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PPP, ATM, MPLS

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Title: Ethernet Author: Z. Morley Mao Last modified by: Z. Morley Mao Created Date: 11/1/2005 4:15:04 AM Document presentation format: On-screen Show – PowerPoint PPT presentation

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Title: PPP, ATM, MPLS


1
PPP, ATM, MPLS
  • EECS 489 Computer Networks
  • http//www.eecs.umich.edu/courses/eecs489/w07
  • Z. Morley Mao
  • Monday March 12, 2007

Acknowledgement Some slides taken from
KuroseRoss
2
More on Switches
  • cut-through switching frame forwarded from input
    to output port without first collecting entire
    frame
  • slight reduction in latency
  • combinations of shared/dedicated, 10/100/1000
    Mbps interfaces

3
Institutional network
mail server
to external network
web server
router
switch
IP subnet
hub
hub
hub
4
Switches vs. Routers
  • both store-and-forward devices
  • routers network layer devices (examine network
    layer headers)
  • switches are link layer devices
  • routers maintain routing tables, implement
    routing algorithms
  • switches maintain switch tables, implement
    filtering, learning algorithms

5
Summary comparison
6
Point to Point Data Link Control
  • one sender, one receiver, one link easier than
    broadcast link
  • no Media Access Control
  • no need for explicit MAC addressing
  • e.g., dialup link, ISDN line
  • popular point-to-point DLC protocols
  • PPP (point-to-point protocol)
  • HDLC High level data link control (Data link
    used to be considered high layer in protocol
    stack!

7
PPP Design Requirements RFC 1557
  • packet framing encapsulation of network-layer
    datagram in data link frame
  • carry network layer data of any network layer
    protocol (not just IP) at same time
  • ability to demultiplex upwards
  • bit transparency must carry any bit pattern in
    the data field
  • error detection (no correction)
  • connection liveness detect, signal link failure
    to network layer
  • network layer address negotiation endpoint can
    learn/configure each others network address

8
PPP non-requirements
  • no error correction/recovery
  • no flow control
  • out of order delivery OK
  • no need to support multipoint links (e.g.,
    polling)

Error recovery, flow control, data re-ordering
all relegated to higher layers!
9
PPP Data Frame
  • Flag delimiter (framing)
  • Address does nothing (only one option)
  • Control does nothing in the future possible
    multiple control fields
  • Protocol upper layer protocol to which frame
    delivered (eg, PPP-LCP, IP, IPCP, etc)

10
PPP Data Frame
  • info upper layer data being carried
  • check cyclic redundancy check for error
    detection

11
Byte Stuffing
  • data transparency requirement data field must
    be allowed to include flag pattern lt01111110gt
  • Q is received lt01111110gt data or flag?
  • Sender adds (stuffs) extra lt 01111110gt byte
    after each lt 01111110gt data byte
  • Receiver
  • two 01111110 bytes in a row discard first byte,
    continue data reception
  • single 01111110 flag byte

12
Byte Stuffing
flag byte pattern in data to send
flag byte pattern plus stuffed byte in
transmitted data
13
PPP Data Control Protocol
  • Before exchanging network-layer data, data link
    peers must
  • configure PPP link (max. frame length,
    authentication)
  • learn/configure network
  • layer information
  • for IP carry IP Control Protocol (IPCP) msgs
    (protocol field 8021) to configure/learn IP
    address

14
Virtualization of networks
  • Virtualization of resources a powerful
    abstraction in systems engineering
  • computing examples virtual memory, virtual
    devices
  • Virtual machines e.g., java
  • IBM VM os from 1960s/70s
  • layering of abstractions dont sweat the details
    of the lower layer, only deal with lower layers
    abstractly

15
The Internet virtualizing networks
  • 1974 multiple unconnected nets
  • ARPAnet
  • data-over-cable networks
  • packet satellite network (Aloha)
  • packet radio network
  • differing in
  • addressing conventions
  • packet formats
  • error recovery
  • routing

satellite net
ARPAnet
"A Protocol for Packet Network Intercommunication"
, V. Cerf, R. Kahn, IEEE Transactions on
Communications, May, 1974, pp. 637-648.
16
The Internet virtualizing networks
  • Gateway
  • embed internetwork packets in local packet
    format or extract them
  • route (at internetwork level) to next gateway

gateway
satellite net
ARPAnet
17
Cerf Kahns Internetwork Architecture
  • What is virtualized?
  • two layers of addressing internetwork and local
    network
  • new layer (IP) makes everything homogeneous at
    internetwork layer
  • underlying local network technology
  • cable
  • satellite
  • 56K telephone modem
  • today ATM, MPLS
  • invisible at internetwork layer. Looks
    like a link layer technology to IP!

18
ATM and MPLS
  • ATM, MPLS separate networks in their own right
  • different service models, addressing, routing
    from Internet
  • viewed by Internet as logical link connecting IP
    routers
  • just like dialup link is really part of separate
    network (telephone network)
  • ATM, MPSL of technical interest in their own
    right

19
Asynchronous Transfer Mode ATM
  • 1990s/00 standard for high-speed (155Mbps to 622
    Mbps and higher) Broadband Integrated Service
    Digital Network architecture
  • Goal integrated, end-end transport of carry
    voice, video, data
  • meeting timing/QoS requirements of voice, video
    (versus Internet best-effort model)
  • next generation telephony technical roots in
    telephone world
  • packet-switching (fixed length packets, called
    cells) using virtual circuits

20
ATM architecture
  • adaptation layer only at edge of ATM network
  • data segmentation/reassembly
  • roughly analogous to Internet transport layer
  • ATM layer network layer
  • cell switching, routing
  • physical layer

21
ATM network or link layer?
  • Vision end-to-end transport ATM from desktop
    to desktop
  • ATM is a network technology
  • Reality used to connect IP backbone routers
  • IP over ATM
  • ATM as switched link layer, connecting IP routers

IP network
ATM network
22
Multiprotocol label switching (MPLS)
  • initial goal speed up IP forwarding by using
    fixed length label (instead of IP address) to do
    forwarding
  • borrowing ideas from Virtual Circuit (VC)
    approach
  • but IP datagram still keeps IP address!

PPP or Ethernet header
IP header
remainder of link-layer frame
MPLS header
label
Exp
S
TTL
5
20
3
1
23
MPLS capable routers
  • a.k.a. label-switched router
  • forwards packets to outgoing interface based only
    on label value (dont inspect IP address)
  • MPLS forwarding table distinct from IP forwarding
    tables
  • signaling protocol needed to set up forwarding
  • RSVP-TE
  • forwarding possible along paths that IP alone
    would not allow (e.g., source-specific routing)
    !!
  • use MPLS for traffic engineering
  • must co-exist with IP-only routers

24
MPLS forwarding tables
in out out label
label dest interface
10 A 0
12 D 0
8 A 1
R6
0
0
D
1
1
R3
R4
R5
0
0
A
R2
R1
25
Chapter 5 Summary
  • principles behind data link layer services
  • error detection, correction
  • sharing a broadcast channel multiple access
  • link layer addressing
  • instantiation and implementation of various link
    layer technologies
  • Ethernet
  • switched LANS
  • PPP
  • virtualized networks as a link layer ATM, MPLS

26
Outline
  • Circuit switching
  • Packet switching vs. circuit switching
  • Virtual circuits
  • MPLS
  • Labels and label-switching
  • Forwarding Equivalence Classes
  • Label distribution
  • MPLS applications
  • Feedback forms
  • Fill out during last 20 minutes

27
Multi-Protocol Label Switching
28
Multi-Protocol Label Switching
  • Multi-Protocol
  • Encapsulate a data packet
  • Could be IP, or some other protocol (e.g., IPX)
  • Put an MPLS header in front of the packet
  • Actually, can even build a stack of labels
  • Label Switching
  • MPLS header includes a label
  • Label switching between MPLS-capable routers

MPLS header
IP packet
29
Pushing, Swapping, and Popping
  • Pushing add the initial in label
  • Swapping map in label to out label
  • Popping remove the out label

30
Forwarding Equivalence Class (FEC)
  • Rule for grouping packets
  • Packets that should be treated the same way
  • 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
  • Src/dest address, src/dest port, and protocol
  • Five-tuple match at entry point
  • Useful for fine-grain control over the traffic
  • Sent by a particular customer site
  • Incoming interface at entry point
  • Useful for virtual private networks

A label is just a locally-significant identifier
for a FEC
31
Label Distribution Protocol
  • Distributing labels
  • Learning the mapping from FEC to label
  • Told by the downstream router
  • Example destination-based forwarding

Im using label 43 for 12.1.1.0/24
Im using label 10 for 12.1.1.0/24
Pick in-label 10 for 12.1.1.0/24
In Link Out 43 to R4 10
R2
Map destinations in 12.1.1.0/24 to out-label 43
and link to R2
12.1.1.0/24
R1
R4
R3
32
Supporting Explicitly-Routed Paths
  • Explicitly routing from ingress to egress
  • Set an explicit path (e.g., based on load)
  • Perhaps reserve resources along the path
  • Extend a protocol for resource reservation
  • Start with ReSource Reservation Protocol (RSVP)
  • Used for reserving resources along an IP path
  • Extensions for label distribution explicit
    routing
  • Extend a protocol for distributing labels
  • Start with Label Distribution Protocol (LDP)
  • Extensions for explicit routing reservation
  • Two competing proposed standards

33
Applications of MPLS
34
TE With Constraint-Based Routing
  • Path calculation
  • Constrained shortest-path first
  • Compute shortest path based on weights
  • But, exclude paths that do not satisfy
    constraints
  • E.g., do not consider links with insufficient
    bandwidth
  • Information dissemination
  • Extend OSPF/IS-IS to carry the extra information
  • E.g., link-state attributes for available
    bandwidth
  • Path signaling
  • Establish label-switched path on explicit route
  • Forwarding MPLS labels

35
Surviving Failures Path Protection
  • Path protection
  • Reserve bandwidth on an alternate route
  • Protect a label-switched path by having a
    stand-by
  • Much better than conventional IP routing
  • Precise control over where the traffic will go
  • Stand-by path can be chosen to be disjoint

36
Surviving Failures Fast Reroute
  • Ensure fast recovery from a link failure
  • Protect a link by having a protection sub-path
  • Much faster recovery than switching paths
  • Affected router can detect the link failure
  • and start redirecting to the protection sub-path

37
BGP-Free Core
iBGP
eBGP
C
12.1.1.0/24
A
R2
R1
R4
B
R3
D
FEC based on the destination prefix
Routers R2 and R3 dont need to speak BGP
38
VPNs With Private Addresses
10.1.0.0/24
10.1.0.0/24
C
A
R2
Two FECs
R1
R4
B
R3
D
Direct traffic to orange
10.1.0.0/24
10.1.0.0/24
MPLS tags can differentiate pink VPN from orange
VPN.
39
Status of MPLS
  • Deployed in practice
  • BGP-free core
  • Virtual Private Networks
  • Traffic engineering
  • Challenges
  • Protocol complexity
  • Configuration complexity
  • Difficulty of collecting measurement data
  • Continuing evolution
  • Standards
  • Operational practices and tools

40
Conclusion
  • MPLS is an overlay
  • Tunneling on top of the network
  • Built on top of an underlying routing algorithm
  • Flexibility in mapping traffic to paths
  • Associating packets with FECs, and then labels
  • New protocols for creating label-switching tables
  • Binding FECs to labels across a path
  • Establishing explicit routes
  • Many open questions
  • Makes operations easier vs. harder?
  • Trade-offs in exploiting the flexibility?
  • Interdomain routing with MPLS?
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