1. Announcement
Written Homework Assignment 2: Due Fri. November 18, 11:59pm
Programming Assignment 3: Due Mon. Nov ember 14, 11:59pm
Group of three students. The shared google sheet is helpful for you to contact your peers.
CSci4211: Data Link Layer

2. Recap: Data Link Layer Data Link Layer Functions
deliver frames over a single link
framing, media access, error checking (error correction), …
Cyclic Redundancy Code for error detection
Local Area Networks (LANs) and MAC Addresses
MAC addresses (vs. IP address)
point-to-point vs. shared access
IP Address Resolution Protocol (ARP) and IP datagram forwarding (revisited!)
Extending LANs & Switched LANs:
Self-learning
Spanning tree algorithms
CSci4211: Data Link Layer

3. ARP Operation Illustration
Here is an illustration of ARP broadcast by W requesting hardware address of Y. As you can see, every node in the local physical network receive this request. Only Y sends the reply and also only to W.
CSci4211: Data Link Layer

4. Forwarding to Another LAN
walkthrough: send datagram from A to B via R
focus on addressing – at IP (datagram) and MAC layer (frame)
assume A knows B’s IP address
assume A knows IP address of first hop router, R (how?)
assume A knows R’s MAC address (how?) R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
C-E8-FF-55 A
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B
CSci4211: Data Link Layer

5. Forwarding to Another LAN
A creates IP datagram with IP source A, destination B
A creates link-layer frame with R's MAC address as destination address, frame contains A-to-B IP datagram
MAC src: C-E8-FF-55
MAC dest: E6-E BB-4B IP
Eth
Phy
IP src:
IP dest: R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
C-E8-FF-55 A
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B
CSci4211: Data Link Layer

6. Forwarding to Another LAN
frame sent from A to R
frame received at R, datagram removed, passed up to IP
MAC src: C-E8-FF-55
MAC dest: E6-E BB-4B
IP src:
IP dest:
IP src:
IP dest: IP
Eth
Phy IP
Eth
Phy R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
C-E8-FF-55 A
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B
CSci4211: Data Link Layer

7. Forwarding to Another LAN
R forwards datagram with IP source A, destination B
R creates link-layer frame with B's MAC address as destination address, frame contains A-to-B IP datagram
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A IP
Eth
Phy
IP src:
IP dest: IP
Eth
Phy R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
C-E8-FF-55 A
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B
CSci4211: Data Link Layer

8. Forwarding to Another LAN
R forwards datagram with IP source A, destination B
R creates link-layer frame with B's MAC address as destination address, frame contains A-to-B IP datagram
IP src:
IP dest:
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A IP
Eth
Phy IP
Eth
Phy R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
C-E8-FF-55 A
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B
CSci4211: Data Link Layer

9. Forwarding to Another LAN
R forwards datagram with IP source A, destination B
R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A
IP src:
IP dest: IP
Eth
Phy B A R
49-BD-D2-C7-56-2A
C-E8-FF-55
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
88-B2-2F-54-1A-0F
CSci4211: Data Link Layer

10. Self-learning: Filtering/Forwarding
when frame received at switch:
1. record incoming link, MAC address of sending host
2. index switch table using MAC destination address
3. if entry found for destination then {
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry }
else flood /* forward on all interfaces except arriving
interface */
CSci4211: Data Link Layer

11. Self-learning, forwarding: example
Source: A
Dest: A’ A A’ B B’ C C’ 1 2 3 4 5 6
A A’
switch learns which hosts can be reached through which interfaces
when frame received, switch “learns” location of sender: incoming LAN segment
records sender/location pair in switch table
MAC addr interface TTL A 1 60
Switch table
(initially empty)
CSci4211: Data Link Layer

12. Self-learning, forwarding: example
Source: A
Dest: A’ A A’ B B’ C C’ 1 2 3 4 5 6
A A’
frame destination, A’, location unknown:
flood
destination A location known:
selectively send
on just one link
A A’
A A’
A A’
A A’
A A’
A’ A
MAC addr interface TTL A 1 60
switch table
(initially empty) A’ 4 60
CSci4211: Data Link Layer

13. CSci4211: Network Data Plane: VLANs, VC and MPLS
VLANs: Motivation
consider:
CS user moves office to EE, but wants connect to CS switch?
single broadcast domain:
all layer-2 broadcast traffic (ARP, DHCP, unknown location of destination MAC address) must cross entire LAN
security/privacy, efficiency issues
Computer
Science
Computer
Engineering
Electrical
Engineering
CSci4211: Network Data Plane: VLANs, VC and MPLS

14. VLANs
port-based VLAN: switch ports grouped (by switch management software) so that single physical switch ……
Virtual Local
Area Network 1 7 9 15 2 8 10 16
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure. … …
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
Electrical Engineering
(VLAN ports 1-8) … 1 8 2 7 9 16 10 15
Computer Science
(VLAN ports 9-16)
… operates as multiple virtual switches
CSci4211: Network Data Plane: VLANs, VC and MPLS

15. Port-based VLAN router
traffic isolation: frames to/from ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints, rather than switch port
router
forwarding between VLANS: done via routing (just as with separate switches)
in practice vendors sell combined switches plus routers 1 7 9 15 2 8 10 16
dynamic membership: ports can be dynamically assigned among VLANs … …
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
CSci4211: Network Data Plane: VLANs, VC and MPLS

16. VLANs Spanning Multiple Switches1 7 9 16 1 15 3 5 7 2 8 10 2 4 6 8 … …
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
Ports 2,3,5 belong to EE VLAN
Ports 4,6,7,8 belong to CS VLAN
trunk port: carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches can’t be vanilla frames (must carry VLAN ID info)
802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports
CSci4211: Network Data Plane: VLANs, VC and MPLS

17. CSci4211: Network Data Plane: VLANs, VC and MPLS
802.1Q VLAN frame format
type
dest.
address
source
address
preamble
data (payload)
CRC
802.1 frame
type
dest.
address
source
preamble
802.1Q frame
data (payload)
CRC
2-byte Tag Protocol Identifier
(value: 81-00)
Recomputed
CRC
Tag Control Information (12 bit VLAN ID field,
3 bit priority field like IP TOS)
CSci4211: Network Data Plane: VLANs, VC and MPLS

18. Virtual Circuit vs. Datagram
Objective of both: move packets through routers from source to destination
Datagram Model:
Routing: determine next hop to each destination a priori
Forwarding: destination address in packet header, used at each hop to look up for next hop
routes may change during “session”
analogy: driving, asking directions at every gas station, or based on the road signs at every turn
Virtual Circuit Model:
Routing: determine a path from source to each destination
“Call” Set-up: fixed path (“virtual circuit”) set up at “call” setup time, remains fixed thru “call”
Data Forwarding: each packet carries “tag” or “label” (virtual circuit id, VCI), which determines next hop
routers maintain ”per-call” state
CSci4211: Network Data Plane: VLANs, VC and MPLS

19. CSci4211: Network Data Plane: VLANs, VC and MPLS
Virtual Circuits
“source-to-dest path behaves much like telephone circuit” (but actually over packet network)
performance-wise
network actions along source-to-dest path
call setup/teardown for each call before data can flow
need special control protocol: “signaling”
every router on source-dest path maintains “state” (VCI translation table) for each passing call
VCI translation table at routers along the path of a call “weaving together” a “logical connection” for the call
link, router resources (bandwidth, buffers) may be reserved and allocated to each VC
to get “circuit-like” performance
Compare w/ transport-layer “connection”: only involves two end systems, no fixed path, can’t reserve bandwidth!
CSci4211: Network Data Plane: VLANs, VC and MPLS

20. CSci4211: Network Data Plane: VLANs, VC and MPLS
VC Implementation
a VC consists of:
path from source to destination
VC numbers, one number for each link along path
entries in forwarding tables in routers along path
packet belonging to VC carries VC number (rather than dest address)
VC number can be changed on each link.
New VC number comes from forwarding table
CSci4211: Network Data Plane: VLANs, VC and MPLS

21. VC Translation/Forwarding Table12 22 32 1 2 3
VC number
interface
number
Forwarding table in
northwest router:
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
… … … …
Routers maintain connection state information!
CSci4211: Network Data Plane: VLANs, VC and MPLS

22. Virtual Circuit: Signaling Protocols
used to setup, maintain teardown VC
used in ATM, frame-relay, X.25
used in part of today’s Internet: Multi-Protocol Label Switching (MPLS) operated at “layer 2+1/2” (between data link layer and network layer) for “traffic engineering” purpose
application
transport
network
data link
physical
1. Initiate call
2. incoming call
3. Accept call
4. Call connected
5. Data flow begins
6. Receive data
CSci4211: Network Data Plane: VLANs, VC and MPLS

23. Virtual Circuit Setup/Teardown
Call Set-Up:
Source: select a path from source to destination
Use routing table (which provides a “map of network”)
Source: send VC setup request control (“signaling”) packet
Specify path for the call, and also the (initial) output VCI
perhaps also resources to be reserved, if supported
Each router along the path:
Determine output port and choose a (local) output VCI for the call
need to ensure that NO two distinct VCs leaving the same output port have the same VCI!
Update VCI translation table (“forwarding table”)
add an entry, establishing an mapping between incoming VCI & port no. and outgoing VCI & port no. for the call
Call Tear-Down: similar, but remove entry instead
Previously we were talking about how to forward packets once the virtual circuit has been set up. For setting a VC, a source has to first select a path and send setup request along that path. We will see later how a source can get information about the network and perform path selection. Each router along the path choose a local VCI for the connection. To be precise, a downstream router selects a VCI to be used as output VCI by the upstream node. Basically we have to make sure that two distinct VCs do not have the same VCI when they flow thru the same port. Once the VCI is chosen, forwarding table is updated to reflect the new mapping from an incoming VCI and port no to outgoing VCI and port no. VC setup is essentially the updation of forwarding tables along a selected path.
The key thing to note here is that VCI has only local significance and that’s why VCI of a packet is changed at each router along the path. If we want VCI to have global meaning, then we would need a larger VCI to identify every connection in the whole network. Moreover, we need to ensure that each VCI is globally unique. On the other hand, with local VCIs a router has to worry only about the VCs passing thru itself which will be much fewer. Also, some other router can also use the same VCI as long as two VCs do not get switched to the same port with same VCI. So with a smaller VCI also it is possible to have many connections in the network.
Another thing to note here is the difference between routing and forwarding. Here the path selection is done the VC setup time, i.e., routing decision is made before any data is sent. And forwarding table along the selected path are updated as part of VC setup. After that each packet gets forwarded by each router/switch along the path as per the routing decision made at the time of VC setup.
CSci4211: Network Data Plane: VLANs, VC and MPLS

24. CSci4211: Network Data Plane: VLANs, VC and MPLS
green call
four “calls” going thru the router, each entry corresponding one call
purple call
blue call
orange call
VCI translation table (aka “forwarding table”), built at call set-up phase 1 2 3 1 2
Here is an example of how VCI translation is done. Forwarding table maps a packet from an input port with input VCI to an output port and output VCI. This forwarding table is set up such that no two packets belonging to different connections (with different input VCIs or from different input ports) get switched to the same output port with same output VCI.
In this example, forwarding table says that a packet coming thru input port 1 with VCI 2 should be sent out on port 4 and VCI of the packet be changed to 1. Similarly packets from port 2 with VCI 1 are sent out on port 3 with VCI 2. You can see that no two output VCIs are same if the output port is also same. 2 1 1
During data packet forwarding phase, input VCI is used to look up the table, and is “swapped” w/ output VCI (VCI translation, or “label swapping”)
CSci4211: Network Data Plane: VLANs, VC and MPLS

25. Virtual Circuit: Example
“call” from host A to host B along path:
host A router 1 router 2  router 3  host B
each router along path maintains an entry for the call in its VCI translation table
the entries piece together a “logical connection” for the call
Exercise: write down the VCI translation table entry for the call at each router
Router 4
Router 1 3 1 2
Router 2 2 3 1 5 11
Host A 7
Router 3 1 3 4
Host B 2
CSci4211: Network Data Plane: VLANs, VC and MPLS

26. 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 20 3 1 8
CSci4211: Network Data Plane: VLANs, VC and MPLS

27. CSci4211: Network Data Plane: VLANs, VC and MPLS
MPLS Capable Routers
a.k.a. label-switched router
forward packets to outgoing interface based only on label value (don’t inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
flexibility: MPLS forwarding decisions can differ from those of IP
use destination and source addresses to route flows to same destination differently (traffic engineering)
re-route flows quickly if link fails: pre-computed backup paths (useful for VoIP)
CSci4211: Network Data Plane: VLANs, VC and MPLS

28. CSci4211: Network Data Plane: VLANs, VC and MPLS
MPLS versus IP paths R6 D R4 R3 R5 A R2
IP routing: path to destination determined by destination address alone
IP router
CSci4211: Network Data Plane: VLANs, VC and MPLS

29. CSci4211: Network Data Plane: VLANs, VC and MPLS
MPLS versus IP paths
entry router (R4) can use different MPLS routes to A based, e.g., on source address R6 D R4 R3 R5 A R2
IP routing: path to destination determined by destination address alone
IP-only
router
MPLS routing: path to destination can be based on source and destination address
fast reroute: precompute backup routes in case of link failure
MPLS and
IP router
CSci4211: Network Data Plane: VLANs, VC and MPLS

30. CSci4211: Network Data Plane: VLANs, VC and MPLS
MPLS Signaling
modify OSPF, IS-IS link-state flooding protocols to carry info used by MPLS routing,
e.g., link bandwidth, amount of “reserved” link bandwidth
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
RSVP-TE R6
modified
link state
flooding D R4 R5 A
CSci4211: Network Data Plane: VLANs, VC and MPLS

31. MPLS Forwarding Tables1 A R6
in out out
label label dest interface A A D A D A A
CSci4211: Network Data Plane: VLANs, VC and MPLS