Network Working Group Pedro Marques
Internet Draft Nischal Sheth
Expiration Date: April 11, 2005 Juniper Networks
Robert Raszuk
Jared Mauch Barry Greene
NTT/Verio Cisco Systems Inc.
Danny McPerson
Arbor Networks
October 2004
Dissemination of flow specification rules
draft-marques-idr-flow-spec-01.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 11, 2005.
Marques, et al. [Page 1]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
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 special treatment that is
desired for sub-components of a particular IP prefix.
Additionally it defines an application of that encoding format to
traffic filtering of inter-domain flows such as what is necessary in
order to mitigate (distributed) denial of service attacks.
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
5 Validation procedure ...................................... 11
6 Traffic Filtering Actions ................................. 12
7 Monitoring ................................................ 12
8 Security considerations ................................... 13
9 Acknowledgments ........................................... 13
10 References ................................................ 13
11 Authors' Addresses ........................................ 13
Marques, et al. [Page 2]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
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
accross the network via routing protocols, there is no agreed upon
mecanism 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
specifc routing entries to an unicast prefix and are expected to
depend upon the existing unicast data information.
For example, 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 prefix is forwarded to a given BGP peer, then the
local system can install more specific flow rules which result in
different forwarding behaviour as requested by this system.
The choice of BGP as the carrier of this control information is also
justified 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
Marques, et al. [Page 3]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
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 substan-
tial advantage of being an incremental addition to deployed mecha-
nisms.
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.
Marques, et al. [Page 4]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
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 two byte NLRI length value in octets followed by a variable length
NLRI value.
+------------------------------+
| NLRI length (2 octets) |
+------------------------------+
| NLRI value (variable) |
+------------------------------+
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 - Route Distinguisher
Encoding: <type (1 octect), RD value (8 octets)>
Route Distinguisher value, encoded as specified in [2547]. This
allows this NLRI to carry information for more than one routing
realm. This value should be ommited when distributing information
for the Internet routing realm. When the RD is present, routes
should also contain the Route Target extended community.
+ Type 2 - 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.
Marques, et al. [Page 5]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
+ Type 3 - Source Prefix
Encoding: <type (1 octet), prefix-length (1 octet), prefix>
Defines the source prefix to match.
+ Type 4 - 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 lenght of value field for this operand is given as (1 <<
len).
-iv. Lt - less than comparisson between data and value.
-v. gt - greater than comparisson 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 5 - 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.
Marques, et al. [Page 6]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
+ Type 6 - 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 7 - 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 8 - 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 9 - 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 10 - 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 |
+---+---+---+---+---+---+---+---+
Marques, et al. [Page 7]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
-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 11 - 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 12 - DSCP
Encoding: <type (1 octet), [op, value]+>
Defines a list of {operation, value} pairs used to match the IP
TOS octect.
+ Type 13 - 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
Marques, et al. [Page 8]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
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
+-------------+--------+-----------+
02 18 0a 01 01 04 81 06 05 81 19 (hex)
Decode for protocol:
0x04 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
+-------------+---------+------------------------+
02 18 0a 01 01 03 08 c0 05 03 89 45 8b 91 1f 90 (hex)
Decode for port:
0x05 type
0x03 value size=1, >=
0x89 value 137
0x45 &, value size=1, <=
0x8b value 139
0x91 end-of-list, value-size=2, =
0x1f90value 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.
Marques, et al. [Page 9]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
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.
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=TBD) along with specific sematic rules. BGP
routing updates containing this identifier use the flow specification
NLRI encoding to convey particular aggregated flows that require spe-
cial treatment.
Marques, et al. [Page 10]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
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.
The concept can be extended, in the case of Flow Specification NLRI,
to allow other validation procedures.
A flow specification NLRI SHOULD be validated such that it is consid-
ered unfeasible if it contains an non-empty AS_PATH and that AS_PATH
does not match the AS_PATH of the best match unicast route that
includes the specified destination address prefix.
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.
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.
Marques, et al. [Page 11]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
6. Traffic Filtering Actions
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 sample-rate 2-byte as#, 4-byte float
0x8008 redirect 6-byte Route Target
0x8009 marking 1-byte DSCP value
A traffic-rate of 0 should result on all traffic that matches the par-
ticular flow to be discarded.
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 low-
est 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 followed by
the DSCP value.
7. 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.
Marques, et al. [Page 12]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
8. 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 corre-
sponding unicast routing paths for the relevant prefixes, the secu-
rity 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.
9. Acknowledgments
The authors would like to thank Yakov Rekhter, Dennis Ferguson and
Chris Morrow for their comments.
10. 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.
11. Authors' Addresses
Pedro Marques
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
Email: roque@juniper.net
Marques, et al. [Page 13]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
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
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any assur-
ances of licenses to be made available, or the result of an attempt
made to obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can
Marques, et al. [Page 14]
Internet Draft draft-marques-idr-flow-spec-01.txt October 2004
be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFOR-
MATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Marques, et al. [Page 15]