PPP

1. Point-to-point protocol (PPP)
   1. PPP frame
   2. Multilink PPP (MLPPP)
      1. MLPPP header (long sequence number)
      2. MLPPP header (short sequence number)
      3. Link fragmentation and interleaving (LFI)
   3. On-demand address pool
      1. IPCP client
      2. IPCP server
2. PPPoE
   1. Client CLI
   2. Server CLI

Point-to-point protocol (PPP)

• can detect loop
• link quality monitoring (LQM):
  • error detection
  • disables link if quality drop lower than threshold
  • quality = 100% × (rcvd pkts/bytes)/(sent pkts/bytes)
• reliable data transmission
• authentication: PAP, CHAP
• supports compression

; CHAP, PASS must match on both ends, secret not allowed
(config)# username <NEIGHBOUR_NAME> password <PASS>

; sends CHAP challenge
(config-if)# ppp authentication chap

; responds to CHAP challenge
(config-if)# ppp chap password <PASS>

; off default, LQM, 
(config-if)# ppp quality <%>

; payload compression
(config-if)# compress <METHOD>

; request IP address from peer
(config-if)# ip address negotiate

; mask matches value, configured on interface 
(config-if)# peer default ip address <IP>

; installs /32 for peer route
(config-if)# peer neighbor-route

PPP frame

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Flag FD    |    Address    |    Control    |  L3 Protocol  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/                              Data                             /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              FCS              |    Flag FD    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Flag FD: 0x7e
Address: 0xff
Control: 0x03

Multilink PPP (MLPPP)

• splits packet and sends pieces over all member links
  • does not use hash to select link – uses all links
  • splits the packet even if fragmentation is not required
• CPU intensive

(config)# interface multilink <M>

; period to calculate average values
(config-if)# load-interval <sec>

(config)# interface serial <N>
(config-if)# ppp multilink group <M>

# show ppp multilink

MLPPP header (long sequence number)

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|B|E|    0x00   |                Sequence number                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/                              Data                             /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

B: beginning fragment
E: ending fragment

MLPPP header (short sequence number)

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|B|E|    0x00   |Sequence number|    Data (variable length)    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Link fragmentation and interleaving (LFI)

• fragment size = fragment delay (ms) × BW
• if packet is larger than fragment size → fragment packet
• if packet is equal or smaller than fragment size ≡ small packet

; enable LFI, off default
(config-if)# ppp multilink interleave

; 16 default, n=1 ≡ disable fragmentation
(config-if)# ppp multilink fragment maximum <N>

; 30ms default
(config-if)# ppp multilink fragment delay <ms>

On-demand address pool

• request IP pool via IPCP to distribute it further (≈ DHCP-PD)

IPCP client

(config)# ip dhcp pool <POOL>
(dhcp-config)# import all
(dhcp-config)# origin ipcp

(config-if)# ip address pool <POOL>
(config-if)# ppp ipcp mask request

IPCP server

(config)# ip local pool <POOL> <START> <END>

(config-if)# peer default ip address pool <POOL>
(config-if)# ppp ipcp mask <MASK>

PPPoE

• phases:
  1. Discovery
     1. PPPoE active discovery initiation (PADI):
        • client-to-server
        • bcast
        • search for DSL access concentrator (DSL-AC)
     2. PPPoE active discovery offer (PADO):
        • server-to-client
        • unicast
     3. PPPoE active discovery request (PADR):
        • client-to-server
        • unicast
     4. PPPoE active discovery session confirmation (PADS):
        • server-to-client
        • unicast
        • passes session ID
     5. PPPoE active discovery termination (PADT):
        • both server and client
        • unicast
        • tears down connection
  2. Session
• VPDN: virtual private dial-up network
• credentials should be stored in NVRAM, otherwise ISP may send Clear and purge entry from config

Client CLI

(config)# interface dialer <N>
(config-if)# encapsulation ppp
(config-if)# mtu 1492
(config-if)# dialer pool <M>
(config-if)# ip address negotiate
(config-if)# ip address pppoe [set-route]

; unidirectional authC
(config-if)# ppp chap hostname <USERNAME>
(config-if)# ppp chap password <PASSWD>

(config)# interface <INTF>
(config-if)# pppoe-client dial-pool-number <M>

Server CLI

(config)# vpdn enable
(config)# vpdn-group <GROUP> request dialout pppoe
(config)# vpdn group <GROUP> ppp authentication <MODE>
(config)# vpdn group <GROUP> localname <USERNAME>
(config)# vpdn username <USERNAME> password <PASSWD> [store-local]

(config-if)# pppoe client vpdn group <GROUP>