| IDR Working Group | S. Hares |
| Internet-Draft | Huawei |
| Obsoletes: 5575,7674 (if approved) | C. Loibl |
| Intended status: Standards Track | Next Layer Communications |
| Expires: October 26, 2019 | R. Raszuk |
| Bloomberg LP | |
| D. McPherson | |
| Verisign | |
| M. Bacher | |
| T-Mobile Austria | |
| April 24, 2019 |
Dissemination of Flow Specification Rules
draft-ietf-idr-rfc5575bis-14
This document defines a 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.
It specifies IPv4 traffic Flow Specifications via a BGP NLRI which carries traffic Flow Specification filter, and an Extended community value which encodes actions a routing system can take if the packet matches the traffic flow filters. The flow filters and the actions are processed in a fixed order. Other drafts specify IPv6, MPLS addresses, L2VPN addresses, and NV03 encapsulation of IP addresses.
This document obsoletes RFC5575 and RFC7674 to correct unclear specifications in the flow filters.
Applications which use the bgp Flow Specification are: 1) application which automate inter-domain coordination of traffic filtering, such as what is required in order to mitigate (distributed) denial-of-service attacks; 2) applications which control traffic filtering in the context of a BGP/MPLS VPN service, and 3) applications with centralized control of traffic in a SDN or NFV context. Some deployments of these three applications can be handled by the strict ordering of the BGP NLRI traffic flow filters, and the strict actions encoded in the extended community Flow Specification actions.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 26, 2019.
Copyright (c) 2019 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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 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:
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.
Experience with previous BGP extensions has also shown that the maximum capacity of BGP speakers has been gradually increased according to expected loads. For example Internet unicast routing as well as other BGP applications increased their maximum capacity 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, this solution has been utilized in deployments because of 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 such as IPv6 families [I-D.ietf-idr-flow-spec-v6],
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
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. This n-tuple is encoded into a BGP NLRI defined below.
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 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 prefix as well as community matching and manipulation, MUST apply to the Flow Specification defined NLRI-type, especially in an inter-domain environment. 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.
We define a "Flow Specification" NLRI type (Figure 1) that may include several components such as destination prefix, source prefix, protocol, ports, and others (see Section 4.2 below).
This NLRI information is encoded using MP_REACH_NLRI and MP_UNREACH_NLRI attributes as defined in [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) |
+------------------------------+
Figure 1: Flow-spec NLRI for IPv4
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 [RFC4760]. The (AFI, SAFI) pair carried in the Multiprotocol Extension Capability MUST be (AFI=1, SAFI=133) for IPv4 Flow Specification, and (AFI=1, SAFI=134) for VPNv4 Flow Specification.
In figure 1 above, values less-than 240 are encoded using two hex digits (0xnn). Values above 239 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.
The encoding of each of the NLRI components begins with a type field (1 octet) followed by a variable length parameter. Section 4.2.1 to Section 4.2.12 define component types and parameter encodings for the IPv4 IP layer and transport layer headers. IPv6 NLRI component types are described in [I-D.ietf-idr-flow-spec-v6].
Flow Specification components must follow strict type ordering by increasing numerical order. A given component type may (exactly once) or may not be present in the specification. If present, it MUST precede any component of higher numeric type value.
All combinations of component types within a single NLRI are allowed, even if the combination makes no sense from a semantical perspective. 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 propagation, this prefix should still be transmitted since BGP route distribution is independent on NLRI semantics.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| e | a | len | 0 |lt |gt |eq |
+---+---+---+---+---+---+---+---+
Numeric operator
The bits lt, gt, and eq can be combined to produce common relational operators such as "less or equal", "greater or equal", and "not equal to".
| lt | gt | eq | Resulting operation |
|---|---|---|---|
| 0 | 0 | 0 | false (independent of the value) |
| 0 | 0 | 1 | == (equal) |
| 0 | 1 | 0 | > (greater than) |
| 0 | 1 | 1 | >= (greater than or equal) |
| 1 | 0 | 0 | < (less than) |
| 1 | 0 | 1 | <= (less than or equal) |
| 1 | 1 | 0 | != (not equal value) |
| 1 | 1 | 1 | true (independent of the value) |
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | e | a | len | 0 | 0 |not| m | +---+---+---+---+---+---+---+---+ Bitmask format
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 0 |LF |FF |IsF|DF | +---+---+---+---+---+---+---+---+
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 | +------------------+----------+----------+
+-------+----------+------------------------------+ | Value | | | +-------+----------+------------------------------+ | 0x03 | type | | | 0x81 | operator | end-of-list, value size=1, = | | 0x06 | value | | +-------+----------+------------------------------+
Decode for protocol:
+------------------+----------+-------------------------+ | destination | source | port | +------------------+----------+-------------------------+ | 0x01 18 0a 01 01 | 02 08 c0 | 04 03 89 45 8b 91 1f 90 | +------------------+----------+-------------------------+
An example of a Flow Specification encoding for: "all packets to 10.1.1/24 from 192/8 and port {range [137, 139] or 8080}".
+--------+----------+------------------------------+ | Value | | | +--------+----------+------------------------------+ | 0x04 | type | | | 0x03 | operator | size=1, >= | | 0x89 | value | 137 | | 0x45 | operator | "AND", value size=1, <= | | 0x8b | value | 139 | | 0x91 | operator | end-of-list, value-size=2, = | | 0x1f90 | value | 8080 | +--------+----------+------------------------------+
Decode for port:
Traffic filtering policies have been traditionally considered to be relatively static. Limitations of the static mechanisms caused this mechanism to be designed for the three new applications of traffic filtering (prevention of traffic-based, denial-of-service (DOS) attacks, traffic filtering in the context of BGP/MPLS VPN service, and centralized traffic control for SDN/NFV networks) requires coordination among service providers and/or coordination among the AS within a service provider. Section 9 has details on the limitation of previous mechanisms and why BGP Flow Specification provides a solution for to prevent DOS and aid BGP/MPLS VPN filtering rules.
This Flow Specification NLRI defined above to convey information about traffic filtering rules for traffic that should be discarded or handled in manner specified by a set of pre-defined actions (which are defined in BGP Extended Communities). 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, this draft specifies two application specific NLRI identifiers that provide traffic filters, and a set of actions encoding in BGP Extended Communities. The two application specific NLRI identifiers are:
Distribution of the IPv4 Flow Specification is described in Section 6, and distibution of BGP/MPLS traffic Flow Specification is described in Section 8. The traffic filtering actions are described in Section 7.
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 consistent 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 (see below) if the types are equal the algorithm continues with the next component.
For IP prefix values (IP destination or source prefix): If the prefixes overlap, the one with the longer prefix-length has higher precedence. If they do not overlap the one with the lowest IP value has higher 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 with equal lengths the lowest string (memcmp) has higher precedence. For strings of different lengths, the common prefix is compared. If the common prefix is not equal the string with the lowest prefix has higher precedence. If the common prefix is equal, the longest string is considered to have higher precedence than the shorter one.
The code below shows a Python3 implementation of the comparison algorithm. The full code was tested with Python 3.6.3 and can be obtained at https://github.com/stoffi92/flowspec-cmp.
<CODE BEGINS>
import itertools
import ipaddress
def flow_rule_cmp(a, b):
for comp_a, comp_b in itertools.zip_longest(a.components,
b.components):
# If a component type does not exist in one rule
# this rule has lower precedence
if not comp_a:
return B_HAS_PRECEDENCE
if not comp_b:
return A_HAS_PRECEDENCE
# higher precedence for lower component type
if comp_a.component_type < comp_b.component_type:
return A_HAS_PRECEDENCE
if comp_a.component_type > comp_b.component_type:
return B_HAS_PRECEDENCE
# component types are equal -> type specific comparison
if comp_a.component_type in (IP_DESTINATION, IP_SOURCE):
# assuming comp_a.value, comp_b.value of type ipaddress
if comp_a.value.overlaps(comp_b.value):
# longest prefixlen has precedence
if comp_a.value.prefixlen > comp_b.value.prefixlen:
return A_HAS_PRECEDENCE
if comp_a.value.prefixlen < comp_b.value.prefixlen:
return B_HAS_PRECEDENCE
# components equal -> continue with next component
elif comp_a.value > comp_b.value:
return B_HAS_PRECEDENCE
elif comp_a.value < comp_b.value:
return A_HAS_PRECEDENCE
else:
# assuming comp_a.value, comp_b.value of type bytearray
if len(comp_a.value) == len(comp_b.value):
if comp_a.value > comp_b.value:
return B_HAS_PRECEDENCE
if comp_a.value < comp_b.value:
return A_HAS_PRECEDENCE
# components equal -> continue with next component
else:
common = min(len(comp_a.value), len(comp_b.value))
if comp_a.value[:common] > comp_b.value[:common]:
return B_HAS_PRECEDENCE
elif comp_a.value[:common] < comp_b.value[:common]:
return A_HAS_PRECEDENCE
# the first common bytes match
elif len(comp_a.value) > len(comp_b.value):
return A_HAS_PRECEDENCE
else:
return B_HAS_PRECEDENCE
return EQUAL
<CODE ENDS>
Flow Specifications received from a BGP peer 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.
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:
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.
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.
The best-match unicast route may change over the time independently of the Flow Specification NLRI. Therefore, a revalidation of the Flow Specification NLRI MUST be performed whenever unicast routes change. Revalidation is defined as retesting that clause a and clause b above are true.
Explanation:
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.
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. Additional actions can be defined as either requiring standards or as vendor specific.
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.
This document defines the following extended communities values shown in Table 2 in the form 0x8xnn where nn indicates the sub-type. Encodings for these extended communities are described below.
| community | action | encoding |
|---|---|---|
| 0x8006 | traffic-rate-bytes | 2-byte ASN, 4-byte float |
| TBD | traffic-rate-packets | 2-byte ASN, 4-byte float |
| 0x8007 | traffic-action | bitmask |
| 0x8008 | rt-redirect AS-2byte | 2-octet AS, 4-octet value |
| 0x8108 | rt-redirect IPv4 | 4-octet IPv4 addres, 2-octet value |
| 0x8208 | rt-redirect AS-4byte | 4-octet AS, 2-octet value |
| 0x8009 | traffic-marking | DSCP value |
Some traffic action communities may interfere with each other. Section 7.6 of this specification provides general considerations on such traffic action interference. Any additional definition of a traffic actions specified by additional standards documents or vendor documents MUST specify if the traffic action interacts with an existing traffic actions, and provide error handling per [RFC7606].
Multiple traffic actions may be present for a single NLRI. The traffic actions are processed in ascending order of the sub-type found in the BGP Extended Communities. If not all of them can be processed the filter SHALL NOT be applied at all (for example: if for a given flow there are the action communities rate-limit-bytes and traffic-marking attached, and the plattform does not support one of them also the other shall not be applied for that flow).
All traffic actions are specified as transitive BGP Extended Communities.
The traffic-rate-bytes extended community uses the 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 maximum 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. On encoding the traffic-rate MUST NOT be negative. On decoding negative values MUST be treated as zero (discard all traffic).
Interferes with: No other BGP Flow Specification traffic action in this document.
The traffic-rate-packets extended community uses the same encoding as the traffic-rate-bytes extended community. The floating point value carries the maximum packet rate in packets per second. A traffic-rate-packets of 0 should result in all traffic for the particular flow to be discarded. On encoding the traffic-rate-packets MUST NOT be negative. On decoding negative values MUST be treated as zero (discard all traffic).
Interferes with: No other BGP Flow Specification traffic action in this document.
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 |
+---+---+---+---+---+---+---+---+
where S and T are defined as:
The use of the Terminal Action (bit 47) may result in more than one filter-rule matching a particular flow. All the flow actions from these rules shall be collected and applied. In case of interfering traffic actions it is an implementation decision which actions are selected. See also Section 7.6.
Interferes with: No other BGP Flow Specification traffic action in this document.
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 allows 3 different encodings formats for the route-target (type 0x80, 0x81, 0x82). Is uses the same encoding as the Route Target extended community [RFC4360].
It should be noted that the low-order nibble of the Redirect's Type field corresponds to the Route Target Extended Community format field (Type). (See Sections 3.1, 3.2, and 4 of [RFC4360] plus Section 2 of [RFC5668].) The low-order octet (Sub-Type) of the Redirect Extended Community remains 0x08 for all three encodings of the BGP Extended Communities (AS 2-byte, AS 4-byte, and IPv4 address).
Interferes with: All other redirect functions.
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.
Interferes with: No other BGP Flow Specification traffic action in this document.
Since traffic actions are represented as BGP extended community values, traffic actions may interfere with each other (ie. there may be more than one conflicting traffic-rate action associated with a single flow-filter). Traffic action interference has no impact on BGP propagation of flow filters (all communities are propagated according to policies).
If a flow filter associated with interfering flow actions is selected for packet forwarding, it is a implementation decision which of the interfering traffic actions are selected. Implementors of this specification SHOULD document the behaviour of their implementation in such cases.
If required, operators are encouraged to make use of the BGP policy framework supported by their implementation in order to achieve a predictable behaviour (ie. match - replace - delete communities on administrative boundaries).
Provider-based Layer 3 VPN networks, such as the ones using a BGP/ MPLS IP VPN [RFC4364] control plane, may have different traffic filtering requirements than Internet service providers. But also Internet service providers may use those VPNs for scenarios like having the Internet routing table in a VRF, resulting in the same traffic filtering requirements as defined for the global routing table environment within this document. 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 above in Section 4.2 of this document. The NLRI length field shall include both the 8 bytes of the Route Distinguisher as well as the subsequent Flow Specification.
+------------------------------+
| length (0xnn or 0xfn nn) |
+------------------------------+
| Route Distinguisher (8 bytes)|
+------------------------------+
| NLRI value (variable) |
+------------------------------+
Flow-spec NLRI for MPLS
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.
The validation procedures are the same as for IPv4.
The traffic action rules are the same as for IPv4.
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 (commonly known as remote triggered blackhole RTBH). 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 is subject to modified forwarding behavior (actions). The actions are defined as extended communities and include (but are not limited to) rate-limiting (including discard), traffic redirection, packet rewriting.
Provider-based Layer 3 VPN networks, such as the ones using a BGP/ MPLS IP VPN [RFC4364] control plane, may 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.
But also Internet service providers may use those VPNs for scenarios like having the Internet routing table in a VRF. Therefore, limitations described in Section 9.1 also apply to this section.
The BGP Flow Specification addresses these limitations.
Traffic filtering applications require monitoring and traffic statistics facilities. While this is an implementation-specific choice, implementations SHOULD provide:
In case BGP encounters an error in a Flow Specification UPDATE message it SHOULD treat this message as Treat-as-withdraw according to [RFC7606] Section 2.
Possible reasons for an error are (for more reasons see also [RFC7606]):
In order to facilitate future extensions of the Flow Specification NLRI, such extensions SHOULD specify a way to encode a "always-true" match condition within the newly introduced components. This match condition can be used to propagate (and apply) certain filters only if a specific extension is known to the implemenation.
This section complies with [RFC7153].
IANA maintains a registry entitled "SAFI Values". For the purpose of this work, IANA updated the registry and allocated two additional SAFIs:
| Value | Name | Reference |
|---|---|---|
| 133 | IPv4 dissemination of Flow Specification rules | [this document] |
| 134 | VPNv4 dissemination of Flow Specification rules | [this document] |
A Flow Specification consists of a sequence of flow components, which are identified by a an 8-bit component type. IANA has created and maintains a registry entitled "Flow Spec Component Types". This document defines the following Component Type Codes:
| Value | Name | Reference |
|---|---|---|
| 1 | Destination Prefix | [this document] |
| 2 | Source Prefix | [this document] |
| 3 | IP Protocol | [this document] |
| 4 | Port | [this document] |
| 5 | Destination port | [this document] |
| 6 | Source port | [this document] |
| 7 | ICMP type | [this document] |
| 8 | ICMP code | [this document] |
| 9 | TCP flags | [this document] |
| 10 | Packet length | [this document] |
| 11 | DSCP | [this document] |
| 12 | Fragment | [this document] |
In order to manage the limited number space and accommodate several usages, the following policies defined by [RFC8126] 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 Spec 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 Extended Community Flow Specification Action types defined in this document consist of two parts:
For the type-part, IANA maintains a registry entitled "BGP Transitive Extended Community Types". For the purpose of this work (Section 7), IANA updated the registry to contain the values listed below:
| Type Value | Name | Reference |
|---|---|---|
| 0x80 | Generic Transitive Experimental Use Extended Community (Sub-Types are defined in the "Generic Transitive Experimental Use Extended Community Sub-Types" registry) | [RFC7153] |
| 0x81 | Generic Transitive Experimental Use Extended Community Part 2 (Sub-Types are defined in the "Generic Transitive Experimental Use Extended Community Part 2 Sub-Types" Registry) | [this document] [See Note-1] |
| 0x82 | Generic Transitive Experimental Use Extended Community Part 3 (Sub-Types are defined in the "Generic Transitive Experimental Use Extended Community Part 3 Sub-Types" Registry) | [this document] [See Note-1] |
Note-1: This document obsoletes RFC7674.
For the sub-type part of the extended community actions IANA maintains and updated the following registries:
| Sub-Type Value | Name | Reference |
|---|---|---|
| 0x06 | Flow spec traffic-rate-bytes | [this document] |
| TBD | Flow spec traffic-rate-packets | [this document] |
| 0x07 | Flow spec traffic-action (Use of the "Value" field is defined in the "Traffic Action Fields" registry) | [this document] [See Note-2] |
| 0x08 | Flow spec rt-redirect AS-2byte format | [this document] |
| 0x09 | Flow spec traffic-remarking | [this document] |
Note-2: This document obsoletes both RFC7674 and RFC5575.
| Sub-Type Value | Name | Reference |
|---|---|---|
| 0x08 | Flow spec rt-redirect IPv4 format | [this document] [See Note-3] |
| Sub-Type Value | Name | Reference |
|---|---|---|
| 0x08 | Flow spec rt-redirect AS-4byte format | [this document] [See Note-3] |
Note-3: This document obsoletes RFC7674, and becomes the only reference for this table.
The "traffic-action" extended community (Section 7.3) 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:
| Bit | Name | Reference |
|---|---|---|
| 47 | Terminal Action | [this document] |
| 46 | Sample | [this document] |
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. However, this document also specifies traffic filtering actions that may need custom additional verification on the receiver side. See Section 14.
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 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.
While the general verification of the traffic filter NLRI is specified in this document (Section 6) the traffic filtering actions received by a third party may need custom verification or filtering. In particular all non traffic-rate actions may allow a third party to modify packet forwarding properties and potentially gain access to other routing-tables/VPNs or undesired queues. This can be avoided by proper filtering of action communities at network borders and by mapping user-defined communities (see Section 7) to expose certain forwarding properties to third parties.
Since verfication of the traffic filtering NLRI is tied to the announcement of the best unicast route, a unfiltered address space hijack (e.g. advertisement of a more specific route) may cause this verification to fail and consequently prevent Flow Specification filters from being accepted by a peer.
Barry Greene, Pedro Marques, Jared Mauch, Danny McPherson, and Nischal Sheth were authors on RFC5575, and therefore are contributing authors on this document.
The authors would like to thank Yakov Rekhter, Dennis Ferguson, Chris Morrow, Charlie Kaufman, and David Smith for their comments for the comments on the original RFC5575. Chaitanya Kodeboyina helped design the flow validation procedure; and Steven Lin and Jim Washburn ironed out all the details necessary to produce a working implementation in the original RFC5575.
A packet rate flowspec action was also discribed in a flowspec extention draft and the authors like to thank Wesley Eddy, Justin Dailey and Gilbert Clark for their work.
Additional the authors would like to thank Alexander Mayrhofer, Nicolas Fevrier, Job Snijders, Jeffrey Haas and Adam Chappell for their comments and review.
| [I-D.ietf-idr-flow-spec-v6] | McPherson, D., Raszuk, R., Pithawala, B., akarch@cisco.com, a. and S. Hares, "Dissemination of Flow Specification Rules for IPv6", Internet-Draft draft-ietf-idr-flow-spec-v6-09, November 2017. |
| [RFC4303] | Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005. |
This document includes numerous editorial changes to RFC5575. It is recommended to read the entire document. The authors, however want to point out the following technical changes to RFC5575: