IDR Working Group C. Loibl, Ed. Internet-Draft next layer Telekom GmbH Intended status: Standards Track R. Raszuk, Ed. Expires: February 13, 2021 Bloomberg LP S. Hares, Ed. Huawei August 12, 2020 Dissemination of Flow Specification Rules for IPv6 draft-ietf-idr-flow-spec-v6-14 Abstract Dissemination of Flow Specification Rules I-D.ietf-idr-rfc5575bis provides a protocol extension for propagation of traffic flow information for the purpose of rate limiting or filtering. I-D.ietf- idr-rfc5575bis specifies those extensions for IPv4 protocol data packets only. This specification extends I-D.ietf-idr-rfc5575bis and defines changes to the original document in order to make it also usable and applicable to IPv6 data packets. Status of This Memo 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 February 13, 2021. Copyright Notice Copyright (c) 2020 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 Loibl, et al. Expires February 13, 2021 [Page 1] Internet-Draft IPv6 Flow Specification August 2020 (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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Definitions of Terms Used in This Memo . . . . . . . . . 3 2. IPv6 Flow Specification encoding in BGP . . . . . . . . . . . 3 3. IPv6 Flow Specification components . . . . . . . . . . . . . 4 3.1. Type 1 - Destination IPv6 Prefix . . . . . . . . . . . . 4 3.2. Type 2 - Source IPv6 Prefix . . . . . . . . . . . . . . . 4 3.3. Type 3 - Next Header . . . . . . . . . . . . . . . . . . 4 3.4. Type 7 - ICMPv6 type . . . . . . . . . . . . . . . . . . 5 3.5. Type 8 - ICMPv6 code . . . . . . . . . . . . . . . . . . 5 3.6. Type 12 - Fragment . . . . . . . . . . . . . . . . . . . 5 3.7. Type 13 - Flow Label (new) . . . . . . . . . . . . . . . 6 3.8. Encoding Example . . . . . . . . . . . . . . . . . . . . 6 4. Ordering of Flow Specifications . . . . . . . . . . . . . . . 8 5. Validation Procedure . . . . . . . . . . . . . . . . . . . . 8 6. IPv6 Traffic Filtering Action changes . . . . . . . . . . . . 8 6.1. Redirect IPv6 (rt-redirect-ipv6) Type/Sub-Type 0x80/TBD . 9 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 11 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 11.1. Normative References . . . . . . . . . . . . . . . . . . 12 11.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Appendix A. Example python code: flow_rule_cmp_v6 . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 1. Introduction The growing amount of IPv6 traffic in private and public networks requires the extension of tools used in the IPv4 only networks to be also capable of supporting IPv6 data packets. In this document authors analyze the differences of IPv6 [RFC8200] flows description from those of traditional IPv4 packets and propose subset of new encoding formats to enable Dissemination of Flow Specification Rules [I-D.ietf-idr-rfc5575bis] for IPv6. Loibl, et al. Expires February 13, 2021 [Page 2] Internet-Draft IPv6 Flow Specification August 2020 This specification should be treated as an extension of base [I-D.ietf-idr-rfc5575bis] specification and not its replacement. It only defines the delta changes required to support IPv6 while all other definitions and operation mechanisms of Dissemination of Flow Specification Rules will remain in the main specification and will not be repeated here. 1.1. Definitions of Terms Used in This Memo AFI - Address Family Identifier. AS - Autonomous System. NLRI - Network Layer Reachability Information. SAFI - Subsequent Address Family Identifier. VRF - Virtual Routing and Forwarding instance. 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. 2. IPv6 Flow Specification encoding in BGP The [I-D.ietf-idr-rfc5575bis] defines new SAFIs 133 (Dissemination of Flow Specification) and 134 (L3VPN Dissemination of Flow Specification) applications in order to carry corresponding to each such application Flow Specification. Implementations wishing to exchange IPv6 Flow Specifications MUST use BGP's Capability Advertisement facility to exchange the Multiprotocol Extension Capability Code (Code 1) as defined in [RFC4760]. While [I-D.ietf-idr-rfc5575bis] specifies Flow Specification for IPv4 (AFI=1) only, the (AFI, SAFI) pair carried in the Multiprotocol Extension Capability MUST be: (AFI=2, SAFI=133) for IPv6 Flow Specification, and (AFI=2, SAFI=134) for VPNv6 Flow Specification. For both SAFIs the indication to which address family they are referring to will be recognized by AFI value (AFI=1 for IPv4 or VPNv4, AFI=2 for IPv6 and VPNv6 respectively). It needs to be observed that such choice of proposed encoding is compatible with filter validation against routing reachability information as described in Section 6 of [I-D.ietf-idr-rfc5575bis]. Loibl, et al. Expires February 13, 2021 [Page 3] Internet-Draft IPv6 Flow Specification August 2020 3. IPv6 Flow Specification components The following components are redefined or added for the purpose of accommodating the IPv6 header encoding. Unless otherwise specified all other components defined in [I-D.ietf-idr-rfc5575bis] Section 4.2.2 also apply to IPv6 Flow Specification. 3.1. Type 1 - Destination IPv6 Prefix Encoding: Defines the destination prefix to match. The offset has been defined to allow for flexible matching on part of the IPv6 address where it is required to skip (don't care) of N first bits of the address. This can be especially useful where part of the IPv6 address consists of an embedded IPv4 address and matching needs to happen only on the embedded IPv4 address. The encoded prefix contains enough octets for the bits used in matching (length minus offset bits). 3.2. Type 2 - Source IPv6 Prefix Encoding: Defines the source prefix to match. The length, offset and prefix are the same as in Section 3.1 3.3. Type 3 - Next Header Encoding: Contains a list of {numeric_op, value} pairs that are used to match the last Next Header ([RFC8200] Section 3) value octet in IPv6 packets. This component uses the Numeric Operator (numeric_op) described in [I-D.ietf-idr-rfc5575bis] Section 4.2.1.1. Type 3 component values SHOULD be encoded as single byte (numeric_op len=00). Note: While IPv6 allows for more then one Next Header field in the packet the main goal of Type 3 flow specification component is to match on the subsequent IP protocol value. Therefore the definition is limited to match only on last Next Header field in the packet. Loibl, et al. Expires February 13, 2021 [Page 4] Internet-Draft IPv6 Flow Specification August 2020 3.4. Type 7 - ICMPv6 type Encoding: Defines a list of {numeric_op, value} pairs used to match the type field of an ICMPv6 packet (see also [RFC4443] Section 2.1). This component uses the Numeric Operator (numeric_op) described in [I-D.ietf-idr-rfc5575bis] Section 4.2.1.1. Type 7 component values SHOULD be encoded as single byte (numeric_op len=00). In case of the presence of the ICMPv6 type component only ICMPv6 packets can match the entire Flow Specification. The ICMPv6 type component, if present, never matches when the packet's last Next Header field value is not 58 (ICMPv6), if the packet is fragmented and this is not the first fragment, or if the system is unable to locate the transport header. Different implementations may or may not be able to decode the transport header. 3.5. Type 8 - ICMPv6 code Encoding: Defines a list of {numeric_op, value} pairs used to match the code field of an ICMPv6 packet (see also [RFC4443] Section 2.1). This component uses the Numeric Operator (numeric_op) described in [I-D.ietf-idr-rfc5575bis] Section 4.2.1.1. Type 8 component values SHOULD be encoded as single byte (numeric_op len=00). In case of the presence of the ICMPv6 code component only ICMPv6 packets can match the entire Flow Specification. The ICMPv6 code component, if present, never matches when the packet's last Next Header field value is not 58 (ICMPv6), if the packet is fragmented and this is not the first fragment, or if the system is unable to locate the transport header. Different implementations may or may not be able to decode the transport header. 3.6. Type 12 - Fragment Encoding: Defines a list of {bitmask_op, bitmask} pairs used to match specific IP fragments. This component uses the Bitmask Operator (bitmask_op) described in [I-D.ietf-idr-rfc5575bis] Section 4.2.1.2. The Type 12 component bitmask MUST be encoded as single byte bitmask (bitmask_op len=00). Loibl, et al. Expires February 13, 2021 [Page 5] Internet-Draft IPv6 Flow Specification August 2020 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 | 0 |LF |FF |IsF| 0 | +---+---+---+---+---+---+---+---+ Figure 1: Fragment Bitmask Operand Bitmask values: IsF - Is a fragment - match if IPv6 Fragment Header ([RFC8200] Section 4.5) Fragment Offset is not 0 FF - First fragment - match if IPv6 Fragment Header ([RFC8200] Section 4.5) Fragment Offset is 0 AND M flag is 1 LF - Last fragment - match if IPv6 Fragment Header ([RFC8200] Section 4.5) Fragment Offset is not 0 AND M flag is 0 0 - MUST be set to 0 on NLRI encoding, and MUST be ignored during decoding 3.7. Type 13 - Flow Label (new) Encoding: Contains a list of {numeric_op, value} pairs that are used to match the 20-bit Flow Label IPv6 header field ([RFC8200] Section 3). This component uses the Numeric Operator (numeric_op) described in [I-D.ietf-idr-rfc5575bis] Section 4.2.1.1. Type 13 component values SHOULD be encoded as 1-, 2-, or 4-byte quantities (numeric_op len=00, len=01 or len=10). 3.8. Encoding Example 3.8.1. Example 1 The following example demonstrates the prefix encoding for: "all packets to ::1234:5678:9A00:0/64-104 from 100::/8 and port 25". +--------+-------------------------+-------------+----------+ | length | destination | source | port | +--------+-------------------------+-------------+----------+ | 0x0f | 01 68 40 12 34 56 78 9A | 02 08 00 01 | 04 81 19 | +--------+-------------------------+-------------+----------+ Decoded: Loibl, et al. Expires February 13, 2021 [Page 6] Internet-Draft IPv6 Flow Specification August 2020 +-------+------------+------------------------------+ | Value | | | +-------+------------+------------------------------+ | 0x0f | length | 16 octets (len<240 1-octet) | | 0x01 | type | Type 1 - Dest. IPv6 Prefix | | 0x68 | length | 104 bit | | 0x40 | offset | 64 bit | | 0x12 | prefix | | | 0x34 | prefix | | | 0x56 | prefix | | | 0x78 | prefix | | | 0x9A | prefix | | | 0x02 | type | Type 2 - Source IPv6 Prefix | | 0x08 | length | 8 bit | | 0x00 | offset | 0 bit | | 0x01 | prefix | | | 0x04 | type | Type 4 - Port | | 0x81 | numeric_op | end-of-list, value size=1, = | | 0x19 | value | 25 | +-------+------------+------------------------------+ This constitutes a NLRI with a NLRI length of 16 octets. 3.8.2. Example 2 The following example demonstrates the prefix encoding for: "all packets to ::1234:5678:9A00:0/65-104". +--------+-------------------------+ | length | destination | +--------+-------------------------+ | 0x08 | 01 68 41 24 68 ac f1 34 | +--------+-------------------------+ Decoded: Loibl, et al. Expires February 13, 2021 [Page 7] Internet-Draft IPv6 Flow Specification August 2020 +-------+------------+------------------------------+ | Value | | | +-------+------------+------------------------------+ | 0x08 | length | 8 octets (len<240 1-octet) | | 0x01 | type | Type 1 - Dest. IPv6 Prefix | | 0x68 | length | 104 bit | | 0x41 | offset | 65 bit | | 0x24 | prefix | starting with the 66ths bit | | 0x68 | prefix | | | 0xac | prefix | | | 0xf1 | prefix | | | 0x34 | prefix | | +-------+------------+------------------------------+ This constitutes a NLRI with a NLRI length of 8 octets. 4. Ordering of Flow Specifications The definition for the order of traffic filtering rules from [I-D.ietf-idr-rfc5575bis] Section 5.1 can be reused with new consideration for the IPv6 prefix offset. As long as the offsets are equal, the comparison is the same, retaining longest-prefix-match semantics. If the offsets are not equal, the lowest offset has precedence, as this flow matches the most significant bit. The code in Appendix A shows a Python3 implementation of the resulting comparison algorithm. The full code was tested with Python 3.7.2 and can be obtained at https://github.com/stoffi92/draft-ietf- idr-flow-spec-v6/tree/master/flowspec-cmp [1]. 5. Validation Procedure The validation procedure is the same as specified in [I-D.ietf-idr-rfc5575bis] Section 6 with the exception that item a) of the validation procedure should now read as follows: a) A destination prefix component with offset=0 is embedded in the Flow Specification 6. IPv6 Traffic Filtering Action changes Traffic Filtering Actions from [I-D.ietf-idr-rfc5575bis] Section 7 can also be applied to IPv6 Flow Specifications. To allow an IPv6 address specific route-target, a new Traffic Filtering Action IPv6 address specific extended community is specified in Section 6.1 below: Loibl, et al. Expires February 13, 2021 [Page 8] Internet-Draft IPv6 Flow Specification August 2020 6.1. Redirect IPv6 (rt-redirect-ipv6) Type/Sub-Type 0x80/TBD The redirect IPv6 address specific extended community allows the traffic to be redirected to a VRF routing instance that lists the specified IPv6 address specific 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 IPv6 address specific Route Target extended community [RFC5701] Section 2 with the high-order octet of the Type always set to 0x80 and the Sub-Type always TBD. Interferes with: All BGP Flow Specification redirect Traffic Filtering Actions (with itself and those specified in [I-D.ietf-idr-rfc5575bis] Section 7.4). 7. Security Considerations No new security issues are introduced to the BGP protocol by this specification over the security considerations in [I-D.ietf-idr-rfc5575bis] 8. IANA Considerations This section complies with [RFC7153] IANA is requested to create and maintain a new registry entitled: "Flow Spec IPv6 Component Types" containing the initial entries as specified in Table 1. Loibl, et al. Expires February 13, 2021 [Page 9] Internet-Draft IPv6 Flow Specification August 2020 +-------+-------------------------+-----------------+ | Value | Name | Reference | +-------+-------------------------+-----------------+ | 1 | Destination IPv6 Prefix | [this document] | | 2 | Source IPv6 Prefix | [this document] | | 3 | Next Header | [this document] | | 4 | Port | [this document] | | 5 | Destination port | [this document] | | 6 | Source port | [this document] | | 7 | ICMPv6 type | [this document] | | 8 | ICMPv6 code | [this document] | | 9 | TCP flags | [this document] | | 10 | Packet length | [this document] | | 11 | DSCP | [this document] | | 12 | Fragment | [this document] | | 13 | Flow Label | [this document] | +-------+-------------------------+-----------------+ Table 1: Registry: Flow Spec IPv6 Component Types In order to manage the limited number space and accommodate several usages, the following policies defined by [RFC8126] are used: +--------------+------------------------+ | Type Values | Policy | +--------------+------------------------+ | 0 | Reserved | | [1 .. 127] | Specification Required | | [128 .. 254] | Expert Review | | 255 | Reserved | +--------------+------------------------+ Table 2: Flow Spec IPv6 Component Types Registration Policy Guidance for Experts: 128-254 requires Expert Review as the registration policy. The Experts are expected to check the clarity of purpose and use of the requested code points. The Experts must also verify that any specification produced in the IETF that requests one of these code points has been made available for review by the IDR working group and that any specification produced outside the IETF does not conflict with work that is active or already published within the IETF. It must be pointed out that introducing new component types may break interoperability with existing implementations of this protocol. Loibl, et al. Expires February 13, 2021 [Page 10] Internet-Draft IPv6 Flow Specification August 2020 IANA maintains a registry entitled "Generic Transitive Experimental Use Extended Community Sub-Types". For the purpose of this work, IANA is requested to assign a new value: +------------+----------------------------------------+-------------+ | Sub-Type | Name | Reference | | Value | | | +------------+----------------------------------------+-------------+ | TBD | Flow spec rt-redirect-ipv6 | [this | | | format | document] | +------------+----------------------------------------+-------------+ Table 3: Registry: Generic Transitive Experimental Use Extended Community Sub-Types 9. Acknowledgements Authors would like to thank Pedro Marques, Hannes Gredler and Bruno Rijsman, Brian Carpenter, and Thomas Mangin for their valuable input. 10. Contributors Danny McPherson Verisign, Inc. Email: dmcpherson@verisign.com Burjiz Pithawala Individual Email: burjizp@gmail.com Andy Karch Cisco Systems 170 West Tasman Drive San Jose, CA 95134 USA Email: akarch@cisco.com 11. References Loibl, et al. Expires February 13, 2021 [Page 11] Internet-Draft IPv6 Flow Specification August 2020 11.1. Normative References [I-D.ietf-idr-rfc5575bis] Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M. Bacher, "Dissemination of Flow Specification Rules", draft-ietf-idr-rfc5575bis-25 (work in progress), May 2020. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006, . [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, March 2006, . [RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol Extensions for BGP-4", RFC 4760, DOI 10.17487/RFC4760, January 2007, . [RFC5701] Rekhter, Y., "IPv6 Address Specific BGP Extended Community Attribute", RFC 5701, DOI 10.17487/RFC5701, November 2009, . [RFC7153] Rosen, E. and Y. Rekhter, "IANA Registries for BGP Extended Communities", RFC 7153, DOI 10.17487/RFC7153, March 2014, . [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Loibl, et al. Expires February 13, 2021 [Page 12] Internet-Draft IPv6 Flow Specification August 2020 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017, . 11.2. URIs [1] https://github.com/stoffi92/draft-ietf-idr-flow-spec- v6/tree/master/flowspec-cmp Appendix A. Example python code: flow_rule_cmp_v6 """ Copyright (c) 2020 IETF Trust and the persons identified as authors of draft-ietf-idr-flow-spec-v6. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Simplified BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info). """ import itertools import collections import ipaddress EQUAL = 0 A_HAS_PRECEDENCE = 1 B_HAS_PRECEDENCE = 2 IP_DESTINATION = 1 IP_SOURCE = 2 FS_component = collections.namedtuple('FS_component', 'component_type value') class FS_IPv6_prefix_component: def __init__(self, prefix, offset=0, component_type=IP_DESTINATION): self.offset = offset self.component_type = component_type # make sure if offset != 0 that non of the # first offset bits are set in the prefix self.value = prefix if offset != 0: Loibl, et al. Expires February 13, 2021 [Page 13] Internet-Draft IPv6 Flow Specification August 2020 i = ipaddress.IPv6Interface( (self.value.network_address, offset)) if i.network.network_address != \ ipaddress.ip_address('0::0'): raise ValueError('Bits set in the offset') class FS_nlri(object): """ FS_nlri class implementation that allows sorting. By calling .sort() on a array of FS_nlri objects these will be sorted according to the flow_rule_cmp algorithm. Example: nlri = [ FS_nlri(components=[ FS_component(component_type=4, value=bytearray([0,1,2,3,4,5,6])), ]), FS_nlri(components=[ FS_component(component_type=5, value=bytearray([0,1,2,3,4,5,6])), FS_component(component_type=6, value=bytearray([0,1,2,3,4,5,6])), ]), ] nlri.sort() # sorts the array accorinding to the algorithm """ def __init__(self, components = None): """ components: list of type FS_component """ self.components = components def __lt__(self, other): # use the below algorithm for sorting result = flow_rule_cmp_v6(self, other) if result == B_HAS_PRECEDENCE: return True else: return False def flow_rule_cmp_v6(a, b): """ Implementation of the flowspec sorting algorithm in draft-ietf-idr-flow-spec-v6. """ Loibl, et al. Expires February 13, 2021 [Page 14] Internet-Draft IPv6 Flow Specification August 2020 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): if comp_a.offset < comp_b.offset: return A_HAS_PRECEDENCE if comp_a.offset < comp_b.offset: return B_HAS_PRECEDENCE # both components have the same offset # assuming comp_a.value, comp_b.value of type # ipaddress.IPv6Network # and the offset bits are reset to 0 (since they are # not represented in the NLRI) 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 Loibl, et al. Expires February 13, 2021 [Page 15] Internet-Draft IPv6 Flow Specification August 2020 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 Authors' Addresses Christoph Loibl (editor) next layer Telekom GmbH Mariahilfer Guertel 37/7 Vienna 1150 AT Phone: +43 664 1176414 Email: cl@tix.at Robert Raszuk (editor) Bloomberg LP 731 Lexington Ave New York City, NY 10022 USA Email: robert@raszuk.net Susan Hares (editor) Huawei 7453 Hickory Hill Saline, MI 48176 USA Email: shares@ndzh.com Loibl, et al. Expires February 13, 2021 [Page 16]