| Network Working Group | S. Previdi, Ed. |
| Internet-Draft | Individual |
| Intended status: Standards Track | C. Filsfils |
| Expires: September 6, 2018 | K. Raza |
| D. Dukes | |
| Cisco Systems, Inc. | |
| J. Leddy | |
| B. Field | |
| Comcast | |
| D. Voyer | |
| D. Bernier | |
| Bell Canada | |
| S. Matsushima | |
| Softbank | |
| I. Leung | |
| Rogers Communications | |
| J. Linkova | |
| E. Aries | |
| T. Kosugi | |
| NTT | |
| E. Vyncke | |
| Cisco Systems, Inc. | |
| D. Lebrun | |
| Universite Catholique de Louvain | |
| D. Steinberg | |
| Steinberg Consulting | |
| R. Raszuk | |
| Bloomberg | |
| March 5, 2018 |
IPv6 Segment Routing Header (SRH)
draft-ietf-6man-segment-routing-header-09
Segment Routing (SR) allows a node to steer a packet through a controlled set of instructions, called segments, by prepending an 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.
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.
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 September 6, 2018.
Copyright (c) 2018 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.
Segment Routing terminology is defined in [I-D.ietf-spring-segment-routing].
The network programming paradigm [I-D.filsfils-spring-srv6-network-programming] defines the basic functions associated to an SRv6 SID.
Segment Routing use cases are described in [RFC7855] 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].
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 an 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.
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 [RFC8200]. The source based routing model offers the support for explicit routing capability.
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 [RFC8200]) 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.
An SRv6 Segment is a 128-bit value. “SRv6 SID” or simply “SID” are often used as a shorter reference for “SRv6 Segment”.
An SRv6-capable node N maintains a “My Local SID Table”. This table contains all the local SRv6 segments explicitly instantiated at node N. N is the parent node for these SID’s.
A local SID of N could be an IPv6 address of a local interface of N but it does not have to. Most often, a local SID is not an address of a local interface of N. The My Local SID Table is thus not populated by default with all the addresses of a node.
An illustration is provided in [I-D.filsfils-spring-srv6-network-programming].
Every SRv6 local SID instantiated has a specific instruction bounded to it.
This information is stored in the "My Local SID Table". The “My Local SID Table” has three main purposes:
A node may receive a packet with an SRv6 SID in the DA without an SRH. In such case the packet should still be processed by the Segment Routing engine.
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:
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.
It is assumed in this document that the SRH is added to the packet by its source, consistently with the source routing model defined in [RFC8200]. For example:
(-------------------------- 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
The following figure illustrates an SR domain consisting of an operator's network infrastructure.
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 such as end-to-end traffic engineering, fast re-route, egress peer engineering, data-center traffic engineering as described in [RFC7855], [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 5) 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.
(--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
The following figure illustrates an SR domain consisting of an overlay network over multiple operator's networks.
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 [RFC8200], MUST NOT inspect the routing extension header since they 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 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.
A new type of the Routing Header (originally defined in [RFC8200]) is defined: the Segment Routing Header (SRH) which has a new Routing Type, (suggested value 4) to be assigned by IANA.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last Entry | Flags | Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[0] (128 bits IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
...
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[n] (128 bits IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// //
// Optional Type Length Value objects (variable) //
// //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|U|P|O|A|H| U |
+-+-+-+-+-+-+-+-+
The Segment Routing Header (SRH) is defined as follows:
This section defines TLVs of the Segment Routing Header.
Type Length Value (TLV) contain optional information that may be used by the node identified in the DA of the packet. It has to be noted that the information carried in the TLVs is not intended to be used by the routing layer. Typically, TLVs carry information that is consumed by other components (e.g.: OAM) than the routing function.
Each TLV has its own length, format and semantic. The code-point allocated (by IANA) to each TLV defines both the format and the semantic of the information carried in the TLV. Multiple TLVs may be encoded in the same SRH.
The "Length" field of the TLV is primarily used to skip the TLV while inspecting the SRH in case the node doesn't support or recognize the TLV codepoint. The "Length" defines the TLV length in octets and not including the "Type" and "Length" fields.
The following TLVs are defined in this document:
Additional TLVs may be defined in the future.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | RESERVED | Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Ingress Node (16 octets) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
The Ingress Node TLV is optional and identifies the node this packet traversed when entered the SR domain. The Ingress Node TLV has following format:
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | RESERVED | Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Egress Node (16 octets) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
The Egress Node TLV is optional and identifies the node this packet is expected to traverse when exiting the SR domain. The Egress Node TLV has following format:
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | RESERVED | Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Opaque Container (16 octets) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
The Opaque Container TLV is optional and has the following format:
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Padding (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Padding (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
The Padding TLV is optional and with the purpose of aligning the SRH on a 8 octet boundary. The Padding TLV has the following format:
The following applies to the Padding TLV:
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | RESERVED | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HMAC Key ID (4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | // | HMAC (32 octets) // | // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
HMAC TLV is optional and contains the HMAC information. The HMAC TLV has the following format:
The Following applies to the HMAC TLV:
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // NSH Carried Object // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
The NSH Carrier TLV is a container used in order to carry TLVs that have been defined in [I-D.ietf-sfc-nsh]. The NSH Carrier TLV has the following format:
The SRH being a new type of the Routing Header, it also has the same properties:
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.
Segment Routing architecture, as defined in [I-D.ietf-spring-segment-routing] defines a segment as an instruction or, more generally, a set of instructions (function).
[I-D.filsfils-spring-srv6-network-programming] defines the basic functions associated with SRv6 SID’s.
In this section we review two of these functions that may be associated to a segment:
More functions are defined in [I-D.filsfils-spring-srv6-network-programming].
The Endpoint function (End) is the most basic function.
1. IF SegmentsLeft > 0 THEN 2. decrement SL 3. update the IPv6 DA with SRH[SL] 4. FIB lookup on updated DA 5. forward accordingly to the matched entry 6. ELSE 7. drop the packet
When the endpoint node receives a packet destined to DA "S" and S is an entry in the MyLocalSID table and the function associated with S in that entry is “End” then the endpoint does:
It has to be noted that:
1. IF SegmentsLeft > 0 THEN
2. decrement SL
3. update the IPv6 DA with SRH[SL]
4. forward to layer-3 adjacency bound to the SID "S"
5. ELSE
6. drop the packet
When the endpoint node receives a packet destined to DA "S" and S is an entry in the MyLocalSID table and the function associated with S in that entry is “End.X” then the endpoint does:
It has to be noted that:
There are different types of nodes that may be involved in segment routing networks: source nodes that originate packets with an SRH, transit nodes that forward packets having an SRH and segment endpoint nodes that MUST process the SRH.
A Source SR Node can be any node originating an IPv6 packet with its IPv6 and Segment Routing Headers. This include either:
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:
The following are the steps of the creation of the SRH:
According to [RFC8200], 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 2.3.2, 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.
The SR segment endpoint node is the node whose MyLocalSID table contains an entry for the DA of the packet.
The segment endpoint node executes the function bound to the SID as per the matched MyLocalSID entry (Section 4).
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 8200 and is:
Per RFC8200, 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 the SRH. Routers whose one interface IPv6 address equals the destination address field of the IPv6 packet MUST parse the SRH and, if supported and if the local configuration allows it, MUST act accordingly to the SRH content.
According to RFC8200, 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:
Using an SRH is similar to source routing, therefore it has some well-known security issues as described in RFC4942 section 2.1.1 and RFC5095:
First of all, the reader must remember this specific part of section 1 of 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) 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):
The main use case for SR consists of the single administrative domain where only trusted nodes with SR enabled and configured participate in SR: this is the same model as in 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 attacks are not applicable.
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’.
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.
Per section 4.4 of RFC8200, 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:
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 RFC8200, the destination node could 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.
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 the SRH are instantiated by the HMAC TLV, containing:
The HMAC field is the output of the HMAC computation (per RFC 2104) using a pre-shared key identified by HMAC Key-id and of the text which consists of the concatenation of:
The purpose of the HMAC TLV 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 TLV 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 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 an 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.
The HMAC field in the HMAC TLV 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 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.
While adding an HMAC to each and every SR packet increases the security, it has a performance impact. Nevertheless, it must be noted that:
With the above points in mind, the performance impact of using HMAC is minimized.
The field HMAC Key-id allows for:
The pre-shared secret distribution can be done:
The intent of this document is NOT to define yet-another-key-distribution-protocol.
An SR domain is defined as a set of interconnected routers where all routers at the perimeter are configured to add and act on SRH. Some routers inside the SR domain can also act on SRH or simply forward IPv6 packets.
The routers inside an 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 an SRH whose HMAC field cannot be validated by local policies. This includes obviously packet with an 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.
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 an HMAC TLV which is computed correctly (see Section 6.2).
When an outside node sends a packet with an SRH and towards an SR domain ingress node, the packet MUST contain the HMAC TLV (with a Key-id and HMAC fields) and the the destination address MUST be an address of an 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 TLV.
If the validation is successful, then the packet is simply forwarded as usual for an 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 TLV 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.
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.
BCP-38, 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 an 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.
Suggested Description Reference Value ---------------------------------------------------------- 4 Segment Routing Header (SRH) This document
This document makes the following registrations in the Internet Protocol Version 6 (IPv6) Parameters "Routing Type" registry maintained by IANA:
This document request IANA to create and maintain a new Registry: "Segment Routing Header TLVs"
Suggested Description Reference Value ----------------------------------------------------- 1 Ingress Node TLV This document 2 Egress Node TLV This document 3 Opaque Container TLV This document 4 Padding TLV This document 5 HMAC TLV This document 6 NSH Carrier TLV This document
This document requests the creation of a new IANA managed registry to identify SRH TLVs. The registration procedure is "Expert Review" as defined in [RFC8126]. Suggested registry name is "Segment Routing Header TLVs”. A TLV is identified through an unsigned 8 bit codepoint value. The following codepoints are defined in this document:
TBD
Dave Barach, John Brzozowski, Pierre Francois, Nagendra Kumar, Mark Townsley, Christian Martin, Roberta Maglione, James Connolly, Aloys Augustin contributed to the content of this document.
The authors would like to thank Ole Troan, Bob Hinden, Fred Baker, Brian Carpenter, Alexandru Petrescu and Punit Kumar Jaiswal for their comments to this document.
| [FIPS180-4] | National Institute of Standards and Technology, "FIPS 180-4 Secure Hash Standard (SHS)", March 2012. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC4303] | Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005. |
| [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. |
| [RFC6407] | Weis, B., Rowles, S. and T. Hardjono, "The Group Domain of Interpretation", RFC 6407, DOI 10.17487/RFC6407, October 2011. |
| [RFC8200] | Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017. |