Network Working Group Pedro Marques
Internet Draft Nischal Sheth
Expiration Date: June 16, 2005 Juniper Networks
Robert Raszuk
Jared Mauch Barry Greene
NTT/Verio Cisco Systems Inc.
Danny McPerson
Arbor Networks
December 2004
Dissemination of flow specification rules
draft-marques-idr-flow-spec-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document defines a new 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
traffic filtering in the context of a BGP/MPLS VPN service.
The information is carried via the Border Gateway Protocol (BGP),
thereby reusing protocol algorithms, operational experience and
administrative processes such as inter-provider peering agreements.
Table of Contents
1 Introduction .............................................. 3
2 Flow specifications ....................................... 4
3 Dissemination of Information .............................. 5
4 Traffic filtering ......................................... 10
4.1 Order of traffic filtering rules .......................... 11
5 Validation procedure ...................................... 11
6 Traffic Filtering Actions ................................. 13
6.1 Traffic-rate .............................................. 13
6.2 Traffic-action ............................................ 14
6.3 Redirect .................................................. 14
7 Traffic filtering in RFC2547bis networks .................. 14
8 Monitoring ................................................ 15
9 Security considerations ................................... 15
10 Acknowledgments ........................................... 16
11 References ................................................ 16
12 Authors' Addresses ........................................ 16
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1. Introduction
Modern IP routers contain both capability to forward traffic accord-
ing to aggregate IP prefixes as well as the capability to identify
and special case particular flows of traffic. The latter are usually
referred to as ACL or firewall engines.
While forwarding information is, typically, dynamically signaled
across the network via routing protocols, there is no agreed upon
mechanism to dynamically signal flows across autonomous-systems.
For several applications, it may be necessary to exchange control
information pertaining to aggregated traffic flow definitions which
cannot be expressed using destination address prefixes only.
An aggregated traffic flow is considered to be an n-tuple consisting
on several matching criteria such as source and destination address
prefixes, IP protocol and transport protocol port numbers.
The intention of this document is to define a general procedure to
encode such flow specification rules as a BGP NLRI which can be
reused for several different control applications. 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 rout-
ing system can take advantage of the ACL/firewall capabilities in the
router's forwarding path. Flow specifications can be seen as more
specific routing entries to an unicast prefix and are expected to
depend upon the existing unicast data information.
A flow specification received from a external autonomous-system will
need to be validated against unicast routing before being accepted.
If the aggregate traffic flow defined by the unicast destination pre-
fix is forwarded to a given BGP peer, then the local system can
install more specific flow rules which result in different forwarding
behavior, as requested by this system.
The choice of BGP as the carrier of this control information is also
justifiable by the fact that the key issues in terms of complexity
are problems which are common to unicast route distribution and have
already been solved in the current environment.
From an algorithmic perspective, the main problem that presents
itself is the distributed loop-free distribution of <key, attribute>
pairs. The key, in this particular instance, being a flow
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specification.
From an operational perspective, the utilization of BGP as the car-
rier for this information, allows a network service provider to reuse
both internal route distribution infrastructure (e.g.: route reflec-
tor 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 sub-
stantial advantage of being an incremental addition to deployed mech-
anisms.
2. Flow specifications
A flow specification is an n-tuple consisting on 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 predeterminate
actions.
A particular application is identified by a specific (AFI, SAFI) pair
and corresponds to a distinct set of RIBs. Those RIBs should be
treated independently from each other in order to assure non-inter-
ference 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.
Standard BGP policy mechanisms, such as UPDATE filtering by NLRI pre-
fix 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.
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3. 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 which a set
of attributes can be associated with.
This NLRI information is encoded using MP_REACH_NLRI and
MP_UNREACH_NLRI attributes as defined in [BGP-MP]. Whenever the cor-
responding 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) |
+------------------------------+
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 a
extended length 2 octet value in which the most significant nibble of
the first byte is all ones.
The Flow Specification NLRI-type consists of several optional subcom-
ponents. A specific packet is considered to match the flow specifica-
tion when it matches the intersection (AND) of all the components
present in the specification.
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>
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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
IP protocol value byte in IP packets.
The operator byte is encoded as:
7 6 5 4 3 2 1 0
+---+---+---+---+---+---+---+---+
| e | a | len | 0 |lt |gt |eq |
+---+---+---+---+---+---+---+---+
-i. End of List bit. Set in the last {op, value} pair in the list.
-ii. 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 evalu-
ating logical expressions.
-iii. The length of value field for this operand is given as (1 <<
len).
-iv. Lt - less than comparison between data and value.
-v. gt - greater than comparison between data and value.
-vi. 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.
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+ Type 5 - Destination port
Encoding: <type (1 octet), [op, value]+>
Defines a list of {operation, value} pairs used to match the des-
tination 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.
+ 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 are encoded using a single byte, using the bit
definitions specified in the TCP header format [rfc793].
This type uses the bitmask operand format, which differs from the
numeric operator format in the lower nibble.
7 6 5 4 3 2 1 0
+---+---+---+---+---+---+---+---+
| e | a | len | 0 | 0 |not| m |
+---+---+---+---+---+---+---+---+
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-i. Top nibble (End of List bit, And bit and Length field), as
defined for in the numeric operator format.
-ii. Not bit. If set, logical negation of operation.
-iii. Match bit. If set this is a bitwise match operation defined as
"(data & value) == value"; if unset (data & value) evaluates to
true if and of the bits in the value mask are set in the data.
+ Type 10 - Packet length
Encoding: <type (1 octet), [op, value]+>
Match on the total IP packet length (excluding L2 but including
IP header). Values are encoded using as 1 or 2 byte quantities.
+ Type 11 - DSCP
Encoding: <type (1 octet), [op, value]+>
Defines a list of {operation, value} pairs used to match the IP
TOS octet.
+ Type 12 - Fragment Encoding: <type (1 octet), [op, bitmask]+>
Uses bitmask operand format defined above.
Bitmask values:
-i. Bit 0 - Dont fragment
-ii. Bit 1 - Is a fragment
-iii. Bit 2 - First fragment
-iv. Bit 3 - Last fragment
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 as the semantics of a logical
AND of all components, if a component is FALSE by definition it can-
not be applied. However for the purposes of BGP route propagation
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this prefix should still be transmitted since BGP route distribution
is independent on NLRI semantics.
Flow specification components are to be interpreted as a bit match at
a given packet offset. When more than one component in a flow speci-
fication tests the same packet offset the behavior is undetermined.
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
+-------------+--------+-----------+
01 18 0a 01 01 03 81 06 04 81 19 (hex)
Decode for protocol:
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
+-------------+---------+------------------------+
01 18 0a 01 01 02 08 c0 04 03 89 45 8b 91 1f 90 (hex)
Decode for port:
0x04 type
0x03 value size=1, >=
0x89 value 137
0x45 &, value size=1, <=
0x8b value 139
0x91 end-of-list, value-size=2, =
0x1f90 value 8080
This constitutes a 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 [BGP-MP]. 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.
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4. 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 other-
wise 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 uni-
cast route advertisements corresponding to a destination prefix being
attacked. One variant of this technique marks such route advertise-
ments with a community that gets translated into a discard next-hop
by the receiving router. Other variants, attract traffic to a partic-
ular 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,
for 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 con-
vey information about traffic filtering rules for traffic that should
be discarded.
This mechanism is designed to, primarily, 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 that goal, we define an application specific NLRI
identifier (AFI=1, SAFI=133) along with specific sematic rules.
BGP routing updates containing this identifier use the flow specifi-
cation NLRI encoding to convey particular aggregated flows that
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require special treatment.
Flow routing information received via this (afi, safi) pair is sub-
ject to the validation procedure detailed bellow.
4.1. Order of traffic filtering rules
With traffic filtering rules, more than one rule may match a particu-
lar 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 specifications rules and must be constant in the
network.
We choose to order traffic filtering rules such that the order of two
flow specifications is given by the comparison of NLRI key byte
strings as defined by the memcmp() function is the ISO C standard.
Given the way that flow specifications are encoded this results in a
flow with a less-specific destination IP prefix being considered
less-than (and thus match before) a flow specification with a more-
specific destination IP prefix.
This matches an application model where the user may want to define a
restriction that affects an aggregate of traffic and a subsequent
rule that applies only to a subset of that.
A flow-specification without a destination IP prefix is considered to
match after all flow-specifications that contain an IP destination
prefix.
5. Validation procedure
Flow specifications received from a BGP peer and which 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 cor-
rect set of attributes to advertise.
The first step of the BGP Route Selection procedure [BGP-BASE] (sec-
tion 9.1.2) 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.
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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 consid-
ered 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 by a dif-
ferent 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 less or equally specific destination
prefix. This, as long as there are no more-specific unicast routes,
received from a different neighbor 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 repre-
sents 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 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.
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6. Traffic Filtering Actions
This specification defines a minimum set of filtering actions that it
standardizes as BGP extended community values. This is not ment to be
an inclusive list of all the possible actions but only a subset that
can be interpreted consistently across the network.
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 differ-
ently in different systems and for which no standard behavior is cur-
rently 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 par-
ticular actions.
type extended community encoding
--------------------------------------------------------
0x8006 traffic-rate 2-byte as#, 4-byte float
0x8007 traffic-action bitmask
0x8008 redirect 6-byte Route Target
6.1. Traffic-rate
The traffic-rate extended community uses the same encoding as the
"Link Bandwidth" extended community defined in [EXT-COMM]. The rate
is is expressed as 4 octets in IEEE floating point format, units
being bytes per second. A traffic-rate of 0 should result on all
traffic for the particular flow to be discarded.
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6.2. 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.
+ Terminal action (bit 0).
When this bit is set the traffic filtering engine will apply any
subsequent filtering rules (as defined by the ordering proce-
dure). If not set the evaluation of the traffic filter stops when
this rule is applied.
+ Sample (bit 1).
Enables traffic sampling and logging for this flow specification.
6.3. Redirect
The redirect extended community allows the traffic to be redirected
to a VRF routing instance that list 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).
The traffic marking extended community instruct 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 fol-
lowed by the DSCP value.
7. Traffic filtering in RFC2547bis networks
Provider-based layer 3 VPN networks, such as the ones using an
BGP/MPLS IP VPN control plane [2547bis], have different traffic fil-
tering 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
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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 encoded defined in this document. The NLRI length field
shall includes the both 8 bytes of the Route Distinguisher as well as
the subsequent flow specification.
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
[2547bis].
Flow specification rules received via this NLRI apply only to traffic
that belongs to the VRF(s) in which it is imported. By default, traf-
fic 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.
8. Monitoring
Traffic filtering applications require monitoring and traffic statis-
tics facilities. While this is an implementation specific choice,
implementations SHOULD provide:
- A mechanism to log the packet header of filtered traffic,
- A mechanism to count the number of matches for a given Flow
Specification rule.
9. Security considerations
Inter-provider routing is based on a web of trust. Neighboring
autonomous-systems are trusted to advertise valid reachability infor-
mation. If this trust model is violated, a neighboring autonomous
system may cause a denial of service attack by advertising reachabil-
ity information for a given prefix for which it does not provide ser-
vice.
As long as traffic filtering rules are restricted to match the
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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 not the case, this would open the door to further denial of
service attacks.
10. Acknowledgments
The authors would like to thank Yakov Rekhter, Dennis Ferguson and
Chris Morrow 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.
11. References
[BGP-BASE] Y. Rekhter, T. Li, S. Hares, "A Border Gateway Protocol 4
(BGP-4)", draft-ietf-idr-bgp4-20.txt, 03/03
[BGP-MP] T. Bates, R. Chandra, D. Katz, Y. Rekhter, "Multiprotocol
Extensions for BGP-4", RFC2858.
[EXT-COMM] S. Sangli, D. Tappan, Y. Rekhter, "BGP Extended Communities
Attribute", draft-ietf-idr-bgp-ext-communities-07.txt, 03/04.
[2547bis] E. Rosen, Y. Rekhter, "BGP/MPLS IP VPNs",
draft-ietf-l3vpn-rfc2547bis-03.txt, 10/04.
12. Authors' Addresses
Pedro Marques
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
Email: roque@juniper.net
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Nischal Sheth
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
E-mail: nsheth@juniper.net
Robert Raszuk
Cisco Systems, Inc.
Al. Jerozolimskie 146C
02-305 Warsaw, Poland
Email: rraszuk@cisco.com
Barry Greene
Cisco Systems, Inc.
Email: bgreene@cisco.com
Jared Mauch
NTT/VERIO
8285 Reese Lane
Ann Arbor, MI, 48103-9753
Email: jmauch@verio.net | jared@puck.nether.net
Danny McPherson
Arbor Networks
Email: danny@arbor.net
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Marques, et al. [Page 18]