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rfc:rfc5575

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.

Marques, et al. Standards Track [Page 17] 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

Marques, et al. Standards Track [Page 18] 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

Marques, et al. Standards Track [Page 19] 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.

Marques, et al. Standards Track [Page 20] 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]

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