Network Working Group                                          J.  Touch
Request for Comments: 4953                                       USC/ISI
Category: Informational                                        July 2007
Network Working Group                                          J.  Touch
Request for Comments: 4953                                       USC/ISI
Category: Informational                                        July 2007

Defending TCP Against Spoofing Attacks


Status of This Memo


This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.


Copyright Notice


Copyright (C) The IETF Trust (2007).




Recent analysis of potential attacks on core Internet infrastructure indicates an increased vulnerability of TCP connections to spurious resets (RSTs), sent with forged IP source addresses (spoofing). TCP has always been susceptible to such RST spoofing attacks, which were indirectly protected by checking that the RST sequence number was inside the current receive window, as well as via the obfuscation of TCP endpoint and port numbers. For pairs of well-known endpoints often over predictable port pairs, such as BGP or between web servers and well-known large-scale caches, increases in the path bandwidth-delay product of a connection have sufficiently increased the receive window space that off-path third parties can brute-force generate a viable RST sequence number. The susceptibility to attack increases with the square of the bandwidth, and thus presents a significant vulnerability for recent high-speed networks. This document addresses this vulnerability, discussing proposed solutions at the transport level and their inherent challenges, as well as existing network level solutions and the feasibility of their deployment. This document focuses on vulnerabilities due to spoofed TCP segments, and includes a discussion of related ICMP spoofing attacks on TCP connections.


Table of Contents


   1. Introduction ....................................................3
   2. Background ......................................................4
      2.1. Review of TCP Windows ......................................5
      2.2. Recent BGP Attacks Using TCP RSTs ..........................6
      2.3. TCP RST Vulnerability ......................................6
      2.4. What Changed - the Ever-Opening Advertised Receive Window ..7
   3. Proposed Solutions and Mitigations .............................10
      3.1. Transport Layer Solutions .................................10
           3.1.1. TCP MD5 Authentication .............................11
           3.1.2. TCP RST Window Attenuation .........................11
           3.1.3. TCP Timestamp Authentication .......................12
           3.1.4. Other TCP Cookies ..................................13
           3.1.5. Other TCP Considerations ...........................13
           3.1.6. Other Transport Protocol Solutions .................14
      3.2. Network Layer (IP) Solutions ..............................14
           3.2.1. Address Filtering ..................................15
           3.2.2. IPsec ..............................................16
   4. ICMP ...........................................................17
   5. Issues .........................................................18
      5.1. Transport Layer (e.g., TCP) ...............................18
      5.2. Network Layer (IP) ........................................19
      5.3. Application Layer .........................................21
      5.4. Link Layer ................................................21
      5.5. Issues Discussion .........................................21
   6. Security Considerations ........................................22
   7. Conclusions ....................................................23
   8. Acknowledgments ................................................23
   9. Informative References .........................................24
   1. Introduction ....................................................3
   2. Background ......................................................4
      2.1. Review of TCP Windows ......................................5
      2.2. Recent BGP Attacks Using TCP RSTs ..........................6
      2.3. TCP RST Vulnerability ......................................6
      2.4. What Changed - the Ever-Opening Advertised Receive Window ..7
   3. Proposed Solutions and Mitigations .............................10
      3.1. Transport Layer Solutions .................................10
           3.1.1. TCP MD5 Authentication .............................11
           3.1.2. TCP RST Window Attenuation .........................11
           3.1.3. TCP Timestamp Authentication .......................12
           3.1.4. Other TCP Cookies ..................................13
           3.1.5. Other TCP Considerations ...........................13
           3.1.6. Other Transport Protocol Solutions .................14
      3.2. Network Layer (IP) Solutions ..............................14
           3.2.1. Address Filtering ..................................15
           3.2.2. IPsec ..............................................16
   4. ICMP ...........................................................17
   5. Issues .........................................................18
      5.1. Transport Layer (e.g., TCP) ...............................18
      5.2. Network Layer (IP) ........................................19
      5.3. Application Layer .........................................21
      5.4. Link Layer ................................................21
      5.5. Issues Discussion .........................................21
   6. Security Considerations ........................................22
   7. Conclusions ....................................................23
   8. Acknowledgments ................................................23
   9. Informative References .........................................24
1. Introduction
1. 介绍

Analysis of the Internet infrastructure has recently demonstrated a new version of a vulnerability in BGP connections between core routers using an attack based on RST spoofing from off-path attackers [9][10][48]. The attack itself is not new, having been documented nearly six years earlier [20]. Such connections, typically using TCP, can be susceptible to off-path third-party reset (RST) segments with forged source addresses (spoofed), which terminate the TCP connection. BGP routers react to a terminated TCP connection in various ways, which can amplify the impact of an attack, ranging from restarting the connection to deciding that the other router is unreachable and thus flushing the BGP routes [37]. This sort of attack affects other protocols besides BGP, involving any long-lived connection between well-known endpoints. The impact on the Internet infrastructure can be substantial (especially for the BGP case), and warrants immediate attention.


TCP, like many other protocols, can be susceptible to these off-path third-party spoofing attacks. Such attacks rely on the increase of commodity platforms supporting public access to previously privileged resources, such as system-level (i.e., root) access. Given such access, it is trivial for anyone to generate a packet with any header desired.


This, coupled with the lack of sufficient address filtering to drop such spoofed traffic, can increase the potential for off-path third-party spoofing attacks [9][10][48]. Proposed solutions include the deployment of existing Internet network and transport security as well as modifications to transport protocols that reduce its vulnerability to generated attacks [13][15][20][36][46].


One way to defeat spoofing is to validate the segments of a connection, either at the transport level or the network level. TCP with MD5 extensions provides this authentication at the transport level, and IPsec provides authentication at the network level [20][24][27]. In both cases, their deployment overhead may be prohibitive, e.g., it may not be feasible for public services, such as web servers, to be configured with the appropriate certificate authorities of large numbers of peers (for IPsec using the Internet Key Exchange Protocol (IKE)), or shared secrets (for IPsec in shared-secret mode, or TCP/MD5), because many clients may need to be configured rapidly without external assistance. Services located on public web servers connecting to large-scale caches or BGP with larger numbers of peers can fall into this category.


The remainder of this document outlines the recent attack scenario in detail and describes and compares a variety of solutions, including existing solutions based on TCP/MD5 and IPsec, as well as recently proposed solutions, including modifications to TCP's RST processing [36], modifications to TCP's timestamp processing [34], and modifications to IPsec and TCP/MD5 keying [45]. This document focuses on spoofing of TCP segments, although a discussion of related spoofing of ICMP packets based on spoofed TCP contents is also discussed.


Note that the description of these attacks is not new; attacks using RSTs on BGP have been known since 1998, and were the reason for the development of TCP/MD5 [20]. The recent attack scenario was first documented by Convery at a NANOG (North American Network Operators' Group) meeting in 2003, but that analysis assumed the entire sequence space (2^32 packets) needed to be covered for an attack to succeed [10]. Watson's more detailed analysis discovered that a single packet anywhere in the current window could succeed at an attack [48]. This document adds the observation that susceptibility to attack is directly proportional to the square of bandwidth, due to the coupling between the linear increase in receive window size and linear increase in rate of a potential attack, as well as comparing the variety of more recent proposals, including modifications to TCP, use of IPsec, and use of TCP/MD5 to resist such attacks.


2. Background
2. 出身背景

The recent analysis of potential attacks on BGP has again raised the issue of TCP's vulnerability to off-path third-party spoofing attacks [9][10][48]. A variety of such attacks have been known for several years, including sending RSTs, SYNs, and even ACKs in an attempt to affect an existing connection or to load down servers. These attacks often combine external knowledge (e.g., to indicate the IP addresses to attack, the destination port number, and sometimes the Initial Sequence Number (ISN)) with brute-force capabilities enabled by modern computers and network bandwidths (e.g., to scan all source ports or an entire window space). Overall, such attacks are countered by the use of some form of authentication at the network (e.g., IPsec), transport (e.g., SYN cookies, TCP/MD5), or other layers. TCP already includes a weak form of such authentication in its check of segment sequence numbers against the current receiver window. Increases in the bandwidth-delay product for certain long connections have sufficiently weakened this type of weak authentication to make reliance on it inadvisable.

最近对BGP潜在攻击的分析再次提出了TCP易受非路径第三方欺骗攻击的问题[9][10][48]。多年来,人们已经知道了各种此类攻击,包括发送RST、SYN,甚至ACK,试图影响现有连接或加载服务器。这些攻击通常将外部知识(例如,指示要攻击的IP地址、目标端口号,有时还包括初始序列号(ISN))与现代计算机和网络带宽启用的暴力能力(例如,扫描所有源端口或整个窗口空间)相结合。总的来说,通过在网络(如IPsec)、传输(如SYN Cookie、TCP/MD5)或其他层使用某种形式的身份验证来反击此类攻击。TCP在根据当前接收器窗口检查段序列号时已经包含了这种弱形式的身份验证。某些长连接的带宽延迟乘积的增加已经充分削弱了这种弱身份验证类型,使依赖它变得不可取。

2.1. Review of TCP Windows
2.1. TCP窗口综述

Before proceeding, it is useful to review the terminology and components of TCP's windowing algorithm. TCP connections have three kinds of windows [1][35]:


o Send window (SND.WND): the latest send window size.

o 发送窗口(SND.WND):最新的发送窗口大小。

o Receive window (RCV.WND): the latest advertised receive window size.

o 接收窗口(RCV.WND):最新公布的接收窗口大小。

o Congestion window (CWND): the window determined by congestion feedback that limits how much of RCV.WND can be in-flight in a round-trip time.

o 拥塞窗口(CWND):由拥塞反馈确定的窗口,限制在往返时间内RCV.WND的飞行量。

For TCP connections in most modern implementations, SND.WND and RCV.WND are the size of the corresponding send and receive socket buffers, and are configurable using socket buffer resizing commands.


CWND determines how much data can be in transit in a round-trip time, SND.WND determines how much data the sender is willing to store on its side for possible retransmission due to loss, and RCV.WND determines the ability of the receiver to accommodate that loss and reorder received packets. CWND never grows beyond RCV.WND.


High bandwidth-delay product networks need CWND to be sufficiently large to accommodate as much data as can be in transit in a round trip time; otherwise, their performance will suffer. As a result, it is recommended that users and various automatic programs increase RCV.WND to at least the size of bandwidth*delay (the bandwidth-delay product) [23][38].


As the bandwidth-delay product of the network increases, however, such increases in the advertised receive window can cause increased susceptibility to spoofing attacks, as the remainder of this document shows. This assumes, however, that the receive window size (e.g., via increased receive socket buffer configuration) is increased with the increased bandwidth-delay product; if not, then connection performance will degrade, but susceptibility to spoofing attacks will increase only linearly (with the rate at which the attacker can send spoofed packets), not as the square of the bandwidth. Note that either increase depends on the receive window itself, and is independent of the congestion state or amount of data transmitted.


2.2. Recent BGP Attacks Using TCP RSTs
2.2. 最近使用TCP RST的BGP攻击

BGP represents a particular vulnerability to spoofing attacks because it uses TCP connectivity to infer routability, so losing a TCP connection with a BGP peer can result in the flushing of routes to that peer [37].


Until six years ago, such connections were assumed difficult to attack because they were described by a few comparatively obscure parameters [20]. Most TCP connections are protected by multiple levels of obfuscation except at the endpoints of the connection:


o Both endpoint addresses are usually not well-known; although server addresses are advertised, clients are somewhat anonymous.

o 两个端点地址通常都不为人所知;虽然公布了服务器地址,但客户机在某种程度上是匿名的。

o Both port numbers are usually not well-known; the server's is usually advertised (representing the service), but the client's is typically sufficiently unpredictable to an off-path third-party.

o 两个端口号通常都不为人所知;服务器的通常是广告的(代表服务),但是客户端的通常对于非路径第三方来说是不可预测的。

o Valid sequence number space is not well-known.

o 有效的序列号空间不是众所周知的。

o Connections are relatively short-lived and valid sequence space changes, so any attempt to guess (e.g., by external knowledge or brute force) the above information is unlikely to be useful.

o 连接是相对短暂且有效的序列空间变化,因此任何猜测(例如,通过外部知识或暴力)上述信息的尝试都不太可能有用。

BGP represents an exception to the above criteria (though not the only case). Both endpoints can be well-known, or guessed using hints from part of an AS path. The destination port is typically fixed to indicate the BGP service. The source port used by a BGP router is sometimes fixed and advertised to enable firewall configuration; even when not fixed, there are only approximately 65,000 valid source ports, which thus may be exhaustively attacked. Connections are long- lived, and, as noted before, some BGP implementations interpret successive TCP connection failures as routing failures, discarding the corresponding routing information. In addition, the valid sequence number space once thought to provide some protection has been significantly weakened by increasing advertised receive window sizes.


2.3. TCP RST Vulnerability
2.3. TCP RST漏洞

TCP has a known vulnerability to third-party spoofed segments. SYN flooding consumes server resources in half-open connections, affecting the server's ability to open new connections [4][11]. ACK spoofing can cause connections to transmit too much data too quickly, creating network congestion and segment loss, causing connections to slow to a crawl. In the most recent attacks on BGP, RSTs cause connections to be dropped. As noted earlier, some BGP


implementations interpret TCP connection termination, or a series of such failures, as a network failure [37]. This causes routers to drop the BGP routing information already exchanged, in addition to inhibiting their ongoing exchanges, thus amplifying the impact of the attack. The result can affect routing paths throughout the Internet.


The dangerous effects of RSTs on TCP have been known for many years, even when used by the legitimate endpoints of a connection. TCP RSTs cause the receiver to drop all connection state; because the source is not required to maintain a TIME_WAIT state, such a RST can cause premature reuse of address/port pairs, potentially allowing segments from a previous connection to contaminate the data of a new connection, known as TIME_WAIT assassination [8]. In this case, assassination occurs inadvertently as the result of duplicate segments from a legitimate source, and can be avoided by blocking RST processing while in TIME_WAIT. However, assassination can be useful to deliberately reduce the state held at servers; this requires that the source of the RSTs go into TIME_WAIT state to avoid such hazards, and that RSTs are not blocked in the TIME_WAIT state [12].

RST对TCP的危险影响已经知道很多年了,即使是在连接的合法端点使用RST时也是如此。TCP RST导致接收器放弃所有连接状态;因为源不需要保持时间等待状态,这样的RST可能会导致地址/端口对的过早重用,可能会允许来自以前连接的段污染新连接的数据,称为时间等待刺杀[8]。在这种情况下,由于来自合法源的重复段而无意中发生刺杀,并且可以通过在TIME_WAIT期间阻塞RST处理来避免刺杀。然而,暗杀有助于故意减少服务器上的状态;这要求RST的源进入TIME_WAIT状态以避免此类危险,并且RST在TIME_WAIT状态下不被阻塞[12]。

Firewalls and load balancers, so-called 'middleboxes', sometimes emit RSTs on behalf of transited connections to optimize server performance, as noted in RFC 3360 [14]. This is effectively an on-path RST attack in which the RSTs are sent for benign or beneficial intent. There are numerous hazards with such use of RSTs, outlined in that RFC.

防火墙和负载平衡器,即所谓的“中间盒”,有时会代表传输连接发出RST以优化服务器性能,如RFC 3360[14]中所述。这实际上是一种路径上RST攻击,其中RST是出于善意或有益的目的而发送的。该RFC中概述了RST的使用存在许多危险。

2.4. What Changed - the Ever-Opening Advertised Receive Window
2.4. 改变了什么-不断打开的广告接收窗口

RSTs represent a hazard to TCP, especially when completely unvalidated. Fortunately, there are a number of obfuscation mechanisms that make it difficult for off-path third parties to forge (spoof) valid RSTs, as noted earlier. We have already shown it is easy to learn both endpoint addresses and ports for some protocols, notably BGP. The final obfuscation is the segment sequence number.


TCP segments include a sequence number, which enables out-of-order receiver processing as well as duplicate detection. The sequence number space is also used to manage congestion, and indicates the index of the next byte to be transmitted or received. For RSTs, this is relevant because legitimate RSTs use the next sequence number in the transmitter window, and the receiver checks that incoming RSTs have a sequence number in the expected receive window. Such processing is intended to eliminate duplicate segments (somewhat moot for RSTs, though), and to drop RSTs that were part of previous connections.


TCP uses two window mechanisms, a primary mechanism for reordering and congestion control (which uses a space of 32 bits), and a secondary mechanism that scales this window [23][35]. The valid advertised receive window is a fraction, not to exceed approximately half, of this space, or ~2 billion (2 * 10^9, i.e., 2E9 or 2 U.S. billion). Under typical configurations, the majority of TCP connections open to a very small fraction of this space, e.g., 10,000-60,000(approximately 5-100 segments). This is because the advertised receive window typically matches the receive socket buffer size. It is recommended that this buffer be tuned to match the needs of the connection, either manually or by automatic external means [38].


On a low-loss path, the advertised receive window should be configured to match the path bandwidth-delay product, including buffering delays (assume 1 packet/hop) [38]. Many paths in the Internet have end-to-end bandwidths of under 1 Mbps, latencies under 100 ms, and are under 15 hops, resulting in fairly small advertised receive windows as above (under 35,000 bytes). Under these conditions, and further assuming that the initial sequence number is suitably (pseudo-randomly) chosen, a valid guessed sequence number would have odds of 1 in 57,000 of falling within the advertised receive window. Put differently, a blind (i.e., off-path) attacker would need to send 57,000 RSTs with suitably spaced sequence number guesses within one round-trip time to successfully reset a connection. At 1 Mbps, 57,000 (40 byte) RSTs would take only 20 seconds to transmit, but this presumes that both IP addresses and both ports are known. Absent knowledge of the source port, an off-path spoofer would need to try at least the entire range of 49152- 65535, or 16,384 different ports, resulting in an attack that would take over 91 hours. Because most TCP connections are comparatively short-lived, even this moderate variation in the source port is sufficient for such environments, although further port randomization may be recommended [29].

在低损耗路径上,广告接收窗口应配置为与路径带宽延迟乘积相匹配,包括缓冲延迟(假设1个包/跳)[38]。Internet中的许多路径的端到端带宽低于1Mbps,延迟低于100ms,跳数低于15跳,因此广告接收窗口非常小(低于35000字节)。在这些条件下,并且进一步假设适当地(伪随机地)选择初始序列号,有效的猜测序列号在广告接收窗口内的概率为57000分之一。换言之,盲(即,非路径)攻击者需要在一个往返时间内发送57000个RST,并进行适当间隔的序列号猜测,以成功重置连接。在1 Mbps时,57000(40字节)RST仅需20秒即可传输,但这假定两个IP地址和两个端口都已知。在不知道源端口的情况下,非路径欺骗者需要尝试至少整个范围内的49152-65535或16384个不同端口,从而导致需要91小时以上的攻击。由于大多数TCP连接的寿命相对较短,因此即使源端口中的这种适度变化也足以用于此类环境,尽管可能建议进一步的端口随机化[29]。

Recent use of high bandwidth paths of 10 Gbps and higher results in bandwidth-delay products over 125 MB -- approximately 1/10 of TCP's overall maximum advertised receive window size (i.e., assuming the receive socket buffers are increased as much as possible) excluding scale, assuming the receiver allocates sufficient buffering (as discussed in Section 2). Even under networks that are ten times slower (1 Gbps), the active advertised receive window covers 1/100th of the overall window size. At these speeds, it takes only 10-100 packets, or less than 32 microseconds, to correctly guess a valid sequence number and kill a connection. A table of corresponding exposure to various amounts of RSTs is shown below, for various line rates, assuming the more conventional 100-ms latencies (though even 100 ms is large for BGP cases):

最近使用的10 Gbps及以上的高带宽路径导致带宽延迟乘积超过125 MB——约为TCP整体最大广告接收窗口大小(即,假设接收套接字缓冲区尽可能增加)的1/10,不包括规模,假设接收器分配了足够的缓冲(如第2节所述)。即使在速度慢十倍(1 Gbps)的网络下,活动的播发接收窗口覆盖了整个窗口大小的1/100。在这些速度下,只需10-100个数据包,或不到32微秒,即可正确猜测有效序列号并终止连接。下面显示了针对不同线路速率的不同RST的对应暴露表,假设更为复杂常规100毫秒延迟(尽管对于BGP病例,即使100毫秒也大):

          BW       BW*delay     RSTs needed     Time needed
       10 Gbps   125       MB          35     1 us (microsecond)
        1 Gbps    12.5     MB         344   110 us
      100 Mbps     1.25    MB       3,436    10 ms (millisecond)
       10 Mbps     0.125   MB      34,360     1 second
        1 Mbps     0.0125  MB     343,598     2 minutes
      100 Kbps     0.00125 MB   3,435,974     3 hours
          BW       BW*delay     RSTs needed     Time needed
       10 Gbps   125       MB          35     1 us (microsecond)
        1 Gbps    12.5     MB         344   110 us
      100 Mbps     1.25    MB       3,436    10 ms (millisecond)
       10 Mbps     0.125   MB      34,360     1 second
        1 Mbps     0.0125  MB     343,598     2 minutes
      100 Kbps     0.00125 MB   3,435,974     3 hours

Figure 1: Time needed to kill a connection


This table demonstrates that the effect of bandwidth on the vulnerability is squared; for every increase in bandwidth, there is a linear decrease in the number of sequence number guesses needed, as well as a linear decrease in the time needed to send a set of guesses. Notably, as inter-router link bandwidths approach 1 Mbps, an 'exhaustive' attack becomes practical. Checking that the RST sequence number is somewhere in the advertised receive window, out of the overall maximum receive window (2^32), is an insufficient obfuscation.


Note that this table makes a number of assumptions:


1. The overall bandwidth-delay product is relatively fixed.

1. 总带宽延迟乘积相对固定。

2. Traffic losses are negligible (insufficient to affect the congestion window over the duration of most of the connection).

2. 流量损失可以忽略不计(不足以在大部分连接期间影响拥塞窗口)。

3. The advertised receive window is a large fraction of the overall maximum receive window size, e.g., because the receive socket buffers are set to match a large bandwidth-delay product.

3. 广告接收窗口是整体最大接收窗口大小的很大一部分,例如,因为接收套接字缓冲区被设置为匹配大带宽延迟产品。

4. The attack bandwidth is similar to the end-to-end path bandwidth.

4. 攻击带宽类似于端到端路径带宽。

Of these assumptions, the last two are more notable. The issue of receive socket buffers was discussed in Section 2. Figure 1 summarized the time to a successful attack based on large advertised receive windows, but many current commercial routers have limits of 128 KB for large devices, 32 KB for medium, and as little as 4 KB for modest ones. Figure 2 shows the time and bandwidths needed to accomplish an attack on BGP sessions in the time shown for 100-ms latencies; for even short-range network latencies (10 ms), these sessions can be still be attacked over short timescales (minutes to hours).

在这些假设中,后两个更值得注意。第2节讨论了接收套接字缓冲区的问题。图1总结了基于大型广告接收窗口的成功攻击的时间,但当前许多商用路由器对大型设备的限制为128 KB,对中型设备的限制为32 KB,对小型设备的限制为4 KB。图2显示了在100毫秒延迟时间内完成对BGP会话的攻击所需的时间和带宽;即使是短距离网络延迟(10毫秒),这些会话仍可能在短时间内(几分钟到几小时)受到攻击。

          BW     Buffer Size  RSTs needed     Time needed
       10 Mbps     0.128 MB        33,555     1 second
        3 Mbps     0.032 MB       134,218    40 seconds
      300 Kbps     0.004 MB     1,073,742     1 hour
          BW     Buffer Size  RSTs needed     Time needed
       10 Mbps     0.128 MB        33,555     1 second
        3 Mbps     0.032 MB       134,218    40 seconds
      300 Kbps     0.004 MB     1,073,742     1 hour

Figure 2: Time needed to kill a connection with limited buffers


The issue of the attack bandwidth is considered reasonable as follows:


1. RSTs are substantially easier to send than data; they can be precomputed and they are smaller than data packets (40 bytes).

1. RST比数据更容易发送;它们可以预先计算,并且比数据包(40字节)小。

2. Although susceptible connections use somewhat less ubiquitous high-bandwidth paths, the attack may be distributed, at which point only the ingress link of the attack is the primary limitation.

2. 尽管易受影响的连接使用不太普遍的高带宽路径,但攻击可能是分布式的,此时只有攻击的入口链路是主要限制。

3. For the purposes of the above table, we assume that the ingress at the attack has the same bandwidth as the path, as an approximation.

3. 出于上表的目的,作为近似值,我们假设攻击处的入口与路径具有相同的带宽。

The previous sections discussed the nature of the recent attacks on BGP due to the vulnerability of TCP to RST spoofing attacks, due largely to recent increases in the fraction of the TCP advertised receive window space in use for a single, long-lived connection.


3. Proposed Solutions and Mitigations
3. 建议的解决方案和缓解措施

TCP currently authenticates received RSTs using the address and port pair numbers, and checks that the sequence number is inside the valid receiver window. The previous section demonstrated how TCP has become more vulnerable to RST spoofing attacks due to the increases in the receive window size. There are a number of current and proposed solutions to this vulnerability, all attempting to provide evidence that a received RST is legitimate.


3.1. Transport Layer Solutions
3.1. 传输层解决方案

The transport layer represents the last place that segments can be authenticated before they affect connection management. TCP has a variety of current and proposed mechanisms to increase the authentication of segments, protecting against both off-path and on-path third-party spoofing attacks. Other transport protocols, such as SCTP and DCCP, also have limited antispoofing mechanisms.


3.1.1. TCP MD5 Authentication
3.1.1. TCP MD5身份验证

An extension to TCP supporting MD5 authentication was developed in 1998 specifically to authenticate BGP connections (although it can be used for any TCP connection) [20]. The extension relies on a pre-shared secret key to authenticate the entire TCP segment, including the data, TCP header, and TCP pseudo-header (certain fields of the IP header). All segments are protected, including RSTs, to be accepted only when their signature matches. This option, although widely deployed in Internet routers, is considered undeployable for widespread use because the need for pre-shared keys [3][30]. It further is considered computationally expensive for either hosts or routers due to the overhead of MD5 [43][44].


There are also concerns about the use of MD5 due to recent collision-based attacks [22]. Similar concerns exist for SHA-1, and the IETF is currently evaluating how these attacks impact the recommendation for using these hashes, both in TCP/MD5 and in the IPsec suite. For the purposes of this discussion, the particular algorithm used in either protocol suite is not the focus, and there is ongoing work to allow TCP/MD5 to evolve to a more general TCP security option [6][47].


3.1.2. TCP RST Window Attenuation
3.1.2. TCP RST窗口衰减

A recent proposal extends TCP to further constrain received RST to match the expected next sequence number [36]. This restores TCP's resistance to spurious RSTs, effectively limiting the receive window for RSTs to a single number. As a result, an attacker would need to send 2^32 different packets to brute-force guess the sequence number (worst case, the average would be half that); this makes TCP's vulnerability to attack independent of the size of the receive window (RCV.WND). The extension further modifies the RST receiver to react to incorrectly-numbered RSTs, by sending a zero-length ACK. If the RST source is legitimate, upon receipt of an ACK, the closed source would presumably emit a RST with the sequence number matching the ACK, correctly resetting the intended recipient. This modification changes TCP's control processing, adding to its complexity and thus potentially affecting its correctness (in contrast to adding MD5 signatures, which is orthogonal to TCP control processing altogether). For example, there may be complications between RSTs of different connections between the same pair of endpoints because RSTs flush the TIME-WAIT (as mentioned earlier). Further, this proposal modifies TCP so that, under some circumstances, a RST causes a reply (an ACK), in violation of generally accepted practice, if not gentle recommendation -- although this can be omitted, allowing timeouts to suffice. The advantage to this proposal is that it can be deployed incrementally and has benefit to the endpoint on which it is


deployed. The other advantage to this proposal is that the window attenuation described here makes the vulnerability to spoofed RST packets independent of the size of the receive window.


A variant of this proposal uses a different value to attenuate the window of viable RSTs. It requires RSTs to carry the initial sequence number rather than the next expected sequence number, i.e., the value negotiated on connection establishment [42][49]. This proposal has the advantage of using an explicitly negotiated value, but at the cost of changing the behavior of an unmodified endpoint to a currently valid RST. It would thus be more difficult, without additional mechanism, to deploy incrementally.


Another variant of this proposal involves increasing TCP's window space, rather than decreasing the valid range for RSTs, i.e., increasing the sequence space from 32 bits to 64 bits. This has the equivalent effect -- the ratio of the valid sequence numbers for any segment to the overall sequence number space is significantly reduced. The use of the larger space, as with current schemes to establish weak authentication using initial sequence numbers (ISNs), is contingent on using suitably random values for the ISN. Such randomness adds additional complexity to TCP both in specification and implementation, and provides only very weak authentication. Such a modification is not obviously backward compatible, and would be thus difficult to deploy.


A converse variant of increasing TCP's window space is to decrease the receive window (RCV.WND) explicitly, which would further reduce the effectiveness of spoofed RSTs with random sequence numbers. This alternative may reduce the throughput of the connection, if the advertised receive window is smaller than the bandwidth-delay product of the connection.


3.1.3. TCP Timestamp Authentication
3.1.3. TCP时间戳认证

Another way to authenticate TCP segments is via its timestamp option, using the value as a sort of authentication [34]. This requires that the receiver TCP discard segments whose timestamp is outside the accepted window, which is derived from the timestamps of other packets from the same connection. This technique uses an existing TCP option, but also requires modified TCP control processing (with the same caveats) and may be difficult to deploy incrementally without further modifications. Additionally, the timestamp value may be easier to guess because it can be derived predictably, either assuming it represents actual time at the host, or by probing the host using unrelated benign traffic.


3.1.4. Other TCP Cookies
3.1.4. 其他TCP Cookies

All of the above techniques are variants of cookies, otherwise meaningless data whose value is used to validate the packet. In the case of MD5 checksums, the cookie is computed based on a shared secret. Note that even a signature can be guessed, and presents a 1 in 2^(signature length) probability of attack. The primary difference is that MD5 signatures are effectively one-time cookies, not predictable based on on-path snooping, because they are dependent on packet data and thus do not repeat. Window attenuation sequence numbers can be guessed by snooping the sequence number of current packets of an existing connection, and timestamps can be guessed even less directly, either by separate benign connections or by assuming they roughly correlate to local time. These variants of cookies are similar in spirit to TCP SYN cookies, again patching a vulnerability to off-path third-party spoofing attacks based on a (fairly weak, excepting MD5) form of authentication. Another form of cookie is the source port itself, which can be randomized but provides only 16 bits of protection (65,000 combinations), which may be exhaustively attacked. This can be combined with destination port randomization as well, but that would require a separate coordination mechanism (so both parties know which ports to use), which is equivalent to (and as infeasible for large-scale deployments as) exchanging a shared secret [39].

上述所有技术都是cookie的变体,否则,这些cookie的值将用于验证数据包的无意义数据。在MD5校验和的情况下,cookie是基于共享秘密计算的。请注意,即使是一个签名也可以猜测,并表示1/2^(签名长度)的攻击概率。主要区别在于MD5签名实际上是一次性cookie,基于路径窥探无法预测,因为它们依赖于数据包数据,因此不会重复。窗口衰减序列号可以通过窥探现有连接的当前数据包的序列号来猜测,时间戳可以通过单独的良性连接或假设它们大致与本地时间相关来猜测,甚至更不直接。这些cookie的变体在精神上类似于TCP SYN cookies,再次修补了一个漏洞,使其能够抵御基于(相当弱,MD5除外)身份验证形式的非路径第三方欺骗攻击。cookie的另一种形式是源端口本身,它可以随机化,但只提供16位保护(65000个组合),可能会受到彻底的攻击。这也可以与目标端口随机化相结合,但这需要一个单独的协调机制(以便双方都知道要使用哪个端口),这相当于(对于大规模部署来说也是不可行的)交换共享秘密[39]。

3.1.5. Other TCP Considerations
3.1.5. 其他TCP注意事项

The analysis of the potential for RST spoofing above assumes that the advertised receive window is opened to the maximum extent suggested by the bandwidth-delay product of the end-to-end path, and that the window is opened to an appreciable fraction of the overall sequence number space. As noted earlier, for most common cases, connections are too brief or over bandwidths too low for such a large window to be useful. Expanding TCP's sequence number space is a direct way to further avoid such vulnerability, even for long connections over emerging bandwidths. If either manual tuning or automatic tuning of the advertised receive window (via receive buffer tuning) is not provided, this is not an issue (although connection performance will suffer) [38].


It may be sufficient for the endpoint to limit the advertised receive window by deliberately leaving it small. If the receive socket buffer is limited, e.g., to the ubiquitous default of 64 KB, the advertised receive window will not be as vulnerable even for very long connections over very high bandwidths. The vulnerability will grow linearly with the increased network speed, but not as the square. The consequence is lower sustained throughput, where only one window's worth of data per round-trip time (RTT) is exchanged.

端点可以通过故意将播发的接收窗口保持较小来限制该窗口。如果接收套接字缓冲区受到限制(例如,普遍默认值为64 KB),则播发的接收窗口即使在非常高的带宽上进行非常长的连接,也不会那么容易受到攻击。该漏洞将随着网络速度的增加而线性增长,但不会呈平方增长。其结果是较低的持续吞吐量,即每个往返时间(RTT)只交换一个窗口的数据。

This will keep the connection open longer; for long-lived connections with continuous sourced data, this may continue to present an attack opportunity, albeit a sparse and slow-moving target. For the most recent case where BGP data is being exchanged between Internet routers, the data is bursty and the aggregate traffic may be small (i.e., unlikely to cover a substantial portion of the sequence space, even if long-lived), so smaller advertised receive windows (via small receiver buffers) may, in some cases, sufficiently address the immediate problem. This assumes that the routing tables can be exchanged quickly enough with bandwidth reduced due to the smaller buffers, or perhaps that the advertised receive window is opened only during a large burst exchange (e.g., via some other signal between the two routers, or a time-based signal, though either would be nonstandard).


3.1.6. Other Transport Protocol Solutions
3.1.6. 其他传输协议解决方案

Segment authentication has been addressed at the transport layer in other protocols. Both SCTP and DCCP include cookies for connection establishment and use them to authenticate a variety of other control messages [28][41]. The inclusion of such mechanism at the transport protocol, although emerging as standard practice, complicates the design and implementation of new protocols [32]. As new attacks are discovered (SYN floods, RSTs, etc.), each protocol must be modified individually to compensate. A network solution may be more appropriate and efficient.


It should be noted that RST attacks, which rely on brute-force, are relatively easy for intrusion detection software to detect at the TCP layer. Any connection that receives a large number of invalid -- outside-window -- RSTs might have subsequent RSTs blocked, to defeat such attacks. This would have the side-effect of blocking legitimate RSTs to that connection, which might then interfere with cleaning up the transport state between the endpoint peers. This side-effect, coupled with the increased monitoring load, might render such solutions undesirable in the general case, but they might usefully be applied to special cases, e.g., for BGP for routers.


3.2. Network Layer (IP) Solutions
3.2. 网络层(IP)解决方案

There are two primary variants of network layer solutions to spoofing: address filtering and IPsec. Address filtering is an indirect system that relies on other parties to filter packets sent upstream of an attack, but does not necessarily require participation of the packet source. IPsec requires cooperation between the endpoints wanting to avoid attack on their connection, which currently involves preexisting shared knowledge of either a shared key or shared certificate authority.


3.2.1. Address Filtering
3.2.1. 地址过滤

Address filtering is often proposed as an alternative to protocol mechanisms to defeat IP source address spoofing [2][13]. Address filtering restricts traffic from downstream sources across transit networks based on the IP source address. A kind of filtering already occurs at the endpoints of a connection, because attack messages must match the socket pair to succeed; again, note that such attacks require knowing the entire socket pair, and are unlikely except in particular cases. This section discusses filtering based on address only, typically done at the borders of an AS.


It can also restrict core-to-edge paths to reject traffic that should have originated further toward the edge. It cannot restrict traffic from edges lacking filtering through the core to a particular edge. As a result, each border router must perform the appropriate filtering for overall protection to result; failure of any border router to filter defeats the protection of all participants inside the border, and potentially those outside as well. Address filtering at the border can protect those inside the border from some kinds of spoofing, i.e., connections among those inside a border, because only interior addresses should originate inside the border. It cannot, however, protect connections including endpoints outside the border (i.e., those that traverse the AS boundary) except to restrict where the traffic enters from, e.g., if it expected from one AS and not another.


As a result, address filtering is not a local solution that can be deployed to protect communicating pairs, but rather relies on a distributed infrastructure of trusted gateways filtering forged traffic where it enters the network. It is not feasible for local, incremental deployment, but may be applicable to connections among those inside the protected border in some scenarios. Applying filtering can also be useful to reduce the network load of spoofed traffic [31].


A more recent variant of address filtering checks the IP TTL (Time to Live) field, relying on the TTL set by the other end of the connection [15]. This technique has been used to provide filtering for BGP. It assumes the connection source TTL is set to 255; packets at the receiver are checked for TTL=255, and others are dropped. This restricts traffic to one hop upstream of the receiver (i.e., a BGP router), but those hops could include other user programs at those nodes (e.g., the BGP router's peer) or any traffic those nodes accept via tunnels -- because tunnels need not decrement TTLs, notably for "bump in the wire" (BITW) or BITW-equivalent scenarios [33] (see also Section 5.1 of [15] and [16]). TTL filtering works only where all traffic from the other end of the tunnel is trusted,

地址过滤的最新变体根据连接另一端设置的TTL检查IP TTL(生存时间)字段[15]。该技术已用于为BGP提供过滤。它假定连接源TTL设置为255;在接收器处检查数据包的TTL=255,并丢弃其他数据包。这将通信量限制在接收器(即BGP路由器)上游的一个跃点,但这些跃点可能包括这些节点(例如BGP路由器的对等节点)上的其他用户程序或这些节点通过隧道接受的任何通信量——因为隧道不必减少TTL,特别是对于“线路中的碰撞”(BITW)或BITW等效场景[33](另请参见[15]和[16]的第5.1节。)TTL过滤仅在隧道另一端的所有流量都可信的情况下工作,

i.e., where it does not originate or transit spoofed traffic. The use of TTL rather than link or network security also assumes an untampered point-to-point link, where no other traffic can be spoofed onto a link.

i、 例如,它不是源于或传输伪造的流量。TTL而非链路或网络安全的使用还假设了无阻尼点对点链路,在该链路上不能欺骗任何其他流量。

This method of filtering works best where traffic originates one hop away, so that the address filtering is based on the trust of only directly-connected (tunneled or otherwise) nodes. Like conventional address filtering, this reduces spoofing traffic in general, but is not considered a reliable security mechanism because it relies on distributed filtering (e.g., the fact that upstream nodes do not terminate tunnels arbitrarily).


3.2.2. IPsec
3.2.2. IPsec

TCP is susceptible to RSTs, but also to other off-path and on-path spoofing attacks, including SYN attacks. Other transport protocols, such as UDP and RTP are equally susceptible. Although emerging transport protocols attempt to defeat such attacks at the transport layer, such attacks take advantage of network layer identity spoofing. The packet is coming from an endpoint that is spoofing another endpoint, either upstream or somewhere else in the Internet. IPsec was designed specifically to establish and enforce authentication of a packet's source and contents in order to most directly and explicitly address this security vulnerability.


The larger problem with IPsec is that of key distribution and use. IPsec is often cumbersome, and has only recently been supported in many end-system operating systems. More importantly, it relies on preshared keys, signed X.509 certificates, or a trusted third-party (e.g., Kerberos) key infrastructure to establish and exchange keying information (e.g., via IKE). Each of these issues presents challenges when using IPsec to secure traffic to a well-known server, whose clients may not support IPsec or may not have registered with a previously-known certificate authority (CA).


These keying challenges are being addressed in the IETF in ways that will enable servers secure associations with other parties without advance coordination [45][46]. This can be especially useful for publicly-available servers, or for protecting connections to servers that -- for whatever reason -- have not or will not deploy conventional IPsec certificates (i.e., core Internet BGP routers).

IETF解决了这些关键问题,使服务器能够在无需事先协调的情况下与其他方安全关联[45][46]。这对于公开可用的服务器特别有用,或者对于保护到服务器的连接(无论出于何种原因)特别有用,因为这些服务器没有或不会部署传统的IPsec证书(即,核心Internet BGP路由器)。


Just as spoofed TCP packets can terminate a connection, so too can spoofed ICMP packets. ICMP can be used to launch a variety of attacks on TCP including connection resets, path-MTU attacks, and can also be used to attack the host with non-TCP 'ping of death' and 'smurf attacks', etc. [40]. ICMP thus represents a substantial threat to TCP, but this is not the focus of this document, although a number of protections are discussed below because some are comparable to TCP anti-spoofing techniques. Note also that ICMP attacks on TCP assume that the socket pair is known by the attacker, which is unlikely except for a subset of services between pairs of widely-known endpoints.


TCP headers can be included inside certain ICMP messages [7]. There have been recent suggestions to validate the sequence number of TCP headers when they occur inside ICMP messages [18]. This sequence checking is similar to checks that would occur for conventional data packets in TCP, but is being proposed in the spirit of the RST window attenuation described in Section 3.1.2.


Some such checks may be reasonable, especially where they parallel the validations already performed by TCP processing, notably where they emulate the semantics of such processing. For example, the TCP checksum should be validated (if the entire TCP segment is contained in the ICMP message) before any fields of the TCP header are examined, to avoid reacting to corrupted packets. Similarly, if the TCP MD5 option is present, its signature should probably be validated before considering the contents of the message. Such validation can ensure that the packet was not corrupted prior to the ICMP generation (checksum), that the packet was one sent by the source (IPsec or TCP/MD5 authenticated), or that the packet was not in the network for an excess of 2*MSL (valid sequence number).

一些这样的检查可能是合理的,特别是当它们与TCP处理已经执行的验证并行时,尤其是当它们模拟这种处理的语义时。例如,在检查TCP头的任何字段之前,应验证TCP校验和(如果整个TCP段包含在ICMP消息中),以避免对损坏的数据包作出反应。类似地,如果存在TCP MD5选项,则在考虑消息内容之前,可能应该验证其签名。这种验证可以确保数据包在ICMP生成(校验和)之前没有损坏,数据包是由源发送的(IPsec或TCP/MD5已验证),或者数据包在网络中的长度不超过2*MSL(有效序列号)。

ICMP presents a particular challenge because some messages can reset a connection more easily -- with less validation -- than even some spoofed TCP segments. One other proposed alternative is to change TCP's reaction to ICMPs after a connection is established; that may leave TCP susceptible during connection establishment and modifies TCP's reaction to certain valid network events [19]. This considers the context-sensitivity of ICMP messages, as does IPsec in some tunneled configurations, but the recommendations are ambiguous regarding such filtering [27].


Ultimately, requiring TCP ICMP messages to be 'in window' may be insufficient protection, as this document shows for spoofed data. ICMP packets can be authenticated when originating at known, trusted endpoints, such as endpoints of connections or routers in known

最终,要求TCP ICMP消息“在窗口中”可能不足以提供足够的保护,正如本文档针对伪造数据所示。ICMP数据包可以在已知的受信任端点(如已知网络中连接或路由器的端点)发起时进行身份验证

domains with preexisting IPsec associations. Unfortunately, they also can originate at other places in the network. In addition, some networks filter all ICMP packets because validation may not be possible, especially because they can be injected from anywhere in a network, and so cannot be easily and locally address filtered [27]. As a result, they are not addressed separately in the issues or security considerations of this document further.


5. Issues
5. 问题

There are a number of existing and proposed solutions addressing the vulnerability of transport protocols in general (and TCP in specific) to off-path third-party spoofing attacks. As shown, these operate at the transport or network layer. Transport solutions require separate modification of each transport protocol, addressing network identity spoofing separately in the context of each transport association. Network solutions require distributed coordination (filtering) or can be computationally intensive and require pervasive registration of certificate authorities with every possible endpoint (authentication). This section explains these observations further.


5.1. Transport Layer (e.g., TCP)
5.1. 传输层(如TCP)

Transport solutions rely on shared cookies to authenticate segments, including data, transport header, and even pseudo-header (e.g., fixed portions of the outer IP header in TCP). Because the Internet relies on stateless network protocols, it makes sense to rely on state establishment and maintenance available in some transport layers not only for the connection but for authentication state. Three-way handshakes and heartbeats can be used to negotiate authentication state in conjunction with connection parameters, which can be stored with connection state easily.


As noted earlier, transport layer solutions require separate modification of all transport protocols to include authentication. Not all transport protocols support negotiated endpoint state (e.g., UDP), and legacy protocols have been notoriously difficult to safely augment. Not all authentication solutions are created equal, either, and relying on a variety of transport solutions exposes end-systems to increased potential for incorrectly specified or implemented solutions. Transport authentication has often been developed piece-wise, in response to specific attacks, e.g., SYN cookies and RST window attenuation [4][36].

如前所述,传输层解决方案需要单独修改所有传输协议,以包括身份验证。并非所有传输协议都支持协商的端点状态(例如UDP),众所周知,传统协议很难安全扩展。并非所有的身份验证解决方案都是平等创建的,并且依赖于各种传输解决方案会使终端系统面临更大的错误指定或实施解决方案的可能性。传输身份验证通常是针对特定的攻击(例如,SYN cookies和RST窗口衰减)分段开发的[4][36]。

Transport layer solutions are not only per-protocol, but often per-connection. This has both advantages and drawbacks. One advantage to transport layer solutions is that they can protect the transport protocol when lower layers have failed, e.g., due to bugs in


implementation. TCP already includes a variety of packet validation mechanisms to protect in these cases, e.g., checking that RSTs are in-window. More strict checks can increase the protections provided, e.g., to protect against misaddressed RSTs that end up in-window (via TCPsecure) or to protect against connection interruption due to RSTs, SYNs, or data injection from misaddressed packets (TCP/MD5) [36].


Another advantage is that transport layer protections can be more specifically limited to a particular connection. Because each connection negotiates its state separately, that state can be more specifically tied to that connection. This is both an advantage and a drawback. It can make it easier to tie security to an individual connection, although in practice a shared secret or certificate will generally be shared across multiple connections.


As a drawback, each transport connection needs to negotiate and maintain authentication state separately. Some overhead is not amortized over multiple connections, e.g., overheads in packet exchanges, whereas other overheads are not amortized over different transport protocols, e.g., design and implementation complexity -- both as would be the case in a network layer solution. Because the authentication happens later in packet processing than is required, additional endpoint resources may be needlessly consumed, e.g., in demultiplexing received packets, indexing connection identifiers, and continuing to buffer spoofed packets, etc., only to be dropped later at the transport layer.


5.2. Network Layer (IP)
5.2. 网络层(IP)

A network layer solution avoids the hazards of multiple transport variants, using a single shared endpoint authentication mechanism early in receiver packet processing to discard unauthenticated packets at the network layer instead. This defeats spoofing entirely because spoofing involves masquerading as another endpoint, and network layer security validates the endpoint as the source of the packets it emits. Such a network level solution protects all transport protocols as a result, including both legacy and emerging protocols, and reduces the complexity of these protocols as well. A shared solution also reduces protocol overhead, and decouples the management (and refreshing) of authentication state from that of individual transport connections. Finally, a network layer solution protects not only the transport layer but the network layer as well, e.g., from IGMP, and some kinds of ICMP (Section 4), spoofing attacks.


The IETF Proposed Standard protocol for network layer authentication is IPsec [27]. IPsec specifies the overall architecture, including header authentication (AH) [25] and encapsulation (ESP) modes [26].


AH authenticates both the IP header and IP data, whereas ESP authenticates only the IP data (e.g., transport header and payload). AH is being phased out since ESP is more efficient and the Security Parameters Index (SPI) includes sufficient information to verify the IP header anyway [27]. These two modes describe the security applied to individual packets within the IPsec system; key exchange and management is performed either out-of-band (via pre-shared keys) or by an automated key exchange protocol, e.g., IKE [24].


IPsec already provides authentication of an IP header and its data contents sufficient to defeat both on-path and off-path third-party spoofing attacks. IKE can configure authentication between two endpoints on a per-endpoint, per-protocol, or per-connection basis, as desired. IKE also can perform automatic periodic re-keying, further defeating crypto-analysis based on snooping (clandestine data collection). The use of IPsec is already commonly strongly recommended for protected infrastructure.


Existing IPsec is not appropriate for many deployments. It is computationally intensive both in key management and individual packet authentication [43]. This computational overhead can be prohibitive, and so often requires additional hardware, especially in commercial routers. As importantly, IKE is not anonymous; keys can be exchanged between parties only if they trust each other's X.509 certificates, trust some other third-party to help with key generation (e.g., Kerberos), or pre-share a key. These certificates provide identification (the other party knows who you are) only where the certificates themselves are signed by certificate authorities (CAs) that both parties already trust. To a large extent, the CAs themselves are the pre-shared keys that help IKE establish security association keys, which are then used in the authentication algorithms.


Alternative mechanisms are under development to address this limitation, to allow publicly-accessible servers to secure connections to clients not known in advance, or to allow unilateral relaxation of identity validation so that the remaining protections of IPsec can be made available [45][46]. In particular, these mechanisms can prevent a client (but without knowing who that client is) from being affected by spoofing from other clients, even when the attackers are on the same communications path.


IPsec, although widely available both in commercial routers and commodity end-systems, is not often used except between parties that already have a preexisting relationship (employee/employer, between two ISPs, etc.). Servers to anonymous clients (e.g., customer/ business) or more open services (e.g., BGP, where routers may have large numbers of peers) are unmanageable, due to the breadth and flux


of CAs. New endpoints cannot establish IPsec associations with such servers unless their own certificate is signed by a CA already trusted by the server. Different servers -- even within the same overall system (e.g., BGP) -- often cannot or will not trust overlapping subsets of CAs in general.


5.3. Application Layer
5.3. 应用层

There are a number of application layer authentication mechanisms, often implicit within end-to-end encryption. Application layer security (e.g., TLS, SSH, or MD5 checksums within a BGP stream) provides the ultimate protection of application data from all intermediaries, including network routers as well as exposure at other layers in the end-systems. This is the only way to ultimately protect the application data.


Application authentication cannot protect either the network or transport protocols from spoofing attacks, however. Spoofed packets interfere with network processing or reset transport connections before the application checks the data. Authentication needs to winnow these packets and drop them before they interfere at these lower layers.


An alternate application layer solution would involve resilience to reset connections. If the application can recover from such connection interruptions, then such attacks have less impact. Unfortunately, attackers still affect the application, e.g., in the cost of restarting connections, delays until connections are restarted, or increased connection establishment messages on the network. Some applications -- notably BGP -- even interpret TCP connection reliability as an indicator of route path stability, which is why attacks on BGP have such substantial consequences.


5.4. Link Layer
5.4. 链路层

Link layer security operates separately on each hop of an Internet. Such security can be critical in protecting link resources, such as bandwidth and link management protocols. Protection at this layer cannot suffice for network or transport layers, because it cannot authenticate the endpoint source of a packet. Link authentication ensures only the source of the current link hop where it is examined.


5.5. Issues Discussion
5.5. 议题讨论

The issues raised in this section suggest that there are challenges with all solutions to transport protection from spoofing attacks. This raises the potential need for alternate security levels. While it is already widely recognized that security needs to occur


simultaneously at many protocol layers, there also may be utility in supporting a variety of strengths at a single layer. For example, IPsec already supports a variety of algorithms (MD5, SHA1, etc., for authentication), but always assumes that:


1. The entire body of the packet is secured.

1. 包的整个主体都是安全的。

2. Security associations are established only where identity is authenticated by a known certificate authority or other pre-shared key.

2. 只有当身份由已知的证书颁发机构或其他预共享密钥进行身份验证时,才会建立安全关联。

3. Both on-path and off-path third-party spoofing attacks must be defeated.

3. 必须击败路径内和路径外的第三方欺骗攻击。

These assumptions are prohibitive, especially in many cases of spoofing attacks. For spoofing, the primary issue is whether packets are coming from the same party the server can reach. Only the IP header is fundamentally in question, so securing the entire packet (1) is computational overkill. It is sufficient to authenticate the other party as "a party you have exchanged packets with", rather than establishing their trusted identity ("Bill" vs. "Bob") as in (2). Finally, many cookie systems use clear-text (unencrypted), fixed cookie values, providing reasonable (1 in 2^{cookie-size}) protection against off-path third-party spoof attacks, but not addressing on-path attacks at all. Such potential solutions are discussed in the Better Than Nothing Security (BTNS) documents [5][45][46]. Note also that NULL Encryption in IPsec applies a variant of this cookie, where the SPI is the cookie, and no further encryption is applied [17].

这些假设是禁止的,特别是在许多欺骗攻击的情况下。对于欺骗,主要问题是数据包是否来自服务器可以到达的同一方。从根本上讲,只有IP报头是有问题的,因此保护整个数据包(1)是计算过度。将另一方认证为“与您交换数据包的一方”就足够了,而不是像(2)中那样建立他们的可信身份(“Bill”vs“Bob”)。最后,许多cookie系统使用明文(未加密)固定cookie值,提供合理(1/2 ^{cookie size})的保护以抵御非路径第三方欺骗攻击,但根本不解决路径攻击。此类潜在解决方案在优于无的安全(BTNS)文件[5][45][46]中进行了讨论。还请注意,IPsec中的NULL加密应用了此cookie的一个变体,其中SPI是cookie,不应用进一步的加密[17]。

6. Security Considerations
6. 安全考虑

This entire document focuses on increasing the security of transport protocols and their resistance to spoofing attacks. Security is addressed throughout.


This document describes a number of techniques for defeating spoofing attacks. Those relying on clear-text cookies, either explicit or implicit (e.g., window sequence attenuation) do not protect from on-path spoofing attacks, since valid values can be learned from prior traffic. Those relying on true authentication algorithms are stronger, protecting even from on-path attacks, because the authentication hash in a single packet approaches the behavior of "one-time" cookies.


The security of various levels of the protocol stack is addressed. Spoofing attacks are fundamentally identity masquerading, so we believe the most appropriate solutions defeat these at the network layer, where end-to-end identity lies. Some transport protocols


subsume endpoint identity information from the network layer (e.g., TCP pseudo-headers), whereas others establish per-connection identity based on exchanged nonces (e.g., SCTP). It is reasonable, if not recommended, to address security at all layers of the protocol stack.


Note that Network Address Translators (NATs) and other middleboxes complicate the design and deployment of techniques to defeat spoofing attacks. Devices such as these, that modify IP and/or TCP headers in-transit, generate traffic equivalent to a spoofing attack, and thus should be inhibited by antispoofing mechanisms. Details of these middlebox-related problems are out of scope for this document, but issues thereof are addressed in RFCs and emerging documents that discuss the interactions between such devices and the Internet architecture, e.g., [21]. Fortunately, many of the most critical TCP-based connections -- in particular, those supporting routing protocols like BGP -- do not traverse such middleboxes, and are not affected by this limitation.


7. Conclusions
7. 结论

This document describes the details of the recent BGP spoofing attacks involving spurious RSTs, which could be used to shutdown TCP connections. It summarizes and discusses a variety of current and proposed solutions at various protocol layers.


8. Acknowledgments
8. 致谢

This document was inspired by discussions in the TCPM WG <> about the recent spoofed RST attacks on BGP routers, including R. Stewart's document (whose author list has since evolved) [36][42]. The analysis of the attack issues, alternate solutions, and the anonymous security proposed solutions were the result of discussions on that list as well as with USC/ISI's T. Faber, A. Falk, G. Finn, and Y. Wang. R. Atkinson suggested the UDP variant of TCP/MD5, P. Goyette suggested using the ISN to seed TCP/MD5, and L. Wood suggested using the ISN to validate RSTs. Other improvements are due to the input of various members of the IETF's TCPM WG, notably detailed feedback from F. Gont, P. Savola, and A. Hoenes.

本文件受TCPM工作组讨论的启发<>关于最近针对BGP路由器的欺骗RST攻击,包括R.Stewart的文档(其作者列表已经演变)[36][42]。对攻击问题、替代解决方案和匿名安全建议解决方案的分析是该列表以及USC/ISI的T.Faber、A.Falk、G.Finn和Y.Wang讨论的结果。R.Atkinson建议使用TCP/MD5的UDP变体,P.Goyette建议使用ISN为TCP/MD5种子,L.Wood建议使用ISN验证RST。其他改进得益于IETF TCPM工作组各成员的投入,特别是F.Gont、P.Savola和A.Hoenes的详细反馈。

This document was prepared using


9. Informative References
9. 资料性引用

[1] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion Control", RFC 2581, April 1999.

[1] Allman,M.,Paxson,V.和W.Stevens,“TCP拥塞控制”,RFC 25811999年4月。

[2] Baker, F. and P. Savola, "Ingress Filtering for Multihomed Networks", BCP 84, RFC 3704, March 2004.

[2] Baker,F.和P.Savola,“多址网络的入口过滤”,BCP 84,RFC 37042004年3月。

[3] Bellovin, S. and A. Zinin, "Standards Maturity Variance Regarding the TCP MD5 Signature Option (RFC 2385) and the BGP-4 Specification", RFC 4278, January 2006.

[3] Bellovin,S.和A.Zinin,“关于TCP MD5签名选项(RFC 2385)和BGP-4规范的标准成熟度差异”,RFC 4278,2006年1月。

[4] Bernstein, D., "SYN cookies", 1997, <>.

[4] Bernstein,D.,“SYN cookies”,1997年<>.

[5] Better Than Nothing Security [BTNS] WG web pages, <>.

[5] 总比没有好安全[BTNS]工作组网页<>.

[6] Bonica, R., Weis, B., Viswanathan, S., Lange, A., and O. Wheeler, "Authentication for TCP-based Routing and Management Protocols", Work in Progress, February 2007.

[6] Bonica,R.,Weis,B.,Viswanathan,S.,Lange,A.,和O.Wheeler,“基于TCP的路由和管理协议的认证”,正在进行的工作,2007年2月。

[7] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989.

[7] Braden,R.,“互联网主机的要求-通信层”,标准3,RFC 1122,1989年10月。

[8] Braden, R., "TIME-WAIT Assassination Hazards in TCP", RFC 1337, May 1992.

[8] Braden,R.,“TCP中的等待时间暗杀危险”,RFC 1337,1992年5月。

[9] CERT alert: "Technical Cyber Security Alert TA04-111A: Vulnerabilities in TCP", April 20, 2004, <>.

[9] 证书警报:“技术网络安全警报TA04-111A:TCP漏洞”,2004年4月20日<>.

[10] Convery, S., and M. Franz, "BGP Vulnerability Testing: Separating Fact from FUD", 2003, <>.

[10] Convery,S.和M.Franz,“BGP漏洞测试:从FUD中分离事实”,2003年<>.

[11] Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations", Work in Progress, May 2007.

[11] Eddy,W.,“TCP SYN洪泛攻击和常见缓解措施”,正在进行的工作,2007年5月。

[12] Faber, T., J. Touch, and W. Yue, "The TIME-WAIT state in TCP and Its Effect on Busy Servers", Proc. Infocom 1999, pp. 1573- 1583, Mar. 1999.

[12] Faber,T.,J.Touch和W.Yue,“TCP中的时间等待状态及其对繁忙服务器的影响”,Proc。《1999年信息通讯》,第1573-1583页,1999年3月。

[13] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, May 2000.

[13] Ferguson,P.和D.Senie,“网络入口过滤:击败利用IP源地址欺骗的拒绝服务攻击”,BCP 38,RFC 2827,2000年5月。

[14] Floyd, S., "Inappropriate TCP Resets Considered Harmful", BCP 60, RFC 3360, August 2002.

[14] Floyd,S.,“不适当的TCP重置被认为是有害的”,BCP 60,RFC 3360,2002年8月。

[15] Gill, V., Heasley, J., and D. Meyer, "The Generalized TTL Security Mechanism (GTSM)", RFC 3682, February 2004.

[15] Gill,V.,Heasley,J.,和D.Meyer,“广义TTL安全机制(GTSM)”,RFC 3682,2004年2月。

[16] Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C. Pignataro, "The Generalized TTL Security Mechanism (GTSM)", Work in Progress, June 2007.

[16] Gill,V.,Heasley,J.,Meyer,D.,Savola,P.,Ed.,和C.Pignataro,“广义TTL安全机制(GTSM)”,正在进行的工作,2007年6月。

[17] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and Its Use With IPsec", RFC 2410, November 1998.

[17] Glenn,R.和S.Kent,“空加密算法及其在IPsec中的使用”,RFC 2410,1998年11月。

[18] Gont, F., "ICMP attacks against TCP", Work in Progress, May 2007.

[18] Gont,F.,“针对TCP的ICMP攻击”,正在进行的工作,2007年5月。

[19] Gont, F., "TCP's Reaction to Soft Errors", Work in Progress, June 2007.

[19] Gont,F.,“TCP对软错误的反应”,正在进行的工作,2007年6月。

[20] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998.

[20] Heffernan,A.,“通过TCP MD5签名选项保护BGP会话”,RFC 2385,1998年8月。

[21] Holdrege, M. and P. Srisuresh, "Protocol Complications with the IP Network Address Translator", RFC 3027, January 2001.

[21] Holdrege,M.和P.Srisuresh,“IP网络地址转换器的协议复杂性”,RFC 3027,2001年1月。

[22] Housley, R., Post to IETF Discussion mailing list regarding his IETF 64 Security Area presentation,, Nov. 24, 2005, < mail-archive/ietf/Current/maillist.html>.

[22] Housley,R.,就其IETF 64安全区域演示向IETF讨论邮件列表发帖,ID=。,2005年11月24日< 邮件存档/ietf/Current/maillist.html>。

[23] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992.

[23] Jacobson,V.,Braden,R.,和D.Borman,“高性能TCP扩展”,RFC 1323,1992年5月。

[24] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005.

[24] Kaufman,C.,编辑,“因特网密钥交换(IKEv2)协议”,RFC 4306,2005年12月。

[25] Kent, S., "IP Authentication Header", RFC 4302, December 2005.

[25] Kent,S.,“IP认证头”,RFC 4302,2005年12月。

[26] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005.

[26] Kent,S.,“IP封装安全有效载荷(ESP)”,RFC 4303,2005年12月。

[27] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005.

[27] Kent,S.和K.Seo,“互联网协议的安全架构”,RFC 43012005年12月。

[28] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

[28] Kohler,E.,Handley,M.和S.Floyd,“数据报拥塞控制协议(DCCP)”,RFC4340,2006年3月。

[29] Larsen, M., and F. Gont, "Port Randomization", Work in Progress, February 2007.

[29] Larsen,M.和F.Gont,“港口随机化”,正在进行的工作,2007年2月。

[30] Leech, M., "Key Management Considerations for the TCP MD5 Signature Option", RFC 3562, July 2003.

[30] Leech,M.,“TCP MD5签名选项的密钥管理注意事项”,RFC 3562,2003年7月。

[31] Moore, D., G. Voelker, and S. Savage, "Inferring Internet Denial-of-Service Activity", Proc. Usenix Security Symposium, Aug. 2001.

[31] Moore,D.,G.Voelker和S.Savage,“推断互联网拒绝服务活动”,Proc。Usenix安全研讨会,2001年8月。

[32] O'Malley, S. and L. Peterson, "TCP Extensions Considered Harmful", RFC 1263, October 1991.

[32] O'Malley,S.和L.Peterson,“TCP扩展被认为是有害的”,RFC 1263,1991年10月。

[33] Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996.

[33] Perkins,C.,“IP内的IP封装”,RFC 2003,1996年10月。

[34] Poon, K., "Use of TCP timestamp option to defend against blind spoofing attack", Work in Progress, October 2004.

[34] Poon,K.,“使用TCP时间戳选项防御盲欺骗攻击”,正在进行的工作,2004年10月。

[35] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981.

[35] 《传输控制协议》,标准7,RFC 793,1981年9月。

[36] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's Robustness to Blind In-Window Attacks", Work in Progress, July 2007.

[36] Ramaiah,A.,Stewart,R.,和M.Dalal,“提高TCP对窗口盲攻击的鲁棒性”,正在进行的工作,2007年7月。

[37] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.

[37] Rekhter,Y.,Ed.,Li,T.,Ed.,和S.Hares,Ed.,“边境网关协议4(BGP-4)”,RFC 42712006年1月。

[38] Semke, J., J. Mahdavi, and M. Mathis, "Automatic TCP Buffer Tuning", ACM SIGCOMM '98/ Computer Communication Review, volume 28, number 4, Oct. 1998.

[38] Semke,J.,J.Mahdavi和M.Mathis,“自动TCP缓冲区调整”,ACM SIGCOMM'98/计算机通信评论,第28卷,第4期,1998年10月。

[39] Shepard, T., "Reassign Port Number option for TCP", Work in Progress, July 2004.

[39] Shepard,T.,“为TCP重新分配端口号选项”,正在进行的工作,2004年7月。

[40] Shirey, R., "Internet Security Glossary, Version 2", Work in Progress, November 2006.

[40] Shirey,R.,“互联网安全词汇表,第2版”,正在进行的工作,2006年11月。

[41] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V. Paxson, "Stream Control Transmission Protocol", RFC 2960, October 2000.

[41] Stewart,R.,Xie,Q.,Morneault,K.,Sharp,C.,Schwarzbauer,H.,Taylor,T.,Rytina,I.,Kalla,M.,Zhang,L.,和V.Paxson,“流控制传输协议”,RFC 29602000年10月。

[42] TCPM: IETF TCPM Working Group and mailing list, <>.

[42] TCPM:IETF TCPM工作组和邮件列表<>.

[43] Touch, J., "Report on MD5 Performance", RFC 1810, June 1995.

[43] Touch,J.,“MD5性能报告”,RFC1810,1995年6月。

[44] Touch, J., "Performance Analysis of MD5", Proc. Sigcomm 1995, pp. 77-86, Mar. 1999.

[44] Touch,J.,“MD5的性能分析”,过程。Sigcomm 1995,第77-86页,1999年3月。

[45] Touch, J., "ANONsec: Anonymous Security to Defend Against Spoofing Attacks", Work in Progress, May 2004.

[45] Touch,J.,“ANONsec:匿名安全以防御欺骗攻击”,正在进行的工作,2004年5月。

[46] Touch, J., Black, D., and Y. Wang, "Problem and Applicability Statement for Better Than Nothing Security (BTNS)", Work in Progress, February 2007.

[46] Touch,J.,Black,D.,和Y.Wang,“比没有更好的安全性(BTNS)的问题和适用性声明”,正在进行的工作,2007年2月。

[47] Touch, J. and A. Mankin, "The TCP Simple Authentication Option", Work in Progress, July 2007.

[47] Touch,J.和A.Mankin,“TCP简单身份验证选项”,正在进行的工作,2007年7月。

[48] Watson, P., "Slipping in the Window: TCP Reset attacks", Presentation at 2004 CanSecWest, <>.

[48] Watson,P.,“在窗口中滑动:TCP重置攻击”,在2004年CanSecWest上的演讲<>.

[49] Wood, L., Post to TCPM mailing list regarding use of ISN in RSTs, ID=Pine.GSO.4.50.0404232249570.5889-, Apr. 23, 2004, < msg00213.html>.

[49] Wood,L.,向TCPM邮寄关于在RSTs中使用ISN的邮件列表,,2004年4月23日< msg00213.html>。

Author's Addresses


Joe Touch USC/ISI 4676 Admiralty Way Marina del Rey, CA 90292-6695 U.S.A.

Joe Touch USC/ISI 4676美国加利福尼亚州玛丽娜·德雷海军部路90292-6695号。

   Phone: +1 (310) 448-9151
   Fax:   +1 (310) 448-9300
   Phone: +1 (310) 448-9151
   Fax:   +1 (310) 448-9300

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