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Futurewei

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This document describes a design for a lightweight decentralized multicast group address allocation protocol (named GAAP and pronounced "gap" as in "mind the gap"). The protocol requires no configuration setup and no centralized services. The protocol runs among group participants which need a unique group address to send and receive multicast packets.

The Group Address Allocation Protocol (GAAP) is a decentralized multicast protocol used by participating applications which send and receive packets to/from a multicast group. The protocol is relatively lightweight, runs with minimized messaging and state so that it can run within a library a multicast application compiles into its executable binary. Other approaches to multicast group allocation have been proposed in the past, they include mDNS , MADCAP , MASC , and IPv6 Allocation Guidelines . However, they require configuration, used on a single subnet, and are not decentralized. This document will describe the protocol operation, protocol message formats, the API definition, and how multicast applications use the API.

is an ASCII string used by applications so they can rendezvous on the same group address. The application is started using this group name parameter. Applications can use multiple group names if they have requirements to use multiple group addresses. is a non-link-local IPv4 multicast group address or an IPv6 multicast group address . is an IANA assigned non-link-local group address the GAAP protocol sends messages to. This address must not come from the GAAP multicast address block allocated by IANA. is a cryptographic hash function which takes the group name as input and produces a hash value as output. The GAAP protocol uses SHA-256 . There are 4 hashed values regarded as "acceptable" for a group name. They are calculated using the SHA-256 hash function on 1 of 4 character strings: "<group-name>", "<group-name>+1", "<group-name>+2", or "<group-name>+3". No GAAP node should run the hash on any other strings for this group name. is the output of a SHA-256 hash function where the low order 32-bits are used to produce a unique network layer multicast group address. Achieving a unique 32-bits allows layer-2 switches to not have MAC multicast address collisions when mapped from multiple network layer multicast group addresses. a network layer group address where the low-order 32-bits of one group address is the same as the low-order 32-bits of another group address. It is desirable that the the low-order 32-bits of a mapped IPv6 group address to a MAC group address not be the same so layer-2 switches do not leak packets to non-group members. This is also true for IPv4 group addresses where the low-order 23-bits must be unique. a GAAP protocol message that allocates a unique group address and claims it among other GAAP nodes on the network.

This section will describe the high-level functionality of the GAAP protocol. Each application runs the GAAP protocol by using the API defined in . An application is started with a group name. The group name is used to create a random allocated group address. A timestamp is taken when the group address is created. A Claim message, see , is sent with group name, group address, and timestamp to determine if the group address has been claimed by any other GAAP nodes. If no Claim message is sent in response, the application can start using the group address. If a Claim message is returned, a collision has occurred and the GAAP node must allocate another group address and send a Claim message for the new group address. Claim messages are sent periodically. They are sent by a single node using a delay-timer suppression mechanism similar to IGMP . See for details. GAAP nodes are not required to cache information from Claim messages. GAAP is designed to be decentralized and stateless. The nodes that participate in the GAAP protocol are responsible for allocating and claiming group addresses. No other entities are needed.

At this time, there is a single message called the Claim message with type value 1. Type value of 0 is reserved. The Claim message is sent in a UDP checksummed packet where the source port is ephemeral and chosen by the sender and the destination port is a well-known port allocated by IANA. GAAP can work behind NAT and firewall devices as long as the GAAP destination port is permitted through filters.

Claim Message Set to 0 and ignored on receipt. The number of records in this Claim message. The fixed bit pattern of 0xAAAAAAAA is required to be set by the sender. The receiver verifies the marker to be 0xAAAAAAAA. If it is not, the packet is dropped. The Marker field is used to indicate to a receiver that the packet may be encrypted. See for details on encrypting GAAP messages. a 32-bit multicast address in network byte order . If all bits are set to 0, there is no IPv4 address being allocated and claimed. a 128-bit multicast address in network byte order . If all bits are set to 0, there is no IPv6 address being allocated and claimed. Standard epoch UTC timestamp according to . A variable length group name the multicast application uses. It is in ASCII format . The string is terminated with a null character.

The GAAP API has the following API calls a multicast application will use. A multicast application imports the library before using it in its code logic. This section documents a python library.

gapp.init() is used to initialize the GAAP API with a application callback function. The callback function is called when a group address has changed (due to collision) for a group name the application allocated.

gaap.allocate() is used when the application needs a group address to send or receive on.

gaap.release() is used when an application is finished using a group address.

gaap.close() is used when an application is finished using the GAAP protocol.

When an application needs a group address it provides the GAAP API with a group name, the group name is used as input to a SHA-256 hash function . Initially, when no group address collision is detected the group name is passed as a string to the hash function and the low-order 32-bits are used for a group address. The following pseudo-code illustrates the functionality:

When the hash function is used to resolve a collision, the following pseudo-code will illustrate how 3 more attempts are used to find a unique group address:

When a group address collision is detected by 2 GAAP nodes, the node with the earliest timestamp for the group address creation wins the collision and keeps using the address. The node with a later timestamp has the responsibility to allocate a new group address to prevent the collision.

When a group address is allocated by a GAAP node it will build and send a Claim message. Included in the Claim message is the group name, group address, and timestamp. If the group address collides with other GAAP nodes already using the address, one of the nodes will send a Claim message to notify the colliding node that it needs to allocate a new group address. A collision is defined to be the same group address allocated to 2 different group names. So if a GAAP node is claiming a group address for its group name and a Claim is received for this group name with the same group address, it is not a collision. It is simply a peer group participant claiming the group address you both agree to be using. Each GAAP node will periodically send Claim messages for all group names for the applications running on the node. It will do this in a multi-record Claim message. The periodic Claim message is sent by setting a rough 1 minute timer. The timer value is set to 1 minute plus a jitter value. The jitter value is a random number in a 10% range of 1 minute (60 to 66 seconds). When the timer expires, a Claim message is sent. Receivers of a Claim message who have their timer running, reset the timer and thereby suppresses sending their own Claim message. This allows only a single GAAP node that is using the group address to keep claiming the group is still in use.

There will be network outage situations where all GAAP nodes may not receive Claim messages. During a partition, duplicate group addresses may be allocated and used by nodes on each side of the partition. During this condition, multicast nodes can operate normally and there is no conflict until the partition heals. When the partition heals, duplicate group addresses will be detected and fixed. The group address with the earliest timestamp is used to determine who keeps the collided group address. All others will have to rehash a new group address and have the applications start using the new address (meaning senders will source using the new group address and receivers will leave the collided group and join the new group).

When applications are no longer sending to a group address or not joined to a group address, they can inform the GAAP API to release the group. When this happens, the GAAP protocol stops claiming the group address in periodic messages and will not respond to a Claim for this address for a different group name. It is important for receiver applications to leave the group before releasing the group address.

It is strongly suggested that the GAAP protocol run over an encrypted multicast channel. The encryption algorithm and key management procedure is not specified in this document. The message Marker is used to indicate if the packet is sent in plaintext or ciphertext. If the Marker is not set to 0xAAAAAAAA and the receiver does not have a shared-key configured, the message MUST be dropped. An open-source GAAP implementation exists where ChaCha20 is used to encrypt GAAP messages. The implementation's key management procedure is a simple shared key that is configured with the application. At this time there is no dynamic rekeying procedure and is left for future work. Therefore, all nodes must be manually rekeyed when a node is removed from the encrypted channel. The following attack threats may exist with possible mitigation techniques: Even when an encrypted channel is used, a bad actor could be claiming a group address not derived from one of the group name inputs used for the Acceptable Group Hash List (see Definition of Terms section). Cooperating nodes should ignore such messages and not try to send Claim messages to correct the bad actor node. A bad actor could send an invalid timestamp giving it tie-breaking priority when a group address collision occurs. If the group address has been prior claimed by another node with a timestamp earlier than the invalid timestamp, cooperating nodes should put the bad actor node on a bad-actor list and ignore future messages from it. If the group name has not been claimed yet, the timestamp will be accepted only if earlier than the current time for the receiving node. A bad actor could send messages too often and is not adhering to the random delay or periodic timer procedures in this document. When this occurs, cooperating nodes should start ignoring messages from the bad actor node and not reset or cancel timers, or send triggered Claim messages

This document makes several requests for IANA. The first is to allocate a well-known UDP port number for the GAAP protocol. The second is to allocate IPv4 and IPv6 multicast addresses the GAAP protocol uses to send messages to. And the third is to allocate a multicast address block where GAAP allocated group addresses come from.

The authors would like to thank the following people for their motivation to start this draft. They include Chris Hopps, Acee Lindem, David Lamparter, Jeff Tantsura, and Nate Karsens.

Submitted February 2023. Updated spec to reflect implementation. Add Marker in message format. Add definition for the Acceptable Group Hash List. Discuss security threats and possible mitigation methods.

Initial posting November 2022.