IPv6 Segment Routing Header (SRH)
The information below is for an old version of the document.
This is an older version of an Internet-Draft that was ultimately published as RFC 8754.
|Authors||Stefano Previdi , Clarence Filsfils , Brian Field , Ida Leung , Jen Linkova , Ebben Aries , Tomoya Kosugi , Éric Vyncke , David Lebrun|
|RFC stream||Internet Engineering Task Force (IETF)|
TSVART Last Call review (of -22) by Joseph Touch Almost Ready
|Additional resources||Mailing list discussion|
|Stream||WG state||WG Document|
|IESG||IESG state||I-D Exists|
|Send notices to||(None)|
Network Working Group S. Previdi, Ed. Internet-Draft C. Filsfils Intended status: Standards Track Cisco Systems, Inc. Expires: June 16, 2016 B. Field Comcast I. Leung Rogers Communications J. Linkova Google E. Aries Facebook T. Kosugi NTT E. Vyncke Cisco Systems, Inc. D. Lebrun Universite Catholique de Louvain December 14, 2015 IPv6 Segment Routing Header (SRH) draft-ietf-6man-segment-routing-header-00 Abstract Segment Routing (SR) allows a node to steer a packet through a controlled set of instructions, called segments, by prepending a SR header to the packet. A segment can represent any instruction, topological or service-based. SR allows to enforce a flow through any path (topological, or application/service based) while maintaining per-flow state only at the ingress node to the SR domain. Segment Routing can be applied to the IPv6 data plane with the addition of a new type of Routing Extension Header. This draft describes the Segment Routing Extension Header Type and how it is used by SR capable nodes. Requirements Language 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]. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Previdi, et al. Expires June 16, 2016 [Page 1] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 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 http://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 June 16, 2016. Copyright Notice Copyright (c) 2015 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 (http://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. Segment Routing Documents . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Data Planes supporting Segment Routing . . . . . . . . . 4 2.2. Segment Routing (SR) Domain . . . . . . . . . . . . . . . 4 2.2.1. SR Domain in a Service Provider Network . . . . . . . 5 2.2.2. SR Domain in a Overlay Network . . . . . . . . . . . 6 3. Segment Routing Extension Header (SRH) . . . . . . . . . . . 7 3.1. SRH and RFC2460 behavior . . . . . . . . . . . . . . . . 11 4. SRH Procedures . . . . . . . . . . . . . . . . . . . . . . . 11 4.1. Segment Routing Node Functions . . . . . . . . . . . . . 11 4.1.1. Source SR Node . . . . . . . . . . . . . . . . . . . 12 4.1.2. SR Domain Ingress Node . . . . . . . . . . . . . . . 13 4.1.3. Transit Node . . . . . . . . . . . . . . . . . . . . 13 4.1.4. SR Segment Endpoint Node . . . . . . . . . . . . . . 13 5. Security Considerations . . . . . . . . . . . . . . . . . . . 14 5.1. Threat model . . . . . . . . . . . . . . . . . . . . . . 14 5.1.1. Source routing threats . . . . . . . . . . . . . . . 15 5.1.2. Applicability of RFC 5095 to SRH . . . . . . . . . . 15 5.1.3. Service stealing threat . . . . . . . . . . . . . . . 16 Previdi, et al. Expires June 16, 2016 [Page 2] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 5.1.4. Topology disclosure . . . . . . . . . . . . . . . . . 16 5.1.5. ICMP Generation . . . . . . . . . . . . . . . . . . . 16 5.2. Security fields in SRH . . . . . . . . . . . . . . . . . 17 5.2.1. Selecting a hash algorithm . . . . . . . . . . . . . 18 5.2.2. Performance impact of HMAC . . . . . . . . . . . . . 18 5.2.3. Pre-shared key management . . . . . . . . . . . . . . 19 5.3. Deployment Models . . . . . . . . . . . . . . . . . . . . 20 5.3.1. Nodes within the SR domain . . . . . . . . . . . . . 20 5.3.2. Nodes outside of the SR domain . . . . . . . . . . . 20 5.3.3. SR path exposure . . . . . . . . . . . . . . . . . . 21 5.3.4. Impact of BCP-38 . . . . . . . . . . . . . . . . . . 21 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 7. Manageability Considerations . . . . . . . . . . . . . . . . 22 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 22 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 10.1. Normative References . . . . . . . . . . . . . . . . . . 22 10.2. Informative References . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 1. Segment Routing Documents Segment Routing terminology is defined in [I-D.ietf-spring-segment-routing]. Segment Routing use cases are described in [I-D.ietf-spring-problem-statement] and [I-D.ietf-spring-ipv6-use-cases]. Segment Routing protocol extensions are defined in [I-D.ietf-isis-segment-routing-extensions], and [I-D.ietf-ospf-ospfv3-segment-routing-extensions]. 2. Introduction Segment Routing (SR), defined in [I-D.ietf-spring-segment-routing], allows a node to steer a packet through a controlled set of instructions, called segments, by prepending a SR header to the packet. A segment can represent any instruction, topological or service-based. SR allows to enforce a flow through any path (topological or service/application based) while maintaining per-flow state only at the ingress node to the SR domain. Segments can be derived from different components: IGP, BGP, Services, Contexts, Locators, etc. The list of segment forming the path is called the Segment List and is encoded in the packet header. Previdi, et al. Expires June 16, 2016 [Page 3] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 SR allows the use of strict and loose source based routing paradigms without requiring any additional signaling protocols in the infrastructure hence delivering an excellent scalability property. The source based routing model described in [I-D.ietf-spring-segment-routing] is inherited from the ones proposed by [RFC1940] and [RFC2460]. The source based routing model offers the support for explicit routing capability. 2.1. Data Planes supporting Segment Routing Segment Routing (SR), can be instantiated over MPLS ([I-D.ietf-spring-segment-routing-mpls]) and IPv6. This document defines its instantiation over the IPv6 data-plane based on the use- cases defined in [I-D.ietf-spring-ipv6-use-cases]. This document defines a new type of Routing Header (originally defined in [RFC2460]) called the Segment Routing Header (SRH) in order to convey the Segment List in the packet header as defined in [I-D.ietf-spring-segment-routing]. Mechanisms through which segment are known and advertised are outside the scope of this document. A segment is materialized by an IPv6 address. A segment identifies a topological instruction or a service instruction. A segment can be either: o global: a global segment represents an instruction supported by all nodes in the SR domain and it is instantiated through an IPv6 address globally known in the SR domain. o local: a local segment represents an instruction supported only by the node who originates it and it is instantiated through an IPv6 address that is known only by the local node. 2.2. Segment Routing (SR) Domain We define the concept of the Segment Routing Domain (SR Domain) as the set of nodes participating into the source based routing model. These nodes may be connected to the same physical infrastructure (e.g.: a Service Provider's network) as well as nodes remotely connected to each other (e.g.: an enterprise VPN or an overlay). A non-exhaustive list of examples of SR Domains is: o The network of an operator, service provider, content provider, enterprise including nodes, links and Autonomous Systems. Previdi, et al. Expires June 16, 2016 [Page 4] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 o A set of nodes connected as an overlay over one or more transit providers. The overlay nodes exchange SR-enabled traffic with segments belonging solely to the overlay routers (the SR domain). None of the segments in the SR-enabled packets exchanged by the overlay belong to the transit networks The source based routing model through its instantiation of the Segment Routing Header (SRH) defined in this document equally applies to all the above examples. While the source routing model defined in [RFC2460] doesn't mandate which node is allowed to insert (or modify) the SRH, it is assumed in this document that the SRH is inserted in the packet by its source. For example: o At the node originating the packet (host, server). o At the ingress node of a SR domain where the ingress node receives an IPv6 packet and encapsulates it into an outer IPv6 header followed by a Segment Routing header. 2.2.1. SR Domain in a Service Provider Network The following figure illustrates an SR domain consisting of an operator's network infrastructure. (-------------------------- Operator 1 -----------------------) ( ) ( (-----AS 1-----) (-------AS 2-------) (----AS 3-------) ) ( ( ) ( ) ( ) ) A1--(--(--11---13--14-)--(-21---22---23--24-)--(-31---32---34--)--)--Z1 ( ( /|\ /|\ /| ) ( |\ /|\ /|\ /| ) ( |\ /|\ /| \ ) ) A2--(--(/ | \/ | \/ | ) ( | \/ | \/ | \/ | ) ( | \/ | \/ | \)--)--Z2 ( ( | /\ | /\ | ) ( | /\ | /\ | /\ | ) ( | /\ | /\ | ) ) ( ( |/ \|/ \| ) ( |/ \|/ \|/ \| ) ( |/ \|/ \| ) ) A3--(--(--15---17--18-)--(-25---26---27--28-)--(-35---36---38--)--)--Z3 ( ( ) ( ) ( ) ) ( (--------------) (------------------) (---------------) ) ( ) (-------------------------------------------------------------) Figure 1: Service Provider SR Domain Figure 1 describes an operator network including several ASes and delivering connectivity between endpoints. In this scenario, Segment Routing is used within the operator networks and across the ASes boundaries (all being under the control of the same operator). In this case segment routing can be used in order to address use cases Previdi, et al. Expires June 16, 2016 [Page 5] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 such as end-to-end traffic engineering, fast re-route, egress peer engineering, data-center traffic engineering as described in [I-D.ietf-spring-problem-statement], [I-D.ietf-spring-ipv6-use-cases] and [I-D.ietf-spring-resiliency-use-cases]. Typically, an IPv6 packet received at ingress (i.e.: from outside the SR domain), is classified according to network operator policies and such classification results into an outer header with an SRH applied to the incoming packet. The SRH contains the list of segment representing the path the packet must take inside the SR domain. Thus, the SA of the packet is the ingress node, the DA (due to SRH procedures described in Section 4) is set as the first segment of the path and the last segment of the path is the egress node of the SR domain. The path may include intra-AS as well as inter-AS segments. It has to be noted that all nodes within the SR domain are under control of the same administration. When the packet reaches the egress point of the SR domain, the outer header and its SRH are removed so that the destination of the packet is unaware of the SR domain the packet has traversed. The outer header with the SRH is no different from any other tunneling encapsulation mechanism and allows a network operator to implement traffic engineering mechanisms so to efficiently steer traffic across his infrastructure. 2.2.2. SR Domain in a Overlay Network The following figure illustrates an SR domain consisting of an overlay network over multiple operator's networks. (--Operator 1---) (-----Operator 2-----) (--Operator 3---) ( ) ( ) ( ) A1--(--11---13--14--)--(--21---22---23--24--)--(-31---32---34--)--C1 ( /|\ /|\ /| ) ( |\ /|\ /|\ /| ) ( |\ /|\ /| \ ) A2--(/ | \/ | \/ | ) ( | \/ | \/ | \/ | ) ( | \/ | \/ | \)--C2 ( | /\ | /\ | ) ( | /\ | /\ | /\ | ) ( | /\ | /\ | ) ( |/ \|/ \| ) ( |/ \|/ \|/ \| ) ( |/ \|/ \| ) A3--(--15---17--18--)--(--25---26---27--28--)--(-35---36---38--)--C3 ( ) ( | | | ) ( ) (---------------) (--|----|---------|--) (---------------) | | | B1 B2 B3 Figure 2: Overlay SR Domain Previdi, et al. Expires June 16, 2016 [Page 6] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 Figure 2 describes an overlay consisting of nodes connected to three different network operators and forming a single overlay network where Segment routing packets are exchanged. The overlay consists of nodes A1, A2, A3, B1, B2, B3, C1, C2 and C3. These nodes are connected to their respective network operator and form an overlay network. Each node may originate packets with an SRH which contains, in the segment list of the SRH or in the DA, segments identifying other overlay nodes. This implies that packets with an SRH may traverse operator's networks but, obviously, these SRHs cannot contain an address/segment of the transit operators 1, 2 and 3. The SRH originated by the overlay can only contain address/segment under the administration of the overlay (e.g. address/segments supported by A1, A2, A3, B1, B2, B3, C1,C2 or C3). In this model, the operator network nodes are transit nodes and, according to [RFC2460], MUST NOT inspect the routing extension header since there are not the DA of the packet. It is a common practice in operators networks to filter out, at ingress, any packet whose DA is the address of an internal node and it is also possible that an operator would filter out any packet destined to an internal address and having an extension header in it. This common practice does not impact the SR-enabled traffic between the overlay nodes as the intermediate transit networks do never see a destination address belonging to their infrastructure. These SR- enabled overlay packets will thus never be filtered by the transit operators. In all cases, transit packets (i.e.: packets whose DA is outside the domain of the operator's network) will be forwarded accordingly without introducing any security concern in the operator's network. This is similar to tunneled packets. 3. Segment Routing Extension Header (SRH) A new type of the Routing Header (originally defined in [RFC2460]) is defined: the Segment Routing Header (SRH) which has a new Routing Type, (suggested value 4) to be assigned by IANA. The Segment Routing Header (SRH) is defined 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 Previdi, et al. Expires June 16, 2016 [Page 7] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len | Routing Type | Segments Left | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | First Segment | Flags | HMAC Key ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Segment List (128 bits IPv6 address) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | ... | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Segment List[n] (128 bits IPv6 address) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Policy List (optional) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Policy List (optional) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Policy List (optional) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Policy List (optional) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | | | HMAC (256 bits) | | (optional) | | | | | Previdi, et al. Expires June 16, 2016 [Page 8] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where: o Next Header: 8-bit selector. Identifies the type of header immediately following the SRH. o Hdr Ext Len: 8-bit unsigned integer, is the length of the SRH header in 8-octet units, not including the first 8 octets. o Routing Type: TBD, to be assigned by IANA (suggested value: 4). o Segments Left. Defined in [RFC2460], it contains the index, in the Segment List, of the next segment to inspect. Segments Left is decremented at each segment. o First Segment: contains the index, in the Segment List, of the first segment of the path which is in fact the last element of the Segment List. o Flags: 16 bits of flags. Following flags are defined: 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |C|P|R|R| Policy Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ C-flag: Clean-up flag. Set when the SRH has to be removed from the packet when packet reaches the last segment. P-flag: Protected flag. Set when the packet has been rerouted through FRR mechanism by a SR endpoint node. R-flags. Reserved and for future use. Policy Flags. Define the type of the IPv6 addresses encoded into the Policy List (see below). The following have been defined: Bits 4-6: determine the type of the first element after the segment list. Bits 7-9: determine the type of the second element. Bits 10-12: determine the type of the third element. Previdi, et al. Expires June 16, 2016 [Page 9] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 Bits 13-15: determine the type of the fourth element. The following values are used for the type: 0x0: Not present. If value is set to 0x0, it means the element represented by these bits is not present. 0x1: SR Ingress. 0x2: SR Egress. 0x3: Original Source Address. 0x4 to 0x7: currently unused and SHOULD be ignored on reception. o HMAC Key ID and HMAC field, and their use are defined in Section 5. o Segment List[n]: 128 bit IPv6 addresses representing the nth segment in the Segment List. The Segment List is encoded starting from the last segment of the path. I.e., the first element of the segment list (Segment List ) contains the last segment of the path while the last segment of the Segment List (Segment List[n]) contains the first segment of the path. The index contained in "Segments Left" identifies the current active segment. o Policy List. Optional addresses representing specific nodes in the SR path such as: SR Ingress: a 128 bit generic identifier representing the ingress in the SR domain (i.e.: it needs not to be a valid IPv6 address). SR Egress: a 128 bit generic identifier representing the egress in the SR domain (i.e.: it needs not to be a valid IPv6 address). Original Source Address: IPv6 address originally present in the SA field of the packet. The segments in the Policy List are encoded after the segment list and they are optional. If none are in the SRH, all bits of the Policy List Flags MUST be set to 0x0. Previdi, et al. Expires June 16, 2016 [Page 10] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 3.1. SRH and RFC2460 behavior The SRH being a new type of the Routing Header, it also has the same properties: SHOULD only appear once in the packet. Only the router whose address is in the DA field of the packet header MUST inspect the SRH. Therefore, Segment Routing in IPv6 networks implies that the segment identifier (i.e.: the IPv6 address of the segment) is moved into the DA of the packet. The DA of the packet changes at each segment termination/completion and therefore the final DA of the packet MUST be encoded as the last segment of the path. 4. SRH Procedures In this section we describe the different procedures on the SRH. 4.1. Segment Routing Node Functions SR packets are forwarded to segments endpoints (i.e.: the segment endpoint is the node representing the segment and whose address is in the segment list and in the DA of the packet when traveling in the segment). The segment endpoint, when receiving a SR packet destined to itself, does: o Inspect the SRH. o Determine the next active segment. o Update the Segments Left field (or, if requested, remove the SRH from the packet). o Update the DA. o Forward the packet to the next segment. The procedures applied to the SRH are related to the node function. Following nodes functions are defined: Source SR Node. SR Domain Ingress Node. Previdi, et al. Expires June 16, 2016 [Page 11] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 Transit Node. SR Endpoint Node. 4.1.1. Source SR Node A Source SR Node can be any node originating an IPv6 packet with its IPv6 and Segment Routing Headers. This include either: A host originating an IPv6 packet A SR domain ingress router encapsulating a received IPv6 packet into an outer IPv6 header followed by a SRH The mechanism through which a Segment List is derived is outside of the scope of this document. As an example, the Segment List may be obtained through: Local path computation. Local configuration. Interaction with a centralized controller delivering the path. Any other mechanism. The following are the steps of the creation of the SRH: Next Header and Hdr Ext Len fields are set according to [RFC2460]. Routing Type field is set as TBD (SRH). The Segment List is built with the FIRST segment of the path encoded in the LAST element of the Segment List. Subsequent segments are encoded on top of the first segment. Finally, the LAST segment of the path is encoded in the FIRST element of the Segment List. In other words, the Segment List is encoded in the reverse order of the path. The final DA of the packet is encoded as the last segment of the path (encoded in the first element of the Segment List). The DA of the packet is set with the value of the first segment (found in the last element of the segment list). The Segments Left field is set to n-1 where n is the number of elements in the Segment List. Previdi, et al. Expires June 16, 2016 [Page 12] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 The First Segment field is set to n-1 where n is the number of elements in the Segment List. The packet is sent out towards the first segment (i.e.: represented in the packet DA). HMAC and HMAC Key ID may be set according to Section 5. 4.1.2. SR Domain Ingress Node The SR Domain Ingress Node is the node where ingress policies are applied and where the packet path (and processing) is determined. After policies are applied and packet classification is done, the result may be instantiated into a Segment List representing the path the packet should take. In such case, the SR Domain Ingress Node instantiate a new outer IPv6 header to which the SRH is appended (with the computed Segment List). The procedures for the creation and insertion of the new SRH are described in Section 4.1.1. 4.1.3. Transit Node According to [RFC2460], the only node who is allowed to inspect the Routing Extension Header (and therefore the SRH), is the node corresponding to the DA of the packet. Any other transit node MUST NOT inspect the underneath routing header and MUST forward the packet towards the DA and according to the IPv6 routing table. In the example case described in Section 2.2.2, when SR capable nodes are connected through an overlay spanning multiple third-party infrastructure, it is safe to send SRH packets (i.e.: packet having a Segment Routing Header) between each other overlay/SR-capable nodes as long as the segment list does not include any of the transit provider nodes. In addition, as a generic security measure, any service provider will block any packet destined to one of its internal routers, especially if these packets have an extended header in it. 4.1.4. SR Segment Endpoint Node The SR segment endpoint node is the node whose address is in the DA. The segment endpoint node inspects the SRH and does: Previdi, et al. Expires June 16, 2016 [Page 13] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 1. IF DA = myself (segment endpoint) 2. IF Segments Left > 0 THEN decrement Segments Left update DA with Segment List[Segments Left] 3. IF Segments Left == 0 THEN IF Clean-up bit is set THEN remove the SRH 4. ELSE continue IPv6 processing of the packet End of processing. 5. Forward the packet out 5. Security Considerations This section analyzes the security threat model, the security issues and proposed solutions related to the new Segment Routing Header. The Segment Routing Header (SRH) is simply another type of the routing header as described in RFC 2460 [RFC2460] and is: o inserted by a SR edge router when entering the segment routing domain or by the originating host itself. The source host can even be outside the SR domain; o inspected and acted upon when reaching the destination address of the IP header per RFC 2460 [RFC2460]. Per RFC2460 [RFC2460], routers on the path that simply forward an IPv6 packet (i.e. the IPv6 destination address is none of theirs) will never inspect and process the content of SRH. Routers whose one interface IPv6 address equals the destination address field of the IPv6 packet MUST to parse the SRH and, if supported and if the local configuration allows it, MUST act accordingly to the SRH content. According to RFC2460 [RFC2460], the default behavior of a non SR- capable router upon receipt of an IPv6 packet with SRH destined to an address of its, is to: o ignore the SRH completely if the Segment Left field is 0 and proceed to process the next header in the IPv6 packet; o discard the IPv6 packet if Segment Left field is greater than 0, it MAY send a Parameter Problem ICMP message back to the Source Address. 5.1. Threat model Previdi, et al. Expires June 16, 2016 [Page 14] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 5.1.1. Source routing threats Using a SRH is similar to source routing, therefore it has some well- known security issues as described in RFC4942 [RFC4942] section 2.1.1 and RFC5095 [RFC5095]: o amplification attacks: where a packet could be forged in such a way to cause looping among a set of SR-enabled routers causing unnecessary traffic, hence a Denial of Service (DoS) against bandwidth; o reflection attack: where a hacker could force an intermediate node to appear as the immediate attacker, hence hiding the real attacker from naive forensic; o bypass attack: where an intermediate node could be used as a stepping stone (for example in a De-Militarized Zone) to attack another host (for example in the datacenter or any back-end server). 5.1.2. Applicability of RFC 5095 to SRH First of all, the reader must remember this specific part of section 1 of RFC5095 [RFC5095], "A side effect is that this also eliminates benign RH0 use-cases; however, such applications may be facilitated by future Routing Header specifications.". In short, it is not forbidden to create new secure type of Routing Header; for example, RFC 6554 (RPL) [RFC6554] also creates a new Routing Header type for a specific application confined in a single network. In the segment routing architecture described in [I-D.ietf-spring-segment-routing] there are basically two kinds of nodes (routers and hosts): o nodes within the SR domain, which is within one single administrative domain, i.e., where all nodes are trusted anyway else the damage caused by those nodes could be worse than amplification attacks: traffic interception, man-in-the-middle attacks, more server DoS by dropping packets, and so on. o nodes outside of the SR domain, which is outside of the administrative segment routing domain hence they cannot be trusted because there is no physical security for those nodes, i.e., they can be replaced by hostile nodes or can be coerced in wrong behaviors. The main use case for SR consists of the single administrative domain where only trusted nodes with SR enabled and configured participate Previdi, et al. Expires June 16, 2016 [Page 15] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 in SR: this is the same model as in RFC6554 [RFC6554]. All non- trusted nodes do not participate as either SR processing is not enabled by default or because they only process SRH from nodes within their domain. Moreover, all SR nodes ignore SRH created by outsiders based on topology information (received on a peering or internal interface) or on presence and validity of the HMAC field. Therefore, if intermediate nodes ONLY act on valid and authorized SRH (such as within a single administrative domain), then there is no security threat similar to RH-0. Hence, the RFC 5095 [RFC5095] attacks are not applicable. 5.1.3. Service stealing threat Segment routing is used for added value services, there is also a need to prevent non-participating nodes to use those services; this is called 'service stealing prevention'. 5.1.4. Topology disclosure The SRH may also contains IPv6 addresses of some intermediate SR- nodes in the path towards the destination, this obviously reveals those addresses to the potentially hostile attackers if those attackers are able to intercept packets containing SRH. On the other hand, if the attacker can do a traceroute whose probes will be forwarded along the SR path, then there is little learned by intercepting the SRH itself. Also the clean-bit of SRH can help by removing the SRH before forwarding the packet to potentially a non- trusted part of the network. 5.1.5. ICMP Generation Per section 4.4 of RFC2460 [RFC2460], when destination nodes (i.e. where the destination address is one of theirs) receive a Routing Header with unsupported Routing Type, the required behavior is: o If Segments Left is zero, the node must ignore the Routing header and proceed to process the next header in the packet. o If Segments Left is non-zero, the node must discard the packet and send an ICMP Parameter Problem, Code 0, message to the packet's Source Address, pointing to the unrecognized Routing Type. This required behavior could be used by an attacker to force the generation of ICMP message by any node. The attacker could send packets with SRH (with Segment Left set to 0) destined to a node not supporting SRH. Per RFC2460 [RFC2460], the destination node could Previdi, et al. Expires June 16, 2016 [Page 16] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 generate an ICMP message, causing a local CPU utilization and if the source of the offending packet with SRH was spoofed could lead to a reflection attack without any amplification. It must be noted that this is a required behavior for any unsupported Routing Type and not limited to SRH packets. So, it is not specific to SRH and the usual rate limiting for ICMP generation is required anyway for any IPv6 implementation and has been implemented and deployed for many years. 5.2. Security fields in SRH This section summarizes the use of specific fields in the SRH. They are based on a key-hashed message authentication code (HMAC). The security-related fields in SRH are: o HMAC Key-id, 8 bits wide; o HMAC, 256 bits wide (optional, exists only if HMAC Key-id is not 0). The HMAC field is the output of the HMAC computation (per RFC 2104 [RFC2104]) using a pre-shared key identified by HMAC Key-id and of the text which consists of the concatenation of: o the source IPv6 address; o First Segment field; o an octet whose bit-0 is the clean-up bit flag and others are 0; o HMAC Key-id; o all addresses in the Segment List. The purpose of the HMAC field is to verify the validity, the integrity and the authorization of the SRH itself. If an outsider of the SR domain does not have access to a current pre-shared secret, then it cannot compute the right HMAC field and the first SR router on the path processing the SRH and configured to check the validity of the HMAC will simply reject the packet. The HMAC field is located at the end of the SRH simply because only the router on the ingress of the SR domain needs to process it, then all other SR nodes can ignore it (based on local policy) because they trust the upstream router. This is to speed up forwarding operations Previdi, et al. Expires June 16, 2016 [Page 17] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 because SR routers which do not validate the SRH do not need to parse the SRH until the end. The HMAC Key-id field allows for the simultaneous existence of several hash algorithms (SHA-256, SHA3-256 ... or future ones) as well as pre-shared keys. The HMAC Key-id field is opaque, i.e., it has neither syntax nor semantic except as an index to the right combination of pre-shared key and hash algorithm and except that a value of 0 means that there is no HMAC field. Having a HMAC Key-id field allows for pre-shared key roll-over when two pre-shared keys are supported for a while when all SR nodes converged to a fresher pre-shared key. It could also allow for interoperation among different SR domains if allowed by local policy and assuming a collision-free HMAC Key Id allocation. When a specific SRH is linked to a time-related service (such as turbo-QoS for a 1-hour period) where the DA, Segment ID (SID) are identical, then it is important to refresh the shared-secret frequently as the HMAC validity period expires only when the HMAC Key-id and its associated shared-secret expires. 5.2.1. Selecting a hash algorithm The HMAC field in the SRH is 256 bit wide. Therefore, the HMAC MUST be based on a hash function whose output is at least 256 bits. If the output of the hash function is 256, then this output is simply inserted in the HMAC field. If the output of the hash function is larger than 256 bits, then the output value is truncated to 256 by taking the least-significant 256 bits and inserting them in the HMAC field. SRH implementations can support multiple hash functions but MUST implement SHA-2 [FIPS180-4] in its SHA-256 variant. NOTE: SHA-1 is currently used by some early implementations used for quick interoperations testing, the 160-bit hash value must then be right-hand padded with 96 bits set to 0. The authors understand that this is not secure but is ok for limited tests. 5.2.2. Performance impact of HMAC While adding a HMAC to each and every SR packet increases the security, it has a performance impact. Nevertheless, it must be noted that: o the HMAC field is used only when SRH is inserted by a device (such as a home set-up box) which is outside of the segment routing domain. If the SRH is added by a router in the trusted segment Previdi, et al. Expires June 16, 2016 [Page 18] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 routing domain, then, there is no need for a HMAC field, hence no performance impact. o when present, the HMAC field MUST only be checked and validated by the first router of the segment routing domain, this router is named 'validating SR router'. Downstream routers may not inspect the HMAC field. o this validating router can also have a cache of <IPv6 header + SRH, HMAC field value> to improve the performance. It is not the same use case as in IPsec where HMAC value was unique per packet, in SRH, the HMAC value is unique per flow. o Last point, hash functions such as SHA-2 have been optimized for security and performance and there are multiple implementations with good performance. With the above points in mind, the performance impact of using HMAC is minimized. 5.2.3. Pre-shared key management The field HMAC Key-id allows for: o key roll-over: when there is a need to change the key (the hash pre-shared secret), then multiple pre-shared keys can be used simultaneously. The validating routing can have a table of <HMAC Key-id, pre-shared secret> for the currently active and future keys. o different algorithms: by extending the previous table to <HMAC Key-id, hash function, pre-shared secret>, the validating router can also support simultaneously several hash algorithms (see section Section 5.2.1) The pre-shared secret distribution can be done: o in the configuration of the validating routers, either by static configuration or any SDN oriented approach; o dynamically using a trusted key distribution such as [RFC6407] The intent of this document is NOT to define yet-another-key- distribution-protocol. Previdi, et al. Expires June 16, 2016 [Page 19] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 5.3. Deployment Models 5.3.1. Nodes within the SR domain A SR domain is defined as a set of interconnected routers where all routers at the perimeter are configured to insert and act on SRH. Some routers inside the SR domain can also act on SRH or simply forward IPv6 packets. The routers inside a SR domain can be trusted to generate SRH and to process SRH received on interfaces that are part of the SR domain. These nodes MUST drop all SRH packets received on an interface that is not part of the SR domain and containing a SRH whose HMAC field cannot be validated by local policies. This includes obviously packet with a SRH generated by a non-cooperative SR domain. If the validation fails, then these packets MUST be dropped, ICMP error messages (parameter problem) SHOULD be generated (but rate limited) and SHOULD be logged. 5.3.2. Nodes outside of the SR domain Nodes outside of the SR domain cannot be trusted for physical security; hence, they need to request by some trusted means (outside of the scope of this document) a complete SRH for each new connection (i.e. new destination address). The received SRH MUST include a HMAC Key-id and HMAC field which is computed correctly (see Section 5.2). When an outside node sends a packet with an SRH and towards a SR domain ingress node, the packet MUST contain the HMAC Key-id and HMAC field and the the destination address MUST be an address of a SR domain ingress node . The ingress SR router, i.e., the router with an interface address equals to the destination address, MUST verify the HMAC field with respect to the HMAC Key-id. If the validation is successful, then the packet is simply forwarded as usual for a SR packet. As long as the packet travels within the SR domain, no further HMAC check needs to be done. Subsequent routers in the SR domain MAY verify the HMAC field when they process the SRH (i.e. when they are the destination). If the validation fails, then this packet MUST be dropped, an ICMP error message (parameter problem) SHOULD be generated (but rate limited) and SHOULD be logged. Previdi, et al. Expires June 16, 2016 [Page 20] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 5.3.3. SR path exposure As the intermediate SR nodes addresses appears in the SRH, if this SRH is visible to an outsider then he/she could reuse this knowledge to launch an attack on the intermediate SR nodes or get some insider knowledge on the topology. This is especially applicable when the path between the source node and the first SR domain ingress router is on the public Internet. The first remark is to state that 'security by obscurity' is never enough; in other words, the security policy of the SR domain MUST assume that the internal topology and addressing is known by the attacker. A simple traceroute will also give the same information (with even more information as all intermediate nodes between SID will also be exposed). IPsec Encapsulating Security Payload [RFC4303] cannot be use to protect the SRH as per RFC4303 the ESP header must appear after any routing header (including SRH). To prevent a user to leverage the gained knowledge by intercepting SRH, it it recommended to apply an infrastructure Access Control List (iACL) at the edge of the SR domain. This iACL will drop all packets from outside the SR-domain whose destination is any address of any router inside the domain. This security policy should be tuned for local operations. 5.3.4. Impact of BCP-38 BCP-38 [RFC2827], also known as "Network Ingress Filtering", checks whether the source address of packets received on an interface is valid for this interface. The use of loose source routing such as SRH forces packets to follow a path which differs from the expected routing. Therefore, if BCP-38 was implemented in all routers inside the SR domain, then SR packets could be received by an interface which is not expected one and the packets could be dropped. As a SR domain is usually a subset of one administrative domain, and as BCP-38 is only deployed at the ingress routers of this administrative domain and as packets arriving at those ingress routers have been normally forwarded using the normal routing information, then there is no reason why this ingress router should drop the SRH packet based on BCP-38. Routers inside the domain commonly do not apply BCP-38; so, this is not a problem. 6. IANA Considerations This document makes the following registrations in the Internet Protocol Version 6 (IPv6) Parameters "Routing Type" registry maintained by IANA: Previdi, et al. Expires June 16, 2016 [Page 21] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 Suggested Description Reference Value ---------------------------------------------------------- 4 Segment Routing Header (SRH) This document 7. Manageability Considerations TBD 8. Contributors Dave Barach, John Leddy, John Brzozowski, Pierre Francois, Nagendra Kumar, Mark Townsley, Christian Martin, Roberta Maglione, James Connolly, Aloys Augustin contributed to the content of this document. 9. Acknowledgements The authors would like to thank Ole Troan, Bob Hinden, Fred Baker, Brian Carpenter and Alexandru Petrescu for their comments to this document. 10. References 10.1. Normative References [FIPS180-4] National Institute of Standards and Technology, "FIPS 180-4 Secure Hash Standard (SHS)", March 2012, <http://csrc.nist.gov/publications/fips/fips180-4/ fips-180-4.pdf>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998, <http://www.rfc-editor.org/info/rfc2460>. [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005, <http://www.rfc-editor.org/info/rfc4303>. [RFC5095] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation of Type 0 Routing Headers in IPv6", RFC 5095, DOI 10.17487/RFC5095, December 2007, <http://www.rfc-editor.org/info/rfc5095>. Previdi, et al. Expires June 16, 2016 [Page 22] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 [RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain of Interpretation", RFC 6407, DOI 10.17487/RFC6407, October 2011, <http://www.rfc-editor.org/info/rfc6407>. 10.2. Informative References [I-D.ietf-isis-segment-routing-extensions] Previdi, S., Filsfils, C., Bashandy, A., Gredler, H., Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS Extensions for Segment Routing", draft-ietf-isis-segment- routing-extensions-05 (work in progress), June 2015. [I-D.ietf-ospf-ospfv3-segment-routing-extensions] Psenak, P., Previdi, S., Filsfils, C., Gredler, H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3 Extensions for Segment Routing", draft-ietf-ospf-ospfv3- segment-routing-extensions-03 (work in progress), June 2015. [I-D.ietf-spring-ipv6-use-cases] Brzozowski, J., Leddy, J., Leung, I., Previdi, S., Townsley, W., Martin, C., Filsfils, C., and R. Maglione, "IPv6 SPRING Use Cases", draft-ietf-spring-ipv6-use- cases-05 (work in progress), September 2015. [I-D.ietf-spring-problem-statement] Previdi, S., Filsfils, C., Decraene, B., Litkowski, S., Horneffer, M., and r. email@example.com, "SPRING Problem Statement and Requirements", draft-ietf-spring-problem- statement-05 (work in progress), October 2015. [I-D.ietf-spring-resiliency-use-cases] Francois, P., Filsfils, C., Decraene, B., and r. firstname.lastname@example.org, "Use-cases for Resiliency in SPRING", draft- ietf-spring-resiliency-use-cases-02 (work in progress), December 2015. [I-D.ietf-spring-segment-routing] Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and r. email@example.com, "Segment Routing Architecture", draft- ietf-spring-segment-routing-06 (work in progress), October 2015. Previdi, et al. Expires June 16, 2016 [Page 23] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 [I-D.ietf-spring-segment-routing-mpls] Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., Litkowski, S., Horneffer, M., firstname.lastname@example.org, r., Tantsura, J., and E. Crabbe, "Segment Routing with MPLS data plane", draft-ietf-spring-segment-routing-mpls-02 (work in progress), October 2015. [RFC1940] Estrin, D., Li, T., Rekhter, Y., Varadhan, K., and D. Zappala, "Source Demand Routing: Packet Format and Forwarding Specification (Version 1)", RFC 1940, DOI 10.17487/RFC1940, May 1996, <http://www.rfc-editor.org/info/rfc1940>. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, <http://www.rfc-editor.org/info/rfc2104>. [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, May 2000, <http://www.rfc-editor.org/info/rfc2827>. [RFC4942] Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/ Co-existence Security Considerations", RFC 4942, DOI 10.17487/RFC4942, September 2007, <http://www.rfc-editor.org/info/rfc4942>. [RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6 Routing Header for Source Routes with the Routing Protocol for Low-Power and Lossy Networks (RPL)", RFC 6554, DOI 10.17487/RFC6554, March 2012, <http://www.rfc-editor.org/info/rfc6554>. Authors' Addresses Stefano Previdi (editor) Cisco Systems, Inc. Via Del Serafico, 200 Rome 00142 Italy Email: email@example.com Previdi, et al. Expires June 16, 2016 [Page 24] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 Clarence Filsfils Cisco Systems, Inc. Brussels BE Email: firstname.lastname@example.org Brian Field Comcast 4100 East Dry Creek Road Centennial, CO 80122 US Email: Brian_Field@cable.comcast.com Ida Leung Rogers Communications 8200 Dixie Road Brampton, ON L6T 0C1 CA Email: Ida.Leung@rci.rogers.com Jen Linkova Google 1600 Amphitheatre Parkway Mountain View, CA 94043 US Email: email@example.com Ebben Aries Facebook US Email: firstname.lastname@example.org Previdi, et al. Expires June 16, 2016 [Page 25] Internet-Draft IPv6 Segment Routing Header (SRH) December 2015 Tomoya Kosugi NTT 3-9-11, Midori-Cho Musashino-Shi, Tokyo 180-8585 JP Email: email@example.com Eric Vyncke Cisco Systems, Inc. De Kleetlaann 6A Diegem 1831 Belgium Email: firstname.lastname@example.org David Lebrun Universite Catholique de Louvain Place Ste Barbe, 2 Louvain-la-Neuve, 1348 Belgium Email: email@example.com Previdi, et al. Expires June 16, 2016 [Page 26]