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rfc:rfc3102

Network Working Group Editors: Request for Comments: 3102 M. Borella Category: Experimental CommWorks

                                                                 J. Lo
                                                  Candlestick Networks
                                                         Contributors:
                                                          D. Grabelsky
                                                             CommWorks
                                                         G. Montenegro
                                                      Sun Microsystems
                                                          October 2001
                    Realm Specific IP: Framework

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2001).  All Rights Reserved.

IESG Note

 The IESG notes that the set of documents describing the RSIP
 technology imply significant host and gateway changes for a complete
 implementation.  In addition, the floating of port numbers can cause
 problems for some applications, preventing an RSIP-enabled host from
 interoperating transparently with existing applications in some cases
 (e.g., IPsec).  Finally, there may be significant operational
 complexities associated with using RSIP.  Some of these and other
 complications are outlined in section 6 of RFC 3102, as well as in
 the Appendices of RFC 3104.  Accordingly, the costs and benefits of
 using RSIP should be carefully weighed against other means of
 relieving address shortage.

Abstract

 This document examines the general framework of Realm Specific IP
 (RSIP).  RSIP is intended as a alternative to NAT in which the end-
 to-end integrity of packets is maintained.  We focus on
 implementation issues, deployment scenarios, and interaction with
 other layer-three protocols.

Borella, et al. Experimental [Page 1] RFC 3102 RSIP: Framework October 2001

Table of Contents

 1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . .  2
 1.1. Document Scope  . . . . . . . . . . . . . . . . . . . . . .  4
 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . .  4
 1.3. Specification of Requirements . . . . . . . . . . . . . . .  5
 2. Architecture  . . . . . . . . . . . . . . . . . . . . . . . .  6
 3. Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  7
 3.1. Host and Gateway Requirements . . . . . . . . . . . . . . .  7
 3.2. Processing of Demultiplexing Fields . . . . . . . . . . . .  8
 3.3. RSIP Protocol Requirements and Recommendations  . . . . . .  9
 3.4. Interaction with DNS  . . . . . . . . . . . . . . . . . . . 10
 3.5. Locating RSIP Gateways  . . . . . . . . . . . . . . . . . . 11
 3.6. Implementation Considerations . . . . . . . . . . . . . . . 11
 4. Deployment  . . . . . . . . . . . . . . . . . . . . . . . . . 12
 4.1. Possible Deployment Scenarios . . . . . . . . . . . . . . . 12
 4.2. Cascaded RSIP and NAT . . . . . . . . . . . . . . . . . . . 14
 5. Interaction with Layer-Three Protocols  . . . . . . . . . . . 17
 5.1. IPSEC . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
 5.2. Mobile IP . . . . . . . . . . . . . . . . . . . . . . . . . 18
 5.3. Differentiated and Integrated Services  . . . . . . . . . . 18
 5.4. IP Multicast  . . . . . . . . . . . . . . . . . . . . . . . 21
 6. RSIP Complications  . . . . . . . . . . . . . . . . . . . . . 23
 6.1. Unnecessary TCP TIME_WAIT . . . . . . . . . . . . . . . . . 23
 6.2. ICMP State in RSIP Gateway  . . . . . . . . . . . . . . . . 23
 6.3. Fragmentation and IP Identification Field Collision . . . . 24
 6.4. Application Servers on RSAP-IP Hosts  . . . . . . . . . . . 24
 6.5. Determining Locality of Destinations from an RSIP Host. . . 25
 6.6. Implementing RSIP Host Deallocation . . . . . . . . . . . . 26
 6.7. Multi-Party Applications  . . . . . . . . . . . . . . . . . 26
 6.8. Scalability . . . . . . . . . . . . . . . . . . . . . . . . 27
 7. Security Considerations . . . . . . . . . . . . . . . . . . . 27
 8. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 27
 9. References  . . . . . . . . . . . . . . . . . . . . . . . . . 28
 10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 29
 11. Full Copyright Statement . . . . . . . . . . . . . . . . . . 30

1. Introduction

 Network Address Translation (NAT) has become a popular mechanism of
 enabling the separation of addressing spaces. A NAT router must
 examine and change the network layer, and possibly the transport
 layer, header of each packet crossing the addressing domains that the
 NAT router is connecting.  This causes the mechanism of NAT to
 violate the end-to-end nature of the Internet connectivity, and
 disrupts protocols requiring or enforcing end-to-end integrity of
 packets.

Borella, et al. Experimental [Page 2] RFC 3102 RSIP: Framework October 2001

 While NAT does not require a host to be aware of its presence, it
 requires the presence of an application layer gateway (ALG) within
 the NAT router for each application that embeds addressing
 information within the packet payload.  For example, most NATs ship
 with an ALG for FTP, which transmits IP addresses and port numbers on
 its control channel.  RSIP (Realm Specific IP) provides an
 alternative to remedy these limitations.
 RSIP is based on the concept of granting a host from one addressing
 realm a presence in another addressing realm by allowing it to use
 resources (e.g., addresses and other routing parameters) from the
 second addressing realm.  An RSIP gateway replaces the NAT router,
 and RSIP-aware hosts on the private network are referred to as RSIP
 hosts.  RSIP requires ability of the RSIP gateway to grant such
 resources to RSIP hosts.  ALGs are not required on the RSIP gateway
 for communications between an RSIP host and a host in a different
 addressing realm.
 RSIP can be viewed as a "fix", of sorts, to NAT.  It may ameliorate
 some IP address shortage problems in some scenarios without some of
 the limitations of NAT.  However, it is not a long-term solution to
 the IP address shortage problem.  RSIP allows a degree of address
 realm transparency to be achieve between two differently-scoped, or
 completely different addressing realms.  This makes it a useful
 architecture for enabling end-to-end packet transparency between
 addressing realms.  RSIP is expected to be deployed on privately
 addresses IPv4 networks and used to grant access to publically
 addressed IPv4 networks.  However, in place of the private IPv4
 network, there may be an IPv6 network, or a non-IP network.  Thus,
 RSIP allows IP connectivity to a host with an IP stack and IP
 applications but no native IP access.  As such, RSIP can be used, in
 conjunction with DNS and tunneling, to bridge IPv4 and IPv6 networks,
 such that dual-stack hosts can communicate with local or remote IPv4
 or IPv6 hosts.
 It is important to note that, as it is defined here, RSIP does NOT
 require modification of applications.  All RSIP-related modifications
 to an RSIP host can occur at layers 3 and 4.  However, while RSIP
 does allow end-to-end packet transparency, it may not be transparent
 to all applications.  More details can be found in the section "RSIP
 complications", below.

Borella, et al. Experimental [Page 3] RFC 3102 RSIP: Framework October 2001

1.1. Document Scope

 This document provides a framework for RSIP by focusing on four
 particular areas:
  1. Requirements of an RSIP host and RSIP gateway.
  1. Likely initial deployment scenarios.
  1. Interaction with other layer-three protocols.
  1. Complications that RSIP may introduce.
 The interaction sections will be at an overview level.  Detailed
 modifications that would need to be made to RSIP and/or the
 interacting protocol are left for separate documents to discuss in
 detail.
 Beyond the scope of this document is discussion of RSIP in large,
 multiple-gateway networks, or in environments where RSIP state would
 need to be distributed and maintained across multiple redundant
 entities.
 Discussion of RSIP solutions that do not use some form of tunnel
 between the RSIP host and RSIP gateway are also not considered in
 this document.
 This document focuses on scenarios that allow privately-addressed
 IPv4 hosts or IPv6 hosts access to publically-addressed IPv4
 networks.

1.2. Terminology

 Private Realm
    A routing realm that uses private IP addresses from the ranges
    (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) specified in
    [RFC1918], or addresses that are non-routable from the Internet.
 Public Realm
    A routing realm with globally unique network addresses.
 RSIP Host
    A host within an addressing realm that uses RSIP to acquire
    addressing parameters from another addressing realm via an RSIP
    gateway.

Borella, et al. Experimental [Page 4] RFC 3102 RSIP: Framework October 2001

 RSIP Gateway
    A router or gateway situated on the boundary between two
    addressing realms that is assigned one or more IP addresses in at
    least one of the realms.  An RSIP gateway is responsible for
    parameter management and assignment from one realm to RSIP hosts
    in the other realm.  An RSIP gateway may act as a normal NAT
    router for hosts within the a realm that are not RSIP enabled.
 RSIP Client
    An application program that performs the client portion of the
    RSIP client/server protocol.  An RSIP client application MUST
    exist on all RSIP hosts, and MAY exist on RSIP gateways.
 RSIP Server
    An application program that performs the server portion of the
    RSIP client/server protocol.  An RSIP server application MUST
    exist on all RSIP gateways.
 RSA-IP: Realm Specific Address IP
    An RSIP method in which each RSIP host is allocated a unique IP
    address from the public realm.
 RSAP-IP: Realm Specific Address and Port IP
    An RSIP method in which each RSIP host is allocated an IP address
    (possibly shared with other RSIP hosts) and some number of per-
    address unique ports from the public realm.
 Demultiplexing Fields
    Any set of packet header or payload fields that an RSIP gateway
    uses to route an incoming packet to an RSIP host.
 All other terminology found in this document is consistent with that
 of [RFC2663].

1.3. Specification of Requirements

 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 documents are to be interpreted as described in [RFC2119].

Borella, et al. Experimental [Page 5] RFC 3102 RSIP: Framework October 2001

2. Architecture

 In a typical scenario where RSIP is deployed, there are some number
 of hosts within one addressing realm connected to another addressing
 realm by an RSIP gateway.  This model is diagrammatically represented
 as follows:
       RSIP Host             RSIP Gateway                    Host
          Xa                    Na   Nb                       Yb
       [X]------( Addr sp. A )----[N]-----( Addr sp. B )-------[Y]
                (  Network   )            (  Network   )
 Hosts X and Y belong to different addressing realms A and B,
 respectively, and N is an RSIP gateway (which may also perform NAT
 functions).  N has two interfaces: Na on address space A, and Nb on
 address space B.  N may have a pool of addresses in address space B
 which it can assign to or lend to X and other hosts in address space
 A.  These addresses are not shown above, but they can be denoted as
 Nb1, Nb2, Nb3 and so on.
 As is often the case, the hosts within address space A are likely to
 use private addresses while the RSIP gateway is multi-homed with one
 or more private addresses from address space A in addition to its
 public addresses from address space B.  Thus, we typically refer to
 the realm in which the RSIP host resides as "private" and the realm
 from which the RSIP host borrows addressing parameters as the
 "public" realm.  However, these realms may both be public or private
 - our notation is for convenience.  In fact, address space A may be
 an IPv6 realm or a non-IP address space.
 Host X, wishing to establish an end-to-end connection to a network
 entity Y situated within address space B, first negotiates and
 obtains assignment of the resources (e.g., addresses and other
 routing parameters of address space B) from the RSIP gateway.  Upon
 assignment of these parameters, the RSIP gateway creates a mapping,
 referred as a "bind", of X's addressing information and the assigned
 resources.  This binding enables the RSIP gateway to correctly de-
 multiplex and forward inbound traffic generated by Y for X.  If
 permitted by the RSIP gateway, X may create multiple such bindings on
 the same RSIP gateway, or across several RSIP gateways.  A lease time
 SHOULD be associated with each bind.
 Using the public parameters assigned by the RSIP gateway, RSIP hosts
 tunnel data packets across address space A to the RSIP gateway.  The
 RSIP gateway acts as the end point of such tunnels, stripping off the
 outer headers and routing the inner packets onto the public realm.
 As mentioned above, an RSIP gateway maintains a mapping of the

Borella, et al. Experimental [Page 6] RFC 3102 RSIP: Framework October 2001

 assigned public parameters as demultiplexing fields for uniquely
 mapping them to RSIP host private addresses.  When a packet from the
 public realm arrives at the RSIP gateway and it matches a given set
 of demultiplexing fields, then the RSIP gateway will tunnel it to the
 appropriate RSIP host.  The tunnel headers of outbound packets from X
 to Y, given that X has been assigned Nb, are as follows:
          +---------+---------+---------+
          | X -> Na | Nb -> Y | payload |
          +---------+---------+---------+
 There are two basic flavors of RSIP: RSA-IP and RSAP-IP.  RSIP hosts
 and gateways MAY support RSA-IP, RSAP-IP, or both.
 When using RSA-IP, an RSIP gateway maintains a pool of IP addresses
 to be leased by RSIP hosts.  Upon host request, the RSIP gateway
 allocates an IP address to the host.  Once an address is allocated to
 a particular host, only that host may use the address until the
 address is returned to the pool.  Hosts MAY NOT use addresses that
 have not been specifically assigned to them.  The hosts may use any
 TCP/UDP port in combination with their assigned address.  Hosts may
 also run gateway applications at any port and these applications will
 be available to the public network without assistance from the RSIP
 gateway.  A host MAY lease more than one address from the same or
 different RSIP gateways.  The demultiplexing fields of an RSA-IP
 session MUST include the IP address leased to the host.
 When using RSAP-IP, an RSIP gateway maintains a pool of IP addresses
 as well as pools of port numbers per address.  RSIP hosts lease an IP
 address and one or more ports to use with it.  Once an address / port
 tuple has been allocated to a particular host, only that host may use
 the tuple until it is returned to the pool(s).  Hosts MAY NOT use
 address / port combinations that have not been specifically assigned
 to them.  Hosts may run gateway applications bound to an allocated
 tuple, but their applications will not be available to the public
 network unless the RSIP gateway has agreed to route all traffic
 destined to the tuple to the host.  A host MAY lease more than one
 tuple from the same or different RSIP gateways.  The demultiplexing
 fields of an RSAP-IP session MUST include the tuple(s) leased to the
 host.

3. Requirements

3.1. Host and Gateway Requirements

 An RSIP host MUST be able to maintain one or more virtual interfaces
 for the IP address(es) that it leases from an RSIP gateway.  The host
 MUST also support tunneling and be able to serve as an end-point for

Borella, et al. Experimental [Page 7] RFC 3102 RSIP: Framework October 2001

 one or more tunnels to RSIP gateways.  An RSIP host MUST NOT respond
 to ARPs for a public realm address that it leases.
 An RSIP host supporting RSAP-IP MUST be able to maintain a set of one
 or more ports assigned by an RSIP gateway from which choose ephemeral
 source ports.  If the host's pool does not have any free ports and
 the host needs to open a new communication session with a public
 host, it MUST be able to dynamically request one or more additional
 ports via its RSIP mechanism.
 An RSIP gateway is a multi-homed host that routes packets between two
 or more realms.  Often, an RSIP gateway is a boundary router between
 two or more administrative domains.  It MUST also support tunneling
 and be able to serve as an end-point for tunnels to RSIP hosts.  The
 RSIP gateway MAY be a policy enforcement point, which in turn may
 require it to perform firewall and packet filtering duties in
 addition to RSIP.  The RSIP gateway MUST reassemble all incoming
 packet fragments from the public network in order to be able to route
 and tunnel them to the proper host.  As is necessary for fragment
 reassembly, an RSIP gateway MUST timeout fragments that are never
 fully reassembled.
 An RSIP gateway MAY include NAT functionality so that hosts on the
 private network that are not RSIP-enabled can still communicate with
 the public network.  An RSIP gateway MUST manage all resources that
 are assigned to RSIP hosts.  This management MAY be done according to
 local policy.

3.2. Processing of Demultiplexing Fields

 Each active RSIP host must have a unique set of demultiplexing fields
 assigned to it so that an RSIP gateway can route incoming packets
 appropriately.  Depending on the type of mapping used by the RSIP
 gateway, demultiplexing fields have been defined to be one or more of
 the following:
  1. destination IP address
  1. IP protocol
  1. destination TCP or UDP port
  1. IPSEC SPI present in ESP or AH header (see [RFC3104])
  1. others
 Note that these fields may be augmented by source IP address and
 source TCP or UDP port.

Borella, et al. Experimental [Page 8] RFC 3102 RSIP: Framework October 2001

 Demultiplexing of incoming traffic can be based on a decision tree.
 The process begins with the examination of the IP header of the
 incoming packet, and proceeds to subsequent headers and then the
 payload.
  1. In the case where a public IP address is assigned for each

host, a unique public IP address is mapped to each RSIP host.

  1. If the same IP address is used for more than one RSIP host,

then subsequent headers must have at least one field that will

       be assigned a unique value per host so that it is usable as a
       demultiplexing field.  The IP protocol field SHOULD be used to
       determine what in the subsequent headers these demultiplexing
       fields ought to be.
  1. If the subsequent header is TCP or UDP, then destination port

number can be used. However, if the TCP/UDP port number is the

       same for more than one RSIP host, the payload section of the
       packet must contain a demultiplexing field that is guaranteed
       to be different for each RSIP host.  Typically this requires
       negotiation of said fields between the RSIP host and gateway so
       that the RSIP gateway can guarantee that the fields are unique
       per-host
  1. If the subsequent header is anything other than TCP or UDP,

there must exist other fields within the IP payload usable as

       demultiplexing fields.  In other words, these fields must be
       able to be set such that they are guaranteed to be unique per-
       host.  Typically this requires negotiation of said fields
       between the RSIP host and gateway so that the RSIP gateway can
       guarantee that the fields are unique per-host.
 It is desirable for all demultiplexing fields to occur in well-known
 fixed locations so that an RSIP gateway can mask out and examine the
 appropriate fields on incoming packets.  Demultiplexing fields that
 are encrypted MUST NOT be used for routing.

3.3. RSIP Protocol Requirements and Recommendations

 RSIP gateways and hosts MUST be able to negotiate IP addresses when
 using RSA-IP, IP address / port tuples when using RSAP-IP, and
 possibly other demultiplexing fields for use in other modes.
 In this section we discuss the requirements and implementation issues
 of an RSIP negotiation protocol.
 For each required demultiplexing field, an RSIP protocol MUST, at the
 very least, allow for:

Borella, et al. Experimental [Page 9] RFC 3102 RSIP: Framework October 2001

  1. RSIP hosts to request assignments of demultiplexing fields
  1. RSIP gateways to assign demultiplexing fields with an

associated lease time

  1. RSIP gateways to reclaim assigned demultiplexing fields
 Additionally, it is desirable, though not mandatory, for an RSIP
 protocol to negotiate an RSIP method (RSA-IP or RSAP-IP) and the type
 of tunnel to be used across the private network.  The protocol SHOULD
 be extensible and facilitate vendor-specific extensions.
 If an RSIP negotiation protocol is implemented at the application
 layer, a choice of transport protocol MUST be made.  RSIP hosts and
 gateways may communicate via TCP or UDP.  TCP support is required in
 all RSIP gateways, while UDP support is optional.  In RSIP hosts,
 TCP, UDP, or both may be supported.  However, once an RSIP host and
 gateway have begun communicating using either TCP or UDP, they MAY
 NOT switch to the other transport protocol.  For RSIP implementations
 and deployments considered in this document, TCP is the recommended
 transport protocol, because TCP is known to be robust across a wide
 range of physical media types and traffic loads.
 It is recommended that all communication between an RSIP host and
 gateway be authenticated.  Authentication, in the form of a message
 hash appended to the end of each RSIP protocol packet, can serve to
 authenticate the RSIP host and gateway to one another, provide
 message integrity, and (with an anti-replay counter) avoid replay
 attacks.  In order for authentication to be supported, each RSIP host
 and the RSIP gateway MUST either share a secret key (distributed, for
 example, by Kerberos) or have a private/public key pair.  In the
 latter case, an entity's public key can be computed over each message
 and a hash function applied to the result to form the message hash.

3.4. Interaction with DNS

 An RSIP-enabled network has three uses for DNS: (1) public DNS
 services to map its static public IP addresses (i.e., the public
 address of the RSIP gateway) and for lookups of public hosts, (2)
 private DNS services for use only on the private network, and (3)
 dynamic DNS services for RSIP hosts.
 With respect to (1), public DNS information MUST be propagated onto
 the private network.  With respect to (2), private DNS information
 MUST NOT be propagated into the public network.

Borella, et al. Experimental [Page 10] RFC 3102 RSIP: Framework October 2001

 With respect to (3), an RSIP-enabled network MAY allow for RSIP hosts
 with FQDNs to have their A and PTR records updated in the public DNS.
 These updates are based on address assignment facilitated by RSIP,
 and should be performed in a fashion similar to DHCP updates to
 dynamic DNS [DHCP-DNS].  In particular, RSIP hosts should be allowed
 to update their A records but not PTR records, while RSIP gateways
 can update both.  In order for the RSIP gateway to update DNS records
 on behalf on an RSIP host, the host must provide the gateway with its
 FQDN.
 Note that when using RSA-IP, the interaction with DNS is completely
 analogous to that of DHCP because the RSIP host "owns" an IP address
 for a period of time.  In the case of RSAP-IP, the claim that an RSIP
 host has to an address is only with respect to the port(s) that it
 has leased along with an address.  Thus, two or more RSIP hosts'
 FQDNs may map to the same IP address.  However, a public host may
 expect that all of the applications running at a particular address
 are owned by the same logical host, which would not be the case.  It
 is recommended that RSAP-IP and dynamic DNS be integrated with some
 caution, if at all.

3.5. Locating RSIP Gateways

 When an RSIP host initializes, it requires (among other things) two
 critical pieces of information.  One is a local (private) IP address
 to use as its own, and the other is the private IP address of an RSIP
 gateway.  This information can be statically configured or
 dynamically assigned.
 In the dynamic case, the host's private address is typically supplied
 by DHCP.  A DHCP option could provide the IP address of an RSIP
 gateway in DHCPOFFER messages.  Thus, the host's startup procedure
 would be as follows: (1) perform DHCP, (2) if an RSIP gateway option
 is present in the DHCPOFFER, record the IP address therein as the
 RSIP gateway.
 Alternatively, the RSIP gateway can be discovered via SLP (Service
 Location Protocol) as specified in [SLP-RSIP].  The SLP template
 defined allows for RSIP service provisioning and load balancing.

3.6. Implementation Considerations

 RSIP can be accomplished by any one of a wide range of implementation
 schemes.  For example, it can be built into an existing configuration
 protocol such as DHCP or SOCKS, or it can exist as a separate
 protocol.  This section discusses implementation issues of RSIP in
 general, regardless of how the RSIP mechanism is implemented.

Borella, et al. Experimental [Page 11] RFC 3102 RSIP: Framework October 2001

 Note that on a host, RSIP is associated with a TCP/IP stack
 implementation.  Modifications to IP tunneling and routing code, as
 well as driver interfaces may need to be made to support RSA-IP.
 Support for RSAP-IP requires modifications to ephemeral port
 selection code as well.  If a host has multiple TCP/IP stacks or
 TCP/IP stacks and other communication stacks, RSIP will only operate
 on the packets / sessions that are associated with the TCP/IP
 stack(s) that use RSIP.  RSIP is not application specific, and if it
 is implemented in a stack, it will operate beneath all applications
 that use the stack.

4. Deployment

 When RSIP is deployed in certain scenarios, the network
 characteristics of these scenarios will determine the scope of the
 RSIP solution, and therefore impact the requirements of RSIP.  In
 this section, we examine deployment scenarios, and the impact that
 RSIP may have on existing networks.

4.1. Possible Deployment Scenarios

 In this section we discuss a number of potential RSIP deployment
 scenarios.  The selection below are not comprehensive and other
 scenarios may emerge.

4.1.1. Small / Medium Enterprise

 Up to several hundred hosts will reside behind an RSIP-enabled
 router.  It is likely that there will be only one gateway to the
 public network and therefore only one RSIP gateway.  This RSIP
 gateway may control only one, or perhaps several, public IP
 addresses.  The RSIP gateway may also perform firewall functions, as
 well as routing inbound traffic to particular destination ports on to
 a small number of dedicated gateways on the private network.

4.1.2. Residential Networks

 This category includes both networking within just one residence, as
 well as within multiple-dwelling units.  At most several hundred
 hosts will share the gateway's resources.  In particular, many of
 these devices may be thin hosts or so-called "network appliances" and
 therefore not require access to the public Internet frequently.  The
 RSIP gateway is likely to be implemented as part of a residential
 firewall, and it may be called upon to route traffic to particular
 destination ports on to a small number of dedicated gateways on the
 private network.  It is likely that only one gateway to the public

Borella, et al. Experimental [Page 12] RFC 3102 RSIP: Framework October 2001

 network will be present and that this gateway's RSIP gateway will
 control only one IP address.  Support for secure end-to-end VPN
 access to corporate intranets will be important.

4.1.3. Hospitality Networks

 A hospitality network is a general type of "hosting" network that a
 traveler will use for a short period of time (a few minutes or a few
 hours).  Examples scenarios include hotels, conference centers and
 airports and train stations.  At most several hundred hosts will
 share the gateway's resources.  The RSIP gateway may be implemented
 as part of a firewall, and it will probably not be used to route
 traffic to particular destination ports on to dedicated gateways on
 the private network.  It is likely that only one gateway to the
 public network will be present and that this gateway's RSIP gateway
 will control only one IP address.  Support for secure end-to-end VPN
 access to corporate intranets will be important.

4.1.4. Dialup Remote Access

 RSIP gateways may be placed in dialup remote access concentrators in
 order to multiplex IP addresses across dialup users.  At most several
 hundred hosts will share the gateway's resources.  The RSIP gateway
 may or may not be implemented as part of a firewall, and it will
 probably not be used to route traffic to particular destination ports
 on to dedicated gateways on the private network.  Only one gateway to
 the public network will be present (the remote access concentrator
 itself) and that this gateway's RSIP gateway will control a small
 number of IP addresses.  Support for secure end-to-end VPN access to
 corporate intranets will be important.

4.1.5. Wireless Remote Access Networks

 Wireless remote access will become very prevalent as more PDA and IP
 / cellular devices are deployed.  In these scenarios, hosts may be
 changing physical location very rapidly - therefore Mobile IP will
 play a role.  Hosts typically will register with an RSIP gateway for
 a short period of time.  At most several hundred hosts will share the
 gateway's resources.  The RSIP gateway may be implemented as part of
 a firewall, and it will probably not be used to route traffic to
 particular destination ports on to dedicated gateways on the private
 network.  It is likely that only one gateway to the public network
 will be present and that this gateway's RSIP gateway will control a
 small number of IP addresses.  Support for secure end-to-end VPN
 access to corporate intranets will be important.

Borella, et al. Experimental [Page 13] RFC 3102 RSIP: Framework October 2001

4.2. Cascaded RSIP and NAT

 It is possible for RSIP to allow for cascading of RSIP gateways as
 well as cascading of RSIP gateways with NAT boxes.  For example,
 consider an ISP that uses RSIP for address sharing amongst its
 customers.  It might assign resources (e.g., IP addresses and ports)
 to a particular customer.  This customer may use RSIP to further
 subdivide the port ranges and address(es) amongst individual end
 hosts.  No matter how many levels of RSIP assignment exists, RSIP
 MUST only assign public IP addresses.
 Note that some of the architectures discussed below may not be useful
 or desirable.  The goal of this section is to explore the
 interactions between NAT and RSIP as RSIP is incrementally deployed
 on systems that already support NAT.

4.2.1. RSIP Behind RSIP

 A reference architecture is depicted below.
                             +-----------+
                             |           |
                             |   RSIP    |
                             |  gateway  +---- 10.0.0.0/8
                             |     B     |
                             |           |
                             +-----+-----+
                                   |
                                   | 10.0.1.0/24
                    +-----------+  | (149.112.240.0/25)
                    |           |  |
    149.112.240.0/24|   RSIP    +--+
    ----------------+  gateway  |
                    |     A     +--+
                    |           |  |
                    +-----------+  | 10.0.2.0/24
                                   | (149.112.240.128/25)
                                   |
                             +-----+-----+
                             |           |
                             |   RSIP    |
                             |  gateway  +---- 10.0.0.0/8
                             |     C     |
                             |           |
                             +-----------+

Borella, et al. Experimental [Page 14] RFC 3102 RSIP: Framework October 2001

 RSIP gateway A is in charge of the IP addresses of subnet
 149.112.240.0/24.  It distributes these addresses to RSIP hosts and
 RSIP gateways.  In the given configuration, it distributes addresses
 149.112.240.0 - 149.112.240.127 to RSIP gateway B, and addresses
 149.112.240.128 - 149.112.240.254 to RSIP gateway C.  Note that the
 subnet broadcast address, 149.112.240.255, must remain unclaimed, so
 that broadcast packets can be distributed to arbitrary hosts behind
 RSIP gateway A.  Also, the subnets between RSIP gateway A and RSIP
 gateways B and C will use private addresses.
 Due to the tree-like fashion in which addresses will be cascaded, we
 will refer to RSIP gateways A as the 'parent' of RSIP gateways B and
 C, and RSIP gateways B and C as 'children' of RSIP gateways A.  An
 arbitrary number of levels of children may exist under a parent RSIP
 gateway.
 A parent RSIP gateway will not necessarily be aware that the
 address(es) and port blocks that it distributes to a child RSIP
 gateway will be further distributed.  Thus, the RSIP hosts MUST
 tunnel their outgoing packets to the nearest RSIP gateway.  This
 gateway will then verify that the sending host has used the proper
 address and port block, and then tunnel the packet on to its parent
 RSIP gateway.
 For example, in the context of the diagram above, host 10.0.0.1,
 behind RSIP gateway C will use its assigned external IP address (say,
 149.112.240.130) and tunnel its packets over the 10.0.0.0/8 subnet to
 RSIP gateway C.  RSIP gateway C strips off the outer IP header.
 After verifying that the source public IP address and source port
 number is valid, RSIP gateway C will tunnel the packets over the
 10.0.2.0/8 subnet to RSIP gateway A.  RSIP gateway A strips off the
 outer IP header.  After verifying that the source public IP address
 and source port number is valid, RSIP gateway A transmits the packet
 on the public network.
 While it may be more efficient in terms of computation to have a RSIP
 host tunnel directly to the overall parent of an RSIP gateway tree,
 this would introduce significant state and administrative
 difficulties.
 A RSIP gateway that is a child MUST take into consideration the
 parameter assignment constraints that it inherits from its parent
 when it assigns parameters to its children.  For example, if a child
 RSIP gateway is given a lease time of 3600 seconds on an IP address,
 it MUST compare the current time to the lease time and the time that
 the lease was assigned to compute the maximum allowable lease time on
 the address if it is to assign the address to a RSIP host or child
 RSIP gateway.

Borella, et al. Experimental [Page 15] RFC 3102 RSIP: Framework October 2001

4.2.2. NAT Behind RSIP

             +--------+      +--------+
             | NAT w/ |      |  RSIP  |
 hosts ------+ RSIP   +------+ gate-  +----- public network
             | host   |      |  way   |
             +--------+      +--------+
 In this architecture, an RSIP gateway is between a NAT box and the
 public network.  The NAT is also equipped with an RSIP host.  The NAT
 dynamically requests resources from the RSIP gateway as the hosts
 establish sessions to the public network.  The hosts are not aware of
 the RSIP manipulation.  This configuration does not enable the hosts
 to have end-to-end transparency and thus the NAT still requires ALGs
 and the architecture cannot support IPSEC.

4.2.3. RSIP Behind NAT

             +--------+      +--------+
 RSIP        |  RSIP  |      |        |
 hosts ------+ gate-  +------+   NAT  +----- public network
             |  way   |      |        |
             +--------+      +--------+
 In this architecture, the RSIP hosts and gateway reside behind a NAT.
 This configuration does not enable the hosts to have end-to-end
 transparency and thus the NAT still requires ALGs and the
 architecture cannot support IPSEC.  The hosts may have transparency
 if there is another gateway to the public network besides the NAT
 box, and this gateway supports cascaded RSIP behind RSIP.

4.2.4. RSIP Through NAT

             +--------+      +--------+
 RSIP        |        |      |  RSIP  |
 hosts ------+   NAT  +------+ gate-  +----- public network
             |        |      |  way   |
             +--------+      +--------+
 In this architecture, the RSIP hosts are separated from the RSIP
 gateway by a NAT.  RSIP signaling may be able to pass through the NAT
 if an RSIP ALG is installed.  The RSIP data flow, however, will have
 its outer IP address translated by the NAT.  The NAT must not
 translate the port numbers in order for RSIP to work properly.
 Therefore, only traditional NAT will make sense in this context.

Borella, et al. Experimental [Page 16] RFC 3102 RSIP: Framework October 2001

5. Interaction with Layer-Three Protocols

 Since RSIP affects layer-three objects, it has an impact on other
 layer three protocols.  In this section, we outline the impact of
 RSIP on these protocols, and in each case, how RSIP, the protocol, or
 both, can be extended to support interaction.
 Each of these sections is an overview and not a complete technical
 specification.  If a full technical specification of how RSIP
 interacts with a layer-three protocol is necessary, a separate
 document will contain it.

5.1. IPSEC

 RSIP is a mechanism for allowing end-to-end IPSEC with sharing of IP
 addresses.  Full specification of RSIP/IPSEC details are in [RSIP-
 IPSEC].  This section provides a brief summary.  Since IPSEC may
 encrypt TCP/UDP port numbers, these objects cannot be used as
 demultiplexing fields.  However, IPSEC inserts an AH or ESP header
 following the IP header in all IPSEC-protected packets (packets that
 are transmitted on an IPSEC Security Association (SA)).  These
 headers contain a 32-bit Security Parameter Index (SPI) field, the
 value of which is determined by the receiving side.  The SPI field is
 always in the clear.  Thus, during SA negotiation, an RSIP host can
 instruct their public peer to use a particular SPI value.  This SPI
 value, along with the assigned IP address, can be used by an RSIP
 gateway to uniquely identify and route packets to an RSIP host.  In
 order to guarantee that RSIP hosts use SPIs that are unique per
 address, it is necessary for the RSIP gateway to allocate unique SPIs
 to hosts along with their address/port tuple.
 IPSEC SA negotiation takes place using the Internet Key Exchange
 (IKE) protocol.  IKE is designated to use port 500 on at least the
 destination side.  Some host IKE implementations will use source port
 500 as well, but this behavior is not mandatory.  If two or more RSIP
 hosts are running IKE at source port 500, they MUST use different
 initiator cookies (the first eight bytes of the IKE payload) per
 assigned IP address.  The RSIP gateway will be able to route incoming
 IKE packets to the proper host based on initiator cookie value.
 Initiator cookies can be negotiated, like ports and SPIs.  However,
 since the likelihood of two hosts assigned the same IP address
 attempting to simultaneously use the same initiator cookie is very
 small, the RSIP gateway can guarantee cookie uniqueness by dropping
 IKE packets with a cookie value that is already in use.

Borella, et al. Experimental [Page 17] RFC 3102 RSIP: Framework October 2001

5.2. Mobile IP

 Mobile IP allows a mobile host to maintain an IP address as it moves
 from network to network.  For Mobile IP foreign networks that use
 private IP addresses, RSIP may be applicable.  In particular, RSIP
 would allow a mobile host to bind to a local private address, while
 maintaining a global home address and a global care-of address.  The
 global care-of address could, in principle, be shared with other
 mobile nodes.
 The exact behavior of Mobile IP with respect to private IP addresses
 has not be settled.  Until it is, a proposal to adapt RSIP to such a
 scenario is premature.  Also, such an adaptation may be considerably
 complex.  Thus, integration of RSIP and Mobile IP is a topic of
 ongoing consideration.

5.3. Differentiated and Integrated Services

 To attain the capability of providing quality of service between two
 communicating hosts in different realms, it is important to consider
 the interaction of RSIP with different quality of service
 provisioning models and mechanisms.  In the section, RSIP interaction
 with the integrated service and differentiated service frameworks is
 discussed.

5.3.1. Differentiated Services

 The differentiated services architecture defined in [RFC2475] allows
 networks to support multiple levels of best-effort service through
 the use of "markings" of the IP Type-of-Service (now DS) byte.  Each
 value of the DS byte is termed a differentiated services code point
 (DSCP) and represents a particular per-hop behavior.  This behavior
 may not be the same in all administrative domains.  No explicit
 signaling is necessary to support differentiated services.
 For outbound packets from an edge network, DSCP marking is typically
 performed and/or enforced on a boundary router.  The marked packet is
 then forwarded onto the public network.  In an RSIP-enabled network,
 a natural place for DSCP marking is the RSIP gateway.  In the case of
 RSAP-IP, the RSIP gateway can apply its micro-flow (address/port
 tuple) knowledge of RSIP assignments in order to provide different
 service levels to different RSIP host.  For RSA-IP, the RSIP gateway
 will not necessarily have knowledge of micro-flows, so it must rely
 on markings made by the RSIP hosts (if any) or apply a default policy
 to the packets.

Borella, et al. Experimental [Page 18] RFC 3102 RSIP: Framework October 2001

 When differentiated services is to be performed between RSIP hosts
 and gateways, it must be done over the tunnel between these entities.
 Differentiated services over a tunnel is considered in detail in
 [DS-TUNN], the key points that need to be addressed here are the
 behaviors of tunnel ingress and egress for both incoming and going
 packets.
 For incoming packets arriving at an RSIP gateway tunnel ingress, the
 RSIP gateway may either copy the DSCP from the inner header to the
 outer header, leave the inner header DSCP untouched, but place a
 different DSCP in the outer header, or change the inner header DSCP
 while applying either the same or a different DSCP to the outer
 header.
 For incoming packets arriving at an RSIP host tunnel egress, behavior
 with respect to the DSCP is not necessarily important if the RSIP
 host not only terminates the tunnel, but consumes the packet as well.
 If this is not the case, as per some cascaded RSIP scenarios, the
 RSIP host must apply local policy to determine whether to leave the
 inner header DSCP as is, overwrite it with the outer header DSCP, or
 overwrite it with a different value.
 For outgoing packets arriving at an RSIP host tunnel ingress, the
 host  may either copy the DSCP from the inner header to the outer
 header, leave the inner header DSCP untouched, but place a different
 DSCP in the outer header, or change the inner header DSCP while
 applying either the same or a different DSCP to the outer header.
 For outgoing packets arriving at an RSIP gateway tunnel egress, the
 RSIP gateway must apply local policy to determine whether to leave
 the inner header DSCP as is, overwrite it with the outer header DSCP,
 or overwrite it with a different value.
 It is reasonable to assume that in most cases, the diffserv policy
 applicable on a site will be the same for RSIP and non-RSIP hosts.
 For this reason, a likely policy is that the DSCP will always be
 copied between the outer and inner headers in all of the above cases.
 However, implementations should allow for the more general case.

5.3.2. Integrated Services

 The integrated services model as defined by [RFC2205] requires
 signalling using RSVP to setup a resource reservation in intermediate
 nodes between the communicating endpoints.  In the most common
 scenario in which RSIP is deployed, receivers located within the
 private realm initiate communication sessions with senders located
 within the public realm.  In this section, we discuss the interaction
 of RSIP architecture and RSVP in such a scenario.  The less common

Borella, et al. Experimental [Page 19] RFC 3102 RSIP: Framework October 2001

 case of having senders within the private realm and receivers within
 the public realm is not discussed although concepts mentioned here
 may be applicable.
 With senders in the public realm, RSVP PATH messages flow downstream
 from sender to receiver, inbound with respect to the RSIP gateway,
 while RSVP RESV messages flow in the opposite direction.  Since RSIP
 uses tunneling between the RSIP host and gateway within the private
 realm, how the RSVP messages are handled within the RSIP tunnel
 depends on situations elaborated in [RFC2746].
 Following the terminology of [RFC2476], if Type 1 tunnels exist
 between the RSIP host and gateway, all intermediate nodes inclusive
 of the RSIP gateway will be treated as a non-RSVP aware cloud without
 QoS reserved on these nodes.  The tunnel will be viewed as a single
 (logical) link on the path between the source and destination.  End-
 to-end RSVP messages will be forwarded through the tunnel
 encapsulated in the same way as normal IP packets.  We see this as
 the most common and applicable deployment scenario.
 However, should Type 2 or 3 tunnels be deployed between the tunneling
 endpoints , end-to-end RSVP session has to be statically mapped (Type
 2) or dynamically mapped (Type 3) into the tunnel sessions.  While
 the end-to-end RSVP messages will be forwarded through the tunnel
 encapsulated in the same way as normal IP packets, a tunnel session
 is established between the tunnel endpoints to ensure QoS reservation
 within the tunnel for the end-to-end session.  Data traffic needing
 special QoS assurance will be encapsulated in a UDP/IP header while
 normal traffic will be encapsulated using the normal IP-IP
 encapsulation.  In the type 2 deployment scenario where all data
 traffic flowing to the RSIP host receiver are given QoS treatment,
 UDP/IP encapsulation will be rendered in the RSIP gateway for all
 data flows.  The tunnel between the RSIP host and gateway could be
 seen as a "hard pipe".  Traffic exceeding the QoS guarantee of the
 "hard pipe" would fall back to the best effort IP-IP tunneling.
 In the type 2 deployment scenario where data traffic could be
 selectively channeled into the UDP/IP or normal IP-IP tunnel, or for
 type 3 deployment where end-to-end sessions could be dynamically
 mapped into tunnel sessions, integration with the RSIP model could be
 complicated and tricky.  (Note that these are the cases where the
 tunnel link could be seen as a expandable soft pipe.)  Two main
 issues are worth considering.
  1. For RSIP gateway implementations that does encapsulation of the

incoming stream before passing to the IP layer for forwarding,

       the RSVP daemon has to be explicitly signaled upon reception of
       incoming RSVP PATH messages.  The RSIP implementation has to

Borella, et al. Experimental [Page 20] RFC 3102 RSIP: Framework October 2001

       recognize RSVP PATH messages and pass them to the RSVP daemon
       instead of doing the default tunneling.  Handling of other RSVP
       messages would be as described in [RFC2746].
  1. RSIP enables an RSIP host to have a temporary presence at the

RSIP gateway by assuming one of the RSIP gateway's global

       interfaces.  As a result, the RSVP PATH messages would be
       addressed to the RSIP gateway.  Also, the RSVP SESSION object
       within an incoming RSVP PATH would carry the global destination
       address, destination port (and protocol) tuples that were
       leased by the RSIP gateway to the RSIP host.  Hence the realm
       unaware RSVP daemon running on the RSIP gateway has to be
       presented with a translated version of the RSVP messages.
       Other approaches are possible, for example making the RSVP
       daemon realm aware.
 A simple mechanism would be to have the RSIP module handle the
 necessary RSVP message translation.  For an incoming RSVP signalling
 flow, the RSIP module does a packet translation of the IP header and
 RSVP SESSION object before handling the packet over to RSVP.  The
 global address leased to the host is translated to the true private
 address of the host.  (Note that this mechanism works with both RSA-
 IP and RSAP-IP.)  The RSIP module also has to do an opposite
 translation from private to global parameter (plus tunneling) for
 end-to-end PATH messages generated by the RSVP daemon towards the
 RSIP host receiver.  A translation on the SESSION object also has to
 be done for RSVP outbound control messages.  Once the RSVP daemon
 gets the message, it maps them to an appropriate tunnel sessions.
 Encapsulation of the inbound data traffic needing QoS treatment would
 be done using UDP-IP encapsulation designated by the tunnel session.
 For this reason, the RSIP module has to be aware of the UDP-IP
 encapsulation to use for a particular end-to-end session.
 Classification and scheduling of the QoS guaranteed end-to-end flow
 on the output interface of the RSIP gateway would be based on the
 UDP/IP encapsulation.  Mapping between the tunnel session and end-
 to-end session could continue to use the mechanisms proposed in
 [RFC2746].  Although [RFC2746] proposes a number of approaches for
 this purpose, we propose using the SESSION_ASSOC object introduced
 because of its simplicity.

5.4. IP Multicast

 The amount of specific RSIP/multicast support that is required in
 RSIP hosts and gateways is dependent on the scope of multicasting in
 the RSIP-enabled network, and the roles that the RSIP hosts will
 play.  In this section, we discuss RSIP and multicast interactions in
 a number of scenarios.

Borella, et al. Experimental [Page 21] RFC 3102 RSIP: Framework October 2001

 Note that in all cases, the RSIP gateway MUST be multicast aware
 because it is on an administrative boundary between two domains that
 will not be sharing their all of their routing information.  The RSIP
 gateway MUST NOT allow private IP addresses to be propagated on the
 public network as part of any multicast message or as part of a
 routing table.

5.4.1. Receiving-Only Private Hosts, No Multicast Routing on

      Private Network
 In this scenario, private hosts will not source multicast traffic,
 but they may join multicast groups as recipients.  In the private
 network, there are no multicast-aware routers, except for the RSIP
 gateway.
 Private hosts may join and leave multicast groups by sending the
 appropriate IGMP messages to an RSIP gateway (there may be IGMP proxy
 routers between RSIP hosts and gateways).  The RSIP gateway will
 coalesce these requests and perform the appropriate actions, whether
 they be to perform a multicast WAN routing protocol, such as PIM, or
 to proxy the IGMP messages to a WAN multicast router.  In other
 words, if one or more private hosts request to join a multicast
 group, the RSIP gateway MUST join in their stead, using one of its
 own public IP addresses.
 Note that private hosts do not need to acquire demultiplexing fields
 and use RSIP to receive multicasts.  They may receive all multicasts
 using their private addresses, and by private address is how the RSIP
 gateway will keep track of their group membership.

5.4.2. Sending and Receiving Private Hosts, No Multicast Routing

      on Private Network
 This scenarios operates identically to the previous scenario, except
 that when a private host becomes a multicast source, it MUST use RSIP
 and acquire a public IP address (note that it will still receive on
 its private address).  A private host sending a multicast will use a
 public source address and tunnel the packets to the RSIP gateway.
 The RSIP gateway will then perform typical RSIP functionality, and
 route the resulting packets onto the public network, as well as back
 to the private network, if there are any listeners on the private
 network.
 If there is more than one sender on the private network, then, to the
 public network it will seem as if all of these senders share the same
 IP address.  If a downstream multicasting protocol identifies sources

Borella, et al. Experimental [Page 22] RFC 3102 RSIP: Framework October 2001

 based on IP address alone and not port numbers, then it is possible
 that these protocols will not be able to distinguish between the
 senders.

6. RSIP Complications

 In this section we document the know complications that RSIP may
 cause.  While none of these complications should be considered "show
 stoppers" for the majority of applications, they may cause unexpected
 or undefined behavior.  Where it is appropriate, we discuss potential
 remedial procedures that may reduce or eliminate the deleterious
 impact of a complication.

6.1. Unnecessary TCP TIME_WAIT

 When TCP disconnects a socket, it enters the TCP TIME_WAIT state for
 a period of time.  While it is in this state it will refuse to accept
 new connections using the same socket (i.e., the same source
 address/port and destination address/port).  Consider the case in
 which an RSIP host (using RSAP-IP) is leased an address/port tuple
 and uses this tuple to contact a public address/port tuple.  Suppose
 that the host terminates the session with the public tuple and
 immediately returns its leased tuple to the RSIP gateway.  If the
 RSIP gateway immediately allocates this tuple to another RSIP host
 (or to the same host), and this second host uses the tuple to contact
 the same public tuple while the socket is still in the TIME_WAIT
 phase, then the host's connection may be rejected by the public host.
 In order to mitigate this problem, it is recommended that RSIP
 gateways hold recently deallocated tuples for at least two minutes,
 which is the greatest duration of TIME_WAIT that is commonly
 implemented.  In situations where port space is scarce, the RSIP
 gateway MAY choose to allocate ports in a FIFO fashion from the pool
 of recently deallocated ports.

6.2. ICMP State in RSIP Gateway

 Like NAT, RSIP gateways providing RSAP-IP must process ICMP responses
 from the public network in order to determine the RSIP host (if any)
 that is the proper recipient.  We distinguish between ICMP error
 packets, which are transmitted in response to an error with an
 associated IP packet, and ICMP response packets, which are
 transmitted in response to an ICMP request packet.
 ICMP request packets originating on the private network will
 typically consist of echo request, timestamp request and address mask
 request.  These packets and their responses can be identified by the
 tuple of source IP address, ICMP identifier, ICMP sequence number,

Borella, et al. Experimental [Page 23] RFC 3102 RSIP: Framework October 2001

 and destination IP address.  An RSIP host sending an ICMP request
 packet tunnels it to the RSIP gateway, just as it does TCP and UDP
 packets.  The RSIP gateway must use this tuple to map incoming ICMP
 responses to the private address of the appropriate RSIP host.  Once
 it has done so, it will tunnel the ICMP response to the host.  Note
 that it is possible for two RSIP hosts to use the same values for the
 tuples listed above, and thus create an ambiguity.  However, this
 occurrence is likely to be quite rare, and is not addressed further
 in this document.
 Incoming ICMP error response messages can be forwarded to the
 appropriate RSIP host by examining the IP header and port numbers
 embedded within the ICMP packet.  If these fields are not present,
 the packet should be silently discarded.
 Occasionally, an RSIP host will have to send an ICMP response (e.g.,
 port unreachable).  These responses are tunneled to the RSIP gateway,
 as is done for TCP and UDP packets.  All ICMP requests (e.g., echo
 request) arriving at the RSIP gateway MUST be processed by the RSIP
 gateway and MUST NOT be forwarded to an RSIP host.

6.3. Fragmentation and IP Identification Field Collision

 If two or more RSIP hosts on the same private network transmit
 outbound packets that get fragmented to the same public gateway, the
 public gateway may experience a reassembly ambiguity if the IP header
 ID fields of these packets are identical.
 For TCP packets, a reasonably small MTU can be set so that
 fragmentation is guaranteed not to happen, or the likelihood or
 fragmentation is extremely small.  If path MTU discovery works
 properly, the problem is mitigated.  For UDP, applications control
 the size of packets, and the RSIP host stack may have to fragment UDP
 packets that exceed the local MTU.  These packets may be fragmented
 by an intermediate router as well.
 The only completely robust solution to this problem is to assign all
 RSIP hosts that are sharing the same public IP address disjoint
 blocks of numbers to use in their IP identification fields.  However,
 whether this modification is worth the effort of implementing is
 currently unknown.

6.4. Application Servers on RSAP-IP Hosts

 RSAP-IP hosts are limited by the same constraints as NAT with respect
 to hosting servers that use a well-known port.  Since destination
 port numbers are used as routing information to uniquely identify an
 RSAP-IP host, typically no two RSAP-IP hosts sharing the same public

Borella, et al. Experimental [Page 24] RFC 3102 RSIP: Framework October 2001

 IP address can simultaneously operate publically-available gateways
 on the same port.  For protocols that operate on well-known ports,
 this implies that only one public gateway per RSAP-IP IP address /
 port tuple is used simultaneously.  However, more than one gateway
 per RSAP-IP IP address / port tuple may be used simultaneously if and
 only if there is a demultiplexing field within the payload of all
 packets that will uniquely determine the identity of the RSAP-IP
 host, and this field is known by the RSIP gateway.
 In order for an RSAP-IP host to operate a publically-available
 gateway, the host must inform the RSIP gateway that it wishes to
 receive all traffic destined to that port number, per its IP address.
 Such a request MUST be denied if the port in question is already in
 use by another host.
 In general, contacting devices behind an RSIP gateway may be
 difficult.  A potential solution to the general problem would be an
 architecture that allows an application on an RSIP host to register a
 public IP address / port pair in a public database.  Simultaneously,
 the RSIP gateway would initiate a mapping from this address / port
 tuple to the RSIP host.  A peer application would then be required to
 contact the database to determine the proper address / port at which
 to contact the RSIP host's application.

6.5. Determining Locality of Destinations from an RSIP Host

 In general, an RSIP host must know, for a particular IP address,
 whether it should address the packet for local delivery on the
 private network, or if it has to use an RSIP interface to tunnel to
 an RSIP gateway (assuming that it has such an interface available).
 If the RSIP hosts are all on a single subnet, one hop from an RSIP
 gateway, then examination of the local network and subnet mask will
 provide the appropriate information.  However, this is not always the
 case.
 An alternative that will work in general for statically addressed
 private networks is to store a list of the network and subnet masks
 of every private subnet at the RSIP gateway.  RSIP hosts may query
 the gateway with a particular target IP address, or for the entire
 list.
 If the subnets on the local side of the network are changing more
 rapidly than the lifetime of a typical RSIP session, the RSIP host
 may have to query the location of every destination that it tries to
 communicate with.

Borella, et al. Experimental [Page 25] RFC 3102 RSIP: Framework October 2001

 If an RSIP host transmits a packet addressed to a public host without
 using RSIP, then the RSIP gateway will apply NAT to the packet (if it
 supports NAT) or it may discard the packet and respond with and
 appropriate ICMP message.
 A robust solution to this problem has proven difficult to develop.
 Currently, it is not known how severe this problem is.  It is likely
 that it will be more severe on networks where the routing information
 is changing rapidly that on networks with relatively static routes.

6.6. Implementing RSIP Host Deallocation

 An RSIP host MAY free resources that it has determined it no longer
 requires.  For example, on an RSAP-IP subnet with a limited number of
 public IP addresses, port numbers may become scarce.  Thus, if RSIP
 hosts are able to dynamically deallocate ports that they no longer
 need, more hosts can be supported.
 However, this functionality may require significant modifications to
 a vanilla TCP/IP stack in order to implement properly.  The RSIP host
 must be able to determine which TCP or UDP sessions are using RSIP
 resources.  If those resources are unused for a period of time, then
 the RSIP host may deallocate them.  When an open socket's resources
 are deallocated, it will cause some associated applications to fail.
 An analogous case would be TCP and UDP sessions that must terminate
 when an interface that they are using loses connectivity.
 On the other hand, this issue can be considered a resource allocation
 problem.  It is not recommended that a large number (hundreds) of
 hosts share the same IP address, for performance purposes.  Even if,
 say, 100 hosts each are allocated 100 ports, the total number of
 ports in use by RSIP would be still less than one-sixth the total
 port space for an IP address.  If more hosts or more ports are
 needed, more IP addresses should be used.  Thus, it is reasonable,
 that in many cases, RSIP hosts will not have to deallocate ports for
 the lifetime of their activity.
 Since RSIP demultiplexing fields are leased to hosts, an
 appropriately chosen lease time can alleviate some port space
 scarcity issues.

6.7. Multi-Party Applications

 Multi-party applications are defined to have at least one of the
 following characteristics:
  1. A third party sets up sessions or connections between two

hosts.

Borella, et al. Experimental [Page 26] RFC 3102 RSIP: Framework October 2001

  1. Computation is distributed over a number of hosts such that the

individual hosts may communicate with each other directly.

 RSIP has a fundamental problem with multi-party applications.  If
 some of the parties are within the private addressing realm and
 others are within the public addressing realm, an RSIP host may not
 know when to use private addresses versus public addresses.  In
 particular, IP addresses may be passed from party to party under the
 assumption that they are global endpoint identifiers.  This may cause
 multi-party applications to fail.
 There is currently no known solution to this general problem.
 Remedial measures are available, such as forcing all RSIP hosts to
 always use public IP addresses, even when communicating only on to
 other RSIP hosts.  However, this can result in a socket set up
 between two RSIP hosts having the same source and destination IP
 addresses, which most TCP/IP stacks will consider as intra-host
 communication.

6.8. Scalability

 The scalability of RSIP is currently not well understood.  While it
 is conceivable that a single RSIP gateway could support hundreds of
 RSIP hosts, scalability depends on the specific deployment scenario
 and applications used.  In particular, three major constraints on
 scalability will be (1) RSIP gateway processing requirements, (2)
 RSIP gateway memory requirements, and (3) RSIP negotiation protocol
 traffic requirements.  It is advisable that all RSIP negotiation
 protocol implementations attempt to minimize these requirements.

7. Security Considerations

 RSIP, in and of itself, does not provide security.  It may provide
 the illusion of security or privacy by hiding a private address
 space, but security can only be ensured by the proper use of security
 protocols and cryptographic techniques.

8. Acknowledgements

 The authors would like to thank Pyda Srisuresh, Dan Nessett, Gary
 Jaszewski, Ajay Bakre, Cyndi Jung, and Rick Cobb for their input.
 The IETF NAT working group as a whole has been extremely helpful in
 the ongoing development of RSIP.

Borella, et al. Experimental [Page 27] RFC 3102 RSIP: Framework October 2001

9. References

 [DHCP-DNS] Stapp, M. and Y. Rekhter, "Interaction Between DHCP and
            DNS", Work in Progress.
 [RFC2983]  Black, D., "Differentiated Services and Tunnels", RFC
            2983, October 2000.
 [RFC3104]  Montenegro, G. and M. Borella, "RSIP Support for End-to-
            End IPSEC", RFC 3104, October 2001.
 [RFC3103]  Borella, M., Grabelsky, D., Lo, J. and K. Taniguchi,
            "Realm Specific IP: Protocol Specification", RFC 3103,
            October 2001.
 [RFC2746]  Terzis, A., Krawczyk, J., Wroclawski, J. and L. Zhang,
            "RSVP Operation Over IP Tunnels", RFC 2746, January 2000.
 [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.J.
            and E. Lear, "Address Allocation for Private Internets",
            BCP 5, RFC 1918, February 1996.
 [RFC2002]  Perkins, C., "IP Mobility Support", RFC 2002, October
            1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to indicate
            requirement levels", BCP 14, RFC 2119, March 1997.
 [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
            Translator (NAT) Terminology and Considerations", RFC
            2663, August 1999.
 [RFC2205]  Braden, R., Zhang, L., Berson, S., Herzog, S. and S.
            Jamin, "Resource Reservation Protocol (RSVP)", RFC 2205,
            September 1997.
 [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
            and W. Weiss, "An Architecture for Differentiated
            Services", RFC 2475, December 1998.
 [RFC3105]  Kempf, J. and G. Montenegro, "Finding an RSIP Server with
            SLP", RFC 3105, October 2001.

Borella, et al. Experimental [Page 28] RFC 3102 RSIP: Framework October 2001

10. Authors' Addresses

 Michael Borella
 CommWorks
 3800 Golf Rd.
 Rolling Meadows IL 60008
 Phone: (847) 262-3083
 EMail: mike_borella@commworks.com
 Jeffrey Lo
 Candlestick Networks, Inc
 70 Las Colinas Lane,
 San Jose, CA 95119
 Phone: (408) 284 4132
 EMail: yidarlo@yahoo.com
 David Grabelsky
 CommWorks
 3800 Golf Rd.
 Rolling Meadows IL 60008
 Phone: (847) 222-2483
 EMail: david_grabelsky@commworks.com
 Gabriel E. Montenegro
 Sun Microsystems
 Laboratories, Europe
 29, chemin du Vieux Chene
 38240 Meylan
 FRANCE
 Phone: +33 476 18 80 45
 EMail: gab@sun.com

Borella, et al. Experimental [Page 29] RFC 3102 RSIP: Framework October 2001

11. Full Copyright Statement

 Copyright (C) The Internet Society (2001).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Borella, et al. Experimental [Page 30]

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