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Keywords: [---------], IDR, Inter-domain routing, BGP, DDOS, Denial of Service, ACL, Firewall, Filter
Network Working Group P. Marques
Request for Comments: 5575 Cisco Systems
Category: Standards Track N. Sheth
Juniper Networks
R. Raszuk
Cisco Systems
B. Greene
Juniper Networks
J. Mauch
NTT America
D. McPherson
Arbor Networks
August 2009
Dissemination of Flow Specification Rules
Abstract
This document defines a new Border Gateway Protocol Network Layer
Reachability Information (BGP NLRI) encoding format that can be used
to distribute traffic flow specifications. This allows the routing
system to propagate information regarding more specific components of
the traffic aggregate defined by an IP destination prefix.
Additionally, it defines two applications of that encoding format:
one that can be used to automate inter-domain coordination of traffic
filtering, such as what is required in order to mitigate
(distributed) denial-of-service attacks, and a second application to
provide traffic filtering in the context of a BGP/MPLS VPN service.
The information is carried via the BGP, thereby reusing protocol
algorithms, operational experience, and administrative processes such
as inter-provider peering agreements.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Marques, et al. Standards Track [Page 1]
RFC 5575 Flow Specification August 2009
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Table of Contents
1. Introduction ....................................................3
2. Definitions of Terms Used in This Memo ..........................5
3. Flow Specifications .............................................5
4. Dissemination of Information ....................................6
5. Traffic Filtering ..............................................12
5.1. Order of Traffic Filtering Rules ..........................13
6. Validation Procedure ...........................................14
7. Traffic Filtering Actions ......................................15
8. Traffic Filtering in BGP/MPLS VPN Networks .....................17
9. Monitoring .....................................................18
10. Security Considerations .......................................18
11. IANA Considerations ...........................................19
12. Acknowledgments ...............................................20
13. Normative References ..........................................21
Marques, et al. Standards Track [Page 2]
RFC 5575 Flow Specification August 2009
1. Introduction
Modern IP routers contain both the capability to forward traffic
according to IP prefixes as well as to classify, shape, rate limit,
filter, or redirect packets based on administratively defined
policies.
These traffic policy mechanisms allow the router to define match
rules that operate on multiple fields of the packet header. Actions
such as the ones described above can be associated with each rule.
The n-tuple consisting of the matching criteria defines an aggregate
traffic flow specification. The matching criteria can include
elements such as source and destination address prefixes, IP
protocol, and transport protocol port numbers.
This document defines a general procedure to encode flow
specification rules for aggregated traffic flows so that they can be
distributed as a BGP [RFC4271] NLRI. Additionally, we define the
required mechanisms to utilize this definition to the problem of
immediate concern to the authors: intra- and inter-provider
distribution of traffic filtering rules to filter (distributed)
denial-of-service (DoS) attacks.
By expanding routing information with flow specifications, the
routing system can take advantage of the ACL (Access Control List) or
firewall capabilities in the router's forwarding path. Flow
specifications can be seen as more specific routing entries to a
unicast prefix and are expected to depend upon the existing unicast
data information.
A flow specification received from an external autonomous system will
need to be validated against unicast routing before being accepted.
If the aggregate traffic flow defined by the unicast destination
prefix is forwarded to a given BGP peer, then the local system can
safely install more specific flow rules that may result in different
forwarding behavior, as requested by this system.
The key technology components required to address the class of
problems targeted by this document are:
1. Efficient point-to-multipoint distribution of control plane
information.
2. Inter-domain capabilities and routing policy support.
3. Tight integration with unicast routing, for verification
purposes.
Marques, et al. Standards Track [Page 3]
RFC 5575 Flow Specification August 2009
Items 1 and 2 have already been addressed using BGP for other types
of control plane information. Close integration with BGP also makes
it feasible to specify a mechanism to automatically verify flow
information against unicast routing. These factors are behind the
choice of BGP as the carrier of flow specification information.
As with previous extensions to BGP, this specification makes it
possible to add additional information to Internet routers. These
are limited in terms of the maximum number of data elements they can
hold as well as the number of events they are able to process in a
given unit of time. The authors believe that, as with previous
extensions, service providers will be careful to keep information
levels below the maximum capacity of their devices.
It is also expected that, in many initial deployments, flow
specification information will replace existing host length route
advertisements rather than add additional information.
Experience with previous BGP extensions has also shown that the
maximum capacity of BGP speakers has been gradually increased
according to expected loads. Taking into account Internet unicast
routing as well as additional applications as they gain popularity.
From an operational perspective, the utilization of BGP as the
carrier for this information allows a network service provider to
reuse both internal route distribution infrastructure (e.g., route
reflector or confederation design) and existing external
relationships (e.g., inter-domain BGP sessions to a customer
network).
While it is certainly possible to address this problem using other
mechanisms, the authors believe that this solution offers the
substantial advantage of being an incremental addition to already
deployed mechanisms.
In current deployments, the information distributed by the flow-spec
extension is originated both manually as well as automatically. The
latter by systems that are able to detect malicious flows. When
automated systems are used, care should be taken to ensure their
correctness as well as to limit the number and advertisement rate of
flow routes.
This specification defines required protocol extensions to address
most common applications of IPv4 unicast and VPNv4 unicast filtering.
The same mechanism can be reused and new match criteria added to
address similar filtering needs for other BGP address families (for
example, IPv6 unicast). The authors believe that those would be best
to be addressed in a separate document.
Marques, et al. Standards Track [Page 4]
RFC 5575 Flow Specification August 2009
2. Definitions of Terms Used in This Memo
NLRI - Network Layer Reachability Information
RIB - Routing Information Base
Loc-RIB - Local RIB
AS - Autonomous System number
VRF - Virtual Routing and Forwarding instance
PE - Provider Edge router
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. Flow Specifications
A flow specification is an n-tuple consisting of several matching
criteria that can be applied to IP traffic. A given IP packet is
said to match the defined flow if it matches all the specified
criteria.
A given flow may be associated with a set of attributes, depending on
the particular application; such attributes may or may not include
reachability information (i.e., NEXT_HOP). Well-known or AS-specific
community attributes can be used to encode a set of predetermined
actions.
A particular application is identified by a specific (Address Family
Identifier, Subsequent Address Family Identifier (AFI, SAFI)) pair
[RFC4760] and corresponds to a distinct set of RIBs. Those RIBs
should be treated independently from each other in order to assure
non-interference between distinct applications.
BGP itself treats the NLRI as an opaque key to an entry in its
databases. Entries that are placed in the Loc-RIB are then
associated with a given set of semantics, which is application
dependent. This is consistent with existing BGP applications. For
instance, IP unicast routing (AFI=1, SAFI=1) and IP multicast
reverse-path information (AFI=1, SAFI=2) are handled by BGP without
any particular semantics being associated with them until installed
in the Loc-RIB.
Marques, et al. Standards Track [Page 5]
RFC 5575 Flow Specification August 2009
Standard BGP policy mechanisms, such as UPDATE filtering by NLRI
prefix and community matching, SHOULD apply to the newly defined
NLRI-type. Network operators can also control propagation of such
routing updates by enabling or disabling the exchange of a particular
(AFI, SAFI) pair on a given BGP peering session.
4. Dissemination of Information
We define a "Flow Specification" NLRI type that may include several
components such as destination prefix, source prefix, protocol,
ports, etc. This NLRI is treated as an opaque bit string prefix by
BGP. Each bit string identifies a key to a database entry with which
a set of attributes can be associated.
This NLRI information is encoded using MP_REACH_NLRI and
MP_UNREACH_NLRI attributes as defined in RFC 4760 [RFC4760].
Whenever the corresponding application does not require Next-Hop
information, this shall be encoded as a 0-octet length Next Hop in
the MP_REACH_NLRI attribute and ignored on receipt.
The NLRI field of the MP_REACH_NLRI and MP_UNREACH_NLRI is encoded as
a 1- or 2-octet NLRI length field followed by a variable-length NLRI
value. The NLRI length is expressed in octets.
+------------------------------+
| length (0xnn or 0xfn nn) |
+------------------------------+
| NLRI value (variable) |
+------------------------------+
flow-spec NLRI
If the NLRI length value is smaller than 240 (0xf0 hex), the length
field can be encoded as a single octet. Otherwise, it is encoded as
an extended-length 2-octet value in which the most significant nibble
of the first byte is all ones.
In the figure above, values less-than 240 are encoded using two hex
digits (0xnn). Values above 240 are encoded using 3 hex digits
(0xfnnn). The highest value that can be represented with this
encoding is 4095. The value 241 is encoded as 0xf0f1.
The Flow specification NLRI-type consists of several optional
subcomponents. A specific packet is considered to match the flow
specification when it matches the intersection (AND) of all the
components present in the specification.
Marques, et al. Standards Track [Page 6]
RFC 5575 Flow Specification August 2009
The following component types are defined:
Type 1 - Destination Prefix
Encoding: <type (1 octet), prefix length (1 octet), prefix>
Defines the destination prefix to match. Prefixes are encoded
as in BGP UPDATE messages, a length in bits is followed by
enough octets to contain the prefix information.
Type 2 - Source Prefix
Encoding: <type (1 octet), prefix-length (1 octet), prefix>
Defines the source prefix to match.
Type 3 - IP Protocol
Encoding: <type (1 octet), [op, value]+>
Contains a set of {operator, value} pairs that are used to
match the IP protocol value byte in IP packets.
The operator byte is encoded as:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| e | a | len | 0 |lt |gt |eq |
+---+---+---+---+---+---+---+---+
Numeric operator
e - end-of-list bit. Set in the last {op, value} pair in the
list.
a - AND bit. If unset, the previous term is logically ORed
with the current one. If set, the operation is a logical
AND. It should be unset in the first operator byte of a
sequence. The AND operator has higher priority than OR
for the purposes of evaluating logical expressions.
len - The length of the value field for this operand is given
as (1 << len).
lt - less than comparison between data and value.
gt - greater than comparison between data and value.
Marques, et al. Standards Track [Page 7]
RFC 5575 Flow Specification August 2009
eq - equality between data and value.
The bits lt, gt, and eq can be combined to produce "less or
equal", "greater or equal", and inequality values.
Type 4 - Port
Encoding: <type (1 octet), [op, value]+>
Defines a list of {operation, value} pairs that matches source
OR destination TCP/UDP ports. This list is encoded using the
numeric operand format defined above. Values are encoded as 1-
or 2-byte quantities.
Port, source port, and destination port components evaluate to
FALSE if the IP protocol field of the packet has a value other
than TCP or UDP, if the packet is fragmented and this is not
the first fragment, or if the system in unable to locate the
transport header. Different implementations may or may not be
able to decode the transport header in the presence of IP
options or Encapsulating Security Payload (ESP) NULL [RFC4303]
encryption.
Type 5 - Destination port
Encoding: <type (1 octet), [op, value]+>
Defines a list of {operation, value} pairs used to match the
destination port of a TCP or UDP packet. Values are encoded as
1- or 2-byte quantities.
Type 6 - Source port
Encoding: <type (1 octet), [op, value]+>
Defines a list of {operation, value} pairs used to match the
source port of a TCP or UDP packet. Values are encoded as 1-
or 2-byte quantities.
Type 7 - ICMP type
Encoding: <type (1 octet), [op, value]+>
Defines a list of {operation, value} pairs used to match the
type field of an ICMP packet. Values are encoded using a
single byte.
Marques, et al. Standards Track [Page 8]
RFC 5575 Flow Specification August 2009
The ICMP type and code specifiers evaluate to FALSE whenever
the protocol value is not ICMP.
Type 8 - ICMP code
Encoding: <type (1 octet), [op, value]+>
Defines a list of {operation, value} pairs used to match the
code field of an ICMP packet. Values are encoded using a
single byte.
Type 9 - TCP flags
Encoding: <type (1 octet), [op, bitmask]+>
Bitmask values can be encoded as a 1- or 2-byte bitmask. When
a single byte is specified, it matches byte 13 of the TCP
header [RFC0793], which contains bits 8 though 15 of the 4th
32-bit word. When a 2-byte encoding is used, it matches bytes
12 and 13 of the TCP header with the data offset field having a
"don't care" value.
As with port specifiers, this component evaluates to FALSE for
packets that are not TCP packets.
This type uses the bitmask operand format, which differs from
the numeric operator format in the lower nibble.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| e | a | len | 0 | 0 |not| m |
+---+---+---+---+---+---+---+---+
e, a, len - Most significant nibble: (end-of-list bit, AND
bit, and length field), as defined for in the
numeric operator format.
not - NOT bit. If set, logical negation of operation.
m - Match bit. If set, this is a bitwise match operation
defined as "(data & value) == value"; if unset, (data &
value) evaluates to TRUE if any of the bits in the value
mask are set in the data.
Marques, et al. Standards Track [Page 9]
RFC 5575 Flow Specification August 2009
Type 10 - Packet length
Encoding: <type (1 octet), [op, value]+>
Match on the total IP packet length (excluding Layer 2 but
including IP header). Values are encoded using 1- or 2-byte
quantities.
Type 11 - DSCP (Diffserv Code Point)
Encoding: <type (1 octet), [op, value]+>
Defines a list of {operation, value} pairs used to match the
6-bit DSCP field [RFC2474]. Values are encoded using a single
byte, where the two most significant bits are zero and the six
least significant bits contain the DSCP value.
Type 12 - Fragment
Encoding: <type (1 octet), [op, bitmask]+>
Uses bitmask operand format defined above.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Reserved |LF |FF |IsF|DF |
+---+---+---+---+---+---+---+---+
Bitmask values:
+ Bit 7 - Don't fragment (DF)
+ Bit 6 - Is a fragment (IsF)
+ Bit 5 - First fragment (FF)
+ Bit 4 - Last fragment (LF)
Flow specification components must follow strict type ordering. A
given component type may or may not be present in the specification,
but if present, it MUST precede any component of higher numeric type
value.
If a given component type within a prefix in unknown, the prefix in
question cannot be used for traffic filtering purposes by the
receiver. Since a flow specification has the semantics of a logical
AND of all components, if a component is FALSE, by definition it
cannot be applied. However, for the purposes of BGP route
Marques, et al. Standards Track [Page 10]
RFC 5575 Flow Specification August 2009
propagation, this prefix should still be transmitted since BGP route
distribution is independent on NLRI semantics.
The <type, value> encoding is chosen in order to account for future
extensibility.
An example of a flow specification encoding for: "all packets to
10.0.1/24 and TCP port 25".
+------------------+----------+----------+
| destination | proto | port |
+------------------+----------+----------+
| 0x01 18 0a 00 01 | 03 81 06 | 04 81 19 |
+------------------+----------+----------+
Decode for protocol:
+-------+----------+------------------------------+
| Value | | |
+-------+----------+------------------------------+
| 0x03 | type | |
| 0x81 | operator | end-of-list, value size=1, = |
| 0x06 | value | |
+-------+----------+------------------------------+
An example of a flow specification encoding for: "all packets to
10.0.1/24 from 192/8 and port {range [137, 139] or 8080}".
+------------------+----------+-------------------------+
| destination | source | port |
+------------------+----------+-------------------------+
| 0x01 18 0a 01 01 | 02 08 c0 | 04 03 89 45 8b 91 1f 90 |
+------------------+----------+-------------------------+
Decode for port:
+--------+----------+------------------------------+
| Value | | |
+--------+----------+------------------------------+
| 0x04 | type | |
| 0x03 | operator | size=1, >= |
| 0x89 | value | 137 |
| 0x45 | operator | &, value size=1, <= |
| 0x8b | value | 139 |
| 0x91 | operator | end-of-list, value-size=2, = |
| 0x1f90 | value | 8080 |
+--------+----------+------------------------------+
Marques, et al. Standards Track [Page 11]
RFC 5575 Flow Specification August 2009
This constitutes an NLRI with an NLRI length of 16 octets.
Implementations wishing to exchange flow specification rules MUST use
BGP's Capability Advertisement facility to exchange the Multiprotocol
Extension Capability Code (Code 1) as defined in RFC 4760 [RFC4760].
The (AFI, SAFI) pair carried in the Multiprotocol Extension
Capability MUST be the same as the one used to identify a particular
application that uses this NLRI-type.
5. Traffic Filtering
Traffic filtering policies have been traditionally considered to be
relatively static.
The popularity of traffic-based, denial-of-service (DoS) attacks,
which often requires the network operator to be able to use traffic
filters for detection and mitigation, brings with it requirements
that are not fully satisfied by existing tools.
Increasingly, DoS mitigation requires coordination among several
service providers in order to be able to identify traffic source(s)
and because the volumes of traffic may be such that they will
otherwise significantly affect the performance of the network.
Several techniques are currently used to control traffic filtering of
DoS attacks. Among those, one of the most common is to inject
unicast route advertisements corresponding to a destination prefix
being attacked. One variant of this technique marks such route
advertisements with a community that gets translated into a discard
Next-Hop by the receiving router. Other variants attract traffic to
a particular node that serves as a deterministic drop point.
Using unicast routing advertisements to distribute traffic filtering
information has the advantage of using the existing infrastructure
and inter-AS communication channels. This can allow, for instance, a
service provider to accept filtering requests from customers for
address space they own.
There are several drawbacks, however. An issue that is immediately
apparent is the granularity of filtering control: only destination
prefixes may be specified. Another area of concern is the fact that
filtering information is intermingled with routing information.
The mechanism defined in this document is designed to address these
limitations. We use the flow specification NLRI defined above to
convey information about traffic filtering rules for traffic that
should be discarded.
Marques, et al. Standards Track [Page 12]
RFC 5575 Flow Specification August 2009
This mechanism is primarily designed to allow an upstream autonomous
system to perform inbound filtering in their ingress routers of
traffic that a given downstream AS wishes to drop.
In order to achieve this goal, we define an application-specific NLRI
identifier (AFI=1, SAFI=133) along with specific semantic rules.
BGP routing updates containing this identifier use the flow
specification NLRI encoding to convey particular aggregated flows
that require special treatment.
Flow routing information received via this (AFI, SAFI) pair is
subject to the validation procedure detailed below.
5.1. Order of Traffic Filtering Rules
With traffic filtering rules, more than one rule may match a
particular traffic flow. Thus, it is necessary to define the order
at which rules get matched and applied to a particular traffic flow.
This ordering function must be such that it must not depend on the
arrival order of the flow specification's rules and must be constant
in the network.
The relative order of two flow specification rules is determined by
comparing their respective components. The algorithm starts by
comparing the left-most components of the rules. If the types
differ, the rule with lowest numeric type value has higher precedence
(and thus will match before) than the rule that doesn't contain that
component type. If the component types are the same, then a type-
specific comparison is performed.
For IP prefix values (IP destination and source prefix) precedence is
given to the lowest IP value of the common prefix length; if the
common prefix is equal, then the most specific prefix has precedence.
For all other component types, unless otherwise specified, the
comparison is performed by comparing the component data as a binary
string using the memcmp() function as defined by the ISO C standard.
For strings of different lengths, the common prefix is compared. If
equal, the longest string is considered to have higher precedence
than the shorter one.
Marques, et al. Standards Track [Page 13]
RFC 5575 Flow Specification August 2009
Pseudocode:
flow_rule_cmp (a, b)
{
comp1 = next_component(a);
comp2 = next_component(b);
while (comp1 || comp2) {
// component_type returns infinity on end-of-list
if (component_type(comp1) < component_type(comp2)) {
return A_HAS_PRECEDENCE;
}
if (component_type(comp1) > component_type(comp2)) {
return B_HAS_PRECEDENCE;
}
if (component_type(comp1) == IP_DESTINATION || IP_SOURCE) {
common = MIN(prefix_length(comp1), prefix_length(comp2));
cmp = prefix_compare(comp1, comp2, common);
// not equal, lowest value has precedence
// equal, longest match has precedence
} else {
common =
MIN(component_length(comp1), component_length(comp2));
cmp = memcmp(data(comp1), data(comp2), common);
// not equal, lowest value has precedence
// equal, longest string has precedence
}
}
return EQUAL;
}
6. Validation Procedure
Flow specifications received from a BGP peer and that are accepted in
the respective Adj-RIB-In are used as input to the route selection
process. Although the forwarding attributes of two routes for the
same flow specification prefix may be the same, BGP is still required
to perform its path selection algorithm in order to select the
correct set of attributes to advertise.
The first step of the BGP Route Selection procedure (Section 9.1.2 of
[RFC4271]) is to exclude from the selection procedure routes that are
considered non-feasible. In the context of IP routing information,
this step is used to validate that the NEXT_HOP attribute of a given
route is resolvable.
Marques, et al. Standards Track [Page 14]
RFC 5575 Flow Specification August 2009
The concept can be extended, in the case of flow specification NLRI,
to allow other validation procedures.
A flow specification NLRI must be validated such that it is
considered feasible if and only if:
a) The originator of the flow specification matches the originator of
the best-match unicast route for the destination prefix embedded
in the flow specification.
b) There are no more specific unicast routes, when compared with the
flow destination prefix, that have been received from a different
neighboring AS than the best-match unicast route, which has been
determined in step a).
By originator of a BGP route, we mean either the BGP originator path
attribute, as used by route reflection, or the transport address of
the BGP peer, if this path attribute is not present.
The underlying concept is that the neighboring AS that advertises the
best unicast route for a destination is allowed to advertise flow-
spec information that conveys a more or equally specific destination
prefix. Thus, as long as there are no more specific unicast routes,
received from a different neighboring AS, which would be affected by
that filtering rule.
The neighboring AS is the immediate destination of the traffic
described by the flow specification. If it requests these flows to
be dropped, that request can be honored without concern that it
represents a denial of service in itself. Supposedly, the traffic is
being dropped by the downstream autonomous system, and there is no
added value in carrying the traffic to it.
BGP implementations MUST also enforce that the AS_PATH attribute of a
route received via the External Border Gateway Protocol (eBGP)
contains the neighboring AS in the left-most position of the AS_PATH
attribute. While this rule is optional in the BGP specification, it
becomes necessary to enforce it for security reasons.
7. Traffic Filtering Actions
This specification defines a minimum set of filtering actions that it
standardizes as BGP extended community values [RFC4360]. This is not
meant to be an inclusive list of all the possible actions, but only a
subset that can be interpreted consistently across the network.
Marques, et al. Standards Track [Page 15]
RFC 5575 Flow Specification August 2009
Implementations should provide mechanisms that map an arbitrary BGP
community value (normal or extended) to filtering actions that
require different mappings in different systems in the network. For
instance, providing packets with a worse-than-best-effort, per-hop
behavior is a functionality that is likely to be implemented
differently in different systems and for which no standard behavior
is currently known. Rather than attempting to define it here, this
can be accomplished by mapping a user-defined community value to
platform-/network-specific behavior via user configuration.
The default action for a traffic filtering flow specification is to
accept IP traffic that matches that particular rule.
The following extended community values can be used to specify
particular actions.
+--------+--------------------+--------------------------+
| type | extended community | encoding |
+--------+--------------------+--------------------------+
| 0x8006 | traffic-rate | 2-byte as#, 4-byte float |
| 0x8007 | traffic-action | bitmask |
| 0x8008 | redirect | 6-byte Route Target |
| 0x8009 | traffic-marking | DSCP value |
+--------+--------------------+--------------------------+
Traffic-rate: The traffic-rate extended community is a non-
transitive extended community across the autonomous-system
boundary and uses following extended community encoding:
The first two octets carry the 2-octet id, which can be
assigned from a 2-byte AS number. When a 4-byte AS number is
locally present, the 2 least significant bytes of such an AS
number can be used. This value is purely informational and
should not be interpreted by the implementation.
The remaining 4 octets carry the rate information in IEEE
floating point [IEEE.754.1985] format, units being bytes per
second. A traffic-rate of 0 should result on all traffic for
the particular flow to be discarded.
Traffic-action: The traffic-action extended community consists of 6
bytes of which only the 2 least significant bits of the 6th byte
(from left to right) are currently defined.
40 41 42 43 44 45 46 47
+---+---+---+---+---+---+---+---+
| reserved | S | T |
+---+---+---+---+---+---+---+---+
Marques, et al. Standards Track [Page 16]
RFC 5575 Flow Specification August 2009
* Terminal Action (bit 47): When this bit is set, the traffic
filtering engine will apply any subsequent filtering rules (as
defined by the ordering procedure). If not set, the evaluation
of the traffic filter stops when this rule is applied.
* Sample (bit 46): Enables traffic sampling and logging for this
flow specification.
Redirect: The redirect extended community allows the traffic to be
redirected to a VRF routing instance that lists the specified
route-target in its import policy. If several local instances
match this criteria, the choice between them is a local matter
(for example, the instance with the lowest Route Distinguisher
value can be elected). This extended community uses the same
encoding as the Route Target extended community [RFC4360].
Traffic Marking: The traffic marking extended community instructs a
system to modify the DSCP bits of a transiting IP packet to the
corresponding value. This extended community is encoded as a
sequence of 5 zero bytes followed by the DSCP value encoded in the
6 least significant bits of 6th byte.
8. Traffic Filtering in BGP/MPLS VPN Networks
Provider-based Layer 3 VPN networks, such as the ones using a BGP/
MPLS IP VPN [RFC4364] control plane, have different traffic filtering
requirements than Internet service providers.
In these environments, the VPN customer network often has traffic
filtering capabilities towards their external network connections
(e.g., firewall facing public network connection). Less common is
the presence of traffic filtering capabilities between different VPN
attachment sites. In an any-to-any connectivity model, which is the
default, this means that site-to-site traffic is unfiltered.
In circumstances where a security threat does get propagated inside
the VPN customer network, there may not be readily available
mechanisms to provide mitigation via traffic filter.
This document proposes an additional BGP NLRI type (AFI=1, SAFI=134)
value, which can be used to propagate traffic filtering information
in a BGP/MPLS VPN environment.
The NLRI format for this address family consists of a fixed-length
Route Distinguisher field (8 bytes) followed by a flow specification,
following the encoding defined in this document. The NLRI length
field shall include both the 8 bytes of the Route Distinguisher as
well as the subsequent flow specification.
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RFC 5575 Flow Specification August 2009
Propagation of this NLRI is controlled by matching Route Target
extended communities associated with the BGP path advertisement with
the VRF import policy, using the same mechanism as described in "BGP/
MPLS IP VPNs" [RFC4364] .
Flow specification rules received via this NLRI apply only to traffic
that belongs to the VRF(s) in which it is imported. By default,
traffic received from a remote PE is switched via an MPLS forwarding
decision and is not subject to filtering.
Contrary to the behavior specified for the non-VPN NLRI, flow rules
are accepted by default, when received from remote PE routers.
9. Monitoring
Traffic filtering applications require monitoring and traffic
statistics facilities. While this is an implementation-specific
choice, implementations SHOULD provide:
o A mechanism to log the packet header of filtered traffic.
o A mechanism to count the number of matches for a given flow
specification rule.
10. Security Considerations
Inter-provider routing is based on a web of trust. Neighboring
autonomous systems are trusted to advertise valid reachability
information. If this trust model is violated, a neighboring
autonomous system may cause a denial-of-service attack by advertising
reachability information for a given prefix for which it does not
provide service.
As long as traffic filtering rules are restricted to match the
corresponding unicast routing paths for the relevant prefixes, the
security characteristics of this proposal are equivalent to the
existing security properties of BGP unicast routing.
Where it is not the case, this would open the door to further denial-
of-service attacks.
Enabling firewall-like capabilities in routers without centralized
management could make certain failures harder to diagnose. For
example, it is possible to allow TCP packets to pass between a pair
of addresses but not ICMP packets. It is also possible to permit
packets smaller than 900 or greater than 1000 bytes to pass between a
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RFC 5575 Flow Specification August 2009
pair of addresses, but not packets whose length is in the range 900-
1000. Such behavior may be confusing and these capabilities should
be used with care whether manually configured or coordinated through
the protocol extensions described in this document.
11. IANA Considerations
A flow specification consists of a sequence of flow components, which
are identified by a an 8-bit component type. Types must be assigned
and interpreted uniquely. The current specification defines types 1
though 12, with the value 0 being reserved.
For the purpose of this work, IANA has allocated values for two
SAFIs: SAFI 133 for IPv4 dissemination of flow specification rules
and SAFI 134 for VPNv4 dissemination of flow specification rules.
The following traffic filtering flow specification rules have been
allocated by IANA from the "BGP Extended Communities Type -
Experimental Use" registry as follows:
0x8006 - Flow spec traffic-rate
0x8007 - Flow spec traffic-action
0x8008 - Flow spec redirect
0x8009 - Flow spec traffic-remarking
IANA created and maintains a new registry entitled: "Flow Spec
Component Types". The following component types have been
registered:
Type 1 - Destination Prefix
Type 2 - Source Prefix
Type 3 - IP Protocol
Type 4 - Port
Type 5 - Destination port
Type 6 - Source port
Type 7 - ICMP type
Type 8 - ICMP code
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RFC 5575 Flow Specification August 2009
Type 9 - TCP flags
Type 10 - Packet length
Type 11 - DSCP
Type 12 - Fragment
In order to manage the limited number space and accommodate several
usages, the following policies defined by RFC 5226 [RFC5226] are
used:
+--------------+-------------------------------+
| Range | Policy |
+--------------+-------------------------------+
| 0 | Invalid value |
| [1 .. 12] | Defined by this specification |
| [13 .. 127] | Specification Required |
| [128 .. 255] | First Come First Served |
+--------------+-------------------------------+
The specification of a particular "flow component type" must clearly
identify what the criteria used to match packets forwarded by the
router is. This criteria should be meaningful across router hops and
not depend on values that change hop-by-hop such as TTL or Layer 2
encapsulation.
The "traffic-action" extended community defined in this document has
46 unused bits, which can be used to convey additional meaning. IANA
created and maintains a new registry entitled: "Traffic Action
Fields". These values should be assigned via IETF Review rules only.
The following traffic-action fields have been allocated:
47 Terminal Action
46 Sample
0-45 Unassigned
12. Acknowledgments
The authors would like to thank Yakov Rekhter, Dennis Ferguson, Chris
Morrow, Charlie Kaufman, and David Smith for their comments.
Chaitanya Kodeboyina helped design the flow validation procedure.
Steven Lin and Jim Washburn ironed out all the details necessary to
produce a working implementation.
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RFC 5575 Flow Specification August 2009
13. Normative References
[IEEE.754.1985] Institute of Electrical and Electronics Engineers,
"Standard for Binary Floating-Point Arithmetic",
IEEE Standard 754, August 1985.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP
Extended Communities Attribute", RFC 4360,
February 2006.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual
Private Networks (VPNs)", RFC 4364, February 2006.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
January 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in RFCs",
BCP 26, RFC 5226, May 2008.
Marques, et al. Standards Track [Page 21]
RFC 5575 Flow Specification August 2009
Authors' Addresses
Pedro Marques
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
US
EMail: roque@cisco.com
Nischal Sheth
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
EMail: nsheth@juniper.net
Robert Raszuk
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
US
EMail: raszuk@cisco.com
Barry Greene
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
EMail: bgreene@juniper.net
Jared Mauch
NTT America
101 Park Ave
41st Floor
New York, NY 10178
US
EMail: jmauch@us.ntt.net
Danny McPherson
Arbor Networks
EMail: danny@arbor.net
Marques, et al. Standards Track [Page 22]