Transcript Long Thin Networks RFC 2757

Internet Networking
Spring 2003
Tutorial 12
Limited Transmit
RFC 3042
Long Thin Networks
RFC 2757
Limited Transmit
RFC 3042
The problem
• cwnd can be smaller than 4
• Due to a large number of packet loss, less
than 3 dup acks will arrive
• This causes a timeout rather than a Fast
Retransmit.
TCP modification
• The sender will send a new segment for
each dup-Ack it receives.
• The amount of data is limited to cwnd+2
• cwnd does NOT change due to a dup-ack.
Example
3
cwnd=3
4
5
ACK(3)
ACK(3)
6
7
Congestion avoidance
ACK(3)
ACK(3)
---- 3
Fast retransmit
Sstresh = 3/2 = 2
Cwnd = sstresh+3=2+3=5
X
ACK(8)
Example #2
cwnd=3
-------
3
4
5
ACK(3)
ACK(3)
6
cwnd=4
-------
7
8
9
ACK(6)
ECN and Limited Transmit
• ECN can inform of congestion without
dropping the packet, thus allowing TCP
with small cwnd to be aware of congestion
while avoiding timeouts.
• ECN requires the co-operation of the
network, while Limited Transmit does not.
• Can be simultaneously used.
(Wireless) Long Thin Networks
RFC 2757
Definitions
• Long: High delay
– Longer RTT.
• Thin: Low bandwidth
– The delay*bw product can be small, what influence does it have
on cwnd ?
• Example: wireless network.
• Counter examples:
– Satellite (Long Fat Networks)
– Wireless LAN (Short Fat Networks)
• More information can be found at:
– http://www.cas.mcmaster.ca/~wmfarmer/SE-4C0302/projects/student_work/mcmahoj.html
– RFC 2757.
Wireless network
BER: Bit Error Rate
•
•
Higher error rate than a wired link
Possible solutions:
1. At the link layer: DLC that ensures FIFO and
reliability (as learned in the previous course)
2. No DLC at the link layer:
•
PEP: Performance Enhancing Proxies (IndirectTCP)
3. Lower MTU
DLC and duplicate retransmission
TCP
RTO
Link layer
timeout
Redundant
transmission
PEP: Performance Enhancing
Proxies.
• Instead of End-to-End TCP:
• User -> Intermediate Node (IN) -> Wire-line
Internet.
IN
internet
Application
Application
TCP
TCP
IP
IP
MAC
MAC
Link
Link
TCP
IP
IP
Link
Link
Indirect TCP
PEP: Pros
– Shield the wire Internet from the wireless
connection and vice-versa.
– TCP for the wireless link can be enhanced,
due to the knowledge we have on that specific
network.
• Different cwnd and RTO
• Since the RTT of the single wireless link is smaller
than the whole route RTT, faster local recovery is
achieved.
– More freedom for the wireless part; Doesn’t
have to be TCP compatible.
PEP: Cons
– TCP end-to-end Semantic is broken
– IN is usually a single point of failure.
– Movement of the client can cause IN change; Since
we are referring to W-LTN and not W-LAN, This has
much less probability.
– Overloading of the IN: much problematic in LFN than
LTN. The IN holds 2 stacks of TCP connections.
– Handles disconnections poorly
– Possible BS buffer overloading. Higher buffer
increases hand-off latency when changing cells.
Indirect TCP
• Split TCP connection into 2 TCPs
• BS acts as a proxy and relays all data
• FH sends a packet
– BS acknowledges this packet and forwards the packet to MH
• Packet is lost on wireless link
– BS can notices faster due to lower RTT and retransmit packet
regular TCP
Internet
Fixed Host (FH)
wireless TCP
Base Station (BS)
Mobile Host (MH)
PEP: retransmission
TCP
RTO
ACK
TCP2
RTO
ACK
TCP-Aware Link Layer Protocol
• Retains local recovery of Split connection approach and link level
retransmission schemes
• Snoop protocol, by Berkley: not in the course material
TCP connection
application
application
application
transport
transport
transport
network
network
network
retrans
link
link
physical
physical
FH
BS
link
physical
wireless
MH