| SFC | M. Boucadair |
| Internet-Draft | Orange |
| Intended status: Standards Track | T. Reddy |
| Expires: December 21, 2020 | McAfee |
| D. Wing | |
| Citrix | |
| June 19, 2020 |
Integrity Protection for the Network Service Header (NSH) and Encryption of Sensitive Context Headers
draft-ietf-sfc-nsh-integrity-00
This specification adds integrity protection and optional encryption of sensitive metadata directly to the Network Service Header (NSH) used for Service Function Chaining (SFC).
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 December 21, 2020.
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 (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.
Many advanced Service Functions (SFs) are invoked for the delivery of value-added services. Typically, SFs are used to meet various service objectives such as IP address sharing, avoiding covert channels, detecting Denial-of-Service (DoS) attacks and protecting network infrastructures against them, network slicing, etc. Because of the proliferation of such advanced SFs together with complex service deployment constraints that demand more agile service delivery procedures, operators need to rationalize their service delivery logics and master their complexity while optimising service activation time cycles. The overall problem space is described in [RFC7498].
[RFC7665] presents a data plane architecture addressing the problematic aspects of existing service deployments, including topological dependence and configuration complexity. It also describes an architecture for the specification, creation, and maintenance of Service Function Chains (SFCs) within a network. That is, how to define an ordered set of SFs and ordering constraints that must be applied to packets/flows selected as a result of traffic classification. [RFC8300] specifies the SFC encapsulation: Network Service Header (NSH).
The NSH data is unauthenticated and unencrypted [RFC8300], forcing a service topology that requires security and privacy to use a transport encapsulation that supports such features. Note that some transport encapsulation (e.g., IPsec) only provide hop-by-hop security between two SFC data plane elements (e.g., two Service Function Forwarders (SFFs), SFF to SF) and do not provide SF-to-SF security of NSH metadata. For example, if IPsec is used, SFFs or SFs within a Service Function Path (SFP) not authorized to access the privacy-sensitive metadata will have access to the metadata. As a reminder, the metadata referred to is an information that is inserted by Classifiers or intermediate SFs and shared with downstream SFs; such information is not visible to the communication endpoints (Section 4.9 of [RFC7665]).
The lack of such capability was reported during the development of [RFC8300] and [RFC8459]. The reader may refer to Section 3.2.1 of [I-D.arkko-farrell-arch-model-t] for a discussion on the need for more awareness about attacks from within closed domains.
This specification fills that gap. Concretely, this document adds integrity protection and optional encryption of sensitive metadata directly to the NSH (Section 4); integrity protects the packet payload, and provides replay protection (Section 7.4). Thus, the NSH does not have to rely upon an underlying transport encapsulation for security and confidentiality.
This specification introduces new Variable-Length Context Headers to carry fields necessary for integrity protected NSH headers and encrypted Context Headers (Section 5), and is therefore only applicable to NSH MD Type 0x02 (Section 2.5 of [RFC8300]).
This specification limits thus access to an information within an SFP to entities that have a need to interpret it. Particularly, SFFs should not act or process the Context Headers.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.
This document makes use of the terms defined in [RFC7665] and [RFC8300].
The document defines the following terms:
Section 2 of [RFC8300] specifies that the NSH data can be spread over three headers:
The NSH allows to share context information (a.k.a., metadata) with downstream SFC-aware data elements on a per SFC/SFP basis. To that aim:
In reference to Figure 1,
Thus, the following requirements:
+----------------+-----------------------------+-------------------+ | | Insert, remove, or replace | Update the NSH | | | the NSH | | | | | | | SFC Data Plane +---------+---------+---------+---------+---------+ | Element | | | |Decrement| Update | | | Insert | Remove | Replace | Service | Context | | | | | | Index |Header(s)| +================+=========+=========+=========+=========+=========+ | | + | | + | | + | | Classifier | | | | | | +----------------+---------+---------+---------+---------+---------+ |Service Function| | + | | | | |Forwarder (SFF) | | | | | | +----------------+---------+---------+---------+---------+---------+ |Service Function| | | | + | + | | (SF) | | | | | | +----------------+---------+---------+---------+---------+---------+ | | + | + | | + | + | | SFC Proxy | | | | | | +----------------+---------+---------+---------+---------+---------+
Figure 1: Summary of NSH Actions
This specification provides the functions described in the following subsections:
The solution allows to encrypt all or a subset of NSH Context Headers by Classifiers, SFC-aware SFs, and SFC proxies.
As depicted in Table 1, SFFs are not involved in data encryption. This document enforces this design approach by encrypting Context Headers with keys that are not supplied to SFFs, thus enforcing this limitation by protocol (rather than requirements language).
| Data Plane Element | Base and Service Headers Encryption | Metadata Encryption |
|---|---|---|
| Classifier | No | Yes |
| SFF | No | No |
| SFC-aware SF | No | Yes |
| SFC Proxy | No | Yes |
| SFC-unaware SF | No | No |
The SFC control plane is assumed to instruct the Classifier(s), SFC-aware SFs, and SFC proxies with the set of Context Headers (privacy-sensitive metadata, typically) that must be encrypted. Encryption keying material is only provided to these SFC data elements.
The control plane may also indicate the set of SFC data plane elements that are entitled to supply a given context header (e.g., in reference to their identifiers as assigned within the SFC-enabled domain). It is out of the scope of this document to elaborate on how such instructions are provided to the appropriate SFC data plane elements, nor to detail the structure used to store the instructions.
The Service Path Header (Section 2 of [RFC8300]) is not encrypted because SFFs use Service Index (SI) in conjunction with Service Path Identifier (SPI) for determining the next SF in the path.
The solution provides integrity protection for the NSH data. Two levels of assurance (LoAs) are supported.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Transport Encapsulation | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+... | Base Header | | +->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ N | | Service Path Header | S | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ H | | Context Header(s) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+... | | Original Packet | +->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +------Scope of integrity protected data
Figure 2: First Level of Assurance
A first level of assurance where all NSH data except the Base Header are integrity protected (Figure 2). In this case, the NSH imposer may be a Classifier, an SFC-aware SF, or an SFC Proxy. SFFs are not thus provided with authentication material. Further details are discussed in Section 5.1.
A second level of assurance where all NSH data, including the Base Header, are integrity protected (Figure 3). In this case, the NSH imposer may be a Classifier, an SFC-aware SF, an SFF, or an SFC Proxy. Further details are provided in Section 5.2.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport Encapsulation |
+->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
| | Base Header | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ N
| | Service Path Header | S
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ H
| | Context Header(s) | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
| | Original Packet |
+->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+----Scope of integrity protected data
Figure 3: Second Level of Assurance
The integrity protection scope is explicitly signaled to SFC-aware SFs and SFC proxies in the NSH by means of a dedicated MD Type (Section 5).
In both levels of assurance, the unencrypted Context Headers and the packet on which the NSH is imposed are subject to integrity protection.
Table 2 lists the roles of SFC data plane elements in providing integrity protection for the NSH.
| Data Plane Element | Integrity Protection |
|---|---|
| Classifier | Yes |
| SFF | No (first LoA); Yes (second LoA) |
| SFC-aware SF | Yes |
| SFC Proxy | Yes |
| SFC-unaware SF | No |
The authenticated encryption algorithm defined in [RFC7518] is used to provide NSH data integrity and to encrypt the Context Headers that carry privacy-sensitive metadata.
The authenticated encryption algorithm provides a unified encryption and authentication operation which turns plaintext into authenticated ciphertext and vice versa. The generation of secondary keys MAC_KEY and ENC_KEY from the secret key (K) is discussed in Section 5.2.2.1 of [RFC7518]:
The advantage of using the authenticated encryption algorithm is that SFC-aware SFs and SFC proxies only need to re-compute the message integrity of the NSH data after decrementing the Service Index (SI) and do not have to re-compute the ciphertext. The other advantage is that SFFs do not have access to the ENC_KEY and cannot act on the encrypted Context Headers and, only in case of the second level of assurance, SFFs do have access to the MAC_KEY. Similarly, an SFC-aware SF or SFC Proxy not allowed to decrypt the Context Headers will not have access to the ENC_KEY.
The authenticated encryption algorithm or HMAC algorithm to be used by SFC data plane elements is typically controlled using the SFC control plane. Mandatory to implement authenticated encryption and HMAC algorithms are listed in Section 4.3.
The authenticated encryption process takes as input four octet strings: a secret key (K), a plaintext (P), Additional Authenticated Data (A) (which contains the data to be authenticated, but not encrypted), and an Initialization Vector (IV). The ciphertext value (E) and the Authentication Tag value (T) are provided as outputs.
In order to decrypt and verify, the cipher takes as input K, IV, A, T, and E. The output is either the plaintext or an error indicating that the decryption failed as described in Section 5.2.2.2 of [RFC7518].
Classifiers, SFC-aware SFs, and SFC proxies MUST implement the AES_128_CBC_HMAC_SHA_256 algorithm and SHOULD implement the AES_192_CBC_HMAC_SHA_384 and AES_256_CBC_HMAC_SHA_512 algorithms.
Classifiers, SFC-aware SFs, and SFC proxies MUST implement the HMAC-SHA-256-128 algorithm and SHOULD implement the HMAC-SHA-384-192 and HMAC-SHA-512-256 algorithms.
SFFs MAY implement the aforementioned cipher suites and HMAC algorithms.
The procedure for the allocation/provisioning of secret keys (K) and authenticated encryption algorithm or MAC_KEY and HMAC algorithm is outside the scope of this specification. As such, this specification does not mandate the support of any specific mechanism.
The documents does not assume nor preclude the following:
In order to accommodate deployments relying upon keying material per SFC/SFP and also the need to update keys after encrypting NSH data for certain amount of time, this document uses key identifier (kid) to unambiguously identify the appropriate keying material. Doing so allows to address the problem of synchronization of keying material.
Additional information on manual vs. automated key management and when one should be used over the other can be found in [RFC4107].
New NSH Variable-Length Context Headers are defined in Section 5 for NSH data integrity protection and, optionally, encryption of Context Headers carrying privacy-sensitive metadata. Concretely, an NSH imposer includes (1) the key identifier to identify the keying material, (2) the timestamp to protect against replay attacks (Section 7.4), and (3) the Message Authentication Code (MAC) for the target NSH data (depending on the integrity protection scope) calculated using the MAC_KEY and optionally Context Headers encrypted using ENC_KEY.
An SFC data plane element that needs to check the integrity of the NSH data uses MAC_KEY and the HMAC algorithm for the key identifier being carried in the NSH.
An SFC-aware SF or SFC Proxy that needs to decrypt some Context Headers uses ENC_Key and the decryption algorithm for the key identifier being carried in the NSH.
Section 7 specifies the detailed procedure.
As discussed in [RFC8459], an SFC-enabled domain (called, upper-level domain) may be decomposed into many sub-domains (called, lower-level domains). In order to avoid maintaining state to restore back upper-lower NSH information at the boundaries of lower-level domains, two NSH levels are used: an Upper-NSH which is imposed at the boundaries of the upper-level domain and a Lower-NSH that is pushed by the Classifier of a lower-level domain in front of the original NSH (Figure 4). As such, the Upper-NSH information is carried along the lower-level chain without modification. The packet is forwarded in the top-level domain according to the Upper-NSH, while it is forwarded according to the Lower-NSH in a lower-level domain.
+---------------------------------+
| Transport Encapsulation |
+->+---------------------------------+
| | Lower-NSH Header |
| +---------------------------------+
| | Upper-NSH Header |
| +---------------------------------+
| | Original Packet |
+->+---------------------------------+
|
|
+----Scope of NSH security protection
provided by a lower-level domain
Figure 4: Encapsulation of NSH within NSH
SFC data plane elements of a lower-level domain includes the Upper-NSH when computing the MAC.
Keying material used at the upper-level domain SHOULD NOT be the same as the one used by a lower-level domain.
This section specifies the format of new Variable-Length Context headers that are used for NSH integrity protection and, optionally, Context Headers encryption.
In particular, this section defines two "MAC and Encrypted Metadata" Context Headers; each having specific deployment constraints. Unlike Section 5.1, the level of assurance provided in Section 5.2 requires sharing MAC_KEY with SFFs. Both Context headers have the same format as shown in Section 5.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metadata Class | Type |U| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Length | Key Identifier (Variable) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Timestamp (8 bytes) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IV Length | Initialization Vector (Variable) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Message Authentication Code and optional Encrypted |
~ Context Headers ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: MAC and Encrypted Metadata Context Header
MAC#1 Context Header is a variable-length TLV that carries the Message Authentication Code (MAC) for the Service Path Header, Context Headers, and the inner packet on which NSH is imposed, calculated using MAC_KEY and optionally Context Headers encrypted using ENC_KEY. The scope of the integrity protection provided by this TLV is depicted in Figure 6.
This MAC scheme does not require sharing MAC_KEY with SFFs. It does not require to re-compute the MAC by each SFF because of TTL processing. Section 8.1 discusses the possible threat associated with this level of assurance.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver|O|U| TTL | Length |U|U|U|U|MD Type| Next Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<--+
| Service Path Identifier | Service Index | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | |
~ Variable-Length Unencrypted Context Headers (opt.) ~ |
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Metadata Class | Type |U| Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Key Length | Key Identifier ~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ Timestamp (8 bytes) ~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| IV Length | Initialization Vector ~ |
+->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ~ Context Header TLVs to encrypt (opt.) ~ |
+->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | | |
| ~ Inner Packet on which NSH is imposed ~ |
| | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<--|
| |
| |
| |
| Integrity Protection Scope ----+
+----Encrypted Data
Figure 6: Scope of MAC#1
In reference to Figure 5, the description of the fields is as follows:
MAC#2 Context Header is a variable-length TLV that carries the MAC for the entire NSH data calculated using MAC_KEY and optionally Context Headers encrypted using ENC_KEY. The scope of the integrity protection provided by this TLV is depicted in Figure 7.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<--+
|Ver|O|U| TTL | Length |U|U|U|U|MD Type| Next Protocol | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Service Path Identifier | Service Index | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | |
~ Variable-Length Unencrypted Context Headers (opt.) ~ |
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Metadata Class | Type |U| Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Key Length | Key Identifier ~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ Timestamp (8 bytes) ~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| IV Length | Initialization Vector | |
+->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ~ Context Header TLVs to encrypt (opt.) ~ |
+->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | | |
| ~ Inner Packet on which NSH is imposed ~ |
| | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<--|
| |
| |
| |
| Integrity Protection Scope ----+
+----Encrypted Data
Figure 7: Scope of MAC#2
In reference to Figure 5, the description of the fields is as follows:
This section follows the template provided in [I-D.ietf-ntp-packet-timestamps].
The format of the Timestamp field introduced in Section 5 is depicted in Figure 8.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fraction |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Timestamp Field Format
Epoch:
Leap seconds:
Resolution:
Wraparound:
Synchronization aspects:
The following subsections describe the processing rules for integrity protected NSH and optionally encrypted Context Headers.
This document adheres to the recommendations in [RFC8300] for handling the Context Headers at both ingress and egress SFC boundary nodes. That is, to strip such context headers.
Failures to inject or validate the Context Headers defined in this document SHOULD be logged locally while a notification alarm MAY be sent to an SFC control element. Similarly, failure to validate the integrity of the NSH data MUST cause that packet to be discarded while a notification alarm MAY be sent to an SFC control element. The details of sending notification alarms (i.e., the parameters affecting the transmission of the notification alarms depend on the information in the context header such as frequency, thresholds, and content in the alarm) SHOULD be configurable by the SFC control plane.
SFC-aware SFs and SFC proxies MAY be instructed to strip some encrypted Context Headers from the packet or to pass the data to the next SF in the service function chain after processing the content of the Context Headers. If no instruction is provided, the default behavior for intermediary SFC-aware nodes is to maintain such Context Headers so that the information can be passed to next SFC-aware hops. SFC-aware SFs and SFC proxies MUST re-apply the integrity protection if any modification is made to the Context Headers (strip a Context Header, update the content of an existing Context Header, insert a new Context Header).
An SFC-aware SF or SFC Proxy that is not allowed to decrypt any Context Headers MUST NOT be given access to the ENC_KEY.
Otherwise, an SFC-aware SF or SFC Proxy that receives encrypted Context Headers, for which it is not allowed to consume a specific Context Header it decrypts (but consumes others), MUST keep that Context Header unaltered when forwarding the packet upstream.
Only one instance of "MAC and Encrypted Metadata" Context Header (Section 5) is allowed. If multiple instances of "MAC and Encrypted Metadata" Context Header are included in an NSH packet, the SFC data element MUST process the first instance and ignore subsequent instances, and MAY log or increase a counter for this event as per Section 2.5.1 of [RFC8300].
MTU and fragmentation considerations are discussed in Section 5 of [RFC8300]. Those considerations are not reiterated here.
If the Context Headers are not encrypted, the HMAC algorithm discussed in [RFC4868] is used to integrity protect the target NSH data. An NSH imposer inserts a "MAC and Encrypted Metadata" Context Header for integrity protection (Section 5).
The NSH imposer computes the message integrity for the target NSH data (depending on the integrity protection scope discussed in Section 5) using MAC_KEY and HMAC algorithm. It inserts the MAC in the "MAC and Encrypted Metadata" Context Header. The length of the MAC is decided by the HMAC algorithm adopted for the particular key identifier.
The Message Authentication Code (T) computation process can be illustrated as follows:
T = HMAC-SHA-256-128(MAC_KEY, A)
An entity in the SFP that intends to update the NSH MUST follow the above behavior to maintain message integrity of the NSH for subsequent validations.
An NSH imposer can encrypt Context Headers carrying privacy-sensitive metadata, i.e., encrypted and unencrypted metadata may be carried simultaneously in the same NSH packet (Figure 9).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver|O|U| TTL | Length |U|U|U|U|MD Type| Next Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Path Identifier | Service Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Variable-Length Unencrypted Context Headers (opt.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Key Identifier ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Timestamp ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ MAC and Encrypted Context Headers ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: NSH with Encrypted and Unencrypted Metadata
In an SFC-enabled domain where pervasive monitoring [RFC7258] is possible, all Context Headers carrying privacy-sensitive metadata MUST be encrypted; doing so privacy-sensitive metadata is not revealed to attackers. Privacy specific threats are discussed in Section 5.2 of [RFC6973].
Using K and authenticated encryption algorithm, the NSH imposer encrypts the Context Headers (as set by the control plane Section 3), computes the message integrity for the target NSH data, and inserts the resulting payload in the "MAC and Encrypted Metadata" Context Header (Section 5). The entire TLV carrying a privacy-sensitive metadata is encrypted (that is, including the MD Class, Type, Length, and associated metadata of each Context Header).
The message Authentication Tag (T) and ciphertext (E) computation process can be illustrated as follows:
MAC_KEY = initial MAC_KEY_LEN octets of K,
ENC_KEY = final ENC_KEY_LEN octets of K,
E = CBC-PKCS7-ENC(ENC_KEY, P),
M = MAC(MAC_KEY, A || IV || E || AL),
T = initial T_LEN octets of M.
MAC and Encrypted Metadata = E || T
As specified in [RFC7518], the octet string (AL) is equal to the number of bits in the Additional Authenticated Data (A) expressed as a 64-bit unsigned big-endian integer.
An authorized entity in the SFP that intends to update the content of an encrypted Context Header or needs to add a new encrypted Context Header MUST also follow the aforementioned behavior.
An SFF or SFC-aware SF or SFC Proxy that only has access to the MAC_KEY, but not the ENC_KEY, computes the message Authentication Tag (T) after decrementing the TTL (by the SFF) or SI (by an SF or SFC Proxy) and replaces the Authentication Tag in the NSH with the computed Authentication Tag. Similarly, an SFC-aware SF (or SFC Proxy) that does not modify the encrypted Context headers also follows the aforementioned behavior.
The message Authentication Tag (T) computation process can be illustrated as follows:
M = MAC(MAC_KEY, A || IV || E || AL),
T = initial T_LEN octets of M.
The received NSH is accepted if the Timestamp (TS) in the NSH is recent enough to the reception time of the NSH (TSrt). The following formula is used for this check:
-Delta < (TSrt ? TS) < +Delta
The RECOMMENDED value for the allowed Delta is 2 seconds. If the timestamp is not within the boundaries, then the SFC data plane element receiving such packet MUST discard the NSH message.
All SFC data plane elements must be synchronized among themselves. These elements may be synchronized to a global reference time.
When an SFC data plane element receives an NSH packet, it MUST first ensure that a "MAC and Encrypted Metadata" Context Header is included. It MUST silently discard the message if the timestamp is invalid (Section 7.4). It MUST log an error at least once per the SPI for which the "MAC and Encrypted Metadata" Context Header is missing.
If the timestamp check is successfuly passed, the SFC data plane element proceeds then with NSH data integrity validation. The SFC data plane element computes the message integrity for the target NSH data (depending on the integrity protection scope discussed in Section 5) using the MAC_KEY and HMAC algorithm for the key identifier. If the value of the newly generated digest is identical to the one enclosed in the NSH, the SFC data plane element is certain that the NSH data has not been tampered and validation is therefore successful. Otherwise, the NSH packet MUST be discarded.
If entitled to consume a supplied encrypted Context Header, an SFC-aware SF or SFC Proxy decrypts metadata using (K) and decryption algorithm for the key identifier in the NSH.
Authenticated encryption algorithm has only a single output, either a plaintext or a special symbol (FAIL) that indicates that the inputs are not authentic (Section 5.2.2.2 of [RFC7518]).
NSH security considerations are discussed in Section 8 of [RFC8300]. The guidelines for cryptographic key management are discussed in [RFC4107].
The interaction between the SFC-aware data plane elements and a key management system MUST NOT be transmitted in clear since this would completely destroy the security benefits of the integrity protection solution defined in this document. The secret key (K) must have an expiration time assigned as the latest point in time before which the key may be used for integrity protection of NSH data and encryption of Context Headers. Prior to the expiration of the secret key, all participating service function nodes SHOULD have the control plane distribute an new key identifier and associated keying material, so that when the secret key is expired those nodes are prepared with the new secret key. This allows the NSH Imposer to switch to the new key identifier as soon as necessary. It is RECOMMENDED that the next key identifier be distributed by the control plane well prior to the secret key expiration time.
NSH data are exposed to several threats:
In an SFC-enabled domain where the above attacks are possible, NSH data MUST be integrity-protected and replay-protected, and privacy-sensitive NSH metadata MUST be encrypted for confidentiality preservation purposes. The Base and Service Path headers are not encrypted.
MACs with two levels of assurance are defined in Section 5. Considerations specific to each level of assurance are discussed in the following subsections.
The attacks discussed in [I-D.nguyen-sfc-security-architecture] are handled owing to the solution specified in this document, except for attacks dropping packets. Such attacks can be detected relying upon statistical analysis; such analysis is out of scope of this document. Also, if SFFs are not involved in the integrity checks, a misbehaving SFF which decrements SI while this should be done by an SF (SF bypass attack) will be detected by an upstream SF because the integrity check will fail.
An active attacker can potentially modify the Base header (e.g., decrement the TTL so the next SFF in the SFP discards the NSH packet). In the meantime, an active attacker can also drop NSH packets. As such, this attack is not considered an attack against the security mechanism specified in the document.
No device other than the SFC-aware SFs in the SFC-enabled domain should be able to update the integrity protected NSH data. Similarly, no device other than the SFC-aware SFs and SFC proxies in the SFC-enabled domain be able to decrypt and update the Context Headers carrying privacy-sensitive metadata. In other words, if the SFC-aware SFs and SFC proxies in the SFC-enabled domain are considered fully trusted to act on the NSH data, only they can have access to privacy-sensitive NSH metadata and the keying material used to integrity protect NSH data and encrypt Context Headers.
SFFs can detect whether an illegitimate node has altered the content of the Base header. Such messages MUST be discarded with appropriate logs and alarms generated.
This document requests IANA to assign the following types from the "NSH IETF-Assigned Optional Variable-Length Metadata Types" (0x0000 IETF Base NSH MD Class) registry available at: https://www.iana.org/assignments/nsh/nsh.xhtml#optional-variable-length-metadata-types.
+-------+-------------------------------+----------------+ | Value | Description | Reference | +=======+===============================+================+ | TBD1 | MAC and Encrypted Metadata #1 | [ThisDocument] | | TBD2 | MAC and Encrypted Metadata #2 | [ThisDocument] | +-------+-------------------------------+----------------+
This document was edited as a follow up to the discussion in IETF#104: https://datatracker.ietf.org/meeting/104/materials/slides-104-sfc-sfc-chair-slides-01 (slide 7).
Thanks to Joel Halpern, Christian Jacquenet, Dirk von Hugo, Tal Mizrahi, Daniel Migault, and Diego Lopez for the comments.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC4107] | Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107, June 2005. |
| [RFC4868] | Kelly, S. and S. Frankel, "Using HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 with IPsec", RFC 4868, DOI 10.17487/RFC4868, May 2007. |
| [RFC7518] | Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, DOI 10.17487/RFC7518, May 2015. |
| [RFC7665] | Halpern, J. and C. Pignataro, "Service Function Chaining (SFC) Architecture", RFC 7665, DOI 10.17487/RFC7665, October 2015. |
| [RFC8174] | Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017. |
| [RFC8300] | Quinn, P., Elzur, U. and C. Pignataro, "Network Service Header (NSH)", RFC 8300, DOI 10.17487/RFC8300, January 2018. |