Multiprotocol Label Switching(MPLS)
中正大學資工系黃仁竑
中正資工 /黃仁竑 2
Outline
Introduction Label Encoding Label Assignment Label Distribution Label Swapping Label Merging Conclusion
中正資工 /黃仁竑 3
Introduction
IETF Multiprotocol Label Switching Working Group created in 1997
Integrates the label swapping forwarding paradigm with network layer routing Use a short, fixed-length label
Examples of MPLS Tag Switching (Cisco)
中正資工 /黃仁竑 4
Why MPLS
MPLS versus Datagram Routed Network Simplified forwarding Efficient explicit routing Traffic engineering QoS routing Complex mapping from IP packets to FEC (Forwarding
Equivalence Class) MPLS versus ATM
Scaling of the routing protocol Common operation over packet and cell media Easier management
中正資工 /黃仁竑 5
Basics of MPLS
Semantics assigned to a stream label Labels are associated with specific streams of data (FEC)
Forwarding Methods Short fixed length labels to identify streams Looking up a label in a table, swapping labels, and
possibly decreasing and checking a TTL May make direct use of layer 2 forwarding (e.g. ATM)
Label Distribution Methods Allow nodes to determine which labels to use for specific
streams May use some sort of control exchange, or be
piggybacked on a routing protocol By LDP
中正資工 /黃仁竑 6
Next Hop Label Forwarding Entry
NHLFE is used when forwarding a labeled packet, it contains next hop operations
replace the label at the top of the label stack with a new label
pop the label stack replace and push one or more new labels data link encapsulation how to encode the label stack other information for properly dispose of the
packet
中正資工 /黃仁竑 7
Label Stack
Label stack A labeled packet may carry a number of labels
A Label A short fixed length significant identifier Based on the stream or forwarding equivalence class (FEC
) Only have local significance
Label encoding MPLS generic encapsulation mechanism ATM SVC, SVP, SVP multipoint encoding methods Others
中正資工 /黃仁竑 8
Label Switched Path (LSP)
Begins with an LSR (LSP Ingress) that pushes on a level m label
Intermediate LSRs make their forwarding decision by label switching on a level m label
Ends (LSP Egress) when forwarding decision is made by label switching on a level m-k label (k>0) or when a forwarding decision is made by non-MPLS forwarding procedures
The label stack may be popped at the penultimate LSR of the LSP, rather than at the LSP Egress reduce times of label lookup at LSP egress
中正資工 /黃仁竑 9
Label Encoding
Generic MPLS encapsulation Between the data link layer and network layer headers Network layer protocol independent A label contains
Label Stack A sequence of label stack entries
Time-to-Live (TTL) Similar to what is provided by IP (e.g. traceroute)
Class of Service (CoS) Allows multiple service classes within the same la
bel
中正資工 /黃仁竑 10
Label Stack Entry
Label(20bits) carries the actual value of the label 0/2:IPv4/v6 Explicit NULL Label
must be sole label stack entry (forward based on IPv4/v6) 1:Router Alert Label;(need software process) 3:Implicit NULL Label
Exp(3bits):reserved S(1bits):Bottom of Stack TTL(8bits):Time to Live
0 20 23 31
Label Exp S TTL
中正資工 /黃仁竑 11
Label Encoding
ATM Switches as LSRs SVC Encoding
Use the VPI/VCI field to encode the label Each LSP is realized as an ATM SVC ATM-LSR cannot perform PUSH or POP
SVP Encoding VPI : Top of label stack VCI : Second label on the stack Permits the use of ATM VP switching can’t include a non-MPLS ATM network
SVP Multipoint Encoding VPI : Top of label stack VCI : Part for the second label on the stack, the remainder to identify th
e LSP ingress Multipoint-to-point VPs
中正資工 /黃仁竑 12
Label Assignment
Topology driven (Tag) In response to normal processing of routing protocol control t
raffic Labels are pre-assigned; no label setup latency at forwarding t
ime Request driven (RSVP)
In response to normal processing of request based control traffic
May require a large number of labels to be assigned Traffic driven (Ipsilon)
The arrival of data at an LSR triggers label assignment and distribution
Label setup latency; potential for packet reordering
中正資工 /黃仁竑 13
Label Distribution
Explicit Label Distribution Downstream label allocation
label allocation is done by the downstream LSR most natural mechanism for unicast traffic
Upstream label allocation label allocation is done by the upstream LSR may be used for optimality for some multicast traffic
A unique label for an egress LSR within the MPLS domain
Any stream to a particular MPLS egress node could use the label of that node.
中正資工 /黃仁竑 14
Label Distribution
Explicit Label Distribution Protocol (LDP) Reliability : by transport protocol (TCP) or as part of LDP Separate routing computation and label distribution
Piggybacking on Other Control Messages Use existing routing/control protocol for
distributing routing/control and label information OSPF, BGP, RSVP, PIM Combine routing and label distribution
Label purge mechanisms By time out Exchange of MPLS control packets
中正資工 /黃仁竑 15
Label Distribution Protocol LDP Peer:
Two LSRs that exchange label/stream mapping information via LDP LDP messages
Discovery messages announce and maintain the presence of LSR via UDP
Session messages maintain session between LDP peers
Advertisement message label operation (Label distribution)
Notification message advisory information and signal error information Error notification:signal fatal errors Advisory notification: status of the LDP session or some previous message
received from the peer.
中正資工 /黃仁竑 16
Label Swapping
Labeled Packet Map the incoming label to
an next hop label, determines where to forward the packet
Encodes the new label stack into the packet, and then forwards it
Unlabeled Packet LSR analyzes the L3 header,
to determine the packet’s stream
Map the stream to a next hop, determines where to forward the packet
Encodes the new label stack into the packet, and then forwards it
L a b e l S w itc h in g R o u te r(L S R )
IP R o u te rM o d u le
1 2
L a b e l
L 3 H e a d e r
P o rt1
L a b e l4
P o rt2
L a b e l6
O u tp u tIn p u t
In c o m in g L a b e l M a p (IL M )
E xam p le : Fo rw ard in g a L ab eled P ack et
D at H 3 6 H 2D at H 3 4 H 2
L 2 H e a d e r
中正資工 /黃仁竑 17
Use of MPLS in a Hierarchy
R 3R 2
R 6R 5R 4
R 1
D om ain 1 D om ain 2
O S P F
B G PL 1
P u shL 2
L 1
L 1S w a p
L 4
L 1P o p
L 3
L 1
S w a pL 3
L 1
L 2
L 1
O U T
L 2
IN
L 3
O U T
L 1
IN
L 1
O U T
L 4
IN
L 2
O U T
L 3
中正資工 /黃仁竑 18
Route Selection
Hop by hop routing Like conventional IP routing Each hop makes independent choice of next hop Repair of a failed route done locally
Explicit routing Manual or based on dynamic routing The LSP next hop is chosen by a single node Useful for policy routing and/or traffic
engineering If an explicit route is specified for an LSP, then
that route must be followed
中正資工 /黃仁竑 19
Loop Handling
Loop Survival minimizes the impact of loops
Loop Detection allows loops to be set up, but detects them
and eliminates them later Loop Prevention
avoiding setting up a loop
中正資工 /黃仁竑 20
Loop Survival
Allow the network to operate well even though short term transient loops may be formed by the routing protocol
Possible solutions Use of TTL to limit the hops that a packet traversed Use of dynamic routing protocol which converges fast
looping packets may cause congestion which may then affect the converge speed of routing protocol
Use of fair queueing to limit the impact of looping packets on normal packets
中正資工 /黃仁竑 21
Loop Detection
Loop may be set up, but will be subsequently detected.
Possible solutions Loop Detection Control Protocol (LDCP)
transmit LDCP packet when route change LDCP is forwarded towards destination until
destination TTL exceeded return to a node which originally transmitted it
Path Vector Control message: list of LSRs on the path hop count to each egress node (like RIP?)
中正資工 /黃仁竑 22
Loop Prevention Ensure loops are never set up Possible solutions
labels are propagated from the egress switch, control packets which propagate the labels also include the path
diffusion mechanism when route changes colored mechanism
a color consist of address of the node that created the color and a local id that is unique within the node
a node that finds a change in the next hop creates a color and passes it to the new next hop
stops when a loop or a loop free path is found explicit routing
configured use routing protocol (link state or path vector)
中正資工 /黃仁竑 23
Diffusion Algorithm
On a route change, R ask N for a label and the associated LSR ID for that stream
R looks in the LSR ID list If R is in the list (route loop), the
old LSP will continue to be used until the route protocol break the loop
If R is not in the list, R will start a diffusion computation
Diffusion computation prunes tree of paths that would loop if R switches to new LSP
When the diffusion completes, R switches to new LSP and discards old LSP
R
N
E
O ld P a thN e w P a th
中正資工 /黃仁竑 24
Diffusion Computation
An extension of Path Vector mechanism An LSR, D, detects the next hop for an FEC has chan
ged, transmits a query message with a Path Vector containing its id to its upstream
A LSR, U, that receives a query will determine if D is the next hop for the given FEC if not, then U return OK message if so, then U checks if the Path Vector already contains it id
if yes, a loop is detected, U responds with a LOOP msg if not, U adds its id to the Path Vector and propagates t
he query message to its upstream neighbors
中正資工 /黃仁竑 25
What to Do if a Loop is Detected
If a loop is know to exist L2 label-swapped path is not setup Packet is forwarding using normal L3 forwarding
Problems : Nodes which are not capable of L3 forwarding
discard packet L2 forwarding faster than L3 forwarding
node will not be capable of forwarding the same volume of traffic at l3, some packets will be discarded
packet lost cause TCP to backoff, which will in turn reduce the load and allow the network to stabilize until the label binding is reestablished again.
中正資工 /黃仁竑 26
Label Merging
Label merging An LSR may want to bind multiple incoming labels to a
particular FEC once packets are transmitted, the information that
they arrived with different labels is not Non-merging LSRs
In ATM, label merging may cause interleaving of cells from various packets
MPLS support procedures which allow ATM switches to function as merging LSRs
中正資工 /黃仁竑 27
Merge over ATM
VP Merge (SVP multipoint encoding) Packet from different sources are distinguished by using dif
ferent VCs within the VP Advantage : no new hardware Disadvantage : requires coordination of the VCI space
VC Merge Switches are required to buffer cells from one packet until t
he entire packet is received Advantage : straightforward application of VC switching Disadvantage :
New hardware (based on per-VC queuing) Delays at the merge points
中正資工 /黃仁竑 28
Conclusion
MPLS A more general forwarding mechanism Cooperates with routing/control protocol Provides Integrated service, Differentiated service Allows flow aggregation (FEC) for QoS routing Support Multicast?