Internet DRAFT - draft-idr-bgp-route-refresh-options
draft-idr-bgp-route-refresh-options
Network Working Group K. Patel
Internet-Draft Arrcus, Inc
Intended status: Standards Track A. Vyavaharkar
Expires: March 2, 2019 Cisco Systems
N. Fazlollahi
Unaffiliated
A. Przygienda
Juniper Networks
August 29, 2018
Extension to BGP's Route Refresh Message
draft-idr-bgp-route-refresh-options-05
Abstract
[RFC2918] defines a route refresh capability to be exchanged between
BGP speakers. BGP speakers that support this capability are
advertising that they can resend the entire BGP Adj-RIB-Out on
receipt of a refresh request. By supporting this capability, BGP
speakers are more flexible in applying any inbound routing policy
changes as they no longer have to store received routes in their
unchanged form or reset the session when an inbound routing policy
change occurs. The route refresh capability is advertised per AFI,
SAFI combination.
There are newer AFI, SAFI types that have been introduced to BGP that
support a variety of route types (e.g. IPv4/MVPN, L2VPN/EVPN).
Currently, there is no way to request a subset of routes in a Route
Refresh message for a given AFI, SAFI. This draft defines route
refresh capability extensions that help BGP speakers to request a
subset of routes for a given address family. This is expected to
reduce the amount of update traffic being generated by route refresh
requests as well as lessen the burden on the router servicing such
requests.
Status of 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 https://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
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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 March 2, 2019.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Use Case Examples . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Route Refresh Options Capability . . . . . . . . . . . . . . 4
4. Route Refresh Sub-Types . . . . . . . . . . . . . . . . . . . 5
5. Route Refresh Option format . . . . . . . . . . . . . . . . . 5
6. Route Refresh Option Length . . . . . . . . . . . . . . . . . 6
7. Route Refresh ID . . . . . . . . . . . . . . . . . . . . . . 6
8. Route Refresh Option Flags . . . . . . . . . . . . . . . . . 7
9. Route Refresh Options . . . . . . . . . . . . . . . . . . . . 8
10. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 11
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
13. Security Considerations . . . . . . . . . . . . . . . . . . . 12
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14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
15.1. Normative References . . . . . . . . . . . . . . . . . . 12
15.2. Information References . . . . . . . . . . . . . . . . . 13
Appendix A. Sequence Number Binary Arithmetic . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
[RFC2918] defines a route refresh capability to be exchanged between
BGP speakers. BGP speakers that support this capability are
advertising that they can resend the entire BGP Adj-RIB-Out on
receipt of a refresh request. By supporting this capability, BGP
speakers are more flexible in applying inbound routing policy changes
as they no longer have to store copies of received routes in their
unchanged form or reset the session when an inbound routing policy
change occurs. The route refresh capability is advertised per AFI,
SAFI combination.
Route refresh allows routers to dynamically request a full Adj-RIB-
Out update from their peers when there's an inbound routing policy
change. This is useful because routers that mutually support this
capability no longer have to flap the peering session or store an
extra copy of received routes in their original form. This helps by
reducing memory requirements as well as eliminating the unnecessary
churn caused by session flaps. [RFC2918] does not define a way for
routers to request a subset of the Adj-RIB-Out for a given AFI, SAFI.
This draft defines new extensions to route refresh that will allow
requesting routers to ask for a subset of the Adj-RIB-Out for a given
AFI, SAFI combination. For example, routers could ask for specific
route types from those address families that support multiple route
types or, they could ask for a specific prefix.
As part of the new extensions, this draft combines elements of
[RFC7313] and [RFC5291] and adds a new set of options to the route
refresh message that will specify filters that can be applied to
limit the scope of the refresh being requested. The new option
format will apply to all new option types that may be defined moving
forward.
1.1. Use Case Examples
The authors acknowledge that while the extensions being proposed in
this draft could potentially be addressed by Route Target Constrain
described in [RFC4684] by using route targets to identify desired
subset of routes, this proposal includes address families where RT
Constrain extension is not supported and avoids the necessity to
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assign and manage the route targets per desired set of routes. The
approach in this draft is intended to be a single-hop refresh only,
i.e., propagation of the refreshes in a way similar to RT Constrain
routes is NOT intended.
Several possible use cases are discernible today:
o The capacity to refresh routes of a certain type within an address
family is needed, e.g., auto discovery routes within the EVPN AF
[RFC7432].
o In VPN scenarios where RT Constrain is not supported or
configured, RDs can be used.
o In BGP LS [RFC7752] cases a speaker may choose to hold only a
subset of routes and depending on configuration request a subset
of routes. This document could provide further filters to support
those use cases.
o On changes in inbound policy, when previously configured filters
have been removed, only the according subset of routes may be
requested.
2. Requirements Language
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 RFC 2119 [RFC2119].
3. Route Refresh Options Capability
A BGP speaker will use the BGP Capabilities Advertisement [RFC5492]
to advertise the Route Refresh Options Capability to its peers. This
new capability will be advertised using the Capability code [TBD]
with a capability length of 0.
By advertising the Route Refresh Options Capability to a peer, a BGP
speaker indicates that it is capable of receiving and processing the
route refresh options described below. This new capability can be
advertised along with the Enhanced Route Refresh Capability described
in [RFC7313]. However, if the Route Refresh Options Capability has
been negotiated by both sides of the BGP session, then it will
override the Enhanced Route Refresh Capability.
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4. Route Refresh Sub-Types
[RFC7313] defines route refresh BGP message sub-types that utilize
the "Reserved" field of the Route Refresh message originally defined
in [RFC2918]. Currently, there are three sub-types defined and this
draft proposes three additional sub-types which will be used to
indicate a Route Refresh message that includes options before any ORF
field of the Route Refresh message as well as BoRR and EoRR Route
Refresh messages with options.
0 - Normal route refresh request [RFC2918]
with/without Outbound Route Filtering (ORF) [RFC5291]
1 - Demarcation of the beginning of a route refresh
(BoRR) operation
2 - Demarcation of the ending of a route refresh
(EoRR) operation
+ 3 - Route Refresh request with options and optional
ORF [RFC5291]
+ 4 - BoRR with options
+ 5 - EoRR with options
255 - Reserved
When the Route Refresh Options Capability has been negotiated by both
sides of a BGP session, both peers MUST use message types 3, 4 and 5.
The requesting speaker MUST use the refresh ID for all refresh
requests including those without any options, i.e., requests for the
full BGP Adj-RIB-Out.
The Route Refresh Request Message with options will now be formatted
as shown below
0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| A F I | Res. | S A F I |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Total Option Length | Refresh ID# | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| One or more Route Refresh Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5. Route Refresh Option format
[RFC2918] defines the route refresh BGP message that includes only
the AFI, SAFI of the routes being requested. This draft proposes
extending the basic message by including options that will indicate
to the remote BGP speaker that a subset of the entire Adj-RIB-Out is
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being requested. The remote BGP speaker will select routes that
match the specified options and the flag settings.
As described in the previous section, the options will be added to
the Route Refresh message before the ORF field of the message.
Outbound Route Filtering is described in [RFC5291]. The options will
assume the following format
0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length Of Options Field | Refresh ID# | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| One or more Route Refresh Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6. Route Refresh Option Length
The Option Length field will occupy the two octets immediately
following the Route Refresh message containing the AFI, SAFI and sub-
type. The purpose of this field is to allow the BGP speaker to
calculate the length of any attached ORF fields by subtracting the
Option Length from the Route Refresh message length.
7. Route Refresh ID
The Refresh ID field will occupy twelve bits following the Route
Refresh Options Length. It is infeasible to use a wide number like a
64-bit unsigned integer since this number must be stored per route
entry associated with any peer supporting this feature, at least
during the time any refresh is pending. It is a value assigned by
the requesting BGP speaker. It MUST be a strictly monotonically
increasing number per peer AFI and SAFI using sequence number
arithmetic based on two-complements given in Appendix A. It is
comparable to the calculations standardized in [RFC1982] but fixes
several of its anomalies. The purpose of this field is to allow the
requesting BGP speaker to correlate concurrent, overlapping refresh
requests and ultimately delete correct stale routes. The Refresh ID
MUST be reflected in the BoRR and EoRR messages sent by the BGP
speaker servicing the refresh request.
A Refresh ID value MUST NOT be reused until an EoRR with this ID has
been received by the requesting speaker or the last resort time has
expired. The behavior is unspecified otherwise. More specifically,
defining the interval [ LID, HID ] by the values
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LID = MAX(lowest requested Refresh ID# without BoRR,
lowest received BoRR without EoRR)
and
HID = highest requested Refresh ID#
the requesting speaker MUST only use values V where V >: LID and V >:
HID as defined by the relation given in Appendix A. Beside that, HID
=>: LID MUST hold by the same algebra.
If no such number V exists, LID must catch up to HID, i.e. no further
requests can be issued. To use a 3 bit example in Appendix A, if LID
was 1 and HID was 4, we cannot progress to unsigned 5 since 1 ? 5.
When LID progresses to unsigned 2 however, we have 5 >: 2 and 5 >: 4
and we can choose a V.
Value of 0 MUST NEVER be used as Refresh ID and is considered an
"invalid" ID.
The sending speaker MUST NOT reorder the BoRR messages on sending in
case it received multiple requests, i.e., the BoRRs MUST follow in
the same sequence as the requested Route Refresh IDs.
8. Route Refresh Option Flags
This draft defines several route refresh option flags:
o 'O'-bit specifies whether the receiving BGP speaker MUST logically
OR the attached options or logically AND them (in case of the bit
being clear). When the flag is clear, the router on the receiving
end SHOULD logically AND the options and only refresh routes that
match all received options. If the option flag is set, the router
SHOULD select routes that match using a logical OR of the options.
In any case the set of routes sent between the according BoRR and
EoRR MUST contain at least the logically requested set.
o 'C' bit indicates that the receiving BGP speaker MUST clear
immediately all the received Route Refresh Requests with Options,
either pending or being processed. EoRRs MUST NOT be sent. The
Refresh ID# on the request MUST be set as the (in unsigned terms)
next possible number L for which LID >: L and HID >: L per
Appendix A or in other words we "wrap around the sequence number
space" on reset. The C flag MUST NOT be set on BoRR or EoRR
messages and CAN be used only with refresh requests.
o by 'S' bit indicate a refresh is being spontaneously originated by
the BGP speaker which received requests and has them pending. The
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receiving BGP speaker MUST immediately clear all their pending
Route Refresh requests with the sending peer. The Refresh ID# on
the request MUST be set as the the largest unsigned number L for
which LID >: L and HID >: L. When this flag is set, the receiving
BGP speaker MUST use this sequence number for its next request.
To use example from Appendix A, if the peer received LID 4 and HID
5 (i.e. it didn't send BoRR for 4 yet but received request for 5
already) it will reset the sequence number to 1 by those rules.
Now, if there is a request with 6 in flight, it will be seen as 1
>: 6 when arriving.
The precise format is indicated below
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| .... |C|O|S|R|
+-+-+-+-+-+-+-+-+
C Clear pending requests and reset Refresh ID# space.
O Use logical OR of attached options
S Synchronize sequence numbers
R Reserved bit
9. Route Refresh Options
This draft introduces new options carried within the Route Refresh
message as shown in the following figure
0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value (cont'd). |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The option Type is a 1 octet field that uniquely identifies
individual options. The Length is a 2 octet field that contains the
length of the option Value field in octets. The option Value is a
variable length field that is interpreted according to the value of
the option Type field.
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The following types are being defined in this draft and additional
types can be defined subsequently as needed
+ 1 - Route Type
+ 2 - NLRI Prefix
+ 3 - Route Distinguisher Prefix
The Route Type option would specify a particular route type that is
being requested. This option applies specifically to those AFI/SAFI
combinations that support multiple route types, e.g. L2VPN/EVPN and
MUST be otherwise ignored. The value field would be the route type
specifying which route type was being requested. The length of the
option depends on the AFI/SAFI.
The NLRI Prefix option would specify a request for all matching
address prefixes with their lengths equal to or greater than the
specified prefix per AFI/SAFI definitions. The value field would
contain the address prefix according to the NLRI specification of the
AFI/SAFI contained in the Route Refresh message. For those AFI/SAFI
combinations that specify NLRIs containing a type and/or RD, the
value field MUST exclude the type and RD and SHOULD only include any
remaining NLRI fields. If the requesting speaker expects its peer to
also match the type and/or RD, the speaker CAN include the type and
RD prefix options accordingly. The length field would contain the
length of the value field in bits.
The Route Distinguisher prefix option would specify an RD prefix that
is being requested for AFs that support it. The receiving BGP
speaker would then refresh all routes in the specified AFI/SAFI that
matched the requested RDs. The Value field would contain the RD, its
length and the mask length of the RD prefix. This option applies
specifically to those AFI/SAFI combinations that support route
distinguishers and MUST be otherwise ignored.
10. Operation
A BGP speaker that understands and supports Route Refresh Options
SHOULD advertise the Route Refresh Options Capability in its Open
message. The following procedures for route refresh are only
applicable if the BGP speaker originating the route refresh has
received the route refresh options capability and supports it.
When originating a Route Refresh message, a BGP speaker SHOULD use
and set these options if it wants to restrict the scope of updates
being refreshed. The specific options being sent will be set
according to the operator's command.
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When a BGP speaker receives a route refresh message that includes any
options, it MUST parse the options and strongly SHOULD use them to
filter outgoing NLRIs when refreshing the Adj-RIB-Out to the
requesting BGP speaker.
If a BGP speaker receives the route refresh message with the message
subtype set to BoRR with options as described above, then it needs to
process all the included options and MUST mark all matching routes as
stale as described in [RFC7313].
If a BGP speaker receives the route refresh message with the message
subtype set to EoRR with options as described above, then it needs to
process all the included options and delete any remaining stale
routes that match the options received with the EoRR as described in
[RFC7313].
A BGP speaker responding to a route refresh request MUST set the
message subtypes of the BoRR and EoRR messages so that each BoRR
message has a matching EoRR message. This means a BoRR message
without options SHOULD only be followed eventually by an EoRR message
without options. Similarly, a BoRR message with options MUST
eventually be followed by an EoRR message with the same options. If
BoRR and EoRR message options do not match, the outcome is
unpredictable as remaining staled routes pending a refresh may get
inadvertently deleted. BGP speakers MUST NOT summarize EoRR messages
by combining options in order to allow the requesting BGP speaker to
uniquely identify the included sets of routes when concurrent
refreshes are originated with overlapping sets of routes.
Observe that overlapping refreshes with different options are
possible and in such case the according BoRR and EoRR messages are
associated by using their Refresh ID#. The BGP speaker responding to
the route refresh requests MAY perform the refreshes in parallel. In
case of concurrent refreshes overlapping same routes, the responding
speaker MUST ensure that the sent advertisements will result in
deletion of the omitted routes at the time all EoRRs have been
received by the remote speaker or it MUST explicitly advertise
withdrawals to correct any anomalies.
The BGP speaker requesting a refresh from its peers SHOULD maintain a
locally configurable upper bound on how long it will keep matching
stale routes once a BoRR has been received. Each subsequent BoRR
SHOULD reset this period so that any remaining stale routes are only
flushed after the last BoRR has been received in case there are
multiple back-to-back refreshes being sent out and the last matching
EoRR is never received or arrives too late. This is an
implementation specific detail.
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A BGP speaker may spontaneously originate a refresh to one or more of
its peers depending on operator intervention, or due to a policy or
configuration change, etc. In such a case, the speaker MUST refresh
the entire Adj-RIB-Out. The speaker MUST also send BoRR/EoRR with the
options field with the 'S' flag set and a sequence number which lies
outside the range of the sequence numbers that are currently in use
with the receiving BGP speaker.
11. Error Handling
The handling of malformed options MUST follow the procedures
mentioned in [RFC7606]. This draft obsoletes some of the error
handling procedures in [RFC7313] if the Route Refresh Options
Capability is sent. In addition, this draft mandates the following
behavior at the receiver of the route refresh request upon detection
of:
Length errors - If the message length minus the fixed-size message
header is less than 4, the procedure in [RFC7313] MUST be followed.
Also, if the overall length of all the options or any individual
option length exceeds the total number of remaining bytes, the same
procedure MUST be followed.
Option type errors - Any unknown option type CAN be ignored for
AND'ed options. In case of OR'ed options the receiving speaker MUST
ignore all the options and de-facto treat it as a full AFI/SAFI Adj-
RIB-Out refresh. Such event SHOULD be logged in either case to
notify the operator.
Option value errors - Length errors which cannot be distinguished
from value field errors at the receiver are treated the same as value
errors. The receiver MUST send a NOTIFICATION message with the Error
Code "ROUTE-REFRESH Message Error" and the subcode of Invalid Message
Length to the peer. The Data field of the NOTIFICATION message MUST
contain the complete ROUTE-REFRESH message.
BoRR with "unknown" or "invalid" Refresh ID# - The receiver MUST
discard all pending requests and issue a Route Refresh Request with
Options. The options MUST be empty and the clear flag MUST be set to
resynchronize the RIBs. "Unknown" means here a BoRR which is not in
the interval
[ MAX(lowest requested Refresh ID# without BoRR,
highest received BoRR+1 respecting sequence number arithmetic),
highest requested Refresh ID# ]
EoRR with unknown Refresh ID# - Those SHOULD be ignored and a warning
or error MUST be logged.
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BoRR or EoRR with incorrect options - analogous to BoRR with unknown
Refresh ID#.
EoRR with known Refresh ID# but without preceding BoRR - analogous to
EoRR with unknown Refresh ID#. Observe that this can be caused by the
peer expiring last resort timer and reusing the ID# for another
request before the EoRR is received. This should be extremely
unlikely given the size of the refresh ID space.
12. IANA Considerations
This draft defines a new route refresh options format for BGP Route
Refresh messages.
This draft defines a new route refresh capability for BGP Route
Refresh messages. We request IANA to record this capability to
create a new registry under BGP Capability Codes as follows:
+74 Route Refresh Options Capability
This draft defines 3 new route refresh message subtypes for BGP Route
Refresh messages. We request IANA to record these subtypes to create
a new registry under BGP Route Refresh Subcodes as follows:
+ 3 - Route Refresh with options
+ 4 - BoRR with options
+ 5 - EoRR with options
13. Security Considerations
This extension to BGP does not change the underlying security issues
inherent in the existing [RFC7313] and [RFC4271].
14. Acknowledgements
The authors would like to thank Anant Utgikar for initial discussions
resulting in this work. John Scudder and Jeff Hass provided further
comments.
15. References
15.1. Normative References
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
DOI 10.17487/RFC1982, August 1996,
<https://www.rfc-editor.org/info/rfc1982>.
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[RFC2918] Chen, E., "Route Refresh Capability for BGP-4", RFC 2918,
DOI 10.17487/RFC2918, September 2000,
<https://www.rfc-editor.org/info/rfc2918>.
[RFC4684] Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
R., Patel, K., and J. Guichard, "Constrained Route
Distribution for Border Gateway Protocol/MultiProtocol
Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC4684,
November 2006, <https://www.rfc-editor.org/info/rfc4684>.
[RFC5291] Chen, E. and Y. Rekhter, "Outbound Route Filtering
Capability for BGP-4", RFC 5291, DOI 10.17487/RFC5291,
August 2008, <https://www.rfc-editor.org/info/rfc5291>.
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <https://www.rfc-editor.org/info/rfc5492>.
[RFC7313] Patel, K., Chen, E., and B. Venkatachalapathy, "Enhanced
Route Refresh Capability for BGP-4", RFC 7313,
DOI 10.17487/RFC7313, July 2014,
<https://www.rfc-editor.org/info/rfc7313>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
15.2. Information References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/info/rfc7432>.
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[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[Wikipedia]
Wikipedia,
"https://en.wikipedia.org/wiki/Serial_number_arithmetic",
2016.
Appendix A. Sequence Number Binary Arithmetic
The only reasonably reference to a cleaner than [RFC1982] sequence
number solution is given in [Wikipedia]. It basically converts the
problem into two complement's arithmetic. Assuming a straight two
complement's substractions on the bit-width of the sequence number
the according >: and =: relations are defined as:
U_1, U_2 are 12-bits aligned unsigned version number
D_f is ( U_1 - U_2 ) interpreted as two complement signed 12-bits
D_b is ( U_2 - U_1 ) interpreted as two complement signed 12-bits
U_1 >: U_2 IIF D_f > 0 AND D_b < 0
U_1 =: U_2 IIF D_f = 0
The >: relationsship is symmetric but not transitive. Observe that
this leaves the case of the numbers having maximum two complement
distance, e.g. ( 0 and 0x800 ) undefined in our 12-bits case since
D_f and D_b are both -0x7ff.
A simple example of the relationship in case of 3-bit arithmetic
follows as table indicating D_f/D_b values and then the relationship
of U_1 to U_2:
U2 / U1 0 1 2 3 4 5 6 7
0 +/+ +/- +/- +/- -/- -/+ -/+ -/+
1 -/+ +/+ +/- +/- +/- -/- -/+ -/+
2 -/+ -/+ +/+ +/- +/- +/- -/- -/+
3 -/+ -/+ -/+ +/+ +/- +/- +/- -/-
4 -/- -/+ -/+ -/+ +/+ +/- +/- +/-
5 +/- -/- -/+ -/+ -/+ +/+ +/- +/-
6 +/- +/- -/- -/+ -/+ -/+ +/+ +/-
7 +/- +/- +/- -/- -/+ -/+ -/+ +/+
Patel, et al. Expires March 2, 2019 [Page 14]
Internet-Draft Extension to BGP's Route Refresh Message August 2018
U2 / U1 0 1 2 3 4 5 6 7
0 = > > > ? < < <
1 < = > > > ? < <
2 < < = > > > ? <
3 < < < = > > > ?
4 ? < < < = > > >
5 > ? < < < = > >
6 > > ? < < < = >
7 > > > ? < < < =
Authors' Addresses
Keyur Patel
Arrcus, Inc
USA
Email: keyur@arrcus.com
Aamod Vyavaharkar
Cisco Systems
821 Alder Drive
Milpitas, CA 95035
USA
Email: avyavaha@cisco.com
Niloofar Fazlollahi
Unaffiliated
USA
Email: Niloofar_fazlollahi@yahoo.com
Tony Przygienda
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
USA
Email: prz@juniper.net
Patel, et al. Expires March 2, 2019 [Page 15]