Hi, I have reviewed this document as part of the Security Directorate's ongoing effort to review all IETF documents being processed by the IESG. ... AND ... I have performed an unofficial Operations Directorate review. (i.e., I wasn't assigned as the OPSDIR reviewer) Background: This memo defines a Virtual Subnet Selection (VSS) option for DHCPv4 and DHCPv6, and a DHCPv4 relay-agent-information sub-option. These are intended for use by DHCP clients, relay agents, and proxy clients in situations where VSS information needs to be passed to the DHCP server for proper address or prefix allocation to take place. ========================= SECDIR Review ========================= The security review comments were written primarily for the benefit of the security area directors. Document editors and WG chairs should treat these comments just like any other last call comments. I found the security considerations section reasonably thorough. Some of the considerations are of the form "there is this known problem; you should do whatever you can to mitigate it." I wonder if some specific mitigation mechanisms might have been described and standardized. In section 4, a relay agent can insert a VSS option into a client request, and then remove it from the server response. I am a little concerned that the server does not actually get to differentiate which entity is making the VSS request, and the relay is thus masquerading as the client. ========================= OPSDIR Review Questions: ========================= The operations review comments were written primarily for the benefit of the OPS area directors. Document editors and WG chairs should treat these comments just like any other last call comments. I do not normally work at the DHCP level, so some of my comments may simply be misunderstandings of DHCP. Is the document readable? yes. Does it contain nits? yes. The document seems to lack a disclaimer for pre-RFC5378 work Is the document class appropriate? yes. Is the problem well stated? yes. Is the problem really a problem? yes. Does the document consider existing solutions? yes. Does the solution break existing technology? possibly. 1) Section 4 says "Deploying relay agents which support and emit VSS sub-options in concert with DHCPv4 servers which do not support the VSS option or sub-option as defined in this document SHOULD NOT be done, as such an ensemble will not operate correctly together because all of the IP addresses will be allocated from the global or default VPN regardless of the VPN on which the client's reside." 2) I have concerns that there are potential conflicts that are not being addressed: " There are many other scenarios which can be created with multiple relay agents each inserting VSS information into different Relay- forward messages, relay agent VSS information conflicting with client VSS information, or DHCP server VSS information conflicting with relay agent and client VSS information. Since these scenarios do not describe situations that are useful today, specifying precisely how to resolve all of these conflicts is unlikely to be valuable in the event that these scenarios actually become practical in the future. The current use of the VSS option and sub-option require that each entity knows the part that it plays in dealing with VPN data. Each entity -- client, relay agent or agents, and server -- SHOULD know through some policy or configuration beyond the scope of this document whether it is responsible for specifying VPN information using the VSS option or sub-option or responsible for responding to VSS information specified by another entity, or simply ignoring any VSS information which it might see. Some simple conflict resolution approaches are discussed below, in the hopes that they will cover simple cases that may arise from scenarios beyond those envisioned today. However, for more complex scenarios, or simple scenarios where appropriate conflict resolution strategies differ from those discussed in this document, a document detailing the usage scenarios and appropriate conflict resolution strategies SHOULD be created and submitted for discussion and approval." 3) section 7 says " In either case above, care should be taken to ensure that a client or relay agent receiving a reply containing a VSS option will correctly understand the VSS option. Otherwise, the client or relay agent will end up using the address as though it were a global address." This could have a negative impact on the existing network deployment. Does the solution preclude future activity? yes. It declares the VPN types 2-254 to be invalid "as of this memo", but doesn't discuss how they could become valid in the future. The IANA section seems to waffle on whether it can or cannot be expanded in the future. Is the solution sufficiently configurable? There is quite a bit of text that says the entity "has been configured in some way" In some cases, the configuration MUST be done correctly for the system to work, but the configuration requirements are not detailed. For example, Section 4 says "It is important to ensure that each entity ... is configured correctly." and "There is no conflict between different entities trying to specify different VSS information -- each entity knows its role through policy or configuration external to this document." and "In the event that there were more than one relay agent involved in this transaction, some external configuration or policy would be needed to inform the DHCPv6 server into which Relay-reply message the VSS option should go." These sound like normative requirements, but there is no attempt to standardize the configuration. Can performance be measured? How? performance measurement is not discussed. Does the solution scale well? I think this should scale as well as DHCP. I suppose that the server might be expected to have lots of storage if it needs to track the address ranges for a large number of VPNs, but I don't think that would be a limiting factor for a DHCP server. general comments: 1. The Terminology section defines upstream and downstream using terms that have not been defined. It is unclear to me whether access concentrator and subscriber are similar to server and client. 2. In section 3.4, might it be better to declare 2-254 as reserved rather than invalid? The text says "invalid as of this memo". Should there be a mechanism to support enterprise-specific VSS? 3. In section 4, it says DHCP can assign the same IP address to nodes in a VPN and in the global IP space, because the address is qualified by the VPN. Is this always true? Is there any potential for conflict, such as in forwarding loops, if the two addresses spaces are handled by the same device? 4. I think section 4 would benefit from sub-sections to separate the scenarios, and all the "in this scenario" phrases could be eliminated. Diagrams of the scenarios would be helpful. 5. Will legacy DHCP entities ignore the VSS option by default? or will the presence of the option somehow impact entity behaviors (e.g., by dropping packets)? 6. In section 5, it says the relay SHOULD insert VSS information into requests from a client. What happens if the client has inserted a VSS option? That isn't discussed. 7. Section 5 says "Anytime a relay agent places a VSS option or sub-option in a DHCP request, it MUST send it only to a DHCP server which supports the VSS option or sub-option." How does it know? is there an option discovery mechanism? 8. In section 5, if a relays requests a specific VPN, but the server returns an address not within that VPN, then the relay should drop the packet. Should the relay let the server know that it dropped the packet and why? Won't the server assume the address has actually been assigned to the client, thus reducing the pool of available addresses? 9. In section 5, "If an IP address that is on the requested VPN is not required, then the relay agent is free to accept the IP address that is not on the VPN that was requested." Then why request it? Under what conditions would it not be required? how does the relay know whether it is or is not required? 10. In section 5, it says "There are only two pieces of information which can be determined ..." but the speciied behaviors could also result from mis-configuration, right? And the relay cannot tell whether it is a deliberate behavior or a mis-configuration. 11. section 5: " Thus, if a DHCPv4 relay agent has a requirement to determine if the address allocated by a DHCPv4 server is on a particular VPN, it must use some other approach than the appearance of the VSS sub-option in the reply packet to make this determination." What other approach works? 12. section 5: " Note that in some environments a relay agent may choose to always place a VSS option or sub-option into packets and messages that it forwards in order to forestall any attempt by a downstream relay agent or client to specify VSS information. In this case, a type field of 255 is used to denote the global, default VPN. When the type field of 255 is used, there MUST NOT be any additional VSS Information in the VSS option." This section does not say that the relay must check to make sure there is no existing VSS option. Section 5 talks about relays inserting VSS options, buit does not specify that the relay must check that no VSS=255 is present in the message already. But section 7.3 talks about resolving conflicting VSS options. I am not sure the potential conflicts abd resolutions are covered adequately. 13. section 5.1 what does "if the relay agent is unable to honor the server requirement" mean? This could be made less ambiguous. 14. section 5.1 " The DHCP server MUST NOT place VSS information in an outgoing packet if the relay agent or DHCP client is unprepared to properly interpret the VSS information." This seems ambiguous. How will the server know if the other entities can properly interpret the VSS information? s/send in/insert/ 15. section 4 says "The DHCP client, in this scenario, does not know the VPN on which it resides." section 6 says " A DHCPv4 or DHCPv6 client will employ the VSS option to communicate VSS information to their respective servers. This information MUST be included in every message concerning any IP address on a different VPN than the global or default VPN." these statements seem to conflict regarding the client's knowledge. 16. section 6: " Clients should be aware that some DHCP servers will return a VSS option with different values than that which was sent in. In addition, a client may receive a response from a DHCP server with a VSS option when none was sent in by the Client." So should subsequent messages from the client use the original VSS information or the server-returned or relay-returned VSS info? Can a return message contain multiple VSS options? If I read the document correctly, multiple relays can add their own VSS options, and a server typically copies all the options. If a client is expected to include the VSS option information in subsequent messages, does it include multiple VSS options? The second paragraph of 6 says that is not allowed. David Harrington dbharrington at comcast.net ietfdbh at comcast.net dharrington at huawei.com