Bridged cryptographic VLAN

[0001] This application is a Continuation-in-Part of U.S. patent application Ser. No. 10/057,566, filed Jan. 25, 2002.

[0002] 1. Technical Field

[0003] The invention relates to VLANs. More particularly, the invention relates to a bridged cryptographic VLAN.

[0004] 2. Description of the Prior Art

Basic VLAN Concepts

[0005] FIG. 1 shows a simple port-based VLAN 10, comprised of two VLANs, i.e. VLAN A 13 and VLAN B 15. The VLAN to which an untagged frame received at a port belongs is determined by the Port VLAN ID (PVID) assigned to the receiving port, or by the VLAN ID (VID) associated with the link-layer protocol carried in the frame (see IEEE Std 802.1v-2001, Virtual Bridged Local Area Networks—Amendment 2: VLAN Classification by Protocol and Port). There must be a way to convey VLAN information between the bridges 12, 14 because they are connected by a trunk link 16 that can carry frames from more than one VLAN. A VLAN tag is added to every frame for this purpose. Such frames are called VLAN-tagged frames.

Trunk Links

[0006] A trunk link is a LAN segment used for VLAN multiplexing between VLAN bridges (see IEEE Std 802.1v-2001, Virtual Bridged Local Area Networks—Amendment 2: VLAN Classification by Protocol and Port). Every device attached to a trunk link must be VLAN-aware. This means that they understand VLAN membership and VLAN frame formats. All frames, including end station frames, on a trunk link are VLAN-tagged, meaning that they carry a non-null VID. There can be no VLAN-unaware end stations on a trunk link.

[0007] The trunk link 16 in FIG. 1 is a multiplexed LAN segment shared by two bridges 12, 14. In general, many VLAN-aware bridges may be attached to a trunk link.

[0008] The access links 11 are LAN segments that do not multiplex VLANs. Instead, each access link carries untagged frames or VLAN-tagged frames belonging to a single VLAN. If frames are tagged then all frames on the segment carry the same VID and end stations on the LAN segment must be VLAN aware.

[0009] Various limitations are encountered with the current state of VLAN art. One problem is that of cryptographic separation of VLANs over trunk links. The introduction of a scheme to solve such problem itself raises the issue of efficient frame transfer between encrypted and unencrypted LAN segments which represent a single VLAN.

[0010] The invention comprises three extensions of the IEEE 802.1Q VLAN bridge model (see IEEE Std 802.1Q-1998, IEEE Standards for Local and Metropolitan Area Networks: Virtual Bridged Local Area Networks). The first extension is the cryptographic separation of VLANs over trunk links. A new LAN segment type, referred to herein as the encapsulated segment type, is introduced. All frames on such a segment are encapsulated according to an encryption and authentication-code scheme. The second extension is the division of a trunk port into inbound and outbound trunk ports. The third extension is a protocol, referred to herein as the Transfer Point Protocol (TPP), that automatically infers for each outbound trunk port in a bridged VLAN, a set of LAN segment types for the port that minimizes the number of transfers between encapsulated and unencapsulated segments required to transport a frame in the bridged VLAN.

LAN Segment Types

[0021] Three types of LAN segments represent a VLAN: untagged, tagged, and encapsulated segments. The IEEE 802.1Q standard addresses only tagged and untagged segment types (see IEEE Std 802.1Q-1998, IEEE Standards for Local and Metropolitan Area Networks: Virtual Bridged Local Area Networks). The standard specifies bridging semantics only for the transfer of traffic between tagged and untagged segments representing the same VLAN. The invention provides a technique that extends the bridging semantics to include transferring traffic between an unencapsulated segment (tagged or untagged) and an encapsulated segment of the same VLAN. In general, any number of LAN segment types can be introduced.

[0022] There is one frame type for each type of segment representing a VLAN. There are three kinds of frames in a bridged, cryptographic VLAN: untagged, VLAN-tagged (also referred to as tagged), and encapsulated. The first two frame types are those of the IEEE 802.1Q standard (see IEEE Std 802.1Q-1998, IEEE Standards for Local and Metropolitan Area Networks: Virtual Bridged Local Area Networks). An encapsulated frame is cryptographically encapsulated. Every encapsulated frame also has a VLAN tag. The tag, however, is different from the tag used within tagged frames belonging to the VLAN. Associated with every VLAN are two unique VLAN tags, VID-T (used within tagged frames of the VLAN) and VID-E (used within encapsulated frames of the VLAN).

[0023] For each VLAN, there is a unique security association comprising a cryptographic authentication code key for checking the integrity and authenticity of frames that are tagged as belonging to the VLAN, and a cryptographic key for ensuring the privacy of all frames belonging to the VLAN.

[0024] The preferred encapsulation scheme is an “encrypt-then-MAC” scheme. In this scheme, the data payload of a frame is encrypted and then a message authentication code is computed over the resulting ciphertext and the frame's sequence number. This scheme has two major advantages: It facilitates forward error correction when used with certain block ciphers and modes of operation, and it permits frame authentication without decryption.

[0025] A tagged set, an untagged set, and an encapsulated set of ports is associated with each VLAN. The security association for a VLAN may be used to verify the authenticity and integrity of every frame tagged as belonging to the VLAN, and received at a port in the VLAN's encapsulated set. The ingress-filtering rule for the port determines whether verification occurs. The association may also be used to encapsulate tagged and untagged frames belonging to the VLAN cryptographically before sending them from a port in the VLAN's encapsulated set.

Trunk Ports

[0026] Every trunk port has an inbound and an outbound port. A trunk link between two trunk ports P1 and P2 connects the inbound port of P1 to the outbound port of P2, and the outbound port of P1 to the inbound port of P2. Therefore the sets of LAN segment types to which an inbound port belongs are exactly those of the outbound port to which it is connected. So, it is sufficient to assign only outbound ports to sets of LAN segment types in order to completely assign all trunk ports in a bridged VLAN to sets of LAN segment types.

[0027] The inbound and outbound ports of a trunk port can belong to different sets of LAN segment types. For instance, the outbound port of a trunk can belong to a VLAN's tagged set, and the inbound port to its encapsulated set, in which case, only encapsulated frames of the VLAN are received on the inbound port, and only tagged frames are ever sent from the outbound port.

[0028] Unlike an access port, the inbound or outbound port of a trunk port can belong to both the tagged and encapsulated sets of a VLAN simultaneously.

[0029] The division of a trunk port into inbound and outbound ports is absent in the 802.1Q standard (see IEEE Std 802.1Q-1998, IEEE Standards for Local and Metropolitan Area Networks: Virtual Bridged Local Area Networks) where, in effect, the inbound and outbound ports are the same port. Inbound and outbound frame types are therefore always the same for a given trunk port in 802.1Q.

[0030] FIG. 2 illustrates a bridged, cryptographic VLAN. Ports P1 (20) and P2 (21) are access ports, one for VLAN A 28, and the other for VLAN B 29, VLANs A and B having access links 30, 31. A trunk link 16 connects the two bridges 12a, 14a via trunk ports P3 (22) and P4 (23). P3 has inbound port P3_i and outbound port P3_o P4 has inbound port P4_i and outbound port P4_o. P4_i is connected to P3_o, and P4_o is connected to P3_i. Frames received at P4 arrive on inbound port P4_i and those sent out P4 leave via outbound port P4_o. Frames received at P3 arrive on inbound port P3_i and those sent out P3 leave via outbound port P3_o.

[0031] Ports P5 (24) and P6 (25) are attached to wireless access links 30. In the preferred embodiment, they are actually virtual ports that share a single radio interface (access point) through which frames are sent and received via RF. VLANs A 28 and B 29 can be represented by different encapsulated segments even though they share the same RF medium. An end station in VLAN A, for example, can receive but not decipher any frame belonging to VLAN B. Therefore, distinct access links 32, 33 are shown for A and B even though their physical separation is only cryptographic.

[0032] Suppose P1 receives only untagged frames, and P2 only tagged frames. Further, suppose the trunk link carries tagged frames in both directions, and the wireless access links only encapsulated frames. Then for VLAN A, the untagged set is {P1}, the tagged set is {P3_o, P3_i, P4_o, P4_i} and the encapsulated set is {P5}; and for B the sets are { }, {P2, P3_o P3_i, P4_o, P4_i}, and {P6} respectively.

[0033] If the ingress-filtering rule at P5 specifies authenticity checking, then a frame received at P5 is authenticated using the security association for VLAN A. If successful, then the frame is determined to be a member of the encapsulated segment for A. Suppose the frame must be forwarded to P4. Then Bridge 2 decapsulates the frame using the same security association. The bridge forwards the decapsulated frame to P4_o with its tag replaced by A-T, thereby transferring the frame from A's encapsulated segment to its tagged segment. Conversely, frames arriving at P4_i, and destined for P5 are encapsulated using the security association for A. Tag A-T is replaced by A-E, which transfers the frame from A's tagged segment to its encapsulated segment.

[0034] There are many variations of the example in FIG. 2. For instance, it may be desirable to protect traffic on VLAN B only. In this case, P5 does not belong to the encapsulated set for A. Only frames received at P6, i.e. frames tagged with B-E, are authenticated, and only B-tagged frames received at P4, and destined for P6 are encapsulated.

Bridging Semantics

[0035] Consider a VLAN bridge having multiple ports. Suppose a frame is received at port P. It is assigned to a VLAN in one of several ways. If P is a trunk port, then the frame must carry a VLAN tag of the form VID-T or VID-E, each of which identifies a VLAN, namely VID. Otherwise, the frame is discarded. If P is not a trunk port, then either port or protocol-based VLAN classification can be used to assign the frame to a VLAN (see IEEE Std 802.1v-2001, Virtual Bridged Local Area Networks—Amendment 2: VLAN Classification by Protocol and Port).

Ingress Filtering

[0036] If P is a trunk port and is not in the tagged or encapsulated sets for VID, then the frame is discarded. The ingress-filter rule for a port may specify authentication and integrity checking for certain VLANs. If P is a port whose ingress filter rule requires authentication and integrity checking for the VLAN VID, then the frame received at P must have a VLAN tag VID-E. Otherwise, the frame is discarded. In the preferred embodiment, an authentication code is computed over the received frame's ciphertext and sequence number using the security association for VID. If it does not match the received authentication code in the frame, then the frame is discarded. Otherwise, the frame is judged to belong to the encapsulated segment for VID.

[0037] If P is not in the tagged set for VID, but it is attached to a VLAN-tagged access link, then the received frame is discarded.

Forwarding Process

[0038] The forwarding process begins by constructing the target port set Q. This is the set of ports to which a frame belonging to a particular VLAN must be forwarded. Suppose a frame received at port P belongs to the VLAN VID. If the frame must be flooded then Q contains any outbound or access port that is a member of the tagged, untagged, or encapsulated sets for VID. The next step is to shrink Q if, and only if, P is an inbound port of a trunk that belongs to both the tagged and encapsulated sets of VID. In this case, every port in the encapsulated set of VID that does not belong to the tagged set of VID is removed from Q if the received frame is a tagged frame, or every port in the tagged or untagged set of VID that does not belong to the encapsulated set of VID is removed from Q if the received frame is encapsulated. Because the inbound port belongs to both sets of LAN segment types for VID, the inbound port must receive a frame of each LAN segment type, and therefore shrinking the target port set is justified. The Transfer Point Protocol has the property that it guarantees shrinking never results in an empty target port set. Shrinking to an empty target set implies the bridge received a frame that it has no reason to receive.

[0039] The next step in the forwarding process is to construct a forwarding set for the received frame. This is the set of frames to be forwarded as a result of receiving the frame belonging to VID at port P. These are the frames necessary to transfer traffic from one LAN segment of the VLAN to another. The table shown in FIG. 3 is used to construct forwarding sets. The frame received at P belongs to a kind K of LAN segment for VID (tagged, untagged, or encapsulated). Likewise, every port in Q belongs to a kind of LAN segment, the kind of port set to which it belongs for VID. Trunk ports may have two kinds of sets: tagged and encapsulated. For every port q in Q, add a frame to the forwarding set according to rule (K, K′) in the table of FIG. 3, where K′ is a kind of port set to which q belongs for VID.

[0040] The rules for constructing the forwarding set for a received frame are as follows:

• • [0041] (1) Add received frame to forwarding set.
  • [0042] (2) Add VLAN tag VID-T to received frame; add the result to forwarding set.
  • [0043] (3) Received frame is cryptographically encapsulated using the security association for VID; resulting frame is VLAN tagged with VID-E and added to forwarding set.
  • [0044] (4) Remove VID-T from received frame; add untagged frame to forwarding set.
  • [0045] (5) Received frame's ciphertext is decrypted using the security association for VID; resulting frame is untagged and added to forwarding set.
  • [0046] (6) Received frame's ciphertext is decrypted using the security association for VID; resulting frame is tagged with VID-T and added to forwarding set.

[0047] In the presently preferred embodiment, there can be at most three frames in any forwarding set, corresponding to the three different kinds of LAN segments that can represent a VLAN. The forwarding process forwards the frames of the forwarding set as follows:

• • [0048] The forwarding process queues for transmission at each port in Q that belongs to the untagged set for VID, the untagged frame, if any, in the forwarding set.
  • [0049] The forwarding process queues for transmission at each port in Q that belongs to the tagged set for VID, the VLAN-tagged frame, if any, in the forwarding set.
  • [0050] The forwarding process queues for transmission at each port in Q that belongs to the encapsulated set for VID, the encapsulated frame, if any, in the forwarding set.
    Frame Transfer

[0051] Within a bridged, cryptographic VLAN, steps are taken to eliminate redundant transfers between LAN segments representing the same VLAN. For instance, it is desirable to avoid transferring an unencapsulated frame to a VLAN's encapsulated segment more than once in a bridged VLAN because each transfer requires encryption. Encapsulation should be done once and shared by all egress ports that belong to the VLAN's encapsulated set across all bridges. Similarly, it is desirable to avoid repeated decapsulation across bridges because each calls for decryption.

[0052] For instance, consider the bridged LAN in FIG. 4. Suppose the ports of Bridges 1 (41) and 2 (42) to which the wireless trunk links 43 are attached belong to the encapsulated set for VLAN B 44. If the trunk link 45 carries only VLAN-tagged frames, then frames belonging to VLAN B that are received at Bridge 1 must be encapsulated at Bridges 1 and 2. However, if the trunk link carries encapsulated frames then encapsulation need only be done at Bridge 1 and shared with Bridge 2.

[0053] There are also situations where encapsulation can be done too early in a bridged LAN, forcing encapsulated frames to be sent over trunk links unnecessarily. There is a transfer point for encapsulation and decapsulation for each VLAN that minimizes cryptographic operations. The Transfer Point Protocol (discussed below) infers this transfer point between segments.

Transfer Point Protocol

[0054] A minimum spanning tree algorithm can reduce any bridged LAN to a spanning tree whose nodes are the bridges and whose edges are trunk links. A spanning tree induces a partial order on bridges. For instance, we can take as the partial order B1<B2, where bridge B1 is the parent of B2 in the spanning tree. The least bridge is the root of the spanning tree. The set of bridges together with the partial order defines a complete, partially ordered set. Every nonempty subset of bridges has a least upper bound.

[0055] Consider frames received at the root of the spanning tree. The least upper bound of all bridges requiring a received frame of a VLAN to belong to one of the LAN segments representing the VLAN is the transfer point for converting received frames to frames for that LAN segment.

[0056] The Transfer Point Protocol (TPP) comprises two link-layer protocols, TPP-T for adding outbound trunk ports to the tagged set of a VLAN, and TPP-E for adding outbound trunk ports to the encapsulated set of a VLAN. The trunk ports are across all bridges that bridge the VLAN. For example, TPP-E determines that the outbound trunk port connecting Bridge 1 to Bridge 2 in FIG. 4 must be a member of the encapsulated set for VLAN B. That way the wireless trunk port at Bridge 2 can share encapsulations performed by Bridge 1 for its outbound wireless trunk port.

[0057] TPP assumes that every access link port has been assigned to the tagged, untagged, or encapsulated set for a VLAN prior to execution because it uses this information to infer the sets to which outbound trunk ports in the bridged VLAN belong. TPP-E can assign an outbound trunk port to the encapsulated set of a VLAN, while TPP-T can assign the same outbound port to the tagged set of the VLAN.

[0058] TPP has two frames types, the announce frame, and the reply frame. Each of these frames contains a VLAN ID and a source bridge routing path, where each entry in the path is a unique pair containing a bridge MAC address and three bits, one bit for each LAN segment type, i.e. tagged, untagged, and encapsulated. The tagged bit is high if and only if the bridge addressed in the pair has an access port in the tagged set of the VLAN named in the frame. The untagged and encapsulated bits are set likewise.

[0059] A bridge sends a TPP announce frame, e.g. a GARP PDU, to a TPP group address, e.g. a GARP application address, through each of its outbound trunk ports for every VLAN known to it. When a bridge receives an announce frame, it appends to the right of the path the pair for itself regarding the named VLAN received, and forwards the frame to each of its enabled, outbound trunk ports except the receiving trunk port. If it has no other such ports, then it sends the final routing path and received VID in a TPP reply frame to the MAC address that precedes it in the routing path. The originating bridge of an announce frame creates a path consisting only of a pair for itself. When a bridge receives a TPP reply frame on an inbound trunk port, it forwards the reply frame to the bridge MAC address that precedes it in the path. If there is none, the frame is discarded.

TPP-E

[0060] When a bridge receives a TPP reply frame on a trunk port, it adds the trunk's outbound port to the encapsulated set for the VID in the frame if, and only if, it is followed by a bridge B in the routing path whose encapsulated bit is high, and either

• • [0061] a) the receiving bridge has a tagged or untagged access port for the VID and no bridge after it in the routing path, up to and including B, has a high tagged or untagged bit; or
  • [0062] b) the receiving bridge has an encapsulated access port for the VID, or is preceded by a bridge in the routing path with a high encapsulated bit.
    TPP-T

[0063] When a bridge receives a TPP reply frame on a trunk port, it adds the trunk's outbound port to the tagged set for the VID in the frame if, and only if, it is followed by a bridge B in the routing path whose tagged or untagged bit is high, and either

• • [0064] a) the receiving bridge has an encapsulated access port for the VID and no bridge after it in the routing path, up to and including B, has a high encapsulated bit; or
  • [0065] b) the receiving bridge has a tagged or untagged access port for the VID, or is preceded by a bridge in the routing path with a high tagged or untagged bit.

[0066] Consider bridging a single VLAN. Each access port therefore is assumed to belong to this VLAN. Thus, VLAN labeling of ports is omitted in the examples. Instead, the outbound trunk ports are labeled with LAN segment types, i.e. T (tagged), U (untagged), and E (encapsulated). If an outbound port is labeled with U, for example, then the port belongs to the untagged set of the VLAN.

[0067] Initially, every access port is labeled according to the kind of set to which the port belongs for the VLAN. Trunk ports are initially unlabeled. It is the job of TPP to infer labels for them. FIG. 5 shows a bridging of a VLAN 50 where two bridges 51, 52 are connected by a trunk 53. Each bridge has two access ports. Because each bridge has both untagged and encapsulated access ports, TPP infers that both outbound ports of the trunk belong to the tagged and encapsulated sets of the VLAN. Each inbound port also belongs to these sets.

[0068] Each outbound port is a member of the tagged set per rule TPP-T (b). Each bridge infers this fact when it initiates a TPP announce frame. Therefore, both the encryption and decryption done by each bridge is shared with the other.

[0069] In FIG. 6, Bridge 1 (61) has an untagged access port and Bridge 2 (62) has an encapsulated access port. Therefore, the outbound port 63 of Bridge 1 is a member of the encapsulated set per rule TPP-E (a) whereas the outbound port 64 of Bridge 2 is a member of the tagged set per rule TPP-T (a).

[0070] FIG. 7 illustrates a purely encapsulated trunk link. All frames over the link are encapsulated, however, no encryption is done at Bridges 2 or 3.

[0071] FIG. 8 shows the TPP message exchange between Bridges 1 (71), 2 (72), and 3 (73) when Bridge 1 (71) of FIG. 7 initiates an announce frame for the VLAN, which we assume for the example is named “B”.

[0072] If Bridges 1 (71) and 2 (72) in FIG. 7 are interchanged, the result is the bridged cryptographic VLAN of FIG. 9.

[0073] FIG. 10 shows a bridge 82 with three trunk ports, each connected to another bridge 81, 83, 84. The outbound port of the trunk from Bridge 4 (84) belongs to the tagged and encapsulated sets, whereas the outbound port of Bridge 2 (82) that is connected to the inbound port of Bridge 4 is only a member of the encapsulated set.

[0074] TPP may run repeatedly to infer changes in transfer points. How frequently it runs and the number of bridges it affects depends on the displacement of access links. For example, if an end station is wireless, then movement of the station with respect to the bridged LAN can result in its encapsulated access link being relocated. Until TPP is rerun, there may be redundant transfers for a VLAN.

[0075] A bridged VLAN may consist of bridges that do not participate in TPP. In general, there may be one or more cryptographic VLAN bridges with trunk ports connected to legacy VLAN bridges. If each such trunk port is viewed instead as a collection of virtual, tagged access ports, one port for each VLAN tag that can be sent over the trunk, then TPP can still be run to infer transfer points among participating bridges. However, there may be redundant transfers across the entire bridged LAN. For example, if a nonparticipating core switch were to separate two cryptographic VLAN bridges, each having an access port in the encapsulated set of the same VLAN, then traffic between these encapsulated segments would be decrypted upon entry to the core and then re-encrypted after exiting. Observe that no encryption or decryption is needed if there are no access ports in the core that belong to the tagged or untagged sets of the VLAN. In this case, TPP can treat the virtual access port for each VLAN tag as an encapsulated access port rather than a tagged access port. Then all traffic between the two encapsulated segments can traverse the core transparently as encapsulated frames because every encapsulated frame is a VLAN-tagged frame.

Group Security

[0076] A cryptographicVLAN v is defined by a group of m stations that has a unique security association. The association consists of the following:

• • [0077] a) an encryption key K_v,
  • [0078] b) an authentication code key K′_v,
  • [0079] c) a distribution key K″_v, and
  • [0080] d) m random values R₁, R₂, . . . , R_m.

[0081] The encryption key is a symmetric key used by v-aware bridges and stations of v to encrypt and decrypt frames belonging to v. All v-aware bridges, and stations of v, compute and verify authentication codes over encrypted frames of v using K′_v.

[0082] There is one random value for each of the m stations. The i^th station of the group knows all m random values except R_i. The m−1 random values it knows are communicated to it by a v-aware bridge. Privacy of the random values is ensured by encryption using distribution key K″_v, while their authenticity is ensured by an authentication code computed over the resulting ciphertext using authentication code key K′_v.

Joining a Cryptographic VLAN

[0083] Joining a cryptographic VLAN is done with a two-step protocol:

• • [0084] adding a new station to the group, and
  • [0085] enabling all other stations in the group to eliminate the new station later.

[0086] A user's station joins a cryptographic VLAN v through a mutual authentication protocol executed between the user, via the station, and an authenticator residing on a v-aware bridge. If mutual authentication succeeds, a secure ephemeral channel is created between the bridge and the new station to transfer K_v, K′_v, and R₁, R₂, . . . , R_m securely from the bridge to the station. Then the second step of the join protocol executes. Otherwise, the protocol terminates immediately. In the second step, the same v-aware bridge chooses a new random value R_m+1 for the new station, and distributes it to all v-aware bridges, and stations comprising v, in a broadcast frame that is encrypted under K″_v and carries an authentication code computed over the ciphertext using K′_v. The bridge then creates a new distribution key for v and distributes it to all v-aware bridges and to members of v, including the new station, in a broadcast frame that is encrypted under K_v and carries an authentication code computed over the ciphertext using K′_v.

[0087] Although the new station can verify the authenticity of the broadcast containing its own random value R_m+1, it is unable to decrypt it because it does not hold key K″_v.

Leaving a Cryptographic VLAN

[0088] A subgroup of stations can simultaneously leave a cryptographic VLAN v, perhaps involuntarily. Suppose stations 1, . . . , k of a group leave. When this happens, it is detected by a v-aware bridge which then announces the departure of stations 1, . . . , k via a single broadcast frame that includes an authentication code computed over the frame using K′_v. The broadcast will notify every v-aware bridge and station in the group that stations 1, . . . , k have left. Each such bridge and station then attempts to rekey the encryption, authentication code, and distribution keys for v, each as a function of the old key and the random values R₁, . . . , R_k. Every v-aware bridge and all remaining stations in v will share a new security association as a result, including k fewer random values.

[0089] Every v-aware bridge always has the current distribution key for v, unlike a station. So every such bridge always has the complete set of random values for any subgroup that leaves v, thereby allowing it to always rekey the keys for v. The situation is different for stations however. Rekeying is a function of the random values for departing stations, values that these stations do not have. Therefore, they are unable to rekey. Furthermore, forward secrecy is guaranteed. A departed station can never become a member of v again as a result of subsequent rekeyings. This is because rekeying is a function of the current keys which means that all keys arrived at thereafter will always be a function of a random value unknown to the station. Only through rejoining v can the station ever become a member of v again.

[0090] Although the invention is described herein with reference to the preferred embodiment, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Accordingly, the invention should only be limited by the Claims included below.