| Internet-Draft | ICN Path Steering | September 2023 |
| Moiseenko & Oran | Expires 31 March 2024 | [Page] |
Path Steering is a mechanism to discover paths to the producers of ICN content objects and steer subsequent Interest messages along a previously discovered path. It has various uses, including the operation of state-of-the-art multipath congestion control algorithms and for network measurement and management. This specification derives directly from the design published in Path Switching in Content Centric and Named Data Networks (4th ACM Conference on Information-Centric Networking - ICN'17) and therefore does not recapitulate the design motivations, implementation details, or evaluation of the scheme. Some technical details are different however, and where there are differences, the design documented here is to be considered definitive.¶
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Path Steering is a mechanism to discover paths to the producers of ICN content objects and steer subsequent Interest messages along a previously discovered path. It has various uses, including the operation of state-of-the-art multipath congestion control algorithms and for network measurement and management. This specification derives directly from the design published in [Moiseenko2017] and therefore does not recapitulate the design motivations, implementation details, or evaluation of the scheme. That publication should be considered a normative reference as it is not likely a reader will be able to understand all elements of this design without first having read the reference. Some technical details are different however, and where there are differences, the design documented here is to be considered definitive.¶
Path discovery and subsequent path steering in ICN networks is facilitated by the symmetry of forward and reverse paths in the CCNx and NDN architectures. Path discovery is achieved by a consumer endpoint transmitting an ordinary Interest message and receiving a Content (Data) message containing an end-to-end path label constructed on the reverse path by the forwarding plane. Path steering is achieved by a consumer endpoint including a path label in the Interest message, which is forwarded to each nexthop through the corresponding egress interfaces in conjunction with longest name prefix match (LNPM) lookup in the Forwarding Information Base (FIB).¶
This document is a product of the IRTF Information-Centric Networking Research Group (ICNRG). It was supported by the ICNRG participants during its development and through Research Group last call. It has received detailed review by experts in both the CCNx and NDN communities.¶
There are a number of important use cases to justify extending ICN architectures such as CCNx [RFC8569] or NDN [NDN] to provide these capabilities. These are summarized as follows:¶
The path discovery machinery described here may (and likely will) discover paths with varying properties. [RFC9217] discusses a number of open questions in path aware networking, among which is how to assess and exploit paths having different properties. Experimenting with ICN path steering may be helpful in further elucidating these questions and perhaps shedding light on which path properties are most useful for the use cases cited above.¶
One nuance compared to other path aware networking approaches is that ICN path steering piggybacks path discovery on the base ICN data exchange, rather than having a separate path advertisement or discovery mechanism. That means when the recorded path comes back in an ICN Data message response, the properties of the path are known only implicitly to the consumer as opposed to being explicitly labeled. That makes the question of what properties a consumer uses to choose a path one of observation or measurement rather than advance selection based on an explicit advertised property (e.g SCION [I-D.dekater-panrg-scion-overview]).¶
The utility and overall technical quality of this path steering capability can be assessed by how well it enables the above use cases and what performance and robustness effects it has on the underlying ICN protocols and their use in various applications. A few of the open questions that should be addressed through experimentation with path steering include:¶
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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This document uses the general ICN terms that are defined in [RFC8793]. In addition we define the following terms specific to path steering:¶
We elucidate the design using CCNx semantics [RFC8569] and extend its Packet Encoding [RFC8609] as defined in Section 3.2. While the terminology is slightly different, this design can be applied also to NDN, by extending its bespoke packet encodings [NDNTLV] (See Section 3.3).¶
End-to-end Path Discovery for CCNx is achieved by creating a path label and placing it as a hop-by-hop TLV in a CCNx Content (Data) message. The path label is constructed hop-by-hop as the message traverses the reverse path of transit CCNx forwarders as shown in the first example in Figure 1. The path label is updated by adding to the existing path label the nexthop label of the interface at which the Content (Data) message has arrived. Eventually, when the Content(Data) message arrives at the consumer, the path label identifies the complete path the Content (Data) message took to reach the consumer. As shown in the second example in the figure, when multiple paths are available, subsequent Interests may be able to discover additional paths by omitting a path steering TLV and obtaining a new path label on the returning interest.¶
Discover and use first path:
Consumer Interest 1 ___ Interest 2
| | ^ |
| | | |
| | | |
Forwarder 1 v | V
| (nexthop 1) (nexthop 1) ^ (nexthop 1)
| | | |
| | | |
Forwarder 2 v | v
(nexthop 3) / \ (nexthop 2) (nexthop 2) ^ (nexthop 2)
/ \ | | |
/ \ | | |
/ \ | | |
/ \ | | |
/ \ | | |
Forwarder 4 Forwarder 3 v | v
(nexthop 5)\ / (nexthop 4) (nexthop 4) ^ (nexthop 4)
\ / | | |
\ / | | |
\ / | | |
\ / | | |
\ / | | |
\ / v | v
Producer ___ Data 1 ___
or
Content Store
Discover and use second path:
Consumer Interest 3 ___ Interest 4
| | ^ |
| | | |
| | | |
Forwarder 1 v | V
| (nexthop 1) (nexthop 1) ^ (nexthop 1)
| | | |
| | | |
Forwarder 2 v | v
(nexthop 3) / \ (nexthop 2) (nexthop 3) ^ (nexthop 3)
/ \ | | |
/ \ | | |
/ \ | | |
/ \ | | |
/ \ | | |
Forwarder 4 Forwarder 3 v | v
(nexthop 5)\ / (nexthop 4) (nexthop 5) ^ (nexthop 5)
\ / | | |
\ / | | |
\ / | | |
\ / | | |
\ / | | |
\ / v | v
Producer ___ Data 2 ___
or
Content Store
Due to the symmetry of forward and reverse paths in CCNx, a consumer application can reuse a discovered path label to fetch the same or similar (e.g. next chunk, or next Application Data Unit, or next pointer in a Manifest [I-D.irtf-icnrg-flic]) Content (Data) message over the discovered network path. This Path Steering is achieved by processing the Interest message's path label at each transit ICN forwarder and forwarding the Interest through the specified nexthop among those identified as feasible by LNPM FIB lookup (Figure 2).¶
----------------------------------------------------------------------
FORWARD PATH
----------------------------------------------------------------------
Interest +---------+ +-----+ (path label) +--------+ (match) Interest
-------->| Content |->| PIT | ------------>| Label |---------------->
| Store | +-----+ | Lookup |
+---------+ | \ (no path label) +--------+
| | \ |\(path label mismatch)
Data | | \ | \
<---------+ v \ | \
aggregate \ | \
\ | \
\ | +-----+ Interest
+--------------|---->| FIB | -------->
| +-----+
Interest-Return (NACK) v | (no route)
<----------------------------------------------+<-------+
----------------------------------------------------------------------
REVERSE PATH
----------------------------------------------------------------------
Interest-return(NACK) +-----+(update path label) Interest-Return(NACK)
<---------------------| |<----------------------------------------
| |
Data +---------+ | PIT | (update path label) Data
<------| Content |<---| |<----------------------------------------
| Store | | |
+---------+ +-----+
|
| (no match)
v
Over time, the state of interfaces and the FIB on forwarders may change such that, at any particular forwarder, a given nexthop is no longer valid for a given prefix. In this case, the path label will point to a now-invalid nexthop. This is detected by failure to find a match between the decoded nexthop ID and the nexthops of the FIB entry after LNPM FIB lookup.¶
On detecting an invalid path label, the forwarder SHOULD respond to the Interest with an Interest-Return. We therefore define a new Invalid path label response code for the Interest Return message and include the current path label as a hop-by-hop header. Each transit forwarder processing the Interest-Return message updates the path label in the same manner as Content (Data) messages, so that the consumer receiving the Interest-Return (NACK) can easily identify which path label is no longer valid.¶
A consumer may alternatively request that a forwarder detecting the inconsistency forward the Interest by means of normal LNPM FIB lookup rather than returning an error. The consumer endpoint, if it cares, can keep enough information about outstanding Interests to determine if the path label sent with the Interest fails to match the path label in the corresponding returned Content (Data), and use that information to replace stale path labels. It does so by setting the FALLBACK MODE flag of the path label TLV in its Interest message.¶
If two or more Interests matching the same PIT entry arrive at a forwarder, under current behavior they will be aggregated whether or not they carry identical Path Labels TLVs. This may or may not be appropriate. For example, multiple Interests with different MODES (e.g. one with DISCOVERY MODE and one without) will get aggregated, and the behavior of the forwarder might therefore be dependent on the arrival order of those Interests. In particular,¶
Multiple Interests intended to discover paths (i.e. by carrying the DISCOVERY MODE flag defined in Section 2.5) might also be aggregated by a forwarder. This limits the ability to discover multiple paths in parallel and instead must be discovered incrementally in subsequent exchanges. In other words, aggregated Interests will all discover only one single path carried by one single Data packet. This has implications for management applications like Traceroute [I-D.irtf-icnrg-icntraceroute] which would likely perform much better if they discover paths in parallel. Hence, it is RECOMMENDED when employing Path Steering that such applications craft their Interests with unique name suffixes in order to avoid being aggregated.¶
[Moiseenko2017] presents various options for how to represent a path label, with different tradeoffs in flexibility, performance and space efficiency. For this specification, we choose the Polynomial encoding which achieves reasonable space efficiency at the cost of establishing a hard limit on the length of paths that can be represented.¶
The polynomial encoding utilizes a fixed-size bit array. Each transit ICN forwarder is allocated a fixed sized portion of the bit array. This design allocates 12 bits (i.e. 4095 as a generator polynomial) to each intermediate ICN forwarder. This matches the scalability of today's commercial routers that support up to 4096 physical and logical interfaces and usually do not have more than a few hundred active ones.¶
+------------------------------------------------------------------+ | Path Label bitmap | +----------+-----------------+-----------------+-------------------+ | index | nexthop label | nexthop label | | +----------+-----------------+-----------------+-------------------+ |<- 8bit ->|<---- 12bit ---->|<---- 12bit ---->|<----------------->|
A forwarder that receives a Content (Data) message encodes the nexthop label in the next available slot and increments label index. Conversely, a forwarder that receives an Interest message reads the current nexthop label and decrements label index. Therefore, the extra computation required at each hop to forward either an interest or Content Object message with a path label is minimized and constitutes a fairly trivial additional overhead compared to FIB lookup and other required operations.¶
This approach results in individual path label TLV instances being of fixed pre-computed size. While this places a hard upper bound on the maximum number of network hops that can be represented, this is not a significant a practical problem in NDN and CCNx, since the size can be pre-set during Content(Data) message encoding based on the exact number of network hops traversed by the Interest message. Even long paths of 24 hops will fit in a path label bitmap of 36 bytes if nexthop label is encoded in 12 bits.¶
A Path label TLV is the tuple: {[Flags], [Path Label Hop Count], [Nexthop Label], [Path label bitmap]}.¶
| Flag | Value (hex) |
|---|---|
| DISCOVERY_MODE | 0x00 |
| FALLBACK_MODE | 0x01 |
| STRICT_MODE | 0x02 |
| Unassigned | 0x03-0xFF |
The Path Label Hop Count (PLHC) MUST be incremented by NDN and CCNx forwarders if the Interest packet carries a path label and DISCOVERY mode flag is set. A producer node or a forwarder with cached data packet MUST use PLHC in calculation of a path label bitmap size suitable for encoding the entire path to the consumer. The Path Label Hop Count (PLHC) MUST be set to zero in newly created Data or Interest-Return (NACK) packets. A consumer node MUST reuse Path Label Hop Count (PLHC) together with the Path label bitmap (PLB) in order to correctly forward the Interest(s) along the corresponding network path.¶
If an NDN or CCNx forwarder supports path labeling, the Nexthop label MUST be used to determine the correct egress interface for an Interest packet carrying either the FALLBACK MODE or STRICT MODE flag. If any particular NDN or CCNx forwarder is configured to decrypt path labels of Interest packets (Section Security Considerations (Section 5)), then the forwarder MUST¶
If any particular NDN or CCNx forwarder is NOT configured to decrypt path labels of Interest packets, then path label decryption SHOULD NOT be performed.¶
The Nexthop label MUST be ignored by NDN and CCNx forwarders if present in Data or Interest-Return (NACK) packets. If any particular NDN or CCNx forwarder is configured to encrypt path labels of Data and Interest-Return (NACK) packets (Section Security Considerations (Section 5)), then the forwarder MUST encrypt existing path label with its own symmetric key, append the nexthop label of the ingress interface to the path label, and increment Path Label Hop Count (PLHC). If any particular NDN or CCNx forwarder is NOT configured to encrypt path labels of Interest packets, then path label encryption SHOULD NOT be performed.¶
NDN and CCNx forwarders MUST fallback to longest name prefix match (LNPM) FIB lookup if an Interest packet carries an invalid nexthop label and the FALLBACK MODE flag is set.¶
CCNx forwarders MUST respond with an Interest Return packet specifying a T_RETURN_INVALID_PATH_LABEL code if Interest packet carries an invalid path label and the STRICT MODE flag is set. This is a new Interrest return code defined herein (see Section 4 for the value allocation).¶
CCNx forwarders MUST respond with an Interest Return packet specifying the existing T_RETURN_MALFORMED_INTEREST code if the Interest packet carries a path label TLV with both FALLBACK MODE and STRICT MODE flags set.¶
Path Label is an optional Hop-by-Hop header TLV that can be present in CCNx Interest, InterestReturn and Content Object packets.¶
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 +---------------+---------------+---------------+---------------+ | T_PATH_LABEL | Length + 4 | +---------------+---------------+---------------+---------------+ | Flags | Path Label | Nexthop Label | | | Hop Count | | +---------------+---------------+---------------+---------------+ / / / Path label bitmap (Length octets) / / / +---------------+---------------+---------------+---------------+
Path Label is an optional TLV for NDN Interest and Data packets which is carried in the NDN Link Adaptation Protocol [NDNLPv2] used to wrap NDN packets for carriage over various link layer protocols. NDNLPv2 was chosen over the NDN packet itself since it can carry hop-by-hop information that potentially mutates at each hop and therefore cannot be included in the secured hash computation or the signature of NDN packets. Further, it can be used instead of the existing NextHopFaceId TLV since it not only can specify the single outgoing face for a consumer, but manages the selection and forwarding over an entire path. The Path Label TLV in NDNLPv2 is defined below:¶
PathLabel = PATH-LABEL-TYPE TLV-LENGTH
PathLabelFlags
PathLabelBitmap
PathLabelFlags = PATH-LABEL-FLAGS-TYPE
TLV-LENGTH ; == 1
OCTET
NexthopLabel = PATH-LABEL-NEXTHOP-LABEL-TYPE
TLV-LENGTH ; == 2
2 OCTET
PathLabelHopCount = PATH-LABEL-HOP-COUNT-TYPE
TLV-LENGTH ; == 1
OCTET
PathLabelBitmap = PATH-LABEL-BITMAP-TYPE
TLV-LENGTH ; == 64
64 OCTET
| Flag | (Suggested) Value (hex) |
|---|---|
| T_PATH_LABEL | 0x0A |
| T_PATH_LABEL_FLAGS | 0x0B |
| T_PATH_LABEL_BITMAP | 0x0D |
| T_PATH_LABEL_NEXTHOP_LABEL | 0x0E |
| T_PATH_LABEL_HOP_COUNT | 0x0F |
IANA is requested to make the following assignments:¶
A path is invalidated by renumbering nexthop label(s). A malicious consumer can attempt to mount an attack by transmitting Interests with path labels which differ only in a single now-invalid nexthop label in order to brute force a valid nexthop label. If such an attack succeeds, a malicious consumer would be capable of steering Interests over a network path that may not match the paths computed by the routing algorithm or learned adaptively by the forwarders.¶
When a label lookup fails, by default an Invalid path label Interest-Return (NACK) message is returned to the consumer. This contains a path label identical to the one included in the corresponding Interest message. A malicious consumer can therefore analyze the message's Hop Count field to infer which specific nexthop label had failed and direct an attack to influence path steering at that hop. This threat can be mitigated by the following countermeasures:¶
Because ICN forwarders maintain per-face state and forwarding state for Interest messages, state inflation attacks are a general concern. The addition of path steering capabilities in Interest and Data messages does not, however, constitute a meaningful increase in susceptibility to such attacks. This is because:¶
ICN protocols can be susceptible to a variety of cache poisoning attacks, where a colluding consumer and producer arrange for bogus content (with either invalid or inappropriate signatures) to populate forwarder caches. These are generally confined to on-path attacks. It is also theoretically possible to launch a similar attack without a cooperating producer such that the caches of on-path routers become poisoned with the content from off-path routers (i.e. physical connectivity, but no route in a FIB for a given prefix). We estimate that without any prior knowledge of the network topology, the complexity of this type of attack is in the ballpark of Breadth-First-Search and Depth-First-Search algorithms with the additional burden of transmitting 2^31 Interests in order to crack a nexthop label on each hop. Relatively short periodic update of nexthop labels and anti- label scan heuristics implemented in the ICN forwarder may successfully mitigate this type of attack.¶
If the countermeasures listed above do not provide sufficient protection against malicious mis-steering of Interests, the path label can be made opaque to the consumer endpoint via hop-by-hop symmetric cryptography applied to the path labels (Figure 6). This method is viable due to the symmetry of forward and reverse paths in CCNx and NDN architectures combined with ICN path steering requiring only reads/writes of the topmost nexthop label (i.e. active nexthop label) in the path label. This way a path steering capable ICN forwarder receiving a Data (Content) message encrypts the current path label with its own non-shared symmetric key prior to adding a new nexthop label to the path label. The Data (Content) message is forwarded downstream with unencrypted topmost (i.e active) nexthop label and encrypted remaining content of the path label. As a result, a consumer endpoint receives a Data (Content) message with a unique path label exposing only the topmost nexthop label as cleartext. A path steering forwarder receiving an Interest message performs label lookup using the topmost nexthop label, decrypts the path label with its own non-shared symmetric key, and forwards the message upstream.¶
Cryptographic protection of a path label does not require any key negotiation among ICN forwarders, and is no more expensive than MACsec or IPsec. It is also quite possible that strict hop-by-hop path label encryption is not necessary and path label encryption only on the border routers of the trusted administrative or routing domains may suffice.¶
Producer
| ^
| |
Path Label TLV | | Path Label TLV
+-----------------------+ | | +-----------------------+
|nexthop label=456 | v | |nexthop label=456 |
|encrypted path label={}| Forwarder 3 |encrypted path label={}|
+-----------------------+ | ^ +-----------------------+
| |
path label is encrypted | | path label is decrypted
with Forwarder 3 | | with Forwarder 3
symmetric key | | symmetric key
| |
| |
| |
| |
| |
Path Label TLV | | Path Label TLV
+-----------------------+ | | +-----------------------+
|nexthop label=634 | v | |nexthop label=634 |
|encrypted path label= | Forwarder 2 |encrypted path label= |
| {456} | | ^ | {456} |
+-----------------------+ | | +-----------------------+
| |
path label is encrypted | | path label is decrypted
with Forwarder 2 | | with Forwarder 2
symmetric key | | symmetric key
| |
| |
| |
| |
| |
Path Label TLV | | Path Label TLV
+-----------------------+ | | +-----------------------+
|nexthop label=912 | v | |nexthop label=912 |
|encrypted path label= | Forwarder 1 |encrypted path label= |
| {634, encrypted path | | ^ | {634, encrypted path |
| label {456}} | | | | label {456}} |
+-----------------------+ | | +-----------------------+
| |
path label is encrypted | | path label is decrypted
with Forwarder 1 | | with Forwarder 1
symmetric key | | symmetric key
| |
| |
| |
| |
v |
Consumer