CS221: IPv6

1
Routing, Addressing, and Naming Switching in
the Internet
Christophe Jelger Post-doctoral
researcher Christophe.Jelger_at_unibas.ch
2
Today's lecture

• MPLS MultiProtocol Label Switching.
• Metropolitan Ethernet.
• The Spanning Tree Protocol (SPT) for Ethernet
  networks.

3
MPLS MultiProtocol Label Switching (1)

• What is it all about?
• The Internet grew from circuit-switched
  (telephone) networks to packet switched networks.
• Telcos were offering leased lines to
  inter-connect networks located at different
  locations (e.g. the world-wide branches of a
  large company).
• Circuit-switching was very well known and
  provided a clear separation of services with
  different levels of quality.
• Circuit-switching was offering a higher level of
  control in the core of the ISPs' networks.
• Technologies like ATM were offering virtual
  circuits and a relatively high level of traffic
  enginneering capabilities.
• With the growth of IP, telcos/ISPs needed a new
  technology to provide this kind of services in an
  IP-friendly manner.

4
MPLS MultiProtocol Label Switching (2)

• History MPLS was hence initially designed to
• Provide a more IP-friendly data-carrying
  technology than ATM.
• Running IP over ATM was complex, and ATM small
  cells (53 bytes) were becoming an overhead when
  carrying potentially large IP packets.
• Allow the creation of simple high-speed "IP
  switches".
• At that time, IP forwarding was not entirely
  feasible in hardware (because of the
  longest-prefix-match forwarding scheme).
• No longer an issue in modern routers, although
  "switching is still faster than routing".
• Create a "shim" 2.5 layer to unify data-carrying
  technologies.
• MPLS used over existing ATM and FrameRelay
  infrastructures.
• IP used directly over MPLS.

5
MPLS MultiProtocol Label Switching (3)

• What is the goal of MPLS today?
• ISPs need to separate/isolate different kinds of
  traffic (IP, voice, video, business-critical
  applications, etc) in their core network(s). In
  practice, MPLS is used to provide
• Virtual Private Networks (VPNs).
• Quality of Services (e.g. guaranteed bandwidth
  between some points in the network).
• Traffic Enginnering (e.g. load balance traffic
  over all links of a network).
• To do this, MPLS introduces the notion of FEC
  Forwarding Equivalence Class.
• A FEC is a group of IP packets which are
  forwarded in the same manner inside an MPLS
  network.
• In practice, a classifier inspects each IP packet
  entering an MPLS network and decides to which
  FEC it belongs.

6
MPLS MultiProtocol Label Switching (4)

• How does it work?
• MPLS uses label switching to forward packets.
• Fowarding is based on "exact match" this is much
  faster than IP's longest-prefix match.
• A label is a short (4 bytes) locally-significant
  identifier used to identify a Forwarding
  Equivalence Class (FEC). MPLS labels have the
  following format
• label value 20 bits, unstructured (flat)
• exp 3 bits, currently used as Class of Service
  (CoS) field
• S bit "bottom of stack" indicator (when labels
  are stacked)
• Time To Live 8 bits.

label value
exp
S
TTL
32 bits
7
MPLS MultiProtocol Label Switching (5)

• The forwarding of packets inside an MPLS network.
• Labels are used to identify Label-Switched Paths
  (LSPs).
• The mapping between IP packets FECs and LSPs is
  done by Label Switched Routers (LSRs) at the
  edges of the MPLS network.

13 ? pop, oif1
Packets for 10.1.2.3.0/24 (blue) 10.1.2.4.0/24
(red)
Forwarding is based on label
1
13
Ingress LSR
subnet 10.1.3.0/24
1
Egress LSR
17
1
21
2
44
subnet 10.1.2.0/24
Assigns each IP packet to the appropriate FEC and
adds appropriate label to IP packet
17 ? swap(13), oif1 21 ? swap(44), oif2
1
subnet 10.1.4.0/24
44 ? pop, oif1
10.1.3.0/24 ? push(17), oif1 10.1.4.0/24 ?
push(21), oif1
8
MPLS MultiProtocol Label Switching (6)

• The forwarding of packets inside an MPLS network.
• FECs can be encasulated inside other FECs we end
  up with stacks of labels. This is useful to
  create "trunks" and reduce state in the core MPLS
  network.

Packets for 10.1.2.3.0/24 (blue) 10.1.2.4.0/24
(red)
13 ? pop, oif1
Forwarding is based on label
1
17
13
subnet 10.1.3.0/24
1
17
11
17
6
1
1
21
6
2
21
11
21
44
6 ? swap(11), oif1
11 ? pop 17 ? swap(13), oif1 21 ? swap(44), oif2
1
17 ? push(6), oif1 21 ? push(6), oif1
subnet 10.1.4.0/24
44 ? pop, oif1
9
MPLS MultiProtocol Label Switching (7)

• The distribution of labels.
• For each hop, the label is chosen by the
  downstream LSR and passed to the upstream LSR.
  Hence labels are distributed "against the flow of
  packets".
• The distribution of labels can be done "in
  collaboration" with an intra-domain routing
  protocol like OSPF or IS-IS.
• There are currently 2 protocols to distribute
  labels
• LDP Label Distribution Protocol.
• RSVP-TE Resource reSerVation Protocol for
  Traffic Engineering.

10
MPLS MultiProtocol Label Switching (8)

• The distribution of labels.
• A simplified example.

The LSR chooses a label
13 ? pop, oif1
Request PATH 10.1.3.0/24
Reply RESV label 13
Reply RESV label 17
Request PATH 10.1.3.0/24
1
Ingress LSR
subnet 10.1.3.0/24
1
Egress LSR
1
2
subnet 10.1.2.0/24
17 ? swap(13), oif1
1
The LSR chooses a label
10.1.3.0/24 ? push(17), oif1
subnet 10.1.4.0/24
11
MPLS MultiProtocol Label Switching (9)

• MPLS in the Internet today.
• MPLS is used extensively by most ISPs. An
  extended version called GMPLS (Generalized MPLS)
  is also used to setup LSPs over optical fiber
  technologies (SONET/SDH and DWDM).
• With "Metro Ethernet" networks, MPLS is used to
  provide "pseudowires" between Ethernet switched
  networks.
• MPLS is still evolving the IETF mpls working
  group is very active, with many internet drafts
  still active and various mechanisms still being
  standardized (e.g. lsp-ping, security, network
  management, etc).

12
Metro/Carrier Ethernet (1)

• According to some studies, 95 of today's
  Internet traffic starts and ends as Ethernet
  (end-sites are using Ethernet networks).
• In the mean time, ISPs/carriers used everything
  but Ethernet in their backbone networks.
• Ethernet is becoming extremely cheap with very
  high data rates.
• In contrast, data carrying technologies
  (SONET/SDH, MPLS) are relatively expensive.
• 10 Gb/s already there, 40 Gb/s and 100 Gb/s are
  on their way.
• However, Ethernet is too "dumb" for carriers.
• Backbone networks require advanced services like
  QoS, network management, traffic engineering, etc.

13
Metro/Carrier Ethernet (2)

• Metro/Carrier Ethernet is a set of technologies
  and products.
• The terms "metro" and "carrier" are more or less
  used to describe the same technologies. However
  "metro" is targeted more at customers networks,
  while "carrier" is targeted more at ISPs.
• Many manufacturers, standards, and deployment
  styles.
• Common denominator is Ethernet for example, one
  typical obejctive is to inter-connect Ethernet
  VLANs via a backbone network (e.g. to
  inter-connect the networks located at different
  branches of a large organization).

14
Metro/Carrier Ethernet (3)

• Metro/Carrier Ethernet some protocols.
• IEEE 802.1Q tunneling, or "tag stacking", or
  "QinQ".
• Very similar to MPLS labeling and label stacking,
  but with Ethernet VLAN tagging technologies the
  goal is to inter-connect customers' VLANs without
  any "collision of VLAN ids/tags".

Image from http//www.cisco.com/warp/public/cc/pd
/si/casi/ca6000/prodlit/65met_wp.htm
15
Metro/Carrier Ethernet (4)

• Metro/Carrier Ethernet some protocols.
• IEEE 802.1Q tunneling, or "tag stacking", or
  "QinQ".
• Also known as 802.1ad or "Provider bridges".

CPE Customer Premises Equipment
PE Provider Edge
Image from http//www.cisco.com/warp/public/cc/pd
/si/casi/ca6000/prodlit/65met_wp.htm
16
Metro/Carrier Ethernet (5)

• Metro/Carrier Ethernet scalability.
• QinQ is limited to 4094 tags/customers, and there
  is a scalability issue with the size of
  forwarding tables.
• To remediate this, new standards have been
  defined
• IEEE 802.1ah or "Backbone Provider Bridges" or
  "MAC-in-MAC".
• Introduces encapsulation techniques of Ethernet
  in Ethernet.
• IEEE 802.1Qay-TE a carrier grade extension of
  802.1ah with traffic engineering, MPLS
  compatibility, deterministic delivery.
• HVLAN proposed extension to introduce
  hierarchical VLAN tagging with a CIDR-style "bast
  match" forwarding.
• Sound like re-inventing the wheel?
• New variants (with new names) of MPLS, IP, SONET,
  ATM?

17
Metro/Carrier Ethernet (6)

• Currently an extremely active area.
• Plenty standards on their way.
• IETF vs. IEEE battle.
• Vendors battle with competing technologies and
  protocols.
• Development seems to be fully driven by the
  market (and not always by technical advances).
• ISPs want to save cost to extend their
  infrastructures.
• Customers want to pay less.
• Vendors want to sell new equipments.
• Network deployments is really becoming "à la
  carte"
• e.g. MPLS over Ethernet? Eth. over MPLS? Eth.
  over MPLS over Eth.?
• A palette of technologies, costs, and services.
  Not clear who wins

18
The spanning tree protocol (SPT).

• A spanning tree of a graph is a sub-graph that
  contains all the vertices (nodes) and is a tree.
• Note that a given graph usually have multiple
  spanning trees.

19
The spanning tree protocol (2).

• In a bridged Ethernet network, the main objective
  of STP is to prevent loops in a topology with
  redundant paths.
• How? Redundant links are de-activated (for
  forwarding).
• One goal is to prevent the "broadcast storm
  problem".

Broadcast loop
Loop is prevented
A
A
ARP REQ B?
ARP REQ B?
B
B
Ethernet switch.
20
The spanning tree protocol (3).

• Another goal is to prevent duplicate frames to be
  received.

A
data sent to B
B
Duplicate frame is received!
Ethernet switch.
21
The spanning tree protocol (4).

• Loops also generate inconsistent and unstable
  states.
• e.g. a switch learns on which port a machine is
  connected by looking at the source MAC address of
  Ethernet frames.

Switch learns A is on right port
Switch learns A is on left port
A
data sent to B
B
Ethernet switch.

• Also note Ethernet frames have no TTL !
• i.e. they can potentially re-circulate forever!

22
The spanning tree protocol (5).

• Centralized algorithms are not desirable in
  practice but are interesting to study the
  problem.
• E.g. Kruskal, Prim, Boruvka, and Dijkstra
  algorithms.
• Challenges for distributed algorithms
• To converge (!) only one active spanning tree
  during steady-state.
• To converge rapidly after topology change (Rapid
  STP).
• Should remain simple for low-cost implementation.
• Very old and well studied algorithm.
• For Ethernet, it is standardized today by IEEE
  802.1D (1990).
• Since 2004, RSTP replaces STP in the standard.

23
The spanning tree protocol (6).

• Basic operation of STP All switches
  participating in STP gather information on other
  switches in the network through an exchange of
  data messages.
• These messages are bridge protocol data units
  (BPDUs). This exchange of messages results in the
  following
• The election of a unique root switch.
• The election of a designated switch for every
  switched LAN segment.
• The removal of loops in the switched network by
  placing redundant switch ports in a backup state.
• The root switch is the logical center of the
  spanning-tree topology. All paths that are not
  needed to reach the root switch from anywhere in
  the switched network are placed in backup mode.

24
The spanning tree protocol (7).

• Electing a root bridge.
• Each switch has a MAC address and a configurable
  priority number both of these numbers make up
  the Bridge Identification or BID.
• The BID is used to elect a root bridge based upon
  the lowest priority number if this is a tie then
  the numerically lowest MAC address wins.
• Upon startup all bridges send BPDUs. Once found,
  only the root bridge sends BPDUs (e.g. every 2
  seconds).
• Typical forwarding algo Forward a BPDU if and
  only if BID lt my_BID.Stop sending my own BPDUs
  if I see BPDUs with BID lt my_BID.

25
The spanning tree protocol (8).

• Format for the BID.

26
The spanning tree protocol (9).

• Finding shortest paths to the root bridge.
• Each bridge must keep one and only one active
  link to the root bridge.
• Link with lowest cost is kept as root link (root
  port).
• Redundant links are blocked.
• Shortest path is based on cumulative link cost.
• Link costs are based on the speed of the link.

27
The spanning tree protocol (10).
Root port
Root port
28
The spanning tree protocol (11).

• Electing a designated port for each segment.
• Port announcing lowest cost is elected as
  designated port for segment.

Root port
Root port
29
The spanning tree protocol (12).

• After convergence is reached there is
• One spanning tree per Ethernet network.
• One root bridge.
• One root port per non-root bridge.
• One designated port per segment.
• All other ports are blocked.
• Note that it's possible to have one spanning tree
  per Ethernet VLAN.

30
The spanning tree protocol (13).

• In 2004 STP is replaced in the standard by Rapid
  STP.
• Convergence of STP takes up to 50 seconds.
• Detection of lost BPDUs 20 seconds (root
  bridge lost).
• Listening phase (no data forwarding) 15
  seconds.
• Learning phase (no data forwarding) 15 seconds.
• Changes introduced by RSTP are
• All bridges periodically generate BPDUs costs
  are updated more rapidly.
• Links are point-to-point, edge-type, shared
  failures are detected more rapidly (e.g. non
  bridge-to-bridge ports are ignored).
• Network convergence is up to 15 seconds.

31
The rapid spanning tree protocol (14).
32
Thank you
Questions?