INTERNET DRAFT S. Bandyopadhyay
draft-shyam-mshn-ipv6-11.txt April 22, 2013
Intended status: Proposed Standard
Expires: October 22, 2013
Mesh Structured Hierarchical Networking and IPv6
draft-shyam-mshn-ipv6-11.txt
Abstract
This document tries to address an approach for reorganization of
entire network in a large address space. It describes how a three-
tier mesh structured hierarchy can be established based on
fragmenting the entire space into some regions and sub regions inside
each of them. It addresses issues which could be relevant to this
architecture in the context of IPv6. This document also tries to come
out with an approach how IP switch based network can perform as good
as ATM network for the processing of real time traffic.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 18, 2013.
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1. Introduction
Transition from IPv4 to IPv6 is in the process. Work has been done to
upgrade individual nodes (workstations) from IPv4 to IPv6. Also,
there are established documents to make router/switches to work to
support IPv4 as well as IPv6 packets simultaneously in order to make
the transition possible [1]. The CIDR[2] based hierarchical
architecture in the existing 32-bit system is supposed to be
continued in IPv6 too with a large address space. There are
documents/concerns over BGP table entries to become too large in the
existing system [3]. There are proposals to upgrade Autonomous System
number to 32-bit from 16-bit to support the demand at the same time
[4]. The challenge relies on how to make the transition smooth from
IPv4 to a real IP world with least changes possible. ATM network
performs faster than the network with IP switches. The difference
becomes more prominent for real time applications. Whereas they have
disadvantages as far as bandwidth usages is concerned compared to the
IP-switch based network. This document tries to address approaches
for IP-switch based network to process real-time applications as fast
as ATM network also a mesh structured hierarchical network with flat
address space for routing convenience. It provides a solution for
site multihoming of stub networks.
2. A Three-tier mesh structured hierarchical network
Existing system is in work with Autonomous System (AS) and inter-AS
layer with the approach of CIDR. In order to meet the need within the
32-bit address space, Autonomous Systems of various sizes maintain
CIDR based hierarchical architecture. With the help of NAT [5], a
stub network can maintain an user ID space as large as a class A
network and can meet its useful need to communicate with the rest of
the world with very few real IP addresses. With the combination of
CIDR and NAT applied in the entire space, most of the part of 32-bit
address space gets effectively used as network ID. This is how,
16-bit 'Autonomous System Number' is realized as insufficient in
order to meet the need of growing customers. If the same gets
continued with a larger network ID, load in the switches will become
too high.
As Autonomous Systems of various sizes are supported, Autonomous
Systems and the nodes inside the Autonomous Systems can be viewed as
graphically lying in the same plane within the address apace. If
network can be viewed as lying in different planes, routing issues
can be made simpler. If network is designed with a fixed length of
prefix for the Autonomous System everywhere, routing information for
the rest will get confined with the other part of the network prefix.
Which means the maximum size of AS gets assigned to all irrespective
of their actual sizes. This can be made possible with the advantage
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of using a large address space and dividing it into number of regions
of fixed sizes inside it. Thus entire network can be viewed as a
network of inter-AS layer nodes. Each node in the inter-AS layer can
act either only as a router in the inter-AS layer or as a router in
the inter-AS layer with an Autonomous System attached to it with a
single point of attachment or as an Autonomous System with multiple
Autonomous System border routers (ASBR) appearing like a mesh. Thus
two tier mesh structured hierarchy gets established between AS layer
and inter-AS layer with each AS having a fixed length of prefix.
Based on the definition of Autonomous System, it is a small area
within the entire network that maintains its own independent identity
that communicates with the rest of the world through some specific
border routers. In the similar manner, if a larger area (say region
or state) can be considered as network of Autonomous Systems, that
can maintain its own identity by communicating with the rest of the
world through some border routers (say, state border router), mesh
structured hierarchy can be established within the inter-AS layer.
The inter-AS layer will be split into inter-AS-top and inter-AS-
bottom. To maintain this hierarchy, each node of inter-AS-top needs
to have multiple regional or state border routers (say, SBR) through
which each one will communicate with the rest of the world in the
similar manner an Autonomous System maintains ASBR. Thus, entire
network will appear as a network of nodes of inter-AS-top layer. To
maintain hierarchy, each node of the inter-AS-top needs to have a
fixed length of prefix. i.e. each node of the inter-AS top will be
assigned a maximum (fixed) number of nodes of Autonomous Systems.
Thus, with three-tier mesh structured hierarchy in the network layer,
network ID can be viewed as A.B.C. If pA, pB and pC be the prefix
lengths of inter-AS-top, inter-AS-bottom and AS layers respectively,
there will be 2^pA nodes at the topmost layer, 2^pB at the inter-AS-
bottom layer and 2^pC nodes at the AS layer. Thus the entire space
gets divided into a fixed number of regions and each region gets
divided into fixed number of sub regions. This division is supposed
to be made based on geography, population density and their demands
and related factors.
Let nMaxInterASTopNodes be the possible maximum number of nodes
assigned at the top most layer and nMaxInterASBottomNodes be that at
the inter-AS-bottom layer and nMaxASNodes at the AS layer. Where
nMaxInterASTopNodes <= 2^pA and nMaxInterASBottomNodes <= 2^pB and
nMaxASNodes <= 2^pC.
2.1. Route propagation
With hierarchy established, routing information that gets established
inside a node of inter-AS-top, does not need to be propagated to
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another node of inter-AS-top. Entire routing information of inter-AS-
top layer needs to be propagated to inter-AS-bottom layer. So, each
router of inter-AS layer will have two tables of information, one for
the inter-AS-top and another for the inter-AS-bottom of the inter-AS-
top node that it belongs to. BGP (with little modification) will work
very well with a trick applied at the SBRs. Each SBR will not
propagate the routing information of inter-AS-bottom layer of its
domain to another SBR of neighboring domain. i.e. SBR of one top
layer node will propagate routing information only of inter-AS-top
layer to SBR of another top layer node. Inside a node of inter-AS-
top, routing information of inter-AS-top and inter-AS-bottom need to
be propagated from one ASBR to another neighboring ASBR. Inside a top
layer node A, routing information of another top layer node B will
have two parts; one for the list of SBRs through which a packet will
traverse from top layer node A to B and another for the list of ASBRs
through which the packet will traverse from one AS to another inside
A. In terms of BGP, AS_PATH attribute will be split into two parts;
one for the information of the top layer and another for the bottom
layer. Within the same node A routing information of one AS to
another AS will not have any top layer information. i.e. the top
layer information will be set to as NULL.
Similarly, each node of the AS layer will have three tables of
routing entries. One for the inter-AS-top, one for the inter-AS-
bottom and another for the routing information inside the Autonomous
System itself.
With traditional CIDR based hierarchy, a node of higher prefix can be
divided into number of nodes with lower prefixes. Each divided node
can further be subdivided with nodes of further lower prefixes. This
process can be continued till no further division is possible. The
point worth noting is at each point the designer of the network has
to preconceive the future expansion of the network with the concept
in the mind that the resource can not be exhausted at any point of
time. This phenomenon leads the designer to allocate resources much
higher than whatever is needed which leads to a space of unused
address space and the concept of H-D (host-density) ratio comes into
play. The problem gets aggravated once resource gets exhausted by any
chance. e.g. a node of prefix /16 can be divided with a number of
nodes of prefixes /24. If any one of the nodes /24 gets exhausted,
resources of other nodes of prefixes /24 can not be used even if they
are available.
Introduction of hierarchy at the inter-AS layer reduces the size of
the routing table substantially. With the availability of hardware
resources if flat address space is maintained at each layer, problems
related to CIDR can be avoided. With flat address space, no
hierarchical relationship needs to be established between any two
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nodes in the same layer. So, all the nodes inside each layer can be
used till they get exhausted. With flat address space (i.e. without
prefix reduction), BGP tables will have nMaxInterASTopNodes +
nMaxInterASBottomNodes entries.
IGP like OSPF has got provision to divide AS into smaller areas. OSPF
hides the topology of an area from the rest of the Autonomous System.
This information hiding enables a significant reduction in routing
traffic. With the support of subnetting, OSPF attaches an IP address
mask to indicate a range of IP addresses being described by that
particular route. With this approach it reduces the size of the
routing traffic instead of describing all the nodes inside it, but
introduces another level of hierarchy. If subnetting concept can be
avoided from the AS layer(with the additional overhead of computation
inside the SPF tree), each area can be configured from a free pool of
addresses based on its requirement dynamically. So, an AS can be
divided into number of areas of heterogeneous sizes with the nodes
from a free pool of address space.
Similarly, the concept of area can be introduced in the inter-AS-
bottom layer the way it works in OSPF. The area border routers in the
inter-AS-bottom layer have to behave exactly in the similar manner
the way an ABR behaves in OSPF. i.e. an area border router will hide
the topology inside an area to the rest of the world and will
distribute the collected information inside the area to the rest. It
will distribute the collected routing information from outside to the
nodes inside as well. In order to implement this, protocol running in
the inter-AS layer (say BGP) will have to introduce a 'cost' factor.
This cost factor can be interpreted as the cost of propagation of a
packet from one AS to another. The protocols running inside AS layer
(RIP/OSPF, etc) will have to the supply the cost information for a
packet to travel from one ASBR to another. All the protocols must
behave in unison for supplying this information. The cost factor is
needed for a remote node while sending a packet to a node inside an
area while more than one area border routers are equidistant from
that remote node. Thus inter-AS-bottom layer (i.e. one inter-AS-top
level node) can be divided into number of areas of heterogeneous
sizes with nodes of AS from a free pool of address space. BGP adopts
a technique called route aggregation. Along with route aggregation it
reduces routing information within a message. In the similar manner,
introduction of area inside inter-AS-bottom layer will not only
reduce the complexity of the protocol, but will reduce the size of a
BGP packet substantially.
With this architecture, each node(router) inside an AS is represented
as A.B.C. Each node may or may not be attached with a network which
acts as a leaf node (i.e. a network will not act as a transit). In
order to make use of user-id space properly and to support customer
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networks of heterogeneous sizes, the user-ID space needs to be
divided as subnet-ID and user-ID. Profoundly, a VLSM (variable length
subnet mask) type of approach has to be adopted at each node of an
AS. So, each node of the AS layer will act as the root of a tree
whose leaves are independent small customer networks which will act
as stub. As the routing information of inter-AS layer as well as AS
layer need not be passed inside any node of the VLSM tree, each
router inside the tree should maintain default route for any address
outside of its network. With this approach, load on each router of
the service providers will become negligible. Protocols that supports
VLSM with MPLS/VPN has to be implemented inside the tree (inside the
VLSM tree, all the physical ports of a switch have to be configured
with the subnet mask. So, mere MPLS on top of static routing table
should do the rest).
The fundamental assumptions based on which this architecture lies can
be summarized as follows:
i) Entire network can be viewed as a network of regions or states
where each region or state can have its own identity by communicating
with the rest of the world through some state border routers. Each
region or state is a network of Autonomous Systems. Each region as
well as each Autonomous System inside them will have a fixed
(maximum) length of prefix.
ii) Availability of hardware resources is such that flat address
space can be maintained at the inter-AS layer.
Introduction of mesh-structured hierarchy at the inter-AS layer will
have several advantages:
o Load at each router will get reduced substantially.
o Concept of CIDR style approach and complexity related to
prefix reduction can be easily avoided.
o Full mesh hierarchy will make traffic evenly distributed.
o Physical cable connection can be optimized.
o Administrative issues will become easier.
2.2. Determination of prefix lengths
With this architecture, IP address can be described as A.B.C.D where
the D part represents the user id. Each router in the inter-AS layer
will have two tables of information, one for the inter-AS-top and
another for the inter-AS-bottom of the inter-AS-top node that it
belongs to. Whereas, each node of the AS layer will have three tables
of routing entries; one for the inter-AS-top, one for the inter-AS-
bottom and another for the routing information inside the Autonomous
System itself. In the worst case. a node inside an AS needs to
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maintain nMaxInterASTopNodes + nMaxInterASBottomNodes + nMaxASNodes
entries in its routing table.
The dynamic nature of allocating an area from a free pool of address
space is more frequent at the AS layer than at the inter-AS-bottom
layer. As OSPF supports all the features needed, it can be considered
as default choice in the AS layer. Existing implementation of OSPF
(Version 2) supports subnetting, by which an entire area can be
represented as a combination of network address and subnet mask. With
this approach, entire routing table gets reduced substantially. With
the removal of subnetting, all the nodes inside an area will have an
entry inside the routing table (OSPF Version 1). So the deterministic
factor is what is the maximum number of nodes inside an AS OSPF can
support once subnetting support gets removed. So the prefix length of
AS layer will be determined by this factor of OSPF.
With the introduction of hierarchy in the inter-AS layer, number of
entries in the BGP routing table will get reduced substantially. Even
if pA and pB both are selected as 16, number of routing entries come
within the admissible range of existing BGP protocol. But, it is the
responsibility of IANA to come out with a scheme how
nMaxInterASTopNodes and nMaxInterASBottomNodes are to be selected.
Each top level node will have nMaxInterASBottomNodes nodes. It will
be a waste of address space if each country gets assigned a top level
nodes (e.g. china has got a population of 1,306,313,800 people where
as Vatican City has got only 920 according to a census of 2006). So a
moderate value of nMaxInterASBottomNodes is desirable, with which
larger countries will have a number of top level nodes. e.g. each
state of USA can be assigned a top level node. With the introduction
of area in the inter-AS-bottom layer, each top level node can be
divided into number of areas of heterogeneous sizes. So, a group of
neighboring countries with less population can share the address
space of a top level node. Similarly, user-id space has to be decided
based on the largest area VLSM tree should be spanned through. All
these issues are completely geo political and have to be decided by
IANA.
2.2.1. A pseudo optimal distribution of prefixes in a 64bit architecture
In order to have optimal use of cable connections, length of the VLSM
tree is expected to be as short as possible. Also any single
organization may prefer to have its user id space to be under the
same network id. So, a 16bit user-id may become insufficient for
places like large university campus, where as 32bit will become too
large. Hence, 24bit user-id will be a moderate one which is the class
A address space in ipv4 (also used as the space for private IP). As
published in 1998 [8], OSPF can support an area with 1600 routers and
30K external LSAs. So, 11 bits are needed to support this space. With
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the assumption that OSPF can support much more address space with the
advancement of hardware technology as well as to keep the space open
for future expansions, 12 bits are assigned for the AS layer. 16 bits
are assigned for the inter-AS-bottom layer. So, if on the average,
16bit equivalent space gets used within the user-id space and 8bit
equivalent nodes gets used inside an AS (16% of 1600), for a top
level node (with 16bit equivalent AS nodes), it will generate 2^40 IP
addresses, which will give 8629 IP addresses per person in Japan
(with a population of 127417200; Japan is at the 10th position from
the top in the population list of the world). So, even if all the
countries with population less than or equal to Japan are assigned a
top level node and all the provinces/states of countries with larger
population are assigned a top level node each, total number of nodes
will come well under 1024. If a number of neighboring countries with
lesser population shares a top level node, total number of top level
nodes will come down further. This suggests that 62 bit equivalent
(10(pA)+16(pB)+12(pC)+24(user-id)) space will be good enough for
unicast addresses. This distribution expects OSPF to support 65K
(64K+1K) external LSAs.
64bit address space may be divided into two 63bit blocks as follows:
i. Global unicast addresses with the most significant bit set to 0.
In order to separate out router address space from the host computers
of customer networks, routers may be assigned a prefix 01 whereas the
host computers will have prefix 00. With three-tier hierarchy,
network ID is represented as A.B.C. Any router inside the VLSM tree
including the root will have an address 01A.B.C.router-id. Where as
a host interface inside a customer network will be represented as
00A.B.C.uid.
As the number of nodes representing routers in the provider network
will be way too less than the user-id space for the customer
networks, in order to keep more space for unicast addresses of
customer networks as well as to keep the option open for future
expansion, entire 63 bit address space with the MSB set to 0 has been
assigned to customer networks for unicast addresses. So, the
distribution will look like 10(pA)+17(pB)+12(pC)+24(user-id). Router
address space will be assigned from the address space with the MSB
set to 1.
ii. Address space with the MSB set to 1 will be distributed within
the rest. This distribution will be based on the requirements and
the work that have already been done in connection to IPv6 with the
additional two requirements:
a) Router address space: Any node in the router address space will be
designated with a prefix followed by A.B.C.router-id. The prefix will
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be determined based on the distribution of the 63 bit address space.
b) Provider independent address space: This space will be used for
the customers who would like to retain their number even after
changing their providers. Each of these addresses has to be mapped
with an address from the global unicast address space. Customers who
would like to have mobility support, the mapped address can be
considered as the "Home Address" of the mobile node as defined in the
specification of "IP Mobility Support"[9].
2.2.2. Whether to go for a two-tier or three-tier hierarchy
Establishment of hierarchy in the inter-AS layer reduces the size of
BGP entries to a great extent, but leads to an improper use of
address space due to geo-political reason. If hierarchy in the inter-
AS space gets removed, entire 26bit (10+16) space will be available
for a single layer and use of inter-AS space will be true to its
sense, but will increase external LSA (and/or number of entries in
the BGP table) dramatically. So, it depends on to what extent OSPF
can support external LSAs. BGP expects the packet length to be
limited to 4096 bytes. BGP manages to make it work with this
limitation with the concept of prefix reduction in the CIDR based
environment. As the number of inter-AS nodes increases, BGP has to
change this limit in order to make it work in flat address space. The
alternate will be to divide the inter-AS space into number of areas
as defined in section 2.1. The area border routers will advertise the
aggregated information to the rest of the world. BGP may have to
incorporate both the options at the same time. As the number of
nodes in the inter-AS layer increases, in order to reduce the number
of entries in the routing table, inter-AS space has to be split into
two separate planes. So, two-tier hierarchy can be considered as an
interim state to go for three-tier hierarchy. If it so happen that
current available data is good enough to support the present need, it
will be worth to look for to what extent it can support in the
future. Assignment of inter-AS nodes in two-tier hierarchy should be
based on the geographical distribution as if it is part of three-tier
hierarchy. Otherwise, introduction of three-tier hierarchy in the
future will become another difficult task to go through. Based on the
report of year 2011, BGP supports ~400,000 entries in the routing
table. With this growing trend, BGP may have to change the limit of
packet length even in a CIDR based environment. With the introduction
of two-tier hierarchy, number of entries in the routing table will
come down drastically and with the three-tier approach, it will come
down further.
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2.3. Issues related to Satellite communications
Establishment of hierarchy in the inter-AS layer expects the only way
any two autonomous systems in two different top level nodes
communicate is through their SBRs. If two autonomous systems inside
the same top level node communicate through satellite, it will be
considered as a direct link between them. Whenever autonomous system
'ASa' of top level node 'A' communicates with autonomous system 'ASb'
of top level node 'B' through satellite, they have to go through
their state border routers. i.e. satellite port inside 'A' that
communicates with a satellite port inside 'B' will be considered as
state border router. If multiple such ports exists inside node 'A',
all of them will be equidistant from any port inside 'B'. Which
expects any satellite port inside 'B' to have prior knowledge of list
of autonomous systems that will be under the purview of any port
inside 'A'. So, all the satellite ports of 'A' have to exchange such
group of information with all the satellite ports of 'B' and vice
versa. These group of autonomous systems can be considered as a
cluster of autonomous systems inside an area of a top level node. If
number of such ports is small, some heuristics can be applied while
assigning AS numbers in order to reduce the processing time during
the circuit establishment phase. It will become difficult to
maintain such heuristics once the number of such ports becomes large.
So, in case of satellite communication, the advantage of establishing
hierarchy inside inter-AS layer diminishes as the number of satellite
ports increases. If any private corporate maintains its own satellite
channel to communicate between its offices at distant locations, all
of these offices are going to be considered as under the user-id
space of its network. Service providers that provide satellite
services to the end-site customers, can operate in the usual manner
as they will provide connection to customer networks which will act
as stub.
2.4. Solution for site multihoming
With mesh structured hierarchical networking, customer networks
appear as the leaf nodes of VLSM tree (i.e. they do not play the role
of a transit network). Whenever a customer network gets service from
more than one service provider, the customer network can be viewed as
having multiple source-id (user-id) space. Each of these IP domain
gets connected to different service providers through different
routers. So each interface of customer network will have IP addresses
as many service providers it is connected with. If it maintains
private IP space it will have an additional one. So, the number of
routing entries in the routing table will (roughly) become a multiple
of IP domains it supports. Communication between any two hosts within
the customer network will follow the traditional routing mechanism.
In order to provide multihoming services it is needed that a host
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computer always forwards packets to the router associated to the same
IP domain while communicating to someone in the outside world. i.e.
if a host computer H receives an IP address A1.B1.C1.u1 and
A2.B2.C2.u2 from two service providers P1 and P2 which are connected
through routers R1 and R2 respectively, host H has to forward a
packet to R1 (or R2) while using its IP address as A1.B1.C1.u1 (or
A2.B2.C2.u2) in order to send packets to the outside world. So, a
host computer as well as the intermediate routers have to use default
routing based on the source domain of the source address in the IP
header.
In order to achieve this, host computers as well as the intermediate
routers need to have information related to its IP domain (net
address/net mask) and the associated default router for all of its IP
domains. They need to have a route entry per IP domain for all of its
default routers. These information should be uploaded at the system
start up time. As each interface is going to have multiple IP
addresses, hosts need to have a provision to select its default IP
domain. Users can select this option based on their need dynamically.
If no source address has been specified by an application, source
address has to be selected based on the outgoing interface and the
'default IP domain' as selected by the user.
UDP based servers that need to support multiple clients
simultaneously need to respond to a client request with the same
source address that the client had specified as the destination
address [10]. In order to satisfy this, system needs to introduce two
system calls along with the existing system calls (i.e. read, write,
send, sendto, recv, recvfrom)
sendwithsrcaddr /* send a response specifying the source address */
recvwithdestaddr /* receive data with destination address as
specified by client */
If no source address is specified by an UDP client, source address
gets selected based on the outgoing interface. As users get provision
to select 'default IP domain' dynamically, source address will get
picked up based on the status of the 'default IP domain'. As client
applications need to use the same source address once selected, it
may be better to maintain a cache for received IP header with the
socket layer. So, if the destination address of the outgoing packet
matches with the source address of the packet received, the source
address has to be selected as the destination address of the packet
received. The best approach will be to introduce another system call
to get the source address based on the destination address.
getsrcaddr(remote IP addr) --> local IP addr[10];
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The client UDP application should bind this address before talking to
the server.
Routing of IP packets (in the ip_output module of the host and in the
ip_forwarding module of the intermediate routers) need to be modified
in the following manner.
If destination address of the IP header falls within any one of its
IP domains, usual routing mechanism has to be followed. The only
place it needs to take care is to select the source address based on
the 'default IP domain' of the outgoing interface if no source
address is specified by the application layer.
If the destination address falls outside of its IP domains, packets
have to be forwarded to any of the default routers. The outgoing
interface has to be selected based on the route look up of the
default router from the routing table.
Following changes are expected with the source code of Net/3.
Introduce ip_domain structure and some parameters as follows:
struct ip_domain {
struct in_addr net_addr;
struct in_addr net_mask;
struct in_addr def_router;
};
#define MAX_IP_DOMAINS 16
short num_ipdomains;
struct ip_domain *ipdomain[MAX_IP_DOMAINS];
Upload IP domain information for all of its IP domains during system
start up (i.e. when the shell script /etc/netstart gets executed;
Introduce an ioctl command (or better to use sysctl as they are going
to be considered as system wide parameters))
If customer network maintains private IP domain (along with the user-
id space provided by the service providers) and expects its
communication to be confined within its own space, def_router field
should be set as NULL.
Introduce another sysctl routine to get the default IP domain which
should be configurable by the users dynamically.
Add a route entry for all the routers connecting to the service
providers during system start up (i.e. when /etc/netstart gets
executed).
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Add an entry in the inpcb structure to restore IP header of the last
packet received.
struct ip inp_pkt_rcvd; /* cached received packet info */
Execute the following steps in the 'ip_output' routine of the IP
stack before it calls 'rtalloc' for route look up.
If the destination address of the IP packet falls outside of its
IP domains {
If source address has been specified,
(i.e. ip->ip_src.s_addr != INADDR_ANY) {
get the def router address based on the source id space it
belongs to.
}
else {
if (ip->ip_dst.s_addr == inp->inp_pkt_rcvd.ip_src.s_addr) {
ip->ip_src.s_addr = inp->inp_pkt_rcvd.ip_dst.s_addr;
get the default router address based on the source IP
domain it belongs to.
}
else {
get default router based on the selected
'default IP domain'
}
}
use 'rtalloc' to get the next hop address for the def router.
if IP address has not been specified, select 'ia', the outgoing
interface and select the IP address based on default IP domain.
Forward the packet to the next hop.
}
else { /* i.e. destination address is inside its IP domains */
follow the usual procedure to forward the packet.
If source address has not been specified, select source address
based on the outgoing interface and the selected
'default IP domain'.
}
In Net/3 the 'ip_forwarding' routine calls 'ip_output', so it should
be left as it is.
udp_input and rip_input routines have to be updated to restore the
header of the packet received.
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2.4.1. Multihoming and IP Mobility
If a mobile node gets a co-located care-of IP address from its
current location[9], usually it selects its address based on its
'home address' while communicating to the correspondent node. As the
multihoming aspect for outgoing packets expect the source domain to
be the deciding factor for packet forwarding, the transport layer of
the mobile node should use IP over IP while forwarding packets. The
inner ip header should be as usual based on the source address as the
home address, the outer ip header should use source address as the
co-located care-of address. If the correspondent node is also mobile,
packets towards the correspondent node will reach the home agent of
the correspondent node. Home agent of the correspondent node should
pop out the outer IP header and replace it with the header to forward
the packets to its final destination in order to avoid further
stacking of IP header. If it so happen that there are applications
that need to use IP over IP and the home agent need to preserve the
stack of the IP header, a new protocol type has to be introduced just
to specify the mobility aspect.
The co-located care-of IP address has to be bound to one of the IP
addresses supported by the service providers (if mobile node
advertises more than one address, the home agent will get confused,
also there are other implications). Transport layer must ensure that
the 'home address' gets tightly coupled with this IP address.
3. Processing of real time packets (QoS issue)
Here is an attempt to come out with a solution for IP switch based
network to operate in the most user-friendly manner to transport data
traffic (IP) as well as real time (RT) traffic (as RTP[6] packet) in
the existing 32-bit system.
In case of IP routing/switching entire packet gets collected at the
intermediate router/switch and forwarded based on the forwarding
table. Inside the switch/router the variable length IP packet gets
fragmented into smaller size frames at the ingress side. The frames
gets transported through the switching fabric with proper priority
mechanism (to support QoS) and then reassembled at the egress side
and passed through the media for the next hop.
In case of ATM, packets get fragmented at the ingress edge devices
into small size cells. Entire packet gets transported as a stream of
cells and gets collected at the egress edge device. The success of
ATM over IP routing as far as speed is concerned is due to the fact
that the latency gets reduced as the entire packet does not get
collected, fragmented and reassembled at the intermediate nodes. So,
in case of IP switch based network, if RT packets can be passed
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without getting fragmented inside the switch, better performance can
be expected. i.e. one RT packet needs to get to fit inside one
internal frame of the switch fabric. Additionally, to make this
approach successful, maximum size of MPLS label stack has to be
defined. Inside the switch all the IP packets will be assumed to
carry same number of MPLS labels whether they are having one or the
maximum in real sense. In fact, to reduce overhead, this limit should
be the minimum number of labels needed to satisfy all sorts of
features supported by MPLS. i.e. label stacking of depth n (without
limit) needs modification.
If minimum frame size is selected to fit one RTP packet, overhead
becomes too high due to very large (40 bytes: 20 bytes IP + 8bytes
UDP + 12 bytes RTP) packet header. Again, if large frame size is
used, fragmentation loss becomes too high for the small size packets
(say, 40 bytes IP packets). So, a compromise is needed that will give
a better result based on the IP packet size distribution. Frame size
is selected based on the minimum value of the overhead due to the
fragmentation loss of data packet as well as the overhead as header
of the RT packets.
Studies show that primarily IP data packets of three different sizes
are found common in nature. Almost
~50% packets of size 40 bytes (TCP ACK),
~20% packets of size 576 bytes (path MTU set by X.25) and
~30% packets of size 1500 bytes (path MTU set by ethernet)
Other packets are less compared to the above three categories and
almost evenly distributed. For the sake of simplicity of calculation,
traffic of the first three categories are only considered. Payload of
the data traffic is the actual IP packet size where as the payload of
RT traffic is the payload inside RTP packet.
If totBytes are to be transported across the internet and dataPcnt be
the %of data traffic,
totBytes*dataPcnt/100 = data traffic and
(100-dataPcnt)*totBytes/100 = RT traffic;
Out of data traffic 50% of 40 bytes length; 20% of 576 bytes length;&
30% of 1500 bytes length.
If totDataPkts be the total data packets,
totDataPkts*(50*40/100 + 20*576/100 + 30*1500/100) =
totBytes*dataPcnt/100;
or, totDataPkts*58520 = totBytes*dataPcnt;
Let totBytes = 58520*100, for the ease of calculation;
i.e. totDataPkt = dataPcnt*100;
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40 bytes packets = 50*totDataPkt/100 i.e. 50*dataPcnt
576 bytes packets = 20*totDataPkt/100 i.e. 20*dataPcnt
1500 bytes packets = 30*totDataPkt/100 i.e. 30*dataPcnt
RT packets = totBytes * (100 - dataPcnt)/100
= 58520 * (100-dataPcnt);
If n is considered to be the depth of MPLS label stack,
inside the switch, actual size of
40 bytes packet = 40+4*n bytes,
576 bytes packet = 576+4*n bytes &
1500 bytes packet = 1500+4*n bytes
Let frameSize be the payload of a frame (excluding the frame header)
inside the switch. If a RT packet fits exactly inside frameSize,
RT packet payload = (frameSize-40-4*n) bytes;
Total overhead = packet header overhead (of RT packets) +
fragmentation overhead (of data packets);
If a plot is drawn for frameSize = 40+4*n+1 to 1500+4*n for different
dataPcnt (with dataPcnt=80 to 100) minimum of overhead are found at
frameSize = (84, 101, 118, 126 and 152) for n==3; frameSize = (119,
127 and 152) for n==4 and at frameSize = (118, 127 and 152) for n==5.
Actual data of the IP traffic has to be collected to get the best
result. As dataPcnt increases minimum values are found at a lower
frameSize and it gives better result with the higher range for lower
dataPcnt. With average IP packet size 585 bytes, switches will
encounter a loss of 4*(n-1) bytes for packets that will need only one
label.
In order to make this scheme work, a standard for maximum label stack
size has to be defined. RTP packet size also has to be standardized.
The same scheme is applicable to all the switching systems where IP
packets get transported in hop by hop basis unlike the way it works
in ATM networks.
3.1. Dual mode operation
Inside ingress as well as in the egress card, packets need to follow
certain functional steps. In order to maximize the output, a series
of processing units work in pipeline mode for these operations.
Ingress service cards need to act in dual mode to process RT packets
and non-RT packets. i.e. the RT packets should follow a direct path
that won't need fragmentation and related complexities before they
are sent to the VOQs (virtual output queues, where from packets gets
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picked up to be sent to the switching fabric). Whereas other packets
need to follow a different path for fragmentation operations. This
will prevent a RT packet to be blocked by the fragmentation procedure
of not-RT packets that arrive in the service card prior to the
arrival of RT packet. So, mere mapping of RT packet size with the
frameSize of switch fabric will not achieve the speed of ATM
switches.
Simulation studies show that significant improvement is achieved once
RT packets are directly sent to VOQs after the operation of label
processing. It will be worth to study by the hardware people to
figure out whether entire set of data can be placed into queues based
on their priorities and segmentation operation is done in each queue
in parallel mode before putting the frames into their respective
VOQs. Entire operation will be lot costlier, but simulation result
shows that in such case, RT packets need not be restricted to fixed
size cells. Standardization of label stack depth need not be imposed
as well.
4. Refinements over existing IPv6 specification
As IPv6 was envisioned long before some of the newer technologies
e.g. MPLS came into picture, some refinements can be made over the
existing specification. These considerations are related to bandwidth
usages and performance inside switches. Previous chapter shows that
smaller packet size gives better result for processing of RT packet.
So, it is desirable to have IP packet header to be as small as
possible.
As described earlier, evaluation of the parameters
nMaxInterASTopNodes, nMaxInterASBottomNodes and nMaxASNodes is geo-
political and have to be decided by IANA. Once these parameters are
determined with mutual agreements, values of pA, pB, pC and prefix
length of user id can be determined. If the total length comes out to
be less than 128, length of IP header will be reduced accordingly.
The 'flow label' field of IPv6 packet header may not be of any use
with MPLS is in use. ATM used to have 4 priority classes. The first
specification of IPv6 RFC-1883 used a 4bit type of service field
along with a 24bits flow label field. These two were modified to a
8bit type of service field and a 20bit flow label field in the
current spec RFC-2460. Too many priority classes may increase
complexities to process inside switches. If type of service field of
IPv6 header may be reduced to be of 4bit length as it was stated in
RFC-1883 and 'flow label' field gets removed, another three bytes may
be reduced from the IPv6 header.
The field 'Hop Limit' has got a 8bit value in the existing spec. The
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role of this field needs to be discussed properly with a large
address space.
4.1. Distributed processing and Multicasting
With the inherent hierarchy involved in this architecture,
distributed applications can also be structured in a suitable manner.
Say, for a commonly used web based application a master level server
will be there at every top level node. Any change that might happen
in the application, has to be synchronized within these master level
servers first. There might be servers at the middle layer (inside
each inter-AS-bottom) inside each top level node. Once the changes
get reflected at the master node, all the servers at the middle layer
needs to update themselves with their master level node. This will
reduce network traffic substantially. Inherent hierarchy in the
architecture will also help establishing multicast tree in the
similar manner. Work on these issues can be progressed only after
this architecture gets approved.
5. Expected changes at the application layer
IP packets with size 576 in most of the cases come out of those TCP
layers that do not process maximum path-MTU and takes the default one
that was set during X.25. The 576 factor can be corrected very easily
with path-MTU set to 1500. With the consideration that label switch
path do not get changed very frequently in between two arbitrary
network points for any particular type of packet, most of the
applications are expected to become UDP based with negative ACK. TCP
in turn might go through changes. Once this comes into effect, 40
bytes packets will come down drastically. Switch fabric frame size
needs to be determined keeping these two factors in mind along with
changes in IP packet header. With the existing 32-bit system, frame
size (excluding the frame header) of 152 and 127 are most viable
solution in general for label stack depth=3,4 &5.
6. IANA Consideration
This is a first level draft for proposed standard. Hence, IANA
actions should come into play at a later stage, if needed.
7. Security Consideration
This document does not include any security related issues.
8. Acknowledgments
The author would like to thank to Professor Amitava Datta of
University of Western Australia for his review and constructive
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comments.
9. Normative References
[1] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for
IPv6 Hosts and Routers", RFC 4213, October 2005.
[2] Fuller V., Li. T., "Classless Inter-Domain Routing (CIDR): The
Internet Address Assignment and Aggregation Plan", RFC 4632,
August 2006.
[3] Huston, G., "Commentary on Inter-Domain Routing in the
Internet", RFC 3221, December 2001.
[4] Q. Vohra, E. Chen., "BGP Support for Four-octet AS Number
Space", RFC 4893, May 2007.
[5] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001.
[6] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson.
"RTP: A Transport Protocol for Real-Time Applications", RFC
3550, July 2003.
[7] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks(VPNs)", RFC 4364, February 2006.
[8] J. Moy., OSPF Standardization Report, RFC 2329, April 1998
[9] C. Perkins, "IP Mobility Support for IPv4, Revised", RFC5944,
November 2010.
[10] R. Braden, "Requiements for Internet Hosts -- Communication
Layers", RFC1122, October 1989.
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10. Informative References
[11] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[12] Rekhter, Y., and T., Li, "A Border Gateway Protocol 4 (BGP-
4)",RFC 1771, March 1995.
[13] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification, RFC 1883, December 1995.
[14] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[15] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[16] Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
11. Author's Address
Shyam Bandyopadhyay
HL No 205/157/7, Inda
Kharagpur 721305
India
Phone: +91 3222 225137
e-mail: shyamb66@gmail.com
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