Review: RFC 4862 - IPv6 Stateless Address Autoconfiguration

RFC 4862 specifies the steps for a host (except routers) to apply in order to auto configure its interface and generate an IPv6 address.

Why Stateless auto configuration?

• Assign a unique address to an interface.
• Hosts in small networks would not require a DHCP server or a router to obtain a unique address. Such hosts should be able to generate unique addresses in the network.
• Large networks will not require DHCP servers for address auto configuration. Hosts should be able to generate global unique addresses.
• Facilitate address renumbering on a site or subnet. A site renumber it's nodes when it switches to a new network service provider (IP prefix modification). When renumbering, old IP address will coexist with the new IP address during a period until the old IP becomes invalid.
  

How stateless auto configuration works?
We can divide the process of auto configuration into the following phases:

1. A node generate a link-local address when its interface becomes enabled. (Reboot, start time, attachment to another link etc). This link-local address is formed by concatenating the interface identifier (generated from the interface MAC address for example) to the well-know link-local prefix 0xFE80::0.
2. The node checks if the address is unique by using Duplication Address Detection (DAD) techniques. If the address is unique then it will be assigned to the interface, if not another interface identifier is required to generate a unique link-local address. Administrators can supply an alternate interface identifier. If not a manual configuration is required. In this phase, the node generates a link-local unique address.
   
3. Nodes will listen to routers advertisements holding information to generate global addresses. Solicitation messages can also be sent by nodes to routers to avoid waiting for advertisement messages. If some specific flags are set then the node can use a prefix carried in the advertisement and apply it to the generated address. (Prefix concatenated with Interface ID). Actually this prefix is usually the subnet's prefix.
4. Duplication Address Detection is required before assigning the global unique address to the interface. Some implementations applies DAD only to link local addresses and assumes it is globally unique if it passes the local test. However new techniques have been developed for privacy protection issues. An interface identifier can be generated randomly for example, then a clash with another global address might occurs [RFC 4941 - Privacy extensions for stateless address autoconfiguration].

Nodes still listens to routers advertisements mainly to reset/increase the prefix lifetime or valid time. When the prefix advertised is different from the one generated, then it is a renumbering case, the node will form a new address (new prefix, Interface ID) and adds it to the list of addresses assigned to the interface.


According to this RFC, the Duplication Address Detection is not fully reliable, it will generate a large overhead when testing a global unique IP address.
Other mechanisms to detect address collision should be pushed further.
Site renumbering is another argument to be added to the list in order to separate a host's name from its identity.
Generating the same suffix every time can expose the identity and location of a node which in some cases can be unwanted.

Link to RFC 4862

Review: 6LowPAN

Why IPv6 can’t be applied directly on LowPANs?
6LoWPAN defines IPv6 protocol over Low power Wireless Personal Area Networks.
LowPAN devices uses IEEE 802.15.4 radios. In order to understand why we
can’t use IPv6 directly on top of LowPAN devices let’s list the characteristics of such
devices [2]:

• Small packet size. 81 bytes for data packets in the networking layer. (The maximum physical layer packet is 127 bytes, consequently the maximum frame size at the media access control layer is 102 octets. 21 bytes at maximum are usedfor securing link-layer communications).
• Support for both 16-bit short or IEEE 64-bit MAC addresses.
• Low cost devices: low bandwidth with data rates of (250 kbps, 40 kbps, 20 kbps) for (2.4 GHz, 915 MHz, 868 MHz). Low power typically battery dependent. Low processing and storage capabilities (8KB RAM, limited buffering, etc).
• Topologies include star and mesh operation.
  
• Large number of devices expected to be deployed.

The main problems with IP for LowPANs are the following:
The term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest PDU that a given layer of a communications protocol can pass to other layers. A higher MTU brings greater efficiency because each packet carries more user data than protocol overheads. Large packets can occupy a slow link for some time, causing greater delays to following packets and increasing lag and minimum latency.
IPv6 protocol uses 128 bits IP address and a header of 40 bytes long. The MTU is at least 1280 bytes in order to maximize the efficiency of the transmission ((useful data)/(overhead data)).
LowPAN devices supports small packets with 81 bytes available for the networking layer and above. Using IPv6 on IEEE 802.15.4 leaves only 41 bytes for transport and applications layers. This is obviously not enough for data exchange, packet fragmentation and reassembly is needed but will use even more bytes. An IP header compression is needed.
Since large number of devices will be deployed, address auto configuration is attractive because it will reduce the overhead between devices. There is a need for a method to generate and assign an Identifier from the EUI-64 bits to a LowPAN device.
Routing protocols in mesh and star networks must be adapted to a small overhead. LowPAN devices have limited resources (memory, bandwidth, CPU, energy), processing 128 bits addresses and large headers will decrease the system’s efficiency and increase data treatment latency.

Why use IP based protocol in LowPANs?
The benefits of using IP based networks are the following [1][2]:

• The hierarchy of naming and addressing in IP networks which simplify the connectivity model.
• IP-based technologies already exist, are well-known, and proven to be working.
  
• IP networking technology is specified in open and freely available specifications.
  
• Use existing tools for diagnostics, management, and debugging of IP networks instead of designing and developing new ones.
• IP-based devices can be connected easily to other IP-based networks, without the need for intermediate entities like translation gateways or proxies.
  

Addressing Modes
IEEE 802.15.4 uses IEEE 64 bit and 16 bit addresses. Short addresses are assigned by a PAN coordinator during an event which means that validity and uniqueness of such addresses are limited by the lifetime of the association, failure of the coordinator, etc [3].
For short addresses (16 bits), a pseudo 48 bit address is formed by concatenating 16 zero bits to the 16 bit PAN ID (Personal Area Network). If no PAN ID assigned then 16 bits of zeros are used and the resulting concatenation is a 32 bits address. These 32 bits are concatenated with the short address in order to obtain a 48 bit address.
From a 48 bit address, a 64 bit interface identifier is formed as in [5] and [6] by adding 0xFFFE in the middle of the 48 bits (24 bits,0xFFFE,24bits). IPv6 local addresses are formed using the 64 interface identifier by appending the prefix FE80::/64. (0xFE80::EUI-64) or global addresses based on information advertised by routers [7].
Routers are the link between IP and LowPAN networks, those routers will handle address transition.

Header types
6LowPAN proposes specific header encoding and compression mechanisms to adapt IPv6 into IEEE 802.15.4 frames. Separating headers will reduce overhead. If a device is sending short packets directly to another node it does not pay for extra fields such as Mesh networking or fragmentation. The overhead reduction is an energy saving.
The header types are the following:

• The Dispatch Header (1 byte), define the type of header to follow. The dispatchheader is identified by the first two bits set to either 00 (non-6LowPAN frames)or 01. The remaining 6 bits indicate if the following field is an uncompressedIPv6 header or an HC1 header (IPv6 compressed header). To accomplish compression[1] the protocol uses a combination of the following facts: the low order64 bits of an IPv6 address (the link local address) can be the device’s MAC address,the 802.15.4 frame carries these MAC addresses, a number of the fields inthe IPv6 header are static.Combining all of these features allows the protocol to compress the standard 40 byte IPv6 header down to just 2 bytes (including the HC1 Header byte) for most intra-PAN unicast communication where source and destination addresses are deleted and generated from Link level frames (IEEE 802.15.4). All of the rest of the fields can be reconstituted without any state information at any of the receiving or intermediate nodes. Additionally by assigning the link local address to the device’s MAC address 6lowpan can use Stateless Address Auto configuration(Zero-conf) and eliminates the need to infrastructure servers like DHCP servers.
• The Mesh Header (4 bytes) is used to encode the hop limit and the source and destination of the packet. It includes two single bit fields to indicate if the originating
  or final address is a short or long address. The “hops left” field is a 4 bit
  field used to limit the number of intermediate hops between the source and destination.
  The value of 0xF was reserved to indicate that an extra byte is included
  allowing for network depths of up to 255 hops.
• The Fragmentation Header (4 bytes for the first fragment and 5 bytes for subsequent
  fragments) supports the fragmentation and reassembly of frames larger
  than the size of the 802.15.4 frame.

Today there is at least 6 implementations of 6LowPAN on multiple 802.15.4 radio platforms. The working group is still continuing to investigate the areas of neighbor discovery: IPv6 network prefix, local routers and other network configurations parameters.
The area of service discovery to locate other sensors and controllers and higher layer services.
Is it a good choice to push further IP based protocols to other areas such as LowPAN while efforts increases to redesign the Internet?
Should we inherit the limitation of IP which merges between naming and addressing to LowPANs?
What about cross-layering violation while compressing Ipv6 headers and regenerating source and destination address for Link level frames? According to the OSI scheme, Layers should be independent and unable to understand other layer’s data.
Even though in some cases header compression is efficient (with layer violation) other compressions remains unoptimized in LowPAN. Additional work should push routing protocols and reduce overhead of such protocols.

References:
1. The 6LoWPAN Architecture, Geoff Mulligan and 6LoWPAN Working Group, EmNets '07: Proceedings of the 4th workshop on Embedded networked sensors
2. RFC 4919: IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals.
3. RFC4944 - Transmission of IPv6 Packets over IEEE 802.15.4 Networks
4. 6LoWPAN: Incorporating IEEE 802.15.4 into the IP architecture Internet Protocol
for Smart Objects (IPSO) alliance.
5. RFC 2464: Transmission of IPv6 Packets over Ethernet Networks
6. http://technet.microsoft.com/en-us/library/cc736439(WS.10).aspx
7. RFC 4862: IPv6 Stateless Address Auto configuration

Review: SENS - a Scalable and Expressive Naming System using CAN Routing Algorithm

This paper proposes a Scalable and Expressive Naming System (SENS) based on CAN routing algorithm. SENS is a descriptive naming scheme that uses a tuple of attribute/value pairs to name each resource. A resource such a computer is named as ( String OS= "Linux", string CPU-name = "Pentium 4", etc). Those informations are stored at a large number of name servers (NS).

Their design claims to achieve scalable and efficient resource information distribution and retrieval. SENS handles exact (MEMORY = 512 MB) and multi-attribute range queries (MEMORY > 512 MB) with small overhead as well as load balancing.

DESIGN OF SENS:
Mapping resource names to resource IDs:
A resource ID is considered as a set of d coordinates of a point in the d-dimensional resource ID space. (In the example below d = 6). A resource name is mapped to a resource ID by assigning the hash value of each attribute/value (a/v) pairs of the resource name to a coordinate value of the resource ID. Name servers are responsible for resource ID sets just like in the CAN system.
Ha is a function that uniformly hashes every attribute from 1 to d and Hv hashes every attribute value in a [1, 2^(m-1)] interval where m is the maximum size of coordinate value in bits.












If multiple attributes in a resource name are hashed to the same value Ha (attr i) = Ha (attr j) then the corresponding attribute values will be mapped to multiple coordinate values in the same dimension. This means that resource names are mapped to multiple resource IDs which are distributed on NSs.


Their mapping scheme is not injective, several resource names can be mapped to the same resource ID. Consequently resource ID is not a unique identifier of resource name, in order to identify a resource, the resource_ID (resulting form Hash) and its name are required to uniquely identify a resource.

In the case of numerical attribute values, a locality preserving hashing function is used. Such hashing function is defined as if (val1 > val2) then (Hval1 > Hval2). the main purpose behind using a locality preserving hashing function is to deal with range queries, in fact it ensures that a resource ID will be in a interval of resource IDs between a min and max value. By doing this they limit the number of NSs responsible for a query range.

If the attribute/value pairs number is lower than d then the resource ID is filled with zeros.
However when the attribute/value pairs number is higher than d then the set of attributes should be divided to multiple sets of attributes which corresponds to multiple resource names. (this aspect of fragmentation is not treated in this paper).

Resource Information Distribution:
Since their scheme is CAN based, zones are assigned to NS, consequently each NS manages resource information according to the resource ID.
In the case of a resource name mapping to multiple resource IDs. If the NS is responsible for several resource IDs of the same resource, only one copy in maintained at the NS. When resource IDs belongs to different zones i.e. different NS, they use a multicast routing algorithm based on Spanning Binomial Trees. Their algorithm sends minimum amount of messages to deliver information to a NS.
Resource IDs corresponding to a resource name construct a hypercube in the resource ID space according to this article (Optimum Broadcasting and Personalized Communication in the Hypercube) . (They don't explain how the hypercube is built, so further reviews will detail the construction algorithm of hypercubes).


The registration message containing information of the resource is first delivered to the NS responsible for the resource ID created from the lowest values of each resource IDs coordinates. (In this example root node is (0.0.0)). This NS becomes the root node and forwards the message to its descendants according to the tree. Those descendants also will forward the registration message to their descendants according to the tree. etc

Query resolution:
SENS supports:

• Exact queries: A query host sends a message to a NS which will map the query resource name to resource IDs and select the nearest destination resource ID. The message arrives to destination using the CAN routing algorithm. The NS responsible for the resource ID will lookup its database to find the queried information and send it back to the initial NS (the one first queried by the host).

• Range queries, in the case of a range query it will be limited by the hash values of the upper and lower limit of the queried value ranges in each dimension. When a host sends a range query message to a NS, the latter will map the query range to a range query segment in the resource ID space. A query message will be broadcasted to all NSs whose zones overlap the segment. They propose a broadcasting algorithm based on the SBT and hypercube in order to reduce the number of messages broadcasted. (Further details will be added once I read the article that treats the hypercube and SBT formation).

Related works:
This article propose a more expressive naming scheme than DNS which offers a value-limited resource name space without the possibility to realize the range query.
Other systems such as the Intentional naming system uses a descriptive name space based also on attribute/value pairs. The message routing for a name query is realized by look up of the query name on forwarding tables. Main limitation of such systems is scalability since the forwarding tables will grow with the number of resource names.
DHT-based routing protocols like Chord, CAN achieve a scalable and efficient lookup by hashing a resource name to a DHT key. Range query is the limitation of such systems, a query may spread to the hole DHT key space.
Other systems proposes range queries like MAAN where nodes are responsible for attribute values of an attribute in the query range. The node responsible for the attribute/value pairs (String OS = "Linux") must keeps information related to Linux OS. The main limitation is load balancing since popular attribute/value pairs may appear in resource names with high probability.

SENS is naming system capable of handling resource information with exact and multi-attribute range queries. They propose a multicast/broadcast algorithm to deliver/retrieve information.
However some issues remains unclear:
Several resource names might be mapped to same resource ID and the unique identification is done by resource ID and resource name. Is it enough to uniquely identify a resource? Is it possible to map 2 different resource names having same attribute values in common into the same resource ID? In 2005 Xiaoyun Wang and Hongbo Yu achieved collision on purpose in the MD5 hashing algorithm (2 different values were hashed to same ID).
Is SENS trying to merge between search engines and DNS-like systems?
Do we really need to merge between such systems? is it faster ? More reliable? More scalable ?
Is it a good approach when a resource is mapped to many resource IDs? Having multiple IDs will accelerate routing and finding the nearest ID? What if we mapped essential and important data to multiple IDs and restrained non important data to a single resource ID? How to design such a ranking system to classify data according to it's importance?
What if I updated my RAM from 512 MB to 1024 MB, how SENS manages such updates?
Is space partition efficient? when a node joins the system, a lot of information is handled to the new joining node ? Consequently huge messages are transmitted to the new node.

Link to the article