1. Intra-Domain Routing and Traffic Engineering
Review of Internet routing paradigm and routing algorithms/protocols
Intra-domain routing design
topology design, convergence, stability, …
Readings: do the required readings.
CSci5221: Intra-Domain Routing Review and Routing Design

2. CSci5221: Intra-Domain Routing Review and Routing Design
Internet Routing
So far we have focused on data plane operations
Forwarding: data plane
Directing a data packet to an outgoing link
Individual router using a forwarding table
Routing: control plane
Computing the paths the packets will follow
Routers talking amongst themselves
Individual router creating a forwarding table
CSci5221: Intra-Domain Routing Review and Routing Design

3. Routing Routing protocol Graph abstraction for routing algorithms:
Goal: determine “good” path
(sequence of routers) thru
network from source to dest. 5 3 B C 2 5 A
Graph abstraction for routing algorithms:
graph nodes are routers
graph edges are physical links
link cost: delay, $ cost, or congestion level 2 1 F 3 1 2 D E 1
“good” path:
typically means minimum cost path
other def’s possible
CSci5221: Intra-Domain Routing Review and Routing Design

4. Two-Tiered Internet Routing System
Interdomain routing: between ASes
Routing policies based on business relationships
No common metrics, and limited cooperation
BGP: policy-based, path-vector routing protocol
Intradomain routing: within an AS
Shortest-path routing based on link metrics
Routers all managed by a single institution
OSPF and IS-IS: link-state routing protocol
RIP and EIGRP: distance-vector routing protocol
CSci5221: Intra-Domain Routing Review and Routing Design

5. Shortest-Path Routing
Path-selection model
Destination-based
Minimum hop count or sum of link weights
Dynamic vs. static link weights 3 2 1 4 5
CSci5221: Intra-Domain Routing Review and Routing Design

6. Distance Vector Routing: Bellman-Ford
Define distances at each node x
dx(y) = cost of least-cost path from x to y
Update distances based on neighbors
dx(y) = min {c(x,v) + dv(y)} over all neighbors v 3 2 1 4 5 u v w x y z s t
du(z) = min{c(u,v) + dv(z),
c(u,w) + dw(z)}
E.g., RIP and EIGRP
CSci5221: Intra-Domain Routing Review and Routing Design

7. Link-State Routing: Dijsktra’s Algorithm
Each router keeps track of its incident links
Link cost, and whether the link is up or down
Each router broadcasts the link state
To give every router a complete view of the graph
Each router runs Dijkstra’s algorithm
To compute shortest paths and forwarding table 3 2 1 4 5
E.g., OSPF and IS-IS
CSci5221: Intra-Domain Routing Review and Routing Design

8. OSPF (Open Shortest Path First)
Link State Protocol
link costs between 0 and 65,535
Cisco recommendation - link cost = 1/(link capacity)
rapid, loop-free convergence, scales well
topology map at each node, route computation using Dijkstra’s algorithm
OSPF advertisement carries one entry per neighbor router, advertisements flooded to entire Autonomous System
multiple equal-cost paths allowed: flow equally split on all outgoing links belonging to shortest paths
IS – IS (intermediate system-intermediate system) similar
CSci5221: Intra-Domain Routing Review and Routing Design

9. CSci5221: Intra-Domain Routing Review and Routing Design
Routing Protocols
Link State
Distance Vector
Path Vector
Dissem-ination
Flood link state advertisements to all routers
Update distances from neighbors’ distances
Update paths based on neighbors’ paths
Algorithm
Dijsktra’s shortest path
Bellman-Ford shortest path
Local policy to rank paths
Converge
Fast due to flooding
Slow, due to count-to-infinity
Slow, due to path exploration
Protocols
OSPF, IS-IS
RIP, EIGRP
BGP
CSci5221: Intra-Domain Routing Review and Routing Design

10. Intra-Domain Routing Today
Link-state routing with static link weights
Static weights: avoid stability problems
Link state: faster reaction to topology changes
Most common protocols in backbones
OSPF: Open Shortest Path First
IS-IS: Intermediate System–Intermediate System
Some use of distance vector in enterprises
RIP: Routing Information Protocol
EIGRP: Enhanced Interior Gateway Routing Protocol
Growing use of Multi-Protocol Label Switching
CSci5221: Intra-Domain Routing Review and Routing Design

11. What do Operators Worry About?
Topology design
Small propagation delay and low congestion
Ability to tolerate node and link failures
Convergence delay (will discuss more later)
Limiting the disruptions during topology changes
E.g., by trying to achieve faster convergence
Scalable routing designs
Avoiding excessive protocol overhead
E.g., by introducing hierarchy in routing
Traffic engineering (focus today)
Limiting propagation delay and congestion
E.g., by carefully tuning the “static” link weights
CSci5221: Intra-Domain Routing Review and Routing Design

12. Topology Design: Intra-AS Topology
Node: router
Edge: link
Hub-and-spoke
Backbone
CSci5221: Intra-Domain Routing Review and Routing Design

13. ISP Topology Design: Points-of-Presence (PoPs)
Inter-PoP links
Long distances
High bandwidth
Intra-PoP links
Short cables between racks or floors
Aggregated bandwidth
Links to other networks
Wide range of media and bandwidth
Inter-PoP
Intra-PoP
Other networks
CSci5221: Intra-Domain Routing Review and Routing Design

14. Topology Design: Abilene Internet2 Backbone
CSci5221: Intra-Domain Routing Review and Routing Design

15. Convergence: Detecting Topology Changes
Beaconing
Periodic “hello” messages in both directions
Detect a failure after a few missed “hellos”
Performance trade-offs
Detection speed
Overhead on link bandwidth and CPU
Likelihood of false detection
“hello”
CSci5221: Intra-Domain Routing Review and Routing Design

16. Convergence: Transient Disruptions
Inconsistent link-state database
Some routers know about failure before others
The shortest paths are no longer consistent
Can cause transient forwarding loops 2 2 1 1 3 3 1 1 4 4 2 2 1 1 5 4 3 4 3
CSci5221: Intra-Domain Routing Review and Routing Design

17. Convergence: Delay for Converging
Sources of convergence delay
Detection latency
Flooding of link-state information
Shortest-path computation
Creating the forwarding table
Performance during convergence period
Lost packets due to blackholes and TTL expiry
Looping packets consuming resources
Out-of-order packets reaching the destination
Very bad for VoIP, online gaming, and video
CSci5221: Intra-Domain Routing Review and Routing Design

18. Convergence: Reducing Convergence Delay
Faster detection
Smaller hello timers
Link-layer technologies that can detect failures
Faster flooding
Flooding immediately
Sending link-state packets with high-priority
Faster computation
Faster processors on the routers
Incremental Dijkstra algorithm
Faster forwarding-table update
Data structures supporting incremental updates
CSci5221: Intra-Domain Routing Review and Routing Design

19. Scalability: Overhead of Link-State Protocols
Protocol overhead depends on the topology
Bandwidth: flooding of link state advertisements
Memory: storing the link-state database
Processing: computing the shortest paths 3 2 1 4 5
CSci5221: Intra-Domain Routing Review and Routing Design

20. Scalability: Improving the Scaling Properties
Dijkstra’s shortest-path algorithm
Simplest version: O(N2), where N is # of nodes
Better algorithms: O(L*log(N)), where L is # links
Incremental algorithms: great for small changes
Timers to pace operations
Minimum time between LSAs for the same link
Minimum time between path computations
More resources on the routers
Routers with more CPU and memory
CSci5221: Intra-Domain Routing Review and Routing Design

21. Scalability: Introducing Hierarchy Through Areas
Divide network into regions
Backbone (area 0) and non-backbone areas
Each area has its own link-state database
Advertise only path distances at area boundaries
Area 0
Area 1
Area 2
Area 3
Area 4
area
border
router
CSci5221: Intra-Domain Routing Review and Routing Design

22. CSci5221: Intra-Domain Routing Review and Routing Design
Traffic Engineering
Goal: configure routes to meet traffic demands
balanced load, low latency, service agreements
operates at coarse timescales
Not to adapt to short-term sudden traffic changes
May take potential failures into consideration
Input to traffic engineering:
Topology: connectivity & capacity of routers & links
Traffic matrix: offered load between points in the network
Traffic Engineering: network-wide optimization
Subject to protocol mechanisms, configurable parameters and other practical constraints, ….
Will come back to this topics later!
CSci5221: Intra-Domain Routing Review and Routing Design

23. Traffic Engineering under Shortest Path Routing: Tuning Link Weights
Problem: congestion along the blue path
Second or third link on the path is overloaded
Solution: move some traffic to bottom path
E.g., by decreasing the weight of the second link 3 2 1 4 5
More on how this is done later!
CSci5221: Intra-Domain Routing Review and Routing Design

24. Limitations of Conventional Intra-Domain Routing
Overhead of hop-by-hop forwarding
Large routing tables and expensive look-ups
Paths depend only on the destination
Rather than differentiating by source or class
Only the shortest path(s) are used
Even if a longer path has enough resources
Transient disruptions during convergence
Cannot easily prepare in advance for changes
Limited control over paths after failure
Depends on the link weights and remaining graph
CSci5221: Intra-Domain Routing Review and Routing Design

25. Multi-Protocol Label Switching (MPLS)
Motivating applications
Small routing tables and fast look-ups
Virtual Private Networks
Traffic engineering
Path protection and fast reroute
Key ideas of MPLS
Label-switched path spans group of routers
Explicit path set-up, including backup paths
Flexible mapping of data traffic to paths
Label Switching
MPLS header includes a label
Label switching between MPLS-capable routers
MPLS header
IP packet
CSci5221: Intra-Domain Routing Review and Routing Design

26. MPLS: Forwarding Based on Labels
Hybrid of packet and virtual circuit switching
Logical circuit between a source and destination
Packets with different labels multiplex on a link
Basic idea of label-based forwarding
Packet: fixed length label in the header
Switch: mapping label to an outgoing link 1 2
1: 7
2: 7
link 7
1: 14
2: 8
link 14
link 8
CSci5221: Intra-Domain Routing Review and Routing Design

27. MPLS Concept: Route at Edge, Switch in CoreIP IP
#L1 IP
#L2 IP
#L3 IP
IP Forwarding
LABEL SWITCHING
IP Forwarding
CSci5221: Intra-Domain Routing Review and Routing Design

28. MPLS: Pushing, Swapping, and Popping
Pushing: add the initial “in” label
Swapping: map “in” label to “out” label
Popping: remove the “out” label IP
Pushing
Popping
Swapping R2 R1 R3 R4
MPLS core A B C D
IP edge
CSci5221: Intra-Domain Routing Review and Routing Design

29. CSci5221: Intra-Domain Routing Review and Routing Design
MPLS Header
IP packet encapsulated in MPLS header and sent down LSP
IP packet restored at end of LSP by egress router
TTL adjusted by default …
IP Packet
32-bit
MPLS Header
MPLS is responsible for directing a flow of IP packets along a predetermined path across a network. This path is called a label-switched path. Label-switched paths are similar to ATM PVCs in that they are simplex in nature; that is, the traffic flows in one direction from the ingress router to a egress router. Duplex traffic requires two label-switched paths; that is, one path to carry traffic in each direction. A label-switched path is created by the concatenation of one or more label-switched hops, allowing a packet to be forwarded from one label-switching router to another label-switching router across the MPLS domain. A label-switching router is a router that supports MPLS-based forwarding.
When an IP packet enters a label-switched path, the ingress router examines the packet and assigns it a label based on its destination, placing the label in the packet’s header. The label transforms the packet from one that is forwarded based on its IP routing information to one that is forwarded based on information associated with the label. The packet is then forwarded to the next router in the label-switched path. The key point in this scheme is that the physical path of the LSP is not limited to what the IGP would choose as the shortest path to reach the destination IP address.
CSci5221: Intra-Domain Routing Review and Routing Design

30. CSci5221: Intra-Domain Routing Review and Routing Design
MPLS Header
Label
EXP S
TTL
label
used to match packet to LSP
experimental bits
carries packet queuing priority (CoS)
stacking bit: can build “stacks” of labels
qoal: nested tunnels!
time to live
copied from IP TTL
An MPLS header consists of:
20-bit label—Used to identify the packet to a particular LSP
Class of service value—Indicates queuing priority through the network. At each hop along the way, the class of service value determines which packets receive preferential treatment within the tunnel.
Stacking bit—Indicates that this MPLS packet has more than one label associated with it. The MPLS implementation in JUNOS Software supports a stacking depth of one.
Time to live value—Contains a limit on the number of router “hops” this MPLS packet may travel through the network. It is decremented at each hop, and if the TTL value drops below one, the packet is discarded.
CSci5221: Intra-Domain Routing Review and Routing Design

31. CSci5221: Intra-Domain Routing Review and Routing Design
MPLS Terminology
LDP: Label Distribution Protocol
LSP: Label Switched Path
FEC: Forwarding Equivalence Class
LSR: Label Switching Router
LER: Label Edge Router (Useful term not in standards)
MPLS “multi-protocol” both in terms of protocols it supports ABOVE and BELOW in protocol stack!
CSci5221: Intra-Domain Routing Review and Routing Design

32. CSci5221: Intra-Domain Routing Review and Routing Design
Example
CSci5221: Intra-Domain Routing Review and Routing Design

33. Label Distribution Protocol (LDP)
label distribution always done from downstream to upstream
downstream-unsolicited: new route => send new label
downstream-on-demand: upstream LSR asks for label
CSci5221: Intra-Domain Routing Review and Routing Design

34. Forwarding Equivalence Class (FEC)
FEC – rule for grouping IP packets
Packets that should be treated the same way,
e.g, forwarded over same path, with same forwarding treatment
Identified just once, at the edge of the network
Example FECs
destination prefix
longest-prefix match in forwarding table at entry point
useful for conventional destination-based forwarding
same src and dst addresses
5-tuple flows (src/dst addr, src/dst port, and protocol)
five-tuple match at entry point
useful for fine-grain control over the traffic
QoS class
A label is just a locally-significant identifier for a FEC
CSci5221: Intra-Domain Routing Review and Routing Design

35. CSci5221: Intra-Domain Routing and TE
Regular IP Forwarding 1
47.1 IP 1 2 IP 3 2 IP 1 3
47.2
47.3 2 IP
IP destination address unchanged in packet header!
CSci5221: Intra-Domain Routing and TE

36. MPLS Label Distribution1
47.1 3
Request: 47.1 3
Request: 47.1 2 1
Mapping: 0.40 1 2
Mapping: 0.50
47.3 3
47.2 2
CSci5221: Intra-Domain Routing and TE

37. Label Switched Path (LSP)IP 1
47.1 3 3 1 2 1 2
47.3 3
47.2 2 IP
CSci5221: Intra-Domain Routing and TE

38. MPLS Forwarding: Example
IP packet destined to /32 arrives to SF
San Francisco has route for /16
next hop is LSP to New York
San
Francisco
New York IP
Santa Fe
/16
1965
1026
CSci5221: Intra-Domain Routing Review and Routing Design

39. MPLS Forwarding Example
San Francisco pre-pends MPLS header onto IP packet, sends packet to first transit router on path
San
Francisco
New York
Santa Fe
/16 IP
1965
CSci5221: Intra-Domain Routing Review and Routing Design

40. MPLS Forwarding Example
because packet arrived to Santa Fe with MPLS header, Santa Fe forwards it using MPLS forwarding table
San
Francisco
New York
Santa Fe
/16 IP
1026
CSci5221: Intra-Domain Routing Review and Routing Design

41. MPLS Forwarding Example
packet arrives from penultimate router with label 0
egress router sees label 0, strips MPLS header
egress router performs standard IP forwarding
San
Francisco
New York
Santa Fe IP
/16
Labels 0 through 15 are reserved labels, as specified in draft-ietf-mpls-label-encaps-07.txt.
A value of 0 represents the "IPv4 Explicit NULL Label". This label value is only legal when it is the sole label stack entry. It indicates that the label stack must be popped, and the forwarding of the packet must then be based on the IPv4 header.
A value of 1 represents the "Router Alert Label". This label value is legal anywhere in the label stack except at the bottom. When a received packet contains this label value at the top of the label stack, it is delivered to a local software module for processing. The actual forwarding of the packet is determined by the label beneath it in the stack. However, if the packet is forwarded further, the Router Alert Label should be pushed back onto the label stack before forwarding. The use of this label is analogous to the use of the "Router Alert Option" in IP packets. Since this label cannot occur at the bottom of the stack, it is not associated with a particular network layer protocol.
A value of 2 represents the "IPv6 Explicit NULL Label".This label value is only legal when it is the sole label stack entry. It indicates that the label stack must be popped, and the forwarding of the packet must then be based on the IPv6 header.
A value of 3 represents the "Implicit NULL Label". This is a label that an LSR may assign and distribute, but which never actually appears in the encapsulation. When an LSR would otherwise replace the label at the top of the stack with a new label, but the new label is "Implicit NULL", the LSR will pop the stack instead of doing the replacement. Although this value may never appear in the encapsulation, it needs to be specified in the Label Distribution Protocol, so a value is reserved.
Values 4-15 are reserved for future use.
CSci5221: Intra-Domain Routing Review and Routing Design

42. CSci5221: Intra-Domain Routing Review and Routing Design
Status of MPLS
Deployed in practice
Small control and data plane overhead in core
Virtual Private Networks
Traffic engineering and fast reroute
Challenges
Protocol complexity
Configuration complexity
Difficulty of collecting measurement data
Continuing evolution
Standards
Operational practices and tools
CSci5221: Intra-Domain Routing Review and Routing Design