ICN Traceroute Protocol Specification

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Table of Contents

3. Traceroute Functionality Challenges and Opportunities in ICN

In the NDN and CCN protocols, the communication paradigm is based exclusively on named objects. An Interest is forwarded across the network based on its name. Eventually, it retrieves a content object either from a producer application or some forwarder's Content Store (CS).¶

An ICN network differs from an IP network in at least 4 important ways:¶

• IP identifies interfaces to an IP network with a fixed-length address, and delivers IP packets to one or more interfaces. ICN identifies units of data in the network with a variable length name consisting of a hierarchical list of segments.¶
• An IP-based network depends on the IP packets having source IP addresses that are used as the destination address for replies. On the other hand, ICN Interests do not have source addresses and they are forwarded based on names, which do not refer to a unique end-point. Data packets follow the reverse path of the Interests based on hop-by-hop state created during Interest forwarding.¶
• An IP network supports multi-path, single destination, stateless packet forwarding and delivery via unicast, a limited form of multi-destination selected delivery with anycast, and group-based multi-destination delivery via multicast. In contrast, ICN supports multi-path and multi-destination stateful Interest forwarding and multi-destination data delivery to units of named data. This single forwarding semantic subsumes the functions of unicast, anycast, and multicast. As a result, consecutive (or retransmitted) ICN Interest messages may be forwarded through an ICN network along different paths, and may be forwarded to different data sources (e.g., end-node applications, in-network storage) holding a copy of the requested unit of data. The ability to discover multiple available (or potentially all) paths towards a name prefix is a desirable capability for an ICN traceroute protocol, since it can be beneficial for congestion control purposes. Knowing the number of available paths for a name can also be useful in cases that Interest forwarding based on application semantics/preferences is desirable.¶
• In the case of multiple Interests with the same name arriving at a forwarder, a number of Interests may be aggregated in a common Pending Interest Table (PIT) entry. Depending on the lifetime of a PIT entry, the round-trip time an Interest-Data exchange might significantly vary (e.g., it might be shorter than the full round-trip time to reach the original content producer). To this end, the round-trip time experienced by consumers might also vary even under constant network load.¶

These differences introduce new challenges, new opportunities and new requirements in the design of ICN traceroute. Following this communication model, a traceroute client should be able to express traceroute requests directed to a name prefix and receive responses.¶

Our goals are the following:¶

• Trace one or more paths towards an ICN forwarder (for troubleshooting purposes).¶
• Trace one or more paths along which an named data of an application can be reached in the sense that Interest packets can be forwarded toward it.¶
• Test whether a specific named object is cached in some on-path CS, and, if so, trace the path towards it and return the identity of the corresponding forwarder.¶
• Perform transit delay network measurements.¶

To this end, a traceroute target name can represent:¶

• An administrative name that has been assigned to a forwarder. Assigning a name to a forwarder implies the presence of a management application running locally, which handles Operations, Administration and Management (OAM) operations.¶
• A name that includes an application's namespace as a prefix.¶
• A named object that might reside in some in-network storage.¶

In order to provide stable and reliable diagnostics, it is desirable that the packet encoding of a traceroute request enable the forwarders to distinguish this request from a normal Interest, while also preserving forwarding behavior as similar as possible to that for an Interest packet. In the same way, the encoding of a traceroute reply should allow for processing as similar as possible to that of a data packet by the forwarders.¶

The term "traceroute session" is used for an iterative process during which an endpoint client application generates a number of traceroute requests to successively traverse more distant hops in the path until it receives a final traceroute reply from a forwarder. It is desirable that ICN traceroute be able to discover a number of paths towards the expressed prefix within the same session or subsequent sessions. To discover all the hops in a path, we need a mechanism (Interest Steering) to steer requests along different paths. Such a capability was initially published in [PATHSTEERING] and has been specified for CCNx and NDN in [I-D.irtf-icnrg-pathsteering].¶

It is also important, in the case of traceroute requests for the same prefix from different sources, to have a mechanism to avoid aggregating those requests in the PIT. To this end, we need some encoding in the traceroute requests to make each request for a common prefix unique, and hence avoid PIT aggregation and further enabling the exact matching of a response with a particular traceroute packet.¶

The packet types and format are presented in Section 4. The procedures, e.g. the procedures for determining and indicating that a destination has been reached, are specified in Section 6.¶

4. ICN Traceroute CCNx Packet Format

In this section, we present the CCNx packet format [RFC8609] of ICN traceroute, where messages exist within outermost containments (packets). Specifically, we propose two types of traceroute packets, a traceroute request and a traceroute reply packet type.¶

 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
 +---------------+---------------+---------------+---------------+
 |               |               |                               |
 |    Version    |   TrRequest   |         PacketLength          |
 |               |               |                               |
 +---------------+---------------+---------------+---------------+
 |               |               |               |               |
 |    HopLimit   |    Reserved   |     Flags     |  HeaderLength |
 |               |               |               |               |
 +---------------+---------------+---------------+---------------+
 /                                                               /
 /                        Path label TLV                         /
 /                                                               /
 +---------------+---------------+---------------+---------------+
 |                                                               |
 |               Traceroute Request Message TLVs                 |
 |                                                               |
 +---------------+---------------+---------------+---------------+

 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
 +---------------+---------------+---------------+---------------+
 |                               |                               |
 |        MessageType = 1        |          MessageLength        |
 |                               |                               |
 +---------------+---------------+---------------+---------------+
 |                                                               |
 |                           Name TLV                            |
 |                                                               |
 +---------------+---------------+---------------+---------------+

 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
 +---------------+---------------+---------------+---------------+
 |                               |                               |
 |        Name_Nonce_Type        |      Name_Nonce_Length = 8    |
 |                               |                               |
 +---------------+---------------+---------------+---------------+
 |                                                               |
 |                                                               |
 |                                                               |
 |                        Name_Nonce_Value                       |
 |                                                               |
 |                                                               |
 +---------------+---------------+---------------+---------------+

 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
 +---------------+---------------+---------------+---------------+
 |               |               |                               |
 |    Version    |    TrReply    |          PacketLength         |
 |               |               |                               |
 +---------------+---------------+---------------+---------------+
 |                               |               |               |
 |            Reserved           |     Flags     | HeaderLength  |
 |                               |               |               |
 +---------------+---------------+---------------+---------------+
 |                                                               |
 |                       Path label TLV                          |
 |                                                               |
 +---------------+---------------+---------------+---------------+
 |                                                               |
 |                 Traceroute Reply Message TLVs                 |
 |                                                               |
 +---------------+---------------+---------------+---------------+

 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
 +---------------+---------------+---------------+---------------+
 |                               |                               |
 |        MessageType = 2        |          MessageLength        |
 |                               |                               |
 +---------------+---------------+---------------+---------------+
 |                                                               |
 |                           Name TLV                            |
 |                                                               |
 +---------------+---------------+---------------+---------------+
 |                                                               |
 |                        PayloadType TLV                        |
 |                                                               |
 +---------------+---------------+---------------+---------------+
 |                                                               |
 |                         ExpiryTime TLV                        |
 |                                                               |
 +---------------+---------------+---------------+---------------+

 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_PAYLOADTYPE_DATA      |             Length            |
 |                               |                               |
 +---------------+---------------+---------------+---------------+
 |                                                               |
 |                      Sender's Name TLV                        |
 |                                                               |
 +---------------+---------------+---------------+---------------+
 |                                                               |
 |                    Sender's Signature TLV                     |
 |                                                               |
 +---------------+---------------+---------------+---------------+
 |                                                               |
 |                       TrReply Code TLV                        |
 |                                                               |
 +---------------+---------------+---------------+---------------+

 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
 +---------------+---------------+---------------+---------------+
 |                               |                               |
 |       TrReply_Code_Type       |    TrReply_Code_Length = 2    |
 |                               |                               |
 +---------------+---------------+---------------+---------------+
 |                                                               |
 |                      TrReply_Code_Value                       |
 |                                                               |
 +---------------+---------------+---------------+---------------+

6. Forwarder Operation

When a forwarder receives a traceroute request, the hop limit value is checked and decremented and the target name (i.e, the name of the traceroute request without the last nonce name segment as well as the suffix "traceroute" and the ParametersSha256DigestComponent in the case of a request with the NDN packet format) is extracted.¶

If the HopLimit has not expired (its value is greater than 0), the forwarder will forward the request upstream based on CS lookup, PIT creation, LPM lookup and the path steering value, if present. If no valid next-hop is found, an InterestReturn indicating "No Route" in the case of CCNx or a network NACK in the case of NDN is sent downstream.¶

If the HopLimit value is equal to zero, the forwarder generates a traceroute reply. This reply includes the forwarder's administrative name and signature, and a Path label TLV. This TLV initially has a null value since the traceroute reply originator does not forward the request and, thus, does not make a path choice. The reply will also include the corresponding TrReply Code TLV.¶

A traceroute reply will be the final reply of a traceroute session if any of the following conditions are met:¶

• If a forwarder has been given one or more administrative names, the target name matches one of them.¶
• The target name exactly matches the name of a content-object residing in the forwarder's CS (unless the traceroute client application has chosen not to receive replies due to CS hits as specified in Appendix A).¶
• The target name matches (in a Longest Prefix Match manner) a FIB entry with an outgoing face referring to a local application.¶

The TrReply Code TLV value of the reply is set to indicate the specific condition that was met. If none of those conditions was met, the TrReply Code is set to 4 to indicate that the hop limit value reached 0.¶

A received traceroute reply will be matched to an existing PIT entry as usual. On the reverse path, the path steering TLV of a reply will be updated by each forwarder to encode its choice of next-hop(s). When included in subsequent requests, this path steering TLV allows the forwarders to steer the requests along the same path.¶

7. Protocol Operation For Locally-Scoped Namespaces

In this section, we elaborate on 2 alternative design approaches in cases that the traceroute target prefix corresponds to a locally-scoped namespace not directly routable from the client's local network.¶

The first approach leverages the NDN Link Object [SNAMP]. Specifically, the traceroute client attaches to the expressed request a LINK Object that contains a number of routable name prefixes, based on which the request can be forwarded across the Internet until it reaches a network region, where the request name itself is routable. A LINK Object is created and signed by a data producer allowed to publish data under a locally-scoped namespace. The way that a client retrieves a LINK Object depends on various network design factors and is out of the scope of the current draft.¶

Based on the current deployment of the LINK Object by the NDN team, a forwarder at the border of the region, where an Interest name becomes routable has to remove the LINK Object from the incoming Interests. The Interest state maintained along the entire forwarding path is based on the Interest name regardless of whether it was forwarded based on this name or a prefix in the LINK Object.¶

The second approach is based on prepending a routable prefix to the locally-scoped name. The resulting prefix will be the name of the traceroute requests expressed by the client. In this way, a request will be forwarded based on the routable part of its name. When it reaches the network region where the original locally-scoped name is routable, the border forwarder rewrites the request name and deletes its routable part. There are two conditions for a forwarder to perform this rewriting operation on a request:¶

1)

the routable part of the request name matches a routable name of the network region adjacent to the forwarder (assuming that a forwarder is aware of those names), and¶

2)

the remaining part of the request name is routable across the network region of this forwarder.¶

The state maintained along the path, where the locally-scoped name is not routable, is based on the routable prefix along with the locally-scoped prefix, while within the network region that the locally-scoped prefix is routable is based only on it. To ensure that the generated replies will reach the client, the border forwarder has also to rewrite the name of a reply and prepend the routable prefix of the corresponding request.¶

9. IANA Considerations

IANA will assign TBD1 to "TrRequest" and TBD2 to "TrReplay" in the CCNx Packet Types registry established by [RFC8609].¶

IANA will assign TBD3 to "Nonce" in the CCNx Name Segment Types registry established by [RFC8609].¶

Appendix A. Traceroute Client Application (Consumer) Operation

This section is an informative appendix regarding the proposed traceroute client operation.¶

The client application is responsible for generating traceroute requests for prefixes provided by users.¶

The overall process can be iterative: the first traceroute request of each session will have a HopLimit of value 1 to reach the first hop forwarder, the second of value 2 to reach the second hop forwarder and so on and so forth.¶

When generating a series of requests for a specific name, the first one will typically not include a Path label TLV, since no TLV value is known. After a traceroute reply containing a Path label TLV is received, each subsequent request might include the received path steering value in the Path label header TLV to drive the requests towards a common path as part of checking the network performance. To discover more paths, a client can omit the Path label TLV in future requests. Moreover, for each new traceroute request, the client has to generate a new nonce and record the time that the request was expressed. It will also set the lifetime of a request, which will have semantics similar to the lifetime of an Interest.¶

Moreover, the client application might not wish to receive replies due to CS hits. In CCNx, a mechanism to achieve that would be to use a Content Object Hash Restriction TLV with a value of 0 in the payload of a traceroute request message. In NDN, the exclude filter selector can be used.¶

When it receives a traceroute reply, the client would typically match the reply to a sent request and compute the round-trip time of the request. It should parse the Path label value and decode the reply's payload to parse the sender's name and signature. The client should verify that both the received message and the forwarder's name have been signed by the key of the forwarder, whose name is included in the payload of the reply (by fetching this forwarder's public key and verifying the contained signature). In the case that the client receives an TrReply Code TLV with a valid value, it can stop sending requests with increasing HopLimit values and potentially start a new traceroute session.¶

In the case that a traceroute reply is not received for a request within a certain time interval (lifetime of the request), the client should time-out and send a new request with a new nonce value up to a maximum number of requests to be sent specified by the user.¶

Authors' Addresses