| Internet-Draft | Packet Content Filter for BGP FlowSpec | August 2024 |
| Cui, et al. | Expires 16 February 2025 | [Page] |
The BGP Flow Specification enables the distribution of traffic filter policies (traffic filters and actions) via BGP, facilitating DDoS traffic filtering. However, the traffic filter in FSv1 and FSv2 predominantly focuses on IP header fields, which may not adequately address volumetric DDoS attack traffic characterized by fixed patterns within the packet content. This document introduces a new flow specification filter type designed for packet content filtering. The match field includes ptype, otype, offset, content-length, content, and mask encoded in the Flowspec NLRI. This new filter aims to leverage network devices such as routers and switches to defend against simple volumetric DDoS attacks, reducing the overall defence cost of carrier network.¶
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 16 February 2025.¶
Copyright (c) 2024 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
BGP flow specification describes the distribution of traffic filter policies through BGP, allowing for efficient traffic management and DDoS attack mitigation. Existing versions, FSv1 and FSv2, primarily offer n-tuple matching conditions for policy enforcement, enabling actions such as packet dropping, re-directing, or limitation, etc. These filter rules can be propagated to all BGP peers simultaneously without necessitating router configuration changes. Despite their utility, existing filters reliance on IP header fields for traffic filtering is increasingly inadequate for new kinds of DDoS attack. DDoS attacks such as ACK flood, UDP flood, ICMP flood and HTTP flood attacks, etc, have been found to potentially characterise a fixed content in the packet.¶
This document delineates a new flow specification filter type that facilitates packet content filtering, leveraging ptype, otype, offset, content-length, content, and mask fields within the FlowSpec NLRI to enhance DDoS defense mechanisms.¶
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.¶
This document specifies a new flow specification filter type that is encoded in the BGP FS NLRI and we follow the FSv1 definition format. The packet content filter is defined as follows:¶
Type TBD – Packet-Content¶
Encoding:< type (1 octet), value>¶
The value field is encoded using ptype, otype, offset, content-length, content and mask.¶
Encoding: < ptype (4 bits), otype (4 bits), offset (2 octets), content-length (1 octets), content (Variable), mask (Variable)>¶
The ptype is defined as a 4-bit unsigned integer that defines the packet type via AFI, because some filters are added to hardware that are IPv4 or IPv6 specific.¶
+-------+-----------------------------+ | Value | Description of Ptype | +-------+-----------------------------+ | 1 | IPv4 | | 2 | IPv6 | +-------+-----------------------------+¶
Figure 1： Ptype field¶
The otype and offset fields define the starting position of the packet content used for matching.¶
To avoid the effect of variable header length on the offset, we use the hierarchical way like [draft-khare-idr-bgp-flowspec-payload-match-08]. The Otype is defined as a 4-bit unsigned integer. The detail are as follows:¶
+-------+-----------------------------+ | Value | Description of Otype | +-------+-----------------------------+ | 0 | IP Header | | 1 | IP Payload | | 2 | TCP/UDP Payload | +-------+-----------------------------+¶
Figure 2： Otype field¶
Otype 0 is defined as the start of the IP header. Otype 1 is defined as the start of the data portion of the IP header after the IP options. Otype 2 is defined as the start of the TCP or UDP data. Type 2 will only be used if it is the Layer 4 transport protocol is TCP (6) or UDP (17). For other IP protocols, type 1 or type 2 can be used.¶
The offset is defined as a 2-octet unsigned integer that specifies the count of octets to be bypassed from the otype's starting position to match the packet content. It is worth noting that packet fragmentation will cause the offset value to change, so it is not enough to filter the fragmented packets through the packet content filter. One possible way is to filter the first packet through the payload filter, and then use its header information along with the fragment filter to filter the subsequent packets.¶
Example:¶
By setting otype 0 and an offset of 0, the match is configured to start precisely at the beginning of the IP header.¶
By setting otype 1 and an offset of 2, the match will start two octets past the initial data portion of the IP header, skipping over any IP options. This configuration, for example, in UDP packets, specifically targets the start of the destination port information.¶
By setting otype 2 and an offset of 10, the match will start ten octets into the content of the TCP/UDP packet.¶
The content-length is a one octet unsigned integer field that contains the length of the value field in octets. Content and mask fields have a same length which defined by the content-length.¶
The content provides a string to be matched. Based on the information provided by equipment vendors and operators, 8 octets is usually sufficient for the identification of DDoS attacks.¶
Mask is a string containing 0 and 1, where 1 represents what will be matched and 0 represents characters that can be ignored.¶
The content and mask are operated AND by bit to get the final content of the packet that needs to be matched.¶
An example of a FlowSpec NLRI encoding for:"all packets to 192.0.2.0/24 and have fixed content 5858 in the tcp payload with offset of 0".¶
| length | destination | packet content |
|---|---|---|
| 0x0b | 01 18 c0 00 02 | TBD 40 12 00 00 02 58 58 ff ff |
Description of each field of the FlowSpec NLRI.¶
| Value | Description | |
|---|---|---|
| 0x0f | length | 15 octets (if len<240, 1 octet) |
| 0x01 | type | Type 1 - Destination Prefix |
| 0x18 | length | 24 bits |
| 0xc0 | prefix | 192 |
| 0x00 | prefix | 0 |
| 0x02 | prefix | 2 |
| TBD | type | Type TBD - Packet Content |
| 0x40 | length | 64 bits |
| 0x12 | ptype, otype | IPv4, TCP payload |
| 0x0000 | offset | 0 octets |
| 0x02 | content-length | 2 octets |
| 0x5858 | content | 0x5858 |
| 0xffff | mask | 0xffff |
To adapt to the updates of flowSpec, this document also gives the definition of the packet content filters in FSv2. The formate is as same as the NLRI formate for extented IP filters, as shown in Figure 3:¶
+--------------------------------+ | NLRI length (2 octets) | +--------------------------------+ | TLVs+ | | +============================+ | | | order (4 octets) | | | +----------------------------+ | | | identifier (4 octets) | | | +----------------------------+ | | | FSv2 Filter type = 2 | | | +----------------------------+ | | | length TLVs (2 octet) | | | +----------------------------+ | | +----------------------------+ | | + +------------------------+ + | | + |FSv2 filters version | + | | + +------------------------+ + | | + | Sub-TLVs | + | | + +------------------------+ + | | +----------------------------+ | +--------------------------------+¶
Figure 3： NLRI formate for extented IP filters¶
The formate of Sub-TLVs is showns in Figure 4:¶
+----------------------------+ | Component Type (1 octet) | +----------------------------+ | Length (1 octet) | + ---------------------------+ | Value (variable) | +----------------------------+¶
Figure 4: formate for Sub-TLV¶
The defination of packet content filter in sub-TLV formate are as follows:¶
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Component Type | Component Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PType | Otype | Offset | Content Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Content |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
¶
Figure 5: Defination of the packet content filter¶
where fileds have the same length as FSv1.¶
Encoding: < ptype (4 bits), otype (4 bits), offset (2 octets), content-length (1 octets), content (Variable), mask (Variable)>¶
Compared to FSv1, FSv2 adds filter ordering function. According to the defination of ordering rules in FSv2, the transmission of SubTLVs within a flow specification rule MUST be sent ascending order by SubTLV type. If the SubTLV types are the same, then the value fields are compared using mechanisms defined in [RFC8955] and [RFC8956] and MUST be in ascending order.¶
However, due to multiple fields in the value of the packet content filter, the mechanisms defined in [RFC8955] and [RFC8956] do not apply. To give the default ordering rules of packet content filters, this document gives the defination as follows:¶
Filters with a larger content-length are implemented first.¶
If they have the same content-length, compare otype and the larger type is implemented first.¶
If they have the same content-lengte and otype, compare offset and the larger value will be implemented first.¶
If they have the same content-length, otype, and offset, compare the content as a binary string using the memcmp() function as defined by [ISO_IEC_9899]. The common prefix is compared. If the common prefix is not equal, the string with the lower prefix has higher precedence.¶
When there are multiple component exist in multiple NLRI, determine the order of the packet content filters according to the above ordering rules.¶
Here is a use case for ordering rules with multiple NLRI and multiple components. There are five components, with the same destination IP and user order, each of which contains a packet content filter with different values:¶
User-Order – 10 FSv2 – NLRI with Extended IP Filters Component 1: Destination IP + Packet content filter (otype 0, offset 50, content-length 2, content 0x1111) + Rate Limit Component 2: Destination IP + Packet content filter (otype 0, offset 50, content-length 3, content 0x111122) + Discard Component 3: Destination IP + Packet content filter (otype 2, offset 70, content-length 2, content 0x1111) + Rate Limit Component 4: Destination IP + Packet content filter (otype 2, offset 70, content-length 3, content 0x111122) + Discard Component 5: Destination IP + Packet content filter (otype 2, offset 70, content-length 3, content 0x111133) + Rate Limit¶
The rules will be installed as:¶
User-Order – 10 Component 4: Destination IP + Packet content filter (otype 2, offset 70, content-length 3, content 0x111122) + Discard Component 5: Destination IP + Packet content filter (otype 2, offset 70, content-length 3, content 0x111133) + Rate Limit Component 2: Destination IP + Packet content filter (otype 0, offset 50, content-length 3, content 0x111122) + Discard Component 3: Destination IP + Packet content filter (otype 2, offset 70, content-length 2, content 0x1111) + Rate Limit Component 1: Destination IP + Packet content filter (otype 0, offset 50, content-length 2, content 0x1111) + Rate Limit¶
In accordance with the procedures outlined for managing the "Flow Spec Component Types" registry, IANA is hereby requested to assign a new Type Value. This assignment is sought from the First Come First Served range, as detailed below:¶
+------------+---------------------------+---------------+ | Type Value | Name | Reference | +------------+---------------------------+---------------+ | TBD | Packet Content filter | this document | +------------+---------------------------+---------------+¶
The introduction of the "Packet Content filter" Type Value is purposed to expand the capability of BGP FSv1 by enabling more granular control over traffic filtering.¶
This is especially pertinent for addressing complex patterns within packet content that are characteristic of Distributed Denial of Service (DDoS) attacks and other security challenges. The proposed Packet Content filter facilitate the specification of detailed criteria for traffic matching, including but not limited to, content inspection at specific packet offsets. In the following update we will add the definition of FSv2.¶
No new security issues are introduced to the BGP protocol by this specification¶
We wish to thank Jeffery Hass and Li Yang for their valuable comments and suggestions on this document. We also wish to thank Rui Xu and Yannan Hu for their contribution in the implementation and validation of the packet content filter software.¶