Dynamic Allocation of Shared IPv4 Addresses

The shortage of available public IPv4 addresses means that it is not always possible for operators to allocate a full IPv4 address to every connected device. This problem is particularly acute whilst an operator is migrating from their existing, native IPv4 network to a native IPv6 network with IPv4 provided as an overlay service. During this phase, public IPv4 addresses are needed to provide for both existing and transition networks. Two main types of solutions have emerged to address the problem (see Appendix A of ): Deploying Carrier Grade Network Address Translation devices (CGNAT, ). Distributing the same public IPv4 address to multiple clients differentiated by non-overlapping layer 4 port sets. This memo focuses on the second category of solutions. introduces a "DHCP 4o6 Server", which offers dynamic leasing for IPv4 addresses to clients as in DHCPv4 but transported within a DHCPv6 message flow. This memo specifies a new DHCPv4 option: OPTION_V4_PORTPARAMS, and describes how it can be used for the dynamic leasing of shared IPv4 addresses. Although DHCPv4 over DHCPv6 is used as the underlying DHCPv4 transport mechanism throughout this document, OPTION_V4_PORTPARAMS as a DHCPv4 option may also be used in other solutions, if required. This extension is only suitable for specific architectures based on the Address plus Port model (A+P) such as and certain configurations of .

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 .

This document makes use of the following terms: An IPv4 address with a restricted layer 4 port set. Connections sourced from the shared address MUST use source ports within the assigned port set. Identifier for a range of ports assigned to a DHCP client.

Functionally, the dynamic allocation of shared IPv4 addresses by the DHCP 4o6 Server is similar to dynamic allocation process for 'full' IPv4 addresses described in . The essential difference is that the DHCP 4o6 Server MAY allocate the same IPv4 address to more than one DHCP 4o6 client simultaneously, providing that each shared address allocation also includes a range of layer 4 source ports unique to that address (i.e., the combined tuple of IPv4 address and Port Set ID MUST be unique for each active lease). The DHCP 4o6 client implements OPTION_V4_PORTPARAMS (described below), which is a DHCPv4 option containing PSID (Port Set ID) information. The client includes this option within the Parameter Request List option in its DHCPv4 DHCPDISCOVER and DHCPREQUEST messages, indicating its support for shared, dynamic address leasing to the DHCP 4o6 server. OPTION_V4_PORTPARAMS is also implemented by the server to identify clients that support shared, dynamic address leasing. With this option, the server can dynamically allocate PSIDs to clients and maintain shared IPv4 address leases. The server then manages unique client leases based the IPv4 address and PSID tuple, instead of using only the IPv4 address. In the event that a dynamic, shared addressing capable client receives more than one DHCP 4o6 offer, where a received offer does not contain OPTION_V4_PORTPARAMS (i.e.,j is an offer for a full IPv4 address), then the client SHOULD prefer the full IPv4 offer over the shared IPv4 address offer(s), unless specifically configured otherwise.

The following DHCPv4 message flow is transported within the DHCPv4-query and DHCPv4-response messages as in DHCPv4 over DHCPv6 . When the client constructs the DHCPv4 DHCPDISCOVER message to be transported within the DHCPv4-query message, the DHCPDISCOVER message MUST include the client identifier option (constructed as per ) and the Parameter Request List (PRL) option with OPTION_V4_PORTPARAMS. DHCP 4o6 Servers that receive the DHCPDISCOVER message and support shared IPv4 addresses respond with a DHCPOFFER message as usual with the shared IPv4 address in the 'yiaddr' field and MUST add an OPTION_V4_PORTPARAMS option containing an available restricted port set. If the DHCPDISCOVER included an OPTION_V4_PORTPARAMS option containing a non-zero PSID-Len field, the DHCP 4o6 Server MAY allocate a port set of the requested size to the client (depending on policy). The DHCPOFFER message is then encapsulated in the DHCPv4-response message and sent to the client. The client evaluates all received DHCPOFFER messages and selects one (e.g., based on the configuration parameters received, such as the size of the offered port set). The client then sends a DHCPREQUEST encapsulated in the DHCPv4-query message containing the corresponding OPTION_V4_PORTPARAMS received in the DHCPOFFER message. The server identified in the DHCPREQUEST message creates a binding for the client. The binding includes the client identifier, the IPv4 address and the PSID. These parameters are used by both the server and the client to identify a lease in any DHCP message. The server MUST respond with a DHCPACK message containing OPTION_V4_PORTPARAMS for the requesting client. On receipt of the DHCPACK message with the configuration parameters, the client MUST NOT perform an in-use probe on the address, such as ARPing for a duplicate allocated address. If the client chooses to relinquish its lease by sending a DHCPRELEASE message, the client MUST include the leased network address and OPTION_V4_PORTPARAMS (with the allocated PSID) to identify the lease to be released. In the case that the client has stored the previously allocated address and restricted port set, the logic described in Section 3.2 of MUST be followed on the condition that the client's source IPv6 address for DHCP 4o6 does not change. Note, this corresponds to INIT-REBOOT state defiend in . The client MUST include the OPTION_V4_PORTPARAMS with the requested port set information in the message flow, which starts with a DHCPREQUEST message. If the client's DHCP 4o6 IPv6 source address is changed for any reason, the client MUST re-initiate the DHCP 4o6 shared-address provisioning process by sending a DHCPDISCOVER message.

A DHCP 4o6 client discovering for a shared IPv4 address MUST include the OPTION_V4_PORTPARAMS option code in the Parameter Request List option. If a client has been successfully allocated and IPv4 address and PSID previously, the client MAY include in the DHCPDISCOVER message the 'requested IP address' option along with an OPTION_V4_PORTPARAMS to request that a specific IPv4 address and PSID be re-assigned. Alternatively, the client MAY omit the 'requested IP address' option, but include an OPTION_V4_PORTPARAMS with a non-zero value in only the PSID-Len field, as a hint to the server for the preferred size of the port set. A client that requests OPTION_V4_PORTPARAMS, but receives DHCPOFFER and DHCPACK messages without OPTION_V4_PORTPARAMS SHOULD proceed as defined in and configure a full IPv4 address with no address sharing. When receiving a DHCPACK message containing OPTION_V4_PORTPARAMS, the client MUST use the received explicit PSID for configuring the interface for which the DHCP 4o6 request was made. The client MUST NOT probe a newly received IPv4 address (e.g., using ARP) to see if it is in use by another host. When the client renews or releases its DHCP lease, it MUST put the values of offset, PSID length and PSID into OPTION_V4_PORTPARAMS, and send it to the server within corresponding DHCPv4 messages that are conveyed through DHCPv4-query message. In the event that the client's DHCP 4o6 IPv6 source address is changed for any reason, the client MUST re-initiate the DHCP 4o6 shared-address provisioning process by sending a DHCPDISCOVER message.

As a single IPv4 address is being shared between a number of different clients, the allocated shared address is only suitable for certain uses. The client MUST implement a function to ensure that only the allocated layer 4 ports of the shared IPv4 address are used for sourcing new connections, or accepting inbound connections. The client MUST apply the following rules for all traffic destined to or originating from the shared IPv4 address: The client MUST use only port-aware protocols (e.g., TCP, UDP, DCCP etc.) or ICMP implementing . All connections originating from the shared IPv4 address MUST use a source port taken from the allocated restricted port set. The client MUST NOT accept inbound connections on ports outside of the allocated restricted port set. In order to prevent addressing conflicts which could arise from the allocation of the same IPv4 address, the client MUST NOT configure the received restricted IPv4 address on-link. The mechanism by which a client implements the above rules is out of the scope of this document. In the event that the DHCPv4 over DHCPv6 configuration mechanism fails for any reason, the client MUST NOT configure an IPv4 link-local address (taken from the 169.254.0.0/16 range).

The DHCP 4o6 Server MUST NOT reply with OPTION_V4_PORTPARAMS unless the client has explicitly listed the option code in the Parameter Request List (Option 55) . The DHCP 4o6 Server SHOULD reply with OPTION_V4_PORTPARAMS if the client includes OPTION_V4_PORTPARAMS in its Parameter Request List. In order to achieve the dynamic management of shared IPv4 addresses, the server MUST implement an address and port-set pool that provides the same function as the address pool in a regular DHCP server. The server MUST use the combination of address and PSID as the key for maintaining the state of a lease, and for searching for an available lease for assignment. The leasing database MUST include the IPv4 address, PSID and client identifier of the requesting client. When a server receives a DHCPDISCOVER message with OPTION_V4_PORTPARAMS in the Parameter Request List option, the server determines an IPv4 address with a PSID for the requesting client. If an IPv4 address with a PSID is available, the server SHOULD follow the logic below to select which specific address and PSID to provision to the client. The logic is similar to that in Section 4.3.1 of . The client's current address with the PSID as recorded in the client's current lease binding, ELSE The client's previous address with PSID as recorded in the client's (expired or released) binding, if that address with PSID is in the server's pool of available addresses and PSIDs, and not already allocated, ELSE The address requested in the 'Requested IP Address' option along with the PSID parameters requested in the OPTION_V4_PORTPARAMS, if that pair of address and PSID is valid and not already allocated, ELSE A new address with a PSID allocated from the server's pool of available addresses and PSIDs. Upon receipt of a DHCPRELEASE message with OPTION_V4_PORTPARAMS, the server searches for the lease using the address in the 'ciaddr' field and the PSID information in the OPTION_V4_PORTPARAMS, and marks the lease as unallocated. The port-set assignment MUST be coupled with the address assignment process. Therefore the server MUST assign the address and port set in the same DHCP message. When defining the pools of IPv4 addresses and PSIDs which are available to lease to clients, the server MUST implement a mechanism to reserve some port ranges (e.g., 'well-known-ports' 0-1023) from allocation to clients. The reservation policy SHOULD be configurable.

A single DHCP 4o6 server may serve clients that do not support OPTION_V4_PORTPARAMS as well as those that do. As the rules for the allocation of shared addresses differ from the rules for full IPv4 address assignment, the DHCP 4o6 server MUST implement a mechanism to ensure that clients not supporting OPTION_V4_PORTPARAMS do not receive shared addresses. For example, two separate IPv4 addressing pools could be used, one of which allocates IPv4 addresses and PSIDs only to clients that have requested them. If the server is only configured with address pools for shared address allocation, it MUST discard requests that do not contain OPTION_V4_PORTPARAMS in the Parameter Request List option.

The DHCPv4 Port Parameters Option uses the same fields as the S46 Port Parameters Option described in Section 4.5 of , implemented as a DHCPv4 option. This is to maintain compatibility with existing port set implementations. The format of OPTION_V4_PORTPARAMS is shown in .

option-code: OPTION_V4_PORTPARAMS (TBA) option-len: 4 offset: (PSID offset) 8 bits long field that specifies the numeric value for the excluded port range/offset bits (A-bits), as per section 5.1 of . Allowed values are between 0 and 15, with the default value being 6 for MAP based implementations. This parameter is unused by a Lightweight 4over6 client and should be set to 0. PSID-len: Bit length value of the number of significant bits in the PSID field (also known as 'k'). When set to 0, the PSID field is to be ignored. After the first 'a' bits, there are k bits in the port number representing the value of PSID. Subsequently, the address sharing ratio would be 2^k. PSID: Explicit 16-bit (unsigned word) PSID value. The PSID value algorithmically identifies a set of ports assigned to a client. The first k-bits on the left of this 2-octets field is the PSID value. The remaining (16-k) bits on the right are padding zeros. Section 5.1 provides a full description of how the PSID is interpreted by the client. In order to exclude the system ports () or ports reserved by ISPs, the former port-sets that contain well-known ports MUST NOT be assigned unless the operator has explicitly configured otherwise (e.g., by allocating a full IPv4 address).

The security considerations in and are to be considered. Additional considerations are elaborated in the following sub-sections.

The solution is vulnerable to DoS attacks when used on a shared medium or when access network authentication is not a prerequisite to IP address assignment. The solution SHOULD only be used on point-to-point links, tunnels, and/or in environments where authentication at the link layer is performed before IP address assignment. It is not suitable for network access over shared mediums.

Preserving port randomization may be more or less difficult depending on the address sharing ratio (i.e., the size of the port space assigned to a client). The host can only randomize the ports inside a fixed port range . More discussion to improve the robustness of TCP against Blind In-Window Attacks can be found at . Other means than the (IPv4) source port randomization to provide protection against attacks should be used (e.g., use to improve the robustness of TCP against Blind In-Window Attacks, use IPv6). A proposal to preserve the entropy when selecting port is discussed in .

IANA is requested to assign the following new DHCPv4 Option Code in the registry maintained in http://www.iana.org/assignments/bootp- dhcp-parameters/: Option Name Value Data length Meaning OPTION_V4_PORTPARAMS TBA 4 This option is used to configure a set of ports bound to a shared IPv4 address.

This document is merged from and . The authors would like to thank Peng Wu, Gabor Bajko, Teemu Savolainen, Ted Lemon, Tina Tsou, Pierre Levis, Cong Liu and Marcin Siodelski, for their contributions to this work.