BWN LAB

1
PNNI (Private Network Node Interface or Private
Network-to-Network Interface)
PNNI is a switch-to-switch protocol developed
within the ATM Forum to support efficient,
dynamic and scalable routing of SVC requests in a
multi-vendor private ATM environment.
2
Internet Routing Protocols (Overview)
IP finds route on a per packet basis. Packet
specifies end system address. Switch picks next
hop.

• A Protocol is run by routers in Internet to
  update routing tables.
• Routing tables are updated automatically on a
  topology change, e.g., a node failure will be
  recognized and avoided.

3
Internet Routing Protocols (Ctd)

• Two well-known approaches (Two Religions)
• Distance Vector Routing Protocols (Distributed)
• Based on Bellman-Ford shortest path algorithm
• (Distributed Version)
• Router maintains best-known distance to each
  destination
• and next hop in the routing table.
• Each router periodically communicates to all
  neighbor
• routers its best-known distance to each
  destination.
• (May take a long time in a large network!!!)
• Routers update distances based on the new
  information.

4
Internet Routing Protocols (Ctd)

• 2. Link-State (Topology Broadcast) Routing
  Protocols (Centralized)
• Each router broadcasts topology information
• (e.g., link states) to all routers.
• Each router independently computes exact shortest
  paths using a centralized algorithm.
• Each router creates then a NETWORK MAP referred
  as
• LINK STATE DATABASE.

5
ATM Routing Protocols
Invoked only for connection setup!!!

• Protocols to route connection requests through
  interconnected network of ATM switches.
• P-NNI Phase 1 completed by ATM Forum in March
  96.
• - Will allow switches from multiple vendors
  to
• interoperate in large ATM networks

6
PNNI (Private Network Node Interface or Private
Network-to-Network Interface)

• PNNI Phase I consists of 2 Protocols
• 1. Routing
• PNNI routing is used to distribute information
  on
• the topology of the ATM network between
  switches
• and groups of switches.
• This information is used by the switch closest
  to the
• SVC requestor to compute a path to the
  destination
• that will satisfy QoS objectives.
• PNNI supports a hierarchical routing structure ?
  
• scalable for large networks.

7
PNNI (Private Network Node Interface or Private
Network-to-Network Interface)

• 2. Signaling
• PNNI signaling uses the topology and resource
• information available at each switch to
  construct
• a source-route path called a Designated
  Transit List (DTL).
• The DTL contains the specific nodes and links
  the
• SVC request will traverse to meet the
  requested
• QoS objectives and complete the connection.
• Crankback and alternate routing are also
  supported
• to route around a failed path.

8
ARCHITECTURE

• Private Network-to-Network Interface
• Private Network-Node-Interface

9
ARCHITECTURE (Cont.)
10
Features of PNNI

• Point-to-point and point-to-multipoint
  connections
• Can treat a cloud as a single logical link
• Multiple levels of hierarchy gt Scalable for
  global networking.
• Reroutes around failed components at connection
  setup.
• Automatic topological discovery gt No manual
  input required
• Connection follows the same route as the setup
  message
• (associated signaling)
• Uses Cost, capacity, link constraints,
  propagation delay
• Also uses Cell delay , cell delay variation,
  current average load,
• current peak load
• Uses both link and node parameters
• Supports transit carrier selection
• Supports anycast

11
Architecture Reference Model ofSwitching System
12
Overview of PNNI Routing Design Concepts

• PNNI uses several formerly known techniques
• Link State Routing
• Hierarchical Routing
• Source Routing

13
CHOICES IN THE BEGINNING

• PNNI is a routing protocol ? requires a routing
  algorithm.
• CHOICE 1. Distance Vector Routing Algorithm used
  in RIP.
• Not selected because
• Not scalable Prone to routing loops Does
  not converge rapidly and uses excessive
  overhead control traffic.
• CHOICE 2 Link-State Routing (such as OSPF).
• Selected because
• Scalable Converges rapidly Generates
  less overhead traffic and is extendible.
  Extendible means that information in addition to
  the status of the links can be exchanged between
  nodes and incorporated into the topology
  database.
• Difference to OSPF Status of an ATM switch is
  advertised in addition to the status of the
  links.

14
1. Concept of Link State Routing

• Each ATM switch uses HELLO protocol and sends
  HELLO packets periodically or on state changes.
• The HELLO packet is flooded to all other switches
  in the network.
• Each ATM switch exchanges updates with its
  neighbor switches on the status of the links, the
  status and resources of the switches, and the
  identity of each others neighbor switches.
• The switch information may include data about
  switch capacity, QoS, and transit time.

15
Concept of Link State Routing (Ctnd)

• This information is important because SVC
  requests are routed over a path that must meet
  its QoS objectives.
• This information is used to build a topology
  database (NETWORK MAP) of the entire network.
• Each ATM switch in the group will have an
  identical copy of the topology database.
• If a change in topology occurs (e.g., link is
  broken), then only that change is propagated
  between the switches.

16
Concept of Link State Routing (Cont.)
17
2. Routing Hierarchy Concept
(Similar to 2-level hierarchy of OSPF)
Can support 104 levels of hierarchy. In practice
we need 3 or 4 levels.
Multilevel Routing Hierarchy
18
Routing Hierarchy Concept (Cont.)

• Peer Groups Switches that share a common
  addressing
• scheme are grouped into an area.
• Members of a peer group will exchange
  information with
• each other about the topology of the peer
  group.

An ATM switch, called the Peer Group Leader
(PGL), then summarizes this information and
exchanges it with other PGLs that represent other
PEER GROUPs of switches in the next higher layer
of the Peer Group.
19
Routing Hierarchy Concept (Cont.)
Example
20
Routing Hierarchy Concept (Cont.)
PNNI Routing Hierarchy
Example
21
Explanation of the Example

• The three peer groups at the bottom of the
  figure represent a topology
• of real ATM switches connected by physical
  links.
• The switches in peer group A, e.g., will
  exchange topology and resource
• information with the other switches in the
  peer group.
• Switch A.1 is elected the PGL and will
  summarize the information about
• peer group A.
• In the next higher-level peer group, N, the PGL
  for A, switch A.1
• will exchange the summarized information with
  the other nodes in N.
• The other PGLs representing B and C will do
  likewise.
• Switch A.1 will then advertise the summarized
  information it has gained
• from the other members of N into its own lower
  level,
• i.e., child peer group A.

.
22

• Remark
• Each switch in a peer group will have complete
  information about the
• topology of the peer group it is part of and
  partial or summarized
• information about the outside or external peer
  groups.
• Hierarchy enables a network to scale by reducing
  the amount of
• information a node is required to maintain.
• It contains the amount of real topology
  information that is transmitted on
• the network to a local area or peer group.
• The information on the network is further
  reduced by the process of
• topology aggregation so that a collection of
  real switches can appear
• as a single node to other peer groups.

23
PNNI Terminology

• PEER GROUP A peer group is a collection of
  nodes that share a common addressing scheme and
  maintains an identical topology database and
  exchange topology and resource information with
  each other. Members of a peer group discover
  their neighbors using a HELLO protocol.

Example Peer groups A, B, and C consist of real
ATM switches connected by physical links. Peer
group N consists of three logical group nodes
(LGN). The LGNs are summarized representations of
the peer groups of actual switches they represent
below them.
24

• PEER GROUP IDENTIFIER Members of the same peer
  group are identified by a common peer group
  identifier. The peer group identifier is defined
  from a unique 20-byte ATM address that is
  manually configured in each switch. (See the
  addressing subsection!)

• LOGICAL NODE. A logical node is any switch or
  group of switches that runs the PNNI routing
  protocol, e.g., all members of PG A and the node
  above it, LGN A are logical nodes.

25

• LOGICAL GROUP NODE (LGN). An LGN is an
  abstract representation of a lower-level peer
  group for the purposes of representing that peer
  group in the next higher-level peer group. In
  other words, representation of a group as a
  single point.

LGN A represents PG A, LGN B represents PG B, and
LGN C represents PG C. Even though an LGN is not
a real switch but a logical representation of a
group of switches, it still behaves as if it was
a real ATM switch.
26

• PARENT PEER GROUP LGN representing peer group
  below it, e.g., PG N is a parent peer group.
• CHILD PEER GROUP Any node at the next lower
  hierarchy level. In other words, a node that is
  part of an LGN in the next higher level peer
  group.

e.g., Peer groups A, B, and C are child peer
groups.
27

• PEER GROUP LEADER (PGL). Within the peer group,
  a PGL is elected to represent the peer group as a
  logical group node in the next higher-level peer
  group. The PGL is responsible for summarizing
  information about the peer group upward and
  passes higher-level information downward.
• SWITCH with the highest leadership priority
  and highest ATM address is elected as a leader.
• Note Continuous process ? Leader may change any
  time.

e.g., Each of the peer groups has a PGL shaded in
gray, i.e., A.1, B.2, C.2 and LGN A.
28

• HELLO PROTOCOL. This is a standard link-state
  procedure used by neighbor
• nodes to discover the existence and identify
  of each other.
• BORDER NODES. A border node is a logical node
  which has a neighbor that belongs to a different
  peer group. This is established when neighbor
  switches exchange hello packets. The links
  connecting two peer groups are called outside
  links.

e.g., Nodes A.4, B.2, B.3, and C.1 are border
nodes.
29

• UPLINKS.
• An uplink is a logical connection from a BORDER
  NODE to a higher-level LGN.
• The existence of an uplink is derived from an
  exchange of
• HELLO PACKETS between BORDER NODES.
• The other members of the peer group are then
  informed about the
• existence of the uplink.
• An uplink is used by the PGL to construct a
  logical link between LGN in
• the next higher-level peer group.

30

• LOGICAL LINK
• A connection between 2 nodes.
• They interconnect the members of PG N.
• Horizontal links are logical links that connect
  nodes in the same peer group

• ROUTING CONTROL CHANNEL
• VPI0, VCI18 is reserved as the VC used to
  exchange routing
• information between logical nodes.
• An RCC that is established between two LGNs
  serves as the logical link
• information needed by LGNs to establish the
  RCC SVC between other
• nodes in the peer group which is derived from
  the existence of uplinks.

31

• TOPOLOGY AGGREGATION
• This is the process of summarizing and
  compressing information at one
• peer group to advertise into the next
  higher-level peer group.
• Topology aggregation is performed by the PGLs.
• Links can be aggregated such that multiple links
  in the child peer group
• may be represented a single link in the parent
  peer group.
• Nodes are aggregated from multiple child nodes
  into a single LGN.

32

• PNNI TOPOLOGY STATE ELEMENT (PTSE)
• This unit of information is used by nodes to
  build and synchronize a
• topology database within the same peer
  group.
• PTSEs are reliably flooded between nodes in a
  peer group and downward from an LGN into the peer
  group it represents.
• PTSEs contain topology state information about
  the links and nodes in the peer group.
• PTSEs are carried in PNNI topology state packets
  (PTSP).
• PTSPs are sent at regular intervals or will be
  sent if triggered by an important change in
  topology.

33

• REMARK (Summary)
• Upon initialization nodes exchange PTSE
  headers.
• e.g., My topology database is dated
  11-March-20011159.
• Node with older database requests more recent
  information.
• After synchronizing the routing databases, they
  advertise the link between them.
• The ad (PTSP) is flooded through the peer group.
• All PTSPs have a lifetime and are unless renewed.
• Only the node that originated a PTSP can reissue
  it.
• PTSPs are issued periodically and also
  event-driven.

34

• UPWARD AND DOWNWARD INFORMATION FLOW
• Fig. shows the information flow during this
  process for PG A and LGN A.
• The PGL in A, A1, is responsible for producing
  information about PG A, summarizing it and then
  representing A as a single LGN in PG N. This is
  the upward flow.
• Note that no PTSEs flow upward.
• PTSEs flow downward and horizontally from the
  PGL.
• This provides the nodes in PG A with visibility
  outside its peer group and enables them to
  intelligently route an SVC request.
• External visibility for nodes in a peer group
  is limited to knowledge about uplinks to other
  LGNs.

35
PNNI Upward/Downward Information Flow
PGLs summarize state information within peer
group communicate to higher level peer group.
Group Leaders also pass summarized topology
information to nodes of lower-level peer groups.
36
Addressing

• The fundamental purpose of PNNI is to compute a
  route from a source to a destination based on a
  called ATM address.
• The called ATM address is an information element
  contained in the SETUP message that is sent over
  UNI from the device to a switch (ATM UNI 3.1
  specification).
• Presumably a switch running PNNI Phase I will
  have in its topology database an entry that will
  match a portion or prefix of the 20-byte ATM
  address that is contained in the SETUP message.
• The switch will then be able to compute a path
  through the network to the destination switch.

37
Addressing (Ctnd)
ATM end system address (20 bytes)

• Addressing and identification of components of
  the PNNI routing hierarchy are based on the use
  of ATM end system addresses.
• PNNI routing works off of the first 19 bytes of
  this address or some prefix of this address.
• The 20th byte is the selector field which only
  has local significance to the end station and is
  ignored by PNNI routing.
• Most significant 13 bytes in ATM address field
  used to define PEER GROUPs.
• Nodes in PEER GROUP have common high-order bits.
• Allows up to 13 ? 8 104 levels in hierarchy.
  (Practice 3 4 levels enough).

PNNI uses 19 bytes
End System Identifier (ESI)
SEL
AFI
DSP
IDP
6 bytes
1 byte
Address prefix (13 bytes)
38
Addressing (Cont.)

• Nodes in a peer group have the same prefix
  address bits in common.

Address Prefix
x Bits
ESID
SEL
Address Prefix
xy Bits
ESID
SEL
Address Prefix
xyz Bits
ESID
SEL
39
Addressing (Cont.)

• At the highest level illustrated, the LGNs that
  make up the high-order LGN have their left x
  high-order bits the same.
• At the next lower level, the three LGNs shown
  have their left xy high order bits the same.
• At the lowest level illustrated, the LGNs have
  their left xyz high order bits the same.(At
  this level, they are all real physical switches.)

40
Peer Group Generation Process

• Two identifiers are used in PNNI to define the
  hierarchy and a node placement in the hierarchy.
• The first is the Peer Group Identifier. This is
  a 14-byte value.
• The first byte is a level indicator which defines
  which of the next 104 left-most bits are shared
  by switches in the peer group. In other words,
  what level in the hierarchy the peer group is in.
  
• Peer group identifiers must be prefixes of ATM
  addresses.

Peer Group Identifier
41
Peer Group Generation Process (Cont.)

• A peer group is identified by its peer group
  identifier.
• Peer group IDs are specified at the
  configuration time.
• Neighboring nodes exchange peer group IDs in
  hello packets.
• If they have the same peer group ID, then they
  belong to the same peer group.
• If the exchanged peer group IDs are different,
  then the nodes belong to different peer groups.
• The Node Identifier is 22 bytes in length and
  consists of a 1-byte level indicator, 1-Byte
  Lowest Level Node Indicator 20-Bytes ATM
  address.
• The Node Identifier is unique for each PNNI node
  in the routing domain. Identifying the
  ACTUAL-PHYSICAL NODE address.
• A PNNI node that advertises topology information
  in PNNI topology state packets will include the
  Node Identifier and the Peer Group Identifier to
  indicate the originator of the information and
  the scope (on which level of the hierarchy it is
  directed to).

42
PNNI Routing Hierarchy
Example
43
Peer Group Generation Process (Cont.)
The process of building PNNI peer groups is
recursive, i.e., the same process is used at each
level in hierarchy. The exceptions are (1) the
lowest level peer groups because the logical
nodes representing actual switches can have no
child nodes and (2) the highest-level peer group
because there is no parent to represent
it. PROCEDURE

• 0. Initiate physical connections or (VPs) between
  switches (at lowest level).
• 1. Exchange HELLO messages with physical peer
  switches by flooding.
• 2. Determine peer group membership (configure
  lowest level peer groups)
• Flood topology-state PTSEs in peer group.
• 3a. Create the Topology Database
• 3b. Determine the BORDER NODES
• 4. Elect peer group leader.

44
PROCEDURE(Cont.)

• 5. Identify UPLINKS from the BORDER NODES (if
  any).
• 6. Build horizontal links between LGNs at the
  next higher level.
• 7. Exchange HELLO messages with adjacent-logical
  nodes (LGNs at that level).
• 8. Determine peer group membership at that level.
• Flood topology-state PTSEs in peer group.
• 9a. Create TOPOLOGY DATABASE
• 9b. Determine the BORDER NODES
• 10. Elect peer group leader
• 11. If highest-level peer group reached, then
  process complete.
• 12. Return to Step 5.

45
PNNI Information Exchange

• A PNNI node will advertise its own direct
  knowledge of the ATM
• network.
• The scope of this advertisement is the peer
  group.
• The information is encoded in TLVs called PNNI
  Topology State
• Elements (PTSE).
• Multiple PTSEs can be carried in a single PNNI
  Topology State
• Packet (PTSP).
• The PTSP is the packet used to send topology
  information to a
• neighbor node in the peer group.

46
PNNI Information Exchange (Ctd)
Each switch advertises the following Nodal
Information This includes the switchs ATM
address, peer group identifier, leadership
priority, and other aspects about the switch
itself. Topology State Information This covers
outbound link and switch resources. Reachability
ATM addresses and ATM address prefixes that the
switch has learned about or is configured with.
47

• PNNI is a topology state protocol ? logical nodes
  will advertise link state and nodal state
  parameters.
• A link state parameter describes the
  characteristics of a specific link and a nodal
  state parameter describes the characteristics of
  a node.
• Together these can form topology state parameters
  that are advertised by PNNI nodes within their
  own peer group.
• Topology state parameters are either metrics or
  attributes.
• A topology state metric (added along the path,
  e.g., delay) is a parameter whose values must be
  combined for all links and nodes in the SVC
  request path to determine if the path is
  acceptable.
• A topological state attribute (considered
  individually on each elements) is a parameter
  that determines if a path is acceptable for an
  SVC request.

48
Topological state attributes can be further
subdivided into two categories
Performance-related and Policy-related.
Performance-related attributes (e.g., capacity)
measure the performance of a particular link or
node. Policy-related attributes (e.g.,
security) provide a measure of conformance level
to a specific policy by a node or link in the
topology.
49
Table PNNI Topology State Parameters Table PNNI Topology State Parameters Table PNNI Topology State Parameters
Metrics Performance/ Resource Attributes Policy Attributes
Cell Delay Variation Cell Loss Ratio for CLP0 Restricted Transit Flag
Maximum Cell Transfer Delay Maximum Cell Rate
Administrative Weight Available Cell Rate
Cell Rate Margin
Variance Factor
Branching Flag
50
Cell Delay Variation (CDV) Expected CDV along
the path relevant for CBR and VBR-rt
traffic. Administrative Weight (AW) Link or
nodal state parameter set by administrator to
indicate preference for A NETWORK LINK. Cell Loss
Ratio (CLR) Describes the expected CLR at a
node or link for CLP0 traffic. Maximum Cell
Rate (MCR) Describes the maximum link or node
capacity. Available Cell Rate (ACR) Measure of
effective available bandwidth in cells
per second, per traffic class.
51
Cell Rate Margin (CRM) A measure of the
difference between effective bandwidth allocation
per traffic class and the allocation for
sustainable cell rate (SCR). A measure of the
safety margin allocated above the aggregate
sustained rate. Variance Factor (VF) A
relative measure of the square of the CRM
normalized by the variance of the aggregate cell
rate on the link. Branching Flag Used to
indicate if a node can branch point-to-multipoint
traffic. Restricted Transit Flag Nodal state
parameter that indicates whether a node supports
transit traffic or not.
52
PNNI Routing Hierarchy

• The process of generating a PNNI routing
  hierarchy is an Automatic Procedure that defines
  how nodes will interact with each other.
• It begins at the lowest level in the hierarchy
  and is based on the information that is exchanged
  between switches.
• The same process is performed at each level of
  the hierarchy.

53
Peer Group B
54

• Switches in peer group A exchange HELLO packets
  with their neighbor switches over a special
  reserved VCC (VPI0, VCI18) called the Routing
  Control Channel (RCC).
• HELLO packets contain
• A nodes ATM end system address,
• node ID, and
• its port ID for the link.
• ? The HELLO protocol makes the neighboring
  nodes known to each other.
• Membership in the peer group is determined based
  on addressing. Those with a matching peer group
  identifier are common peer group members.

55

• Topology information in the form of PTSEs is
  reliably flooded in the peer group over the
  Routing Control Channel.
• PTSEs are the smallest collection of PNNI routing
  information that is flooded as a unit among all
  logical nodes within a peer group.
• A nodes topology database consists of a
  collection of all PTSEs received, which represent
  that nodes present view of the PNNI routing
  domain.
• The topology database provides all the
  information required to compute a route from the
  given node to any address reachable through that
  routing domain.

56

• A peer group leader (PGL) is elected based on the
  leadership priority configured in the switch.
• The PGL represents the peer group as a logical
  group node in the next higher-level peer group.
• PGLs summarize and circulate info in the parent
  group.
• Switch A.1 is the PGL for peer group A.
• A logical group node (LGN) is an abstract
  representation of a lower-level peer group for
  the purposes of representing that peer group in
  the next higher-layer peer group.
• LGN A represents peer group A in the next
  higher-level peer group, i.e., peer group N.

57

• Because PNNI is recursive, LGN A behaves just
  like it was a switch in a peer group which in
  this case is peer group N.
• It is also the responsibility for the PGL to
  advertise PTSEs that it has collected in
  higher-level peer groups.
• This enables the switches in peer group A to have
  at least a partial picture of the entire network.
• Identify uplink and build horizontal links
  between LGNs.
• An uplink is a connection to an adjacent peer
  group.
• This is discovered when border switches exchange
  HELLOs and determine that they are not in the
  same peer group.
• From the perspective of a switch in peer group A
  an uplink is a connection to an LGN in a
  higher-level peer group.

58

• A horizontal link is a logical connection between
  LGNs in the next higher-level peer group. It is
  in actuality an SVC between PGLs.
• So the horizontal link that connects LGN A and
  LGN B in peer group N is an SVC between switches
  A.1 and B.2. It functions as a RCC so that nodes
  in peer group N can exchange topology
  information.
• The same process of exchanging HELLOs and
  flooding PTSEs is performed in peer group N,
  i.e., PTSEs flow horizontally through the peer
  group and downward through children.
• Border nodes do not exchange databases (different
  peer groups).

59
Generic Connection Admission Control (GCAC)

• CAC is the function performed by ATM switches
  that determines whether a connection request can
  be accepted or not.
• This is performed by every switch in the SVC
  request path.
• But CAC is not standardized, so it is up to
  individual switch to decide if a connection
  request and its associated QoS can be supported.
• PNNI uses information stored in the originating
  nodes topology database along with the
  connections traffic characteristics and QoS
  requirements, to compute a path.
• But again, CAC is a local switch process that the
  originating node cannot realistically keep track
  of.

60
Generic Connection Admission Control (GCAC)

• Therefore, PNNI invokes a Generic Connection
  Admission Control. (GCAC) procedure during the
  path selection process which provides the
  originating node with an estimate of whether each
  switchs local CAC process will accept the
  connection.
• Generic Call Admission Control (GCAC)
• Run by a switch in choosing source route
• Determines which path can probably support the
  call
• Actual Call Admission Control (ACAC)
• Run by each switch
• Determines if it can support the call

61
Generic Connection Admission Control (Ctd)

• Ingress Switch performs GCAC to check QoS-based
  or route information available.
• Individual switches on path perform actual CAC on
  receipt of SETUP message.
• When local admission fails, request backtracked
  to previous switch in path (crankback)

Runs ACAC
Runs ACAC
Runs ACAC Runs GCAC Chooses Path
Runs ACAC
62
3. Source Routing Concept

• A switch that receives an SVC request from a
  user-device over a UNI connection will compute
  and generate the entire path through the network
  based on its knowledge of the network.
• Since QoS metrics are advertised and contained in
  the topology-state database, the first switch has
  a good idea about what path to take.
• The first switch will designate which switches
  the SVC request should pass through. This is
  called Designated Transit List (DTL).
• Note that the Intermediate Switches along the
  path do not need to perform any path
  computations.

63
Source Routing Concept (Ctnd)

• They only perform CAC and forward SVC request by
  following the information in the source-route
  path.
• If the SVC request is destined for a switch in
  another peer group, it will specify all external
  peer groups the SVC should travel through and
  direct it to a border switch in an adjacent peer
  group.
• It will be up to the entry or border switch of
  the adjacent or intermediate peer group to
  generate a DTL for its peer group.
• Advantage of source routing gt Prevents loops!!!

64
Source Routing Concept (Cont.)
Example
65
Designated Transit Lists

• PNNI uses source routing to forward an SVC
  request across one or more
• peer groups in a PNNI routing hierarchy.
• The PNNI term for the source route vector is
  designated transit list (DTL)
• which is a vector of information that defines a
  complete path from the
• source node to the destination node across a peer
  group in the routing
• hierarchy.
• A DTL is computed by the source node or the first
  node in a peer group
• to receive an SVC request.
• Based on the source nodes knowledge of the
  network, it computes a path
• to the destination that will satisfy the QoS
  objectives of the request.
• Nodes then simply obey the DTL and forward the
  SVC request through
• the network.

66
Designated Transit Lists

• A DTL is implemented as an information element
  (IE) that is added to the
• PNNI signaling messages SETUP and ADD PARTY.
• One DTL is computed for each peer group and
  contains the complete
• path across the peer group.
• In other words, it is a list of nodes and links
  that the SVC request must
• visit on its way to the destination.
• A series of DTLs is combined into a stack with
  the lowest-level peer group
• on top and highest at the bottom.
• A pointer is also included to indicate the DTL
  node currently visited
• by SVC.
• When the pointer reaches end of DTL, the DTL is
  removed from
• stack and next DTL is processed.
• If the SVC request enters a new lowest-level peer
  group, then a new DTL
• will be generated by the ingress switch and
  placed at the top of the DTL
• stack for processing.

67
DTL ? EXAMPLE
Suppose User A wishes to establish an SVC
with user C and for policy reasons the SVC
request can only traverse the path shown in the
Figure.
68
DTL ? EXAMPLE (Cont.)

• The SVC request is signaled across the UNI to
  node A.2.
• The node A.2 will use DIJKSTRAs shortest path
  algorithm to find the path to the destination.
  VIEW from A.2!!!!
• Node A.2. knows that User C is reachable through
  LGN C and that LGN C is reachable through LGN B.
• Node A.2 constructs two DTLs, one to provide a
  path across PG A and another across PG N. The SVC
  request is forwarded.
• Not shown but included in a pointer that
  indicates which node in the DTL is currently
  being visited.
• When the last node in the DTL is reached, node
  A.4, is removed and the next DTL in the stack is
  processed.
• When the SVC request reaches node B.2, a new DTL
  (B.2,B.3) is popped on top of the stack.
• Node B.2 simply adds a DTL that enables the SVC
  request to traverse PG B.
• When the SVC request reaches the end of the
  current DTL (B.2,B.3), it is removed and the next
  one in the stack is processed.
• When the SVC request reaches node C.1, a new DTL
  (C.1,C.2,C.3) is popped on top and the call is
  forwarded to the destination.
• .

69
CRANKBACK ALTERNATE ROUTING

• Nodes that generate DTLs (A2, B.2, C.1) in the
  previous example use information in the topology
  and resource database that may change while the
  SVC request is being forwarded.
• This may cause the SVC request to be blocked.
• Short of going all the way back to User A and
  attempting to reestablish the connection, PNNI
  invokes a technique called crankback with
  alternate routing .
• When the SVC request cannot be forwarded
  according to the DTL, it is cleared back to the
  originator of the DTL with an indication of the
  problem. This is the crankback mechanism.

70
CRANKBACK ALTERNATE ROUTING (Cont.)
New DTL of (B.2, B.1, B.3)
71
CRANKBACK ALTERNATE ROUTING (Cont.)

• At that point a new DTL (alternate routing) may
  be constructed that bypasses the nodes or links
  that blocked the SVC request but which must match
  the higher-level DTLs which are further down in
  the DTL stack.
• If no path can be found, then the request is
  cranked back to the previous DTL originator.
• If the DTL originator is original source node,
  then the crankback message is translated into a
  REJECT and USER A must attempt another connection
  request.

72
CRANKBACK ALTERNATE ROUTING (Cont.)

• In our example, suppose the port on node B.3
  that connects the link to node B.2 experienced
  congestion while the SVC request was being
  forwarded.
• Node B.3 would realize, after running CAC, that
  the SVC request could not be satisfied over this
  port.
• A crankback message would then be sent back to
  node B.2 indicating a problem with the specified
  port on node B.3.
• Node B.2 would then recompute a new DTL as shown
  and forward the SVC request around the failed
  resource.
• This is illustrated in the Figure.