ls-distribution-10
(work in progress), January 2015.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.

[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, April 2006.

[RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
January 2007.

[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760, January
2007.

8.2. Informative References

[I-D.raszuk-bgp-optimal-route-reflection]
Raszuk, R., Cassar, C., Aman,

[RFC7313] Patel, K., Chen, E., and B. Decraene, "BGP
Optimal Venkatachalapathy, "Enhanced
Route Reflection (BGP-ORR)",
draft-raszuk-bgp-optimal-route-reflection-01 (work in
progress), March 2011. Refresh Capability for BGP-4", RFC 7313, July 2014.

8.2. Informative References

[RFC2918] Chen, E., "Route Refresh Capability for BGP-4", RFC 2918,
September 2000.

Appendix A. USAGE SCENARIOS

A.1. Trivial case

--+---NetA---+--
| |
r1 r2
| |
R1--RR-----R2
| \ |
| +------R4
R3

In this scenario r1 and r3 along with NetA are part of AS1; and R1-R4
along with RR are in AS2.

If RR implements non-optimized route-reflection, then it will choose
path to NetA via R1 and advertise it to both R3 and R4. Such choice
is good from R3 perspective, but it results in suboptimal traffic
flow from R4 to NetA.

Using NH SAFI the proposed BGPLS extensions, the route-reflector will learn
that cost from R4 to R1 is 8 whereas to R2 it's only 1. RR will
announce NetA to R4 with next-hop set to R2, while its announce to R3
will still have R1 as next-hop. Both R3 and R4 now will send traffic
to NetA via closest exit, achieving same behaviour as if full iBGP
mesh would have been configured.

A.2. Non-IGP based cost

When it's desirable to direct traffic over an exit other than the one
with smallest IGP cost, NH SAFI BGPLS extensions can be used to convey cost
which is not based on IGP. For example, network operator may arrange
exit points in order of administrative preference and configure
routers to send this instead of IGP cost. Route reflector then will
then calculate best path based on administrative preference rather
than IGP metrics.

Network operators should excercise care to ensure that all routers up
to and including exit point do not devert packets on to a different
path, otherwise routing loops may occur. One way to achieve this is
to have consistent administrative preference among all routers.
Another option is to use a tunneling mechanism (e.g. MPLS-TE tunnel)
between source and the exit point, provided that the router serving
as exit point will send packets out of the network rather than
diverting them to another exit point.

A.3. Multiple route-reflectors

This example demonstrates that NH SAFI peerings BGPLS extensions are necessary only
between routers that already exchange other AFI/SAFI.

In the above network the routers R1-R4 are clients of RR1, and R5-R8
are clients of RR2. RR1 and RR2 also peer with each other and use
ADDPATH.

RR2 learns about NetA from R7 and R8. Since it sends not just best-
path but all prefixes to RR1, there is no need for RR2 to learn cost
information from R1 and R2 towards R7 and R8. On the other hand RR1
does exchange NH SAFI cost information using BGPLS with R1 and R2 so that
each of them can receive routes, which are best from their
perspective.

As addition to ADDPATH a mechanism could be devised that would allow
RR2 to learn how many alternative routes does it need to send to RR1.
For example, if NetA would also be connected to R9 (not shown) but
all clients of RR1 prefer R7 as exit point and R9 as next-best, then
there is no need for RR2 to send NetA routes with next-hop R8 to RR1.

Discussion: authors would like to solicit discussion whether there is
sufficient interest in such mechanism.

A.4. Inter-AS MPLS VPN

Previous example could be transposed to Inter-AS MPLS VPN Option C
scenario. In this case route reflectors RR1 and RR2 can be from
different autonomous system. Essentially the behaviour of routers
remains as already described.

A.5. Corner case

--+---NetA--+--
| |
RR---R1 R2
\ /
R3---R4

In the above network cost from R3 to R1 is 10, all other costs are 1.
If RR advertises NetA to R3 based on cost information received from
R3, but uses its own cost when advertising NetA to R4, there will be
a loop formed. This is the reason why section "BGP best path
selection modification" requires RR to have next-hop cost information
for every next-hop and every peer.

Note that the problem is the same as if RR would not use extensions
described in this document and R3 would peer directly with R1 and R2,
while R4 would peer only with RR.

Authors' Addresses

Ilya Varlashkin
Easynet Global Services
Google

Email: ilya.varlashkin@easynet.com ilya@nobulus.com

Robert Raszuk
NTT MCL
Mirantis Inc.
101 S Ellsworth Avenue Suite 350
San Mateo,
615 National Ave. #100
Mt View, CA 94401
US 94043
USA

Email: robert@raszuk.net

Keyur Patel
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95124 95134
USA

Email: keyupate@cisco.com
Manish Bhardwaj
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95124 95134
USA

Email: manbhard@cisco.com

Serpil Bayraktar
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95124 95134
USA

Email: serpil@cisco.com