Label Switching

Label Switching

Motivation

Issues related to IP based routing

Lookup is rather complex
- Longest matching prefix $\rightarrow$ high performance forwarding needed
Shortest path routing selects shortest path to destination
- Multiple paths to destination can not be utilized concurrently $\rightarrow$ traffic engineering desirable
Strictly packet based
- Each IP datagram is handled individually – no support for data streams (flows) 🤪

Flows

What is a flow?

A flow is a sequence of packets traversing a network that share a set of header field values.

Different levels of granularity possible, e.g.,

All packets belonging to a particular TCP connection
HTTPS traffic
VoIP traffic
- Of a particular sender
- Within a network

Example

Flow Based Forwarding

Fundamental concept, independent of certain layers
- Can span multiple layers
Incorporates classic routing/forwarding concepts
Goes beyond classic concepts

Aggregation

Micro-flows
- Consider a single “connection” e.g., a TCP connection
- Fine grained control
- High number of flows possible
Macro-flows
- Higher level of aggregation
- Aggregation of several “connections”
  - e.g., IP destination address in specific subnet
- Lower number of flows

Classification of Communication Networks

Label Switching

Combination of
- Packet switching
  - Packets are forwarded individually (data path is NOT fixed)
  - Packets include metadata needed for forwarding decision
- Circuit switching
  - Paths established for flows through the network (data path is fixed)
  - Simple forwarding decision
  - Differentiation of flows possible
    - Load balancing
    - Quality of service (QoS)
Implementation
- Switching at layer 2, Instead of routing at layer 3
- Labels: Identification which is only locally valid
- Virtual circuits: Sequence of labels

Label

Short unstructured identification of fixed length
- Does NOT carry any layer-3-information
- Unique: only locally at the corresponding switch
- Label swapping: Mapping from input label to output label
Virtual circuit: Identified through sequence of labels at the path

Transport of Label

Label must be transported within the packet

Additional „header“ in the packet, between headers of layer 2 and layer 3 $\rightarrow$ layer 2.5
Alternative: In specialized fields within existing packet headers
- IPv6: flow label (20 bit field in IPv6 header, to identify micro flows more easily)

Label Switching Domain

Basic architecture

Border of the domain (edge devices)
- Add / remove label
- Map flow to forwarding class
- Access control
- …
Within the domain (switching device)
- Forward packets based on label information
- Label swapping

Label Forwarding Information Base

Forwarding table in case of label switching: Efficient access through label (NO longest prefix matching needed).

Example:

Multiprotocol Label Switching (MPLS)

General Aspects

MPLS

Based on label switching
Originally: data plane optimization
Standardized within the IETF
Increasingly applied in larger autonomous systems
Main Features
- Fast forwarding (due to reduced amount of packet processing)
- QoS support
  - Guarantees on latency and capacity, e.g., for voice traffic
- Traffic engineering
  - Supports load balancing in order to optimize network utilization …
- Virtual private networks
  - Isolate traffic from other packets on the Internet
- Multiple networks support
  - Usable on different network technologies, e.g., IP, ATM …

👍 Advantages

Clear separation of forwarding (label switching) and control (manipulation of label binding)
Not limited to IP
Support of metrics
Versatile concept
Scales

Architecture, Components and Basic Operation

Architecture

Components

Label-switching router (LSR)
- MPLS-capable IP router
  - Can forward packets based on both, IP prefixes and MPLS labels
  - Typically: IP for control plane and MPLS for data plane
- Architecture:
Label edge router (LER)
- Router at the edge of an MPLS domain
  - Each LSR with a non-MPLS capable neighbor is an LER
  - Also called: label ingress router resp. label egress router
- Classifies packets that enter the MPLS domain
  - Forwarding equivalency class (FEC)
MPLS-Node: General term for MPLS-capable intermediate systems, like LSRs

Forwarding Equivalence Classs

Class of packets that should be treated equally
- Same path through the network
- Same QoS properties
Basis for label assignment
MPLS-specific term, roughly comparable to „flow“
Example
- Same address prefix and same type-of-service field
- Same IP addresses and same port numbers
- VoIP traffic with destination address in subnet X
Granularity
- Coarse-grained: Important for quick forwarding and scalability
- Fine-grained: Important for differentiated treatment of packets or flows

Example 1: Very fine granular FEC (“micro flow”)

A single TCP connection, identified by 5-tuple

Example 2: data streams differentiation

Traffic engineering
- Usage of different paths
- Goals
  - Load balancing
  - Utilization of all available resources
  - Prioritization of individual data streams
  (realized through separate virtual connections)
Support of quality of service
Different quality of service for different data streams

Label Switched Path

Virtual connection: Sequence of labels on a path through MPLS domain.

Example:

MPLS-Label

Encapsulation: Between headers of layer 2 (Data Link layer) and layer 3 (Network layer)

Label: the label itself
Exp: Bits for experimental usage
S: Stack-bit
TTL: Time-to-live

Label Distribution

Label Binding
- Associate specific label to FEC
- Stored in label forwarding information base
  - Used as incoming label
Label distribution
- Label binding is distributed to neighboring routers
- Stored in label forwarding information base
  - Used as outgoing label

Types of Label Distribution

“Roles” of a label-switching router
- Downstream LSR: In direction of data flow
- Upstream LSR: Against direction of data flow
Unsolicited downstream
- Router generates label bindings as soon as it is ready to forward MPLS packets of the respective FEC
  - Upstream neighbors (according to IP routing): update forwarding tables
    - Label used as outgoing label
  - Non-upstream neighbors can store label for later use
    - Quicker reactions on route changes
Downstream on demand
- Downstream router generates label binding on demand
- Upstream router has to request label binding for FEC

Label Distribution Protocol

RSVP (Resource ReserVation Protocol)

🎯 Goal: bandwidth reservation for end-to-end data streams
Soft state principle
- Establish a session and periodically signal that session is still alive
- In case of failure state is automatically removed after some time
Signaling
- Path message
  - From sender to receiver
  - Find path to receiver
  - Each hop is recorded in the message
- Resv message
  - From receiver to sender
  - Bandwidth reservation on return path

RSVP-TE (Traffic Engineering)

Extension to RSVP to support label distribution
- Many additional fields and functionality, e.g., fast reroute
Signaling
- Path message
  - From upstream LER to downstream LER
  - Label request
  - Source route (“explicit route”) [optional]
- Resv message
  - In response to path message
  - From downstream LER to upstream LER
  - Label binding (hop-per-hop)

Virtual Private Networks

MPLS is useful for virtual private networks (VPNs)
Use case: VPN traffic engineering
- Customer with sites at different locations (e.g., different cities) wants to lease seamless “network” service
- Requirements
  - Connect physically remote locations
  - Carry IP-based intranet traffic
  - Each customer has obtained an IP address block
  - Guaranteed bandwidth / SLAs
- Options
  - “Dark fibre” provider
  - VPN backbone provider

Example: Private Networks over “Dark Fibre”

Suppose that three companies have sites at remote locations

Company A: Karlsruhe, Paris, Zürich
Company B: Karlsruhe, Paris
Company C: Karlsruhe, Paris

Each company runs a private network

Different subnet for each site from customers IP address space
Router connects site to other site(s)
Data is transported over leased fiber optic cables (“dark fibre”)
- Capacity 155 Mbit/s, utilization marked in graph

A provider uses MPLS to offer virtual private networks

Has „points of presence (PoP)“ in all three cities
Offers bandwidth at arbitrary rates
Is cheaper than leasing fiber optic cables

Question: Can the provider serve the need of all three companies?

The answer is: YES! By utilizing non-shortest paths!

We can achieve that using VPNs implemented by Label Switching

Outer label: identifies path to LER
Inner label: identifies VPN instance / customer

For company A:

Inner label $5$: Indicates that this packet belongs to company A\
Outer labels $2, 7, 1$: Label switching/Swapping

For company B:

For company C:

Label Distribution

Recall VPN example from above

LSP for customer B (Karlsruhe $\rightarrow$ Paris) should take a “detour” over Zürich) to match bandwidth requirements
Setup of LSPs over explicitly given route with RSVP-TE
- Example: LSP “Karlsruhe to Paris over Zürich”
  - RSVP-TE signaling initiated at upstream LER (LER-KA)
  - Note: LSPs are unidirectional!

How are the labels distributed?