20. Switched Local Area Networks
n
Addressing in LANs (ARP)
nSpanning tree algorithm
n
Forwarding in switched Ethernet LANs
n
Virtual LANs
n
Layer 3 switching
n
Datacenter networks
Virtual LANs (VLAN)
n
Allows hosts to be divided among
different VLANs
»
Ethernet packets do not propagate beyond VLAN boundaries
»
to go between VLANs, packet must pass through a router
• but many switches support router-like functions that handle this • VLANs often correspond to IP subnets, but need not
n
VLANs can increase network’s traffic capacity
n
Packet’s VLAN is identified by VLAN id carried in packet
»
12 bit VLAN “tag” inserted just before the “ethertype” field
»
packets with VLAN id X are sent only on ports that belong to X
• “host ports” typically belong to one VLAN
• VLAN tag is typically added/removed by switches at “host ports”
»
routers and servers often participate in multiple VLANs
C D E A F B id=3 id=7
Ethernet Frame With VLAN Tag
preamble (7 bytes) start of frame destination address source address x8100 pri d vlan id type (2 bytes) data (46-1500 bytes) CRCn
Tag starts with two-byte value x8100
»
takes place of type field, allowing packet to
be identified as tagged packet
n
Priority field (3 bits)
»
0 for best-effort, 7 for highest priority
n
Drop Eligible Bit
»
indicates packet that can be preferentially
discarded during congestion
n
VLAN Identifier (12 bits)
»
value of 0 means “no vlan”
n
Double tagging
»
a vlan tag that starts with 0x9100, identifies
the first in a pair of tags
»
allows ISPs to use VLAN tags while carrying
VLANs and Overlay Networks
n
VLANs can be used to implement
virtual links joining routers
» configured by network managers
» can be “provisioned” to provide
guaranteed bandwidth
» support for “private WANs”
n
Routers treat these much like
physical links
» link rates can be configured so several overlay links can share
physical links – no constraint on individual link rates
• and several overlay links can share a single router port
» makes it easy to add new links between routers, as needed
» overlay link rates can be changed in response to traffic
• may require re-routing of some overlay links
» effectively replaces router ports with cheaper switch ports
Ethernet switches VLAN paths
“overlay” links
Some New-ish Developments
n
Faster STP response to topology changes
»
original protocol can take nearly a minute to converge on new
spanning tree after a link fails
»
Rapid Spanning Tree Protocol cuts time to under 10 seconds
n
Computing multiple spanning trees
»
when VLANs were first introduced, all VLANs used the same
spanning tree
• manual configuration required to use all available links
»
Multiple Spanning Tree Protocol allows automatic configuration
of multiple subtrees (that is, may restrict a tree to a region)
• VLANs are mapped to trees (so several VLANs may share a tree)
n
Shortest Path Bridging (standardized in 2012)
»
link-state protocol (based on IS-IS)
• switches distribute topology information, compute shortest-path-trees and configure local routing tables
MPLS
n
Multiprotocol Label Switching
extends IP to provide “virtual links”
within IP networks
» MPLS-only switches are potentially less
expensive than routers
» MPLS features often added to standard routers
» allows finer-grained management of traffic than IP routing alone
n
MPLS headers added by “edge routers”
(before IP header)
n
Core routers switch packets using “labels” in MPLS headers
» labels used to select entries in MPLS routing tables
» packets may contain multiple “stacked” headers
» routing table entries can be configured to select output based on
label, replace label value with another, push/pop headers
MPLS header edge router
core router
Base MPLS Label Distribution
n
Relies on fact that all routers in a domain share the
same routing table
»
Routers distribute labels together with route entries
• Bandwidth can be included in modified routing protocols
»
IP packets are “pre-pended” with corresponding label by ingress
routers
»
Routers swap labels at each hop based on downstream mapping
»
Egress router removes label before forwarding the IP packet
D R4 R5 A R6 modified link state flooding
Base MPLS Label Distribution
R2 D R3 R4 R5 0 1 0 0 A R6 1 0 Src Inlabel Out label Dest Out iface
R6 10 A 0
12 D 0
R5 8 A 1
In
label Out label Dest Out iface
10 6 A 1
12 9 D 0
In
label Out label Dest Out iface
6 - A 0
In
label Out label Dest Out iface
Base MPLS Label Distribution
R2 D R3 R4 R5 0 1 0 0 A R6 1 0 Inlabel Out label Dest Out iface
10 6 A 1
12 9 D 0
In
label Out label Dest Out iface
6 - A 0
In
label Out label Dest Out iface
8 6 A 0 DA=A DA=A L=8,DA=A L=10,DA=A L=6,DA=A L=6,DA=A DA=A DA=A DA=A DA=A Src In
label Out label Dest Out iface
R6 10 A 0
12 D 0
Base MPLS Label Distribution
n
Relies on fact that all
routers in a domain share
the same routing table
»
Routers distribute labels
together with route entries
»
IP packets are “pre-pended”
with corresponding label by
ingress routers
»
Routers swap labels at each
hop based on downstream
mapping
»
Egress router removes label
before forwarding the IP
packet
<L2,R1> <L6,R1> <L5,R1> <L2,R1> <L6,R1> <L4,R1> <L2,R1> <L6,R1> <L4,R1> <L4,R1> <L2,R1> R1:158.130.0.0/16Base MPLS Packet Forwarding
n
Relies on fact that all
routers in a domain share
the same routing table
»
Routers distribute labels
together with route entries
»
IP packets are “pre-pended”
with corresponding label by
ingress routers
»
Routers swap labels at each
hop based on downstream
mapping
»
Egress router removes label
before forwarding the IP
packet
<L2, 158.130.14.67> Packet to: 158.130.14.67 <158.130.14.67> <L6, 158.130.14.67> <L4, 158.130.14.67> <L2, 158.130.14.67> <L6, 158.130.14.67> <L4, 158.130.14.67> <L2, 158.130.14.67> <L6, 158.130.14.67> <L4, 158.130.14.67> <L2, 158.130.14.67> <L4, 158.130.14.67>Explicit Routing
n
Overcome the shortest
path, destination-based
constraint of standard
IP forwarding
»
Greater flexibility and
control in distributing
traffic across links
Packets to: 158.130.0.0/16
<158.130.14.67>
IP Forwarding
Explicit Routing
n
Overcome the shortest
path, destination-based
constraint of standard
IP forwarding
»
Greater flexibility and
control in distributing
traffic across links
Packets to: 158.130.0.0/16
<158.130.14.67>
MPLS Forwarding
R1 R2 R3
Layer 3 Switching
n
Many switches have extensive support for IP routing
» routing features first added to connect subnets in different VLANs
» feature sets have expanded as way to “add value” to products
n
Example features
» IP forwarding, ARP, ICMP, DHCP, RIP, ...
» Diffserv QoS with 8 queues per link
» IGMP support (snooping and querier functions)
» Access Control lists (firewall functions)
n
Getting harder to distinguish routers and switches
» routers support different kinds of layer 2 links (not just Ethernet)
and support multiple L3 protocols (not just IP)
» routers have more extensive feature sets, more configurable
» routers have larger routing tables & buffers, flexible queueing
Data Center Networks
n
10’s to 100’s of thousands of hosts, often closely
coupled, in close proximity:
»
e-business (e.g. Amazon)
»
content-servers (e.g., YouTube, Akamai, Apple, Microsoft)
»
search engines, data mining (e.g., Google)
n
Challenges
»
multiple applications, each
serving many clients
»
managing/balancing load
»
multiple “tenants”
• must keep tenants isolated
• actions of one tenant must not interfere with another
Scaling Up
n Example
» Each rack has
• 32 servers each with 32 cores
• TOR switch with 1G and/or 10G links
» 32 rack pod has 1024
servers, and 32K cores
» 64 pod configuration has
32K servers, 1M cores
n Networking challenges
» addressing – too many L2 addresses for typical switches
» must balance load across many paths
• Ethernet routing too restrictive (even with advanced routing features) • basic options: per packet load-balancing, flow-based load balancing
» requires new class of data center switches
n Some analysis of Amazon’s AWS scale:
» http://www.enterprisetech.com/2014/11/14/rare-peek-massive-scale-aws/ servers Top-Of-Rack switch
...
...
... ... pod switch . . . pod backbone switchBlurring the L2/L3 Boundary
n
Market forces are pushing Ethernet beyond the LAN
» traditionally, large volume of switch market has kept prices low
» smaller sales volume for routers kept prices high
n
Technology improvements support greater capability
» early switches were simple and cheap (no VLANs, a few thousand
routing table entries)
» modern switches can have >100K routing table entries and support
thousands of VLANs, extensive IP features, ...
» market expectations have kept prices moderate, even as feature
sets have ballooned
n
Not clear how successful some advanced features will be
» big attraction of Ethernet is simple operation, but advanced features
compromise simplicity
Putting It Together – An End-to-End Scenario
Comcast network 68.80.0.0/13 Google’s network 64.233.160.0/19 web server DNS server school network 68.80.2.0/24 web page browserrouter (runs DHCP)
A day in the life… connecting to the Internet
v
connecting laptop
needs to get its own
IP address, addr of
first-hop router, addr
of DNS server: use
DHCP
DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP vDHCP request
encapsulated
in
UDP
,
encapsulated in
IP
,
encapsulated in
802.3
Ethernet
vEthernet frame
broadcast
(dest:
FFFFFFFFFFFF) on LAN,
received at router
running
DHCP
server
vEthernet
demuxed
to
IP demuxed, UDP
router (runs DHCP) n DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop
router for client, name & IP address of DNS server DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP v encapsulation at DHCP
server, frame forwarded
(switch learning)
through LAN,
demultiplexing at client
Client now has IP address, knows name & addr of DNS
server, IP address of its first-hop router
v DHCP client receives
DHCP ACK reply
router (runs DHCP)
A day in the life… ARP (before DNS,
before HTTP)
v
before sending
HTTP
request,
need IP address of
www.google.com:
DNS
DNS UDP IP Eth Phy DNS DNSDNS v DNS query created, encapsulated
in UDP, encapsulated in IP, encapsulated in Eth. To send
frame to router, need MAC address of router interface: ARP
v ARP query broadcast, received
by router, which replies with ARP
reply giving MAC address of
router interface
v client now knows MAC address
of first hop router, so can now send frame containing DNS query ARP query Eth Phy ARP ARP ARP reply
router (runs DHCP) DNS UDP IP Eth Phy DNS DNS DNS DNS DNS v
IP datagram containing
DNS query forwarded via
LAN switch from client to
1
sthop router
v
IP datagram forwarded from
campus network into comcast
network, routed (tables
created by
OSPF, IS-IS,
EIGRP
and/or
BGP
routing
protocols) to DNS server
v
demux’ed to DNS server
v
DNS server replies to
client with IP address of
Comcast network 68.80.0.0/13 DNS server DNS UDP IP Eth Phy DNS DNS DNS DNS
router (runs DHCP)
A day in the life…TCP connection carrying HTTP
HTTP TCP IP Eth Phy HTTP v
to send HTTP request,
client first opens
TCP
socket
to web server
v
TCP
SYN segment
(step 1 in
3-way handshake) inter-domain
routed to web server
v
TCP
connection established!
64.233.169.105 web server SYN SYN SYN SYN TCP IP Eth Phy SYN SYN SYN SYNACK SYNACK SYNACK SYNACK SYNACK SYNACK SYNACKv
web server responds with
TCP
SYNACK
(step 2 in 3-way
router (runs DHCP)
A day in the life… HTTP request/reply
HTTP TCP IP Eth Phy HTTP
v
HTTP request
sent into
TCP socket
vIP datagram containing
HTTP request routed to
www.google.com
vIP datagram containing
web server HTTP TCP IP Eth Phyv
web server responds
with
HTTP reply
(containing web page)
HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP
v
web page
finally (!!!)
Exercise
1. In the diagram at right, suppose that host d is on
vlan 7 at switch D, host f is on vlan 3 at switch F and router c is on switch C and has connections to both VLANs. What sequence of links is used by a
packet going from d to f assuming no other routers?
C D E A F B id=3 id=7 f c d
Exercise
1. In the diagram at right, suppose that host d is on
vlan 7 at switch D, host f is on vlan 3 at switch F and router c is on switch C and has connections to both VLANs. What sequence of links is used by a
packet going from d to f assuming no other routers?
The packet would need to be delivered to router c so that it can be forwarded from vlan 7 to vlan 3. Assuming that host d knows the MAC address of router c and that entries are present in the switch forwarding tables of vlan 7 for that MAC address, the packet from host d is forwarded on links D-F, F-E, E-B, and B-C.
Assuming that router c knows the MAC address of host f and that that entries are present in the switch forwarding tables of vlan 3 for that MAC address, the packet from host d is forwarded on links C-A, A-E, and E-F.
C D E A F B id=3 id=7 f c d
Exercise
2. The diagram at right represents a core network for
some ISP. Assume all the nodes are MPLS switches and that each connects to one or more edge routers. Describe how MPLS can be used to distribute traffic
between switches C and F to use two different paths. Show MPLS routing table entries for all the switches along these paths, using different labels on each hop. Can you spread the load like this if the nodes were all conventional routers, using OSPF-routing? C D A E F B
Exercise
2. The diagram at right represents a core network for
some ISP. Assume all the nodes are MPLS switches and that each connects to one or more edge routers. Describe how MPLS can be used to distribute traffic
between switches C and F to use two different paths. Show MPLS routing table entries for all the switches along these paths, using different labels on each hop. Can you spread the load like this if the nodes were all conventional routers, using OSPF-routing?
Two link disjoint paths between C and F are C-E-D-F and C-B-A-F.
Two realize those paths, we need two distinct labels (or sets of labels).
C would have two labels, say L1 and L2, associated with subnets connected to F. Each label would point to a different next hop, e.g., L1 points to E and L2 to B. Next would need associated label mappings at the intermediate MPLS switches on each path. Specifically, B would have an entry mapping incoming label L2 to
next hop A with an outgoing label of L2A (which could be equal to L2), similarly, A
would have an entry mapping incoming label L2A to next hop F and outgoing label
L2F. A similar set of mappings would be present on the path C-E-D-F for label L1.
In this case, a similar outcome could be realized with OSPF, if both paths were equal cost shortest paths. In this case, traffic would be load-balanced across
C D
A E
F B
