1
Chapter 7 Protocols for QoS Support
7.0 Increased Demands
Need to incorporate bursty and stream traffic in TCP/IP
architecture
Increase capacity
Faster links,switches,routers
Intelligent routing policies
End-to-end flow control
Multicasting
Quality of Service (QoS) capability
Transport protocol for streaming
2
7.1 Resource Reservation - Unicast
Prevention as well as reaction to congestion required
Can do this by resource reservation
Unicast
End users agree on QoS for task and request from network
May reserve resources
Routers pre-allocate resources
If QoS not available,may wait or try at reduced QoS
3
7.1 Resource Reservation – Multicast
Generate vast traffic
High volume application like video
Lots of destinations
Can reduce load
Some members of group may not want current transmission
,Channels” of video
Some members may only be able to handle part of transmission
Basic and enhanced video components of video stream
Routers can decide if they can meet demand
4
Resource Reservation Problems on an Internet
Must interact with dynamic routing
Reservations must follow changes in route
Soft state – a set of state information at a router that expires
unless refreshed
End users periodically renew resource requests
5
7.1.1 Resource ReSerVation Protocol (RSVP) Design Goals
Enable receivers to make reservations
Different reservations among members of same multicast group
allowed
Deal gracefully with changes in group membership
Dynamic reservations,separate for each member of group
Aggregate for group should reflect resources needed
Take into account common path to different members of group
Receivers can select one of multiple sources (channel selection)
Deal gracefully with changes in routes
Re-establish reservations
Control protocol overhead
Independent of routing protocol
6
RSVP Characteristics
Unicast and Multicast
Simplex
Unidirectional data flow
Separate reservations in two directions
Receiver initiated
Receiver knows which subset of source transmissions it wants
Maintain soft state in internet
Responsibility of end users
Providing different reservation styles
Users specify how reservations for groups are aggregated
Transparent operation through non-RSVP routers
Support IPv4 (ToS field) and IPv6 (Flow label field)
7
7.1.2 Data Flows - Session
Data flow identified by destination
Resources allocated by router for duration of session
Defined by
Destination IP address
Unicast or multicast
IP protocol identifier
TCP,UDP etc.
Destination port
May not be used in multicast
8
Flow Descriptor
Reservation Request
Flow spec
Desired QoS
Used to set parameters in node’s packet scheduler
Service class,Rspec (reserve),Tspec (traffic)
Filter spec
Set of packets for this reservation
Source address,source prot
9
Treatment of Packets of One Session at One Router
10
RSVP Operation Diagram
11
7.1.3 RSVP Operation
G1,G2,G3 members of multicast group
S1,S2 sources transmitting to that group
Heavy black line is routing tree for S1,heavy grey line for S2
Arrowed lines are packet transmission from S1 (black) and S2
(grey)
All four routers need to know reservation s for each multicast
address
Resource requests must propagate back through routing tree
12
Filtering
G3 has reservation filter spec including S1 and S2
G1,G2 from S1 only
R3 delivers from S2 to G3 but does not forward to R4
G1,G2 send RSVP request with filter excluding S2
G1,G2 only members of group reached through R4
R4 doesn’t need to forward packets from this session
R4 merges filter spec requests and sends to R3
R3 no longer forwards this session’s packets to R4
Handling of filtered packets not specified
Here they are dropped but could be best efforts delivery
R3 needs to forward to G3
Stores filter spec but doesn’t propagate it
13
Reservation Styles
Determines manner in which resource requirements from members
of group are aggregated
Reservation attribute
Reservation shared among senders (shared)
Characterizing entire flow received on multicast address
Allocated to each sender (distinct)
Simultaneously capable of receiving data flow from each
sender
Sender selection
List of sources (explicit)
All sources,no filter spec (wild card)
14
Reservation Attributes and Styles
Reservation Attribute
Distinct
Sender selection explicit = Fixed filter (FF)
Sender selection wild card = none
Shared
Sender selection explicit= Shared-explicit (SE)
Sender selection wild card = Wild card filter (WF)
15
Wild Card Filter Style
If used by all receivers,shared pipe whose capacity is largest of
resource requests from receivers downstream from any point on
tree
Single resource reservation shared by all senders to this address
Independent of number of senders using it
Propagated upstream to all senders
WF(*{Q})
* = wild card sender
Q = flowspec
Audio teleconferencing with multiple sites
16
Fixed Filter Style
Distinct reservation for each sender
Explicit list of senders
FF(S1{Q!},S2{Q2},… )
Video distribution
17
Shared Explicit Style
Single reservation shared among specific list of senders
SE(S1,S2,S3,… {Q})
Multicast applications with multiple data sources but unlikely to
transmit simultaneously
18
Reservation Style Examples
19
7.1.4 RSVP Protocol Mechanisms
Two message types
Resv
Originate at multicast group receivers
Propagate upstream
Merged and packet when appropriate
Create soft states
Reach sender
Allow host to set up traffic control for first hop
Path
Provide upstream routing information
Issued by sending hosts
Transmitted through distribution tree to all destinations
20
RSVP Host Model
21
7.2 Multiprotocol Label Switching (MPLS)
Enhancements provide direct support
Routing algorithms provide support for performance goals
Distributed and dynamic
React to congestion
Load balance across network
Based on metrics
Develop information that can be used in handling different
service needs
IS,DS,RSVP
Nothing directly improves throughput or delay
MPLS tries to match ATM QoS support
22
7.2.1 Background
IP switching (Ipsilon)
Efforts to marry IP and ATM
Tag switching (Cisco)
Aggregate route based IP switching (IBM)
Cascade (IP navigator)
All use standard routing protocols to define paths between end
points
Assign packets to path as they enter network
Use ATM switches to move packets along paths
ATM switching (was) much faster than IP routers
Use faster technology
23
Developments
IETF working group in 1997,proposed standard 2001
Routers developed to be as fast as ATM switches
Remove the need to provide both technologies in same network
MPLS does provide new capabilities
QoS support
Traffic engineering
Virtual private networks
Multiprotocol support
24
Connection Oriented QoS Support
Guarantee fixed capacity for specific applications
Control latency/jitter
Ensure capacity for voice
Provide specific,guaranteed quantifiable SLAs
Configure varying degrees of QoS for multiple customers
MPLS imposes connection oriented framework on IP based
internets
25
Traffic Engineering
Ability to dynamically define routes,plan resource commitments
based on known demands and optimize network utilization
Basic IP allows primitive traffic engineering
E.g,dynamic routing
MPLS makes network resource commitment easy
Able to balance load in face of demand
Able to commit to different levels of support to meet user traffic
requirements
Aware of traffic flows with QoS requirements and predicted
demand
Intelligent re-routing when congested
26
VPN Support
Traffic from a given enterprise or group passes transparently
through an internet
Segregated from other traffic on internet
Performance guarantees
Security
27
Multiprotocol Support
MPLS can be used on different network technologies
IP
Requires router upgrades
Coexist with ordinary routers
ATM
Enables and ordinary switches co-exist
Frame relay
Enables and ordinary switches co-exist
Mixed network
28
MPLS Terminology
29
7.2.2 MPLS Operation
Label switched routers capable of switching and routing packets
based on label appended to packet
Labels define a flow of packets between end points or multicast
destinations
Each distinct flow (forward equivalence class – FEC) has specific
path through LSRs defined
Connection oriented
Each FEC has QoS requirements
IP header not examined
Forward based on label value
30
MPLS Operation Diagram
31
Explanation - Setup
Labelled switched path established prior to routing and delivery of
packets
QoS parameters established along path
Resource commitment
Queuing and discard policy at LSR
Interior routing protocol e.g,OSPF used
Labels assigned
Local significance only
Manually or using Label distribution protocol (LDP) or
enhanced version of RSVP
32
Explanation – Packet Handling
Packet enters domain through edge LSR
Processed to determine QoS
LSR assigns packet to FEC and hence LSP
May need co-operation to set up new LSP
Append label
Forward packet
Within domain LSR receives packet
Remove incoming label,attach outgoing label and forward
Egress edge strips label,reads IP header and forwards
33
Notes
MPLS domain is contiguous set of MPLS enabled routers
Traffic may enter or exit via direct connection to MPLS router or
from non-MPLS router
FEC determined by parameters,e.g.
Source/destination IP address or network IP address
Port numbers
IP protocol id
Differentiated services codepoint
IPv6 flow label
Forwarding is simple lookup in predefined table
Map label to next hop
Can define PHB at an LSR for given FEC
Packets between same end points may belong to different FEC
34
MPLS Packet Forwarding
35
7.2.3 Label Stacking
Packet may carry number of labels
LIFO (stack)
Processing based on top label
Any LSR may push or pop label
Unlimited levels
Allows aggregation of LSPs into single LSP for part of route
C.f,ATM virtual channels inside virtual paths
E.g,aggregate all enterprise traffic into one LSP for access
provider to handle
Reduces size of tables
36
7.2.4 Label Format Diagram
Exp,3 bit reserved for experimental use
Label value,Locally significant 20 bit
E.g,DS information or PHB guidance
S,1 for oldest entry in stack,zero otherwise
Time to live (TTL),hop count or TTL value
37
Time to Live Processing
Needed to support TTL since IP header not read
First label TTL set to IP header TTL on entry to MPLS domain
TTL of top entry on stack decremented at internal LSR
If zero,packet dropped or passed to ordinary error processing
(e.g,ICMP)
If positive,value placed in TTL of top label on stack and packet
forwarded
At exit from domain,(single stack entry) TTL decremented
If zero,as above
If positive,placed in TTL field of Ip header and forwarded
38
Label Stack
Appear after data link layer header,before network layer header
Bottom of stack is earliest (closest to network layer header)
Network layer packet follows label stack entry with S=1
Over connection oriented services
Topmost label value in ATM header VPI/VCI field
Facilitates ATM switching
Top label inserted between cell header and IP header
In DLCI field of Frame Relay
Note,TTL problem
39
Position of MPLS Label Stack
40
7.2.5 FECs,LSPs,and Labels
Traffic grouped into FECs
Traffic in a FEC transits an MLPS domain along an LSP
Packets identified by locally significant label
At each LSR,labelled packets forwarded on basis of label.
LSR replaces incoming label with outgoing labe
Each flow must be assigned to a FEC
Routing protocol must determine topology and current conditions
so LSP can be assigned to FEC
Must be able to gather and use information to support QoS
LSRs must be aware of LSP for given FEC,assign incoming label to
LSP,communicate label to other LSRs
41
Topology of LSPs
Unique ingress and egress LSR
Single path through domain
Unique egress,multiple ingress LSRs
Multiple paths,possibly sharing final few hops
Multiple egress LSRs for unicast traffic
Multicast
42
Route Selection
Selection of LSP for particular FEC
Hop-by-hop
LSR independently chooses next hop
Ordinary routing protocols e.g,OSPF
Doesn’t support traffic engineering or policy routing
Explicit
LSR (usually ingress or egress) specifies some or all LSRs in
LSP for given FEC
Selected by configuration,or dynamically
43
Constraint Based Routing Algorithm
Take in to account traffic requirements of flows and resources
available along hops
Current utilization,existing capacity,committed services
Additional metrics over and above traditional routing protocols
(OSPF)
Max link data rate
Current capacity reservation
Packet loss ratio
Link propagation delay
44
Label Distribution
Setting up LSP
Assign label to LSP
Inform all potential upstream nodes of label assigned by LSR to
FEC
Allows proper packet labelling
Learn next hop for LSP and label that downstream node has
assigned to FEC
Allow LSR to map incoming to outgoing label
45
7.3 Real Time Transport Protocol
TCP not suited to real time distributed application
Point to point so not suitable for multicast
Retransmitted segments arrive out of order
No way to associate timing with segments
UDP does not include timing information nor any support for real
time applications
Solution is real-time transport protocol RTP
46
7.3.1 RTP Architecture
Close coupling between protocol and application layer functionality
Framework for application to implement single protocol
Application level framing
Integrated layer processing
47
Application Level Framing
Recovery of lost data done by application rather than transport
layer
Application may accept less than perfect delivery
Real time audio and video
Inform source about quality of delivery rather than
retransmit
Source can switch to lower quality
Application may provide data for retransmission
Sending application may recompute lost values rather than
storing them
Sending application can provide revised values
Can send new data to,fix” consequences of loss
Lower layers deal with data in units provided by application
Application data units (ADU)
48
Integrated Layer Processing
Adjacent layers in protocol stack tightly coupled
Allows out of order or parallel functions from different layers
49
RTP Architecture Diagram
50
7.3.2 RTP Data Transfer Protocol
Transport of real time data among number of participants in a
session,defined by:
RTP Port number
UDP destination port number if using UDP
RTP Control Protocol (RTCP) port number
Destination port address used by all participants for RTCP
transfer
IP addresses
Multicast or set of unicast
51
Multicast Support
Each RTP data unit includes:
Source identifier
Timestamp
Payload format
52
Relays
Intermediate system acting as receiver and transmitter for given
protocol layer
Mixers
Receives streams of RTP packets from one or more sources
Combines streams
Forwards new stream
Translators
Produce one or more outgoing RTP packets for each incoming
packet
E.g,convert video to lower quality
53
RTP Header
54
7.3.3 RTP Control Protocol (RTCP)
RTP is for user data
RTCP is multicast provision of feedback to sources and session
participants
Uses same underlying transport protocol (usually UDP) and
different port number
RTCP packet issued periodically by each participant to other
session members
55
RTCP Functions
QoS and congestion control
Identification
Session size estimation and scaling
Session control
56
RTCP Transmission
Number of separate RTCP packets bundled in single UDP
datagram
Sender report
Receiver report
Source description
Goodbye
Application specific
57
RTCP Packet Formats
58
Packet Fields (All Packets)
Version (2 bit) currently version 2
Padding (1 bit) indicates padding bits at end of control
information,with number of octets as last octet of padding
Count (5 bit) of reception report blocks in SR or RR,or source
items in SDES or BYE
Packet type (8 bit)
Length (16 bit) in 32 bit words minus 1
In addition Sender and receiver reports have:
Synchronization Source Identifier
59
Packet Fields (Sender Report) Sender Information Block
NTP timestamp,absolute wall clock time when report sent
RTP Timestamp,Relative time used to create timestamps in RTP
packets
Sender’s packet count (for this session)
Sender’s octet count (for this session)
60
Packet Fields (Sender Report) Reception Report Block
SSRC_n (32 bit) identifies source refered to by this report block
Fraction lost (8 bits) since previous SR or RR
Cumulative number of packets lost (24 bit) during this session
Extended highest sequence number received (32 bit)
Least significant 16 bits is highest RTP data sequence number
received from SSRC_n
Most significant 16 bits is number of times sequence number
has wrapped to zero
Interarrival jitter (32 bit)
Last SR timestamp (32 bit)
Delay since last SR (32 bit)
61
Receiver Report
Same as sender report except:
Packet type field has different value
No sender information block
62
Source Description Packet
Used by source to give more information
32 bit header followed by zero or more additional information
chunks
E.g.:
0 END End of SDES list
1 CNAME Canonical name
2 NAME Real user name of source
3 EMAIL Email address
63
Goodbye (BYE)
Indicates one or more sources no linger active
Confirms departure rather than failure of network
64
Application Defined Packet
Experimental use
For functions & features that are application specific
Chapter 7 Protocols for QoS Support
7.0 Increased Demands
Need to incorporate bursty and stream traffic in TCP/IP
architecture
Increase capacity
Faster links,switches,routers
Intelligent routing policies
End-to-end flow control
Multicasting
Quality of Service (QoS) capability
Transport protocol for streaming
2
7.1 Resource Reservation - Unicast
Prevention as well as reaction to congestion required
Can do this by resource reservation
Unicast
End users agree on QoS for task and request from network
May reserve resources
Routers pre-allocate resources
If QoS not available,may wait or try at reduced QoS
3
7.1 Resource Reservation – Multicast
Generate vast traffic
High volume application like video
Lots of destinations
Can reduce load
Some members of group may not want current transmission
,Channels” of video
Some members may only be able to handle part of transmission
Basic and enhanced video components of video stream
Routers can decide if they can meet demand
4
Resource Reservation Problems on an Internet
Must interact with dynamic routing
Reservations must follow changes in route
Soft state – a set of state information at a router that expires
unless refreshed
End users periodically renew resource requests
5
7.1.1 Resource ReSerVation Protocol (RSVP) Design Goals
Enable receivers to make reservations
Different reservations among members of same multicast group
allowed
Deal gracefully with changes in group membership
Dynamic reservations,separate for each member of group
Aggregate for group should reflect resources needed
Take into account common path to different members of group
Receivers can select one of multiple sources (channel selection)
Deal gracefully with changes in routes
Re-establish reservations
Control protocol overhead
Independent of routing protocol
6
RSVP Characteristics
Unicast and Multicast
Simplex
Unidirectional data flow
Separate reservations in two directions
Receiver initiated
Receiver knows which subset of source transmissions it wants
Maintain soft state in internet
Responsibility of end users
Providing different reservation styles
Users specify how reservations for groups are aggregated
Transparent operation through non-RSVP routers
Support IPv4 (ToS field) and IPv6 (Flow label field)
7
7.1.2 Data Flows - Session
Data flow identified by destination
Resources allocated by router for duration of session
Defined by
Destination IP address
Unicast or multicast
IP protocol identifier
TCP,UDP etc.
Destination port
May not be used in multicast
8
Flow Descriptor
Reservation Request
Flow spec
Desired QoS
Used to set parameters in node’s packet scheduler
Service class,Rspec (reserve),Tspec (traffic)
Filter spec
Set of packets for this reservation
Source address,source prot
9
Treatment of Packets of One Session at One Router
10
RSVP Operation Diagram
11
7.1.3 RSVP Operation
G1,G2,G3 members of multicast group
S1,S2 sources transmitting to that group
Heavy black line is routing tree for S1,heavy grey line for S2
Arrowed lines are packet transmission from S1 (black) and S2
(grey)
All four routers need to know reservation s for each multicast
address
Resource requests must propagate back through routing tree
12
Filtering
G3 has reservation filter spec including S1 and S2
G1,G2 from S1 only
R3 delivers from S2 to G3 but does not forward to R4
G1,G2 send RSVP request with filter excluding S2
G1,G2 only members of group reached through R4
R4 doesn’t need to forward packets from this session
R4 merges filter spec requests and sends to R3
R3 no longer forwards this session’s packets to R4
Handling of filtered packets not specified
Here they are dropped but could be best efforts delivery
R3 needs to forward to G3
Stores filter spec but doesn’t propagate it
13
Reservation Styles
Determines manner in which resource requirements from members
of group are aggregated
Reservation attribute
Reservation shared among senders (shared)
Characterizing entire flow received on multicast address
Allocated to each sender (distinct)
Simultaneously capable of receiving data flow from each
sender
Sender selection
List of sources (explicit)
All sources,no filter spec (wild card)
14
Reservation Attributes and Styles
Reservation Attribute
Distinct
Sender selection explicit = Fixed filter (FF)
Sender selection wild card = none
Shared
Sender selection explicit= Shared-explicit (SE)
Sender selection wild card = Wild card filter (WF)
15
Wild Card Filter Style
If used by all receivers,shared pipe whose capacity is largest of
resource requests from receivers downstream from any point on
tree
Single resource reservation shared by all senders to this address
Independent of number of senders using it
Propagated upstream to all senders
WF(*{Q})
* = wild card sender
Q = flowspec
Audio teleconferencing with multiple sites
16
Fixed Filter Style
Distinct reservation for each sender
Explicit list of senders
FF(S1{Q!},S2{Q2},… )
Video distribution
17
Shared Explicit Style
Single reservation shared among specific list of senders
SE(S1,S2,S3,… {Q})
Multicast applications with multiple data sources but unlikely to
transmit simultaneously
18
Reservation Style Examples
19
7.1.4 RSVP Protocol Mechanisms
Two message types
Resv
Originate at multicast group receivers
Propagate upstream
Merged and packet when appropriate
Create soft states
Reach sender
Allow host to set up traffic control for first hop
Path
Provide upstream routing information
Issued by sending hosts
Transmitted through distribution tree to all destinations
20
RSVP Host Model
21
7.2 Multiprotocol Label Switching (MPLS)
Enhancements provide direct support
Routing algorithms provide support for performance goals
Distributed and dynamic
React to congestion
Load balance across network
Based on metrics
Develop information that can be used in handling different
service needs
IS,DS,RSVP
Nothing directly improves throughput or delay
MPLS tries to match ATM QoS support
22
7.2.1 Background
IP switching (Ipsilon)
Efforts to marry IP and ATM
Tag switching (Cisco)
Aggregate route based IP switching (IBM)
Cascade (IP navigator)
All use standard routing protocols to define paths between end
points
Assign packets to path as they enter network
Use ATM switches to move packets along paths
ATM switching (was) much faster than IP routers
Use faster technology
23
Developments
IETF working group in 1997,proposed standard 2001
Routers developed to be as fast as ATM switches
Remove the need to provide both technologies in same network
MPLS does provide new capabilities
QoS support
Traffic engineering
Virtual private networks
Multiprotocol support
24
Connection Oriented QoS Support
Guarantee fixed capacity for specific applications
Control latency/jitter
Ensure capacity for voice
Provide specific,guaranteed quantifiable SLAs
Configure varying degrees of QoS for multiple customers
MPLS imposes connection oriented framework on IP based
internets
25
Traffic Engineering
Ability to dynamically define routes,plan resource commitments
based on known demands and optimize network utilization
Basic IP allows primitive traffic engineering
E.g,dynamic routing
MPLS makes network resource commitment easy
Able to balance load in face of demand
Able to commit to different levels of support to meet user traffic
requirements
Aware of traffic flows with QoS requirements and predicted
demand
Intelligent re-routing when congested
26
VPN Support
Traffic from a given enterprise or group passes transparently
through an internet
Segregated from other traffic on internet
Performance guarantees
Security
27
Multiprotocol Support
MPLS can be used on different network technologies
IP
Requires router upgrades
Coexist with ordinary routers
ATM
Enables and ordinary switches co-exist
Frame relay
Enables and ordinary switches co-exist
Mixed network
28
MPLS Terminology
29
7.2.2 MPLS Operation
Label switched routers capable of switching and routing packets
based on label appended to packet
Labels define a flow of packets between end points or multicast
destinations
Each distinct flow (forward equivalence class – FEC) has specific
path through LSRs defined
Connection oriented
Each FEC has QoS requirements
IP header not examined
Forward based on label value
30
MPLS Operation Diagram
31
Explanation - Setup
Labelled switched path established prior to routing and delivery of
packets
QoS parameters established along path
Resource commitment
Queuing and discard policy at LSR
Interior routing protocol e.g,OSPF used
Labels assigned
Local significance only
Manually or using Label distribution protocol (LDP) or
enhanced version of RSVP
32
Explanation – Packet Handling
Packet enters domain through edge LSR
Processed to determine QoS
LSR assigns packet to FEC and hence LSP
May need co-operation to set up new LSP
Append label
Forward packet
Within domain LSR receives packet
Remove incoming label,attach outgoing label and forward
Egress edge strips label,reads IP header and forwards
33
Notes
MPLS domain is contiguous set of MPLS enabled routers
Traffic may enter or exit via direct connection to MPLS router or
from non-MPLS router
FEC determined by parameters,e.g.
Source/destination IP address or network IP address
Port numbers
IP protocol id
Differentiated services codepoint
IPv6 flow label
Forwarding is simple lookup in predefined table
Map label to next hop
Can define PHB at an LSR for given FEC
Packets between same end points may belong to different FEC
34
MPLS Packet Forwarding
35
7.2.3 Label Stacking
Packet may carry number of labels
LIFO (stack)
Processing based on top label
Any LSR may push or pop label
Unlimited levels
Allows aggregation of LSPs into single LSP for part of route
C.f,ATM virtual channels inside virtual paths
E.g,aggregate all enterprise traffic into one LSP for access
provider to handle
Reduces size of tables
36
7.2.4 Label Format Diagram
Exp,3 bit reserved for experimental use
Label value,Locally significant 20 bit
E.g,DS information or PHB guidance
S,1 for oldest entry in stack,zero otherwise
Time to live (TTL),hop count or TTL value
37
Time to Live Processing
Needed to support TTL since IP header not read
First label TTL set to IP header TTL on entry to MPLS domain
TTL of top entry on stack decremented at internal LSR
If zero,packet dropped or passed to ordinary error processing
(e.g,ICMP)
If positive,value placed in TTL of top label on stack and packet
forwarded
At exit from domain,(single stack entry) TTL decremented
If zero,as above
If positive,placed in TTL field of Ip header and forwarded
38
Label Stack
Appear after data link layer header,before network layer header
Bottom of stack is earliest (closest to network layer header)
Network layer packet follows label stack entry with S=1
Over connection oriented services
Topmost label value in ATM header VPI/VCI field
Facilitates ATM switching
Top label inserted between cell header and IP header
In DLCI field of Frame Relay
Note,TTL problem
39
Position of MPLS Label Stack
40
7.2.5 FECs,LSPs,and Labels
Traffic grouped into FECs
Traffic in a FEC transits an MLPS domain along an LSP
Packets identified by locally significant label
At each LSR,labelled packets forwarded on basis of label.
LSR replaces incoming label with outgoing labe
Each flow must be assigned to a FEC
Routing protocol must determine topology and current conditions
so LSP can be assigned to FEC
Must be able to gather and use information to support QoS
LSRs must be aware of LSP for given FEC,assign incoming label to
LSP,communicate label to other LSRs
41
Topology of LSPs
Unique ingress and egress LSR
Single path through domain
Unique egress,multiple ingress LSRs
Multiple paths,possibly sharing final few hops
Multiple egress LSRs for unicast traffic
Multicast
42
Route Selection
Selection of LSP for particular FEC
Hop-by-hop
LSR independently chooses next hop
Ordinary routing protocols e.g,OSPF
Doesn’t support traffic engineering or policy routing
Explicit
LSR (usually ingress or egress) specifies some or all LSRs in
LSP for given FEC
Selected by configuration,or dynamically
43
Constraint Based Routing Algorithm
Take in to account traffic requirements of flows and resources
available along hops
Current utilization,existing capacity,committed services
Additional metrics over and above traditional routing protocols
(OSPF)
Max link data rate
Current capacity reservation
Packet loss ratio
Link propagation delay
44
Label Distribution
Setting up LSP
Assign label to LSP
Inform all potential upstream nodes of label assigned by LSR to
FEC
Allows proper packet labelling
Learn next hop for LSP and label that downstream node has
assigned to FEC
Allow LSR to map incoming to outgoing label
45
7.3 Real Time Transport Protocol
TCP not suited to real time distributed application
Point to point so not suitable for multicast
Retransmitted segments arrive out of order
No way to associate timing with segments
UDP does not include timing information nor any support for real
time applications
Solution is real-time transport protocol RTP
46
7.3.1 RTP Architecture
Close coupling between protocol and application layer functionality
Framework for application to implement single protocol
Application level framing
Integrated layer processing
47
Application Level Framing
Recovery of lost data done by application rather than transport
layer
Application may accept less than perfect delivery
Real time audio and video
Inform source about quality of delivery rather than
retransmit
Source can switch to lower quality
Application may provide data for retransmission
Sending application may recompute lost values rather than
storing them
Sending application can provide revised values
Can send new data to,fix” consequences of loss
Lower layers deal with data in units provided by application
Application data units (ADU)
48
Integrated Layer Processing
Adjacent layers in protocol stack tightly coupled
Allows out of order or parallel functions from different layers
49
RTP Architecture Diagram
50
7.3.2 RTP Data Transfer Protocol
Transport of real time data among number of participants in a
session,defined by:
RTP Port number
UDP destination port number if using UDP
RTP Control Protocol (RTCP) port number
Destination port address used by all participants for RTCP
transfer
IP addresses
Multicast or set of unicast
51
Multicast Support
Each RTP data unit includes:
Source identifier
Timestamp
Payload format
52
Relays
Intermediate system acting as receiver and transmitter for given
protocol layer
Mixers
Receives streams of RTP packets from one or more sources
Combines streams
Forwards new stream
Translators
Produce one or more outgoing RTP packets for each incoming
packet
E.g,convert video to lower quality
53
RTP Header
54
7.3.3 RTP Control Protocol (RTCP)
RTP is for user data
RTCP is multicast provision of feedback to sources and session
participants
Uses same underlying transport protocol (usually UDP) and
different port number
RTCP packet issued periodically by each participant to other
session members
55
RTCP Functions
QoS and congestion control
Identification
Session size estimation and scaling
Session control
56
RTCP Transmission
Number of separate RTCP packets bundled in single UDP
datagram
Sender report
Receiver report
Source description
Goodbye
Application specific
57
RTCP Packet Formats
58
Packet Fields (All Packets)
Version (2 bit) currently version 2
Padding (1 bit) indicates padding bits at end of control
information,with number of octets as last octet of padding
Count (5 bit) of reception report blocks in SR or RR,or source
items in SDES or BYE
Packet type (8 bit)
Length (16 bit) in 32 bit words minus 1
In addition Sender and receiver reports have:
Synchronization Source Identifier
59
Packet Fields (Sender Report) Sender Information Block
NTP timestamp,absolute wall clock time when report sent
RTP Timestamp,Relative time used to create timestamps in RTP
packets
Sender’s packet count (for this session)
Sender’s octet count (for this session)
60
Packet Fields (Sender Report) Reception Report Block
SSRC_n (32 bit) identifies source refered to by this report block
Fraction lost (8 bits) since previous SR or RR
Cumulative number of packets lost (24 bit) during this session
Extended highest sequence number received (32 bit)
Least significant 16 bits is highest RTP data sequence number
received from SSRC_n
Most significant 16 bits is number of times sequence number
has wrapped to zero
Interarrival jitter (32 bit)
Last SR timestamp (32 bit)
Delay since last SR (32 bit)
61
Receiver Report
Same as sender report except:
Packet type field has different value
No sender information block
62
Source Description Packet
Used by source to give more information
32 bit header followed by zero or more additional information
chunks
E.g.:
0 END End of SDES list
1 CNAME Canonical name
2 NAME Real user name of source
3 EMAIL Email address
63
Goodbye (BYE)
Indicates one or more sources no linger active
Confirms departure rather than failure of network
64
Application Defined Packet
Experimental use
For functions & features that are application specific
