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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 4541

MAGMA Working Group                                       M. Christensen
Internet Draft                               morten@jagd-christensen.com
June 2002                                                    F. Solensky
Expiration Date: December 2002                                Premonitia

                     IGMP and MLD snooping switches

Status of this Memo

     This document is an Internet-Draft and is in full conformance with
     all provisions of Section 10 of RFC2026 [RFC2026].

     Internet-Drafts are working documents of the Internet Engineering
     Task Force (IETF), its areas, and its working groups. Note that
     other groups may also distribute working documents as Internet-

     Internet-Drafts are draft documents valid for a maximum of six
     months and may be updated, replaced, or obsoleted by other docu¡
     ments at any time. It is inappropriate to use Internet-Drafts as
     reference material or to cite them other than as "work in

     The list of current Internet-Drafts can be accessed at

     The list of Internet-Draft Shadow Directories can be accessed at


     This memo describes the requirements for IGMP and MLD snooping
     switches. The requirements for IGMPv2 snooping switches are based
     on best current practices. IGMPv3 and MLDv2 snooping are also cov¡
     ered in this draft although we are not aware of any such implemen¡
     tations at the time of writing.  Areas which are of relevance to
     IGMP and MLD snooping switches, such as link layer topology changes
     and Ethernet specific encapsulation issues are also considered.

     Interoperability issues that arise betweed different versions of
     IGMP are not discussed in this document.  Interested readers are
     directed to [IGMPv3] for a thorough description of problem area.

     This document is intended as an accompanying document to the IGMPv3
     and MLDv2 specifications.

Christensen, Solensky                                           [Page 1]

RFC DRAFT                                                      June 2002

11..  IInnttrroodduuccttiioonn

     When a packet with a broadcast or multicast destination address is
     received, the switch will forward a copy into each of the remaining
     network segments in accordance with [BRIDGE].  Eventually, the
     packet is made accessible to all nodes connected to the network.

     This approach works well for broadcast packets that are intended to
     be seen or processed by all connected nodes.  In the case of multi¡
     cast packets, however, this approach could lead to less efficient
     use of network bandwidth, particularly when the packet is intended
     for only a small number of nodes.  Packets will be flooded into
     network segments where no node has any interest in receiving the
     packet.  While nodes will rarely incur any processing overhead to
     filter packets addressed to unrequested group addresses, they are
     unable to transmit new packets onto the shared media for the period
     of time that the multicast packet is flooded.

     The problem of wasting bandwidth is even worse when the LAN segment
     is not shared, for example in Full Duplex links.  Full Duplex is
     standard today for most switches operating at 1Gbps or above.  In
     this case the bandwidth that is wasted is proportional to the num¡
     ber of attached nodes.

     In recent years, a number of commercial vendors have introduced
     products described as "IGMP snooping switches" to the market.
     These devices do not adhere to the conceptual model that provides
     the strict separation of functionality between different communica¡
     tions layers in the ISO model and instead utilizes information in
     the upper level protocol headers as factors to be considered in the
     processing at the lower levels.  This is analogous to the manner in
     which a router can act as a firewall by looking into the transport
     protocol's header before allowing a packet to be forwarded to its
     destination address.

     In the case of multicast traffic, an IGMP snooping switch provides
     the benefit of conserving bandwidth on those segments of the net¡
     work where no node has expressed interest in receiving packets
     addressed to the group address. This is in contrast to normal
     switch behaviour where multicast traffic is typically forwarded on
     all interfaces.

     Many switch datasheets state support for IGMP snooping, but no
     requirements for this exist today. It is the authors hope that the
     information presented in this draft will supply this information.

     The requirements presented here is based on the following informa¡
     tion sources: The IGMP specifications [RFC112][RFC2236][IGMPv3],

Christensen, Solensky                                           [Page 2]

RFC DRAFT                                                      June 2002

     vendor supplied technical documents [CISCO], bug reports [MSOFT],
     discussions with people invovled in design of IGMP snooping
     switches, MAGMA mailinglist discussions, and on replies by switch
     vendors to an implementation questionnaire.

     The discussions in this document are based on IGMP which applies to
     IPv4 only. For IPv6 we must use MLD instead. Because MLD is based
     on IGMP we do not repeat the whole discussion and requirements for
     MLD snooping switches. Instead we point out the few cases where
     there is a difference compared to IGMP.

22..  IIGGMMPP SSnnooooppiinngg RReeqquuiirreemmeennttss

     The following sections list the requirements for an IGMP snooping
     switch.  The requirement is stated and is supplemented by a discus¡
     sion. All implementation discussions are examples only and there
     may well be other ways to achieve the same functionality.

22..11..  FFoorrwwaarrddiinngg rruulleess

     The IGMP snooping functionality is here separated in a control sec¡
     tion (IGMP forwarding) and data section (Data forwarding).

22..11..11..  IIGGMMPP FFoorrwwaarrddiinngg RRuulleess

     1)  A snooping switch SHOULD forward IGMP Membership Reports only
         to those ports where multicast routers are attached.  Alterna¡
         tively stated: A snooping switch SHOULD NOT forward IGMP Mem¡
         bership Reports to ports on which only hosts are attached.  An
         administrative control MAY be provided to override this
         restriction, allowing the report messages to be flooded to
         other ports.

         This is the main IGMP snooping functionality.  Sending member¡
         ship reports (as described in IGMP versions 1 and 2) to other
         hosts can result in unintentionally preventing a host from
         joining a specific multicast group.  This is not a problem in
         an IGMPv3 only network because there is no cancellation of IGMP
         Membership reports.

         The administrative control allows IGMP Membership Report mes¡
         sages to be processed by network monitoring equipment such as
         packet analysers or port replicators.

Christensen, Solensky                                           [Page 3]

RFC DRAFT                                                      June 2002

         The switch supporting IGMP snooping MUST maintain a list of
         multicast routers and the ports on which they are attached.
         This list can be constructed in any combination of the follow¡
         ing ways:

         a)  This list SHOULD be built using IGMP Multicast Router Dis¡
             covery [MRDISC] by the snooping switch sending Multicast
             Router Solicitation messages on its own.  It MAY also snoop
             Multicast Router Advertisement messages sent by and to
             other nodes.

         b)  The arrival port for IGMP Queries (sent by multicast
             routers) where the source where the address is not

         c)  A list of ports configured by management as described in
             the previous step.

     2)  IGMP snooping switches MAY implement "proxy-reporting" in which
         reports received from downstream hosts are summarized and used
         to build internal membership states as described in [PROXY].
         An IGMP proxy-reporting switch would then report its own state
         in response to upstream queriers.  If the switch does not
         alreay have an IP address it SHOULD use the address as
         source in these reports.

         An IGMP proxy-reporting switch may act as Querier for the down¡
         stream hosts while proxy reporting to the 'real' upstream

         It should be noted that there may be multiple IGMP proxy-
         reporting switches in the network all using the source
         IP address. In this case the switches can be uniquely identi¡
         fied through their link layer source MAC address.

         IGMP membership reports MUST NOT be rejected because of a
         source IP address of; however, these messages MUST NOT
         be included the election process so that a snooping switch does
         not elected over an active router.

     3)  The switch that supports IGMP snooping MUST flood all unrecog¡
         nized IGMP messages to all other ports and MUST NOT attempt to
         make use of any information beyond the end of the network layer
         header.  In particular, messages where any reserved fields in
         the IGMP header are non-zero MUST NOT be subject to "normal"
         snooping since this could indicate an incompatible change to
         the IGMP message format.

Christensen, Solensky                                           [Page 4]

RFC DRAFT                                                      June 2002

     4)  An IGMP snooping switch SHOULD be aware of link layer topology
         changes.  Following a topology change the switch SHOULD initi¡
         ate the transmission of a General Query on all ports in order
         to reduce network convergence time.

     5)  An IGMP snooping switch MUST NOT make use of information in
         IGMP packets where the IP or IGMP headers have checksum or
         intregity errors.  The switch SHOULD NOT flood such packets but
         if it does, it SHOULD take some note of the event (i.e.: incre¡
         ment a counter).  These errors and their processing are further
         discussed in [IGMPv3], [MLD] and [MLDv2].

22..11..22..  DDaattaa FFoorrwwaarrddiinngg RRuulleess

     1)  Packets with a destination IP (DIP) address in the 224.0.0.X
         range which are not IGMP MUST be forwarded on all ports.

         This requirement is based on fact that many hosts exist today,
         which does not Join IP multicast addresses in this range before
         sending or listening to IP multicast. Furthermore since the
         224.0.0.X address range is defined as link local (not to be
         routed) it seems unnecessary to keep state for each address in
         this range.

     2)  Packets with a destination IP address outside 224.0.0.X which
         are not IGMP SHOULD be forwarded according to group based port
         membership tables and MUST also be forwarded on router ports.

         This is the core IGMP snooping requirement for the data path.

         Discussion: An implementation could maintain separate member¡
         ship and multicast router tables in software and then "merge"
         these tables into a current forwarding cache.

     3)  If a switch receives a non-IGMP multicast packet without having
         first processed Membership Reports for the group address, it
         MAY forward the packet on all ports, but it MUST forward the
         packet on router ports.  A switch MAY forward an unregistered
         packet only on router ports, but the switch MUST have a config¡
         uration option that suppresses this restrictive operation and
         forces flooding of unregistered packets on all ports.

     4)  IGMP snooping switches MAY maintain forwarding tables based on
         either MAC addresses or IP addresses.  If a switch supports
         both types of forwarding tables then the default behavior
         SHOULD be to use IP addresses.

Christensen, Solensky                                           [Page 5]

RFC DRAFT                                                      June 2002

         Discussion: Forwarding based on MAC addresses is subject to the
         problem associated with the 32-fold IP address to 1 MAC address

     5)  Switches which rely on information in the IP header SHOULD ver¡
         ify that the IP header checksum is correct.  If the checksum
         fails, the information in the packet MUST NOT be incorporated
         into the forwarding table.  Further, the packet SHOULD be dis¡

22..22..  IIGGMMPP ssnnooooppiinngg rreellaatteedd pprroobblleemmss

     A special problem arise in the network consisting of IGMPv3 routers
     as well as IGMPv2 and IGMPv3 hosts interconnected by a IGMPv2
     snooping switch.  IGMPv3 has a mechanism to fall back to IGMPv2
     when receiving IGMPv2 membership reports. This means that the net¡
     work will converged on IGMPv2 eventually. However, the convergence
     time will lead to supression of v3 Hosts for several minutes.

     Therefore it is recommended that in such a network, the multicast
     router is configured to use IGMPv2.

33..  IIPPvv66 CCoonnssiiddeerraattiioonnss

     In order to avoid confusion, the previous discussions have been
     based on IGMPv3 functionality which only applies to IPv4 multicast.
     In the case of IPv6 most of the above discussions are still valid
     with a few exceptions which we will describe here.

     In IPv6 the protocol for multicast group maintenance is called Mul¡
     ticast Listener Discovery (MLDv2).  IPv6 is not widely deployed
     today and neither is IPv6 multicast.  However, it is anticipated
     that at some time IPv6 switches capable of MLD snooping will

     The three main differences between IGMPv3 and MLDv2 are:

     -  MLDv2 uses ICMPv6 message types instead of IGMP message types.

     -  The ethernet encapsulation is a mapping of 32 bits of the 128
        bit DIP addresses into 48 bit DMAC addresses [IPENCAPS].

     -  Multicast router discovery is done using Neighbor Discovery Pro¡
        tocol (NDP) for IPv6. NDP uses ICMPv6 message types.

Christensen, Solensky                                           [Page 6]

RFC DRAFT                                                      June 2002

     The IPv6 packet header does not include a checksum field.  Never¡
     theless, the switch SHOULD detect other packet integrity issues.
     When the snooping switch detects such an error, it MUST NOT include
     information from the corresponding packet in the IGMP forwarding
     table.  The forwarding code SHOULD drop the packet and take further
     reasonable actions as advocated above.

     The fact that MLDv2 is using ICMPv6 adds new requirements to a
     snooping switch because ICMPv6 has multiple uses aside from MLD.
     This means that it is no longer sufficient to detect that the next-
     header field of the IP header is ICMPv6 in order to redirect pack¡
     ets to the CPU.  If this was the case the CPU queue assigned for
     MLD would potentially be filled with non-MLD related packets. Fur¡
     thermore ICMPv6 packets destined for other hosts would not reach
     their destination.  A solution is either to require that the snoop¡
     ing switch looks further into the packets or to be able to detect a
     multicast DMAC address in conjunction with ICMPv6.  The first solu¡
     tion is desirable only if it is configurable which message types
     should trigger a CPU redirect and which should not. The reason is
     that a hardcoding of message types is unflexible for the introduc¡
     tion of new message types.  The second solution introduces the risk
     of new protocols, which are not related to MLD but uses ICMPv6 and
     multicast DMAC addresses wrongly being identified as MLD. It is
     suggested that solution one is the preferred if the switch is capa¡
     ble of triggering CPU redirects on individual ICMPv6 message types.
     If this is not the case then use solution two.

     The mapping from IP multicast addresses to multicast DMAC addresses
     introduces a potentially enormous overlap. The structure of an IPv6
     multicast address is shown in figure 5.  Theoretically 2**80, two
     to the power of 80 (128 - 8 - 4 - 4 - 32) unique DIP addresses
     could map to one DMAC address. This should be compared to 2**5 in
     the case of IPv4.

     Initial allocation of IPv6 multicast addresses, however, uses only
     the lower 32 bits of group ID. This eliminates the address ambigu¡
     ity for the time being but it should be noted that the allocation
     policy may change in the future.

       |   8    |  4 |  4 |             112 bits                  |
       |11111111|flgs|scop|             group ID                  |
                                Figure 5

     In the case of IPv6 forwarding can be made on the basis of DMAC

Christensen, Solensky                                           [Page 7]

RFC DRAFT                                                      June 2002

     addresses in the forseable future.

     Finally, we mention the reserved address range FF02::/96.  This
     range is similar to 224.0.0.X for IPv4 and is reserved to routing
     protocols and resource discovery [RFC2375].  In the case of IPv6 it
     is suggested that packets in this range are forwarded on all ports
     if they are not MLD packets.

44..  SSeeccuurriittyy CCoonnssiiddeerraattiioonnss

     Security considerations for IGMPv3 are accounted for in [IGMPv3].
     The introduction of IGMP snooping switches adds the following con¡
     siderations with regard to IP multicast.

     The exclude source failure which could cause traffic from sources
     that are 'black listed' to reach hosts that have requested other¡
     wise.  This can also occur in certain network topologies without
     IGMP snooping.

     It is possible to generate packets which make the switch wrongly
     believe that there is a multicast router on the segment on which
     the source is attached. This will potentially lead to excessive
     flooding on that segment.  The authentication methods discussed in
     [IGMPv3] will also provide protection in this case.

     IGMP snooping switches which rely on the IP header of a packet for
     their operation and which do not validate the header checksum
     potentially will forward packets on the wrong ports. Even though
     the IP headers are protected by the ethernet checksum this is a
     potential vulnerability.

     Generally though, it is worth to stress that IP multicast must so
     far be considered insecure until the work of for example the sug¡
     gested Multicast Security (MSEC) working group or similar is com¡
     pleted or at least has matured.

55..  IIGGMMPP QQuueessttiioonnnnaaiirree

     As part of this work the following questions were asked both on the
     MAGMA discussion list and sent to known switch vendors implementing
     IGMP snooping.  The individual contributions have been anonymized
     upon request and do not necessarily apply to all of the vendors'

Christensen, Solensky                                           [Page 8]

RFC DRAFT                                                      June 2002

     The questions were:

     Q1  Does your switches perform IGMP Join aggregation?  In other
         words, are IGMP joins intercepted, absorbed by the hard¡
         ware/software so that only one Join is forwarded to the

     Q2  Is multicast forwarding based on MAC addresses?  Would data¡
         grams addressed to multicast IP addresses and be forwarded on the same ports-groups?

     Q3  Is it possible to forward multicast datagrams based on IP
         addresses (not routed). In other words, could and be forwarded on different port-groups with unal¡
         tered TTL?

     Q4  Are multicast datagrams within the range to forwarded on all ports whether or not IGMP Joins
         have been sent?

     Q5  Are multicast frames within the MAC address range
         01:00:5E:00:00:01 to 01:00:5E:00:00:FF forwarded on all ports
         whether or not IGMP joins have been sent?

     Q6  Does your switch support forwarding to ports on which IP multi¡
         cast routers are attached in addition to the ports where IGMP
         Joins have been received?

     Q7  Is your IGMP snooping functionality fully implemented in hard¡

     Q8  Is your IGMP snooping functionality partly software imple¡

     Q9  Can topology changes (for example spanning tree configuration
         changes) be detected by the IGMP snooping functionality so that
         for example new queries can be sent or tables can be updated to
         ensure robustness?

Christensen, Solensky                                           [Page 9]

RFC DRAFT                                                      June 2002

     The answers were:

                                     |     Switch Vendor     |
                                     | 1 | 2 | 3 | 4 | 5 | 6 |
          Q1 Join aggregation        | x | x | x |   | x | x |
          Q2 Layer-2 forwarding      | x | x | x | x |(1)|   |
          Q3 Layer-3 forwarding      |(1)|   |(1)|   |(1)| x |
          Q4 224.0.0.X aware         |(1)| x |(1)|(2)| x | x |
          Q5 01:00:5e:00:00:XX aware | x | x | x |(2)| x | x |
          Q6 Mcast router list       | x | x | x | x | x | x |
          Q7 Hardware implemented    |   |   |   |   |   |   |
          Q8 Software assisted       | x | x | x | x | x | x |
          Q9 Topology change aware   | x | x | x | x |   |(2)|
           x  Means that the answer was Yes.
          (1) In some products (typically high-end) Yes, in others No.
          (2) Currently no, but will be real soon.

66..  RReeffeerreenncceess

     [BRIDGE]   IEEE 802.1D, "Media Access Control (MAC) Bridges"

     [CISCO]    Cisco Tech Notes, "Multicast In a Campus Network: CGMP
                and IGMP snooping", http://www.cisco.com/warp/pub¡

     [IANA]     Internet Assigned Numbers Authority, "Internet Multicast
                Addresses", http://www.isi.edu/in-notes/iana/assign¡

     [IGMPv3]   Cain, B., "Internet Group Management Protocol, Version
                3", draft-ietf-idmr-igmp-v3-11.txt, May 2002.

     [IPENCAPS] Crawford, M., "Transmission of IPv6 Packets over Ether¡
                net Networks", RFC2464, December 1998.

     [MLD]      Deering, S., Fenner, B., and Haberman, B.  "Multicast
                Listener Discovery (MLD) for IPv6", RFC2710, October

     [MLDv2]    Vida, R., "Multicast Listener Discovery Version 2
                (MLDv2) for IPv6", draft-vida-mld-v2-03.txt, June 2002.

Christensen, Solensky                                          [Page 10]

RFC DRAFT                                                      June 2002

     [MRDISC]   Biswas, S.  "IGMP Multicast Router Discovery", draft-
                ietf-idmr-igmp-mrdisc-08.txt, January 2002.

     [MSOFT]    Microsoft support article Q223136, "Some LAN Switches
                with IGMP Snooping Stop Forwarding Multicast Packets on
                RRAS Startup", http://support.microsoft.com/sup¡

     [PROXY]    Fenner, B. et al, "IGMP-based Multicast Forwarding (IGMP
                Proxying)", draft-ietf-magma-proxy-02(?).txt.

     [RFC1112]  Deering, S., "Host Extensions for IP Multicasting", RFC
                1112, August 1989.

     [RFC2026]  Bradner, S.  "The Internet Standards Process -- Revision
                3", RFC2026, October 1996.

     [RFC2236]  Fenner, W., "Internet Group Management Protocol, Version
                2", RFC2236, November 1997.

     [RFC2375]  Hinden, R.  "IPv6 Multicast Address Assignments",
                RFC2375, July 1998.

77..  AAcckknnoowwlleeddggeemmeennttss

     We would like to thank Martin Bak, Les Bell, Yiqun Cai, Paul Cong¡
     don, Toerless Eckert, Bill Fenner, Brian Haberman, Edward Hilquist,
     Hugh Holbrook, Kevin Humphries, Karen Kimball and Jaff Thomas for
     comments and suggestions on this document.

     Furthermore, the following companies are acknowledged for their
     contributions: 3Com, Alcatel, Cisco Systems, Enterasys Networks,
     Hewlett-Packard, Vitesse Semiconductor Corporation.  The ordering
     of these names do not necessarily correspond to the column numbers
     in the response table.

88..  RReevviissiioonn HHiissttoorryy

     This section, while incomplete, is provided as a convenience to the
     working group members.  It will be removed when the document is
     released in its final form.

     draft-ietf-magma-snoop-01.txt: January 2002

          Extensive restructuring of the original text.

Christensen, Solensky                                          [Page 11]

RFC DRAFT                                                      June 2002

     draft-ietf-magma-snoop-02.txt: June 2002

          Status section removes document history; moved into this sec¡
          tion instead.

          Introduction restores text from the -00 revision that
          describes snooping and its goals

          IGMP flooding rules eased, allowing management option to
          broaden beyond "routers only".

          Removed a SHOULD/MAY inconsistancy between IPv4 Forwarding and
          IPv6 processing of checksums.

          IGMP Forwarding Rules: clarify text describing processing of
          non-zero reserved fields.

          Data Forwarding Rules, item 3 is changed from "MUST forward to
          all ports" to "MAY"; item 4 default changes from "MUST" to
          "SHOULD use network addresses".

          Added two sets of additional responses to the questionnaire
          and text indicating that responses don't cover all products.

          Removed (commented out) description of IPR issues: IESG is
          aware of them.

     The next revision:

          In the interest of getting this version of the draft released
          before the deadline (less than seven hours from the moment
          this paragraph is being typed), we briefly summarize some of
          the comments on the previous version that need to be included
          in the next one.  We believe that other comments have been
          addressed in this draft; please let the authors know if this
          they have either not been included or need to be corrected.

          IGMP Forwarding rules:

               Add a reference to and become consistant with the next
               revision of the IGMP proxy draft,

               In item 'b': include a description on how the switch
               determines that a Query came from the router and not
               another switch.  Is there some way to make this distinc¡
               tion beyond the source address?

               Proxy reporting: further analysis of the impact on the

Christensen, Solensky                                          [Page 12]

RFC DRAFT                                                      June 2002

               election process when using as the source address
               in membership report messages.  Also consider the case
               where the switch assumes the role of Querier when no
               routers are detected and forfeits the role as soon as one
               is announced.

               Include some discussion about how entries are to be aged
               from the list, perhaps similar to spanning tree algorithm
               for unicast MAC address processing.

          Data Forwarding rules:

               Link-local range to mention the problem is due to routing
               protocols not sending Report Messages for their respec¡
               tive multicast addresses.

               Expand discussion of non-IGMP packet forwarding for data
               that matches an IGMPv3 record.  Do snooping switches add
               intelligence to recognize SSM versus ASM groups?

          IPv6 Considerations:

               Is having MLD a subset of ICMPv6 an issue?  Should MLDv2
               be a separate protocol?

               Add reference to ICMPv6 specification for message pro¡
               cessing rules.

99..  AAuutthhoorr''ss AAddddrreesssseess

     Morten Jagd Christensen
     email: morten@jagd-christensen.com

     Frank Solensky
     Premonitia, Inc.
     1 Nanog Park
     Acton, MA  01720
     email: fsolensky@premonitia.com

Christensen, Solensky                                          [Page 13]

RFC DRAFT                                                      June 2002

                              TTaabbllee ooff CCoonntteennttss

     1. Introduction . . . . . . . . . . . . . . . . . . . . . . . .   2
     2. IGMP Snooping Requirements . . . . . . . . . . . . . . . . .   3
     2.1. Forwarding rules . . . . . . . . . . . . . . . . . . . . .   3
     2.1.1. IGMP Forwarding Rules  . . . . . . . . . . . . . . . . .   3
     2.1.2. Data Forwarding Rules  . . . . . . . . . . . . . . . . .   5
     2.2. IGMP snooping related problems . . . . . . . . . . . . . .   6
     3. IPv6 Considerations  . . . . . . . . . . . . . . . . . . . .   6
     4. Security Considerations  . . . . . . . . . . . . . . . . . .   8
     5. IGMP Questionnaire . . . . . . . . . . . . . . . . . . . . .   8
     6. References . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  11
     8. Revision History . . . . . . . . . . . . . . . . . . . . . .  11
     9. Author's Addresses . . . . . . . . . . . . . . . . . . . . .  13

Christensen, Solensky                                           [Page i]

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