Network Working Group
Request for Comments: 1620
Category: Informational
B. Braden
J. Postel
Y. Rekhter
IBM Research
May 1994

Internet Architecture Extensions for Shared Media

Status of This Memo

This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited.


The original Internet architecture assumed that each network is labeled with a single IP network number. This assumption may be violated for shared media, including "large public data networks" (LPDNs). The architecture still works if this assumption is violated, but it does not have a means to prevent multiple host- router and router-router hops through the shared medium. This memo discusses alternative approaches to extending the Internet architecture to eliminate some or all unnecessary hops.

Table of Contents

   1. INTRODUCTION ..................................................  2
   2. THE ORIGINAL INTERNET ARCHITECTURE ............................  2
   3. THE PROBLEMS INTRODUCED BY SHARED MEDIA .......................  4
   4. SOME SOLUTIONS TO THE SM PROBLEMS .............................  7
      4.1  Hop-by-Hop Redirection ...................................  7
      4.2  Extended Routing ......................................... 11
      4.3  Extended Proxy ARP ....................................... 13
      4.4  Routing Query Messages ................................... 14
      4.5  Stale Routing Information ................................ 14
      4.6  Implications of Filtering (Firewalls) .................... 15
   5. SECURITY CONSIDERATIONS ....................................... 16
   6. CONCLUSIONS ................................................... 17
   7. ACKNOWLEDGMENTS ............................................... 17
   8. REFERENCES .................................................... 18
   Authors' Addresses ............................................... 19


This memo concerns the implications of shared medium networks for the architecture of the TCP/IP protocol suite. General familiarity with the TCP/IP architecture and the IP protocol is assumed.

The Internet architecture is founded upon what was originally called the "Catenet model" [PSC81]. Under this model, the Internet (originally dubbed "the Catenet") is formed using routers (originally called "gateways") to interconnect distinct and perhaps diverse networks. An IP host address (more correctly characterized as a network interface address) is formed of the pair (net#,host#). Here "net#" is a unique IP number assigned to the network (or subnet) to which the host is attached, and "host#" identifies the host on that network (or subnet).

The original Internet model made the implicit assumptions that each network has a single IP network number and that networks with different numbers may interchange packets only through routers. These assumptions may be violated for networks implemented using a common "shared medium" (SM) at the link layer (LL). For example, network managers sometimes configure multiple IP network numbers (usually subnet numbers) on a single broadcast-type LAN such as an Ethernet. The large (switched) public data networks (LPDNs), such as SMDS and B-ISDN, form a potentially more important example of shared medium networks. Any two systems connected to the same shared medium network are capable of communicating directly at the LL, without IP layer switching by routers. This presents an opportunity to optimize performance and perhaps lower cost by eliminating unnecessary LL hops through the medium.

This memo discusses how unnecessary hops can be eliminated in a shared medium, while retaining the coherence of the existing Internet architecture. This issue has arisen in a number of IETF Working Groups concerned with LPDNs, including IPLPDN, IP over ATM, IDRP for IP, and BGP. It is time to take a careful look at the architectural issues to be solved. This memo first summarizes the relevant aspects of the original Internet architecture (Section 2), and then it explains the extra-hop problems created by shared media networks (Section 3). Finally, it discusses some possible solutions (Section 4).


We very briefly review the original architecture, to introduce the terminology and concepts. Figure 1 illustrates a typical set of four networks A, ... D, represented traditionally as clouds, interconnected by routers R2, R3, and R4. Routers R1 and R5 connect to other parts of the Internet. Ha, ... Hd represent hosts connected to these networks.

It is not necessary to distinguish between network and subnet in this memo. We may assume that there is some address mask associated with each "network" in Figure 1, allowing a host or router to divide the 32 bits of an IP address into an address for the cloud and a host number that is defined uniquely only within that cloud.

Ha Hb Hc Hd

              |            |            |            |
              |            |            |            |
             _|_          _|_          _|_          _|_
            (   )        (   )        (   )        (   )
            (Net)        (Net)        (Net)        (Net)
            ( A )        ( B )        ( C )        ( D )
     - R1 --(   )-- R2 --(   )-- R3 --(   )-- R4 --(   )-- R5 --
            (   )        (   )        (   )        (   )
            (___)        (___)        (___)        (___)

Figure 1. Example Internet Fragment

   An Internet router is connected to local network(s) as a special kind
   of host.  Indeed, for network management purposes, a router plays the
   role of a host by originating and terminating datagrams.  However,
   there is an important difference between a host and a router:  the
   routing function is mostly centralized in the routers, allowing hosts
   to be "dumb" about routing.  Internet hosts are required [RFC-1122]
   to make only one simple routing decision: is the destination address
   local to the connected network?  If the address is not local, we say
   it is "foreign" (relative to the connected network or to the host).

A host sends a datagram directly to a local destination address or (for a foreign destination) to a first-hop router. The host initially uses some "default" router for any new destination address. If the default is the wrong choice, that router returns a Redirect message and forwards the datagram. The Redirect message specifies the preferred first-hop router for the given destination address. The host uses this information, which it maintains in a "routing cache" [RFC-1122], to determine the first-hop address for subsequent datagrams to the same destination.

To actually forward an IP datagram across a network hop, the sender must have the link layer (LL) address of the target. Therefore, each host and router must have some "address resolution" procedure to map IP address to an LL address. ARP, used for networks with broadcast capability, is the most common address resolution procedure

[Plummer82]. If there is no LL broadcast capability (or if it is too expensive), then there are two other approaches to address resolution: local configuration tables, and "address-resolution servers" (AR Servers).

If AR Servers are used for address resolution, hosts must be configured with the LL address(es) of one or more nearby servers. The mapping information provided by AR Servers might itself be collected using a protocol that allows systems to register their LL addresses, or from static configuration tables. The ARP packet format and the overall ARP protocol structure (ARP Request/ARP Reply) may be suitable for the communications between a host and an AR server, even in the absence of the LL broadcast capabilities; this would ease conversion of hosts to using AR Servers.

The examples in this memo use ARP for address resolution. At least some of the LPDN's that are planned will provide sufficient broadcast capability to support ARP. It is important to note that ARP operates at the link layer, while the Redirect and routing cache mechanisms operate at the IP layer of the protocol stack.


Figure 2 shows the same configuration as Figure 1, but now networks A, B, C, and D are all within the same shared medium (SM), shown by the dashed box enclosing the clouds. Networks A, ... D are now logical IP networks (called LIS's in [Laubach93]) rather than physical networks. Each of these logical networks may (or may not) be administratively distinct. The SM allows direct connectivity between any two hosts or routers connected to it. For example, host Ha can interchange datagrams directly with host Hd or with router R4. A router that has some but not all of its interfaces connected to the shared medium is called a "border router"; R1 and R5 are examples.

Figure 2 illustrates the "classical" model [Laubach93] for use of the Internet architecture within a shared medium, i.e., simply applying the original Internet architecture described earlier. This will provide correct but not optimal operation. For example, in the case of two hosts on the same logical network (not shown in Figure 2), the original rules will clearly work; the source host will forward a datagram directly in a single hop to a host on the same logical network. The original architectural rules will also work for communication between any pair of hosts shown in Figure 2; for example, host Ha would send a datagram to host Hd via the four-hop path Ha -> R2 -> R3 -> R4 -> Hd. However, the classical model does not take advantage of the direct connectivity Ha -> Hd allowed by the shared medium.

           Ha           Hb           Hc           Hd
           |            |            |            |
      ---- | ---------- | ---------- | ---------- | ----
     |   __|__        __|__        __|__        __|__   |
        (     )      (     )      (     )      (     )
     |  (     )      (     )      (     )      (     )  |
        ( Net )      ( Net )      ( Net )      ( Net )
     |  (  A  )      (  B  )      (  C  )      (  D  )  |
        (     )      (     )      (     )      (     )
     |  (     )      (     )      (     )      (     )  |
        (_____)      (_____)      (_____)      ( ____)
     |    | |          | |          | |          | |    |
      --- | | -------- | | -------- | | -------- | | ---
          | |          | |          | |          | |
    - R1 -   --- R2 ---   --- R3 ---   --- R4 ---   --- R5 ---

Figure 2. Logical IP Networks in Shared Medium

This memo concerns mechanisms to achieve minimal-hop connectivity when it is desired. We should note that is may not always be desirable to achieve minimal-hop connectivity in a shared medium. For example, the "extra" hops may be needed to allow the routers to act as administrative firewalls. On the other hand, when such firewall protection is not required, it should be possible to take advantage of the shared medium to allow this datagram to use shorter paths. In general, it should be possible to choose between firewall security and efficient connectivity. This is discussed further in Section 4.6 below.

We also note that the mechanisms described here can only optimize the path within the local SM. When the SM is only one segment of the path between source and receiver, removing hops locally may limit the ability to switch to globally more optimal paths that may become available as the result of routing changes. Thus, consider Ha- >...Hx, where host Hx is outside the SM to which host Ha is attached. Suppose that the shortest global path to Hx is via some border router Rb1. Local optimization using the techniques described below will remove extra hops in the SM and allow Ha->Rb1->...Hx. Now suppose that a later route change outside the SM makes the path Ha->Rb2- >...Hx more globally optimum, where Rb2 is another border router. Since Ha does not participate in the routing protocol, it does not know that it should switch to Rb2. It is possible that Rb2 may not realize it either; this is the situation:

     GC(Ha->Rb2->...Hx) < GC(Ha->Rb1->Rb2->...Hx) < GC(Ha->Rb1->...Hx)

where GC() represents some global cost function of the specified path.

Note that ARP requires LL broadcast. Even if the SM supports broadcast, it is likely that administrators will erect firewalls to keep broadcasts local to their LIS.

There are three cases to be optimized. Suppose H and H' are hosts and Rb and Rb' are border routers connected to the same same SM. Then the following one-hop paths should be possible:

         H -> H':  Host to host within the SM

H -> Rb: Host to exit router

Rb -> Rb': Entry border router to exit border router,

for transit traffic.

We may or not be able to remove the extra hop implicit in Rb -> R -> H, where Rb, R, and H are within the same SM, but the ultimate source is outside the SM. To remove this hop would require distribution of host routes, not just network routes, between the two routers R and Rb; this would adversely impact routing scalability.

There are a number of important requirements for any architectural solution to these problems.

   *    Interoperability

Modified hosts and routers must interoperate with unmodified nodes.

   *    Practicality

Minimal software changes should be required.

   *    Robustness

The new scheme must be at least as robust against errors in

software, configuration, or transmission as the existing


   *    Security

The new scheme must be at least as securable against subversion as the existing architecture.

The distinction between host and router is very significant from an engineering viewpoint. It is considered to be much harder to make a global change in host software than to change router software, because there are many more hosts and host vendors than routers and router vendors, and because hosts are less centrally administered than routers. If it is necessary to change the specification of what a host does (and it is), then we must minimize the extent of this change.


Four different approaches have been suggested for solving these SM problems.

(1) Hop-by-Hop Redirection

In this approach, the host Redirect mechanism is extended to collapse multiple-hop paths within the same shared medium, hop- by-hop. A router is to be allowed to send, and a host allowed to accept, a Redirect message that specifies a foreign IP address within the same SM. We refer to this as a "foreign Redirect". Section 4.1 analyzes this approach in some detail.

(2) Extended Routing

Routing protocols can be modified to know about the SM and to provide LL addresses.

(3) Extended Proxy ARP

This is a form of the proxy ARP approach, in which the routing problem is solved implicitly by an extended address resolution mechanism at the LL. This approach has been described by Heinanen [Heinanen93] and by Garrett et al [Garratt93].

(4) Route Query Messages

This approach has been suggested by Halpern [Halpern93]. Rather than adding additional information to routing, this approach would add a new IP-layer mechanism using end-to-end query and reply datagrams.

These four are discussed in the following four subsections.

   4.1  Hop-by-Hop Redirection

The first scheme we consider would operate at the IP layer. It would cut out extra hops one by one, with each router in the path operating on local information only. This approach requires both host and router changes but no routing protocol changes.

The basic idea is that the first-hop router, upon observing that the next hop is within the same SM, sends a foreign Redirect to the source, redirecting it to the next hop. Successive application of this algorithm at each intermediate router will eventually result in a direct path from source host to destination host, if both are within the same SM.

Suppose that Ha wants to send a datagram to Hd. We use the notation IP.a for the IP address of entity a, and LL.a for the corresponding LL address. Each line in the following shows an IP datagram and the path that datagram will follow, separated by a colon. The notation "Redirect( h, IP.a)" means a Redirect specifying IP.a as the best next hop to reach host h.

         (1)  Datagram 1: Ha -> R2 -> R3 -> R4 -> Hd
         (2)  Redirect(Hd, IP.R3): R2 -> Ha
         (3)  Datagram 2: Ha -> R3 -> R4 -> Hd
         (4)  Redirect(Hd, IP.R4): R3 -> Ha
         (5)  Datagram 3: Ha -> R4 -> Hd
         (6)  Redirect(Hd, IP.Hd): R4 -> Ha
         (7)  Datagram 4: Ha -> Hd

There are three problems to be solved to make hop-by-hop redirection work; we label them HH1, HH2, and HH3.


Each router must be able to resolve the LL address of the source Ha, to send a (foreign) Redirect.

Let us assume that the link layer provides the source LL address when an IP datagram arrives. If the router determines that a Redirect should be sent, then it will be sent to the source LL address of the received datagram.


A source host must be able to perform address resolution to obtain the LL address of each router to which it is redirected.

It would be possible for each router R, upon sending a Redirect to Ha, to also send an unsolicited ARP Reply point- to-point to LL.Ha, updating Ha's ARP cache with LL.R. However, there is not guarantee that this unsolicited ARP Reply would be delivered. If it was lost, there would be a forwarding black hole. The host could recover by starting over from the original default router; however, this may be too inefficient a solution.

A much more direct and efficient solution would introduce an extended ICMP Redirect message (call it XRedirect) that carries the LL address as well as the IP address of the target. This would remove the issue of reliable delivery of the unsolicited ARP described earlier, because the fate of the LL address would be shared with the IP target address; both would be delivered or neither would. (An XRedirect is essentially the same as a Redirect in the OSI ES-IS protocol).

Using XRedirect, the previous example becomes:

(1) Datagram 1: Ha -> R2 -> R3 -> R4 -> Hd

(2) XRedirect(Hd, IP.R3, LL.R3): R2 -> Ha

              (3)  Datagram 2: Ha -> R3 -> R4 -> Hd

(4) XRedirect(Hd, IP.R4, LL.R4): R3 -> Ha

              (5)  Datagram 3: Ha -> R4 -> Hd

(6) XRedirect(Hd, IP.Hd, LL.Hd): R4 -> Ha

              (7)  Datagram 4: Ha -> Hd


Each router should be able to recognize when it is the first hop in the path, since a Redirect should be sent only by the first hop router. Unfortunately this will be possible only if the LL address corresponding to the IP source address has been cached from an earlier event; a router in this chain determines the LL address of the source from the arriving datagram (see HH1 above). If it cannot determine whether it is the first hop, a router must always send an [X]Redirect, which will be spurious if the router is not the first hop.

Such spurious [X]Redirects will be sent to the IP address of the source host, but using the LL address of the previous-hop router. The propagation scope of [X]Redirects can be limited to a single IP hop (see below), so they will go no further than the previous-hop router, where they will be discarded.

However, there will be some router overhead to process these useless [X]Redirects

Next, we discuss the changes in hosts and in routers required for hop-by-hop redirection.

      o    Host Changes

The Host Requirements RFC [RFC-1122] specifies the host mechanism for routing an outbound datagram in terms of sending the datagram directly to a local destination or else to the first hop router (to reach a foreign destination) [RFC-1122 3.3.1]. Although this mechanism assumes a local address, a foreign address for a first-hop router should work equally well.

The target address contained in the routing cache is updated by Redirect messages. There is currently a restriction on what target addresses may be accepted in Redirect messages [RFC-1122], which would prevent foreign Redirects from working:

A Redirect message SHOULD be silently discarded if the new router address it specifies is not on the same connected (sub-) net through which the Redirect arrived, or if the source of the Redirect is not the current first-hop router for the specified destination.

To support foreign Redirects requires simply removing the first validity check. The second check, which requires an acceptable Redirect to come from the node to which the datagram that triggered the Redirect was sent, is retained. The same validity check would be used for XRedirects.

In order to send a datagram to the target address found in the routing cache, a host must resolve the IP address into a LL address. No change should be necessary in the host's IP- to-LL resolution mechanism to handle a foreign rather than a local address.

The Hop-by-Hop redirection requires changes to the semantics of the IP address that an ICMP Redirect is allowed to carry. Under the present definition [Postel81b], an ICMP Redirect message is only allowed to carry an IP address of a router. In order for the hop-by-hop redirection mechanism to eliminate all router hops, allowing two hosts connected to the same SM to communicate directly, a [X]Redirect message must be able to carry the IP address of the destination host.

      o    Router Changes

The router changes required for hop-by-hop redirection are much more extensive than the host changes. The examples given earlier showed the additional router functions that would be needed.

Consider a router that is connected to an SM. When it receives a datagram from the SM, it tests whether the next hop is on the same SM, and if so, it sends a foreign XRedirect to the source host, using the link layer address with which the datagram arrived.

A router should avoid sending more than a limited number of successive foreign Redirects to the same host. This is necessary because an unmodified host may legitimately ignore a Redirect to a foreign network and continue to forward datagrams to the same router. A router can accomplish this limitation by keeping a cache of foreign Redirects sent.

Note that foreign Redirects generated by routers according to these rules, like the current local Redirects, may travel exactly one link-layer hop. It is therefore reasonable and desirable to set their TTL to 1, to ensure they cannot stray outside the SM.

The extra check needed to determine whether to generate a Redirect may incur additional processing and thus result in a performance degradation; to avoid this, a router may not perform the check at all but just forward the packet. The scheme with [X]Redirects is not applicable to such a router.

Finally, note that the hop-by-hop redirection scheme is only applicable when the source host is connected to an SM, since routers do not listen to Redirects. To optimize the forwarding of transit traffic between entry and exit border routers, an extension to routing is required, as discussed in the following section. Conversely, an extension to the routing protocol cannot be used to optimize forwarding traffic from a host connected to the SM, since a host should not listen to routing protocols.

   4.2  Extended Routing

The routing protocols may be modified to carry additional information that is specific to the SM. The router could use the attribute "SameSM" for a route to deduce the shortest path to be reported to its neighbors. It could also carry the LL addresses with each router IP address.

For example, the extended routing protocol would allow R2 to know that R4 is the best next-hop to reach the destination network in the same SM, and to know both IP.R4 and LL.R4, leading to the path Ha->R2->R4->Hb. Further optimization cannot be done with extended routing alone, since the host does not participate in routing, and because we want the routing protocol to handle only per-network information, not per-host information. Hop-by-hop redirection could then be used to eliminate all router hops, as in the following sequence:

          (1) Datagram 1: Ha -> R2 -> R4 -> Hd
          (2) XRedirect(Hd, IP.R4, LL.R4): R2 -> Ha
          (3) Datagram 2: Ha -> R4 -> Hd
          (4) XRedirect(Hd, IP.Hd, LL.Hd): R4 -> Ha
          (5) Datagram 3: Ha -> Hd

There are three aspects to the routing protocol extension:

(1) the ability to pass "third-party" information -- a router

should be able to specify the address (IP address and perhaps LL address) of some other router as the next-hop;

(2) knowledge of the "SameSM" attribute for routes; and

(3) knowledge of LL addresses corresponding to IP addresses of

routers within the same SM.

A router must be able to determine that a particular IP address (e.g., the source address) is in the same SM. There are several possible ways to make this information available to a router in the SM.

(1) A router may use a single physical interface to an SM; this

implies that all its logical interfaces lie within the same SM.

(3) There might be some administrative structure in the IP

addresses, e.g., all IP addresses within a particular

national SM might have a common prefix string.

(3) There might be configuration information, either local to the

router or available from some centralized server (e.g, the

DNS). Note that a router could consult this server in the background while continuing to forward datagrams without delay. The only consequence of a delay in obtaining the "SameSM" information would be some unnecessary (but temporary) hops.

   4.3  Extended Proxy ARP

The approach of Heinanen [Heinanen93] was intended to solve the problem of address resolution in a shared medium with no broadcast mechanism ("Non-Broadcast, MultiAccess" or NBMA). Imagine that the shared medium has a single IP network number, i.e., it is one network "cloud". Heinanen envisions a set of AR Servers within this medium. These AR Servers run some routing protocol among themselves. A source host issues an ARP Request (via a point-to- point LL transmission) to an AR Server with which it is associated. This ARP Request is forwarded hop-by-hop at the link layer through the AR Servers, towards the AR Server that is associated with the destination host. That AR Server resolves the address (using information learned from either host advertisement or a configuration file), and returns an ARP Reply back through the AR Servers to the source host.

Ha Hb Hc Hd

              |            |            |            |
         ---- | ---------- | ---------- | ---------- | ----
        (                                                  )
        (        Shared Medium (One Logical Network)       )
        (                                                  )
             |  |         |            |          |    |
       - R1 -   |         |            |          |    --- R5 ---
            ____|__     __|____      __|____     _|_____
           | AR Sa |   | AR Sb |    | AR Sc |   | AR Sd |
           |_______|   |_______|    |_______|   |_______|
            Figure 3.  Single-Cloud Shared Medium

Figure 3 suggests that each of the hosts Ha, ... Hd is associated with a corresponding AR Server "AR Sa", ..."AR Sd".

This same scheme could be applied to the LIS model of Figure 2. The AR Servers would be implemented in the routers, and if the medium supports broadcast then the hosts would be configured for proxy ARP. That is, the host would be told that all destinations are local, so it will always issue an ARP request for the final destination. The set of AR Servers would resolve this request.

Since routing loops are a constant possibility, Heinanen's proposal includes the addition of a hop count to ARP requests and replies.

Like all proxy ARP schemes, this one has a seductive simplicity. However, solving the SM problem at the LL has several costs. It requires a complete round-trip time before the first datagram can flow. It requires a hop count in the ARP packet. This seems like a tip-off that the link layer may not be the most appropriate place to solve the SM problem.

   4.4  Routing Query Messages

This scheme [Halpern93] introduces a new IP level mechanism: SM routing query and reply messages. These messages are forwarded as IP datagrams hop-by-hop in the direction of the destination address. The exit router can return a reply, again hop-by-hop, that finally reaches the source host as an XRedirect. It would also be possible (but not necessary) to modify hosts to initiate these queries.

The query/reply pair is supplying the same information that we would add to routing protocols under Extended Routing. However, the Query/Reply messages would allow us to keep the current routing protocols unchanged, and would also provide the extra information only for the routes that are actually needed, thus reducing the routing overhead. Note that the Query/Reply sequence can happen in parallel with forwarding the initial datagram hop- by-hop, so it does not add an extra round-trip delay.

   4.5  Stale Routing Information

We must consider what happens when the network topology changes. The technique of extended routing (Section 4.2) is capable of providing sufficient assurances that stale information will be purged from the system within the convergence time associated with a particular routing protocol being used.

However, the three other techniques (hop-by-hop redirection, extended Proxy ARP, and routing query messages) may be expected to provide minimal-hop forwarding only as long as the network topology remains unchanged since the time such information was acquired. Changes in the topology may result in a change in the minimal-hop path, so that the first-hop router may no longer be the correct choice. If the host that is using this first-hop router is not aware of the changes, then instead of a minimal-hop path the host could be using a path that is now suboptimal, perhaps highly sub-optimal, with respect to the number of hops.

Futhermore, use of the information acquired via either extended Proxy ARP or routing query messages to optimize routing between routers attached to the same SM is highly problematic, because presence of stale information on routers could result in forwarding loops that might persist as long as the information isn't purged; neither approach provides suitable handling of stale information.

   4.6  Implications of Filtering (Firewalls)

For a variety of reasons an administrator of a LIS may erect IP Layer firewalls (perform IP-layer filtering) to constrain LL connectivity between the hosts/routers within the LIS and hosts/routers in other LISs within the same SM. To avoid disruption in forwarding, the mechanisms described in this document need to take into account such firewalls.

Using [X]Redirects requires a router that generates an [X]Redirect to be cognizant of possible Link Layer connectivity constraints between the router that is specified as the Next Hop in the Redirect and the host that is the target of the Redirect.

Using extended routing requires a router that originates and/or propagates "third-party" information be cognizant of the possible Link Layer connectivity constraints. Specifically, a router should not propagate "third-party" information when there is a lack of Link Layer connectivity between the router depicted by the information and the router which is the immediate recipient of that information.

Using extended proxy ARP requires an ARP Server not to propagate an ARP Request to another ARP server if there are Link Layer connectivity constraints between the originator of the ARP Request and the other ARP server.

Using SM routing query and reply messages requires the routers that pass the messages to be aware of the possible Link Layer connectivity constraints. The flow of messages need to reflect these constraints.


We should discuss the security issues raised by our suggested changes. We should note that we are not talking about "real" security here; real Internet security will require cryptographic techniques on an end-to-end basis. However, it should not be easy to subvert the basic delivery mechanism of IP to cause datagrams to flow to unexpected places.

With this understanding, the security problems arise in two places: the ICMP Redirect messages and the ARP replies.

   *    ICMP Redirect Security

We may reasonably require that the routers be secure. They are generally under centralized administrative control, and we may assume that the routing protocols will contain sufficient authentication mechanisms (even if it is not currently true). Therefore, a host will reasonably be able to trust a Redirect that comes from a router.

However, it will NOT be reasonable for a host to trust another host. Suppose that the target host in the examples of Section 4.1 is untrustworthy; there is no way to prevent its issuing a new Redirect to some other destination, anywhere in the Internet. On the other hand, this exposure is no worse than it was; the target host, once subverted, could always act as a hidden router to forward traffic elsewhere.

   *    ARP Security

Currently, an ARP Reply can come only from the local network, and a physically isolated network can be administrative secured from subversion of ARP. However, an ARP Reply can come from anywhere within the SM, and an evil-doer can use this fact to divert the traffic flow from any host within the SM [Bellovin89].

The XRedirect closes this security hole. Validating the XRedirect (as coming from the node to which the last datagram was sent) will also validate the LL address.

Another approach is to validate the source address from which the ARP Reply was received (assuming the link layer protocol carries the source address and the driver supplies it). An acceptable ARP reply for destination IP address D can only come from LL address x, where the routing cache maps D -> E and the ARP cache gives x as the translation of E. This validation, involving both routing and ARP caches, might be ugly to implement in a strictly-layered implementation. It would be natural if layering were already violated by combining the ARP cache and routing cache.

It is possible for the link layer to have security mechanisms that could interfere with IP-layer connectivity. In particular, there could possible be non-transitivity of logical interconnection within a shared medium. In particular, some large public data networks may include configuration options that could allow Net A to talk to Net B and Net B to talk to Net C, but prevent A from talking directly to C. In this case, the routing protocols have to be sophisticated enough to handle such anomalies.


We have discussed four possible extensions to the Internet architecture to allow hop-efficient forwarding of IP datagrams within shared media, when this optimization is allowed by IP-layer firewalls. We do not draw any conclusions in this paper about the best mechanisms.

Our suggested extensions are evolutionary, leaving intact the basic ideas of the current Internet architecture. It would be possible to make (and some have suggested) much more radical changes to accommodate shared media. In the extreme, one could entirely abolish the inner clouds in Figure 2, so that there would be no logical network structure within the SM. The IP addresses would then be logical, and some mechanism of distributed servers would be needed to find routes within this random haze. We think this approach ignores all the requirements for management and security in today's Internet. It might make a good research paper, but it would not be good Internet design strategy.


We are grateful to Keith McGloghrie, Joel Halpern, and others who rubbed our noses in this problem. We also acknowledge Tony Li (cisco), Greg Minshall (Novell), and John Garrett (AT&T) for their review and constructive comments. We are also grateful to Gerri Gilliland who supplied the paper tablecloth, colored crayons, and fine food that allowed these ideas to be assembled initially.


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 [Postel81a]  Postel, J., "Internet Protocol - DARPA Internet Program
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 [Postel81b]  Postel, J., "Internet Control Message Protocol- DARPA
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Authors' Addresses

Bob Braden
Information Sciences Institute
University of Southern California
4676 Admiralty Way
Marina del Rey, CA 90292

     Phone: (310) 822-1511
     EMail: Braden@ISI.EDU

Jon Postel
Information Sciences Institute
University of Southern California
4676 Admiralty Way
Marina del Rey, CA 90292

     Phone: (310) 822-1511
     EMail: Postel@ISI.EDU
     Yakov Rekhter
     Office 32-017
     T.J. Watson Research Center, IBM Corp.
     P.O. Box 218,
     Yorktown Heights, NY 10598
     Phone: (914) 945-3896
     EMail: Yakov@WATSON.IBM.COM